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Stem cell-based therapy for human diseases

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• Review Article
• Open access
• Published: 06 August 2022

Stem cell-based therapy for human diseases

• Duc M. Hoang, 
• Phuong T. Pham, 
• Trung Q. Bach, 
• Anh T. L. Ngo, 
• Quyen T. Nguyen, 
• Trang T. K. Phan, 
• Giang H. Nguyen, 
• Phuong T. T. Le, 
• Van T. Hoang, 
• Nicholas R. Forsyth, 
• Michael Heke & 
• Liem Thanh Nguyen 

Signal Transduction and Targeted Therapy volume 7, Article number: 272 (2022) Cite this article

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Abstract

Recent advancements in stem cell technology open a new door for patients suffering from diseases and disorders that have yet to be treated. Stem cell-based therapy, including human pluripotent stem cells (hPSCs) and multipotent mesenchymal stem cells (MSCs), has recently emerged as a key player in regenerative medicine. hPSCs are defined as self-renewable cell types conferring the ability to differentiate into various cellular phenotypes of the human body, including three germ layers. MSCs are multipotent progenitor cells possessing self-renewal ability (limited in vitro) and differentiation potential into mesenchymal lineages, according to the International Society for Cell and Gene Therapy (ISCT). This review provides an update on recent clinical applications using either hPSCs or MSCs derived from bone marrow (BM), adipose tissue (AT), or the umbilical cord (UC) for the treatment of human diseases, including neurological disorders, pulmonary dysfunctions, metabolic/endocrine-related diseases, reproductive disorders, skin burns, and cardiovascular conditions. Moreover, we discuss our own clinical trial experiences on targeted therapies using MSCs in a clinical setting, and we propose and discuss the MSC tissue origin concept and how MSC origin may contribute to the role of MSCs in downstream applications, with the ultimate objective of facilitating translational research in regenerative medicine into clinical applications. The mechanisms discussed here support the proposed hypothesis that BM-MSCs are potentially good candidates for brain and spinal cord injury treatment, AT-MSCs are potentially good candidates for reproductive disorder treatment and skin regeneration, and UC-MSCs are potentially good candidates for pulmonary disease and acute respiratory distress syndrome treatment.

요약

최근 줄기세포 기술의 발전은 

아직 치료법이 없는 질병과 장애로 고통받는 환자들에게 

새로운 치료의 문을 열어주고 있습니다. 

인간 만능 줄기세포(hPSC)와 다능 중간엽 줄기세포(MSC)를 

포함한 줄기세포 기반 치료는

최근 재생 의학의 핵심으로 부상하고 있습니다.

human pluripotent stem cells (hPSCs) and

multipotent mesenchymal stem cells (MSCs)

hPSC는

세 개의 생식 세포층을 포함하여

인체의 다양한 세포 표현형으로 분화할 수 있는 능력을 부여하는

자가 재생 가능한 세포 유형으로 정의됩니다.

국제 세포 및 유전자 치료 학회(ISCT)에 따르면

MSC는

자가 재생 능력(시험관 내에서 제한적)과 중간엽 계통으로의 분화 잠재력을 지닌

다능성 전구세포입니다.

이 리뷰에서는

신경계 질환, 폐 기능 장애, 대사/내분비 관련 질환, 생식 장애, 피부 화상, 심혈관 질환 등

인간 질환 치료를 위해

골수(BM), 지방 조직(AT) 또는 탯줄(UC)에서 유래한

hPSC 또는 MSC를 사용한 최근 임상 적용 사례에 대한 최신 정보를 제공합니다.

또한

임상 환경에서 MSC를 이용한 표적 치료법에 대한 자체 임상 시험 경험을 논의하고,

재생 의학의 중개 연구를 임상 응용 분야로 촉진하는 것을 궁극적인 목표로

MSC 조직 기원 개념과 다운스트림 응용 분야에서

MSC의 역할에 기여할 수 있는 방법에 대해 제안하고 논의합니다.

여기서 논의되는 메커니즘은

BM-MSC가 뇌 및 척수 손상 치료에,

AT-MSC가 생식 장애 치료 및 피부 재생에,

UC-MSC가 폐 질환 및 급성호흡곤란증후군 치료에

잠재적으로 좋은 후보라는 제안된 가설을 뒷받침합니다.

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Introduction

The successful approval of cancer immunotherapies in the US and mesenchymal stem cell (MSC)-based therapies in Europe have turned the wheel of regenerative medicine to become prominent treatment modalities.1,2,3 Cell-based therapy, especially stem cells, provides new hope for patients suffering from incurable diseases where treatment approaches focus on management of the disease not treat it. Stem cell-based therapy is an important branch of regenerative medicine with the ultimate goal of enhancing the body repair machinery via stimulation, modulation, and regulation of the endogenous stem cell population and/or replenishing the cell pool toward tissue homeostasis and regeneration.4 Since the stem cell definition was introduced with their unique properties of self-renewal and differentiation, they have been subjected to numerous basic research and clinical studies and are defined as potential therapeutic agents. As the main agenda of regenerative medicine is related to tissue regeneration and cellular replacement and to achieve these targets, different types of stem cells have been used, including human pluripotent stem cells (hPSCs), multipotent stem cells and progenitor cells.5 However, the emergence of private and unproven clinics that claim the effectiveness of stem cell therapy as “magic cells” has raised highly publicized concerns about the safety of stem cell therapy. The most notable case involved the injection of a cell population derived from fractionated lipoaspirate into the eyes of three patients diagnosed with macular degeneration, resulting in the loss of vision for these patients.6 Thus, as regenerative medicine continues to progress and evolve and to clear the myth of the “magic” cells, this review provides a brief overview of stem cell-based therapy for the treatment of human diseases.

Stem cell therapy is a novel therapeutic approach that utilizes the unique properties of stem cells, including self-renewal and differentiation, to regenerate damaged cells and tissues in the human body or replace these cells with new, healthy and fully functional cells by delivering exogenous cells into a patient.7 Stem cells for cell-based therapy can be of (1) autologous, also known as self-to-self therapy, an approach using the patient’s own cells, and (2) allogeneic sources, which use cells from a healthy donor for the treatment.8 The term “stem cell” were first used by the eminent German biologist Ernst Haeckel to describe the properties of fertilized egg to give rise to all cells of the organism in 1868.9 The history of stem cell therapy started in 1888, when the definition of stem cell was first coined by two German zoologists Theodor Heinrich Boveri and Valentin Haecker,9 who set out to identify the distinct cell population in the embryo capable of differentiating to more specialized cells (Fig. 1a).

소개

미국에서는 암 면역 요법이,

유럽에서는 중간엽 줄기세포(MSC) 기반 치료법이 성공적으로 승인되면서

재생 의학이 주목받는 치료법으로 부상했습니다.1,2,3

세포 기반 치료,

특히 줄기세포는 질병의 치료가 아닌 관리에 초점을 맞춘 치료법으로

난치병으로 고통받는 환자들에게 새로운 희망을 선사합니다.

줄기세포 기반 치료는

내인성 줄기세포 집단의 자극,

조절 및 조절을 통해 신체 복구 메커니즘을 강화하거나

조직 항상성과 재생을 위해 세포 풀을 보충하는 것을 궁극적인 목표로 하는

재생 의학의 중요한 분야입니다.4

줄기세포는

자기 재생과 분화라는 독특한 특성을 가지고 도입된 이후

수많은 기초 연구와 임상 연구를 거쳐 잠재적인 치료제로 정의되고 있습니다.

재생 의학의 주요 의제는

조직 재생 및 세포 대체와 관련이 있으며

이러한 목표를 달성하기 위해 인간 만능 줄기세포(hPSC),

다능 줄기세포 및 전구 세포 등

다양한 유형의 줄기세포가 사용되었습니다.5

그러나

줄기세포 치료의 효과를 '마법의 세포'라고 주장하는

검증되지 않은 사설 클리닉이 등장하면서

줄기세포 치료의 안전성에 대한 우려가 크게 제기되고 있습니다.

가장 주목할 만한 사례는

황반변성 진단을 받은 환자 3명의 눈에

지방흡인분획물에서 추출한 세포군을 주입하여 시력을 잃게 한 사건입니다.6

이처럼 재생 의학이 계속 발전하고 진화하면서

'마법' 세포에 대한 신화를 없애기 위해

이 리뷰에서는 인간 질병 치료를 위한 줄기세포 기반 치료에 대해 간략히 살펴봅니다.

줄기세포 치료는

자가 재생 및 분화 등 줄기세포의 고유한 특성을 활용하여

인체의 손상된 세포와 조직을 재생하거나

외인성 세포를 환자에게 전달함으로써

이러한 세포를 새롭고 건강하며 완전한 기능을 갖춘 세포로 대체하는 새로운 치료법입니다.7

세포 기반 치료용 줄기세포는

(1) 환자 자신의 세포를 사용하는 접근 방식인 자가 치료라고도 하는 자가 세포와

(2) 건강한 기증자의 세포를 치료에 사용하는 동종이식이 될 수 있습니다.8

“줄기세포"라는 용어는 1868년 독일의 저명한 생물학자 에른스트 헤켈(Ernst Haeckel)이 생물의 모든 세포를 생성하는 수정란의 특성을 설명하기 위해 처음 사용했습니다.9 줄기세포 치료의 역사는 1888년 독일의 동물학자 테오도르 하인리히 보베리와 발렌틴헤커9가 배아에서 보다 특화된 세포로 분화할 수 있는 고유한 세포 집단을 확인하기 위해 줄기세포의 정의를 처음 만들면서 시작되었습니다(그림 1a)에서부터 시작되었습니다.

In 1902, studies carried out by the histologist Franz Ernst Christian Neumann, who was working on bone marrow research, and Alexander Alexandrowitsch Maximov demonstrated the presence of common progenitor cells that give rise to mature blood cells, a process also known as haematopoiesis.10 From this study, Maximov proposed the concept of polyblasts, which later were named stem cells based on their proliferation and differentiation by Ernst Haeckel.11 Maximov described a hematopoietic population presented in the bone marrow. In 1939, the first case report described the transplantation of human bone marrow for a patient diagnosed with aplastic anemia. Twenty years later, in 1958, the first stem cell transplantation was performed by the French oncologist George Mathe to treat six nuclear researchers who were accidentally exposed to radioactive substances using bone marrow transplantation.12 Another study by George Mathe in 1963 shed light on the scientific community, as he successfully conducted bone marrow transplantation in a patient with leukemia. The first allogeneic hematopoietic stem cell transplantation (HSCT) was pioneered by Dr. E. Donnall Thomas in 1957.13 In this initial study, all six patients died, and only two patients showed evidence of transient engraftment due to the unknown quantities and potential hazards of bone marrow transplantation at that time. In 1969, Dr. E. Donnall Thomas conducted the first bone marrow transplantation in the US, although the success of the allogeneic treatment remained exclusive. In 1972, the year marked the discovery of cyclosporine (the immune suppressive drug),14 the first successes of allogeneic transplantation for aplastic anemia and acute myeloid leukemia were reported in a 16-year-old girl.15 

1902년 골수 연구를 수행하던 조직학자 프란츠 에른스트 크리스티안 노이만과 알렉산드로비치 막시모프는 조혈이라고도 알려진 성숙한 혈액 세포를 생성하는 공통 전구 세포의 존재를 입증했습니다.10 이 연구에서 막시모프는 다분화세포의 개념을 제안했고, 이후 에른스트 헤켈이 증식 및 분화를 기반으로 줄기세포로 명명했습니다.11 막시모프는 골수에서 나타나는 조혈 세포군을 설명했습니다.

1939년 재생불량성 빈혈 진단을 받은 환자에게

인간 골수를 이식한 최초의 사례 보고가 발표되었습니다.

20년 후인 1958년,

프랑스 종양학자인 조지 마테가

우연히 방사성 물질에 피폭된 6명의 원자력 연구원을 골수 이식을 통해 치료하기 위해

최초의 줄기세포 이식을 시행했습니다.12

1963년 조지 마테가

백혈병 환자에게 골수 이식을 성공적으로 시행하면서

과학계의 주목을 받기도 했습니다.

최초의 동종 조혈모세포 이식(HSCT)은

1957년 E. 도널 토마스 박사에 의해 개척되었습니다.13

이 초기 연구에서는 6명의 환자가 모두 사망했으며,

당시에는 골수 이식의 양과 잠재적 위험성을 알 수 없었기 때문에

두 명의 환자만 일시적 생착의 증거를 보였습니다.

1969년 E. 도널 토마스 박사가

미국에서 최초의 골수 이식을 시행했지만,

동종 치료의 성공은 여전히 독점적이었습니다.

1972년은 사이클로스포린(면역 억제제)이 발견된 해로,14

재생 불량성 빈혈과 급성 골수성 백혈병에 대한 동종 이식이

16세 소녀에게 처음으로 성공했다는 보고가 있었습니다.15

From the 1960s to the 1970s, series of works conducted by Friendenstein and coworkers on bone marrow aspirates demonstrated the relationship between osteogenic differentiation and a minor subpopulation of cells derived from bone marrow.16 These cells were later proven to be distinguishable from the hematopoietic population and to be able to proliferate rapidly as adherent cells in tissue culture vessels. Another important breakthrough from Friendenstein’s team was the discovery that these cells could form the colony-forming unit when bone marrow was seeded as suspension culture following by differentiation into osteoblasts, adipocytes, and chondrocytes, suggesting that these cells confer the ability to proliferate and differentiate into different cell types.17 In 1991, combined with the discovery of human embryonic stem cells (hESCs), which will be discussed in the next section, the term “mesenchymal stem cells”, previously known as stromal stem cells or “osteogenic” stem cells, was first coined in Caplan and widely used to date.18 Starting with bone marrow transplantation 60 years ago, the journey of stem cell therapy has developed throughout the years to become a novel therapeutic agent of regenerative medicine to treat numerous incurable diseases, which will be reviewed and discussed in this review, including neurological disorders, pulmonary dysfunctions, metabolic/endocrine-related diseases, reproductive disorders, skin burns, and cardiovascular conditions).

1960년대부터 1970년대까지 프렌덴슈타인과 동료들은

골수 흡인물에 대한 일련의 연구를 통해

골 형성 분화와 골수 유래 세포의 작은 하위집단 사이의 관계를 입증했습니다.16

이 세포는 나중에

조혈 세포 집단과 구별할 수 있고 조직 배양 용기에서

부착 세포로서 빠르게 증식할 수 있다는 것이 입증되었습니다.

프렌덴슈타인 연구팀의 또 다른 중요한 혁신은

골수를 현탁 배양하여

조골세포, 지방세포, 연골세포로 분화시킨 후

이 세포들이 콜로니 형성 단위를 형성할 수 있다는 사실을 발견하여

이 세포들이 다른 세포 유형으로 증식 및 분화할 수 있는 능력을 부여한다는 것을 시사한 것입니다.17

1991년,

다음 섹션에서 설명할

인간 배아 줄기세포(hESC)의 발견과 함께 이전에

기질 줄기세포 또는 “골 형성” 줄기세포로 알려진 “중간엽 줄기세포”라는 용어가

Caplan에 의해 처음 만들어져 현재까지 널리 사용되고 있습니다.18

60년 전 골수 이식을 시작으로 줄기세포 치료의 여정은

수년에 걸쳐 발전하여

신경계 질환, 폐 기능 장애, 대사/내분비 관련 질환, 생식 장애, 피부 화상, 심혈관 질환 등

이 리뷰에서 검토 및 논의될 수많은 난치병을 치료하는 새로운 재생 의학 치료제로 자리 잡았습니다).

Fig. 1

Stem cell-based therapy: the history and cell source. a The timeline of major discoveries and advances in basic research and clinical applications of stem cell-based therapy. The term “stem cells” was first described in 1888, setting the first milestone in regenerative medicine. The hematopoietic progenitor cells were first discovered in 1902. In 1939, the first bone marrow transplantation was conducted in the treatment of aplasmic anemia. Since then, the translation of basic research to preclinical studies to clinical trials has driven the development of stem cell-based therapy by many discoveries and milestones. The isolations of “mesenchymal stem cells” in 1991 following by the discovery of human pluripotent stem cells have recently contributed to the progress of stem cell-based therapy in the treatment of human diseases. b Schematic of the different cell sources that can be used in stem cell-based therapy. (1) Human pluripotent stem cells, including embryonic stem cells (derived from inner cell mass of blastocyst) and induced pluripotent stem cells confer the ability to proliferate indefinitely in vitro and differentiate into numerous cell types of the human body, including three germ layers. (2) Mesenchymal stem cells are multipotent stem cells derived from mesoderm possessing self-renewal ability (limited in vitro) and differentiation potential into mesenchymal lineages. The differentiated/somatic cells can be reprogrammed back to the pluripotent stage using OSKM factors to generate induced pluripotent stem cells. It is important to note that stem cells show a relatively higher risk of tumor formation and lower risk of immune rejection (in the case of mesenchymal stem cells) when compared to that of somatic cells.

줄기세포 기반 치료: 역사와 세포 공급원

a. 줄기세포 기반 치료의 기초 연구 및 임상 적용에 대한 주요 발견과 발전의 타임라인 .

'줄기세포'라는 용어는 1888년에 처음 설명되어 재생 의학의 첫 번째 이정표를 세웠습니다.

조혈 전구세포는 1902년에 처음 발견되었습니다.

1939년에는 재생불량성 빈혈 치료를 위해 최초의 골수 이식이 시행되었습니다.

그 이후로 기초 연구에서 전임상 연구, 임상 시험으로 전환되면서

많은 발견과 이정표가 줄기세포 기반 치료법의 발전을 이끌었습니다.

1991년 인간 만능 줄기세포의 발견에 이은 중간엽 줄기세포의 분리는

최근 인간 질병 치료에 있어 줄기세포 기반 치료의 발전에 기여하고 있습니다.

b 줄기세포 기반 치료에 사용할 수 있는 다양한 세포 공급원의 모식도.

(1) 배아 줄기세포(배반포의 내부 세포 덩어리에서 유래)와 유도 만능 줄기세포를 포함한

인간 만능 줄기세포는

시험관 내에서 무한히 증식하고

세 개의 생식층을 포함한 인체의 다양한 세포 유형으로 분화할 수 있는 능력을 부여합니다.

(2) 중간엽 줄기세포는

중배엽에서 유래한 다능성 줄기세포로서 자기 재생 능력(시험관 내에서 제한적)과

중간엽 계통으로 분화할 수 있는 잠재력을 가지고 있습니다.

분화/체세포는

OSKM 인자를 사용하여 유도만능줄기세포를 생성하기 위해

다시 만능 단계로 리프로그래밍할 수 있습니다.

줄기세포는

체세포에 비해 종양 형성 위험이 상대적으로 높고

면역 거부 반응의 위험이 낮다는 점(중간엽 줄기세포의 경우)에 유의해야 합니다.

The figure was created with BioRender.com

Full size image

In this review, we described the different types of stem cell-based therapies (Fig. 1b), including hPSCs and MSCs, and provided an overview of their definition, history, and outstanding clinical applications. In addition, we further created the first literature portfolio for the “targeted therapy” of MSCs based on their origin, delineating their different tissue origins and downstream applications with an in-depth discussion of their mechanism of action. Finally, we provide our perspective on why the tissue origin of MSCs could contribute greatly to their downstream applications as a proposed hypothesis that needs to be proven or disproven in the future to further enhance the safety and effectiveness of stem cell-based therapy.

이 리뷰에서는

중간엽줄기세포와 중간엽줄기세포를 포함한 다양한 유형의 줄기세포 기반 치료법(그림 1b)에 대해 설명하고,

그 정의, 역사 및 뛰어난 임상 적용 사례에 대한 개요를 제공했습니다.

또한,

MSC의 기원을 기반으로 한 '표적 치료'에 대한

최초의 문헌 포트폴리오를 작성하여

다양한 조직 기원과 하위 응용 분야를 설명하고

그 작용 메커니즘에 대한 심층적인 논의를 진행했습니다.

마지막으로,

줄기세포 기반 치료의 안전성과 효과를 더욱 향상시키기 위해

향후 입증 또는 반증되어야 할 가설로서

MSC의 조직 기원이 다운스트림 응용에 크게 기여할 수 있는 이유에 대한 관점을 제시합니다.

Stem cell-based therapy: an overview of current clinical applications

Cardiovascular diseases

The clinical applications of stem cell-based therapies for heart diseases have been recently discussed comprehensively in the reviews19,20 and therefore will be elaborated in this study as the focus discussions related to hPSCs and MSCs in the following sections. In general, the safety profiles of stem cell-based therapies are supported by a large body of preclinical and clinical studies, especially adult stem cell therapy (such as MSC-based products). However, clinical trials have not yet yielded data supporting the efficacy of the treatment, as numerous studies have shown paradoxical results and no statistically significant differences in infarct size, cardiac function, or clinical outcomes, even in phase III trials.21 The results of a meta-analysis showed that stem cells derived from different sources did not exhibit any therapeutic effects on the improvement of myocardial contractility, cardiovascular remodeling, or clinical outcomes.22 The disappointing results obtained from the clinical trials thus far could be explained by the fact that the administered cells may exert their therapeutic effects via an immune modulation rather than regenerative function. Thus, well-designed, randomized and placebo-controlled phase III trials with appropriate cell-preparation methods, patient selection, follow-up schedules and suitable clinical measurements need to be conducted to determine the efficacy of the treatments. In addition, concerns related to optimum cell source and dose, delivery route and timing of administration, cell distribution post administration and the mechanism of action also need to be addressed. In the following section of this review, we present clinical trials related to MSC-based therapy in cardiovascular disease with the aim of discussing the contradictory results of these trials and analyzing the potential challenges underlying the current approaches.

줄기세포 기반 치료: 현재 임상 적용에 대한 개요

심혈관 질환

심장 질환에 대한 줄기세포 기반 치료의 임상 적용은

최근 리뷰19,20에서 포괄적으로 논의되었으므로

본 연구에서는 다음 섹션에서 hPSC 및 MSC와 관련된 중점 논의로 상세히 설명합니다.

일반적으로

줄기세포 기반 치료제의 안전성 프로파일은 많은 전임상 및 임상 연구,

특히 성체 줄기세포 치료(예: MSC 기반 제품)에 의해 뒷받침됩니다.

그러나

임상시험에서는 아직 치료의 효능을 뒷받침하는 데이터가 나오지 않았으며,

수많은 연구에서 역설적인 결과가 나타났고

3상 시험에서도 경색 크기, 심장 기능 또는 임상 결과에서 통계적으로 유의미한 차이가 나타나지 않았습니다.21

메타 분석 결과에 따르면

다양한 출처에서 유래한 줄기세포는

심근 수축성, 심혈관 리모델링 또는 임상 결과 개선에 치료 효과를 나타내지 않았습니다.22

지금까지 임상 시험에서 얻은 실망스러운 결과는 투여된 세포가 재생 기능보다는 면역 조절을 통해 치료 효과를 발휘할 수 있다는 사실로 설명할 수 있습니다. 따라서 치료의 효능을 확인하기 위해서는 적절한 세포 준비 방법, 환자 선정, 추적 관찰 일정 및 적절한 임상 측정을 통해 잘 설계된 무작위 배정 및 위약 대조 3상 임상시험을 실시해야 합니다.

또한 최적의 세포 공급원 및 용량, 전달 경로 및 투여 시기, 투여 후 세포 분포 및 작용 메커니즘과 관련된 문제도 해결해야 합니다. 이 리뷰의 다음 섹션에서는 이러한 임상시험의 모순된 결과를 논의하고 현재 접근법의 근간이 되는 잠재적 과제를 분석하기 위해 심혈관 질환에 대한 MSC 기반 치료와 관련된 임상시험을 소개합니다.

Digestive system diseases

Gastrointestinal diseases are among the most diagnosed conditions in the developed world, altering the life of one-third of individuals in Western countries. The gastrointestinal tract is protected from adverse substances in the gut environment by a single layer of epithelial cells that are known to have great regenerative ability in response to injuries and normal cell turnover.23 These epithelial cells have a rapid turnover rate of every 2–7 days under normal conditions and even more rapidly following tissue damage and inflammation. This rapid proliferation ability is possible owing to the presence of a specific stem cell population that is strictly compartmentalized in the intestinal crypts.24 The gastrointestinal tract is highly vulnerable to damage, tissue inflammation and diseases once the degradation of the mucosal lining layer occurs. The exposure of intestinal stem cells to the surrounding environment of the gut might result in the direct destruction of the stem cell layer or disruption of intestinal functions and lead to overt clinical symptoms.25 In addition, the accumulation of stem cell defects as well as the presence of chronic inflammation and stress also contributes to the reduction of intestinal stem cell quality.

In terms of digestive disorders, Crohn’s disease (CD) and ulcerative colitis are the two major forms of inflammatory bowel disease (IBD) and represent a significant burden on the healthcare system. The former is a chronic, uncontrolled inflammatory condition of the intestinal mucosa characterized by segmental transmural mucosal inflammation and granulomatous changes.26 The latter is a chronic inflammatory bowel disease affecting the colon and rectum, characterized by mucosal inflammation initiating in the rectum and extending proximal to the colon in a continuous fashion.27 Cellular therapy in the treatment of CD can be divided into haematopoietic stem cell-based therapy and MSC-based therapy. The indication and recommendation of using HSCs for the treatment of IBD were proposed in 1995 by an international committee with four important criteria: (1) refractory to immunosuppressive treatment; (2) persistence of the disease conditions indicated via endoscopy, colonoscopy or magnetic resonance enterography; (3) patients who underwent an imminent surgical procedure with a high risk of short bowel syndromes or refractory colonic disease; and (4) patients who refused to treat persistent perianal lesions using coloproctectomy with a definitive stroma implant.28 In the standard operation procedure, patents’ HSCs were recruited using cyclophosphamide, which is associated with granulocyte colony-stimulating factor (G-CSF), at two different administered concentrations (4 g/m2 and 2 g/m2). Recently, it was reported that high doses of cyclophosphamide do not improve the number of recruited HSCs but increase the risk of cardiac and bladder toxicity. An interest in using HSCTs in CD originated from case reports that autologous HSCTs can induce sustained disease remission in some29,30 but not all patients31,32,33 with CD. The first phase I trial was conducted in Chicago and recruited 12 patients with active moderate to severe CD refractory to conventional therapies. Eleven of 12 patients demonstrated sustained remission after a median follow-up of 18.5 months, and one patient developed recurrence of active CD.31 

The ASTIC trial (the Autologous Stem Cell Transplantation International Crohn Disease) was the first randomized clinical trial with the largest cohort of patients undergoing HSCT for refractory CD in 2015.34 The early report of the trial showed no statistically significant improvement in clinical outcomes of mobilization and autologous HSCT compared with mobilization followed by conventional therapy. In addition, the procedure was associated with significant toxicity, leading to the suggestion that HSCT for patients with refractory CD should not be widespread. Interestingly, by using conventional assessments for clinical trials for CD, a group reassessed the outcomes of patients enrolled in the ASTIC trial showing clinical and endoscopic benefits, although a high number of adverse events were also detected.35 A recent systematic review eval‎uated 18 human studies including 360 patients diagnosed with CD and showed that eleven studies confirmed the improvement of Crohn’s disease activity index between HSCT groups compared to the control group.36 Towards the cell sources, HSCs are the better sources as they afforded more stable outcomes when compared to that of MSC-based therapy.37 Moreover, autologous stem cells were better than their allogeneic counterparts.36 The safety of stem cell-based therapy in the treatment of CD has attracted our attention, as the risk of infection in patients with CD was relatively higher than that in those undergoing administration to treat cancer or other diseases. During the stem cell mobilization process, patient immunity is significantly compromised, leading to a high risk of infection, and requires carefully nursed and suitable antibiotic treatment to reduce the development of adverse events. Taken together, stem cell-based therapy for digestive disease reduced inflammation and improved the patient’s quality of life as well as bowel functions, although the high risk of adverse events needs to be carefully monitored to further improve patient safety and treatment outcomes.

소화기 질환

소화기 질환은

선진국에서 가장 많이 진단되는 질환 중 하나로,

서구 국가의 3분의 1에 해당하는 사람들의 삶을 변화시킵니다.

위장관은

손상과 정상적인 세포 회전율에 반응하여 재생 능력이 뛰어난 것으로 알려진

상피 세포의 단일 층에 의해 장내 환경의 유해 물질로부터 보호됩니다.23

이 상피 세포는 정상적인 조건에서

2~7일마다 빠르게 교체되며

조직 손상과 염증 후에는 더욱 빠르게 교체됩니다.

이러한 빠른 증식 능력은

장의 지하실에 엄격하게 구획된 특정 줄기세포 집단이 존재하기 때문에 가능합니다.24

위장관은

점막 내벽층의 퇴화가 발생하면

손상, 조직 염증 및 질병에 매우 취약합니다.

장 줄기세포가 장 주변 환경에 노출되면

줄기세포층이 직접 파괴되거나 장 기능에 장애가 발생하여 명

백한 임상 증상이 나타날 수 있습니다.25

또한

줄기세포 결함의 축적과 만성 염증 및 스트레스도

장 줄기세포의 질 저하를 유발합니다.

소화기 질환의 경우

크론병(CD)과 궤양성 대장염은

염증성 장 질환(IBD)의 두 가지 주요 형태이며

의료 시스템에 상당한 부담을 주고 있습니다.

전자는

장 점막의 만성적이고 조절되지 않는 염증성 질환으로,

분절성 경막염증과 육아종성 변화가 특징입니다.26

후자는

결장과 직장에 영향을 미치는 만성 염증성 장 질환으로,

직장에서 시작하여 결장 근위부로 지속적으로 점막 염증이 확장되는 것이 특징입니다.27

CD 치료에서 세포 치료는

조혈 줄기세포 기반 치료와 MSC 기반 치료로 나눌 수 있습니다.

조혈모세포를 IBD 치료에 사용하는 적응증과 권장 사항은

1995년 국제 위원회에서 네 가지 중요한 기준으로 제안되었습니다:

(1) 면역 억제 치료에 불응성,

(2) 내시경, 대장 내시경 또는 자기공명 장 조영술을 통해 나타난 질병 상태의 지속성,

(3) 단장 증후군 또는 불응성 대장 질환의 위험이 높은 임박한 수술 절차를 받은 환자,

(4) 결정적 기질 이식술을 이용한 대장 절제술로 지속적인 항문 주위 병변 치료를 거부한 환자.28

표준 수술 절차에서는 과립구 콜로니 자극 인자(G-CSF)와 관련된 사이클로포스파마이드를 두 가지 투여 농도(4 g/m2 및 2 g/m2)로 사용하여 특허의 HSC를 모집했습니다. 최근 고용량의 사이클로포스파마이드는 모집된 HSC의 수를 개선하지 않고 심장 및 방광 독성 위험을 증가시킨다는 보고가 있었습니다. CD에 HSCT를 사용하는 것에 대한 관심은 자가 HSCT가 일부29,30 환자에서 지속적인 질병 관해를 유도할 수 있다는 사례 보고에서 비롯되었지만 모든 CD 환자31,32,33에게 해당되는 것은 아닙니다. 첫 번째 임상 1상 시험은 시카고에서 실시되었으며, 기존 치료법에 불응하는 활동성 중등도에서 중증의 CD 환자 12명을 모집했습니다. 12명의 환자 중 11명이 18.5개월의 중앙 추적 관찰 후 지속적인 관해 상태를 보였으며, 1명의 환자에서 활동성 CD가 재발했습니다.31

ASTIC 시험(자가 줄기세포 이식 국제 크론병 임상시험)은 2015년에 난치성 CD로 HSCT를 받은 환자 코호트가 가장 큰 최초의 무작위 임상시험이었습니다.34 이 시험의 초기 보고에 따르면 동원 및 자가 HSCT의 임상 결과는 동원 후 기존 치료와 비교하여 통계적으로 유의미한 개선이 없는 것으로 나타났습니다. 또한 이 시술은 상당한 독성과 관련이 있었기 때문에 불응성 CD 환자에 대한 HSCT가 널리 보급되어서는 안 된다는 제안으로 이어졌습니다. 흥미롭게도, 한 연구 그룹은 CD에 대한 임상시험에 대한 기존의 평가를 사용하여 ASTIC 시험에 등록된 환자의 결과를 재평가하여 임상 및 내시경적 이점을 보여주었지만 많은 수의 부작용도 발견되었습니다.35

최근 체계적 문헌고찰에서는

CD로 진단받은 환자 360명을 포함한 18건의 인간 연구를 평가한 결과

11건의 연구에서 대조군에 비해 HSCT 그룹 간의 크론병 활성 지수 개선이 확인되었습니다.36

세포 공급원의 경우,

HSC는 MSC 기반 치료와 비교했을 때

더 안정적인 결과를 제공했기 때문에 더 나은 공급원입니다.37

또한 자가 줄기세포가 동종 줄기세포보다 더 우수했습니다.36

CD 환자의 감염 위험이

암이나 다른 질병 치료를 위해 투여하는 환자보다 상대적으로 높기 때문에

CD 치료에서 줄기세포 기반 치료의 안전성이 주목받고 있습니다.

줄기세포를 동원하는 과정에서

환자의 면역력이 크게 저하되어 감염 위험이 높기 때문에

부작용 발생을 줄이기 위해 세심한 간호와 적절한 항생제 치료가 필요합니다.

종합하면,

줄기세포 기반 소화기 질환 치료는 염증을 줄이고

환자의 삶의 질과 장 기능을 개선했지만,

환자의 안전과 치료 결과를 더욱 개선하기 위해서는 부작용의 높은 위험을 주의 깊게 모니터링해야 합니다.

Liver diseases

The liver is the largest vital organ in the human body and performs essential biological functions, including detoxification of the organism, metabolism, supporting digestion, vitamin storage, and other functions.38 The disruption of liver homeostasis and function might lead to the development of pathological conditions such as liver failure, cirrhosis, cancer, alcoholic liver disease, nonalcoholic fatty liver disease (NAFLD), and autoimmune liver disease (ALD). Orthotropic liver transplantation is the only effective treatment for severe liver diseases, but the number of available and suitable donor organs is very limited. Currently, stem cell-based therapies in the treatment of liver disease are associated with HSCs, MSCs, hPSCs, and liver progenitor cells.

Liver failure is a critical condition characterized by severe liver dysfunctions or decompensation caused by numerous factors with a relatively high mortality rate. Stem cell-based therapy is a novel alternative approach in the treatment of liver failure, as it is believed to participate in the enhancement of liver regeneration and recovery. The results of a meta-analysis including four randomized controlled trials and six nonrandomized controlled trials in the treatment of acute-on-chronic liver failure (ACLF) demonstrated that clinical outcomes of stem cell therapy were achieved in the short term, requiring multiple doses of stem cells to prolong the therapeutic effects.39,40 Interestingly, although MSC-based therapies improved liver functions, including the model of end-stage liver disease score, albumin level, total bilirubin, and coagulation, beneficial effects on survival rate and aminotransferase level were not observed.41 A randomized controlled trial illustrated the improvement of liver functions and reduction of severe infections in patients with hepatitis B virus-related ACLF receiving allogeneic bone marrow-derived MSCs (BM-MSCs) via peripheral infusion.42 HSCs from peripheral blood after the G-CSF mobilization process were used in a phase I clinical trial and exhibited an improvement in serum bilirubin and albumin in patients with chronic liver failure without any specific adverse events related to the administration.43 Taken together, an overview of stem cell-based therapy in the treatment of liver failure indicates the potential therapeutic effects on liver functions with a strong safety profile, although larger randomized controlled trials are still needed to assure the conclusions.

Liver cirrhosis is one of the major causes of morbidity and mortality worldwide and is characterized by diffuse nodular regeneration with dense fibrotic septa and subsequent parenchymal extinction leading to the collapse of liver vascular structure.44 In fact, liver cirrhosis is considered the end-stage of liver disease that eventually leads to death unless liver transplantation is performed. Stem cell-based therapy, especially MSCs, currently emerges as a potential treatment with encouraging results for treating liver cirrhosis. In a clinical trial using umbilical cord-derived MSCs (UC-MSCs), 45 chronic hepatitis B patients with decompensated liver cirrhosis were divided into two groups: the MSC group (n = 30) and the control group (n = 15).45 The results showed a significant reduction in ascites volume in the MSC group compared with the control. Liver function was also significantly improved in the MSC groups, as indicated by the increase in serum albumin concentration, reduction in total serum bilirubin levels, and decrease in the sodium model for end-stage liver disease score.45 Similar results were also reported from a phase II trial using BM-MSCs in 25 patients with HCV-induced liver cirrhosis.46 Consistent with these studies, three other clinical trials targeting liver cirrhosis caused by hepatitis B and alcoholic cirrhosis were conducted and confirmed that MSC administration enhanced and recovered liver functions.47,48,49 With the large cohort study as the clinical trial conducted by Fang, the safety and potential therapeutic effects of MSC-based therapies could be further strengthened and confirmed the feasibility of the treatment in virus-related liver cirrhosis.49 In terms of delivery route, a randomized controlled phase 2 trial suggested that systemic delivery of BM-MSCs does not show therapeutic effects on patients with liver cirrhosis.50 MSCs are not the only cell source for liver cirrhosis. Recently, an open-label clinical trial conducted in 19 children with liver cirrhosis due to biliary atresia after the Kasai operation illustrated the safety and feasibility of the approach by showing the improvement of liver function after bone marrow mononuclear cell (BMNC) administration assessed by biochemical tests and pediatric end-stage liver disease (PELD) scores.51 Another study using BMNCs in 32 decompensated liver cirrhosis patients illustrated the safety and effectiveness of BMNC administration in comparison with the control group.52 Recently, a long-term analysis of patients receiving peripheral blood-derived stem cells indicated a significant improvement in the long-term survival rate when compared to the control group, and the risk of hepatocellular carcinoma formation did not increase.53 CD133+ HSC infusion was performed in a multicentre, open, randomized controlled phase 2 trial in patients with liver cirrhosis; the results did not support the improvement of liver conditions, and cirrhosis persisted.54 Notably, these results are in line with a previous randomized controlled study, which also reported that G-CSF and bone marrow-derived stem cells delivered via the hepatic artery did not introduce therapeutic potential as expected.55 Thus, stem cell-based therapy for liver cirrhosis is still in its immature stage and requires larger trials with well-designed experiments to confirm‎ the efficacy of the treatment.

Nonalcoholic fatty liver disease (NAFLD) is the most common medical condition caused by genetic and lifestyle factors and results in a severe liver condition and increased cardiovascular risk.56 NAFLD is the hidden enemy, as most patients are asymptomatic for a long time, and their routine life is unaffected. Thus, the detection, identification, and management of NAFLD conditions are challenging tasks, as patients diagnosed with NAFLD often develop nonalcoholic steatohepatitis, cirrhosis, and hepatocellular carcinoma.57 Although preclinical studies have shown that stem cell administration could enhance liver function in NAFLD models, a limited number of clinical trials were performed in human subjects. Recently, a multi-institutional clinical trial using freshly isolated autologous adipose tissue-derived regenerative cells was performed in Japan to treat seven NAFLD patients.58 The results illustrated the improvement in the serum albumin level of six patients and prothrombin activity of five patients, and no treatment-related adverse events or severe adverse events were observed. This study illustrates the therapeutic potential of stem cell-based therapy in the treatment of NAFLD.

Autoimmune liver disease (ALD) is a severe liver condition affecting children and adults worldwide, with a female predominance.59 The condition occurs in genetically predisposed patients when a stimulator, such as virus infection, leads to a T-cell-mediated autoimmune response directed against liver autoantigens. As a result, patients with ALD might develop liver cirrhosis, hepatocellular carcinoma, and, in severe cases, death. To date, HSCT and bone marrow transplantation are the two common stem cell-based therapies exhibiting therapeutic potential for ALD in clinical trials. An interesting report illustrated that haploidentical HSCTs could cure ALD in patients with sickle cells.60 This report is particularly important, as it illustrates the potential therapeutic approach of using haploidentical HSCTs to treat patients with both sickle cells and ALD. Another case report described a 19-year-old man with a 4-year history of ALD who developed acute lymphoblastic leukemia and required allogeneic bone marrow transplantation from this wholesome brother.61 The clinical data showed that immunosuppressive therapy for transplantation generated ALD remission in the patient.62 However, the data also provided valid information related to the sustained remission and the normalization of ASGPR-specific suppressor-inducer T-cell activity following bone marrow transplantation, suggesting that these suppressor functions originated from donor T cells.61 Thus, it was suggested that if standard immunosuppressive treatment fails, alternative cellular immunotherapy would be a viable option for patients with ALD. Primary biliary cholangitis (PBC), usually known as primary biliary cirrhosis, is a type of ALD characterized by a slow, progressive destruction of small bile ducts of the liver leading to the formation of cirrhosis and accumulation of bile and other toxins in the liver. A pilot, single-arm trial from China recruited seven patents with PBC who had a suboptimal response to ursodeoxycholic acid (UDCA) treatment.63 These patients received UDCA treatment in combination with three rounds of allogeneic UC-MSCs at 4-week intervals with a dose of 0.5 × 106 cells/kg of patient body weight via the peripheral vein. No treatment-related adverse events or severe adverse events were observed throughout the course of the study. The clinical data indicated significant improvement in liver function, including reduction of serum ALP and gamma-glutamyltransferase (GGT) at 48 weeks post administration. The common symptoms of PBC, including fatigue, pruritus, and hypogastric ascites volume, were also reduced, supporting the feasibility of MSC-based therapy in the treatment of PBC, although major limitations of the study were nonrandomized, no control group and small sample size. Another study was conducted in China with ten PBC patients who underwent incompetent UDCA treatment for more than 1 year. These patients received a range of 3–5 allogeneic BM-MSCs/kg body weight by intravenous infusion.64 Although these early studies have several limitations, such as small sample size, nonrandomization, and no control group, their preliminary data related to safety and efficacy herald the prospects and support the feasibility of stem cell-based therapy in the treatment of ALD.

In summary, the current number of trials for liver disease using stem cell-based therapy has provided fundamental data supporting the safety and potential therapeutic effects in various liver diseases. Unfortunately, due to the small number of trials, several obstacles need to be overcome to prove the effectiveness of the treatments, including (1) stem cell source and dose, (2) administration route, (3) time of intervention, and (4) clinical assessments during the follow-up period. Only by addressing these challenges we will be able to prove, facilitate and promote stem cell-based therapy as a mainstream treatment for liver diseases.

간 질환

간은 인체에서 가장 큰 생명 기관으로 해독, 신진대사, 소화 지원, 비타민 저장 등 필수적인 생물학적 기능을 수행합니다.38 간 항상성과 기능이 파괴되면 간부전, 간경변, 암, 알코올성 간 질환, 비알코올성 지방간 질환(NAFLD), 자가 면역 간 질환(ALD) 등의 병리적 질환이 발병할 수 있습니다. 동종 간 이식은 중증 간 질환에 대한 유일한 효과적인 치료법이지만, 이용 가능하고 적합한 기증 장기의 수는 매우 제한적입니다. 현재 간 질환 치료에 사용되는 줄기세포 기반 치료법은 간엽줄기세포, 중간엽줄기세포, 유도만능줄기세포, 간 전구세포와 관련이 있습니다.

간부전은

다양한 요인에 의해 발생하는 심각한 간 기능 장애 또는 보상 기능 저하를 특징으로 하는 심각한 질환으로

사망률이 상대적으로 높습니다.

줄기세포 기반 치료는

간 재생과 회복을 촉진하는 것으로 알려져

간부전 치료의 새로운 대안으로 떠오르고 있습니다.

급성-만성 간부전(ACLF) 치료에 대한 4건의 무작위 대조 시험과 6건의 비무작위 대조 시험을 포함한 메타 분석 결과에 따르면 줄기세포 치료의 임상 결과는 단기간에 달성되었으며, 치료 효과를 연장하기 위해서는 줄기세포를 여러 번 투여해야 합니다.39,40 흥미로운 점은 MSC 기반 치료가 말기 간 질환 점수, 알부민 수치, 총 빌리루빈, 응고를 포함한 간 기능을 개선했지만 생존율과 아미노전달효소 수치에는 유익한 효과가 관찰되지 않았습니다.41 무작위 대조 시험에서 말초 주입을 통해 동종 골수 유래 MSC(BM-MSC)를 투여받은 B형 간염 바이러스 관련 만성 간부전 환자에서 간 기능이 개선되고 중증 감염이 감소한 것으로 나타났습니다.42 G-CSF 동원 과정 후 말초 혈액에서 얻은 HSC를 1상 임상 시험에 사용한 결과 투여와 관련된 특별한 부작용 없이 혈청 빌리루빈 및 알부민이 개선된 것으로 나타났습니다.43 종합하면, 간부전 치료에서 줄기세포 기반 치료의 개요는 강력한 안전성 프로파일과 함께 간 기능에 대한 잠재적 치료 효과를 나타내지만, 결론을 확신하기 위해서는 대규모 무작위 대조 시험이 여전히 필요합니다.

간경변증은 전 세계적으로 이환율과 사망률의 주요 원인 중 하나이며, 섬유화 격막이 치밀하게 형성된 확산성 결절 재생과 그에 따른 실질 소멸로 간 혈관 구조가 붕괴되는 것이 특징입니다.44 실제로 간경변증은 간 이식을 하지 않으면 결국 사망에 이르는 간 질환의 말기 단계로 간주됩니다. 줄기세포 기반 치료, 특히 중간엽줄기세포는 현재 간경변증 치료에 고무적인 결과를 보이며 잠재적인 치료법으로 떠오르고 있습니다. 탯줄 유래 중간엽줄기세포(UC-MSC)를 이용한 임상시험에서 비대상성 간경변증을 가진 만성 B형 간염 환자 45명을 MSC 투여군(n= 30)과 대조군(n= 15)으로 나누었습니다.45 그 결과 대조군에 비해 MSC 투여군에서 복수의 양이 유의하게 감소한 것으로 나타났습니다. 혈청 알부민 농도 증가, 총 혈청 빌리루빈 수치 감소, 말기 간 질환 점수에 대한 나트륨 모델 감소로 알 수 있듯이 간 기능도 MSC 그룹에서 유의미하게 개선되었습니다.45 HCV로 인한 간경변증 환자 25명을 대상으로 BM-MSC를 사용한 2상 임상시험에서도 유사한 결과가 보고되었습니다.46 이러한 연구와 일관되게, B형 간염 및 알코올성 간경변증으로 인한 간경변증을 대상으로 한 다른 3건의 임상시험이 진행되어 MSC 투여가 간 기능을 개선하고 회복시키는 것을 확인했습니다.47,48,49 Fang이 수행한 임상시험과 같은 대규모 코호트 연구를 통해 MSC 기반 치료의 안전성과 잠재적 치료 효과를 더욱 강화하고 바이러스 관련 간경변증에서 치료 가능성을 확인할 수 있었습니다.49 전달 경로 측면에서 무작위 대조 2상 시험에서는 BM-MSC의 전신 전달이 간경변증 환자에게 치료 효과를 보이지 않는 것으로 나타났습니다.50 MSC가 간경변증의 유일한 세포 공급원은 아닙니다. 최근 카사이 수술 후 담도 폐쇄증으로 인한 간경변증 어린이 19명을 대상으로 실시한 오픈 라벨 임상시험에서 생화학 검사 및 소아 말기 간 질환(PELD) 점수로 평가한 골수 단핵 세포(BMNC) 투여 후 간 기능이 개선된 것으로 나타나 이 접근법의 안전성과 타당성을 입증했습니다.51 32명의 비대상성 간경변 환자를 대상으로 BMNC를 사용한 또 다른 연구에서는 대조군에 비해 BMNC 투여의 안전성과 효과를 입증했습니다.52 최근에는 말초혈액 유래 줄기세포를 투여받은 환자를 장기 분석한 결과 대조군에 비해 장기 생존율이 유의하게 개선되었으며 간세포암종 형성 위험은 증가하지 않은 것으로 나타났습니다.53 간경변 환자를 대상으로 한 다기관, 공개, 무작위 대조 2상 임상시험에서 CD133+ HSC 주입을 실시한 결과 간 상태가 개선되지 않았고 간경변이 지속되는 것으로 나타났습니다.54 특히 이러한 결과는 간동맥을 통해 전달된 G-CSF와 골수 유래 줄기세포가 기대만큼 치료 효과를 나타내지 못했다는 이전의 무작위 대조 연구와 일치합니다.55 따라서 줄기세포 기반 간경변 치료는 아직 미성숙 단계에 있으며 치료 효과를 확인하기 위해 잘 설계된 실험을 통한 대규모 임상시험이 필요합니다.

비알코올성 지방간 질환(NAFLD)은 유전적 및 생활습관 요인으로 인해 발생하는 가장 흔한 질환으로 심각한 간 상태와 심혈관 위험을 초래합니다.56 대부분의 환자는 오랫동안 증상이 없고 일상 생활에 영향을 받지 않기 때문에 NAFLD는 숨겨진 적입니다. 따라서 비알코올성 지방간염, 간경변, 간세포암종으로 발전하는 경우가 많기 때문에 NAFLD 상태를 감지, 식별, 관리하는 것은 어려운 과제입니다.57 전임상 연구에서 줄기세포 투여가 NAFLD 모델에서 간 기능을 향상시킬 수 있다는 사실이 밝혀졌지만, 사람을 대상으로 한 임상 시험은 제한적으로 수행되었습니다. 최근 일본에서 갓 분리한 자가 지방 조직 유래 재생 세포를 사용한 다기관 임상 시험이 7명의 NAFLD 환자를 대상으로 실시되었습니다.58 그 결과 6명의 환자 혈청 알부민 수치와 5명의 환자 프로트롬빈 활성도가 개선되었으며 치료 관련 이상 반응이나 심각한 부작용은 관찰되지 않았습니다. 이 연구는 비알코올성 지방간염 치료에 있어 줄기세포 기반 치료의 치료 가능성을 보여줍니다.

자가면역성 간질환(ALD)은 전 세계 어린이와 성인에게 영향을 미치는 중증 간 질환으로 여성이 더 많이 걸립니다.59 이 질환은 바이러스 감염과 같은 자극제가 간 자가 항원에 대한 T세포 매개 자가 면역 반응을 유발할 때 유전적 소인이 있는 환자에서 발생합니다. 그 결과, ALD 환자는 간경변증, 간세포암종, 심한 경우 사망에 이를 수 있습니다. 현재까지 임상시험에서 ALD에 대한 치료 가능성을 보여주는 두 가지 일반적인 줄기세포 기반 치료법은 HSCT와 골수 이식입니다. 한 흥미로운 보고서는 동형일치 HSCT가 겸상 적혈구 환자의 ALD를 치료할 수 있음을 보여주었습니다.60 이 보고서는 겸상 적혈구와 ALD를 모두 가진 환자를 치료하기 위해 동형일치 HSCT를 사용하는 잠재적 치료 접근법을 보여주기 때문에 특히 중요합니다. 또 다른 사례 보고에서는 급성 림프모구 백혈병이 발병하여 건강한 형으로부터 동종 골수 이식이 필요한 19세 남성에 대해 설명했습니다.61 임상 데이터에 따르면 이식을 위한 면역 억제 요법이 환자에게서 ALD 관해가 발생했습니다.62 그러나 이 데이터는 또한 골수 이식 후 지속적인 관해와 ASGPR 특이적 억제자 유도 T 세포 활성의 정상화와 관련된 유효한 정보를 제공하여 이러한 억제 기능이 기증자 T 세포에서 비롯되었음을 시사했습니다.61 따라서 표준 면역 억제 치료에 실패하면 대체 세포 면역 치료가 ALD 환자를위한 실행 가능한 옵션이 될 수 있다고 제안되었습니다. 일반적으로 원발성 담즙성 간경변으로 알려진 원발성 담즙성 담관염(PBC)은 간의 작은 담관이 천천히 점진적으로 파괴되어 간경변을 일으키고 담즙과 기타 독소가 간에 축적되는 것을 특징으로 하는 ALD의 한 유형입니다. 중국에서 실시된 파일럿 단일군 임상시험에서는 우르소데옥시콜산(UDCA) 치료에 차선책으로 반응한 간경변 환자 7명을 모집했습니다.63 이 환자들은 4주 간격으로 환자 체중의 0.5 × 106 세포/kg의 용량으로 세 차례의 동종 UC-MSC를 말초 정맥을 통해 투여하면서 UDCA 치료를 병행했습니다. 연구 기간 동안 치료 관련 이상 반응이나 심각한 이상 반응은 관찰되지 않았습니다. 임상 데이터에 따르면 투여 48주 후 혈청 ALP와 감마글루타밀전달효소(GGT)가 감소하는 등 간 기능이 유의하게 개선된 것으로 나타났습니다. 피로, 가려움증, 위 복수량 감소 등 PBC의 일반적인 증상도 감소하여 PBC 치료에서 MSC 기반 치료의 가능성을 뒷받침했지만 연구의 주요 한계는 비무작위, 대조군 없음, 표본 크기가 작다는 점입니다. 또 다른 연구는 중국에서 1년 이상 UDCA 치료를 받지 않은 10명의 PBC 환자를 대상으로 실시되었습니다. 이 환자들은 정맥 주입을 통해 체중당 3~5개의 동종 BM-MSC를 투여받았습니다.64 이러한 초기 연구에는 표본 규모가 작고, 비무작위 배정, 대조군 없음 등 몇 가지 한계가 있지만, 안전성 및 효능과 관련된 예비 데이터는 ALD 치료에서 줄기세포 기반 치료의 가능성을 예고하고 가능성을 뒷받침하고 있습니다.

요약하면,

현재 줄기세포 기반 치료법을 사용한 간 질환 임상시험은

다양한 간 질환에서 안전성과 잠재적 치료 효과를 뒷받침하는 기초 데이터를 제공하고 있습니다.

안타깝게도 임상시험 수가 적기 때문에

(1) 줄기세포 출처 및 용량,

(2) 투여 경로,

(3) 개입 시간,

(4) 추적 기간 동안의 임상 평가 등 치료 효과를 입증하기 위해

극복해야 할 몇 가지 장애물을 극복해야 합니다.

이러한 과제를 해결해야만 줄기세포 기반 치료법을 간 질환의 주류 치료법으로 입증하고, 촉진하고, 홍보할 수 있을 것입니다.

Arthritis

Arthritis is a general term describing cartilage conditions that cause pain and inflammation of the joints. Osteoarthritis (OA) is the most common form of arthritis caused by persistent degeneration and poor recovery of articular cartilage.65 OA affects one or several diarthrodial joints, such as small joints at the hand and large joints at the knee and hips, leading to severe pain and subsequent reduction in the mobility of patients. There are two types of OA: (1) primary OA or idiopathic OA and secondary OA caused by causative factors such as trauma, surgery, and abnormal joint development at birth.66 As conventional treatments for OA are not consistent in their effectiveness and might cause unbearable pain as well as long-term rehabilitation (in the case of joint replacement), there is a need for a more reliable, less painful, and curative therapy targeting the root of OA.67 Thus, stem cell therapy has recently emerged as an alternative approach for OA and has drawn great attention in the regenerative field.

The administration of HSCs has been proven to reduce bone lesions, enhance bone regeneration and stimulate the vascularization process in degenerative cartilage. Attempts were made to eval‎uate the efficacy of peripheral blood stem cells in ten OA patients by three intraarticular injections. Post-administration analysis indicated a reduction in the WOMAC index with a significant reduction in all parameters. All patients completed 6-min walk tests with an increase of more than 54 meters. MRI analysis indicated an improvement in cartilage thickness, suggesting that cartilage degeneration was reduced post administration. To further enhance the therapeutic potential of HSCT, CD34+ stem cells were proposed to be used in combination with the rehabilitation algorithm, which included three stages: preoperative, hospitalization and outpatient periods.68 Currently, a large wave of studies has been directed to MSC-based therapy for the treatment of OA due to their immunoregulatory functions and anti-inflammatory characteristics. MSCs have been used as the main cell source in several multiple and small-scale trials, proving their safety profile and potential effectiveness in alleviating pain, reducing cartilage degeneration, and enhancing the regeneration of cartilage structure and morphology in some cases. However, the best source of MSCs, whether from bone marrow, adipose tissue, or umbilical cord, for the management of OA is still a great question to be answered. A systematic review investigating over sixty-one of 3172 articles with approximately 2390 OA patients supported the positive effects of MSC-based therapy on OA patients, although the study also pointed out the fact that these therapeutic potentials were based on limited high-quality evidence and long-term follow-up.69

 Moreover, the study found no obvious evidence supporting the most effective source of MSCs for treating OA. Another systematic review covering 36 clinical trials, of which 14 studies were randomized trials, provides an interesting view in terms of the efficacy of autologous MSC-based therapy in the treatment of OA.70 In terms of BM-MSCs, 14 clinical trials reported the clinical outcomes at the 1-year follow-up, in which 57% of trials reported clinical outcomes that were significantly better in comparison with the control group. However, strength analysis of the data set showed that outcomes from six trials were low, whereas the outcomes of the remaining eight trials were extremely low. Moreover, the positive evidence obtained from MRI analysis was low to very low strength of evidence after 1-year post administration.70 Similar results were also found in the outcome analysis of autologous adipose tissue-derived MSCs (AT-MSCs). Thus, the review indicated low quality of evidence for the therapeutic potential of MSC therapy on clinical outcomes and MRI analysis. The low quality of clinical outcomes could be explained by the differences in interventions (including cell sources, cell doses, and administration routes), combination treatments (with hyaluronic acid,71 peripheral blood plasma,72 etc.), control treatments and clinical outcome measurements between randomized clinical trials.73 In addition, the data of the systematic analysis could not prove the better source of MSCs for OA treatment. Taken together, although stem cell-based therapy has been shown to be safe and feasible in the management of OA, the authors support the notion that stem cell-based therapy could be considered an alternative treatment for OA when first-line treatments, such as education, exercise, and body weight management, have failed.

관절염

관절염은 관절에 통증과 염증을 일으키는 연골 질환을 총칭하는 용어입니다. 골관절염(OA)은 관절 연골의 지속적인 퇴행과 회복 부진으로 인해 발생하는 가장 흔한 형태의 관절염입니다.65 OA는 손의 작은 관절, 무릎과 엉덩이의 큰 관절 등 하나 또는 여러 관절에 영향을 미치며 심한 통증과 그에 따른 환자의 이동성 저하를 초래합니다. OA에는 (1) 원발성 OA 또는 특발성 OA와 외상, 수술, 출생 시 관절 발달 이상과 같은 원인 요인으로 인해 발생하는 이차성 OA의 두 가지 유형이 있습니다.66 기존의 OA 치료법은 효과가 일정하지 않고 견딜 수 없는 통증과 장기간의 재활(관절 치환술의 경우)을 유발할 수 있기 때문에 보다 안정적이고 통증이 적으며 OA의 근원을 표적으로 하는 치료법이 필요합니다.67 따라서 최근 줄기세포 치료가 OA의 대안으로 부상하며 재생 분야에서 큰 관심을 끌고 있습니다.

HSC를 투여하면 뼈 병변을 줄이고 뼈 재생을 촉진하며 퇴행성 연골의 혈관 형성 과정을 자극하는 것으로 입증되었습니다. 10명의 퇴행성 관절염 환자를 대상으로 말초혈액 줄기세포의 효능을 평가하기 위해 세 차례 관절 내 주사를 시행했습니다. 투여 후 분석 결과, 모든 매개변수에서 유의미한 감소와 함께 WOMAC 지수가 감소한 것으로 나타났습니다. 모든 환자가 6분 걷기 테스트를 완료하여 54미터 이상 증가했습니다. MRI 분석 결과 연골 두께가 개선되어 투여 후 연골 퇴행이 감소했음을 시사했습니다. HSCT의 치료 가능성을 더욱 높이기 위해 수술 전, 입원 및 외래 기간의 3단계로 구성된 재활 알고리즘과 함께 CD34+ 줄기세포를 사용하는 것이 제안되었습니다.68 현재 면역 조절 기능과 항염증 특성으로 인해 OA 치료를 위한 MSC 기반 치료에 대한 많은 연구가 진행 중입니다. MSC는 여러 대규모 및 소규모 임상시험에서 주요 세포 공급원으로 사용되어 통증 완화, 연골 퇴행 감소, 연골 구조 및 형태 재생 향상에 대한 안전성 프로파일과 잠재적 효과를 입증했습니다. 그러나 골수, 지방 조직 또는 탯줄에서 채취한 중간엽줄기세포가 골관절염 관리에 가장 적합한지는 여전히 풀어야 할 숙제입니다. 3172개 논문 중 약 2390명의 OA 환자를 대상으로 한 61개 이상의 논문을 조사한 체계적 문헌고찰에서는 OA 환자에 대한 MSC 기반 치료의 긍정적인 효과를 지지했지만, 이러한 치료 잠재력이 제한된 양질의 증거와 장기 추적 관찰에 기반하고 있다는 사실도 지적했습니다.69

또한 이 연구에서는 OA 치료에 가장 효과적인 MSC 공급원을 뒷받침하는 명백한 증거를 찾지 못했습니다. 36건의 임상시험을 대상으로 한 또 다른 체계적 문헌고찰(이 중 14건의 연구는 무작위 배정 임상시험)은 OA 치료에서 자가 MSC 기반 치료의 효능에 대한 흥미로운 관점을 제공합니다.70 BM-MSC의 경우, 14건의 임상시험에서 1년 추적 관찰 시점의 임상 결과를 보고했는데, 57%의 임상시험에서 대조군과 비교하여 유의하게 개선된 임상 결과를 보고했습니다. 그러나 데이터 세트의 강도 분석 결과 6건의 임상시험의 결과는 낮은 반면 나머지 8건의 임상시험의 결과는 매우 낮은 것으로 나타났습니다. 또한, MRI 분석에서 얻은 긍정적인 증거는 투여 후 1년 후 증거 강도가 낮거나 매우 낮았습니다.70 자가 지방 조직 유래 MSC(AT-MSC)의 결과 분석에서도 유사한 결과가 발견되었습니다. 따라서 이 검토에서는 임상 결과 및 MRI 분석에서 MSC 치료의 치료 가능성에 대한 근거의 질이 낮다고 지적했습니다. 임상 결과의 낮은 품질은 무작위 임상시험 간의 중재(세포 공급원, 세포 용량 및 투여 경로 포함), 병용 치료(히알루론산,71 말초 혈장,72 등), 대조 치료 및 임상 결과 측정의 차이로 설명할 수 있습니다.73 또한 체계적 분석 데이터는 OA 치료에 더 나은 MSC 공급원을 증명할 수 없었습니다.

종합하면,

줄기세포 기반 치료가 OA 관리에 안전하고 실현 가능한 것으로 나타났지만

저자들은 교육, 운동 및 체중 관리와 같은 일차 치료가 실패한 경우

줄기세포 기반 치료가 OA의 대체 치료로 간주될 수 있다는 개념을 지지합니다.

Cancer treatment

Stem cell therapy in the treatment of cancer is a sensitive term and needs to be used and discussed with caution. Clinicians and researchers should protect patients with cancer from expensive and potentially dangerous or ineffective stem cell-based therapy and patients without a cancer diagnosis from the risk of malignancy development. In general, unproven stem cell clinics employed three cell-based therapies for cancer management, including autologous HSCTs, stromal vascular fraction (SVF), and multipotent stem cells, such as MSCs. Allogeneic HSCTs confer the ability to generate donor lymphocytes that contribute to the suppression and regression of hematological malignancies and select solid tumors, a specific condition known as “graft-versus-tumor effects”.74 However, stem cell clinics provide allogeneic cell-based therapy for the treatment of solid malignancies despite limited scientific evidence supporting the safety and efficacy of the treatment. High-quality evidence from the Cochrane library shows that marrow transplantation via autologous HSCTs in combination with high-dose chemotherapy does not improve the overall survival of women with metastatic breast cancer. In addition, a study including more than 41,000 breast cancer patients demonstrated no significant difference in survival benefits between patients who received HSCTs following high-dose chemotherapy and patients who underwent conventional treatment.75 Thus, the use of autologous T-cell transplants as monotherapy and advertising stem cell-based therapies as if they are medically approved or preferred treatment of solid tumors is considered untrue statements and needs to be alerted to cancer patients.76

Over the past decades, many preclinical studies have demonstrated the potential of MSC-based therapy in cancer treatment due to their unique properties. They confer the ability to migrate toward damaged sites via inherent tropism controlled by growth factors, chemokines, and cytokines. MSCs express specific C–X–C chemokine receptor type 4 (CXCR4) and other chemokine receptors (including CCR1, CCR2, CCR4, CCR7, etc.) that are essential to respond to the surrounding signals.77 In addition, specific adherent proteins, including CD49d, CD44, CD54, CD102, and CD106, are also expressed on the MSC surface, allowing them to attach, rotate, migrate, and penetrate the blood vessel lumen to infiltrate the damaged tissue.78 Similar to damaged tissues, tumors secrete a wide range of chemoattractant that also attract MSC migration via the CXCL12/CXCR4 axis. Previous studies also found that MSC migration toward the cancer site is tightly controlled by diffusible cytokines such as interleukin 8 (IL-8) and growth factors including transforming growth factor-beta 1 (TGF-β1),79 platelet-derived growth factor (PDGF),80 fibroblast growth factor 2 (FGF-2),81 vascular endothelial growth factor (VEGF),81 and extracellular matrix molecules such as matrix metalloproteinase-2 (MMP-2).82 Once MSCs migrate successfully to cancerous tissue, accumulating evidence demonstrates the interaction between MSCs and cancer cells to exhibit their protumour and antitumour effects, which are the major concerns of MSC-based therapy. MSCs are well-known for their regenerative effects that regulate tissue repair and recovery. This unique ability is also attributed to the protumour functions of these cells. A previous study reported that breast cancer cells induce MSC secretion of chemokine (C–C motif) ligand 5 (CCL-5), which regulates the tumor invasion process.83,84 Other studies also found that MSCs secrete a wide range of growth factors (VEGF, basic FGF, HGF, PDGF, etc.) that inhibits apoptosis of cancer cells.85 Moreover, MSCs also respond to signals released from cancer cells, such as TGF-β,86 to transform into cancer-associated fibroblasts, a specific cell type residing within the tumor microenvironment capable of promoting tumorigenesis.87 Although MSCs have been proven to be involved in protumour activities, they also have potent tumor suppression abilities that have been used to develop cancer treatments. It has been suggested that MSCs exhibit their tumor inhibitory effects by inhibiting the Wnt and AKT signaling pathways,88 reducing the angiogenesis process,89 stimulating inflammatory cell infiltration,90 and inducing tumor cell cycle arrest and apoptosis.91 To date, the exact functions of MSCs in both protumour and antitumor activities are still a controversial issue across the stem cell field. Other approaches exploit gene editing and tissue engineering to convert MSCs into “a Trojan horse” that could exhibit antitumor functions. In addition, MSCs can also be modified to express specific anticancer miRNAs exhibiting tumor-suppressive behaviors.92 However, genetically modified MSCs are still underdeveloped and require intensive investigation in the clinical setting.

To date, ~25 clinical trials have been registered on ClinicalTrials.gov aimed at using MSCs as a therapeutic treatment for cancer.93 These trials are mostly phase 1 and 2 studies focusing on eval‎uating the safety and efficacy of the treatment. Studies exploiting MSC-based therapy have combined MSCs with an oncolytic virus approach. Oncolytic viruses are specific types of viruses that can be genetically engineered or naturally present, conferring the ability to selectively infect cancer cells and kill them without damaging the surrounding healthy cells.94 A completed phase I/II study using BM-MSCs infected with the oncolytic adenovirus ICOVIR5 in the treatment of metastatic and refractory solid tumors in children and adult patients demonstrated the safety of the treatment and provided preliminary data supporting their therapeutic potential.95 The same group also reported a complete disappearance of all signs of cancer in response to MSC-based therapy in one pediatric case three years post administration.96 A reported study in 2019 claimed that adipose-derived MSCs infected with vaccinia virus have the potential to eradicate resistant tumor cells via the combination of potent virus amplification and senitization of the tumor cells to virus infection.97 However, in a recently published review, a valid question was posed regarding the 2019 study that “do these reported data merit inclusion in the publication record when they were collected by such groups using a dubious therapeutic that was eventually confiscated by US Marshals?”76

Taken together, cancer research and therapy have entered an innovative and fascinating era with advancements in traditional therapies such as chemotherapy, radiotherapy, and surgery on one hand and stem cell-based therapy on the other hand. Although stem cell-based therapy has been considered a novel and attractive therapeutic approach for cancer treatment, it has been hampered by contradictory results describing the protumour and antitumour effects in preclinical studies. Despite this contradictory reality, the use of stem cell-based therapy, especially MSCs, offers new hope to cancer patients by providing a new and more effective tool in personalized medicine. The authors support the use of MSC-based therapy as a Trojan horse to deliver specific anticancer functions toward cancer cells to suppress their proliferation, eradicate cancer cells, or limit the vascularization process of cancerous tissue to improve the clinical safety and efficacy of the treatment.

암 치료

암 치료에서 줄기세포 치료는 민감한 용어이므로 신중하게 사용하고 논의해야 합니다. 임상의와 연구자들은 암 환자를 고가의 잠재적으로 위험하거나 효과가 없는 줄기세포 기반 치료로부터 보호하고 암 진단을 받지 않은 환자를 악성 종양 발생의 위험으로부터 보호해야 합니다. 일반적으로 검증되지 않은 줄기세포 클리닉에서는 암 관리를 위해 자가 줄기세포, 기질 혈관 분획(SVF), 다능성 줄기세포(MSC) 등 세 가지 세포 기반 치료법을 사용했습니다. 동종 HSCT는 혈액 악성 종양의 억제 및 퇴행에 기여하는 기증자 림프구를 생성하고 특정 고형 종양을 선별하는 능력을 부여하며, 이를 “이식편대종양 효과”라고 합니다.74 그러나 줄기세포 클리닉은 치료의 안전성과 효능을 뒷받침하는 과학적 증거가 제한적임에도 고형 악성 종양 치료를 위한 동종 세포 기반 치료를 제공합니다. 코크레인 라이브러리의 고품질 증거에 따르면 고용량 화학요법과 함께 자가 조혈모세포이식을 통한 골수 이식이 전이성 유방암 여성의 전체 생존율을 개선하지 못하는 것으로 나타났습니다. 또한 41,000명 이상의 유방암 환자를 대상으로 한 연구에서는 고용량 화학요법 후 HSCT를 받은 환자와 기존 치료를 받은 환자 간에 생존 혜택에 큰 차이가 없는 것으로 나타났습니다.75 따라서 자가 T 세포 이식을 단독 요법으로 사용하고 줄기세포 기반 치료법이 의학적으로 승인되거나 고형암의 선호 치료인 것처럼 광고하는 것은 사실이 아닌 내용으로 간주되며 암 환자들에게 경고해야 할 필요가 있습니다.76

지난 수십 년 동안 많은 전임상 연구를 통해 암 치료에서 MSC 기반 치료의 고유한 특성으로 인한 잠재력이 입증되었습니다. MSC는 성장 인자, 케모카인 및 사이토카인에 의해 제어되는 고유한 영양성을 통해 손상된 부위로 이동하는 능력을 부여합니다. MSC는 주변 신호에 반응하는 데 필수적인 특정 C-X-C 케모카인 수용체 4형(CXCR4) 및 기타 케모카인 수용체(CCR1, CCR2, CCR4, CCR7 등 포함)를 발현합니다.77 또한 CD49d, CD44, CD54, CD102, CD106을 포함한 특정 부착 단백질도 MSC 표면에 발현되어 혈관 내강에 부착, 회전, 이동, 침투하여 손상된 조직에 침투할 수 있습니다.78 손상된 조직과 마찬가지로 종양은 CXCL12/CXCR4 축을 통해 MSC 이동을 유인하는 광범위한 화학 유인 물질을 분비합니다. 이전 연구에서도 인터루킨 8(IL-8)과 같은 확산성 사이토카인과 형질 전환 성장 인자 베타 1(TGF-β1),79 혈소판 유래 성장 인자(PDGF),80 섬유아세포 성장 인자 2(FGF-2),81 혈관 내피 성장 인자(VEGF)81 및 매트릭스 메탈로프로테아제-2(MMP-2) 같은 세포외 기질 분자에 의해 암 부위를 향한 MSC 이동이 엄격히 제어된다는 사실이 밝혀졌습니다. 82 일단 MSC가 암 조직으로 성공적으로 이동하면, 축적된 증거는 MSC 기반 치료의 주요 관심사인 MSC와 암 세포 간의 상호작용을 통해 종양 억제 및 항종양 효과를 나타낸다는 것을 입증합니다. MSC는 조직 복구와 회복을 조절하는 재생 효과로 잘 알려져 있습니다. 이 독특한 능력은 또한 이러한 세포의 종양 보호 기능에 기인합니다. 이전 연구에서는 유방암 세포가 종양 침입 과정을 조절하는 케모카인(C-C 모티브) 리간드 5(CCL-5)의 MSC 분비를 유도한다고 보고했습니다.83,84 다른 연구에서도 MSC가 암세포의 세포 사멸을 억제하는 광범위한 성장 인자(VEGF, 기본 FGF, HGF, PDGF 등)를 분비하는 것으로 밝혀졌습니다.85 또한, MSC는 TGF-β와 같은 암세포에서 방출되는 신호에반응하여86 종양 미세 환경 내에 존재하는 특정 세포 유형인 암 관련 섬유아세포로 전환하여 종양 형성을 촉진할 수 있습니다.87 MSC는 종양 활동에 관여하는 것으로 입증되었지만 암 치료제 개발에 사용되는 강력한 종양 억제 능력도 가지고 있습니다. MSC는 Wnt 및 AKT 신호 경로를 억제하고,88 혈관 신생 과정을 줄이고,89 염증 세포 침윤을 자극하고,90 종양 세포 주기 정지 및 세포 사멸을 유도함으로써 종양 억제 효과를 나타낸다고 제안되었습니다.91 현재까지 줄기세포 분야에서 종양 및 항종양 활동에서 MSC의 정확한 기능은 여전히 논란의 여지가 있는 문제입니다. 다른 접근법은 유전자 편집과 조직 공학을 활용하여 MSC를 항종양 기능을 발휘할 수 있는 '트로이의 목마'로 전환하는 것입니다. 또한 종양 억제 작용을 나타내는 특정 항암 miRNA를 발현하도록 MSC를 변형할 수도 있습니다.92 그러나 유전자 변형 MSC는 아직 개발이 미진하며 임상 환경에서 집중적인 연구가 필요합니다.

현재까지 암 치료제로 MSC를 사용하기 위한 약 25건의 임상시험이 ClinicalTrials.gov에 등록되었습니다.93 이러한 임상시험은 대부분 치료의 안전성과 효능을 평가하는 데 초점을 맞춘 1상 및 2상 연구입니다. MSC 기반 치료법을 활용하는 연구에서는 MSC와 종양 용해 바이러스 접근법을 결합했습니다. 종양 용해 바이러스는 유전적으로 조작되거나 자연적으로 존재할 수 있는 특정 유형의 바이러스로, 암세포를 선택적으로 감염시켜 주변의 건강한 세포를 손상시키지 않고 죽일 수 있는 능력을 부여합니다.94 소아 및 성인 환자의 전이성 및 난치성 고형 종양 치료에 종양 용해성 아데노바이러스 ICOVIR5에 감염된 BM-MSC를 사용한 완료된 1상/2상 연구에서 치료의 안전성을 입증하고 치료 가능성을 뒷받침하는 예비 데이터를 제공했습니다.95 또한 같은 그룹은 투여 3년 후 한 소아 사례에서 MSC 기반 치료에 반응하여 암의 모든 징후가 완전히 사라진 것으로 보고했습니다.96 2019년에 보고된 한 연구에서는 백시니아 바이러스에 감염된 지방 유래 MSC가 강력한 바이러스 증폭과 종양 세포의 바이러스 감염에 대한 노화(senitization)의 조합을 통해 내성 종양 세포를 근절할 가능성이 있다고 주장했습니다.97 그러나 최근 발표된 리뷰에서 2019년 연구에 대해 “이러한 보고 데이터는 결국 미국 연방 보안관에 의해 압수된 의심스러운 치료제를 사용하여 해당 그룹에서 수집한 것인데 출판 기록에 포함할 가치가 있는가”라는 유효한 질문이 제기되었습니다.76

종합하면,

암 연구와 치료는 한편으로는

화학 요법, 방사선 요법, 수술과 같은 전통적인 요법의 발전과 다른 한편으로는

줄기세포 기반 치료의 발전으로 혁신적이고 매혹적인 시대에 접어들었습니다.

줄기세포 기반 치료는 암 치료를 위한 새롭고 매력적인 치료법으로 여겨져 왔지만,

전임상 연구에서 종양 및 항종양 효과에 대한 모순된 결과로 인해 어려움을 겪어 왔습니다.

이러한 모순된 현실에도 불구하고 줄기세포 기반 치료, 특히 중간엽줄기세포의 사용은 개인 맞춤형 의학에서 새롭고 효과적인 도구를 제공함으로써 암 환자에게 새로운 희망을 제공합니다. 저자는 암세포에 특정 항암 기능을 전달하여 암세포의 증식을 억제하고 암세포를 제거하거나 암 조직의 혈관 형성 과정을 제한하여 치료의 임상적 안전성과 효능을 향상시키는 트로이 목마로서 MSC 기반 치료법을 사용하는 것을 지지합니다.

Human pluripotent stem cell-based therapy: a growing giant

The discovery of hPSCs, including human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs), has revolutionized stem cell research and cell-based therapy.98 hESCs were first isolated from blastocyst-stage embryos in 1998,99 followed by breakthrough reprogramming research that converted somatic cells into hiPSCs using just four genetic factors.100,101 Methods have been developed to maintain these cells long-term in vitro and initiate their differentiation into a wide variety of cell types, opening a new era in regenerative medicine, particularly cell therapy to replace lost or damaged tissues.

인간 만능 줄기세포 기반 치료: 성장하는 거인

인간 배아 줄기세포(hESC)와

인간 유도 만능 줄기세포(hiPSC)를 포함한 hPSC의 발견은

줄기세포 연구와 세포 기반 치료에 혁신을 가져왔습니다.98

hESC는 1998년 배반포 단계 배아에서 처음 분리되었고,99

단 4개의 유전 인자를 사용하여 체세포를 hiPSC로 전환하는

획기적인 리프로그래밍 연구가 이어졌습니다.100,101

이러한 세포를 체외에서 장기간 유지하고 다양한 세포 유형으로 분화시키는 방법이 개발되어

재생 의학, 특히 손실되거나 손상된 조직을 대체하기 위한 세포 치료의 새로운 시대를 열었습니다.

History of hPSCs

hPSCs are defined as self-renewable cell types that confer the ability to differentiate into various cellular phenotypes of the human body, including three germ layers.102 Historically, the first pluripotent cell lines to be generated were embryonic carcinoma (EC) cell lines established from human germ cell tumors103 and murine undifferentiated compartments.104 Although EC cells are a powerful tool in vitro, these cells are not suitable for clinical applications due to their cancer-derived origin and aneuploidy genotype.105 The first murine ESCs were established in 1981 based on the culture techniques obtained from EC research.106 Murine ESCs are derived from the inner cell mass (ICM) of the pre-implantation blastocyst, a unique biological structure that contains outer trophoblast layers that give rise to the placenta and ICM.107 In vivo ESCs only exist for a short period during the embryo’s development, and they can be isolated and maintained indefinitely in vitro in an undifferentiated state. The discovery of murine ESCs has dramatically changed the field of biomedical research and regenerative medicine over the last 40 years. Since then, enormous investigations have been made to isolate and culture ESCs from other species, including hESCs, in 1998.99 The success of Thomson et al. in 1998 triggered the great controversy in media and ethical research boards across the globe, with particularly strong objections being raised to the use of human embryos for research purposes.108 Several studies using hESCs have been conducted demonstrating their therapeutic potential in the clinical setting. However, the use of hESCs is limited due to (1) the ethical barrier related to the destruction of human embryos and (2) the potential risk of immunological rejection, as hESCs are isolated from pre-implantation blastocysts, which are not autologous in origin. To overcome these two great obstacles, several research groups have been trying to develop technology to generate hESCs, including nuclear transfer technology, the well-known strategy that creates Dolly sheep, although the generation of human nuclear transfer ESCs remains technically challenging.109 Taking a different approach, in 2006, Yamanaka and Takahashi generated artificial PSCs from adult and embryonic mouse somatic cells using four transcription factors (Oct-3/4, Sox2, Klf4, and c-Myc, called OSKM) reduced from 24 factors.100 Thereafter, in 2007, Takahashi and colleagues successfully generated the first hiPSCs exhibiting molecular and biological features similar to those of hESCs using the same OSKM factors.101 Since then, hiPSCs have been widely studied to expand our knowledge of the pathogenesis of numerous diseases and aid in developing new cell-based therapies as well as personalized medicine.

hPSC의 역사

hPSC는 세 개의 생식층을 포함하여 인체의 다양한 세포 표현형으로 분화할 수 있는 능력을 부여하는 자가 재생 가능한 세포 유형으로 정의됩니다.102 역사적으로 최초로 생성된 만능 세포주는 인간 생식세포 종양103 및 쥐 미분화 구획에서 확립된 배아 암종(EC) 세포주였습니다.104 EC 세포는 체외에서 강력한 도구이지만 암 유래 기원 및 이형성 유전자형으로 인해 임상 적용에는 적합하지 않습니다.105 최초의 쥐 ESC는 EC 연구에서 얻은 배양 기술을 기반으로 1981년에 확립되었습니다.106 쥐 ESC는 태반과 ICM을 생성하는 외부 영양막 층을 포함하는 독특한 생물학적 구조인 착상 전 배반포의 내부 세포 덩어리(ICM)에서 유래합니다.107 생체 내 ESC는 배아 발달 중 짧은 기간 동안만 존재하며 미분화 상태로 체외에서 분리 및 무기한 유지될 수 있습니다. 쥐 ESC의 발견은 지난 40년 동안 생물의학 연구와 재생 의학 분야를 극적으로 변화시켰습니다. 이후 1998년 hESC를 포함한 다른 종으로부터 ESC를 분리하고 배양하기 위한 방대한 연구가 진행되었습니다.99 1998년 톰슨 등의 성공은 전 세계 언론과 윤리 연구 위원회에서 큰 논란을 불러 일으켰으며, 특히 연구 목적으로 인간 배아를 사용하는 것에 대해 강한 반대가 제기되었습니다.108 hESC를 사용한 여러 연구가 임상 환경에서 치료 가능성을 입증하는 것으로 수행되었습니다. 그러나 (1) 인간 배아의 파괴와 관련된 윤리적 장벽과 (2) 자가 배아가 아닌 착상 전 배반포에서 분리된 hESC이기 때문에 면역학적 거부 반응의 잠재적 위험으로 인해 hESC의 사용은 제한적입니다. 이 두 가지 큰 장애물을 극복하기 위해 여러 연구 그룹이 돌리양을 만드는 잘 알려진 전략인 핵이식 기술을 포함하여 hESC를 생성하는 기술을 개발하기 위해 노력해 왔지만 인간 핵이식 ESC의 생성은 기술적으로 여전히 어렵습니다.109 다른 접근 방식을 취하여 2006년에 야마나카와 타카하시는 성체 및 배아 마우스 체세포에서 4개의 전사인자(Oct-3/4, Sox2, Klf4, 그리고 OSKM이라고 불리는 c-Myc)를 사용하여 24개에서 줄어든 인공 PSC를 생성했습니다.100 그 후, 2007년에 타카하시와 동료들은 동일한 OSKM 인자를 사용하여 hESC와 유사한 분자 및 생물학적 특징을 나타내는 최초의 hiPSC를 성공적으로 생성했습니다.101 그 이후로 hiPSC는 수많은 질병의 발병 기전에 대한 지식을 넓히고 새로운 세포 기반 치료법과 개인 맞춤형 의약품 개발에 도움을 주기 위해 광범위하게 연구되고 있습니다.

Clinical applications of hPSCs

Since its beginning 24 years ago, hPSC research has evolved momentously toward applications in regenerative medicine, disease modeling, drug screening and discovery, and stem cell-based therapy. In clinical trial settings, the uses of hESCs are restricted by ethical concerns and tight regulation, and the limited preclinical data support their therapeutic potential. However, it is important to acknowledge several successful outcomes of hESC-based therapies in treating human diseases. In 2012, Steven Schwartz and his team reported the first clinical evidence of using hESC-derived retinal pigment epithelium (RPE) in the treatment of Stargardt’s macular dystrophy, the most common pediatric macular degeneration, and an individual with dry age-related macular degeneration.110,111 With a differentiation efficiency of RPE greater than 99%, 5 × 104 RPEs were injected into the subretinal space of one eye in each patient. As the hESC source of RPE differentiation was exposed to mouse embryonic stem cells, it was considered a xenotransplantation product and required a lower dose of immunosuppression treatment. This study showed that hESCs improved the vision of patients by differentiating into functional RPE without any severe adverse events. The trial was then expanded into two open-label, phase I/II studies with the published results in 2015 supporting the primary findings.112 In these trials, patients were divided into three groups receiving three different doses of hESC-derived RPE, including 10 × 104, 15 × 104 and 50 × 104 RPE cells per eye. After 22 months of follow-up, 19 patients showed improvement in eyesight, seven patients exhibited no improvement, and one patient experienced a further loss of eyesight. The technical challenge of hESC-derived RPE engraftment was an unbalanced proliferation of RPE post administration, which was observed in 72% of treated patients. A similar approach was also conducted in two South Korean patients diagnosed with age-induced macular degeneration and two patients with Stargardt macular dystrophy.113 The results supported the safety of hESC-derived RPE cells and illustrated an improvement in visual acuity in three patients. Recently, clinical-graded hESC-derived RPE cells were also developed by Chinese researchers under xeno-free culture conditions to treat patients with wet age-related degeneration.114 As hESC development is still associated with ethical concerns and immunological complications related to allogeneic administration, hiPSC-derived RPE cells have emerged as a potential cell source for macular degeneration. Although RPE differentiation protocols have been developed and optimized to improve the efficacy of hiPSC-derived RPE cells, they are still insufficient, time-consuming and labor intensive.115,116 For clinical application, an efficient differentiation of “primed” to “naïve” state hiPSCs toward the RPE was developed using feeder-free culture conditions utilizing the transient inhibition of the FGF/MAPK signaling pathway.117 Overexpression‎ of specific transcription factors in hiPSCs throughout the differentiation process is also an interesting approach to generate a large number of RPE cells for clinical use. In a recent study, overexpression‎ of three eye-field transcription factors, including OTX2, PAX6, and MITF, stimulated RPE differentiation in hiPSCs and generated functional RPE cells suitable for transplantation.118 To date, although reported data from phase I/II clinical trials have been produced enough to support the safety of hESC-derived RPE cells, the treatment is still in its immature stage. Thus, future studies should focus on the development of the cellular manufacturing process of RPE and the subretinal administration route to further improve the outcomes of RPE fabrication and engraftment into the patient’s retina (recommended review119).

Numerous studies have demonstrated that hESC-derived cardiomyocytes exhibit cardiac transcription factors and display a cardiomyocyte phenotype and immature electrical phenotype. In addition, using hPSC-derived cardiomyocytes could provide a large number of cells required for true remuscularization and transplantation. Thus, these cells can be a promising novel therapeutic approach for the treatment of human cardiovascular diseases. In a case report, hESC-derived cardiomyocytes showed potential therapeutic effects in patients with severe heart failure without any subsequent complications.120 This study was a phase I trial (ESCORT [Transplantation of Human Embryonic Stem Cell-derived Progenitors in Severe Heart Failure] trial) to eval‎uate the safety of cardiomyocyte progenitor cells derived from hESCs seeded in fibrin gel scaffolds for 10 patients with severe heart failure (NCT02057900). The encouraging results from this study demonstrated the feasibility of producing hESC-derived cardiomyocyte progenitor cells toward clinical-grade standards and combining them with a tissue-engineered scaffold to treat severe heart disease (the first patient of this trial has already reached the 7-year follow-up in October 2021).121 Currently, the two ongoing clinical trials using hPSC-derived cardiomyocytes have drawn great attention, as their results would pave the way to lift the bar for approving therapies for commercial use. The first trial was conducted by a team led by cardiac surgeon Yoshiki Sawa at Osaka University using hiPSC-derived cardiomyocytes embedded in a cell sheet for engraftment (jRCT2052190081). The trials started first with three patients followed by ten patients to assess the safety of the approach. Once safety is met, the treatment can be sold commercially under Japan’s fast-track system for regenerative medicine.122 Another trial used a collagen-based construct called BioVAT-HF to contain hiPSC-derived cardiomyocytes. The trial was divided into two parts to eval‎uate the cell dose: (Part A) recruiting 18 patients and (Part B) recruiting 35 patients to test a broad range of engineered human myocardium (EHM) doses. The expected results from this study will provide the “proof-of-concept” for the use of EHM in the stimulation of heart remuscularization in humans. To date, no adverse events or severe adverse events have been reported from these trials, supporting the safety of the procedure. However, as the number of treated patients was relatively small, limitations in drawing conclusions regarding efficacy are not yet possible.21,123

One of the first clinical trials using hPSC-based therapy was conducted by Geron Corporation in 2010 using hESC-derived oligodendrocyte progenitor cells (OPC1) to treat spinal cord injury (SCI). The results confirmed the safety one year post administration in five participants, and magnetic resonance imaging demonstrated improvement of spinal cord deterioration in four participants.124 Asterias Biotherapeutic (AST) continued the Geron study by conducting the SCiStar Phase I/IIa study to eval‎uate the therapeutic effects of AST-OPC1 (NCT02302157). The trial’s results published in clinicaltrials.gov demonstrated significant improvement in running speed, forelimb stride length, forelimb longitudinal deviations, and rear stride frequency. Interestingly, the recently published data of a phase 1, multicentre, nonrandomized, single-group assignment, interventional trial illustrated no evidence of neurological decline, enlarging masses, further spinal cord damage, or syrinx formation in patients 10 years post administration of the OPC1 product.125 This data set provides solid evidence supporting the safety of OPC1 with an event-free period of up to 10 years, which strengthens the safety profile of the SCiStar trial.

Analysis of the global trends in clinical trials using hPSC-based therapy showed that 77.1% of studies were observational (no cells were administered into patient), and only 22.9% of studies used hPSC-derived cells as interventional treatment.126 The number of studies using hiPSCs was relatively higher than that using hESCs, which was 74.8% compared to 25.2%, respectively. The majority of observational studies were performed in developed countries, including the USA (41.6%) and France (16.8%), whereas interventional studies were conducted in Asian countries, including China (36.7%), Japan (13.3%), and South Korea (10%). The trends in therapeutic studies were also clear in terms of targeted diseases. The three most studied diseases were ophthalmological conditions, circulatory disorders, and nervous systems.127 However, it is surprising that the clinical applications of hPSCs have achieved little progress since the first hESCs were discovered worldwide. The relatively low number of clinical trials focusing on using iPSCs as therapeutic agents to administer into patients could be ascribed to the unstable genome of hiPSCs,128 immunological rejection,129 and the potential for tumor formation.130

hPSC의 임상 적용

24년 전 시작된 이래로 hPSC 연구는 재생 의학, 질병 모델링, 약물 스크리닝 및 발견, 줄기세포 기반 치료 분야에 적용하는 방향으로 빠르게 발전해 왔습니다. 임상시험 환경에서는 윤리적 우려와 엄격한 규제로 인해 hESC의 사용이 제한되고 있으며, 전임상 데이터가 제한되어 있어 치료 잠재력을 뒷받침하지 못하고 있습니다. 그러나 인간 질병을 치료하는 데 있어 hESC 기반 요법의 몇 가지 성공적인 결과를 인정하는 것이 중요합니다. 2012년 스티븐 슈워츠와 그의 팀은 가장 흔한 소아 황반변성인 스타가르트 황반이상증과 건성 연령 관련 황반변성 환자의 치료에 hESC 유래 망막 색소 상피(RPE)를 사용한 최초의 임상 증거를 보고했습니다.110,111 99% 이상의 RPE 분화 효율로 각 환자의 한쪽 눈 망막하 공간에 5 × 104개의 RPE를 주입했습니다. RPE 분화의 원천이 마우스 배아 줄기세포에 노출된 hESC는 이종 이식 제품으로 간주되어 더 적은 용량의 면역 억제 치료가 필요했습니다. 이 연구에서는 hESC가 심각한 부작용 없이 기능적인 RPE로 분화하여 환자의 시력을 개선하는 것으로 나타났습니다. 그 후 이 임상시험은 두 개의 공개 라벨, 1상/2상 연구로 확장되어 2015년에 1차 연구 결과를 뒷받침하는 결과가 발표되었습니다.112 이 임상시험에서 환자들은 세 그룹으로 나뉘어 눈당 10×104, 15×104, 50×104 RPE 세포 등 세 가지 용량의 hESC 유래 RPE를 투여받았습니다. 22개월의 추적 관찰 결과, 19명의 환자는 시력이 개선되었고, 7명의 환자는 시력이 개선되지 않았으며, 1명의 환자는 시력을 더 잃었습니다. hESC 유래 RPE 생착의 기술적 난제는 투여 후 RPE의 불균형한 증식으로, 치료받은 환자의 72%에서 관찰되었습니다. 노화로 인한 황반변성 진단을 받은 한국인 환자 2명과 스타가르트 황반이상증 환자 2명에게도 유사한 접근법이 시행되었습니다.113 그 결과 hESC 유래 RPE 세포의 안전성을 뒷받침하고 3명의 환자에서 시력이 개선되는 것을 보여주었습니다. 최근에는 중국 연구진에 의해 습성 연령 관련 변성 환자를 치료하기 위한 임상 등급 hESC 유래 RPE 세포가 이종 무배양 조건에서 개발되었습니다.114 hESC 개발은 여전히 동종 투여와 관련된 윤리적 우려 및 면역학적 합병증과 관련이 있기 때문에, hiPSC 유래 RPE 세포가 황반변성의 잠재적 세포 공급원으로 부상했습니다. RPE 분화 프로토콜이 개발되고 최적화되어 hiPSC 유래 RPE 세포의 효능을 개선했지만, 여전히 불충분하고 시간이 많이 소요되며 노동 집약적입니다.115,116 임상 적용을 위해 FGF/MAPK 신호 경로의 일시적 억제를 활용하여 피더가 없는 배양 조건을 사용하여 “프라이밍” 상태에서 “나이브” 상태의 RPE로 효율적으로 분화하는 방법이 개발되었습니다.117 분화 과정에서 특정 전사인자를 hiPSC에 과발현하는 것도 임상용으로 많은 수의 RPE 세포를 생성하는 흥미로운 접근 방식입니다. 최근 연구에서 OTX2, PAX6 및 MITF를 포함한 세 가지 안구 전사인자의 과발현은 hiPSC에서 RPE 분화를 자극하고 이식에 적합한 기능성 RPE 세포를 생성했습니다.118 현재까지 임상 1/II상 시험의 보고 데이터는 hESC 유래 RPE 세포의 안전성을 뒷받침하기에 충분하지만 치료는 아직 미숙한 단계에 머물러 있습니다. 따라서 향후 연구에서는 RPE의 세포 제조 공정과 망막하 투여 경로의 개발에 초점을 맞추어 RPE 제조 및 환자 망막으로의 생착 결과를 더욱 개선해야 합니다(권장 리뷰119).

수많은 연구에서 hESC 유래 심근세포가 심장 전사인자를 나타내며 심근세포 표현형과 미성숙 전기 표현형을 나타낸다는 사실이 입증되었습니다. 또한 hPSC 유래 심근세포를 사용하면 진정한 심근 재생 및 이식에 필요한 많은 수의 세포를 제공할 수 있습니다. 따라서 이러한 세포는 인간 심혈관 질환 치료를 위한 유망한 새로운 치료법이 될 수 있습니다. 한 사례 보고에 따르면, hESC 유래 심근세포는 중증 심부전 환자에서 후속 합병증 없이 잠재적인 치료 효과를 보였습니다.120 이 연구는 중증 심부전 환자 10명을 대상으로 피브린 젤 스캐폴드에 시드된 hESC 유래 심근세포 전구세포의 안전성을 평가하기 위한 1상 시험(ESCORT [Transplantation of Human Embryonic Stem Cell-derived Progenitors in Severe Heart Failure] 시험)이었습니다(NCT02057900). 이 연구의 고무적인 결과는 임상 수준의 표준으로 hESC 유래 심근세포 전구세포를 생산하고 이를 조직 공학 스캐폴드와 결합하여 중증 심장 질환을 치료할 수 있다는 가능성을 보여주었습니다(이 시험의 첫 번째 환자는 2021년 10월에 이미 7년 추적 관찰에 도달했습니다).121 현재 진행 중인 두 건의 hPSC 유래 심근세포 임상시험은 그 결과가 상업적 사용을 위한 치료제 승인 기준을 높일 수 있는 길을 열어줄 수 있어 큰 관심을 끌고 있습니다. 첫 번째 임상시험은 오사카 대학의 심장외과 의사 요시키 사와가 이끄는 연구팀이 생착용 세포 시트에 삽입된 hiPSC 유래 심근세포를 사용하여 수행했습니다(jRCT2052190081). 이 임상시험은 먼저 3명의 환자를 대상으로 시작하여 10명의 환자를 대상으로 이 접근법의 안전성을 평가했습니다. 안전성이 충족되면 일본의 재생의료 패스트트랙 제도에 따라 이 치료법은 상업적으로 판매될 수 있습니다.122 또 다른 시험에서는 BioVAT-HF라는 콜라겐 기반 구조물을 사용하여 hiPSC 유래 심근세포를 포함했습니다. 이 시험은 세포 용량을 평가하기 위해 18명의 환자를 모집하는 (파트 A)와 35명의 환자를 모집하여 광범위한 엔지니어링 인간 심근(EHM) 용량을 테스트하는 (파트 B) 두 부분으로 나뉘어 진행되었습니다. 이 연구에서 기대되는 결과는 인간의 심장 재근육화 자극에 EHM을 사용하기 위한 '개념 증명'을 제공할 것입니다. 현재까지 이 임상시험에서 부작용이나 심각한 부작용은 보고되지 않았으며, 이는 시술의 안전성을 뒷받침합니다. 그러나 치료받은 환자 수가 상대적으로 적었기 때문에 효능에 대한 결론을 내리는 데는 아직 한계가 있습니다.21,123

hPSC 기반 치료법을 사용한 최초의 임상시험 중 하나는 2010년 제론(Geron)사에서 척수손상 치료를 위해 hESC 유래 희소돌기아교세포 전구세포(OPC1)를 사용하여 수행한 임상시험입니다(SCI). 그 결과 5명의 참가자에게 투여 1년 후 안전성이 확인되었고, 자기공명영상에서 4명의 참가자에게서 척수 손상 악화가 개선된 것으로 나타났습니다.124 아스테리아스 바이오테라퓨틱(AST)은 AST-OPC1(NCT02302157)의 치료 효과를 평가하는 SCiStar 1상/IIa 연구를 진행하여 제론 연구를 이어나갔습니다. 임상시험 결과, 임상시험.gov에 발표된 임상시험 결과에서는 달리기 속도, 앞다리 보폭 길이, 앞다리 종방향 편차, 뒷다리 보폭 빈도가 유의하게 개선된 것으로 나타났습니다. 흥미롭게도 최근에 발표된 1상, 다기관, 비무작위, 단일 그룹 배정, 중재 시험의 데이터에 따르면 OPC1 제품 투여 후 10년이 지난 환자에서 신경학적 기능 저하, 종괴 확대, 추가 척수 손상 또는 척수 공동 형성의 증거가 나타나지 않았습니다.125 이 데이터 세트는 최대 10년의 무사건 기간으로 OPC1의 안전성을 뒷받침하는 확실한 증거를 제공하여 SCiStar 시험의 안전성 프로필을 강화합니다.

hPSC 기반 치료법을 사용한 임상시험의 글로벌 동향을 분석한 결과, 77.1%의 연구가 관찰 연구(환자에게 세포를 투여하지 않음)였으며, 22.9%의 연구만이 중재 치료로 hPSC 유래 세포를 사용했습니다.126 hESC를 사용한 연구의 수는 74.8%로 각각 25.2%에 비해 상대적으로 더 많았습니다. 관찰 연구는 미국(41.6%), 프랑스(16.8%) 등 선진국에서 많이 수행된 반면, 중재 연구는 중국(36.7%), 일본(13.3%), 한국(10%) 등 아시아 국가에서 많이 수행되었습니다. 치료 연구 동향은 표적 질환 측면에서도 뚜렷하게 나타났습니다. 가장 많이 연구된 3대 질환은 안과 질환, 순환기 질환, 신경계 질환이었습니다.127 그러나 전 세계적으로 hESC가 처음 발견된 이후 임상 적용이 거의 이루어지지 않았다는 점은 놀랍습니다. 환자에게 투여하는 치료제로서 iPSC를 사용하는 데 초점을 맞춘 임상시험의 수가 상대적으로 적은 것은 hiPSC의 불안정한 게놈,128 면역학적 거부반응,129 종양 형성 가능성 때문일 수 있습니다.130

Mesenchymal stem/stromal cell-based therapy: is it time to consider their origin toward targeted therapy?

Approximately 55 years ago, fibroblast-like, plastic-adherent cells, later named mesenchymal stem cells (MSCs) by Arnold L. Caplan,18 were discovered for the first time in mouse bone marrow (BM) and were later demonstrated to be able to form colony-like structures, proliferate, and differentiate into bone/reticular tissue, cartilage, and fat.131 Protocols were subsequently established to directly culture this subpopulation of stromal cells from BM in vitro and to stimulate their differentiation into adipocytes, chondroblasts, and osteoblasts.132 Since then, MSCs have been found in and derived from different human tissue sources, including adipose tissue (AT), the umbilical cord (UC), UC blood, the placenta, dental pulp, amniotic fluid, etc.133 To standardize and define MSCs, the International Society for Cell and Gene Therapy (ISCT) set minimal identification criteria for MSCs derived from multiple tissue sources.134 Among them, MSCs derived from AT, BM, and UC are the most commonly studied MSCs in human clinical trials,135 and they constitute the three major tissue sources of MSCs that will be discussed in this review.

The discovery of MSCs opened an era during which preclinical studies and clinical trials have been performed to assess the safety and efficacy of MSCs in the treatment of various diseases. The major conclusion of these studies and trials is that MSC-based therapy is safe, although the outcomes have usually been either neutral or at best marginally positive in terms of the clinically relevant endpoints regardless of MSC tissue origin, route of infusion, dose, administration duration, and preconditioning.136 It is important to note that a solid background of knowledge has been generated from all these studies that has fueled the recent translational research in MSC-based therapy. As MSCs have been intensively studied over the last 55 years and have become the subject of multiple reviews, systematic reviews, and meta-analyses, the objective of this paper is not to duplicate these publications. Rather, we will discuss the questions that both clinicians and researchers are currently exploring with regard to MSC-based therapy, diligently seeking answers to the following:

• “With a solid body of data supporting their safety profiles derived from both preclinical and clinical studies, does the tissue origin of MSCs also play a role in their downstream clinical applications in the treatment of different human diseases?”

• “Do MSCs derived from AT, BM, and UC exhibit similar efficacy in the treatment of neurological diseases, metabolic/endocrine-related disorders, reproductive dysfunction, skin burns, lung fibrosis, pulmonary disease, and cardiovascular conditions?”

To answer these questions, we will first focus on the most recently published clinical data regarding these targeted conditions, including neurological disorders, pulmonary dysfunctions, metabolic/endocrine-related diseases, reproductive disorders, skin burns, and heart-related diseases, to analyze the potential efficacy of MSCs derived from AT, BM, and UC. Based on the level of clinical improvement observed in each trial, we analyzed the potential efficacy of MSCs derived from each source to visualize the correlation between patient improvement and MSC sources. We will then address recent trends in the exclusive use of MSC-based products, focusing on the efficacy of treatment with MSCs from each of the abovementioned sources, and we will analyze the relationship between the respective efficacies of MSCs from these sources in relation to the targeted disease conditions. Finally, we propose a hypothesis and mechanism to achieve the currently still unmet objective of eval‎uating the use of MSCs from AT, BM, and UC in regenerative medicine.

중간엽 줄기세포/기질세포 기반 치료: 표적 치료로 나아가기 위해 중간엽 줄기세포의 기원을 고려할 때인가?

약 55년 전, 섬유아세포와 유사한 플라스틱 부착성 세포(나중에 아놀드 L. 캐플란에 의해 중간엽 줄기세포(MSC)로 명명)18가 마우스 골수(BM)에서 처음으로 발견되었으며, 이후 군집과 같은 구조를 형성하고 증식하며 뼈/망상 조직, 연골 및 지방으로 분화할 수 있다는 것이 입증되었습니다.131 이후 BM에서 이 기질 세포의 하위 집단을 시험관 내에서 직접 배양하고 지방 세포, 연골 세포 및 조골 세포로의 분화를 자극하는 프로토콜이 확립되었습니다.132 그 이후로 지방 조직(AT), 탯줄(UC), UC 혈액, 태반, 치아 치수, 양수 등 다양한 인체 조직 공급원에서 MSC가 발견되고 유래되었습니다.133 국제 세포 및 유전자 치료학회(ISCT)는 MSC를 표준화하고 정의하기 위해 여러 조직 공급원에서 유래된 MSC에 대한 최소 식별 기준을 설정했습니다.134 그 중 AT, BM 및 UC에서 유래한 MSC는 인간 임상시험에서 가장 일반적으로 연구되는 MSC이며,135 이 리뷰에서 논의할 MSC의 세 가지 주요 조직 공급원을 구성합니다.

MSC의 발견으로 다양한 질병의 치료에서 MSC의 안전성과 효능을 평가하기 위한 전임상 연구와 임상시험이 수행되는 시대가 열렸습니다. 이러한 연구와 임상시험의 주요 결론은 MSC 기반 치료가 안전하다는 것이지만, 결과는 일반적으로 MSC 조직 기원, 주입 경로, 용량, 투여 기간 및 사전 조건에 관계없이 임상 관련 평가변수 측면에서 중립적이거나 기껏해야 약간 긍정적인 결과를 보였습니다.136 이러한 모든 연구를 통해 탄탄한 지식 배경이 생성되어 최근 MSC 기반 치료의 중개 연구를 촉진했다는 점에 주목하는 것이 중요합니다. MSC는 지난 55년 동안 집중적으로 연구되어 왔으며 여러 리뷰, 체계적 고찰 및 메타 분석의 대상이 되었기 때문에 이 백서의 목적은 이러한 출판물을 중복하는 것이 아닙니다. 그보다는 임상의와 연구자 모두가 현재 MSC 기반 치료와 관련하여 탐구하고 있는 질문에 대해 논의하며 다음과 같은 질문에 대한 답을 부지런히 찾고자 합니다:

• “전임상 및 임상 연구 모두에서 도출된 안전성 프로파일을 뒷받침하는 탄탄한 데이터를 바탕으로, MSC의 조직 기원이 다양한 인간 질병 치료의 후속 임상 적용에도 중요한 역할을 할 수 있는가?”
• “신경계 질환, 대사/내분비 관련 장애, 생식 기능 장애, 피부 화상, 폐 섬유증, 폐 질환 및 심혈관 질환의 치료에서 AT, BM 및 UC에서 유래한 MSC가 유사한 효능을 나타내는가?”

이러한 질문에 답하기 위해 먼저 신경 장애, 폐 기능 장애, 대사/내분비 관련 질환, 생식 장애, 피부 화상, 심장 관련 질환 등 이러한 표적 질환에 대해 가장 최근에 발표된 임상 데이터에 집중하여 AT, BM, UC에서 추출한 MSC의 잠재적 효능을 분석할 것입니다. 각 임상시험에서 관찰된 임상적 개선 수준을 바탕으로 각 공급원에서 추출한 MSC의 잠재적 효능을 분석하여 환자 개선과 MSC 공급원 간의 상관관계를 시각화했습니다. 그런 다음 위에서 언급한 각 출처에서 추출한 MSC를 사용한 치료의 효능에 초점을 맞춰 MSC 기반 제품의 독점적 사용에 대한 최근 동향을 다루고, 목표 질병 상태와 관련하여 이러한 출처에서 추출한 각 효능 간의 관계를 분석할 것입니다. 마지막으로, 재생 의학에서 AT, BM, UC에서 유래한 MSC의 사용을 평가하는 현재 아직 충족되지 않은 목표를 달성하기 위한 가설과 메커니즘을 제안합니다.

An overview of MSC tissue origins and therapeutic potential

In general, MSCs are reported to be isolated from numerous tissue types, but all of these types can be organized into two major sources: adult137 and perinatal sources138 (Fig. 2). Adult sources of MSCs are defined as tissues that can be harvested or obtained from an individual, such as dental pulp,139 BM, peripheral blood,140 AT,141 lungs,142 hair,143 or the heart.144 Adult MSCs usually reside in specialized structures called stem cell niches, which provide the microenvironment, growth factors, cell-to-cell contacts and external signals necessary for maintaining stemness and differentiation ability.145 BM was the first adult source of MSCs discovered by Friedenstein131 and has become one of the most documented and largely used MSC sources to date, followed by AT. BM-MSCs are isolated and cultured in vitro from BM aspirates using a Ficoll gradient-centrifugation method146 or a red blood cell lysate buffer to collect BM mononuclear cell populations, whereas AT-MSCs are obtained from stromal vascular fractions of enzymatically digested AT obtained through liposuction,141 lipoplasty, or lipectomy procedures.147 These tissue collection procedures are invasive and painful for the patient and are accompanied by a risk of infection, although BM aspiration and adipose liposuction are considered safe procedures for BM and AT biopsies. The number of MSCs that can be isolated from these adult tissues varies significantly in a tissue-dependent manner. The percentage of MSCs in BM mononuclear cells ranges from 0.001 to 0.01% following gradient centrifugation.132 

The number of MSCs in AT is at least 500 times higher than that in BM, with approximately 5,000 MSCs per 1 g of AT. Perinatal sources of MSCs consist of UC-derived components, such as UC, Wharton’s jelly, and UC blood, and placental structures, such as the placental membrane, amnion, chorion membrane, and amniotic fluid.138 The collection of perinatal MSCs, such as UC-MSCs, is noninvasive, as the placenta, UC, UC blood, and amnion are considered waste products that are usually discarded after birth (with no ethical barriers).148 Although MSCs represent only 10−7% the cells found in UC, their higher proliferation rate and rapid population doubling time allow these cells to rapidly replicate and increase in number during in vitro culture.149 Under standardized xeno-free and serum-free culture platforms, AT-MSCs show a faster proliferation rate and a higher number of colony-forming units than BM-MSCs.149 UC-MSCs have the fastest population doubling time compared to AT-MSCs and BM-MSCs in both conventional culture conditions and xeno- and serum-free environments.149 MSCs extracted from AT, BM and UC exhibit all minimal criteria listed by the ISCT, including morphology (plastic adherence and spindle shape), MSC surface markers (95% positive for CD73, CD90 and CD105; less than 2% negative for CD11, CD13, CD19, CD34, CD45, and HLR-DR) and differentiation ability into chondrocytes, osteocytes, and adipocytes.150

MSC 조직 기원과 치료 잠재력에 대한 개요

일반적으로 MSC는 다양한 조직 유형에서 분리되는 것으로 보고되지만, 이러한 모든 유형은 크게 성인137 및 주산기 공급원138의 두 가지 주요 공급원으로 분류할 수 있습니다(그림 2). 성체 공급원은 치아 치수,139 BM, 말초 혈액,140 AT,141 폐,142 모발,143 또는 심장과 같이 개인으로부터 채취하거나 얻을 수 있는 조직으로 정의됩니다.144 성체 MSC는 일반적으로 줄기세포 틈새라는 특수 구조에 존재하며, 줄기 및 분화 능력을 유지하는 데 필요한 미세 환경, 성장 인자, 세포 간 접촉 및 외부 신호를 제공합니다.145 BM은 프리덴슈타인131이 최초로 발견된 성체 MSC 공급원으로 현재까지 가장 많이 기록되고 널리 사용되고 있는 MSC 공급원 중 하나이며 그 다음으로 AT가 그 뒤를 잇고 있습니다. BM-MSC는 BM 단핵 세포 집단을 수집하기 위해 Ficoll 구배 원심분리법146 또는 적혈구 용해 완충액을 사용하여 BM 흡인물로부터 체외에서 분리 및 배양하는 반면, AT-MSC는 지방 흡입,141 지방 성형술 또는 지방 절제술을 통해 얻은 효소 소화된 AT의 기질 혈관 분획에서 얻습니다.147. 이러한 조직 채취 절차는 환자에게 침습적이고 고통스러우며 감염 위험이 수반되지만, BM 흡인 및 지방 지방 흡입술은 BM 및 AT 생검을 위한 안전한 절차로 간주됩니다. 이러한 성인 조직에서 분리할 수 있는 MSC의 수는 조직에 따라 크게 달라집니다. BM 단핵 세포에서 MSC의 비율은 구배 원심분리 후 0.001~0.01%입니다.132

AT의 MSC 수는 BM보다 최소 500배 이상 많으며, AT 1g당 약 5,000개의 MSC가 있습니다. 주산기 MSC의 공급원은 UC, 와튼젤리, UC 혈액과 같은 UC 유래 성분과 태반막, 양막, 융모막, 양수와 같은 태반 구조물로 구성됩니다.138 태반, UC, UC 혈액 및 양수는 일반적으로 출생 후 폐기되는 폐기물로 간주되기 때문에(윤리적 장벽이 없음) UC-MSC와 같은 주산기 MSC의 수집은 비침습적입니다.148 MSC는 UC에서 발견되는 세포의 10-7%에 불과하지만 높은 증식률과 빠른 인구 배가 시간 덕분에 체외 배양 중에 빠르게 복제하고 그 수를 늘릴 수 있습니다.149 표준화된 제노 프리 및 혈청 프리 배양 플랫폼에서 AT-MSC는 BM-MSC보다 더 빠른 증식 속도와 더 많은 수의 콜로니 형성 유닛을 보여줍니다.149 UC-MSC는 기존 배양 조건과 제노 및 혈청 프리 환경 모두에서 AT-MSC 및 BM-MSC에 비해 가장 빠른 인구 배가 시간을 가집니다.149 AT, BM 및 UC에서 추출한 MSC는 형태(플라스틱 부착성 및 스핀들 모양), MSC 표면 마커(CD73, CD90 및 CD105에 95% 양성, CD11, CD13, CD19, CD34, CD45 및 HLR-DR에 2% 미만 음성) 및 연골세포, 조골세포 및 지방세포로의 분화 능력을 포함하여 ISCT에서 제시한 최소 기준을 모두 충족합니다.150

Fig. 2

The two major sources of MSCs: adult and perinatal sources. The adult sources of MSCs are specific tissue in human body where MSCs could be isolated, including bone marrow, adipose tissue, dental pulp, peripheral blood, menstrual blood, muscle, etc. The perinatal sources of MSCs consist of umbilical cord-derived components, such as umbilical cord, Wharton’s jelly, umbilical cord blood, and placental structures, such as placental membrane, amnion, chorion membrane, amniotic fluid, etc. The figure was created with BioRender.com

MSC의 두 가지 주요 공급원은 성인과 주산기 공급원입니다. 성인의 MSC 공급원은 골수, 지방 조직, 치아 치수, 말초 혈액, 생리혈, 근육 등 MSC를 분리할 수 있는 인체의 특정 조직입니다. 주산기 MSC의 공급원은 탯줄, 와튼젤리, 제대혈과 같은 탯줄 유래 성분과 태반막, 양막, 융모막, 양수 등과 같은 태반 구조물로 구성되어 있으며, 이 그림에서는 태반의 구조물과 태반막, 양막, 융모막, 양수 등 태반 유래 성분이 어떻게 분리되는지 설명합니다. 

이 그림은 BioRender.com으로 제작되었습니다.

Full size image

In fact, although MSCs derived from either adult or perinatal sources exhibit similar morphology and the basic characteristics of MSCs, studies have demonstrated that these cells also differ from each other. Regarding immunophenotyping, AT-MSCs express high levels of CD49d and low levels of Stro-1. An analysis of the expression‎ of CD49d and CD106 showed that the former is strongly expressed in AT-MSCs, in contrast to BM-MSCs, whereas CD106 is expressed in BM-MSCs but not in AT-MSCs.151 Increased expression‎ of CD133, which is associated with stem cell regeneration, differentiation, and metabolic functions,152 was observed in BM-MSCs compared to MSCs from other sources.153 A recent study showed that CD146 expression‎ in UC-MSCs was higher than that in AT- and BM-MSCs,153 supporting the observation that UC-MSCs have a stronger attachment and a higher proliferation rate than MSCs from other sources, as CD146 is a key cell adhesion protein in vascular and endothelial cell types.154 In terms of differentiation ability, donor-matched BM-MSCs exhibit a higher ability to differentiate into chondrogenic and osteogenic cell types than AT-MSCs, whereas AT-MSCs show a stronger capacity toward the adipogenic lineage.150 The findings from an in vitro differentiation study indicated that BM-MSCs are prone to osteogenic differentiation, whereas AT-MSCs possess stronger adipogenic differentiation ability, which can be explained by the fact that the epigenetic memory obtained from either BM or AT drives the favored MSC differentiation along an osteoblastic or adipocytic lineage.155 Interestingly, although UC-MSCs have the ability to differentiate into adipocytes, osteocytes, or chondrocytes, their osteogenic differentiation ability has been proven to be stronger than that of BM-MSCs.156 The most interesting characteristic of MSCs is their immunoregulatory functions, which are speculated to be related to either cell-to-cell contact or growth factor and cytokine secretion in response to environmental/microenvironmental stimuli. MSCs from different sources almost completely inhibit the proliferation of peripheral blood mononuclear cells (PBMCs) at PBMC:MSC ratios of 1:1 and 10:1.149 At a higher ratio, BM-MSCs showed a significantly higher inhibitory effect than AT- or UC-MSCs.153 Direct analysis of the immunosuppressive effects of BM- and UC-MSCs has revealed that these cells exert similar inhibitory effects in vitro with different mechanisms involved.157 With these conflicting data, the mechanism of action related to the immune response of MSCs from different sources is still poorly understood, and long-term investigations both in preclinical studies and in clinical trial settings are needed to shed light on this complex immunomodulation function.

The great concern in MSC-based therapy is the fate of these cells post administration, especially through different delivery routes, including systemic administration via an intravenous (IV) route or tissue-specific administration, such as dorsal pancreatic administration. It is important to understand the distribution of these cells after injection to expand our understanding of the underlying mechanisms of action of treatments; in addition, this knowledge is required by authorized bodies (the Food and Drug Administration (FDA) in the United States or the regulation of advanced-therapy medicinal products in Europe, No. 1394/2007) prior to using these cells in clinical trials. The preclinical data using various labeling techniques provide important information demonstrating that MSCs do not have unwanted homing that could lead to the incorrect differentiation of the cells or inappropriate tumor formation. In a mouse model, human BM-MSCs and AT-MSCs delivered via an IV route are rapidly trapped in the lungs and then recirculate through the body after the first embolization process, with a small number of infused cells found mainly in the liver after the second embolization.158 Using the technetium-99 m labeling method, intravenously infused human cells showed long-term persistence up to 13 months in the bone, BM compartment, spleen, muscle, and cartilage.159 A similar result was reported in baboons, confirm‎ing the long-term homing of human MSCs in various tissues post administration.160 Although the retainment of MSCs in the lungs might potentially reduce their systemic therapeutic effects,161 it provides a strong advantage when these cells are used in the treatment of respiratory diseases. Local injection of MSCs also revealed their tissue-specific homing, as an injection of MSCs via the renal artery route resulted in the majority of the injected cells being found in the renal cortex.162 Numerous studies have been conducted to track the migration of administered MSCs in human subjects. Henriksson and his team used MSCs labeled with iron sucrose in the treatment of intervertebral disc degeneration.163 Their study showed that chondrocytes differentiated from infused MSCs could be detected at the injured intervertebral discs at 8 months but not at 28 months. A study conducted in a patient with hemophilia A using In-oxine-labeled MSCs showed that the majority of the cells were trapped in the lungs and liver 1 h post administration, followed by a reduction in the lungs and an increase in the number of cells in the liver after 6 days.164 Interestingly, a small proportion of infused MSCs were found in the hemarthrosis site at the right ankle after 24 h, suggesting that MSCs are attracted and migrate to the injured site. The distribution of MSCs was also reported in the treatment of 21 patients diagnosed with type 2 diabetes using 18-FDG-tagged MSCs and visualized using positron emission tomography (PET).165 The results illustrated that local delivery of MSCs via an intraarterial route is more effective than delivery via an IV route, as MSCs home to the pancreatic head (pancreaticoduodenal artery) or body (splenic artery). Therefore, although the available data related to the biodistribution of infused MSCs are still limited, the results obtained from both preclinical and clinical studies illustrate a comparable set of data supporting results on homing, migration to the injured site, and the major organs where infused MSCs are located. The following comprehensive and interesting reviews are highly recommended.166,167,168

To date, 1426 registered clinical trials spanning different trial phases have used MSCs for therapeutic purposes, which is four times the number reported in 2013.169,170 As supported by a large body of preclinical studies and advancements in conducting clinical trials, MSCs have been proven to be effective in the treatment of numerous diseases, including nervous system and brain disorders, pulmonary diseases,171 cardiovascular conditions,172 wound healing, etc. The outcomes of MSC-based therapy have been the subject of many intensive reviews and systematic analyses with the solid conclusion that these cells exhibit strong safety profiles and positive outcomes in most tested conditions.173,174,175 In addition, the available data have revealed several potential mechanisms that could explain the beneficial effects of MSCs, including their homing efficiency, differentiation potential, production of trophic factors (including cytokines, chemokines, and growth factors), and immunomodulatory abilities. However, it is still not known which MSC types should be used for which diseases, as it seems to be that MSCs exhibit beneficial effects regardless of their sources.169

실제로 성체 또는 주산기 유래 MSC는 유사한 형태와 MSC의 기본 특성을 나타내지만, 연구에 따르면 이러한 세포는 서로 다르다는 것이 입증되었습니다. 면역 표현형과 관련하여 AT-MSC는 높은 수준의 CD49d를 발현하고 낮은 수준의 Stro-1을 발현합니다. CD49d와 CD106의 발현을 분석한 결과, 전자는 BM-MSC와 달리 AT-MSC에서 강하게 발현되는 반면, CD106은 BM-MSC에서는 발현되지만 AT-MSC에서는 발현되지 않습니다.151 줄기세포 재생, 분화 및 대사 기능과 관련된 CD133의 발현 증가는 다른 출처의 MSC에 비해 BM-MSC에서 관찰되었습니다.153 최근 연구에 따르면 UC-MSC의 CD146 발현이 AT-MSC 및 BM-MSC보다 높았으며,153 이는 CD146이 혈관 및 내피 세포 유형에서 핵심 세포 부착 단백질이기 때문에 UC-MSC가 다른 출처의 MSC보다 더 강한 부착력과 높은 증식 속도를 가지고 있다는 관찰을 뒷받침합니다.154 분화 능력 측면에서 공여자와 일치하는 BM-MSC는 AT-MSC보다 연골 형성 및 골 형성 세포 유형으로 분화하는 능력이 더 높은 반면, AT-MSC는 지방 형성 계통으로 분화하는 능력이 더 강합니다.150 체외 분화 연구 결과에 따르면 BM-MSC는 골형성 분화 경향이 있는 반면 AT-MSC는 지방형성 분화 능력이 더 강한 것으로 나타났는데, 이는 BM 또는 AT에서 얻은 후성유전학적 기억이 조골세포 또는 지방세포 계통을 따라 선호하는 MSC 분화를 유도한다는 사실로 설명할 수 있습니다.155 흥미롭게도 UC-MSC는 지방세포, 조골세포 또는 연골세포로 분화할 수 있지만, 골 형성 분화 능력이 BM-MSC보다 더 강한 것으로 입증되었습니다.156 MSC의 가장 흥미로운 특징은 면역 조절 기능으로, 이는 세포 간 접촉 또는 환경/미세 환경 자극에 대한 성장 인자 및 사이토카인 분비와 관련이 있을 것으로 추측됩니다. 서로 다른 출처의 MSC는 1:1 및 10:1의 PBMC:MSC 비율에서 말초 혈액 단핵 세포(PBMC)의 증식을 거의 완벽하게 억제합니다.149 이보다 높은 비율에서는 BM-MSC가 AT 또는 UC-MSC보다 훨씬 높은 억제 효과를 보였습니다.153 BM-MSC와 UC-MSC의 면역 억제 효과를 직접 분석한 결과 이들 세포는 서로 관련된 메커니즘이 다르지만 체외에서 유사한 억제 효과를 발휘하는 것으로 밝혀졌습니다.157 이러한 상충되는 데이터로 인해 서로 다른 출처의 MSC의 면역 반응과 관련된 작용 메커니즘은 아직 제대로 이해되지 않았으며, 이 복잡한 면역 조절 기능을 밝히기 위해서는 전임상 연구와 임상 시험 환경에서 장기간의 조사가 필요합니다.

MSC 기반 치료에서 가장 큰 관심사는 투여 후, 특히 정맥(IV) 경로를 통한 전신 투여 또는 등쪽 췌장 투여와 같은 조직별 투여를 포함한 다양한 전달 경로를 통한 이러한 세포의 운명입니다. 치료제의 근본적인 작용 메커니즘에 대한 이해를 넓히기 위해서는 주사 후 세포의 분포를 이해하는 것이 중요하며, 이러한 지식은 임상시험에 세포를 사용하기 전에 승인 기관(미국 식품의약국(FDA) 또는 유럽의 첨단 치료 의약품 규정, 1394/2007호)에서 요구하고 있습니다. 다양한 라벨링 기법을 사용한 전임상 데이터는 MSC가 세포의 잘못된 분화 또는 부적절한 종양 형성으로 이어질 수 있는 원치 않는 홈을 가지고 있지 않음을 입증하는 중요한 정보를 제공합니다. 마우스 모델에서 정맥을 통해 전달된 인간 BM-MSC와 AT-MSC는 1차 색전술 후 폐에 빠르게 갇힌 다음 체내를 재순환하며, 2차 색전술 후 소수의 주입된 세포가 주로 간에서 발견되었습니다.158 테크네튬-99m 표지 방법을 사용하여 정맥 주입된 인간 세포는 뼈, BM 구획, 비장, 근육 및 연골에서 최대 13개월까지 장기적 지속성을 보였습니다.159 개코원숭이에서도 유사한 결과가 보고되어 투여 후 다양한 조직에서 인간 MSC가 장기적으로 유지되는 것을 확인했습니다.160 폐에 MSC가 유지되면 전신 치료 효과가 감소할 수 있지만,161 이러한 세포가 호흡기 질환 치료에 사용될 때 강력한 이점을 제공합니다. 또한 신장 동맥 경로를 통해 MSC를 주입한 결과 주입된 세포의 대부분이 신장 피질에서 발견되는 등 조직별 이동 경로가 밝혀졌습니다.162 인간 대상에서 투여된 MSC의 이동을 추적하기 위한 수많은 연구가 수행되었습니다. 헨릭슨과 그의 팀은 추간판 퇴행 치료에 자당으로 표지된 MSC를 사용했습니다.163 그들의 연구에 따르면 주입된 MSC에서 분화된 연골세포가 8개월 후에는 손상된 추간판에서 발견될 수 있었지만 28개월 후에는 발견되지 않았습니다. A형 혈우병 환자를 대상으로 인옥신 표지 MSC를 사용한 연구에 따르면 투여 후 1시간 후 대부분의 세포가 폐와 간에 갇혀 있었고, 6일 후에는 폐가 감소하고 간에서 세포 수가 증가했습니다.164 흥미롭게도 24시간 후 오른쪽 발목의 혈전증 부위에서 소량의 주입된 MSC가 발견되어 MSC가 손상된 부위로 유인되어 이동하는 것을 시사하는 것으로 나타났습니다. 2형 당뇨병으로 진단받은 21명의 환자를 대상으로 18-FDG 태그가 부착된 MSC를 사용하여 양전자 방출 단층촬영(PET)을 통해 시각화한 결과에서도 MSC의 분포가 보고되었습니다.165 그 결과 췌장 머리(췌십이지장 동맥) 또는 몸(비장 동맥)에 MSC를 동맥 내 경로로 전달하는 것이 정맥 경로를 통한 전달보다 더 효과적이라는 사실이 입증되었습니다. 따라서 주입된 MSC의 생체 분포와 관련된 이용 가능한 데이터는 아직 제한적이지만, 전임상 및 임상 연구에서 얻은 결과는 귀환, 손상 부위로의 이동 및 주입된 MSC가 위치한 주요 장기에 대한 결과를 뒷받침하는 비교 가능한 데이터 세트를 보여줍니다. 다음과 같은 포괄적이고 흥미로운 리뷰를 적극 권장합니다.166,167,168

현재까지 다양한 시험 단계에 걸쳐 1426건의 등록된 임상시험에서 치료 목적으로 MSC를 사용했으며, 이는 2013년에 보고된 수의 4배에 달하는 수치입니다.169,170 많은 전임상 연구와 임상시험 수행의 발전이 뒷받침되면서 MSC는 신경계 및 뇌 질환, 폐 질환,171 심혈관 질환,172 상처 치유 등을 포함한 수많은 질병의 치료에 효과가 있음이 입증되었습니다. MSC 기반 치료의 결과는 많은 집중적인 검토와 체계적인 분석의 대상이 되어 왔으며, 이러한 세포는 대부분의 테스트 조건에서 강력한 안전성 프로파일과 긍정적인 결과를 보인다는 확실한 결론을 내렸습니다.173,174,175 또한, 이용 가능한 데이터는 MSC의 귀환 효율성, 분화 가능성, 영양 인자(사이토카인, 케모카인 및 성장 인자 포함) 생성 및 면역 조절 능력 등 MSC의 유익한 효과를 설명할 수 있는 몇 가지 잠재적 메커니즘을 밝혀냈습니다. 그러나 MSC는 출처에 관계없이 유익한 효과를 나타내는 것으로 보이기 때문에 어떤 질병에 어떤 유형의 MSC를 사용해야 하는지는 아직 알려지지 않았습니다.169

Acquired brain and spinal cord injury treatment: BM-MSCs have emerged as key players

The theory that brain cells can never regenerate has been challenged by the discovery of newly formed neurons in the human adult hippocampus or the migration of stem cells in the brain in animal models.176 These observations have triggered hope for regeneration in the context of neuronal diseases by using exogenous stem cell sources to replenish or boost the stem cell population in the brain. Moreover, the limited regenerative capacity of the brain and spinal cord is an obstacle for traditional treatments of neurodegenerative diseases, such as autism, cerebral palsy, stroke, and spinal cord injury (SCI). As current treatments cannot halt the progression of these diseases, studies throughout the world have sought to exploit cell-based therapies to treat neurodegenerative diseases on the basis of advances in the understanding and development of stem cell technology, including the use of MSCs. Successful stem cell therapy for treating brain disease requires therapeutic cells to reach the injured sites, where they can repair, replace, or at least prevent the deteriorative effects of neuronal damage.177 Hence, the gold standard of cell-based therapy is to deliver the cells to the target site, stimulate the tissue repair machinery, and regulate immunological responses via either cell-to-cell contact or paracrine effects.178 Among 315 registered clinical trials using stem cells for the treatment of brain diseases, MSCs and hematopoietic stem cells (HSCs; CD34+ cells isolated from either BM aspirate or UC blood) are the two main cell types investigated, whereas approximately 102 clinical trials used MSCs and 62 trials used HSCs for the treatment of brain disease (main search data from clinicaltrial.gov). MSCs are widely used in almost all clinical trials targeting different neuronal diseases, including multiple sclerosis,179 stroke,180 SCI,181 cerebral palsy,182 hypoxic-ischemic encephalopathy,183 autism,184 Parkinson’s disease,185 Alzheimer’s disease185 and ataxia. Among these trials in which MSCs were the major cells used, nearly two-thirds were for stroke, SCI, or multiple sclerosis. MSCs have been widely used in 29 registered clinical trials for stroke, with BM-MSCs being used in 16 of these trials. With 26 registered clinical trials, SCI is the second most common indication for using MSCs, with 16 of these trials using mainly expanded BM-MSCs. For multiple sclerosis, 15 trials employed BM-MSCs among a total of 23 trials conducted for the treatment of this disease. Hence, it is important to note that in neuronal diseases and disorders, BM-MSCs have emerged as the most commonly used therapeutic cells among other MSCs, such as AT-MSCs and UC-MSCs.

The outcomes of the use of BM-MSCs in the treatment of neuronal diseases have been widely reported in various clinical trial types. A review by Zheng et al. indicated that although the treatments appeared to be safe in patients diagnosed with stroke, there is a need for well-designed phase II multicentre studies to confirm‎ the outcomes.173 One of the earliest trials using autologous BM-MSCs was conducted by Bang et al. in five patients diagnosed with stroke in 2005. The results supported the safety and showed an improved Barthel index (BI) in MSC-treated patients.186 In a 2-year follow-up clinical trial, 16 patients with stroke received BM-MSC infusions, and the results showed that the treatment was safe and improved clinical outcomes, such as motor impairment scale scores.187 A study conducted in 12 patients with ischemic stroke showed that autologous BM-MSCs expanded in vitro using autologous serum improved the patient’s modified Rankin Scale (mRS), with a mean lesion volume reduced by 20% at 1 week post cell infusion.188 In 2011, a modest increase in the Fugl Meyer and modified BI scores was observed after autologous administration of BM-MSCs in patients with chronic stroke.189 More recently, a prospective, open-label, randomized controlled trial with blinded outcome eval‎uation was conducted, with 39 patients and 15 patients in the BM-MSC administration and control groups, respectively. The results of this study indicated that autologous BM-MSCs with autologous serum administration were safe, but the treatment led to no improvements at 3 months in modified Rankin Scale (mRS) scores, although leg motor improvement was observed.180 Researchers explored whether the efficacy of BM-MSC administration was maintained over time in a 5-year follow-up clinical trial. Patients (85) were randomly assigned to either the MSC group or the control group, and follow-ups on safety and efficacy were performed for 5 years, with 52 patients being examined at the end of the study. The MSC group exhibited a significant improvement in terms of decreased mRS scores, whereas the number of patients with an mRS score increase of 0–3 was statistically significant.187 Although autologous BM-MSCs did not improve the Basel index, mRS, or National Institutes of Health Stroke Scale (NIHSS) score 2 years post infusion, patients who received BM-MSC therapy showed improvement in their motor function score.190 In addition, a prospective, open-label, randomized controlled trial by Lee et al. showed that autologous BM-MSCs primed with autologous “ischemic” serum significantly improved motor functions in the MSC-treated group. Neuroimaging analysis also illustrated a significant increase in interhemispheric connectivity and ipsilesional connectivity in the MSC group.191 Recently, a single intravenous infection of allogeneic BM-MSCs has been proven to be safe and feasible in patients with chronic stroke with a significant improvement in BI score and NIHSS score.192

In two systematic reviews using MSCs for the treatment of SCI, BM-MSCs (n = 16) and UC-MSCs (n = 5) were reported to be safe and well-tolerated.193,194 The results indicated significant improvements in the stem cell administration groups compared with the control groups in terms of a composite of the American Spinal Injury Association (ASIA) impairment scale (AIS) grade, AIS grade A, and ASIA sensory scores and bladder function (Table 1). However, larger experimental groups with a randomized and multicentre design are needed for further confirmation of these findings. For multiple sclerosis, several early-phase (phase I/II) registered clinical studies have used BM-MSCs. A study compared the potential efficacy of BM-MSC and BM mononuclear cell (BMMNC) transplantation in 105 patients with spastic cerebral palsy.195 The results showed that the GMFM (gross motor function measure) and the FMFM (fine motor function measure) scores of the BM-MSC transplant group were higher than those of the BMNNC transplant group at 3, 6, and 12 months of assessment. In terms of autism spectrum disorder, a review of 254 children after BMMNC transplantation found that over 90% of patients’ ISAA (Indian Scale for Assessment of Autism) and CARS (Childhood Autism Rating Scale) scores improved. Young patients and those in whom autism spectrum disorder was detected early generally showed better improvement.196

획득한 뇌 및 척수 손상 치료: BM-MSC가 핵심 플레이어로 부상하다

뇌세포는 재생이 불가능하다는 이론은 인간의 성인 해마에서 새로 형성된 뉴런이 발견되거나 동물 모델에서 뇌의 줄기세포가 이동하는 것을 발견함으로써 도전받고 있습니다.176 이러한 관찰은 외인성 줄기세포 공급원을 사용하여 뇌의 줄기세포 개체군을 보충하거나 강화함으로써 신경 질환의 맥락에서 재생에 대한 희망을 불러 일으켰습니다. 또한 뇌와 척수의 제한된 재생 능력은 자폐증, 뇌성마비, 뇌졸중, 척수 손상과 같은 신경 퇴행성 질환의 기존 치료에 장애물이 되고 있습니다. 현재의 치료법으로는 이러한 질환의 진행을 막을 수 없기 때문에 전 세계의 연구자들은 줄기세포 기술에 대한 이해와 발전의 진전을 바탕으로 신경 퇴행성 질환을 치료하기 위해 세포 기반 치료법을 활용하고자 노력해 왔으며, 여기에는 중간엽줄기세포의 사용도 포함됩니다. 뇌 질환 치료를 위한 성공적인 줄기세포 치료는 치료 세포가 손상된 부위에 도달하여 신경 세포 손상의 악영향을 복구, 대체 또는 최소한 예방할 수 있어야 합니다.177 따라서 세포 기반 치료의 표준은 세포를 목표 부위에 전달하고 조직 복구 메커니즘을 자극하며 세포 간 접촉 또는 파라크린 효과를 통해 면역학적 반응을 조절하는 것입니다.178 뇌 질환 치료를 위해 줄기세포를 사용한 등록된 315건의 임상시험 중, 조사된 두 가지 주요 세포 유형은 MSC와 조혈 줄기세포(HSC; BM 흡인 또는 UC 혈액에서 분리된 CD34+ 세포)이며, 약 102건의 임상시험에서 MSC를, 62건의 임상시험에서 HSC를 뇌 질환 치료에 사용했습니다(clinicaltrial.gov의 주요 검색 데이터). 다발성 경화증,179 뇌졸중,180 SCI,181 뇌성마비,182 저산소성 허혈성 뇌병증,183 자폐증,184 파킨슨병,185 알츠하이머병185 운동 실조증 등 거의 모든 신경 질환을 대상으로 하는 임상시험에서 MSC가 널리 사용되고 있으며, 그 종류는 다양합니다. MSC가 주요 세포로 사용된 임상시험 중 거의 3분의 2가 뇌졸중, SCI 또는 다발성 경화증에 대한 임상시험이었습니다. 뇌졸중은 29건의 등록된 임상시험에서 MSC가 널리 사용되었으며, 이 중 16건의 임상시험에서 BM-MSC가 사용되었습니다. 등록된 임상시험이 26건으로 두 번째로 많은 적응증은 SCI이며, 이 중 16건은 주로 확장된 BM-MSC를 사용하고 있습니다. 다발성 경화증의 경우, 이 질환의 치료를 위해 수행된 총 23건의 임상시험 중 15건의 임상시험에서 BM-MSC를 사용했습니다. 따라서 신경계 질환 및 장애에서 BM-MSC가 AT-MSC 및 UC-MSC와 같은 다른 MSC 중에서 가장 일반적으로 사용되는 치료 세포로 부상했다는 점에 주목하는 것이 중요합니다.

신경 질환 치료에 BM-MSC를 사용한 결과는 다양한 임상 시험 유형에서 널리 보고되었습니다. Zheng 등의 검토에 따르면 뇌졸중 진단을 받은 환자에서 치료가 안전한 것으로 보이지만 결과를 확인하기 위해 잘 설계된 2상 다기관 연구가 필요하다고 지적했습니다.173 자가 BM-MSC를 사용한 최초의 시험 중 하나는 2005년에 뇌졸중 진단을 받은 환자 5명을 대상으로 Bang 등이 수행한 것입니다. 그 결과 안전성을 뒷받침하는 결과와 함께 MSC 치료 환자에서 바텔 지수(BI)가 개선된 것으로 나타났습니다.186 2년 추적 임상시험에서 16명의 뇌졸중 환자가 BM-MSC 주입을 받았으며 그 결과 치료가 안전하고 운동 장애 척도 점수 등 임상 결과가 개선된 것으로 나타났습니다.187 허혈성 뇌졸중 환자 12명을 대상으로 실시한 연구에 따르면 자가 혈청을 사용하여 체외에서 확장된 자가 BM-MSC가 환자의 수정 랭킨 척도(mRS)를 개선했으며 세포 주입 후 1주째 평균 병변 부피가 20% 감소한 것으로 나타났습니다.188 2011년에 만성 뇌졸중 환자에게 BM-MSC를 자가 투여한 후 푸글 마이어 및 수정된 BI 점수가 약간 증가한 것으로 관찰되었습니다.189 최근에는 맹검 결과 평가가 포함된 전향적, 공개 라벨, 무작위 대조 시험이 실시되었으며, BM-MSC 투여군과 대조군에서 각각 39명의 환자와 15명의 환자가 참여했습니다. 이 연구 결과에 따르면 자가 혈청을 이용한 자가 BM-MSC 투여는 안전했지만, 다리 운동 개선이 관찰되었지만 3개월 후 수정 랭킨 척도(mRS) 점수는 개선되지 않았습니다.180 연구자들은 5년 추적 임상시험에서 BM-MSC 투여의 효능이 시간이 지나도 유지되는지 여부를 조사했습니다. 환자(85명)를 무작위로 MSC 그룹 또는 대조군에 배정하여 5년 동안 안전성과 효능에 대한 추적 관찰을 실시했으며, 연구 종료 시점에 52명의 환자를 검사했습니다. MSC 그룹은 mRS 점수 감소 측면에서 유의미한 개선을 보였으며, mRS 점수가 0-3으로 증가한 환자 수는 통계적으로 유의미했습니다.187 자가 BM-MSC는 주입 2년 후 바젤 지수, mRS 또는 국립보건원 뇌졸중 척도(NIHSS) 점수를 개선하지 않았지만 BM-MSC 치료를 받은 환자는 운동 기능 점수가 개선된 것으로 나타났습니다.190 또한 Lee 등이 실시한 전향적, 공개 라벨, 무작위 대조 시험에 따르면 자가 “허혈성” 혈청으로 프라이밍된 자가 BM-MSC가 MSC 치료 그룹의 운동 기능을 크게 개선한 것으로 나타났습니다. 또한 신경 영상 분석 결과 MSC 그룹에서 반구 간 연결성과 동측 연결성이 유의하게 증가한 것으로 나타났습니다.191 최근에는 동종 BM-MSC의 단일 정맥 감염이 만성 뇌졸중 환자에서 안전하고 실행 가능한 것으로 입증되어 BI 점수와 NIHSS 점수가 크게 개선되었습니다.192

SCI 치료를 위해 MSC를 사용한 두 건의 체계적 문헌고찰에서 BM-MSC(n= 16)와 UC-MSC(n= 5)는 안전하고 내약성이 좋은 것으로 보고되었습니다.193,194 그 결과 줄기세포 투여군은 미국 척추손상협회(ASIA) 장애 척도(AIS) 등급, AIS 등급, ASIA 감각 점수 및 방광 기능의 종합적인 측면에서 대조군에 비해 유의미한 개선을 보였습니다(표 1)에서 확인할 수 있습니다. 그러나 이러한 결과를 추가로 확인하려면 무작위 및 다기관 설계의 대규모 실험 그룹이 필요합니다. 다발성 경화증의 경우, 여러 초기 단계(1상/2상) 등록 임상 연구에서 BM-MSC를 사용했습니다. 한 연구에서는 경직성 뇌성마비 환자 105명을 대상으로 BM-MSC와 BM 단핵세포(BMMNC) 이식의 잠재적 효능을 비교했습니다.195 그 결과 3, 6, 12개월 평가 시 BM-MSC 이식 그룹의 총 운동 기능 측정(GMFM) 및 미세 운동 기능 측정(FMFM) 점수가 BMNNC 이식 그룹보다 더 높은 것으로 나타났습니다. 자폐 스펙트럼 장애의 경우, BMMNC 이식 후 254명의 어린이를 대상으로 조사한 결과 환자의 90% 이상이 ISAA(인도 자폐증 평가 척도)와 CARS(아동기 자폐증 평가 척도) 점수가 개선된 것으로 나타났습니다. 어린 환자와 자폐 스펙트럼 장애를 조기에 발견한 환자일수록 일반적으로 더 나은 개선 효과를 보였습니다.196

Table 1 The reported clinical trials using MSCs from AT, BM, and UC in the treatment of brain-related injuries and neurological disorders

Full size table

One of the biggest limitations when using BM-MSCs is the bone marrow aspiration process, as it is an invasive procedure that can introduce a risk of complications, especially in pediatric and elderly patients.197 Therefore, UC-MSCs have been suggested as an alternative to BM-MSCs and are being studied in clinical trials for the treatment of neurological diseases in approximately 1550 patients throughout the world; however, only three studies have been completed, with data published recently.198 A recent study showed that UC-MSC administration improved both gross motor function and cognitive skills, assessed using the Activities of Daily Living (ADL), Comprehensive Function Assessment (CFA), and GMFM, in patients diagnosed with cerebral palsy. The improvements peaked 6 months post administration and lasted for 12 months after the first transplantation.199 In a single-targeted phase I/II clinical trial using UC-MSCs for the treatment of autism, Riordan et al. reported decreases in Autism Treatment Eval‎uation Checklist (ATEC) and CARS scores for eight patients, but the paper has been retracted due to a violation of the journal’s guidelines.200 In an open-label, phase I study, UC-MSCs were used as the main cells to treat 12 patients with autism spectrum disorder via IV infusions. It is important to note that five participants developed new class I anti-human leukocyte antigen in response to the specific lot of manufactured UC-MSCs, although these responses did not exhibit any immunological response or clinical manifestations. Only 50% of participants showed improvements in at least two autism-specific measurements.201 Although not as widely used as BM-MSCs, these trials have demonstrated the efficacy of using UC-MSCs in the treatment of SCIs. In a pilot clinical study, Yang et al. showed that the use of UC-MSCs has the potential to improve disease status through an increase in total ASIA and SCI Functional Rating Scale of the International Association of Neurorestoratology (IANR-SCIFRS) scores, as well as an improvement in pinprick, light touch, motor and sphincter scores.202 A study of 22 patients with SCIs showed a potential therapeutic effect in 13 patients post UC-MSC infusion.203 AT-MSCs were also used to treat SCI, with a single case report indicating an improvement in neurological and motor functions in a domestic ferret patient.204 However, a result obtained from another phase I trial using AT-MSCs showed mild improvements in neurological function in a small number of patients.205 A phase II, randomized, double-blind, placebo-controlled, single-center, pilot clinical trial using AT-MSCs in the treatment of acute ischemic stroke published a data set that supports the safety of the therapy, although patients who received AT-MSCs showed a nonsignificant improvement after 24 months of follow-up.206 In all of the above studies, the safety of using either AT-MSCs or UC-MSCs was eval‎uated, and no significant reactions were reported after infusion.

Therefore, based on the number of recovered patients post-transplantation and the number of recruited patients in large-scale trials using BM-MSCs, it seems that BM-MSCs are the prominent cells in regard to treating neurodegenerative disease with potentially good outcomes (Table 1). It is important to note that we do not negate the fact that AT- and UC-MSCs also show positive outcomes in the treatment of neuronal diseases, with numerous ongoing large-scale, multicentre, randomized, and placebo-control trials,207,208 but we suggest alternative and thoughtful decisions regarding which sources of MSCs are best for the treatment of neuronal diseases and degenerative disorders.

Respiratory disease and lung fibrosis: clinical data support UC as a good source of MSCs

In the last decade, significant increases in respiratory disease incidence due to air pollution, smoking behavior, population aging, and recently, respiratory virus infections such as coronavirus disease 2019 (COVID-19)209 have been observed, leading to substantial burdens on public health and healthcare systems worldwide. Respiratory inflammatory diseases, including bronchopulmonary dysplasia (BPD), chronic obstructive pulmonary disease (COPD), and acute respiratory distress syndrome (ARDS), have recently emerged as three preval‎ent pulmonary diseases in children and adults. These conditions are usually associated with inflammatory cell infiltration, a disruption of alveolar structural integrity, a reduction in alveolar fluid clearance ability, cytokine release and associated cytokine storms, airway remodeling, and the development of pulmonary fibrosis. Traditional treatments are focused on relieving symptoms and preventing disease progression using surfactants, artificial respiratory support, mechanical ventilation, and antibiotic/anti-inflammatory drugs, with limited effects on the damaged airway, alveolar fluid clearance, and other detrimental effects caused by the inflammatory response. MSCs are known for their immunomodulatory abilities, showing potential in injury reduction and aiding lung recovery after injury. According to ClinicalTrials.gov, from 2017 to date, there have been 159 studies testing the application of MSCs in the treatment of pulmonary diseases, including but not limited to BPD, COPD, and ARDS, suggesting a trend in the use of MSCs as an alternative approach for the treatment of respiratory diseases, especially MSCs from UC as an “off-the-shelf” and allogeneic source.

Extremely premature infants are born with arrested lung development at the canalicular-saccular phases prior to alveolarization and before pulmonary maturation occurs, which results in the development of BPD.210 These infants require intensive care during the first three months of life using postnatal interventions, including positive pressure mechanical ventilation, external oxygen support, and surfactant infusions, and the newborns have recurrent infections that further compromise normal lung development.211 To date, 13 clinical trials have been proposed to use UC-MSCs in the treatment of BPD, recruiting ~566 premature infants throughout the world, including Vietnam, Korea, the United States, Spain, Australia, and China. The majority of these trials use UC-derived stem cells for phases I and II, focusing on eval‎uating the safety and efficacy of stem cell-based therapy.212 Human UC tissue and its derivative components are considered the most attractive cell sources for MSCs in the treatment of BPD due to the ease of obtaining them, being readily available, with no ethical concerns, low antigenicity, a high cell proliferation rate, and superior regenerative potential. Chang et al. used MSCs derived from UC blood in a phase I dose-escalation clinical trial to treat 9 preterm infants via intratracheal administration to prevent the development of BPD.213 All 9 preterm infants survived, and only three developed BPD; these infants had significantly decreased BPD severity compared with the historically matched control group. A follow-up study of the same patients after 24 months indicated that only one infant had an E. cloacae infection after discharge at 4 months, with subsequent disseminated intravascular coagulation, which was later proven to be unrelated to the intervention. The remaining eight patients survived with normal pulmonary development and function, suggesting that the therapy was safe. MSCs from UC blood were also used for the treatment of 12 extremely low birthweight preterm patients using the same administration route, which further confirmed the safety of the therapy in the treatment of BPD, although ten of 12 infants still developed severe BPD at 36 weeks.214 Our group also reported the safety and potential efficacy of using UC-MSCs in the treatment of four preterm infants, and the results supported the safety of UC-MSCs and demonstrated that patients could be weaned from oxygen supply and develop normal lung structure and function.215 A phase II clinical trial of 66 infants born at 23–28 weeks with a birthweight of 500–1250 g who were recruited and randomized into an MSC-administration group and a control group was conducted. Although the results supported the safety of MSC administration in preterm infants, the efficacy of the treatment was not supported by statistical analysis, potentially due to the small sample size. Subgroup analysis showed that patients with severe BPD born at 23–24 weeks showed a significant improvement in BPD severity, but those born at 25–28 weeks did not.216 Hence, it is important to conduct controlled phase II clinical trials with larger cohort sizes to further substantiate the efficacy of UC blood-derived MSCs in the treatment of infants with BPD.

With more than 65 million patients worldwide, COPD was the third-leading cause of death in 2020, according to World Health Organization records. COPD is classified as a chronic inflammatory and destructive pulmonary disease characterized by a progressive reduction in lung function. Averyanov et al. performed a randomized, placebo-controlled phase I/IIa study in 20 patients with mild to moderate idiopathic pulmonary fibrosis (IPF). Treatment group patients received two IV doses of allogeneic MSCs (2 × 108 cells) every 3 months, and the second group received a placebo.217 Eval‎uation tests were performed at weeks 13, 26, 39, and 52. The 6-min walking test distance (6MWTD) results showed that patient fitness improved from week 13 onwards and was maintained until up to the 52nd week. Pulmonary function indicators improved markedly before and after treatment in the treated group but did not change significantly in the placebo group. The goal of MSC therapy in the treatment of COPD is to promote the regeneration of parenchymal cells and alveolar structure and the restoration of lung function. Based on the results of a phase I trial of a commercial BM-MSC product, ProchymalTM, which led to improvements in pulmonary function in treated patients, a multicentre, double-blind, placebo-controlled phase II trial was conducted in 62 patients diagnosed with COPD to determine the safety and potential efficacy of the product. Although the results supported the safety of BM-MSCs, their effectiveness in the treatment of COPD was not assured. No statistically significant differences in FEV1 or FEV1%, total lung capacity, or carbon monoxide diffusing capacity were detected after 2 years of follow-up between the two treatment groups. To date, there have been five clinical trials using BM-MSCs as the main stem cells for the treatment of COPD, but the overall clinical outcomes did not demonstrate the potential therapeutic effects of the treatment.218,219,220,221,222 In clinical trial NCT001110252, the results showed that there was an overall reduction in the process of COPD pathological development 3 years after the administration of BM-MSCs, although the trial had a phase I design, with no control group, and eval‎uated only a small cohort (four patients).219 To alleviate local inflammatory progression in COPD, Oliveira et al. studied the combination treatment of one-way endobronchial valve (EBV) and BM-MSC intubation.223 Ten GOLD (Global Initiative for Obstructive Lung Disease) stage C or D patients were equally divided into 2 groups: one group received a dose of 108 cells before valve insertion, and the other group received a normal saline infusion. The follow-up time was 90 days. Inflammation was significantly improved as assessed by the CRP (C-reactive protein) index at 30 and 90 days after infusion. In addition, improvements in St. George’s Respiratory Questionnaire (SGRQ) scores indicated improved patient quality of life. Furthermore, an investigation into the homing ability of MSCs in vivo was performed on 9 GOLD patients, from stage A to stage D. Each patient received two 2 × 106 BM-MSC/kg IV infusions 1-week apart.224 The marking of MSCs with indium-111 showed that MSCs were retained in the pulmonary vasculature longer in patients with mild COPD and that the levels of inflammatory mediators improved after 7 days of treatment. The results of the eval‎uation survey conducted after 1 year showed that the number of COPD exacerbations decreased to six times/year compared to 11 times/year before treatment. In addition, AT-MSCs present in the stromal vascular fraction were used to treat patients with COPD, and no adverse events were observed after 12 months of follow-up, but the clinical improvements post administration were not clear.225 The results from a phase I clinical trial using AT-MSCs in eight patients with COPD also reported no significant change in pulmonary function test parameters.226 A study eval‎uating the use of AT-MSCs as adjunctive therapy for COPD in 12 patients was performed.227 AT was obtained using standard liposuction, MSCs were isolated, and 150–300 million cells were intravenously infused. The patients showed improvements in quality of life, with improved SGRQ scores after 3 and 6 months of treatment. Recently, UC-MSCs have emerged as potential allogeneic stem cell candidates for the treatment of COPD.228 In a pilot clinical study, it was demonstrated that allogeneic administration of UC-MSCs in the treatment of COPD was safe and potentially effective.229 In one study, 20 patients, including 9 at stage C and 11 at stage D per the GOLD classification, with histories of smoking were recruited and received cell-based therapy. The patients who received UC-MSC treatment showed significant reductions in Modified Medical Research Council scores, COPD assessment test scores, and the number of pulmonary exacerbations 6 months post administration. The results of the second trial using UC-MSCs showed that the mean FEV1/FVC ratios were increased along with improvements in SGRQ scores and 6MWTDs at three months post administration.230 Although thorough assessments of the effectiveness of UC-MSCs are still in the early stages, the number of trials using UC-MSCs for the treatment of COPD is increasing steadily, with larger sample sizes and stronger designs (randomized or matched case–control studies), providing a data set strongly supporting the future applications of UC-MSCs.231

The ongoing pandemic of the 21st century, the COVID-19 pandemic, emerged as a major pulmonary health problem worldwide, with a relatively high mortality rate. Numerous studies, reviews, and systematic analyses have been conducted to discuss and expand our knowledge of the virus and propose different mechanisms by which the virus could alter the immune system.232 One of the most critical mechanisms is the generation of cytokine storms, which result from the initiation of hyperreactions of the adaptive immune response to viral infection.233 These cytokine storms are formed by the establishment of waves of hypercytokinaemia generated from overreactive immune cells, which enhance their expression‎ of TNF-α, IL-6, and IL-10, preventing T-lymphocyte recruitment and proliferation and culminating in T-lymphocyte apoptosis and T-cell exhaustion. In COVID-19, once a cytokine storm is formed, it spreads from an initial focal area through the body via circulation, which has been discussed in a comprehensive review by Jamilloux et al.234 At the time of writing this review, there were 74 clinical trials using MSCs from UC (29 trials; including WJ-derived MSCs (WJ-MSCs) and placenta-derived MSCs (PL-MSCs)), AT (15 trials), and BM (11 trials) (comprehensive review171,235). Hence, UC-MSCs have emerged as the most common MSCs for the treatment of COVID-19, with a total of 1047 patients participating in these trials. Among these trials, 15 completed trials using UC-MSCs (including WJ- and PL-MSCs) have been reported, with clinical data from approximately 600 recruited patients.232 Eight of these 15 studies used allogenic UC-MSC transplantation to treat critically ill patients.236 A list of case reports using UC-MSCs showed that the treatments were safe and well-tolerated in 14 patients with COVID-19, with the primary outcomes including increased percentages and numbers of T cells,237,238 improved respiratory and renal functions,239 reductions in inflammatory biomarker levels,240 and positive outcomes in the PaO2/FiO2 ratio.240 In a pilot study conducted in ten patients with severe COVID-19, a single dose of UC-MSCs was safe and improved clinical outcomes, although the study did not investigate whether multiple doses of UC-MSCs could further improve the outcomes.241 Two trials without a control group were conducted in 47 patients, and the results indicated that UC-MSCs were safe and feasible for the treatment of patients with COVID-19.235,242 A single-center, open-label, individually randomized, standard treatment-controlled trial was performed in 41 patients (12 patients assigned to the UC-MSC group), and the results showed that significant improvements in C-reactive protein levels, IL-6 levels, oxygen indices, and lymphocyte numbers were found in the MSC groups. Chest computed tomography (CT) illustrated significant reductions in lung inflammatory responses as reflected by CT findings, the number of lobes involved, and pulmonary consolidation.238 In a phase I trial conducted in 18 hospitalized patients with COVID-19, UC-MSCs were administered via an IV route in nine patients (five patients with moderate COVID-19 and 4 patients with severe COVID-19) at days 0, 3, and 6, with no treatment-related adverse events or severe adverse events.243 Only one patient in the UC-MSC group required mechanical ventilation, compared to four patients in the control group. However, the clinical outcomes, such as COVID-19 symptoms, laboratory test results, CT findings of lung damage, and pulmonary function test parameters, were improved in both groups. Interestingly, a 1-year follow-up of the same sample revealed that the patients who received UC-MSC administration improved in terms of whole-lung lesion volume compared to the control group.244 Moreover, chest CT at 12 months showed significant regeneration of lung tissue in the MSC-administered groups, whereas lung fibrosis was found in all patients in the control group. This finding is of interest because it indicates that a long time is needed to detect the regenerative functions of MSC-based therapy, as the biological process to enhance lung tissue regeneration occurs relatively slowly and requires multiple steps. The effects of UC-MSCs in the attenuation and prevention of the development of cytokine storms were illustrated in an interventional, prospective, three-parallel arm study with two control arms conducted in 30 patients in moderate and critical clinical conditions.245 The results indicated a significant decrease in proinflammatory cytokines (IFNγ, IL-6, IL-17A, IL-2, and IL-12) and an increase in anti-inflammatory cytokines (IL-10, IL-13, and IL-1ra), suggesting that UC-MSCs might participate in the prevention of cytokine storm development. Lanzoni et al. performed a double-blind, randomized, controlled trial and found that UC-MSC infusions significantly decreased cytokine levels at day 6 and improved survival in patients with COVID-19 with ARDS. In this trial, 24 patients were randomized and assigned 1:1 to receive either MSCs or placebo.246 MSC treatment was associated with a significant improvement in the survival rate without serious adverse events. To date, other trials conducted using UC-MSCs as the main MSCs provide a solid data set on their safety and efficacy in preventing the development of cytokine storms, reducing the inflammatory response, improving pulmonary function, reducing intensive care unit (ICU) stay duration, enhancing lung tissue regeneration, and reducing lung fibrosis progression.240,247,248,249 In two large cohort studies (phase I with 210 patients and phase II with 100 patients), the volume of lung lesions and solid component injuries of patients’ lungs were reduced significantly after the administration of UC-MSCs,250 and clinical symptoms and inflammatory levels were improved.251 Of the 26 reported clinical trials for the treatment of COVID-19 with MSCs, 1 study used AT-MSCs as the main MSCs.236 Thirteen COVID-19 adult patients under invasive mechanical ventilation who had received previous antiviral and/or anti-inflammatory treatments (including steroids, lopinavir/ritonavir, hydroxychloroquine, and/or tocilizumab, among others) were treated with allogeneic AT-MSCs. With a mean follow-up time of 16 days after infusion, 9/13 patients’ clinical symptoms improved, and 7/13 patients were intubated. A decrease in inflammatory cytokines and an increase in immunoregulatory cells were also observed in patients, especially in the group of patients with overall clinical improvement. Although there is a lack of clinical efficacy data supporting the use of AT-MSCs in the treatment of patients with COVID-19, AT-MSCs are still potential candidates for inhibiting COVID-19 due to their high secretory activity, strong immune-modulatory effects, and homing ability.252,253,254

For ARDS, in a phase IIa trial, 60 patients with moderate to severe disease were randomized into 2 groups. A group of 40 patients received a single infusion of BM-MSCs at a dose of 1 × 106 cells/kg body weight, and another 20 patients received a placebo.255 After 6 and 24 h of infusion, the decrease in plasma inflammatory cytokine levels in the MSC group was significantly greater than that in the placebo group. For severe pulmonary hypertension (PH) associated with BPD (BPD-PH), in a small trial, two preterm infants born at 26–27 weeks of age were intravenously administered heterologous BM-MSCs at a dose of 5 × 106 cells per kg of body weight; the treatment reduced oxygen requirements and supported respiration in the infants.256 The administration of allogeneic AT-MSCs in the treatment of ARDS appeared to be safe and well-tolerated in 12 adult patients, but clinical outcomes were not observed.257 The results of two patients who received BM-MSCs showed that both patients had improved respiratory function and hemodynamic function and a reduction in multiorgan failure.258 Although the safety of BM-MSCs was confirmed in a multicentre, open-label, dose-escalation, phase I clinical trial (The Stem cells for ARDS treatment—START trial),259 no significant improvements were found in a phase II trial, including in respiratory function and ARDS conditions.260 The safety profile of UC-MSCs is also supported by the findings of a previous phase I clinical trial conducted in 9 patients, which showed that a single IV administration of UC-MSCs was safe and led to positive outcomes in terms of respiratory function and a reduction in the inflammatory response.261 The findings of this study were also supported by those of the REALIST (Repair of Acute Respiratory Distress with Stromal Cell Administration) trial, which further confirmed the maximum tolerated dose of allogeneic UC-MSCs in patients with moderate to severe ARDS.262

Although AT- and BM-MSCs have demonstrated therapeutic potential with similar mechanisms of action, UC-MSCs have emerged as potential candidates in the treatment of pulmonary diseases due to their ease of production as “off-the-shelf” products, rapid proliferation, noninvasive isolation methods, and supreme immunological regulation as well as anti-inflammatory effects.263 However, it is important to note that there is a need to conduct phase III clinical trials with larger cohorts and trials with at least two sources of MSCs in the treatment of pulmonary conditions to further confirm‎ this speculation.264 Table 2 summarizes several clinical trials with published results discussed in this review.

Table 2 The reported clinical trials using MSCs from AT, BM, and UC in the treatment of respiratory diseases

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Endocrine disorders, infertility/reproductive function recovery, and skin burns: should we consider AT-MSCs as the main MSCs based on their origin?

Endocrine disorders

The human body maintains function and homeostatic regulation via a complex network of endocrine glands that synthesize and release a wide range of hormones. The endocrine system regulates body functions, including heartbeat, bone regeneration, sexual function, and metabolic activity. Endocrine system dysregulation plays a vital role in the development of diabetes, thyroid disease, growth disorder, sexual dysfunction, reproductive malfunction, and other metabolic disorders. The central dogma of regenerative medicine is the use of adult stem cells as a footprint for tissue regeneration and organ renewal. The functions of these stem cells are tightly regulated by microenvironmental stimuli from the nervous system (rapid response) and endocrine signals via hormones, growth factors, and cytokines. This harmonized and orchestrated system creates a symphony of signals that directly regulate tissue homeostasis and repair after injury. The disruption of these complex networks results in an imbalance of tissue homeostasis and regeneration that can lead to the development of endocrine disorders in humans, such as diabetes, sexual hormone deficiency, premature ovarian failure (POF), and Asherman syndrome.

In recent years, obesity and diabetes (type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM)) have been the two biggest challenges in endocrinology research, and the application of MSCs has emerged as a novel approach for therapeutic consideration. T1DM is characterized by the autoimmune destruction of pancreatic β-cells, whereas T2DM is defined as a combination of insulin resistance and pancreatic insulin-producing cell dysfunction. Regenerative medicine seeks to provide an exogenous cell source for replacing damaged or lost β-cells to achieve the goal of stabilizing patients’ blood glucose levels. To date, there are 28 clinical trials using MSCs in the treatment of T1DM (http://www.clinicaltrials.gov, searched in October 2021), among which three trials were completed using autologous BM-MSCs (NCT01068951), allogeneic BM-MSCs (NCT00690066), and allogeneic AT-MSCs (NCT03920397). Interestingly, UC-MSCs were the most favored MSCs for the remaining trials. All published studies confirmed the safety of MSC therapy in the treatment of T1DM with no adverse events. The first study using autologous BM-MSCs showed that patients who were randomized into the MSC-administration group showed an increase in C-peptide levels in response to a mixed-meal tolerance test (MMTT) in comparison to the control group.265 Unfortunately, there was no significant improvement in C-peptide levels, HbA1C or insulin requirements. The use of autologous AT-MSCs in combination with vitamin D was safe and improved HbA1C levels 6 months post administration.266 WJ-MSCs were used as the main MSCs for the treatment of new-onset T1DM, which showed a significant improvement in both HbA1C and C-peptide levels when compared to those of the control group at three and six months post administration.267,268 The combination of allogeneic WJ-MSCs with autologous BM-derived mononuclear cells improved insulin secretion and reduced insulin requirements in patients with T1DM.269 In terms of T2DM, 23 studies were registered on clinicaltrials.gov (searched in October 2021), with six completed studies (three studies used BM-MSCs and three studies used allogeneic UC-MSCs). Although the number of studies using MSCs for the treatment of T2DM is small, their findings support the safety of MSCs, with no severe adverse events observed during the course of these studies.270 It was confirmed that MSC therapy potentially reduced fasting blood glucose and HbA1C levels and increased C-peptide levels. However, these effects were short-term, and multiple doses were required to maintain the MSC effects. Interestingly, the autologous MSC approach in the treatment of patients with diabetes in general is hampered, as both BM-MSCs and AT-MSCs isolated from patients with diabetes showed reduced stemness and functional characteristics.271,272 In addition, the durations of diabetes and obesity are strongly associated with autologous BM-MSC metabolic function, especially mitochondrial respiration, and the accumulation of mitochondrial DNA, which directly interfere with the functions of BM-MSCs and reduce the effectiveness of the therapy.271 Therefore, the allogeneic approach using MSCs from healthy donors provides an alternative approach for stem cell therapy in the treatment of patients with diabetes.

Infertility and reproductive function recovery

Modern society is increasingly facing the problem of infertility, which is defined as the inability to become pregnant after more than 1 year of unprotected intercourse.273 This problem has emerged as an important worldwide health issue and social burden. Assisted reproductive techniques and in vitro fertilization technology have recently become the most effective methods for the treatment of infertility in humans, but the use of these approaches is limited, as they cannot be applied in patients with no sperm or those who are unable to support implantation during pregnancy, they are associated with complications, they are time-consuming and expensive, and they are associated with ethical issues in certain territories.274 Numerous conditions are related to infertility, including POF, nonobstructive azoospermia, endometrial dysfunction, and Asherman syndrome. Recent progress has been illustrated in preclinical studies for the potential applications of stem cell-based therapy for reproductive function recovery, especially recent studies in the field of MSCs, which provide new hope for patients with infertility and reproductive disorders.275

POF is characterized by a loss of ovarian activity during middle age (before 40 years old) and affects 1–2% of women of reproductive age.276 Patients diagnosed with POF exhibit oligo-/amenorrhea for at least 4 months, with increased levels of follicle-stimulating hormone (FSH) (>25 IU/L) on two occasions more than 1 month apart.277 Diverse factors, such as genetic backgrounds, autoimmune disorders, environmental conditions, and iatrogenic and idiopathic situations, have been reported to be the cause of POF.278 POF can be treated with limited effectiveness via psychosocial support, hormone replacement intervention, and fertility management.279 MSCs from AT, BM, and UC have been used in the treatment of POF, with improvements in ovarian function in preclinical studies using chemotherapy-induced POF animal models. The early published POF study using BM-MSCs as the main cell source is a single case report in which a perimenopausal woman showed an improvement in follicular regeneration, and increased AMH levels resulted in a successful pregnancy followed by delivery of a healthy infant.280 A report using autologous BM-MSCs in two women with POF illustrated an increase in baseline estrogen levels and the volume of the treated ovaries along with amelioration of menopausal symptoms.281 The clinical procedures used in this early trial were invasive, as patients underwent two operations: (1) BM aspiration and (2) laparoscopy. A similar approach was used in two trials conducted in 10 women with POF (age range from 26–33 years old) and 30 patients (age from 18 to 40 years old).282 A later study investigated two different routes of cell delivery, including laparoscopy and the ovarian artery, but the results have not been reported at this time.282 Based on the positive outcomes of the mouse model, an autologous stem cell ovarian transplantation (ASCOT) trial was deployed using BM-derived stem cells with encouraging observations of improved ovarian function, as determined by elevated levels of AMH and AFC in 81.3% of participants, six pregnancies, and the successful delivery of three healthy babies.283 A randomized trial (NCT03535480) was conducted in 20 patients with POF aged less than 39 years to further elaborate on the results of the ASCOT trial.284 To date, there are no completed trials using AT-MSCs or UC-MSCs in the treatment of patients with POF, limiting the eval‎uation of these MSCs in the treatment of POF. The speculated reason is that POF is a rare disease, affecting 1% of women younger than 40 years, and with improvements in assisted productive technology, patients have several alternative options to enhance the recovery of reproductive function.285

Wound healing and skin burns

Burns are the fourth most common injury worldwide, affecting ~11 million people, and are a major cause of death (180,000 patients annually). The severity of burns is defined based on the percentage of surface area burned, burn depth, burn location and patient age, and burns are usually classified into first-, second-, third-, and fourth-degree burns on the basis of their severity.286 Postburn recovery depends on the severity of the burn and the effectiveness of treatment. Rapid healing may occur over weeks, while alternatively, healing can take months, with the ultimate result being scar formation and disability in patients with severe burns. Different from mechanical injury, burn injury is an invasive progression of damage to tissue at the burn site, including both mechanical damage to the skin surface and biological damage caused by natural apoptosis that prolongs excessive inflammation, oxidative stress, and impaired tissue perfusion.287 To date, completely reversing the devastating damage of severe burns remains unachievable in medicine, and stem cell therapy provides an alternative option for patients with burn injury. The first case report of the use of BM-MSCs to treat a 45-year-old patient with burns on 40% of their body demonstrated the safety of the therapy and showed partial improvements in vascularization at the wound site and reduced coarse cicatrices.288,289 Later, patients with second- and third-degree burns as well as deep burns were treated using either autologous BM-MSCs or allogeneic BM-MSCs by spraying the MSCs onto the burn sites or adding MSCs over a dermal matrix sheet to cover the wound. The results in these case reports revealed the potential efficacy of MSC-based therapy, which not only enhanced the speed of wound recovery but also reduced pain and improved blood supply without introducing infection.288,290,291 In 2017, a study conducted in 60 patients with 10–25% of their total body surface areas burned treated with either autologous BM-MSCs or UC-MSCs showed that both MSC types improved the rate of healing and reduced the hospitalization period.292 The drawback of BM-MSCs in the treatment of burns is the invasive harvesting method, which causes pain and possible complications in patients. Hence, treatment with allogeneic MSCs obtained from healthy donors is the method of choice, and AT- and UC-MSCs are two suitable candidates for this option. To date, a limited number of clinical trials have been conducted using MSC therapy. These trials have several limitations in trial design, such as a lack of a negative control group and blinding, small sample sizes, and the use of standardized measurement tools for burn injury and wound healing. Currently, AT-MSCs are being used in seven ongoing phase I and II trials in the treatment of burns. Hence, it is important to note that among the most widely studied MSCs, AT-MSCs have advantages over BM-MSCs when obtained from an allogeneic source, while their abilities in burn treatment remain to be determined. The main MSCs that should be used in the regeneration of burn tissue remain undefined (Table 3), and we observed the trend that AT-MSCs are more suitable candidates due to their biological nature, which contributes to the generation of keratinocytes and secretion profiles that strongly enhance the skin regeneration process.293,294,295,296

Table 3 The reported clinical trials using MSCs from AT, BM, and UC in the treatment of the endocrinological disorder, reproductive disease, and skin healing

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MSC applications in cardiovascular disease: a promising but still controversial field

In the last two decades, great advancements have been achieved in the development of novel regenerative medicine and cardiovascular research, especially stem cell technology.297 The discovery of human embryonic stem cells and human induced pluripotent stem cells (hiPSCs) opened a new door for basic research and therapeutic investigation of the use of these cells to treat different diseases.298 However, the clinical path of hiPSCs and hiPSC-derived cardiomyocytes in the treatment of cardiovascular diseases is limited due to the potential for teratoma formation with hiPSCs and the immaturity of hiPSC-derived cardiomyocytes, which might pose a risk of cancer formation,299 arrhythmia, and cardiac arrest to patients.300 A recently emerged stem cell type is adult stem cells/progenitor cells, including MSCs, which can stimulate myocardial repair post administration due to their paracrine effects. Promising results of MSC-based therapy obtained from preclinical studies of cardiac diseases enhance the knowledge and strengthen the clinical research to investigate the safety and efficacy in a clinical trial setting. There are papers that discuss the importance of MSC therapy in the treatment of cardiovascular diseases, with the following references being highly recommended.301,302,303,304,305,306 To date, 36 trials have eval‎uated the therapeutic potential of MSCs in different pathological conditions, with the most preval‎ent types being BM-MSCs (25 trials), followed by UC-MSCs (7 trials) and AT-MSCs (4 trials).303 However, the reported results are contradictory and create controversy about the efficacy of the treatments.

One of the first trials using MSCs in the treatment of chronic heart failure was the Cardiopoietic Stem Cell Therapy in Heart Failure (C-CURE) trial, a multicentre, randomized clinical trial that recruited 47 patients. The trial findings supported the safety of BM-MSC therapy and provided a data set that demonstrated improvements in cardiovascular scores along with New York Heart Association functional class, quality of life, and general physical health.307 Despite these encouraging results in the phase I trial, the treatment failed to achieve the primary outcomes in the phase II/III trial (CHART-1 trial), including no significant improvements in cardiac structure or function or patient quality of life.308 A positive outcome was also found in a phase I/II, randomized pilot study called the POSEIDON trial, which was the first trial to demonstrate the superior effectiveness of the administration of allogeneic BM-MSCs compared to allogeneic MSCs from other sources.309,310 Published results from the MSC-HF study, with 4 years of follow-up results,311,312 and the TRIDENT study313 illustrated the positive outcomes of BM-MSCs in the treatment of heart failure. However, a contradictory result from the recently published CONCERT-HF trial demonstrated that the administration of autologous BM-MSCs to patients diagnosed with chronic ischemic heart failure did not improve left ventricular function or reduce scar size at 12 months post administration, but the patient’s quality of life was improved.314 This observation is similar to that of the TAC-HFT trial315 but completely different from the reported results of the MSC-HF trial. A comprehensive investigation is still needed to determine the reasons behind these contradictory results. The largest clinical trial to date using BM-MSCs is the DREAM-HF study, which was a randomized, double-blind, placebo-controlled, phase III trial that was conducted at 55 sites across North America and recruited a total of 565 patients with ischemic and nonischaemic heart failure.172 Although recent reports from the sponsor confirmed that the trial missed its primary endpoint (a reduction in recurrent heart failure-related hospitalization), other prespecified endpoints were met, such as a reduction in overall major adverse cardiac events (including death, myocardial infarction, and stroke).306 Thus, a complete report from the DREAM-HF trial will provide pivotal data supporting the therapeutic potential of BM-MSCs in the treatment of heart failure and open a new path for the FDA to approve cell-based therapy for cardiovascular diseases.

The early trial using AT-derived cells was the PRECISE trial, which was a phase I, randomized, placebo-controlled, double-blind study that examined the safety and efficacy of adipose-derived regenerative cells (ADRCs) in the treatment of chronic ischemic cardiomyopathy.316 ADRCs are a homogenous population of cells obtained from the vascular stromal fraction of AT, which contains a small proportion of AT-MSCs.317 Although the study supported the safety of ADRC administration and illustrated a preserved functional capacity (peak VO2) in the treated group and improvements in heart wall motion, neither poor left ventricle (LV) volume nor poor left ventricular ejection fraction (LVEF) was ameliorated. The follow-up trial of the PRECISE trial, called the ATHENA trial, was conducted in 31 patients, although the study was terminated prematurely because two cerebrovascular events occurred, which were not related to the cell product itself.318 The results of the study illustrated increases in functional capacity, hospitalization rate, and MLHFQ scores, but the LV volume and LVEF were not significantly different between the two groups. Kastrup and colleagues conducted the first in vitro expanded AT-MSC trial in ten patients with ischemic heart disease and ischemic heart failure in 2017. The results confirmed that ready-to-use AT-MSCs were well-tolerated and potentially effective in the treatment of ischemic heart disease and heart failure.319 Comparable results of AT-MSCs were also reported from the MyStromalCell Trial, which was a randomized placebo-controlled study. In this trial, 61 patients were randomized at a 2:1 ratio into two groups, with the results showing no significant difference in the primary endpoint, which was a change in the maximal bicycle exercise tolerance test (ETT) score from baseline to 6 months post administration.320 A 3-year follow-up report from the MyStromalCell Trial confirmed that patients who received AT-MSC administration maintained their preserved exercise capacity and their cardiac symptoms improved, whereas the control group experienced a significant reduction in exercise performance and a worsened cardiovascular condition.321

UC-MSCs are potential allogeneic cells for the treatment of cardiovascular disease, as they are “ready to use” and easy to isolate, they rapidly proliferate, and they secrete hepatocyte growth factors,322 which are involved in cardioprotection and cardiovascular regeneration.323 The pilot study using UC-MSCs in 30 patients with heart failure, called the RIMECARD trial, was the first reported trial for which the results supported the effectiveness of UC-MSCs, as seen in the improved ejection fraction, left ventricular function, functional status, and quality of life in patients administered UC-MSCs.324 Encouraging results reported from a phase I/II HUC-HEART trial325 showed improvements in LVEF and reductions in the size of the injured area of the myocardium. However, the opposite observations were also reported from a recently published phase I randomized trial using a combination of UC-MSCs and a collagen scaffold in patients with ischemic heart conditions, in which the size of fibrotic scar tissue was not significantly reduced.326

Although MSCs from AT, BM, and UC have proven to be safe and feasible in the treatment of cardiovascular diseases, the correlation between the MSC types and their therapeutic potentials is still uncertain because different results have been reported from different clinical trials (Table 4). The mechanisms by which MSCs participate in recovery and enhance myocardial regeneration have been discussed comprehensively in a recently published review;305,327 therefore, they will not be discussed in this review. In fact, the challenges of MSC-based therapy in cardiovascular diseases have been clearly described previously,328 including (1) the lack of an in vitro eval‎uation of the transdifferentiation potential of MSCs to functional cardiac and endothelial cells,329 (2) the uncontrollable differentiation of MSCs to undesirable cell types post administration,330 and (3) the undistinguishable nature of MSCs derived from different sources with various levels of differentiation potential.331 Therefore, the applications of MSC-based therapy in cardiovascular disease are still in their immature stage, with potential benefits to patients. Thus, there is a need to conduct large-scale, well-designed randomized clinical trials not only to confirm‎ the therapeutic potential of MSCs from various sources but also to enhance our knowledge of cardiovascular regeneration post administration.

Table 4 The reported clinical trials using MSCs from AT, BM, and UC in the treatment of cardiovascular diseases

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Proposed mechanism of BM-MSCs in the treatment of acquired brain and spinal injury

Bones are complex structures constituting a part of the vertebrate skeleton, and they play a vital role in the production of blood cells from HSCs. Similar to the functions of most vertebrate organs, bone function is tightly regulated by its constituents and by long-range signaling from AT and the adrenal glands, parathyroid glands, and nervous system.332 The central nervous system (CNS) orchestrates the voluntary and involuntary input transmitted by a network of peripheral nerves, which act as the bridge between the nervous system and target organs. The CNS controls involuntary responses via the autonomic nervous system (ANS), consisting of the sympathetic nervous system and the parasympathetic nervous system, and voluntary responses via the somatic nervous system. The ANS penetrates deep into the BM cavity, reaching the regions of hematopoietic activity to deliver neurotransmitters that tightly regulate BM stem cell niches.333 The BM microenvironment consists of various cell types that participate in the maintenance of HSC niches, which are composed of specialized cells, including BM-MSCs (Fig. 3a). The release of a specific neurotransmitter, circadian norepinephrine, from the sympathetic nervous system at nerve terminals leads to a reduction in the circadian expression‎ of C–X-C chemokine ligand 12 (CXCL12, which is also known as stromal cell-derived factor-1 (SDF-1)) by Nestin+/NG22+ BM-MSCs, resulting in the secretion of HSCs into the peripheral bloodstream.334,335 In fact, BM-MSCs play a significant role in the regulation of HSC quiescence and are closely associated with arterioles and sympathetic nervous system nerve fibers. Nestin-expressing BM-MSCs have been shown to express high levels of SDF-1, stem cell factor (SCF), angiopoietin-1 (Ang-1), interleukin-7, vascular cell adhesion molecule 1 (VCAM-1), and osteopontin (OPN), which are directly involved in the regulation and maintenance of HSC quiescence.336 The depletion of BM-MSCs in BM leads to the mobilization of HSCs into the peripheral bloodstream and spleen. The findings from a previous study demonstrated that reduced SDF-1 expression‎ in norepinephrine-treated BM-MSCs resulted in the mobilization of CXCR4+ HSCs into circulation.337 The ability of BM-MSCs to produce SDF-1 is tightly related to their neuronal protective functions.338 SDF-1 is a member of a chemokine subfamily that orchestrates an enormous diversity of pathways and functions in the CNS, such as neuronal survival and proliferation. The chemokine has two receptors, CXCR4 and CXCR7, that are involved in the pathogenic development of neurodegenerative and neuroinflammatory diseases.339 In the damaged brain, SDF-1 functions as a stem cell homing signal, and in acquired immune deficiency syndrome (AIDS), SDF-1 has been reported to be involved in the protection of damaged neurons by preventing apoptosis. In a traumatic brain injury model, SDF-1 was found to function as an inhibitor of the caspase-3 pathway by upregulating the Bcl-2/Bax ratio, which in turn protects neurons from apoptosis.340 Moreover, the release of SDF-1 also facilitates cell recruitment, cell migration, and the homing of neuronal precursor cells in the adult CNS by activating the CXCR4 receptor.341,342 Existing data support that SDF-1 acts as the guiding signal for the regeneration of axon growth in damaged neurons and enhances spinal nerve regeneration.343,344 Hence, the ability of BM-MSCs to express SDF-1 in response to the neuronal environment provides a unique neuronal protective effect that could explain the potential therapeutic efficacy of BM-MSCs in the treatment of neurodegenerative diseases (Fig. 3b).

Fig. 3

The nature of the “stem niche” of bone marrow-derived mesenchymal stem cells (BM-MSCs) supports their therapeutic potential in neuron-related diseases. a Bone marrow is a complex stem cell niche regulated directly by the central nervous system to maintain bone marrow homeostasis and haematopoietic stem cell (HSC) functions. MSCs in bone marrow respond to the environmental changes through the release of norepinephrine (NE) from the sympathetic nerves that regulate the synthesis of SDF-1 and the migration of HSCs through the sinusoids. The secretion of stem cell factors (SCFs), VCAM-1 and angiotensin-1 from MSCs also plays a significant role in the maintenance of HSCs. b BM-MSCs have the ability to produce and release SDF-1, which directly contributes to neuroprotective functions at the damaged site through interaction with its receptors CXCR4/7, located on the neuronal membrane. c Neuronal protection and the functional remyelination induced by BM-MSCs are also modulated by the release of a wide range of growth factors, including VEGF, BDNF, and NGF, by the BM-MSCs. d BM-MSCs also have the ability to regulate neuronal immune responses by direct interaction or paracrine communication with microglia. Figure was created with BioRender.com

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The migration of exogenous MSCs after systemic administration to the brain is limited by the physical blood–brain barrier (BBB), which is a selective barrier formed by CNS endothelial cells to restrict the passage of molecules and cells. The mechanism of molecular movement across the BBB is well established, but how stem cells can bypass the BBB and home to the brain remains unclear. Recent studies have reported that MSCs are able to migrate through endothelial cell sheets by paracellular or transcellular transport followed by migration to the injured or inflammatory site of the brain.345,346 During certain injuries or ischemic events, such as brain injury, stroke, or cerebral palsy, the integrity and efficiency of BBB protection is compromised, which allows MSC migration across the BBB via paracellular transport through the transient formation of interendothelial gaps.347 CD24 expression‎ has been detected in human BM-MSCs, which are regulated by TGF-β3,348 allowing them to interact with activated endothelial cells via P-selectin and initiate the tethering and rolling steps of MSCs.349 Additionally, BM-MSCs express high levels of CXCR4 or CXCR7,350,351 which bind to integrin receptors, such as VLA-4, to activate the integrin-binding process and allow the cells to anchor to endothelial cells, followed by the migration of MSCs through the endothelial cell layer and basement membrane in a process called transmigration.352 This process is facilitated by the secretion of matrix metalloproteinases (MMPs), which degrade the endothelial basement membrane, allowing BM-MSCs to enter the brain environment.353,354 BM-MSCs can also regulate the integrity of the BBB via the secretion of tissue inhibitor of matrix metalloproteinase-3 (TIMP3), which has been shown to ameliorate the effects of a compromised BBB in traumatic brain injury.355 The secretion of TIMP3 from MSCs directly blocked vascular endothelial growth factor a (VEGF-a)-induced breakdown of endothelial cell adherent junctions, demonstrating the potential mechanism of BM-MSCs in the regulation of BBB integrity.

The therapeutic applications of BM-MSCs in neurodegenerative conditions have been significantly increased by the demonstration of BM-MSC involvement in axonal and functional remyelination processes. Remyelination is a spontaneous regenerative process occurring in the human CNS to protect oligodendrocytes, neurons, and myelin sheaths from neuronal degenerative diseases.356 Remyelination is considered a neuroprotective process that limits axonal degeneration by demyelination and neuronal damage. The first mechanism of action of BM-MSCs related to remyelination is the activation of the JAK/STAT3 pathway to regulate dorsal root ganglia development.357 It was reported that BM-MSCs secrete vascular endothelial growth factor-A (VEGF-A),358 brain-derived neurotrophic factor (BDNF), interleukin-6, and leukemia inhibitor factor (LIF), which directly function in neurogenesis and neurite growth.357 VEGF-A is a key regulator of hemangiogenesis during development and bone homeostasis. Postnatally, osteoblast- and MSC-derived VEGF plays a critical role in maintaining and regulating bone homeostasis by stimulating MSC differentiation into osteoblasts and suppressing their adipogenic differentiation.359,360,361 To balance osteoblast and adipogenic differentiation, VEGF forms a functional link with the nuclear envelope protein laminin A, which in turn directly regulates the osteoblast and adipocyte transcription factors Runx2 and PPARγ, respectively.361,362 In the brain, VEGF is a potent growth factor mediating angiogenesis, neural migration, and neuroprotection. VEGF-A, secreted from BM-MSCs under in vitro xeno- and serum-free culture conditions, is the most studied member of the VEGF family and is suggested to play a protective role against cognitive impairment, such as in the context of Alzheimer’s disease pathology or stroke.363,364,365 Recently, it was reported that the neurotrophic and neuroprotective function of VEGF is mediated through VEGFR2/Flk-1 receptors, which are expressed in the neuroproliferative zones and extend to astroglia and endothelial cells.366 In animal models of intracerebral hemorrhage and cerebral ischemia, the transfusion of Flk-1-positive BM-MSCs promotes behavioral recovery and anti-inflammatory and angiogenic effects.367,368 Moreover, supplementation with VEGF-A in neuronal disorders enhances intraneural angiogenesis, improves nerve regeneration, and promotes neurotrophic capacities, which in turn increase myelin thickness via the activation of the prosurvival transcription factor nuclear factor-kappa B (NF-kB). This activation, together with the downregulation of Mdm2 and increased expression‎ of the pro-apoptotic transcription factor p53, is considered to be the neuroprotective process associated with an increased VEGF-A level.369,370,371 An analysis of microRNA (miRNA) in extracellular vesicles (EVs) secreted from BM-MSCs revealed that BM-MSCs release substantial amounts of miRNA133b, which suppresses the expression‎ of connective tissue growth factor (CTGF) and protects hippocampal neurons from apoptosis and inflammatory injury372,373,374 (Fig. 3c).

In terms of immunoregulatory functions, the administration of human BM-MSCs into immunocompetent mice subjected to SCI or brain ischemia showed that BM-MSCs exhibited a short-term neuronal protective function against neurological damage (Fig. 3d). Further investigation demonstrated the ability of BM-MSCs to directly communicate with host microglia/macrophages and convert them from phenotypic polarization into alternative activated microglia/macrophages (AAMs), which are key players in axonal extension and the reconstruction of neuronal networks.375 Other studies have also illustrated that the administration of AAMs directly to the injured spinal cord induced axonal regrowth and functional improvement.376 The mechanism by which BM-MSCs activate the conversion of microglia/macrophages occurs through two representative macrophage-related chemokine axes, CCL2/CCR2 and CCL-5/CCR5, both of which exhibit acute or chronic elevation following brain injury or SCI.377 The CCL2/CCR2 axis contributed to the enhancement of inflammatory function, and BM-MSC-mediated induction of CCL2 did not alter the total granulocyte number (Fig. 3d). Although the chemokine-mediated mechanism of BM-MSCs in the activation of AAMs and enhanced axonal regeneration at the damage sites is evident, the direct mechanism by which the communication between BM-MSCs and the target cells results in these phenomena remains unclear, and further investigation is needed.

BM-MSCs also confer the ability to regulate the inflammatory regulation of the immune cells present in the brain by (1) promoting the polarization of macrophages toward the M2 type, (2) suppressing T-lymphocyte activities, (3) stimulating the proliferation and differentiation of regulatory T cells (Tregs), and (4) inhibiting the activation of natural killer (NK) cells. BM-MSCs secrete glial cell line-derived neurotrophic factor (GDNF), a specific growth factor that contributes directly to the transition of the microglial destructive M1 phenotype into the regenerative M2 phenotype during the neuroinflammatory process.378 A similar result was also found in AT-379 and UC-MSCs380 under neuroinflammation-associated conditions, suggesting that AT-, BM-, and UC-MSCs share the same mechanism in promoting macrophage polarization. In terms of T-lymphocyte suppression, compared to MSCs from AT and BM, UC-MSCs show the strongest potential to inhibit the proliferation of T-lymphocytes by promoting cell cycle arrest (G0/G1 phase) and apoptosis.381 In addition, UC-MSCs have been proven to be more effective in promoting the proliferation of Tregs382 and inhibiting NK activation.383 Although MSCs are well-known for their inflammatory regulatory ability, the mechanism is not exclusive to BM-MSCs, especially in neurological disorders.384

Proposed mechanism of UC-MSCs in the treatment of pulmonary diseases and lung fibrosis

In contrast to AT-MSCs and BM-MSCs, UC-MSCs have lower expression‎ of major histocompatibility complex I (MHC I) and no expression‎ of MHC II, which prevents the complications of immune rejection.385 Moreover, as UC is considered a waste product after birth, with the option of noninvasive collection, UC-MSCs are easier to obtain and culture than AD- and BM-MSCs.386 These advantages of UC-MSCs have contributed to their use in the treatment of pulmonary diseases, especially during the rampant COVID-19 pandemic, as “off-the-shelf” products. Numerous pulmonary diseases have been the subject of applications of UC-MSCs, including BPD, COPD, ARDS, and COVID-19-induced ARDS. In BPD, premature infants are born before the alveolarization process, resulting in arrested lung development and alveolar maturation. Upon administration via an IV route, the majority of exogenous UC-MSCs reach the immature lung and directly interact with immune cells to exert their immunomodulatory properties via cell-to-cell interaction mechanisms (Fig. 4a). UC-MSCs interact with T cells via the PD-L1 ligand, which binds to the PD-1 inhibitory molecule on T cells, resulting in the suppression of CD3+ T-cell proliferation and effector T-cell responses.387 In addition, UC-MSCs also express CD54 (ICAM-1), which plays a crucial role in the immunomodulatory functions of T cells.388 Direct contact between UC-MSCs and macrophages via CD54 expression‎ on UC-MSCs promotes the immune regulation of UC-MSCs via the regulation of phagocytosis by monocytes.389 Moreover, the contact of UC-MSCs with macrophages during proinflammatory responses increases the secretion of TSG-6 by UC-MSCs, which in turn promotes the inhibitory regulation of CD3+ T cells, macrophages, and monocytes by MSCs.390 Recently, upregulation of SDF-1 was described in neonatal lung injury, especially in layers of the respiratory epithelium.391 SDF-1 has been shown to participate in the migration and initiation of the homing process of MSCs via the CXCR4 receptors on their surface.392 It was reported that UC-MSCs express low levels of CXCR4, allowing them to induce SDF-1-associated migration processes via the Akt, ERK, and p38 signal transduction pathways.393 Hence, in BPD, the upregulation of SDF-1 together with the homing ability of UC-MSCs strongly supports the therapeutic effects of UC-MSCs in the treatment of BPD. Furthermore, UC-MSCs have the ability to communicate with immune cells via cell-to-cell contact to reduce proinflammatory responses and the production of proinflammatory cytokines (such as TGF-β, INF-γ, macrophage MIF, and TNF-α). The modulation of the human innate immune system by UC-MSCs is mediated by cell–cell interactions via CD54-LFA-1 that switch macrophage polarization processes, promoting the proliferation of M2 macrophages, which in turn reduce inflammatory responses in the immature lung.394 Moreover, UC-MSCs also have the ability to produce VEGF and hepatocyte growth factors (HGFs), promoting angiogenesis and enhancing lung maturation.395

Fig. 4

Adipose tissue-derived mesenchymal stem cells (AT-MSCs) and the nature of their tissue of origin support their use in therapeutic applications. a Adipose tissue is considered an endocrine organ, supporting and regulating various functions, including appetite regulation, immune regulation, sex hormone and glucocorticoid metabolism, energy production, the orchestration of reproduction, the control of vascularization, and blood flow, the regulation of coagulation, and angiogenesis and skin regeneration. b In terms of metabolic disorders, such as type 2 diabetes mellitus (T2DM), as adipose tissue is directly involved in the metabolism of glucose and lipids and the regulation of appetite, the detrimental effects of T2DM also alter the functions of AT-MSCs, which in turn, hampers their therapeutic effects. Hence, the use of autologous AT-MSCs is not recommended for the treatment of metabolic disorders, including T2DM, suggesting that allogeneic AT-MSCs from healthy donors could be a better alternative approach. c AT-MSCs are suitable for the treatment of reproductive disorders due to their unique ability to mobilize and home to the thecal layer of the injured ovary, enhance the regeneration and maturation of thecal cells, increase the structure and function of damaged ovaries via exosome-activated SMAD, decrease oxidative stress and autophagy, and increase the proliferation of granulosa cells via PI3K/AKT pathways. These functions are regulated specifically by growth hormones produced by AT-MSCs in response to the surrounding environment, including HGF, TGF-β, IGF-1, and EGF. d AT-MSCs are also good candidates for skin healing and regeneration as their growth factors strongly support neovascularization and angiogenesis by reducing PLL4, increase anti-apoptosis via the activation of PI3K/AKT pathways, regulate inflammation by downregulating NADPH oxidase isoform 1, and increase immunoregulation through the inhibition of NF-κB activation. The figure was created with BioRender.com

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COPD is characterized by an increase in hyperinflammatory reactions in the lung, compromising lung function and increasing the development of lung fibrosis. The mechanism by which UC-MSCs contribute to the response to COPD is inflammatory regulation (Fig. 4b). The administration of UC-MSCs prevented the infiltration of inflammatory cells in peribronchiolar, perivascular, and alveolar septa and switched macrophage polarization to M2.396 A significant reduction in proinflammatory cytokines, including IL-1β, TNF-α, and IL-8, was also observed following UC-MSC administration.224 MSCs, including UC-MSCs, have been reported to trigger the production of secretory leukocyte protease inhibitors in epithelial cells through the secretion of HGF and epidermal growth factor (EGF), which is believed to have beneficial effects on COPD.397,398 In addition to their inflammatory regulation ability, UC-MSCs exhibit antimicrobial effects through the inhibition of bacterial growth and the alleviation of antibiotic resistance during Pseudomonas aeruginosa infection.399 The combination of the regulation of the host immune response and the antimicrobial effects of UC-MSCs may be relevant for the prevention and treatment of COPD exacerbations, as inflammation and bacterial infections are important risk factors that significantly contribute to the morbidity and mortality of patients with COPD. In terms of regenerative functions, UC-MSCs were reported to be able to differentiate into type 2 alveolar epithelial cells in vitro and alleviate the development of pulmonary fibrosis via β-catenin-regulated cell apoptosis.400 Furthermore, UC-MSCs enhanced alveolar epithelial cell migration and proliferation by increasing matrix metalloproteinase-2 levels and reduced their endogenous inhibitors, tissue inhibitors of matrix metalloproteinases, providing a potential mechanism underlying their anti-pulmonary-fibrosis effects.401,402

In ARDS, especially that associated with COVID-19, the proinflammatory state is initiated by increases in plasma concentrations of proinflammatory cytokines, such as IL-1 beta, IL-7, IL-8, IL-9, IL-10, bFGF, granulocyte colony-stimulating factor (G-CSF), GM-CSF, IFN-γ, and TNF-α. The significant increases in the concentrations of these cytokines in patient plasma suggest the development of a cytokine storm, which is a leading cause of COVID-induced mortality. In addition to the immunomodulatory functions regulated via cell-to-cell interactions between UC-MSCs and immune cells, such as macrophages, monocytes, and T cells, UC-MSCs exert their functions via paracrine effects through the secretion of growth factors, cytokines, and exosomes (Fig. 4c). The most relevant immunomodulatory function of UC-MSCs is considered to be their inhibition of effector T cells via the induction of T-cell apoptosis and cell cycle arrest by the production of indoleamine 2,3- dioxygenase (IDO), prostaglandin E2 (PGE-2), and TGF-β. Elevated levels of PGE-2 in patients with COVID-19 are reported to be a crucial factor in the initiation of inflammatory regulation by UC-MSCs post administration and prevent the development of cytokine storms by direct inhibition of T- and B lymphocytes.403 UC-MSCs exert these inhibitory activities through a PGE-2-dependent mechanism.404 It was reported that UC-MSCs confer the ability to secrete tolerogenic mediators, including TGF-β1, PGE-2, nitric oxide (NO), and TNF-α, which are directly involved in their immunoregulatory mechanism. The secretion of NO from UC-MSCs is reported to be associated with the desensitization of T cells via the IFN-inducible nitric oxide synthase (iNOS) pathways and to stimulate the migration of T cells in close proximity to MSCs that subsequently suppress T-cell sensitivities via NO.405 Lung infection with viruses usually leads to impairments in alveolar fluid clearance and protein permeability. The administration of UC-MSCs enhances alveolar protection and restores fluid clearance in patients with COVID-19. UC-MSCs secrete growth factors associated with angiogenesis and the regeneration of pulmonary blood vessels and micronetworks, including angiotensin-1, VEGF, and HGF, which also reduce oxidative stress and prevent fibrosis formation in the lungs. These trophic factors have been identified as key players in the modulation of the microenvironment and promote pulmonary repair. Additionally, UC-MSCs are more effective than BM-MSCs in the restoration of impaired alveolar fluid clearance and the permeability of airways in vitro, supporting the use of UC-MSCs in the treatment of patients with pulmonary pneumonia.406 In the context of pulmonary regeneration, UC-MSCs were shown to inhibit apoptosis and fibrosis in pulmonary tissue by activating the PI3K/AKT/mTOR pathways via the secretion of HGF, which also acts as an inhibitory stimulus that blocks alveolar epithelial-to-mesenchymal transition.407,408 Moreover, UC-MSCs can reverse the process of fibrosis via enhanced expression‎ of macrophage matrix-metallopeptidase-9 for collagen degradation and facilitate alveolar regeneration via Toll-like receptor-4 signaling pathways.409 UC-MSCs were shown to communicate with CD4+ T cells through HGF induction not only to inhibit their differentiation into Th17 cells, reducing the secretion of IL-17 and IL-22 but also to switch their differentiation into regulatory T cells.410,411 In addition, UC-MSCs conferred the ability to facilitate the number of M2 macrophages and reduce M1 cells via the control of the macrophage polarization process.412

There are several potential mechanisms of UC-MSCs in the treatment of patients with pulmonary diseases and pneumonia, including the regulation of immune cell function, immunomodulation, the enhancement of alveolar fluid clearance and protein permeability, the modulation of endoplasmic reticulum stress, and the attenuation of pulmonary fibrosis. Hence, based on these discussions, UC-MSCs are recommended as suitable candidates for the treatment of pulmonary disease both in pediatric and adult patients.

Proposed mechanism of AT-MSCs in the treatment of endocrinological diseases, reproductive disorders, and skin burns

Human AT was first viewed as a passive reservoir for energy storage and later as a major site for sex hormone metabolism, the production of endocrine factors (such as adipsin and leptin), and a secretion source of bioactive peptides known as adipokines.413 It is now clear that AT functions as a complex and highly active metabolic and endocrine organ, orchestrating numerous different biological features414 (Fig. 5a). In addition to adipocytes, AT contains hematopoietic-derived progenitor cells, connective tissue, nerve tissue, stromal cells, endothelial cells, MSCs, and pericytes. AT-MSCs and pericytes mobilize from their perivascular locations to aid in healing and tissue regeneration throughout the body. As AT is involved directly in energy storage and metabolism, AT-MSCs are also mediated and regulated by growth factors related to these pathways. In particular, interleukin-6 (IL-6), IL-33, and leptin regulate the maintenance of metabolic activities by increasing insulin sensitivity and preserving homeostasis related to AT. Nevertheless, in the development of obesity and diabetes, omental and subcutaneous AT maintains a low-grade state of inflammation, resulting in the impairment of glucose metabolism and potentially contributing to the development of insulin resistance.415 In normal AT, direct regulation of Pre-B-cell leukemia homeobox (Pbx)-regulating protein-1 (PREP1) by leptin and thyroid growth factor-beta 1 (TGF-β1) in AT-MSCs and mature adipocytes is involved in the protective function and maintenance of AT homeostasis. However, under diabetic conditions, the balance between the expression‎ of leptin and the secretion of TGF-β1 is compromised, resulting in the malfunction of AT-MSC metabolic activity and the proliferation, differentiation, and maturation of adipocytes. Therefore, the use of autologous AT-MSCs in the treatment of diabetic conditions is not a suitable option, as the functions of AT-MSCs are directly altered by diabetic conditions, which reduces their effectiveness in cell-based therapy (Fig. 5b).

Fig. 5

Umbilical cord-derived mesenchymal stem cells (UC-MSCs) are good candidates for the treatment of pulmonary diseases. a Lung immaturity and fibrosis are the major problems of patients with bronchopulmonary dysplasia and lead to increased levels of SDF-1, the development of fibrosis, the induction of the inflammatory response, and the impairment of alveolarization. UC-MSCs are attracted to the damaged lung via the chemoattractant SDF-1, which is constantly released from the immature lung via SDF-1 and CXCR4 communication. Moreover, UC-MSCs reduce the level of proinflammatory cytokines (TGF-β, INF-γ, macrophage MIF, and TNF-α) via a cell-to-cell contact mechanism. The ability of UC-MSCs to produce and secrete VEGF also involves in the regeneration of the immature lung through enhanced angiogenesis. b Upon an exacerbation of chronic obstructive pulmonary disease (COPD), UC-MSCs respond to the surrounding stimuli by reducing IL-8 and TNF-α levels, resulting in the inhibition of the inflammatory response but an increase in the secretion of growth factors participating in the protection of alveoli, fluid clearance and reduced oxidative stress and lung fibrosis, including HGF, TGF-β, IGF-1, and exosomes. c In a similar manner, UC-MSCs prevent the formation of cytokine storms in coronavirus disease 2019 (COVID-19) by inhibiting CD34+ T-cell differentiation into Th17 cells and enhancing the number of regulatory T cells. Moreover, UC-MSCs also have antibacterial activity by secreting LL-3717 and lipocalin. Figure was created with BioRender.com

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Preclinical studies and clinical trials have revealed the therapeutic effects of MSCs, in general, and AT-MSCs, in particular, in the management of POF, with relatively high efficacy and enhanced regeneration of the ovaries. Understanding the molecular and cellular mechanisms underlying these effects is the first step in the development of suitable MSC-based therapies for POF. One of the mechanisms by which MSCs exert their therapeutic effects is their ability to migrate to sites of injury, a process known as “homing”. Studies have shown that MSCs from different sources have the ability to migrate to different compartments of the injured ovary. For example, BM-MSCs administered through IV routes migrated mostly to the ovarian hilum and medulla,416 whereas a significant number of UC-MSCs were found in the medulla.417 Interestingly, AT-MSCs were found to be engrafted in the theca layers of the ovary but not in the follicles, where they acted as supportive cells to promote follicular growth and the regeneration of thecal layers.418 The structure and function of the thecal layer have a great impact on fertility, which has been reviewed elsewhere.419 In brief, the thecal layer consists of two distinct parts, the theca interna, which contains endocrine cells, and the theca externa, which is an outer fibrous layer. The thecal layer contains not only endocrine-derived cells but also vascular- and immune-derived cells, whose functions are to maintain the structural integrity of the follicles, transport nutrients to the inner compartment of the ovary and produce key reproductive hormones such as androgens (testosterone and dihydrotestosterone) and growth factors (morphogenic proteins, e.g., BMPs and TGF-β).420 As AT-MSCs originate from an endocrine organ, their ability to sense signals and migrate to the thecal layer is anticipated. Additionally, secretome analysis of AT-MSCs showed a wide range of growth factors, including HGF, TBG-β, VEGF, insulin-like growth factor-1 (IGF-1), and EGF,421 that are directly involved in the restoration of the structure and function of damaged ovaries by stimulating cell proliferation and reducing the aging process of oocytes via the activation of the SIRT1/FOXO1 pathway, a key regulator of vascular endothelial homeostasis.422,423 In POF pathology, autophagy and its correlated oxidative stress contribute to the development of POF throughout a patient’s life. Recently, AT-MSCs were shown to be able to improve the structure and function of mouse ovaries by reducing oxidative stress and inflammation, providing essential data supporting the mechanism of AT-MSCs in the treatment of POF.424 Several studies have illustrated that AT-MSCs secrete biologically active EVs that regulate the proliferation of ovarian granulosa cells via the PI3K/AKT pathway, resulting in the enhancement of ovarian function.425 Direct regulation of ovarian cell proliferation modulates the state of these cells, which in turn restores the ovarian reserve.426 Other mechanisms supporting the effectiveness of MSCs have been carefully reviewed, confirm‎ing the therapeutic potential of MSCs derived from different sources426 (Fig. 5c).

In the last decade, the number of clinical trials using AT-MSCs in the treatment of chronic skin wounds and skin regeneration has exponentially increased, with data supporting the enhancement of the skin healing processes, the reduction of scar formation, and improvements in skin structure and quality. Several mechanisms are directly linked to the origin of AT-MSCs, including differentiation ability, neovascularization, anti-apoptosis, and immunological regulation. AT is a connective and supportive tissue positioned just beneath the skin layers. AT-MSCs have a strong ability to differentiate into adipocytes, endothelial cells,427 epithelial cells428 and muscle cells.429 The adipogenic differentiation of AT-MSCs is one of the three mesoderm lineages that defines MSC features, and AT-MSCs are likely to be the best MSC type harboring this ability compared to BM- and UC-MSCs. Recent reports detailed that AT-MSCs accelerated diabetic wound tissue closure through the recruitment and differentiation of endothelial cell progenitor cells into endothelial cells mediated by the VEGF-PLCγ-ERK1/ERK2 pathway.430 Upon injury, the skin must be healed as quickly as possible to prevent inflammation and excessive blood loss. The reparation process occurs through distinct overlapping phases and involves various cell types and processes, including endothelial cells, keratinocyte proliferation, stem cell differentiation, and the restoration of skin homeostasis.431 Hence, the differentiation ability of AT-MSCs plays a critical role in their therapeutic effect on skin wound regeneration and healing processes. AT-MSCs accelerate wound healing via the production of exosomes that serve as paracrine factors. It was reported that AT-MSCs responded to skin wound injury stimuli by increasing their expression‎ of the lncRNA H19 exosome, which upregulated SOX9 expression‎ via miR-19b, resulting in the acceleration of human skin fibroblast proliferation, migration, and invasion.432 In addition, the engraftment of AT-MSCs supported wound bed blood flow and epithelialization processes.433 Anti-apoptosis plays a critical role in AT-MSC-based therapy, as without a microvascular supply network established within 4 days post injury, adipocytes undergo apoptosis and degenerate. Exogenous sources of AT-MSCs mediate anti-apoptosis via IGF-1 and exosome secretion by triggering the activation of PI3K signaling pathways.434 Another mechanism supporting the therapeutic potential of AT-MSCs is their anti-inflammatory function, which results in the reduction of proinflammatory factors, such as tumor necrosis factor (TNF) and interferon-γ (IFN-γ), and increases the production of the anti-inflammatory factors IL-10 and IL-4. Exosomes from AT-MSCs in response to a wound environment were found to contain high levels of Nrf2, which downregulated wound NADPH oxidase isoform 1 (NOX1), NADPH oxidase isoform 4 (NOX4), IL-1β, IL-6, and TNF-α expression‎. The anti-inflammatory functions of AT-MSCs are also regulated by their immunomodulatory ability, partially through the inhibition of NF-κB activation in T cells via the PD-L1/PD-1 and Gal-9/TIM-3 pathways, providing a novel target for the acceleration of wound healing435 (Fig. 5d).

Therefore, as an endocrine organ in the human body, AT and its derivative stem cells, including AT-MSCs, have shown great potential in the treatment of reproductive disorders and skin diseases. Their potential is supported by mechanisms that are directly related to the nature of AT-MSCs in the maintenance of tissue homeostasis, angiogenesis, anti-apoptosis, and the regulation of inflammatory responses.

The current challenges for MSC-based therapies

Over the past decades, MSC-based research and therapy have made tremendous advancements due to their advantages, including immune evasion, diverse tissue sources for harvesting, ease of isolation, rapid expansion, and cryopreservation as “off-the-shelf” products. However, several important challenges have to be addressed to further enhance the safety profile and efficacy of MSC-based therapy. In our opinion, the most important challenge of MSC-based therapy is the fate of these cells post administration, especially the long-term survival of allogeneic cells in the treatment of certain diseases. Although reported data confirm‎ that the majority of MSCs are trapped in the lung and rapidly removed from the circulation, caution has been raised related to the occurrence of embolism events post infusion, which was proven to be related to MSC-induced innate immune attack (called instant blood-mediated inflammatory reaction).436 Another related challenge is the homing ability of infused cells, as successful homing at targeted tissue might result in long-term benefits to patients. Other concerns related to MSC-based therapy are the number of dead cells infused into the patients. An interesting study reported that dead MSCs alone still exerted the same immunomodulatory property as live MSCs by releasing phosphatidylserine.437 This is an interesting observation, as there is always a certain number of dead cells present in the cell-based product, and concerns are always raised related to their effects on the patient’s health. Finally, the hypothesis presented in this review is also a great challenge of the field, which has been proposed for future studies to answer the question: “What is the impact of MSC sources on their downstream application?”. Tables 5 and 6 illustrate the comparative studies that were conducted in preclinical and clinical settings to address the MSC source challenge. Other challenges of MSC-based therapies have been discussed in several reviews and systematic studies,135,185,438,439 which are highly recommended.

Table 5 Comparative analysis of the effectiveness of MSC sources in a preclinical setting

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Table 6 Clinical trials comparing the efficacy of MSCs derived from different sources in the treatment of pulmonary diseases and cardiovascular conditions

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Limitations of the current hypothesis

The proposed hypothesis presented in this review was made based on (1) the calculated number of recovered patients from published clinical trials; (2) the empirical experience of the authors in the treatment of brain-related diseases,440 pulmonary disorders,215 and endocrinological conditions;271,441 and (3) the proposed mechanisms by which each type of MSC exhibits its best potential for downstream applications. The authors understand that the approach that we used has a certain level of research bias, as a comprehensive meta-analysis is needed to first confirm‎ the correlation between the origins of MSCs and their downstream clinical outcomes before a complete hypothesis can be made. However, to date, a limited number of clinical trials have been conducted to directly compare the efficacy of MSCs from different sources in treating the same disease, which in turn dampened our analysis to prove this hypothesis. In addition, MSC-based therapy is still in its early stages, as controversy and arguments are still present in the field, including (1) the name of MSCs (medicinal signaling cells vs. MSCs or mesenchymal stromal cells),442,443 (2) the existence of “magic cells” (one cell type for the treatment of all diseases),444,445 (3) the conflicting results from large-scale clinical trials,135 and (4) the dangerous issues of unauthorized, unproven stem cell therapies and clinics.446,447 Therefore, our hypothesis is proposed at this time to encourage active researchers and clinicians to either prove or disprove it so that future research can strengthen the uses of MSC-based therapies with solid mechanistic study results and clarify results for “one cell type for the treatment of all diseases”.

Another limitation is the knowledge coverage in the field of MSC-based regenerative medicine, as discussed in this study. First, the abovementioned diseases were narrowed to four major disease categories for which MSC-based therapy is widely applied, including neuronal, pulmonary, cardiovascular, and endocrinological conditions. In fact, other diseases also receive great benefits from MSC therapy, including liver cirrhosis,448 bone regeneration,360 plastic surgery,449 autoimmune disease,450 etc., which are not fully discussed in this review and included in our hypothesis. Recently, the secretome profile of MSCs and its potential application in clinical settings have emerged as a new player in the field, with a recently published comprehensive review including MSC-derived exosomes.451,452 To date, the therapeutic potential of MSCs is believed to be strongly influenced by their secretomes, including growth factors, cytokines, chemokines, and exosomes.453 However, this body of knowledge is also not fully included in our discussion, as this review focuses on the function and potency of MSCs as a whole with considerations derived from published clinical data. Therefore, the authors believe in and support the future applications of the secreted components derived from MSCs, including exosomes, in the treatment of human diseases. In fact, this potential approach could elevate the uses of MSCs to the next level, where the sources of MSCs could be neglected with advancements in the development of protocols that allow strict control of the secretome profiles of MSCs under specific conditions.454,455,456 Finally, strategies that could potentially enhance the therapeutic outcomes of MSC-based therapy, such as the “priming” process, are not discussed in this review. The idea of “priming” MSCs is based on the nature of MSCs, which is similar to the immune cells,457 that MSCs have proven to be able to “remember” the stimulus from the surrounding environment.458,459 Thus, activating or priming MSCs using certain conditions, such as hypoxia, matrix mechanics, 3D environment, hormones, or inflammatory cytokines, could trigger the memory mechanism of the MSCs in vitro so that these cells are ready to function towards specific therapeutic activities without the need for in vivo activation.3,460

Conclusion

From a cellular and molecular perspective and from our own experience in a clinical trial setting, AD-, BM- and UC-MSCs exhibit different functional activities and treatment effectiveness across a wide range of human diseases. In this paper, we have provided up-to-date data from the most recently published clinical trials conducted in neuronal diseases, endocrine and reproductive disorders, skin regeneration, pulmonary dysplasia, and cardiovascular diseases. The implications of the results and discussions presented in this review and in a very large body of comprehensive and excellent reviews as well as systematic analyses in the literature provide a different aspect and perspective on the use of MSCs from different sources in the treatment of human diseases. We strongly believe that the field of regenerative medicine and MSC-based therapy will benefit from active discussion, which in turn will significantly advance our knowledge of MSCs. Based on the proposed mechanisms presented in this review, we suggest several key mechanistic issues and questions that need to be addressed in the future:

1. 1.

   The confirmation and demonstration of the mechanism of action prove that tissue origin plays a significant role in the downstream applications of the originated MSCs.

2. 2.

   Is it required that MSCs derived from particular cell sources need to have certain functionalities that are unique to or superior in the original tissue sources?

3. 3.

   As mechanisms may rely on the secretion of factors from MSCs, it is important to identify the specific stimuli from the wound environments to understand how MSCs from different sources can exhibit similar functions in the same disease and whether or not MSCs derived from a particular source have stronger effects than their counterparts derived from other tissue sources.

4. 4.

   Should we create “universal” MSCs that could be functionally equal in the treatment of all diseases regardless of their origin by modeling their genetic materials?

5. 5.

   Can new sources of MSCs from either perinatal or adult tissues better stimulate the innate mechanisms of specific cell types in our body, providing a better tool for MSC-based treatment?

6. 6.

   A potential ‘priming’ protocol that allows priming, activating, and switching the potency of MSCs from one source to another with a more appropriate clinical phenotype to treat certain diseases. This idea is potentially relevant to our suggestion that each MSC type could be more beneficial in downstream applications, and the development of such a “priming” protocol would allow us to expand the bioavailability of specific MSC types.

From our clinical perspective, the underlying proposal in our review is to no longer use MSCs for applications while disregarding their sources but rather to match the MSC tissue source to the application, shifting from one cell type for the treatment of all diseases to cell source-specific disease treatments. Whether the application of MSCs from different sources still shows their effectiveness to a certain extent in the treatment of diseases or not, the transplantation of MSCs derived from different sources for each particular disease needs to be further investigated, and protocols need to be established via multicentre, randomized, placebo-controlled phase II and III clinical trials (Fig. 6).

Fig. 6

The tissue sources of mesenchymal stem cells (MSCs) contribute greatly to their therapeutic potential, as all MSC types share safety profiles and overlapping efficacy. Although a large body of data and their review and systematic analysis indicated the shared safety and potential efficacy of MSCs derived from different tissue sources, targeted therapies considering MSC origin as an important factor are imperative to enhance the downstream therapeutic effects of MSCs. We suggest that bone marrow-derived MSCs (BM-MSCs) are good candidates for the treatment of brain and spinal cord injury, adipose tissue-derived MSCs (AT-MSCs) are suitable for the treatment of reproductive disorders and skin regeneration, and umbilical cord-derived MSCs (UC-MSCs) could be alternatives for the treatment of pulmonary diseases and acute respiratory distress syndrome (ARDS). Figure was created with BioRender.com

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Data availability

All data generated or analyzed in this study are included in this published article.

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