위장관은 음식과 pathogen을 어떻게 구분하는가? 패턴인식수용체 2021 nature

[문형철]

저는 라면, 국수, 파스타를 엄청 좋아했었습니다. 1주일에 한두번 씩 라면을 먹었죠. 글루텐 문제의 심각성(글루텐 intolerance는 최소 30%이상, 10명중 3명은 글루텐 소화효소가 없음)을 알고 나서 밀, 보리 음식을 완전히 끊었습니다. 6개월정도 끊었다가 시험삼아 설탕없는 베이글 빵 1/4조각을 먹어봤죠. 정확히 10시간 후에 배가 살살 아프고, 방귀가 나오더라구요. ... 2주 후 베이글 빵 1/2이상을 먹어봤습니다. 정확히 10시간 후 배가 아프더니 그날 설사를 했습니다....

더 놀라운 것은 2~3년이 지난후부터는 라면 1개를 다 먹어도, 베이글빵 하나를 다 먹어도 약간 방귀만 나올뿐 배가 아프거나 설사를 하지 않습니다. 이유가 뭘까? 

쇼그렌 증후군 환자가 돼지고기만 먹으면 눈이 붉어지고 배가 아펐습니다. 그분에게 제가 말했습니다. 아마 나중에 먹을 수 있게 될거에요. 6개월이 지난 후 조금씩 먹어도 괜찮더라 라는 소리를 들었습니다. 이유가 뭘까?

만성염증이 치유되면서 점막을 회복하고, 위산 농도가 정상화되는 거 다 맞아요. 위장관에서 음식과 pathogen을 어떻게 구분하는가? 이 과정을 이해하고 싶은 거에요. 그러기 위해서는 PRR(pattern recognition receptor, 패턴인식수용체)가 PAMP(pathogen association molecule pattern, 병원체관련 분자패턴)을 인식해야 하죠. 만성염증인 사람들은 DAMP(damage association molecular pattern 손상관련 분자패턴)이 많아 정상적인 음식조차도 적(pathogen)으로 인식을 하구요. 

군복으로 비유해볼께요. 건생병사로 건강한 몸일때 면역세포는 명확하게 패턴인식수용체로 적군을 가려내고 제거합니다. 만성염증일때는 패턴으로 인식하는 면역세포는 적군인지 아군인지 가려내지 못하고 적군을 막사안으로 들이기도하고, 진흙이 묻어있는 아군을 적으로 간주해 죽이기도 하는 거죠. 그리고 염증의 역치를 넘어서 자가면역 상태가 되면 패턴인식 수용체는 특정 아군(자신의 조직)을 적으로 인식해서 죽이려고 지속적으로 염증을 일으키는 거이다. 이렇게 비유할 수 있겠네요

쇼그렌 환자가 2주, 4주 3개월 시간축으로 건생병사 4과제를 엄격하게 실천해서 2주만에 염증이 줄어들기 시작하고, 4주면 염증이 확연히 줄어들어 증상이 호전되기 시작하죠. 3개월정도면 비타민 미네랄 결핍이 해소되고 건강한 세포가 만들어지면서 .. 면역세포의 패턴인식 수용체가 적군과 아군을 구별하기 시작할 것으로 추정합니다. ... 

위 글을 읽은 후에 이 영상 다시 보시면 조금더 이해될거에요.. 

https://www.youtube.com/watch?v=yKfjG6RMqlE

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• Review Article
• Open access
• Published: 04 August 2021

Pattern recognition receptors in health and diseases

• Danyang Li & 
• Minghua Wu 

Signal Transduction and Targeted Therapy volume 6, Article number: 291 (2021) Cite this article

•  
•  
•  

Abstract

Pattern recognition receptors (PRRs) are a class of receptors that can directly recognize the specific molecular structures on the surface of pathogens, apoptotic host cells, and damaged senescent cells. PRRs bridge nonspecific immunity and specific immunity. Through the recognition and binding of ligands, PRRs can produce nonspecific anti-infection, antitumor, and other immunoprotective effects. Most PRRs in the innate immune system of vertebrates can be classified into the following five types based on protein domain homology:

Toll-like receptors (TLRs), nucleotide oligomerization domain (NOD)-like receptors (NLRs), retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs), C-type lectin receptors (CLRs), and absent in melanoma-2 (AIM2)-like receptors (ALRs). PRRs are basically composed of ligand recognition domains, intermediate domains, and effector domains. PRRs recognize and bind their respective ligands and recruit adaptor molecules with the same structure through their effector domains, initiating downstream signaling pathways to exert effects. In recent years, the increased researches on the recognition and binding of PRRs and their ligands have greatly promoted the understanding of different PRRs signaling pathways and provided ideas for the treatment of immune-related diseases and even tumors. This review describes in detail the history, the structural characteristics, ligand recognition mechanism, the signaling pathway, the related disease, new drugs in clinical trials and clinical therapy of different types of PRRs, and discusses the significance of the research on pattern recognition mechanism for the treatment of PRR-related diseases.

요약

패턴 인식 수용체(PRR)는 

병원체, 세포자멸사 숙주세포, 손상된 노화세포 표면의 특정 분자 구조를 

직접 인식할 수 있는 수용체입니다. 

PRR은 비특이적 면역과 특이적 면역을 연결하는 역할을 합니다. 리간드의 인식과 결합을 통해 PRR은 비특이적 항감염, 항종양, 기타 면역 보호 효과를 생성할 수 있습니다. 

척추동물의 선천성 면역체계에 존재하는 대부분의 PRR은 

단백질 도메인 상동성에 따라 

다음의 다섯 가지 유형으로 분류할 수 있습니다. 

Toll-like receptors (TLRs), 

nucleotide oligomerization domain (NOD)-like receptors (NLRs), 

retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs), 

C-type lectin receptors (CLRs), 

absent in melanoma-2 (AIM2)-like receptors (ALRs). 

PRRs는 

기본적으로 리간드 인식 영역, 중간 영역, 그리고 이펙터 영역으로 구성되어 있습니다. 

ligand recognition domains, intermediate domains, and effector domains

PRR은 각각의 리간드를 인식하고 결합하며, 이펙터 도메인을 통해 동일한 구조를 가진 어댑터 분자를 모집하여 다운스트림 신호 전달 경로를 시작하여 효과를 발휘합니다. 최근 몇 년 동안 PRR과 그 리간드의 인식과 결합에 대한 연구가 증가하면서 다양한 PRR 신호 전달 경로를 이해하는 데 큰 도움이 되었고, 면역 관련 질병과 심지어 종양 치료에 대한 아이디어를 제공했습니다. 이 리뷰에서는 PRR의 역사, 구조적 특징, 리간드 인식 메커니즘, 신호 전달 경로, 관련 질병, 임상 시험 중인 신약, 다양한 유형의 PRR에 대한 임상 치료에 대해 자세히 설명하고, PRR 관련 질병의 치료를 위한 패턴 인식 메커니즘 연구의 중요성에 대해 논의합니다.

Article 15 February 2021

Introduction

The first line of defense against pathogens that gradually evolved in organisms is innate immunity,1 which is divided into two levels: first, the skin, mucosal tissue, blood–brain barrier, and chemical barrier (e.g. fatty acid, pH, enzyme, and complement system) of the host can effectively resist the invasion of general pathogenic microorganisms;2,3,4 second, the innate immune system of vertebrates protects the organism through nonspecific immune defense and surveillance by innate immune cells. Innate immune cells mainly include monocytes, neutrophils, macrophages, dendritic cells, natural killer (NK) cells, mast cells, eosinophils, and basophils.5,6 Unlike T cells and B cells, which have high specificity, innate immune cells do not express specific antigen recognition receptors. Through the recognition and binding of some common molecules on the surface of pathogens, apoptotic host cells, and damaged senescent cells, pattern recognition receptors (PRRs) induce immunoprotective effects, such as anti-infection and antitumor effects, and participate in the initiation and effect process of specific immune response.7,8,9

In the 1990s, the hypothesis of pathogen-associated molecular patterns (PAMPs) and PRRs that recognize PAMPs was proposed by Janeway, which was of epoch-making significance and changed research on innate immunity.10 The main point of this hypothesis is the connection between the innate immune signal and the initiation of the adaptive immune response. Some unique and conserved components of pathogenic microorganisms can induce the second signal required to activate T cells, so as to control the adaptive immunity from being activated under normal conditions.11,12 In addition, there are a class of receptors in the host that can recognize pathogenic microorganisms and activate the second signal in time, which are independent of gene rearrangement. In vertebrates, innate immunity recognizes pathogenic microorganisms and assists in the activation and expression‎ of second signals that activate the adaptive immunity.13

소개

점진적으로 진화한 병원체에 대한 첫 번째 방어선은 선천성 면역입니다.1

선천성 면역은 두 가지 수준으로 나뉩니다.

첫째, 숙주의 피부, 점막 조직, 혈액-뇌 장벽, 화학적 장벽(예: 지방산, pH, 효소, 보체 시스템)은 일반적인 병원성 미생물의 침입을 효과적으로 막을 수 있습니다.2,3,4

둘째, 척추동물의 선천성 면역 체계는 선천성 면역 세포에 의한 비특이적 면역 방어 및 감시를 통해 유기체를 보호합니다.

선천성 면역 세포는

주로 단핵구, 호중구, 대식세포, 수지상 세포, 자연살해(NK) 세포, 비만세포, 호산구, 호염기구 등을

포함합니다.5,6

특이성이 높은 T세포와 B세포와는 달리,

선천성 면역 세포는 특정한 항원 인식 수용체를 발현하지 않습니다.

병원체, 세포자멸사 숙주세포, 손상된 노화세포 표면의 일부 공통 분자를 인식하고 결합함으로써, 패턴 인식 수용체(PRR)는 항감염 및 항종양 효과와 같은 면역 보호 효과를 유도하고, 특정 면역 반응의 개시 및 효과 과정에 참여합니다.7,8,9

1990년대, 잰웨이(Janeway)는

병원체 관련 분자 패턴(PAMPs)과 PAMPs를 인식하는 PRR에 대한 가설을 제안했는데,

이는 획기적인 의미를 지니고 선천성 면역에 대한 연구를 변화시켰습니다.10

이 가설의 핵심은 선천성 면역 신호와 적응성 면역 반응의 개시 사이의 연관성입니다. 병원성 미생물의 일부 고유하고 보존된 구성 요소는 T 세포를 활성화하는 데 필요한 두 번째 신호를 유도하여 정상적인 조건에서 적응 면역이 활성화되는 것을 제어할 수 있습니다.11,12 또한 숙주에는 병원성 미생물을 인식하고 제 시간에 두 번째 신호를 활성화할 수 있는 수용체 클래스가 있는데, 이는 유전자 재배열과 무관합니다.

척추동물에서 선천성 면역은

병원성 미생물을 인식하고 적응 면역을 활성화하는

두 번째 신호의 활성화와 발현을 돕습니다.13

Toll-like receptors (TLRs) are one of the earliest PRRs discovered in the innate immune system, which plays an important role in inflammatory responses.14,15 Therefore, here is a brief description of the development history of PRRs with TLRs as a representative. TLRs were first found in Drosophila in the form of genes in 1994. Studies have shown that the function of this gene is related to the formation of the dorsal–ventral axis during the embryonic development of Drosophila.16 In 1988, Hashimoto et al. discovered that the Toll gene encodes a transmembrane protein and clarified the structure of the Toll protein.17 In 1991, Gay et al. found that Toll protein had structural homology with interleukin-1 (IL-1), a natural immune molecule in mammals, suggesting that the function of Toll may be related to immunity.18 In 1996, Hoffmann team found that Toll plays a role in the resistance of Drosophila to fungal infection. Toll-activated mutants persistently express antifungal peptides, while Toll-deletion mutants, on the contrary, lose their ability to arrest fungal infection. It has been found that Toll can recognize spatzle (an important protein in the development of the dorsal and abdomen of Drosophila) and initiate a series of signal transduction to activate the expression‎ of antifungal peptide.19 

In 1997, Janeway et al. cloned human TLR4. TLR4 can induce the activation of nuclear factor (NF)-κB and the expression‎ of the co-stimulatory molecule CD80. This proves that innate immunity recognizes pathogenic microorganisms and activates the expression‎ of the second signal, which is indispensable for the activation of adaptive immunity.20 Since the discovery of TLR4, many PRRs and their corresponding ligands have been discovered. PRRs can be divided into the following five types based on protein domain homology: TLRs, nucleotide oligomerization domain (NOD)-like receptors (NLRs), retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs), C-type lectin receptors (CLRs), and absent in melanoma-2 (AIM2)-like receptors (ALRs) (Table 1).21 PRRs are representative of immune receptors in innate immunity and exist in various forms. PRRs are not only expressed on the cell membrane but also widely distributed in intracellular compartment membranes and the cytoplasm.22 Membrane-bound PRRs and PRRs in the cytoplasm are basically composed of ligand recognition domains, intermediate domains, and effector domains.23,24 PRRs activate downstream signaling pathways through recognition of their ligands. The activation of downstream signaling pathways can produce many effects: recruiting and releasing cytokines, chemokines, hormones, and growth factors; inducing chronic inflammation; forming an inflammatory microenvironment; initiating innate immune killing and subsequent acquired immune response,9 maintaining the balance of host microecology; and eliminating dead or mutated cells.

Toll-like receptors (TLRs)는

선천성 면역 체계에서 발견된 가장 초기의 PRRs 중 하나이며,

염증 반응에서 중요한 역할을 합니다.14,15

따라서,

여기에서는 TLRs를 대표적인 것으로 하여

PRRs의 발전 역사를 간략하게 설명합니다.

TLR은 1994년 초파리에서 유전자 형태로 처음 발견되었습니다. 연구에 따르면 이 유전자의 기능은 초파리의 배아 발달 과정에서 배의 앞뒤 축의 형성과 관련이 있는 것으로 나타났습니다.16

1988년, 하시모토(Hashimoto) 등은 톨(Toll) 유전자가 막 관통 단백질을 암호화한다는 사실을 발견하고 톨 단백질의 구조를 규명했습니다. 17

1991년 Gay 등은 Toll 단백질이 포유류에 존재하는 자연 면역 분자인 인터루킨-1(IL-1)과 구조적 상동성을 가지고 있다는 사실을 발견했습니다. 이 사실은 Toll의 기능이 면역과 관련이 있을 수 있음을 시사합니다.18 1

996년 Hoffmann 팀은 Toll이 초파리의 곰팡이 감염에 대한 저항성에 중요한 역할을 한다는 사실을 발견했습니다. 톨(Toll) 활성화 돌연변이체는 항진균성 펩타이드를 지속적으로 발현하는 반면, 톨(Toll) 결실 돌연변이체는 곰팡이 감염을 억제하는 능력을 상실합니다. 톨(Toll)은 스파츨(Spatzle, 초파리의 등쪽과 복부 발달에 중요한 단백질)을 인식하고 일련의 신호 전달을 시작하여 항진균성 펩타이드의 발현을 활성화할 수 있는 것으로 밝혀졌습니다.19

1997년, Janeway 등은 인간 TLR4를 복제했습니다.

TLR4는

핵 인자(NF)-κB의 활성화와 공동 자극 분자 CD80의 발현을

유도할 수 있습니다.

이것은

선천성 면역이 병원성 미생물을 인식하고

적응성 면역의 활성화에 필수적인

두 번째 신호의 발현을 활성화한다는 것을 증명합니다.20

TLR4가 발견된 이후로, 많은 PRR과 그 대응하는 리간드가 발견되었습니다. PRR은 단백질 도메인 상동성에 따라 다음의 다섯 가지 유형으로 나눌 수 있습니다.

TLRs,

뉴클레오티드 올리고머화 도메인(NOD)-유사 수용체(NLRs),

레티노산 유도 유전자-I(RIG-I)-유사 수용체(RLRs),

C형 렉틴 수용체(CLRs),

흑색종-2(AIM2)-유사 수용체(ALRs) (표 1).21

PRRs는

선천성 면역의 대표적인 면역 수용체이며

다양한 형태로 존재합니다.

PRR은

세포막에 존재할 뿐만 아니라

세포 내 소포체 막과 세포질에도 널리 분포되어 있습니다.22

세포막에 존재하는 PRR과 세포질에 존재하는 PRR은

기본적으로 리간드 인식 영역, 중간 영역, 효과 영역으로 구성되어 있습니다.23,24

PRR은

리간드를 인식함으로써

하류 신호 전달 경로를 활성화합니다.

하류 신호 경로의 활성화는

사이토카인, 케모카인, 호르몬, 성장 인자의 모집과 방출, 만성 염증 유발, 염증성 미세환경 형성,

선천성 면역 살해 및 후천성 면역 반응 개시,

숙주 미세생태계의 균형 유지,

죽은 세포 또는 돌연변이 세포 제거 등 다양한 효과를 가져올 수 있습니다.

Table 1 Common PRRs in innate immunity

Table 1 Common PRRs in innate immunity

From: Pattern recognition receptors in health and diseases

ItemsPRRDomainsCellular distributionPAMPSourcesSignaling pathways

Toll-like receptors (TLRs)	TLR1 (TLR1–TLR2)	LRR domain–transmembrane domain–TIR domain (extracellular to intracellular)	Mo, DC, Ma, Eo, Ba	Triacyl lipopeptide	Bacteria	Most TLRs: MyD88-dependent pathways; TLR3: TRIF-dependent pathways; TLR4: MyD88-dependent pathways and TRIF-dependent pathways
	TLR2 (TLR1–TLR2, TLR2–TLR6)		Mo, DC, Ma, Eo, Ba	Lipoteichoic acid	Bacteria
				Arabinomannan	Mycobacterium
				Peptidoglycan	Bacteria
				Zymosan	Fungi
				Lipoprotein	Mycoplasma
				Pore protein	Neisseria
	TLR3		Mφ, DC, IEC	dsRNA	Virus
	TLR4 (MD-2/CD14)		Mφ, DC, Ma, Eo	Lipopolysaccharides	Bacteria
				Heat-shock proteins	Host
	TLR5		IEC	Flagellin	Bacteria
	TLR6 (TLR2–TLR6)		Mo, DC, Ma, Eo, Ba	Lipoteichoic acid	Bacteria
				Peptidoglycan	Bacteria
	TLR7		pDC, Mφ, Eo	ssRNA	Virus
				Imidazoquinoline	Artificially synthesized
	TLR8		Mφ, N	ssRNA	Virus
	TLR9		pDC, Eo, Ba	Non-methylated CpG DNA	Bacteria, Virus
	TLR10 (human)		pDC, Eo, Ba	dsRNA	Virus
	TLR11 (mouse)		Mφ, DC	Profilin and related proteins	Toxoplasma gondii
	TLR12 (mouse)		DC	Profilin and related proteins	Toxoplasma gondii
	TLR13 (mouse)		Unknown	23s ribosomal RNA	Bacteria
Nucleotide-binding oligomerization domain-like receptors (NLRs)	NOD1	LRR domain–NBD–effector domains	IEC, cytosol of Mφ	iE-DAP	Gram negative bacteria	RIP2-TAK1-NF-κB pathways
	NOD2			MDP	Gram-negative bacteria, Gram-positive bacteria
RIG-I-like receptors (RLRs)	RIG-I	(RD)-CTD-DexD/H helicase domain–CARD	Cytosol	5’-triphosphorylated RNA, short-chain dsRNA	Virus	MAVS-TRAF6-NF-κB/TBK1 pathways
	MDA5			poly IC, long-chain dsRNA	Virus
	LGP2			dsRNA	Virus
C-type lectin receptors (CLRs)	Dectin-1	CTLD–ITAM	DC, Mφ	β-Glucan	Fungus	Tyrosine kinase-dependent and non-tyrosine kinase-dependent pathways
	Dectin-2			α-Mannan	Fungus
Absent in melanoma-2-like receptors (ALRs)	ALRs	HIN-200-PYD	Cytosol	dsDNA	Bacteria	Inflammasome–pyroptosis

1. LRR leucine-rich repeat, TIR Toll/IL-1R domain, NBD nucleotide-binding domain, RD repressor domain, CTD C-terminal domain, CARD caspase activation and recruitment domain, CTLD C-type lectin-like domains, ITAM immunoreceptor tyrosine-based activation motif, PYD pyrin domain, Mo monocyte, DC dendritic cell, Ma mastocyte, Eo eosinophils, Ba basophils, pDC plasmacytoid dendritic cell, IEC intestinal epithelial cell, N neutrophil, dsRNA double-stranded RNA, ssRNA single-stranded RNA, iE-DAP γ-D-glu-meso-diaminopimelic acid, MDP muramyl dipeptide, MyD88 myeloid differentiation factor 88, TRIF TIR domain-containing adaptor protein-inducing interferon β, RIP2 receptor-interacting serine–threonine protein 2, TAK1 transforming growth factor-β-activated kinase 1, NF-κB nuclear factor κB, MAVS mitochondrial antiviral signaling protein, TRAF6 tumor necrosis factor receptor-associated factor, TBK1 TANK-binding kinase 1

Full size table

PAMPs are the specific and highly conserved molecular structures shared by the same kind of pathogenic microorganisms,25,26 including lipids, proteins, and nucleic acids, such as lipopolysaccharides (LPS), lipoteichoic acid (LTA), and bacterial DNA.27,28 PAMPs are essential for pathogen survival and usually have unique molecular or subcellular characteristics that are not found in host cells. Therefore, innate immune cells can recognize PAMPs via PRRs, distinguish “self” and “non-self,”29 and respond to pathogens and their products. However, the host will produce some proteins and metabolites after being stimulated by its own tissue damage, cell necrosis, and other factors.30 These molecules are called damage-associated molecular pattern (DAMP).31 PRRs can also recognize such molecules, activate natural immunity, and cause inflammation.32

With advances in research on PRRs structure and distribution at different levels, the role of PRRs in the innate immune regulatory network has become clearer. The recognition and binding of PRRs to their ligands is critical in initiating the innate immune response. Therefore, the study of PRR-mediated pattern recognition mechanisms will help to elucidate the signaling pathways and mechanisms of disease and provide new targets and methods for the treatment of diseases. In this review, we describe the structural characteristics, ligand recognition mechanism, the signaling pathway, the related disease, new drugs in clinical trials, and clinical therapy of different types of PRRs in detail. We focus on the different domains and ligand recognition mechanisms between PRRs, which can not only provide new ideas for the definition, role, and clinical application of PRRs but also promote the study of the role of the innate immune system in related diseases and even tumors.

PAMP는

같은 종류의 병원성 미생물이 공유하는

특이적이고 고도로 보존된 분자 구조로,

지질, 단백질, 핵산(예: 리포폴리사카라이드(LPS), 리포테이코산(LTA), 박테리아 DNA)을 포함하며,25,2627,28

PAMP는 병원체의 생존에 필수적이며,

일반적으로 숙주 세포에서는 발견되지 않는

독특한 분자적 또는 세포내 특성을 가지고 있습니다.

따라서

선천성 면역 세포는

PRR을 통해 PAMP를 인식하고, “자기”와 “비자기”를 구별하며29,

병원체와 그 산물에 반응할 수 있습니다.

그러나 숙주는

자신의 조직 손상, 세포 괴사 및 기타 요인에 의해 자극을 받은 후

일부 단백질과 대사 산물을 생성합니다.30

이러한 분자를

손상 관련 분자 패턴(DAMP)이라고 합니다.31

PRR은

이러한 분자를 인식하고,

자연 면역을 활성화하며, 염증을 유발할 수 있습니다.32

PRR의 구조와 분포에 대한 연구가 다양한 수준에서 진행되면서,

선천성 면역 조절 네트워크에서

PRR의 역할이 더욱 명확해졌습니다.

PRR이 리간드에 결합하는 것은

선천성 면역 반응을 시작하는 데 매우 중요합니다.

따라서,

PRR 매개 패턴 인식 메커니즘에 대한 연구는

질병의 신호 전달 경로와 메커니즘을 밝히는 데 도움이 될 것이며,

질병 치료를 위한 새로운 목표와 방법을 제공할 것입니다.

이 리뷰에서는 구조적 특징, 리간드 인식 메커니즘, 신호 전달 경로, 관련 질병, 임상 시험 중인 신약, 다양한 유형의 PRR의 임상 치료에 대해 자세히 설명합니다. 우리는 PRR의 정의, 역할, 임상 적용에 대한 새로운 아이디어를 제공할 수 있을 뿐 아니라 관련 질병과 종양에서 선천성 면역계의 역할에 대한 연구를 촉진할 수 있는 PRR 간의 다양한 도메인과 리간드 인식 메커니즘에 초점을 맞춥니다.

PRRs and ligand-recognition mechanisms

Toll-like receptors

TLRs are membrane-bound signal receptors and are important PRRs in the innate immune system of vertebrates.15,33 Such receptor molecules usually have two functions, one is to bind specifically to the ligand, and the other is to transmit signals. The corresponding signal transduction will amplify the effect of anti-pathogen infection, so that the immune cells active in the inflammatory response can be activated through the transcription of genes, and produce and secrete a variety of pro-inflammatory and antiviral factors.34,35,36 Up to now, 10 functional TLRs (TLR1–10) have been found in humans and 12 (TLR1–9 and TLR11–13) in mice.37,38,39,40,41 TLR10 in mice is not functional due to the insertion of reverse transcriptase.42 TLRs recognize PAMPs in different subcellular structures. The cellular localization of TLRs determines the types of ligands and the recognition mechanism. Some TLRs (TLR1, 2, 4, 5, 6, 10) are expressed on the surface of immune cells in the form of heterodimers or homodimers, mainly recognizing the membrane components of pathogenic microorganisms, such as lipids, lipoproteins, and proteins; others (TLR3, 7, 8, 9) are expressed in the form of homodimers, which mainly recognize the nucleic acids of microorganisms (Fig. 1).43

PRR과 리간드 인식 메커니즘

톨 유사 수용체(Toll-like receptors, TLRs)

TLR은

막에 결합된 신호 수용체이며

척추동물의 선천성 면역체계에서 중요한 PRR입니다.15,33

이러한 수용체 분자는 일반적으로 두 가지 기능을 가지고 있는데,

하나는 리간드에 특이적으로 결합하는 것이고,

다른 하나는 신호를 전달하는 것입니다.

해당 신호 전달은 항병원체 감염의 효과를 증폭시켜,

염증 반응에서 활성화된 면역 세포가 유전자 전사를 통해 활성화되고,

다양한 전염증 및 항바이러스 인자를 생성 및 분비할 수 있도록 합니다. 34,35,36

지금까지 인간에게는 10개의 기능적 TLR(TLR1-10)이 발견되었고,

쥐에게는 12개(TLR1-9와 TLR11-13)가 발견되었습니다.37,38,39,40,41

쥐의 TLR10은 역전사효소의 삽입으로 인해 기능하지 않습니다.42 TLR은 다양한 세포 내 구조에서 PAMP를 인식합니다. TLR의 세포 내 위치는 리간드의 유형과 인식 메커니즘을 결정합니다. 일부 TLR(TLR1, 2, 4, 5, 6, 10)은 면역 세포 표면에서 이종이합체 또는 동종이합체 형태로 발현됩니다. 주로 지질, 지단백질, 단백질과 같은 병원성 미생물의 막 구성 요소를 인식하는 반면, 다른 것(TLR3, 7, 8, 9)은 주로 미생물의 핵산을 인식하는 동형 이량체 형태로 발현됩니다(그림 1).43

Fig. 1

The signal transduction pathways and structure of TLR-binding ligand complex. TLRs can recognize one or more PAMPs through LRR domain. They usually dimerize themselves and recruit adaptor molecules with the same TIR domain to transmit signals

Full size image

TLRs are type I transmembrane glycoproteins and are composed of an extracellular region, a transmembrane region, and an intracellular region.44 The extracellular region contains leucine-rich repeats (LRRs), which are responsible for the recognition of specific ligands and perform extracellular pattern recognition. The intracellular domain contains the same Toll/IL-1R (TIR) domain as IL-1R, which plays a role in signal transduction. The extracellular region of TLRs contains LRRs, which mediate the pattern recognition of TLRs (Fig. 1).45 In 2007, researchers used X-ray crystal diffraction to analyze and determine the structure of the TLR–ligand complex,46 which provided a deeper understanding of the LRR domain. The LRR domain is shaped like a horseshoe, and each module consists of a conserved leucine motif and a variable region. The “LxxLxLxxN (L leucine, x any amino acid, N asparagine)” motif is composed of 20–30 amino acids and is on the concave surface of the horseshoe-like structure.47,48,49,50 The horseshoe-shaped N-terminus and C-terminus contain disulfide bridges formed by cysteine clusters51,52 to protect the hydrophobic core. After TLRs recognize and bind the corresponding PAMPs and endogenous ligands, the TIR domains conduct signals by binding to different receptor adaptor proteins in the cytoplasmic region.53,54 The TIR domain has three conserved amino acid sequences, which are called 1,2,3 cassettes. Depending on the different adaptor proteins, TLRs signaling can be divided into myeloid differentiation factor 88 (MyD88)-dependent and MyD88-independent pathways (Fig. 1).55,56

Exploring the pattern recognition mechanisms of TLRs is very valuable for understanding innate immunity and some tumorigenesis mechanisms. Therefore, researchers used X-ray crystal diffraction to determine the crystallographic structure of the extracellular domain of TLRs and the ligand complex. Although the ligand complexes have different structures, all these complexes have similar M-type crystal structures (Fig. 2).50,51 TLR1 or TLR6 can form TLR1/TLR2 and TLR6/TLR2 heterodimers with TLR2 to recognize tri-acylated lipopeptide and di-acylated lipopeptide,57 respectively. After recognizing the appropriate ligands, TLR2 can form an M-type structure with the extracellular region of TLR1 and TLR6, and the pocket structure formed binds to the ligand.58,59,60,61 The crystal structure of TLR1–TLR2–tri-acylated lipopeptide complex is similar to that of TLR2–TLR6–di-acylated lipopeptide complex, but there are important structural differences between TLR1 and TLR6 in the ligand-binding site and dimerization surface. The ligand-binding pocket of TLR1–TLR2 is located in the interface between the central and C-terminal domain, and TLR1–TLR2–tri-acylated lipopeptide is stabilized by non-covalent bonds such as hydrogen bonds, hydrophobic interactions, and ionic interactions near the ligand-binding pocket.62 In TLR6, the side chains of amino acid residues block the ligand-binding pocket, resulting in a pocket less than half the length of TLR1. In addition, the TLR2–TLR6 heterodimer is mainly regulated by the surface exposed residues of the LRR11–14 module (Fig. 2a, b).63 Researches on the structure of ligand complexes can significantly promote the discovery of small molecule agonists/antagonists targeting PRRs. A recent study revealed the activation mechanism of atypical agonists for TLR1–TLR2. Diprovocim is a recently found small molecule activator for TLR1–TLR2, but it has no structural similarity with the tri-acylated lipopeptide complex. It also interacts with TLR1–TLR2 in the same binding pocket as typical lipopeptide ligand.64 Crystal structure analysis revealed that double-stranded RNA (dsRNA) binds to the LRR domains of the N-terminus and C-terminus of TLR3.65,66 Different from the way that other TLRs directly recognize ligands,67,68,69 TLR4 specifically recognizes LPS in combination with two auxiliary molecules, myeloid differentiation factor 2 (MD2) and the LRR structural protein CD14. LPS is transported by LPS-binding protein to CD14 on the cell membrane of monocytes and macrophages to form a complex and then interacts with TLR4/MD2.70 After LPS binds with the TLR4/MD2 complex, the hydrophobic pocket of MD2 is used to bridge the two TLR4–MD2–LPS complexes to form a spatially symmetrical M-type TLR4–MD2–LPS dimer,71 and then conformational changes affect their respective functional domains and transmit signals (Fig. 2c). In addition to binding to LPS, TLR4 is also involved in the recognition of natural products (carnosic acid, paclitaxel) and pneumolysin.72,73,74 TLR5 is the most conserved and important PRR, which is usually stimulated by bacterial flagellin. In the form of homodimer, TLR5 plays a major role in the primary defense of invasive pathogens and immune homeostasis regulation.75 

Although the heterodimeric structure of TLR5a–TLR5b in zebrafish and the crystallographic structure of TLR5–flagellin complex has been clearly reported, the lack of biochemical and structural information of fish TLR hinders the understanding of flagellin-based therapies. In the future, experimental TLR5–flagellin complex structure modeling and computational simulation should be used to study flagellin-mediated interactions between various pathogens and host immune receptors (Fig. 2d).76 It has been reported that TLR1–6 each exist as monomers in solution, and dimerization occurs only when the ligand is bound; in contrast, TLR8 and TLR9 exist as preformed dimers, and the binding of ligands induces conformational changes in preformed dimers (Fig. 1).49,77,78 Lee revealed that TLR10 binds dsRNA in vitro at endosomal pH, indicating that dsRNA is a ligand of TLR10. The recognition of dsRNA by TLR10 recruits MyD88, thereby transducing signals and inhibiting interferon regulatory factor 7 (IRF7)-dependent type I interferon (IFN) production.79 In mice, TLR11 and TLR12 are the main effector molecules to recognize Toxoplasma gondii. The recognition of T. gondii profilin by TLR11 depends on the parasite-specific, surface-exposed motif in TgPRF consisting of an acidic loop and a β-hairpin.80,81,82

TLR은

I형 막 횡단 당단백질이며,

세포외 영역, 막 횡단 영역, 세포내 영역으로 구성되어 있습니다.44

세포외 영역에는

특정 리간드를 인식하고

세포외 패턴 인식을 수행하는 류신-풍부 반복(LRR)이 포함되어 있습니다.

세포내 영역에는

신호 전달에 중요한 역할을 하는

IL-1R과 동일한 Toll/IL-1R(TIR) 영역이 포함되어 있습니다.

TLR의 세포외 영역에는 TLR의 패턴 인식을 매개하는 LRR이 포함되어 있습니다(그림 1).45 2007년, 연구자들은 X-선 결정 회절을 사용하여 TLR-리간드 복합체의 구조를 분석하고 결정했으며,46 이를 통해 LRR 도메인에 대한 더 깊은 이해를 할 수 있었습니다. LRR 도메인은 말굽 모양이며, 각 모듈은 보존된 류신 모티프와 가변 영역으로 구성되어 있습니다. “LxxLxLxxN(L 류신, x 임의의 아미노산, N 아스파라긴)” 모티프는 20-30개의 아미노산으로 구성되어 있으며, 말굽 모양의 구조의 오목한 표면에 있습니다.47,48,49,50 말굽 모양의 N-말단과 C-말단에는 소수성 코어를 보호하기 위해 시스테인 클러스터에 의해 형성된 이황화 결합이 포함되어 있습니다51,52. TLR이 해당 PAMP와 내인성 리간드를 인식하고 결합한 후, TIR 도메인은 세포질 영역의 다양한 수용체 어댑터 단백질에 결합하여 신호를 전달합니다.53,54 TIR 도메인에는 1,2,3 카세트라고 불리는 세 개의 보존된 아미노산 서열이 있습니다. 다양한 어댑터 단백질에 따라, TLR 신호 전달은 골수성 분화 인자 88(MyD88) 의존 경로와 MyD88 독립 경로로 나눌 수 있습니다(그림 1).55,56

TLR의 패턴 인식 메커니즘을 탐구하는 것은 선천성 면역과 일부 종양 발생 메커니즘을 이해하는 데 매우 유용합니다. 따라서 연구자들은 X-선 결정 회절을 사용하여 TLR의 세포외 영역과 리간드 복합체의 결정학적 구조를 결정했습니다. 리간드 복합체는 구조가 다르지만, 이 복합체들은 모두 유사한 M형 결정 구조를 가지고 있습니다(그림 2).50,51 TLR1 또는 TLR6은 TLR2와 TLR1/TLR2 및 TLR6/TLR2 이종 이합체를 형성하여 각각 트라이아실화 리포펩티드와 디아실화 리포펩티드를 인식할 수 있습니다57. 적절한 리간드를 인식한 후, TLR2는 TLR1과 TLR6의 세포외 영역과 M형 구조를 형성할 수 있으며, 형성된 포켓 구조는 리간드에 결합합니다. 58,59,60,61 TLR1-TLR2-트리아실화 리포펩티드 복합체의 결정 구조는 TLR2-TLR6-디아실화 리포펩티드 복합체의 결정 구조와 유사하지만, 리간드 결합 부위와 이합체화 표면에는 TLR1과 TLR6 사이에 중요한 구조적 차이가 있습니다. TLR1-TLR2의 리간드 결합 포켓은 중심과 C-말단 도메인 사이의 경계면에 위치하고 있으며, TLR1-TLR2-트리아실화 리포펩타이드는 리간드 결합 포켓 근처의 수소 결합, 소수성 상호작용, 이온 상호작용과 같은 비공유 결합에 의해 안정화됩니다. 62 TLR6에서 아미노산 잔기의 측쇄는 리간드 결합 포켓을 막아 TLR1의 절반보다 작은 포켓을 만들어 냅니다. 또한, TLR2-TLR6 이종이합체는 주로 LRR11-14 모듈의 표면 노출 잔기들에 의해 조절됩니다(그림 2a, b).63 리간드 복합체의 구조에 대한 연구는 PRR을 표적으로 하는 저분자 작용제/길항제의 발견을 크게 촉진할 수 있습니다. 최근의 한 연구는 TLR1-TLR2에 대한 비정형 작용제의 활성화 메커니즘을 밝혀냈습니다. Diprovocim은 최근 발견된 TLR1-TLR2를 활성화하는 작은 분자이지만, 트라이아실화 리포펩티드 복합체와 구조적 유사성이 없습니다. 또한 일반적인 리포펩티드 리간드와 동일한 결합 포켓에서 TLR1-TLR2와 상호 작용합니다.64 결정 구조 분석 결과, 이중 가닥 RNA(dsRNA)가 TLR3의 N-말단과 C-말단의 LRR 도메인에 결합하는 것으로 나타났습니다. 65,66 다른 TLR이 리간드를 직접 인식하는 방식과는 달리,67,68,69 TLR4는 두 개의 보조 분자인 골수성 분화 인자 2(MD2)와 LRR 구조 단백질 CD14와 함께 LPS를 특이적으로 인식합니다. LPS는 LPS 결합 단백질에 의해 단핵구와 대식세포의 세포막에 있는 CD14로 운반되어 복합체를 형성한 다음, TLR4/MD2와 상호 작용합니다. 70 LPS가 TLR4/MD2 복합체와 결합한 후, MD2의 소수성 포켓이 두 개의 TLR4-MD2-LPS 복합체를 연결하는 데 사용되어 공간적으로 대칭적인 M형 TLR4-MD2-LPS 이량체를 형성하고,71 그 후, 구조적 변화가 각각의 기능 영역에 영향을 미치고 신호를 전달합니다(그림 2c). LPS에 결합하는 것 외에도 TLR4는 천연물(카르노산, 파클리탁셀)과 뉴모라이신(pneumolysin)의 인식에도 관여합니다.72,73,74 TLR5는 가장 보존성이 높고 중요한 PRR이며, 보통 세균성 편모에 의해 자극됩니다. TLR5는 동종 이량체 형태로 침입 병원체의 1차 방어와 면역 항상성 조절에 중요한 역할을 합니다.75

제브라피시의 TLR5a-TLR5b 이종이합체 구조와 TLR5-플라겔린 복합체의 결정학적 구조가 명확하게 보고되었지만, 생화학 및 구조적 정보가 부족하여 플라겔린 기반 치료법에 대한 이해가 어렵습니다. 앞으로 실험적인 TLR5-플라젤린 복합 구조 모델링과 전산 시뮬레이션을 사용하여 다양한 병원체와 숙주 면역 수용체 사이의 플라젤린 매개 상호작용을 연구해야 합니다(그림 2d). 76 TLR1-6은 각각 용액에서 단량체로 존재하며, 리간드가 결합될 때만 이량체화가 발생한다고 보고되었습니다. 반면, TLR8과 TLR9는 미리 형성된 이량체로 존재하며, 리간드의 결합은 미리 형성된 이량체의 형태 변화를 유도합니다(그림 1). 49,77,78 Lee는 TLR10이 엔도솜 pH에서 dsRNA를 결합한다는 사실을 밝혀냈으며, 이는 dsRNA가 TLR10의 리간드임을 나타냅니다. TLR10에 의한 dsRNA의 인식은 MyD88을 모집하여 신호를 전달하고 인터페론 조절 인자 7(IRF7) 의존형 I형 인터페론(IFN) 생산을 억제합니다.79 생쥐에서 TLR11과 TLR12는 톡소플라즈마 곤디를 인식하는 주요 이펙터 분자입니다. TLR11에 의한 T. gondii 프로파일린의 인식은 TgPRF의 표면에 노출된 기생충 특이적 모티프에 의존하며, 이 모티프는 산성 루프와 β-헤어핀으로 구성되어 있습니다.80,81,82

Fig. 2

Crystal structure of TLRs with ligands. a Crystal structure of the TLR1–TLR2 heterodimer induced by binding of a tri-acylated lipopeptide (PDB 2Z7X). TLR2 initiates immune responses by recognizing di-acylated and tri-acylated lipopeptides. The ligand specificity of TLR2 is controlled by whether it heterodimerizes with TLR1 or TLR6. Binding of the tri-acylated lipopeptide (red) induced the formation of M-type crystal structures of the TLR1 (pale yellow) and TLR2 (slate) ectodomains. b Crystal structure of TLR2–TLR6–Pam2CSK4 complex (PDB 3A79). Binding of the di-acylated lipopeptide, Pam2CSK4 (red), induced the formation of M-type crystal structures of the TLR2 (slate) and TLR6 (pale green) ectodomains. c Crystal structure of mouse TLR4/MD2/LPS complex (PDB 3VQ2). After LPS (red) binds with the TLR4 (yellow)/MD2 (gray) complex, the hydrophobic pocket of MD2 is used to bridge the two TLR4–MD2–LPS complexes to form a spatially symmetrical M-type structure. Mouse TLR4/MD2/LPS exhibited an complex similar to the human TLR4/MD2/LPS complex. d Crystal structure of the N-terminal fragment of zebrafish TLR5 in complex with Salmonella flagellin (PDB 3V47). Two TLR5 (cyan)–flagellin (firebrick) 1:1 heterodimers assemble into a 2:2 tail-to-tail signaling complex to function

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NOD-like receptors

The growth cycle of some pathogenic microorganisms involves infection of the cytoplasm. For example, viral genes are often transcribed and translated in the cytoplasm, and virus particles are assembled. In addition, some bacteria and parasites have a series of escape mechanisms, such as making holes in the phagosome membrane and entering the cytoplasm. Therefore, pathogens and their components, as well as other components produced by infection and injury, will appear in the cytoplasm,83 which requires the recognition of PRRs in the body. NLRs are intracellular PRRs, composed of three domains:84,85 one is the central nucleotide-binding domain (NBD), also known as the NACHT domain (synthesized by the abbreviations of the following four kinds of NLR members: NAIP, CIITA, HETE, TP1), which is shared by the NLR family and is very important for nucleic acid binding and oligomerization of NLRs (Fig. 3); LRRs at the C-terminus, which are used to identify ligands; and the N-terminal effector domain, which is the protein interaction domain, such as the caspase activation and recruitment domain (CARD) or the pyrin domain (PYD).86,87,88,89 According to the different N-terminal effector domains, the NLRs family can be divided into five subfamilies: the NLRC subfamily, which contain CARDs; the NLRP subfamily, which contain PYDs; the NLRB subfamily, which contain baculovirus inhibitor of apoptosis protein repeats; the NLRA subfamily, which contain acidic activation domains; and the NLRX subfamily containing other NLR effector domains.85

NOD 수용체

일부 병원성 미생물의 성장 주기에는 세포질의 감염이 포함됩니다.

예를 들어,

바이러스 유전자는

종종 세포질에서 전사 및 번역되고, 바이러스 입자가 조립됩니다.

또한, 일부 박테리아와 기생충은

식세포막에 구멍을 뚫고 세포질로 들어가는 것과 같은

일련의 탈출 메커니즘을 가지고 있습니다.

따라서

병원체와 그 구성 요소, 그리고 감염과 부상으로 인해 생성된 다른 구성 요소들이 세포질에 나타나게 되는데,

이를 위해서는 체내에서 PRRs를 인식해야 합니다.

NLR은 세포 내 PRRs로, 세 개의 도메인으로 구성되어 있습니다.83,84 하나는 중심 뉴클레오티드 결합 도메인(NBD)으로, NACHT 도메인이라고도 합니다(다음 네 종류의 NLR 구성원의 약어로 합성됨). NLR 패밀리에서 공유되고, 핵산 결합과 NLR의 올리고머화에 매우 중요한 NAIP, CIITA, HETE, TP1); 리간드를 식별하는 데 사용되는 C-말단 LRR; 그리고 단백질 상호작용 도메인인 N-말단 이펙터 도메인(카스파제 활성화 및 모집 도메인(CARD) 또는 피린 도메인(PYD) 등). 86,87,88,89 N-말단 이펙터 도메인에 따라 NLR 패밀리는 다섯 개의 하위 패밀리로 나눌 수 있습니다: CARD를 포함하는 NLRC 하위 패밀리, PYD를 포함하는 NLRP 하위 패밀리, baculovirus inhibitor of apoptosis protein repeats를 포함하는 NLRB 하위 패밀리, 산성 활성화 도메인을 포함하는 NLRA 하위 패밀리, 그리고 다른 NLR 이펙터 도메인을 포함하는 NLRX 하위 패밀리. 85

Fig. 3

The ligand recognition mechanism of NLRs. The combination of PAMP and LRR changes the conformation of NLRs from self-inhibition to activation

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Among the NLRs family, the most in-depth study has focused on NOD1 and NOD2 proteins. NOD1 mainly recognizes the diaminopimelic acid (γ-D-glu-meso-diaminopimelic acid (iE-DAP)) of the cell wall of Gram-negative bacteria.90,91 In addition to recognizing muramyl dipeptide (MDP) in all bacterial cell walls, NOD2 can also recognize single-stranded RNA (ssRNA) of the virus, but it must be a complete viral ssRNA.92 The basic process of NOD2 activation and signal transduction is as follows: after pathogenic bacteria are phagocytosed by macrophages, they first form phagosomes, and then fuse with lysosomes to become phagolysosomes. Under the action of lysosomal enzymes, bacterial cell wall components are decomposed into peptidoglycan, which can be degraded into a cell wall peptide with immunomodulatory activity and enter the cytosol, thereby activating NOD2.93 In general, the LRR domain of the NLR molecule folds to form a U-shaped configuration with the central NACHT domain, which inhibits its multimerization and makes the NLRs inactive.94 Once PAMPs directly or indirectly bind to the LRRs, the NLR molecule change their conformation, exposing the NACHT oligomerization domain, which triggers oligomerization, and the NLR molecule is activated.95 At the same time, the N-terminal effector domain is exposed, and through homotypic interactions, downstream adaptor molecules and signaling proteins with the same structure are recruited to initiate the corresponding signal transduction (Fig. 3).96 Although NOD1 and NOD2 do not have transmembrane domains, studies have shown that they are recruited into the plasma membrane and endosomal membrane, which is necessary for signal transduction.97 In this process, palmitoylation plays a vital role. The modification of NOD1/2 protein under the action of palmitoyltransferase ZDHHC5, which makes NOD1/2, possess the characteristics of rapid and reversible localization changes, which is necessary for membrane recruitment and inflammatory signal transduction.98 This study gives us a good enlightenment that the modification of PRRs may play a key role in the regulation of host innate immune signal.

NLR 계열 중에서 가장 심도 있는 연구는 NOD1과 NOD2 단백질에 초점을 맞춘 것입니다. NOD1은 주로 그람 음성균의 세포벽에 있는 디아미노피멜산(γ-D-글루메소디아미노피멜산(iE-DAP))을 인식합니다. 90,91 NOD2는 모든 세균 세포벽에 존재하는 뮤라마이드 디펩티드(MDP)를 인식할 수 있을 뿐 아니라, 바이러스의 단일 가닥 RNA(ssRNA)도 인식할 수 있지만, 이때는 완전한 바이러스 ssRNA여야 합니다.92

NOD2 활성화와 신호 전달의 기본 과정은 다음과 같습니다:

병원성 세균이 대식세포에 의해 식균작용을 거친 후,

먼저 식세포체를 형성한 다음,

리소좀과 융합하여 식세포-리소좀 복합체를 형성합니다.

리소좀 효소의 작용으로

세균 세포벽 구성요소가 펩티도글리칸으로 분해되어

면역 조절 작용을 하는 세포벽 펩티드로 분해되어 세포질에 들어가

NOD2를 활성화합니다.93

일반적으로 NLR 분자의 LRR 도메인은 중앙의 NACHT 도메인과 함께 U자형 구조를 형성하여 다중화를 억제하고 NLR을 비활성화시킵니다. 94 PAMPs가 직접 또는 간접적으로 LRR에 결합하면, NLR 분자는 그 형태를 변화시켜 NACHT 올리고머화 도메인을 노출시킵니다. 이 도메인은 올리고머화를 촉발하고, NLR 분자는 활성화됩니다.95 동시에, N-말단 이펙터 도메인이 노출되고, 동형 상호작용을 통해 동일한 구조를 가진 다운스트림 어댑터 분자와 신호 전달 단백질이 모집되어 해당 신호 전달을 시작합니다(그림 3). 96 NOD1과 NOD2에는 트랜스막 도메인이 없지만, 연구에 따르면 이 두 가지가 신호 전달에 필요한 원형질막과 엔도소말막에 모집된다는 사실이 밝혀졌습니다.97 이 과정에서 팔미토일화가 중요한 역할을 합니다. NOD1/2 단백질의 팔미토일트랜스퍼라제 ZDHHC5의 작용에 의한 변형은 NOD1/2가 막 모집과 염증 신호 전달에 필요한 신속하고 가역적인 국소화 변화를 특징으로 합니다.98 이 연구는 PRR의 변형이 숙주 선천성 면역 신호의 조절에 중요한 역할을 할 수 있다는 것을 보여줍니다.

RIG-I-like receptors

RLRs are also intracellular PRRs. In innate antiviral immunity, in addition to the recognition of viral nucleic acids by TLR7 and TLR9, most other types of cells recognize viral nucleic acids through RLRs to induce antiviral immune responses.99,100 The currently discovered RLR family members mainly include three: RIG-I, melanoma differentiation-associated gene 5 (MDA5), and laboratory of genetics and physiology 2 (LGP2) (Fig. 4).101

Fig. 4

Structural features and ligand recognition mechanism of RLRs. The structure and functions of MDA5 are similar to those of RIG-I. However, MDA5 lacks the repressor domain, so it does not have self-inhibitory functions. LGP2 does not have CARD, and so it cannot transmit signals. The combination of viral RNA and CTD changes the conformation of RLRs

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RIG-I was first discovered in acute promyelocytic leukemia cells induced by retinoic acid. In 2004, it was found that RIG-I could induce the expression‎ of a reporter gene in the IFN-β promoter region, which confirmed its antiviral activity.102 The structure of the RIG-I protein consists of three parts.103,104,105 The middle part is the DexD/H helicase domain, which is the common domain of the RLR family, and has ATPase and helicase activities.106,107,108 The N-terminus of the RIG-I protein is composed of two caspase activation and recruitment domains in series,109 which are responsible for transmitting signals downstream.110 The C-terminus is composed of the repressor domain (RD) and the C-terminal domain (CTD), which can regulate its own state.106,111 The former can inhibit the activation of the receptor, and the latter is responsible for the recognition of viral RNA.112,113 In the resting state, CARD, CTD, and the helicase domain are folded, and RIG-I is in a self-inhibited state. During viral infection, the CTD of RIG-I recognizes viral RNA and undergoes a conformational change.114 RIG-I uses ATP hydrolase activity to expose and activate the CARD and multimerize, thereby recruiting downstream signaling linker molecules (Fig. 4).115,116,117

The structure and functions of MDA5 are similar to those of RIG-I, with the DexD/H helicase domain in the middle, two CARD at the N-terminus, and a CTD at the C-terminus; however, MDA5 lacks the RD, and so it does not have self-inhibitory functions. In contrast to other RLRs, LGP2 does not have CARD,118,119 and so it cannot recruit molecules of the same structure to transmit signals, but it can regulate the recognition of viral nucleic acids by RIG-I and MDA5, thereby preventing RLR-mediated resistance.120,121,122,123 LGP2 can negatively regulate RIG-I-mediated recognition of viral dsRNA, reduce the production of IFNs and inflammatory factors, and ultimately inhibit the antiviral innate immune response.124 LGP2 is also critical in the antiviral response mediated by MDA5.125 LGP2 exhibits a concentration-dependent conversion between MDA5-specific enhancement and interference.126 The latest research revealed a mechanistic basis for LGP2-mediated regulation of MDA5 antiviral innate immune responses. LGP2 facilitates MDA5 fiber assembly and is incorporated into the fibers, forming hetero-oligomers with MDA5.127 In addition, LGP2 can significantly induce the exposure of the CARD domain of MDA5.128 Under bacterial infection of the Indian major carp Labeo rohita, LGP2 gene expression‎ was significantly increased after dsRNA and various PAMPs were stimulated, indicating that LGP2 can act as an antiviral and antibacterial cytoplasmic receptor.129

Although the RLR family members have similar structures, they recognize the RNA of different viruses through ligand-recognition domains.130 Both RIG-I and MDA5 can recognize viral dsRNA, but their recognition depends on the length of the dsRNA.131 RIG-I mainly recognizes viruses with relatively short dsRNA (<1000 bp), while MDA5 tends to recognize long-chain dsRNA (>1000 bp).132 Additionally, RIG-I mediates the antiviral response by recognizing the 5’-triphosphate RNA of viruses.133 The 5’-terminal triphosphate group can be recognized by RIG-I as a non-self component, but after posttranslational modification, this molecule cannot be recognized by RIG-I.134 Because host cell RNA needs to undergo different degrees of processing and modification after synthesis in the nucleus, these results indicate that RIG-I can distinguish viral dsRNA from endogenous RNA. In the cell, RIG-I mainly recognizes influenza virus,135 vesicular stomatitis virus,136 Sendai virus, and Japanese encephalitis virus,137,138 while MDA5 mainly recognizes small RNA viruses, such as poliovirus.139,140 MDA5 also participates in the synthesis of the dsRNA analog polycytidylic acid (poly I:C). Previous studies have shown that filamentous fibers are formed during the recognition of ligands by RIG-I and MDA5, and signaling pathways are initiated from the tail and inside of the viral dsRNA, respectively.141

Although it is mentioned in the “Toll-like receptors” section that TLR3, TLR7, TLR8, and TLR9 specifically recognize virus-derived nucleic acid molecules and bacterial nuclear components, they mainly appear in the endosomal membrane. RLRs can not only be expressed in cells infected by various viruses but also can directly recognize and perceive the virus products and virus particles that exist in the cytosol. Its antiviral significance cannot be ignored.142

C-type lectin receptors

CLRs, which belong to phagocytic PRRs, are also a popular type of receptor under study.143 The function of phagocytic receptor is different from the receptor that activates cells by signal transduction. It recognizes and binds to PAMPs through PRRs and places pathogens in cytoplasmic vesicles for direct digestion and elimination to control infection.144 CLRs are a class of receptors that recognize carbohydrates on the surface of pathogenic microorganisms with the participation of Ca+.145 It is expressed on macrophages, dendritic cells (DCs), and certain tissue cells. The ability of CLRs to recognize carbohydrates existing on self and non-self structures is mediated by carbohydrate recognition domain (CRD).146 The CRD of CLRs is a compact spherical structure, and this region is called C-type lectin-like domain (CTLD).147,148 Depending on the location of the protein on the cell membrane, CLRs are divided into transmembrane receptors and secretory receptors.146,149,150 The main representative of secretory receptors is collagen lectin family (under the “Extracellular pattern recognition molecules”).151 Transmembrane receptors can be divided into type I and type II according to their topological structure.152,153 The N-terminal of type I receptors points to extracellular and contains multiple CRDs, while the N-terminal of type II receptors points to intracellular and contains only one CRD.154,155 It has been shown that the vast majority of CLRs are involved in the presentation of antigens as active membrane-associated receptors, and CLRs are mainly expressed on antigen-presenting cells such as DCs and macrophages.145 CLRs are circular structures connected by two disulfide bonds.156 CLRs contain at least one CTLD outside the cell, while the intracellular domain is different.

Mannose receptors (MRs) belong to membrane CLRs, which are single-chain transmembrane molecules.157,158,159 The extracellular segment of MR consists of two parts: one is the proximal membrane end with eight consecutive CTLDs, which is responsible for the endocytosis and transport of the ligand; the other is the distal membrane end of the cysteine-rich lectin domain, which recognizes sulfation of carbohydrate conjugates.160 The endogenous ligands of MR are lysosomal hydrolase and myeloperoxidase, as well as the mannan-rich structure expressed by pathogens.161,162

Dendritic cell-associated C-type lectin (Dectin)-1 and Dectin-2 are typical representatives of the CLR family.163,164 Dectin-1 is a type II transmembrane protein expressed in DCs, macrophages, neutrophils, and monocytes.165 The extracellular region is a CTLD. The intracellular tail is connected to an immunoreceptor tyrosine-based activation motif (ITAM),166 indicating that the receptor also has a signal transduction function. Dectin-1 can identify a variety of fungi,167 including yeast,168 Candida albicans,169,170 Pneumocystis carinii,171,172 Cryptococcus,173,174 and Aspergillus.175,176 The ligand of Dectin-1 is β-1,3-glucan, which can activate downstream signals through tyrosine kinase-dependent and tyrosine kinase-independent pathways after recognition and binding of the ligand.177,178 Glycosylation is an important modification of the posttranslational modification of proteins (including antibodies),179 which can significantly change the structure and function of proteins or antibodies, so it is also a key mechanism for the immune system to regulate biological activity.180 Abnormal glycosylation is usually associated with malignant tumors.179 Therefore, the identification of molecules that bind glycosylated glycans can provide a new way for the treatment of human infectious and malignant diseases. Studies have found that Dectin-1 can recognize aromatic amino acids adjacent to the N-terminal asparagine at the glycosylation site as well as the core fucose on IgG antibodies, which do not compete for the same protein binding site for β-glucan, so Dectin-1 can regulate the immune response induced by IgG by combining with core fucose.181

Dectin-2, which is different from Dectin-1, does not contain the ITAM sequence and has no signal transduction function.182 Dectin-2 mainly recognizes α-mannan in the fungal cell wall and recognizes the Schistosoma mansoni egg antigen.183,184 The molecular mechanism by which Dectin-2 recognizes the binding ligand has always been the focus of research. Decout et al.185 found that the stimulation of Dectin-2 by purified Mycobacterium tuberculosis mannose-capped lipoarabinomannan requires the (α1 → 2)-linked mannosides forming the cap. Besides, Dectin-2 can also recognize lipoglycans from other bacterial species.185,186 From the perspective of the relationship between the structure and function of the above two ligands, dimannoside caps and multivalent interaction are necessary for Dectin-2 to recognize binding ligands and conduct signals.187

AIM2-like receptors

ALRs are a new type of PRRs that can recognize intracellular DNA.188,189 The C-terminus is the DNA-binding domain HIN-200, and the N-terminus is the PYD.189,190,191,192 The HIN-200 domain recognizes double-stranded DNA and binds to it. The N-terminal PYD binds to the PYD of apoptosis-associated speck-like protein containing CARD (ASC),193,194 thereby promoting the formation of inflammasomes and the maturation and release of IL-1β and IL-18.195 Both the DNA-binding affinity of AIM2 and the activity of its inflammasome depend on dsDNA, and it can assemble into filamentous structures along dsDNA. However, without dsDNA, it can also form filaments at high protein concentrations.196,197,198 ALRs can not only participate in the innate immune response but also regulate apoptosis, which is related to the occurrence and development of tumors.199

Extracellular soluble pattern recognition molecules

The initiation of the innate immune response depends on the recognition of PAMPs by pattern recognition molecules (PRMs), including cell PRRs and extracellular soluble PRMs. They are a class of free receptors that can play an antibacterial effect in serum.200 Although the pattern recognition of innate immunity does not have the antigen specificity of the adaptive immune response, some PRMs produced by the body after infection by pathogenic microorganisms will exist in the serum. Once the new pathogens invade, they can also bind to the pathogen like an antibacterial molecule and play an effective function. Unlike cell-related PRRs, extracellular soluble PRMs are an important part of non-specific humoral immunity.201 Extracellular soluble PRMs are composed of different molecular families, mainly including pentraxin,202 collectin, and ficolin.203 They generally function in two ways: one is that they recognize various pathogenic factors and eliminate them through complement activation,204,205 opsonization,206 aggregation, and neutralization of inflammatory regulation; the other is that they interact with cell-related PRRs and regulate their functions to jointly regulate innate immune response.207

Pentraxin is characterized by the aggregation of five molecules and is highly conserved in evolution, including two families of short molecules and long molecules.208,209,210,211 The family of short molecules is called acute phase proteins, which is represented by C-reactive protein (CRP)212,213,214 and serum amyloid P component215,216 in humans and mice, respectively. These molecules are mainly produced by the liver under the stimulation of inflammatory signals and interleukins. They are non-specific proteins that reflect the systemic inflammatory response. Serum levels increase rapidly after the body is infected or injured. CRP generally binds to phosphocholine expressed on the surface of pathogenic microorganisms in a Ca+-dependent manner.217 SAA can bind to the outer membrane protein A of bacteria and interact with TLRs.218,219 In clinic, SAA and CRP are usually used as auxiliary diagnostic indicators for infectious diseases, but studies have shown that they also have diagnostic value in non-infectious diseases and can be used as disease classification markers.220,221 The representative of the pentraxin long molecule family is PTX3,222 which is unique in that it has a long N-terminal domain. PTX3 is produced by dendritic cells, monocyte macrophages, epithelial cells, smooth muscle cells (SMCs), and endothelial cells under the regulation of a variety of inflammatory factors.223 PTX3 is involved in the defense of selected pathogens and the regulation of inflammation.224,225,226 Due to its expression‎ increases sharply under the conditions of inflammatory stimulation, PTX3 can become a biomarker of general acute inflammation and a variety of tumors.227 In coronavirus disease 2019 (COVID-19) patients, circulating and lung bone marrow monocytes and endothelial cells express high levels of PTX3, and PTX3 plasma concentration can serve as an independent strong prognostic indicator of short-term mortality in COVID-19.228,229

Collectin mainly includes mannose-binding lectin (MBL) and surfactant protein (SP).151,230 MBL is formed by connecting multiple homotrimers. Each component of the trimer includes a CRD, an alpha helix, and a main stem formed by spirals of collagen.231,232 The main stem of collagen gathers each trimer into bundles. MBL is composed of six CRDs.151 The end of CRD can identify the sugar structure on the surface of various pathogens, such as mannose, fucose, glucose, etc.233,234,235 The pathogens involved include yeast, parasites, Gram bacteria, and so on.236,237,238,239,240 When the distance of each CRD between the same trimer or adjacent trimers is 45 Å, it is most conducive to ligand binding.241 The other family members include A and D,242 which exist on the surface of the alveoli and are important innate immune defense molecules in the lungs. Both of them are composed of N-terminal region, CRD, neck region, collagen-like region, and other parts.243 CRD recognizes and binds glycosyl groups. The biological significance is that they can selectively identify microbial carbohydrate structures that are harmful to themselves.244,245

The domain of ficolin is similar to collectin, but it recognizes a variety of bacteria with a fibrinogen-type carbohydrate recognition structure.246,247 Its ligands are N-acetylglucosamine and LTA, a cell wall component of Gram-positive bacteria.248,249

세포 외 가용성 패턴 인식 분자

면역 반응의 시작은 세포

PRR과 세포 외 가용성 PRM을 포함한 패턴 인식 분자(PRM)에 의한

PAMP의 인식에 달려 있습니다.

이들은 혈청에서

항균 효과를 발휘할 수 있는

일종의 유리 수용체입니다.200 선

천성 면역의 패턴 인식은

적응성 면역 반응의 항원 특이성을 갖지 않지만,

병원성 미생물에 감염된 후 체내에서 생성되는 일부 PRM은 혈청에 존재합니다.

일단 새로운 병원체가 침입하면,

항균 분자처럼 병원체에 결합하여

효과적인 기능을 수행할 수 있습니다.

세포 관련 PRR과는 달리,

세포 외 가용성 PRM은

비특이적 체액성 면역의 중요한 부분입니다.201

세포 외 가용성 PRM은

주로 펜트락신,202 콜렉틴, 피콜린을 포함한 다양한 분자 군으로 구성되어 있습니다. 203

일반적으로 두 가지 방식으로 작용합니다.

하나는 다양한 병원성 인자를 인식하고 보체 활성화,204,205 오프소화,206 응집, 염증 조절의 중화를 통해 제거하는 것이고,

다른 하나는 세포 관련 PRR과 상호 작용하고 그 기능을 조절하여 선천성 면역 반응을 공동으로 조절하는 것입니다.207

펜트락신은 5개의 분자가 모여서 형성되는 특징을 가지고 있으며, 진화 과정에서 매우 잘 보존되어 왔습니다. 여기에는 두 종류의 짧은 분자와 긴 분자가 포함됩니다.208,209,210,211 짧은 분자 계열은 급성기 단백질이라고 불리며, 인간과 생쥐의 경우 각각 C-반응성 단백질(CRP)212,213,214와 혈청 아밀로이드 P 성분215,216으로 대표됩니다. 이 분자들은 주로 염증 신호와 인터루킨의 자극을 받아 간에서 생성됩니다. 이들은 전신 염증 반응을 반영하는 비특이적 단백질입니다. 혈청 수치는 신체가 감염되거나 부상을 입은 후에 급격하게 증가합니다. CRP는 일반적으로 Ca+ 의존적 방식으로 병원성 미생물의 표면에 발현된 포스포콜린에 결합합니다.217 SAA는 박테리아의 외막 단백질 A에 결합하고 TLR과 상호 작용할 수 있습니다. 218,219 병원에서는 SAA와 CRP가 일반적으로 감염성 질환의 보조 진단 지표로 사용되지만, 연구 결과에 따르면 비감염성 질환에서도 진단적 가치가 있으며 질병 분류 표지로 사용될 수 있는 것으로 나타났습니다.220,221 펜트락신 긴 분자 계열의 대표적인 물질은 PTX3이며,222 N-말단 영역이 길다는 점에서 독특합니다. PTX3는 다양한 염증 인자의 조절 하에 수지상 세포, 단핵구 대식세포, 상피 세포, 평활근 세포(SMC), 내피 세포에 의해 생성됩니다.223 PTX3는 특정 병원체의 방어와 염증 조절에 관여합니다.224,225,226 염증 자극 조건 하에서 급격하게 증가하는 PTX3는 일반적인 급성 염증과 다양한 종양의 바이오마커가 될 수 있습니다. 227 코로나바이러스 감염증 2019(COVID-19) 환자에서 순환 및 폐 골수 단핵구와 내피 세포는 높은 수준의 PTX3를 발현하며, PTX3 혈장 농도는 COVID-19의 단기 사망률에 대한 독립적인 강력한 예후 지표가 될 수 있습니다.228,229

콜렉틴은 주로 만노스 결합 렉틴(MBL)과 계면활성단백질(SP)을 포함합니다.151,230 MBL은 여러 개의 동종 삼량체를 연결하여 형성됩니다. 삼량체의 각 구성 요소에는 CRD, 알파 나선, 콜라겐 나선으로 형성된 주 줄기가 포함됩니다.231,232 콜라겐의 주 줄기는 각 삼량체를 묶음으로 모읍니다. MBL은 6개의 CRD로 구성되어 있습니다.151 CRD의 끝부분은 다양한 병원체의 표면에 있는 당 구조를 식별할 수 있습니다. 예를 들어, 만노스, 푸코스, 글루코스 등입니다.233,234,235 관련된 병원체에는 효모, 기생충, 그람 음성균 등이 포함됩니다. 236,237,238,239,240 동일한 삼량체 또는 인접한 삼량체들 사이의 CRD들 사이의 거리가 45Å일 때, 리간드 결합에 가장 도움이 됩니다.241 다른 가족 구성원으로는 폐포 표면에 존재하며 폐에서 중요한 선천성 면역 방어 분자인 A와 D가 있습니다. 둘 다 N-말단부, CRD, 목부, 콜라겐 유사부, 그리고 다른 부분들로 구성되어 있습니다.243 CRD는 글리코실기를 인식하고 결합합니다. 생물학적으로 중요한 것은 이것들이 자신에게 해로운 미생물 탄수화물 구조를 선택적으로 식별할 수 있다는 것입니다.244,245

ficolin의 도메인은 collectin과 비슷하지만, 피브리노겐형 탄수화물 인식 구조를 통해 다양한 박테리아를 인식합니다.246,247 그 리간드는 N-아세틸글루코사민과 그람 양성균의 세포벽 구성 요소인 LTA입니다.248,249

Signaling pathways of PRRs

There are three main types of molecules involved in signal transduction: protein kinases, adaptor proteins, and transcription factors. Although PRRs are activated by their respective ligands in different subcellular structures with different mechanisms, the three main types of molecules involved in signal transduction have similar structures and functions, and the signals they transmit are cross-talking, which can converge into several common signaling pathways.

The NF-κB signaling

The transcription factor NF-κB is named after it was first discovered to be involved in the transcription of B cell κ chain genes.250 NF-κB is a heterodimer composed of two molecules, p50 and p65, and is inactive due to binding to the inhibitory protein IκB under normal conditions. NF-κB plays a key role in the process of cellular inflammation and immune response,251,252 and its mediated signal pathways are commonly seen in the activation of various immune cells, including signal transduction initiated by PRRs in innate immunity (Fig. 5).253

PRR의 신호 전달 경로

신호 전달에 관여하는 분자는 세 가지 주요 유형으로 나뉩니다:

단백질 키나아제, 어댑터 단백질, 전사 인자.

PRR은

서로 다른 메커니즘을 가진 서로 다른 세포 내 구조에서 각각의 리간드에 의해 활성화되지만,

신호 전달에 관여하는 세 가지 주요 유형의 분자는 유사한 구조와 기능을 가지고 있으며,

이들이 전달하는 신호는 교차 작용을 일으켜 여러 가지 공통적인 신호 전달 경로로 수렴될 수 있습니다.

NF-κB 신호 전달

전사 인자 NF-κB는 B 세포 κ 사슬 유전자의 전사에 관여하는 것으로 처음 발견된 이후로 그 이름을 얻게 되었습니다.250개의 NF-κB는 두 개의 분자, p50과 p65로 구성된 이종 이량체로, 정상적인 조건에서는 억제 단백질 IκB에 결합하여 비활성화됩니다.

NF-κB는

세포 염증과 면역 반응 과정에서 핵심적인 역할을 하며,251,252,

이 매개 신호 경로는 선천성 면역에서 PRRs에 의해 시작되는 신호 전달을 포함하여

다양한 면역 세포의 활성화에서 흔히 볼 수 있습니다(그림 5).253

Fig. 5

Pattern recognition receptor-mediated NF-κB signaling. The NF-κB protein can regulate gene expression‎ and affect various biological processes, including innate and adaptive immunity, inflammation, stress response, B cell development, and lymphoid organ formation. TLRs, NLRs, RLRs, and CLRs can generally phosphorylate IκB protein, which inhibits the activation of NF-κB protein, thereby promoting the transcription and activation of inflammatory genes

Full size image

In the signal transduction initiated by TLRs,45 after TLRs recognize and bind the corresponding PAMPs and DAMPs, the TIR domains conduct signals by binding to different receptor adaptor proteins in the cytoplasmic region.254,255 Depending on the different adaptor proteins, TLR signaling can be divided into MyD88-dependent and MyD88-independent pathways.256 MyD88 has a TIR domain at the C-terminus and a death domain at the N-terminus and is the linker molecule in most TLR signal transduction pathways.257 The current research indicated that, in the MyD88-dependent pathway, MyD88 signaling mainly leads to the production of pro-inflammatory cytokines, such as tumor necrosis factor (TNF), IL-6, IL-1, and chemokines.258,259,260 The C-terminus of MyD88 binds to the intracellular TIR domain of TLRs, and the N-terminus of MyD88 recruits IL-1R-related kinase 4 (IRAK4)261 and activates IRAK1 and IRAK2 through autophosphorylation of its central kinase domain. Then ubiquitin ligase TNF receptor-associated factor 6 (TRAF6) is recruited to form a complex with transforming growth factor (TGF)-β-activated kinase 1 (TAK1) and two TAK-binding proteins (TAB1 and TAB4). TRAF6 is degraded due to its own ubiquitination.262,263 The TAK1–TAB1–TAB4 complex activates the IκB kinase (IKK) complex through phosphorylation. The latter phosphorylates IκB and degrades itself by ubiquitination. NF-κB is released and translocated to the nucleus, thereby regulating the transcription of inflammatory genes.264,265

I

n the signal pathway mediated by NLRs, when the bacterial component invades the cell, NOD1 and NOD2 recognize the bacterial iE-DAP and MDP, respectively.266,267 And then NOD-like receptors are activated, self-dimerize, and recruit downstream receptor-interacting serine–threonine protein 2 (RIP2) through its CARD.268 Activated RIP2 gathers downstream TAK1, TAK1-binding protein 1, and the NF-κB essential modulator/IKKα/IKKβ complex, and the former activates IKKα/IKKβ,269 thereby activating the transcription of NF-κB and promoting the release of pro-inflammatory factors.

When virus invades cells, RIG-I and MDA5 recognize the corresponding viral RNA through the CTD and undergo conformation changes.270 Activated RIG-I and MDA-5 induce downstream signal transduction by binding with mitochondrial antiviral signaling protein (MAVS). MAVS is an important adaptor protein for downstream signal transduction. The N-terminus contains a CARD-like domain, which binds to RIG-I and MDA-5 through the CARD–CARD interaction.271,272 The proline-enriched domain in MAVS can interact with a series of downstream signaling molecules, such as TRAF3 and 6,273 and activate the protein kinase IKK, which causes phosphorylation of IκB,265 and then IκB is ubiquitinated and degraded by proteases, activating the NF-κB pathway.274

Different from other typical PRR-mediated signaling pathways, spleen tyrosine kinase (Syk) can be activated by associating with the phosphorylated ITAM motif of CLRs.275 In the Dectin-1/Syk pathway, Syk activates protein kinase C-δ, which mediates the phosphorylation of CARD9.276 This allows CARD9 to bind to B cell lymphoma 10277 and para-aspase mucosa-associated lymphoid tissue lymphoma translocation protein 1, forming a three molecular structure that can typically activate NF-κB.278

TLR에 의해 시작되는 신호 전달에서,

TLR이 해당 PAMP와 DAMP를 인식하고 결합한 후45,

TIR 도메인은 세포질 영역의 다양한 수용체 어댑터 단백질에 결합하여 신호를 전달합니다.254,255

다양한 어댑터 단백질에 따라,

TLR 신호 전달은 MyD88 의존 경로와 MyD88 독립 경로로 나눌 수 있습니다. 256 MyD88은 C-말단에 TIR 도메인과 N-말단에 death 도메인을 가지고 있으며, 대부분의 TLR 신호 전달 경로에서 링커 분자입니다.257 현재 연구에 따르면, MyD88 의존 경로에서 MyD88 신호는 주로 종양 괴사 인자(TNF), IL-6, IL-1, 케모카인과 같은 전염증성 사이토카인의 생성을 유도합니다.

258,259,260 MyD88의 C-말단은 TLR의 세포 내 TIR 도메인에 결합하고, MyD88의 N-말단은 IRAK4(IL-1R-related kinase 4)를 모집261하고, 중앙 키나제 도메인의 자가인산화를 통해 IRAK1과 IRAK2를 활성화합니다. 그런 다음, 유비퀴틴 리가제 TNF 수용체-관련 인자 6(TRAF6)이 모집되어 변형 성장 인자(TGF)-β 활성화 키나제 1(TAK1)과 두 개의 TAK 결합 단백질(TAB1 및 TAB4)과 복합체를 형성합니다. TRAF6은 자체적인 유비퀴틴화에 의해 분해됩니다.262,263 TAK1-TAB1-TAB4 복합체는 인산화 작용을 통해 IκB 키나아제(IKK) 복합체를 활성화시킵니다. IKK는 IκB를 인산화시키고, 유비퀴틴화에 의해 스스로 분해됩니다. NF-κB는 방출되어 핵으로 이동함으로써 염증 유전자의 전사를 조절합니다.264,265

NLR에 의해 매개되는 신호 전달 경로에서, 세균성 성분이 세포에 침입하면 NOD1과 NOD2가 각각 세균성 iE-DAP와 MDP를 인식합니다.266,267 그리고 나서 NOD-like 수용체가 활성화되고, 자가 이합체화되며, CARD를 통해 하류 수용체 상호작용 세린-트레오닌 단백질 2(RIP2)를 모집합니다. 268 활성화된 RIP2는 하류 TAK1, TAK1 결합 단백질 1, 그리고 NF-κB 필수 조절제/IKKα/IKKβ 복합체를 모은다. 그리고 전자는 IKKα/IKKβ를 활성화하여269, NF-κB의 전사를 활성화하고 전염증성 인자의 방출을 촉진한다.

바이러스가 세포에 침입하면, RIG-I와 MDA5는 CTD를 통해 해당 바이러스 RNA를 인식하고 형태 변화를 겪습니다.270 활성화된 RIG-I와 MDA-5는 미토콘드리아 항바이러스 신호 전달 단백질(MAVS)과 결합하여 다운스트림 신호 전달을 유도합니다. MAVS는 다운스트림 신호 전달을 위한 중요한 어댑터 단백질입니다. N-말단에는 CARD와 유사한 도메인이 포함되어 있으며, 이 도메인은 CARD-CARD 상호작용을 통해 RIG-I와 MDA-5에 결합합니다. 271,272 MAVS의 프로라인이 풍부한 도메인은 TRAF3 및 6과 같은 일련의 하류 신호 분자와 상호 작용할 수 있으며,273 단백질 키나아제 IKK를 활성화하여 IκB의 인산화를 유발하고,265 IκB는 유비퀴틴화되어 프로테아제에 의해 분해되어 NF-κB 경로를 활성화합니다.274

다른 전형적인 PRR 매개 신호 전달 경로와는 달리, 비장 티로신 키나아제(Syk)는 CLR의 인산화된 ITAM 모티프와 결합함으로써 활성화될 수 있습니다. 275 Dectin-1/Syk 경로에서, Syk는 CARD9의 인산화를 매개하는 단백질 키나아제 C-δ를 활성화합니다.276 이로 인해 CARD9는 B 세포 림프종 10277 및 파라-아스파제 점막 관련 림프 조직 림프종 전좌 단백질 1에 결합하여, 일반적으로 NF-κB를 활성화할 수 있는 3분자 구조를 형성합니다.278

The mitogen-activated protein kinase (MAPK) signaling

MAPK is a group of serine–threonine protein kinases that can be activated by different extracellular stimuli,279 such as cytokines, neurotransmitters, hormones, cell stress, and cell adhesion. The MAPK pathway is one of the common intersections of signal transduction pathways, such as cell proliferation, stress, inflammation, differentiation, functional synchronization, transformation, and apoptosis.280,281 It is an important transmitter of signals from the cell surface to the inside of the nucleus.

In the MyD88-dependent pathway of TLRs, IRAK-1 is activated by phosphorylation and interacts with TRAF6. In addition to activating the IKK complex, it can also cause the activation of MAPKs (c-Jun N-terminal kinase (JNK), p38 MAPK).282 In addition, when bacterial components invade cells, NLRs are activated, recruiting downstream CARD9, thereby activating p38, JNK, and finally activating the MAPK pathway283 to promote the release of pro-inflammatory factors.

미토겐 활성화 단백질 키나아제(MAPK) 신호 전달

MAPK는

사이토카인, 신경전달물질, 호르몬, 세포 스트레스, 세포 부착 등

다양한 세포외 자극에 의해 활성화될 수 있는

세린-트레오닌 단백질 키나아제 그룹입니다.

MAPK 경로는

세포 증식, 스트레스, 염증, 분화, 기능적 동기화, 변형, 세포 사멸과 같은

신호 전달 경로의 공통 교차점 중 하나입니다.280,281

세포 표면에서 핵 내부로 신호를 전달하는 중요한 전달자입니다.

MyD88 의존성 TLR 경로에서 IRAK-1은 인산화 작용에 의해 활성화되고 TRAF6과 상호 작용합니다. IKK 복합체의 활성화 외에도, MAPKs(c-Jun N-terminal kinase(JNK), p38 MAPK)의 활성화도 유발할 수 있습니다.282 또한, 세균성 성분이 세포에 침입하면 NLR이 활성화되어 다운스트림 CARD9를 모집함으로써 p38, JNK를 활성화하고, 마지막으로 MAPK 경로를 활성화하여 전염증성 인자의 방출을 촉진합니다283.

The TBK1–IRF-3 signaling

IRF-3 is a key transcription factor that promotes the synthesis of type I IFN and plays an important role in the antiviral innate immune response.284 IRF-3 can be activated through two innate immune antiviral signal pathways, TLR3/TLR4-TIR domain-containing adaptor protein-inducing interferon β (TRIF) and RIG-I-MAVS,285 and then dimerize and merge into the nucleus to work (Fig. 6).286

Fig. 6

Pattern recognition receptor-mediated TBK1-IRF-3 signaling. Intracellular induction of pathogens is carried out through the detection of foreign molecular components (including cytoplasmic viral and bacterial nucleic acids). Once detected, the innate immune system induces type I interferon (IFN) production through the TANK-binding kinase 1 (TBK1)-interferon regulatory factor-3/7 (IRF-3/7) pathway. IRF-3/7 can be activated through two innate immune antiviral signal pathways, TLR3/TLR4-TIR domain-containing adaptor protein-inducing interferon β (TRIF) and RIG-I-MAVS, and then dimerize and merge into the nucleus to work

Full size image

The adaptor protein in the MyD88-independent pathway is TRIF. The TRIF axis mainly induces the expression‎ of type I IFNs.287 After the receptor is recognized and combined with the ligand, the pathway is activated by TRIF and TRAF3, leading to the recruitment of IKKε/TANK-binding kinase 1 (TBK1),288 phosphorylation of IRF3, and the activation of type I IFN genes, which promotes the expression‎ of IFN-α and IFN-β, and exerts antiviral effects (Fig. 6).289,290,291

RLRs such as RIG-I and MDA5 can detect viral nucleic acid. MDA5 and RIG-I will interact with the shared caspase recruitment domain to induce MAVS to dimerize and bind to TRAF3.134,140,292 In turn, TRAF3 recruits the adaptor proteins TANK, NAP1, and SINTBAD. TANK connects upstream RLR signal transduction to TBK1, which induces phosphorylation of IRF-3. IRF-3 phosphorylation and subsequent dimerization induce IRF-3 nuclear translocation, leading to type I IFN gene expression‎ (Fig. 6).192,293,294

The inflammasome signaling

Inflammasome is the multi-protein complex assembled by PRRs in the cytoplasm and is an important part of the innate immune system.295 The inflammasome can recognize PAMPs or DAMPs and recruit and activate Caspase-1. The activated Caspase-1 spliced proIL-1β/proIL-18 into the corresponding mature cytokine.193,296 There are five main types of inflammasomes that have been discovered, namely, NLRP1 inflammasome,297 NLRP3 inflammasome,298 NLRC4 inflammasome,299,300 IPAF inflammasome, and AIM2 inflammasome.301 Known inflammasomes generally contain ASC, caspase protease, and a protein of the NLR family (e.g., NLRP3) or HIN-200 family protein (e.g., AIM2). Taking NLRP3 as an example,302 the dimerization of NLRP3 under the action of intracellular PAMPs or DAMPs makes the two PYDs to polymerize. With the help of homotype interaction, NLRP3 binds and activates the ASC complex with both PYD and CARD domains, which reactivates the effector complex composed of CARD and caspase-1. In this way, NLRP3 (LRR + NACHT + PYD), ASC (PYD + CARD), and the effector complex (CARD + Caspase-1) together constitute the inflammasome, which produces important pro-inflammatory factors.303,304,305 After AIM2 recognizes cytoplasmic dsDNA, it also uses inflammasomes to produce IL-1β and IL-18. After AIM2 recognizes cytoplasmic dsDNA, it also produces IL-1β and IL-18 through the inflammasome pathway (Fig. 7).306

인플라마솜 신호 전달

인플라마솜은

세포질에 있는 PRR에 의해 조립된 다중 단백질 복합체이며,

선천성 면역체계의 중요한 부분입니다.295

인플라마솜은

PAMP 또는 DAMP를 인식하고

카스파제-1을 모집 및 활성화할 수 있습니다.

활성화된 카스파제-1은 프로IL-1β/프로IL-18을 해당 성숙 사이토카인으로 스플라이스했습니다.193,296

지금까지 발견된 염증성 복합체의 주요 유형은 다섯 가지입니다.

NLRP1 염증성 복합체,297

NLRP3 염증성 복합체,298

NLRC4 염증성 복합체,299,300

IPAF 염증성 복합체,

AIM2 염증성 복합체입니다. 301

알려진 염증성 전구단백질은 일반적으로 ASC, 카스파제 프로테아제, NLR 패밀리(예: NLRP3) 또는 HIN-200 패밀리 단백질(예: AIM2)의 단백질을 포함합니다. NLRP3를 예로 들면,302 세포 내 PAMP 또는 DAMP의 작용으로 NLRP3의 이합체가 두 개의 PYD가 중합되도록 합니다. 동형 상호작용의 도움으로 NLRP3는 PYD와 CARD 도메인을 모두 가지고 ASC 복합체를 결합하고 활성화시킵니다. 이로 인해 CARD와 caspase-1로 구성된 이펙터 복합체가 재활성화됩니다. 이런 식으로 NLRP3(LRR+NACHT+PYD), ASC(PYD+CARD), 그리고 이펙터 복합체(CARD+Caspase-1)가 함께 중요한 전염증성 인자를 생성하는 인플라마솜을 구성합니다.303,304,305 AIM2가 세포질 dsDNA를 인식한 후, 인플라마솜을 사용하여 IL-1β와 IL-18을 생성합니다. AIM2가 세포질 dsDNA를 인식한 후에는, 인플라마솜 경로를 통해 IL-1β와 IL-18도 생성합니다(그림 7).306

Fig. 7

Pattern recognition receptor-mediated inflammasome signaling. One way for pathogenic microorganisms to induce inflammation is by activating inflammasomes, which are multi-protein complexes assembled by PRRs in the cytoplasm and activate caspase-1 and subsequent activation of pro-inflammatory cytokines IL-1β and IL-18. The inflammasome complex usually contains cytoplasmic PRRs, adaptor protein (ASC), and pro-caspase-1. Many different inflammasome complexes have been detected, each with unique PRRs and activation triggers

Full size image

Innate immunity not only plays a role in controlling the infection and spread of pathogens in the early stage of infection but also plays an important role in initiating and regulating adaptive immunity.12,307 Innate immune cells produce different types of cytokines through signal transduction initiated after PRRs recognize PAMPs, which directly affect the differentiation of T helper type 1 (Th1), Th2, Th17, and other subgroups in adaptive immunity.15 For example, pathogenic microorganisms activate macrophages to secrete IL-6, TGF-β, IL-23, and other cytokines, which promote Th17 response, leading to excessive immune-inflammatory effects and tissue damage, or activate NK cells to secrete IFN-γ, and then activate macrophages to secrete IL-12, promote the differentiation of Th0 into Th1, promote cellular immune response, and effectively eliminate viral infections.13,308 Therefore, the immune system is a system of mutual influence. Any anti-infection process is completed by mutually activating or inhibiting of different components. These components are as small as each cytokine and as large as the immune system.

선천성 면역은

감염 초기 단계에서 병원체의 감염과 확산을 제어하는 역할을 할 뿐만 아니라

적응성 면역의 시작과 조절에도 중요한 역할을 합니다.12,307

선천성 면역 세포는

PRR이 PAMP를 인식한 후 시작되는 신호 전달을 통해

다양한 유형의 사이토카인을 생성하며,

이는 적응성 면역에서 T 헬퍼 1형(Th1), Th2, Th17 및 기타 하위 그룹의 분화에 직접적인 영향을 미칩니다. 15

예를 들어,

병원성 미생물은

대식세포를 활성화시켜 IL-6, TGF-β, IL-23 및 기타 사이토카인을 분비하게 하고,

이 사이토카인들은 Th17 반응을 촉진하여

과도한 면역 염증 효과와 조직 손상을 유발하거나,

NK 세포를 활성화시켜 IFN-γ를 분비하게 하고,

그 다음 대식세포를 활성화시켜 IL-12를 분비하게 하고,

Th0의 Th1로의 분화를 촉진하고,

세포 면역 반응을 촉진하고,

바이러스 감염을 효과적으로 제거합니다. 13,308

따라서 면역 체계는 상호 영향을 미치는 시스템입니다. 모든 항감염 과정은 서로 다른 구성 요소의 상호 활성화 또는 억제에 의해 완료됩니다. 이러한 구성 요소는 각 사이토카인만큼 작을 수도 있고 면역 체계만큼 클 수도 있습니다.

PRR-related diseases

PRRs and cancers

The inflammatory microenvironment of tumor constitutes the barrier for tumor growth, which is conducive to tumor formation and development.309 PRRs are widely expressed in a variety of tumor tissues, such as colon cancer, lung cancer, breast cancer, gastric cancer, melanoma, and so on.310,311 The activation of PRRs on the surface of tumor cells can induce the expression‎ of a large number of cytokines, chemokines, hormones, and vascular-promoting factors, which is one of the important factors to induce the formation of tumor inflammatory microenvironment and promote the development of tumor.312,313 At the same time, the activation of PRRs on immune cells can induce antigen-presenting cells including DCs, tumor-associated macrophages, and B cells to activate tumor-specific T cell responses or enhance the antitumor effects of phagocytes. These also indicate that the role of PRRs in immunotherapy against tumors is very important and might represent a new strategy for patients with tumors.314,315

We can see from the data in GEPIA316 (http://gepia.cancer-pku.cn/) that TLR level is significantly increased in tumors, including glioblastoma multiforme, brain lower-grade glioma, kidney renal clear cell carcinoma, acute myeloid leukemia (AML), and pancreatic adenocarcinoma (PAAD). NOD1/2 are highly expressed in AML and PAAD, while RLRs are highly expressed in AML, PAAD, diffuse large B cell lymphoma, head and neck squamous cell carcinoma, and thymoma.

PRR 관련 질병

PRR과 암

종양의 염증성 미세환경은

종양 성장의 장벽을 구성하며,

이는 종양 형성 및 발달에 도움이 됩니다. 309

PRR은

대장암, 폐암, 유방암, 위암, 흑색종 등

다양한 종양 조직에서 광범위하게 발현됩니다.310,311

종양 세포 표면의 PRR 활성화는

다수의 사이토카인, 케모카인, 호르몬, 혈관 촉진 인자의 발현을 유도할 수 있으며,

이는 종양 염증성 미세환경의 형성을 유도하고

종양의 발달을 촉진하는 중요한 요인 중 하나입니다. 312,313

동시에 면역세포에서 PRR의 활성화는

DC, 종양 관련 대식세포, B 세포를 포함한 항원 제시 세포를 활성화시켜

종양 특이적 T 세포 반응을 유도하거나

식세포의 항암 효과를 강화할 수 있습니다.

이는 또한 종양 면역요법에서 PRR의 역할이 매우 중요하며,

종양 환자를 위한 새로운 전략이 될 수 있음을 보여줍니다.314,315

GEPIA316(http://gepia.cancer-pku.cn/)의 데이터에 따르면, 다형성 교모세포종, 뇌하수체 저급성 신경교종, 신장 투명세포암, 급성 골수성 백혈병(AML), 췌장 선암(PAAD)을 포함한 종양에서 TLR 수준이 상당히 증가하는 것을 알 수 있습니다. NOD1/2는 AML과 PAAD에서 높게 발현되는 반면, RLR은 AML, PAAD, 미만성 거대 B세포 림프종, 두경부 편평상피세포암, 흉선종에서 높게 발현됩니다.

Colorectal cancer (CRC)

CRC, including colon cancer and rectal cancer, is one of the most common gastrointestinal malignancies in clinical practice, and it is also one of the cancers that seriously endanger human health.317,318 Intestinal mucosal epithelial cells and immune cells recognize intestinal microorganisms and their products through TLRs.319,320,321,322 TLR2 can recognize peptidoglycans and lipopeptides that infect intestinal epithelial bacteria and produce anti-infection and other immune-protective effects.323 Studies have shown that the expression‎ level of TLR2 protein in colon cancer is significantly upregulated compared with normal epithelial tissues,324 and the use of TLR2 agonists significantly enhances the proliferation, migration, and invasion capabilities of colon cancer cells.325 TLR4 is highly expressed on the surface of colon cancer cells. After stimulation and activation, it can induce a variety of immunosuppressive factors, thus promoting the proliferation and immune escape of colon cancer cells.326 First, TLR4 can produce trophic factors and vascular growth factors through the TLR4/MyD88/NF-κB signaling pathway, thereby promoting tumor cell invasion.327,328 Second, TLR4 can promote tumor proliferation through TLR4/Cyclooxygenase 2 (COX2)/prostaglandin E2 (PGE2). PGE2 is an important cell growth and regulatory factor. After binding to specific receptors, it plays a key role in mediating a series of cell activities, such as cell proliferation, differentiation, and apoptosis, and has immunosuppressive and anti-inflammatory effects. COX2 is the rate-limiting enzyme of prostaglandin synthesis, and it is also highly expressed in inflammation, tumor, and other pathological states.329 Hsu et al.330 found that knocking out the mouse TLR4 gene significantly reduced the expression‎ of COX2 and PGE2 in the intestinal mucosa; after administration of PGE2, the expression‎ of COX2 in the intestinal mucosa increased significantly and promoted the occurrence of intestinal tumors. After administration of PGE2, the expression‎ of COX2 in the intestinal mucosa increased significantly and promoted the occurrence of intestinal tumors. At the same time, it has also been found to promote the expression‎ of amphiregulin and epidermal growth factor receptor (EGFR) in the intestinal mucosa. Finally, the study showed that the abnormal expression‎ of TLR4 in CRC caused by chronic inflammation of the intestine can significantly enhance the expression‎ of PGE2, the upregulation of COX2, and the phosphorylation of EGFR in intestinal mucosal cells, thereby positive feedback promotes the proliferation of tumor cells. TLR5 also plays an important role in tumor immunotherapy.331 In mouse xenograft models of human colon cancer, flagellin around the tumor activates TLR5 to inhibit tumor growth and promote tumor apoptosis.332 In addition, TLR9 is expressed on the surface of the mesentery, which maintains intestinal homeostasis and repairs intestinal damage by generating an immune response.333 TLR9 relies on the MyD88 pathway to induce downstream signals to recruit many inflammatory factors, such as IL-8, TGF-β, PGE2, and other immunosuppressive molecules,334 leading to the continuous development of inflammation, resulting in immune escape, and promoting the unlimited proliferation of tumor cells. After TLR9 recognizes the exogenous ligand, it upregulates the expression‎ of NF-κB signaling factor. Once this pathway is opened, it may induce the secretion of matrix metalloproteinase-13 (MMP-13) and the activation of intercellular adhesion molecule-1, thus promoting the metastasis of tumor cells.335,336 At the same time, the metastatic tumor cells are better adapted and combined with the cell matrix at the metastasis, and the stability of tumor cell metastasis is enhanced. Although TLRs have been shown to enhance colon cancer metastasis, inhibiting these receptors cannot completely hinder tumor progression. Surprisingly, NOD1 is highly expressed in human CRC and its cell lines. After being activated by C12-iE-DAP, it mainly enhances the adhesion, migration, and metastasis of CRC cells through the p38 MAPK pathway.337

Hepatocellular carcinoma (HCC)

HCC is the most common type of primary liver cancer. Among its many influencing factors, inflammation is one of the main reasons that induce liver cancer.338 The expression‎ of TLR2 in liver cancer tissues is significantly higher than that in normal liver tissues, and the expression‎ of TLR2 protein is related to some mutant genes that lead to the occurrence of HCC, such as p53, PIK3CA, and β-catenin.339,340,341,342 In addition, Chew et al. revealed that the expression‎ of TLR3 has independent effects on tumor parenchyma and infiltrating NK cells, and the expression‎ of these two parts is related to inhibiting tumor cell proliferation, promoting tumor cell death, and prolonging the survival rate of patients.343 This indicates that TLR3 may directly act on tumor parenchymal cells, promote the recruitment and activation of NK cells, and exert antitumor effects. More and more evidences show that LPS plays a role in the development of HCC. Zhou et al.344 found that LPS activates the TLR4–AKT–SOX2 signaling pathway of liver cancer cell lines to improve the ability of cancer stem cells; Lin et al.345 found that there is a positive feedback loop of COX-2/PGE2/signal transducer and activator of transcription factor 3 (STAT3) activated by LPS in liver cancer cells, which regulates the expression‎ of genes related to tumor proliferation, differentiation, and apoptosis. In the latest research on the treatment of liver cancer, it is found that the antitumor effect of TLR9 agonist combined with anti-PD-1 antibody or anti-PD-L1 is significantly better than single-agent therapy.346 The activation of TLR9 inherent in liver cancer cells regulates the autoarylation and ubiquitination of poly(ADP-ribose) polymerase-1 and the phosphorylation of STAT3, which together upregulate the expression‎ of PD-L1 and eventually induce immune escape. Although TLRs have been reported to be associated with chronic inflammation of the liver, whether they promote the development of HCC remains uncertain. Song et al.347 found that the deficiency of TLR4, TLR9, and their downstream molecule MyD88 in a mouse model characterized by hepatic deletion of TAK1 could block the liver inflammation–fibrosis–cancer axis and reduce liver injury and tumor growth. For TLR3, the downregulation of TLR3 in HCC patients leads to poor prognosis (e.g., defective immune cell recruitment and lack of killing of transformed hepatocytes), leading to protection of transformed hepatocytes from apoptosis, thereby promoting the occurrence of liver cancer.348 Therefore, the expression‎ of TLR3 may become a useful clinical treatment monitoring marker.

In addition to the above reasons, there is now more and more evidence that the imbalance of the gut–hepatic axis may also play a role in the occurrence of HCC.349 Zhou et al.350 discovered that NOD2 acts as a bacterial sensor, linking gut-derived microorganisms to the occurrence of HCC through a known mechanism and a newly discovered mechanism. The known mechanism is that NOD2 activates NF-κB, JAK2/STAT3, and MAPK pathways in a RIP2-dependent manner, leading to liver inflammation.351 It is worth noting that activated NOD2 can also act as the initiator of the nuclear autophagy pathway that does not depend on RIP2, thereby promoting the degradation of the nuclear component lamin A/C, leading to damage to DNA damage repair mechanisms and increased genomic instability, which eventually leads to the occurrence of HCC.350 Meanwhile, the study found that the expression‎ of ALRs was negatively correlated with tumor volume, stage, and metastasis of HCC patients. The researchers proposed that overexpression‎ of ALRs in HCC cells could increase the expression‎ of caspase-1 and IL-1a, and the release of lactate dehydrogenase was also observed,352 which was a marker of the initiation of apoptosis. Thus, ALRs may play an antitumor role by promoting tumor cell apoptosis.

Breast cancer

Breast cancer is one of the most common malignant tumors in the female population. It has a strong ability to invade and metastasize.353 It can metastasize to the liver, lung, brain, bone, and other organs, forming complications and increasing the difficulty of treatment. Studies have shown that the promotion of the TLR2 signaling pathway on the metastasis and invasion ability of human breast cancer cells is achieved by upregulating the secretion of inflammatory cytokines.354 LTA, a TLR2 specific ligand, can significantly promote the secretion of tumor metastasis-related factors IL-6, TGF-β, and vascular endothelial growth factor (VEGF) in breast cancer cells,355 thereby promoting the proliferation and metastatic invasion of breast cancer cells, and this promotion is related to the level of TLR2 expression‎.356,357 In addition, the activation of TLR4 can increase the secretion of IL-6 and IL-10 of cancer cells and induce the production of more MMP-2, MMP-9, and VEGF,358,359 which can significantly enhance the invasion ability of breast cancer. It has been reported that activation of TLR4 on metastatic breast cancer cells can regulate the expression‎ of integrin, which can promote its adhesion and invasion.360

Head and neck squamous cell carcinoma

Squamous cell carcinoma, also known as epidermal carcinoma, is a malignant tumor occurring in the epidermis or adnexal cells.361,362 It is more common in the parts covered by squamous epithelium, such as skin, mouth, lip, esophagus, cervix, vagina, etc.363,364 TLR2, TLR4, and TLR9 are expressed in primary tumors, neck metastases, and recurrent tumors of oral tongue squamous cell carcinoma (OTSCC), and their expression‎ varies from the tumor surface to the invasive front, which may be one of the important factors to promote the invasion of OTSCC.365,366 NOD1 and NOD2 genes are expressed in the human oral squamous cell carcinoma (OSCC) cell line YD-10B, and they may trigger immune responses through the MAPK pathway. Surprisingly, the study revealed that stimulation by the NOD2 agonist MDP can inhibit cell growth by inducing apoptosis. These findings provide the potential value of MDP as a new candidate for OSCC antitumor drugs.367

Respiratory diseases

Aspergillus fumigatus is a fungus widely distributed in nature. It can easily invade the respiratory tract and cause bronchitis and pneumonia in patients.368,369 For the allergic lung inflammation caused by Aspergillus, the recognition of PAMPs by the body’s dendritic cells is mainly negatively regulated through the TLR2-MyD88 pathway. The results showed that PAMPs recognized by TLR2 upregulated IL-10 and decreased the recruitment of pulmonary eosinophils, thus downregulating Th2 response.370,371 In allergic asthma, TLR9–IL-2 affects the Th2 response by regulating the expression‎ of IL-17A, so small molecule inhibitors targeting TLR9 may become a new treatment strategy.372

Nervous diseases

The connection between innate immunity and nervous system is becoming more and more complex and close.373,374,375 Studies have shown that NOD1/NOD2 may be a new target for the treatment of stress-related gut–brain diseases.376 The gut–brain axis is a biochemical signal of the digestive tract and central nervous system, which affects all events from brain development to the progression of neurological diseases. The hypothalamic–pituitary–adrenal axis (HPA) is one of the main pathways of gut–brain axis signal transmission.377 It has been reported that the immune system plays a key role in brain function and stress response. NLRs are PRRs expressed in the gut and brain. The lack of NOD1 and NOD2 affects the serotonergic signaling of gut and brain, the proliferation of hippocampal cells, and the maturation of neurons, which makes the mice lacking both of them vulnerable to HPA overactivation under stimulation, thus showing anxiety, cognitive impairment, and depression.378,379,380 TLR4 is very important in the process of neuropathic pain caused by infection and sterile neuronal injury. TLR4 exerts its effects through the activation and nuclear localization of NF-κB and the production of pro-inflammatory cytokines, which can activate pain receptors to cause neuropathic pain.381,382 Relevant studies have revealed for the first time that lysozyme acts as an endogenous ligand for activating TLR4 in sterile nerve injury, thereby promoting neuronal excitement and neuropathic pain.383 The identification of lysozyme as DAMPs has improved our understanding of neuroinflammation and opened up prospects for the treatment of neuropathic pain.

신경 질환

선천성 면역과 신경계의 관계는 점점 더 복잡해지고 밀접해지고 있습니다.373,374,375 연구에 따르면 NOD1/NOD2가 스트레스 관련 장-뇌 질환 치료의 새로운 표적이 될 수 있다고 합니다.376 장-뇌 축은 소화관과 중추 신경계의 생화학 신호로, 뇌 발달부터 신경 질환의 진행에 이르기까지 모든 사건에 영향을 미칩니다. 시상하부-뇌하수체-부신 축(HPA)은 장-뇌 축 신호 전달의 주요 경로 중 하나입니다.377 면역 체계가 뇌 기능과 스트레스 반응에 중요한 역할을 한다는 사실이 보고되었습니다. NLR은 장과 뇌에서 발현되는 PRR입니다. NOD1과 NOD2의 결핍은 장과 뇌의 세로토닌 신호 전달, 해마 세포의 증식, 뉴런의 성숙에 영향을 미치며, 이로 인해 두 가지 모두 결핍된 생쥐는 자극에 대한 HPA 과잉 활성화에 취약해져 불안, 인지 장애, 우울증을 보입니다.378,379,380 TLR4는 감염과 무균성 신경 손상으로 인한 신경병성 통증의 과정에서 매우 중요합니다. TLR4는 NF-κB의 활성화와 핵 내 위치 이동, 그리고 통증 수용체를 활성화하여 신경병성 통증을 유발할 수 있는 전염증성 사이토카인의 생성을 통해 그 효과를 발휘합니다.381,382 관련 연구에 따르면, 리소자임은 무균성 신경 손상 시 TLR4를 활성화하는 내인성 리간드 역할을 함으로써 신경 흥분과 신경병성 통증을 촉진한다는 사실이 처음으로 밝혀졌습니다. 383 라이소자임의 DAMP로서의 확인은 신경염에 대한 이해를 향상시켰고, 신경병성 통증의 치료에 대한 전망을 열어주었습니다.

Digestive diseases

Newborns with chronic obstructive jaundice make their livers prone to cholestatic liver disease, and biliary atresia (BA) accounts for half of the cases.384 Viruses have always been considered as the causative pathogen of this disease, and the role of TLRs in the pathogenesis and progression of BA has been determined.385 Subsequent studies have shown that activation of TLR7 can induce type 1 IFN signal transduction, apoptosis, and dysplasia of the neonatal liver and biliary system. This new discovery reveals the pathogenesis of neonatal cholestatic liver disease.386

The expression‎ of TLR5 is closely related to various infectious diseases caused by bacteria.387,388 The lack of TLR5 can cause changes in the intestinal flora and cause colitis.389 The protein–protein interaction between TLR5 and flagellin plays an important role in pathogen defense, immune diseases, and tumors. 4-((4-benzyl-5-(pyridin4yl)-4H-1,2,4-triazol-3-yl)thio)pyrido[3’,2’:4,5]thieno[3,2-d] Pyrimidine (TH1020) is a small molecule inhibitor identified through high-throughput screening, which can disrupt the association between TLR5 and flagellin, and provides a lead compound for new therapies against TLR5.390,391

Alcoholic liver disease is a liver disease caused by long-term heavy drinking. The initial stage usually manifests as fatty liver, which can then develop into alcoholic hepatitis, liver fibrosis, and cirrhosis.392,393 Patients with advanced alcoholic cirrhosis are more susceptible to infection.394 This phenomenon is related to multiple organ failure and immunodeficiency and is usually manifested as insufficient antibacterial activity of neutrophils.395 The neutrophil function to resist microbial infections needs to generate reactive oxygen species through NADPH oxidase 2. Rolas et al. found that, in patients with alcoholic liver cirrhosis, the lack of catalytic core flavocytochrome b558 (gp91phox, p22phox) and p47phox of the NADPH enzyme may be a new factor that patients are susceptible to infection. What is surprising is that the activation of TLR7/8 can reverse the expression‎ and activity of the deficient gp91phox, which provides a direction for restoring the antibacterial response of immunodeficiency patients.396,397

Cardiovascular diseases

Atherosclerosis is a chronic inflammatory disease caused by plaques composed of lipids, cholesterol, calcium, and other substances in blood vessels.398,399 It has been reported that TLRs are extensively and deeply involved in the process of atherosclerosis.400,401 TLR7 has been identified as a good prognosis marker for patients with severe atherosclerosis. TLR7 in the plaque produced in atherosclerotic lesions will secrete IL-10 and TNF-α after being stimulated by ligand. Studies have found that TLR7 may regulate inflammation in atherosclerosis by inhibiting the effects of pro-inflammatory cytokines.402 In addition to the production of plaque, cell-free DNA (cfDNA) is also released in atherosclerotic lesions. TLR9 recognizes cfDNA and plays a key role in the development of vascular inflammation and atherosclerosis by promoting the pro-inflammatory activation of macrophages.403 The formation of initial atherosclerotic plaques is caused by the interaction of macrophages and endothelial cells, macrophage infiltration, and other factors that lead to an increase in neointima.404,405 It is worth noting that studies have shown that TLR5-mediated activation of NADPH oxidase 4 (Nox4) can regulate the migration of SMCs and promote the expression‎ of pro-inflammatory cytokines, which may contribute to the formation of atherosclerotic plaques. This indicates that the flagellin–TLR5–Nox4 cascade is of great significance in atherosclerotic intimal hyperplasia.406,407 In addition, TLR2 activates p38 and extracellular signal-regulated kinase 1/2 signals, thereby upregulating IL-6-mediated receptor activator of NF-κB ligand and downregulating osteoprotegerin. These will make the cartilage formation of vascular SMC transdifferentiate, leading to vascular calcification.408 The expression‎ of TLR3 is reduced in the lung tissue and endothelial cells of patients with pulmonary hypertension, and its deficiency increases the susceptibility to apoptosis and pulmonary hypertension.409 In addition to TLRs, the lack of NOD1 and NOD2 can lead to lipid deposition of atherosclerotic plaques and the reduction of inflammatory cell infiltration, so it has been identified as pre-disease factors.410

Human brain pericytes (HBPs) are an important component of the microvascular wall and contribute to the integrity of the blood–brain barrier (BBB).411 It has been found that TLR4 and NOD1 are expressed in HBP, which also reveals that HBP has the ability to sense systemic infection or blood-borne PAMPs. HBP can perceive Gram-negative bacteria to protect BBB through different pathways mediated by TLR4 and NOD1, or it can trigger paracrine signaling pathways by releasing chemokines and cytokines, leading to the destruction of BBB.412,413 These are all new insights that PRRs are involved in the body’s inflammatory response and may have an impact on the treatment of diseases.

Endocrine diseases

In order to adapt to the constantly changing internal and external environment and maintain the relative stability of the internal environment, the human body must rely on the cooperation and regulation of the nervous, endocrine, and immune systems,414,415 so that the activities of various organs and systems are coordinated, and jointly shoulder all the life phenomena of the body. The immune system is not only regulated by the other two systems but also affects them through chemical information molecules and receptors.416,417

In patients with autoimmune thyroid disease, the expression‎ and activation of TLR2, 3, and 9 are significantly increased.418,419 In addition, it has been reported that TLR9 negatively regulates pancreatic islet development and β cell differentiation, providing new directions for diabetes prevention and treatment strategies.420 In diet-induced obesity, TLR2 and TLR4 inhibit the replication of β cells and affect the nuclear abundance of the cell cycle regulators cyclin D2 and Cdk4. Therefore, targeting TLR2–TLR4 may alleviate the failure of β cells in diabetic patients.421

One of the most common complications of diabetes is diabetic foot ulcers, and the reason why it is not easy to get better is the inflammation of the wound.422,423 Singh et al. believe that changes in the expression‎ level of intracellular TLRs may be one of the reasons for the continuous inflammation of chronic wounds, such as diabetic foot ulcers, and may hinder wound healing in patients with type 2 diabetes mellitus (T2DM).424 The basis of these pro-inflammatory effects is that intracellular TLRs activate dendritic cells and B cells to produce IFN I and III, aggravating the inflammatory response.425,426,427 The inflammatory phase of the wound healing cascade is the decisive stage of wound development, and studies have found that certain members of innate immunity play an important role in the pathogenesis of chronic wound healing abnormalities.428,429 The signaling pathway of intracellular TLRs is shown to be involved in some chronic inflammatory diseases, such as systemic lupus erythematosus (SLE), multiple sclerosis, hepatitis, and T2DM.430

Skeletal diseases

The expression‎ level of NOD2 in human osteoarthritis (OA) cartilage is significantly higher than that of normal cartilage, and its combined action with TLR2 contributes to the pro-catabolic gene expression‎ induced by 29-kDa amino terminal (matrix degradation product in synovial fluid of patients with OA) in human chondrocytes.431,432 This indicates that the NOD2 and TLR2 cross-regulatory pathway may be a target to prevent the development of arthritis. More and more evidences indicate that the interaction between different signal pathways may be a new way for the occurrence and development of diseases. The latest research combines three biological processes in skeletal muscle during exercise, innate immune response, autophagy protein homeostasis, and adenosine monophosphate-activated protein kinase (AMPK) activation, revealing that TLR9 can regulate the energy metabolism of skeletal muscle during exercise, and TLR9 regulates exercise-induced skeletal muscle AMPK activation by interacting with the core autophagy protein beclin1.433,434

Clinical therapy of PRRs

Based on the important role that PRRs play in innate immunity, they have received extensive attention in the fields of immunology and drug research.310,435 There are many types of PRRs and a wide range of ligands. They can be used as drug targets for tumor, inflammation, autoimmune disease, pathogenic microbial infection, and other diseases, which are important entry points for immunotherapy.314 The activation of PRRs brings double-sided effects: on the one hand, it stimulates innate immunity and adaptive immunity to resist pathogenic microorganisms; on the other hand, it promotes the expression‎ of a large number of cytokines, forms an inflammatory microenvironment, and causes tissue damage.29,436 Therefore, the treatment strategy targeting PRRs is mainly to use ligand analogs to activate PRRs,435 use antagonists to inhibit their activation, or use antibodies and small molecules to inhibit PRR signaling pathways.437 It is reported that most of the agonists and antagonists of TLRs are only in the clinical development stage, which are studied comprehensively in PRRs (Table 2). In addition, microRNA (miRNA), exosomes, and combination therapy also have certain potential in this field.438,439

Table 2 Clinical trials investigating the use of TLR agonists and antagonists in diseases

Full size table

PRR agonists for therapy

Imiquimod is the first targeted drug for TLRs. It is a specific agonist of the TLR7 receptor. It can induce the production of IFN-α, IL-6, and TNF-α to achieve the purpose of regulating immunity and treating tumors.440 Selgantolimod, a novel TLR8 agonist, can activate TLR8 and elicit cytokine responses in patients with chronic hepatitis B infection.441,442 Further studies with longer durations of selgantolimod treatment are required to eval‎uate efficacy. The ligand of TLR9 is unmethylated DNA, and CpG oligodinucleotide (CpG ODN) that mimics this structure has good antitumor potential (e.g., IMO-2055, MGN-1703, MGN-1704).443 The research of MGN-1703 on the treatment of advanced CRC is in phase III. MGN-1703 can induce a potent type I IFN response in the intestine, which is related to the subtle changes of intestinal flora.444 Clinically, PRR agonists can be potentially used not only as therapeutic agents to treat but also as adjuvants in conjunction with other immunotherapies.445,446 Due to the instability and drug resistance of existing drugs, some adjuvants targeting PRRs emerge as the times require. A TLR3-specific adjuvant, ARNAX, can enhance DC priming and CTL proliferation without cytokine toxicity.447 And the conjugated STING and TLR1/2 agonist Pam3CSK4-CDGSF can be used as an effective adjuvant for constructing vaccines to enhance antitumor immunotherapy.448 NLR agonists can promote the processing and secretion of IL-1β, which is very important for activating many immune cells.449,450 Therefore, NLR agonists can be served as effective components of vaccines and immune stimulants.

The class II major histocompatibility complex transactivator (CIITA) is a single member of the NLRA subfamily.451 As the main regulator of major histocompatibility complex II, its own modification is very important in the occurrence and development of tumors. Both the deacetylation of histones and the demethylation of DNA will downregulate the transcription of CIITA.452 Decitabine (5-aza-2’-deoxycytidine), Entinostat (MS-275), and Trichostatin A target CIITA to promote its expression‎ recovery in tumors. In addition, andrographolide, arglabin, formononetin, mangiferin, and other botanicals can be used to treat tumors by inhibiting NLRP3 inflammasome.453,454,455,456,457 Representative NLR agonists like Mifamurtide have been approved for clinical trials of non-metastatic sarcoma.

RLRs are involved in the recognition of viral infection by the innate immune system, and their agonists include poly I:C.99 Studies have revealed the link between energy metabolism and innate immunity, indicating that lactate may act as a natural inhibitor of RLR signal transduction by targeting the adaptor protein MAVS.458 The latest research shows that activating RIG-I before diffusing alpha-emitting radiation therapy for metastatic and low immunogenic tumors is a more effective combination therapy technique that can inhibit tumor growth and metastasis.459

PRR antagonists for therapy

PRR antagonists belong to immunosuppressants, which are mainly used to inhibit the function of receptors and block the connection between receptors and ligands.460 They can be used to treat diseases with abnormal activation of the immune system. The abnormal activation of TLR7, 8, and 9 contributes to the onset and maintenance of inflammatory diseases.461,462 The inhibitors IMO-3100 and IMO-8400 targeting them can significantly reduce the expression‎ of IL-17A induced by IL-23, which has a potential role in the inflammatory cascade.463,464 In addition, inhibition of TLR7 and TLR9 with IRS-954 or chloroquine may be a new treatment for HCC.465 In addition to the immune inhibitory oligonucleotides that can be used as TLR inhibitors, CPG-52364, a derivative of the small molecular weight chemical compound quinazoline, can also block ligand-induced activation of TLR7, 8, and 9.462 FP7 is a synthetic glycolipid, which can be used as TLR4 inhibitor to treat septic shock.466 Studies have found that inhibitory ODN-containing phosphorothioate modifications of the backbone have potential therapeutic effects for SLE and rheumatoid arthritis. ODN1411 directly binds to the extracellular domain of TLR8 and competitively inhibits its signal transduction.467

In the NLR family, the NLRP3 inflammasome is closely related to the pathophysiology of a variety of inflammatory diseases, so it is usually used as an inhibitor target.298,304,305 MCC950 is a specific NLRP3 antagonist, which can prevent its activation or maintain its activation state.468 β-Carotene has been shown to directly bind to the PYD of NLRP3, thereby blocking the association between NLRP3 and its adaptor protein and ultimately inhibiting the activation of NLRP3 inflammasomes.469 This result further complements the new pharmacological strategy to prevent NLRP3 inflammasome-driven gouty arthritis. The development of small molecule inhibitors for NLRP3 has been in full swing.470,471 Dai et al. reported a series of tetrahydroquinoline inhibitors. In further studies in vitro, it was found that compound 6 directly binds to the NACHT domain of NLRP3 but not to the PYD or LRR domains, which inhibits the assembly and activation of NLRP3 inflammasomes. In vivo, compound 6 can significantly inhibit the expression‎ of IL-1β and alleviate the symptoms of colitis.472 Agarwal et al. identified alkenyl sulfonylurea derivatives as a novel, oral, and biologically effective NLRP3 inhibitor. Compound 7 of alkenyl sulfonylurea derivatives has good pharmacokinetic characteristics and high oral bioavailability.473

Other potential therapies via PRRs

Emerging evidence suggests that the interaction between miRNA and PRR pathway and immunoregulation are intimately interwoven.438 The interaction between the two is particularly significant in the pathogenesis of rheumatoid arthritis (RA).474,475 RA is a common autoimmune disease, which is characterized by synovial hyperplasia and irreversible bone destruction caused by chronic synovitis. Rheumatoid arthritis synovial fibroblast (RASF) plays a central role in this process.476,477,478 The expression‎ of miR-146a and miR-155 is upregulated in RASF. miR-146a inhibits the expression‎ of TRAF-6 and IRAK-1 in the TLR signaling pathway, thereby inhibiting the production of key adaptor molecules downstream of TLRs in RA.479 And miR-155 negatively regulates the activation of TLRs/IL-1R inflammatory pathway. Both of them will eventually reduce RA inflammation.480 In addition, miR19a/b, miR-20a, and miR-10a are downregulated in RASF. Downregulated miR19a/b directly targets TLR2 to increase its expression‎481; miR-20a inhibits the expression‎ of ASK1, a key component of the TLR4 pathway482; miR-10a targets IRAK4 and TAK1 to accelerate IκB degradation and NF-κB activation.483 These effects promote the inflammatory response of RA. All these evidences indicate that miRNAs can surpass their conventional functions and act as physiological ligands for TLRs, thereby regulating the expression‎ of cytokines in the inflammatory response. miRNA can also target other PRRs. miR-485 can target RIG-I mRNA for degradation, leading to enhancement of virus replication and inhibition of antiviral response.484 miRNA-155 can aggravate the brain tissue inflammation in neonatal rats with hypoxia and ischemia by regulating the NOD1/NF-κB signaling pathway, thereby causing hypoxia and ischemic brain damage in neonatal rats.485,486

In addition, exosomes are also involved in the complex immune regulatory network of miRNA and PRRs.487 Exosomes are small membrane vesicles containing complex RNA and protein. A variety of cells can secrete exosomes under normal and pathological conditions.488,489 There is evidence that exosomes can regulate innate immunity by combining with TLRs, which provides a new method for the body to resist pathogens and treat diseases.487 Brain exosomes are rich in amyloid-beta (Aβ), the main component of neuritic plaques, which can activate TLR2, TLR4, and TLR9 signaling, thus alleviating the early symptoms of Alzheimer’s disease.490 It was found that the expression‎ of miR-216a-5p in the plasma of patients with syphilis is negatively correlated with the expression‎ of inflammatory cytokines. Exosomes containing miR-216a-5p inhibit the inflammatory response induced by recombinant Tp17 by targeting the TLR4–MyD88 pathway.491 Melphalan, a genotoxic agent, can enhance the ability of multiple myeloma (MM) cells to release exosomes in MM. Hsp70 on the surface of MM-derived exosomes activates TLR2 in NK cells, thereby inducing the production of INFγ.492 This enlightens us that exosomes with significant exposure to DAMPs can link chemotherapy with antitumor innate immune response.

However, whether they are therapeutic agents, adjuvants, or others, it still needs a long time to further improve the research in vitro and in vivo before clinical application in tumor treatment.493

Conclusions and future perspectives

The innate immune system has multiple recognition mechanisms in different cell types with different subcellular structures. Recently, great progress has been made in the study of the interaction between PRRs and ligands. A preliminary understanding of the crystal structure of some PRRs and the commonalities and differences in the structural composition of different types of PRRs has been made. We understand the existing forms of PRRs alone in solution and after binding to ligands, as well as the molecules involved in the process of PRR identification and ligand binding. As the initiation step of the innate immune response and antiviral response, PRR recognition and binding of ligands is critical. PRRs recognize targets through their respective ligand-recognition domains: LRRs, CTDs, CTLDs, and a variety of DNA- or RNA-binding domains. Ligand binding induces activation and transmission of the signal, which is transmitted through cascade amplification by their respective domains—TIR domain, protein interaction domain, CARD, ITAM, and PYD. In some cases, one kind of PRRs can recognize multiple PAMPs, and one kind of PAMPs can also be recognized by different PRRs, which can induce an inflammatory response and adaptive immune response in a synergistic manner.494

PRRs that are difficult to find and identify and complex ligand-recognition mechanisms have hindered the study of the role of the innate immune system in related diseases and even tumors. In previous studies, computational modeling and mutational analysis of human TLR10 showed that the structure of TLR10 was modeled with the explicit inclusion of TLR2 and lipopeptide structures.495 Recently, RRGPredictor, a tool mainly based on text mining and set theory to predict new plant PRRs, makes the identification of PRRs more effective, specific, and sensitive than other available tools.496 And a random forest-based method was proposed to identify PRRs, which is superior to other machine learning methods for PRRs. This method constructs a benchmark database, uses the amino acid composition and the composition transition distribution to formulate the sequences in the dataset, and then uses the maximum relevance maximum distance to select the best features. However, due to the small amount of data and the low sensitivity of the method, the identification results for PRRs are still unsatisfactory.497 Therefore, the ability to develop a vertebrate PRR predictor based on protein domain similarity and homology modeling is becoming an important part of this story. In addition, the future studies on the how membrane PRRs are secreted into body fluids and act as soluble PRRs will undoubtedly further our understandings of their roles in antimicrobial immune defense and autoimmune and autoinflammatory disorders. Therefore, the study of PRR-mediated pattern recognition mechanisms will help to elucidate the signaling pathways and mechanisms of disease, provide new targets and methods for the treatment of diseases, and promote the development of immunology and oncology research and improvements in their theoretical systems.

결론과 미래 전망

선천성 면역 체계는

서로 다른 세포 유형과 세포 내 구조를 가진

여러 가지 인식 메커니즘을 가지고 있습니다.

최근

PRR과 리간드 사이의 상호 작용에 대한 연구가

크게 진전되었습니다.

일부 PRR의 결정 구조에 대한 예비적 이해와 다양한 유형의 PRR의 구조적 구성의 공통점과 차이점에 대한 이해가 이루어졌습니다.

우리는 PRR의 기존 형태를 이해하고,

리간드에 결합한 후,

PRR 식별 및 리간드 결합 과정에 관여하는 분자를 이해합니다.

선천성 면역 반응과 항바이러스 반응의 시작 단계로서,

PRR 인식과 리간드 결합은 매우 중요합니다.

PRR은

각각의 리간드 인식 도메인(LRR, CTD, CTLD, 그리고 다양한 DNA 또는 RNA 결합 도메인)을 통해

표적을 인식합니다.

리간드 결합은

활성화와 신호 전달을 유도하며,

이 신호는 각각의 도메인(TIR 도메인, 단백질 상호작용 도메인, CARD, ITAM, PYD)을 통해

캐스케이드 증폭을 통해 전달됩니다.

어떤 경우에는, 한 종류의 PRR이 여러 종류의 PAMP를 인식할 수 있고, 한 종류의 PAMP가 다른 PRR에 의해 인식될 수도 있습니다. 이 경우, 시너지 효과를 통해 염증 반응과 적응성 면역 반응을 유도할 수 있습니다.494

발견과 식별이 어려운 PRR과 복잡한 리간드 인식 메커니즘은

관련 질병과 심지어 종양에서 선천성 면역체계의 역할에 대한 연구를 방해해 왔습니다.

이전 연구에서 인간 TLR10의 전산 모델링과 돌연변이 분석은 TLR10의 구조가 TLR2와 리포펩티드 구조를 명시적으로 포함하는 것으로 모델링되었음을 보여주었습니다. 495

최근에, 새로운 식물 PRR을 예측하기 위해 주로 텍스트 마이닝과 집합 이론에 기반을 둔 도구인 RRGPredictor는 PRR을 식별하는 작업을 다른 이용 가능한 도구들보다 더 효과적이고, 구체적이며, 민감하게 만들어 줍니다.496

그리고 PRR을 식별하기 위해 랜덤 포레스트 기반의 방법이 제안되었는데, 이 방법은 PRR을 위한 다른 기계 학습 방법보다 우수합니다. 이 방법은 벤치마크 데이터베이스를 구축하고, 아미노산 구성과 구성 전이 분포를 사용하여 데이터 세트의 시퀀스를 공식화한 다음, 최대 관련성 최대 거리를 사용하여 최상의 특징을 선택합니다.

그러나 데이터의 양이 적고 이 방법의 민감도가 낮기 때문에 PRR의 식별 결과는 여전히 만족스럽지 않습니다.497 따라서 단백질 도메인 유사성과 상동 모델링을 기반으로 척추동물 PRR 예측기를 개발하는 능력이 이 이야기에서 중요한 부분이 되고 있습니다. 또한, 멤브레인 PRR이 체액으로 분비되어 수용성 PRR로 작용하는 방법에 대한 향후 연구는 항균 면역 방어 및 자가면역 및 자가염증성 질환에서 PRR의 역할에 대한 이해를 더욱 증진시킬 것입니다.

따라서, PRR 매개 패턴 인식 메커니즘에 대한 연구는 질병의 신호 전달 경로와 메커니즘을 밝히고, 질병 치료를 위한 새로운 표적과 방법을 제공하며, 면역학 및 종양학 연구의 발전과 이론적 체계의 개선을 촉진하는 데 도움이 될 것입니다.

References

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