Molecular Sensing of Chitin by Toll-like Receptors

真菌感染和过敏性哮喘导致全球数百万患者死亡和痛苦，与模式识别受体(PRR)介导的免疫控制密切相关。Dectin-1是微生物相关分子模式(MAMP)β-1，3-葡聚糖的细胞表面PRR，是真菌中的主要结构细胞壁成分，可触发真菌病原体的吞噬和保护性炎症反应。然而，几丁质是真菌细胞壁和尘螨变应原中的一种β-1，4-N-乙酰-氨基葡萄糖(NAG)成分，其PRR仍然存在争议，到目前为止还没有直接的结合研究发表。已知TLR2通过识别与TLR6或TLR1形成的复合体中的二酰化和三酰化脂肽来启动有效的免疫应答。尽管到目前为止还没有发现真菌TLR2配体，但TLR2已被强烈地与患有主要病原体曲霉和念珠菌以及过敏性哮喘的患者和人类的小鼠感染模型有关。我们现在已经确定TLR2是直接与甲壳素结合并发出信号的PRR。我们观察到，尽管含有6个以上NAG的几丁质链可以有效地激活各种人和小鼠的免疫细胞，但只含有4或5个NAG的片段在免疫上是无效的，但具有拮抗活性。然而，几丁质NAG通过TLR2激化或拮抗的机制基础和免疫学意义仍然难以捉摸，我们将在此加以阐述。此外，目前尚不清楚TLR2脂肽和几丁质识别如何在TLR1、TLR6或潜在的TLR10、共受体的使用以及刺激或拮抗的免疫结果方面显示出不同和/或相似之处。由于甲壳素是自然界中含量第二丰富的多糖，是与多种人类疾病相关的免疫激活成分，影响适应性免疫反应的Th1/Th2偏斜，是一种潜在的新型疫苗佐剂，因此解开这些甲壳素识别、信号和免疫调节的原理具有很高的基础和治疗意义。

Fungal infections and allergic asthma cause death and suffering in millions of patients worldwide and critically involve pattern recognition receptors (PRR)-mediated immune control. Dectin-1 is a cell-surface PRR for the microbe-associated molecular pattern (MAMP) beta-1,3-glucan, which is a major structural cell wall component in fungi and triggers phagocytosis of fungal pathogens and protective inflammatory responses. However, the PRR for chitin, a beta-1,4-N-acetyl-glucosamine (NAG) component of fungal cell walls and house dust mite allergen has remained controversial and so far no direct binding studies have been published. TLR2 is known to initiate potent immune responses by recognizing di- and triacylated lipopeptides in complex with TLR6 or TLR1, respectively. Although no fungal TLR2 ligand has been identified to date, TLR2 has been strongly implicated in murine infection models of and human patients suffering from the major pathogens Aspergillus and Candida, as well as allergic asthma. We have now identified TLR2 as the PRR directly binding and signaling to chitin. We observe that, whereas chitin chains containing more than 6 NAGs potently activate various human and murine immune cells, fragments containing only 4 or 5 NAGs are immunologically inactive but possess antagonistic activity. However, the mechanistic basis and immunological significance for agonism or antagonism of chitin NAGs via TLR2 remains elusive and is to be addressed here. Additionally, it remains unclear how TLR2 lipopeptide and chitin recognition may show differences and/or similarities in TLR1, TLR6, or potentially TLR10, co-receptor usage and the immunological outcomes of stimulation or antagonism. Since chitin represents the second most abundant polysaccharide in nature, is an immuno-activatory component linked to multiple human disease, affects Th1/Th2 skewing of adaptive immune responses and represents a potential novel vaccine adjuvant unraveling these principles of chitin recognition, signaling and immuno-regulation is of high basic and therapeutic interest.