TRIF signalling drives homeostatic intestinal epithelial antimicrobial peptide expression. TRIF drives homeostatic signalling in IECs

Recent results indicate a significant contribution of innate immune signalling to maintain mucosal homeostasis but the precise underlying signal transduction pathways are ill-defined. By comparative analysis of intestinal epithelial cells isolated from conventionally raised and germ-free mice as well as animals deficient in the adaptor molecules MyD88 and TRIF, the Toll-like receptors (TLR) 3 and 4, as well as the type I and III interferon (IFN) receptors, we demonstrate significant TLR-mediated signalling under homeostatic conditions. Surprisingly, homeostatic expression of Reg3γ and Paneth cell enteric antimicrobial peptides critically relied on TRIF and in part TLR3 but was independent of IFN receptor signalling. Reduced antimicrobial peptide expression was associated with signicantly lower numbers of Paneth cells and a reduced Paneth cell maturation and differentiation factor expression in TRIF mutant compared to wildtype epithelium. This phenotype was not transferred to TRIF sufficient germ-free animals during cohousing. Low antimicrobial peptide expression in TRIF deficient mice caused reduced immediate killing of orally administered bacteria but was not associated with significant alterations in the overall composition of the enteric microbiota. The phenotype was rapidly restored in a TRIF-independent fashion after transient epithelial damage. Our results identify TRIF signalling as truly homeostatic pathway to maintain intestinal epithelial barrier function revealing fundamental differences in the innate immune signalling between mucosal homeostasis and tissue repair.

近期研究表明，固有免疫信号通路在维持黏膜稳态中发挥重要作用，但其确切的下游信号转导通路仍未明确。本研究通过对常规饲养、无菌小鼠以及衔接分子MyD88和TRIF、Toll样受体（TLR）3与4、I型及III型干扰素（IFN）受体缺陷小鼠的分离肠上皮细胞进行比较分析，证实了稳态条件下存在显著的TLR介导的信号通路激活。令人意外的是，Reg3γ及潘氏细胞肠源性抗菌肽的稳态表达高度依赖TRIF，且部分依赖TLR3，但与IFN受体信号通路无关。与野生型上皮相比，TRIF突变体中抗菌肽表达降低，同时伴有潘氏细胞数量显著减少，以及潘氏细胞成熟与分化因子表达水平下降。该表型在同笼饲养过程中并未传递至TRIF功能完整的无菌小鼠体内。TRIF缺陷小鼠体内抗菌肽表达水平低下，导致经口接种细菌的即时清除能力下降，但并未引起肠道菌群整体组成发生显著改变。在一过性上皮损伤后，该表型可通过TRIF非依赖途径快速恢复。本研究结果证实，TRIF信号通路是维持肠上皮屏障功能的真正稳态调控通路，同时揭示了黏膜稳态与组织修复过程中固有免疫信号通路的根本性差异。