TRIF signalling drives homeostatic intestinal epithelial antimicrobial peptide expression

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 Reg3? 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.