Epithelial Antimicrobial Peptide Expression TRIF Signaling Drives Homeostatic Intestinal

Recent results indicate a significant contribution of innate immune signaling 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 TLR3 and TLR4, as well as the type I and III IFN receptors, we demonstrate significant TLR-mediated signaling under homeostatic conditions. Surprisingly , homeostatic expression of Reg3g and Paneth cell enteric antimicrobial peptides critically relied on TRIF and, in part, TLR3 but was independent of IFN receptor signaling. Reduced antimicrobial peptide expression was associated with significantly lower numbers of Paneth cells and a reduced Paneth cell maturation and differentiation factor expression in TRIF mutant compared with wild-type 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 signaling as a truly homeo-static pathway to maintain intestinal epithelial barrier function revealing fundamental differences in the innate immune signaling between mucosal homeostasis and tissue repair. I nnate immune receptors of the family of TLRs recognize conserved microbial structures and activate host defense mechanisms upon infection with pathogenic microorganisms. They are expressed by professional immune cells but also intestinal epithelial cells (IECs), although at a somewhat restricted spectrum (1, 2). Recent reports indicate that also in the absence of infectious challenge, innate immune stimulation of mucosal cells by ligands released from the gut microbiota occurs and contributes to maintenance of the intestinal epithelial barrier. A number of studies examined the role of epithelial innate immune signaling mediators and regulatory molecules in experimental models of intestinal inflammation and observed a significant influence on disease outcome (3–12). Only few studies, however, noted spontaneous mucosal dysfunction in gene-deficient animals, and these were restricted to central intermediates or regulators of the downstream TLR signal transduction pathway or induced effector molecules (6, 9, 12–18). Thus, the functional importance of TLR signaling under unchallenged, homeostatic conditions for the expression of epi-thelial effector functions of the mucosal barrier has remained largely undefined. TLR recognition of the respective microbial ligand initiates signal transduction through two major pathways. Recruitment of MyD88 leads to …