Regulation of mucosal barrier functions by gut microbes and host immune cells. Mucosal barrier function is modulated by gut microbes and host immune cells. SFB colonization or C. rodentium infection promotes the induction of helper T cells producing IL-17 and simulates ILC3 to secrete IL-22. Both cytokines enhance the production of antimicrobials such as AMPs and Reg3γ from IECs. In the case of parasite infection, activated tuft cells produce IL-25, which stimulates ILC2 to secrete IL-13. IL-13 promotes the proliferation of goblet cells and mucus production from them. Metabolites from gut microbes also directly influence the mucosal barrier function of IECs. SCFA promotes mucus production from goblet cells, and indole upregulates the expression of cell junction-related molecules through PXR activation
SFB: segmented filamentous bacteria, SAA: serum amyloid A, ILC: innate lymphoid cell, TLR: Toll-like receptor, NOD2: nucleotide-binding oligomerization domain-containing 2, AMP: antimicrobial peptide, IEC: intestinal epithelial cell, SCFA: short-chain fatty acid, PXR: Pregnane X receptor.