Fig. 1: Diagrammatic representation of the interaction between
the immune system, mucus layer, epithelial barrier, and microbiota in a
homeostatic gut environment. A. Intact mucus layer, glycocalyx, and
epithelial tight junctions restrict bacterial access to the epithelial
barrier. Mucin glycans aid in colonization by acting as attachment
points and as sources of nutrients. Mucin glycan sialylation protects
the integrity of the mucus layer from bacterial proteolytic degradation.
Fucosylation of mucin through FUT2 promotes positive growth. B.IL-4, IL-15, IL-22 secretion promotes mucus secretion, and IL-10
secretion prevents MUC2 misfolding in goblet cells. C .
Secretory IgA secreted by IgA-secreting plasma cells regulates bacterial
concentration in the mucosal layer. D. MAMPs trigger TLR-Myd88
signaling to induce antimicrobial peptides (i.e., RegIIIγ and
α-defensins), which limit bacterial access to mucosal and epithelial
layers. E. Mincle recognition of commensal bacteria in the gut
inhibits IL-17 cytokine and Th17 cell responses through IL-6 and
IL-23p19 cytokines. F. Anti-inflammatory responses maintain the
homeostatic gut environment through Treg cell activity, Siglec activity,
tolerogenic dendritic cells and macrophages, and galectin-1 activation.G. Commensal bacteria digest dietary (i.e., fiber) and host
glycans through CAZymes to generate monosaccharides, disaccharides, and
SCFAs, which are subsequently used as a nutrient and energy resource by
the microbiota and host.