Colonic Microbiota Modulate Epithelial Barrier Function through Intraepithelial Lymphocyte Produced IL-6

Microbial colonization is critical for the development of both the epithelial barrier and mucosal immune function.  Antibiotic use is known to influence epithelial homeostasis and increase risk of infection by pathogens through unknown mechanisms. We evaluated the impact of broad-spectrum antibiotics on mucosal immune components and epithelial function in C57Bl/6 mice. Antibiotic administration was associated with profound decreases in intraepithelial lymphocytes (IELs) and increased colonic permeability. Therefore, we hypothesized that colonic bacteria modulate epithelial barrier function through IELs. Our data demonstrate the major subpopulation of IELs in the colon are CD3⁺, CD4^-CD8^- (52 ± 3.5% of total IELs), and TCRβ⁺ (71 ± 4.5% of total IELs), express cell surface markers consistent with activated lymphocytes (CD44+ CD69+ CD62L-), and produce large amounts of IL-6. Administration of antibiotics to mice significantly decreased the number of activated, IL-6-secreting IELs (p<0.0001). The influence of antibiotics was reversible, as recolonization resulted in normalization of the IEL numbers, IL-6 secretion, and epithelial barrier. IL-6 was found to signal in epithelial cells, and resulted in increased epithelial barrier integrity in model epithelia. Interestingly, IL-6^-/- mice were found to have a profound barrier defect, similar to antibiotic treated mice. We conclude that modulation of host microbiota by antibiotics impairs epithelial barrier function through effects of IEL function.