Background/Purpose:  Dysbiosis of colon bacteria has emerged as a likely contributor to diseases such as rheumatoid arthritis and spondyloarthropathies. Interactions between the microbiota and distal gut are critical for human health. These interactions are focused at the level of the colonic mucosa, and intestinal epithelial homeostasis is regulated by multiple signals emanating from both microbes and immune cells. As intraepithelial lymphocytes (IELs) are positioned within the epithelial barrier, we hypothesized that colon bacteria can modulate epithelial barrier function through interactions with IELs. 

Methods:  Epithelial cells were harvested from wild type C57Bl/6 mice, and lymphocytes evaluated by flow cytometry for T cell surface markers or magnetically sorted and stimulated ex vivo with mitogen for evaluation of cytokine production by ELISA. To model dysbiosis, mice were treated with ampicillin, metronidazole, neomycin, and vancomycin in the drinking water. Recolonization of mice occurred by cohousing antibiotic treated mice with unmanipulated littermates. Epithelial barrier integrity was evaluated by paracellular permeability of indigestible FITC-dextran in vivo and in vitro using the model human colonic cell line T84. Contact of host bacteria with the epithelium was performed by FISH using a universal bacterial probe. Protein and RNA analyses were done by Western blot and qRT-PCR, respectively.

Results: Our data demonstrate the major subpopulation of IELs in the mouse colon are CD3+, CD4-CD8-, and TCRβ+, express cell surface markers consistent with activated lymphocytes, and produce large amounts of IL-6 under mitogenic stimulation. Administration of significantly decreased the number of activated, IL-6-secreting IELs. This was reversible, as recolonization resulted in normalization of the IEL numbers, IL-6 secretion, and epithelial barrier integrity. The epithelial barrier in IL-6^-/- and antibiotic-treated mice was noted to have increased paracellular permeability and closer interaction with luminal bacteria. IL-6 was found to signal in colonic epithelial cells and resulted in increased epithelial barrier integrity and claudin 1 expression in model epithelia. 

Conclusion:  We conclude that the host microbiota provides a homeostatic role through regulation of IEL derived IL-6. In turn, IL-6 participates in a barrier protective role in which it stimulates increased tight junction protein expression and decreased contact between luminal bacteria and the host. We postulate that dysbiosis, through the loss of IEL-derived IL-6, in combination with genetic risk, results in increased immune activation at the mucosal barrier, leading to downstream autoimmune responses.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/colonic-intraepithelial-lymphocytes-produce-il-6-in-response-to-resident-bacteria-to-modulate-epithelial-barrier-function/