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P002. Intestinal homeostasis is critically regulated by the transcription factor T-bet

N. Powell1, E. Stolarczyk1, J. Lo1, J. Canavan1, E. Marks1, T. MacDonald2, G. Lord1, 1King's College London, Experimental Immunobiology, London, 2Queen Mary University, Immunity and Infectious Diseases, London, United Kingdom

Background

Intestinal immunity is tightly regulated to allow host defence mechanisms to engage pathogens, yet restraining host responses to luminal commensals. Loss of the immune transcription factor T-bet (encoded by the Tbx21 gene) in the innate immune compartment results in loss of intestinal homeostasis and the emergence of chronic colitis. Although deregulation of TNFα expression by dendritic cells (DCs) is important in early disease, the innate immune mechanisms responsible for driving chronic IBD in TRUC (Tbx21−/− Rag2−/− Ulcerative Colitis) mice have yet to be resolved.

Methods

TRUC and agonistic anti-CD40 mouse models of IBD were interrogated. Explant cultures, multiparameter flow cytometry and qPCR were used to evaluate immune pathways involved. Human lamina propria mononuclear cells (LPMC) were also analysed.

Results

In addition to increased production of TNFα in the colon, TRUC IBD was also characterized by increased innate production of IL17A. The chief cellular source of IL17A was IL7R+ RORγt+ CCR6+ innate lymphoid cells (ILCs). These cells were functionally relevant in disease, since in vivo depletion of ILCs significantly abrogated disease. We also show that CD11b+ CD103 DCs are key producers of TNFα in the colon, which synergises with IL23 to induce IL17A production by ILCs. Notably, unlike wild type ILCs which favour IFNγ production, Tbx21−/− ILC were poor producers of IFNγ and instead expressed high levels of IL17A. Antibody blockade of the IL23/IL17A axis also reversed IBD in TRUC mice.

Analysis of gut LPMCs from IBD patients also demonstrated the presence of a non-T-cell RORγt+source of IL17A in which T-bet expression was reduced, indicating that loss of T-bet mediated trancriptional control of innate IL17A production may be relevant in human IBD.

Conclusion

These data confirm that IL23 responsive intestinal ILCs selectively expressing IL17A are responsible for mediating chronic IBD in TRUC mice. For the first time we have shown that TNFα derived from CD11b+ CD103 colonic DCs synergized with IL23 to potently induce IL17A production by ILCs, highlighting a previously unrecognised level of cellular cross talk between DCs and ILCs. In the absence of T-bet there is increased production of TNFα and IL17A which appears to be a potently colitogenic cytokine milieu. In conclusion, T-bet deficiency results in loss of intestinal homeostasis and the emergence of chronic innate immune mediated inflammation, in which cross talk between DCs and ILCs plays a central role.