c-MAF-dependent regulatory T cells mediate immunological tolerance to a gut pathobiont

Both microbial and host genetic factors contribute to the pathogenesis of autoimmune diseases. There is accumulating evidence that microbial species that potentiate chronic inflammation, as in inflammatory bowel disease, often also colonize healthy individuals. These microorganisms, including the Helicobacter species, can induce pathogenic T cells and are collectively referred to as pathobionts. However, how such T cells are constrained in healthy individuals is not yet understood. Here we report that host tolerance to a potentially pathogenic bacterium, Helicobacter hepaticus, is mediated by the induction of RORγt⁺FOXP3⁺ regulatory T (iT_reg) cells that selectively restrain pro-inflammatory T helper 17 (T_H17) cells and whose function is dependent on the transcription factor c-MAF. Whereas colonization of wild-type mice by H. hepaticus promoted differentiation of RORγt-expressing microorganism-specific iT_reg cells in the large intestine, in disease-susceptible IL-10-deficient mice, there was instead expansion of colitogenic T_H17 cells. Inactivation of c-MAF in the T_reg cell compartment impaired differentiation and function, including IL-10 production, of bacteria-specific iT_reg cells, and resulted in the accumulation of H. hepaticus-specific inflammatory T_H17 cells and spontaneous colitis. By contrast, RORγt inactivation in T_reg cells had only a minor effect on the bacteria-specific T_reg and T_H17 cell balance, and did not result in inflammation. Our results suggest that pathobiont-dependent inflammatory bowel disease is driven by microbiota-reactive T cells that have escaped this c-MAF-dependent mechanism of iT_reg-T_H17 homeostasis.