RORγt Promotes Foxp3 Expression by Antagonizing the Effector Program in Colonic Regulatory T Cells

ROR gamma t is the master transcription factor for the Th17 cells. Paradoxically, in the intestine, ROR gamma t is coexpressed in peripherally induced regulatory T cells (pTregs) together with Foxp3, the master transcription factor for Tregs. Unexpectedly, by an unknown mechanism, colonic ROR gamma t(+) Tregs show an enhanced suppressor function and prevent intestinal inflammation more efficiently than ROR gamma t-nonexpressing pTregs. Although studies have elucidated the function of ROR gamma t in Th17 cells, how ROR gamma t regulates pTreg function is not understood. In our attempt to understand the role of ROR gamma t in controlling Treg function, we discovered a ROR gamma t-driven pathway that modulates the regulatory (suppressor) function of colonic Tregs. We found that ROR gamma t plays an essential role in maintaining Foxp3 expression. ROR gamma t-deficient Tregs failed to sustain Foxp3 expression with concomitant upregulation of T-bet and IFN-gamma expressions. During colitis induced by adoptive transfer of CD45RBhi cells in Rag1(-/-) mice, ROR gamma t-deficient colonic Tregs transitioned to a Th1-like effector phenotype and lost their suppressor function, leading to severe colitis with significant mortality. Accordingly, Foxp3-expressing, ROR gamma t-deficient Tregs showed impaired therapeutic efficacy in ameliorating colitis that is not due to their reduced survival. Moreover, using the Treg-specific ROR gamma t and T-bet double-deficient gene knockout mouse, we demonstrate that deletion of T-bet from ROR gamma t-deficient Tregs restored Foxp3 expression and suppression function as well as prevented onset of severe colitis. Mechanistically, our study suggests that ROR gamma t-mediated repression of T-bet is critical to regulating the immunosuppressive function of colonic Tregs during the inflammatory condition.