The role of RORγt in T17-mediated allo- and autoimmunity

Introduction: IL-17-producing T cells (T17) play a major role in allograft rejection and beyond, as T17 cells in Th1-deficient mice mediate pro-inflammatory immune responses resulting in accelerated allograft rejection and autoimmunity. However, the mechanisms underlying T17-mediated immune responses in transplantation and autoimmunity are poorly understood, particularly with respect to regulation by the hallmark T17 transcription factor RORγt.

Specific aims and methods: To precisely explore the role of RORγt in T17 alloimmunity in the absence of Th1, we created RORγt reporter and knockout mice on a T-bet-deficient (T-bet^-/-) background by crossbreeding T-bet^-/- with Rorγt^gfp/wt reporter and Rorγt^gfp/gfp knockout mice, respectively.

Results: First, we explored the fate-mapping capacity of our reporter model by polarizing T cells from Rorγt^gfp/wtT-bet^-/- mice towards T17 and measuring GFP expression in vitro. We found that GFP was clearly up-regulated in T17-polarized T cells from Rorγt^gfp/wtT-bet^-/- mice and that the expression of GFP paralleled the expression of RORγt, as shown by RT-PCR and FACS. Importantly, the T17-polarized T cells from the reporter mice were as capable of producing IL-17 as T cells from controls, as proven by ELISA and FACS, thus demonstrating the efficacy of our reporter model. Next, we tested the hypothesis that RORγt significantly contributes to the differentiation of alloreactive T17 cells by studying T cells from Rorγt^gfp/gfpT-bet^-/- double-knockout mice under T17-polarizing conditions, in terms of expression of T17 effector cytokines in vitro. As expected, we found that IL-17 expression was substantially lower in the double-knockout mice when compared to Rorγt^gfp/wtT-bet^-/- and T-bet^-/- controls, as indicated by RT-PCR, ELISA, and FACS. Thus, RORγt seems to play a major, but not exclusive, role in driving IL-17-producing T cells in the absence of T-bet.

Conclusion: In conclusion, our novel model is an unprecedented tool in studying RORγt⁺ T17 cells in immune responses. In transplantation and autoimmunity, our studies point to an important and as yet understudied role of RORγt.