Foxo1 Is a T Cell–Intrinsic Inhibitor of the RORγt-Th17 Program

An uncontrolled exaggerated Th17 response can drive the onset of autoimmune and inflammatory diseases. In this study, we show that, in T cells, Foxo1 is a negative regulator of the Th17 program. Using mixed bone marrow chimeras and Foxo1-deficient mice, we demonstrate that this control is effective in vivo, as well as in vitro during differentiation assays of naive T cells with specific inhibitor of Foxo1 or inhibitors of the PI3K/Akt pathway acting upstream of Foxo1. Consistently, expressing this transcription factor in T cells strongly decreases Th17 generation in vitro as well as transcription of both IL-17A and IL-23R RORγt-target genes. Finally, at the molecular level, we demonstrate that Foxo1 forms a complex with RORγt via its DNA binding domain to inhibit RORγt activity. We conclude that Foxo1 is a direct antagonist of the RORγt-Th17 program acting in a T cell-intrinsic manner.