Foxp3 expression in induced regulatory T cells is stabilized by C/EBP in inflammatory environments

Proper control of immune responses by Foxp3 + regulatory T cells at inflamed sites is crucial for the prevention of immunopathology. TGF‐β‐induced Foxp3 + regulatory T (T reg ) cells are generated in inflammatory environments as well as in steady‐state conditions. Inflammatory cytokines such as IFN‐γ and IL‐4 have an antagonistic effect on T reg cell conversion. However, it is not known how naive CD4 + T cells overcome the inhibitory environment in inflamed sites to differentiate into T reg cells. Here, we show that CCAAT/enhancer‐binding protein (C/EBP) functions as a safeguard that enhances T reg cell generation by dampening the inhibitory effect of IFN‐γ and IL‐4 on Foxp3 expression. We find that C/EBPβ is induced by retinoic acid and binds to the methyl‐CRE sequence in the Foxp3 TSDR to sustain its expression. C/EBPβ‐transduced iT reg cells show more potent suppressive activity in mouse disease models. We also reveal that C/EBPβ‐transduced human iT reg cells exhibit more enhanced suppressor function. These results establish C/EBP as a new molecular target for enhancing the formation and stability of T reg cells in inflammatory environments. Synopsis Naive CD4 + T cells need to overcome the inhibitory environment at inflamed sites to differentiate into Tregs. C/EBP enhances Treg generation by dampening the inhibitory effect of IFN‐γ and IL‐4 on Foxp3 expression. C/EBP is induced by retinoic acid and counteracts the inhibitory effect of IFN‐γ and IL‐4 on iTreg generation. C/EBP stabilizes Foxp3 expression by binding to the methylated TSDR of the Foxp3 gene. Both mouse and human iTregs transduced with C/EBPβ exhibit increased suppressive activity. Graphical Abstract Naive CD4 + T cells need to overcome the inhibitory environment at inflamed sites to differentiate into Tregs. C/EBP enhances Treg generation by dampening the inhibitory effect of IFN‐γ and IL‐4 on Foxp3 expression.