Foxo1 restrains naive CD4 T cell differentiation into type 1 and type 2 effector cells by limiting early cytokine production

Over the past 15 years, Foxo1 has emerged as a key transcription factor in T cell biology, regulating survival, trafficking, and the establishment of memory responses. Its role in naive CD4 T cell differentiation into effector cells, however, appears more complex. Foxo1 has been reported to be essential for the generation of induced regulatory T cells and TH9 cells, while restraining the differentiation of TH17 and TFH lineages. Surprisingly, its contribution to TH1 and TH2 differentiation has remained poorly defined. Here, we show that memory CD4 T cells from Foxo1 ^TKO mice exhibited enhanced capacity to produce type 1 and type 2 cytokines ex vivo , associated with increased T-bet and GATA3 expression. Foxo1-deficient naive CD4 T cells displayed a strong bias toward TH1 and TH2 differentiation in vitro , even under nonpolarizing conditions, driven by rapid and robust production of IL-4, IL-13 and IFN-γ upon activation. Coculture experiments further revealed that Foxo1-sufficient cells benefited from cytokines produced by Foxo1-deficient cells, while competition for IFN-γ limited the differentiation of the latter. Mechanistically, IL-4 stimulation elicited increased STAT1 and STAT3 phosphorylation in Foxo1-deficient naive CD4 T cells. Pharmacological inhibition of STAT3, but not STAT1, markedly reduced their TH1/TH2 polarization capacity, although residual differentiation suggested the involvement of additional mechanisms.