Foxo1 Serine 209 Is a Critical Regulatory Site of CD8 T Cell Differentiation and Survival

Foxo1 is an essential transcription factor required for the survival and differentiation of memory CD8 T cells, yet it is unclear whether these Foxo1-dependent functions are inherently coupled. To address this question, we examined the effects of different Foxo1 posttranslational modifications. Phosphorylation of Foxo1 by Akt kinases at three distinct residues is well characterized to inhibit Foxo1 transcriptional activity. However, the effect of Foxo1 phosphorylation within its DNA-binding domain at serine 209 by Mst1 kinase is not fully understood. In this study, we show that an S209A phospho-null Foxo1 exhibited Akt-dependent nuclear trafficking in mouse CD8 T cells and augmented the expression of canonical Foxo1 target genes such as Il7r and Sell In contrast, an S209D phosphomimetic Foxo1 (SD-Foxo1) was largely excluded from the nucleus of CD8 T cells and failed to transactivate these genes. RNA sequencing analysis revealed that SD-Foxo1 was associated with a distinct Foxo1-dependent transcriptional profile, including genes mediating CD8 effector function and cell survival. Despite defective transactivation of canonical target genes, SD-Foxo1 promoted IL-15-mediated CD8 T cell survival in vitro and survival of short-lived effector cells in vivo in response to Listeria monocytogenes infection. However, SD-Foxo1 actively repressed CD127 expression and failed to generate memory precursors and long-lived memory T cells. Together, these data indicate that S209 is a critical residue for the regulation of Foxo1 subcellular localization and for balancing CD8 T cell differentiation and survival.