Elucidating the role of the Mst1/Foxo1 axis in CD8+ memory T cell development

Abstract

Abstract The Foxo1 transcription factor is required for naïve T cell homeostasis and trafficking, T regulatory cell development, and CD8 memory T cell differentiation. Two kinases, Akt1 and Mst1 differentially control Foxo1 activity. Akt1-induced Foxo1 phosphorylation at T24, S256, and S319 causes association with 14-3-3 adapters, nuclear export, and degradation, resulting in decreased transcription of Foxo1 targets. Akt1-mediated control of Foxo1 activity in CD8 T cells promotes terminal differentiation into effector cells. In contrast, Mst1 may prevent Foxo transcription factors from binding 14-3-3, promoting Foxo nuclear localization and transcriptional activity. These data suggest that Akt1 and Mst1 have opposing effects on Foxo1 activity in T cells. However, how Mst1 regulates Foxo1 activity to impact CD8 memory T cell differentiation has not been directly evaluated. We hypothesize that, while phosphorylation of Foxo1 by Akt1 causes cytosolic localization and promotes effector CD8 differentiation, phosphorylation of Foxo1 by Mst1 increases Foxo1 stability and promotes CD8 central memory differentiation. To test this hypothesis, we have generated Mst1 phospho-null and phospho-mimetic Foxo1 variants (S209A and S209D). Surprisingly, Foxo1 mutations at the Mst1 phosphorylation site differentially altered Foxo1 subcellular localization. Current studies utilizing Lm--Ova to study the antigen-specific memory response of OT-I CD8 T cells expressing WT, S209A, or S209D Foxo1 variants in vivo revealed skewed effector and central memory differentiation. Together, these studies indicate that coordinated regulation of Foxo1 by Akt1 and Mst1 kinases determine the extent of CD8 terminal effector and memory cell differentiation.