Convergence of TCR and cytokine signaling leads to FOXO3a phosphorylation and drives the survival of CD4(+) central memory T cells

The molecular events involved in the establishment and maintenance of CD4(+) central memory and effector memory T cells (T(CM) and T(EM), respectively) are poorly understood. In this study, we demonstrate that ex vivo isolated T(CM) are more resistant to both spontaneous and Fas-induced apoptosis than T(EM) and have an increased capacity to proliferate and persist in vitro. Using global gene expression profiling, single cell proteomics, and functional assays, we show that the survival of CD4(+) T(CM) depends, at least in part, on the activation and phosphorylation of signal transducer and activator of transcription 5a (STAT5a) and forkhead box O3a (FOXO3a). T(CM) showed a significant increase in the levels of phosphorylation of STAT5a compared with T(EM) in response to both IL-2 (P < 0.04) and IL-7 (P < 0.002); the latter is well known for its capacity to enhance T cell survival. Moreover, ex vivo T(CM) express higher levels of the transcriptionally inactive phosphorylated forms of FOXO3a and concomitantly lower levels of the proapoptotic FOXO3a target, Bim. Experiments aimed at blocking FOXO3a phosphorylation confirmed the role of this phosphoprotein in protecting T(CM) from apoptosis. Our results provide, for the first time in humans, an insight into molecular mechanisms that could be responsible for the longevity and persistence of CD4(+) T(CM).