Transient FOXP3 upregulation protects human conventional T cells from restimulation-induced cell death during activation-induced proliferation

Abstract An efficient adaptive immune response relies on the rapid clonal expansion of effector T cells, followed by timely disposal of those cells in order to avoid damage to host tissues. Effector T cell proliferation is constrained by the action of forkhead box P3 (FOXP3)+ regulatory T cells (Tregs) and restimulation-induced cell death (RICD), an apoptotic pathway triggered by repeated stimulation through the T cell receptor (TCR) in the presence of interleukin-2 (IL-2). Constitutive FOXP3 expression renders Tregs resistant to RICD, in part through repression of the signaling adaptor molecule SLAM-associated protein (SAP) and FAS ligand (FASL). Interestingly, FOXP3 is also expressed transiently in human conventional T cells (Tcons) during initial rounds of activation-induced proliferation, but its function in this context remains unclear. Here we uncover a new role for FOXP3 induction in protecting both CD4+ and CD8+ human Tcons from premature RICD during the expansion phase. SiRNA-mediated silencing of FOXP3 sensitized early effector Tcons to RICD. This increase in RICD was completely dependent on de novo transcription, but FOXP3 knockdown did not alter SAP or FASL expression in these cells. Instead, FOXP3 silencing increased glycolysis and reduced autophagy in Tcons, which we and others have linked to changes in apoptosis sensitivity. Surprisingly, FOXP3-dependent autophagy specifically protected CD4+ but not CD8+ Tcons from RICD. Our results suggest a fundamentally different mechanism connecting transient FOXP3 expression to metabolic activity and RICD resistance in CD4+ versus CD8+ T cells.