FOXP3 protects human T cells from restimulation-induced cell death via CD48 upregulation and autophagy

Abstract

Abstract The adaptive immune response relies on specific apoptosis programs to regulate effector T cell expansion and maintain homeostasis. Restimulation-induced cell death (RICD) is an apoptotic pathway triggered by repeated stimulation through the T cell receptor (TCR), which ensures that effector T cell proliferation remains in check. Constitutive fork head box P3 (FOXP3) expression renders regulatory T cells (Tregs) resistant to RICD. Although FOXP3 is also expressed transiently in human conventional T cells (Tcons) during initial rounds of activation-induced proliferation, its function in this context remains unclear. Here we describe a novel role for FOXP3 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, and was dependent on de novo transcription. RNA-Seq analysis revealed FOXP3-dependent effects on genes involved in metabolic pathways, autophagy, and cell surface receptors. FOXP3 silencing increased glycolysis and reduced autophagy in Tcons, which has been linked to changes in apoptosis sensitivity. Indeed, blocking autophagy increased RICD in control Tcons without augmenting death of FOXP3-silenced cells, suggesting FOXP3 reduces RICD sensitivity through an autophagy-dependent mechanism. Additionally, FOXP3 knockdown markedly reduced expression of CD48, a SLAM-family receptor. We now show that CD48 protects expanding Tcons from RICD. Surprisingly, CD48 also promoted protective autophagy, illuminating a novel connection between SLAM receptor signaling, autophagy regulation, and RICD sensitivity that is governed by FOXP3 expression in early Tcons.