FOXP3, CD48 and autophagy confer the protection of CD4 and CD8 human T cells from T cell receptor restimulation-induced cell death

Abstract

Abstract The proliferation and contraction of activated effector T cells must be carefully coordinated to maintain immune homeostasis. This is achieved in part through restimulation-induced cell death (RICD), a pre-programmed apoptosis program triggered by antigen restimulation through the T cell receptor (TCR). Forkhead box P3 (FOXP3)+ regulatory T cells (Tregs) also constrain conventional T cell (Tcon) responses, but can resist RICD themselves despite frequent TCR stimulation. We previously showed that FOXP3 protects Tregs from RICD by suppressing SLAM-associated protein (SAP), a key adaptor protein that amplifies TCR signal strength. Mysteriously, FOXP3 expression is also transiently induced in human Tcons after activation, with a kinetic expression profile that correlates inversely with acquired RICD sensitivity. Hence we asked whether FOXP3 protects expanding human effector T cells from premature RICD by modulating SAP expression. Our results show that although siRNA-mediated FOXP3 knockdown sensitizes early effector CD4 and CD8 T cells to RICD, SAP expression remains unaffected. Unlike late stage effector T cells, a low level of RICD in expanding Tcons was entirely dependent on de novo transcription, and knockdown of SAP failed to reduce death. Subsequent RNA-Seq analyses revealed that CD48, a SLAM family receptor, was markedly reduced upon FOXP3 knockdown. Blockade or knockdown of CD48 also increased RICD in CD4 and CD8 T cells. We now show that CD48 protects early effector T cells both by promoting autophagy and upregulation of pro-survival genes such as BATF. These findings implicate FOXP3 as the central governor of a distinct transcriptional program that promotes RICD resistance early in the effector T cell response.