FOXP3 protects conventional human T cells from premature restimulation-induced cell death

Abstract

The adaptive immune response relies on specific apoptotic programs to maintain homeostasis. Conventional effector T cell (Tcon) expansion is constrained by both forkhead box P3 (FOXP3) +-regulatory T cells (Tregs) and restimulation-induced cell death (RICD), a propriocidal apoptosis pathway triggered by repeated stimulation through the T-cell receptor (TCR). Constitutive FOXP3 expression protects Tregs from RICD by suppressing SLAM-associated protein (SAP), a key adaptor protein that amplifies TCR signaling strength. The role of transient FOXP3 induction in activated human CD4 and CD8 Tcons remains unresolved, but its expression is inversely correlated with acquired RICD sensitivity. Here, we describe a novel role for FOXP3 in protecting human Tcons from premature RICD during expansion. Unlike FOXP3-mediated protection from RICD in Tregs, FOXP3 protects Tcons through a distinct mechanism requiring de novo transcription that does not require SAP suppression. Transcriptome profiling and functional analyses of expanding Tcons revealed that FOXP3 enhances expression of the SLAM family receptor CD48, which in turn sustains basal autophagy and suppresses pro-apoptotic p53 signaling. Both CD48 and FOXP3 expression reduced p53 accumulation upon TCR restimulation. Furthermore, silencing FOXP3 expression or blocking CD48 decreased the mitochondrial membrane potential in expanding Tcons with a concomitant reduction in basal autophagy. Our findings suggest that FOXP3 governs a distinct transcriptional program in early-stage effector Tcons that maintains RICD resistance via CD48-dependent protective autophagy and p53 suppression.

Molecular and temporal control of restimulation-induced cell death (RICD) in T lymphocytes

“…Because FOXP3 is expressed in activated expanding T cons when antigen and IL-2 are abundant, we reasoned that transient FOXP3 upregulation may protect activated Tcons from premature RICD by suppressing SAP expression, mirroring its function in Tregs. We showed that siRNA-mediated knockdown of FOXP3 significantly enhanced RICD sensitivity during in vitro clonal expansion of human T cells, a finding not conferred in latestage, terminally differentiated T cells (Voss et al, 2019b). However, FOXP3 silencing did not impact SAP expression, and SAP and NTB-A were not required for residual RICD at this early stage.…”

TIM-3 drives temporal differences in restimulation-induced cell death sensitivity in effector CD8+ T cells in conjunction with CEACAM1

“…Several factors govern RICD sensitivity in late-stage effector T cells, but little is known about mechanisms of RICD resistance in early expanding T cells. A recent study from our group showed that early expanding conventional human effector T cells are less susceptible to RICD in part due to the influence of the transcription factor FOXP3 and autophagy 10 . It is also plausible that newly activated T cells avoid RICD via downregulation of TCR/CD3 expression 68 .…”

“…A number of mechanisms have been described which sensitize T cells to RICD. We and other groups have shown that the cell cycle phase 4 , IL-2 signaling 5 , metabolic state 6,7 , SAP and natural killer (NK), T, B cell antigen (NTB-A) expression 8 , diacylglycerol kinase alpha (DGKα) activity 9 , and transient expression of transcription factors such as forkhead box protein 3 (FOXP3) 10 can all influence when and how conventional T cells are susceptible to RICD. One key driver of differential RICD sensitivity in late-stage effector T cells is relative TCR signal strength 11 ; only T cells that reach a critical TCR signal threshold via strong, repeated restimulation will succumb to RICD.…”

FOXP3 deficiency, from the mechanisms of the disease to curative strategies

“…Collectively, these data suggest that the FOXP3 deficiency has different impacts on T cell function in variously polarized Th cells. More recently, FOXP3 has been shown to play a role in preventing activation‐induced cell death in conventional T cells 74 …”