TIM-3-dependent regulation of restimulation-induced cell death sensitivity flips changes in early versus late-stage human effector T cells

Abstract

In healthy individuals, the balance between immune-mediated pathogen clearance and tissue damage is tightly regulated through multiple processes. One such mechanism is restimulation-induced cell death (RICD), a propriocidal apoptotic mechanism that constrains T cell responses. Activated T cells must surpass a high threshold of TCR signal strength to induce pro-apoptotic genes responsible for executing RICD. Because co-receptors such as TIM-3 are thought to modulate TCR signal strength, we hypothesized that TIM-3 can shape the T cell response by tuning RICD sensitivity. We utilized siRNA knockdowns and transfection of wild-type and mutant TIM-3 expression constructs to assay for changes in RICD sensitivity in both immortalized and primary human T cells in vitro. Strikingly, we observed that the effect of altering TIM-3 expression varied temporally during the effector T cell response. Whereas TIM-3 promoted RICD resistance in newly activated T cells, TIM-3 enhanced RICD sensitivity in late-stage effector T cells by boosting TCR-induced FASL and BIM expression in a ligand-independent fashion. Preliminary evidence suggests that this dichotomy may relate to substantial temporal changes in the expression, cellular localization, and glycosylation of TIM-3 in early versus late-stage effector T cells. Overall, these results indicate that TIM-3 influences apoptosis sensitivity differently at distinct phases of the human T cell response, with important implications for the use of checkpoint blockade immunotherapies targeting TIM-3. Our findings may also help to clarify discrepancies in the literature regarding TIM-3 signaling and function in T cells.