A20 intrinsically influences human effector T-cell survival and function by regulating both NF-κB and JNK signaling.

A20 is a dual-function ubiquitin-editing enzyme that maintains immune homeostasis by restraining inflammation. Although A20 serves a similar negative feedback function for T-cell receptor (TCR) signaling, the molecular mechanisms utilized and their ultimate impact on human T-cell function remain unclear. TCR engagement triggers the assembly of the CARD11-BCL10-MALT1 (CBM) protein complex, a signaling platform that governs the activation of downstream transcription factors including NF-κB and c-Jun/AP-1. Utilizing WT and A20 knockout Jurkat T cells, we found that A20 is required to negatively regulate NF-κB and JNK. Utilizing a novel set of A20 mutants in NF-κB and AP-1-driven reporter systems, we discovered the ZnF7 domain is crucial for negative regulatory capacity, while deubiquitinase activity is dispensable. Successful inactivation of A20 in human primary effector T cells congruently conferred sustained NF-κB and JNK signaling, including enhanced upregulation of activation markers, and increased secretion of several cytokines including IL-9. Finally, loss of A20 in primary human T cells resulted in decreased sensitivity to restimulation-induced cell death and increased sensitivity to cytokine withdrawal-induced death. These findings demonstrate the importance of A20 in maintaining T-cell homeostasis via negative regulation of both NF-κB and JNK signaling.

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

For effective adaptive immunity, T lymphocytes must rapidly expand and contract in an antigen-specific manner to effectively control invading pathogens and preserve immunological memory, without sustaining excessive collateral damage to host tissues. Starting from initial antigen encounter, carefully calibrated programmed cell death pathways are critical for maintaining homeostasis over distinct phases of the T cell response. Restimulation-induced cell death (RICD), a self-regulatory apoptosis pathway triggered by re-engagement of the T cell receptor (TCR), is particularly important for constraining effector T cell expansion to preclude overt immunopathology; indeed, genetic disorders affecting key molecules involved in RICD execution can manifest in excessive lymphoproliferation, malignancy, and autoimmunity. Herein we review our current knowledge of how RICD sensitivity is ultimately regulated over the course of an immune response, including recent revelations on molecules that tune RICD by enforcing resistance or promoting susceptibility in expanding versus mature effector T cells, respectively. Detailed dissection of the molecular and temporal control of RICD also illuminates novel therapeutic strategies for correcting abnormal T cell responses noted in various immune disorders by ultimately tuning RICD sensitivity.