A20 is a negative regulator of both NF-κB and JNK signaling following T cell receptor stimulation in humans

Abstract A20 is an NF-κB-induced, dual function ubiquitin editing enzyme that negatively regulates NF-κB signaling induced by TNF and other innate immune receptors, constraining inflammation. Indeed, A20 haploinsufficiency drives a severe autoinflammatory disease in humans. 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. We employed an extensive set of mutant expression constructs to better elucidate how A20 regulates TCR signaling output in Jurkat T cells. Intriguingly, A20 knockout cells showed enhanced, sustained NF-κB and JNK signaling after stimulation, which was reversed upon wild-type (WT) A20 complementation. A20 deubiquitinase function was entirely dispensable for NF-κB and JNK regulation, whereas zinc finger domains that confer E3 ligase activity (ZnF4) and linear ubiquitin-binding capacity (ZnF7) were required. Abolishing MALT1-dependent proteolytic cleavage of A20 did not affect suppressive function, although neither N- nor C-terminal A20 cleavage fragments retained regulatory capacity. We also found that CRISPR-mediated deletion of A20 adversely affects the survival and expansion of primary human T cells. Ongoing studies will determine how A20 deletion affects human T cell effector responses after stimulation, and decipher how dysregulated NF-κB and/or JNK signaling contributes to T cell-intrinsic abnormalities that perturb adaptive immune homeostasis.