Abstract 61: Transient "rest" reinvigorates exhausted CAR T cells via epigenetic remodeling

Abstract

Abstract The efficacy of chimeric antigen receptor (CAR) T cell therapy is limited by T cell exhaustion, whereby continuous CAR signaling induces hierarchical loss of effector function. Current methods to mitigate exhaustion in cancer, like immune checkpoint blockade, are not efficacious when combined with CAR T cells. Thus, new therapeutic approaches that target CAR T cell exhaustion are needed. To test the hypothesis that transient cessation of CAR signaling reverses CAR T cell exhaustion, we engineered human T cells to express a drug-regulatable CAR that demonstrates a high degree of antigen-independent tonic signaling and induces hallmarks of exhaustion within 10 days. Using this in vitro model of CAR T cell exhaustion, we discovered that transient CAR downregulation and concomitant cessation of tonic CAR signaling (i.e. “rest”) resulted in profound phenotypic changes, including decreased inhibitory receptor expression and increased memory-associated markers. Upon CAR re-expression and tumor challenge, rested CAR T cells exhibited improved killing, cytokine secretion, and sensitivity to antigen in vitro, as well as enhanced in vivo potency following adoptive transfer into tumor-bearing mice. Similar results were achieved via pharmacologic treatment with the Src kinase inhibitor dasatinib, which potently and reversibly inhibits CAR signaling. RNA- and ATAC-sequencing revealed that rest induces global transcriptional and epigenetic reprogramming, wherein rested CAR T cells closely resembled healthy, non-exhausted CAR T cells. Notably, epigenetic reprogramming and functional reinvigoration in rested cells were dependent on the activity of the histone methyltransferase EZH2, indicating that epigenetic alterations are required for functional reversal of CAR T cell exhaustion. Finally, providing multiple periods of rest in vivo by toggling CAR expression or pulsing with dasatinib reversed hallmarks of CAR T cell exhaustion and lead to more durable anti-tumor responses in xenograft murine models. This work demonstrates that transient cessation of CAR T cell signaling can enhance anti-tumor potency by preventing or reversing exhaustion and challenges the paradigm that exhaustion is an epigenetically fixed state. These results also raise the prospect that targeting proximal TCR/CAR signaling kinases might represent a novel immunotherapeutic strategy for mitigating exhaustion in human CAR T cells. Citation Format: Evan W. Weber, Crystal Mackall. Transient "rest" reinvigorates exhausted CAR T cells via epigenetic remodeling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 61.