Science: Intermittent

Chimeric antigen receptor T cell therapy (referred to as"CAR-T") is increasingly being applied to the treatment of cancer patients. CAR-T cell therapy has shown encouraging results in patients with hematologic cancers, but its anticancer activity may be limited by the functional effectiveness of CAR-T cells. In a recent study, Professor Crystal L. Mackall's group from Stanford University School of Medicine characterized phenotypic and epigenomic changes associated with CAR-T cell failure caused by continuous activity, as well as the beneficial effects of brief rest for restoring its function. The authors tested different types of "intermittent rest" treatments, such as temporary inhibition of T cell activity using the drug dasatinib, which helps prevent CAR-T cell failure and is able to effectively improve CAR-T cell antitumor activity in mouse models. The results were published in the recent issue of Science.

More than 50% of patients with B-cell malignancies treated with chimeric antigen receptor (CAR)-T cells still experience further development of cancer, and this therapy has not shown a stabilizing effect on solid tumors. The efficacy of CAR-T cells is usually limited by T cell failure, and changes in their transcription and epigenetics drive the overexpression of immunosuppressive proteins and reduce T cell function. Current treatments for T cell failure, including immune checkpoint inhibitors, do not affect associated epigenetic trends.

Previous studies have demonstrated that improving CAR signaling activity is thought to induce and maintain the function of human T cells, so the authors hypothesized that inhibition of CAR signaling or mandatory "rest" can prevent or even possibly reverse the failure of the CAR-T cell population. Dasatinib, a clinically used tyrosine kinase inhibitor that reversibly inhibits kinase activity downstream of CAR signaling, inhibits CAR-T cell activity. Based on the above results, the authors hope to be able to comprehensively analyze the phenotype, functionality, and transcriptional epigenetic markers before and after CAR-T cell failure to understand the effect of "intermittent rest" on CAR-T cell function.

First, the authors found that CAR-T cells further showed "exhausting"-related phenotypes, transcriptional and epigenetic hallmarks after expansion in a state of continuous stimulation, while CAR-T cells developed "memory" cell-related features after dasatinib-induced "rest", which also showed a memory-like phenotype and more excellent antitumor activity after adoptive transfer into mice with tumor grafts.

In addition, the authors transferred their fate from failure to a memory-like state by inducing intermittent resting of "failing" CAR-T cells. In CAR-T cells, which already have "exhausted" features, resting induction reverses their phenotype in only 4 days and induces transcriptional reprogramming and global epigenetic remodeling. In addition, the antitumor activity of "exhausted" CAR-T cells was fully restored after rest, and the degree of functional recovery was related to the length of rest, and was associated with reduced expression of the fatigue-related transcription factor TOX and increased expression of the memory-related transcription factors LEF1 and TCF1.

Afterwards, the authors found that the reversion of CAR-T cell function depended on the activity of the histone methyltransferase EZH2, which is compatible with the phenomenon of epigenetic remodeling. By using a mouse tumor xenograft model, the authors found that CAR-T cells that induce intermittent "tipping" in vivo by oscillations in CAR expression or dasatinib exhibited better antitumor ability and higher survival compared to control CAR-T cells. Single-cell analysis suggests that stimulation with dasatinib is sufficient to induce a "failing" tumor-infiltrating CAR-T cell to generate a memory phenotype and enhance its anti-tumor function.

In summary, the results of this study suggest that inhibition of CAR signaling can enhance the adaptability of CAR-T cells by preventing their "failure", which in turn enhances the therapeutic efficacy. In addition, transient inhibition or resting treatment of CAR signaling restores its function and leads to global epigenetic remodeling in CAR-T cells that have already developed features of failure. The findings raise the prospect of immunotherapeutic strategies that target CAR or TCR downstream signaling kinases that may help mitigate the shape of T cell failure.