TOX2 and TOX at the intersection of central memory and exhaustion in human CAR T cells

CAR T therapy has provided a breakthrough in the treatment of hematologic malignancies, showing remarkable efficacy in cancers such as leukemia and lymphoma. However, the success of CAR T therapy in treating solid tumors has been limited due to T cell exhaustion, which limits their proliferation and cytotoxicity when chronically stimulated with antigen, such as in cancer. Genetic modification of CAR T cells is one approach to improve tumor killing capacity. We profiled a patient with chronic lymphocytic leukemia (CLL) who responded exceedingly well to CAR T therapy, finding increased transcription and chromatin opening at the transcription factor TOX2. We show, in an in vitro CAR T model, that TOX2 is required for the development of central memory T cells (TCMs), the preponderance of which underlay the positive therapeutic response. Our results show that TOX2 can induce a TCM phenotype and gene expression program by binding to the promoters of many TCM genes. In contrast, some studies in mice position TOX2 as a positive regulator of T cell exhaustion, like the closely related transcription factor TOX. We characterize TOX as a master regulator of T cell exhaustion in mice. We show that TOX works with a chromatin regulator to bind chromatin and positively regulate transcription of other exhaustion-related genes in mice, such as TOX2. Despite this similarity in mouse models of exhaustion, while TOX2 promotes TCM in our model of human CAR T therapy, we find that TOX is not required for TCMs, since TCMs increase when TOX expression is lost. However, when TOX2 is overexpressed at very high levels, it begins to induce exhaustion gene signatures and bind to exhaustion-related loci. We propose a model for TOX2 as a regulator of both central memory and exhaustion—two T cell subsets that are surprisingly similar—and we posit that the exact level of TOX2 expression could be responsible for the balance between them in certain contexts. Our results present a new role for TOX2 as a regulator of central memory, which distinguishes it from TOX, and introduce the potential for TOX2 overexpression to improve the potency of CAR T therapy.