ER stress-induced mediator C/EBP homologous protein thwarts effector T cell activity in tumors through T-bet repression.

Understanding the intrinsic mediators that render CD8 T cells dysfunctional in the tumor microenvironment is a requirement to develop more effective cancer immunotherapies. Here, we report that C/EBP homologous protein (Chop), a downstream sensor of severe endoplasmic reticulum (ER) stress, is a major negative regulator of the effector function of tumor-reactive CD8 T cells. Chop expression is increased in tumor-infiltrating CD8 T cells, which correlates with poor clinical outcome in ovarian cancer patients. Deletion of Chop in T cells improves spontaneous antitumor CD8 T cell immunity and boosts the efficacy of T cell-based immunotherapy. Mechanistically, Chop in CD8 T cells is elevated primarily through the ER stress-associated kinase Perk and a subsequent induction of Atf4; and directly represses the expression of T-bet, a master regulator of effector T cell function. These findings demonstrate the primary role of Chop in tumor-induced CD8 T cell dysfunction and the therapeutic potential of blocking Chop or ER stress to unleash T cell-mediated antitumor immunity.