GITR is a costimulatory receptor currently undergoing phase I clinical trials. Efficacy of anti-GITR therapy in syngeneic mouse models requires regulatory T-cell depletion and CD8⁺ T-cell costimulation. It is increasingly appreciated that immune cell proliferation and function are dependent on cellular metabolism. Enhancement of diverse metabolic pathways leads to different immune cell fates. Little is known about the metabolic effects of GITR agonism; thus, we investigated whether costimulation via GITR altered CD8⁺ T-cell metabolism. We found activated, GITR-treated CD8⁺ T cells upregulated nutrient uptake, lipid stores, glycolysis, and oxygen consumption rate (OCR) in vitro Using MEK, PI3Kδ, and metabolic inhibitors, we show increased metabolism is required, but not sufficient, for GITR antibody (DTA-1)-induced cellular proliferation and IFNγ production. In an in vitro model of PD-L1-induced CD8⁺ T-cell suppression, GITR agonism alone rescued cellular metabolism and proliferation, but not IFNγ production; however, DTA-1 in combination with anti-PD-1 treatment increased IFNγ production. In the MC38 mouse tumor model, GITR agonism significantly increased OCR and IFNγ and granzyme gene expression in both tumor and draining lymph node (DLN) CD8⁺ T cells ex vivo, as well as basal glycolysis in DLN and spare glycolytic capacity in tumor CD8⁺ T cells. DLN in GITR-treated mice showed significant upregulation of proliferative gene expression compared with controls. These data show that GITR agonism increases metabolism to support CD8⁺ T-cell proliferation and effector function in vivo, and that understanding the mechanism of action of agonistic GITR antibodies is crucial to devising effective combination therapies. Cancer Immunol Res; 6(10); 1199-211. ©2018 AACR.

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