Treatment With Agonist Anti-Gitr Antibody After Chemotherapy Enhances Tumor Immunity

Stimulation of glucocorticoid-induced tumor necrosis factor receptor (GITR) has been shown to enhance antitumor immunity by stimulating effector CD4 + and CD8 + T cells and attenuating suppression and depleting by CD4 + Foxp3 + regulatory T cells (T reg ). However, GITR monotherapy does not effectively control tumor growth. We hypothesize that cyclophosphamide (CTX), a cytotoxic chemotherapeutic agent with key immunomodulatory properties, can enhance the potency of GITR engagement anti-tumor effects. Further, we hypothesize that CTX can induce neoantigen formation by increasing tumor mutation burden. In this study, we administered high-dose CTX (250 mg/kg) followed the next day by an agonist anti-GITR antibody (DTA-1) to C56BL/6 mice bearing poorly immunogenic B16 melanoma tumors. We found that the combination therapy can cause regression of established tumors whereas the single agents did not. Mechanistically, we found that CTX administration results in tumor antigen release, which enhances the antitumor T cell response after treatment with DTA-1. This effect is lost when lower doses of CTX are given or CTX is administered prior to tumor inoculation, suggesting that tumor cell death and cross-presentation of tumor antigens to T cells is necessary. Furthermore, T reg cells are an important target of combination therapy. Intratumoral T reg levels were substantially reduced in treated mice, which greatly increased the ratio of T effector/T reg in the tumor. In conclusion, CTX and agonist anti-GITR antibody slows tumor progression and can induce regression of palpable, poorly immunogenic B16 melanoma tumors, likely by enhancing cross-priming and increasing the T effector/T reg ratio in the tumor. Thus, this combination warrants further investigation as a novel immunotherapy approach and provides rational for exploring combination of GITR agonist antibodies with chemotherapeutic treatments in clinical settings. Citation Format: Allison Betof Warner, Daniel Hirschhorn, Adam Cohen, Cailian Liu, Walter Newman, Cyndi Sirard, Jedd Wolchok, Taha Merghoub. Treatment with agonist anti-GITR antibody after chemotherapy enhances tumor immunity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1699.