Targeting Phosphatidylserine Enhances the Anti-Tumor Response to Tumor-Directed Radiation Therapy in a Preclinical Model of Melanoma

Phosphatidylserine (PS) is a phospholipid that is exposed on surface of apoptotic cells and has been shown to promote immunosuppressive signals in the tumor microenvironment (TME). Antibodies that block PS interaction with its receptors have been shown to repolarize tumor associated macrophages (TAMs) into a pro-inflammatory (M1) state, reduce the number of myeloid derived suppressor cells (MDSCs) in tumors and promote maturation of dendritic cells (DCs). Radiation therapy (RT) is an effective focal treatment of isolated solid tumors but is less effective in controlling metastatic cancers as a monotherapy. Tumor-directed RT is known to induce immunogenic cell death and enhance tumor-specific T cell infiltration into treated tumors. We found that a single dose of 15 Gy of tumor-directed RT on mouse B16 melanoma causes an increase in expression of PS on the surface of viable immune infiltrates. We hypothesize that expression of PS on immune cells may act similar to immune checkpoint molecules and provide a negative feedback mechanism to immune cells. Therefore, we posit that blocking PS may enhance the efficacy of tumor-directed RT. Indeed, we found that treatment with an antibody that targets PS (mch1N11) enhanced the anti-tumor efficacy of tumor-directed RT and improved overall survival. This combination led to an increase in M1 TAMs and a corresponding decrease in M2 TAMs which translated to a more pro-inflammatory TME. The addition of anti-PD-1 to RT and mch1N11 led to even greater anti-tumor efficacy and overall survival. Anti-PD-1 exerted its effects primarily by enhancing CD8+ T cell infiltration, activation, and effector function in the triple combination. Consistent with the mouse studies, we found increased PS expression on several immune subsets in the blood in patients with metastatic melanoma after receiving tumor-directed RT. These findings highlight the potential of combining PS targeting with RT and PD-1 pathway blockade to improve outcomes in patients with advanced-stage cancers and may inform the design of clinical studies combining PS-targeting antibodies with RT and/or checkpoint blockade.