Abstract 4289: Targeting of phosphatidylserine by monoclonal antibodies enhances the activity of immune checkpoint inhibitors in breast tumors

Abstract

Abstract Phosphatidylserine (PS) is a phospholipid normally residing in the inner leaflet of the plasma membrane that becomes exposed on tumor vascular endothelial cells (ECs) and tumor cells. PS exposure becomes enhanced in response to chemotherapy, irradiation, and oxidative stresses in the tumor microenvironment. PS exposure in tumors promotes an immunosuppressive microenvironment which includes the recruitment of myeloid derived suppressor cells (MDSCs), immature dendritic cells and M2-like macrophages, as well as, the production of anti-inflammatory cytokines. Binding of PS targeting antibodies on tumor endothelial cells, tumor cells and their secreted microparticles triggers an Fc-FcR mediated pro-inflammatory cellular and cytokine response that reverses the immunosuppressive PS meditated checkpoint, thereby enhancing anti-tumor immunity. A chimeric PS-targeting antibody, bavituximab, is being used in combination with chemotherapy to treat patients with solid tumors in multiple late-stage clinical trials. Using breast tumors in immune competent mice, we demonstrate PS targeting antibodies enhance the anti-tumor activity of combination therapies including anti-PD-1 antibodies. Tumor growth inhibition correlates with statistically significant increases in the infiltration of CD8+ T cells and a reduction of myeloid-derived suppressor cells (MDSCs). The combination of these mechanisms promotes strong, localized, anti-tumor responses without the side-effects of systemic immune activation. Citation Format: Jian Gong, Shen Yin, Van Nguyen, Jeff Hutchins, Bruce D. Freimark. Targeting of phosphatidylserine by monoclonal antibodies enhances the activity of immune checkpoint inhibitors in breast tumors. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4289. doi:10.1158/1538-7445.AM2015-4289