Abstract 574: Phosphatidylserine targeting antibody in combination with tumor radiation and immune checkpoint blockade promotes anti-tumor activity in mouse B16 melanoma
Authors: Budhu Sadna , Henau Olivier De , Zappasodi Roberta , Giese Rachel , Campesato Luis F. , Barker Christopher , Freimark Bruce , Hutchins Jeff , Wolchok Jedd D. , Merghoub Taha 
Published: July 2017

Abstract
Phosphatidylserine (PS) is a phospholipid that is exposed on surface of apoptotic cells, tumor cells and tumor endothelium. PS has been shown to promote immunosuppressive signals in the tumor microenvironment. Antibodies that target PS have been shown to reactivate anti-tumor immunity by polarizing tumor associated macrophages into a pro-inflammatory M1 phenotype, reducing the number of MDSCs in tumors and promoting the maturation of dendritic cells into functional APCs. In a mouse B16 melanoma model, targeting PS in combination with immune checkpoint blockade promoted greater anti-tumor activity than either agent alone. This combination was shown to enhance CD4+ and CD8+ T cell infiltration and activation in the tumors of treated animals. Radiation therapy (RT) is an effective focal treatment of primary solid tumors, but is less effective in treating metastatic solid tumors as a monotherapy. There is evidence that RT induces immunogenic tumor cell death and enhances tumor-specific T cell infiltration in treated tumors. The abscopal effect, a phenomenon in which tumor regression occurs outside the site of RT, has been observed in both preclinical and clinical trials when RT is combined with immunotherapy. In this study, we show that irradiation treatment of B16 melanoma causes an increase in PS expression on the surface of viable tumor and immune infiltrates. We subsequently examined the effects of combining RT with an antibody that targets PS (mch1N11) and immune checkpoint blockade (anti-PD-1) in B16 melanoma. We found that treatment with mch1N11 synergizes with RT to improve anti-tumor activity and overall survival in tumor bearing mice. In addition, the triple combination of mch1N11, RT and anti-PD-1 treatment displayed even greater anti-tumor and survival benefit. Analysis of local immune responses in the tumors of treated animals revealed an increase in tumor-associated macrophages with a shift towards a pro-inflammatory M1 phenotype after treatment with RT and mch1N11. In addition, analysis of the systemic immune responses in the spleen and tumor draining lymph nodes revealed an increase in CD8 T cell activation, effector cytokine production and differentiation into effector memory cells in the triple combination. This finding highlights the potential of combining these three agents to improve outcome in patients with advanced-stage melanoma and other cancers and may inform the design of clinical studies combining PS-targeting antibodies with RT and/or checkpoint blockade.
Citation Format: Sadna Budhu, Olivier De Henau, Roberta Zappasodi, Rachel Giese, Luis F. Campesato, Christopher Barker, Bruce Freimark, Jeff Hutchins, Jedd D. Wolchok, Taha Merghoub. Phosphatidylserine targeting antibody in combination with tumor radiation and immune checkpoint blockade promotes anti-tumor activity in mouse B16 melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 574. doi:10.1158/1538-7445.AM2017-574