Abstract 3651: Phosphatidylserine-targeting antibody induces differentiation of myeloid-derived suppressor cells into M1-like macrophages

Abstract

Abstract Introduction: The phosphatidylserine (PS)-targeting antibody, bavituximab, is currently in Phase IIb clinical trials in patients with lung cancer. Bavituximab, and its murine counterpart, 2aG4, induce the attack of monocytes and macrophages on PS-expressing tumor vascular endothelium and tumor cells and inhibit the immunosuppressive effects of PS in the tumor microenvironment. Myeloid-derived suppressor cells (MDSC) are one of the major cells responsible for the immunosuppressed state in tumors. In this study, we tested the influence of 2aG4 on the differentiation of MDSC into M1-like tumor associated macrophages (TAMs). Methods: MDSC were isolated from 4T1 tumor bearing mice with anti-Gr1-coated magnetic beads. The purified MDSC were then cultured for 5 days in the presence of 2aG4 or control C44 antibody. For in vivo studies, PC3 tumor-bearing mice were treated with 2aG4 for 30 days and MDSC and TAMs in tumors and spleens were analyzed by FACS and immunohistochemistry. Results: 2aG4 treatment of purified MDSC switched their cytokine production from an immunosuppressive IL-10-dominated response to a pro-inflammatory IL-12- and TNFα -dominated response. The percentage of Gr1+ cells decreased to 8% in the 2aG4-treated cultures (P<0.0001) but only to 50 – 57% in the PBS and C44-treated cultures. Treatment with 2aG4 induced the differentiation of MDSC into M1-like macrophages that expressed lower CD206 and produced more NO than control cultures. Treatment of mice bearing PC3 prostate tumors with 2aG4 decreased the percentage of MDSC from 7% to 4% (P<0.001) in the tumors and from 28% to 20% (P<0.001) in the spleens. The antibody treatment also increased the ratio of M1 to M2 TAMs in PC3 tumors from 0.7% to 1.4% (P<0.001). Conclusion: Taken together, our results suggest that 2aG4 causes the differentiation of MDSCs into macrophages having an M1-like phenotype. 2aG4 treatment decreased IL-10 production and increased IL-12 and TNFα-production. These results suggest that 2aG4 treatment reactivates innate immunity in tumors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3651. doi:10.1158/1538-7445.AM2011-3651