Abstract PR09: Antibody-mediated blockade of phosphatidylserine synergizes with immune checkpoint blockade by inhibiting multiple immune suppressive mechanisms
Authors: Huang Xianming , Gong Jian , Gray Michael , Nguyen Van , Parks Ryan , Hughes Chris , Hutchins Jeff , Brekken Rolf , Freimark Bruce 
Published: October 2016

Abstract
Background: The expression of phosphatidylserine (PS) on cell surfaces drives immunosuppressive mechanisms associated with tolerogenic cell death. In the tumor microenvironment, PS is exposed on tumor cells and tumor vascular endothelial cells treated with conventional therapies. PS signals through multiple immune cell signaling receptors where it drives the expansion of myeloid-derived suppressor cells (MDSCs), regulatory T cells, M2 macrophages, and stimulates their production of immunosuppressive cytokines (e.g., TGFβ and IL-10), rendering the tumor microenvironment non-responsive to immune activation.
PS targeting antibodies have significant anti-tumor effects in multiple tumor xenografts, syngeneic tumors, and genetically engineered preclinical tumor models through mechanisms that re-program the tumor microenvironment from immunosuppressive to immune potentiating.
Methods: The combination of PS-targeting and anti-PD1 antibodies was compared to single agent therapy in multiple pre-clinical mouse models, including syngeneic melanoma and breast tumors, and a genetically engineered mouse model (GEMM) of pancreatic cancer. Mice were treated weekly IP at 5 mg/kg with a PS targeting antibody, anti-PD1, or the combination for the duration of each experiment; tumor and spleen tissue microenvironments were immune profiled by FACS, ELISPOT, and immunohistochemistry.
Results: In all tumor models examined, the anti-tumor effect of combination therapy was significantly superior to single agent therapy. In B16 and K1735 melanoma tumors, the combination more than doubled the anti-tumor effect of anti-PD-1 alone. Combination therapy significantly inhibited tumor growth by over 90% in E077.1 breast tumors and prolonged animal survival by 30% (n&gt;25 each group) in the GEMM of pancreatic cancer despite the fact that anti-PD1 therapy alone was ineffective in each of these models. Analysis of immune cell subsets in the tumor microenvironment indicated that the combination of antibody-mediated blockade of PS and PD-1 significantly enhanced the effector function of tumor infiltrating CD8+ T cells and increased the ratio of T effector to T regulatory cells, as demonstrated by significant increases in TILs producing IFNγ, TNFα, IL-2, granzyme B, and Ki-67. Combination therapy re-conditioned the tumor microenvironment to favor immune potentiation, as demonstrated by significant decreases in the frequency of MDSC, the ratio of M2 to M1 macrophages, the expression of surface PD-L1, and reductions in TGFβ and IL-10. Furthermore, combination treatment induced systemic tumor specific CD8 T cell immunity, as mice demonstrating complete responses (in the K1735 and E077.1 models) rejected tumor cells upon re-challenge. Splenocytes from these mice had significantly higher numbers of tumor-specific IFNγ-producing cells in ELISPOT assay. Studies were completed without toxicity in any setting.
Conclusions: Antibody-mediated PS blockade inhibits PS-mediated immune suppression and stimulates FcRγ activation. This results in immune activation that enables an otherwise non-responsive tumor microenvironment to respond to checkpoint inhibition.
This abstract is also presented as Poster B08.
Citation Format: Xianming Huang, Jian Gong, Michael Gray, Van Nguyen, Ryan Parks, Chris Hughes, Jeff Hutchins, Rolf Brekken, Bruce Freimark. Antibody-mediated blockade of phosphatidylserine synergizes with immune checkpoint blockade by inhibiting multiple immune suppressive mechanisms. [abstract]. In: Proceedings of the AACR Special Conference: Function of Tumor Microenvironment in Cancer Progression; 2016 Jan 7–10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2016;76(15 Suppl):Abstract nr PR09.