Abstract A46: Antibody-mediated phosphatidylserine blockade significantly enhances the efficacy of downstream immune checkpoint inhibition in K1735 and B16 mouse melanoma
Authors: Freimark Bruce , Gong Jian , Ye Dan , Nguyen Van , Yin Shen , Archer Rich , Hughes Chris , Hutchins Jeff , Hutchins Jeff , Schroit Alan , Brekken Rolf , Huang Xianming 
Published: October 2015

Abstract
Background: The underlying cause for the failure of immune checkpoint blockade is the overwhelming, persistent and multifocal immune suppression in the tumor microenvironment. This is due to the absence of pre-existing antitumor Teff because of the action of important upstream immune checkpoints that recruit tumor infiltrating myeloid-derived suppressor cells (MDSCs), regulatory T cells (Tregs) and M2 macrophages, which also produce potent immunosuppressive cytokines (e.g., TGF-beta and IL-10). Phosphatidylserine (PS) is an upstream immune checkpoint that drives global immunosuppression. Previous work has shown that PS targeting agents can override PS-driven immunosuppression and re-program the tumor microenvironment from immunosuppressive to immunosupportive, break tumor immune tolerance, and elicit potent de novo antitumor T-cell immunity.
Methods: In the present study, the antitumor effect of the combination of a PS-targeting antibody with antibodies that inhibit the immune checkpoints programmed death receptor-1 (PD-1) or cytotoxic T-lymphocyte associated antigen (CTLA-4) in the K1735 mouse melanoma model was examined. Tumor-bearing mice were treated with each antibody alone or the combination at 5 to 10 mg/kg, twice a week. Tumor growth, survival, as well as intra-tumoral and peripheral immune cell profiles after treatment were assessed by FACS and IHC.
Results: Here we demonstrate that combination of PS-targeting antibody (mch1N11) with anti-PD-1 or anti-CTLA-4 antibodies significantly improved therapeutic efficacy in mice bearing K1735 melanoma compared to single agent treatment. Similar results were seen in the B16 mouse melanoma model. Importantly, we found that the level of MDSCs in the spleen was significantly reduced by combination treatment, 9.9% in the combination treated group, as compared with 18.27% and 14.69% in anti-PD-1 (p&lt;0.0005) or control C44 (p&lt;0.05) treated group, respectively. In addition, combination therapy had the highest ratio of tumor-infiltrating immune effector to suppressor cells and the most IL-2 and IFNγ-secreting T cells in the spleen in response to non-specific stimulation. The percentage of IFNγ-secreting CD8+ T cells in the combination group was 5.85%, as compared with 4.1% and 4.0% in anti-PD-1 or control C44 group, respectively (p&lt;0.05). The percentage of IL-2-secreting CD8+ T cells in the combination group was 2.4%, as compared with 1.29% and 1.17% in anti-PD-1 or control C44 group, respectively (p&lt;0.05). The percentage of IL-2-secreting CD4+ T cells in the combination group was 6.5%, as compared with 2.9% and 3.2% in anti-PD-1 or control C44 group, respectively (p&lt;0.05). Finally, combination of PS-targeting with downstream immune checkpoint blockade did not induce any observable toxicity following multiple treatment doses.
Conclusions: Our findings indicate that the combination of PS-blockade with blockade of downstream immune checkpoints (e.g., PD-1 and CTLA-4) represents a promising strategy to enhance the efficacy of cancer immunotherapy.
Citation Format: Bruce Freimark, Jian Gong, Dan Ye, Van Nguyen, Shen Yin, Rich Archer, Chris Hughes, Jeff Hutchins, Jeff Hutchins, Alan Schroit, Rolf Brekken, Xianming Huang. Antibody-mediated phosphatidylserine blockade significantly enhances the efficacy of downstream immune checkpoint inhibition in K1735 and B16 mouse melanoma. [abstract]. In: Proceedings of the AACR Special Conference: Tumor Immunology and Immunotherapy: A New Chapter; December 1-4, 2014; Orlando, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2015;3(10 Suppl):Abstract nr A46.