Bruce Freimark

Phosphatidylserine is a global immunosuppressive signal in efferocytosis, infectious disease, and cancer.

Apoptosis is an evolutionarily conserved and tightly regulated cell death modality. It serves important roles in physiology by sculpting complex tissues during embryogenesis and by removing effete cells that have reached advanced age or whose genomes have been irreparably damaged. Apoptosis culminates in the rapid and decisive removal of cell corpses by efferocytosis, a term used to distinguish the engulfment of apoptotic cells from other phagocytic processes. Over the past decades, the molecular and cell biological events associated with efferocytosis have been rigorously studied, and many eat-me signals and receptors have been identified. The externalization of phosphatidylserine (PS) is arguably the most emblematic eat-me signal that is in turn bound by a large number of serum proteins and opsonins that facilitate efferocytosis. Under physiological conditions, externalized PS functions as a dominant and evolutionarily conserved immunosuppressive signal that promotes tolerance and prevents local and systemic immune activation. Pathologically, the innate immunosuppressive effect of externalized PS has been hijacked by numerous viruses, microorganisms, and parasites to facilitate infection, and in many cases, establish infection latency. PS is also profoundly dysregulated in the tumor microenvironment and antagonizes the development of tumor immunity. In this review, we discuss the biology of PS with respect to its role as a global immunosuppressive signal and how PS is exploited to drive diverse pathological processes such as infection and cancer. Finally, we outline the rationale that agents targeting PS could have significant value in cancer and infectious disease therapeutics.

Phosphatidylserine Sensing by TAM Receptors Regulates AKT-dependent Chemoresistance and PD-L1 Expression

Tyro3, Axl, and Mertk (collectively TAM receptors) are three homologous receptor tyrosine kinases that bind vitamin K–dependent endogenous ligands, Protein S (ProS), and growth arrest–specific factor 6 (Gas6), and act as bridging molecules to promote phosphatidylserine (PS)-mediated clearance of apoptotic cells (efferocytosis). TAM receptors are overexpressed in a vast array of tumor types, whereby the level of expression correlates with the tumor grade and the emergence of chemo- and radioresistance to targeted therapeutics, but also have been implicated as inhibitory receptors on infiltrating myeloid-derived cells in the tumor microenvironment that can suppress host antitumor immunity. In the present study, we utilized TAM-IFNγR1 reporter lines and expressed TAM receptors in a variety of epithelial cell model systems to show that each TAM receptor has a unique pattern of activation by Gas6 or ProS, as well as unique dependency for PS on apoptotic cells and PS liposomes for activity. In addition, we leveraged this system to engineer epithelial cells that express wild-type TAM receptors and show that although each receptor can promote PS-mediated efferocytosis, AKT-mediated chemoresistance, as well as upregulate the immune checkpoint molecule PD-L1 on tumor cells, Mertk is most dominant in the aforementioned pathways. Functionally, TAM receptor–mediated efferocytosis could be partially blocked by PS-targeting antibody 11.31 and Annexin V, demonstrating the existence of a PS/PS receptor (i.e., TAM receptor)/PD-L1 axis that operates in epithelial cells to foster immune escape. These data provide a rationale that PS-targeting, anti–TAM receptor, and anti–PD-L1-based therapeutics will have merit as combinatorial checkpoint inhibitors. Implications: Many tumor cells are known to upregulate the immune checkpoint inhibitor PD-L1. This study demonstrates a role for PS and TAM receptors in the regulation of PD-L1 on cancer cells. Mol Cancer Res; 15(6); 753–64. ©2017 AACR.

Antibody-Mediated Phosphatidylserine Blockade Enhances the Antitumor Responses to CTLA-4 and PD-1 Antibodies in Melanoma.

Blocking phosphatidylserine (PS) with antibodies reprograms the tumor microenvironment from immunosuppressive to immunosupportive and reactivates innate and adaptive antitumor immunity. Combining PS targeting with immune checkpoint blockade improved the therapeutic efficacy of both approaches in two preclinical tumor models. In tumor-bearing animals, the membrane phospholipid phosphatidylserine (PS) suppresses immune responses, suggesting that PS signaling could counteract the antitumor effect of antibody-driven immune checkpoint blockade. Here, we show that treating melanoma-bearing mice with a PS-targeting antibody enhances the antitumor activity of downstream checkpoint inhibition. Combining PS-targeting antibodies with CTLA-4 or PD-1 blockade resulted in significantly greater inhibition of tumor growth than did single-agent therapy. Moreover, combination therapy enhanced CD4+ and CD8+ tumor-infiltrating lymphocyte numbers; elevated the fraction of cells expressing the proinflammatory cytokines IL2, IFNγ, and TNFα; and increased the ratio of CD8 T cells to myeloid-derived suppressor cells and regulatory T cells in tumors. Similar changes in immune cell profiles were observed in splenocytes. Taken together, these data show that antibody-mediated PS blockade enhances the antitumor efficacy of immune checkpoint inhibition. Cancer Immunol Res; 4(6); 531–40. ©2016 AACR.

Antibody-mediated phosphatidylserine blockade significantly enhances the efficacy of downstream immune checkpoint inhibition in K1735 mouse melanoma

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. In the present study, the antitumor effect of the combination of a PS-targeting antibody with antibodies that inhibit the downstream immune checkpoints PD-1 or CTLA-4 antibody 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. Combination therapy potently suppressed tumor growth and improved overall survival compared to single agent treatment. Flow cytometry revealed that combination therapy induced the highest ratio of tumor-infiltrating immune effector to suppressor cells. Importantly, combination treatment also significantly decreased the levels of myeloid-derived suppressor cells (MDSC) in the spleen. In addition, inhibition of PS and PD-1 or CTLA-4 resulted in significantly more IL-2 and IFNg-secreting splenic CD4+ and CD8+ T cells than any single agent treatment. Finally, combined immune checkpoint blockade did not induce any observable toxicity following multiple treatment doses. In summary, our findings demonstrate that the combination of antibody-mediated PS blockade with an inhibition of established immune checkpoints (e.g., PD-1 and CTLA-4) represents a promising strategy for cancer immunotherapy.

Abstract 574: Phosphatidylserine targeting antibody in combination with tumor radiation and immune checkpoint blockade promotes anti-tumor activity in mouse B16 melanoma

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

Abstract B27: Targeting of phosphatidylserine by monoclonal antibody ch1N11 enhances the antitumor activity of immune checkpoint inhibitor PD-1/PD-L1 therapy in orthotopic murine breast cancer models

Introduction: Phosphatidylserine (PS) is a phospholipid that resides in the plasma membrane inner leaflet in many types of cells, including both tumor and tumor associated endothelial cells. Conditions that cause cellular stress, including those that occur from oxygen radicals, hypoxia, irradiation, and chemotherapy, cause a dramatic shift in PS localization in both tumor and tumor associated endothelial cells. This change in localization results in PS shifting to the outer plasma membrane, allowing its recognition by components of the tumor microenvironment. Recognition of PS promotes an immunosuppressive environment that encourages tumor growth, in part by promoting the recruitment of myeloid derived suppressor cells, immature dendritic cells, and M2-like macrophages, in addition to inducing production of anti-inflammatory cytokines. Currently the chimeric PS-targeting antibody, bavituximab, is being studied in combination with chemotherapies to treat patients with solid tumors in multiple late-stage clinical trials, where it is believed to help augment the efficacy of chemotherapeutics by blockade of PS-mediated immunosuppression and triggering an Fc-FcR mediated pro-inflammatory response in the tumor microenvironment. The results with PS targeting therapies and chemotherapeutics are encouraging, and the effectiveness of PS targeting therapies in combination with therapies directed towards immune checkpoint regulators warrants further attention. Methods: Immune competent mice bearing established syngeneic EMT-6 or E0771 breast tumors were subjected to treatments comprising of a PS targeting antibody (ch1N11) and an anti-PD-1 antibody (to interrupt the PD-1/PD-L1 signaling axis) either as single or combination therapy. The anti-tumor effects of treatments were determined by measuring primary tumor growth inhibition and specific immunity was determined by tumor re-challenge. Results: In both models, which showed distinct sensitivity to therapy, the combination of ch1N11 with an anti-PD-1 checkpoint blocking antibody had a significantly greater anti-tumor affect than single arm treatments. In the E0771 model, complete tumor regression was observed in 6 of 10 animals treated with combination treatment. Animals with no tumor growth for 30 days post study end were resistant to tumor re-challenge indicating the development of tumor-specific immunity. Conclusions: These results suggest that the combination of PS targeting antibodies in conjunction with checkpoint inhibitors has the potential to block tumor immunosuppression in breast cancer and promote a durable antitumor immune response. Citation Format: Michael J. Gray, Jian Gong, Van Nguyen, Michaela Schuler-Hatch, Chris Hughes, Jeff Hutchins, Bruce Freimark. Targeting of phosphatidylserine by monoclonal antibody ch1N11 enhances the antitumor activity of immune checkpoint inhibitor PD-1/PD-L1 therapy in orthotopic murine breast cancer models. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr B27.

Targeting of phosphatidylserine by monoclonal antibodies augments the activity of paclitaxel and anti-PD1/PD-L1 therapy in the murine breast model E0771

The phospholipid lipid phosphatidylserine (PS) normally resides in the inner plasma membrane leaflet in most mammalian cells, including tumor and tumor associated vascular cells. Inducers of cellular stress, such as hypoxia and oxygen radicals encountered in tumors, and treatments by cytotoxic therapies promote PS relocation to the outer leaf of the plasma membrane. In tumors this re-localization allows PS recognition by a number of receptors on myeloid and lymphoid cells in the microenvironment, promoting tumor growth and metastatic disease through the development of an immunosuppressive environment. Currently the PS-targeting antibody bavituximab is being used to treat patients with solid tumors in multiple late-stage clinical trials. Bavituximabs anti-tumor properties are attributed in part through alleviating PS-receptor mediated immunosuppression and assisting in generating an Fc-FcR mediated pro-inflammatory response.

Antibody-mediated blockade of phosphatidylserine enhances the anti-tumor activity of targeted therapy and immune checkpoint inhibitors by affecting myeloid and lymphocyte populations in the tumor microenvironment

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 immunosuppressive cytokines (e.g., TGF-beta and IL-10) and tumor infiltrating myeloid-derived suppressor cells (MDSCs), regulatory T cells (Tregs) and M2 macrophages that can occupy up to 50% of the tumor mass. The membrane phospholipid, phosphatidylserine (PS), is an upstream immune checkpoint. In normal non-tumorigenic cells, PS is segregated to the inner leaflet of the plasma membrane but becomes externalized to the outer leaflet of the plasma membrane in cells in the tumor microenvironment. PS is recognized and bound by PS receptors on immune cells where it induces and maintains immune suppression. PS-targeting agents block PS-mediated immunosuppression by multifocal reprograming of the immune cells in the tumor microenvironment to support immune activation. Antibody-mediated PS blockade reduces the levels of MDSC, TGF-beta, and IL-10 and increases the levels of TNF-alpha and IL-12. PS blockade also re-polarizes tumor-associated macrophages (TAMs) from predominant M2 to predominant M1 phenotype, promotes the maturation of dendritic cells (DCs) and induces potent adaptive antitumor T cell immunity. In a Phase II clinical study, immunohistochemical evaluation of HCC tumor tissues post combination treatment indicated an increase of immune infiltrates; raising the potential of a clinically meaningful anti-tumor immune response. Pre-clinically, we demonstrate that PS targeting agents enhance the anti-tumor activity of anti-CTLA-4 and anti-PD-1 antibodies in immunocompetent models of melanoma (B16 and K1735) and breast (EMT-6) cancer and that tumor growth inhibition correlates with an increase in the infiltration of activated T cells and myeloid cells and the induction of adaptive immunity. In summary, PS blockade in combination with targeted therapy and other immune checkpoint inhibitors promotes a robust, localized, anti-tumor response and represents a promising strategy to enhance cancer immunotherapy.

Abstract 2850: Predicting anti-tumor responses to phosphatidylserine targeting antibodies using tumor imaging.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC Phosphatidylserine (PS) is a phospholipid normally residing in the inner leaflet of the plasma membrane and becomes exposed on tumor vascular endothelial cells and tumor cells in response to chemotherapy, irradiation and oxidative stresses in the tumor microenvironment. Binding of antibodies targeting PS on the tumor endothelial cells and tumors induces the recruitment of immune cells and engages the immune system to destroy tumor vasculature. The antibodies also enhance anti-tumor immunity by blocking the immunosuppressive action of PS. A chimeric PS-targeting antibody, bavituximab, is being used in combination with chemotherapy to treat patients with solid tumors in Phase II trials. Fully human antibody PGN635 binds PS through the interaction of beta-2-glycoprotein 1 (β2GP1) in the same manner as bavituximab. Using human PC-3 prostate tumor xenografts in SCID mice, we demonstrate that targeting of PS in tumors by PGN635 is enhanced by chemotherapy. Combination of PGN635 with docetaxel inhibited tumor growth compared to the control IgG plus docetaxel group (p<0.05). Near-infrared optical imaging of PS in tumors with PGN650, an F(ab’)2 antibody fragment of PGN635, showed tumor growth inhibition in mice treated with docetaxel correlates with PS expression levels in the tumors. Maximal uptake of the PS imaging was observed when chemotherapy was given 24 hours before the imaging probe. Citation Format: Jian Gong, Richard Archer, Van Nguyen, Christopher C.W. Hughes, Jeff Hutchins, Bruce Freimark. Predicting anti-tumor responses to phosphatidylserine targeting antibodies using tumor imaging. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2850. doi:10.1158/1538-7445.AM2013-2850

Antibody targeting of phosphatidylserine for the detection and immunotherapy of cancer

Phosphatidylserine (PS) is a negatively charged phospholipid in all eukaryotic cells that is actively sequestered to the inner leaflet of the cell membrane. Exposure of PS on apoptotic cells is a normal physiological process that triggers their rapid removal by phagocytic engulfment under noninflammatory conditions via receptors primarily expressed on immune cells. PS is aberrantly exposed in the tumor microenvironment and contributes to the overall immunosuppressive signals that antagonize the development of local and systemic antitumor immune responses. PS-mediated immunosuppression in the tumor microenvironment is further exacerbated by chemotherapy and radiation treatments that result in increased levels of PS on dying cells and necrotic tissue. Antibodies targeting PS localize to tumors and block PS-mediated immunosuppression. Targeting exposed PS in the tumor microenvironment may be a novel approach to enhance immune responses to cancer.