Abstract 5846: A potent STING agonist induces endothelial PD-L1 and enhances antitumor efficacy of a novel PD-L1/PD-L2 antibody

Background: The activation of stimulator of interferon genes (STING) has emerged as a promising strategy to reprogram the tumor microenvironment (TME) and enhance antitumor immunity. We previously demonstrated that combining a potent STING agonist (8803, IMGS-203) with a novel dual-specific anti-PD-L1/PD-L2 monoclonal antibody (27907) produced curative responses in checkpoint-refractory tumor models. In this study, we investigated the mechanisms underlying these effects by analyzing the TME of tumors treated locally with 8803 and systemically with 27907 using immunohistochemistry and flow cytometry. Objective: To evaluate the necrobiotic effects of the STING agonist 8803 combined with the cytotoxic anti-PD-L1/PD-L2 antibody 27907 across multiple tumor models by characterizing the TME and tumor vascularization. Methods: The efficacy of 8803 was assessed in vivo in combination with 27907 in murine melanoma (B16F10 expressing mouse PD-L2) and mammary adenocarcinoma (TS/A) models. Mice with established tumors received intratumor injections of 8803 (10 ug/dose, twice) and systemic administration of 27907 (10 mg/kg, twice weekly for three weeks). Tumors were harvested 32 days post-challenge and analyzed via H&E staining, IHC, and FACS to evaluate necrobiosis, immune cell infiltration, and vascularization. Results: The combination of 8803 with 27907 demonstrated robust antitumor activity in both melanoma and breast cancer models. IHC and FACS profiling revealed a significant increase in infiltrating myeloid cells alongside a notable reduction of the immunosuppressive M2 macrophage phenotype (CD206+). Intratumor 8803 markedly upregulated PD-L1 expression in endothelial cells, that were effectively disrupted by systemic 27907, as evidenced by increased caspace-3 and CD62p co-expression in endothelial cells within treated tumors. This endothelial disruption was absent in tumors treated with 8803 alone. Further, western blot and FACS analyses of human and murine endothelial cells co-cultured with 8803 confirmed upregulation of PD-L1 and PD-L2. Critically, PD-L1+/PD-L2+ endothelial cells were selectively eliminated by monocytic-enriched bone marrow-derived myeloid cells and PBMCs in the presence of 27907, suggesting a direct effect of the antibody on tumor vasculature. Conclusion: Tumors treated with the combination of 8803 and 27907 exhibited extensive necrobiosis and endothelial damage, uniquely observed in the combination treatment group and correlating with curative outcomes. This enhanced antitumor response was associated with increased myeloid infiltration, a shift away from M2 macrophage phenotype, and an elevated effector T-cell population. These findings underscore the potent therapeutic synergy of 8803 with 27907 and support their further development for clinical applications. Citation Format: Ahmad Salameh, Manuela iezzi, Andrew Lewis, Christine Gagliardi, Laura Conti, Elisabetta Bolli, Chiara Cossu, Martina CECI, Paul Blezinger, Michael A. Curran, Federica Cavallo, James Barlow, Federica Pericle. A potent STING agonist induces endothelial PD-L1 and enhances antitumor efficacy of a novel PD-L1/PD-L2 antibody [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 5846.