Abstract 3967: Targeting Siglec-10 interactions enhances macrophage-mediated phagocytosis of pancreatic cancer

Background: Pancreatic ductal adenocarcinoma (PDAC) is one of the most aggressive and immunotherapy-resistant cancers. Tumor-associated macrophages (TAMs) contribute to this resistance by failing to phagocytose cancer cells. Cancer cells evade immune detection by upregulating sialic acid on surface glycoproteins like CD24, which bind to inhibitory Siglec receptors such as Siglec-10 on TAMs, triggering signals that block phagocytosis. While blocking Siglec-10/CD24 interactions with αCD24 antibodies has improved phagocytosis in ovarian and breast cancers, the role of Siglec-10 interactions with CD24 or other glycoproteins in PDAC immune evasion, as well as targeting Siglec-10 to enhance macrophage phagocytosis in PDAC, remains unexplored. Methods: Flow cytometry with Siglec-10 protein and affinity pulldown, followed by mass spectrometry, were used to identify glycoprotein ligands interacting with Siglec-10 on various human PDAC cell lines (PANC-1, MIAPaCa-2, Hs766T, and AsPC-1). Surface plasmon resonance (SPR) assay validated binding of these ligands to Siglec-10. To assess the role of Siglec-10 interactions in PDAC cells' evasion of macrophage-mediated phagocytosis, live-imaging immune-killing assays were performed in vitro with αCD24 or Siglec-10 blocking antibodies. Finally, the efficacy of Siglec-10 blockade in enhancing TAM-mediated killing of PDAC cells in vivo was evaluated using a subcutaneous PDAC xenograft model in immunodeficient mice injected with human monocyte-derived TAMs and treated with an isotype control or Siglec-10 blocking antibody. Results: Most PDAC cells (∼95%) bind Siglec-10, indicating sialic acid ligands. However, unlike ovarian and breast cancers, most PDAC cells do not express CD24 (∼81%), suggesting PDAC cells bind Siglec-10 through other glycoproteins. Mass spectrometry identified integrin alpha-3 (ITGA3) as a novel Siglec-10 ligand on PDAC cells. ITGA3 binds strongly to Siglec-10, as assessed by SPR, and this interaction was abrogated when sialic acid was removed from ITGA3 using sialidase enzyme. Analysis of PDAC tissue single-cell datasets confirmed that PDAC cells express significantly higher ITGA3 levels than normal cells (p<0.0001). Our newly developed antibodies against Siglec-10, but not αCD24, significantly increased macrophage phagocytosis of PDAC cells in vitro (p<0.05). In vivo, Siglec-10 blockade reduced tumor size by 36% compared to controls (p=0.0002). Conclusion: We identified ITGA3 as a novel Siglec-10 ligand enabling PDAC immune evasion from macrophage-mediated phagocytosis. Glycan-based immunotherapies, such as Siglec-10 blocking antibodies, are promising for enhancing phagocytosis and countering PDAC’s immunosuppressive environment. Citation Format: Pratima Saini, Gauri Mirji, Sajad A. Bhat, Rahul S. Shinde, Mohamed Abdel-Mohsen. Targeting Siglec-10 interactions enhances macrophage-mediated phagocytosis of pancreatic cancer [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 3967.