Abstract B061: Sialylation-driven MDSC generation shapes the ovarian tumor immune microenvironment

Abstract Background: Cancer cells employ various mechanisms to establish a tolerogenic tumor immune microenvironment (TIME). Identifying key pathways that facilitate this immune suppression is critical for developing effective therapeutic strategies. This is especially important for ovarian cancer (OC), which showed limited response to current immune therapies. Sialylation, the addition of sialic acid to terminal ends of glycans, has been suggested to play a role in this process. In this study, we investigate the impact of cancer cell sialylation on tumor dynamics and the TIME. Our findings demonstrate that sialylation is a crucial regulator of the ovarian TIME, primarily influencing the expansion of myeloid-derived suppressor cells (MDSC). Methods: Hyper- and hyposialylated OC cells were established from mCherry+ Triple knockout (p53LSL-R172H/Dicerflox/flox/Ptenflox/flox) mouse OC cells by FACS using Sambucus Nigra (SNA) lectin, which recognizes ⍺2,6-linked sialic acids. Cells were injected intra-peritoneally (i.p.) in female C57BL/6 mice and tumor burden was measured by live imaging while seeding was determined by microscopy. Single cell RNA sequencing was performed on omental tumors and immune phenotype was characterized on peritoneal lavage by flow cytometry. MDSC was depleted using anti-Gr1. Results: We compared tumor distribution, kinetics, and TIME in mice with hyper- or hyposialylated ovarian tumors. On day 14, mCherry+ OC cells were detected in the omentum of both groups, indicating successful seeding. Tumors from hypersialylated OC cells were detected at day 25, while hyposialylated OC cells showed no measurable disease up to 80 days, although histology revealed micrometastases and significant immune cell infiltration at that time. Single-cell sequencing showed enrichment in hypersialylated tumors of cells expressing immunosuppressive genes (S100a8/9, Hdc, IL1r2) with enriched GO processes related to myeloid differentiation and reactive oxygen species metabolism, suggesting MDSC. In contrast, hyposialylated tumors had more cells expressing CD3, Itk, Skap1, and Trbc2, with enriched GO processes linked to T cell proliferation and TCR signaling, suggesting T cell infiltration and activation. Flow cytometry showed a progressive increase in CD11b+/Ly6Clow/Ly6G+/Arg1+ MDSC only in hypersialylated tumors. Depleting MDSC in hypersialylated tumors significantly slowed tumor growth and improved overall survival. The depletion also altered the TIME, increasing CD11b+MHCII+ M1 macrophages, CD8+IFNγ+ cytotoxic T cells, and NK1.1+ IFNγ+ active NK cells compared to controls. Conclusion: Sialylation plays a crucial role in shaping the immune suppressive ovarian TIME. Notably, the expansion of MDSC appears to be a key factor influencing both immune responses and tumor progression. Depleting MDSC in hypersialylated tumors was sufficient to shift the environment from a tolerogenic state to one characterized by an active cytotoxic immune response. This highlights the potential of targeting sialylation to modulate immune dynamics in ovarian cancer. Citation Format: Ayesha Alvero, Alexandra Fox, Aditi Singh, Nicholas Adzibolosu, Sapna Sharma, Garry Leonard, Charlie Fehl, Radhika Gogoi, Gil Mor. Sialylation-driven MDSC generation shapes the ovarian tumor immune microenvironment [abstract]. In: Proceedings of the AACR IO Conference: Discovery and Innovation in Cancer Immunology: Revolutionizing Treatment through Immunotherapy; 2025 Feb 23-26; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Immunol Res 2025;13(2 Suppl):Abstract nr B061.