Aberrant sialylation functions as an important modulator of all steps of malignant transformation. Therefore, targeting sialylation regulators, such as sialyltransferases and neuraminidases, is a potential strategy for treating cancer. Here, we found that elevated α2,3-sialyltransferase III (St3gal3) was associated with dismal prognosis in high-grade serous ovarian carcinoma (HGSC). St3gal3 knockdown antagonized subcutaneous tumor growth in immunocompetent, but not immunodeficient mice, with enhanced accumulation of functional CD8+ T cells and antitumor immune gene signatures. St3gal3 knockdown inhibited intraperitoneal tumor growth and repolarized tumor-associated macrophages from a protumorigenic M2-like to a tumor-suppressive M1-like phenotype. In vitro, St3gal3 knockdown tumor cells guided bone marrow–derived macrophages (BMDM) toward the M1-like phenotype under both direct contact and distant Transwell coculture conditions. Depletion of macrophages rescued the suppressed tumor growth induced by St3gal3 knockdown and completely suppressed infiltration of functional CD8+ T cells that rely on macrophage-derived CXCL10. St3gal3 engendered an immunosuppressive HGSC microenvironment characterized by an abundance of pro-tumorigenic macrophages and reduced cytotoxic T-cell infiltration. In vivo, St3gal3 knockdown improved effectiveness of dual immune checkpoint blockade (ICB) with αPD-1 and αCTLA4 antibodies. Preclinical inhibition of sialylation with ambroxol resulted in decreased tumor growth and prolonged the survival of tumor-bearing mice, which was enhanced by the addition of dual ICB. These findings indicate that altered sialylation induced by St3gal3 upregulation promotes a tumor-suppressive microenvironment in HGSC and targeting α2,3-sialylation may reprogram the immunosuppressive tumor microenvironment and improve the efficacy of immunotherapy.

Significance:

Blocking sialylation augments antitumor immunity and enhances response to immune checkpoint blockade therapy, highlighting a potential therapeutic approach for treating patients with high-grade serous ovarian cancer.

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