Siglec-9⁺ tumor-associated macrophages delineate an immunosuppressive subset with therapeutic vulnerability in patients with high-grade serous ovarian cancer

Background: The potent immunosuppressive properties of sialic acid-binding immunoglobulin-like lectin-9 (Siglec-9) on myeloid cells and lymphocytes provide a strong rationale for serving as a therapeutic target. However, the expression profile and critical role of Siglec-9 in high-grade serous ovarian cancer (HGSC) remain obscure. This study aimed to elucidate the prognostic significance of Siglec-9 expression and its predictive value for immunotherapy in HGSC.

Methods: Study enrolled two cohorts, consisting of 120 tumor microarray specimens of HGSC for immunohistochemistry (IHC) and 40 fresh tumor specimens for flow cytometry (FCM). Expression profile of Siglec-9 in immune cells was analyzed by both bioinformatics analysis and FCM. Role of Siglec-9 was studied to identify that Siglec-9⁺TAMs linked with an immunosuppressive phenotype by IHC and FCM, and block Siglec-9 was sensitive to immunotherapy by ex vivo and in vitro assays.

Results: Siglec-9 is predominantly expressed on tumor-associated macrophages (TAMs). High Siglec-9⁺TAMs were associated with inferior overall survival (OS). Both tumor-conditioned medium (TCM) and tumor ascites induced enrichment of Siglec-9⁺TAMs with protumorigenic phenotypes. Siglec-9⁺TAMs were associated with immunosuppressive tumor microenvironment (TME) characterized by exhausted CD8⁺T cells and increased immune checkpoint expression. Blockade of Siglec-9 suppressed phosphorylation of the inhibitory phosphatase SHP-1 and repolarized TAMs to antitumorigenic phenotype and retrieved cytotoxic activity of CD8⁺T cells in vitro and ex vivo. Responders toward antiprogrammed death receptor-1 (anti-PD-1) therapy present more Siglec-9⁺TAMs than non-responders. Furthermore, blockade Siglec-9 synergized with anti-PD-1 antibody to enhance the cytotoxic activity of CD8⁺T cells in tissues with higher Siglec-9⁺TAMs.

Conclusions: Siglec-9⁺TAMs may serve as an independent prognostic of poor survival but a predictive biomarker for anti-PD-1/antiprogrammed death ligand-1 immunotherapy in HGSC. In addition, the potential of immunosuppressive Siglec-9⁺TAMs as a therapeutic target is worth further exploration.