Abstract A087: High grade serous ovarian cancer ascites exhibit an impaired immune environment - immunosuppressive & protumoral – detrimental to the immune response

Abstract Background: Malignant ovarian cancer (OC) ascites are increasingly recognized to play a prominent role in tumorigenesis and cancer progression, via complex processes of interactions between tumor cells and their surrounding environment, including immune cells. Thus, the characterization of the immune and metabolic composition of OC ascites and the evaluation of their biological effects on tumor and immune cells are critical for understanding how ascitic milieu influences tumor behavior and immune response and to what extent ascites components are involved. Methods: Ascites samples were collected from advanced OC patients. Through respective multiplexed approaches (cell flow cytometry, LEGENDplex, metabolomics), immune and metabolic contexts were profiled by investigating immune cell composition and levels of a plethora of soluble factors, including cytokines/chemokines, as well as metabolome, respectively. Furthermore, ascites fluids were functionally screened for their biological effects on healthy monocytes, undifferentiated or undergoing M1 polarization, on healthy PBMCs and on ovarian cancer cells, in baseline and under chemotherapy. Results: OC ascites were mostly “enriched” in regulatory/immunosuppressive immune cell subsets including T and myeloid populations. Moreover, T cells and Tregs were found to highly express immune checkpoints such as PD1 and TIGIT, respectively, while CD163+ macrophages were shown to express CSF1R, CCR8 and CCR2, as well as certain more M1-related markers like Arg1 and iNOS. Furthermore, LEGENDplex and metabolomics highlighted most of ascites as displaying unbalanced chemokinic/cytokinic landscapes and metabolically-immunosuppressive profiles. Such an environment was shown to harbor elevated CCL18, IL6, VEGF, CCL2, and LIF levels, as well as amino acid metabolism alterations. Functionally, OC ascites screening allowed to demonstrate that not only ascites basically polarized monocytes into M2 macrophages, but even antagonized with their M1 polarization to ultimately tilt to M2 status. In addition, OC ascites screening highlighted an anti-immune effect of OC ascites and a pro-tumor action promoting tumor cell proliferation and insensitivity to chemotherapy. Conclusions: Taken together, our data show that ascites fluids most favorable to the M2 phenotype were associated with a highly unbalanced immune & metabolic landscape where a mixture of cells and soluble factors interact together leading to functional alterations, i.e. dysfunctional immune cells, in favor of tumor cell proliferation and resistance to chemotherapy. These translational findings support the need to delineate ascites-mediated signaling pathways occurring in tumor and immune cells, which may be responsible for drug resistance and metastatic processes. Malignant OC ascites appear as a valuable insightful tool for target identification and development of targeted-therapies. Citation Format: Rian Mc Neill, Coriolan Lebreton, Dominique Bodet, Assia Chaibi, Guillaume Babin, Alban Bessede, Imane Nafia. High grade serous ovarian cancer ascites exhibit an impaired immune environment - immunosuppressive & protumoral – detrimental to the immune response [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 A087.