Abstract High grade serous ovarian cancer (HGSOC) is the most common and aggressive type of ovarian carcinoma with a 5-year survival rate below 50% and a very low (∼10%) response rate to current immunotherapies. One of the hallmarks of HGSOC is the build-up of ascites fluid in the peritoneal cavity, which is associated with increased metastasis and decreased survival. Immune studies on patient-derived ascites fluid have indicated decreased functionality of CD8+ T cells in this unique microenvironment. B cells can infiltrate ascites fluid; however, it is unclear if cellular and acellular immunosuppressive cues reduce their metabolic programming and anti-tumor potential. Our findings indicate that ascites-infiltrating B cells are hyporesponsive to antigen stimulation at baseline but display increased differentiation to antibody secreting cells (ASCs) i. e. plasmablasts and plasma cells in vitro. We aimed to test the impact of patient-derived ascites on B cell differentiation into antibody-secreting plasma cells. We isolated naïve and memory B cell populations from healthy donors and cultured them in acellular patient-derived ascites. We noted an increase in viability, activation, and differentiation of B cells to ASCs compared to culture conditions without ascites and with benign intra-peritoneal washes. We observed a similar trend with ascites-infiltrating B cells from HGSOC patients. We also found increased cytokines in the fluid that are often associated with B cell survival and differentiation i. e. IL15, IL23 and IL27. These data suggest that ascites play a critical role in B cell effector fate, which in turn can determine the clinical outcomes of HGSOC patients. We hypothesize that patient-derived ascites promotes the differentiation of B cells into ASCs with reduced effector functions. To support this hypothesis, we found that ascites-infiltrating B cells had reduced glucose uptake but increased mitochondrial mass both ex-vivo and in vitro when cultured in the presence of matched ascites fluid. When cultured with varying concentrations of ascites, these B cells also demonstrated increased mitochondrial stress and substantial loss of spare respiratory capacity – a measure of mitochondrial reserve, indicating that metabolic stress in ascites blunts B cell mitochondrial respiration and their overall function. Thus, these data suggest that despite increased differentiation of B cells to ASCs within ascites, these ASCs maybe nonfunctional. Preliminary data suggests that BCR signaling is indeed blunted within ascites. We are now interrogating if increased differentiation to ASCs is contributing or hindering a functional immune response in ascites by measuring their IgG and IgA production. We will then test their tumor-specificity and their ability to mediate antibody-dependent cellular cytotoxicity (ADCC). Completion of these studies will fill a critical knowledge gap on B cell education and function outside of tertiary lymphoid structures and within a metastatic site, thus providing rationale for designing B cell directed immunotherapies for patients. Citation Format: Rufiaat Rashid, Ian MacFawn, Noor Nader, Lan Coffman, Tullia Bruno. Ovarian cancer-induced ascites fluid impacts B cell differentiation fate and function in women with high grade serous ovarian cancer abstract. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Ovarian Cancer Research; 2025 Sep 19-21; Denver, CO. Philadelphia (PA): AACR; Cancer Res 2025;85 (18Suppl): Abstract nr B064.
Rashid et al. (Fri,) studied this question.