BRCA1/2 -stratified immune profiling of treatment-naive high-grade serous ovarian cancer ascites identifies a Treg-enriched ascitic immune phenotype

Background: Malignant ascites is a common presentation in advanced high-grade serous ovarian carcinoma (HGSOC), yet its immune composition and genetic correlates remain poorly defined. This study explored the immune microenvironment of ascitic fluid in ovarian carcinoma and its association with BRCA mutation status, chemotherapy response, and survival. Methods: Ascitic fluid from 133 patients (33 non-malignant controls, 31 non-ovarian carcinoma, and 69 HGSOC cases) was analyzed using multiparameter flow cytometry to quantify lymphocyte and macrophage subsets, and programmed cell death 1/programmed cell death ligand 1 immune-checkpoint marker expression. Targeted sequencing of TP53 and BRCA1/2 was performed in HGSOC, and findings were correlated to immune parameters, chemotherapy response, progression-free and overall survival. Results: Ovarian carcinoma ascites, compared with non-malignant effusions, showed increased CD3+ and CD8+ T-cell frequencies, higher regulatory T cell (Treg)-associated populations, and reduced CD19+CD20+ B-cell levels. Targeted sequencing identified TP53 mutations in 97.4% and BRCA1/2 mutations in 35.9% of sequenced HGSOC cases. BRCA-wild-type cases showed significantly lower CD4+ T-cell and B-cell levels, higher Treg levels, and shorter progression-free and overall survival than BRCA1/2-mutated cases. Higher CD3+ and CD8+ T-cell levels were associated with poor chemotherapy response. In exploratory multivariable Cox models, BRCA-wild-type status remained significantly associated with poorer overall survival, whereas immune-cell groups were not independently significant. Conclusions: HGSOC ascites showed a Treg-enriched immune profile, with relatively higher Treg levels in BRCA-wild-type cases. BRCA-wild-type status was associated with poorer outcomes, while the evaluated immune-cell subsets were not independently prognostic in exploratory analysis. Ascitic fluid-based immune and molecular profiling may provide prognostic information worthy of validation in larger independent cohorts.