Ovarian cancer (OC) patients develop ascites, an accumulation of ascitic fluid in the peritoneal cavity anda sign of tumour dissemination within the peritoneal cavity. This body fluid is under-researched, mainly regarding the ascites formed during tumour progression that have no diagnostic value and, therefore, are discarded. We performed a discovery proteomics study to identify new biomarkers in the ascites supernatant of OC patients. In this preliminary study, we analyzed a small amount of OC ascites to highlight the importance of not discarding such biological material during treatment, which could be valuable for OC management. Our findings reveal that OC malignant ascitic fluid (MAF) displays a proliferative environment that promotes the growth of OC cells that shift the metabolic pathway using alternative sources of nutrients, such as the cholesterol pathway. Also, OC ascites drained from patients during treatment showed an immunosuppressive environment, with up-regulation of proteins from the signaling pathways of IL-4 and IL-13 and down-regulation from the MHC-II. This preliminary study pinpointed a new protein (Transmembrane Protein 132A) in the OC context that deserves to be better explored in a more extensive cohort of patients’ samples. The proteomic profile of MAF from OC patients provides a unique insight into the metabolic kinetics of cancer cells during disease progression, and this information can be used to develop more effective treatment strategies.

Background/Objectives: High-grade serous carcinoma (HGSC) represents the most frequent subtype of epithelial ovarian cancer (OC) and is associated with malignant ascitic fluid (MAF), which fosters a pro-inflammatory microenvironment that supports tumor progression. Given its rich cellular and soluble content, MAF offers a valuable window into tumor–host interactions and disease dynamics. This study examines the immune landscape of MAF samples from 22 newly diagnosed treatment-naïve HGSC patients. Methods: Immune cell phenotypes and cytokine profiles were analyzed via flow cytometry. Patients were stratified according to time to death or recurrence (TDR), using a six-month interval from diagnosis to either documented recurrence or disease-specific death as the cutoff: TDR < 6 months defined as the worse prognosis group, whereas TDR ≥ 6 months defined the better prognosis group. The expression of immune checkpoint molecules on T cells and the effects of PD-1 blockade with Pembrolizumab were also assessed. Results: Patients with worst prognosis exhibited a marked pro-inflammatory cytokine milieu, with elevated levels of TNFα, IL-1β, IL-23, and IFNγ. Concurrently, their CD4⁺ and CD8⁺ T cells displayed evidence of a functional exhaustion-associated phenotype, marked by heightened expression of the inhibitory receptors TIM-3, PD-1, and LAG-3. Notably, treatment with Pembrolizumab (a PD-1 checkpoint inhibitor) significantly enhance T cell effector function. Conclusions: These results underscore a potential immunotherapeutic approach in anti-tumor immunity among selected HGSC patients, presenting a compelling direction for advancing OC treatment.