Introduction/Background High-grade serous carcinoma of the ovary, fallopian tube and peritoneum (HGSC), the most common type of ovarian cancer, ranks among the deadliest malignancies in women. Many HGSC patients present with ascites at the time of diagnosis. Malignant ascites is a complex tumor microenvironment (TME) containing various cells, proteins and extracellular vesicles (EVs). EVs are small membrane-bound particles that convey proteins, lipids and nucleic acids between cells and their cargo reflects the cell of origin. EVs play important role in cancerogenesis and hold great promise as biomarkers. Small size and polydispersity of EVs bring various challenges to their isolation and characterization, including method-dependent enrichment of different EV subtypes as well as contaminants.

Methodology We isolated EVs from malignant ascites of eleven patients by two orthogonal methods and analyzed them by tandem mass spectrometry (MS/MS). We also included MS/MS analysis of EVs from related non-malignant control patient fluids and analysis of ascitic cells by spectral flow cytometry (FC).

Results We identified a set of ‘core ascitic EV proteins’ and also described proteins present only on EVs from HGSC patients. We believe this list contains important players of HGSC progression as well as potential biomarkers. Using single cell RNA sequencing data, we mapped the origin of EVs to different types of cells present in ascites. EVs in ascites did not come predominantly from tumor cells, but rather from a variety of non-malignant cell types. FC of ascitic cells in combination with the analysis of EV cells of origin incriminate critical contribution of macrophages to the ascitic TME, including the association of macrophage-derived EVs with patient prognosis.

Conclusion This is the first study attempting to link EV composition to the cell types producing it. As such it opens numerous avenues both for a better understanding of EV role in tumor promotion/prevention and for improved HGSC diagnostics.

Disclosures The authors disclose no potential conflict of interest.