Interleukin (IL)-18 is a proinflammatory and immune-enhancing cytokine, which exerts antitumor effects in vivo, mediated by the induction of interferon (IFN)γ. We previously reported that IL-18 processing is defective in epithelial ovarian carcinoma (EOC) cells, which secrete an inactive precursor (pro-IL-18) in vitro. In addition, IL-18 was reported as a potential biomarker of EOC. Here, we further investigated its role as a serological marker in human EOC and addressed its possible biological activity in vivo. Our data indicate that immunoreactive IL-18 is increased in EOC patients' sera at diagnosis as compared with age-matched healthy women. IL-18 levels were higher in the ascitic fluids than in sera, suggesting a local production in the peritoneal cavity. Indeed, immunohistochemical analysis of tumors showed IL-18 expression in cytokeratine-positive neoplastic cells, although also scattered histiocytes and some lymphoid cells stained for IL-18. The detection of human IL-18 in sera and ascitic fluids of immunodeficient mice, orthotopically implanted with human EOC cells, further suggested that circulating IL-18 is tumor-derived. However, IL-18 is not an EOC specific biomarker, as increased serum levels were found also in some endometrial cancer patients. By means of a new monoclonal antibody, we characterized IL-18 present in the ascitic fluid as pro-IL-18, which is biologically inactive. Accordingly, IFNγ was not increased in EOC patients' sera and ascitic fluids and showed no correlation with IL-18 levels. Altogether these data indicate that IL-18 in EOC fluids is predominantly tumor-derived and that its lack of biological activity may represent a mechanism of tumor-escape.
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