Role of tumor-derived exosomal arginase-1 in avoiding immune responses by ovarian cancer

Aim/Background: In patients with ovarian cancer (OvCa) exosomes released by tumor cells (TEX) are abundant in body fluids, including plasma or ascites and carry proteins that are expressed on OvCa cells. Arginases are enzymes involved in degradation of L-Arg and are linked to tumor-induced inflammation and immune escape. Based on our preliminary results showing strong arginase-1 (Arg-1) expression in primary OvCa lesions and established OvCa cell lines we hypothesize that through the release of Arg-1 expressing TEX, which become systemically distributed through the bloodstream, tumor cells achieve a systemic T-cell dysfunction leading to immune escape. The aim of the study is to: (1) assess the expression/activity of Arg-1 in OvCa-derived exosomes isolated from ascites, (2) determine the clinical significance of Arg-1 expression in OvCa, (3) elucidate the effects of TEX-associated Arg-1 on immune cells in vitro. Methods: TEX are isolated by ultracentrifugation or exclusion chromatography and verified by Western blotting, NanoSight and electron microscopy. The expression of Arg-1 in TEX is determined by Western blotting and the enzymatic activity by measuring the conversion of arginine into urea. Arg-1 expression data in primary OvCa lesions is correlated to clinicopathological characteristics. Effects of exosomal Arg-1 on immune cells are analyzed by in vitro proliferation assay and flow cytometry. Results: Enzymatically active Arg-1 was detected in TEX derived from patients' ascites. In comparison to fluid from benign ovarian cysts OvCa ascites contained higher levels of exosomal Arg-1. Arg-1 positive TEX inhibited T-cell proliferation and T-cell receptor expression in vitro. High Arg-1 expression in primary lesions was associated with poor survival and correlated negatively with intratumoral T-cell infiltrates and CD3-zeta expression. Conclusions: Our findings will provide first evidence for the role of Arg-1 in the creation of an immunosuppressive microenviroment in OvCa, however further in vivo studies in an OvCa mouse model will be needed. In future, the inhibition of Arg-1 activity may be a target for novel anti-cancer strategies, especially in combination with classical chemotherapy. Legal entity responsible for the study: Medical University of Warsaw Funding: Narodowe Centrum Nauki (National Science Center), OPUS 6 Program 2013/11/B/NZ6/02790 Disclosure: All authors have declared no conflicts of interest.