Objective: Indoleamine 2,3-dioxygenase (IDO) is a tryptophan-catabolizing enzyme that induces immune tolerance. The purpose of the present study is to investigate IDO expression and its functional role in ovarian cancer cells in vitro and in vivo. Methods: IDO expression was immunohistochemically scored in surgically-resected ovarian cancer tissues (n = 60), and its association with tumor-infiltrating lymphocyte (TIL) count or patient survival was analyzed. Next, IDO cDNA was transfected into the human ovarian carcinoma cell line SKOV3, establishing stable clones of IDO-overexpressing cells (SK-IDO). SK-IDO cells were characterized in vitro as well as in vivo using a nude mouse xenograft model. Results: High IDO expression in tumor cells was found in 34 (56.7%) cases and was correlated with a reduced number of CD8+ TIL. Patients with high IDO expression had significantly impaired overall and progression-free survival compared to patients with no or low IDO expression. There were no significant differences in in vitro cell proliferation, migration, invasion, or chemosensitivity to paclitaxel between the SK-IDO and control vector-transfected (SK-pcDNA) cells. However, tumor peritoneal dissemination was significantly increased in SK-IDO-xenografted mice compared to SK-pcDNA-xenografted mice. This tumor-progressive effect in SK-IDO-xenografted mice was abrogated by oral administration of the IDO inhibitor 1-methyl-tryptophan (1-MT). Finally, treatment with weekly i.p. paclitaxel combined with daily administration of 1-MT significantly prolonged the survival of the SK-IDO-xenografted mice compared to treatment with paclitaxel alone. Conclusions: These results suggest that IDO is involved in ovarian cancer progression in vivo and may be a promising therapeutic target for advanced ovarian cancer.
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