The catabolism of the essential amino-acid tryptophan by indoleamine 2,3-dioxygenase (IDO1) is a central pathway that contributes to an immunosuppressive microenvironment in many types of cancer. Our lab has previously shown that IDO1 expression in human ovarian tumor correlates with poor prognosis and poor tumor infiltration by CD8+ T cells, and that improved survival is associated with increased tumor-infiltrating lymphocytes. Thus, IDO1 inhibition represents an attractive target for cancer immunotherapy.

To establish the mechanism by which IDO1 inhibition augments immune responses in a murine model of metastatic ovarian cancer, we generated a stable IDO1-overexpressing cell line (IE9mp1-mIDO1) and confirmed functional IDO1 enzyme activity. C57BL/6 mice were challenged intraperitoneally with IE9mp1-mIDO1 or IE9mp1-Empty Vector as a control. IE9mp1-mIDO1 tumor-bearing mice displayed earlier onset of tumor burden and decreased overall survival.

To delineate the role of host- and tumor-derived IDO1 on immune cell infiltration in the tumor, IDO1 genetic knockout and C57BL/6 mice were challenged with IE9mp1-EV or -mIDO1 tumor cells. All mice challenged with IE9mp1-mIDO1 demonstrate decreased CD8+ T cells in the tumor compared to IDO1KO mice challenged with IE9mp1-EV tumor cells. Moreover, tumor-derived IDO1 mediates increased CD11b+Gr1+ myeloid derived suppressor cells (MDSC) in tumor ascites.

From these data, we conclude that abrogation of IDO1 enzyme function will promote antitumor immune responses by permitting increased effector T cells decrease MDSCs in the tumor microenvironment. Ongoing experiments will characterize the mechanism by which IDO1 inhibition may augment vaccine-induced immune responses in ovarian cancer.

This abstract is also being presented as Poster A19.

Citation Format: Adaobi E. Amobi, Takemasa Tsuji, Robert McGray, Kunle Odunsi. Tumor-derived indoleamine 2,3-dioxygenase regulates density of tumor-infiltrating CD8+ T cells and myeloid-derived suppressor cells in a murine model of ovarian cancer. [abstract]. In: Proceedings of the AACR Conference: Addressing Critical Questions in Ovarian Cancer Research and Treatment; Oct 1-4, 2017; Pittsburgh, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(15_Suppl):Abstract nr PR04.