Figure 1: Possible mechanisms of IDO-induced immune tolerance within the tumor microenvironment. IDO expressed by antigen-presenting dendritic cells within tumor-draining lymph nodes induces tolerance to tumor-derived antigens, while IDO expressed by tumor cells within the tumor microenvironment blocks effectors of adaptive immunity (CD8+ T-cells) and innate immunity (NK cells), in cooperation with Treg, MDSC, and immunosuppressive cytokines. This immune tolerogenic microenvironment leads to tumor growth and progression. IDO: Indoleamine 2,3-dioxygenase; LN: Lymph node; DC: Dendritic cell; NK: Natural killer cell; TIL: Tumor-infiltrating lymphocyte; Treg: Regulatory T cell; MDSC: Myeloid-derived suppressor cell; IL-10: Interleukine-10; IL-6: Interleukine-6; TGF-b: Transforming growth factor-b; VEGF: Vascular endothelial growth factor.