IDO1 and neoplasm: Interferon-γ induces CD8+ T cells to antigen (Ag)-specific CTLs and CD4+ T cell differentiation [67, 68], however, continuous exposure to interferon-γ may induce T cell exhaustion and tumor progression [69] by inducing immune escape mediators, including PD-L1, STAT3, and IDO1 [70, 71].