IDO1 and major depressive disorder: It is widely accepted that type I IFNs have a great impact on both tumor inhibition and stimulating antitumor immune responses, while the systemic administration of type I IFNs is accompanied by many adverse outcomes, including fatigue, nausea, anorexia, flu­like symptoms, dizziness, hepatotoxicity, severe depression, leukopenia, and possibly the expression of immunosuppressive enzyme indoleamine 2,3-dioxygenase 1 (IDO1) [71, 72].