IDO1 and major depressive disorder: IDO is activated by proinflammatory cytokines, such as interferon (IFN)-α, IFN-γ, and tumor necrosis factor (TNF)-α6, 9, 10, 11, 12, which in addition to other inflammatory molecules such as interleukins, chemokines, and prostaglandins have been associated with major depressive disorder (MDD)2, 6, 11, 13, 14, 15.