In addition to its broad immunosuppressive activity in autoimmune diseases [157,158,159], chronic infections [160], and cancer [161], IDO-derived kynurenine exposure induces NK cell apoptosis through a reactive oxygen species-mediated pathway and impairs NK cell function by suppressing surface expression of the NK activating receptors NKp46 and NKG2D (Figure 2C) [162,163,164]. Here, IDO1 is linked to cancer.