Defects in MHC I proteins [162], infiltration of suppressive cells such as MDSCs [163], and secretion of immunosuppressive cytokines and chemokines such as IL-10, TGF-β, and indoleamine 2,3-dioxygenase (IDO) [164] are the main mechanisms of cancer immune evasion that reduce the effectiveness of cancer vaccines. Here, IDO2 is linked to cancer.