TDO2 and glioblastoma: GBM induces immunosuppression through upregulation of idoelamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO), enzymes that catalyze the degradation of Trp following accumulation of kynurenine pathway (KP)-derived catabolites that have immunosuppressive functions.