IDO2 and glioblastoma: GBM cells increase conversion of tryptophan to kynurenine via upregulation of indoleamine 2,3-dioxygenase (IDO1/IDO2) and tryptophan 2,3-dioxygenase (TDO2), depleting local levels of tryptophan and impairing T-cell functioning (107, 108).