To recruit Treg cells, gliomas overproduce factors like indoleamine 2,3-dioxygenase 1 (IDO-1), and glioma stem cell (GSC)-derived pericytes secrete CCL5, whereas cerebral stromal cells produce immunosuppressive cytokines, namely TGF-β and IL-10 [50]. Here, IDO1 is linked to central nervous system cancer.