IDO1 and central nervous system cancer: Glioma highly inhibits tumor immunity, and glioma cells produce immunosuppressive factors such as TGF-β, IL-10 and indoleamine–pyrrole 2, 3-dioxygenase (IDO) (Tran et al., 2007; Preusser et al., 2015) Glioma also expresses immune checkpoint ligand to inhibit immune response (Garber et al., 2016; Hodges et al., 2017), so it is difficult to use immunotherapy to treat glioma.