IDO1 and oligodendroglioma: The solid TME also has many factors that promote antitumor immunity, including indoleamine 2,3 dioxygenase 1 (IDO1), TGF-β, VEGFRA, etc. A lack of T cells may indicate a lack of tumor immunogenicity, however, relatively infiltrative CD4/CD8 + T cells in oligodendroglioma do not indicate a considerable checkpoint blockade response (Egen et al. 2020).