IDO1 and neoplasm: Glioma is also regarded as an immunosuppressive tumor, with the tumor microenvironment expressing and secreting a large number of immunosuppressive factors such as programmed cell death ligand-1 (PD-L1), cytotoxic T lymphocyte-associated protein 4 (CTLA-4), and Indolamine 2,3-dioxygenase (IDO), among others (9–11).