IDO1 and acute myeloid leukemia: T-cell inhibitory enzymes such as indoleamine 2,3-dioxygenase 1 (IDO1) and arginase, of immune checkpoints (e.g., cytotoxic T-lymphocyte antigen-4 (CTLA-4) and programmed death ligand-1 (PD-L1)) and the production of nitric oxide, galectins, and ROS have also been shown to promote an immunosuppressive microenvironment in AML [52,57,58].