IDO1 and melanoma: Although T cells can infiltrate into melanoma tumor sites directed by chemokines, they subsequently become functionally inhibited by the immunosuppressive microenvironment, including the high expression of programmed cell death ligand 1 (PD-L1) on tumor cells, the high infiltration with Treg cells or MDSCs, and the highly secreted factors TGF-β, IL-10, and IDO.