IDO1 and neoplasm: It has been proposed that OSCC exhibits an immunosuppressive microenvironment [7]–[8] and tumor cells could exhibit proficient capacity to express Fas ligand to induce apoptosis of T cells [16] or to produce indoleamine 2,3-dioxygenase (IDO) to inhibit the local proliferation of effector T cells and induce the regulatory T cells [36]–[37].