IDO1 and neoplasm: Combining vaccines with therapeutic strategies that are designed to inhibit or alleviate the immunosuppressive microenvironment such as imatinib [118] (which inhibits IDO), sunitinib [119] (which antagonizes MDSCs and TReg cells), cyclophosphamide [120] (kills TReg cells) and gemcitabine [106] (kills MDSCs) may enhance the effect of immunotherapy and promote anti-tumor immune responses (Figure 1).