IDO1 and neoplasm: Strategies that primarily address additional immunosuppressive mechanisms in the tumor microenvironment, such as indoleamine 2,3-dioxgenase (IDO)-inhibition, TGF-β blockade, regulatory T cell (Treg) depletion, and angiogenesis inhibition may be particularly effective to enhance or rescue tumor responses achieved with anti-PD-1/PD-L1 monotherapy.