IDO1 and neoplasm: In addition, TGF-β1 and other immunosuppressive factors produced by tumor cells have been shown to suppress NK cell effector functions [105] (Figure 2) and recruit immunosuppressive cells to tumor tissue, such as regulatory T cells (Tregs) [106], myeloid-derived suppressor cells (MDSCs) [107], CD11b+Ly6G+ neutrophils [108], and indoleamine 2, 3-dioxygenase 1 (IDO1)-expressing dendritic cells (DCs) [109].