IDO2 and neoplasm: The tumor microenvironment, which possesses immunosuppressive cells, such as regulatory T cells (Tregs), myeloid-derived suppressor cells (MDSCs), tumor-associated macrophages (TAMs) and tolerogenic DCs as well as immunosuppressive factors such as transforming growth factor (TGF)-β, IL-10 and indoleamine 2,3 dioxygenase (IDO), is another major limitation to the effectiveness of NK cells in AML [60, 61].