IDO1 and neoplasm: In the TME, several tumor and tumor-associated cells produce and secrete factors that directly or indirectly prevent NK cell activation, including interleukin (IL)-6, IL-10, transforming growth factor-β (TGF-β), prostaglandin E2 (PGE2), and idoleamine 2,3-dioxygenase (IDO) (4, 5).