IDO1 and neoplasm: Finally, studies involving melanoma, hepatocellular and colorectal carcinoma-derived fibroblasts have shown that CAFs can decrease the expression of several NK activating receptors (including NKp30, NKp44 and NKG2D) on the NK cell surface through the secretion of prostaglandin E2 (PGE2) and/or indoleamine-2,3-dioxygenase (IDO) [34–36] leading to an attenuate cytotoxic activity of NK cells against their tumor target cells.