IDO2 and hepatocellular carcinoma: An early in vitro study illustrated that CAFs deactivate natural killer (NK) cells and impair their cytotoxicity against HCC cells by secreting prostaglandin E2 (PGE2) and indoleamine 2,3-dioxygenase (IDO), and interestingly, the release of these factors depends on the presence of NK cells, implying plausible crosstalk between CAFs and NK cells in the HCC TME.56