IDO1 and hepatocellular carcinoma: Notably, the fact that HCV can potentiate the suppressive function of natural Treg cells as well as induce Tr1 cells may help to explain the cellular mechanism by which HCV escapes the immune system to promote the progression of hepatitis C infection to cirrhosis and HCC.45 However, the molecular mechanisms mediating HCV immune evasion, potentially including IDO/TDO, Kyn and AhR activation, remain to be further characterized.