Indoleamine 2,3-dioxygenase: As a potential prognostic marker and immunotherapeutic target for hepatocellular carcinoma

Figure 1 Immunosuppressive role of indoleamine 2,3-dioxygenase in human hepatocellular carcinoma. Indoleamine 2, 3-dioxygenase (IDO) expression is constitutively induced in both IDO⁺ tumor cells and plasmacytoid dendritic cells (DCs) by exposure to IFN-γ. The effector function of the antigen-specific T cells is impaired because of tryptophan degradation via IDO at tumor site, which in turn affects the immune-mediated control of tumor growth. A tolerogenic microenvironment is created by regulatory DCs resulting in the increased number of T-regs and reduced number of antigen-specific T cells leading to cell cycle arrest and apoptosis. IDO⁺ tumor cells lead to down-regulation of activating NK cell receptors.