In contrast, immune cells themselves may trigger immunosuppression associated with cancer, as shown recently in co-culture of immune and liver cancer cells, in which immune cells induce tumor cells to produce Indoleamine 2, 3-Dioxygenase 1 (IDO1) [75], which induces T cell anergy and promotes polarization of CD4+ cells to the immunosuppressive Treg phenotype. Here, IDO1 is linked to liver cancer.