IDO1 and neoplasm: In addition, tumor cells secrete factors [e.g., transforming growth factor beta (TGF-β), indoleamine 2,3-dioxygenase (IDO)] that directly inhibit effector immune cell functions or recruit regulatory cells, tumor-associated macrophages, and myeloid-derived suppressor cells (MDSC) to intensify an immunosuppressive microenvironment (Figure 1) (1).