IDO2 and neoplasm: Immunosuppressive mediators such as indoleamine 2,3-dioxygenase (IDO), interleukin-10 (IL-10) and TGF-β are also highly expressed in CCAs; these mediators can suppress the function of effector, cytotoxic T lymphocytes (CTL) and increase the migration of innate immune cells such as tumor-associated neutrophils (TANs), tumor-associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs) into the tumor [35].