Immunosuppression in tumor microenvironment. Tumors secrete various factors such as VEGF, IL-6, IL-10, TGF- , Fas-L, and IDO, all of which promote the accumulation of heterogeneous populations of tumor-associated macrophages (TAMs), myeloid-derived suppressor cells (MDSCs), or immature DCs. These immunosuppressive cells inhibit antitumor immunity by various mechanisms, including depletion of arginine and elaboration of reactive oxygen species (ROS) and nitrogen oxide (NO). The tumor microenvironment also promotes the accumulation of regulatory T cells (Tregs) that suppress CD8+ CTL function through secretion of IL-10 or TGF- from Tregs and tumors.