Many tumor-infiltrating macrophages, referred to myeloid-derived suppressor cells (MDSCs), have an immune-suppressive phenotype [119] . These macrophages are abundant in many tumors arising in both humans and mice and can exert powerful anti-inflammatory effects. In addition to MDSCs, regulatory T cells (Tregs) also heavily infiltrate many tumors [120] . These cells, characterized by the expression of the transcription factor FoxP3 as well as CD4 and CD25, play a key role in the regulation of adaptive immunity. Tregs can suppress immune responses through the secretion of suppressive cytokines like TGF-β and IL-35 [120, 121] . Tregs are a potential barrier to developing productive immune therapies for cancer, and they represent an attractive target for enhancing antitumor immunity.