Myeloid Derived Suppressor Cells: Subsets, Expansion, and Role in Cancer Progression

Cancer immunotherapies have shown considerable promise in pre-clinical studies, but the potency of these interventions has often proved disappointing in vivo. This is in part due to tumor infiltration by myeloid cells, which are usually associated with less favorable clinical outcomes. In the past decade, several distinct subsets of tumor-infiltrating myeloid cells have been described (Movahedi et al., 2010), among which myeloid-derived suppressor cells (MDSC) have been subject to particular scrutiny for exerting a critical role in cancer progression (Bronte, 2009; Gabrilovich and Nagaraj, 2009; Ostrand-Rosenberg and Sinha, 2009; Ribechini et al., 2010). MDSC have been studied intensively in the context of cancer, and the weight of evidence indicates that these cells accumulate in most human cancers and also in experimental animal models with transplanted or spontaneous tumors (Eruslanov et al., 2011; Gabitass et al., 2011; Movahedi et al., 2008; Peranzoni et al., 2010; Raychaudhuri et al., 2011; Youn et al., 2008). MDSC also have significant roles to play in numerous other pathologies, including bacterial infections (Delano et al., 2007), parasitic infections (Brys et al., 2005; Goni et al., 2002), chemotherapy outcomes (Angulo et al., 2000), experimental autoimmunity (Arora et al., 2011; Kerr et al., 2008; Moline-Velazquez et al., 2011; Zhu et al., 2007), inflammatory bowel diseases (Haile et al., 2008), obesity (Xia et al., 2011), transplant rejection (Hock et al., 2011), and stress responses (Makarenkova et al., 2006).