Fig. 1

Differentiation and accumulation of MDSCs. MDSCs are differentiated in the bone marrow from hematopoietic stem cells (HSCs) through common myeloid progenitors (CMPs) and granulocyte–macrophage progenitors (GMPs). In response to multiple tumor-induced cytokines, GMPs differentiate to form myeloblasts (MBs), monocytes/macrophages and dendritic cells (MDPs). Most MBs and MDPs can further differentiate into neutrophils and monocytes. However, under pathological conditions, immature myeloid cells expand and transform into immunosuppressive MDSCs. In cancer patients, neutrophils and monocytes, as well as pathologically activated MDSCs, coexist at any given time and accumulate more MDSCs during tumor progression