Figure 1.

General overview of MDSC immunosuppressive mechanisms and expansion in hematological malignancies and during stem cell transplantation (SCT). (A) MDSC suppress the immune system by distinct mechanisms including increased macrophage differentiation and regulatory T cell (Treg) proliferation, direct actions of MDSC on T cells by increased NO, nitrotyrosine and ROS secretion, and decreased l-arginine production. (B) MDSC originate from common myeloid progenitors (CMP), which arise from hematopoeitic stem cells (HSC). M-MDSC and G-MDSC are formed and proliferate in the presence of distinct factors including GM-CSF, IL-6, VEGF, and IL-1β. Factors involved in the proliferation and survival of MDSC are S100A9, Cyclin D1, Bcl-xL Myc, and Survivin.