The roles of myeloid-derived suppressor cells in transplantation.

CD11b(+)Gr1(+) myeloid-derived suppressor cells (MDSCs) are an important regulatory innate cell population and have significant inhibitory effect on T cell-mediated responses. In addition to their negative role in cancer development, MDSCs also exert strong regulatory effects on transplantation and autoimmunity. In many transplantation models, such as bone marrow transplant, renal transplant, heart transplant and skin transplant settings, MDSCs accumulate and have inhibitory effect on graft rejection. However, the inducing factors, detailed phenotype and functional molecular mediators of MDSCs are significantly different in various transplant models. With their strong suppressive activity, MDSCs could become a potential clinical therapy during transplantation tolerance induction and the combination of the MDSCs with other immunoregulatory cells or immunosuppressive drugs is an intriguing protocol in the future. In this review, we will summarize MDSC expansion, activation and induction in different transplantation models and discuss the effects of immunoregulatory cells and immunosuppressive drugs on MDSCs in transplant settings.