Phenotypes, accumulation, and functions of myeloid-derived suppressor cells and associated treatment strategies in cancer patients

Myeloid-derived suppressor cells (MDSCs) comprise a group of heterogeneous and immature myeloid-derived cells. MDSCs accumulate in the blood, lymphoid organs, spleens and tumor tissues under different pathogenic conditions such as infection, trauma, hematosepsis, and especially oncogenesis. MDSCs can suppress both adaptive and innate immunities through multiple mechanisms. However, most of our knowledge of MDSCs is based on pre-clinical studies. Clinical observations have shown that the number of MDSCs in the peripheral blood of patients is closely related to tumor stage, tumor burden, remote metastasis and prognosis, though inconsistencies in MDSC phenotypes among cancer patients mean that results have been inconclusive, and subsequent research progress has been slow. This review summarizes recent studies that have investigated MDSCs in cancer patients.