Myeloid-derived suppressor cells--their role in haemato-oncological malignancies and other cancers and possible implications for therapy.

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells at different stages of maturation that play a role in cancer tolerance and function as an immune-suppressive cell subpopulation. They utilize different mechanisms to block both innate and adaptive arms of anti-tumour immunity, mostly through inhibition of T cell activation and expansion. Further advances in our understanding of this cell population in both murine models and humans has enabled more accurate characterization of their phenotype and the recognition of two major classes of MDSCs: granulocytic and monocytic. Recently, the mechanism of action and clinical importance of MDSCs has been more clearly defined and their interactions with cancer cells have been shown to be among the factors influencing tumour development and induction of tolerance. Most of the earlier studies were performed using murine models, but recent clinical investigations have shown their potential role in human cancers. Here, we review the origin of MDSCs, their mechanisms of action, the factors influencing their production and related signalling pathways. We focus on their role in human solid tumours and haemato-oncological malignancies, and relate to possible novel therapeutic approaches targeting MDSCs which could be considered together with other anticancer strategies in the not too distant future.