Myeloid-derived suppressor cells: Important contributors to tumor progression and metastasis.

Myeloid-derived suppressor cells (MDSCs) are traditionally considered among the major components of the immunosuppressive tumor microenvironment (TME). However, there is currently increasing evidence indicating that MDSCs in addition to suppression of immune surveillance is also involved in an array of non-immunological functions like augmenting metastatic potential of tumor cells. Indeed, MDSCs can promote metastasis in animal models and cancer patients through promoting premetastatic niche formation, tumor angiogenesis and invasion. Moreover, MDSC frequency and function have been associated with progressive disease and correlated with clinical outcome. This review will summarize and discusses the data demonstrating the role for MDSCs in tumor metastasis.