Myeloid-Derived Suppressor Cells in the Tumor Microenvironment: Current Knowledge and Future Perspectives

The current knowledge on tumor-infiltrating myeloid-derived suppressor cells (MDSCs) is based mainly on the extensive work performed in murine models. Data obtained for human counterparts are generated on the basis of tumor analysis from patient samples. Both sources of information led to determination of the main suppressive mechanisms used by these cell subsets in tumor-bearing hosts. As a result of the identification of protein targets responsible for MDSCs suppressive activity, different therapeutics agents have been used to eliminate/reduce their adverse effect. In the present work, we review the current knowledge on suppressive mechanisms of MDSCs and therapeutic treatments that interfere with their differentiation, expansion or activity. Based on the accumulation of new evidences supporting their importance for tumor progression and metastasis, the interest in these cell types is increasing. We revise the methods of MDSC generation/differentiation ex vivo that may help in overcoming problems associated with limited numbers of cells available from animals and patients for their study.