Myeloid-Derived Suppressor Cells in the Tumor Microenvironment

Myeloid-derived suppressor cells (MDSCs) represent a heterogenous population of immature myeloid cells capable of modulating immune responses. In the context of cancer, MDSCs are abnormally produced and recruited to the tumor microenvironment (TME) to aid in the establishment of an immunosuppressive TME that facilitates tumor escape. Additionally, MDSCs contribute to non-immunologic aspects of tumor biology, including tumor angiogenesis and metastasis. The clinical significance of MDSCs has recently been appreciated as numerous studies have suggested a correlation between circulating and intratumoral MDSC frequencies and tumor stage, progression, and treatment resistance. In this chapter, we review MDSC characterization, development, expansion, and mechanisms that facilitate immunosuppression and tumor progression. Furthermore, we highlight studies demonstrating the clinical significance of MDSCs in various disease states in addition to strategies that modulate various aspects of MDSC biology for therapeutic gain.