Myeloid-Derived Suppressor Cells: Function, Migration, and Therapeutic Opportunities in Glioblastoma

Myeloid-derived suppressor cells (MDSCs) are a class of immature, heterogenous, and functionally immunosuppressive myeloid progenitors that are expanded in malignant disease including glioblastoma (GBM). Extensive preclinical evaluation of GBM has revealed that MDSCs express multiple different chemokine and cytokine receptors that facilitate their entry, infiltration, expansion and immunosuppression of antitumor immunity in the tumor microenvironment. Additionally, translational investigation of approaches that target MDSCs directly or indirectly through immune remodeling has yielded promising effects that are under clinical trial investigation. Given the immunosuppressive phenotype of high-grade gliomas like GBM, the removal of MDSCs represents a clinically relevant strategy to enhance immune responses against neoplastic cells. In this review, we provide a comprehensive summary of MDSCs in GBM, emphasizing clinical observations and large-scale multi-omics studies that position MDSCs at the nexus of GBM immunosuppression. Next, we provide detailed coverage of multiple chemokines, cytokines, and growth factors that are relevant to MDSC migration, survival and expansion in GBM along with commentary on the associated receptors. Lastly, we discuss therapeutic approaches that directly target MDSCs as a strategy to improve immune responses against malignant brains and observations on the changes to MDSCs in the tumor microenvironment after immunotherapy. Our review serves as a valuable resource for the neuro-oncology research space, updating scientists and clinicians on a cell central to the biology and therapeutic targeting of GBM.