Myeloid‐derived suppressor cells: the dark knight or the joker in viral infections?

Summary Myeloid derived suppressor cells ( MDSC s) are immature cells of myeloid origin, frequently found in tumor microenvironments and in the blood of cancer patients. In recent years, MDSC s have also been found in non‐cancer settings, including a number of viral infections. The evasion of host immunity employed by viruses to establish viral persistence strikingly parallels mechanisms of tumor escape, prompting investigations into the generation and function of MDSC s in chronic viral infections. Importantly, analogous to the tumor microenvironment, MDSC s effectively suppress antiviral host immunity by limiting the function of several immune cells including T cells, natural killer cells, and antigen‐presenting cells. In this article, we review studies on the mechanisms of MDSC generation, accumulation, and survival in an effort to understand their emergent importance in viral infections. We include a growing list of viral infections in which MDSC s have been reported. Finally, we discuss how MDSC s might play a role in establishing chronic viral infections and identify potential therapeutics that target MDSC s.