Myeloid-derived suppressor cells (MDSC): When good intentions go awry

Abstract

MDSC are a heterogeneous population of immature myeloid cells that are released by biological stress such as tissue damage and inflammation. Conventionally, MDSC are known for their detrimental role in chronic inflammation and neoplastic conditions. However, their intrinsic functions in immunoregulation, wound healing, and angiogenesis are intended to protect from over-reactive immune responses, maintenance of immunotolerance, tissue repair, and homeostasis. Paradoxically, under certain conditions, MDSC can impair protective immune responses and exacerbate the disease. The transition from protective to harmful MDSC is most likely driven by environmental and epigenetic mechanisms induced by prolonged exposure to unresolved inflammatory triggers. Here, we review several examples of the dual impact of MDSC in conditions such as maternal-fetal tolerance, self-antigens immunotolerance, obesity-associated cancer, sepsis and trauma. Moreover, we also highlighted the evidence indicating that MDSC have a role in COVID-19 pathophysiology. Finally, we have summarized the evidence indicating epigenetic mechanisms associated with MDSC function.

Keywords: COVID-19; Cancer; Chronic inflammation; Epigenetic regulation; Homeostasis; Immune tolerance; Immunosuppression; MDSC; Obesity; Wound healing.

Figure 1.. Comparison of the conditions where MDSC may have protective effects or cause abnormal pathological outcomes.

While clearance of initial insult results in the disappearance of MDSC and restoration of homeostasis, the unresolved inflammatory stimulus perpetuates the expansion and function of MDSC enhancing the inflammatory response, creating an immunosuppressive microenvironment, and facilitating angiogenesis and tissue damage. PAMPs stand for Pathogen-associated molecular patterns; DAMPs, Damage-associated molecular patterns; ROS, Reactive oxygen species; NO, Nitric oxide; MDSC, myeloid-derived suppressor cells; DC, dendritic cells, PMN, polymorphonuclear leukocytes.