The biology of myeloid-derived suppressor cells: the blessing and the curse of morphological and functional heterogeneity

Abstract

Myeloid-derived suppressor cells (MDSC) play an important role in the cellular network regulating immune responses in cancer, chronic infectious diseases, autoimmunity, and in other pathological conditions. Morphological, phenotypic and functional heterogeneity is a hallmark of MDSC. This heterogeneity demonstrates the plasticity of this immune suppressive myeloid compartment, and shows how various tumors and infectious agents can have similar biological effects on myeloid cells despite the differences in the factors that they produce to influence the immune system; however, such a heterogeneity creates ambiguity in the definition of MDSC as well as confusion regarding the origin and fate of these cells. In this review, we will discuss recent findings that help to better clarify these issues and to determine the place of MDSC within the myeloid cell lineage.

Figure 1. Schematic of possible pathway of MDSC differentiation in cancer

Blue dashed lines denote myeloid cell differentiation under physiologic conditions. Black solid line shows proposed development of MDSC in tumor-bearing host. HSC – hematopoietic stem cells, CMOP – common myeloid progenitor, MΦ – macrophages, DC –dendritic cells, T – T cells. See detailed explanation in the text.