Myeloid cells in cancer progression: unique subtypes and their roles in tumor growth, vascularity, and host immune suppression

Abstract

Leukocytic infiltrates, particularly myeloid cells, can stimulate an anti-tumor immune response, but more often they stimulate tumor development, including promoting invasion, tumor growth, angiogenesis, and metastasis. Distinct myeloid phenotypes are being characterized that have been shown to promote tumor growth, angiogenesis, and metastasis. This review provides an overview of myeloid differentiation and spotlights specific pro-tumorogenic myeloid populations and their role in cancer progression. Efforts to characterize these pro-tumorogenic myeloid cell immunophenotypes may lead to novel targets for cancer therapy.

Keywords: Myeloid derived suppressor cells; Tie-2 expressing monocytes; Tumor associated macrophages; Tumor microenvironment; Vascular leukocytes.

Fig. 1

Scheme of hematopoietic lineage differentiation. Depiction of the stages of myelopoiesis and associated surface antigen expression. Abbreviations: HSCs, Hematopoietic Stem Cells; MPPs, MultiPotential Progenitors; CLPs, Common Lymphoid Progenitors; GMPs, Granulocyte/Macrophage Progenitors; MEPs, Megakaryocyte/Erythroid Progenitors; CMPs, Common Myeloid Progenitors; MDPs, Macrophage and Dendritic-cell Progenitors. See [, –, , , , , , , , –120]

Fig. 2

Schematic of interactions between tumor and bone marrow derived myeloid subtypes. Noted are reported effects of the specific population on tumor growth. Factors that are involved in recruitment to tumor site are denoted for some populations, (i.e. TEMs and MDSCs) that are present in the circulation of tumor-bearing animals. By contrast, vascular leukocytes and TAMs are primarily described within tumor tissues. Hence, factors involved in their generation in situ are noted