Endothelial progenitor cell: a blood cell by many other names may serve similar functions

Abstract

The first reports of circulating cells that displayed the capacity to repair and regenerate damaged vascular endothelial cells as progenitor cells for the endothelial lineage (EPC) were met with great enthusiasm. However, the cell surface antigens and colony assays used to identify the putative EPC were soon found to overlap with those of the hematopoietic lineage. Over the past decade, it has become clear that specific hematopoietic subsets play important roles in vascular repair and regeneration. This review will provide some overview of the hematopoietic hierarchy and methods to segregate distinct subsets that may provide clarity in identifying the proangiogenic hematopoietic cells. This review will not discuss those circulating viable endothelial cells that play a role as EPC and are called endothelia colony-forming cells. The review will conclude with identification of some roadblocks to progress in the field of identification of circulating cells that participate in vascular repair and regeneration.

Figure 1

Models of Lineage Determination in Mouse and Human Hematopoietic Hierarchies. The major classes of stem and progenitor cells are defined by cell surface phenotypes, which are listed next to each population and in the gray bars below each schematic. Terminally differentiated cells are shown on the right, and inferred lineage relationships are depicted with arrows. In mice (A), HSCs can be separated into long-term (LT), intermediate-term (IT), and short-term (ST) classes based on the duration of repopulation. In humans (B), HSCs are defined by the expression of CD49f and other markers, but their heterogeneity has not been investigated. In mice, differentiation of HSCs gives rise to transiently engrafting multipotent progenitors (MPPs), and a series of immature lymphoid-biased progenitors (such as LMPPs) that undergo gradual lymphoid specification. In humans, MPPs can be identified by the loss of CD49f expression; however, only one population of immature lymphoid progenitors (MLPs) has been described. Both mice and humans have well-defined populations of common myeloid progenitors (CMPs), granulocyte macrophage progenitors (GMPs), and megakaryocyte erythroid progenitors (MEPs). Lin: cocktail containing cell surface markers for all terminally differentiated populations (B cell; T cell; NK; dendritic cell, monocyte, granulocyte, megakaryocyte, and erythrocyte). Modified from Doulatov S, et al. Hematopoiesis: A human perspective. Cell Stem Cell 10:124, 2012.

Figure 2

Frequency analysis of CD31⁺CD34^brightCD45^dimAC133⁺ cells. Uncompensated raw data was collected on a digital flow cytometer, compensated after acquisition by using FlowJo software, and visualized in plots with bi-exponential scaling. Mononuclear cells were identified on a forward versus side scatter plot and then CD14⁻ cells (orange gate in a) were identified. All CD14⁻ cells were then assessed for viability and glycophorin A (CD235a) expression (b). CD14⁻ glyA⁻ViViD⁻ (viable cells do not retain this molecule) cells (pink gate in b) were subgated onto a bivariant antigen plot to identify CD14⁻glyA⁻ViViD⁻CD34^brightAC133⁺ cells (dark blue gate in c). Viable CD14⁻glyA⁻CD34^brightAC133⁺ cells are further subgated to identify the CD45^dim subpopulation (light blue gate in d). CD31 expression on the resulting viable cells was confirmed on a CD31 histogram (e). Figure modified from Estes ML, et al. Application of polychromatic flow cytometry to identify novel subsets of circulating cells with angiogenic potential. Cytometry Part A 77A:835, 2010.

Figure 3

Ratio of circulating progenitor subsets denotes disease state in peripheral arterial disease patients. The ratio of proangiogenic (CD31⁺CD34^brightCD45^dimAC133⁺) to nonangiogenic (CD31⁺CD34^brightCD45^dimAC133⁻) cells is depicted and patients with peripheral arterial disease (PAD) display a significant decrease when compared to age and gender matched control subjects. Figure modified from Estes ML, et al. Application of polychromatic flow cytometry to identify novel subsets of circulating cells with angiogenic potential. Cytometry Part A 77A:837, 2010.