Human endothelial progenitor cells

Abstract

Human endothelial progenitor cells (EPCs) have been generally defined as circulating cells that express a variety of cell surface markers similar to those expressed by vascular endothelial cells, adhere to endothelium at sites of hypoxia/ischemia, and participate in new vessel formation. Although no specific marker for an EPC has been identified, a panel of markers has been consistently used as a surrogate marker for cells displaying the vascular regenerative properties of the putative EPC. However, it is now clear that a host of hematopoietic and vascular endothelial subsets display the same panel of antigens and can only be discriminated by an extensive gene expression analysis or use of a variety of functional assays that are not often applied. This article reviews our current understanding of the many cell subsets that constitute the term EPC and provides a concluding perspective as to the various roles played by these circulating or resident cells in vessel repair and regeneration in human subjects.

Figure 1.

A late neural plate stage embryo. (A) Late neural plate stage mouse embryo with maturing Flk-1⁺ vascular plexus (green) and distinct band of CD41⁺ primitive erythroid progenitor cells in the proximal yolk sac (red). Note the paucity of angioblasts (green) in the blood band region. (B) Higher-magnification cross section of the blood band. The Flk-1⁺ angioblasts (green cells identified by asterisks) are located between the primitive erythroid progenitor cells and the yolk sac visceral endoderm (blue cells indicated by endo) cells. (C) Blood island image depicting an angioblastic cord (a) from the same stage embryo as in panels A and B. Note the similarity of the cross section of the tissue in panel B to the panel C. The primary difference is the evidence that the cells highlighted in red in panel B are now known to be primitive erythroid cells and not mesoderm (angioblastic) cords as once thought (C). (Figure adapted from Ferkowitz 2005; reprinted, with permission, from Elsevier © 2005.)

Figure 2.

Formation of capillary-like structures in Matrigel coated plates. Photomicrographs (20× magnification) of freshly sorted cord blood (CB)– or mobilized peripheral blood (mPB)–derived CD34⁺AC133⁺ VEGFR2⁺ cells and early passage endothelial colony–forming cells (ECFCs) plated over Matrigel. The triple positive CB- and mPB-derived cells failed to form capillary-like structures, whereas the ECFC formed numerous lumenized structures. (Figure adapted from Case 2007; reprinted, with permission, from Elsevier © 2007.)

Figure 3.

Frequency analysis of CD31⁺CD34^brightCD45^dimAC133⁺ cells using two distinct methods of analysis. In the first strategy (A–D), manually compensated data collected on a digital flow cytometer were visualized on plots with logarithmic scaling. Mononuclear cells (MNCs) (red gate in A) were identified on a forward and side scatter (FSC/SSC) plot and subgated onto a bivariant antigen plot to identify CD34^brightAC133⁺ cells (dark blue gate in B). CD34^brightAC133⁺ cells were further gated to identify the CD45^dim subpopulation (light blue gate in C). CD31 expression on the resulting CD34^brightAC133⁺ CD45^dim cells was confirmed on a CD31 histogram (D). In this first strategy (A–D), gate boundaries were set using Boolean gating and negative isotype controls. In the second strategy (E–I), uncompensated data was collected on a digital flow cytometer, compensated after acquisition by using software, and visualized in plots with biexponential scaling (linear and logarithmic). MNCs (red gate in A) were identified on a FSC/SSC plot and then CD14⁻ cells (orange gate in E) were identified. All CD14⁻ events were then assessed for viability (ViViD) and glycophorin A (GlyA) (F). The CD14⁻GlyA⁻ ViViD⁻ (pink gate in F) were subgated onto a bivariant antigen plot to identify CD14⁻GlyA⁻ViViD⁻ CD34^brightAC133⁺ cells (dark blue gate in G). CD14⁻GlyA⁻ViViD⁻CD34^brightAC133⁺ cells were further subgated to identify the CD45^dim subpopulation (light blue gate in H). CD31 expression was confirmed on a CD31 histogram. In the second approach (E–I), fluorescence minus one gating controls were used to set gate boundaries. (Figure adapted from Estes et al. 2010; reprinted, with permission, from John Wiley & Sons © 2010.)