Circulating endothelial precursors: identification of functional subpopulations

Comment on Elsheikh et al, page 2347

The characterization of circulating endothelial progenitor cells is unclear. A recent study now provides evidence that the expression of the VEGF receptor 2 identifies a subpopulation of cells with endothelial regeneration activity within the CD14⁺ monocytic cells.

Endothelial progenitor cells (EPCs) contribute to neovascularization after ischemia, tumorangiogenesis, and recovery of the endothelial monolayer after injury. Infusion of bone marrow-derived or circulating EPC augments neovascularization (for review, see Urbich and Dimmeler¹ ). However, the precise characterization of EPCs is still an open question. Increasing evidence suggests that different subsets of EPCs exist within the circulating blood. Cells with endothelial surface marker expression and functional characteristics of endothelial cells can be grown out of hematopoietic stem cells expressing the markers CD34 or CD133, but also from monocytic cells.

In this issue of Blood, Elsheikh and colleagues aimed to define the subpopulation within the monocytic cells, which exerts “EPC quality.” The authors isolated CD14⁺ monocytic cells and purified the cells that expressed the vascular endothelial growth factor receptor 2 (VEGF-R2, or KDR). Expression of endothelial marker proteins and functional activity of the isolated cells was then determined. CD14⁺VEGF-R2⁺ cells express higher levels of endothelial marker proteins and formed tubes in vitro (see figure). Moreover, CD14⁺VEGF-R2⁺ but not CD14⁺ VEGF-R2^- cells contributed to re-endothelialization after infusion of the cells in mice after denuding injury. These results are in accordance with previous studies demonstrating that CD34⁺VEGF-R2⁺ cells showed a better improvement of neovascularization after hind-limb ischemia compared with CD34⁺VEGF-R2^- cells.²  These data implicate that VEGF-R2 expression is a major criterion to identify cells with a capacity to generate endothelial cells independent of whether they originate directly from hematopoietic stem cells or from monocytic cells.

One may speculate that the expression of VEGF-R2 is simply required to mediate signalling induced by its ligand, VEGF. However, CD14⁺VEGF-R⁺ as well as CD34⁺VEGF-R2⁺ also exhibited additional functional features. VEGF-R2⁺ cells showed a higher proliferation, migration, and growth factor release,²  (see figure) indicating that selection of VEGF-R2⁺ cells may enrich cells with a specific functional profile and potential “stemness” features. Indeed, pluripotent stem cells were restricted to the CD34⁺VEGF-R2⁺-cell fraction.³  Moreover, a specific subfraction of circulating CD14⁺ monocytic cells was recently shown to express the stem-cell markers Nanog and octamer-binding transcription factor 4 (oct-4).⁴  These Nanog⁺ monocytic cells were enriched in the VEGF-R2⁺ fraction and showed low expression of CD34. Whether these CD14⁺/CD34^low/Nanog⁺ cells are representing the active fraction of CD14⁺/VEGF-R2⁺ cells isolated by Elsheikh et al remains to be elucidated. ▪