Redefining endothelial progenitor cells

Since 1997, post‐natal vasculogenesis has been purported to be an important alternative to angiogenesis via marrow‐derived circulating endothelial progenitor cells (EPCs). But, EPCs are poorly defined and thus heterogenous, and comparison of results among clinical studies is confusing. We have used a commercially available assay to identify circulating human EPC that enumerates “colony forming unit‐endothelial cell” (CFU‐EC) activity and we correlated CFU‐EC formation with assays of hematopoietic and endothelial cells and these results were simultaneously compared to endothelial colony forming cells (ECFC) isolated from the same blood samples. CFU‐ECs display evidence of a monocyte/macrophage precursor origin by retaining expression of c‐fms and non‐specific esterase activity. CFU‐EC can be plucked and replated in hematopoietic CFC assays to give rise to colony forming unit‐macrophages, however, cells plucked from a CFU‐EC colonies are not capable of proliferation at a single cell level. In contrast, ECFC display a hierarchy of proliferative potential at the single cell level, fail to display monocyte/macrophage antigens, and do not form hematopoietic CFC. CFU‐EC lack robust in vitro capillary tube formation, however, CFU‐EC co‐localize with ECFC formed capillary tubes. Human ECFC, but not CFU‐EC, implanted subcutaneously in immunodeficient mice spontaneously form vessels that function to carry murine blood cells. Thus, CFU‐EC are myeloid progenitors that give rise to macrophages in vitro and are not EPCs. We propose a revised model where CFU‐EC participate in neoangiogenesis without becoming endothelial cells and ECFC function as the vessel forming EPCs that circulate and/or are recruited from nearby vessels.