Redefining endothelial progenitor cells

Abstract

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.