Defining endothelial progenitor cells

Abstract

New vessel formation occurs via vasculogenesis, angiogenesis, or arteriogenesis. Since 1997, post-natal vasculogenesis has been purported to be an important mechanism for angiogenesis via marrow-derived circulating endothelial progenitor cells (EPCs). Emerging evidence suggests that EPC are derived from monocytes. We have used a commercially available assay for circulating human EPC that enumerates “colony forming unit-endothelial cell” (CFU-EC) activity and we correlated CFU-EC formation with markers for monocytes, macrophages, and endothelium and these results were compared to endothelial colony forming cells (ECFC) isolated from the same blood samples. CFU-ECs display evidence of their monocyte/macrophage lineage 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 colony are not capable of proliferation at a single cell level. In contrast, ECFC display a hierarchy of proliferative potentials 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. A whole cell proteomics analysis indicates differences in expression of several hundred proteins between CFU-EC and ECFC. 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 macrophages. We propose a revised model for considering the cellular events involved in new vessel formation emphasizing the interacting role of macrophages and ECFC.