CFU-EC: how they were originally defined

To the editor:

We read with interest the article “Redefining endothelial progenitor cells via clonal analysis and hematopoietic stem/progenitor cell principles” by Yoder et al.¹  The authors compared 2 types of colony assays that have been shown to identify endothelial progenitor cells (EPCs)^2,3  and studied the function of the cells derived from these assay systems. They found that a commercially available kit, which is widely used to quantify EPCs, actually detects cells of the monocytic lineage, whereas an assay developed by the authors unequivocally measures the colony-forming potential of true EPCs. Although we agree with the authors that it is time to clearly define the term EPC and the results presented are consistent with our own experience, we have some comments regarding the nomenclature of the endothelial colonies.

In the May 15, 2000 issue of Blood, we reported the identification of human endothelial progenitor cells expressing CD133 (previously known as AC133).⁴  We described a suspension culture system that induces differentiation of CD133-positive (+) cells into functional endothelial cells and a methylcellulose-based colony assay for EPCs. In this assay, CD133⁺ progenitor cells give rise to colonies with a unique morphology, which is different from CD133-derived hematopoietic colonies grown in methylcellulose. We showed that these colonies are composed of small-sized cells that express the endothelial cell antigen von Willebrand factor. Because our colony assay for EPCs was developed in analogy to the standard colony assay for hematopoietic stem and progenitor cells, we called the EPC-derived colonies “colony-forming unit endothelial cells (CFU-EC)” and introduced this term to scientific readership for the first time.

Meanwhile, numerous studies have been published claiming that they investigated the clonogenic potential of EPCs. We are aware that some of these studies adopted the term CFU-EC, although they used different culture systems.⁵  However, we would like to stress that the colonies presented by Hill et al have not been referred to as CFU-EC by the authors themselves.³  With regard to the already existing confusion about the definition of EPC and their progeny, we propose to refer to the myeloid colonies occurring in the Hill assay as “CFU-Hill” according to nomenclature used by the manufacturer of this assay. The term CFU-EC should be reserved for colonies derived from CD133⁺ EPC as described in our study.⁴ 

In addition, we would like to note that the study by Reyes et al, cited as reference 11 in the article by Yoder and colleagues, did not use colony assays.⁶