The limited vessel-forming capacity of infused endothelial progenitor cells (EPCs) into patients with cardiovascular dysfunction may be related to a misunderstanding of the biologic potential of the cells. EPCs are generally identified by cell surface antigen expression or counting in a commercially available kit that identifies "endothelial cell colony-forming units" (CFU-ECs). However, the origin, proliferative potential, and differentiation capacity of CFU-ECs is controversial. In contrast, other EPCs with blood vesselforming ability, termed endothelial colonyforming cells (ECFCs), have been isolated from human peripheral blood. We compared the function of CFU-ECs and ECFCs and determined that CFU-ECs are derived from the hematopoietic system using progenitor assays, and analysis of donor cells from polycythemia vera patients harboring a Janus kinase 2 V617F mutation in hematopoietic stem cell clones. Further, CFU-ECs possess myeloid progenitor cell activity, differentiate into phagocytic macrophages, and fail to form perfused vessels in vivo. In contrast, ECFCs are clonally distinct from CFU-ECs, display robust proliferative potential, and form perfused vessels in vivo. Thus, these studies establish that CFUECs are not EPCs and the role of these cells in angiogenesis must be re-examined prior to further clinical trials, whereas ECFCs may serve as a potential therapy for vascular regeneration. IntroductionNew blood vessel formation occurs via angiogenesis, vasculogenesis, or arteriogenesis. 1,2 Since 1997, postnatal vasculogenesis has been purported to be an important mechanism for angiogenesis via marrow-derived circulating endothelial progenitor cells (EPCs). 3 Based on this paradigm, EPCs have been extensively studied as biomarkers of cardiovascular disease and as a cell-based therapy for repair of damaged blood vessels. [4][5][6] However, administration of EPCs or bone marrow-derived cell populations enriched for EPCs into subjects with cardiovascular disease has had limited efficacy, with regard to new vessel formation. Many investigators speculate that the paracrine effects of cultured EPCs are responsible for the modest effects in patients because there is no evidence of long-term engraftment of EPCs into newly formed vessels. 7-9 These clinical observations are surprising given animal studies where EPC administration partially rescued cardiovascular dysfunction following ischemic hind limb or myocardial injury with some evidence for EPC contribution to new vessel growth. 5,9 In most studies, EPCs are identified and enumerated via flow cytometric identification of cells expressing CD34, CD133, or the VEGF receptor 2 (KDR). 3,10,11 Because these molecules are also expressed on hematopoietic stem/progenitor populations, 12-15 the presence of hematopoietic contamination of EPCs should be expected. EPCs are also quantitated by counting in a commercially available kit that identifies "endothelial cell colony-forming units" (CFU-ECs). Identification of CFU-ECs from peripheral blood by use of colony-forming ...

Endothelial progenitor cells (EPCs) circulate in the peripheral blood and reside in blood vessel walls. A hierarchy of EPCs exists where progenitors can be discriminated based on their clonogenic potential. EPCs are exposed to oxidative stress during vascular injury as residents of blood vessel walls or as circulating cells homing to sites of neovascularization. Given the links between oxidative injury, endothelial cell dysfunction, and vascular disease, we tested whether EPCs were sensitive to oxidative stress using newly developed clonogenic assays. Strikingly, in contrast to previous reports, we demonstrate that the most proliferative EPCs (high proliferative potential-endothelial colony-forming cells and low proliferative potential-endothelial colony-forming cells) had decreased clonogenic capacity after oxidant treatment. In addition, EPCs exhibited increased apoptosis and diminished tube-forming ability in vitro and in vivo in response to oxidative stress, which was directly linked to activation of a redox-dependent stress-induced kinase pathway. Thus, this study provides novel insights into the effect of oxidative stress on EPCs. Furthermore, this report outlines a framework for understanding how oxidative injury leads to vascular disease and potentially limits the efficacy of transplantation of EPCs into ischemic tissues enriched for reactive oxygen species and oxidized metabolites.

Neurofibromatosis type I (NF1) is a genetic disorder caused by mutations in the NF1 tumor suppressor gene. Neurofibromin is encoded by NF1 and functions as a negative regulator of Ras activity. NF1 patients develop renal artery stenosis and arterial occlusions resulting in cerebral and visceral infarcts. Further, NF1 patients develop vascular neurofibromas where tumor vessels are invested in a dense pericyte sheath. Although it is well established that aberrations in Ras signaling lead to human malignancies, emerging data generated in genetically engineered mouse models now implicate perturbations in the Ras signaling axis in vascular smooth muscular cells (VSMCs) as central to the initiation and progression of neointimal hyperplasia and arterial stenosis. Despite these observations, the function of neurofibromin in regulating VSMC function and how Ras signals are terminated in VSMCs is virtually unknown. Utilizing VSMCs harvested from Nf1+/- mice and primary human neurofibromin-deficient VSMCs, we identify a discrete Ras effector pathway, which is tightly regulated by neurofibromin to limit VSMC proliferation and migration. Thus, these studies identify neurofibromin as a novel regulator of Ras activity in VSMCs and provide a framework for understanding cardiovascular disease in NF1 patients and a mechanism by which Ras signals are attenuated for maintaining VSMC homeostasis in blood vessel walls.

Genetic inactivation of tumor suppressor genes initiates human cancers. However, interaction of accessory cells with the tumor-initiating cell within the microenvironment is often required for tumor progression. This paradigm is relevant to understanding neurofibroma development in neurofibromatosis type I patients. Somatic inactivation of the Nf1 tumor suppressor gene, which encodes neurofibromin, is necessary but not sufficient to initiate neurofibroma development. In contrast, neurofibromas occur with high penetrance in mice in which Nf1 is ablated in Schwann cells in the context of a heterozygous mutant (Nf1+/-) microenvironment. Neurofibromas are highly vascularized, and recent studies suggest that Nf1+/- mice have increased angiogenesis in vivo. However, the function of neurofibromin in human endothelial cells (ECs) and the biochemical mechanism by which neurofibromin regulates neoangiogenesis are not known. Utilizing Nf1+/- mice, primary human ECs and endothelial progenitor cells harvested from NF1 patients, we identified a discrete Ras effector pathway, which alters the proliferation and migration of neurofibromin-deficient ECs in response to neurofibroma-derived growth factors both in vitro and in vivo. Thus, these studies identify a unique biochemical pathway in Nf1+/- ECs as a potential therapeutic target in the neurofibroma microenvironment.

We recently identified a novel hierarchy of human endothelial progenitor cells (EPCs), which are functionally defined by their proliferative and clonogenic potential (Blood, 2004). Emerging evidence suggests that EPCs may be used as angiogenic therapies, or as biomarkers to assess cardiovascular disease risk. Thus, identification of animal models, which phenocopy the human EPC hierarchy, is an important priority for preclinical testing of experimental therapeutics. Given the importance of the Rhesus Macaque as a preclinical model, we tested whether EPCs could be isolated from the peripheral blood of the Rhesus Macaque and compared to EPCs isolated from human adult peripheral blood. Mononuclear cells were isolated from 20 ml of Rhesus peripheral blood and cultured in EGM-2 medium, which promotes the formation of EPC colonies. After 7 days in culture, we identified approximately 20 endothelial cell colonies (n=9), which appeared identical to human EPC colonies. We subcultured the endothelial cell colonies into monolayers for immunophenotyping and functional analysis. Endothelial cells (ECs) derived from the Rhesus EPC colonies formed vessels in matrigel, and demonstrated uptake of acetylated LDL, which are characteristics of ECs. Similar to ECs derived from human EPCs, Rhesus ECs expressed the endothelial cell antigens, CD31, CD144, CD105, CD146, and Flk1. Importantly, Rhesus ECs did not express the hematopoietic cell specific antigens, CD45 and CD14. Similar to ECs derived from human peripheral blood EPC colonies, Rhesus ECs could be serially passaged for at least 40 population doublings without signs of cellular senescence. A hallmark of stem and progenitor cells is their ability to proliferate and give rise to functional progeny. Analogous to a paradigm established in the hematopoietic cell system, we recently developed a single cell deposition assay to reproducibly identify the following human EPCs: (1) high proliferative potential - endothelial colony forming cells (HPP-ECFC), which form macroscopic colonies that form secondary and tertiary colonies upon replating, (2) low proliferative potential - endothelial colony forming cells (LPP-ECFC), which form colonies greater than 50 cells, but do not form secondary colonies upon replating, (3) endothelial cell clusters (EC-clusters) that contain less than 50 cells, and (4) mature terminally differentiated endothelial cells (EC), which do not divide (Blood, 2004). To determine whether these different populations of EPCs could be identified in the ECs derived from Rhesus EPCs, we performed single cells deposition assays on 1,000 cells. All types of EPCs could be identified in the Rhesus ECs (Table I). Further, ECs derived from the Rhesus EPCs rapidly form chimeric vessels with human ECs derived from adult blood, implying that the molecular mechanisms critical for vessel formation are conserved between the two species. Finally, while the murine model is an animal model widely used for studying EPCs, a similar hierarchy of EPCs could not be established from the peripheral blood of mice. Thus, given the diversity of therapeutic applications of EPCs for treating a variety of human diseases, these studies establish the Rhesus Macaque as an important preclinical model. Percent of 1,000 Single Cells Plated Mature EC EC-Cluster LPP-ECFC HPP-ECFC Rhesus ECs 85.8±2.1 4.2±1.1 7.8±0.5 1.3±0.5 Human ECs 80.8±9.6 8.6±1.4 12.4±8.1 0.2±0.2