Role of endothelial progenitor cells in acute vascular injury in man

Percutaneous coronary intervention (PCI) acutely improves coronary blood flow and myocardial perfusion but at the expense of endovascular laceration and endothelial denudation. PCI associated vascular injury is associated with intense inflammation and a loss of vascular function that may lead to significant in-stent restenosis (ISR), and the potentially catastrophic, acute stent thrombosis. Reendothelialisation is essential to the restoration of normal homeostasis and facilitating vascular healing. Attention has recently focused on a novel mechanism of reendothelialisation mediated by bone marrow-derived precursor or stem cells: endothelial progenitor cells (EPC). EPC are thought to home to, and reendothelialise sites of endothelial denudation, and therefore offer the potential to provide exciting new developments in the management of cardiovascular disease. Understanding the role of EPC following vascular injury may help us to enhance vascular repair following PCI. The following studies were performed to clarify the relationships between putative EPC and vascular injury associated with PCI. In studies of patients undergoing elective PCI for stable anginal symptoms I found that concentrations of traditional circulating phenotypic EPC expressing CD34+VEGFR-2+ were unaffected, unlike CD34+CD45- cell concentrations, which were transiently increased six hours following PCI, subsequently returning to normal by 24 hours, notably without an increase in CD34+ adhesion molecule expression or VEGF-A production. However, the purported progeny of CD34+VEGFR-2+ cells, endothelial cell-colony forming units (EC-CFU), were mobilised at 24 hours, commensurate with a systemic inflammatory response. Interestingly the concentration of circulating CD34+VEGFR-2+ cells and EC-CFU were unrelated to each other, emphasising the distinction between these two cell populations. Although EC-CFU contained proliferating cells and exhibited some endothelial characteristics, EC-CFU predominantly expressed the leukocyte antigen CD45 in addition to the lymphocyte markers CD4 and CD8, and most intensely, the surface markers CD68 and CD105, epitopes commonly expressed on macrophages. Notably, EC-CFU were a potent stimulus for the migration of mononuclear cells. However, despite being mobilised in the context of an acute systemic inflammatory response and being composed of leukocytes, isolated systemic inflammation in healthy volunteers (induced by Salmonella Typhus vaccination) in the absence of vascular injury did not cause selective mobilisation of EC-CFU or indeed of putative phenotypic EPC. It is therefore likely that EC-CFU mobilisation is a relatively specific inflammatory response to cardiovascular injury. In a cohort of 201 patients undergoing coronary angiography, traditional circulating phenotypic EPC (CD34+VEGFR-2+ and CD34+VEGFR-2+CD133+) were very rare indeed and were not increased in response to an acute coronary syndrome (ACS). Furthermore traditional EPC concentrations bore no relation to atheroma burden or clinical outcome. In contrast, concentrations of CD34+CD45- cells were increased in patients with coronary artery disease compared to those with normal coronary arteries and were increased in association with more severe coronary disease. Increased concentrations of circulating CD34+CD45- cells were also associated with a shorter cumulative event-free survival. Both EC-CFU and angiogenic monocytes expressing Tie-2 and VEGFR-2 were increased following acute myocardial infarction but did not relate to coronary atheroma or clinical outcome. These studies examine the behavior of putative EPC in response to both discrete vascular injury and myocardial infarction, and isolated inflammation in the absence of vascular injury. I have identified novel characteristics of the EC-CFU assay and determined that specific factors associated with cardiovascular injury likely trigger EC-CFU mobilisation. The clinical relevance of the traditional phenotypic EPC population is uncertain, but a novel CD34+CD45- population is mobilised acutely following discrete vascular injury and is significantly associated with coronary atheroma and clinical events. It is probable that the circulating CD34+CD45- concentration reflects vascular injury and atheroma burden, and I suggest that CD34+CD45- cells are released directly from the vessel wall following PCI, and do not reflect a reparatory response. In order to determine the impact of EPC populations on vascular healing, prospective studies examining the impact of periprocedural EPC concentrations on vascular healing following PCI are required.