Endothelial progenitor cells, neointimal hyperplasia, and hemodialysis vascular access dysfunction: novel therapies for a recalcitrant clinical problem.

Vascular stenosis as a result of neointimal hyperplasia is a major clinical problem that has an impact on multiple and diverse disciplines, including cardiology (coronary restenosis), cardiothoracic and vascular surgery (saphenous vein and polytetrafluoroethylene [PTFE] graft failure), neurology (carotid stenosis), nephrology (dialysis access dysfunction), and transplant medicine (chronic allograft rejection in hearts and kidneys). The traditional response to injury hypothesis on the pathogenesis of neointimal hyperplasia focuses on the migration of medial smooth muscle cells from the media into the intima.1 Recently, there has been a great deal of excitement about the role of circulating smooth muscle progenitor cells in the pathogenesis of neointimal hyperplasia. These cells have been identified in a variety of experimental models of vascular injury,2 and interventions that reduce the number of these cells can attenuate neointimal hyperplasia. In marked contrast to the deleterious effects of smooth muscle progenitor cells on neointimal hyperplasia, circulating endothelial progenitor cells (EPCs) are believed to play an important role in vascular repair and in the inhibition of neointimal hyperplasia.3