Читать книгу The SAGE Encyclopedia of Stem Cell Research - Группа авторов - Страница 139

The development of adult stem cells for clinical application has undergone a paradigm shift compared to the rest of the stem cell therapeutic approaches. The adult stem cells are present in the circulation, bone marrow, or within specific tissue types as residents. Neovascularization therapies have included mainly circulating or bone marrow derived stem cell approaches. The application of adult stem cells in the vasculogenesis was set up with the discovery of a subpopulation of vasculogenic endothelial progenitor cells (EPCs) by Asahara in 1997. The idea has gone on to be used in recent clinical trials. The commonly used surface markers for EPC identification include the markers that are not endothelial lineage specific like CDR10 and CD133.

The standardization of EPC culturing, purifying, and harvesting are not yet fully elucidated. Thus, methodological variation compounds semantic confusion. The EPCs may be considered a mixed population of different lineage of progenitor cells. True endothelial progenitors constitute this population of cells that gets incorporated within the vascular network while the secretion of angiogenic cytokines makes up the contribution of the hematopoietic progenitors. The EPCs give rise to “late outgrowth” or “early outgrowth” colonies in cell culture. Cells originating from the early outgrowth colonies express hematopoietic lineage markers and are quite evidently not of endothelial origin. These cells are different from the late outgrowth cells morphologically. The late outgrowth cells are reminiscent of endothelial cells and grow in a cobblestone like pattern. The early endothelial progenitor distinguishing surface markers are yet to be clearly established. The best possible approach for morphologically defining the endothelial lineage is by the use of tubulogenesis assays. In matrigel, the endothelial progenitors form tubular networks and get incorporated into the networks made by the differentiated endothelial cells. It has been generally accepted that the EPCs are formed from earlier progenitors known as hemangioblast that also act as the progenitors for hematopoietic stem cells. Pre-clinical studies have indicated that the EPCs circulate at very low levels in the blood (less than 0.01% of white circulating cells) and reside in the marrow (adhered to the stem cell niche constituting supporting stromal cells). Their presence in the blood changes with the type of stimuli. VEGF expression gets increased by ischemia resulting in the release of EPCs (CD34+/cKit+) by activating the matrix metalloproteinases and cleaving the kit-ligand. The EPCs so mobilized home to the ischemic site after entering the circulation. Ischemia mobilized bone marrow derived cells get incorporated within the vasculature and differentiate into pericytes, endothelial cells or smooth muscle cells. Besides, they may induce local angiogenesis through the paracrine factor elaboration. A number of animal studies including coronary- and hind limb-ischemia models suggest that the EPCs can be expanded and harvested ex vivo and administered to stimulate perfusion, capillary density, and organ function. In humans, G-CSF expanded peripheral blood mononuclear cells (PBMNCs) and autologous bone marrow mononuclear cells (BMNCs) have been contemplated for vascular regeneration in patients with peripheral and coronary arterial disease. Phase I and II trials are now completed with acute chronic ischemic and myocardial infarction patients. Currently, to substantiate the initial findings, several international Phase II trials are underway that can determine the efficacy and safety of the earlier results.