Characterisation of the interaction between circulating and in vitro cultivated endothelial progenitor cells and the endothelial barrier

In vitro cultured endothelial progenitor cells (cEPC) are used for intracoronary cell therapy in cardiac regeneration. The aim of this study was to investigate whether cEPC and circulating mononuclear cells (MNC), which include a small number of in vivo circulating EPC, are able to transmigrate through the endothelial barrier into the cardiac tissue. MNC and EPC were isolated from the peripheral blood from healthy male volunteers (n = 13, 25+/-6 years) and stained with a fluorescent marker. The cells were perfused in vitro through organs with endothelial layers of different phenotypes (rat aorta, human umbilical vein, isolated mouse heart). The endothelium and the basal lamina were then stained by immunofluorescence and the cryo-sections analysed using a confocal laser scanning microscope. After perfusion through the rat aorta, an adhesion/integration of MNC was observed at the endothelial layer and the basal lamina beneath endothelial cells. However, no migration of MNC over the endothelial barrier was found. This remained true even when the cell numbers were increased (from 0.5 to 10 million cells/h), when the time of perfusion was prolonged (1.5-4 h) and when the aorta was cultivated for 24 h. In the Langendorff-perfused mouse heart with intact endothelium, no migration of MNC (1 x 10(7)) or cEPC (1 x 10(6)) was observed after 0.5 and 2 h. In conclusion, MNC and cEPC do not possess any capacity to transmigrate the endothelial barrier. In the context of stem cell therapy, these cells may therefore serve as endothelial regenerators but not as cardiomyocyte substitutes.