Summary

Endothelial progenitor cells (EPCs) mobilize from the bone marrow in response to tissue injury and participate in vascular repair. However, there is limited data about the homing mechanisms of EPCs to vascular injury sites. Recently animal experiments indicated that platelets play a role in recruitment of EPCs to injury sites. However, data on the possible interaction between platelets and EPCs within the human system are limited. We, therefore, examined in-vitro human platelet-EPC interaction under static and flow conditions. Human EPCs were isolated from donated buffy coats by magnetic microbeads and flow cytometry cell sorting using CD133 and VEGFR-2, respectively, as markers. Platelets were tested in the form of washed platelets, platelet rich plasma or whole blood.EPCs formed heterotypic aggregates with resting platelets under static conditions,an interaction that was greatly enhanced when platelets were actiendothelial regeneration (8). Furthermore, transplantation of isolated peripheral blood EPCs into balloon-injured arteries led to rapid endothelialization of the denuded vessels (10). In humans, a rapid increase in the levels of circulating EPCs was observed following acute vascular insults (11) or coronary artery stenting (12). There is limited data about the homing mechanisms of EPCs to areas of vascular injury, where endothelial cells are denuded and subendothelium is exposed. Since platelets circulate near the vessel wall and are the first cells to tether and subsequently adhere to subendothelium exposed by injury, we hypothesized that they play a central role in attracting and homing EPCs to injury sites. Several lines of evidence suggest an interaction between EPCs and platelets. Human CD34+ cells isolated from peripheral