Role of Platelets in Homing of Endothelial Progenitor Cells to the Site of Vascular Injury.

Endothelial Progenitor Cells (EPCs) are undifferentiated bone marrow cells that are rapidly mobilized upon vascular or tissue injury. The circulating EPCs home to and differentiate at the site of vessel injury to enhance repair of damaged endothelium. Much has been learnt of how EPCs adhere to and transmigrate on endothelial cells during angiogenesis, but little is know about how EPCs home to the site where endothelial cells are denuded and subendothelium is exposed following vascular injury. We hypothesized that platelets mediate EPC homing because they are the first cells that tether and adhere to subendothelium exposed by vascular injury. To test this hypothesis, we examined the platelet-EPC interaction under static and flow conditions. Human EPCs were purified from buffy coats by cell sorting using CD133 and VEGFR-2 as the cell markers. The EPCs were then incubated with washed platelets that were either at resting state or activated by 5 mg/ml of collagen. EPC-platelet interaction was measured in the form of heterotypic aggregation between two types of cells by flow cytometry (dual labeled with FITC-CD42a and PE-CD133). We found the EPCs attached to resting platelets, however, the percentage of platelets binding to EPCs significantly increased when platelets were activated by collagen (1.02% vs. 2.76%, p < 0.05). EPC-platelet aggregation was similarly detected in whole blood and platelet-rich plasma. When perfused over a monolayer of activated platelets under a shear stress of 2.5 dyn/cm², the CD133 positive EPCs tethered to the platelet monolayer. The tethered EPCs either firmly adhered immediately or rolled a short distance before becoming adherent. This EPC-platelet interaction was calcium-dependent and inhibited by up to 60±11.5% by a polyclonal P-selectin antibody. In comparison, the monoclonal GP Iba antibody AK2, which blocks GP Ib-VWF interaction, had no effect. These results demonstrate for the first time a significant interaction between EPCs and activated platelets both at static and flow conditions. Our findings suggest that platelets may function as carrier cells that direct EPCs to the site of vascular injury. In addition to homing, platelets, which release several growth factors known to affect EPCs, may provide a supporting matrix on which adherent EPCs proliferate and differentiate.