Platelets Provide Signals for CD34+ Progenitor Cells to Home to Sites of Vascular Injury.

Introduction Accumulating evidence indicates that vascular function is modulated by circulating progenitor cells (PCs). Circulating PCs have been shown to enhance angiogenesis, promote vascular repair, improve endothelial function, inhibit atherosclerosis and increase ventricular function after myocardial infarction. How circulating PCs home to their target sites is not known.

Hypothesis Since vascular damage, ischemia and atherosclerosis coincides with the activation of coagulation and platelets we hypothesized that fibrin, activated platelets and platelet products might trigger circulating PCs to home to sites of injury. Homing involves (1) escape of PCs from flowing blood, (2) migration towards an injury and (3) differentiation into an endothelial cell phenotype.

Methods & Results (1) Escape from flowing blood was tested in a flow model using tissue factor expressing endothelial cells (ECs) on which a fibrin network containing thrombi was formed under flow. The platelet thrombi offered flowing CD34⁺ cells a landing site leading to deceleration and firm attachment to the surrounding ECs. Tethering of the CD34⁺ was dependent on P-Selectin expressed by the platelets, since blokkage of P-Selectin or P-Selectin glycoprotein ligand-1, expressed by the CD34⁺, prevented deceleration of the CD34⁺ cells. (2) Migration of PCs was tested in a Boyden chamber in which fibrin, fibrin+platelets or platelets alone was used to attract CD34⁺ cells. Fibrin+platelets induced migration of CD34⁺ cells, while fibrin or platelets had no effect. (3) Differentiation of PCs was tested in a culture system using a fibrin clot containing platelets and a culture medium lacking serum and additional growth factors. In this way, secreted platelet products like VEGF, transforming growth factor, platelet activating factor or lysophosphatidic acid determine the culture conditions. After 10 days of culture on fibrin+platelets, CD34⁺ PCs showed uptake of acetylated LDL, expression of eNOS, VEGF receptor 2 and CD31, indicating that the cells exhibited endothelial-like features.

Conclusion Our results indicate that activation of the coagulation system and consequently activation of platelets, may offer circulating PCs a way to efficiently home to sites of vascular injury and thus contribute to neovascularization.