Abstract 969: Importance of CXCR2 in the Homing of Human Peripheral Blood Endothelial Progenitor Cells to Sites of Vascular Injury
Circulating endothelial progenitor cells (EPCs) may contribute to endothelial regeneration, however, the mechanisms underlying their homing remain elusive. We have investigated the contribution of two key angiogenic chemokine receptors, CXCR2 and CXCR4, to vascular homing of human EPCs. EPCs were isolated from peripheral blood mononuclear cells of healthy volunteers and cultured on fibronectin-coated plates in endothelial cell specific medium for 7 days. Flow cytometric analysis revealed the expression of both CXCR2 and CXCR4 together with KDR, CD31, and VE-cadherin on isolated EPCs. By contrast, mature endothelial cells expressed CXCR4 but little CXCR2. This was further confirmed by analyzing mRNA levels of both chemokine receptors. Assays on adhesion molecules and extracellular matrix (ECM) proteins under flow conditions showed that EPCs preferentially adhere to beta2-integrin ligands (ICAM-1 and fibrinogen), and that firm adhesion of EPCs to fibronectin was enhanced by co-immobilizing the CXCR2 ligand CXCL1 but not the CXCR4 ligand CXCL12. Conversely, CXCL12 but not CXCL1 increased EPC transmigration. Notably, blockade of CXCR2 but not that of CXCR4 significantly reduced EPC adhesion on platelet-coated endothelial ECM in flow. This was corroborated by a predominant involvement of CXCR2 in EPC recruitment to denuded areas of murine carotid arteries ex vivo but also in vivo. Immunofluorescence exhibited reactivity for the beta-thromboglobulin isoform and CXCR2 ligand CXCL7 in platelets and denuded medial smooth muscle cells (SMCs) early after wire-injury. In isolated human arterial SMCs, CXCL7 mRNA transcripts were detected in addition to those of the CXCR2 ligands CXCL1 and CXCL8. Finally, we identified CXCR2+/CD34+/KDR+ and CXCR2+/CD14+/KDR+ cells in human peripheral blood, implying the presence of two potential EPC subsets expressing CXCR2 in vivo. Thus, our data indicate that CXCR2 plays a critical role in the homing of EPCs to sites of arterial injury.