Title: Soluble E-Selectin Effects on Homing of Ex Vivo Expanded Endothelial Progenitor Cells (EPC) for Ischemic Neovasculogenesis Background: E-selectin is an endothelial membrane glycoprotein best known for its ability to promote adhesion of leukocytes to cytokine-activated endothelial cells and transendothelial migration of human hematopoietic progenitor cells. Given the close relationship between hematopoietic stem cells and EPC, we investigated the effect of soluble E-Selectin on EPC-mediated vasculogenesis in murine hindlimb ischemic model. Methods and Results: In mouse, BM and spleen, in which homing of EPC were occurred mainly, expressed E-selectin highly than muscle on immunofluorescence. FACS analysis showed that each E-selectin and E-selectin ligand were expressed in murine endothelial cell line, MS-1, and EPC. To investigate the interaction between EPC and soluble E-selectin, we mixed EPC with soluble E-selectin or vehicle in vitro and then executed FACS analysis of the mixture with anti-E-selectin antibody. In this way, we confirmed the binding of the soluble E-selectin to EPC. To demonstrate the interaction in flow system, we labeled the EPC with fluorescent dye, DiI, and added the cells into the flow system which had either E-selectin-coated membrane or control immunoglobulin-coated one. The number of the attached cells on the E-selectin-coated membrane was 1012짹215/cm2 and that on the control membrane, 534짹149/cm2 (n=4, p=0.014). To investigate the effect of E-selectin in vivo, we locally injected soluble E-selectin into ischemic hindlimb muscle of athymic nude mice combined with human EPC transplantation to determine whether E-selectin augmented EPC homing. Fluorescence microscopic examination disclosed enhanced accumulation of fluorescence labeled EPC in ischemic muscle in the E-selectin treatment group (control versus E-selectin = 82짹14 versus 132짹17 cells/mm2, P=0.027, n=8). Conclusion: These findings suggested that low rate of homing of EPC to the ischemic muscle might be due to low expression level of E-selecin in the muscle and local enhancement of E-selectin in ischemic organ might increase neovasculogenesis through enhanced homing of EPC.