Abstract 4953: Del-1, a New Beta2-integrin Ligand, Which Inhibits Mobilization, Adhesion and Homing of Progensitor Cells

Progenitor cells (PC) are recruited to ischemic tissues and improve neovascularization. Beta2-integrins are essential for adhesion and homing of PC to ischemic tissues. Developmental Endothelial Locus-1 (Del-1) is an extracellular matrix protein that binds alphaVbeta3-and beta2-integrins and is up-regulated during ischemia. Therefore, we investigated the role of Del-1 for angiogenesis and homing functions of PC. Del-1-deficient mice (Del-1−/−) displayed a significantly increased angiogenic response in ischemic muscles in comparison to the wild type (WT) mice in the model of hind limb ischemia. However, when we assessed the role of Del-1 in HUVEC in vitro, silencing of Del-1 by siRNA did not affect angiogenic sprouting. Moreover, the ischemic muscles of Del-1−/− mice displayed a higher infiltration with CD45+ cells than WT mice, suggesting that Del-1-deficiency may induce activation of homing of PC and inflammatory cells to ischemic tissues. Interestingly, in adhesion assays human endothelial progenitor cells (EPC) and murine Lin- PC bound to Del-1 via beta2-integrins, but not via the alphaVbeta3-integrins. Furthermore, soluble Del-1 significantly inhibited the adhesion of EPC to HUVEC monolayers and to the major beta2-integrin-ligand, ICAM-1. Moreover, WT murine bone marrow (BM) mononuclear cells displayed higher adhesion rates on Del-1-deficient murine lung endothelial cells (LEC) than on WT LEC. In order to investigate the role of Del-1 for in vivo homing of PC, we intravenously injected murine fluorescence-labeled WT Lin- BM PC in WT and Del-1−/− mice 2 days after the induction of hind limb ischemia. Interestingly, the homing of injected Lin- cells to ischemic muscles was significantly increased in Del-1−/− in comparison to WT mice (200±30 % increase). Moreover, Del-1−/− mice displayed a significantly increased GCSF-induced mobilization of Sca1+ cKit+ PC in the peripheral blood in comparison to WT mice. Taken together, Del-1 is a new beta2-integrin ligand, which blocks PC mobilization and beta2-integrin-dependent adhesion and homing of PC to ischemic tissues. It is conceivable, that endogenous Del-1 may reduce ischemia-induced neovascularization through an inhibitory effect on the beta2-integrins of PC and inflammatory cells.