Abstract 3915: Del-1, A New β2-Integrin Ligand, Which Inhibits Adhesion and Homing of Progenitor Cells

Progenitor cells (PC) are recruited to ischemic tissues and improve neovascularization. Beta2-integrins are essential for adhesion, transmigration and homing of PC to ischemic tissues. Developmental Endothelial Locus-1 (Del-1) is an extracellular matrix protein, that binds alphaVbeta3- and alphaVbeta5-integrins and is up-regulated during ischemia. Therefore, we investigated the role of endogenous Del-1 for angiogenesis and homing functions of PC. The 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 + hematopoietic cells than WT mice, suggesting that Del-1 may have an inhibitory effect on homing of PC and inflammatory cells to ischemic tissues. Interestingly, in adhesion assays human endothelial progenitor cells (EPC) and murine Lin − progenitor cells bound to Del-1 via beta2-integrins, but not via the alphaVbeta3- and alphaVbeta5- integrins. Furthermore, soluble Del-1 significantly inhibited the adhesion of EPC to HUVEC monolayers and to the major beta2-integrin-ligand, ICAM-1, raising the possibility that Del-1 is a beta2-integrin-inhibitor. Indeed, WT murine bone marrow 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 − bone marrow 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). Taken together, endogenous Del-1 is a new beta2-integrin ligand, which blocks 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 progenitor and inflammatory cells.