Abstract 1299: Thrombospondin-1 is Essential for Bone Marrow Mononuclear Cell Recruitment to Sites of Vascular Injury
Bone marrow-mononuclear cells (BMC) contain endothelial progenitor cell precursors (EPC) useful in cell therapy to enhance post-ischemic neovascularization. There is growing interest in the role of thrombosis in EPC engraftment and function. We hypothesized that thrombospondin-1 (TSP1) which makes up to 25% of total platelet degranulated proteins may modulate BMC proliferation, survival and EPC differentiation. Treating mouse BMC suspensions with recombinant or purified TSP1 at near-physiological doses (1 to 50 μg/ml) for 16h upregulated their adhesion to fibrin (3 fold increase over control, p<0,05). In contrast, TSP1 failed to modulate EPC differentiation, as assessed by UEA-1 lectin and Dil-acetylated LDL staining. TSP1 treatment for up to 5 days did not modify BMC survival or proliferation by FACS and direct cell counts, in contrast with mature endothelial cells (HUVEC). We collected the degranulation products of wild type and TSP1−/− platelets after in vitro aggregation. Treatment of BMC with platelet products stimulated adhesion to fibrin in vitro (+36.5%; p<0.05), but TSP1−/− platelet products had no significant effect. In a model of FeCl3-induced intravascular injury and thrombosis combined with systemic administration of fluorescent BMC and intra-vital microscopy, we showed specific recruitment of BMC to sites of intra-vascular thrombosis, with increased BMC-to-vessel wall interactions (3 to 4 fold increase; p<0.05), and decreased rolling speed. BMC recruitment was nearly entirely abolished in TSP1−/− vessels, with similar interactions and rolling speeds with or without thrombosis. In summary, we showed that TSP1 stimulates strong BMC adhesion and recruitment to sites of thrombosis without affecting differentiation into EPC. Contrary to expectations, TSP1 may play a critical role in EPC recruitment and engraftment to sites of vascular injury and may thus contribute to vasculogenesis. These observations may help design novel methods to enhance pro-angiogenic cell therapy, and may contribute to redefine the role of TSP1 in the onset of tissue neovascularization.