Phosphatidylserine (PS)-Positive Erythrocytes Bind to Both Immobilized and Soluble Thrombospondin-1 (TSP-1) Via Its Heparin Binding Domain.

Phosphatidylserine (PS)-dependent erythrocyte adhesion to both cultured endothelial cells and the components of sub-endothelial matrix appears to be mediated in part via thrombospondin-1 (TSP). While TSP exhibits multiple cell-binding domains, the PS-binding site on TSP has not been identified. Since a cell-binding domain for anionic heparin is located at the amino-terminal domain of TSP, we hypothesized that anionic PS-positive (PS^+ve) red cells bind to this domain. In a recent preliminary study, using a flow adhesion system and PS^+ve erythrocytes (prepared by treating control AA red cells with A23187), we have demonstrated that heparin inhibited PS^+ve erythrocyte adhesion to immobilized TSP in a concentration-dependent manner with 58 to 77% inhibition noted at concentrations between 1 and 50 U/ml (n=9, P<0.001). Other anionic polysaccharides including high molecular weight dextran sulfate and chondroitin sulfate A also inhibited PS^+ve erythrocyte adhesion to immobilized TSP with the magnitude of the inhibitory effects comparable to heparin. These results suggested that the heparin-binding domain on TSP may be involved in PS-mediated red cell adhesion to immobilized TSP. We have extended these studies to characterize the PS-binding site on TSP using monoclonal antibodies directed against specific cell-binding domains on the molecule and also using specific TSP peptides. We demonstrate that pre-incubation of immobilized TSP with an antibody directed against the heparin-binding domain on TSP (TSP-Ab9, Lab Vision) blocked PS-mediated red cell adhesion to immobilized TSP (80% inhibition compared to an isotype-matched negative control antibody, n=7, P<0.001), whereas an antibody that recognizes the collagen-binding domain on TSP (TSP-Ab4) did not affect this process. In addition, incubation of PS^+ve erythrocytes with a TSP peptide containing the specific heparin-binding motif, KKTRG, inhibited PS-mediated red cell adhesion by 55% (P<0.001, n=6), whereas a peptide lacking the binding motif had no effect. Since protein confirmation of immobilized TSP appears to be different from that of soluble TSP, we next investigated whether soluble TSP, like immobilized TSP, also interacted with PS^+ve erythrocytes. Erythrocytes containing 8 to 10% PS^+ve cells were incubated in the absence or the presence of increasing concentrations of soluble TSP (0.1 to 10 μg/ml), and then analyzed by flow cytometry for surface bound TSP using both adhesion blocking (TSP-Ab9) and non-blocking (TSP-Ab4) anti-TSP antibodies. We demonstrate that soluble TSP binds to PS^+ve erythrocytes in a concentration-dependent manner with 3 to 11% TSP-positive (TSP^+ve) red cells measured at soluble TSP concentrations between 1 to 10 μg/ml (n=4). In addition, TSP binding could be detected only with the non-adhesion blocking antibody TSP-Ab4, which recognizes the collagen-binding domain on TSP. The adhesion blocking antibody TSP-Ab9 that interacts with the heparin binding domain, failed to detect any TSP^+ve red cells. No TSP^+ve erythrocytes were detected when PS-negative control red cells were evaluated in binding assays. In parallel adhesion experiments, soluble TSP inhibited PS^+ve erythrocyte adhesion to immobilized TSP at concentrations at which significant TSP binding to erythrocytes occurred (43% and 44% inhibition at 5 and 10 μg of soluble TSP per ml, n=4). These results conclusively demonstrate that PS-positive erythrocytes interact with both immobilized and fluid phase TSP through the heparin-binding domain, and that heparin blocks this interaction.