Adherence of phosphatidylserine-exposing erythrocytes to endothelial matrix thrombospondin

Phospholipid asymmetry is well maintained in erythrocyte (RBC) membranes wi th phosphatidylserine (PS) exclusively present in the inner leaflet. The ap pearance of PS on the surface of the cell can have major physiologic conseq uences, including increased cell-cell interactions. Because increased adher ence of PS-exposing RBCs to endothelial cells (ECs) may be pathologically i mportant in hemoglobinopathies such as sickle cell disease and thalassemia, we studied the role of PS exposure in calcium ionophore-treated normal RBC adherence to human umbilical vein endothelial cell (HUVEC) monolayers. Whe n HUVEC monolayers were incubated with these PS-exposing RBCs, the ECs retr acted and the RBCs adhered primarily in the gaps opened between the ECs, A linear correlation was found between the number of PS-exposing RBCs in the population and the number of adhering RBCs to the monolayer, Pretreatment o f RBCs with annexin V significantly decreased adherence by shielding PS on the RBCs, Similarly, PS-containing lipid vesicles decreased RBC binding by competing for the PS binding sites in the monolayer, PS-exposing RBCs and P S-containing lipid vesicles adhered to immobilized thrombospondin (TSP) and matrix TSP, respectively, and adherence of PS-exposing RBCs to EC monolaye rs was reduced by antibodies to TSP and to its EC receptor, alpha(v)beta(3) . Together, these results indicate a role for PS and matrix TSP in the adhe rence of PS-exposing RBCs to EC monolayers, and suggest an important contri bution of PS-exposing RBCs in pathologies with reported vascular damage, su ch as sickle cell anemia, (C) 2000 by The American Society of Hematology.