Sickle erythrocytes adhere to fibronectin-thrombospondin-integrin complexes exposed by thrombin-induced endothelial cell contraction

Vascular damage appears to be associated with sickle erythrocyte (SS RBC) adherence to the endothelium. Thrombin, which has been found in abnormal levels in many sickle patients, causes endothelial cell (EC) retraction and increased SS RBC adherence, and SS RBC adhere in the gaps opened between the EC. Our objective was to elucidate the mechanism of adherence to activated EC monolayers and to determine whether the matrix proteins thrombospondin (TSP) and fibronectin (FN) are mediators of this adherence. Thrombin activation elicited the same 2.5-fold increase in adherence whether 10 or 35% of the matrix was exposed, and the majority of the RBC adhered at the edges of the EC regardless of the extent of matrix exposed. Using static adherence assays we investigated whether TSP, FN, or the integrins alpha(v)beta(3) and alpha(5)beta(1) mediated adherence. Blocking antibodies to any of these four had no effect on adherence to untreated monolayers. However, all the increased adherence elicited by thrombin was abrogated by each one, whereas control antibodies had no effect. Immunofluorescent microscopy demonstrated that both integrins were present on the luminal surface of confluent EC. Neither TSP nor FN was exposed in confluent cultures but they both became available as receptors after EC retraction. These data suggest that SS RBC adhere to a complex of matrix TSP and FN maintained in an adhesive conformation by interactions with both integrins.