ADHESION OF BLOOD-PLATELETS IS INHIBITED BY VCL, A RECOMBINANT FRAGMENT (LEUCINE(504) TO LYSINE(728)) OF VON-WILLEBRAND-FACTOR

VCL, fragment Leu(504) to Lps(728) Of Von Willebrand factor (VWF) expr essed in Escherichia coli, contains the glycoprotein (GP) Ib-binding d omain of VWF. This fragment inhibited ristocetin-induced platelet aggr egation with an IC50 of 0.2 mu mol/L and botrocetin-induced aggregatio n with an ICS, of 0.08 mu mol/L. We studied the antiadhesive profile o f VCL by adding it to blood that was circulated over various adhesive surfaces. VCL inhibited adhesion to endothelial cell matrix, which ser ved as a model of the vessel wall. Maximal inhibition at a high shear rate of 1600 s(-1) was stronger (60%) than at a low shear rate of 300 s(-1) (40%). Half maximal inhibition was found to be 1.5 mu mol/L at b oth shear rates. The role of various adhesive molecules was investigat ed in more detail by coating glass coverslips with collagen type I, la minin, fibronectin, or vWF. Fibrinogen was studied as well. Platelet a dhesion to laminin and VWF was not inhibited by VCL. Adhesion to colla gen, fibronectin, and fibrinogen was particularly inhibited al a high shear rate. VCL coated to a coverslip caused a concentration-dependent adhesion that was blocked by antibodies against GPIb, which block int eraction with vWF. Binding studies showed a nonsaturable ristocetin bi nding of VCL to platelets that was blocked by vWF or inhibitory antibo dies against GPIb. Binding to collagen was weak, and VCL did not inhib it binding of vWF at a 5000-fold excess. From these data, we conclude that VCL inhibits adhesion in all cases in which adhesion is VWF depen dent by competing for vWF binding to activated GPIb. The lack of inhib ition of adhesion to vWF as a single molecule may be explained by assu ming that this adhesion is determined by interaction of nonactivated G PIb, with vWF that has been changed in conformation by adsorption. Stu dies investigating thrombus formation on the connective tissue of an a therosclerotic plaque in a human coronary artery showed that VCL was a ble to partially prevent this thrombus formation. VCL may be of value in preventing adhesion and thrombus formation under conditions in whic h these processes are dependent on VWF.