SummaryThe adhesion of human fixed washed platelets (FWP) to collagen was decreased after treatment with Serratia marcescens protease (SP), which removed 95% of the glycocalicin from platelet membrane glycoprotein (GP) lb. However, the diminished adhesion of SP treated FWP to collagen could still be increased in the presence of purified von Willebrand factor (vWF). This ability of vWF to increase FWP adhesion to collagen is defined as collagen cofactor (CCo). The adhesion of FWP to collagen was not affected by a monoclonal antibody (MAb) to GP Ilb/IIIa (10E5), that inhibits ADP and collagen induced platelet aggregation. On the other hand, it was decreased by 50% by a MAb to GP lb (6D1), that inhibits ristocetin induced platelet aggregation. Adhesion of FWP in buffer to collagen was completely inhibited by Ricinus communis agglutinin I or concanavalin A, while Lens culinalis agglutinin and wheat germ agglutinin showed 50% inhibition. The FWP adhesion to collagen in the presence of vWF (normal plasma) was unaffected by MAbs to GP Ilb/IIIa (10E5, P2, HPL1) but was decreased to 32-38% by MAbs to GP lb (6D1, AN51, HPL11). A MAb to vWF (CLB-RAg 35), that inhibits ristocetin induced binding of vWF to platelets, decreased the CCo of normal plasma by 70%. The MAb, CLB-RAg 201, that inhibits the binding of vWF to collagen, completely inhibited the CCo of normal plasma. In conclusion, our data suggest that (1) GP lb has a partial role in FWP adhesion to collagen; (2) the binding of vWF to collagen is required for the expression of CCo; (3) CCo is partly mediated through GP lb; but (4) other platelet membrane protein(s) besides GP lb or GP Ilb/IIIa must also be involved in FWP-vWF-collagen interactions.
