Increased platelet sensitivity to ristocetin is predicted by the binding characteristics of a GPIb/IX determinant

Platelets from patients with platelet-type von Willebrand disease (vWD) were used as immunogens for the production of murine monoclonal antibodies (MoAbs). One such MoAb, C-34, inhibited ristocetin-induced aggregation of patient or normal platelets, but not aggregation induced by other aggregating agents. As demonstrated by crossed-immunoelectrophoresis, C-34 recognized an epitope within the GPIb/IX complex. In indirect immunofluorescence studies on fresh platelets, the ratio of any of four different anti-GPIb MoAbs to one another was near unity (0.88-1.14) both for normals and for patients. In contrast, the ratio of the binding of C-34 to such a MoAb (AP-1) was 0.31 +/- 0.02 (means +/- SE) for normal platelets and significantly increased to 0.54 +/- 0.01 for patient platelets (P less than 0.001). In NP-40 lysates of 3H-labelled platelets, saturating concentrations of C-34 produced much fainter bands than did AS-2 or other anti-GPIb MoAbs in the GPIb and GPIX regions. In contrast to the other anti-GPIb MoAbs, C-34 did not bind to the purified 125I-labelled glycocalicin fragment of GPIb, to the glycocalicin derivative identified by crossed-immunoelectrophoresis, or to the amino-terminal approximately 40 kDa portion of GPIb alpha cleaved from 3H-labelled platelets by trypsin. C-34 appears to be the first MoAb that is quantitatively informative in identifying the abnormal platelets in platelet-type vWD. The observed differences between the patient and normal platelets may reflect an abnormality in the primary structure of the GPIb/IX complex. Alternatively, patient platelets may have an abnormality of other structures near this region that impose less of a steric hindrance upon binding of antibody to the C-34 epitope.