Aggregation of chymotrypsin-treated thrombasthenic platelets is mediated by fibrinogen binding to glycoproteins IIb and IIIa.

Previous experiments demonstrated that chymotrypsin, but not adenosine diphosphate (ADP), exposed fibrinogen binding sites on platelets from patients with Glanzmann's thrombasthenia. Three of these patients have been reexamined, and previous observations were confirmed. The quantity of iodine 125-labeled glycoprotein IIb (GPIIb) and glycoprotein IIIa (GPIIIa) on the platelets of these patients was considerably less than normal but was detectable by immunoprecipitation, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and autoradiography. The amount of residual GPIIb and GPIIIa as measured by binding studies with radiolabeled monoclonal antibodies was between 3% and 12% of the normal value. Platelet suspensions from these patients did not aggregate with fibrinogen and did not bind 125I-fibrinogen on stimulation with ADP. However, incubation of these platelets with chymotrypsin or pronase resulted in fibrinogen binding and platelet aggregation. Monoclonal antibodies specific for the GPIIb-GPIIIa complex blocked both the fibrinogen binding and the aggregation of enzyme-treated platelets. The treatment of washed platelets of a fourth thrombasthenic patient with ADP or with chymotrypsin failed to result in fibrinogen binding and aggregation. However, the level of GPIIb and GPIIIa on these platelets as measured by a Western blot technique and by monoclonal antibody binding amounted to less than 0.35% to 0.5% of normal values. In conclusion, fibrinogen binding sites exposed on thrombasthenic platelets by chymotrypsin are derived from GPIIb-GPIIIa molecules. Aggregation of chymotrypsin-treated thrombasthenic platelets by fibrinogen appears to represent a sensitive test for detection of functionally active GPIIb-GPIIIa complex on the platelet surface.