Relationship between fibrinogen binding and the platelet glycoprotein deficiencies in Glanzmann's thrombasthenia type I and type II.

We have studied the fibrinogen binding to the platelets of four type I thrombasthenic two type II thrombasthenic patients and 15 normal subjects. The amount of [125I]fibrinogen bound to washed platelets in the presence or absence of 10 micrometers ADP was measured and the results expressed as the percentage of the total radioactivity added. The mean value for specific binding to the platelets of normal subjects was 3.7 (range 1--7.25) at 15 min after the addition of ADP. Type I thrombasthenic patients lack glycoproteins (GP) IIb and IIIa in the platelet membrane and intra-platelet fibrinogen. In all four patients studied, little or no specific fibrinogen binding was detected. Platelets of type II thrombasthenic patients contain detectable, although markedly reduced amounts of GP IIb and IIIa, and subnormal or normal levels of fibrinogen. In these platelets, significant specific fibrinogen binding was observed, the values being slightly below or at the lower end of the normal range. Fibrinogen binding to normal platelets was also measured after the addition of an IgG purified from the serum of a multi-transfused thrombasthenic type I patient. This antibody was previously shown to be directed against the GP IIb/IIIa complex as located by crossed immunoelectrophoresis. Specific fibrinogen-binding was blocked and the extent of the inhibition was directly proportional to the concentration of the antibody added.