SummaryIn quinine- and quinidine-induced thrombocytopenic purpura IgG antibodies are known to react in a drug-dependent manner with different combinations of surface glycoproteins (GP) Ib , IIb, IIIa and IX. Because endothelial cells share a number of properties of platelets, including the presence of GP Ilia and GPIb-like proteins, we have compared these two cell types for their quinine-dependent IgG binding abilities. By immunoblotting of endothelial cells, quinine-dependent IgG binding from four patient sera was observed only to a 93 kDa component corresponding to GP Ilia. Strong IgG binding independent of the drug was found at 170–180 kDa. Thus endothelial cells express the GP Ilia quinine-dependent epitope on platelet GP IIIa , but not those on other platelet glycoproteins.

The molecular nature of platelet receptors for quinine- and quinidine- dependent antiplatelet antibodies (Q.Ab and Qd.Ab) was studied by immunoblotting. One Q.Ab caused quinine-dependent IgG binding to platelet proteins with molecular weights (mol wts) of 174 Kd and 93 Kd and another to only a 93-Kd protein. A third Q.Ab caused binding to 174- , 140-, 93-, and 57-Kd proteins, while a fourth Q.Ab and a Qd.Ab caused IgG binding to 174- and 18-Kd proteins. Using platelets from patients with Glanzmann's thrombasthenia or Bernard Soulier syndrome and purified GPIIIa, these proteins were shown to be GPIb, GPIIb, GPIIIa, GPIX, and an unidentified 57-Kd protein missing in Bernard Soulier syndrome. Binding to the 93-Kd protein was independent of the PIA1 antigen. Absorption of one Q.Ab with Glanzmann's thrombasthenia platelets revealed different populations of antibodies with different specificities within the one patient. Thus Q.Ab and Qd.Ab are heterogeneous and may be directed toward different epitopes on major platelet glycoproteins.

Quinine-dependent (Q) IgG antibodies (Q.Ab) in drug-induced immune thrombocytopenia are heterogeneous and bind to different platelet surface glycoproteins (GP), namely GPIb, IX, lIb, IlIa and an unidentified 57-kDa membrane proteins. Although both the Q-dependent epitope on GPIIIa and the P1^A1 antigen require intact disulphide bonds for their expression, they are distinct because Q.Ab bind to GPIIIa lacking P1^A1. Epitopes for both antigens were examined on Western blots of either intact washed human platelets or purified GPIIIa. When intact platelets were digested with trypsin and washed and solubilised prior to electrophoresis, membrane-associated fragments of GPIIIa of 78 kDa were found to be reactive with both antibodies. In addition, 60- and 68-kDa fragments bound anti-P1^A1 but not Q.Ab. Similar digestion with chymotrypsin produced only 60-kDa fragments containing both epitopes. Digestion of purified GPIIIa with chymotrypsin produced 60-kDa peptides reactive with Q.Ab and anti-P1^A1 in immunoblotting studies. Similar digestion with elastase produced 58-kDa fragments also containing the epitopes for both antibodies. Longer digestion times or sequential digestion with different enzymes did not reveal extra fragments. However, immunoprecipitation of trypsin-digested ¹²⁵I-labelled GPIIIa with affinity-purified Q.Ab produced a 17-kDa fragment containing the Q-dependent epitope.

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