Fab-mediated binding of glycoprotein Ib/IX and IIb/IIIa specific antibodies in chronic idiopathic thrombocytopenic purpura.

The antibody domain responsible for the interactions between platelet glycoproteins (GP) and serum IgG autoantibodies in patients with chronic idiopathic thrombocytopenic purpura (ITP) was studied. Sera from nine non-transfused ITP patients and 20 normal controls and a serum containing an anti-PlA1 antibody were employed. Serum, purified IgG and F(ab')2 fragments were prepared and their binding to platelet GPIb/IX and GPIIb/IIIa were analysed using a modified MAIPA assay and an antigen capture ELISA. In all experiments most of the autoantibodies studied behaved identically to the anti-PlA1 antibody in that the IgG-F(ab')2 fragments retained their ability to bind to the respective glycoprotein. Substituting the enzyme-conjugated secondary antibody (Fab specific), in the MAIPA assay, with an Fc specific antibody removed all reactivities observed against platelet GPs, produced by IgG-F(ab')2 fragments. Furthermore, in an antigen-capture ELISA, IgG autoantibodies against platelet GPIb/IX and/or GPIIb/IIIa were blocked preferentially by pre-incubating the ITP sera with a goat anti-human IgG (F(ab')2 specific) antibody, but not with an anti-Fc antibody. We conclude that these ITP patients produced antibodies specific for platelet GPIb/IX and/or GPIIb/IIIa, and that the autoantibody-platelet interaction was mediated by the classic Fab binding.