IMMUNOGLOBULINS TARGETING BOTH GPIIB IIIA AND GPIB/IX IN CHRONIC IDIOPATHIC THROMBOCYTOPENIC PURPURA (ITP) - EVIDENCE FOR AT LEAST 2 DIFFERENT IGG ANTIBODIES/

Antiplatelet antibodies in chronic idiopathic thrombocytopenic purpura (ITP) mainly target glycoprotein (GP) IIb/IIIa and GPIb/IX. Previous studies, employing modern antigen-specific assays, indicate that serum reactive with both GPIIb/IIIa and GPIb/IX is not an uncommon finding in chronic ITP. However, the mechanism behind this dual reactivity rem ains unclear. We studied sera from 72 patients with chronic ITP using modified GPIIb/IIIa- and GPIb/IX-specific MAIPA assays. Among the 34 p ositive sera, seven showed strong reactivity against both GPIIb/IIIa a nd GPIb/IX. These seven dual reactive ITP sera were further analysed b y absorption studies. It was found that sera absorbed with immobilize GPIb/IX lost nearly all serum IgG specific for GPIb/IX but fully retai ned the IgG specific for GPIIb/IIIa. Conversely, sera absorbed with im mobilized GPIIb/IIIa retained their reactivity only with GPIb/IX. Thes e findings demonstrate that ITP sera, reactive with both GPIIb/IIIa an d GPIb/IX, contain at least two different IgG antibody populations, ea ch reactive with only one of the GP complexes.