Reactivity of autoantibodies from chronic ITP patients with recombinant glycoprotein IIIa peptides

Chronic immune thrombocytopenia is an autoimmune disorder characterized by destructive thrombocytopenia due to the formation of autoantibodies against platelet-associated antigens. Most antiplatelet autoantibodies react with either the platelet glycoprotein IIb/IIIa or Ib/IX complex, whereas some plasma autoantibodies react with glycoprotein IIIa. Previous studies from our laboratory suggested that most platelet-associated autoantibodies to platelet GPIIb/IIIa, which bind to the intact complex, bind much less avidly to the EDTA-dissociated complex, suggesting that the epitopes were complex-dependent. To evaluate this further we have studied the binding of platelet-associated autoantibody and plasma auto- and alloantibody eluates to large recombinant GPIIIa peptides: peptide 1 (GPIIIa Gly1-Val200); peptide 2 (GPIIIa Arg150-Glu400); peptide 3 (GPIIIa Lys350-Asp550); peptide 4 (GPIIIa Asn450-Val700) and peptide 5 (GPIIIa Trp715-Thr762, cytoplasmic fragment). Of the 33 platelet-associated antibody eluates tested, all bound avidly to the GPIIb/IIIa complex, but only one showed significant binding (>3 SD above control values) to one of the immobilized peptides (peptide 3). Conversely, antibodies known to bind to specific regions of GPIIIa (murine monoclonal antibody, anti-LIBS2; plasma autoantibody against the GPIIIa cytoplasmic fragment and anti-P1A1 antibody) all bound avidly to the GPIIIa peptide containing the appropriate epitope. Based on these and our previous results, we conclude that platelet-associated antibodies from chronic ITP patients rarely bind to epitopes localized to GPIIIa alone.