REACTIVITY OF AUTOANTIBODIES FROM CHRONIC ITP PATIENTS WITH RECOMBINANT GLYCOPROTEIN IIIA PEPTIDES

Chronic immune thrombocytopenia is an autoimmune disorder characterize d by destructive thrombocytopenia due to the formation of autoantibodi es against platelet-associated antigens, Most antiplatelet autoantibod ies react with either the platelet glycoprotein IIb/IIIa or Ib/IX comp lex, whereas some plasma autoantibodies react: with glycoprotein IIIa. Previous studies from our laboratory suggested that most platelet-ass ociated autoantibodies to platelet GPIIb/IIIa, which bind to the intac t complex, bind much less avidly to the EDTA-dissociated complex, sugg esting that the epitopes were complex-dependent. To evaluate this furt her we have studied the binding of platelet-associated autoantibody an d plasma auto- and alloantibody eluates to large recombinant GPIIIa pe ptides: peptide 1 (GPIIIa Gly(1)-Val(200)); peptide 2 (GPIIIa Arg(150) -Glu(400)); peptide 3 (GPIIIa Lys(350)-Asp(550)); peptide 4 (GPIIIa As n(450)-Val(700)) and peptide 5 (GPIIIa Trp(715)-Thr(762), cytoplasmic fragment). Of the 33 platelet-associated antibody eluates tested, all bound avidly to the GPIIb/IIIa complex, but only one showed significan t binding (>3 SD above control values) to one of the immobilized pepti des (peptide 3), Conversely, antibodies known to bind to specific regi ons of GPIIIa (murine monoclonal antibody, anti-LIBS2; plasma autoanti body against the GPIIIa cytoplasmic fragment and anti-Pl(Al) antibody) all bound avidly to the GPIIIa peptide containing the appropriate epi tope, Based on these and our previous results, we conclude that platel et-associated antibodies from chronic ITP patients rarely bind to epit opes localized to GPIIIa alone.