Platelet-associated antibody to glycoprotein IIb/IIIa from chronic immune thrombocytopenic purpura patients often binds to divalent cation-dependent antigens.
Chronic immune thrombocytopenic purpura (ITP) is a syndrome of destructive thrombocytopenia due to autoantibodies against platelet-associated antigens. These antigens are most commonly located on the platelet glycoprotein (GP) IIb/IIIa complex. In the present studies, we show that many platelet-associated anti-GPIIb/IIIa autoantibodies from chronic ITP patients depend on conformationally intact GPIIb/IIIa for maximal binding. We studied anti-GPIIb/IIIa autoantibodies from 19 ITP patients (15 platelet-associated, 8 plasma) and alloantibodies from three patients with posttransfusion purpura (anti-PIA1). Antibodies were preincubated with purified intact GPIIb/IIIa, EDTA-dissociated GPIIb/IIIa, GPIIIa, or GPIIb for 2 hours and then residual antibody was measured in an antigen capture assay. The binding results were compared with those obtained using antibody preincubated in buffer. Of the 15 platelet-associated autoantibodies studied, the intact GPIIb/IIIa complex resulted in greater inhibition of antibody binding than the EDTA-dissociated complex, with a mean inhibition ratio (intact/dissociated) of 7.9 (range, 1.4 to 30.3). Little inhibition was noted using either GPIIb or GPIIIa. Conversely, plasma anti-PIA1 alloantibodies or plasma autoantibodies from ITP patients against the c-terminal region of GPIIIa were more efficiently inhibited by the dissociated complex or purified GPIIIa. We conclude that platelet-associated anti-GPIIb/IIIa autoantibodies in chronic ITP are frequently directed to cation-dependent conformational antigens.
Fujisawa
K
K
Department of Molecular and Experimental Medicine, Scripps Research Institute, La Jolla, CA 92037.
Tani
P
P
McMillan
R
R
D001323:Autoantibodies / Q000276:immunology*; D001792:Blood Platelets / Q000276:immunology*; D002413:Cations, Divalent; D002908:Chronic Disease; D004492:Edetic Acid / Q000494:pharmacology; D000939:Epitopes / Q000276:immunology*; D006801:Humans; D007518:Isoantibodies / Q000276:immunology; D010980:Platelet Membrane Glycoproteins / Q000276:immunology* / Q000302:isolation & purification; D016553:Purpura, Thrombocytopenic, Idiopathic / Q000276:immunology*
D001323:Autoantibodies; D002413:Cations, Divalent; D000939:Epitopes; D007518:Isoantibodies; D010980:Platelet Membrane Glycoproteins; D004492:Edetic Acid