Many αIIbβ3 autoepitopes in chronic immune thrombocytopenic purpura are localized to αIIb between amino acids L1 and Q459

• Published in: British journal of haematology Vol. 118; no. 4; pp. 1132 - 1136
• Main Authors: Mcmillan, Robert, Wang, Lei, Lopez‐Dee, Jennifer, Jiu, Sandra, Loftus, Joseph C.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Science Ltd 01.09.2002
• Subjects: antiplatelet antibody; autoantibody; autoimmune; chronic ITP
• ISSN: 0007-1048; 1365-2141
• DOI: 10.1046/j.1365-2141.2002.03751.x

In chronic immune thrombocytopenic purpura (ITP), autoantibodies bind to platelet surface proteins, particularly αIIb, resulting in platelet destruction by the reticulo‐endothelial system. In order to better localize the autoepitopes on αIIb, we studied the binding of antibodies to Chinese hamster ovary (CHO) cells expressing either αIIbβ3 or αIIb‐αvβ3 chimaeras in which a segment of αIIb (either amino acids L1‐Q459, L1‐F223 or F223‐Q459) was substituted for that portion of αv. We evaluated platelet‐associated autoantibodies from 14 ITP patients with αIIb‐dependent antibodies. Ten of 14 bound to αIIb (L1‐Q459)‐αvβ3, showing that autoepitopes were often localized to this region of αIIb. In addition, each of the autoantibodies binding to αIIb (L1‐Q459)‐αvβ3, also bound to CHO cells expressing either αIIb(L1‐F223)‐αvβ3 or αIIb(F223‐Q459)‐αvβ3). In two of the three eluates tested, > 95% of the autoantibody binding to αIIb could be adsorbed using CHO cells expressing any of the three chimaeras, showing that the epitope(s) have contact points on either side of amino acid F223; in the third eluate, only a portion (∼40%) could be adsorbed by the chimaeric cell lines showing that, in this patient, an additional antibody was also present, directed to a site distal to amino acid Q459. The remaining four eluates bound to CHO cells expressing αIIbβ3 but to none of the chimaeras, suggesting that these epitopes are also distal to amino acid Q459. We conclude that the binding of many anti‐αIIbβ3 autoantibodies is dependent on the presence of αIIb amino acids L1‐Q459.