Many α_IIbβ₃ autoepitopes in chronic immune thrombocytopenic purpura are localized to α_IIb between amino acids L1 and Q459

Abstract

Summary. In chronic immune thrombocytopenic purpura (ITP), autoantibodies bind to platelet surface proteins, particularly a IIb , resulting in platelet destruction by the reticulo-endothelial system. In order to better localize the autoepitopes on a IIb , we studied the binding of antibodies to Chinese hamster ovary (CHO) cells expressing either a IIb b 3 or a IIb -a v b 3 chimaeras in which a segment of a IIb (either amino acids L1-Q459, L1-F223 or F223-Q459) was substituted for that portion of a v . We evaluated plateletassociated autoantibodies from 14 ITP patients with a IIb -dependent antibodies. Ten of 14 bound to a IIb (L1-Q459) -a v b 3 , showing that autoepitopes were often localized to this region of a IIb . In addition, each of the autoantibodies binding to a IIb (L1-Q459) -a v b 3 , also bound to CHO cells expressing either a IIb(L1-F223) -a v b 3 or a IIb(F223-Q459) -a v b 3 ). In two of the three eluates tested, > 95% of the autoantibody binding to a IIb 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 a IIb b 3 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-a IIb b 3 autoantibodies is dependent on the presence of a IIb amino acids L1-Q459.

Recognition of highly restricted regions in the β-propeller domain of αIIb by platelet-associated anti-αIIbβ3 autoantibodies in primary immune thrombocytopenia

“…13,15 In this study, we found that samples from 15 patients with primary ITP harbored PA anti-␣IIb␤3 Abs that mainly recognized the N-terminal half of the ␤-propeller domain (L1-W235) of ␣IIb. A systematic examination with human-mouse ␣IIb chimeras found 3 main recognition sites: (1) a conformational epitope composed of W1:1-2 and W2:3-4 loops, (2) a region containing the W1:2-3 loop, and (3) a region containing the W3:4-1 loop.…”

“…We and others previously reported that, in chronic ITP, PA anti-␣IIb␤3 Abs frequently bound to cation-dependent conformational antigens and did not react with ␣v␤3. [11][12][13] Those data suggested that, in primary ITP, the target epitopes of anti-␣IIb␤3 Abs may be localized mainly on ␣IIb; in contrast, in HIVassociated ITP, the target epitopes appeared to be localized to the 49-66 residues of ␤3. 14 Moreover, we previously reported that, in one-third of patients with ITP (11 of 34), PA anti-␣IIb␤3 Ab binding was markedly impaired against KO variant ␣IIb␤3, which had 2 amino acids inserted between residues 160 and 161 in the W3:4-1 loop of the ␤-propeller domain.…”

Emerging Concepts in Immune Thrombocytopenia

“…Subsequent studies have shown that autoantibodies particularly bind to the IIb subunit ( 117 , 118 ), or contradictory, the IIIa subunit of the dimer ( 119 ). Eventually, several investigators have demonstrated that specific portions of the protein are preferred autoepitopes in ITP, often near ligand binding sites ( 81 , 120 ). The vitronectin (α v β 3) receptor shares the β 3 integrin with GPIIb/IIIa and was shown to be an important autoantigen in ITP as well ( 121 ).…”

Autoantibody against integrin αvβ3 contributes to thrombocytopenia by blocking the migration and adhesion of megakaryocytes

“…Integrin α v β 3 , which is broadly expressed on cancer and normal cells , is also a special platelet integrin antigen for GP‐specific IgG. Anti‐α v β 3 autoantibody was detected in chronic ITP patients , whereas its role in ITP is unknown.…”