PAICA - A METHOD FOR CHARACTERIZING PLATELET-ASSOCIATED ANTIBODIES - ITS APPLICATION TO THE STUDY OF IDIOPATHIC THROMBOCYTOPENIC PURPURA AND TO THE DETECTION OF PLATELET-BOUND C7E3

In idiopathic thrombocytopenic purpura (ITP), autoantibodies reacting with antigens on the platelet membrane bring about accelerated platele t destruction. We now report PAICA (''Platelet-Associated IgG Characte rization Assay''), a method for detecting autoantibodies bound to spec ific membrane glycoproteins in total platelet lysates. This monoclonal antibody (MAb) capture assay takes into account the fact that antibod ies on circulating platelets may be translocated to internal pools as well as being on the surface. A total of twenty ITP patients were exam ined by PAICA, and the results compared with those obtained by measuri ng (i) serum antibodies bound to paraformaldehyde-fixed control platel ets by ELISA, (ii) IgG bound to the surface of the patient's own plate lets by flow cytometry (PSIgG), (iii) total platelet-associated IgG (P AIgG) by ELISA and (iv) serum antibodies reacting with control platele ts by MAIPA (''Monoclonal Antibody-specific Immobilization of Platelet Antigens''). Of twelve patients with elevated PAIgG, nine had increas ed PSIgG yet eleven reacted positively in PAICA. Of these, eight posse ssed antibodies directed against GP IIb-IIIa, two against GP Ib-IX and one patient possessed antibodies directed against GP IIb-IIIa and GP Ia-IIa respectively. Only seven of the patients possessed serum antibo dies detectable by MAIPA. PAICA was also able to detect platelet-assoc iated c7E3 (the chimeric form of Fab fragments of the MAb 7E3) followi ng its infusion during antithrombotic therapy, when it proved more sen sitive over a seven-day period than a MAIPA assay adapted for assessin g surface-bound antibody. We propose that PAICA provides added sensiti vity to the detection of platelet-associated antibodies in immune thro mbocytopenias or following therapy with humanized MAbs.