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Anti-von Willebrand factor (vWf) antibody mediated platelet activation was studied using 2 monoclonal anti-vWf antibodies promoting the binding of vWf to GPIbalpha: 1C1E7 (IgG2a) reacting with the vWf N-terminus and 75H4B12 (IgM), characterized in this paper and studied in association with 1C1E7.75H4B12 binds to an N-terminal epitope in vWf, different from that reacting with 1C1E7. When com-bined, 1C1E7 and 75H4B12 promoted vWf binding to isolated GPIb under static conditions, even in the absence of ristocetin or botrocetin, and induced platelet aggregation synergistically in the presence of zero to subthreshold ristocetin concentrations. Specific inhibitors of GPIbalpha-vWf interactions prevented vWf binding to GPIbalpha in ELISA and during platelet aggregation. In addition, the 1C1E7 dependent platelet aggregation involved Fc receptor mediated platelet activation, a phenomenon even more pronounced when 1C1E7 and 75H4B12 were combined. A 75H4B12 binding phage expressing a peptide homologous with vWf sequence 88-95 neutralized the antibody induced platelet activation. However, at arterial shear rates, both 1C1E7 and 75H4B12 potently prolonged cartridge closure times in the PFA-100, compatible with inhibition of platelets by vWf, unfolded by the combined action of shear stress and antibodies.We conclude that antibodies directed against different epitopes in the N-terminus of vWf modify the folded vWf structure synergistically and enhance A1 domain mediated vWf binding to platelet GPIb at low shear forces. In addition, once platelet-bound, IgG antibodies potently activate platelets via the FcgammaII receptor. Thus, such antibodies may promote immune mediated thrombosis at low shear rates, typical of the venous circulation. In contrast, at arterial shear rates, anti-vWf antibodies may rather compromise platelet function following enhanced binding of the unfolded vWf multimers to platelets, shielding platelets from interacting with subendothelial and soluble ligands. |
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