Vibi C

Factor VIII:C (FVIII:C) inhibitors are pathologic circulating antibodies that reduce FVIII:C activity. They can arise either as alloantibodies in some congenital hemophiliacs, or as autoantibodies in nonhemophilic patients with acquired inhibitors. The reason for development of such antibodies is not known, but their basic biochemical and clinical characteristics have been reviewed extensively in the past several years. This article surveys recent immunochemical studies to characterize the FVIII:C molecule and antibodies against it. For instance, epitope (antigenic site) mapping investigations conducted to date suggest that binding sites for most inhibitory antibodies can be localized to limited regions of the FVIII:C molecule. Further progress in the understanding of immune responses against FVIII:C is likely to provide a sound basis for the design of new therapeutic approaches to patients suffering the sequelae of FVIII:C inhibitor antibodies.

Factor VIII C-domains are believed to have specific functions in cofactor activity and in interactions with von Willebrand factor. We have previously shown that factor VIII is co-targeted with von Willebrand factor to the Weibel-Palade bodies in blood outgrowth endothelial cells, even when factor VIII carries mutations in the light chain that are associated with defective von Willebrand factor binding. In this study, we addressed the contribution of individual factor VIII C-domains in intracellular targeting, von Willebrand factor binding and cofactor activity by factor VIII/V C-domain swapping. Blood outgrowth endothelial cells were transduced with lentivirus encoding factor V, factor VIII or YFP-tagged C-domain chimeras, and examined by confocal microscopy. The same chimeras were produced in HEK293-cells for in vitro characterization and chemical foot-printing by mass spectrometry. In contrast to factor VIII, factor V did not target to Weibel-Palade bodies. The chimeras showed reduced Weibel-Palade body targeting, suggesting that this requires the factor VIII C1-C2 region. The factor VIII/V-C1 chimera did not bind von Willebrand factor and had reduced affinity for activated factor IX, whereas the factor VIII/V-C2 chimera showed a minor reduction in von Willebrand factor binding and normal interaction with activated factor IX. This suggests that mainly the C1-domain carries factor VIII-specific features in assembly with von Willebrand factor and activated factor IX. Foot-printing analysis of the chimeras revealed increased exposure of lysine residues in the A1/C2- and C1/C2-domain interface, suggesting increased C2-domain mobility and disruption of the natural C-domain tandem pair orientation. Apparently, this affects intracellular trafficking, but not extracellular function.