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The factor VIII C2 domain is a highly immunogenic domain, whereby inhibitory antibodies develop following factor VIII replacement therapy for congenital hemophilia A patients. Inhibitory antibodies also arise spontaneously in cases of acquired hemophilia A. The structural basis for molecular recognition by 2 classes of anti-C2 inhibitory antibodies that bind to factor VIII simultaneously was investigated by x-ray crystallography. The C2 domain/3E6 FAB/G99 FAB ternary complex illustrates that each antibody recognizes epitopes on opposing faces of the factor VIII C2 domain. The 3E6 epitope forms direct contacts to the C2 domain at 2 loops consisting of Glu2181-Ala2188 and Thr2202-Arg2215, whereas the G99 epitope centers on Lys2227 and also makes direct contacts with loops Gln2222-Trp2229, Leu2261-Ser2263, His2269-Val2282, and Arg2307-Gln2311. Each binding interface is highly electrostatic, with positive charge present on both C2 epitopes and complementary negative charge on each antibody. A new model of membrane association is also presented, where the 3E6 epitope faces the negatively charged membrane surface and Arg2320 is poised at the center of the binding interface. These results illustrate the potential complexities of the polyclonal anti-factor VIII immune response and further define the "classical" and "nonclassical" types of antibody inhibitors against the factor VIII C2 domain.

The most significant complication for patients with severe cases of congenital or acquired hemophilia A is the development of inhibitor antibodies against coagulation factor VIII (fVIII). The C2 domain of fVIII is a significant antigenic target of anti-fVIII antibodies. Here, we have utilized small angle x-ray scattering (SAXS) and biochemical techniques to characterize interactions between two different classes of anti-C2 domain inhibitor antibodies and the isolated C2 domain. Multiple assays indicated that antibodies 3E6 and G99 bind independently to the fVIII C2 domain and can form a stable ternary complex. SAXS-derived numerical estimates of dimensional parameters for all studied complexes agree with the proportions of the constituent proteins. Ab initio modeling of the SAXS data results in a long kinked structure of the ternary complex, showing an angle centered at the C2 domain of ∼130°. Guided by biochemical data, rigid body modeling of subunits into the molecular envelope of the ternary complex suggests that antibody 3E6 recognizes a C2 domain epitope consisting of the Arg(2209)-Ser(2216) and Leu(2178)-Asp(2187) loops. In contrast, antibody G99 recognizes the C2 domain primarily through the Pro(2221)-Trp(2229) loop. These two epitopes are on opposing sides of the fVIII C2 domain, are consistent with the solvent accessibility in the context of the entire fVIII molecule, and provide further structural detail regarding the pathogenic immune response to fVIII.

The factor VIII C2 domain is a highly immunogenic domain, whereby inhibitory antibodies develop following factor VIII replacement therapy for congenital hemophilia A patients. Inhibitory antibodies also arise spontaneously in cases of acquired hemophilia A. The structural basis for molecular recognition by two classes of anti-C2 inhibitory antibodies that bind to factor VIII simultaneously has been investigated by small angle X-ray scattering and X-ray crystallography. The C2 domain/3E6 FAB/G99 FAB stable ternary complex, both in solution and in its crystalline state, illustrates that each antibody epitope resides on opposing faces of the factor VIII C2 domain. The 3E6 epitope is a classical antibody that forms direct contacts to the C2 domain at two loops consisting of Glu2181-Ala2188 and Thr2202-Arg2215, which inhibits the binding of the C2 domain to von Willebrand Factor and phospholipid surfaces. The G99 is a non-classical antibody that prevents proteolytic activation of factor VIII, and its epitope centers on Lys2227 and also makes direct contacts with loops Gln2222-Trp2229, Leu2261-Ser2263, His2269-Val2282 and Arg2307-Gln2311. Each binding interface is highly electrostatic, with positive charges present on both C2 epitopes and complementary negative charges on each antibody. A new model of phospholipid membrane association is also presented, where the 3E6 epitope faces the negatively charged membrane surface and Arg2320 is poised at the center of the binding interface. Furthermore, a 1.7 Å X-ray crystal structure of the porcine factor VIII C2 domain has also been determined, which supports the presented model for phospholipid binding. These results illustrate the complex nature of the polyclonal immune response against the factor VIII C2 domain, and further define the epitopes for both classical and non-classical inhibitory antibodies. Disclosures: No relevant conflicts of interest to declare.