Approximately 30% of patients with severe hemophilia A will develop inhibitory antibodies to factor VIII (fVIII inhibitors). In addition, autoimmune antibodies to fVIII can develop in non-hemophiliacs, producing acquired hemophilia A, which frequently produces life- or limb-threatening bleeding. Patients with congenital hemophilia who develop inhibitors usually have a polyclonal antibody response directed against the A2 and C2 domains of fVIII. Patients with acquired hemophilia typically have a more limited B-cell epitope response with antibodies directed against the A2 or C2 domain not both. Classical anti-C2 antibodies inhibit the binding of fVIII to phospholipid membranes and to von Willebrand factor. We recently have identified anti-C2 antibodies that inhibit the activation of fVIII, but do not inhibit the binding of fVIII to phospholipid membranes or to von Willebrand factor. These non-classical inhibitors are found in the plasmas of most inhibitor patients (

Meeks, S.L. et al.
). The pathogenicity of classical and non-classical murine anti-human fVIII monoclonal antibodies (MAbs) was tested in a murine in vivo bleeding model. MAbs were injected into the tail veins of –hemophilia A mice to a peak plasma concentration of 60 nM followed by an injection human B domain-deleted fVIII to a concentration of 2 nM. At 2 hours the mice were anesthetized and a 4 mm tail snip was made. The amount of blood lost into a collection tube of normal saline over 40 minutes was measured. 4A4 is a type I anti-A2 inhibitor with an inhibitory titer of 40,000 Bethesda units (BU)/mg IgG. I54 and 1B5 are classical type I anti-C2 inhibitors with inhibitory titers of 1300 and 930 BU/mg IgG, respectively. 2-77 is a non-classical type II anti-C2 inhibitor that produces a residual fVIII level of 40% at saturating concentrations and whose titer is 21,000 BU/mg IgG. 2-117 is a non-classical anti-C2 MAb with inhibitory activity less than 0.4 BU/mg IgG. All of these MAbs except 2-117 significantly increased blood loss over control mice injected with fVIII alone (p= 0.01-0.02, Mann-Whitney Test) (Fig .1). The amount of blood loss was similar at these saturating concentrations of antibody despite inhibitory titers ranging from 930-40,000 BU/mg IgG. Increasing the dose of fVIII to 4 nM could overcome the bleeding diathesis produced by the non-classical MAb 2-77, but not the type I antibodies, 4A4 and I54. Similar results were seen in the in vitro Bethesda assay where 4A4 completely inhibited both 1 U/ml and 3 U/ml fVIII at saturating concentrations, while 2-77 had 40% residual activity with either 1 or 3 U/ml fVIII (0.4 U and 1.2 U respectively) (Fig. 2). These results suggest that high-dose fVIII rather than bypassing agents may be warranted in patients with an inhibitor response dominated by non-classical anti-C2 antibodies.

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