Autoantibody to factor VIII that has less reactivity to factor VIII/von Willebrand factor complex

To determine the difference in reactivity of factor VIII (FVIII) inhibitor to FVIII/von Willebrand Factor (vWF) complex and FVIII free of vWF, an autoantibody to FVIII light chain was tested. A patient (1-3) suffered from autoimmune hemolytic anemia with autoantibody to FVIII. Epitope specificity of the patient's IgG (I-3 IgG) was shown to be the C2 domain of FVIII light chain (2170-2332) by Western blotting using recombinant FVIII deletions expressed in Escherichia coli. The inhibitory effect on FVIII procoagulant activity (VIII:C) of I-3 IgG was tested against a conventional FVIII concentrate; Haemate P, a monoclonal antibody-purified FVIII concentrate; Hemofil M, and a recombinant FVIII (rFVIII); Kogenate. I-3 IgG showed only 1.3 BU/mgIgG for Haemate P, in contrast to 20 BU/mgIgG for both Hemofil M and Kogenate. The ratio of VIII:C/vWF:Ag in Haemate P and Hemofil M was 1/3.43 and 1/0.01, respectively, while Kogenate did not contain vWF. The inhibitory effect of the I-3 IgG was then compared with Kogenate and its complex with vWF. The inhibitory effect was decreased against the rFVIII by forming a complex with vWF from 22 BU/mgIgG to 0.5 BU/mgIgG. Fab from the I-3 IgG had the same effect. In addition, vWF showed a protective effect on FVIII inactivation by the I-3 IgG in a dose dependent manner. Fifty-nine percent of residual VIII:C was retained in the presence of 8 U/ml of vWF after 1 hr incubation with I-3 IgG. These results suggested that vWF could compete with the I-3 IgG for binding to FVIII.