[Identification of IgG subclass and FVIII binding epitope of an acquired FVIII inhibitor in a bullous pemphigoid patient].

Objective: To identify the clinical and laboratory diagnosis of a bullous pemphigoid patient with acquired hemophilia A (AH-A). To identify FVIII binding epitope and IgG subclass of the FVIII inhibitor, and explore the molecular mechanism for AH-A pathogenesis.

Methods: Plasma FVIII activity( FVIII: C) was determined by one-stage assay, the titre of FYIII inhibitor by Bethesda Unit (BU). IgG purification of patient plasma or normal pooled plasma was finished by protein A-agarose column chromatography. Activated partial thromboplastin time (APTT) was assayed for uncovering FVIII inhibitor effect on FVIII in vivo. Combined Western blot analysis by anti-IgG1, IgG2, IgG3 and IgG4 antibodies was used to determine the relative concentration of patient' s IgG subclass. IgG subclass concentrations were quantified by nephelometric method. Solid-phase binding assay of FVIII and FVIII inhibitor, combined with Western blot was used to recognize the binding epitope at which the FVIII inhibitor bound to FVIII.

Results: (1) Plasma APTT value of patient was prolonged evidently and could not be corrected by normal pooled plasma. Patient's FVIII: C was < 1.5%. The titre of FVIII inhibitor in patient plasma was 147.8 BU. (2) The purified patient IgG was able to inhibit FVIII: C of normal pooled plasma significantly with a dose dependent manner, and the patient plasma could prolong rabbit plasma APTT markedly with a time dependent manner. (3) The FVIII inhibitor was predominantly then of IgG4 subtype with a minority IgG1, and the concentration of IgG4 and IgG1 in the patient was higher than that in normal. The FVIII inhibitor reacted with FVIII 44 x 10(3) fragment epitope.

Conclusions: The inhibiting effect of FVIII inhibitors on FVIII: C in the bullous pemphigoid patient with AH-A is determined and the IgG subclass of the FVIII inhibitor is identified. A binding epitope for the FVIII inhibitor is a FVIII 44 x 10(3) fragment. The results provides evidence for understanding the pathogenesis of AH-A.