Summary
We recently described an outbreak of anti-factor VIII (FVIII) antibodies in a population
of haemophilia A patients non-responsive to FVIII (1). To find out what part of the
FVIII molecule had been altered, we purified specific anti-FVIII antibodies from the
plasma of the five patients showing high titres of inhibitors. An average of 100 µg
antibodies per ml of initial plasma was recovered by immunoadsorption on insolubilised
FVIII. The antibodies followed the normal isotypic distribution, including the presence
of specific IgG2 antibodies; the relative increase in IgG4 that is usually observed
in patients with long-standing inhibitors, was not present. The regions of FVIII to
which human antibodies bound were determined by a competition assay using a panel
of murine monoclonal antibodies: two major regions were identified, one located in
the A2 heavy chain domain, and the other made of determinants of both the A3 and C2
light chain domains. Affinity-purified antibodies inhibited the function of FVIII
as determined in a chromo- genic assay. However, variations existed in the affinities
with which antibodies bound to soluble FVIII. This study shows that the immu- nogenicity
of two particular regions of FVIII has been altered. A screening for alterations located
in these two regions should possibly be included in the preclinical evaluation of
FVIII concentrates.
