HEALTHY-SUBJECTS PRODUCE BOTH ANTI-FACTOR-VIII AND SPECIFIC ANTIIDIOTYPIC ANTIBODIES

Anti-Factor VIII (FVIII) antibodies were prepared by a combination of salt precipitation, gel filtration chromatography, and specific adsorp tion over insolubilized FVIII from the serum of 10 healthy subjects wi th normal levels of FVIII. Antibody specificity was confirmed by the c apacity to recognize soluble and insolubilized FVIII and to neutralize FVIII cofactor activity in FX activation. Epitope mapping was carried out using a competition ELISA in which affinity-purified human antibo dies inhibited the binding of labeled monoclonal antibodies. In most c ases, a single region of the A3 domain of the FVIII light chain was re cognized by the antibodies, while the reactivity toward heavy chain ep itopes differed from one antibody preparation to the other. Sera or Ig G fractions of the serum before immunoadsorption over insolubilized FV III did not bind to FVIII. The IgG fraction that was not retained on t he FVIII immunosorbent contained IgG that bound to the variable part o f anti-FVIII mouse monoclonal antibodies and inhibited the binding of labeled FVIII; in addition, the IgG fraction inhibited the binding of affinity-purified human antibodies to FVIII, thereby strongly suggesti ng the presence of anti-idiotypic antibodies. These findings indicate that the presence of anti-FVIII antibodies is a more universal phenome non than previously thought and that anti-idiotypic antibodies capable of inhibiting the binding of anti-FVIII antibodies to FVIII are produ ced spontaneously.