Gonococcal protein I-specific opsonic IgG in normal human serum

Pooled normal human serum (NHS), as well as 10 individual NHS samples, markedly inhibited the reaction between monoclonal antibodies and their cognate epitopes on protein I of serum-sensitive, serum-resistant, and disseminated gonococcal infection-associated strains of Neisseria gonorrhoeae, as determined by ELISA inhibition. IgG was the immunoglobulin class responsible for the inhibition. Only the Fab fragment of IgG was inhibitory, making it likely that the IgG reacted specifically with protein I. After absorption with purified protein I, NHS did not inhibit the binding of a protein III-specific monoclonal antibody, thus excluding the possibility that protein III-specific antibodies in NHS masked epitopes on protein I. In addition, lipopolysaccharide-specific IgG in NHS did not appear to contribute to the inhibition of monoclonal antibody binding to protein I. The IgG from NHS was opsonic; opsonization was prevented by coating gonococci with the Fab fragment of protein I-specific monoclonal antibodies.