Neisseria gonorrhoeae were cultured from the urethra of male patients and from the cervix and urethra of female contacts. Isolates from a given group of individuals were of the same strain but differed considerably in terms of the molecular weight of both protein II and pili. Radioimmunoprecipitation assays showed that most patients produced serum antibodies to protein I. Antibodies to pili, when present, showed limited cross-reactivity with the different pili produced by a single strain. Antibodies to protein II were highly specific, reacting with only one of the protein II-types produced by a single strain; this observation suggested that the host immune response may be an important factor in antigenic variation. Several sera also contained antibodies to a common surface protein with a molecular weight of 43,000 that was present in all strains tested.
Antibodies were detected by an enzyme-linked immunosorbent assay (ELISA) in sera from rabbits immunized with outer membranes from colonial opacity variants of Neisseria gonorrhoeae P9. ELISA-inhibition experiments with purified antigens revealed approximately equal proportions of antibodies directed against each of the three major surface antigens, lipopolysaccharide, the major outer membrane protein (protein I) and protein 11, the variable protein associated with colonial opacity. Inhibition experiments with intact gonococci showed considerable surface antigenic diversity which could be correlated with differences between the protein I1 species present. Despite their considerable structural homology, different protein I1 species from colonial variants of the same strain showed little cross-reactivity with specific anti-protein I1 sera, thus demonstrating the considerable variation in that part of the antigen which is exposed on the surface of the gonococcus and is closely involved in pathogenic mechanisms.
Strains of meningococci, which were shown to be pilated by electron microscopy, could be divided into two groups on the basis of antigenicity and subunit Mr. Strains from group 1 which reacted with monoclonal antibodies directed against gonococcal pili, had pili with subunit Mr similar to that of gonococci which could be detected by radioimmune precipitation or electroblotting. Strains from group 2 failed to react with the monoclonal antibodies and had pili with lower subunit Mr which could only be detected by radioimmune precipitation using polyclonal antipilus antiserum and not by electroblotting.
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