Elaboration of a 3.6-kilodalton lipooligosaccharide, antibody against which is absent from human sera, is associated with serum resistance of Neisseria gonorrhoeae.

Neisseria gonorrhoeae strains that resist lysis by normal human sera (NHS) do so, in part, because NHS contain immunoglobulin M (IgM) specific for lipooligosaccharide (LOS) antigens of serum-sensitive strains, but lack antibodies for LOS antigens that can serve as loci for immune lysis of serum-resistant (serr) strains. We used a monoclonal antibody (McAb), specific for an epitope within a 3.6-kilodalton (kDa) component of Neisseria meningitidis L8 LOS, that binds a 3.6-kDa gonococcal LOS component so that we could explore further serr gonococcal strains. The McAb bound to the LOS of 6 of 7 serr of strains but not to the LOS of 0 of 14 serum-sensitive and serum-intermediate gonococcal strains of diverse origin. We studied three serr strains further. Strain 7134 does not elaborate the 3.6-kDa LOS component and does not bind the McAb; strains WR220 and WR302 do elaborate the 3.6-kDa LOS component. The titer (log2) at which the McAb, diluted in NHS, lysed strain WR220 was 7.7; for WR302 it was 3.7, and for 7134 it was 0. Addition of McAb to NHS caused increased classical and alternative-pathway C3 deposition onto strain WR220, but only classical-pathway-activated C3 deposition onto strain WR302. The difference in lytic effectiveness of the McAb for the two strains, therefore, may result from differences in alternative-pathway augmentation of McAb-dependent classical-pathway activation on their surfaces. None of 40 randomly selected normal young adults had serum antibody that could compete with the McAb for binding to WR220 LOS in a solid-phase RIA. We conclude that the 3.6-kDa LOS component is commonly expressed by serr strains of N. gonorrhoeae and that antibody to it would be lytic if present in human serum, but that it is infrequently, if ever, present. As a result, strains elaborating this LOS are resistant to lysis by NHS.