Lysis of Neisseria gonorrhoeae initiated by binding of normal human IgM to a hexosamine-containing lipooligosaccharide epitope(s) is augmented by strain-specific, properdin-binding-dependent alternative complement pathway activation.

We studied the specificity of naturally acquired IgM bactericidal for strains of Neisseria gonorrhoeae that varied in sensitivity to the lytic action of normal human serum (NHS) and the relative ability of these strains to deplete the classical (CP) and alternative (ACP) C pathways. Lysis of both highly sensitive and relatively insensitive strains was inhibited by the same gonococcal lipooligosaccharides (LOS), as well as by Salmonella minnesota Re LOS and three hexosamine-containing glycose polymers. A polymer of N-acetylgalactosamine phosphate was the most inhibitory; a polymer of N-acetylglucosamine phosphate only partially inhibited. Neither 3-deoxy-D-manno-octulosonic acid (dOc1A) nor a polymer that contained dOc1A but not hexosamine inhibited NHS lysis. A co-polymer of N-acetylgalactosamine-dOc1A inhibited both bactericidal activity and the binding of IgM to the LOS of a highly serum-sensitive (sers) gonococcal strain. Carboxyl reduction of the dOc1A in this polymer did not affect its inhibitory capacity for gonococcal antibody, but abolished its binding to homologous antibody induced by vaccination. CP activity was not affected by vaccination. CP activity was not affected by absorption of NHS with gonococcal strains, whereas ablation of CP activity markedly but variously diminished lytic activity for highly sers strains. ACP activity was variously depleted by gonococcal strains, and the proportion of bacteria that could be lysed through the ACP varied among strains and among different populations of a given strain. The titer at which a strain was sensitive to NHS lysis was a function of its ACP consumption (p = 0.006), which accounted for 70% of the differences in titer among strains. Analyses of the absorbed sera revealed that the gonococci had variously depleted properdin from NHS as assessed by using an Ag-capture solid-phase RIA. Addition of purified properdin to absorbed sera restored ACP activity to normal levels. Western immunoblots of gonococcal lysates showed that purified properdin bound directly to a 39-kDa outer membrane protein. We conclude that both CP activation by IgM binding to LOS epitopes, one of which contains hexosamine, and ACP activation, which is a function of strain-specific direct binding of properdin, can initiate lysis of sers strains and that ACP activation, also enhances lysis and accounts for variations in sensitivity of sers strains.