Antibodies to N‐terminal peptides of gonococcal porin are bactericidal when gonococcal lipopolysaccharide is not sialylated

Summary Six synthetic 25‐mer peptides corresponding to certain presumed surfaced‐exposed regions of gonococcal porin protein I (PI) were made from strains FA19 (PIA) and MS11 (PIB). Four peptides were immunogenic in rabbits. Affinity‐purified antisera against both PIA and PIB N ‐terminal peptides were bactericidal for homologous gonococci and many heterologous PI serovars. However, sialylation of gonococcal lipopolysaccharide (LPS) by growth of gonococci in the presence of cytidine monophosphate–neuraminic acid (CMP–NANA) abrogated the bactericidal activity of these antisera. Binding of anti‐PI monoclonal antibodies to whole gonococci was reduced two‐ to fourfold by sialylation of LPS, suggesting that sialylation may inhibit bactericidal activity by masking porin epitopes. However, binding of anti‐PII (Opa) monoclonal antibodies was not inhibited, yet complement‐mediated killing was inhibited by sialylated LPS. Binding of complement components C3 and C9 was inhibited in the presence of either anti‐PI or anti‐PII monoclonals when gonococci were grown in the presence of CMP–NANA. Thus sialylation inhibited both anti‐PI antibody binding and complement deposition, with a resultant decrease in bactericidal activity.