Immunization of guinea pigs with epitope C-rich lipopolysaccharide from Neisseria gonorrhoea; in vivo selection of gonococcal variants

The immunogenic potential of lipopolysaccharide (LPS) of a variant of Neisseria gonorrhoeae strain Gc 40 selected by growth in vivo (vivo variant) was investigated in guinea pigs. LPS extracts obtained from the water (WLPS) and the phenol (PLPS) phases of a hot phenol-water extraction were compared for their antigenic capacity and protective effect against infection in subcutaneous chambers. Immunization with PLPS induced significant levels of anti-LPS and anti-epitope C antibodies but WLPS was not antigenic. Two days after challenge, all guinea pigs immunized with WLPS had infections similar to those seen in unimmunized control animals while most animals immunized with PLPS and challenged with either 10(3) or 10(5) gonococci per ml showed low numbers of or no viable gonococci in their chambers. Five days after challenge, however, the same animals had chamber infections with high viable counts similar to controls. Gonococci reisolated from three such animals had physically and antigenically altered lipopolysaccharide and showed patterns of serum sensitivity to pre-challenge chamber fluid from immunized animals which were different from those of the parent vivo variant used for immunization and challenge. The results demonstrate that selection of LPS variants occurs in vivo. This could constitute an immune evasion mechanism.