Vibrio cholerae O139 capsular polysaccharide confers complement resistance in the absence or presence of antibody yet presents a productive target for cell lysis: implications for detection of bactericidal antibodies.

Vibrio cholerae O139 variants with different surface phenotypes have been compared for their resistance to complement and also for their susceptibility to antibody-dependent, complement-mediated bacteriolysis (ACB). While both capsular polysaccharide (CPS) and lipopolysaccharide (LPS) contribute to complement resistance in the absence of antibody, the relative survival of variants expressing only one of these surface polysaccharides reveals CPS to be of much greater significance in this respect. Variants with LPS+/CPS- or LPS-/CPS+ surface phenotypes were both susceptible to ACB, showing that antibody binding to either of the O139 polysaccharides can initiate ACB. Demonstration of the lytic potential of antibodies bound to the capsule is novel and necessitates a revision of the suggested mechanism by which CPS confers partial protection of wild-type V. cholerae O139 against ACB. This finding also raised the possibility that there may be CPS-restricted epitopes which can function as substrates for ACB, which would invalidate the common practice of using unencapsulated O139 variants for estimating bactericidal responses. Since our data did not support this scenario, we evaluated several unencapsulated strains - including variants producing normal or elongated polysaccharide chains, and a hybrid O1/O139 strain - for their utility as indicators of bactericidal antibodies to V. cholerae O139. Our findings support the use of the recently-isolated CIRS134, which ensures reproducible titrations and allows assignment of high lytic titres in assays requiring only 2% complement. However low-level, cross-serogroup lytic activity was detected when CIRS134 was used to assay responses of mice injected with O1 serogroup V. cholerae, suggesting that this indicator should be used with caution when evaluating the immunogenicity of bivalent O1/O139 vaccines.