Differential immunogenicity of Vibrio cholerae O139 variants expressing different combinations of naturally occurring and atypical forms of the serogroup polysaccharide

Field testing of an inactivated bivalent O1/O139 cholera vaccine suggests that Vibrio cholerae O1 is more immunogenic than V. cholerae O139. To investigate whether this might be partly attributable to the production of capsular polysaccharide (CPS) by O139 isolates, we have compared the immunogenicity of variant strains expressing different combinations of lipopolysaccharide (LPS) and CPS. These studies indicate that the core-linked LPS structure is of paramount importance for induction of antibodies to the serogroup antigen. By contrast CPS was minimally immunogenic. Significantly the presence of CPS did not modulate the immunogenicity of the underlying LPS. To examine whether differences in LPS structure might contribute to the differing immunogenicities of the O1 and O139 serogroups, an attempt was made to modify the normal O139 LPS structure by provision of one of several heterologous wzz genes. The resulting variants displayed additional, atypical surface polysaccharide, whose modal length was characteristic for the particular wzz gene. By immunoblotting this novel material showed a ladder-like banding pattern typical of LPS, but its failure to be stained by silver indicated that it was not core-associated and was therefore more like truncated CPS. Consistent with our earlier findings, studies using systemic or mucosal routes of immunization failed to demonstrate any consistent enhancement of antibody responses associated with production of these aberrant polysaccharide polymers.