Analysis of human serum immunoglobulin G against O-acetyl-positive and O-acetyl-negative serogroup W135 meningococcal capsular polysaccharide

The capsular polysaccharide of Neisseria meningitidis serogroup W135 is expressed in both O-acetyl-positive (OA⁺) and O-acetyl-negative (OA^-) forms. This study investigates the impact of OA status (OA ⁺ versus OA^-) on serological measurements of anti-W135 immunoglobulin G (IgG) antibodies in immunized adults. W135-specific serum antibody assignments were made for 28 postimmunization sera from adults by enzyme-linked immunosorbent assay using the meningococcal standard reference serum CDC1992. The established IgG concentration in micrograms per milliliter ([IgG] μg/ml) for CDC1992 against OA⁺ antigen (16.2 μg/ml) was used as a reference to assign a concentration of 10.13 μg/ml IgG against OA^- antigen by cross-standardization. Overall, the IgG assignments for these sera were higher against OA⁺ antigen (geometric mean concentration [GMC] = 7.16 μg/ml) than against OA^- antigen (GMC = 2.84 μg/ml). However, seven sera showed higher specific [IgG] μg/ml values against the OA⁺ antigen than against the OA^- antigen. These sera were also distinguished by the inability of fluid-phase OA ^- antigen to compete for antibody binding to OA⁺ solid-phase antigen. Although there was no overall difference in functional activity measured by complement-mediated serum bactericidal assay (SBA) against OA⁺ and OA^- target bacteria (geometric mean titers of 9,642 and 9,045, respectively), three serum specimens showed a large difference in SBA antibody titers against OA⁺ versus OA^- W135 target bacteria, which may reflect different epitope specificities for these sera. Our data indicate that, for some sera, the agreement in anti-OA⁺ versus anti-OA^- W135 IgG assignments is serum specific and does not reflect the functional (killing) activity in vitro.