Clonotypic analysis of human antibodies specific for Neisseria meningitidis polysaccharides A and C in adults

Serum antibodies to the capsular polysaccharides A and C (PSA and PSC) of N. meningitidis in healthy adults before and after vaccination with the sole polysaccharides were analysed by isoelectric focusing (IEF). Before vaccination, 49% and 28% had naturally acquired antibodies against PSA and PSC, respectively, whereas 18 days after vaccine administration 84% and 91%, respectively, showed a detectable spectrotypic pattern. Oligoclonality appeared to be the main feature of naturally acquired and vaccine‐induced antibodies for both polysaccharides. In all subjects the anti‐PSA response, showing dominant bands at the same pH position, was more homogeneous than anti‐PSC one. Most subjects with naturally acquired antibodies (25 out of 38 for PSA and 20 out of 22 for PSC) showed a spectrotypic pattern after vaccination, similar to that observed before vaccination (any differences were just related to band intensity), suggesting that PSA and PSC are able to recruit the same B cell clones previously primed with a T‐dependent form of the antigen, i.e. the whole bacterium. However, in one‐third of subjects with naturally acquired anti‐PSA antibodies, the appearance of new alkaline bands after vaccination was observed. Furthermore, in subjects with absence of detectable natural antibodies, the vaccine‐induced antibody response started in correspondence of alkaline pH areas, subsequently extending to neutral and acidic areas. Therefore, it may be hypothesized that alkaline antibody‐secreting B cell clones are the first to be recruited. The final spectrotype in these subjects was similar to that observed in subjects with naturally acquired antibodies. This observation, together with the above reported data, allow us to conclude that natural (T‐dependent pathway) and vaccine (T‐independenl pathway) immunization induce the expression of the same antibody repertoire, for both meningococcal PSA and PSC.