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Neisseria meningitidis is a commensal of the human nasopharynx but is also a major cause of septicemia and meningitis. The meningococcal factor H binding protein (fHbp) binds human factor H (fH), enabling downregulation of complement activation on the bacterial surface. fHbp is a component of two serogroup B meningococcal vaccines currently in clinical development. Here we characterize 12 fHbp subvariants for their level of surface exposure and ability to bind fH, to mediate serum resistance, and to induce bactericidal antibodies. Flow cytometry and Western analysis revealed that all strains examined expressed fHbp on their surface to different extents and bound fH in an fHbp-dependent manner. However, differences in fH binding did not always correlate with the level of fHbp expression, indicating that this is not the only factor affecting the amount of fH bound. To overcome the issue of strain variability in fHbp expression, the MC58Δ fHbp strain was genetically engineered to express different subvariants from a constitutive heterologous promoter. These recombinant strains were characterized for fH binding, and the data confirmed that each subvariant binds different levels of fH. Surface plasmon resonance revealed differences in the stability of the fHbp-fH complexes that ranged over 2 orders of magnitude, indicating that differences in residues between and within variant groups can influence fH binding. Interestingly, the level of survival in human sera of recombinant MC58 strains expressing diverse subvariants did not correlate with the level of fH binding, suggesting that the interaction of fHbp with fH is not the only function of fHbp that influences serum resistance. Furthermore, cross-reactive bactericidal activity was seen within each variant group, although the degree of activity varied, suggesting that amino acid differences within each variant group influence the bactericidal antibody response.

NadA is a novel vaccine candidate recently identified in Neisseria meningitidis and involved in adhesion to host tissues. The nadA gene has been found in approximately 50% of the strains isolated from patients and in three of the four hypervirulent lineages of non-serogroup A strains. Here we investigated the presence of the nadA gene in 154 meningococcal strains isolated from healthy people (carrier strains). Only 25 (16.2%) of the 154 carrier isolates harbored the nadA gene. The commensal species Neisseria lactamica was also found not to have the nadA gene. Eighteen of the carrier strains belonged to the ET-5 and ET-37 hypervirulent clusters, indicating that only the 5.1% of the genuine carrier population actually harbored nadA (7 of 136 strains). Five of the seven strains harbored a novel allele of the nadA gene that was designated nadA4 . The NadA4 protein was present on the bacterial surface as heat-stable high-molecular-weight oligomers. Antibodies against the recombinant NadA4 protein were bactericidal against homologous strains, whereas the activity against other NadA alleles was weak. In conclusion, the nadA gene segregates differently in the population of strains isolated from healthy individuals and in the population of strains isolated from patients. The presence of NadA can therefore be used as a tool to study the dynamics of meningococcal infections and understand why this bacterium, which is mostly a commensal, can become a severe pathogen.