Role of the NMB0342-NMB0348 locus in meningococcal pathogenesis and investigation of NMB0345 as a vaccine candidate
Neisseria meningitidis is the most common cause of bacterial meningitis in the Western world and the second leading cause of mortality in 1-5 year-olds in the United Kingdom. A signature-tagged mutagenesis screen of approximately 3,000 insertional mutants of a serogroup B isolate of N. meningitidis, C311⁺, identified 73 genes required for pathogenesis in an infant rat model of meningococcal septicaemia. Homology-based searches indicate that two of the genes identified, NMB0342 and NMB0345, have homologues in other pathogenic bacteria and exist within a potential operon of seven genes (NMB0342-NMB0348). NBM0342 is a homologue of ispA (intracellular septation protein) of Shigella flexneri, required for effective intercellular spread and plaque formation in epithelial cells. NMB0345 is a homologue of cbf, a Campylobacter jejuni gene that encodes a 29 kDa protein which is a major antigenic peptide. PCR and Southern analyses of genes in the NMB0342-NMB0348 locus show that they are conserved across a wide range of pathogenic isolates and serogroups of N. meningitidis. However, these genes are present only in a subset of commensal strains. N. meningitidis mutants with transpoon insertions in NMB0342 and NMB0345 are highly attenuated when directly competed with the wild-type bacterium in the infant rat model of infection. Mutations have been introduced into other genes within the locus and the resulting mutants analyzed for their ability to cause systemic disease. Mutants with transpoon insertions in the NMB0343 and NMB0344 genes are significantly attenuated, while mutants with insertions in NMB0347 and NMB0348 are attenuated to a lesser degree. Complementation of the NMB0342 mutation by ectopic chromosomal integration of a wild-type copy of NMB0342 almost completely restores the virulence of the bacterium. In vitro cell-culture analyses and microscopic analysis of mutants in association with host cells demonstrate that a mutant with a transpoon insertion in NMB0345 is significantly reduced in its adherence to Chang epithelial cells compared to the wild-type bacterium. Whole blood and serum-sensitivity assays show that the NMB0342 and NMB0345 mutants are highly serum-sensitive compared to the wild-type bacterium. Preliminary experiments have demonstrated that immunization of mice with recombinant NMB0345 protein confers protection against challenge with the serogroup B isolate MC58. In vitro assays demonstrate that recombinant NMB0345 is an active peptidyl polyl cistrans isomerase (PPIase). Together, the results from these investigations support a role for genes of the NMB0342-NMB0348 locus in the pathogenesis of N. meningitidis infections. Further investigation is needed to assess the potential of NMB0345 as a vaccine candidate.