Neisseria meningitidis

Neisseria meningitidis (the meningococcus) is a gram-negative human pathogen of major clinical importance. The meningococcus is an asymptomatic coloniser of the human nasopharynx. In rare cases, meningococci can cross the mucosal barrier leading to a systemic infection, which can further progress to septicaemia (‘blood poisoning’) or severe meningitis.

Conjugate vaccines (i.e. the capsular polysaccharide linked to a carrier protein such as the tetanus toxoid) are available for protection against disease caused by N. meningitidis serogroups A, C, W, and Y. Additionally there are recombinant protein-based vaccines (Bexsero and Trumenba) available for protection against some strains of serogroup B, the major group of strains responsible for disease in the UK. Bexsero is a component of the UK infant immunisation schedule and provides protection to infants against disease caused by ~88% of MenB strains. Because coverage is not universal, on-going monitoring of disease cases in infants is required to determine whether there is a lack of coverage, vaccine escape, and an increase in prevalence of non-vaccine serotypes.

N. meningitidis has a remarkable ability to evade the human immune response. This is in part due to the rapid generation of genotypic and phenotypic diversity afforded by phase variation – an ON/OFF switching of gene expression afforded by localised hyper-mutation within simple sequence repeats. Phase variation can be assessed by bioinformatics analyses of whole genome sequences, PCR-based methods and using immunological reagents.

N. meningitidis can also generate genetic variation by hyper-recombination, normal mutation and recombination following natural transformation. The sequencing of whole genomes is utilised to monitor the frequencies of these processes.

Work on N. meningitidis at Leicester

Dr Chris Bayliss, from the Department of Genetics, Genomics and Cancer Sciences, and his group are studying the genetics underpinning the success of N. meningitidis as a major human pathogen. The major areas of study are epidemiological studies of strain distributions and spread, bioinformatics of whole genome sequences, phenotypic analysis of strains and vaccinology. A key focus is on phase variation in this species.

Epidemiology

Dr. Bayliss' group has been involved in generating large collections of meningococcal isolates from university students who are asymptomatically colonised with N. meningitidis. Observations:- carriage of a single strain for 6 months occurs in 45% of carriers; detection of a high prevalence of MenY strains in 2008-2011; carriage of MenW strains in vaccinated individuals and rapid amplification of a MenW ST-11 clone during the first few months of university life in 2015-2016.

Bioinformatics

Dr. Bayliss' group developed PhasomeIt to detect phase-variable genes in whole genome sequences. Using this tool, pathogenic Neisseria species (the meningococcus and gonococcus) and the commensal species N. lactamica were found to have many more phase-variable genes than other Neisserial species indicating that PV contributes to host colonisation and persistence. This group has also shown that a MenW ST-11 sub-clone evolved longer repeat tracts in multiple genes encoding surface proteins, a phenomenon that is likely to have increased the PV rate and facilitated strain transmission.

Phenotypic analyses

Key tools:- cell culture, whole blood culture and serum bactericidal assays. Key observations:- demonstration that the transferrin-binding protein is required for growth of meningococci in human blood; repetitive tract in PorA, a major outer membrane protein, mediates escape from PorA-specific bactericidal antibodies by altering surface expression of this protein.

Phase variation

Epidemiological studies found an association between persistent carriage and PV-mediated reductions in expression of single-copy surface proteins, probably driven by host immune responses. These studies also showed that PV of the multiple-copy Opa proteins leads to switching between antigenically-divergent proteins during carriage whilst maintaining the adhesive properties of these proteins. Meningococcal disease isolates were also found to have both PilC proteins switched OFF whilst persistent carriage is correlated with a switch from PilC1 to PilC2.

Vaccinology

Bexsero®, a MenB vaccine, was introduced into the childhood immunisation schedule at the end of 2015. Funded by the Meningitis Research Foundation, Dr. Bayliss and his group have been assessing how to monitor the Bexsero® immunisation programme. By focussing on the sequences controlling expression of the factor H binding protein (one of the vaccine antigens), new approaches have been generated to help with understanding and monitoring performance of this vaccine in all cases of disease.

Collaborators

• Dr David Turner, Dr. Neil Oldfield and Prof. Dlawer Ala’Aldeen, University of Nottingham (epidemiological studies)
• Professor Ray Borrow, head of the Meningococcal Reference Unit, part of Public Health England (analysis of meningococcal disease isolates)
• Professor Martin Maiden, University of Oxford (analysis of meningococcal genomes).