Investigation of the Distribution, Antigenic Variation and the Biological Role of Phase Variation of the Haemoglobin Receptors of Neisseria Meningitidis

Neisseria meningitidis is the major cause of bacterial meningitis worldwide. The genome of this pathogen contains >40 phase variable loci whose expression is regulated by tandem DNA repeat tracts. Majority of these loci encode OMPs, which are potential targets for host innate and adaptive immune responses. An analysis for the distribution, frequency and role of PV of these genes is relevant in determining their virulence association and suitability as a future vaccine candidate. The project investigated the combined distribution, frequency and PV status of two important genes, hpuAB and hmbR, in disease (n=221) and carriage (n=305) isolates. Strains with both genes or only hmbR were present at similar frequencies among disease isolates as compared with carriage isolates. However, >90 % of isolates from CC5, CC8 and CC11 (CCs with the highest disease to carriage ratios) contained both genes. Strains with only hpuAB gene were under-represented among disease isolates, possibly due to the receptor having a high level of immunogenicity or being inefficient in iron acquisition during systemic spread. Absence of hpuAB resulted from either complete deletion or replacement by an insertion element. Further, one or both genes were found in an ON state in 96 % of disease and 66 % of carriage isolates. This suggests that expression of at least one Hb receptor is of major importance for disease, and that the presence of both receptors contributes to virulence in some strains. The experimental findings also revealed the ability of strain 8047 to escape from MAb P1.2 mediated killing due to PV in porA promoter sequences. We conclude that incubation in the presence of antibody selected for phase variants (10C or 9C) with >3 fold lower PorA surface expression than wild type. This escape in the presence of a specific immune response is an elegant demonstration of the importance of PV for adaptation, and have implications in the development of PorA based vaccines. Finally, the bactericidal activity of HpuA polyclonal antisera generated during this project was evaluated and possible reasons for a lack of its activity are discussed.