The role of phase-variable expression of meningococcal surface proteins during carriage and disease

Neisseria meningitidis is the most common cause of bacterial meningitis, a disease that kills thousands of people yearly. Asymptomatic colonisation of the oropharynx (i. e. carriage) occurs in 10% – 30% of humans. Significant features of meningococcal genomes are simple sequence repeats (SSR), which have been shown to control gene expression in a reversible process known as phase variation (PV). This study investigated the consequence of “switching-OFF” of two haemoglobin-acquisition systems (hpuAB and hmbR) during disease and also explored potential associations between PV state of the immunodominant meningococcal antigen, PorA, and the adaptive immune response during carriage. Using an ex vivo human whole blood model, hmbR-OFF mutants of strain MC58 (hpuAB-negative) exhibited a pattern of growth similar to wild-type. Conversely, an inability to utilise transferrin (ΔtbpBA) significantly affected growth but not survival in blood. Five recombinant versions of these Hb receptors were prepared in this study and subsequently used as antigens for the generation of polyclonal and monoclonal antibodies in mice. The polyclonal antirHpuA, anti-rHpuB and anti-rHmbR antisera were reactive with homologous receptors in lysates of diverse meningococcal strains. Surface expression of HpuA and HmbR was detected by flow cytometry but all antisera were incapable of mediating killing of iron-restricted meningococci. An immunodetection assay employed in this study revealed the induction of variant-specific anti-PorA IgG antibodies following acquisition of carriage. These antibodies may have contributed to subsequent loss of carriage but a role for PV in immune escape in vivo was not established. This study posits that HmbR is less important than TbpBA and HpuAB during disease and that phase variable expression of surface receptors is irrelevant for immune evasion during carriage. Further studies are recommended to confirm the proposed importance of HpuAB over HmbR during invasive disease and to investigate the opsonophagocytic property of the anti-HpuA and anti-HmbR antisera.