Investigating the regulation of the vaccine antigen Factor H binding protein in Neisseria meningitidis

Neisseria meningitidis is a strictly human pathogen and is a major cause of septicemia and meningitis worldwide. Factor H binding protein (fHbp) is a surface-exposed lipoprotein that binds human factor H (hfH) allowing the bacterium to evade the host innate immunity response. Of note, fHbp is a key antigen in two vaccines against N. meningitidis serogroup B. Although the fHbp gene is present in most circulating meningococcal strains, its level of expression varies among isolates and may influence strain susceptibility to anti-fHbp antisera. The aim of the study was to understand the sequence determinants that control fHbp expression in globally circulating strains. We analyzed the upstream fHbp intergenic region (fIR) of a panel of 105 invasive strains and we identified nine fIR sequence types which represent 77% of the isolates. By mass spectrometry we obtained an absolute quantification of fHbp in the same panel of strains and found a correlation between the fIR type and fHbp amounts. By the generation of a series of isogenic recombinant strains, where fHbp expression was under the control of each of the nine fIR types, we were able to confirm that the fIR sequence determines a specific level of expression and investigate the major determinants involved. The quantity of fHbp on the surface of the bacteria correlated directly with the susceptibility to killing mediated by anti-fHbp antibodies in immune sera. The influence of fHbp to mediate the evasion from generic complement-mediated killing presumably through binding of hfH was assessed and survival in human non-immune serum was less correlated with protein amounts measured from an in vitro growth culture. Overall, we demonstrated that the expression level of this antigen can be inferred by the DNA sequence of the fHbp intergenic region. Therefore, our findings can contribute to understand and predict vaccine coverage mediated by fHbp.