Investigating the Molecular Mechanisms of Neisseria Meningitidis Antigen Regulation: Determining a Switch between Colonization and Invasion

NHBA is a surface-exposed lipoprotein which is expressed by all N. meningitidis strains in different isoforms. We determined that temperature controlled the expression of NHBA in all strains tested. NHBA expression was significantly increased at 30 compared to 37°C, both at protein and RNA levels and was also reflected by a higher surface exposure. A detailed molecular analysis indicated that multiple molecular mechanisms are responsible for the thermoregulated NHBA expression. RNA stability/translatability resulted to be increased at lower temperatures and, protein stability was also impacted. Increased NHBA expression resulted in more efficient killing as shown by serum bactericidal assay (SBA). We investigated the NHBA expression in response to the presence of serum. The presence of human serum has contrasting effects on NHBA expression, resulting in transient up-regulation of NHBA mRNA, however the protein is rapidly processed. We propose a model in which NHBA regulation in response to temperature downshift might reflect the bacterial adaptation during the initial step of host-bacterial interaction and might also explain higher susceptibility to anti-NHBA antibodies in the nasopharynx niche. On the other hand, the initial up-regulation and the high processing of NHBA might play a role during the first steps of invasive disease. In the second part of the thesis, we compared genetically engineered outer membrane vesicles and recombinant proteins, as delivery systems of protective antigen. Using NadA as model antigen, we determined that OMV overexpressing NadA produced by homologous (MenB) or heterologous (E.coli) bacterial strains, are able to elicit a higher functional antibody response respect to the recombinant protein per se. The differences in functionality might be due to different IgG subclasses distribution. Moreover, OMV overexpressing NadA are able to elicit antibodies that inhibit NadA-mediated adhesion on the host cells surface, in a much more efficient way respect to the recombinant protein formulation.