VACCINE POTENTIAL OF MENINGOCOCCAL FRPB - STUDIES ON SURFACE EXPOSURE AND FUNCTIONAL ATTRIBUTES OF COMMON EPITOPES

Neisseria meningitidis expresses several novel outer membrane proteins (OMPs) in vivo and when grown under iron limitation in vitro. One of the most prominent is a 70 kDa ken-regulated protein (FrpB). FrpB was purified by elution from SDS-polyacrylamide gels and rabbit polyclonal antiserum (R-70) was raised against it. R-70 was bactericidal against homologous, but not heterologous, strains in the presence of human complement. The bactericidal activity was retained when R-70 was adsorbed with formaldehyde-fixed iron-replete cells (i.e. not expressing FrpB), but lost when adsorbed with fixed iron-restricted cells (which express FrpB). A murine monoclonal anti-FrpB antibody (mAb M70) was raised against a common epitope which showed complete cross-reaction on Western blots of OMPs from other serogroups and serotypes of N. meningitidis and some commensal Neisseriae species. However, it failed to kill the organism. Immunogold electron microscopy on ultrathin sections, using the R-70 antiserum adsorbed with fixed iron-replete cells, showed labelling on 40% of the cells, whereas the R-70 adsorbed with fixed iron-restricted cells and mAb M70 failed to label. However, none of these sera labelled whole cells, suggesting lack of surface accessibility. It appears that the highly conserved cross-reactive epitopes of FrpB only become exposed in the process of generating the antigen, whereas the surface-exposed epitopes recognized in killing assays are immunologically variable among different strains. Therefore, despite the strong immunogenicity of the common epitopes of FrpB in vivo, their apparent lack of surface exposure and the failure of antibodies against them to kill meningococci do not support consideration of this protein alone as the sole component of a broadly cross-protective vaccine.