Certain pathogens recruit host complement inhibitors such as factor H (fH) to evade the immune system. Microbial complement inhibitor-binding molecules can be promising vaccine targets by eliciting antibodies that neutralize this microbial defense mechanism. One such antigen, meningococcal fH-binding protein (fHbp), was used in clinical trials before the protein was discovered to bind fH. The potential effect of fH binding on vaccine immunogenicity had not been assessed in experimental animals because fHbp binds human fH specifically. In this study, we developed a human fH transgenic mouse model. Transgenic mice immunized with fHbp vaccine had 4- to 8-fold lower serum bactericidal antibody responses than control mice whose native fH did not bind the vaccine. In contrast, antibody responses were unimpaired in transgenic mice immunized with a control meningococcal group C polysaccharide-protein conjugate vaccine. In transgenic mice, immunization with an fH non-binding mutant of fHbp elicited antibodies with higher bactericidal activity than fHbp vaccination itself. Antibodies elicited by the mutant fHbp more effectively blocked fH binding to wild-type fHbp than antibodies elicited by fHbp that bound fH. Thus, a mutant fHbp vaccine that does not bind fH, but which retains immunogenicity, is predicted to be superior in humans than an fHbp vaccine that binds human fH. In the case of mutant fHbp vaccination, the resultant antibody responses may be directed more at epitopes in or near the fH-binding site, which result in greater complement-mediated serum bactericidal activity; these epitopes may be obscured when human fH is bound to the wild-type fHbp vaccine.

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No broadly protective vaccine is available for the prevention of group B meningococcal disease. One promising candidate is factor H-binding protein (fHbp), which is present in all strains but often sparsely expressed. We prepared seven murine immunoglobulin G monoclonal antibodies (

Meningococcal factor H binding protein (fHbp) is a promising vaccine candidate that elicits serum bactericidal antibodies in humans. Based on sequence variability of the entire protein, fHbp has been divided into three variant groups or two sub-families. We recently reported that the fHbp architecture was modular, consisting of five variable segments, each encoded by genes from one of two lineages. Based on combinations of segments from different lineages, all 70 known fHbp sequence variants could be classified into one of six modular groups. In this study, we analyzed sequences of 172 new fHbp variants that were available from public databases. All but three variants could be classified into one of the six previously described modular groups. Among systematically collected invasive group B isolates from the U.S. and Europe, modular group I overall was most common (60%) but group IV (natural chimeras) accounted for 23% of UK isolates and <1% of U.S. isolates (P<0.0001). Mouse antisera to recombinant fHbp from each of the modular groups showed modular group-specific bactericidal activity against strains with low fHbp expression but had broader activity against strains with higher fHbp expression. Thus both modular group and relative expression of fHbp affected strain susceptibility to anti-fHbp bactericidal activity. The results confirmed the modular architecture of fHbp and underscored its importance for the design of broadly protective group B vaccines in different regions.

Meningococcal factor H binding protein (fHbp) is a promising vaccine antigen that binds the human complement downregulatory molecule factor H (fH), and this binding enhances the survival of the organism in serum. Based on sequence variability of the entire protein, fHbp has been divided into three variant groups or two subfamilies. Here, we present evidence based on phylogenetic analysis of 70 unique fHbp amino acid sequences that the molecular architecture is modular. From sequences of natural chimeras we identified blocks of two to five invariant residues that flanked five modular variable segments. Although overall, 46 % of the fHbp amino acids were invariant, based on a crystal structure, the invariant blocks that flanked the modular variable segments clustered on the membrane surface containing the amino-terminal lipid anchor, while the remaining invariant residues were located throughout the protein. Each of the five modular variable segments could be classified into one of two types, designated a or b, based on homology with segments encoded by variant 1 or 3 fHbp genes, respectively. Forty of the fHbps (57 %) comprised only a (n533) or b (n57) type segments. The remaining 30 proteins (43 %) were chimeras and could be classified into one of four modular groups. These included all 15 proteins assigned to the previously described variant 2 in subfamily A. The modular segments of one chimeric modular group had 96 % amino acid identity with those of fHbp orthologs in Neisseria gonorrhoeae. Collectively, the data suggest that recombination between Neisseria meningitidis and N. gonorrhoeae progenitors generated a family of modular, antigenically diverse meningococcal fHbps.

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The binding of human complement inhibitors to vaccine antigens in vivo could diminish their immunogenicity. A meningococcal ligand for the complement down-regulator, factor H (fH), is fH-binding protein (fHbp), which is specific for human fH. Vaccines containing recombinant fHbp or native outer membrane vesicles (NOMV) from mutant strains with over-expressed fHbp are in clinical development. In a previous study in transgenic mice, the presence of human fH impaired the immunogenicity of a recombinant fHbp vaccine. In the present study, we prepared two NOMV vaccines from mutant group B strains with over-expressed wild-type fHbp or an R41S mutant fHbp with no detectable fH binding. In wild-type mice in which mouse fH did not bind to fHbp in either vaccine, the NOMV vaccine with wild-type fHbp elicited 2-fold higher serum IgG anti-fHbp titers (P = 0.001) and 4-fold higher complement-mediated bactericidal titers against a PorA-heterologous strain than the NOMV with the mutant fHbp (P = 0.003). By adsorption, the bactericidal antibodies were shown to be directed at fHbp. In transgenic mice in which human fH bound to the wild-type fHbp but not to the R41S fHbp, the NOMV vaccine with the mutant fHbp elicited 5-fold higher serum IgG anti-fHbp titers (P = 0.002), and 19-fold higher bactericidal titers than the NOMV vaccine with wild-type fHbp (P = 0.001). Thus, in mice that differed only by the presence of human fH, the respective results with the two vaccines were opposite. The enhanced bactericidal activity elicited by the mutant fHbp vaccine in the presence of human fH far outweighed the loss of immunogenicity of the mutant protein in wild-type animals. Engineering fHbp not to bind to its cognate complement inhibitor, therefore, may increase vaccine immunogenicity in humans.

Susceptibility to bactericidal activity can be predicted, in part, on the basis of fHBP phenotypes. Both vaccines have the potential to prevent most group B disease in the United States.