Defining a protective epitope on factor H binding protein, a key meningococcal virulence factor and vaccine antigen.

Mapping of epitopes recognized by functional monoclonal antibodies (mAbs) is essential for understanding the nature of immune responses and designing improved vaccines, therapeutics, and diagnostics. In recent years, identification of B-cell epitopes targeted by neutralizing antibodies has facilitated the design of peptide-based vaccines against highly variable pathogens like HIV, respiratory syncytial virus, and Helicobacter pylori; however, none of these products has yet progressed into clinical stages. Linear epitopes identified by conventional mapping techniques only partially reflect the immunogenic properties of the epitope in its natural conformation, thus limiting the success of this approach. To investigate antigen-antibody interactions and assess the potential of the most common epitope mapping techniques, we generated a series of mAbs against factor H binding protein (fHbp), a key virulence factor and vaccine antigen of Neisseria meningitidis. The interaction of fHbp with the bactericidal mAb 12C1 was studied by various epitope mapping methods. Although a 12-residue epitope in the C terminus of fHbp was identified by both Peptide Scanning and Phage Display Library screening, other approaches, such as hydrogen/deuterium exchange mass spectrometry (MS) and X-ray crystallography, showed that mAb 12C1 occupies an area of ∼1,000 Å(2) on fHbp, including >20 fHbp residues distributed on both N- and C-terminal domains. Collectively, these data show that linear epitope mapping techniques provide useful but incomplete descriptions of B-cell epitopes, indicating that increased efforts to fully characterize antigen-antibody interfaces are required to understand and design effective immunogens.

Two cross-reactive monoclonal antibodies recognize overlapping epitopes on Neisseria meningitidis factor H binding protein but have different functional properties.

Factor H binding protein (fHbp) is one of the main antigens of the 4-component meningococcus B (4CMenB) multicomponent vaccine against disease caused by serogroup B Neisseria meningitidis (MenB). fHbp binds the complement down-regulating protein human factor H (hfH), thus resulting in immune evasion. fHbp exists in 3 variant groups with limited cross-protective responses. Previous studies have described the generation of monoclonal antibodies (mAbs) targeting variant-specific regions of fHbp. Here we report for the first time the functional characterization of two mAbs that recognize a wide panel of fHbp variants and subvariants on the MenB surface and that are able to inhibit fHbp binding to hfH. The antigenic regions targeted by the two mAbs were accurately mapped by hydrogen-deuterium exchange mass spectrometry (HDX-MS), revealing partially overlapping epitopes on the N terminus of fHbp. Furthermore, while none of the mAbs had bactericidal activity on its own, a synergistic effect was observed for each of them when tested by the human complement serum bactericidal activity (hSBA) assay in combination with a second nonbactericidal mAb. The bases underlying fHbp variant cross-reactivity, as well as inhibition of hfH binding and cooperativity effect observed for the two mAbs, are discussed in light of the mapped epitopes.

Nonfunctional variant 3 factor H binding proteins as meningococcal vaccine candidates.

Neisseria meningitidis is a human-specific pathogen and leading cause of meningitis and septicemia. Factor H binding protein (fHbp), a virulence factor which protects N. meningitidis from innate immunity by binding the human complement regulator factor H (fH) with high affinity, is also a key antigen in vaccines being developed to prevent meningococcal disease. fHbp can be divided into three variant groups (V1, V2, and V3) that elicit limited immunological cross-reactivity. The interaction of fH with fHbp could impair the immunogenicity of this antigen by hindering access to the antigenic epitopes in fHbp, providing the rationale for the development of nonfunctional fHbps as vaccine candidates. Here, we characterized the two nonfunctional V3 fHbps, fHbp(T286A) and fHbp(E313A), which each contains a single amino acid substitution that leads to a marked reduction in affinity for fH without affecting the folding of the proteins. The immunogenicity of the nonfunctional fHbps was assessed in transgenic mice expressing a single chimeric fH containing domains of human fH involved in binding to fHbp. No differences in anti-V3 fHbp antibody titers were elicited by the wild-type V3 fHbp, V3 fHbp(T286A), and V3 fHbp(E313A), demonstrating that the nonfunctional fHbps retain their immunogenicity. Furthermore, the nonfunctional V3 fHbps elicit serum bactericidal activity that is equivalent to or higher than that observed with the wild-type protein. Our findings provide the basis for the rational design of next-generation vaccines containing nonfunctional V3 fHbps.

Adjuvanticity of the oil-in-water emulsion MF59 is independent of Nlrp3 inflammasome but requires the adaptor protein MyD88.

Oil-in-water emulsions have been successfully used to increase the efficacy, immunogenicity, and cross-protection of human vaccines; however, their mechanism of action is still largely unknown. Nlrp3 inflammasome has been previously associated to the activity of alum, another adjuvant broadly used in human vaccines, and MyD88 adaptor protein is required for the adjuvanticity of most Toll-like receptor agonists. We compared the contribution of Nlrp3 and MyD88 to the adjuvanticity of alum, the oil-in-water emulsion MF59, and complete Freund's adjuvant in mice using a three-component vaccine against serogroup B Neisseria meningitidis (rMenB). Although the basal antibody responses to the nonadjuvanted rMenB vaccine were largely dependent on Nlrp3, the high-level antibody responses induced by alum, MF59, or complete Freund's adjuvant did not require Nlrp3. Surprisingly, we found that MF59 requires MyD88 to enhance bactericidal antibody responses to the rMenB vaccine. Because MF59 did not activate any of the Toll-like receptors in vitro, we propose that MF59 requires MyD88 for a Toll-like receptor-independent signaling pathway.