Epitope mapping of a mAb against the factor H binding protein (fHbp) of N. meningitidis by phage display

No broadly protective vaccine is yet available for the prevention of group B meningococcal disease. One promising vaccine lead is the factor H binding protein (fHbp), a surface-exposed lipoprotein, which is present in all strains of Neisseria meningitidis. This protein binds human factor H (fH), contributing to the ability of these bacteria to avoid complement-mediated killing. The fHbp is a component of a serogroup B meningococcal vaccine currently in late-phase clinical development. Based on the sequence variability of the entire protein, fHbp has been divided into 3 variant groups or 2 sub-families and its architecture is modular, consisting of 5 variable segments, amino acid residues in position between 100 and 255 have been identified as important targets of variant-specific bactericidal antibodies. A panel of anti-fHbp mAbs has been produced from mice immunized with each of the 3 variants of fHbp and all their epitopes have been mapped, except that of the mAb designated JAR 36. This antibody was isolated from a mouse immunized with fHbp of the antigenic variant 3 group, but it cross-reacts with all fHbp sequences in the variant 2 group and elicits complement-mediated cooperative bactericidal activity with JAR 13 and JAR 11. We have screened phage displayed random peptide libraries to identify amino acid residues composing the JAR 36 epitope, putative mapping hypotheses have been then validated by constructing recombinant fHbps single-point variants through site-specific mutagenesis, and analyzing their reactivity with the mAb. Moreover, by using a computational tool, a total of seventeen residues have been identified as possibly constituting the epitope of JAR 36. Eight of these residues are located in a segment of fHbp previously recognized to play an important role in the binding of JAR 36 and four of them are common in variant groups 2 and 3. Both the amino acid identified by phage display and the †œin silico†�-predicted epitope are located in the proximity of the fH-binding site. This finding is according with previous experimental data, of JAR 36 partially inhibiting binding of fH to fHbp. The data reported will help in characterizing the molecular mechanism of immune protection against MenB, toward the development of safer and effective protein-based vaccines for the prevention of group B meningitis.