Epitope mapping of a cross-reactive monoclonal antibody to meningococcal factor H-binding protein

Meningococcal factor H-binding protein (fHbp) is a surface-exposed lipoprotein that binds human factor H (fH), enabling down-regulation of complement activation on the bacterial surface. Binding of fH leads to bacterial evasion of host-mediated immunity. The protein has been divided into 3 variant groups or 2 sub-families. A panel of anti- fHbp mAbs has been produced from mice immunized with the 3 variants of fHbp and their epitopes were previously mapped, except for the mAb designated JAR36, a murine IgG mAb isolated from a mouse immunized with variant 3. We now report epitope mapping of JAR36, this mAb cross-reacts with all fHbp sequences in V.2 and V.3 groups, binds to the bacterial surface and elicits complement-mediated bactericidal activity in combinations with other anti-fHbp mAbs. We screened bacteriophage-displayed random peptide libraries to identify amino acid residues contributing to the JAR36 epitope. Mapping predictions were validated by constructing, through site-specific mutagenesis, corresponding rfHbps single-point variants, and analyzing their reactivity with the mAb.