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Infect. Immun. doi:10.1128/IAI.00367-08
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Fine Antigenic Specificity and Cooperative Bactericidal Activity of Monoclonal Antibodies Directed at the Meningococcal Vaccine Candidate, Factor H-Binding Protein

Center for Immunobiology and Vaccine Development, Children's Hospital Oakland Research Institute, Oakland, CA; and Novartis Vaccines, Siena, Italy

^* To whom correspondence should be addressed. Email: dgranoff{at}chori.org.

	Abstract

No broadly protective vaccine is available for 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 7 murine IgG mAbs against fHbp from antigenic variant (v.) groups 2 or 3 (40% of group B strains). Although none of the mAbs individually elicited bactericidal activity with human complement, all had activity in different combinations. We used mAb reactivity with strains expressing fHbp polymorphisms and site-specific mutagenesis to identify residues important for epitopes recognized by six of the v.2 or 3 mAbs, and by two v.1 mAbs previously characterized. Residues affecting v.2 or v.3 epitopes resided between amino acids 174 and 216, which formed an 8-stranded beta-barrel in the C domain, while residues affecting the v.1 epitopes included amino acids 121-122 of the B domain. Pairs of mAbs were bactericidal when their respective epitopes involved residues separated by 16-20 Å, and when at least one of the mAbs inhibited binding of fH, a down-regulatory complement protein. In contrast, there was no cooperative bactericidal activity when the distance between residues was 27 Å, or <14 Å, which correlated with inhibition of binding of one mAb by the other mAb. Thus, a model for anti-fH mAb bactericidal activity against low fHbp-expressing strains requires binding of two mAbs directed at non-overlapping epitopes, which activates the classical complement pathway, as well as inhibits fH binding. The latter increases susceptibility of the organism to complement-mediated bacteriolysis.