Role of factor H binding protein in Neisseria meningitidis virulence and its potential as a vaccine candidate to broadly protect against meningococcal disease

Abstract

Neisseria meningitidis is a Gram-negative microorganism that exists exclusively in humans and can cause devastating invasive disease. Although capsular polysaccharide-based vaccines against serogroups A, C, Y, and W135 are widely available, the pathway to a broadly protective vaccine against serogroup B has been more complex. The last 11 years has seen the discovery and development of the N. meningitidis serogroup B (MnB) outer membrane protein factor H binding protein (fHBP) as a vaccine component. Since the initial discovery of fHBP, a tremendous amount of work has accumulated on the diversity, structure, and regulation of this important protein. fHBP has proved to be a virulence factor for N. meningitidis and a target for functional bactericidal antibodies. fHBP is critical for survival of meningococci in the human host, as it is responsible for the primary interaction with human factor H (fH). Binding of hfH by the meningococcus serves to downregulate the host alternative complement pathway and helps the organism evade host innate immunity. Preclinical studies have shown that an fHBP-based vaccine can elicit serum bactericidal antibodies capable of killing MnB, and the vaccine has shown very encouraging results in human clinical trials. This report reviews our current knowledge of fHBP. In particular, we discuss the recent advances in our understanding of fHBP, its importance to N. meningitidis, and its potential role as a vaccine for preventing MnB disease.

Fig 1

Biochemical approach resulting in the discovery of fHBP. The traditional biochemical approach is a proven method for the identification of vaccine candidates that provide broad protection against heterologous strains. The initial approach starts with the isolation of cell membranes containing surface antigens (top left). The proteins are extracted biochemically into separate fractions, with each fraction containing a different composition of membrane proteins. Each fraction is used to immunize mice, and the sera are tested for PorA-independent bactericidal antibody activity (SBA) against heterologous MnB strains (top right). Fractions that contain broadly reactive SBA activity are further fractionated and retested. The final fraction contains very few proteins but is highly enriched for SBA activity. These proteins are size separated and their amino acid sequences determined (bottom right). Using available genomic sequences, the corresponding gene is identified, cloned, and expressed in E. coli and the recombinant protein evaluated for SBA activity. The final proteins that elicit broad protection in preclinical studies are then candidates for clinical studies (bottom left).

Fig 2

Phylogenetic analysis of fHBP protein sequences. (A) Nomenclature used to describe fHBP, including Pfizer and Novartis subfamily designations, N- and C-terminal domains, and Pajon modular architecture assignations (20, 21, 29, 31). (B) Neighbor-joining tree of 569 fHBP variants from http://pubmlst.org/neisseria/fHbp/. The tree was generated in ClustalW (145) with 500 bootstraps and drawn using MEGA 5.05 (146). Subfamily designations are defined by Murphy et al. and Masignani et al. (20, 29). Subfamily A (variants 2 and 3) can be further subdivided into 4 major groupings based on the N- and C-terminal domains of fHBP (N1C1, N2C2, N1C2, and N2C1), and subfamily B can be divided into 3 groupings (N4, N5, and N6). Pajon et al. have made similar groupings based on five variable segments, dividing all fHBP variants into 9 modular groups (31). The six major modular groups are indicated by the roman numerals (the minor groups VIII and IX, each represented by a single variant, are not labeled on the tree). Not shown are several hybrid sequences that are composed of both subfamily A and subfamily B regions. N2C1 and N2C2 together are equivalent to variant 2 (v.2), and N1C2 and N1C1 are equivalent to variant 3 (v.3). The bar indicates genetic distance.

Fig 3

Schematic depiction of fHBP anchored on the surface of N. meningitidis. Shown is the B01 fHBP structure colored as follows: white, N-terminal flexible stem; green, N-terminal domain; blue, C-terminal domain; pink, linker between the β-structures of two domains. Inset, top view of fHBP demonstrating the surface proposed to face away from the bacterial surface.

Fig 4

fHBP conservation within and across the A and B subfamilies. The residues defining the A and B subfamilies (gold) and the residues conserved in all A and B variants (purple) are shown as spheres on the B01 structure. Other residues are dimmed and colored according to structural regions: green, N-terminal domain; blue, C-terminal domain; pink, linker between the β-structures of two domains. The membrane-anchoring N-terminal flexible stem of fHBP is behind the structure.

Fig 5

fH binding on fHBP. Shown is the cocrystal structure of human fH₆₇ bound to B24 fHBP (52). fH₆₇ is colored in orange, while fHBP is colored by domain: green, N-terminal domain; blue, C-terminal domain; pink, linker between the β-structures of two domains.

Fig 6

Antibody-fHBP interactions. (A) The binding residues on fHBP of bactericidal antibodies. Published bactericidal monoclonal antibody epitopes are highlighted as red spheres on the secondary structure diagram of B01 fHBP. fHBP domains are colored by domain (green, N-terminal domain; blue, C-terminal domain; pink, linker between the β-structures of two domains), with fH binding residues highlighted in purple. (B) Interaction between surface-exposed fHBP and IgG. Depicted schematically in the cell membrane are fHBP (cyan) and PorA (gold) expressed on the N. meningitidis surface; on the top, an IgG is drawn to scale. The fH binding residues on fHBP and the complementarity-determining regions on IgG are highlighted in purple and blue, respectively.

Fig 7

Demonstration of the breadth of coverage afforded by the bivalent fHBP vaccine: cumulative coverage of fHBP variants. Variants shown to be killed by anti-fHBP bivalent immune sera from adults (23) are indicated on the x axis. The prevalence of these variants in a systematically collected set of 1,263 invasive MnB disease isolates from the United States and Europe (29) is shown on the left y axis, with the cumulative prevalence indicated by the red line (right y axis).