Meningococcal factor H-binding protein variants expressed by epidemic capsular group A, W-135, and X strains from Africa

Abstract

Background: Meningococcal epidemics in Africa are generally caused by capsular group A strains, but W-135 or X strains also cause epidemics in this region. Factor H-binding protein (fHbp) is a novel antigen being investigated for use in group B vaccines. Little is known about fHbp in strains from other capsular groups.

Methods: We investigated fHbp in 35 group A, W-135, and X strains from Africa.

Results: The 22 group A isolates, which included each of the sequence types (STs) responsible for epidemics since 1963, and 4 group X and 3 group W-135 isolates from recent epidemics had genes encoding fHbp in antigenic variant group 1. The remaining 6 W-135 isolates had fHbp variant 2. Within each fHbp variant group, there was 92%-100% amino acid identity, and the proteins expressed conserved epitopes recognized by bactericidal monoclonal antibodies. Serum samples obtained from mice vaccinated with native outer membrane vesicle vaccines from mutants engineered to express fHbp variants had broad bactericidal activity against group A, W-135, or X strains.

Conclusions: Despite extensive natural exposure of the African population, fHbp is conserved among African strains. A native outer membrane vesicle vaccine that expresses fHbp variants can potentially elicit protective antibodies against strains from all capsular groups that cause epidemics in the region.

Figure 1

Map of Africa showing the source countries and nos. of isolates that were examined in this study. Sub-Saharan Africa is depicted in gray.

Figure 2

Phylogram of the factor H–binding protein (fHbp) polypeptide sequences from African strains. The relative distance between respective peptides is shown on the horizontal line; the scale bar denotes 5 changes/100 amino acids. One fHbp amino acid sequence from each sequence type (ST) is represented, except for ST-11 (group W-135), for which data from strains with fHbp variant 1 (v.1) or 2 (v.2) are shown. N, the no. of isolates with identical capsular group, ST, and fHbp polypeptide sequence. Peptide identifiers for each strain are shown in table 1.

Figure 3

Expression of factor H–binding protein (fHbp) in outer membrane vesicle (OMV) preparations of representative isolates, as measured by Western blot analysis. Monoclonal antibodies (MAbs) JAR 1 and JAR 5 were raised against fHbp variant 1 (v.1) (gene from strain MC58); MAb JAR 31 was raised against fHbp variant 3 (v.3) (gene from strain M1239), but it cross-reacts with fHbp variant 2 (v.2) and v.3 proteins. OMV from control strains included MC58 wt (fHbp v.1), MC58 fHbp knockout (ΔfHbp) mutant, and RM1090 WT (fHbp v.2). The sequence type (ST) of each isolate is shown in parentheses. rfHbp, recombinant fHbp.

Figure 4

Surface-accessibility of factor H–binding protein (fHbp) on live bacterial cells, as measured by flow cytometry with mouse anti-fHbp variant 1 (v.1) (A) or 2 (v.2) (B) antisera. The H44/76 positive control is known to express high levels of fHbp v.1 [14]. The RM1090 positive control is known to express low levels of fHbp v.2 [21] with 99% amino acid identity to fHbp v.2 of the group W-135 test strains. White and black histograms denote the respective binding of anti-fHbp antisera (1:250) to wild-type and fHbp knockout strains. For strains for which fHbp knockout mutants were not available, the gray histograms denote binding of a 1:50 dilution of a negative control antiserum with the wild-type strain. Strain designations are those shown in table 1.

Figure 5

Serum bactericidal titers against epidemic group A, W-135 and X strains. Eight strains were from Africa and 2 (LNP17592 and LNP19995) were Hajj-related sequence type (ST)–11, W-135 case isolates from France. All of the strains had heterologous PorA from the vaccine strains. The multilocus STs are shown in parentheses. The vaccine groups were as follows: Native outer membrane vesicle (OMV), Mutant (with “Mutant” denoting LpxL1 knockout strains of H44/76 and NZ98/254 expressing fHbp variant 1 and variant 2); Detergent OMV, WT (detergent-treated OMV from respective wild-type strain); and Al(OH)₃ (aluminum hydroxide only).