Role of transferrin receptor from a Neisseria meningitidis tbpB isotype II strain in human transferrin binding and virulence

Neisseria meningitidis acquires iron through the action of the transferrin (Tf) receptor, which is composed of the Tf-binding proteins A and B (TbpA and TbpB). Meningococci can be classified into isotype I and II strains depending on whether they harbor a type I or II form of TbpB. Both types of TbpB have been shown to differ in their genomic, biochemical, and antigenic properties. Here we present a comparative study of isogenic mutants deficient in either or both Tbps from the isotype I strain B16B6 and isotype II strain M982. We show that TbpA is essential in both strains for iron uptake and growth with iron-loaded human Tf as a sole iron source. No growth has also been observed for the TbpB- mutant of strain B16B6, as shown previously, whereas the growth of the analogous mutant in M982 was similar to that in the wild type. This indicates that TbpB in the latter strain plays a facilitating but not essential role in iron uptake, which has been observed previously in similar studies of other bacteria. These data are discussed in relation to the fact that isotype II strains represent more than 80% of serogroup B meningococcal strains. The contribution of both subunits in the bacterial virulence of strain M982 has been assessed in a murine model of bacteremia. Both the TbpB- TbpA- mutant and the TbpA- mutant are shown to be nonvirulent in mice, whereas the virulence of the TbpB- mutant is similar to that of the wild type.

Assessment of Helicobacter pylori gene expression within mouse and human gastric mucosae by real-time reverse transcriptase PCR

Despite increasing knowledge on the biology of Helicobacter pylori, little is known about the expression pattern of its genome during infection. While mouse models of infection have been widely used for the screening of protective antigens, the reliability of the mouse model for gene expression analysis has not been assessed. In an attempt to address this question, we have developed a quantitative reverse transcriptase PCR (RT-PCR) that allowed the detection of minute amounts of mRNA within the gastric mucosa. The expression of four genes, 16S rRNA, ureA (encoding urease A subunit), katA (catalase), and alpA (an adhesin), was monitored during the course of a 6-month infection of mice and in biopsy samples from of 15 infected humans. We found that the selected genes were all expressed within both mouse and human infected mucosae. Moreover, the relative abundance of transcripts was the same (16S rRNA > ureA > katA > alpA), in the two models. Finally, results obtained with the mouse model suggest a negative effect of bacterial burden on the number of transcripts of each gene expressed per CFU (P < 0.05 for 16S rRNA, alpA, and katA). Overall, this study demonstrates that real-time RT-PCR is a powerful tool for the detection and quantification of H. pylori gene expression within the gastric mucosa and strongly indicates that mice experimentally infected with H. pylori provide a valuable model for the analysis of bacterial gene expression during infection.

Allelic diversity of the two transferrin binding protein B gene isotypes among a collection of Neisseria meningitidis strains representative of serogroup B disease: implication for the composition of a recombinant TbpB-based vaccine

The distribution of the two isotypes of tbpB in a collection of 108 serogroup B meningococcal strains belonging to the four major clonal groups associated with epidemic and hyperendemic disease (the ET-37 complex, the ET-5 complex, lineage III, and cluster A4) was determined. Isotype I strains (with a 1.8-kb tbpB gene) was less represented than isotype II strains (19.4 versus 80.6%). Isotype I was restricted to the ET-37 complex strains, while isotype II was found in all four clonal complexes. The extent of the allelic diversity of tbpB in these two groups was studied by PCR restriction analysis and sequencing of 10 new tbpB genes. Four major tbpB gene variants were characterized: B16B6 (representative of isotype I) and M982, BZ83, and 8680 (representative of isotype II). The relevance of these variants was assessed at the antigenic level by the determination of cross-bactericidal activity of purified immunoglobulin G preparations raised to the corresponding recombinant TbpB (rTbpB) protein against a panel of 27 strains (5 of isotype I and 22 of isotype II). The results indicated that rTbpB corresponding to each variant was able to induce cross-bactericidal antibodies. However, the number of strains killed with an anti-rTbpB serum was slightly lower than that obtained with an anti-TbpA(+)B complex. None of the sera tested raised against an isotype I strain was able to kill an isotype II strain and vice versa. None of the specific antisera tested (anti-rTbpB or anti-TbpA(+)B complex) was able to kill all of the 22 isotype II strains tested. Moreover, using sera raised against the C-terminus domain of TbpB M982 (amino acids 352 to 691) or BZ83 (amino acids 329 to 669) fused to the maltose-binding protein, cross-bactericidal activity was detected against 12 and 7 isotype II strains, respectively, of the 22 tested. These results suggest surface accessibility of the C-terminal end of TbpB. Altogether, these results show that although more than one rTbpB will be required in the composition of a TbpB-based vaccine to achieve a fully cross-bactericidal activity, rTbpB and its C terminus were able by themselves to induce cross-bactericidal antibodies.

Human antibody responses to A and C capsular polysaccharides, IgA1 protease and transferrin-binding protein complex stimulated by infection with Neisseria meningitidis of subgroup IV-1 or ET-37 complex

The protein sequences of the IgA1 protease, TbpA and TbpB proteins differ between meningococci representative of serogroup A, subgroup IV-1 from epidemic disease in The Gambia and serogroup C, ET-37 complex from endemic disease in Mali. The uniformity of restriction endonuclease sites was determined for the iga, tbpA and thpB genes among strains of both clonal lineages. Rare isolates had acquired a variant thpAB operon by horizontal genetic exchange but all other strains were uniform within each clonal lineage. The quantitative levels of IgG to capsular polysaccharide, IgA1 protease and TBP complex were measured in paired acute phase and convalescent phase sera from The Gambia and from Mali using antigens from the homologous clonal lineages. IgG levels to these antigens were also measured in paired sera from healthy Gambians who permanently carried meningococci in the nasopharynx or did not. The results showed that disease stimulated IgG to each antigen in Mali and to all but TBP complex in The Gambia. Similarly, higher levels of IgG were found in sera from permanent carriers than in sera from permanent non-carriers. Acute phase sera from Mali contained low levels of IgG to C capsular polysaccharide (geometric mean value of 0.3 microg ml(-1)) while such sera from The Gambia contained higher and potentially protective levels of IgG to A polysaccharide (geometric mean of 5.5 microg ml(-1)). The concentrations of IgG to TBP complex in acute phase sera were higher and IgG to IgA1 protease was even higher, suggesting that intermediate levels of IgG to these proteins do not protect against disease.

Both the full-length and the N-terminal domain of the meningococcal transferrin-binding protein B discriminate between human iron-loaded and apo-transferrin

We have readdressed the ability of the transferrin-binding protein B (TbpB) from Neisseria meningitidis to discriminate between the iron-loaded and the iron-free human transferrin (hTf) by using the BIAcore technology, a powerful experimental technique for the observation of direct interactions between a receptor and its ligands, without the use of labels. Recombinant full-length TbpB from five N. meningitidis strains were produced and purified from Escherichia coli as fusion proteins. They showed a preference for the binding to iron-loaded hTf. As for the full-length molecule, we have demonstrated that the minimal N-terminal hTf binding domain of meningococcal TbpB from B16B6 and M982 strains was able to discriminate between both hTf forms.

Identification of human transferrin-binding sites within meningococcal transferrin-binding protein B

Transferrin-binding protein B (TbpB) from Neisseria meningitidis binds human transferrin (hTf) at the surface of the bacterial cell as part of the iron uptake process. To identify hTf binding sites within the meningococcal TbpB, defined regions of the molecule were produced in Escherichia coli by a translational fusion expression system and the ability of the recombinant proteins (rTbpB) to bind peroxidase-conjugated hTf was characterized by Western blot and dot blot assays. Both the N-terminal domain (amino acids [aa] 2 to 351) and the C-terminal domain (aa 352 to 691) were able to bind hTf, and by a peptide spot synthesis approach, two and five hTf binding sites were identified in the N- and C-terminal domains, respectively. The hTf binding activity of three rTbpB deletion variants constructed within the central region (aa 346 to 543) highlighted the importance of a specific peptide (aa 377 to 394) in the ligand interaction. Taken together, the results indicated that the N- and C-terminal domains bound hTf approximately 10 and 1000 times less, respectively, than the full-length rTbpB (aa 2 to 691), while the central region (aa 346 to 543) had a binding avidity in the same order of magnitude as the C-terminal domain. In contrast with the hTf binding in the N-terminal domain, which was mediated by conformational epitopes, linear determinants seemed to be involved in the hTf binding in the C-terminal domain. The host specificity for transferrin appeared to be mediated by the N-terminal domain of the meningococcal rTbpB rather than the C-terminal domain, since we report that murine Tf binds to the C-terminal domain. Antisera raised to both N- and C-terminal domains were bactericidal for the parent strain, indicating that both domains are accessible at the bacterial surface. We have thus identified hTf binding sites within each domain of the TbpB from N. meningitidis and propose that the N- and C-terminal domains together contribute to the efficient binding of TbpB to hTf with their respective affinities and specificities for determinants of their ligand.

Heterogeneity of tbpB, the transferrin-binding protein B gene, among serogroup B Neisseria meningitidis strains of the ET-5 complex

ET-5 complex strains of Neisseria meningitidis were traced intercontinentally and have been causing hyperendemic meningitis on a worldwide scale. In an attempt to develop a fully broad cross-reactive transferrin-binding protein B (TbpB)-based vaccine, we undertook to assess the extent of variability of TbpB proteins among strains of this epidemiological complex. For this purpose, a PCR-based method was developed to study the heterogeneity of the tbpB genes from 31 serogroup B N. meningitidis strains belonging to the ET-5 complex. To define adequate primers, the tbpB gene from an ET-5 complex strain, 8680 (B:15:P1.3; isolated in Chile in 1987), was cloned and the nucleotide sequence was determined and compared to two other previously published tbpB sequences. A tbpB fragment was amplified from genomic DNA from each of the 31 strains. By this method, heterogeneity in size was observed and further characterized by restriction pattern analysis with four restriction enzymes and by sequencing tbpB genes from three other ET-5 complex strains. Four distinct tbpB gene types were identified. Fifty-five percent of the strains studied (17/31) harbored tbpB genes similar to that of strain BZ83 (B:15:-) isolated in The Netherlands in 1984. Ten of the 31 strains (32.2%) had tbpB genes close to that of strain M982. Only 3 of the 31 (9.6%) were found to harbor tbpB genes close to that of strain 8680, and finally one strain, 8710 (B:15:P1.3; isolated in Chile in 1987), was found to harbor a tbpB gene different from all the others. These results demonstrated a pronounced variability among tbpB alleles within a limited number of ET-5 complex strains collected over a 19-year period. Despite the genetic heterogeneity observed, specific antisera raised to purified Tbps from ET-5 complex strains showed broad cross-reactivity between different TbpBs both by Western blot analysis and bactericidal assay, confirming that a limited number of TbpB molecules included in a vaccine are likely to induce broadly cross-reactive antibodies against the different strains.

Evaluation of recombinant transferrin-binding protein B variants from Neisseria meningitidis for their ability to induce cross-reactive and bactericidal antibodies against a genetically diverse collection of serogroup B strains

Transferrin-binding protein B (TbpB) is a surface-exposed protein, variable among strains of Neisseria meningitidis, that has been considered as a vaccine candidate. To define a TbpB molecule that would give rise to broadly cross-reactive antibodies with TbpB of many strains, specific antisera were produced against three recombinant TbpB variants from strain M982: one corresponding to the full-length TbpB; one in which stretches of amino acids located in the central part of the molecule, described as hypervariable, have been deleted; and one corresponding to the N-terminal half of the molecule, described as the human transferrin binding domain. The reactivity of these antisera against 58 serogroup B strains with a 2.1-kb tbpB gene representing different genotypes, serotypes, and subtypes and different geographic origins was tested on intact meningococcal cells. In parallel, the bactericidal activity of the antisera was evaluated against 15 of the 58 strains studied. Of the 58 strains, 56 (98%) reacted with the antiserum specific for the N-terminal half of TbpB M982; this antiserum was bactericidal against 9 of 15 strains (60%). On the other hand, 43 of 58 strains reacted with the antiserum raised to full-length TbpB while 12 of 15 (80%) were killed with this antiserum. The antiserum specific to TbpB deleted of its central domain gave intermediate results, with 53 of 58 strains (91.3%) recognized and 10 of 15 (66.6%) killed. These results indicate that the N-terminal half of TbpB was sufficient to induce cross-reactive antibodies reacting with the protein on meningococcal cells but that the presence of the C-terminal half of the protein is necessary for the induction of cross-bactericidal antibodies.

Molecular characterization of hybrid Tbp2 proteins from Neisseria meningitidis

Transferrin-binding protein 2 (Tbp2) from Neisseria is an outer membrane-associated extracellular lipoprotein that is involved in iron capture within the infected host. The analysis of tbp2 clones isolated from various bacterial strains revealed extensive divergences throughout the open reading frame (ORF), with predicted amino acid (aa) sequences displaying 47% to 83% identity. Such a variability is likely to have resulted from the selective pressure exerted by the host immune system, but raises questions regarding the existing constraints for conservation of protein function. Indeed, the neisserial Tbp2s include a large structured domain, extending throughout the N-terminal half of the protein (approximately 270-290 aa), which is extremely stable and whose conformational integrity is required for efficient binding to human transferrin (hTf). In this work, a functional study of Tbp2s encoded by hybrid genes constructed by reassorting highly divergent tbp2 sequences in the region of the ORF encoding this structured domain was performed. The data demonstrate that the determinant intramolecular interactions allowing formation of a stable Tbp2 structure able to interact efficiently with hTf or/and that the Tbp2 residues involved in the interaction with hTf are not well conserved. However, a number of rearrangements appeared to generate genes encoding proteins which have retained structural stability and hTf-binding capacity. This suggested that despite the extreme aa sequence divergence and the conformational constraints, horizontal genetic exchanges, which are known to occur in neisserial populations, may have contributed significantly to the generation of sequence variation within tbp2 ORFs. The analysis of two tbp2 clones characterized in this work supports this hypothesis.

Variable sequences in a mosaic-like domain of meningococcal tbp2 encode immunoreactive epitopes

Transferrin-binding proteins from Neisseria meningitidis vary among different isolates. We have identified and studied a hypervariable region adjacent to the carboxyl-end of the transferrin-binding domain of the Tbp2 molecule. The tbp2 genes from six strains of N. meningitidis were cloned and sequenced in this particular region. Sequence analysis of these regions along with five other sequences available from pathogenic Neisseria showed a common organisation of seven highly variable nucleotide stretches interspersed with six conserved nucleotide stretches. The variable regions correlated with the location of immunoreactive epitopes in polyclonal antisera raised to transferrin-binding proteins identified by peptide pin technology. Sequence analysis suggested a mosaic-like organisation of the tbp2 genes. Taken together, these data suggest that the antigenic variation in this part of the protein may result from a strong host immune pressure.

Diversity of the transferrin-binding protein Tbp2 of Neisseria meningitidis

In order to investigate the genetic basis for the observed polymorphism amongst meningococcal transferrin-binding proteins, Tbp2, the corresponding genes of different Neisseria meningitidis strains were cloned and sequenced. Comparison of the deduced amino acid (aa) sequences indicated that the Tbp2 were 76.6 to 81.2% homologous. Several stretches of aa have been found repeated both in the N- and C-terminal halves of the molecule.

Evaluation of transferrin-binding protein 2 within the transferrin-binding protein complex as a potential antigen for future meningococcal vaccines

Because the meningococcal transferrin receptor was shown to elicit bactericidal and protective antibodies in laboratory animals, we undertook a study of the protective role of each of the polypeptides within the Tbp1-Tbp2 complex. We developed a procedure to purify from Neisseria meningitidis B16B6 the two proteins in milligram amounts and raised specific antisera in rabbits and mice. Only antisera specific for Tbp2 displayed bactericidal activity against the parent strain. Mice immunized with purified Tbp2 survived a lethal challenge to a similar degree as animals immunized with the Tbp1-Tbp2 complex, demonstrating that Tbp2 played an important role in the protective activity observed with the complex. Both Tbp1- and Tbp2-specific antisera inhibited transferrin binding to the purified receptor in a solid-phase binding assay, suggesting that the antibodies were able to interact with the Tbp1 molecule only when it was removed from its membrane environment. Finally, Tbp2-specific immunoglobulins were able to lower the growth rate of the meningococci when human transferrin was their sole iron source. Therefore, in all four different systems tested, Tbp2 or antibodies specific for Tbp2 displayed biological characteristics close to those of the Tbp1-Tbp2 complex. This suggests that Tbp2 plays an important role in the protective activity of the complex, eliciting antibodies that are not only bactericidal but also inhibitory for meningococcal growth.

Transferrin-binding proteins isolated from Neisseria meningitidis elicit protective and bactericidal antibodies in laboratory animals

Transferrin-binding proteins (Tbps) were affinity-isolated from group B Neisseria meningitidis strain B16B6 and used to raise specific antisera. Administration of the antisera to mice loaded with human transferrin before bacterial challenge significantly protected the animals from death. In active immunization studies, mice received three 25 micrograms injections of purified Tbps over a period of 28 days, 7 days after which they were challenged with N. meningitidis. The survival rate in immunized mice was much higher than in control groups. In both active and passive immunization experiments mice were protected against at least 100 LD50. A specific Tbp antiserum was highly bactericidal against the parent strain and against approximately half of the strains tested.

Cloning and characterization of Neisseria meningitidis genes encoding the transferrin-binding proteins Tbp1 and Tbp2

Genes tbp1 and tbp2, encoding the transferrin-binding proteins Tbp1 and Tbp2, have been isolated from two strains of Neisseria meningitidis. The tbp2 and tbp1 open reading frames are tandemly arranged in the genome with an 87-bp intergenic region, and the DNA region upstream from the tbp2-coding sequence contains domains homologous to Escherichia coli promoter consensus motives. Nucleotide sequence analysis suggests the existence of a Tbp1 precursor carrying an N-terminal signal peptide with a peptidase I cleavage site and of a Tbp2 precursor with N-terminal homology to lipoproteins, including a peptidase II cleavage site. Comparison of the Tbp1 deduced amino acid (aa) sequences from both strains showed about 76% aa homology, while those of Tbp2 revealed only about 47% aa homology. These comparisons should be extended to other Neisseria strains in order to evaluate further this genetic divergence further.

Identification of two major families of transferrin receptors among Neisseria meningitidis strains based on antigenic and genomic features

The transferrin receptor or transferrin-binding proteins (Tbps) of 50 strains of Neisseria meningitidis belonging to different serogroups were examined by Western blotting using two rabbit antisera raised against Tbp purified from N. meningitidis strains B16B6 and M982. On the basis of the reactivity of Tbp2 with the antisera two patterns were observed and allowed the classification of 74% of the strains in group I (M982-like strains) and 26% in group II (B16B6-like strains). Southern blot analysis was performed on the genomic DNA of 16 meningococcal strains and showed that under stringent conditions, the tbp2 probes were specific for each group identified. Both immunological and genomic analyses have led to the identification within N. meningitidis strains of two major families distinguished on the basis of the characteristics of Tbp2 molecules, independently of serogroup, type or subtype.

Bordetella pertussis and Bordetella parapertussis: two immunologically distinct species

Bordetella pertussis and Bordetella parapertussis are closely related species. Both are responsible for outbreaks of whooping cough in humans and produce similar virulence factors, with the exception of pertussis toxin, specific to B. pertussis. Current pertussis whole-cell vaccine will soon be replaced by acellular vaccines containing major adhesins (filamentous hemagglutinin and pertactin) and major toxin (pertussis toxin). All of these factors are antigens that stimulate a protective immune response in the murine respiratory model and in clinical assays. In the present study, we examined the protective efficacies of these factors, and that of adenylate cyclase-hemolysin, another B. pertussis toxin, against B. parapertussis infection in a murine respiratory model. As expected, pertussis toxin did not protect against B. parapertussis infection, since this bacterium did not express this protein, but the surprising result was that none of the other factors were protective against B. parapertussis infection. Furthermore, B. parapertussis adenylate cyclase-hemolysin, although it protected against B. parapertussis infection, did not protect against B. pertussis infection. Despite a high degree of homology between both B. pertussis and B. parapertussis species, no cross-protection was observed. Our results outline the fact that, as in other gram-negative bacteria, Bordetella surface proteins vary immunologically.

Acellular pertussis vaccines: evaluation of reversion in a nude mouse model

An animal model has been developed to assess the safety of acellular pertussis vaccines in terms of reversion to toxicity. Adsorbed pertussis toxoid preparations, alone or combined in a DTP formulation, were administered to nude mice intraperitoneally. In parallel, groups of positive and negative control mice received pertussis toxin and buffer, respectively. The circulating white blood cells of the animals were monitored for 28 days. Mice immunized with glutaraldehyde toxoid preparations did not develop a lymphocytosis during the observation period, whereas mice immunized with an experimental formalin pertussis toxoid vaccine exhibited a high lymphocytosis six days after vaccine administration, demonstrating, in this model, a reversion of the toxoid. The nude mouse model thus appears to reveal the in-vivo reversion of pertussis toxoids and could be included in the quality control panel for the assessment of the safety of acellular pertussis vaccine.