Antigenic and molecular heterogeneity of the transferrin-binding protein ofNeisseria meningitidis

Meningococcal protein antigens and vaccines

The development of a comprehensive vaccine against meningococcal disease has been challenging. Recent developments in molecular genetics have provided both explanations for these challenges and possible solutions. Since genome sequence data became available there has been a marked increase in number of protein antigens that have been suggested as prospective vaccine components. This review catalogues the proposed vaccine candidates and examines the evidence for their inclusion in potential protein vaccine formulations.

Mapping of monoclonal antibodies specific to P64k: A common antigen of several isolates of Neisseria meningitidis

P64k is a minor outer membrane protein from Neisseria meningitidis. This protein has been produced at high levels in Escherichia coli. We generated a group of monoclonal antibodies (mAbs) against recombinant P64k, which recognise four non-overlapping epitopes, as shown using competition assays with biotinylated mAbs. The P64k sequences involved in mAbs binding were mapped with synthetic overlapping peptides derived from the P64k protein, and located in the previously determined three-dimensional structure of the protein. These antibodies were also characterised by whole-cell ELISA and bactericidal tests against N. meningitidis. Only two of the recognised epitopes were exposed on the bacterial surface, and none of the mAbs showed bactericidal activity. The relationship between these results and the structural data on the epitopes bound by the mAbs is discussed.Key words: Neisseria meningitidis, P64k, monoclonal antibodies, epitope mapping.

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.

Analysis of the human Ig isotype response to individual transferrin binding proteins A and B from Neisseria meningitidis

Subcapsular antigens, including transferrin binding proteins, are being considered as potential vaccines against serogroup B meningococci. This study examined the human isotype antibody responses in cases of meningococcal disease to meningococcal TbpA (transferrin binding protein A) and TbpB (transferrin binding protein B) from two strains (SD and B16B6) expressing high and low molecular mass TbpB respectively. TbpA isolated from both strains were recognised more frequently and higher durable ELISA absorbance values were detected than those detected against TbpB from either strain. These antibody responses to Tbps were independent of the infecting meningococcal strain type. The antibody response to the four proteins was highly variable between individuals and differed significantly against all four antigens. The variability of immune responses to each Tbp from the two strains suggests that a successful vaccine would need to include TbpA and TbpB from a number of strains.

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.

Sequence analysis of the structural tbpA gene: protein topology and variable regions within neisserial receptors for transferrin iron acquisition

The gene coding for the 98-kDa meningococcal outer membrane transferrin binding protein 1 (TbpA) from strain B385 was cloned and sequenced. Sequence comparison among its deduced aminoacid sequence and those from TbpA and the closely related LbpA (lactoferrin binding protein) gene from three different meningococcal strains, and four isolates from two other bacterial pathogens, showed that TbpA variability is confined to five specific segments, designated VR1 (199-287), VR2 (306-381), VR3 (480-546), VR4 (618-651) and VR5 (681-708). The third VR was the most variable among strains both at the nucleotide and amino acid levels. Six additional tbpA genes from different meningococcal strains were cloned and its VR3 sequence determined. On the basis of this data we were able to cluster tbpA genes in two groups: D (bearing a deletion in VR3) and N (nondeleted); all N and D strains belonging to the groups of high or low molecular weight transferrin receptor isotype, respectively. However, by phenogram analysis, the prototypical strain M982 (Group II) was clustered with M990 (B16B6 isotype, Group I). These results point to the existence of important exposed regions as well as to the possibility of horizontal gene exchange involving this locus. A topology model with 14 exposed loops and 28 membrane spanning segments was postulated. According to this tentative analysis, TbpA as well as LbpA proteins should form a gated channel in the neisserial outer membrane. The variable regions were located in the fifth, sixth, eighth, 10th and 11th loops respectively. Among TbpAs VR1, VR2, and VR3 resulted the most relevant regions.

Antigenicity, cross-reactivity and surface exposure of the Neisseria meningitidis 37 kDa protein (Fbp)

The 37 kDa iron-repressible protein, Fbp, was purified from two Neisseria meningitidis strains by metal-affinity chromatography and used to obtain mouse monospecific polyclonal immune sera. Dot-blot, immunoblotting and whole cell ELISA results demonstrate that the Fbp is present in all 16 N. meningitidis and four commensal Neisseria species tested, is highly antigenic in mouse when injected in pure form, and shows intra- and inter-species antigenic homogeneity, anti-Fbp antibodies being fully cross-reactive using the techniques mentioned. We also found that Fbp molecules (or parts of them) are surface exposed, in disagreement with the proposed exclusively periplasmic localization, although anti-Fbp antibodies seem unable to block iron uptake or to induce complement-mediated killing of the meningococci. Taken along with the high immunogenicity of the purified protein and the complete cross-reactivity of the antibodies elicited, this suggests that the protective effect of the purified Fbp must be further studied to evaluate its inclusion in future vaccine trials.

Mapping of functional regions on the transferrin-binding protein (TfbA) of Actinobacillus pleuropneumoniae

Actinobacillus pleuropneumoniae can use porcine transferrin as the sole source of iron. Two proteins with molecular masses of approximately 60 kDa (TfbA) and 110 kDa have been shown to specifically bind porcine transferrin; from the TfbA protein, three isoforms from A. pleuropneumoniae serotypes 1, 5, and 7 have been identified and characterized by nucleotide sequence analysis. Here we defined the transferrin-binding region(s) of the TfbA protein of A. pleuropneumoniae serotype 7 by TnphoA mutagenesis, random mutagenesis, and peptide spot synthesis. The amino-terminal half of the TfbA molecule, which has only 36% amino acid sequence identity among the three isoforms, was shown to be responsible for transferrin binding by TnphoA mutagenesis. This result was confirmed by analysis of six random mutants with decreased transferrin binding affinity. The subsequent analysis of overlapping 16-mer peptides comprising the amino-terminal half of the TfbA molecule revealed three domains of 13 or 14 amino acids in length with transferrin-binding activity. They overlapped, or were very close to, point mutations decreasing transferrin-binding ability. The first and third domains were unique to the TfbA protein of A. pleuropneumoniae serotype 7. In contrast, the sequence of the second domain was present in almost identical forms (12 of 14 residues) in the TfbA proteins of A. pleuropneumoniae serotypes 1 and 5; in addition, a sequence consisting of functionally homologous amino acids was present in the otherwise completely distinct small transferrin-binding proteins of Neisseria gonorrhoeae (TbpB), N. meningitidis (Tbp2), and Haemophilus influenzae (Tbp2).

Human antibody response to meningococcal transferrin binding proteins: evidence for vaccine potential

During iron-limited growth Neisseria meningitidis expresses two transferrin binding proteins, TBP1 and TBP2, with molecular masses of approximately 98 and 65-90 kDa depending on strain. Mixtures of TBP1 and TBP2 (TBP1 + 2) from three meningococcal strains were purified using affinity chromatography and used to determine anti-TBP antibodies in human sera by ELISA. Sera were obtained from healthy individuals, asymptomatic carriers of N. meningitidis and cases of meningococcal disease. Healthy individuals had little detectable antibody to TBPs but sera from carriers and cases exhibited a response demonstrating that TBPs are expressed in vivo during both carriage and disease. The ELISA absorbances produced by each of the individual sera to TBPs from the three meningococcal strains were compared and very high correlation coefficients were obtained, indicating that human anti-TBP antibodies, in contrast to mouse and rabbit antibodies, are cross-reactive between strains. Antibodies to separately purified TBP1 and TBP2 were also detected in both cases and carriers. The IgG and IgM response to TBP1 + 2 was greater in cases than carriers but the mean IgA response was the same. This demonstration of an antibody response that is cross-reactive between TBP types greatly strengthens the case for inclusion of TBPs in a meningococcal vaccine to protect against all serogroups and serotypes.

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.

Reliability of laboratory models in the analysis of TBP2 and other meningococcal antigens

The lack of experimental models suitable for the study of meningococcal pathogenicity led us to investigate if those actually in use (culture in iron-restricted media and animal models) provide results comparable with the responses observed in vivo during infection. In this work we studied three invasive strains cultured both in laboratory media and in human plasma, analysing the immune responses elicited in mice against membrane antigens and comparing them with those seen using homologous human convalescent sera. Outer membrane protein profiles observed after culture in plasma were different and more complex than those obtained after growth in laboratory media. Analogous differences were observed in the antigenic profiles, detecting some antigens recognized by human, but not mouse sera, and vice versa. However, the response to one of the major iron-regulated outer membrane antigens, the transferrin binding protein 2 (TBP2), was unaffected by the culture medium or the model, human or mouse, used for the analysis. In conclusion, we have found that results of antigenic analysis change depending on the culture conditions and animal models used. For the meningococcal antigen TBP2, growth in iron-restricted laboratory media and a mouse model provide results which correlate well with those observed using convalescent human serum from individuals recovered from infections. We suggest that careful analysis and evaluation of experimental results and their comparison with in vivo elicited immune responses are essential in order to get accurate extrapolations for experimental vaccine designs.

Characterization of a highly structured domain in Tbp2 from Neisseria meningitidis involved in binding to human transferrin

The binding of iron-loaded human transferrin at the surface of Neisseria meningitidis is mediated by two polypeptides, Tbp1 and Tbp2. Predicted Tbp amino acid sequences from N. meningitidis strains are highly divergent. This variability is particularly pronounced throughout the Tbp2 polypeptide. In this study, a highly structured and extremely stable Tbp2 domain of about 270 to 290 amino acids which is involved in the binding to transferrin and whose position is well conserved has been characterized. The conservation of such a remarkable structure in a very divergent protein domain (there is only 43% amino acid identity within this region) suggests that is plays an essential biological role and raises a number of questions regarding tbp2 evolution.

In vivo human immune response to transferrin-binding protein 2 and other iron-regulated proteins of Neisseria meningitidis

When grown under iron restriction, Neisseria meningitidis expresses new outer-membrane proteins, some of which are antigenic and potentially useful as vaccine components. This is particularly relevant to N. meningitidis serogroup B, against which neither polysaccharide nor conjugate vaccines are effective. We investigated recognition of N. meningitidis serogroup B outer-membrane antigens by three sera from patients recovered from meningitis. Recognition of antigens from the homologous strain provided information on in vivo expression during infection and immunogenicity, while cross-reactivity with outer membrane proteins from the other two strains and from another five strains in our collection allowed evaluation of antigenic heterogeneity. Our results demonstrate that transferrin-binding protein 2 (TBP2) is immunogenic in humans, to varying degrees depending on the strain, and that TBP2s (like the equivalent proteins of Haemophilus influenzae type b) are among the most important iron-regulated outer membrane antigens expressed during infection. Other immunogenic outer membrane proteins (some iron-regulated) are also expressed during infection; in a previous study in mouse, three of these proteins (with M(r) of 50, 70 and 77 kDa) did not induce an immune response. Our cross-reactivity data provide some support for Robki et al.'s two-group classification of N. meningitidis strains, and provide evidence against the possibility that the antigenic domains shared by the TBP2s of all N. meningitidis strains induce immune responses in vivo.

Iron uptake mechanisms of pathogenic bacteria

Most of the iron in a mammalian body is complexed with various proteins. Moreover, in response to infection, iron availability is reduced in both extracellular and intracellular compartments. Bacteria need iron for growth and successful bacterial pathogens have therefore evolved to compete successfully for iron in the highly iron-stressed environment of the host's tissues and body fluids. Several strategies have been identified among pathogenic bacteria, including reduction of ferric to ferrous iron, occupation of intracellular niches, utilisation of host iron compounds, and production of siderophores. While direct evidence that high affinity mechanisms for iron acquisition function as bacterial virulence determinants has been provided in only a small number of cases, it is likely that many if not all such systems play a central role in the pathogenesis of infection.

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.

Characterization of the transferrin-iron uptake system in Neisseria meningitidis

The transferrin-iron uptake system of six Neisseria meningitidis strains was characterized using 125I-transferrin in receptor assays and 55Fe-loaded transferrin in uptake assays. Receptors for transferrin varied among the strains both in number (from 700 to 4700 receptors per cell) and in their affinity constants for the protein (Ka ranged from 0.7 x 10(7) to 4.0 x 10(7) l mol-1). Neither receptor numbers nor affinity constants were significantly different in carrier and invasive strains, although the Ka seem to be somewhat higher in the latter. Iron uptake from transferrin was also variable among the strains, but showed the same lack of correlation with their origin.

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.

Neisseria meningitidis transferrin-binding protein 1 expressed in Escherichia coli is surface exposed and binds human transferrin

A gene library of Neisseria meningitidis B15 P1.16 DNA was established in lambda Zap II and clones containing DNA encoding transferrin binding protein 1 (TBP-1) identified following hybridisation with a 63-bp DNA probe based on the codon assignment for the first 21 N-terminal amino acids of TBP-1. Sequencing of the cloned DNA demonstrated that all of the intergenic DNA (i.e. upstream of tbp-1 running through to the 3' end of the transferrin-binding protein 2 gene) and approx. 15% of tbp-1 had been cloned. The complete gene was generated using a polymerase chain reaction, with the primer for the 3' end being based on tbp-A of N. gonorrhoeae, and the approx. 2.9-kb DNA product cloned into pGem-3Z. The expressed protein (approx. 100 kDa) reacted with antiserum to an N-terminal peptide of TBP-1. In addition, the native product was surface-expressed by Escherichia coli and bound human transferrin.

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.

Purification of the Neisseria meningitidis transferrin binding protein-2 (TBP2) to homogeneity using column chromatography

A method for purifying TBP2 from N. meningitidis has been developed using affinity chromatography on Tf-agarose followed by ion exchange chromatography; the Tf-binding activity of fractions was evaluated by a dot-blot technique. This method allowed the purification of the TBP2 in milder conditions than those used previously and to a high degree of homogeneity as was demonstrated by Coomassie brilliant blue or Silver staining after SDS-PAGE electrophoresis. The SDS-PAGE profile of the material obtained in the Tf-agarose affinity chromatography step shows only two detectable proteins, identified as the TBP1 and the TBP2, with a small amount of contamination. Passage through a MonoQ HR anion exchange column, allowed the isolation of TBP2 in the absence of TBP1. Our results demonstrate the conservation of the antigenic reactivity of this protein, which produces monospecific serum with the antibodies elicited reacting exclusively with the TBP2 in whole outer membrane vesicles.

Antigenic relationships of transferrin-binding proteins from Neisseria meningitidis, N. gonorrhoeae and Haemophilus influenzae: cross-reactivity of antibodies to NH2-terminal peptides

The NH2-terminal amino sequence through the first 20 amino acids was obtained for transferrin-binding protein (TBP)1 from three strains of Neisseria meningitidis. These were identical except for a glutamine to a glycine substitution at residue 6 in one case. The sequences of the NH2-terminal 20 amino acids of TBP2 from the same three strains were also determined; one TBP2 had a M(r) of 68,000 and the other two of 78,000. Sequences were identical up to residue 13 in all three proteins. Peptides based on the NH2-terminal sequences of TBP1 and 2 were synthesized, linked to Keyhole Limpet haemocyanin and used to raise antibodies in rabbits. Anti-peptide antibodies cross-reacted on immunoblotting with the respective TBPs from all meningococcal strains tested, as well as with those from N. gonorrhoeae, suggesting that the NH2-terminals of these proteins are well conserved in the Neisseria. Neither anti-peptide serum reacted with the analogous TBP1 and 2 from Haemophilus influenzae, although common epitopes have previously been shown to exist.

Evidence for in vivo expression of transferrin-binding proteins in Haemophilus influenzae type b

Haemophilus influenzae type b acquires transferrin-bound iron via a siderophore-independent mechanism involving direct contact between the human iron-binding glycoprotein and the bacterial cell surface. Evidence has accumulated to show that the transferrin receptor consists of at least two iron-regulated outer membrane transferrin-binding proteins (TBPs), of which one has a molecular mass of around 100 kDa (TBP1) and the other has a molecular mass of 60 to 90 kDa (TBP2). In H. influenzae type b strain Eagan, proteins of 76, 90, and 107 kDa appear to be involved in transferrin binding. To determine whether these TBPs are expressed during growth in vivo, strain Eagan was recovered without subculture from the intraperitoneal cavities of infected infant rats. By using a dot blot assay, outer membranes prepared from these in vivo-grown bacteria, unlike those grown in iron-sufficient broth, bound human transferrin and produced the 76-, 90-, and 107-kDa TBPs. Immunoblotting experiments using convalescent sera from infected rats also revealed the presence of antibodies to the 76- and 90-kDa strain Eagan TBPs. In addition, convalescent sera from three of four patients recovering from H. influenzae type b meningitis contained antibodies to the 90- and 105-kDa TBPs of the corresponding infecting strain. Furthermore, fresh clinical isolates of H. influenzae type b recovered from blood and cerebrospinal fluid showed constitutive expression of the TBPs, which became iron regulated only after prolonged in vitro subculture on iron-sufficient medium. This contrasted with the laboratory-adapted Eagan strain, in which the TBPs remained iron regulated even after animal passage. These findings indicate that the H. influenzae type b transferrin receptor is expressed during experimental animal and human infections.

Effect of porphyrins and host iron transport proteins on outer membrane protein expression in Porphyromonas (Bacteroides) gingivalis: identification of a novel 26 kDa hemin-repressible surface protein

Porphyromonas gingivalis is capable of in vitro growth when iron sources are either complexed to hemin or host iron transport proteins, or exist in an inorganic form. This study examined the effect of these iron sources on outer membrane protein (OMP) expression in P. gingivalis W50. Hemin (iron) starved P. gingivalis was transferred into growth medium containing hemin, hemoglobin, hemin-saturated human serum albumin, hemin-free human serum albumin, transferrin, lactoferrin, or inorganic iron. Surface proteins were identified by 125I-labeling and resolved by SDS-PAGE and autoradiography. When grown under hemin starved conditions, P. gingivalis W50 and related strains expressed a major 26 kDa OMP, as revealed by 125I-autoradiography. Autoradiographic analysis demonstrated the absence of this 26 kDa OMP from the P. gingivalis surface in hemin-containing environments. Growth of P. gingivalis W50 in the presence of host iron transport proteins (hemin-free) or inorganic iron resulted in surface expression of a 26 kDa OMP. The presence of protoporphyrin IX or substitution of hemin-associated iron with zinc, resulted in continued surface expression of the 26 kDa OMP, indicating that repressibility of this OMP required the coordination of iron to the protoporphyrin IX molecule (i.e. hemin). A survey of 125I-labeled OMPs from several hemin starved P. gingivalis and related strains, demonstrated that a hemin-repressible 26 kDa OMP occurred only in P. gingivalis. We report here a newly described 26 kDa hemin-regulated surface protein occurring in several strains of P. gingivalis which is expressed on the cell surface in hemin starved conditions and is lost from the cell surface in response to an environment containing iron coordinated specifically to protoporphyrin IX (i.e. hemin).

Immunogenicity and antigenic heterogeneity of a human transferrin-binding protein in Neisseria meningitidis

Growing Neisseria meningitidis on an iron restriction medium induces the synthesis of new outer membrane proteins, some of them true iron-regulated outer membrane proteins (IROMPs) and others synthesized because of the stress produced by the iron restriction. Some of these proteins are antigenic and can be considered for the development of vaccines; this is especially desirable in the case of N. meningitidis serogroup B, for which polysaccharide vaccines are not efficient. The antigenicity of N. meningitidis 37- and 70-kDa IROMPs has been studied previously; in this work, we studied the immunogenicity and antigenic heterogeneity of another IROMP, the human transferrin-binding protein 2 (TBP2), which seems to be indispensable for meningococcal growth inside the host. Mice were inoculated with purified outer membrane vesicles (blebs) from 5 selected N. meningitidis strains, and the five serum samples obtained were analyzed for anti-TBP2 antibodies by using the homologous strain and for cross-reactivity with the TBP2 of the 4 other selected strains and another 35 heterologous N. meningitidis strains. The TBP2s of the 5 strains tested were all immunogenic in mice to various degrees depending on the strain, and all five TBP2s shared one or more epitopes with heterologous strains (as shown by the cross-reactivities of the five serum samples), although the number of cross-reacting strains was very variable, ranging from 2 for strain V002 to 35 for strain P391. This suggests that the TBP2 epitopes of different strains differ in nature or in their accessibility to the immune system. Under the iron restriction conditions used, all strains synthesized a non-TBP2 antigenic 56-kDa protein thought to be a stress protein.

Common antigenic domains in transferrin-binding protein 2 of Neisseria meningitidis, Neisseria gonorrhoeae, and Haemophilus influenzae type b

There is now considerable evidence to show that in the Neisseria and Haemophilus species, membrane receptors specific for either transferrin or lactoferrin are involved in the acquisition of iron from these glycoproteins. In Neisseria meningitidis, the transferrin receptor appears to consist of two proteins, one of which (TBP 1) has an M(r) of 95,000 and the other of which (TBP 2) has an M(r) ranging from 68,000 to 85,000, depending on the strain; TBP 2 binds transferrin after sodium dodecyl sulfate-polyacrylamide gel electrophoresis and electroblotting, but TBP 1 does not do so. The relative contributions of these two proteins to the binding reaction observed with intact cells and to iron uptake are presently unknown. However, they are being considered as potential components of a group B meningococcal vaccine. Analogous higher- and lower-molecular-weight proteins associated with transferrin binding have been found in N. gonorrhoeae and Haemophilus influenzae. Previous work with polyclonal antibodies raised in mice with whole cells of iron-restricted N. meningitidis showed that the meningococcal TBP 2 exhibits considerable antigenic heterogeneity. Here, we report that antiserum against purified TBP 2 from one strain of N. meningitidis cross-reacts on immunoblotting with the TBP 2 of all meningococcal isolates examined, as well as with the TBP 2 of N. gonorrhoeae. This antiserum also cross-reacted with the TBP 2 of several strains of H. influenzae type b, thus showing the presence of common antigenic domains among these functionally equivalent proteins in different pathogens; no cross-reaction was detected with a purified sample of the human transferrin receptor.

Transferrins and heme-compounds as iron sources for pathogenic bacteria

The low concentration of free iron in body fluids creates bacteriostatic conditions for many microorganisms and is therefore an important defense factor of the body against invading bacteria. Pathogenic bacteria have developed several mechanisms for acquiring iron from the host. Siderophore-mediated iron uptake involves the synthesis of low molecular weight iron chelators called siderophores which compete with the host iron-binding glycoproteins lactoferrin (LF) and transferrin (TF) for iron. Other ways to induce iron uptake, without the mediation of siderophores, are the possession of outer membrane protein receptors that actually recognize the complex of TF or LF with iron, resulting in the internalization of this metal, and the use of heme-compounds released into the circulation after lysis of erythrocytes. In this review, the nonsiderophore-mediated iron-uptake systems used by certain pathogenic bacteria are emphasized. The possible contribution of these iron-uptake systems to the virulence of pathogens is also discussed.

Analysis of the molecular mass heterogeneity of the transferrin receptor in Neisseria meningitidis and commensal Neisseria

Peroxidase-conjugated transferrin was used to detect transferrin receptors both in intact outer membrane vesicles (OMVs) from Neisseria species in a dot blot assay, and in SDS-PAGE-separated OMV proteins after transferring to nitrocellulose membranes. All N. meningitidis strains produced transferrin receptors after culturing in either iron sufficiency or iron restriction although expression was higher in the latter case, whereas only six N. lactamica and two N. sicca (among 20 commensal species) were able to bind transferrin. Molecular mass (MM) of the receptors were mainly between 78 kDa and 85 kDa (87.5% of strains), 12.5% had receptors with MM close to 70 kDa, and 5% showed receptors with MM over 85 kDa. Our results confirm the molecular mass heterogeneity of the transferrin receptors in N. meningitidis, completely disagree with the 'universal' 98 kDa receptor proposed by some authors, and show a low expression of the receptor in commensal Neisseria.

Iron and bacterial virulence--a brief overview

Iron is now recognized as playing a vital role in infection. Not only does it restricted availability in tissue fluids present microbial pathogens with the problem of acquiring sufficient for multiplication in vivo, but it also constitutes a major environmental signal which co-ordinately regulates the expression of a number of virulence and metabolic genes. Progress in understanding the strategies used by pathogens for acquiring iron in vivo, and their responses to iron restriction, is providing a fresh insight into microbial pathogenicity.