Production of Neisseria meningitidis transferrin-binding protein B by recombinant Bordetella pertussis

Neisseria meningitidis serogroup B infections are among the major causes of fulminant septicemia and meningitis, especially severe in young children, and no broad vaccine is available yet. Because of poor immunogenicity of the serogroup B capsule, many efforts are now devoted to the identification of protective protein antigens. Among those are PorA and, more recently, transferrin-binding protein B (TbpB). In this study, TbpB of N. meningitidis was genetically fused to the N-terminal domain of the Bordetella pertussis filamentous hemagglutinin (FHA), and the fha-tbpB hybrid gene was expressed in B. pertussis either as a plasmid-borne gene or as a single copy inserted into the chromosome. The hybrid protein was efficiently secreted by the recombinant strains, despite its large size, and was recognized by both anti-FHA and anti-TbpB antibodies. A single intranasal administration of recombinant virulent or pertussis-toxin-deficient, attenuated B. pertussis to mice resulted in the production of antigen-specific systemic immunoglobulin G (IgG), as well as local IgG and IgA. The anti-TbpB serum antibodies were of the IgG1, IgG2a, and IgG2b isotypes and were found to express complement-mediated bactericidal activity against N. meningitidis. These observations indicate that recombinant B. pertussis may be a promising vector for the development of a mucosal vaccine against serogroup B meningococci.

The heparin-binding haemagglutinin of M. tuberculosis is required for extrapulmonary dissemination

Tuberculosis remains the world's leading cause of death due to a single infectious agent, Mycobacterium tuberculosis, with 3 million deaths and 10 million new cases per year. The infection initiates in the lungs and can then spread rapidly to other tissues. The availability of the entire M. tuberculosis genome sequence and advances in gene disruption technologies have led to the identification of several mycobacterial determinants involved in virulence. However, no virulence factor specifically involved in the extrapulmonary dissemination of M. tuberculosis has been identified to date. Here we show that the disruption of the M. tuberculosis or Mycobacterium bovis Bacille Calmette-Guérin (BCG) hbhA gene encoding the heparin-binding haemagglutinin adhesin (HBHA) markedly affects mycobacterial interactions with epithelial cells, but not with macrophage-like cells. When nasally administered to mice, the mutant strains were severely impaired in spleen colonization, but not in lung colonization. Coating wild-type mycobacteria with anti-HBHA antibodies also impaired dissemination after intranasal infection. These results provide evidence that adhesins such as HBHA are required for extrapulmonary dissemination, and that interactions with non-phagocytic cells have an important role in the pathogenesis of tuberculosis. They also suggest that antibody responses to HBHA may add to immune protection against tuberculosis.

Biochemical characterization of acyl carrier protein (AcpM) and malonyl-CoA:AcpM transacylase (mtFabD), two major components of Mycobacterium tuberculosis fatty acid synthase II

Malonyl coenzyme A (CoA)-acyl carrier protein (ACP) transacylase (MCAT) is an essential enzyme in the biosynthesis of fatty acids in all bacteria, including Mycobacterium tuberculosis. MCAT catalyzes the transacylation of malonate from malonyl-CoA to activated holo-ACP, to generate malonyl-ACP, which is an elongation substrate in fatty acid biosynthesis. To clarify the roles of the mycobacterial acyl carrier protein (AcpM) and MCAT in fatty acid and mycolic acid biosynthesis, we have cloned, expressed, and purified acpM and mtfabD (malonyl-CoA:AcpM transacylase) from M. tuberculosis. According to the culture conditions used, AcpM was produced in Escherichia coli in two or three different forms: apo-AcpM, holo-AcpM, and palmitoylated-AcpM, as revealed by electrospray mass spectrometry. The mtfabD gene encoding a putative MCAT was used to complement a thermosensitive E. coli fabD mutant. Expression and purification of mtFabD resulted in an active enzyme displaying strong MCAT activity in vitro. Enzymatic studies using different ACP substrates established that holo-AcpM constitutes the preferred substrate for mtFabD. In order to provide further insight into the structure-function relationship of mtFabD, different mutant proteins were generated. All mutations (Q9A, R116A, H194A, Q243A, S91T, and S91A) completely abrogated MCAT activity in vitro, thus underlining the importance of these residues in transacylation. The generation and characterization of the AcpM forms and mtFabD opens the way for further studies relating to fatty acid and mycolic acid biosynthesis to be explored in M. tuberculosis. Since a specific type of FabD is found in mycobacterial species, it represents an attractive new drug target waiting to be exploited.

[Cellular and acellular anti-pertussis vaccines]

Since 1940, vaccination with whole cell vaccines, composed of Bordetella pertussis extracts, has dramatically reduced the incidence of whooping cough. However, the occurrence of side effects has stimulated research that has resulted in the design of new, acellular vaccines. These vaccines are composed of adhesins, such as filamentous haemagglutinin and pertactin, and detoxified pertussis toxin. Detoxification can be achieved by genetic engineering resulting in alterations of specific amino acid residues that are involved in the enzymatic activity or in the target-cell receptor binding activity of the toxin. Assessment on laboratory animals and in clinical trials have indicated that these acellular vaccines are almost as protective as the whole cell vaccines, and that they are much less reactogenic. However, these vaccines are more expensive and still need multiple injections to achieve optimal protection. An interesting alternative would be the construction of attenuated strains able to protect after a single intranasal administration. In addition, the construction of recombinant attenuated strains would perhaps simultaneously protect against whooping cough and other infectious or parasitic diseases.

Expression of the putative siderophore receptor gene bfrZ is controlled by the extracytoplasmic-function sigma factor BupI in Bordetella bronchiseptica

A new gene from Bordetella bronchiseptica, bfrZ encoding a putative siderophore receptor, was identified in a Fur-repressor titration assay. A bfrZ null mutant was constructed by allelic exchange. The protein profile of this mutant is similar to that of the wild-type parent strain. The BfrZ(-)-BfrZ(+) isogenic pair was tested for utilization of 132 different siderophores as iron sources. None of these iron sources acted as a ligand for BfrZ. Translational bfrZ::phoA and transcriptional bfrZ::lacZ fusions were introduced into the B. bronchiseptica bfrZ locus. No alkaline phosphatase or beta-galactosidase activity was detected. Sequence analysis of the bfrZ upstream region revealed the presence of two tightly linked genes, bupI and bupR. Both of these genes are located downstream from a Fur-binding sequence. BupI is homologous to Escherichia coli FecI and Pseudomonas putida PupI and belongs to the family of extracytoplasmic-function sigma factors involved in transcription of genes with extracytoplasmic functions. BupR is homologous to the FecR and PupR antisigma factors and is predicted to be localized in the inner membrane. Similar to the surface signaling receptors FecA and PupB, BfrZ bears an N-terminal extension. We found that bfrZ is not transcribed when bupI and bupR are expressed at the same level. However, overexpression of bupI from a multicopy plasmid triggers bfrZ transcription, and under these conditions BfrZ was detected in membrane fractions. By analogy with the FecI-FecR-FecA and PupI-PupR-PupB systems, our data suggest that bfrZ expression is inducible by binding of the cognate ligand to BfrZ and transduction of a signal through the envelope.

Two-partner secretion in Gram-negative bacteria: a thrifty, specific pathway for large virulence proteins

A collection of large virulence exoproteins, including Ca2+-independent cytolysins, an iron acquisition protein and several adhesins, are secreted by the two-partner secretion (TPS) pathway in various Gram-negative bacteria. The hallmarks of the TPS pathway are the presence of an N-proximal module called the 'secretion domain' in the exoproteins that we have named the TpsA family, and the channel-forming beta-barrel transporter proteins we refer to as the TpsB family. The genes for cognate exoprotein and transporter protein are usually organized in an operon. Specific secretion signals are present in a highly conserved region of the secretion domain of TpsAs. TpsBs probably serve as specific receptors of the TpsA secretion signals and as channels for the translocation of the exoproteins across the outer membrane. A subfamily of transporters also mediates activation of their cognate cytolysins upon secretion. The exoproteins are synthesized as precursors with an N-terminal cleavable signal peptide, and a subset of them carries an extended signal peptide of unknown function. According to our current model, the exoproteins are probably translocated across the cytoplasmic membrane in a Sec-dependent fashion, and their signal peptide is probably processed by a LepB-type signal peptidase. The N-proximal secretion domain directs the exoproteins towards their transporters early, so that translocation across both membranes is coupled. The exoproteins transit through the periplasm in an extended conformation and fold progressively at the cell surface before eventually being released into the extracellular milieu. Several adhesins also undergo extensive proteolytic processing upon secretion. The genes of many new TpsAs and TpsBs are found in recently sequenced genomes, suggesting that the TPS pathway is widespread.

Characterization of the type III secretion locus of Bordetella pertussis

Multiple sequence comparisons of proteins of the LcrD/FlbF family allowed the design of primers that specifically amplify sequences coding for type III secretion components. Amplification of Bordetella pertussis DNA with these primers yielded a fragment that was further used as a probe for screening a genomic library. The nucleotide sequence of a positive clone revealed a 2100-bp gene, called bcrD, which specifies a 75-kDa polypeptide homologous to the Yersinia LcrD protein. Chromosome walking allowed the characterization of a 35-kb DNA segment that contains the entire locus and flanking housekeeping genes. The B. pertussis type III secretion locus consists of more than 30 open reading frames (ORFs), most of which are identical to annotated genes of Bordetella spp and share similarities with known type III secretion genes of related bacteria. In order to assess the function of this locus, we engineered a bcrD null mutant. However, none of the tested phenotypes, such as protein secretion, cellular invasion, cytotoxicity or mouse lung colonization, differentiated the mutant from its parental strain. Studies of bcrD and bscN expressions indicated that, under our experimental conditions, these genes are not expressed in vitro. Restriction analyses on pulsed-field gel electrophoresis allowed the type III locus mapping at coordinate position 1,590 kb on the Tohama I strain chromosome.

Interaction of Bordetella pertussis with mast cells, modulation of cytokine secretion by pertussis toxin

Together with macrophages and dendritic cells, mast cells have recently been shown to interact with certain pathogenic bacteria and present microbial antigens to the immune system. We show here that Bordetella pertussis can adhere to and be phagocytosed by mast cells. In addition, mast cells are able to process and present B. pertussis antigens to T lymphocytes. Furthermore, exposure of mast cells to B. pertussis induced the release of the proinflammatory cytokines tumour necrosis factor alpha (TNF-alpha) and interleukin 6 (IL-6). The release of IL-6 was strongly reduced by pertussis toxin expressed by B. pertussis. The production of IL-10, but not that of IL-4, by mast cells was also inhibited by pertussis toxin. Depletion of mast cells in vivo resulted in significant reduction of early TNF-alpha production in bronchoalveolar lavage (BAL) fluids of B. pertussis-infected mice. These data suggest that mast cells may play a role in the induction of immune responses against B. pertussis through the release of cytokines, especially TNF-alpha.

High-resolution minisatellite-based typing as a portable approach to global analysis of Mycobacterium tuberculosis molecular epidemiology

The worldwide threat of tuberculosis to human health emphasizes the need to develop novel approaches to a global epidemiological surveillance. The current standard for Mycobacterium tuberculosis typing based on IS6110 restriction fragment length polymorphism (RFLP) suffers from the difficulty of comparing data between independent laboratories. Here, we propose a high-resolution typing method based on variable number tandem repeats (VNTRs) of genetic elements named mycobacterial interspersed repetitive units (MIRUs) in 12 human minisatellite-like regions of the M. tuberculosis genome. MIRU-VNTR profiles of 72 different M. tuberculosis isolates were established by PCR analysis of all 12 loci. From 2 to 8 MIRU-VNTR alleles were identified in the 12 regions in these strains, which corresponds to a potential of over 16 million different combinations, yielding a resolution power close to that of IS6110-RFLP. All epidemiologically related isolates tested were perfectly clustered by MIRU-VNTR typing, indicating that the stability of these MIRU-VNTRs is adequate to track outbreak episodes. The correlation between genetic relationships inferred from MIRU-VNTR and IS6110-RFLP typing was highly significant. Compared with IS6110-RFLP, high-resolution MIRU-VNTR typing has the considerable advantages of being fast, appropriate for all M. tuberculosis isolates, including strains that have a few IS6110 copies, and permitting easy and rapid comparison of results from independent laboratories. This typing method opens the way to the construction of digital global databases for molecular epidemiology studies of M. tuberculosis.

Bordetella pertussis, molecular pathogenesis under multiple aspects

Recent studies, including those based on genomics, have demonstrated that besides toxins and adhesins, Bordetella pertussis uses many additional virulence determinants. Most of them are part of the BvgAS regulon, although some, in particular iron-uptake systems, are independent of BvgAS. They are regulated by iron, although in one case, the production of a siderophore receptor could be linked to the BvgAS regulon.

Immunostimulatory effect of IL-18-encoding plasmid in DNA vaccination against murine Schistosoma mansoni infection

In vivo delivery of DNA encoding antigens is a simple tool to induce immune responses against pathogens. This approach to vaccination also offers the possibility to codeliver plasmids encoding immunomodulatory molecules in order to drive immune responses towards optimal protective effects. In the murine model of Schistosoma mansoni infection, vaccination inducing a Th1 profile has been shown to be protective. In this study, we used a plasmid encoding the Th1-promoting cytokine IL-18, since we observed that percutaneous infection of Balb/c mice strongly induced the production of IL-18 mRNA in the skin. Intradermal injection of the IL-18-encoding plasmid prior to infection did not interfere with parasite migration through the skin although it led to a local and transient cellular infiltration. When the IL-18-encoding plasmid was codelivered with a S. mansoni glutathione S-transferase (Sm28GST)-encoding plasmid, a 30-fold increase of antigen-specific IFN-gamma secretion by spleen cells was observed in comparison to spleen cells from mice that had received only the Sm28GST-encoding plasmid. This immunostimulatory effect was related to a significant protective effect (28% reduction in egg laying and 23% reduction in worm burden) which was attributed to a cooperative effect between both plasmids. Therefore, this study shows that codelivery of an IL-18-encoding plasmid with an antigen-encoding plasmid can stimulate specific cellular responses and induce protective effects against S. mansoni infection.

Mycobacterium smegmatis laminin-binding glycoprotein shares epitopes with Mycobacterium tuberculosis heparin-binding haemagglutinin

Mycobacterium tuberculosis, the causative agent of tuberculosis, produces a heparin-binding haemagglutinin adhesin (HBHA), which is involved in its epithelial adherence. To ascertain whether HBHA is also present in fast-growing mycobacteria, Mycobacterium smegmatis was studied using anti-HBHA monoclonal antibodies (mAbs). A cross-reactive protein was detected by immunoblotting of M. smegmatis whole-cell lysates. However, the M. tuberculosis HBHA-encoding gene failed to hybridize with M. smegmatis chromosomal DNA in Southern blot analyses. The M. smegmatis protein recognized by the anti-HBHA mAbs was purified by heparin-Sepharose chromatography, and its amino-terminal sequence was found to be identical to that of the previously described histone-like protein, indicating that M. smegmatis does not produce HBHA. Biochemical analysis of the M. smegmatis histone-like protein shows that it is glycosylated like HBHA. Immunoelectron microscopy demonstrated that the M. smegmatis protein is present on the mycobacterial surface, a cellular localization inconsistent with a histone-like function, but compatible with an adhesin activity. In vitro protein interaction assays showed that this glycoprotein binds to laminin, a major component of basement membranes. Therefore, the protein was called M. smegmatis laminin-binding protein (MS-LBP). MS-LBP does not appear to be involved in adherence in the absence of laminin but is responsible for the laminin-mediated mycobacterial adherence to human pneumocytes and macrophages. Homologous laminin-binding adhesins are also produced by virulent mycobacteria such as M. tuberculosis and Mycobacterium leprae, suggesting that this adherence mechanism may contribute to the pathogenesis of mycobacterial diseases.

New virulence-activated and virulence-repressed genes identified by systematic gene inactivation and generation of transcriptional fusions in Bordetella pertussis

An in silico scan of the partially completed genome sequence of Bordetella pertussis and analyses of transcriptional fusions generated with a new integrational vector were used to identify new potential virulence genes. The genes encoding a putative siderophore receptor, adhesins, and an autotransporter protein appeared to be regulated in a manner similar to Bordetella virulence genes by the global virulence regulator BvgAS. In contrast, the gene encoding a putative intimin-like protein appeared to be repressed under conditions of virulence.

Genomics of Bordetella pertussis toxins

Bordetella pertussis, the etiologic agent of whooping cough, produces numerous toxins including pertussis toxin (PTX), adenylate cyclase toxin (AC), dermonecrotic toxin (DNT) and tracheal cytotoxin (TCT). PTX is composed of five different subunits organised in a typical A-B type structure of which the A part possesses an enzymatic ADP-ribosyltransferase activity and the B moiety expresses receptor-binding activity. The secretion of this toxin requires nine other genes (ptl) organised in an operon together with the five structural genes of PTX. To further characterise the genetic locus of this major virulence factor, we analysed the ptx/ptl upstream and downstream sequences. Comparison of these regions between three species of Bordetella (B. pertussis, Bordetella parapertussis and Bordetella bronchiseptica) revealed differences in the upstream region. Analysis of two strains of B. bronchiseptica naturally lacking the ptx genes showed that only the ptx/ptl genes were deleted in these strains, and that the upstream and downstream regions were conserved. Upstream of the PTX structural genes and the promoter, an open reading frame (bugT) was identified, the product of which is homologous with putative proteins from several other Gram-negative organisms. Detailed analysis of the genome of B. pertussis which is currently sequenced at the Sanger Centre revealed the presence of 90 genes coding for proteins homologous to BugT, which qualifies the bug gene family as the most populated one of Bordetella. These bug genes are located in various genetic environments, including the proximities of genes coding for other toxins, such as DNT and AC. The Bug proteins are highly conserved in terms of size and periodicity of predicted secondary structure elements, but have also a high variability in their amino acid composition reflected in their wide range of isoelectric points. The function of these genes which is currently unknown is under investigation. To characterise the expression and regulation of these genes, as well as of novel putative B. pertussis virulence factors, we designed a transcriptional fusion vector to be inserted in precise locations of the B. pertussis chromosome by homologous recombination. The reporter gene present in this vector allowed us to show that at least some of the bug genes are expressed.

Intracellular trafficking and membrane translocation of pertussis toxin into host cells

The translocation of the pertussis toxin (PTX) S1 subunit into the cytoplasm of host cells was analysed in CHO cells producing S1 fused to a signal peptide. This protein channelled into the endoplasmic reticulum (ER) by the signal peptide, was found to ADP-ribosylate its target G proteins, suggesting that membrane translocation can occur from the ER and does not require the B oligomer. Similar results were obtained with a C-terminally truncated S1 subunit, indicating that this hydrophobic tail is not involved in the translocation mechanism. We also analysed the activity of two PTX mutants in which the S3 and S2 subunits were substituted for each other. The mutant protein containing two S3 subunits (PTXAS2) presented a decreased binding to fetuin or haptoglobin but higher in vivo activity than the wild-type PTX, suggesting that replacement of S2 by S3 favours the targeting of PTX to the compartment where translocation occurs and/or the dissociation of S1 from the B oligomer, thereby leading to a better translocation of S1 into the cytoplasm.

Novel topological features of FhaC, the outer membrane transporter involved in the secretion of the Bordetella pertussis filamentous hemagglutinin

Many pathogenic Gram-negative bacteria secrete virulence factors across the cell envelope into the extracellular milieu. The secretion of filamentous hemagglutinin (FHA) by Bordetella pertussis depends on the pore-forming outer membrane protein FhaC, which belongs to a growing family of protein transporters. Protein alignment and secondary structure predictions indicated that FhaC is likely to be a beta-barrel protein with an odd number of transmembrane beta-strands connected by large surface loops and short periplasmic turns. The membrane topology of FhaC was investigated by random insertion of the c-Myc epitope and the tobacco etch virus protease-specific cleavage sequence. FhaC was fairly permissive to short linker insertions. Furthermore, FhaC appeared to undergo conformational changes upon FHA secretion. Surface detection of the inserted sequences indicated that several predicted loops in the C-terminal moiety as well as the N terminus of the protein are exposed. However, a large surface-predicted region in the N-terminal moiety of FhaC was inaccessible from the surface. In addition, the activity and the stability of the protein were affected by insertions in that region, indicating that it may have important structural and/or functional roles. The surface exposure of the N terminus and the presence of an odd number of beta-strands are novel features for beta-barrel outer membrane proteins.

Activation of the pro-drug ethionamide is regulated in mycobacteria

The anti-tuberculosis drug ethionamide (ETH), which is a structural analog of isoniazid (INH), is known to strongly inhibit mycolic acid synthesis in Mycobacterium tuberculosis. Although several targets have been identified for INH, only speculative information is available concerning ETH. Mutations within the promoter and the coding region of enoyl-acyl carrier protein reductase (InhA) were found to confer resistance to both drugs, thus leading to the impression that INH and ETH may share a common mode of action. However, a notable distinction between the two drugs lies in the lack of cross-resistance in clinical isolates. This may be attributed in part to the fact that the pro-drug INH must be activated via KatG, and no activation step for ETH has yet been described. Here we report the identification of an activator for ETH. The ETH activator (Rv3854c), which we have termed EthA, was found to be homologous to various monooxygenases and induced ETH sensitivity when overexpressed in mycobacteria. Interestingly, the neighboring open reading frame (Rv3855), which was found homologous to transcriptional repressors of the tetR family, led to ETH resistance when overexpressed. In addition, chromosomal inactivation of this gene by transposition led to ETH hypersensitivity. These data strongly suggest that Rv3855, which we have termed EthR, regulates the production of EthA, which subsequently activates the pro-drug ETH. This study opens up new avenues of research relating to ETH activation in mycobacteria, possibly leading to an improved efficacy of ETH and to the generation of new anti-mycobacterial agents.

Interleukin-18 modulates immune responses induced by HIV-1 Nef DNA prime/protein boost vaccine

Many different HIV-1 vaccine strategies have been developed, but as yet none has been completely successful. Promising results from combined DNA prime/protein boost vaccines have been reported. Specific immune responses generated by DNA vaccines can be modulated by the co-delivery of genes coding for cytokines. In this study, we have used the intradermal route by needle injection of a plasmid coding for the HIV-1 Nef accessory protein. We show that DNA prime/protein boost vaccine combinations increase the humoral and cellular immune responses against HIV-1 Nef and that the co-injection of DNA encoding Interleukin-18 (IL-18) modulates the specific immune response towards a Th1 type.

Ineffective cellular immune response associated with T-cell apoptosis in susceptible Mycobacterium bovis BCG-infected mice

It has previously been reported that inhibition of delayed-type hypersensitivity-mediating functions of T cells during mycobacterial infection in mice is haplotype dependent. In the present study, we show that Mycobacterium bovis BCG infection induced, in susceptible C57BL/6 and BALB/c mice but not in resistant C3H/HeJ and DBA/2 mice, an important splenomegaly. An in vitro defect in T-cell proliferation in response to T-cell receptor (TCR) stimulation with mitogens or anti-CD3 antibodies was associated with enhanced levels of CD4(+) and CD8(+) T-cell apoptosis in susceptible but not in resistant mice 2 weeks after infection. Further investigations of C57BL/6 and C3H/HeJ mice revealed that in vivo splenomegaly was associated with destruction of the lymphoid tissue architecture, liver cellular infiltrates, and increased numbers of apoptotic cells in both spleen and liver tissue sections. Infection of C57BL/6 mice but not of C3H/HeJ mice induced massive production of tumor necrosis factor alpha (TNF-alpha) in serum, as well as an increase in Fas and Fas ligand (FasL) expression in T cells. In vitro addition of neutralizing anti-TNF-alpha antibodies led to a significant reduction in CD3-induced T-cell apoptosis of both CD4(+) and CD8(+) T cells of C57BL/6 mice, while the blockade of Fas-FasL interactions reduced apoptosis only in CD4(+) but not in CD8(+) T cells. Together, these results suggest that TNF-alpha and Fas-FasL interactions play a role in the activation-induced cell death (AICD) process associated with a defect in T-cell proliferation of the susceptible C57BL/6 mice. T-cell death by apoptosis may represent one of the important components of the ineffective immune response against mycobacterium-induced immunopathology in susceptible hosts.

Characterization of the heparin-binding site of the mycobacterial heparin-binding hemagglutinin adhesin

The mycobacterial adhesin heparin-binding hemagglutinin (HBHA) contains several lysine-rich repeats at its carboxyl-terminal end. Using truncated recombinant HBHA forms and hybrid proteins containing HBHA repeats grafted onto the Escherichia coli maltose-binding protein (MBP), we found that these repeats are responsible for heparin binding. Immunofluorescence microscopy studies revealed that their deletion abrogates binding of HBHA to human pneumocytes. Conversely, when fused to MBP, the HBHA repeats confer pneumocyte adherence properties to the hybrid protein. Treatment of pneumocytes with glycosaminoglycan-degrading enzymes showed that HBHA binding depends on the presence of heparan sulfate chains on the cell surface. The epitope of a monoclonal antibody that inhibits mycobacterial adherence to epithelial cells was mapped within the lysine-rich repeats, confirming their involvement in mycobacterial adherence to epithelial cells. Surface plasmon resonance analyses showed that recombinant HBHA binds to immobilized heparin with fast association kinetics (k(a) = 5.62 (+/- 0.10) x 10(5) m(-1) s(-1)), whereas the dissociation kinetics were slower (k(d) = 0.015 (+/- 0.002) s(-1)), yielding a K(D) value of 26 nm. Similar analyses with grafted MBP indicated similar kinetic constants, indicating that the carboxyl-terminal repeats contain the entire heparin-binding site of HBHA. The molecular characterization of the interactions of HBHA with epithelial glycosaminoglycans should help to better understand mycobacterial adherence within the lungs and may ultimately lead to new approaches for therapy or immunoprophylaxis.

Variable human minisatellite-like regions in the Mycobacterium tuberculosis genome

Mycobacterial interspersed repetitive units (MIRUs) are 40-100 bp DNA elements often found as tandem repeats and dispersed in intergenic regions of the Mycobacterium tuberculosis complex genomes. The M. tuberculosis H37Rv chromosome contains 41 MIRU loci. After polymerase chain reaction (PCR) and sequence analyses of these loci in 31 M. tuberculosis complex strains, 12 of them were found to display variations in tandem repeat copy numbers and, in most cases, sequence variations between repeat units as well. These features are reminiscent of those of certain human variable minisatellites. Of the 12 variable loci, only one was found to vary among genealogically distant BCG substrains, suggesting that these interspersed bacterial minisatellite-like structures evolve slowly in mycobacterial populations.

Bordetella pertussis TonB, a Bvg-independent virulence determinant

In gram-negative bacteria, high-affinity iron uptake requires the TonB/ExbB/ExbD envelope complex to release iron chelates from their specific outer membrane receptors into the periplasm. Based on sequence similarities, the Bordetella pertussis tonB exbB exbD locus was identified on a cloned DNA fragment. The tight organization of the three genes suggests that they are cotranscribed. A putative Fur-binding sequence located upstream from tonB was detected in a Fur titration assay, indicating that the tonB exbB exbD operon may be Fur-repressed in high-iron growth conditions. Putative structural genes of the beta-subunit of the histone-like protein HU and of a new two-component regulatory system were identified upstream from tonB and downstream from exbD, respectively. A B. pertussis DeltatonB exbB::Km(r) mutant was constructed by allelic exchange and characterized. The mutant was impaired for growth in low-iron medium in vitro and could not use ferrichrome, desferal, or hemin as iron sources. Levels of production of the major bacterial toxins and adhesins were similar in the TonB(+)/TonB(-) pair. The DeltatonB exbB mutant was still responsive to chemical modulators of virulence; thus, the BvgA/BvgS two-component system is not TonB dependent. Nevertheless, in vivo in the mouse respiratory infection model, the colonization ability of the mutant was reduced compared to the parental strain.

Genital antibody responses in mice after intranasal infection with an attenuated candidate vector strain of Bordetella pertussis

Intranasal administration of live attenuated Bordetella pertussis, from which the pertussis toxin gene has been deleted, has previously been shown to give rise to high levels of serum immunoglobulin G (IgG) antibodies against both the protective antigen filamentous hemagglutinin (FHA) and heterologous antigens genetically fused to FHA. Here, we extend these results by demonstrating that anti-FHA IgA and IgG antibodies are also produced in the genital tract of mice, both in the vagina and in the uterus, after a single intranasal administration of B. pertussis. By comparing the immune responses induced after infection with wild-type virulent B. pertussis with that induced by infection with an attenuated pertussis toxin-deficient strain, we conclude that pertussis toxin produced by the virulent bacteria does not modify antibody production to FHA in the genital tract of B. pertussis-infected mice. The intranasal infection with either the attenuated or the virulent B. pertussis strain also led to the development of immunologic memory that could be efficiently boosted with purified FHA administered either intranasally or intravaginally to give rise to a significant increase in the levels of specific IgA and IgG produced locally in the genital tract, as well as of specific antibodies in the serum. These observations suggest that attenuated B. pertussis could be a promising vector for intranasal administration to induce antibody responses against antigens from sexually transmitted pathogens fused to FHA.

Channel formation by FhaC, the outer membrane protein involved in the secretion of the Bordetella pertussis filamentous hemagglutinin

Many virulence factors of pathogenic microorganisms are presented at the cell surface. However, protein secretion across the outer membrane of Gram-negative bacteria remains poorly understood. Here we used the extremely efficient secretion of the Bordetella pertussis filamentous hemagglutinin (FHA) to decipher this process. FHA secretion requires a single specific accessory protein, FhaC, the prototype of a family of proteins necessary for the extracellular localization of various virulence proteins in Gram-negative bacteria. We show that FhaC is heat-modifiable and localized in the outer membrane. Circular dichroism spectra indicated that FhaC is rich in beta-strands, in agreement with structural predictions for this protein. We further demonstrated that FhaC forms pores in artificial membranes, as evidenced by single-channel conductance measurements through planar lipid bilayers, as well as by liposome swelling assays and patch-clamp experiments using proteoliposomes. Single-channel conductance appeared to fluctuate very fast, suggesting that the FhaC channels frequently assume a closed conformation. We thus propose that FhaC forms a specific beta-barrel channel in the outer membrane for the outward translocation of FHA.