Functional domains present in the mycobacterial hemagglutinin, HBHA

Immunogenicity and cytoadherence of recombinant heparin binding haemagglutinin (HBHA) of Mycobacterium avium subsp. paratuberculosis: functional promiscuity or a role in virulence?

Mycobacterium avium subsp. paratuberculosis (MAP) is the causative agent of Johne's Disease, a chronic granulomatous enteritis of ruminants. Recently, an association of MAP bacilli with Crohn's disease in humans has been proposed. Due to genetic similarities and serological cross-reactivity of the M. avium complex with other mycobacteria, functional analysis of species-specific proteins may allow new insights into the pathogenesis of mycobacterial diseases. We report production and molecular characterization of the recombinant HBHA from the MAP complex bacilli. The HBHA was expressed in Escherichia coli and Mycobacterium smegmatis using efficient expression vector systems. The recombinant HBHA was found to be immunogenic and therefore induced antibody responses in cattle against the MAP bacilli with a possible cross reactivity with M. bovis infection. The MAP complex HBHA was thus found to be a target of the host humoral responses in Johne's disease. The recombinant HBHA protein was also found to be adherent to the Caco2 cell lines in-vitro, a significant observation to understand possible virulence mechanisms. Since M. tuberculosis HBHA was earlier shown to be involved in dissemination of the tubercle bacilli, the immunogenicity and cytoadherent nature of this MAP protein possibly suggests functional promiscuity.

Patients with pulmonary tuberculosis develop a strong humoral response against methylated heparin-binding hemagglutinin

Reactivities of human sera against selected recombinant Mycobacterium tuberculosis antigens were assessed by enzyme-linked immunosorbent assay. The results obtained indicate that patients with tuberculosis (TB) do not develop a strong humoral response against PE_PGRS and PPE proteins or against the Ag85B and heparin-binding hemagglutinin (HBHA) recombinant antigens. Conversely, purified methylated HBHA was strongly recognized by sera obtained from TB patients compared to controls.

Nanoscale mapping and functional analysis of individual adhesins on living bacteria

Although much progress has been made in the identification and characterization of adhesins borne by pathogenic bacteria, the molecular details underlying their interaction with host receptors remain largely unknown owing to the lack of appropriate probing techniques. Here we report a method, based on atomic force microscopy (AFM) with tips bearing biologically active molecules, for measuring the specific binding forces of individual adhesins and for mapping their distribution on the surface of living bacteria. First, we determined the adhesion forces between the heparin-binding haemagglutinin adhesin (HBHA) produced by Mycobacterium tuberculosis and heparin, used as a model sulphated glycoconjugate receptor. Both the adhesion frequency and adhesion force increased with contact time, indicating that the HBHA-heparin complex is formed via multiple intermolecular bridges. We then mapped the distribution of single HBHA molecules on the surface of living mycobacteria and found that the adhesin is not randomly distributed over the mycobacterial surface, but concentrated into nanodomains.

The mycobacterial heparin-binding hemagglutinin is a protective antigen in the mouse aerosol challenge model of tuberculosis

The heparin-binding hemagglutinin (HBHA) of Mycobacterium tuberculosis is a surface-expressed adhesin that can affect binding to host cells via a unique, methylated, carboxyl-terminal, lysine-, alanine-, and proline-rich repeat region. It has been implicated in extrapulmonary dissemination of M. tuberculosis from the lung following the initial infection of the host. To assess the vaccine potential of this protein, purified preparations of HBHA were emulsified in a dimethyldioctadecylammonium bromide-monophosphoryl lipid A adjuvant and tested for the ability to reduce M. tuberculosis infection in the mouse aerosol challenge model for tuberculosis. The HBHA-containing vaccine gave a approximately 0.7-log reduction in CFU in both mouse lungs and spleens compared to adjuvant controls 28 days following challenge. Although a notable level of serum antibody to HBHA was elicited after three immunizations and the antibodies were able to bind to the surface of M. tuberculosis, passive immunization with monoclonal antibodies directed against HBHA did not protect in the challenge model. Compared to adjuvant controls, an elevated gamma interferon response was generated by splenic and lymph node-derived T cells from immunized mice in the presence of macrophages pulsed with purified HBHA or infected with live M. tuberculosis, suggesting that the effective immunity may be cell mediated. Efforts to construct effective recombinant HBHA vaccines in fast-growing Mycobacterium smegmatis have been unsuccessful so far, which indicates that distinctive posttranslational modifications present in the HBHA protein expressed by M. tuberculosis are critical for generating effective host immune responses. The vaccine studies described here demonstrate that HBHA is a promising new vaccine candidate for tuberculosis.

Expression and purification of recombinant methylated HBHA inMycobacterium smegmatis

The Heparin-Binding Haemagglutinin (HBHA) is a mycobacterial adhesin involved in the dissemination of Mycobacterium tuberculosis from the site of primary infection and a potential candidate for the development of a new vaccine against tuberculosis. Methylation of HBHA is a novel post-translational event that imparts important immunological properties to the protein. Since recombinant HBHA expressed in Escherichia coli is not methylated, we investigated the possibility of producing recombinant methylated HBHA in fast growing mycobacteria for use in immunological and biochemical studies. The complete coding sequence of HBHA was cloned in the plasmid pMV206, under the control of a strong promoter (hsp60) or its own promoter. The constructs generated were electroporated into Mycobacterium smegmatis and the recombinant strains obtained were analyzed for the presence of the HBHA protein using the anti-HBHA monoclonal antibodies D2 and E4. Our results indicate that expression of high amounts of intact protein can be toxic for the mycobacteria, that methylated HBHA can be obtained in M. smegmatis only when using a promoter sequence weaker than hsp60 and that the expression of the complete structural gene is required in order to obtain methylated HBHA. We constructed a recombinant M. smegmatis strain (pMV3-38) that expresses a histidine-tagged methylated HBHA that can be easily purified. The use of fast-growing strains of M. smegmatis to obtain significant amounts of purified HBHA protein within a short timeframe, should be an effective strategy for the evaluation of a new HBHA-based vaccine candidate for tuberculosis.

Extracellular Mycobacterial DNA-binding Protein 1 Participates in Mycobacterium-Lung Epithelial Cell Interaction through Hyaluronic Acid

Mycobacterium tuberculosis infects not only host macrophages but also nonprofessional phagocytes, such as alveolar epithelial cells. Glycosaminoglycans (GAGs) are considered as the component of mycobacterial adherence to epithelial cells. Here we show that extracellularly occurring mycobacterial DNA-binding protein 1 (MDP1) promotes mycobacterial infection to A549 human lung epithelial cells through hyaluronic acid (HA). Both surface plasmon resonance analysis and enzyme-linked immunosorbent assay revealed that MDP1 bound to HA, heparin, and chondroitin sulfate. Utilizing synthetic peptides, we next defined heparin-binding site of 20 amino acids from 31 to 50 of MDP1, which is responsible for the specific DNA-binding site of MDP1. MDP1 bound to A549 cells, and exogenous DNA and HA interfered with the interaction. The binding was also abolished by treatment of A549 cells with hyaluronidase, suggesting that HA participates in the MDP1-A549 cell interaction. Adherence of bacillus Calmette-Guérin (BCG) and M. tuberculosis to A549 cells was inhibited by addition of HA, DNA, and anti-MDP1 antibody, showing that MDP1 participates in the interaction between mycobacteria-alveolar epithelial cells. Simultaneous treatment of intratracheal BCG-infected mice with HA reduced the growth of BCG in vivo. Taken together, theses results suggest that HA participates in Mycobacterium-lung epithelium interaction and has potential for therapeutic and prophylactic interventions in mycobacterial infection.

Methylation-dependent T cell immunity to Mycobacterium tuberculosis heparin-binding hemagglutinin

Although post-translational modifications of protein antigens may be important componenets of some B cell epitopes, the determinants of T cell immunity are generally nonmodified peptides. Here we show that methylation of the Mycobacterium tuberculosis heparin-binding hemagglutinin (HBHA) by the bacterium is essential for effective T cell immunity to this antigen in infected healthy humans and in mice. Methylated HBHA provides high levels of protection against M. tuberculosis challenge in mice, whereas nonmethylated HBHA does not. Protective immunity induced by methylated HBHA is comparable to that afforded by vaccination with bacille Calmette et Guérin, the only available anti-tuberculosis vaccine. Thus, post-translational modifications of proteins may be crucial for their ability to induce protective T cell-mediated immunity against infectious diseases such as tuberculosis.

Decreased infectivity despite unaltered C3 binding by a DeltahbhA mutant of Mycobacterium tuberculosis

HbhA of Mycobacterium tuberculosis is a multifunctional binding protein, binding to both sulfated sugars such as heparin and to human complement component C3. HbhA may therefore interact with host molecules and/or host cells during M. tuberculosis infection and play a role in the pathogenesis of this bacterium. The purpose of this study was to use allelic exchange to create an M. tuberculosis strain deficient in expression of HbhA to determine whether this protein's C3-binding activity plays a role in the pathogenesis of M. tuberculosis. An in-frame, 576-bp unmarked deletion in the hbhA gene was created using sacB as a counterselectable marker. Southern blotting and PCR analyses confirmed deletion of hbhA in the DeltahbhA mutant. The DeltahbhA mutant strain grew at a rate similar to that of the parent in broth culture and in J774.A1 murine macrophage-like cells but was deficient in growth compared to the parent strain in the lungs, liver, and spleen of infected mice. In addition, the DeltahbhA mutation did not reduce binding of M. tuberculosis to human C3 or to J774.A1 cells in the presence or absence of serum, suggesting that in the absence of HbhA, other molecules serve as C3-binding molecules on the M. tuberculosis surface. Taken together, these data indicate that HbhA is important in the infectivity of M. tuberculosis, but its ability to bind C3 is not required for mycobacterial adherence to macrophage-like cells. Using the DeltahbhA mutant strain, a second M. tuberculosis C3-binding protein similar in size to HbhA was identified as HupB, but the role of HupB as a C3-binding protein in intact organisms remains to be determined.

Mycobacterial heparin-binding hemagglutinin and laminin-binding protein share antigenic methyllysines that confer resistance to proteolysis

Mycobacterium tuberculosis and Mycobacterium bovis bacillus Calmette-Guérin produce a heparin-binding hemagglutinin adhesin (HBHA) required for extrapulmonary dissemination and a laminin-binding protein (LBP) involved in cytoadherence through laminin recognition. These adhesins bear posttranslational modifications that are not present when the proteins are produced in a recombinant (r) form in Escherichia coli. Mass spectrometry analysis of HBHA revealed that the posttranslational modifications are borne by the C-terminal moiety, which comprises the heparin-binding domain made of repeated lysine-rich motifs. Amino acid sequencing showed that these modifications consist of mono- and dimethyllysines within these motifs. The methyllysine-containing repeats were recognized by mAb 4057D2 and were also detected in LBP, which is equally recognized by mAb 4057D2. This Ab does not recognize the recombinant forms of these proteins. However, when rHBHA and rLBP were subjected to NaBH(4) and formalin treatment to induce lysine methylation, reactivity with mAb 4057D2 was recovered. Methylated rHBHA displayed enhanced resistance to proteolysis compared with rHBHA, as previously observed for native HBHA. S-adenosylmethionine-dependent HBHA methyltransferase activity was detected in the cell-wall fractions of M. bovis bacillus Calmette-Guérin and of Mycobacterium smegmatis, a species that produces LBP but naturally lacks hbhA, suggesting that the same enzyme(s) methylate(s) both LBP and HBHA. This hypothesis was confirmed by the fact that HBHA produced by recombinant M. smegmatis was also methylated. These results show that mycobacteria use enzymatic methylation of lysines to ensure greater stability of their adhesins.

Enhanced bacterial virulence through exploitation of host glycosaminoglycans

Present in the extracellular matrix and membranes of virtually all animal cells, proteoglycans (PGs) are among the first host macromolecules encountered by infectious agents. Because of their wide distribution and direct accessibility, it is not surprising that pathogenic bacteria have evolved mechanisms to exploit PGs for their own purposes, including mediating attachment to target cells. This is achieved through the expression of adhesins that recognize glycosaminoglycans (GAGs) linked to the core protein of PGs. Some pathogens, such as Bordetella pertussis and Chlamydia trachomatis, may express more than one GAG-binding adhesin. Bacterial interactions with PGs may also facilitate cell invasion or systemic dissemination, as observed for Neisseria gonorrhoeae and Mycobacterium tuberculosis respectively. More-over, pathogenic bacteria can use PGs to enhance their virulence via a shedding of PGs that leads to there lease of effectors that weaken the host defences. The exploitation of PGs by pathogenic bacteria is thus a multifaceted mechanistic process directly related to the potential virulence of a number of microorganisms.

Mycobacterial protein HbhA binds human complement component C3

Mycobacterium tuberculosis and Mycobacterium avium are facultative intracellular pathogens that are able to survive and replicate in mononuclear phagocytes. Human complement component C3 has previously been shown to mediate attachment and phagocytosis of these bacteria by mononuclear phagocytes. In this study, a C3 ligand affinity blot protocol was used to identify a 30-kDa C3-binding protein in M. tuberculosis and Mycobacterium smegmatis and a 31-kDa C3-binding protein in M. avium. The C3-binding proteins in M. tuberculosis and M. avium localized to the cell membrane fraction and partitioned to the detergent fraction during Triton X-114 phase partitioning. The C3-binding protein from M. tuberculosis was partially purified using a cation exchange column and was shown to bind concanavalin A. The N terminus and an internal fragment of the partially purified C3-binding protein were subjected to amino acid sequence analysis. The resulting amino acid sequences matched the M. tuberculosis heparin-binding hemagglutinin (HbhA) protein. Recombinant full-length HbhA and the C terminus of HbhA fused to maltose-binding protein, but not recombinant HbhA lacking the C-terminal region, bound human C3. Recombinant full-length HbhA coated on polystyrene beads, was found to enhance the adherence and/or phagocytosis of the coated beads to J774.A1 cells in both the presence and absence of human serum. The presence of complement-sufficient serum increased the adherence of the HbhA-coated beads to the J774.A1 cells in a C3-dependent manner. If HbhA within the bacterial cell membrane functions similarly to isolated HbhA, this protein may enhance the adherence and phagocytosis of M. tuberculosis and M. avium to mononuclear phagocytes through the binding of C3 and interaction with C3 receptors on mononuclear phagocytes.

Evidence that mycobacterial PE_PGRS proteins are cell surface constituents that influence interactions with other cells

The elucidation of the genomic sequence of Mycobacterium tuberculosis revealed the presence of a novel multigene family designated PE/PE_PGRS that encodes numerous, highly related proteins of unknown function. In this study, we demonstrate that a transposon insertion in a PE_PGRS gene (1818(PE_PGRS)) found in Mycobacterium bovis BCG Pasteur, which is the BCG homologue of the M. tuberculosis H37Rv gene Rv1818c, introduces new phenotypic properties to this BCG strain. These properties include dispersed growth in liquid medium and reduced infection of macrophages. Complementation of the 1818(PE_PGRS)::Tn5367 mutant with the wild-type gene restores both aggregative growth (clumping) in liquid medium and reestablishes infectivity of macrophages to levels equivalent to those for the parent BCG strain. Western blot analysis using antisera raised against the 1818(PE_PGRS) protein shows that PE_PGRS proteins are found in cell lysates of BCG and M. tuberculosis H37Ra and in the cell wall fraction of M. tuberculosis H37Rv. Moreover, immunofluorescent labeling of mycobacteria indicates that certain PE_PGRS proteins are localized at the cell surface of BCG and M. tuberculosis. Together these results suggest that certain PE_PGRS proteins may be found at the surface of mycobacteria and influence both cell surface interactions among mycobacteria as well as the interactions of mycobacteria with macrophages.

Comparative immune response to PE and PE_PGRS antigens of Mycobacterium tuberculosis

Sequencing of the entire genome of Mycobacterium tuberculosis identified a novel multigene family composed of two closely related subfamilies designated PE and PE_PGRS. The major difference between these two families is the presence of a domain containing numerous Gly-Ala repeats extending to the C terminus of the PE_PGRS genes. We have used a representative PE_PGRS gene from M. tuberculosis, Rv1818c (1818PE_PGRS), and its amino-terminal PE region (1818PE), to investigate the immunological response to these proteins during experimental tuberculosis and following immunization with DNA constructs. During infection of mice with M. tuberculosis, a significant humoral immune response was observed against recombinant 1818PE_PGRS but not toward the 1818PE protein. Similarly, immunization with a 1818PE_PGRS DNA construct induced antibodies directed against 1818PE_PGRS but not against 1818PE proteins, and no humoral response was induced by 1818PE DNA. These results suggest that certain PE_PGRS genes are expressed during infection of the host with M. tuberculosis and that an antibody response is directed solely against the Gly-Ala-rich PGRS domain. Conversely, splenocytes from 1818PE-vaccinated mice but not mice immunized with 1818PE_PGRS secreted gamma interferon following in vitro restimulation and demonstrated protection in the mouse tuberculosis challenge model. These results suggest that the PE vaccine can elicit an effective cellular immune response and that immune recognition of the PE antigen is influenced by the Gly-Ala-rich PGRS domain.

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.

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.

Negligible genetic diversity of mycobacterium tuberculosis host immune system protein targets: evidence of limited selective pressure

A common theme in medical microbiology is that the amount of amino acid sequence variation in proteins that are targets of the host immune system greatly exceeds that found in metabolic enzymes or other housekeeping proteins. Twenty-four Mycobacterium tuberculosis genes coding for targets of the host immune system were sequenced in 16 strains representing the breadth of genomic diversity in the species. Of the 24 genes, 19 were invariant and only six polymorphic nucleotide sites were identified in the 5 genes that did have variation. The results document the highly unusual circumstance that prominent M. tuberculosis antigenic proteins have negligible structural variation worldwide. The data are best explained by a combination of three factors: (i) evolutionarily recent global dissemination in humans, (ii) lengthy intracellular quiescence, and (iii) active replication in relatively few fully immunocompetent hosts. The very low level of amino acid diversity in antigenic proteins may be cause for optimism in the difficult fight to control global tuberculosis.