Construction of a modified vector for efficient purification of recombinant Mycobacterium tuberculosis proteins expressed in Escherichia coli

Immunological Characterization of Proteins Expressed by Genes Located in Mycobacterium tuberculosis-Specific Genomic Regions Encoding the ESAT6-like Proteins

The 6 kDa early secreted antigen target (ESAT6) is a low molecular weight and highly immunogenic protein of Mycobacterium tuberculosis with relevance in the diagnosis of tuberculosis and subunit vaccine development. The gene encoding the ESAT6 protein is located in the M. tuberculosis-specific genomic region known as the region of difference (RD)1. There are 11 M. tuberculosis-specific RDs absent in all of the vaccine strains of BCG, and three of them (RD1, RD7, and RD9) encode immunodominant proteins. Each of these RDs has genes for a pair of ESAT6-like proteins. The immunological characterizations of all the possible proteins encoded by genes in RD1, RD7 and RD9 have shown that, besides ESAT-6 like proteins, several other proteins are major antigens useful for the development of subunit vaccines to substitute or supplement BCG. Furthermore, some of these proteins may replace the purified protein derivative of M. tuberculosis in the specific diagnosis of tuberculosis by using interferon-gamma release assays and/or tuberculin-type skin tests. At least three subunit vaccine candidates containing ESAT6-like proteins as antigen components of multimeric proteins have shown efficacy in phase 1 and phase II clinical trials in humans.

Mycobacterium tuberculosis-Specific Antigen Rv3619c Effectively Alleviates Allergic Asthma in Mice

Despite significant advances, asthma remains a cause of premature death, and current treatments are suboptimal. Antigen-specific Th2 cells and their cytokines are primary mediators of the pathophysiological changes seen in asthma. Studies in animal models have shown that mycobacteria can suppress the asthma phenotype by alteration of the Th1/Th2 cytokines ratio. In this study, utilizing a Th1 delivery system to modulate the allergic airway inflammation in a Th2-driven model of asthma, we evaluated the efficacy of immunization with Mycobacterium tuberculosis-specific antigen Rv3619c, either alone or in combination with low dose dexamethasone. The rv3619c gene was cloned in an expression plasmid pGES-TH-1, expressed in Escherichia coli, and the recombinant protein Rv3619c was purified to homogeneity using affinity chromatography. Mice were immunized with the recombinant protein emulsified in Freund's Incomplete Adjuvant (IFA) alone and in combination with low dose dexamethasone, and then challenged with ovalbumin (OVA). Airway inflammation was assessed by quantifying airway cytology, histological changes and Th2 cytokine (IL-5) secretion from splenocytes. OVA-specific IgE, IgG and IgG1 from sera was assessed, as well as pERK1/2 expression in the lung tissue. Immunization with recombinant Rv3619c alone inhibited the OVA-induced increase in total cell counts, eosinophil airway cell infiltration in BAL fluid, perivascular and peribronchial inflammation and fibrosis, and goblet cell hyper/metaplasia. In addition, Rv3619c/IFA inhibited the OVA-induced IL-5 in spleen cells, OVA-specific IgE, IgG, and IgG1 levels in sera, and pERK1/2 expression in lung tissue. Immunization with Rv3619c/IFA in combination with low dose dexamethasone resulted in an enhanced effect on some but not all the asthma features. Taken together, this study demonstrates that immunization with Rv3619c/IFA, alone or in combination with dexamethasone, may be an effective treatment strategy for the prevention of asthma.

The effect of adjuvants and delivery systems on Th1, Th2, Th17 and Treg cytokine responses in mice immunized with Mycobacterium tuberculosis-specific proteins

Tuberculosis (TB) is a major health problem of global concern. The control of this disease requires appropriate preventive measures, including vaccines. In TB, T helper (Th)1 cytokines provide protection whereas Th2 and T regulatory (Treg) cytokines contribute to the pathogenesis and Th17 cytokines play a role in both protection and pathogenesis. Previous studies with Mycobacterium tuberculosis-specific proteins have identified seven low molecular weight proteins, PE35, ESXA, ESXB, Rv2346c, Rv2347c, Rv3619c, and Rv3620c, as immunodominant antigens inducing Th1-cell responses in humans following natural infection with M. tuberculosis. The aim of this study was to characterize the cytokine responses induced in mice immunized with these proteins, using various adjuvants and delivery systems, i.e. chemical adjuvants (Alum and IFA), non-pathogenic mycobacteria (M. smegmatis and M. vaccae) and a DNA vaccine plasmid (pUMVC6). The immune responses were monitored by quantifying the marker cytokines secreted by Th1 (IFN-ɣ), Th2 (IL-5), Treg (IL-10), and Th17 (IL-17A) cells. DNA corresponding to pe35, esxa, esxb, rv2346c, rv2347c, rv3619c, and rv3620c genes were cloned into the expression vectors pGES-TH-1, pDE22 and pUMVC6 for expression in Escherichia coli, mycobacteria and eukaryotic cells, respectively. Mice were immunized with the recombinants using different adjuvants and delivery systems, and spleen cells were stimulated in vitro with peptides of immunizing proteins to investigate antigen-specific secretion of Th1 (IFN-ɣ), Th2 (IL-5), Treg (IL-10), and Th17 (IL-17A) cytokines. The results showed that spleen cells, from mice immunized with all antigens, secreted the protective Th1 cytokine IFN-ɣ, except ESXB, with one or more adjuvants and delivery systems. However, only Rv3619c consistently induced Th1-biased responses, without the secretion of significant concentrations of Th2, Th17 and Treg cytokines, with all adjuvants and delivery systems. Rv3619c also induced antigen-specific IgG antibodies in immunized mice.

Fusion Molecules of Heat Shock Protein HSPX with Other Antigens of Mycobacterium tuberculosis Show High Potential in Serodiagnosis of Tuberculosis

Variable individual response against the antigens of Mycobacterium tuberculosis necessitates detection of multiple antibodies for enhancing reliability of serodiagnosis of tuberculosis. Fusion molecules consisting of two or more antigens showing high sensitivity would be helpful in achieving this objective. Antigens of M. tuberculosis HSPX and PE35 were expressed in a soluble form whereas tnPstS1 and FbpC1 were expressed as inclusion bodies at 37°C. Heat shock protein HSPX when attached to the N-termini of the antigens PE35, tnPstS1 and FbpC1, all the fusion molecules were expressed at high levels in E. coli in a soluble form. ELISA analysis of the plasma samples of TB patients against HSPX-tnPstS1 showed 57.7% sensitivity which is nearly the same as the expected combined value obtained after deducting the number of plasma samples (32) containing the antibodies against both the individual antigens. Likewise, the 54.4% sensitivity of HSPX-PE35 was nearly the same as that expected from the combined values of the contributing antigens. Structural analysis of all the fusion molecules by CD spectroscopy showed that α-helical and β-sheet contents were found close to those obtained through molecular modeling. Molecular modeling studies of HSPX-tnPstS1 and HSPX-PE35 support the analytical results as most of the epitopes of the contributing antigens were found to be available for binding to the corresponding antibodies. Using these fusion molecules in combination with other antigenic molecules should reduce the number of antigenic proteins required for a more reliable and economical serodiagnosis of tuberculosis. Also, HSPX seems to have potential application in soluble expression of heterologous proteins in E. coli.

Cellular immune responses to recombinant Mycobacterium bovis BCG constructs expressing major antigens of region of difference 1 of Mycobacterium tuberculosis

Besides being the most widely used vaccine directed against tuberculosis (TB) worldwide, Mycobacterium bovis BCG is also the most controversial vaccine in current use. Its protective efficacy varies widely in different parts of the world. One approach to improving the current BCG vaccine might be to produce recombinant BCG strains that express major antigens encoded by genes that are present in the M. tuberculosis-specific region of difference 1 (RD1), such as pe35, cfp10, and esat6. In this study, pe35, cfp10, and esat6 genes were cloned into shuttle plasmid pDE22 to generate the recombinant plasmids PDE22-PE35, PDE22-CFP10, and PDE22-ESAT6, which were electroporated into BCG to generate recombinant BCGs (rBCGs). The cellular immune responses (antigen-induced proliferation and secretion of selected T helper 1 [Th1], Th2, and anti-inflammatory cytokines, i.e., gamma interferon [IFN-γ], interleukin 5 [IL-5], and IL-10, respectively) that are specific to the proteins of cloned genes were studied by using spleen cells from mice immunized with native BCGs and rBCGs and synthetic peptides covering the protein sequence of the cloned genes. The results showed that the spleen cells did not secrete IL-5, whereas IL-10 was secreted in response to peptides of all three proteins from mice immunized with rBCGs only, suggesting expression of the cloned genes and in vivo priming of spleen cells to the expressed proteins. However, in Th1 cell assays that correlate with protective cellular immune responses, i.e., antigen-induced proliferation and IFN-γ secretion, only mice immunized with rBCG-pDE22-PE35 yielded positive responses to the peptides of PE35. These results suggest that rBCG-PDE22-PE35 is the only one of the three vaccines used in this work that is worthy of consideration as a new vaccine candidate against TB.

In silico analysis and experimental validation of Mycobacterium tuberculosis -specific proteins and peptides of Mycobacterium tuberculosis for immunological diagnosis and vaccine development

Comparative analyses of the Mycobacterium tuberculosis genome with the genomes of other mycobacteria have led to the identification of several genomic regions of difference (RDs) between M. tuberculosis and M. bovis BCG. The identification of immunodominant and HLA-promiscuous antigens and peptides encoded by these RDs could be useful for diagnosis and the development of new vaccines against tuberculosis. The analysis of RD proteins and peptides by in silico methods (using computational programs to predict major and HLA-promiscuous antigenic proteins and peptides) and experimental validations (using peripheral blood mononuclear cells and sera from tuberculosis patients and BCG-vaccinated healthy subjects to assess antigen-specific cellular and humoral immune responses in vitro) identified several major antigens and peptides. To evaluate the in vivo potentials, the genes of immunodominant antigens were cloned and expressed in DNA vaccine vectors. Immunizations of experimental animals with the recombinant constructs induced antigen-specific cellular responses. Further experiments showed that each of these proteins had several T and B cell epitopes scattered throughout their sequence, which confirmed their strong immunogenicity. In conclusion, the bioinformatics-based in silico identification of promiscuous antigens and peptides of M. tuberculosis is a useful approach to identify new candidates important for diagnosis and vaccine applications.

Cellular immune responses in mice induced by M. tuberculosis PE35-DNA vaccine construct

The PE35 (Rv3872) gene of Mycobacterium tuberculosis is present in the region of difference (RD) one that is deleted in all vaccine strains of Mycobacterium bovis bacillus Calmette Guerin. The aim of this study was to clone PE35 DNA into a DNA vaccine plasmid with CMV promoter and interleukin-2 secretory signal and evaluate the recombinant plasmid for induction of antigen-specific cellular responses in mice. DNA corresponding to PE35 was PCR amplified from the genomic DNA of M. tuberculosis H(37) Rv, cloned into pGEMT-Easy vector and sub-cloned into the DNA vaccine vector pUMVC6. BALB/c mice were immunized with recombinant pUMVC6/PE35 and spleen cells were tested for T-helper (Th)1-type (antigen-induced proliferation and secretion of IFN-γ) and Th2-type (IL-5), and anti-inflammatory (IL-10) cytokine responses to pure recombinant PE35 protein and its synthetic peptides. Mice immunized with the recombinant plasmid DNA (pUMVC6/PE35) showed positive Th1-type cellular responses to pure PE35, but not to an irrelevant antigen, i.e. PPE68 (Rv3873). However, the vaccine construct did not induce antigen-specific Th2-type (IL-5) or anti-inflammatory (IL-10) reactivity to PE35. Testing with synthetic peptides showed that Th1-type cells recognizing various epitopes of PE35 were induced in mice immunized with pUMVC6/PE35 DNA. These results suggest that pUMVC6/PE35 may be useful as a safer vaccine candidate against TB.

Polyclonal antibody against conserved sequences of mce1A protein blocks MTB infection in macrophages

The pathogenesis of Mycobacterium tuberculosis is largely due to its ability to enter and survive within human macrophages. It is suggested that a specific protein namely mammalian cell entry protein is involved in the pathogenesis and the specific gene for this protein mce1A has been identified in several pathogenic organisms such as Rickettsia, Shigella, Escherichia coli, Helicobacter, Streptomyces, Klebsiella, Vibrio, Neisseria, Rhodococcus, Nocardioides, Saccharopolyspora erthyrae, and Pseudomonas. Analysis of mce1 operons in the above mentioned organisms through bioinformatics tools has revealed the presence of unique sequences (conserved regions) suggesting that these sequences may be involved in the process of infection. Presently, the mce1A full-length (1,365 bp) region from Mycobacterium bovis and its conserved regions (303 bp) were cloned in to an expression vector and the purified expressed proteins of molecular weight ~47 and ~11 kDa, respectively, were injected to rabbits to raise the polyclonal antibodies. The purified polyclonal antibodies were checked for their ability to inhibit the Mycobacterium infection in cultured human macrophages. In macrophage invasion assay, when antibody added at high concentration, decrease in viable counts was observed in all cell cultures within the first 5 days after infection, where the intracellular bacterial CFU obtained from the infected MTB increased by the 3rd day at low concentration of antibody. The macrophage invasion assay has indicated that the purified antibodies of mce1A conserved region can inhibit the infection of Mycobacterium.

DNA vaccine constructs expressing Mycobacterium tuberculosis-specific genes induce immune responses

RD1 PE35, PPE68, EsxA, EsxB and RD9 EsxV genes are present in Mycobacterium tuberculosis genome but deleted in Mycobacterium bovis BCG. The aim of this study was to clone these genes into DNA vaccine vectors capable of expressing them in eukaryotic cells as fusion proteins, fused with immunostimulatory signal peptides of human interleukin-2 (hIL-2) and tissue plasminogen activator (tPA), and evaluate the recombinant DNA vaccine constructs for induction of antigen-specific cellular immune responses in mice. DNA corresponding to the aforementioned RD1 and RD9 genes was cloned into DNA vaccine plasmid vectors pUMVC6 and pUMVC7 (with hIL-2 and tPA signal peptides, respectively), and a total of 10 recombinant DNA vaccine constructs were obtained. BALB/c mice were immunized with the parent and recombinant plasmids and their spleen cells were tested for antigen-induced proliferation with antigens of M. tuberculosis and pure proteins corresponding to the cloned genes. The results showed that antigen-specific proliferation responses were observed for a given antigen only with spleen cells of mice immunized with the homologous recombinant DNA vaccine construct. The mice immunized with the parent plasmids did not show positive immune responses to any of the antigens of the cloned genes. The ability of the DNA vaccine constructs to elicit cellular immune responses makes them an attractive weapon as a safer vaccine candidate for preventive and therapeutic applications against tuberculosis.

Molecular cloning, expression, purification and immunological characterization of three low-molecular weight proteins encoded by genes in genomic regions of difference of mycobacterium tuberculosis

The aim of this study was to clone, express and purify three major antigenic proteins, i.e. Rv3874, Rv3875 and Rv3619c, encoded by genes located in regions of difference of Mycobacterium tuberculosis and characterize them for immunogenicity in rabbits. The respective genes were amplified using gene-specific primers and genomic DNA of M. tuberculosis by polymerase chain reaction. The amplified DNA were cloned into pGEM-T Easy and subcloned into pGES-TH-1 vector for high-level expression in Escherichia coli and efficient purification. The results showed that the three fusion proteins, i.e. glutathione-S-transferase (GST)-Rv3874, GST-Rv3875 and GST-Rv3619c, were expressed at high levels and were purified (free of the GST fusion partner) to homogeneity using glutathione-Sepharose and Ni-NTA agarose affinity matrix after cleavage of the column-bound fusion proteins by thrombin protease. The purified recombinant Rv3874, Rv3875 and Rv3619c proteins were immunogenic and induced antigen-specific antibodies in rabbits. Testing of the rabbit sera with overlapping synthetic peptides showed that the antibodies were induced to several epitopes that were scattered throughout the sequence of each protein. These results show immunogenicity of all the proteins for inducing antigen-specific antibodies in rabbits and demonstrate the usefulness of pGES-TH-1 vector for obtaining purified recombinant proteins of M. tuberculosis for immunological characterization.

Comparative evaluation of INNO-LiPA HBV assay, direct DNA sequencing and subtractive PCR-RFLP for genotyping of clinical HBV isolates

Genotypes (A to H) of hepatitis B virus (HBV) influence liver disease progression and response to antiviral therapy in HBV-infected patients. Several methods have been developed for rapid genotyping of HBV strains. However, some of these methods may not be suitable for developing countries. The performance of INNO-LiPA HBV Genotyping assay (LiPA), direct DNA sequencing and subtractive PCR-RFLP of genotype-specific HBV genome regions were evaluated for accurately determining the HBV genotypes by analyzing sera (n = 80) samples from chronic HBV patients. Both, LiPA and DNA sequencing identified 63, 4 and 13 HBV strains as belonging to genotype D, genotype A and mixed genotype A and D, respectively. On the contrary, the PCR-RFLP-based method correctly identified all 4 genotype A but only 56 of 63 genotype D strains. Seven genotype D strains yielded indeterminate results. DNA sequence comparisons showed that a single nucleotide change in the target region generated an additional restriction site for Nla IV that compromised the accuracy of this method. Furthermore, all the mixed genotype A and D strains were identified only as genotype A strains. The data show that the PCR-RFLP-based method incorrectly identified some genotype D strains and failed to identify mixed genotype infections while LiPA and DNA sequencing yielded accurate results.

Expression, purification, and characterization of recombinant human beta-amyloid42 peptide in Escherichia coli

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by a progressive loss of cognitive function. Evidence indicates that abnormal processing and extracellular deposition of the beta-amyloid42 peptide, the longer form of proteolytic derivative of the transmembrane glycoprotein-amyloid precursor protein (APP), is a key step in the pathogenesis of AD. Since it is convenient and economical to obtain such a peptide biologically, in this study, we report for the first time a method to express in E. coli and purify beta-amyloid42 using glutathione-S-transferase (GST) fusion system. beta-Amyloid42 gene was inserted into a vector pGEX-4T-1 to construct a GST-fusion protein. The fusion protein GST-beta-amyloid42, expressed in BL21 (DE3) strain, was purified with GSH-affinity chromatography followed by thrombin cleavage. The digested product was further purified with an additional GSH-affinity and a Benzamidine chromatography step. After cleavage and purification, the beta-amyloid42 moiety showed the expected size of 4.5 kDa on Tricine-SDS-PAGE, and was further confirmed by Western blot. Moreover, the fibrillar recombinant beta-amyloid42 exhibited great aggregation activity and showed neurotoxicity on neuron cells in vitro. These results suggest that our method will be useful in obtaining a large quantity of recombinant beta-amyloid42 peptide for further physiological and biochemical studies.

Internalization by HeLa cells of latex beads coated with mammalian cell entry (Mce) proteins encoded by the mce3 operon of Mycobacterium tuberculosis

The mammalian cell entry (Mce) operon 3 (mce3) is one of four homologous mce operons of Mycobacterium tuberculosis, encoding six (Mce3A-F) invasin-like membrane-associated proteins. Previous studies have shown that recombinant expression of Mce1A encoded by the mce1 operon in Escherichia coli allows this non-pathogenic bacterium to invade and survive inside macrophages, and latex beads coated with Mce1A are internalized by non-phagocytic HeLa cells. However, the role of other mce1 operon proteins (Mce1B-F) and proteins encoded by the operons mce2-4 in facilitating the internalization of M. tuberculosis in mammalian cells has not been studied. This study was carried out to determine whether Mce proteins encoded by the mce3 operon also facilitated the internalization of latex beads by HeLa cells. Recombinant pure Mce3A and lipoprotein LprM (Mce3E) were expressed and purified from E. coli cells. Mce1A expressed as a fusion protein with glutathione S-transferase (GST-Mce1A) and GST alone, purified similarly from E. coli cells, were used as control proteins. Fluorescent latex beads coated with purified proteins were used to study their uptake by HeLa cells using fluorescence microscopy, flow cytometry and electron microscopy. Fluorescence microscopy and flow cytometry showed an association of HeLa cells with beads coated with both Mce3A and LprM, whilst GST-Mce1A and GST yielded the expected results. Transmission electron microscopy confirmed the uptake of beads coated with Mce3A or LprM by HeLa cells. The data showed that Mce3A encoded by the mce3 operon facilitated the uptake and internalization of latex beads by HeLa cells. The data also showed, for the first time, the role of another Mce protein (LprM/Mce3E) in facilitating the interaction and internalization of M. tuberculosis by mammalian cells.

Cloning, expression and rapid purification of active recombinant mycothiol ligase as B1 immunoglobulin binding domain of streptococcal protein G, glutathione-S-transferase and maltose binding protein fusion proteins in Mycobacterium smegmatis

Mycothiol ligase (MshC) is a key enzyme in the biosynthesis of mycothiol, a small molecular weight thiol found in Mycobacteria spp. and other actinomycetes. Mycothiol plays a fundamental role in these organisms by helping to provide protection from the effects of reactive oxygen species and electrophiles, including many antibiotics. It has recently been demonstrated that the MshC gene and more generally the production of mycothiol are essential to Mycobacterium tuberculosis, indicating that MshC may represent a novel target for new classes of antituberculars. Because MshC cannot be expressed heterologously in Escherichia coli and isolation from Mycobacterium smegmatis is impractical, we have optimized the E. coli-M. smegmatis shuttle vector pACE for cloning and recombinant expression of MshC (under control of an acetamidase-inducible promoter). To improve expression levels and simplify purification, we further constructed three N-terminal-MshC fusion proteins where N-terminal tags included the B1 domain of streptococcal protein G (to give GB1-MshC), glutathione-S-transferase (to give GST-MshC) and maltose binding protein (to give MBP-MshC), for expression in M. smegmatis. By expressing all three fusion proteins in a mutant strain of M. smegmatis mc(2)155, namely I64 L205P MshC M. smegmatis which lacks mycothiol ligase activity, we demonstrate in vivo mycothiol ligase activity for each construct. Recombinant GST-MshC and MBP-MshC were isolated in one step by affinity chromatography in a yield of 0.7 and 1.2 mg fusion protein/L and exhibited specific activities of 9 nmolmin(-1)mg(-1) and 25 nmolmin(-1)mg(-1), respectively.

Recombinant and synthetic peptides to identify Mycobacterium tuberculosis antigens and epitopes of diagnostic and vaccine relevance

The failures of Bacillus Calmette Guerin (BCG) as a vaccine and purified protein derivative as a diagnostic reagent in controlling the worldwide prevalence of tuberculosis (TB) have accelerated the research to identify Mycobacterium tuberculosis-specific antigens that could be useful as new vaccines and diagnostic reagents against TB. In the recent years, the comparative analyses of M. tuberculosis genome with the genomes of other mycobacteria have led to the identification of several genomic regions of M. tuberculosis that are deleted in BCG and other mycobacteria. These deleted regions (RDs) are predicted to encode over 100 proteins. If found immunologically reactive, the proteins encoded by M. tuberculosis-specific RDs could be useful in the specific diagnosis of TB and developing new vaccines. Among the approaches available for immunological characterization of the predicted M. tuberculosis-specific proteins are the evaluations of recombinant proteins and/or overlapping synthetic peptides, covering the sequence of each protein, for antibody and/or Th1 cell reactivity. These approaches have resulted into the identification of several antigenic proteins of M. tuberculosis encoded by genes located in RD1 with potentials in specific diagnosis of TB in low endemic areas and/or development of new vaccines, e.g. ORF14, ESAT6, CFP10, PE, PPE proteins, etc. In addition, prediction programs to identify peptides that could bind several HLA molecules, and presented to T-cells in a promiscuous manner, have been developed. These programs have been used, on a limited scale, to identify the promiscuous peptides encoded by the genes spanning the M. tuberculosis-specific sequence. The promiscuous antigens/peptides recognized by T-cells in cell mediated immunity assays may have potentials in developing peptide-based vaccines and diagnostic reagents against TB.

The six mammalian cell entry proteins (Mce3A-F) encoded by the mce3 operon are expressed during in vitro growth of Mycobacterium tuberculosis

The pathogenesis of Mycobacterium tuberculosis is largely due to its ability to enter and survive within human macrophages. The mammalian cell entry (mce)3 operon is one of four homologous mce operons that encodes six putative invasin-like exported proteins (Mce3A-F), possibly involved in entry and survival of M. tuberculosis inside macrophages. We have recently shown that Mce3A, Mce3D and Mce3E are expressed and elicit antibody responses in a majority of human subjects during natural infection with M. tuberculosis. In this study, we demonstrate the expression of Mce3A-F proteins and their mRNA during in vitro growth of M. tuberculosis. To demonstrate the expression of mce3A-F proteins, the antibodies were raised in rabbits against three pure proteins (Mce3A, Mce3D and Mce3E), and their specificity was checked by immunoblotting with recombinant Mce1A-F proteins encoded by mce1 operon. The antibodies were also generated against all the six Mce3 proteins, which were expressed and purified as fusion proteins with glutathione S-transferase (GST) as the fusion partner (GST-Mce3A-F). The antibodies reacted, in each case, with a protein of expected molecular mass (Mr) for the corresponding Mce3 protein in the cell wall fraction but not in the soluble fraction of in vitro-grown M. tuberculosis cells. The presence of mRNA for mce3A-F genes was also shown by using mce3A-F gene-specific primers, and total RNA isolated from in vitro-grown M. tuberculosis cells by reverse transcription-polymerase chain reaction (RT-PCR). Pretreatment of the RNA preparation with RNase A abolished amplification in RT-PCR confirming that mce3A-F mRNA rather than genomic DNA was being amplified. The data show that Mce3A-F encoded by the mce3 operon are expressed during in vitro growth of M. tuberculosis.

ProPred analysis and experimental evaluation of promiscuous T-cell epitopes of three major secreted antigens of Mycobacterium tuberculosis

In the search for safe vaccine candidates against tuberculosis (TB), subunit vaccines including peptide-based candidates deserve consideration. However, an important requirement for such vaccine candidates is their promiscuous presentation to Th1 cells mediating protective immunity against TB, i.e. Th1 cells secreting IFN-gamma. The aim of the present study was to identify promiscuous Th1 cell epitopes of three major secreted antigens of Mycobacterium tuberculosis, i.e. ESAT-6, CFP10 and MPT70 by using a virtual matrix-based prediction program (ProPred) for peptide binding to 51 HLA-DR alleles. The ProPred analysis of these proteins was performed using the server (http:www.imtech.res.in/raghava/ProPed/). The peptides predicted to bind > 50% HLA-DR alleles included in the ProPred were considered promiscuous for binding predictions. Based on this criteria, one region in ESAT-6 (aa 69-77), two regions in CFP10 (aa 55-66 and aa 76-84) and four regions in MPT70 (aa 1-11, aa 81-95, aa 124-140 and aa 182-191) were considered promiscuous HLA-DR binders. The experimental evaluation of these regions, by using overlapping synthetic peptides for presentation to T-cells, confirmed the promiscuous nature of peptides covering the regions aa 69-77, aa 76-84 and aa 182-191 of ESAT-6, CFP10 and MPT70, respectively. These results demonstrate that the ProPred analysis can facilitate the selection of promiscuous peptides recognized by Th1 cells, and thus it can be useful in the identification of peptide-based vaccine candidates against TB.

HLA-DR binding prediction and experimental evaluation of T-cell epitopes of mycolyl transferase 85B (Ag85B), a major secreted antigen of Mycobacterium tuberculosis

OBJECTIVE

To identify T-cell epitopes of Ag85B by analysis of its sequence for prediction to bind HLA-DR alleles and evaluate the predicted peptides for recognition by T cells in antigen-induced proliferation assays.

MATERIALS/SUBJECTS AND METHODS

The complete sequence of Ag85B was analyzed for HLA-DR binding prediction to 51 HLA-DR alleles by using a virtual matrix-based prediction program (ProPred). Synthetic peptides covering the sequence of mature Ag85B were also analyzed for binding to HLA-DR alleles, and evaluated for recognition in antigen-induced proliferation assays with Ag85B-specific T-cell lines established from the peripheral blood mononuclear cells of 10 HLA-DR-heterogeneous tuberculosis patients.

RESULTS

The ProPred analysis of the full-length Ag85B (325 aa), signal peptide (40 aa) and the mature protein (285 aa) predicted their binding to 100, 76 and 98% of the 51 HLA-DR alleles, respectively. The analysis of 31 synthetic peptides for binding to HLA-DR alleles showed that 4 of them could bind >50% HLA-DR alleles, and were considered promiscuous. Testing of Ag85B-specific T-cell lines with synthetic peptides showed that all of the T-cell lines responded to one or more peptides of Ag85B, and 9 of the 10 cell lines responded to one or more of the four peptides considered promiscuous for binding to HLA-DR alleles.

CONCLUSION

The ProPred program was useful in predicting the HLA-DR alleles binding regions of Ag85B and identifying the promiscuous peptides recognized by T cells.

Mycobacterial gene cloning and expression, comparative genomics, bioinformatics and proteomics in relation to the development of new vaccines and diagnostic reagents

Recent advances in molecular and genomic techniques have facilitated research on several aspects of mycobacteriology, such as diagnosis and the identification of new vaccines and therapeutic targets for various diseases, including tuberculosis. The aim of this review was to analyze the implications of advances in molecular and genomic techniques on the development of new vaccines for tuberculosis as well as immunological reagents to diagnose the disease. Gene cloning and expression, DNA and protein sequencing, polymerase chain reaction, comparative genomics, bioinformatics, proteomics and DNA and peptide synthesis coupled with the application of cellular immunology techniques have led to the identification of several antigens of Mycobacterium tuberculosis, which have potential for diagnosis and vaccine applications. For example, cross-reactive mycobacterial antigens like heat shock proteins, MTB32 and MTB39, have been identified as new vaccine candidates, and antigens encoded by M. tuberculosis-specific genomic regions as new reagents for diagnosis.

Mammalian Cell-Entry Proteins Encoded by the mce3 Operon of Mycobacterium tuberculosis are Expressed During Natural Infection in Humans

The mammalian cell-entry (mce)3 operon is one of four homologous mce operons on Mycobacterium tuberculosis genome that encodes six putative invasin/ adhesin-like proteins (Mce3A-F) possibly involved in the entry and survival of this bacterium inside macrophages. To study the in vivo expression of the mce3 operon-encoded proteins during natural human infection, the genes encoding Mce3A-F were cloned and expressed in Escherichia coli as fusion proteins with glutathione-S-transferase (GST) at the N-terminal and a x6 histidine (His) tag at the C-terminal end. The recombinant proteins appeared as major cellular proteins in SDS-PAGE gels and reacted with anti-GST and antipenta-His antibodies at the expected molecular mass of 70, 61, 68, 71, 66 and 72 [corrected] kDa for GST-Mce3A, GST-Mce3B, GST-Mce3C, GST-Mce3D, GST-Mce3E and GST-Mce3F, respectively. In Western immunoblots, all the six fusion proteins, particularly GST-Mce3A, GST-Mce3C, GST-Mce3D and GST-Mce3E, reacted with antibodies in combined human serum from 11 tuberculosis (TB) patients. Pure Mce3A, Mce3D and Mce3E could be isolated by specific proteolytic cleavage by thrombin protease of the respective purified fusion protein followed by preparative SDS-PAGE. The pure Mce3A, Mce3D and Mce3E reacted to various extents with antibodies in serum samples from TB patients. The Mce3E reacted with 51 of 55 (93%) and all the three proteins reacted with 34 of 55 (62%) serum samples. The Mce3A, Mce3D and Mce3E proteins also reacted, albeit at lower frequency, with one of 23 (4%) serum sample obtained from M. bovis bacillus Calmette-Guérin-vaccinated healthy subjects and four of 18 (22%) serum samples from long-term contacts of TB patients showing reactivity with all the three Mce3 proteins. The data show that Mce3A, Mce3D and Mce3E encoded by mce3 operon of M. tuberculosis are expressed and elicit antibody responses in humans during natural infection with this pathogen.