Cloning of the gene encoding a 22-kilodalton cell surface antigen of Mycobacterium bovis BCG and analysis of its potential for DNA vaccination against tuberculosis

An update on Mycobacterium tuberculosis lipoproteins

Almost 3% of the proteins of Mycobacterium tuberculosis (M. tuberculosis), the main causative agent of human tuberculosis, are lipoproteins. These lipoproteins are characteristic of the mycobacterial cell envelope and participate in many mechanisms involved in the pathogenesis of M. tuberculosis. In this review, the authors provide an updated analysis of M. tuberculosis lipoproteins and categorize them according to their demonstrated or predicted functions, including transport of compounds to and from the cytoplasm, biosynthesis of the mycobacterial cell envelope, defense and resistance mechanisms, enzymatic activities and signaling pathways. In addition, this updated analysis revealed that at least 40% of M. tuberculosis lipoproteins are glycosylated.

S16 and T18 mannosylation sites of LppX are not essential for its activity in phthiocerol dimycocerosates localization at the surface of Mycobacterium tuberculosis

LppX is an important virulence factor essential for surface localization of phthiocerol dimycocerosates (DIM) in Mycobacterium tuberculosis. Based on Concanavalin A recognition, M. tuberculosis LppX (LppX-tb) was initially proposed to be glycosylated in M. tuberculosis and more recently this glycosylation was characterized by mass spectrometry analysis on LppX-tb expressed and purified from Corynebacterium glutamicum. Here, using this model organism and Mycobacterium smegmatis, we show that S16 and T18 residues of LppX-tb are indeed glycosylated with several hexoses units. Interestingly this glycosylation is strictly dependent on the mannosyl transferase PMT which, in M. tuberculosis, has been reported to be crucial for virulence. Using a site directed mutagenesis approach, we were able to show that the absence of S16 and T18 glycosylation does not alter phthiocerol dimycocerosates (DIM) localization in M. tuberculosis.

Role of the unique, non-essential phosphatidylglycerol::prolipoprotein diacylglyceryl transferase (Lgt) in Corynebacterium glutamicum

Bacterial lipoproteins are secreted proteins that are post-translationally lipidated. Following synthesis, preprolipoproteins are transported through the cytoplasmic membrane via the Sec or Tat translocon. As they exit the transport machinery, they are recognized by a phosphatidylglycerol::prolipoprotein diacylglyceryl transferase (Lgt), which converts them to prolipoproteins by adding a diacylglyceryl group to the sulfhydryl side chain of the invariant Cys₊₁ residue. Lipoprotein signal peptidase (LspA or signal peptidase II) subsequently cleaves the signal peptide, liberating the α-amino group of Cys₊₁, which can eventually be further modified. Here, we identified the lgt and lspA genes from Corynebacterium glutamicum and found that they are unique but not essential. We found that Lgt is necessary for the acylation and membrane anchoring of two model lipoproteins expressed in this species: MusE, a C. glutamicum maltose-binding lipoprotein, and LppX, a Mycobacterium tuberculosis lipoprotein. However, Lgt is not required for these proteins' signal peptide cleavage, or for LppX glycosylation. Taken together, these data show that in C. glutamicum the association of some lipoproteins with membranes through the covalent attachment of a lipid moiety is not essential for further post-translational modification.

Identification of Secreted O-Mannosylated Proteins From BCG and Characterization of Immunodominant Antigens BCG_0470 and BCG_0980

Bacterial glycoproteins have been investigated as vaccine candidates as well as diagnostic biomarkers. However, they are poorly understood in Mycobacterium bovis strain bacille Calmette-Guérin (BCG), a non-pathogenic model of Mycobacterium tuberculosis. To understand the roles of secreted O-mannosylated glycoproteins in BCG, we conducted a ConA lectin-affinity chromatography and mass spectra analysis to identify O-mannosylated proteins in BCG culture filtrate. Subsequent screening of antigens was performed using polyclonal antibodies obtained from a BCG-immunized mouse, with 15 endogenous O-mannosylated proteins eventually identified. Of these, BCG_0470 and BCG_0980 (PstS3) were revealed as the immunodominant antigens. To examine the protective effects of the antigens, recombinant antigens proteins were first expressed in Mycobacterium smegmatis and Escherichia coli, with the purified proteins then used to boost BCG primed-mice. Overall, the treated mice showed a greater delayed-type hypersensitivity response in vivo, as well as stronger Th1 responses, including higher level of IFN-γ, TNF-α, and specific-IgG. Therefore, mannosylated proteins BCG_0470 and BCG_0980 effectively amplified the immune responses induced by BCG in mice. Together, our results suggest that the oligosaccharide chains containing mannose are the antigenic determinants of glycoproteins, providing key insight for future vaccine optimization and design.

Immune responses elicited by the recombinant Erp, HspR, LppX, MmaA4, and OmpA proteins from Mycobacterium tuberculosis in mice

Immunogenic potency of the recombinant Erp, HspR, LppX, MmaA4, and OmpA proteins from Mycobacterium tuberculosis (MTB), formulated with Montanide ISA 720 VG adjuvant, was evaluated in BALB/c mice for the first time in this study. The five vaccine formulations, adjuvant, and BCG vaccine were subcutaneously injected into mice, and the sera were collected at days 0, 15, 30, 41, and 66. The humoral and cellular immune responses against vaccine formulations were determined by measuring serum IgG and serum interferon-gamma (IFN-γ) and interleukin-12 (IL-12) levels, respectively. All formulations significantly increased IgG levels post-vaccination. The highest increase in IFN-γ level was provided by MmaA4 formulation. The Erp, HspR, and LppX formulations were as effective as BCG in enhancement of IFN-γ level. The most efficient vaccine boosting the IL-12 level was HspR formulation, especially at day 66. Erp formulation also increased the IL-12 level more than BCG at days 15 and 30. The IL-12 level boosted by MmaA4 formulation was found to be similar to that by BCG. OmpA formulation was inefficient in enhancement of cellular immune responses. This study showed that MmaA4, HspR, and Erp proteins from MTB are successful in eliciting both humoral and cellular immune responses in mice.

Tuberculosis vaccine candidates based on mycobacterial cell envelope components

Even after decades searching for a new and more effective vaccine against tuberculosis, the scientific community is still pursuing this goal due to the complexity of its causative agent, Mycobacterium tuberculosis (Mtb). Mtb is a microorganism with a robust variety of survival mechanisms that allow it to remain in the host for years. The structure and nature of the Mtb envelope play a leading role in its resistance and survival. Mtb has a perfect machinery that allows it to modulate the immune response in its favor and to adapt to the host's environmental conditions in order to remain alive until the moment to reactivate its normal growing state. Mtb cell envelope protein, carbohydrate and lipid components have been the subject of interest for developing new vaccines because most of them are responsible for the pathogenicity and virulence of the bacteria. Many indirect evidences, mainly derived from the use of monoclonal antibodies, support the potential protective role of Mtb envelope components. Subunit and DNA vaccines, lipid extracts, liposomes and membrane vesicle formulations are some examples of technologies used, with encouraging results, to evaluate the potential of these antigens in the protective response against Mtb.

Mycobacterium tuberculosis effectors involved in host-pathogen interaction revealed by a multiple scales integrative pipeline

BACKGROUND

Mycobacterium tuberculosis (Mtb) has evolved multiple strategies to counter host immunity. Proteins are one important player in the host-pathogen interaction. A comprehensive list of such proteins will benefit our understanding of pathogenesis of Mtb.

METHODS

A genome-scale dataset was created from different sources of published data: global gene expression studies in disease models; genome-wide insertional mutagenesis defining gene essentiality under different conditions; genes lost in clinical isolates; subcellular localization analysis and non-homology analysis. Using data mining and meta-analysis, expressed proteins critical for intracellular survival of Mtb are first identified, followed by subcellular localization analysis, finally filtering a series of subtractive channel of analysis to find out promising drug target candidates.

RESULTS

The analysis found 54 potential candidates essential for the intracellular survival of the pathogen and non-homologous to host or gut flora, and might be promising drug targets.

CONCLUSION

Based on our meta-analysis and bioinformatics analysis, 54 hits were found from Mtb around 4000 open reading frames. These hits can be good candidates for further experimental investigation.

Virulence factors of the Mycobacterium tuberculosis complex

The Mycobacterium tuberculosis complex (MTBC) consists of closely related species that cause tuberculosis in both humans and animals. This illness, still today, remains to be one of the leading causes of morbidity and mortality throughout the world. The mycobacteria enter the host by air, and, once in the lungs, are phagocytated by macrophages. This may lead to the rapid elimination of the bacillus or to the triggering of an active tuberculosis infection. A large number of different virulence factors have evolved in MTBC members as a response to the host immune reaction. The aim of this review is to describe the bacterial genes/proteins that are essential for the virulence of MTBC species, and that have been demonstrated in an in vivo model of infection. Knowledge of MTBC virulence factors is essential for the development of new vaccines and drugs to help manage the disease toward an increasingly more tuberculosis-free world.

Coexpression of PPE 34.9 Antigen of Mycobacterium avium subsp. Paratuberculosis with Murine Interferon Gamma in HeLa Cell Line and Study of Their Immunogenicity in Murine Model

Mycobacterium avium subsp. paratuberculosis (Map) is the causative agent of johne's disease whose immunopathology mainly depends on cell mediated immuneresponse. Genome sequencing revealed various PPE (Proline-Proline-Glutamic acid) protein family of Map which are immunologically importance candidate genes In present study we have developed a bicistrionic construct pIR PPE/IFN containing a 34.9 kDa PPE protein (PPE 34.9) of Map along with a cytokine gene encoding murine gamma Interferon gene (IFNγ) and a monocistrionic construct pIR PPE using a mammalian vector system pIRES 6.1. The construct were transfected in HeLa cell line and expression were studied by Western blot as well as Immunefluroscent assay using recombinant sera. Further we have compared the immunereactivity of these two constructs in murine model by means of DTH study, LTT, NO assay and ELISA. DTH response was higher in pIR PPE/IFN than pIR PPE group of mice, similar finding also observed in case of LTT and NO production assay . ELISA titer of the pIR PPE/IFN was less than that with PPE only. These preliminary finding can revealed a CMI response of this PPE protein of Map and IFNγ having synergistic effect on this PPE protein to elicit a T cell based immunity in mice.

Protegen: a web-based protective antigen database and analysis system

Protective antigens are specifically targeted by the acquired immune response of the host and are able to induce protection in the host against infectious and non-infectious diseases. Protective antigens play important roles in vaccine development, as biological markers for disease diagnosis, and for analysis of fundamental host immunity against diseases. Protegen is a web-based central database and analysis system that curates, stores and analyzes protective antigens. Basic antigen information and experimental evidence are curated from peer-reviewed articles. More detailed gene/protein information (e.g. DNA and protein sequences, and COG classification) are automatically extracted from existing databases using internally developed scripts. Bioinformatics programs are also applied to compute different antigen features, such as protein weight and pI, and subcellular localizations of bacterial proteins. Presently, 590 protective antigens have been curated against over 100 infectious diseases caused by pathogens and non-infectious diseases (including cancers and allergies). A user-friendly web query and visualization interface is developed for interactive protective antigen search. A customized BLAST sequence similarity search is also developed for analysis of new sequences provided by the users. To support data exchange, the information of protective antigens is stored in the Vaccine Ontology (VO) in OWL format and can also be exported to FASTA and Excel files. Protegen is publically available at http://www.violinet.org/protegen.

Differential immunogenicity and protective efficacy of DNA vaccines expressing proteins of Mycobacterium tuberculosis in a mouse model

BALB/c mice were vaccinated three times (2-week intervals) with plasmid DNA separately encoding antigen Ag85B, ESAT-6 or Ag85A from Mycobacterium tuberculosis. The protective efficacy of these DNA vaccines against intravenous M. tuberculosis H37Rv challenge infection was measured by counting bacterial loads in spleen and lung and recording changes in lung pathology. The splenocyte proliferative response to the corresponding antigens and antigen-specific interferon (IFN)-gamma secreted by splenocytes of the vaccinated mice were also detected. We found a clear hierarchy of protective efficacies among the three DNA vaccines tested in this study. Plasmid DNA encoding Ag85A provided the strongest protection and showed the least change in lung pathology, followed by plasmid DNAs encoding Ag85B and ESAT-6. However, DNA-85B reduced comparative bacterial load in lung tissue, as did DNA-85A. Compared to the control group, protective efficacies conferred by different DNA vaccines were consistent with the lymphoproliferative responses to the corresponding antigens as well as the secretions of antigen-specific IFN-gamma. Our study demonstrates that both Ag85A and Ag85B are the most promising of the candidate antigens tested for future TB vaccine development.

Tuberculosis subunit vaccine development: Impact of physicochemical properties of mycobacterial test antigens

Tuberculosis caused by Mycobacterium tuberculosis continues to be one of the major public health problems in the world. The eventual control of this disease will require the development of a safe and effective vaccine. One of the approaches receiving a great deal of attention recently is subunit vaccination. An efficacious antituberculous subunit vaccine requires the identification and isolation of key components of the pathogen that are capable of inducing a protective immune response. Clues to identify promising subunit vaccine candidates may be found in their physicochemical and immunobiological properties. In this article, we review the evidence that the physicochemical properties of mycobacterial components can greatly impact the induction of either protective or deleterious immune response and consequently influence the potential utility as an antituberculous subunit vaccine.

Association of the Rv0679c protein with lipids and carbohydrates in Mycobacterium tuberculosis/Mycobacterium bovis BCG

The Rv0679c gene in Mycobacterium tuberculosis H37Rv encodes a protein with a predicted molecular mass of 16,586 Da consisting of 165 amino acids which contains a putative N-terminal signal sequence and a consensus lipoprotein-processing motif. Globomycin treatment, Triton X-114 separation and mass spectrometry analyses clarified a property of the Rv0679c protein as a lipoprotein. In addition, trifluoromethanesulphonic acid treatment of the lysate revealed an association of the recombinant Rv0679c protein with carbohydrates. The Rv0679c protein homolog of Mycobacterium bovis BCG was also expressed as the protein associated with lipids and carbohydrates. In Western blot analysis, each of the protein homolog and Lipoarabinomannan (LAM) was detected as a similar pattern by anti-Rv0679c and anti-LAM antibodies, respectively. Interestingly, the Rv0679c protein was detected in commercially available LAM purified from M. tuberculosis. Inhibition assay of LAM synthesis in M. bovis BCG by ethambutol showed an altered migration pattern of the Rv0679c protein to low molecular mass similar to that of LAM. The results suggest that the Rv0679c protein exists as a tight complex with LAM in M. tuberculosis/M. bovis BCG.

Tuberculosis subunit vaccine design: the conflict of antigenicity and immunogenicity

The attempts to find an effective antituberculous subunit vaccine are based on the assumption that it must drive a Th1 response. In the absence of effective correlates of protection, a vast array of mycobacterial components are being evaluated worldwide either on the basis of their ability to be recognized by T lymphocytes in in vitro assays during early stage of animal or human infection (antigenicity) or their capacity to induce T cell response following immunization in animal models (immunogenicity). The putative vaccine candidates selected using either of these strategies are then subjected to challenge studies in different animal models to evaluate the protective efficacy. Here we review the outcome of this current scheme of selection of vaccine candidates using an 'antigenicity' or 'immunogenicity' criterion on the actual protective efficacy observed in experimental animal models. The possible implications for the success of some of the leading vaccine candidates in clinical trials will also be discussed.

LppX is a lipoprotein required for the translocation of phthiocerol dimycocerosates to the surface of Mycobacterium tuberculosis

Cell envelope lipids play an important role in the pathogenicity of mycobacteria, but the mechanisms by which they are transported to the outer membrane of these prokaryotes are largely unknown. Here, we provide evidence that LppX is a lipoprotein required for the translocation of complex lipids, the phthiocerol dimycocerosates (DIM), to the outer membrane of Mycobacterium tuberculosis. Abolition of DIM transport following disruption of the lppX gene is accompanied by an important attenuation of the virulence of the tubercle bacillus. The crystal structure of LppX unveils an U-shaped beta-half-barrel dominated by a large hydrophobic cavity suitable to accommodate a single DIM molecule. LppX shares a similar fold with the periplasmic molecular chaperone LolA and the outer membrane lipoprotein LolB, which are involved in the localization of lipoproteins to the outer membrane of Gram-negative bacteria. Based on the structure and although an indirect participation of LppX in DIM transport cannot yet be ruled out, we propose LppX to be the first characterized member of a family of structurally related lipoproteins that carry lipophilic molecules across the mycobacterial cell envelope.

Identification and characterization of an immunogenic 22 kDa exported protein of Mycobacterium avium subspecies paratuberculosis

An exported 22 kDa putative lipoprotein was identified in an alkaline phosphatase gene fusion library ofMycobacterium aviumsubsp.paratuberculosisand expressed inMycobacterium smegmatis. The full nucleic acid sequence of the gene encoding P22 was determined and the ORF was cloned into a mycobacterial expression vector, enabling full-length P22 to be produced as a C-terminal polyhistidine-tagged protein inM. smegmatis. N-terminal sequencing of the recombinant protein confirmed cleavage of a signal sequence. Native P22 was detected in culture supernatants and cell sonicates ofM. aviumsubsp.paratuberculosisstrain 316F using rabbit antibody raised to recombinant P22. Investigation of the presence of similar genes in other mycobacterial species revealed that the gene was present inMycobacterium aviumsubsp.aviumand similar genes existed inMycobacterium intracellulareandMycobacterium scrofulaceum. Database searches showed that P22 belonged to the LppX/LprAFG family of mycobacterial lipoproteins also found inMycobacterium lepraeand in members of theMycobacterium tuberculosiscomplex. P22 shared less than 75 % identity to these proteins. Recombinant P22 was able to elicit interferon-gamma secretion in blood from eight of a group of nine sheep vaccinated with a live attenuated strain ofM. aviumsubsp.paratuberculosis(strain 316F) compared to none from a group of five unvaccinated sheep. Antibody to P22 was detected by Western blot analysis in 10 out of 11 vaccinated sheep, in two out of two clinically affected cows and in 11 out of 13 subclinically infected cows.

Lipoproteins ofMycobacterium tuberculosis: an abundant and functionally diverse class of cell envelope components

Mycobacterium tuberculosis remains the predominant bacterial scourge of mankind. Understanding of its biology and pathogenicity has been greatly advanced by the determination of whole genome sequences for this organism. Bacterial lipoproteins are a functionally diverse class of membrane-anchored proteins. The signal peptides of these proteins direct their export and post-translational lipid modification. These signal peptides are amenable to bioinformatic analysis, allowing the lipoproteins encoded in whole genomes to be catalogued. This review applies bioinformatic methods to the identification and functional characterisation of the lipoproteins encoded in the M. tuberculosis genomes. Ninety nine putative lipoproteins were identified and so this family of proteins represents ca. 2.5% of the M. tuberculosis predicted proteome. Thus, lipoproteins represent an important class of cell envelope proteins that may contribute to the virulence of this major pathogen.

Towards more accurate diagnosis of bovine tuberculosis using defined antigens

Diagnostic accuracy is of paramount importance in test-and-slaughter programmes for the eradication of bovine tuberculosis (TB). Currently applied methods, such as in vivo skin testing and in vitro interferon-gamma (IFN- gamma) testing, utilize purified protein derivatives (PPDs), which are poorly-defined mixtures containing many individual antigenic components. It is known that false-positive responses to these reagents can occur in cattle which are not infected with TB, largely because of that antigenic complexity. This paper reviews recent approaches to the characterization of more precisely defined diagnostic tools which can be used to develop tests with greater specificity. For example, the low mass secreted protein ESAT-6 has been shown to be capable of differentiating TB-infected cattle from those which develop responsiveness to PPD through contact with environmental mycobacteria or vaccination with BCG. The information which has accumulated in recent years has shown that the increased specificity is associated with some decrease in test sensitivity, but the overall advantages of being able to make precise diagnostic decisions will have significant advantages in many situations.