Activity of mycobacterial promoters during intracellular and extracellular growth

Utility of a Mycobacterium leprae molecular viability assay for clinical leprosy: An analysis of cases from the Philippines, Ethiopia, and Nepal

Mycobacterium leprae is a slow-growing species of mycobacteria that cannot be cultured in axenic media. This presents a number of challenges for monitoring treatment efficacy and advancing new drugs and regimens for treating leprosy. We previously developed a molecular viability assay (MVA) which measures expression of hsp18 and esxA transcripts to determine viability of M. leprae directly from infected tissue. The objective of the current study was to determine the utility of the MVA for practical use on clinical specimens. Leprosy cases from the Philippines (N = 199), Ethiopia (N = 40), and Nepal (N = 200) were diagnosed by clinical examination, slit-skin smears (SSS) from index sites, and/or histopathology. Biopsy specimens for MVA were collected from an active lesion and stored in 70% ethanol. DNA and RNA were extracted from the tissue, and M. leprae were enumerated on the DNA fraction via RLEP qPCR. Based on this count, DNased RNA was normalized to the equivalent of 3x103M. leprae per reverse transcription reaction, and hsp18 and esxA transcripts were amplified by PCR on the resulting cDNA. There was a strong correlation between RLEP enumeration on the specific biopsy specimen for MVA and the average SSS bacterial index (BI) in all three cohorts (p < 0.001). The MVA could be performed on most biopsies with an average SSS BI ≥ 2 and showed a decrease in M. leprae viability with increasing duration of leprosy multidrug therapy (R2 = 0.81, p < 0.001). The MVA also detected viable M. leprae in relapse patients where it showed significant correlation with the mouse footpad assay (p = 0.018). The MVA is a M. leprae-specific, sensitive, and relatively quick test. Clinically, the MVA would likely be most useful to monitor treatment, confirm suspected relapse cases, and determine efficacy of new leprosy drugs in clinical trials.

Role of ATP-Small Heat Shock Protein Interaction in Human Diseases

Adenosine triphosphate (ATP) is an important fuel of life for humans and Mycobacterium species. Its potential role in modulating cellular functions and implications in systemic, pulmonary, and ocular diseases is well studied. Plasma ATP has been used as a diagnostic and prognostic biomarker owing to its close association with disease’s progression. Several stresses induce altered ATP generation, causing disorders and illnesses. Small heat shock proteins (sHSPs) are dynamic oligomers that are dominantly β-sheet in nature. Some important functions that they exhibit include preventing protein aggregation, enabling protein refolding, conferring thermotolerance to cells, and exhibiting anti-apoptotic functions. Expression and functions of sHSPs in humans are closely associated with several diseases like cataracts, cardiovascular diseases, renal diseases, cancer, etc. Additionally, there are some mycobacterial sHSPs like Mycobacterium leprae HSP18 and Mycobacterium tuberculosis HSP16.3, whose molecular chaperone functions are implicated in the growth and survival of pathogens in host species. As both ATP and sHSPs, remain closely associated with several human diseases and survival of bacterial pathogens in the host, therefore substantial research has been conducted to elucidate ATP-sHSP interaction. In this mini review, the impact of ATP on the structure and function of human and mycobacterial sHSPs is discussed. Additionally, how such interactions can influence the onset of several human diseases is also discussed.

Recombinant Mycobacterium bovis BCG is a promising platform to develop vaccines against Trypansoma cruzi infection

Chagas disease, caused by the hemoflagelate parasite Trypanosoma cruzi, is one of the most prevalent endemic parasitoses, affecting 7-8 million people. Due to the complexity of the infection, no vaccines are available at present. The extraordinary adjuvant capacity of bacille Calmette-Guérin (BCG) was explored in this work to develop a vaccine candidate to protect against T. cruzi infection using the recombinant BCG (rBCG) vaccine platform. Three antigens of the parasite corresponding to the N and C terminal fragments of the enzyme trans-sialidase (NT-TS and CT-TS, respectively) and a fragment of the cruzipain enzyme (CZf) were cloned into the vectors pUS997 and pUS2000 and transformed into the BCG Pasteur strain. In vaccinated mice, rBCG expressing NT-TS in pUS2000 plasmid provided the highest protection and the lowest parasitemia after challenging BALB/c mice with a 50% lethal dose of parasites. When mice vaccinated with pUS2000-NT-TS were challenged with a 100% lethal dose of parasite, high levels of protection were also obtained, together with a low degree of cardiac lesions 120 days after infection. In immunized mice with pUS2000-NT-TS/rBCG clone, the proliferation of CD4⁺ cells from splenocytes stimulated with the TS antigen was significant; this stimulation increased interferon (IFN)-γ and interleukin (IL)-17 within CD4⁺ T lymphocytes (LTCD4⁺ ) cells and IFN-γ and CD107 expression within LTCD8⁺ cells. Therefore, pUS2000-NT-TS/rBCG conferred high levels of protection, which correlated with an immune response orientated towards a T helper type 1 (Th1)/Th17 profile, together with an LTC-specific response, indicating that rBCG is a promising platform to develop vaccines against T. cruzi.

Characterisation of alternative expression vectors for recombinant Bacillus Calmette-Guérin as live bacterial delivery systems

BACKGROUND

Bacillus Calmette-Guérin (BCG) is considered a promising live bacterial delivery system. However, several proposals for rBCG vaccines have not progressed, mainly due to the limitations of the available expression systems.

OBJECTIVES

To obtain a set of mycobacterial vectors using a range of promoters with different strengths based on a standard backbone, previously shown to be stable.

METHODS

Mycobacterial expression vectors based on the pLA71 vector as backbone, were obtained inserting different promoters (P_AN, P_αAg, P_Hsp60, P_BlaF* and P_L5) and the green fluorescence protein (GFP) as reporter gene, to evaluate features such as their relative strengths, and the in vitro (inside macrophages) and in vivo stability.

FINDINGS

The relative fluorescence observed with the different vectors showed increasing strength of the promoters: P_AN was the weakest in both Mycobacterium smegmatis and BCG and P_BlaF* was higher than P_Hsp60 in BCG. The relative fluorescence observed in a macrophage cell line showed that P_BlaF* and P_Hsp60 were comparable. It was not possible to obtain strains transformed with the extrachromosomal expression vector containing the P_L5 in either species.

MAIN CONCLUSION

We have obtained a set of potentially stable mycobacterial vectors with a arrange of expression levels, to be used in the development of rBCG vaccines.

Structure, stability and chaperone function of Mycobacterium leprae Heat Shock Protein 18 are differentially affected upon interaction with gold and silver nanoparticles

Gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) have several biomedical applications. However, the effective usage of these two nanoparticles is impeded due to limited understanding of their interaction with proteins including small heat shock proteins (sHSPs). Specifically, no evidences of interaction of these two nanoparticles with HSP18 (an antigenic protein) which is an important factor for the growth and survival of M. leprae (the causative organism of leprosy) are available in the literature. Here, we report for the first time evidences of "HSP18-AuNPs/AgNPs interaction" and its impact on the structure and chaperone function of HSP18. Interaction of citrate-capped AuNPs/AgNPs (~20 nm diameter) to HSP18 alters the secondary and tertiary structure of HSP18 in a distinctly opposite manner; while "HSP18-AuNPs interaction" leads to oligomeric association, "HSP18-AgNPs interaction" results in oligomeric dissociation of the protein. Surface hydrophobicity, thermal stability, chaperone function of HSP18 and survival of thermally stressed E. coli harbouring HSP18 are enhanced upon AuNPs interaction, while all of them are reduced upon interaction with AgNPs. Altogether, our study reveals that HSP18 is an important drug target in leprosy and its chaperone function may possibly plays a vital role in the growth and survival of M. leprae pathogen in infected hosts.

Recombinant BCG vaccines: molecular features and their influence in the expression of foreign genes

Recombinant Mycobacterium bovis BCG vaccines (rBCG) were first developed in the 1990s as a means of expressing antigens from multiple pathogens. This review examines the key structural factors of recombinant M. bovis that influence the expression of the heterologous antigens and the generation of genetic and functional stability in rBCG, which are crucial for inducing strong and lasting immune responses. The fundamental aim of this paper is to provide an overview of factors that affect the expression of recombinant proteins in BCG and the generation of the immune response against the target antigens, including mycobacterial promoters, location of foreign antigens, and stability of the vectors. The reporter systems that have been employed for evaluation of these molecular features in BCG are also reviewed here.

Stable expression of Mycobacterium bovis antigen 85B in auxotrophic M. bovis bacillus Calmette-Guérin

BACKGROUND

Bovine tuberculosis (TB) is a zoonotic disease caused by Mycobacterium bovis, responsible for causing major losses in livestock. A cost effective alternative to control the disease could be herd vaccination. The bacillus Calmette-Guérin (BCG) vaccine has a limited efficacy against bovine TB, but can improved by over-expression of protective antigens. The M. bovis antigen 85B demonstrates ability to induce protective immune response against bovine TB in animal models. However, current systems for the construction of recombinant BCG expressing multiple copies of the gene result in strains of low genetic stability that rapidly lose the plasmid in vivo. Employing antibiotic resistance as selective markers, these systems also compromise vaccine safety. We previously reported the construction of a stable BCG expression system using auxotrophic complementation as a selectable marker.

OBJECTIVES

The fundamental aim of this study was to construct strains of M. bovis BCG Pasteur and the auxotrophic M. bovis BCG ΔleuD expressing Ag85B and determine their stability in vivo.

METHODS

Employing the auxotrophic system, we constructed rBCG strains that expressed M. bovis Ag85B and compared their stability with a conventional BCG strain in mice. Stability was measured in terms of bacterial growth on the selective medium and retention of antigen expression.

FINDINGS

The auxotrophic complementation system was highly stable after 18 weeks, even during in vivo growth, as the selective pressure and expression of antigen were maintained comparing to the conventional vector.

MAIN CONCLUSION

The Ag85B continuous expression within the host may generate a stronger and long-lasting immune response compared to conventional systems.

The Chromosomal parDE2 Toxin-Antitoxin System of Mycobacterium tuberculosis H37Rv: Genetic and Functional Characterization

Mycobacterium tuberculosis H37Rv escapes host-generated stresses by entering a dormant persistent state. Activation of toxin-antitoxin modules is one of the mechanisms known to trigger such a state with low metabolic activity. M. tuberculosis harbors a large number of TA systems mostly located within discernible genomic islands. We have investigated the parDE2 operon of M. tuberculosis H37Rv encoding MParE2 toxin and MParD2 antitoxin proteins. The parDE2 locus was transcriptionally active from growth phase till late stationary phase in M. tuberculosis. A functional promoter located upstream of parD2 GTG start-site was identified by 5'-RACE and lacZ reporter assay. The MParD2 protein transcriptionally regulated the P parDE2 promoter by interacting through Arg16 and Ser15 residues located in the N-terminus. In Escherichia coli, ectopic expression of MParE2 inhibited growth in early stages, with a drastic reduction in colony forming units. Live-dead analysis revealed that the reduction was not due to cell death alone but due to formation of viable but non-culturable cells (VBNCs) also. The toxic activity of the protein, identified in the C-terminal residues Glu98 and Arg102, was neutralized by the antitoxin MParD2, both in vivo and in vitro. MParE2 inhibited mycobacterial DNA gyrase and interacted with the GyrB subunit without affecting its ATPase activity. Introduction of parE2 gene in the heterologous M. smegmatis host prevented growth and colony formation by the transformed cells. An M. smegmatis strain containing the parDE2 operon also switched to a non-culturable phenotype in response to oxidative stress. Loss in colony-forming ability of a major part of the MParE2 expressing cells suggests its potential role in dormancy, a cellular strategy for adaptation to environmental stresses. Our study has laid the foundation for future investigations to explore the physiological significance of parDE2 operon in mycobacterial pathogenesis.

Conformational perturbation, hydrophobic interactions and oligomeric association are responsible for the enhanced chaperone function of Mycobacterium leprae HSP18 under pre-thermal condition

Chaperone function of HSP18 is enhanced upon pre-heating at 60 °C and above which may be due to structural alterations.

Role of Subunit Exchange and Electrostatic Interactions on the Chaperone Activity of Mycobacterium leprae HSP18

Mycobacterium leprae HSP18, a major immunodominant antigen of M. leprae pathogen, is a small heat shock protein. Previously, we reported that HSP18 is a molecular chaperone that prevents aggregation of different chemically and thermally stressed client proteins and assists refolding of denatured enzyme at normal temperature. We also demonstrated that it can efficiently prevent the thermal killing of E. coli at higher temperature. However, molecular mechanism behind the chaperone function of HSP18 is still unclear. Therefore, we studied the structure and chaperone function of HSP18 at normal temperature (25°C) as well as at higher temperatures (31–43°C). Our study revealed that the chaperone function of HSP18 is enhanced significantly with increasing temperature. Far- and near-UV CD experiments suggested that its secondary and tertiary structure remain intact in this temperature range (25–43°C). Besides, temperature has no effect on the static oligomeric size of this protein. Subunit exchange study demonstrated that subunits of HSP18 exchange at 25°C with a rate constant of 0.018 min^-1. Both rate of subunit exchange and chaperone activity of HSP18 is found to increase with rise in temperature. However, the surface hydrophobicity of HSP18 decreases markedly upon heating and has no correlation with its chaperone function in this temperature range. Furthermore, we observed that HSP18 exhibits diminished chaperone function in the presence of NaCl at 25°C. At elevated temperatures, weakening of interactions between HSP18 and stressed client proteins in the presence of NaCl results in greater reduction of its chaperone function. The oligomeric size, rate of subunit exchange and structural stability of HSP18 were also found to decrease when electrostatic interactions were weakened. These results clearly indicated that subunit exchange and electrostatic interactions play a major role in the chaperone function of HSP18.

Interaction of ATP with a small heat shock protein from Mycobacterium leprae: effect on its structure and function

Adenosine-5'-triphosphate (ATP) is an important phosphate metabolite abundantly found in Mycobacterium leprae bacilli. This pathogen does not derive ATP from its host but has its own mechanism for the generation of ATP. Interestingly, this molecule as well as several antigenic proteins act as bio-markers for the detection of leprosy. One such bio-marker is the 18 kDa antigen. This 18 kDa antigen is a small heat shock protein (HSP18) whose molecular chaperone function is believed to help in the growth and survival of the pathogen. But, no evidences of interaction of ATP with HSP18 and its effect on the structure and chaperone function of HSP18 are available in the literature. Here, we report for the first time evidences of "HSP18-ATP" interaction and its consequences on the structure and chaperone function of HSP18. TNP-ATP binding experiment and surface plasmon resonance measurement showed that HSP18 interacts with ATP with a sub-micromolar binding affinity. Comparative sequence alignment between M. leprae HSP18 and αB-crystallin identified the sequence 49KADSLDIDIE58 of HSP18 as the Walker-B ATP binding motif. Molecular docking studies revealed that β4-β8 groove/strands as an ATP interactive region in M. leprae HSP18. ATP perturbs the tertiary structure of HSP18 mildly and makes it less susceptible towards tryptic cleavage. ATP triggers exposure of additional hydrophobic patches at the surface of HSP18 and induces more stability against chemical and thermal denaturation. In vitro aggregation and thermal inactivation assays clearly revealed that ATP enhances the chaperone function of HSP18. Our studies also revealed that the alteration in the chaperone function of HSP18 is reversible and is independent of ATP hydrolysis. As the availability and binding of ATP to HSP18 regulates its chaperone function, this functional inflection may play an important role in the survival of M. leprae in hosts.

The use of directed evolution to create a stable and immunogenic recombinant BCG expressing a modified HIV-1 Gag antigen

Numerous features make Mycobacterium bovis BCG an attractive vaccine vector for HIV. It has a good safety profile, it elicits long-lasting cellular immune responses and in addition manufacturing costs are affordable. Despite these advantages it is often difficult to express viral antigens in BCG, which results in genetic instability and low immunogenicity. The aim of this study was to generate stable recombinant BCG (rBCG) that express high levels of HIV antigens, by modification of the HIV genes. A directed evolution process was applied to recombinant mycobacteria that expressed HIV-1 Gag fused to the green fluorescent protein (GFP). Higher growth rates and increased GFP expression were selected for. Through this process a modified Gag antigen was selected. Recombinant BCG that expressed the modified Gag (BCG[pWB106] and BCG[pWB206]) were more stable, produced higher levels of antigen and grew faster than those that expressed the unmodified Gag (BCG[pWB105]). The recombinant BCG that expressed the modified HIV-1 Gag induced 2 to 3 fold higher levels of Gag-specific CD4 T cells than those expressing the unmodified Gag (BCG[pWB105]). Mice primed with 10⁷ CFU BCG[pWB206] and then boosted with MVA-Gag developed Gag-specific CD8 T cells with a frequency of 1343±17 SFU/10⁶ splenocytes, 16 fold greater than the response induced with MVA-Gag alone. Levels of Gag-specific CD4 T cells were approximately 5 fold higher in mice primed with BCG[pWB206] and boosted with MVA-Gag than in those receiving the MVA-Gag boost alone. In addition mice vaccinated with BCG[pWB206] were protected from a surrogate vaccinia virus challenge.

Double recombinant Mycobacterium bovis BCG strain for screening of primary and rationale-based antimycobacterial compounds

Conventional antimycobacterial screening involves CFU analysis, which poses a great challenge due to slow growth of mycobacteria. Recombinant strains carrying reporter genes under the influence of constitutive promoters allow rapid and wide screening of compounds but without revealing their modes of action. Reporter strains using pathway-specific promoters provide a better alternative but allow a limited screening of compounds interfering with only a particular metabolic pathway. This reduces these strains to merely a second-line screening system, as they fail to identify even the more potent compounds if they are not inhibiting the pathway of interest. In this study, we have generated a double recombinant Mycobacterium bovis BCG strain carrying firefly and Renilla luciferase genes as two reporters under the control of a constitutive and an inducible mycobacterial promoter. The presence of dual reporters allows simultaneous expression and analysis of two reporter enzymes within a single system. The expression profile of the firefly luciferase gene, rendered by a constitutive mycobacterial promoter, coincides with the decline in bacterial growth in response to a wide range of antimycobacterial drugs, while the enhanced expression of Renilla luciferase mirrors the selective induction of the reporter gene expression as a result of pathway-specific inhibition. Thus, the double recombinant strain allows the screening of both primary and rationally synthesized antimycobacterial compounds in a single assay. The inhibiting response of drugs was monitored with a dual-luciferase reporter assay which can be easily adapted in high-throughput mode.

Protective role of Mycobacterium leprae small heat-shock protein in heterologous hosts, Escherichia coli and Mycobacterium smegmatis, grown under stress

The aim of this study is to examine the in vivo role of a small heat-shock protein (sHsp18) from Mycobacterium leprae in the survival of heterologous recombinant hosts carrying the gene encoding this protein under different environmental conditions that are normally encountered by M. leprae during its infection of the human host. Using an Escherichia coli system where shsp18 expression is controlled by its native promoter, we show that expression of shsp18 is induced under low oxygen tension, nutrient depletion and oxidative stress, all of which reflect the natural internal environment of the granulomas where the pathogen resides for long periods. We demonstrate the in vivo chaperone activity of sHsp18 through its ability to confer survival advantage to recombinant E. coli at heat-shock temperatures. Additional evidence for the protective role of sHsp18 was obtained when Mycobacterium smegmatis harbouring a copy of shsp18 was found to multiply better in human macrophages. Furthermore, the autokinase activity of sHsp18 protein demonstrated for what is believed to be the first time in this study implies that some of the functions of sHsp18 might be controlled by the phosphorylation state of this protein. Results from this study suggest that shsp18 might be one of the factors that facilitate the survival and persistence of M. leprae under stress and autophosphorylation of sHsp18 protein could be a mechanism used by this protein to sense changes in the external environment.

The complex architecture of mycobacterial promoters

The genus Mycobacterium includes a variety of species with differing phenotypic properties, including growth rate, pathogenicity and environment- and host-specificity. Although many mycobacterial species have been extensively studied and their genomes sequenced, the reasons for phenotypic variation between closely related species remain unclear. Variation in gene expression may contribute to these characteristics and enable the bacteria to respond to changing environmental conditions. Gene expression is controlled primarily at the level of transcription, where the main element of regulation is the promoter. Transcriptional regulation and associated promoter sequences have been studied extensively in E. coli. This review describes the complex structure and characteristics of mycobacterial promoters, in comparison to the classical E. coli prokaryotic promoter structure. Some components of mycobacterial promoters are similar to those of E. coli. These include the predominant guanine residue at the transcriptional start point, conserved -10 hexamer, similar interhexameric distances, the use of ATG as a start codon, the guanine- and adenine-rich ribosome binding site and the presence of extended -10 (TGn) motifs in strong promoters. However, these components are much more variable in sequence in mycobacterial promoters and no conserved -35 hexamer sequence (clearly defined in E. coli) can be identified. This may be a result of the high G+C content of mycobacterial genomes, as well as the large number of sigma factors present in mycobacteria, which may recognise different promoter sequences. Mycobacteria possess a complex transcriptional regulatory network. Numerous regulatory motifs have been identified in mycobacterial promoters, predominantly in the interhexameric region. These are bound by specific transcriptional regulators in response to environmental changes. The combination of specific promoter sequences, transcriptional regulators and a variety of sigma factors enables rapid and specific responses to diverse conditions and different stages of infection. This review aims to provide an overview of the complex architecture of mycobacterial transcriptional regulation.

Assaying promoter activity using LacZ and GFP as reporters

The ability of bacteria to survive in a variety of different niches is due, in part, to their ability to respond and adapt to the environment. Extracellular signals are recognized by bacilli, and their responses are generally conducted at the transcript level. RNA polymerases recognize specific promoter regions on the genome and initiate transcription. Therefore, the analysis of gene expression is paramount to understanding the biology of an organism. In the case of pathogens, gene expression can alter during the course of the infection, and, therefore, specific targets can be identified for drug development. Promoter activity can be determined by cloning a promoter sequence upstream of a reporter gene and assaying the reporter activity, either from whole cells or from cell lysates. This chapter describes two reporter systems (GFP and LacZ) used for determining promoter activity that have been widely used in mycobacteria.

Optimisation of bioluminescent reporters for use with mycobacteria

Background

Mycobacterium tuberculosis, the causative agent of tuberculosis, still represents a major public health threat in many countries. Bioluminescence, the production of light by luciferase-catalyzed reactions, is a versatile reporter technology with multiple applications both in vitro and in vivo. In vivo bioluminescence imaging (BLI) represents one of its most outstanding uses by allowing the non-invasive localization of luciferase-expressing cells within a live animal. Despite the extensive use of luminescent reporters in mycobacteria, the resultant luminescent strains have not been fully applied to BLI.

Methodology/Principal Findings

One of the main obstacles to the use of bioluminescence for in vivo imaging is the achievement of reporter protein expression levels high enough to obtain a signal that can be detected externally. Therefore, as a first step in the application of this technology to the study of mycobacterial infection in vivo, we have optimised the use of firefly, Gaussia and bacterial luciferases in mycobacteria using a combination of vectors, promoters, and codon-optimised genes. We report for the first time the functional expression of the whole bacterial lux operon in Mycobacterium tuberculosis and M. smegmatis thus allowing the development of auto-luminescent mycobacteria. We demonstrate that the Gaussia luciferase is secreted from bacterial cells and that this secretion does not require a signal sequence. Finally we prove that the signal produced by recombinant mycobacteria expressing either the firefly or bacterial luciferases can be non-invasively detected in the lungs of infected mice by bioluminescence imaging.

Conclusions/Significance

While much work remains to be done, the finding that both firefly and bacterial luciferases can be detected non-invasively in live mice is an important first step to using these reporters to study the pathogenesis of M. tuberculosis and other mycobacterial species in vivo. Furthermore, the development of auto-luminescent mycobacteria has enormous ramifications for high throughput mycobacterial drug screening assays which are currently carried out either in a destructive manner using LuxAB or the firefly luciferase.

The small heat-shock protein HspL is a VirB8 chaperone promoting type IV secretion-mediated DNA transfer

Agrobacterium tumefaciens is a plant pathogen that utilizes a type IV secretion system (T4SS) to transfer DNA and effector proteins into host cells. In this study we discovered that an alpha-crystallin type small heat-shock protein (alpha-Hsp), HspL, is a molecular chaperone for VirB8, a T4SS assembly factor. HspL is a typical alpha-Hsp capable of protecting the heat-labile model substrate citrate synthase from thermal aggregation. It forms oligomers in a concentration-dependent manner in vitro. Biochemical fractionation revealed that HspL is mainly localized in the inner membrane and formed large complexes with certain VirB protein subassemblies. Protein-protein interaction studies indicated that HspL interacts with VirB8, a bitopic integral inner membrane protein that is essential for T4SS assembly. Most importantly, HspL is able to prevent the aggregation of VirB8 fused with glutathione S-transferase in vitro, suggesting that it plays a role as VirB8 chaperone. The chaperone activity of two HspL variants with amino acid substitutions (F98A and G118A) for both citrate synthase and glutathione S-transferase-VirB8 was reduced and correlated with HspL functions in T4SS-mediated DNA transfer and virulence. This study directly links in vitro and in vivo functions of an alpha-Hsp and reveals a novel alpha-Hsp function in T4SS stability and bacterial virulence.

Experimental models used to study human tuberculosis

Mycobacterium tuberculosis causes more deaths in humans than any other bacterial pathogen. The most recent data from the World Health Organization reveal that over 9million new cases of tuberculosis occur each year and that the incidence appears to be increasing with population growth. Despite the global burden of tuberculosis, we are still reliant on relatively dated measures to prevent, diagnose, and treat the disease. New, more effective tools are needed to diminish the incidence of tuberculosis. M. tuberculosis lacks a natural host beyond humans and, hence, surrogate models have been employed in the study of the pathogen. The discovery and development of new vaccines, diagnostics, or antitubercular drugs are dependent upon the validity of any experimental model used and its relevance to tuberculosis in humans. In this review, a range of experimental models, from in vitro studies with fast-growing low-pathogenic species of mycobacteria to the infection of nonhuman primates with virulent M. tuberculosis, will be discussed.

Recombinant Mycobacterium bovis BCG

The Bacillus Calmette-Guerin (BCG) is an attenuated strain of Mycobacterium bovis that has been broadly used as a vaccine against human tuberculosis. This live bacterial vaccine is able to establish a persistent infection and induces both cellular and humoral immune responses. The development of mycobacterial genetic systems to express foreign antigens and the adjuvanticity of BCG are the basis of the potential use of this attenuated mycobacterium as a recombinant vaccine. Over the years, a range of strategies has been developed to allow controlled and stable expression of viral, bacterial and parasite antigens in BCG. Herein, we review the strategies developed to express heterologous antigens in BCG and the immune response elicited by recombinant BCG constructs. In addition, the use of recombinant BCG as an immunomodulator and future perspectives of BCG as a recombinant vaccine vector are discussed.

Small heat-shock protein HspL is induced by VirB protein(s) and promotes VirB/D4-mediated DNA transfer in Agrobacterium tumefaciens

Agrobacterium tumefaciens is a Gram-negative plant-pathogenic bacterium that causes crown gall disease by transferring and integrating its transferred DNA (T-DNA) into the host genome. We characterized the chromosomally encoded alpha-crystallin-type small heat-shock protein (alpha-Hsp) HspL, which was induced by the virulence (vir) gene inducer acetosyringone (AS). The transcription of hspL but not three other alpha-Hsp genes (hspC, hspAT1, hspAT2) was upregulated by AS. Further expression analysis in various vir mutants suggested that AS-induced hspL transcription is not directly activated by the VirG response regulator but rather depends on the expression of VirG-activated virB genes encoding components of the type IV secretion system (T4SS). Among the 11 virB genes encoded by the virB operon, HspL protein levels were reduced in strains with deletions of virB6, virB8 or virB11. VirB protein accumulation but not virB transcription levels were reduced in an hspL deletion mutant early after AS induction, implying that HspL may affect the stability of individual VirB proteins or of the T4S complex directly or indirectly. Tumorigenesis efficiency and the VirB/D4-mediated conjugal transfer of an IncQ plasmid RSF1010 derivative between A. tumefaciens strains were reduced in the absence of HspL. In conclusion, increased HspL abundance is triggered in response to certain VirB protein(s) and plays a role in optimal VirB protein accumulation, VirB/D4-mediated DNA transfer and tumorigenesis.

Functional insights by comparison of modeled structures of 18kDa small heat shock protein and its mutant in Mycobacterium leprae

In this work we are proposing Homology modeled structures of Mycobacterium leprae 18kDa heat shock protein and its mutant. The more closely related structure of the small heat shock protein (sHSP) belonging to the eukaryotic species from wheat sHSP16.9 and 16.3kDa ACR1 protein from Mycobacterium tuberculosis were used as template structures. Each model contains an N-terminal domain, alpha-crystalline domain and a C-terminal tail. The models showed that a single point mutation from serine to proline at 52(nd) position causes structural changes. The structural changes are observed in N-terminal region and alpha-crystalline domains. Serine in 52(nd) position is observed in beta4 strand and Proline in 52(nd) position is observed in loop. The number of residues contributing alpha helix at N-terminal region varies in both models. In 18S more number of residues is present in alpha helix when compared to 18P. The loop regions between beta3 and beta4 strands of both models vary in number of residues present in it. Number of residues contributing beta4 strand in both models vary. beta6 strand is absent in both models. Major functional peptide region of alpha crystalline domains of both models varies. These differences observed in secondary structures support their distinct functional roles. It also emphasizes that a point mutation can cause structural variation.

Functional characterization of a small heat shock protein from Mycobacterium leprae

BACKGROUND

Small heat shock proteins are ubiquitous family of stress proteins, having a role in virulence and survival of the pathogen. M. leprae, the causative agent of leprosy is an uncultivable organism in defined media, hence the biology and function of proteins were examined by cloning M. leprae genes in heterologous hosts. The study on sHsp18 was carried out as the knowledge about the functions of this major immunodominant antigen of M. leprae is scanty.

RESULTS

The gene encoding Mycobacterium leprae small heat shock protein (sHsp18) was amplified from biopsy material of leprosy patients, and cloned and expressed in E. coli. The localization and in vitro characterization of the protein are detailed in this report. Data show that major portion of the protein is localized in the outer membrane of E. coli. The purified sHsp18 functions as an efficient chaperone as shown by their ability to prevent thermal inactivation of restriction enzymes SmaI and NdeI. Physical interaction of the chaperone with target protein is also demonstrated. Size exclusion chromatography of purified protein shows that the protein can form multimeric complexes under in vitro conditions as is demonstrated for several small heat shock proteins.

CONCLUSION

The small heat shock protein sHsp18 of M. leprae is a chaperone and shows several properties associated with other small heat shock proteins. Membrane association and in vitro chaperone function of sHsp18 shows that the protein may play a role in the virulence and survival of M. leprae in infected host.

A novel differential expression system for gene modulation in Mycobacteria

Tuberculosis (TB) remains a major global health problem, and successful genetic manipulation of mycobacteria is crucial for developing new approaches to study the mechanism of pathogenesis of Mycobacterium tuberculosis (M.tb) and to combat TB. In this study, a series of M.tb furA gene operator/promoter (pfurA) mutants were generated aiming at optimization of the promoter activities in mycobacterial strains. Measured by the lacZ gene-fusion reporter system, change of the initial codon GTG to the preferred ATG resulted in a double increase of beta-galactosidase activity, while a 6-bp substitution in the conserved FurA binding AT-rich region upstream of furA gene led to 4-6 folds increase of the activity. It is significant that combination of both mutations showed about 10 folds of beta-galactosidase activity higher than that of the prototype pfurA. Furthermore, all of the furA promoters were expressed continuously in vivo during intracellular growth of Mycobacterium bovis BCG, and were induced early upon infection in macrophages. Employing the series of pfurA-based differential expression vectors, M.tb chimeric antigen Ag856A2 known for its excellent immunogenicity, was shown to be expressed at different levels in the recombinant Mycobacterium smegmatis and BCG strains. These results indicated that this differential expression system is feasible to express any target antigen of interest in a modular fashion for the study of gene regulation in mycobacterial strains, and also for the development of different recombinant BCG vaccine candidates against TB or other infectious diseases, which would be beneficial for elicitation of optimal immune response.

B subunit ofEscherichia coliheat-labile enterotoxin as adjuvant of humoral immune response in recombinant BCG vaccination

The B subunit of Escherichia coli heat-labile enterotoxin (LTB), a nontoxic molecule with potent biological properties, is a powerful mucosal and parenteral adjuvant that induces a strong immune response against co-administered or coupled antigens. In this paper, the effect of LTB on the humoral immune response to recombinant BCG (rBCG) vaccination was evaluated. Isogenic mice were immunized with rBCG expressing the R1 repeat region of the P97 adhesin of Mycoplasma hyopneumoniae alone (rBCG/R1) or fused to LTB (rBCG/LTBR1). Anti-R1 systemic antibody levels (IgG1, IgG2a, IgG2b, IgG3, IgM, and IgA) were measured by ELISA using recombinant R1 as antigen. With the exception of IgM, LTB doubled the anti-R1 antibody levels in rBCG vaccination. The IgG1/IgG2a mean ratio showed that both rBCG/LTBR1 and rBCG/R1 induced a mixed Th1/Th2 immune response. Interestingly, anti-R1 serum IgA was induced only by rBCG/LTBR1. These results demonstrate that LTB has an adjuvant effect on the humoral immune response to recombinant antigens expressed in BCG.

Recombinant Mycobacterium bovis BCG expressing the LipL32 antigen of Leptospira interrogans protects hamsters from challenge

The bacillus Calmette--Guerin (BCG), a live attenuated Mycobacterium bovis strain is considered a promising candidate as a vector system for delivery of foreign antigens to the immune system. The gene coding for the Leptospira interrogans external membrane protein LipL32, a highly immunogenic antigen found in all pathogenic leptospira, was cloned into several mycobacterial vectors for expression in BCG. Hamsters immunized with recombinant BCG (rBCG) expressing LipL32 were protected against mortality (P<or=0.05) upon challenge with a lethal inoculum of L. interrogans serovar Copenhageni. Autopsy examination did not reveal macroscopic or histological evidence of disease in rBCG immunized hamsters that survived lethal challenge. The data presented here further enhance the status of LipL32 as a promising candidate antigen for use in the control of leptospirosis, when presented to the immune system by an appropriate delivery system.

Auxotrophic complementation as a selectable marker for stable expression of foreign antigens in Mycobacterium bovis BCG

Mycobacterium bovis BCG has the potential to be an effective live vector for multivalent vaccines. However, most mycobacterial cloning vectors rely on antibiotic resistance genes as selectable markers, which would be undesirable in any practical vaccine. Here we report the use of auxotrophic complementation as a selectable marker that would be suitable for use in a recombinant vaccine. A BCG auxotrophic for the amino acid leucine was constructed by knocking out the leuD gene by unmarked homologous recombination. Expression of leuD on a plasmid not only allowed complementation, but also acted as a selectable marker. Removal of the kanamycin resistance gene, which remained necessary for plasmid manipulations in Escherichia coli, was accomplished by two different methods: restriction enzyme digestion followed by re-ligation before BCG transformation, or by Cre-loxP in vitro recombination mediated by the bacteriophage P1 Cre Recombinase. Stability of the plasmid was evaluated during in vitro and in vivo growth of the recombinant BCG in comparison to selection by antibiotic resistance. The new system was highly stable even during in vivo growth, as the selective pressure is maintained, whereas the conventional vector was unstable in the absence of selective pressure. This new system will now allow the construction of potential recombinante vaccine strains using stable multicopy plasmid vectors without the inclusion of antibiotic resistance markers.

Regulation of pyr gene expression in Mycobacterium smegmatis by PyrR-dependent translational repression

Regulation of pyrimidine biosynthetic (pyr) genes by a transcription attenuation mechanism that is mediated by the PyrR mRNA-binding regulatory protein has been demonstrated for numerous gram-positive bacteria. Mycobacterial genomes specify pyrR genes and contain obvious PyrR-binding sequences in the initially transcribed regions of their pyr operons, but transcription antiterminator and attenuation terminator sequences are absent from their pyr 5' leader regions. This work demonstrates that repression of pyr operon expression in Mycobacterium smegmatis by exogenous uracil requires the pyrR gene and the pyr leader RNA sequence for binding of PyrR. Plasmids containing the M. smegmatis pyr promoter-leader region translationally fused to lacZ also displayed pyrR-dependent repression, but transcriptional fusions of the same sequences to a lacZ gene that retained the lacZ ribosome-binding site were not regulated by PyrR plus uracil. We propose that PyrR regulates pyr expression in M. smegmatis, other mycobacteria, and probably in numerous other bacteria by a translational repression mechanism in which nucleotide-regulated binding of PyrR occludes the first ribosome-binding site of the pyr operon.

Evaluation of recombinant BCG expressing rotavirus VP6 as an anti-rotavirus vaccine

Recombinant BCG expressing rotavirus VP6 was explored as an anti-rotavirus vaccine in a mouse model. Three promoters and five ribosome-binding sites were used in episomal and integrative E. coli-mycobacterium shuttle vectors to express VP6 in BCG. The VP6 gene was configured for accumulation within the BCG cytoplasm, secretion from the BCG cell or targeting to the BCG cell membrane. Vectors were assessed in terms of stability, levels of antigen production, immunogenicity and protection in mice. Gross instability occurred in episomal vectors utilizing the hsp60 promoter. However, three integrative vectors using the same expression system and two episomal vectors using inducible promoters were successfully recovered from BCG. Growth rates of the former were not detectably reduced. Growth rates of the latter were considerably reduced, implying the existence of a significant metabolic load. In the absence of selection, loss rate of these plasmids was high. VP6 production levels (0.04-1.78% of total cytoplasmic protein) were on the lower end of the range reported for other rBCG. One episomal and one integrated vaccine reduced viral shedding in intraperitoneally vaccinated mice challenged with rotavirus. Compared to controls, infection-associated faecal shedding of virus was reduced by 66% and 62%, respectively. These protective vectors differ in promoter, ribosome-binding site and antigen production level, but both link the VP6 protein to the 19kDa lipoprotein signal sequence, suggesting that transport of VP6 to the BCG membrane is important for induction of a protective immune response. Protection occurred in the absence of detectable anti-rotavirus antibody in serum or faeces, implicating cellular immunity in protection.

Mycobacterium bovis BCG as a delivery system for the RAP-1 antigen from Babesia bovis

Babesia bovis is the causative agent of babesiosis, a tick-borne disease that is a major cause of loss to livestock production in Latin America. Vaccination against Babesia species represents a major challenge against cattle morbidity and mortality in enzootic areas. The aim of this study was to evaluate the capacity of Bacille Calmette-Guerin (BCG) to deliver the rhoptry associated protein (RAP-1) antigen of B. bovis and to stimulate specific cellular and humoral immune responses in mice. Two of five mycobacterial expression vectors efficiently expressed the antigen. These constructs were subsequently studied in vivo following three immunization protocols. The construct with the greatest in vivo stability proved to be the one that induced the strongest immune responses. Our data support the hypothesis that specific T lymphocyte priming by rBCG can be employed as a component of a combined vaccine strategy to induce long-lasting humoral and cellular immune responsiveness towards B. bovis and encourage further work on the application of rBCG to the development of Babesia vaccines.

Use of the immunodominant 18-kiloDalton small heat shock protein as a serological marker for exposure to Mycobacterium ulcerans

While it is well established that proximity to wetlands is a risk factor for contracting Buruli ulcer, it is not clear what proportion of a population living in an area where the etiologic agent, Mycobacterium ulcerans, is endemic is actually exposed to this disease. Immunological cross-reactivity among mycobacterial species complicates the development of a specific serological test. Among immunodominant proteins recognized by a panel of anti-M. ulcerans monoclonal antibodies, the M. ulcerans homologue of the M. leprae 18-kDa small heat shock protein (shsp) was identified. Since this shsp has no homologues in M. bovis and M. tuberculosis, we evaluated its use as a target antigen for a serological test. Anti-18-kDa shsp antibodies were frequently found in the sera of Buruli ulcer patients and of healthy household contacts but rarely found in controls from regions where the infection is not endemic. The results indicate that only a small proportion of M. ulcerans-infected individuals contract the clinical disease.

Mycobacterial transcriptional signals: requirements for recognition by RNA polymerase and optimal transcriptional activity

Majority of the promoter elements of mycobacteria do not function well in other eubacterial systems and analysis of their sequences has established the presence of only single conserved sequence located at the −10 position. Additional sequences for the appropriate functioning of these promoters have been proposed but not characterized, probably due to the absence of sufficient number of strong mycobacterial promoters. In the current study, we have isolated functional promoter-like sequences of mycobacteria from the pool of random DNA sequences. Based on the promoter activity in Mycobacterium smegmatis and score assigned by neural network promoter prediction program, we selected one of these promoter sequences, namely A₃₇ for characterization in order to understand the structure of housekeeping promoters of mycobacteria. A₃₇–RNAP complexes were subjected to DNase I footprinting and subsequent mutagenesis. Our results demonstrate that in addition to −10 sequences, DNA sequence at −35 site can also influence the activity of mycobacterial promoters by modulating the promoter recognition by RNA polymerase and subsequent formation of open complex. We also provide evidence that despite exhibiting similarities in −10 and −35 sequences, promoter regions of mycobacteria and Escherichia coli differ from each other due to differences in their requirement of spacer sequences between the two positions.

Immunogenicity of Mycobacterium bovis BCG expressing Anaplasma marginale MSP1a antigen

Humoral and cellular immune responses of mice inoculated with recombinant Mycobacterium bovis BCG expressing the MSP1a antigen of Anaplasma marginale were evaluated. The msp1a gene was amplified by PCR and cloned into the mycobacterial expression vectors pUS2000 and pMIP12. Immunization of isogenic BALB/c mice with the rBCG/pUS2000-msp1a construct induced significant seroconversion to MSP1a (p<0.001), which was 26 times above pre-immunization levels at day 63 post-initial immunization and which remained stable for the duration of the experiment (6 months). In contrast, rBCG/pMIP12-msp1a induced seroconversion at a level of 6 times above pre-immunization values, which peaked at day 63. Western blot analysis showed that sera derived from mice vaccinated with either rBCG construct recognized both native and recombinant forms of A. marginale MSP1a. In contrast to the humoral response data, immunization with rBCG/pMIP12-msp1a was found to induce a markedly stronger cellular response than that recorded for BCG/pUS2000-msp1a. These observations clearly demonstrated the immunogenicity of recombinant BCG expressing the MSP1a antigen and suggested that the immune responses were influenced by the level of antigen expression. The results of this research warrant studies of recombinant M. bovis BCG expressing MSP1a in cattle to test for protective antibody production for control of bovine anaplasmosis.

Truncated hemoglobin GlbO from Mycobacterium leprae alleviates nitric oxide toxicity

As a consequence of reductive genome evolution, the obligate intracellular pathogen Mycobacterium leprae has minimized the repertoire of genes implicated in defense against reactive oxygen and nitrogen species. Genes for multiple hemoglobin types coexist in mycobacterial genomes, but M. leprae has retained only glbO, encoding a group-II truncated hemoglobin. Mycobacterium tuberculosis GlbO has been involved in oxygen transfer and respiration during hypoxia, but a role in protection from nitric oxide (NO) has not been documented yet. Here, we report that the in vitro reaction of oxygenated recombinant M. leprae GlbO with NO results in an immediate stoichiometric formation of nitrate, concomitant with heme-protein oxidation. Overexpression of GlbO alleviates the growth inhibition of Escherichia colihmp (flavohemoglobin gene) mutants in the presence of NO-donors, partly complementing the defect in Hmp synthesis. A promoter element upstream of glbO was predicted in silico, and confirmed by using a glbO::lacZ transcriptional fusion in the heterologous Mycobacterium smegmatis system. The glbO::lacZ fusion was expressed through the whole growth cycle of M. smegmatis, and moderately induced by NO. We propose that M. leprae, by retaining the unique truncated hemoglobin GlbO, may have coupled O2 delivery to the terminal oxidase with a defensive mechanism to scavenge NO from respiratory enzymes. These activities would help to sustain the obligate aerobic metabolism required for intracellular survival of leprosy bacilli.

rmlB and rmlC genes are essential for growth of mycobacteria

The rhamnose-GlcNAc disaccharide linker is fundamental to the structural integrity of mycobacterial cell wall. The donor dTDP-rhamnose is synthesized by four enzymes (RmlA, B, C, and D) beginning with dTTP and glucose-1-phosphate. We generated M. smegmatis rmlB gene knockout mutant (transcription of downstream rmlC gene was blocked because of a polar effect) by homologous recombination. When the Mycobacterium tuberculosis (Tb) rmlB rescue plasmid carrying a temperature-sensitive replication origin and Tb rmlC bearing plasmid with a normal replication origin were present in the mc(2)155 rmlB knockout mutant, the mutant was unable to grow at non-permissive temperature (42 degrees C) where the Tb rmlB rescue plasmid is lost. While the Tb rmlC rescue plasmid carrying a temperature-sensitive replication origin and Tb rmlB bearing plasmid with a normal replication origin were present in the mc(2)155 rmlB knock out mutant, this mutant was also unable to grow at the non-permissive temperature where the Tb rmlC rescue plasmid is lost. These results demonstrate that rmlB and rmlC genes are essential for mycobacterial growth, therefore, RmlB and RmlC are essential targets to develop new anti-tuberculosis drugs.

Identification and characterization of rel promoter element of Mycobacterium tuberculosis

The rel gene is responsible for the maintenance of the level of (p)ppGpp in bacteria under nutrient starvation. This phenomenon known as stringent response plays an important role during survival of the microorganisms in stationary phase. We have cloned 1.6 kb upstream sequence of rel gene of Mycobacterium tuberculosis in a shuttle vector pSD5B containing promoterless lacZ gene and promoter activity was observed in Mycobacterium smegmatis cells by blue/white selection and was measured by beta-galactosidase assay. In order to delineate the minimal promoter element of rel gene, a 200 bp fragment from this 1.6 kb upstream sequence was further cloned in promoterless lacZ shuttle vector pSD5B and promoter activity was observed in M. smegmatis cells in similar way. The 200 bp promoter fragment was found to be mycobacterium specific and did not respond when transformed in Escherichia coli. The +1 transcription start site was determined by primer extension method. The -10 promoter region was identified to be TATCCT. The three T bases when mutated, showed a remarkable decrease in the lacZ expression thus confirming the -10 region. The translation start site has also been identified by site directed frame shift mutagenesis. It appears that this rel promoter can be used for expression of proteins in mycobacteria.

Factors influencing the immune response to foreign antigen expressed in recombinant BCG vaccines

A wide range of recombinant BCG vaccine candidates containing foreign viral, bacterial, parasite or immunomodulatory genetic material have been developed and evaluated, primarily in animal models, for immune response to the foreign antigen. This review considers some of the factors that may influence the immunogenicity of these vaccines. The influence of levels and timing of expression of the foreign antigen and the use of targeting sequences are considered in the first section. Genetic and functional stability of rBCG is reviewed in the second section. In the last section, the influence of dose and route of immunization, strain of BCG and the animal model used are discussed.

Characterization of the two Mycobacterium tuberculosis recA promoters

The recA gene of Mycobacterium tuberculosis is unusual in that it is expressed from two promoters, one of which, P1, is DNA damage inducible independently of LexA and RecA, while the other, P2, is regulated by LexA in the classical way (E. O. Davis, B. Springer, K. K. Gopaul, K. G. Papavinasasundaram, P. Sander, and E. C. Böttger, Mol. Microbiol. 46:791-800, 2002). In this study we characterized these two promoters in more detail. Firstly, we localized the promoter elements for each of the promoters, and in so doing we identified a mutation in each promoter which eliminates promoter activity. Interestingly, a motif with similarity to Escherichia coli sigma(70) -35 elements but located much closer to the -10 element is important for optimal expression of P1, whereas the sequence at the -35 location is not. Secondly, we found that the sequences flanking the promoters can have a profound effect on the expression level directed by each of the promoters. Finally, we examined the contribution of each of the promoters to recA expression and compared their kinetics of induction following DNA damage.

Role of 5'-TGN-3' motif in the interaction of mycobacterial RNA polymerase with a promoter of 'extended -10' class

In a systematic approach to understand the transcriptional machinery of mycobacteria, we had previously isolated and characterized mycobacterial promoter regions. In this study, we have investigated molecular interactions between mycobacterial RNA polymerase holoenzyme, reconstituted with different sigma subunits and the promoter element of the Mycobacterium tuberculosis gene pknH (Rv1266c), a representative of promoters belonging to the 'extended -10' class. In vitro transcription assays using the pknH promoter and reconstituted RNA polymerase holoenzyme demonstrated that transcription from the pknH promoter is specifically initiated by sigmaA, the principal sigma factor of mycobacteria. DNase I protection assay and deletion studies with the pknH promoter revealed that the minimal region required for optimal transcription carries the sequence from position -37 to position +6. Moreover, mutation in the TGN motif of the pknH promoter resulted in the loss of >75% of its activity. Binding of RNA polymerase with wild-type promoter as well as its TG- mutant revealed that the TGN motif is required for the transition from a close complex into an open complex. Further, it was observed that the presence of the TGN motif reduces the thermal energy required for the conversion of a close complex into an open complex, necessary for initiation of transcription.

Expression of foreign genes in Mycobacterium bovis BCG strains using different promoters reveals instability of the hsp60 promoter for expression of foreign genes in Mycobacterium bovis BCG strains

SETTING

Optimization of BCG as a vehicle for live recombinant vaccines requires improved strategies for stable antigen expression.

OBJECTIVES

To investigate the effects of various combinations of post-translational signals and promoters on expression and stability in different BCG strains.

DESIGN

Plasmids were constructed using mycobacterial promoters (hsp60, 19-kDa antigen, 85A antigen--from the Mycobacterium tuberculosis complex--and the 18-kDa antigen from Mycobacterium leprae) and post-translation signals (85A antigen secretion and 19-kDa antigen acylation signals), coupled with reporter genes.

RESULTS

The 19-kDa acylation signal had little effect on expression, while the 85A secretion signal enhanced markedly the levels of cell-associated product. Inclusion of the hsp60 promoter caused plasmid instability; various deletions affecting the promoter region occurred during or soon after transformation, but not during subsequent growth of the transformants, nor with other promoters. BCG Moreau appeared to be more susceptible to deletions than other BCG strains.

CONCLUSIONS

The 85A signal may prove useful in optimizing gene expression in BCG, irrespective of secretion of the product. Deletions associated with the hsp60 promoter may be due to a transient lethal induction of the hsp60 promoter associated with electroporation. With intact plasmid there was no marked difference in expression between BCG strains.

Mycobacterium tuberculosis pathogenesis and molecular determinants of virulence

Tuberculosis (TB), one of the oldest known human diseases. is still is one of the major causes of mortality, since two million people die each year from this malady. TB has many manifestations, affecting bone, the central nervous system, and many other organ systems, but it is primarily a pulmonary disease that is initiated by the deposition of Mycobacterium tuberculosis, contained in aerosol droplets, onto lung alveolar surfaces. From this point, the progression of the disease can have several outcomes, determined largely by the response of the host immune system. The efficacy of this response is affected by intrinsic factors such as the genetics of the immune system as well as extrinsic factors, e.g., insults to the immune system and the nutritional and physiological state of the host. In addition, the pathogen may play a role in disease progression since some M. tuberculosis strains are reportedly more virulent than others, as defined by increased transmissibility as well as being associated with higher morbidity and mortality in infected individuals. Despite the widespread use of an attenuated live vaccine and several antibiotics, there is more TB than ever before, requiring new vaccines and drugs and more specific and rapid diagnostics. Researchers are utilizing information obtained from the complete sequence of the M. tuberculosis genome and from new genetic and physiological methods to identify targets in M. tuberculosis that will aid in the development of these sorely needed antitubercular agents.

Mycobacterium tuberculosis gene expression in macrophages

This review provides a discussion on the current information about the response of Mycobacterium tuberculosis to the environment encountered in the macrophage. We focus on the types of genes shown to be upregulated when the pathogen grows in macrophages and discuss the possible roles of these genes in adaptation to the conditions in the eukaryotic cell, in the context of enhancing the survival of the pathogen during infection.

Comparative evaluation of Mycobacterium vaccae as a surrogate cloning host for use in the study of mycobacterial genetics

Mycobacterium vaccae represents an alternative mycobacterial cloning host that has been largely overlooked to date. The main reason for this may be the reported non-transformability of this species, specifically the so-called Stanford strain (NCTC 11659), with expression vectors that use kanamycin resistance as a selection method. However, this strain can be transformed using hygromycin resistance as an alternative selectable phenotype. The present study has shown that in contrast to previous reports, M. vaccae (ATCC 15483) is capable of being transformed with a range of vectors encoding kanamycin resistance as the selectable marker. Thereafter, the expression of the lacZ reporter gene in M. vaccae, Mycobacterium bovis BCG and Mycobacterium smegmatis mc(2)155 was evaluated using a range of characterized mycobacterial promoter sequences (hsp60, hsp70, PAN, 18kDa and 16S rRNA) cloned in the same promoter probe vector. In general, the promoters showed similar levels of activity in the three species, demonstrating that existing expression systems can readily be employed with M. vaccae (ATCC 15483). This was further confirmed by the observation that M. vaccae was capable of stable, in vitro expression of recombinant S1 subunit of pertussis toxin at levels equivalent to those obtained with BCG and M. smegmatis. Analysis of structural and functional stability of a range of vectors demonstrated that the incidence of instability noted for M. vaccae was lower than that recorded for M. smegmatis. Taken together, the results indicate that M. vaccae is an additional cloning host which may prove useful for specific aspects of mycobacterial biology and provide increased flexibility to the field of recombinant protein technology for mycobacteria.