Auxotrophic vaccines for tuberculosis

Sex-Specific Effects of Environmental Exposure to the Antimicrobial Agents Benzalkonium Chloride and Triclosan on the Gut Microbiota and Health of Zebrafish (Danio rerio)

The use of disinfectants containing benzalkonium chloride (BAC) has become increasingly widespread in response to triclosan (TCS) restrictions and the COVID-19 pandemic, leading to the increasing presence of BAC in aquatic ecosystems. However, the potential environmental health impacts of BAC on fish remain poorly explored. In this study, we show that BAC and TCS can induce the gut dysbiosis in zebrafish (Danio rerio), with substantial effects on health. Breeding pairs of adult zebrafish were exposed to environmentally relevant concentrations of BAC and TCS (0.4-40 μg/L) for 42 days. Both BAC and TCS exposure perturbed the gut microbiota, triggering the classical NF-κB signaling pathway and resulting in downstream pathological toxicity associated with inflammatory responses, histological damage, inhibited ingestion, and decreased survival. These effects were dose-dependent and sex-specific, as female zebrafish were more susceptible than male zebrafish. Furthermore, we found that BAC induced toxicity to a greater extent than the restricted TCS at environmentally relevant concentrations, which is particularly concerning. Our results suggest that environmental exposure to antimicrobial chemicals can have ecological consequences by perturbing the gut microbiota, a previously underappreciated target of such chemicals. Rigorous ecological analysis should be conducted before widely introducing replacement antimicrobial compounds into disinfecting products.

Development in the Concept of Bacterial Polysaccharide Repeating Unit-Based Antibacterial Conjugate Vaccines

The surface of cells is coated with a dense layer of glycans, known as the cell glycocalyx. The complex glycans in the glycocalyx are involved in various biological events, such as bacterial pathogenesis, protection of bacteria from environmental stresses, etc. Polysaccharides on the bacterial cell surface are highly conserved and accessible molecules, and thus they are excellent immunological targets. Consequently, bacterial polysaccharides and their repeating units have been extensively studied as antigens for the development of antibacterial vaccines. This Review surveys the recent developments in the synthetic and immunological investigations of bacterial polysaccharide repeating unit-based conjugate vaccines against several human pathogenic bacteria. The major challenges associated with the development of functional carbohydrate-based antibacterial conjugate vaccines are also considered.

Auxotrophic Mycobacterium bovis BCG: Updates and Perspectives

Mycobacterium bovis BCG has been used for a century as the only licensed vaccine against tuberculosis. Owing to its strong adjuvant properties, BCG has also been employed as an oncological immunotherapeutic as well as a live vaccine vector against other pathogens. However, BCG vaccination has limited efficacy in protecting against adult forms of tuberculosis (TB), raises concerns about its safety in immunocompromised populations, compromises the diagnosis of TB through the tuberculin test and lacks predictability for successful antigen expression and immune responses to heterologous antigens. Together, these factors propelled the construction and evaluation of auxotrophic BCG strains. Auxotrophs of BCG have been developed from mutations in the genes required for their growth using different approaches and have shown the potential to provide a model to study M. tuberculosis, a more stable, safe, and effective alternative to BCG and a vector for the development of recombinant live vaccines, especially against HIV infection. In this review, we provide an overview of the strategies for developing and using the auxotrophic BCG strains in different scenarios.

A D-Alanine auxotrophic live vaccine is effective against lethal infection caused by Staphylococcus aureus

Staphylococcus aureus infections are becoming a major global health issue due to the rapid emergence of multidrug-resistant strains. Therefore, there is an urgent need to develop an effective vaccine to prevent and control these infections. In order to develop a universal immunization strategy, we constructed a mutant derivative of S. aureus 132 which lacks the genes involved in D-alanine biosynthesis, a structural component of cell wall peptidoglycan. This unmarked deletion mutant requires the exogenous addition of D-alanine for in vitro growth. The aim of this study was to examine the ability of this D-alanine auxotroph to induce protective immunity against staphylococcal infection. Our findings demonstrate that this deletion mutant is highly attenuated, elicits a protective immune response in mice and generates cross-reactive antibodies. Moreover, the D-alanine auxotroph was completely eliminated from the blood of mice after its intravenous or intraperitoneal injection. We determined that the protective effect was dependent on antibody production since the adoptive transfer of immune serum into naïve mice resulted in effective protection against S. aureus bacteremia. In addition, splenocytes from mice immunized with the D-alanine auxotroph vaccine showed specific production of IL-17A after ex vivo stimulation. We conclude that this D-alanine auxotroph protects mice efficiently against virulent staphylococcal strains through the combined action of antibodies and IL-17A, and therefore constitutes a promising vaccine candidate against staphylococcal disease, for which no licensed vaccine is available yet.

Identification of a new alanine racemase in Salmonella Enteritidis and its contribution to pathogenesis

BACKGROUND

Non-typhoidal Salmonella (NTS) infections caused primarily by S. Enteritidis and S. Typhimurium particularly in immunocompromised hosts have accounted for a large percentage of fatalities in developed nations. Antibiotics have revolutionized the cure of enteric infections but have also led to the rapid emergence of pathogen resistance. New powerful therapeutics involving metabolic enzymes are expected to be potential targets for combating microbial infections and ensuring effective health management. Therefore, the need for new antimicrobials to fight such health emergencies is paramount. Enteric bacteria successfully evade the gut and colonize their hosts through specialized virulence strategies. An important player, alanine racemase is a key enzyme facilitating bacterial survival.

RESULTS

This study aims at understanding the contribution of alanine racemase genes alr, dadX and SEN3897 to Salmonella survival in vitro and in vivo. We have shown SEN3897 to function as a unique alanine racemase in S. Enteritidis which displayed essential alanine racemase activity. Interestingly, the sole presence of this gene in alr dadX double mutant showed a strict dependence on d-alanine supplementation both in vitro and in vivo. However, Alr complementation in d-alanine auxotrophic strain restored the alanine racemase deficiency. The Km and Vmax of SEN3897 was 89.15 ± 10.2 mM, 400 ± 25.6 µmol/(min mg) for l-alanine and 35 ± 6 mM, 132.5 ± 11.3 µmol/(min mg) for d-alanine, respectively. In vitro assays for invasion and survival as well as in vivo virulence assays involving SEN3897 mutant showed attenuated phenotypes. Further, this study also showed attenuation of d-alanine auxotrophic strain in vivo for the development of potential targets against Salmonella that can be investigated further.

CONCLUSION

This study identified a third alanine racemase gene unique in S. Enteritidis which had a potential effect on survival and pathogenesis in vitro and in vivo. Our results also confirmed that SEN3897 by itself wasn't able to rescue d-alanine auxotrophy in S. Enteritidis which further contributed to its virulence properties.

Gene Transfer in Mycobacterium tuberculosis: Shuttle Phasmids to Enlightenment

Infectious diseases have plagued humankind throughout history and have posed serious public health problems. Yet vaccines have eradicated smallpox and antibiotics have drastically decreased the mortality rate of many infectious agents. These remarkable successes in the control of infections came from knowing the causative agents of the diseases, followed by serendipitous discoveries of attenuated viruses and antibiotics. The discovery of DNA as genetic material and the understanding of how this information translates into specific phenotypes have changed the paradigm for developing new vaccines, drugs, and diagnostic tests. Knowledge of the mechanisms of immunity and mechanisms of action of drugs has led to new vaccines and new antimicrobial agents. The key to the acquisition of the knowledge of these mechanisms has been identifying the elemental causes (i.e., genes and their products) that mediate immunity and drug resistance. The identification of these genes is made possible by being able to transfer the genes or mutated forms of the genes into causative agents or surrogate hosts. Such an approach was limited in Mycobacterium tuberculosis by the difficulty of transferring genes or alleles into M. tuberculosis or a suitable surrogate mycobacterial host. The construction of shuttle phasmids-chimeric molecules that replicate in Escherichia coli as plasmids and in mycobacteria as mycobacteriophages-was instrumental in developing gene transfer systems for M. tuberculosis. This review will discuss M. tuberculosis genetic systems and their impact on tuberculosis research.

The Mycobacterium tuberculosis H37Ra gene MRA_1916 causes growth defects upon down-regulation

D-amino acid oxidases play an important role in converting D-amino acids to their corresponding α-keto acids. MRA_1916 of Mycobacterium tuberculosis H37Ra (Mtb-Ra) is annotated to be a D-amino acid oxidase (DAO). However, not much information is available about its physiological role during Mtb-Ra growth and survival. The present study was taken-up to understand the role of DAO during different stages of growth and effect of its down-regulation on growth. Recombinant Mtb-Ra strains with DAO and GlcB (malate synthase: MRA_1848) gene knockdown were developed and their growth was studied using Microtiter Alamar Blue Assay (MABA) with glycerol, acetate and glycine as a carbon source. Ethyl bromopyruvate (BrP) was used as an inhibitor of GlcB. MABA study showed inhibition of wild-type (WT) and knockdowns in the presence of BrP (2.5mM). However, growth inhibition of WT was less noticeable at lower concentrations of BrP. Mtb-Ra with DAO knockdown showed poor utilization of glycine in the presence of BrP. The DAO localization study showed its prominent distribution in cytosolic fraction and to some extent in cell wall and membrane fractions. Growth profile of WT under oxygen and nutritional stress showed changes in expression of DAO, GlcB, PckA (phosphoenolpyruvate carboxykinase: MRA_0219) and GlyA1 (serine hydroxymethyltransferase: MRA_1104).

Prospects in Mycobacterium bovis Bacille Calmette et Guérin (BCG) vaccine diversity and delivery: why does BCG fail to protect against tuberculosis?

Mycobacterium tuberculosis (M.tb) infection leads to active tuberculosis (TB), a disease that kills one human every 18s. Current therapies available to combat TB include chemotherapy and the preventative vaccine Mycobacterium bovis Bacille Calmette et Guérin (BCG). Increased reporting of drug resistant M.tb strains worldwide indicates that drug development cannot be the primary mechanism for eradication. BCG vaccination has been used globally for protection against childhood and disseminated TB, however, its efficacy at protecting against pulmonary TB in adult and aging populations is highly variable. In this regard, the immune response generated by BCG vaccination is incapable of sterilizing the lung post M.tb infection as indicated by the large proportion of individuals with latent TB infection that have received BCG. Although many new TB vaccine candidates have entered the development pipeline, only a few have moved to human clinical trials; where they showed no efficacy and/or were withdrawn due to safety regulations. These trials highlight our limited understanding of protective immunity against the development of active TB. Here, we discuss current vaccination strategies and their impact on the generation and sustainability of protective immunity against TB.

Dietary pyridoxine controls efficacy of vitamin B6-auxotrophic tuberculosis vaccine bacillus Calmette-Guérin ΔureC::hly Δpdx1 in mice

The only tuberculosis (TB) vaccine in use today, bacillus Calmette-Guérin (BCG), provides insufficient protection and can cause adverse events in immunocompromised individuals, such as BCGosis in HIV(+) newborns. We previously reported improved preclinical efficacy and safety of the recombinant vaccine candidate BCG ΔureC::hly, which secretes the pore-forming listeriolysin O of Listeria monocytogenes. Here, we evaluate a second-generation construct, BCG ΔureC::hly Δpdx1, which is deficient in pyridoxine synthase, an enzyme that is required for biosynthesis of the essential cofactor vitamin B6. This candidate was auxotrophic for vitamin B6 in a concentration-dependent manner, as was its survival in vivo. BCG ΔureC::hly Δpdx1 showed markedly restricted dissemination in subcutaneously vaccinated mice, which was ameliorated by dietary supplementation with vitamin B6. The construct was safer in severe combined immunodeficiency mice than the parental BCG ΔureC::hly. A prompt innate immune response to vaccination, measured by secretion of interleukin-6, granulocyte colony-stimulating factor, keratinocyte cytokine, and macrophage inflammatory protein-1α, remained independent of vitamin B6 administration, while acquired immunity, notably stimulation of antigen-specific CD4 T cells, B cells, and memory T cells, was contingent on vitamin B6 administration. The early protection provided by BCG ΔureC::hly Δpdx1 in a murine Mycobacterium tuberculosis aerosol challenge model consistently depended on vitamin B6 supplementation. Prime-boost vaccination increased protection against the canonical M. tuberculosis H37Rv laboratory strain and a clinical isolate of the Beijing/W lineage. We demonstrate that the efficacy of a profoundly attenuated recombinant BCG vaccine construct can be modulated by external administration of a small molecule. This principle fosters the development of safer vaccines required for immunocompromised individuals, notably HIV(+) infants.

IMPORTANCE

Mycobacterium tuberculosis can synthesize the essential cofactor vitamin B6, while humans depend on dietary supplementation. Unlike the lipophilic vitamins A, D, and E, water-soluble vitamin B6 is well tolerated at high doses. We generated a vitamin B6 auxotroph of the phase II clinical tuberculosis vaccine candidate bacillus Calmette-Guérin ΔureC::hly. The next-generation candidate was profoundly attenuated compared to the parental strain. Adaptive immunity and protection in mice consistently depended on increased dietary vitamin B6 above the daily required dose. Control of vaccine efficacy via food supplements such as vitamin B6 could provide a fast track toward improved safety. Safer vaccines are urgently needed for HIV-infected individuals at high risk of adverse events in response to live vaccines.

Role of a highly conserved proline-126 in ThDP binding of Mycobacterium tuberculosis acetohydroxyacid synthase

Acetohydroxyacid synthase (AHAS) of Mycobacterium tuberculosis is a promising target for the development of anti-tuberculosis agents. With the absence of an available bacterial AHAS crystal structure, that of M. tuberculosis, site-directed mutagenesis has been a useful tool for determining its structural and functional features. In this study, a highly conserved proline residue (P126 of M. tuberculosis AHAS) was selected, and the possible role was evaluated by site-directed mutagenesis. P126 was replaced by valine, threonine, alanine, and glutamate to yield P126V, P126T, P126A, and P126E, respectively. All variants were expressed in their soluble forms in Escherichia coli and purified to near homogeneity. The molecular mass (SDS-PAGE) of the purified variants was ∼68 kDa, which is similar to that of wild-type AHAS. The P126V, P126T, and P126A variants exhibited significantly lower activity than wild-type AHAS, whereas P126E was inactive under the tested assay conditions. Furthermore, the P126V and P126T variants showed a significantly decreased preference toward pyruvate and ThDP as substrate and cofactor respectively, whereas the P126A showed similar kinetics to that of wild-type AHAS. Like in AHAS from yeast Saccharomyces cerevisiae (PDB ID: 1N0H), residue P126 is located in the ThDP binding pocket of M. tuberculosis AHAS homology model. Collectively, these results suggest that the conserved P126 plays a significant role in the ThDP binding of M. tuberculosis AHAS.

Pre-existing immunity against vaccine vectors--friend or foe?

Over the last century, the successful attenuation of multiple bacterial and viral pathogens has led to an effective, robust and safe form of vaccination. Recently, these vaccines have been evaluated as delivery vectors for heterologous antigens, as a means of simultaneous vaccination against two pathogens. The general consensus from published studies is that these vaccine vectors have the potential to be both safe and efficacious. However, some of the commonly employed vectors, for example Salmonella and adenovirus, often have pre-existing immune responses in the host and this has the potential to modify the subsequent immune response to a vectored antigen. This review examines the literature on this topic, and concludes that for bacterial vectors there can in fact, in some cases, be an enhancement in immunogenicity, typically humoral, while for viral vectors pre-existing immunity is a hindrance for subsequent induction of cell-mediated responses.

Vaccines against tuberculosis: where are we and where do we need to go?

In this review we discuss recent progress in the development, testing, and clinical evaluation of new vaccines against tuberculosis (TB). Over the last 20 years, tremendous progress has been made in TB vaccine research and development: from a pipeline virtually empty of new TB candidate vaccines in the early 1990s, to an era in which a dozen novel TB vaccine candidates have been and are being evaluated in human clinical trials. In addition, innovative approaches are being pursued to further improve existing vaccines, as well as discover new ones. Thus, there is good reason for optimism in the field of TB vaccines that it will be possible to develop better vaccines than BCG, which is still the only vaccine available against TB.

Regulation of proline metabolism in mycobacteria and its role in carbon metabolism under hypoxia

Genes with a role in proline metabolism are strongly expressed when mycobacterial cells are exposed to nutrient starvation and hypoxia. Here we show that proline metabolism in mycobacteria is mediated by the monofunctional enzymes Δ(1) -pyrroline-5-carboxylate dehydrogenase (PruA) and proline dehydrogenase (PruB). Proline metabolism was controlled by a unique membrane-associated DNA-binding protein PruC. Under hypoxia, addition of proline led to higher biomass production than in the absence of proline despite excess carbon and nitrogen. To identify the mechanism responsible for this enhanced growth, microarray analysis of wild-type Mycobacterium smegmatis versus pruC mutant was performed. Expression of the DNA repair machinery and glyoxalases was increased in the pruC mutant. Glyoxalases are proposed to degrade methylglyoxal, a toxic metabolite produced by various bacteria due to an imbalance in intermediary metabolism, suggesting the pruC mutant was under methylglyoxal stress. Consistent with this notion, pruB and pruC mutants were hypersensitive to methylglyoxal. Δ(1) -pyrroline-5-carboxylate is reported to react with methylglyoxal to form non-toxic 2-acetyl-1-pyrroline, thus providing a link between proline metabolism and methylglyoxal detoxification. In support of this mechanism, we show that proline metabolism protects mycobacterial cells from methylglyoxal toxicity and that functional proline dehydrogenase, but not Δ(1) -pyrroline-5-carboxylate dehydrogenase, is essential for this protective effect.

Identification and characterization of inhibitors of Haemophilus influenzae acetohydroxyacid synthase

Acetohydroxyacid synthase (AHAS), a potential target for antimicrobial agents, catalyzes the first common step in the biosynthesis of branched-chain amino acids. The gene coding for the AHAS catalytic subunit from Haemophilus influenzae (Hi) was cloned, overexpressed in Escherichia coli, and purified. To identify new inhibitory scaffolds, we used a high-throughput screen to test 221 small diverse chemical compounds against Hi-AHAS. Compounds were selected for their ability to inhibit AHAS in vitro. The screen identified 3 compounds, each representing a structural class, as Hi-AHAS inhibitors with an IC(50) in the low micromolar range (4.4-14.6 μM). The chemical scaffolds of the three compounds were oxa-1-thia-4-aza-cyclopenta[b]naphthalene (KHG25229), phenyl-2,3-dihydro-isothiazole (KHG25386), and phenyl-pyrrolidine-3-carboxylic acid phenylamide (KHG25056). Further, molecular docking of the two most potent chemicals, KHG25229 and KHG25386, in Hi-AHAS yielded binding energies of -10.41 and -9.21 kcal/mol, respectively. The binding modes were consistent with inhibition mechanisms, as both chemicals oriented outside the active site. As the need for novel antibiotic classes to combat drug resistant bacteria increases, screening compounds that act against Hi-AHAS may assist in the identification of potential new anti-Hi drugs.

Thiamin (vitamin B1) biosynthesis and regulation: a rich source of antimicrobial drug targets?

Drug resistance of pathogens has necessitated the identification of novel targets for antibiotics. Thiamin (vitamin B1) is an essential cofactor for all organisms in its active form thiamin diphosphate (ThDP). Therefore, its metabolic pathways might be one largely untapped source of antibiotics targets. This review describes bacterial thiamin biosynthetic, salvage, and transport pathways. Essential thiamin synthetic enzymes such as Dxs and ThiE are proposed as promising drug targets. The regulation mechanism of thiamin biosynthesis by ThDP riboswitch is also discussed. As drug targets of existing antimicrobial compound pyrithiamin, the ThDP riboswitch might serves as alternative targets for more antibiotics.

Downregulation of Rv0189c, encoding a dihydroxyacid dehydratase, affects growth of Mycobacterium tuberculosis in vitro and in mice

Dihydroxyacid dehydratase (DHAD), a key enzyme involved in branched-chain amino acid (BCAA) biosynthesis, catalyses the synthesis of 2-ketoacids from dihydroxyacids. In Mycobacterium tuberculosis, DHAD is encoded by gene Rv0189c, and it shares 40 % amino acid sequence identity and conserved motifs with DHAD of Escherichia coli encoded by ilvD. In this study, Rv0189c was overexpressed in E. coli and the resultant protein was characterized as a homodimer (∼155 kDa). Functional characterization of Rv0189c was established by biochemical testing and by genetic complementation of an intron-disrupted ilvD-auxotrophic mutant of E. coli to prototrophy. Growth of M. tuberculosis, E. coli BL21(DE3) and recombinant E. coli BL21(DE3) ΔilvD carrying Rv0189c was inhibited by transient nitric oxide (NO) exposure in minimal medium but growth was restored if the medium was supplemented with BCAA (isoleucine, leucine and valine). This suggested that inactivation of Rv0189c by NO probably inhibited bacterial growth. The role of Rv0189c in M. tuberculosis was elucidated by antisense and sense RNA constructs. Growth of M. tuberculosis transformed with a plasmid encoding antisense mRNA was markedly poor in the lungs of infected mice and in Middlebrook 7H9 broth compared to that of sense and vector-alone transformants, but growth was normal when the medium was supplemented with BCAA. Upregulation of Rv0189c was observed during the early exponential phase of growth, under acid stress and ex vivo, suggesting that Rv0189c has a role in the survival of M. tuberculosis during normal and stress conditions. It may be concluded that the DHAD encoded by Rv0189c is essential for the survival of M. tuberculosis and could be a potential drug/vaccine target, as it is absent in mammals.

Biochemical and transcription analysis of acetohydroxyacid synthase isoforms in Mycobacterium tuberculosis identifies these enzymes as potential targets for drug development

Acetohydroxyacid synthase (AHAS) is a biosynthetic enzyme essential for de novo synthesis of branched-chain amino acids. The genome sequence of Mycobacterium tuberculosis revealed genes encoding four catalytic subunits, ilvB1 (Rv3003c), ilvB2 (Rv3470c), ilvG (Rv1820) and ilvX (Rv3509c), and one regulatory subunit, ilvN (Rv3002c), of AHAS. All these genes were found to be expressed in M. tuberculosis growing in vitro. Each AHAS subunit gene was cloned and expressed in Escherichia coli. AHAS activity of IlvB1 and IlvG was found in cell-free lysates and with recombinant purified proteins. Kinetic studies with purified IlvG revealed positive cooperativity towards substrate and cofactors. To understand the role of the catalytic subunits in the biology of M. tuberculosis, expression of AHAS genes was analysed in different physiological conditions. ilvB1, ilvB2 and ilvG were differentially expressed. The role of ilvB1 in persistence is known, but the upregulation of ilvB2 and ilvG in extended stationary phase, ex vivo, and in acid stress and hypoxic environments, suggests the relevance of AHAS enzymes in the metabolism and survival of M. tuberculosis by functioning as catabolic AHAS. These enzymes are therefore potential targets for drug development.

Future vaccination strategies against tuberculosis: thinking outside the box

With almost a dozen vaccine candidates in clinical trials, tuberculosis (TB) research and development is finally reaping the first fruits of its labors. Vaccine candidates in clinical trials may prevent TB disease reactivation by efficiently containing the pathogen Mycobacterium tuberculosis (Mtb). Future research should target vaccines that achieve sterile eradication of Mtb or even prevent stable infection. These are ambitious goals that can be reached only by highly cooperative engagement of basic immunologists, vaccinologists, and clinical researchers--or in other words, by translation from basic immunology to vaccine research and development, as well as reverse translation of insights from clinical trials back to hypothesis-driven research in the basic laboratory. Here, we review current and future strategies toward the rational design of novel vaccines against TB, as well as the progress made thus far, and the hurdles that need to be overcome in the near and distant future.

Characterization of acetohydroxyacid synthase I from Escherichia coli K-12 and identification of its inhibitors

The first step in branched-chain amino acid biosynthesis is catalyzed by acetohydroxyacid synthase (EC 2.2.1.6). This reaction involves decarboxylation of pyruvate followed by condensation with either an additional pyruvate molecule or with 2-oxobutyrate. The enzyme requires three cofactors, thiamine diphosphate (ThDP), a divalent ion, and flavin adenine dinucleotide (FAD). Escherichia coli contains three active isoenzymes, and acetohydroxyacid synthase I (AHAS I) large subunit is encoded by the ilvB gene. In this study, the ilvB gene from E. coli K-12 was cloned into expression vector pETDuet-1, and was expressed in E. coli BL21 (DH3). The purified protein was identified on a 12% SDS-PAGE gel as a single band with a mass of 65 kDa. The optimum temperature, buffer, and pH for E. coli K-12 AHAS I were 37 °C, potassium phosphate buffer, and 7.5. Km values for E. coli K-12 AHAS I binding to pyruvate, Mg(+2), ThDP, and FAD were 4.15, 1.26, 0.2 mM, and 0.61 µM respectively. Inhibition of purified AHAS I protein was determined with herbicides and new compounds.

Novel tuberculosis vaccination strategies based on understanding the immune response

The current tuberculosis (TB) vaccine bacillus Calmette-Guérin (BCG) fails to protect against adult pulmonary TB. Yet, its capacity to control miliary TB in newborn infants forms the basis for development of novel vaccine candidates. These either exploit genetic modification of BCG to create a viable replacement vaccine or use BCG to prime the immune response followed by boost with a novel subunit vaccine. This could ultimately result in a combination vaccination schedule comprising a prime with a live BCG replacement followed by a subunit vaccine boost. Ultimately, vaccination strategies that achieve sterile eradication of, or prevent infection with, tubercle bacilli would be an ambitious highly promising goal.

Evaluation of substituted triazol-1-yl-pyrimidines as inhibitors of Bacillus anthracis acetohydroxyacid synthase

Acetohydroxyacid synthase (AHAS), a potential target for antimicrobial agents, catalyzes the first common step in the biosynthesis of the branched-chain amino acids. The genes of both catalytic and regulatory subunits of AHAS from Bacillus anthracis (Bantx), a causative agent of anthrax, were cloned, overexpressed in Escherichia coli, and purified to homogeneity. To develop novel anti-anthracis drugs that inhibit AHAS, a chemical library was screened, and four chemicals, AVS2087, AVS2093, AVS2387, and AVS2236, were identified as potent inhibitors of catalytic subunit with IC(50) values of 1.0 +/- 0.02, 1.0 +/- 0.04, 2.1 +/- 0.12, and 2.0 +/- 0.08 microM, respectively. Further, these four chemicals also showed strong inhibition against reconstituted AHAS with IC(50) values of 0.05 +/- 0.002, 0.153 +/- 0.004, 1.30 +/- 0.10, and 1.29 +/- 0.40 microM, respectively. The basic scaffold of the AVS group consists of 1-pyrimidine-2-yl-1H-[1,2,4]triazole-3-sulfonamide. The potent inhibitor, AVS2093 showed the lowest binding energy, -8.52 kcal/mol and formed a single hydrogen bond with a distance of 1.973 A. As the need for novel antibiotic classes to combat bacterial drug resistance increases, the screening of new compounds that act against Bantx-AHAS shows that AHAS is a good target for new anti-anthracis drugs.

Mutagenesis of the enolase-phosphatase gene in Xanthomonas oryzae pv. oryzae affects growth on methylthioadenosine and in vivo S-adenosylmethionine pools

Enolase-phosphatase (E1), as an enzyme, is involved in methionine salvage pathway in many prokaryotic and eukaryotic organisms. But the identity and function of E1 in Xanthomonas oryzae pv. oryzae (Xoo) remain undetermined. Here, we report the cloning and characterization of E1 gene, named xep, from Xoo. Sequence analysis shows that XEP is highly conserved among the six Xoo strains we investigated and all other Xanthomonas species. The strain with an insertion mutation in xep could not grow when methylthioadenosine (MTA) was used as the sole sulfur source, but its growth in rice leaves was comparable to that of wild-type strain. Furthermore, the mutant also showed less S-adenosylmethionine (SAM) and lower gene expression of sulfate reduction gene raxQ, compared to wild-type bacterial cells. Introduction of wild-type xep gene to the mutant resulted in the full restoration of growth on MTA, the SAM quantity and the expression level of raxQ. The results demonstrate that xep is involved in the predicted methionine salvage pathway and an inactive form of this gene results in a decreased SAM level in vivo. Our data also indicate that SAM may play a role in the regulation of sulfur reduction at the transcriptional level in Xoo.

Impairment of d-alanine biosynthesis in Mycobacterium smegmatis determines decreased intracellular survival in human macrophages

d-Alanine is a structural component of mycobacterial peptidoglycan. The primary route of d-alanine biosynthesis in eubacteria is the enantiomeric conversion from l-alanine, a reaction catalysed by d-alanine racemase (Alr). Mycobacterium smegmatis alr insertion mutants are not dependent on d-alanine for growth and display a metabolic pattern consistent with an alternative pathway for d-alanine biosynthesis. In this study, we demonstrate that the M. smegmatis alr insertion mutant TAM23 can synthesize d-alanine at lower levels than the parental strain. The insertional inactivation of the alr gene also decreases the intracellular survival of mutant strains within primary human monocyte-derived macrophages. By complementation studies, we confirmed that the impairment of alr gene function is responsible for this reduced survival. Inhibition of superoxide anion and nitric oxide formation in macrophages suppresses the differential survival. In contrast, for bacteria grown in broth, both strains had approximately the same susceptibility to hydrogen peroxide, acidified sodium nitrite, low pH and polymyxin B. In contrast, TAM23 exhibited increased resistance to lysozyme. d-Alanine supplementation considerably increased TAM23 viability in nutritionally deficient media and within macrophages. These results suggest that nutrient deprivation in phagocytic cells combined with killing mediated by reactive intermediates underlies the decreased survival of alr mutants. This knowledge may be valuable in the construction of mycobacterial auxotrophic vaccine candidates.

Protective efficacy of recombinant BCG Tokyo (Ag85A) in rhesus monkeys (Macaca mulatta) infected intratracheally with H37Rv Mycobacterium tuberculosis

We have reported previously that recombinant BCG Tokyo (Ag85A) (rBCG-Ag85A[Tokyo]) shows promise as a tuberculosis vaccine, demonstrating protective efficacy in cynomolgus monkeys. As a next step, rhesus monkeys were utilized because they are also susceptible to Mycobacterium tuberculosis and show a continuous course of infection resembling human tuberculosis. The recombinant BCG vaccine (5x10(5) CFU per monkey) was administered once intradermally into the back skin to three groups of rhesus monkeys, and its protective efficacy was compared for 4months with that of its parental BCG Tokyo strain. Eight week vaccination of the monkeys with rBCG-Ag85A[Tokyo] resulted in a reduction of tubercle bacilli CFU (p<0.01) and lung pathology in animals infected intratracheally with 3000 CFU H37Rv M. tuberculosis. Vaccination prevented an increase in the old tuberculin test after challenge with M. tuberculosis and reaction of M. tuberculosis-derived antigen. Thus, it was shown that even in rhesus monkeys rBCG-Ag85A[Tokyo] induced higher protective efficacy than BCG Tokyo.

A Replication-Limited Recombinant Mycobacterium bovis BCG vaccine against tuberculosis designed for human immunodeficiency virus-positive persons is safer and more efficacious than BCG

Tuberculosis is the leading cause of death in AIDS patients, yet the current tuberculosis vaccine, Mycobacterium bovis bacillus Calmette-Guérin (BCG), is contraindicated for immunocompromised individuals, including human immunodeficiency virus-positive persons, because it can cause disseminated disease; moreover, its efficacy is suboptimal. To address these problems, we have engineered BCG mutants that grow normally in vitro in the presence of a supplement, are preloadable with supplement to allow limited growth in vivo, and express the highly immunoprotective Mycobacterium tuberculosis 30-kDa major secretory protein. The limited replication in vivo renders these vaccines safer than BCG in SCID mice yet is sufficient to induce potent cell-mediated and protective immunity in the outbred guinea pig model of pulmonary tuberculosis. In the case of one vaccine, rBCG(mbtB)30, protection was superior to that with BCG (0.3-log fewer CFU of M. tuberculosis in the lung [P < 0.04] and 0.6-log fewer CFU in the spleen [P = 0.001] in aerosol-challenged animals [means for three experiments]); hence, rBCG(mbtB)30 is the first live mycobacterial vaccine that is both more attenuated than BCG in the SCID mouse and more potent than BCG in the guinea pig. Our study demonstrates the feasibility of developing safer and more potent vaccines against tuberculosis. The novel approach of engineering a replication-limited vaccine expressing a recombinant immunoprotective antigen and preloading it with a required nutrient, such as iron, that is capable of being stored should be generally applicable to other live vaccine vectors targeting intracellular pathogens.

Recombinant BCG Tokyo (Ag85A) protects cynomolgus monkeys (Macaca fascicularis) infected with H37Rv Mycobacterium tuberculosis

One tuberculosis vaccine candidate that has shown a promising degree of protective efficacy in guinea pigs is recombinant BCG Tokyo (Ag85A)(rBCG-Ag85A[Tokyo]). As a next step, cynomolgus monkeys were utilized because they are susceptible to Mycobacterium tuberculosis and develop a continuous course of infection that resembles that in humans both clinically and pathologically. The recombinant BCG vaccine was administered once intradermally in the back skin to three groups of cynomolgus monkeys, and its protective efficacy was compared for 4 months with that of its parental BCG Tokyo strain. Vaccination of the monkeys with the rBCG-Ag85A[Tokyo] resulted in a reduction of tubercle bacilli CFU (p<0.01) and lung pathology in animals challenged intratracheally with 3000 CFU H37Rv M. tuberculosis. Vaccination prevented an increase in the old tuberculin test after challenge with M. tuberculosis and reaction of M. tuberculosis-derived antigen. Thus, it was shown in monkeys that rBCG-Ag85A[Tokyo] induced higher protective efficacy than BCG Tokyo. This warrants further clinical evaluation.

Identification of the catalytic subunit of acetohydroxyacid synthase in Haemophilus influenzae and its potent inhibitors

Acetohydroxyacid synthase (AHAS; EC 2.2.1.6) is a thiamin diphosphate- (ThDP)- and FAD-dependent enzyme that catalyzes the first common step in the biosynthetic pathway of the branched-amino acids (BCAAs) leucine, isoleucine, and valine. The gene from Haemophilus influenzae that encodes the AHAS catalytic subunit was cloned, overexpressed in Escherichia coli BL21(DE3), and purified to homogeneity. The purified H. influenzae AHAS catalytic subunit (Hin-AHAS) appeared as a single band on SDS-PAGE gel, with a molecular mass of approximately 63 kDa. The enzyme catalyzes the condensation of two molecules of pyruvate to form acetolactate, with a K(m) of 9.2mM and the specific activity of 1.5 micromol/min/mg. The cofactor activation constant (K(c)=13.5 microM) and the dissociation constant (K(d)=3.3 microM) of ThDP were also determined by enzymatic assay and tryptophan fluorescence quenching studies, respectively. We screened a chemical library to discover new inhibitors of the Hin AHAS catalytic subunit. Through which, AVS-2087 (IC(50)=0.53 microM), KSW30191 (IC(50)=1.42 microM), and KHG20612 (IC(50)=4.91 microM) displayed potent inhibition as compare to sulfometuron methyl (IC(50)=276.31 microM).

Exploring functional genomics for the development of novel intervention strategies against tuberculosis

Tuberculosis (TB) remains a serious threat to humankind, and humans have encountered the causative agent of TB, Mycobacterium tuberculosis (MTB), for more than 10,000 years. Despite rapid advances in technology, efforts to besiege this robust pathogen seem to fail. The availability of genome sequences of several MTB complex strains open a new era of MTB research, the functional genomics, which will provide guidelines for novel control measures. In recent years, a series of methods have been developed to explore the mechanisms employed by MTB to persist and cause disease in the host. DNA array technology enables us to perform comparative genomics of different MTB strains and to examine the gene expression profiles of MTB growing under diverse living conditions. The generated transcriptome data can be exploited for design of new drugs, especially against multidrug-resistant (MDR) strains, development of more efficient vaccines, and identification of biomarkers for better diagnosis.

A tuberculosis vaccine based on phosphoantigens and fusion proteins induces distinct gammadelta and alphabeta T cell responses in primates

Phosphoantigens are mycobacterial non-peptide antigens that might enhance the immunogenicity of current subunit candidate vaccines for tuberculosis. However, their testing requires monkeys, the only animal models suitable for gammadelta T cell responses to mycobacteria. Thus here, the immunogenicity of 6-kDa early secretory antigenic target-mycolyl transferase complex antigen 85B (ESAT-6-Ag85B) (H-1 hybrid) fusion protein associated or not to a synthetic phosphoantigen was compared by a prime-boost regimen of two groups of eight cynomolgus. Although phosphoantigen activated immediately a strong release of systemic Th1 cytokines (IL-2, IL-6, IFN-gamma, TNF-alpha), it further anergized blood gammadelta T lymphocytes selectively. By contrast, the hybrid H-1 induced only memory alphabeta T cell responses, regardless of phosphoantigen. These latter essentially comprised cytotoxic T lymphocytes specific for Ag85B (on average + 430 cells/million PBMC) and few IFN-gamma-secreting cells (+ 40 cells/million PBMC, equally specific for ESAT-6 and for Ag85B). Hence, in macaques, a prime-boost with the H-1/phosphoantigen subunit combination induces two waves of immune responses, successively by gammadelta T and alphabeta T lymphocytes.

Current status of TB vaccines

During last 10 years, there has been extensive work for the development of potential tuberculosis vaccine candidates using the mice and guinea pig models. Though till date several promising candidates have been identified and at least eight vaccines have entered clinical evaluation. These recent advances in the clinical testing of new TB vaccines are very exciting and promising. However, there is a need to continue the search for additional vaccine candidates or vaccination strategies.

Protective efficacy of recombinant (Ag85A) BCG Tokyo with Ag85A peptide boosting against Mycobacterium tuberculosis-infected guinea pigs in comparison with that of DNA vaccine encoding Ag85A

A recombinant form of BCG Tokyo with an Ag85A gene insert was administered once subcutaneously to guinea pigs and its protective efficacy was compared with that of a DNA vaccine encoding Ag85A from Mycobacterium tuberculosis administered twice to guinea pigs by epidermal gene gun bombardment. Vaccination with either the recombinant BCG Tokyo or Ag85A DNA significantly reduced the severity of pulmonary pathology and the number of pulmonary and splenic colony-forming units (cfu) (p<0.001). The recombinant BCG Tokyo was better than Ag85A DNA in terms of protective efficacy against M. tuberculosis. When immunogenic synthetic Ag85A peptide was further used as a booster together with recombinant BCG Tokyo (Ag85A) or Ag85A DNA, lung pathology was improved significantly and the number of pulmonary CFU was reduced significantly. Neither recombinant BCG Tokyo, Ag85A DNA, nor the parental BCG Tokyo protected the guinea pigs from hematogenous spread of tubercle bacilli to the spleen because splenic granulomas without central necrosis were recognized. The spleen tissues from guinea pigs vaccinated with recombinant BCG Tokyo or Ag85A DNA expressed IFN-gamma and IL-2 mRNA at significantly high levels (p<0.001) as evaluated by reverse transcription polymerase chain reaction. It is concluded that peptide boosting is important for the induction of higher protective efficacy by recombinant BCG Tokyo or a tuberculosis DNA vaccine and both recombinant BCG Tokyo (Ag85A) and Ag85A DNA vaccine induce Th2 cytokine mRNA expression significantly.

Advances in tuberculosis vaccine strategies

Tuberculosis (TB), an ancient human scourge, is a growing health problem in the developing world. Approximately two million deaths each year are caused by TB, which is the leading cause of death in HIV-infected individuals. Clearly, an improved TB vaccine is desperately needed. Heterologous prime-boost regimens probably represent the best hope for an improved vaccine regimen to prevent TB. This first generation of new vaccines might also complement drug treatment regimens and be effective against reactivation of TB from the latent state, which would significantly enhance their usefulness.

Tuberculosis: from genome to vaccine

The availability of mycobacterial genome sequences has paved the way to identifying potential tuberculosis vaccine candidates in order to replace the currently used bacillus Calmette-Guérin (BCG) vaccines that show variable protective efficacy in adults. Genomics provides the basis for bioinformatic, transcriptomic and proteomic analysis, increases screening efficiency and enables valuable information concerning the biology and virulence of the mycobacterial species to be extracted by comparative genomics. Although in silico results must be confirmed in vitro and in vivo, bioinformatic analysis of the genomes is highlighting candidates for testing. For designing subunit vaccines, attenuated or improved recombinant whole-cell live vaccines, information from the genomes of the human host and pathogenic mycobacterial species is of great help.