Mycobacterium tuberculosis growth following aerobic expression of the DosR regulon

The Mycobacterium tuberculosis regulator DosR is induced by multiple stimuli including hypoxia, nitric oxide and redox stress. Overlap of these stimuli with conditions thought to promote latency in infected patients fuels a model in which DosR regulon expression is correlated with bacteriostasis in vitro and a proxy for latency in vivo. Here, we find that inducing the DosR regulon to wildtype levels in aerobic, replicating M. tuberculosis does not alter bacterial growth kinetics. We conclude that DosR regulon expression alone is insufficient for bacterial latency, but rather is expressed during a range of growth states in a dynamic environment.     

Human T-cell responses to 25 novel antigens encoded by genes of the dormancy regulon of Mycobacterium tuberculosis

The dormancy (DosR) regulon of Mycobacterium tuberculosis is expressed in vitro during hypoxia and low-dose nitric oxide stimulation. Tubercle bacilli are thought to encounter these conditions in humans during latent infection. In this study, immune responses were evaluated to 25 most strongly induced DosR-regulon-encoded proteins, referred to as latency antigens. Proliferation assays were performed using M. tuberculosis-specific T-cell lines and peripheral blood mononuclear cells (PBMC) from tuberculosis (TB) patients, tuberculin skin test positive (TST+) individuals and uninfected controls. All 25 latency antigens were able to induce production of interferon-gamma (IFN-gamma) by T-cell lines. Eighteen latency antigens were also recognized by PBMC of M. tuberculosis-infected individuals, which indicates expression of the DosR-regulon during natural infection. Differential analysis showed that TST+ individuals recognized more latency antigens and with a stronger cumulative IFN-gamma response than TB patients, while the opposite profile was found for culture filtrate protein-10. In particular Rv1733c, Rv2029c, Rv2627c and Rv2628 induced strong IFN-gamma responses in TST+ individuals, with 61%, 61%, 52% and 35% responders, respectively. In conclusion, several new M. tuberculosis antigens were identified within the DosR-regulon. Particularly strong IFN-gamma responses to latency antigens were observed in latently infected individuals, suggesting that immune responses against these antigens may contribute to controlling latent M. tuberculosis infection.     

Codominance of TLR2-dependent and TLR2-independent modulation of MHC class II in Mycobacterium tuberculosis infection in vivo

Mycobacterium tuberculosis is an exceptionally successful human pathogen. A major component of this success is the ability of the bacteria to infect immunocompetent individuals and to evade eradication by an adaptive immune response that includes production of the macrophage-activating cytokine, IFN-gamma. Although IFN-gamma is essential for arrest of progressive tuberculosis, it is insufficient for efficacious macrophage killing of the bacteria, which may be due to the ability of M. tuberculosis to inhibit selected macrophage responses to IFN-gamma. In vitro studies have determined that mycobacterial lipoproteins and other components of the M. tuberculosis cell envelope, acting as agonists for TLR2, inhibit IFN-gamma induction of MHC class II. In addition, M. tuberculosis peptidoglycan and IL-6 secreted by infected macrophages inhibit IFN-gamma induction of MHC class II in a TLR2-independent manner. To determine whether TLR2-dependent inhibition of macrophage responses to IFN-gamma is quantitatively dominant over the TLR2-independent mechanisms in vivo, we prepared mixed bone marrow chimeric mice in which the hemopoietic compartment was reconstituted with a mixture of TLR(+/+) and TLR2(-/-) cells. When the chimeric mice were infected with M. tuberculosis, the expression of MHC class II on TLR2(+/+) and TLR2(-/-) macrophages from the lungs of individual infected chimeric mice was indistinguishable. These results indicate that TLR2-dependent and -independent mechanisms of inhibition of responses to IFN-gamma are equivalent in vivo, and that M. tuberculosis uses multiple pathways to abrogate the action of an important effector of adaptive immunity. This work was supported by National Institutes of Health Grants AI 065357-AI 020010.     

Stress responses in mycobacteria

Mycobacterium tuberculosis is a successful pathogen that overcomes numerous challenges presented by the immune system of the host. This bacterium usually establishes a chronic infection in the host where it may silently persist inside a granuloma until, a failure in host defenses, leads to manifestation of the disease. None of the conventional anti-tuberculosis drugs are able to target these persisting bacilli. Development of drugs against such persisting bacilli is a constant challenge since the physiology of these dormant bacteria is still not understood at the molecular level. Some evidence suggests that the in vivo environment encountered by the persisting bacteria is anoxic and nutritionally starved. Based on these assumptions, anaerobic and starved cultures are used as models to study the molecular basis of dormancy. This review outlines the problem of persistence of M. tuberculosis and the various in vitro models used to study mycobacterial latency. The basis of selecting the nutritional starvation model has been outlined here. Also, the choice of M. smegmatis as a model suitable for studying mycobacterial latency is discussed. Lastly, general issues related to oxidative stress and bacterial responses to it have been elaborated. We have also discussed general control of OxyR-mediated regulation and emphasized the processes which manifest in the absence of functional OxyR in the bacteria. Lastly, a new class of protein called Dps has been reviewed for its important role in protecting DNA under stress.     

Comparative pathogenesis of Mycobacterium marinum and Mycobacterium tuberculosis

A thorough understanding of Mycobacterium tuberculosis pathogenesis in humans has been elusive in part because of imperfect surrogate laboratory hosts, each with its own idiosyncrasies. Mycobacterium marinum is the closest genetic relative of the M. tuberculosis complex and is a natural pathogen of ectotherms. In this review, we present evidence that the similar genetic programmes of M. marinum and M. tuberculosis and the corresponding host immune responses reveal a conserved skeleton of Mycobacterium host–pathogen interactions. While both species have made niche-specific refinements, an essential framework has persisted. We highlight genetic comparisons of the two organisms and studies of M. marinum in the developing zebrafish. By pairing M. marinum with the simplified immune system of zebrafish embryos, many of the defining mechanisms of mycobacterial pathogenesis can be distilled and investigated in a tractable host/pathogen pair.     

Mycothiol biosynthesis is essential for ethionamide susceptibility in Mycobacterium tuberculosis

Spontaneous mutants of Mycobacterium tuberculosis that were resistant to the anti-tuberculosis drugs ethionamide and isoniazid were isolated and found to map to mshA , a gene encoding the first enzyme involved in the biosynthesis of mycothiol, a major low-molecular-weight thiol in M. tuberculosis . Seven independent missense or frameshift mutations within mshA were identified and characterized. Precise null deletion mutations of the mshA gene were generated by specialized transduction in three different strains of M. tuberculosis . The mshA deletion mutants were defective in mycothiol biosynthesis, were only ethionamide-resistant and required catalase to grow. Biochemical studies suggested that the mechanism of ethionamide resistance in mshA mutants was likely due to a defect in ethionamide activation. In vivo , a mycothiol-deficient strain grew normally in immunodeficient mice, but was slightly defective for growth in immunocompetent mice. Mutations in mshA demonstrate the non-essentiality of mycothiol for growth in vitro and in vivo , and provide a novel mechanism of ethionamide resistance in M. tuberculosis.     

The effects of reactive nitrogen intermediates on gene expression in Mycobacterium tuberculosis

Nitric oxide (NO) and related reactive nitrogen intermediates (RNI) are effective antimycobacterial agents and signal-transducing molecules. The present study uses microarray analysis to examine the effects of RNI on Mycobacterium tuberculosis gene expression. A common set of 53 genes was regulated by two chemically distinct nitric oxide donors. For a subset of the RNI-inducible genes, evidence exists suggesting that they may play a role in promoting survival of the tubercle bacillus in the host. Results obtained from studies based on a murine experimental tuberculosis model involving nos2-deficient mice suggest that RNI could regulate M. tuberculosis gene expression in vivo. Finally, there is a remarkable overlap between the RNI-inducible regulon and that previously reported to be regulated by hypoxia; and both reactive nitrogen species and anaerobicity upregulate the expression of one and the same putative two-component regulatory response system. Together, the results of this study provide evidence suggesting that (i) RNI play a role in regulating M. tuberculosis gene expression in vivo; (ii) the reactive nitrogen species upregulate genes that may be conducive to the survival of the tubercle bacillus in the infected host; and (iii) RNI and hypoxia may regulate mycobacterial gene expression via overlapping signal transduction pathways.     

Direct ex vivo analysis of antigen-specific IFN-gamma-secreting CD4 T cells in Mycobacterium tuberculosis-infected individuals: associations with clinical disease state and effect of treatment.

The wide spectrum of clinical outcomes following infection with Mycobacterium tuberculosis is largely determined by the host immune response; therefore, we studied several clinically defined groups of individuals (n = 120) that differ in their ability to contain the bacillus. To quantitate M. tuberculosis-specific T cells directly ex vivo, we enumerated IFN-gamma-secreting CD4 T cells specific for ESAT-6, a secreted Ag that is highly specific for M. tuberculosis, and a target of protective immune responses in animal models. We found that frequencies of circulating ESAT-6 peptide-specific IFN-gamma-secreting CD4 T cells were higher in latently infected healthy contacts and subjects with minimal disease and low bacterial burdens than in patients with culture-positive active pulmonary tuberculosis (p = 0.009 and p = 0.002, respectively). Importantly, the frequency of these Ag-specific CD4 T cells fell progressively in all groups with treatment (p = 0.005), suggesting that the lower responses in patients with more extensive disease were not due to tuberculosis-induced immune suppression. This population of M. tuberculosis Ag-specific Th1-type CD4 T cells appears to correlate with clinical phenotype and declines during successful therapy; these features are consistent with a role for these T cells in the containment of M. tuberculosis in vivo. Such findings may assist in the design and evaluation of novel tuberculosis vaccine candidates.     

Decreased outer membrane permeability protects mycobacteria from killing by ubiquitin-derived peptides

Ubiquitin-derived peptides are bactericidal in vitro and contribute to the mycobactericidal activity of the lysosome. To further define interactions of ubiquitin-derived peptides with mycobacteria, we screened for mutants with increased resistance to the bactericidal activity of the synthetic ubiquitin-derived peptide Ub2. The four Ub2-resistant Mycobacterium smegmatis mutants were also resistant to the bactericidal action of other antimicrobial peptides and macrophages. Two mutants were in the mspA gene encoding the main M. smegmatis porin. Using a translocation-deficient MspA point mutant, we showed that susceptibility of M. smegmatis to Ub2 was independent of MspA channel activity. Instead, the M. smegmatis Ub2-resistant mutants shared a common phenotype of decreased cell wall permeability compared with wild-type bacteria. Expression of mspA rendered Mycobacterium tuberculosis CDC1551 more susceptible both to ubiquitin-derived peptides in vitro and to lysosomal killing in macrophages. Finally, biochemical assays designed to assess membrane integrity indicated that Ub2 treatment impairs membrane function of M. smegmatis and M. tuberculosis cells . The M. smegmatis Ub2-resistant mutants were more resistant than wild-type M. smegmatis to this damage. We conclude that Ub2 targets mycobacterial membranes and that reduced membrane permeability provides mycobacteria intrinsic resistance against antimicrobial compounds including bactericidal ubiquitin-derived peptides.     

M. tuberculosis induces potent activation of IDO-1, but this is not essential for the immunological control of infection

Indoleamine 2,3-dioxygenesae-1 (IDO-1) catalyses the initial, rate-limiting step in tryptophan metabolism, thereby regulating tryptophan availability and the formation of downstream metabolites, including picolinic and quinolinic acid. We found that Mycobacterium tuberculosis infection induced marked upregulation of IDO-1 expression in both human and murine macrophages in vitro and in the lungs of mice following aerosol challenge with M. tuberculosis. The absence of IDO-1 in dendritic cells enhanced the activation of mycobacteria-specific T cells in vitro. Interestingly, IDO-1-deficiency during M. tuberculosis infection in mice was not associated with altered mycobacteria-specific T cell responses in vivo. The bacterial burden of infected organs, pulmonary inflammatory responses, and survival were also comparable in M. tuberculosis-infected IDO-1 deficient and wild type animals. Tryptophan is metabolised into either picolinic acid or quinolinic acid, but only picolinic acid inhibited the growth of M. tuberculosis in vitro. By contrast macrophages infected with pathogenic mycobacteria, produced quinolinic, rather than picolinic acid, which did not reduce M. tuberculosis growth in vitro. Therefore, although M. tuberculosis induces robust expression of IDO-1 and activation of tryptophan metabolism, IDO-1-deficiency fails to impact on the immune control and the outcome of the infection in the mouse model of tuberculosis.     

Cloning of the gene encoding a protective Mycobacterium tuberculosis secreted protein detected in vivo during the initial phases of the infectious process

The existence of therapeutic agents and the bacille Calmette-Guérin (BCG) vaccine have not significantly affected the current tuberculosis pandemic. BCG vaccine protects against serious pediatric forms of tuberculosis but not against adult pulmonary tuberculosis, the most common and contagious form of the disease. Several vaccine candidates, including Mycobacterium tuberculosis recombinant proteins formulated in newer adjuvants or delivered in bacterial plasmid DNA have recently been described. An attractive source of vaccine candidates has been M. tuberculosis Ags present in culture supernatants of the initial phases of the bacterial growth in vitro. In this study we describe an Ag discovery approach to select for such Ags produced in vivo during the initial phases of the infection. We combined RP-HPLC and mass spectrometry to identify secreted or shed M. tuberculosis proteins eliminated in animal urine within 14 days after the infection. A peptide containing sequence homology with a hypothetical M. tuberculosis protein was identified and the recombinant protein produced in Escherichia coli. The protein was recognized by Ab (IgG2a and IgG1) and T cells (Th1) of mice infected with M. tuberculosis and by lymphoid cells from healthy donors who had a positive purified protein derivative skin test but not from tuberculosis patients. Moreover, this Ag induced protection in mice against M. tuberculosis at levels comparable to protection induced by BCG vaccine. These results validate the Ag discovery approach of M. tuberculosis proteins secreted or shed in vivo during the early phases of the infection and open new possibilities for the development of potential vaccine candidates or of markers of active mycobacterial multiplication and therefore active disease.     

Lung-selective gene responses to alveolar hypoxia: potential role for the bone morphogenetic antagonist gremlin in pulmonary hypertension

Pulmonary hypoxia is a common complication of chronic lung diseases leading to the development of pulmonary hypertension. The underlying sustained increase in vascular resistance in hypoxia is a response unique to the lung. Thus we hypothesized that there are genes for which expression is altered selectively in the lung in response to alveolar hypoxia. Using a novel subtractive array strategy, we compared gene responses to hypoxia in primary human pulmonary microvascular endothelial cells (HMVEC-L) with those in cardiac microvascular endothelium and identified 90 genes (forming 9 clusters) differentially regulated in the lung endothelium. From one cluster, we confirmed that the bone morphogenetic protein (BMP) antagonist, gremlin 1, was upregulated in the hypoxic murine lung in vivo but was unchanged in five systemic organs. We also demonstrated that gremlin protein was significantly increased by hypoxia in vivo and inhibited HMVEC-L responses to BMP stimulation in vitro. Furthermore, significant upregulation of gremlin was measured in lungs of patients with pulmonary hypertensive disease. From a second cluster, we showed that CXC receptor 7, a receptor for the proangiogenic chemokine CXCL12, was selectively upregulated in the hypoxic lung in vivo, confirming that our subtractive strategy had successfully identified a second lung-selective hypoxia-responsive gene. We conclude that hypoxia, typical of that encountered in pulmonary disease, causes lung-specific alterations in gene expression. This gives new insights into the mechanisms of pulmonary hypertension and vascular loss in chronic lung disease and identifies gremlin 1 as a potentially important mediator of vascular changes in hypoxic pulmonary hypertension.     

Autoluminescent Mycobacterium tuberculosis for rapid, real-time, non-invasive assessment of drug and vaccine efficacy

Preclinical efforts to discover and develop new drugs and vaccines for tuberculosis are hampered by the reliance on colony-forming unit (CFU) counts as primary outcomes for in vivo efficacy studies and the slow growth of Mycobacterium tuberculosis. The utility of bioluminescent M. tuberculosis reporter strains for real-time in vitro and ex vivo assessment of drug and vaccine activity has been demonstrated but a simple, non-invasive, real-time surrogate marker to replace CFU counts for real-time evaluation of drug and vaccine efficacy in vivo has not been described. We describe the development of a fully virulent and stable autoluminescent strain of M. tuberculosis and proof-of-concept experiments demonstrating its utility for in vivo bioluminescence imaging to assess the efficacy of new drugs and vaccines for tuberculosis in a mouse model. Relative light unit (RLU) counts paralleled CFU counts during the active phase of bacterial growth, with a lower limit of detection of approximately 10(6) CFU in live, anesthetized mice. Experiments distinguishing active from inactive anti-tuberculosis drugs and bacteriostatic drug effects from bactericidal effects were completed in less than 5 days. The ability of a recombinant BCG vaccine to limit bacterial growth was demonstrated within 3 weeks. Use of this autoluminescent reporter strain has the potential to drastically reduce the time, effort, animals and costs consumed in the evaluation of drug activity in vitro and the in vivo assessment of drug and vaccine efficacy.     

Characterization of Mycobacterium smegmatis expressing the Mycobacterium tuberculosis fatty acid synthase I (fas1) gene

Unlike most other bacteria, mycobacteria make fatty acids with the multidomain enzyme eukaryote-like fatty acid synthase I (FASI). Previous studies have demonstrated that the tuberculosis drug pyrazinamide and 5-chloro-pyrazinamide target FASI activity. Biochemical studies have revealed that in addition to C(16:0), Mycobacterium tuberculosis FASI synthesizes C(26:0) fatty acid, while the Mycobacterium smegmatis enzyme makes C(24:0) fatty acid. In order to express M. tuberculosis FASI in a rapidly growing Mycobacterium and to characterize the M. tuberculosis FASI in vivo, we constructed an M. smegmatis Deltafas1 strain which contained the M. tuberculosis fas1 homologue. The M. smegmatis Deltafas1 (attB::M. tuberculosis fas1) strain grew more slowly than the parental M. smegmatis strain and was more susceptible to 5-chloro-pyrazinamide. Surprisingly, while the M. smegmatis Deltafas1 (attB::M. tuberculosis fas1) strain produced C(26:0), it predominantly produced C(24:0). These results suggest that the fatty acid elongation that produces C(24:0) or C(26:0) in vivo is due to a complex interaction among FASI, FabH, and FASII and possibly other systems and is not solely due to FASI elongation, as previously suggested by in vitro studies.     

T lymphocytes from healthy individuals with specificity to self-epitopes shared by the mycobacterial and human 65-kilodalton heat shock protein

The immune response to mycobacterial pathogens comprises a significant percentage of T cells with specificity for a 65-kDa heat shock protein (hsp) which is highly conserved in bacteria and man. PBMC were activated in vitro with killed Mycobacterium tuberculosis and afterward tested for CTL activity on autologous target cells primed with 1) killed M. tuberculosis, 2) intact recombinant 65-kDa hsp of Mycobacterium bovis/M. tuberculosis; or 3) tryptic fragments of the recombinant 65-kDa hsp. Strong CTL activity was observed on targets primed with killed M. tuberculosis or with tryptic fragments of the 65-kDa hsp, but not on those primed with the intact 65-kDa hsp. M. tuberculosis activated T cells from 2/13 donors tested exerted killer activity against unprimed targets. To assess whether T cell responses were directed against self-epitopes shared by the mycobacterial and human 65-kDa hsp, four peptides of at least 10 amino acids length were synthesized corresponding to fully or almost identical regions of these molecules. Peripheral blood T cells from 8/9 individuals tested, after activation with killed M. tuberculosis, expressed strong CTL activity toward autologous targets primed with one or more of these synthetic peptides. By using HLA-DR transfected murine L cells we found that the epitopes were recognized in the context of histocompatible HLA-DR (class II) molecules. We conclude that the demonstration of T cells with specificity to self-epitopes in vitro is not indicative for autoimmune disease. However, if at certain stages of infection such T cells are activated by crossreactive microbial epitopes they could cause autoimmune responses.