Mycobacterial growth and sensitivity to H2O2 killing in human monocytes in vitro

Comparative Study of the Susceptibility to Oxidative Stress between Two Types of Mycobacterium bovis BCG Tokyo 172

Genomic analysis revealed that the vaccine seed lot of Mycobacterium bovis bacillus Calmette-Guérin (BCG) Tokyo 172 contains two subclones (types I and II), but their phenotypic differences have not been elucidated. In this study, we compared the susceptibility of bacilli types I and II to oxidative stress in vitro and within host cells. Notably, the subclones displayed similar superoxide dismutase activity; however, foam height in the catalase test and lysate catalase/peroxidase activity were higher for type I bacilli than for type II bacilli. Additionally, type I bacilli were less susceptible to hydrogen peroxide (H₂O₂) than type II bacilli. After exposure to H₂O₂, antioxidative stress response genes katG, ahpC, sodA, and trxA were more strongly induced in type I bacilli than in type II bacilli. Further, we investigated cell survival in macrophages. Fewer type II bacilli were recovered than type I bacilli. However, in the presence of apocynin, a specific inhibitor of NADPH oxidase, type II recovery was greater than that of type I. The production of interleukin 1β (IL-1β), IL-12 p40, and tumor necrosis factor alpha (TNF-α) was higher in type I bacillus-infected macrophages than in type II bacillus-infected macrophages. The proportions of type I and type II bacilli in vaccine lots over 3 years (100 lots) were 97.6% ± 1.5% and 2.4% ± 1.5%, respectively. The study results illustrated that type I bacilli are more resistant to oxidative stress than type II bacilli. Overall, these findings provide important information in terms of the quality control and safety of BCG Tokyo 172 vaccine.

IMPORTANCE This study revealed the difference of in vivo and in vitro antioxidative stress properties of BCG Tokyo 172 types I and II as one of the bacteriological characteristics. In particular, the bacilli exhibited differences in catalase/peroxidase activity, which could explain their different protective effects against infection. The differences correlated with survival in the host cell and the production of proinflammatory cytokines to protect against infection by Mycobacterium tuberculosis. The proportion of bacilli types I and II in all commercial lots of BCG Tokyo 172 over 3 years (100 lots) was constant. The findings also highlighted the importance of analyzing their content for quality control during vaccine production.

Efferocytosis of Apoptotic Neutrophils Enhances Control of Mycobacterium tuberculosis in HIV-Coinfected Macrophages in a Myeloperoxidase-Dependent Manner

Tuberculosis remains a big threat, with 1.6 million deaths in 2017, including 0.3 million deaths among patients with HIV. The risk of developing active disease increases considerably during an HIV coinfection. Alveolar macrophages are the first immune cells to encounter the causative agent Mycobacterium tuberculosis, but during the granuloma formation other cells are recruited in order to combat the bacteria. Here, we have investigated the effect of efferocytosis of apoptotic neutrophils by M. tuberculosis and HIV-coinfected macrophages in a human in vitro system. We found that the apo-ptotic neutrophils enhanced the control of M. tuberculosis in single and HIV-coinfected macrophages, and that this was dependent on myeloperoxidase (MPO) and reactive oxygen species in an autophagy-independent manner. We show that MPO remains active in the apoptotic neutrophils and can be harnessed by infected macrophages. In addition, MPO inhibition removed the suppression in M. tuberculosis growth caused by the apoptotic neutrophils. Antimycobacterial components from apoptotic neutrophils could thus increase the microbicidal activity of macrophages during an M. tuberculosis/HIV coinfection. This cooperation between innate immune cells could thereby be a way to compensate for the impaired adaptive immunity against M. tuberculosis seen during a concurrent HIV infection.

Biological and Epidemiological Consequences of MTBC Diversity

Tuberculosis is caused by different groups of bacteria belonging to the Mycobacterium tuberculosis complex (MTBC). The combined action of human factors, environmental conditions and bacterial virulence determine the extent and form of human disease. MTBC virulence is a composite of different clinical phenotypes such as transmission rate and disease severity among others. Clinical phenotypes are also influenced by cellular and immunological phenotypes. MTBC phenotypes are determined by the genotype, therefore finding genotypes responsible for clinical phenotypes would allow discovering MTBC virulence factors. Different MTBC strains display different cellular and clinical phenotypes. Strains from Lineage 5 and Lineage 6 are metabolically different, grow slower, and are less virulent. Also, at least certain groups of Lineage 2 and Lineage 4 strains are more virulent in terms of disease severity and human-to-human transmission. Because phenotypic differences are ultimately caused by genotypic differences, different genomic loci have been related to various cellular and clinical phenotypes. However, defining the impact of specific bacterial genomic loci on virulence when other bacterial determinants, human and environmental factors are also impacting the phenotype would contribute to a better knowledge of tuberculosis virulence and ultimately benefit tuberculosis control.

Antioxidants protect keratinocytes against M. ulcerans mycolactone cytotoxicity

BACKGROUND

Mycobacterium ulcerans is the causative agent of necrotizing skin ulcerations in distinctive geographical areas. M. ulcerans produces a macrolide toxin, mycolactone, which has been identified as an important virulence factor in ulcer formation. Mycolactone is cytotoxic to fibroblasts and adipocytes in vitro and has modulating activity on immune cell functions. The effect of mycolactone on keratinocytes has not been reported previously and the mechanism of mycolactone toxicity is presently unknown. Many other macrolide substances have cytotoxic and immunosuppressive activities and mediate some of their effects via production of reactive oxygen species (ROS). We have studied the effect of mycolactone in vitro on human keratinocytes--key cells in wound healing--and tested the hypothesis that the cytotoxic effect of mycolactone is mediated by ROS.

METHODOLOGY/PRINCIPAL FINDINGS

The effect of mycolactone on primary skin keratinocyte growth and cell numbers was investigated in serum free growth medium in the presence of different antioxidants. A concentration and time dependent reduction in keratinocyte cell numbers was observed after exposure to mycolactone. Several different antioxidants inhibited this effect partly. The ROS inhibiting substance deferoxamine, which acts via chelation of Fe(2+), completely prevented mycolactone mediated cytotoxicity.

CONCLUSIONS/SIGNIFICANCE

This study demonstrates that mycolactone mediated cytotoxicity can be inhibited by deferoxamine, suggesting a role of iron and ROS in mycolactone induced cytotoxicity of keratinocytes. The data provide a basis for the understanding of Buruli ulcer pathology and the development of improved therapies for this disease.

Does M. tuberculosis genomic diversity explain disease diversity?

The outcome of tuberculosis infection and disease is highly variable. This variation has been attributed primarily to host and environmental factors, but better understanding of the global genomic diversity in the M. tuberculosis complex (MTBC) suggests that bacterial factors could also be involved. Review of nearly 100 published reports shows that MTBC strains differ in their virulence and immunogenicity in experimental models, but whether this phenotypic variation plays a role in human disease remains unclear. Given the complex interactions between the host, the pathogen and the environment, linking MTBC genotypic diversity to experimental and clinical phenotypes requires an integrated systems epidemiology approach embedded in a robust evolutionary framework.

Impaired activation of Stat1 and c-Jun as a possible defect in macrophages of patients with active tuberculosis

Studies of patients with active tuberculosis (TB) and infected healthy individuals have shown that interferon (IFN)-gamma is present in sites of Mycobacterium tuberculosis infection in comparable levels. This suggests that there is a deficiency in the macrophage response to IFN-gamma in TB patients. We used recombinant human IFN-gamma to stimulate adherent monocyte-derived macrophages from three groups of people: patients with active tuberculosis (TBP), their healthy household contacts (HHC) and healthy uninfected controls from the community (CC). We then evaluated the ability of the macrophages to inhibit the growth of M. tuberculosis H37Rv as well as their cytokine profile at early in infection (48 h). After IFN-gamma treatment, macrophages of healthy individuals (HHC and CC) controlled M. tuberculosis growth and produced mainly nitric oxide (NO) and interleukin (IL)-12p70, whereas TBP macrophages did not kill M. tuberculosis. Additionally, TBP macrophages produced low levels of NO and IL-12p70 and high levels of tumour necrosis factor (TNF)-alpha and IL-10. Transforming growth factor (TGF)-beta levels were similar among all three groups. M. tuberculosis infection had little effect on the cytokine response after IFN-gamma stimulus, but infection alone induced more IL-10 and TGF-beta in TBP macrophages. There were no differences in Stat1 nuclear translocation and DNA binding between the groups. However, the phosphorylated Stat1 and c-Jun (AP-1) in nuclear protein extracts was diminished in TBP macrophages compared to macrophages of healthy individuals. These results indicate an impairment of Stat1-dependent and Stat1-independent IFN-gamma signalling in macrophages of people with active tuberculosis, suggesting a different molecular regulation that could impact macrophage functionality and disease outcome.

Tactics of Mycobacterium avium subsp. paratuberculosis for intracellular survival in mononuclear phagocytes

Johne's disease is a condition that refers to chronic granulomatous enteritis in ruminants. It is believed that survival and replication of Mycobacterium (M.) paratuberculosis in mononuclear phagocytes plays an important role in the pathogenesis of Johne's disease. However, it is not clear how M. paratuberculosis survives for long time periods in mononuclear phagocytes, nor is it clear which factors trigger multiplication of these bacilli and result in the development of Johne's disease. Investigating the intracellular fate of M. paratuberculosis is challenging because of its very slow growth (more than two months to form visible colonies on media). Existing animal models also have limitations. Despite those obstacles, there has been progress in understanding the intracellular survival tactics of M. paratuberculosis and the host response against them. In this review, we compare known aspects of the intracellular survival tactics of M. paratuberculosis with those of other mycobacterial species, and consider possible mycobactericidal mechanisms of mononuclear phagocytes.

Transcriptional analysis of Mycobacterium fortuitum cultures upon hydrogen peroxide treatment using the novel standard rrnA-P1

BACKGROUND

The ability of an intracellular pathogen to establish infection depends on the capacity of the organism to survive and replicate inside the host. Mycobacterium fortuitum is a bacteria that contains genes involved in the detoxification of the oxygen reactive species such as those produced by the host during the infection. In this work, we investigate the effects of hydrogen peroxide on the transcription and expression of these genes by developing a real time quantitative PCR technique (qRT-PCR) using the ribosomal promoter region (rrnA-P1) as reference product for quantification of the mRNA levels.

RESULTS

M. fortuitum cultures were treated with different hydrogen peroxide concentrations (0.02 to 20 mM) during several periods of time (30 to 120 minutes). The activity of the enzymes KatGII and SodA, and the transcription of corresponding genes were evaluated. The transcriptional regulator furAII gene was also studied. The ribosomal promoter region rrnA-P1 was validated as referential product under the stress conditions checked by qRT-PCR. Minor changes were observed under the conditions tested except when bacteria were incubated in the presence of 20 mM hydrogen peroxide. Under those conditions, the levels of transcription of the three genes under study increased at 30 minutes of treatment. The viability of the bacteria was not influenced under the conditions tested.

CONCLUSION

In this work, we have quantified transcriptional responses to stress suggesting that, the opportunistic pathogen M. fortuitum is more resistant and differs in behaviour in the presence of hydrogen peroxide, when compared to the major pathogen Mycobacterium tuberculosis and the saprophyte Mycobacterium smegmatis. Besides, we demonstrate the mycobacterial non-coding region rrnA-P1 to be a suitable reference product in the analysis of qRT-PCR transcriptional data of M. fortuitum.

Yeast of the oral cavity is the reservoir of Heliobacter pylori

BACKGROUND

Frequent occurrence of Helicobacter pylori in the human gastrointestinal tract and its persistence due to unsuccessful antimicrobial therapy might be related to a stage in the life cycle of H. pylori in which the bacterium establishes itself as an intracellular symbiont in yeast. In this study, occurrence of non-culturable H. pylori in the oral yeast was assessed by targeting vacuolating cytotoxin A (vacA s1s2) and ureAB genes in the total DNAs of yeasts.

METHODS

DNAs were extracted from 13 oral yeasts in which bacterium-like bodies, suspected to be H. pylori, were observed microscopically. Primers were recruited to amplify vacA s1s2 and ureAB genes. DNAs from H. pylori and E. coli were used as controls. The amplicons from one yeast and H. pylori were sequenced. Yeasts were identified as Candida albicans.

RESULTS

Fragments of vacA s1s2 and ureAB genes were amplified from 13 yeasts. The size of PCR products was 286 bp for vacA s1s2 gene and 406 bp for ureAB gene. Similar bands were obtained from the control H. pylori, and the results for E. coli were negative. The data from sequencing of PCR products showed about 98% homology between the genes amplified from yeast and those from H. pylori.

CONCLUSIONS

The results of this study showed the intracellular occurrence of H. pylori in yeast. This endosymbiotic relationship might explain the persistence of H. pylori in the oral cavity, the consequence of which could be reinoculation of the stomach by the bacterium and spread of infection among human populations.

Impaired Generation of Reactive Oxygen Species during Differentiation of Dendritic Cells (DCs) byMycobacterium tuberculosisSecretory Antigen (MTSA) and Subsequent Activation of MTSA-DCs by Mycobacteria Results in Increased Intracellular Survival

We investigated the role of reactive oxygen species (ROS) in dendritic cell (DC) differentiation by 10-kDa Mycobacterium tuberculosis secretory Ag (MTSA) and survival of mycobacteria therein. Compared with GM-CSF, MTSA induced lower ROS production during DC differentiation from precursors. This result correlated with higher superoxide dismutase 1 expression in MTSA stimulated precursors as compared with GM-CSF stimulation. Furthermore, a negative regulation of protein kinase C (PKC) activation by ROS was observed during DC differentiation. ROS inhibited the rapid and increased phosphorylation of PKCalpha observed during DC differentiation by MTSA. In contrast, ROS inhibition increased the weak and delayed PKCalpha phosphorylation by GM-CSF. Similar to DC differentiation, upon activation with either M. tuberculosis cell extract (CE) or live Mycobacterium bovis bacillus Calmette-Guérin (BCG), DCs differentiated with MTSA (MTSA-DCs) generated lower ROS levels when compared with DCs differentiated with GM-CSF (GM-CSF-DCs). Likewise, a negative regulation of PKCalpha phosphorylation by ROS was once again observed in DCs activated with either M. tuberculosis CE or live M. bovis BCG. However, a reciprocal positive regulation between ROS and calcium was observed. Compared with MTSA-DCs, stimulation of GM-CSF-DCs with M. tuberculosis CE induced a 2-fold higher ROS-dependent calcium influx. However, pretreatment of MTSA-DCs with H(2)O(2) increased calcium mobilization. Finally, lower ROS levels in MTSA-DCs correlated with increased intracellular survival of M. bovis BCG when compared with survival in GM-CSF-DCs. Although inhibiting ROS in GM-CSF-DCs increased M. bovis BCG survival, H(2)O(2) treatment of MTSA-DCs decreased survival of M. bovis BCG. Overall our results suggest that DCs differentiated with Ags such as MTSA may provide a niche for survival and/or growth of mycobacteria following sequestration of ROS.

Evidence for positive selection on Mycobacterium tuberculosis within patients

BACKGROUND

While the pathogenesis and epidemiology of tuberculosis are well studied, relatively little is known about the evolution of the infectious agent Mycobacterium tuberculosis, especially at the within-host level. The insertion sequence IS6110 is a genetic marker that is widely used to track the transmission of tuberculosis between individuals. This and other markers may also facilitate our understanding of the disease within patients.

RESULTS

This article presents three lines of evidence supporting the action of positive selection on M. tuberculosis within patients. The arguments are based on a comparison between empirical findings from molecular epidemiology, and population genetic models of evolution. Under the hypothesis of neutrality of genotypes, 1) the mutation rate of the marker IS6110 is unusually high, 2) the time it takes for substitutions to occur within patients is too short, and 3) the amount of polymorphism within patients is too low.

CONCLUSIONS

Empirical observations are explained by the action of positive selection during infection, or alternatively by very low effective population sizes. I discuss the possible roles of antibiotic treatment, the host immune system and extrapulmonary dissemination in creating opportunities for positive selection.

Mycobacterium tuberculosis-specific CD8+ T cells preferentially recognize heavily infected cells

Both CD4+ and CD8+ T cells are important for successful immunity to tuberculosis and have redundant effector functions, such as cytolysis and release of potent antimycobacterial cytokines such as interferon-gamma and tumor necrosis factor-alpha. We hypothesized that CD8+ T cells play a unique role in host defense to Mycobacterium tuberculosis infection as well. Possibilities include preferential and/or enhanced release of granular constituents and/or preferential recognition of heavily infected cells. Utilizing human, Mycobacterium tuberculosis-specific, CD4+ and CD8+ T cell clones, we demonstrate that, after recognition of antigen-presenting cells displaying peptide antigen, CD4+ T cells preferentially release interferon-gamma, whereas CD8+ T cells preferentially lyse antigen-presenting cells. Furthermore, utilizing dendritic cells infected with Mycobacterium tuberculosis expressing green fluorescent protein, we show that CD8+ T cells preferentially recognize heavily infected cells that constitute the minority of infected cells. These data support the hypothesis that the central role of CD8+ T cells in the control of infection with Mycobacterium tuberculosis may be that of surveillance; in essence, recognition of cells in which the containment of Mycobacterium tuberculosis is no longer effective.

Identification and characterization of a diamide sensitive mutant of Mycobacterium smegmatis

A mutant, T7, highly sensitive to oxidative stress as caused by diamide was isolated from a Mycobacterium smegmatis mc(2)155 transposon mutant library. While wild-type M. smegmatis is able to grow well on solid media supplemented with 10 mM diamide, T7 is only able to grow on solid media containing up to 1 mM diamide. This mutant is also sensitive to other thiol modifying agents such as iodoacetamide and chlorodinitrobenzene. By sequencing the genomic DNA flanking the transposon, T7 was found to be mutated in the region upstream of the homolog of M. tuberculosis Rv0274 open reading frame. Sequence analysis revealed that Rv0274 is a member of a superfamily of metalloenzymes comprising enzymes such as extradiol dioxygenases, glyoxalases, and fosfomycin resistant glutathione transferases. Cloning and epichromosomal expression of M. tuberculosis Rv0274 in the mutant resulted in complementation of the sensitivity to diamide.

Reactive nitrogen intermediates have a bacteriostatic effect on Mycobacterium tuberculosis in vitro

Susceptibility of six isolates of Mycobacterium tuberculosis (CB3.3, CDC1551, RJ2E, C.C.13, H37Rv, and H37Ra) and two isolates of Mycobacterium bovis (Ravenel and BCG) to reactive oxygen intermediates (ROI) and reactive nitrogen intermediates (RNI) was determined by standard in vitro survival assays. After 21 days of incubation, the survival of most strains exposed to either acidified sodium nitrite (ASN) or hydrogen peroxide (H(2)O(2)) was significantly lower than the same strains unexposed to these RNI or ROI products. However, after 50 days of incubation, these differences in susceptibility became less apparent for strains exposed to ASN but not for strains exposed to H(2)O(2). The recovery of these strains after exposure to RNI suggests that the effect of RNI on M. tuberculosis is bacteriostatic. The in vitro concentrations of ROI and RNI used in these assays were higher than those expected in vivo. These observations suggest that, in vivo, RNI expression at physiologically achievable concentrations may keep M. tuberculosis from proliferating but that removal of RNI may allow the organisms to proliferate. Furthermore, the ability of some M. tuberculosis strains to cause rapidly progressive disease may relate to their intrinsic levels of RNI and ROI resistance.

Mycobacterium tuberculosis CDC1551 is resistant to reactive nitrogen and oxygen intermediates in vitro

Resistance to reactive oxygen intermediates and reactive nitrogen intermediates in vitro of a clinical isolate of Mycobacterium tuberculosis (CDC1551) that caused a large outbreak of tuberculosis was compared to that of M. tuberculosis strains CB3.3, H37Rv, H37Ra, Erdman, RJ2E, C.C. 13, and C.C. 22 as well as M. bovis strains Ravenel and BCG. CDC1551 and CB3.3 were significantly more resistant to both hydrogen peroxide (H(2)O(2)) and acidified sodium nitrite than were the other strains tested. This biological phenotype may serve as an in vitro marker for clinical strains of M. tuberculosis likely to cause a large outbreak of tuberculosis.

IRF and tuberculosis

Tuberculosis is the most prevalent infectious disease and causes more deaths than any other, yet only 5%-10% of people infected by the causative agent, Mycobacterium tuberculosis, will develop the disease. Thus, natural resistance among humans is the norm. Fundamental immune responses to M. tuberculosis are being elucidated, including induction of interferon regulatory factor-1 (IRF-1). Moreover, IRF-1 has been found necessary for normal resistance to infection by mycobacteria in mice. Roles for IRF-1 in a plethora of immune system functions have been described. This review considers molecular responses to infection by M. tuberculosis that might account for induction of IRF-1 and highlights putative connections between immunomodulatory functions of IRF-1 and immune responses relevant to infection by M. tuberculosis. However, the complexity inherent in pleiotropy and redundancy limits the ability to draw firm conclusions. In many cases, it remains to be demonstrated that a particular function of IRF-1 is the basis for a known response to infection. For example, although IRF-1 is required for a Th1 cell-mediated, adaptive immune response in some circumstances, it is not known if the Th1 response to infection by M. tuberculosis requires IRF-1. Conversely, some known contributions by IRF-1 to fundamental aspects of the immune system are not yet proven relevant in the host response to infection. For example, it is not known if control of T cell subset development by IRF-1 is significant for host defense against M. tuberculosis. Functions of other IRF that overlap with or are distinct from the functions of IRF-1 also could be important for the immune response to M. tuberculosis.

Feedback regulation of an Agrobacterium catalase gene katA involved in Agrobacterium-plant interaction

Catalases are known to detoxify H2O2, a major component of oxidative stress imposed on a cell. An Agrobacterium tumefaciens catalase encoded by a chromosomal gene katA has been implicated as an important virulence factor as it is involved in detoxification of H2O2 released during Agrobacterium-plant interaction. In this paper, we report a feedback regulation pathway that controls the expression of katA in A. tumefaciens cells. We observed that katA could be induced by plant tissue sections and by acidic pH on a minimal medium, which resembles the plant environment that the bacteria encounter during the course of infection. This represents a new regulatory factor for catalase induction in bacteria. More importantly, a feedback regulation was observed when the katA-gfp expression was studied in different genetic backgrounds. We found that introduction of a wild-type katA gene encoding a functional catalase into A. tumefaciens cells could repress the katA-gfp expression over 60-fold. The katA gene could be induced by H2O2 and the encoded catalase could detoxify H2O2. In addition, the katA-gfp expression of one bacterial cell could be repressed by other surrounding catalase-proficient bacterial cells. Furthermore, mutation at katA caused a 10-fold increase of the intracellular H2O2 concentration in the bacteria grown on an acidic pH medium. These results suggest that the endogenous H2O2 generated during A. tumefaciens cell growth could serve as the intracellular and intercellular inducer for the katA gene expression and that the acidic pH could pose an oxidative stress on the bacteria. Surprisingly, one mutated KatA protein, exhibiting no significant catalase activity as a result of the alteration of two important residues at the putative active site, could partially repress the katA-gfp expression. The feedback regulation of the katA gene by both catalase activity and KatA protein could presumably maintain an appropriated level of catalase activity and H2O2 inside A. tumefaciens cells.

M. tuberculosis: immunology and vaccination

Tuberculosis is increasing. Current treatment regimens require at least 6 months, because latent or stationary phase organisms are difficult to kill. Such regimens do not achieve full compliance, and "directly observed therapy short course" (DOTS) is having less impact than expected. This worrying situation is aggravated by coinfection with human immunodeficiency virus (HIV), and by the increase in drug-resistant strains. We need new insights that lead to more rapid therapies and immunotherapies, and more reliable vaccines. Recent insights have come from: understanding of the relationship between Mycobacterium tuberculosis and macrophages; the multiple T cell types that recognise mycobacterial peptides, lipids and glycolipids; the critical role of interferon-gamma (IFNgamma) and interleukin-12 (IL-12) in human mycobacterial infection revealed by genetically defective children; quantitation of the presence and importance of Th2 lymphocyte activation in human tuberculosis; the role of local conversion of inactive cortisone to active cortisol in the lesions; the recognition that some effective prophylactic vaccines also work as immumotherapeutics whereas others do not. In the longer term the recent sequencing of the M. tuberculosis genome will lead to further advances. In the short term, effective immunotherapy remains the most accessible breakthrough in the management of tuberculosis. The types of practical advance that will result from sequencing the genome are discussed speculatively, but cannot yet be predicted with certainty.

Recent advances in our understanding of human host responses to tuberculosis

Tuberculosis remains one of the world's greatest public health challenges: 2 billion persons have latent infection, 8 million people develop active tuberculosis annually, and 2-3 million die. Recently, significant advances in our understanding of the human immune response against tuberculosis have occurred. The present review focuses on recent work in macrophage and T-cell biology that sheds light on the human immune response to tuberculosis. The role of key cytokines such as interferon-gamma is discussed, as is the role of CD4+ and CD8+ T cells in immune regulation in tuberculosis, particularly with regard to implications for vaccine development and evaluation.

Necrosis of lung epithelial cells during infection with Mycobacterium tuberculosis is preceded by cell permeation

Mycobacterium tuberculosis establishes infection, progresses towards disease, and is transmitted from the alveolus of the lung. However, the role of the alveolar epithelium in any of these pathogenic processes of tuberculosis is unclear. In this study, lung epithelial cells (A549) were used as a model in which to examine cytotoxicity during infection with either virulent or avirulent mycobacteria in order to further establish the role of the lung epithelium during tuberculosis. Infection of A549 cells with M. tuberculosis strains Erdman and CDC1551 demonstrated significant cell monolayer clearing, whereas infection with either Mycobacterium bovis BCG or Mycobacterium smegmatis LR222 did not. Clearing of M. tuberculosis-infected A549 cells correlated to necrosis, not apoptosis. Treatment of M. tuberculosis-infected A549 cells with streptomycin, but not cycloheximide, demonstrated a significant reduction in the necrosis of A549 cell monolayers. This mycobacterium-induced A549 necrosis did not correlate to higher levels of intracellular or extracellular growth by the mycobacteria during infection. Staining of infected cells with propidium iodide demonstrated that M. tuberculosis induced increased permeation of A549 cell membranes within 24 h postinfection. Quantitation of lactate dehydrogenase (LDH) release from infected cells further demonstrated that cell permeation was specific to M. tuberculosis infection and correlated to A549 cellular necrosis. Inactivated M. tuberculosis or its subcellular fractions did not result in A549 necrosis or LDH release. These studies demonstrate that lung epithelial cell cytotoxicity is specific to infection by virulent mycobacteria and is caused by cellular necrosis. This necrosis is not a direct correlate of mycobacterial growth or of the expression of host cell factors, but is preceded by permeation of the A549 cell membrane and requires infection with live bacilli.

Mycobacterium tuberculosis CDC1551 Induces a More Vigorous Host Response In Vivo and In Vitro, But Is Not More Virulent Than Other Clinical Isolates

Mycobacterium tuberculosis CDC1551, a clinical isolate reported to be hypervirulent and to grow faster than other isolates, was compared with two other clinical isolates (HN60 and HN878) and two laboratory strains (H37Rv and Erdman). The initial (1–14 days) growth of CDC1551, HN60, HN878, and H37Rv was similar in the lungs of aerosol-infected mice, but growth of Erdman was slower. Thereafter, the growth rate of CDC1551 decreased relative to the other strains which continued to grow at comparable rates up to day 21. In the lungs of CDC1551-infected mice, small well-organized granulomas with high levels of TNF-α, IL-6, IL-10, IL-12, and IFN-γ mRNA were apparent sooner than in lungs of mice infected with the other strains. CDC1551-infected mice survived significantly longer. These findings were confirmed in vitro. The growth rates of H37Rv and CDC1551 in human monocytes were the same, but higher levels of TNF-α, IL-10, IL-6, and IL-12 were induced in monocytes after infection with CDC1551 or by exposure of monocytes to lipid fractions from CDC1551. CD14 expression on the surface of the monocytes was up-regulated to a greater extent by exposure to the lipids of CDC1551. Thus, CDC1551 is not more virulent than other M. tuberculosis isolates in terms of growth in vivo and in vitro, but it induces a more rapid and robust host response.

Differential Sensitivity of Distinct Chlamydia trachomatis Isolates to IFN-γ-Mediated Inhibition

Resistance to the mouse pneumonitis (MoPn) strain of Chlamydia trachomatis has been mapped to MHC class II-restricted, IL-12-dependent CD4+ T cells that secrete a type 1 profile of proinflammatory cytokines, which includes IFN-γ and TNF-α. The relative contribution of IFN-γ is controversial, however, due to variation in results presented by different laboratories. To determine whether C. trachomatis strain differences contributed to this apparent conflict, the relative resistance of IFN-γ-deficient mice to murine and human strains of C. trachomatis was compared. All human serovars were much more sensitive to the direct inhibitory actions of IFN-γ than the MoPn strain. Furthermore, genital clearance of human serovar D in the C57BL/6 mouse was mediated by class II-independent mechanisms that probably involved local production of IFN-γ by cells of the innate immune system. TNF-α also contributed indirectly to host resistance against all strains tested. The differential susceptibility of distinct C. trachomatis strains to effector cytokines such as IFN-γ could not have been predicted by interstrain biologic variation or by the profile of cytokines stimulated during infection. These findings indicate that strain variation should be considered in situations where related isolates of a given parasite produce conflicting data in models of infection and immunity. They also suggest that stimulation of mucosal IFN-γ activity is a relevant goal for a human chlamydial vaccine.

IFN-γ-Mediated Control of Coxiella burnetii Survival in Monocytes: The Role of Cell Apoptosis and TNF

The treatment of infectious diseases caused by intracellular bacteria, such as Q fever, may benefit from cytokines acting on macrophages. Monocytic THP-1 cells were infected with Coxiella burnetii, the etiological agent of Q fever, and then treated with IFN-γ. While C. burnetii multiplied in untreated monocytes, IFN-γ reduced bacterial viability after 24 h of treatment and reached maximum inhibition after 96 h. IFN-γ also affected the viability of infected cells. Cell death resulted from apoptosis; occurring 24 h after the addition of IFN-γ, it reached a maximum after 48 h and was followed by necrosis. Reactive oxygen intermediates were not required for C. burnetii killing, since monocytes from patients with chronic granulomatous disease were microbicidal in response to IFN-γ. The role of cytokines was also investigated. IFN-γ elicited a moderate release of IL-1β in infected monocytes. Moreover, the IL-1 receptor antagonist did not affect C. burnetii survival, suggesting that IL-1β was not involved in the bacterial killing induced by IFN-γ. TNF was involved in IFN-γ-induced killing of C. burnetii and cell death. IFN-γ induced mRNA expression and sustained secretion of TNF. Neutralizing Abs to TNF as well as Abs directed against TNF receptors I and II, significantly prevented IFN-γ-dependent killing of C. burnetii and cell death. These results suggest that IFN-γ promotes the killing of C. burnetii in monocytes through an apoptotic mechanism mediated in part by TNF.

Generation of multinucleated giant cells in vitro by culture of human monocytes with Mycobacterium bovis BCG in combination with cytokine-containing supernatants

Multinucleated giant cells (MGC), a characteristic feature of tuberculous granulomas, form by fusion of monocytes or macrophages, but little is known about the mechanism of the fusion process itself. Several studies report an indirect effect of mycobacteria, i.e., induction of a soluble lymphocyte-derived fusion factor following stimulation by mycobacteria or mycobacterial products. The aim of our study was to determine whether contact with mycobacteria can induce MGC formation from human monocytes in vitro. Stimulation of monocytes with Mycobacterium bovis bacillus Calmette-Guérin (BCG) in combination with cytokine-containing supernatants of herpesvirus saimiri-transformed human T-cell clones (T-SN) led to MGC formation with fusion rates of about 27%. In contrast, stimulation with one component alone induced only low fusion rates of up to 10%. Heat-killed BCG in combination with T-SN induced monocyte fusion to the same extent as live mycobacteria. BCG culture supernatant, BCG lysate, or inert particles in combination with T-SN did not induce MGC formation. Experiments using transwell plates containing a semipermeable membrane revealed that induction of the fusion process is dependent on direct contact of monocytes and mycobacteria. MGC formation induced by BCG plus T-SN could be inhibited by addition of monoclonal antibodies to gamma interferon (but not tumor necrosis factor alpha) as well as to the beta chain (CD18) of beta2-integrins. These results demonstrate that contact with mycobacteria in combination with cytokine-containing supernatants is able to induce human monocytes to form MGC and that membrane-bound molecules of mycobacteria and monocytes are involved in the fusion process.

Mycobacterium tuberculosis catalase and peroxidase activities and resistance to oxidative killing in human monocytes in vitro

Mycobacterium tuberculosis has a relatively high resistance to killing by hydrogen peroxide and organic peroxides. Resistance may be mediated by mycobacterial catalase-peroxidase (KatG) and possibly by alkyl hydroperoxide reductase (AhpC). To determine the interrelationship between sensitivity to H2O2, catalase and peroxidase activities, and bacillary growth rates measured both intracellularly in human monocytes and in culture medium, we examined one laboratory strain, two clinical isolates, and three recombinant strains of M. tuberculosis with differing levels of KatG and AhpC. Five of the mycobacterial strains had intracellular doubling times of 27 to 32 h, while one KatG-deficient clinical isolate (ATCC 35825) doubled in approximately 76 h. Killing of mycobacteria by exogenously added H2O2 was more pronounced for intracellular bacilli than for those bacilli derived from disrupted monocytes. Strains with no detectable KatG expression or catalase activity were relatively sensitive to killing (43 to 67% killing) by exogenous H2O2. However, once even minimal catalase activity was present, mycobacterial catalase activity over a 10-fold range (0.56 to 6.2 U/mg) was associated with survival of 85% of the bacilli. Peroxidase activity levels correlated significantly with resistance of the mycobacterial strains to H2O2-mediated killing. An endogenous oxidative burst induction by 4beta-phorbol 12beta-myristate 13alpha-acetate treatment of infected monocytes reduced the viability of the KatG null strain (H37Rv Inhr) but not the KatG-overexpressing strain [H37Rv(pMH59)]. These results suggest that mycobacterial resistance to oxidative metabolites (including H2O2 and other peroxides) may be an important mechanism of bacillary survival within the host phagocyte.

Oxidative stress response and characterization of the oxyR-ahpC and furA-katG loci in Mycobacterium marinum

Oxidative stress response in pathogenic mycobacteria is believed to be of significance for host-pathogen interactions at various stages of infection. It also plays a role in determining the intrinsic susceptibility to isoniazid in mycobacterial species. In this work, we characterized the oxyR-ahpC and furA-katG loci in the nontuberculous pathogen Mycobacterium marinum. In contrast to Mycobacterium smegmatis and like Mycobacterium tuberculosis and Mycobacterium leprae, M. marinum was shown to possess a closely linked and divergently oriented equivalents of the regulator of peroxide stress response oxyR and its subordinate gene ahpC, encoding a homolog of alkyl hydroperoxide reductase. Purified mycobacterial OxyR was found to bind to the oxyR-ahpC promoter region from M. marinum and additional mycobacterial species. Mobility shift DNA binding analyses using OxyR binding sites from several mycobacteria and a panel of in vitro-generated mutants validated the proposed consensus mycobacterial recognition sequence. M. marinum AhpC levels detected by immunoblotting, were increased upon treatment with H2O2, in keeping with the presence of a functional OxyR and its binding site within the promoter region of ahpC. In contrast, OxyR did not bind to the sequences upstream of the katG structural gene, and katG expression did not follow the pattern seen with ahpC. Instead, a new open reading frame encoding a homolog of the ferric uptake regulator Fur was identified immediately upstream of katG in M. marinum. The furA-katG linkage and arrangement are ubiquitous in mycobacteria, suggesting the presence of additional regulators of oxidative stress response and potentially explaining the observed differences in ahpC and katG expression. Collectively, these findings broaden our understanding of oxidative stress response in mycobacteria. They also suggest that M. marinum will be useful as a model system for studying the role of oxidative stress response in mycobacterial physiology, intracellular survival, and other host-pathogen interactions associated with mycobacterial diseases.