Dual oxidase 1 is dispensable during Mycobacterium tuberculosis infection in mice

Introduction

Mycobacterium tuberculosis (Mtb) is the primary cause of human tuberculosis (TB) and is currently the second most common cause of death due to a singleinfectious agent. The first line of defense against airborne pathogens, including Mtb, is the respiratory epithelium. One of the innate defenses used by respiratory epithelial cells to prevent microbial infection is an oxidative antimicrobial system consisting of the proteins, lactoperoxidase (LPO) and Dual oxidase 1 (Duox1), the thiocyanate anion (SCN-) and hydrogen peroxide (H2O2), which together lead to the generation of antimicrobial hypothiocyanite (OSCN-) in the airway lumen. OSCN- kills bacteria and viruses in vitro, but the role of this Duox1-based system in bacterial infections in vivo remains largely unknown. The goal of this study was to assess whether Duox1 contributes to the immune response against the unique respiratory pathogen, Mtb.

Methods

Duox1-deficient (Duox1 KO) and wild-type (WT) mice were infected with Mtb aerosols and bacterial titers, lung pathology, cytokines and immune cell recruitment were assessed.

Results and discussion

Mtb titers in the lung, spleen and liver were not different 30 days after infection between WT and Duox1 KO mice. Duox1 did not affect lung histology assessed at days 0, 30, and 90 post-Mtb infection. Mtb-infected Duox1 KO animals exhibited enhanced production of certain cytokines and chemokines in the airway; however, this response was not associated with significantly higher numbers of macrophages or neutrophils in the lung. B cell numbers were lower, while apoptosis was higher in the pulmonary lesions of Mtb-infected Duox1 KO mice compared to infected WT animals. Taken together, these data demonstrate that while Duox1 might influence leukocyte recruitment to inflammatory cell aggregates, Duox1 is dispensable for the overall clinical course of Mtb lung infection in a mouse model.

Ferrets as a model for tuberculosis transmission

Even with the COVID-19 pandemic, tuberculosis remains a leading cause of human death due to a single infectious agent. Until successfully treated, infected individuals may continue to transmit Mycobacterium tuberculosis bacilli to contacts. As with other respiratory pathogens, such as SARS-CoV-2, modeling the process of person-to-person transmission will inform efforts to develop vaccines and therapies that specifically impede disease transmission. The ferret (Mustela furo), a relatively inexpensive, small animal has been successfully employed to model transmissibility, pathogenicity, and tropism of influenza and other respiratory disease agents. Ferrets can become naturally infected with Mycobacterium bovis and are closely related to badgers, well known in Great Britain and elsewhere as a natural transmission vehicle for bovine tuberculosis. Herein, we report results of a study demonstrating that within 7 weeks of intratracheal infection with a high dose (>5 x 10³ CFU) of M. tuberculosis bacilli, ferrets develop clinical signs and pathological features similar to acute disease reported in larger animals, and ferrets infected with very-high doses (>5 x 10⁴ CFU) develop severe signs within two to four weeks, with loss of body weight as high as 30%. Natural transmission of this pathogen was also examined. Acutely-infected ferrets transmitted M. tuberculosis bacilli to co-housed naïve sentinels; most of the sentinels tested positive for M. tuberculosis in nasal washes, while several developed variable disease symptomologies similar to those reported for humans exposed to an active tuberculosis patient in a closed setting. Transmission was more efficient when the transmitting animal had a well-established acute infection. The findings support further assessment of this model system for tuberculosis transmission including the testing of prevention measures and vaccine efficacy.

Genetic Diversity of Mycobacterium tuberculosis Isolates From an Amerindian Population in Chiapas, México

This is the first report of the genetic diversity of the Mycobacterium tuberculosis complex isolates found in a Mexican-Amerindian setting. In this study, we analyzed isolates collected from the Highlands region of Chiapas, Mexico, by using spoligotyping and whole-genome sequencing analyses. Seventy-three M. tuberculosis isolates were analyzed initially by spoligotyping; no new spoligotypes were identified. Nineteen percent of the isolates were identified as SIT53 (T1) (n = 14), followed by SIT42 (14%, n = 10, LAM9) and SIT119 (11%; n = 8, X1). SIT53, SIT42, and orphan isolates (16.4%, n = 12) constituted about 50% of the isolates studied and were subjected to whole-genome sequencing (WGS) analysis. Most SIT53 (10/12) isolates belonged to the Euro-American sub-lineage 4.8. Most SIT42 isolates (4/7) as .well as most orphan isolates (5/8) belonged to the lineage 4.3.3 LAM group. By comparing the single-nucleotide polymorphism (SNP) patterns of the SIT53 isolates, we found one clone (<7 SNPs) and four clustered isolates (<15 SNPs). In isolates from the SIT42 and orphan groups, we did not find any clones or clusters. This work demonstrates the success of sub-lineage 4.8 to predominate in Mexico and confirms the dominion of sub-lineage 4.3.3 in Central and South America.

CtpB Facilitates Mycobacterium tuberculosis Growth in Copper-Limited Niches

Copper is required for aerobic respiration by Mycobacterium tuberculosis and its human host, but this essential element is toxic in abundance. Copper nutritional immunity refers to host processes that modulate levels of free copper to alternately starve and intoxicate invading microbes. Bacteria engulfed by macrophages are initially contained within copper-limited phagosomes, which fuse with ATP7A vesicles that pump in toxic levels of copper. In this report, we examine how CtpB, a P-type ATPase in M. tuberculosis, aids in response to nutritional immunity. In vitro, the induced expression of ctpB in copper-replete medium inhibited mycobacterial growth, while deletion of the gene impaired growth only in copper-starved medium and within copper-limited host cells, suggesting a role for CtpB in copper acquisition or export to the copper-dependent respiration supercomplex. Unexpectedly, the absence of ctpB resulted in hypervirulence in the DBA/2 mouse infection model. As ctpB null strains exhibit diminished growth only in copper-starved conditions, reduced copper transport may have enabled the mutant to acquire a "Goldilocks" amount of the metal during transit through copper-intoxicating environments within this model system. This work reveals CtpB as a component of the M. tuberculosis toolkit to counter host nutritional immunity and underscores the importance of elucidating copper-uptake mechanisms in pathogenic mycobacteria.

Characterization of the Proportion of Clustered Tuberculosis Cases in Guatemala: Insights from a Molecular Epidemiology Study, 2010-2014

There is little information about the amount of recent tuberculosis transmission in low-income settings. Genetic clustering can help identify ongoing transmission events. A retrospective observational study was performed on Mycobacterium tuberculosis isolates from persons living with HIV (PLHIV) and HIV-seronegative participants who submitted samples to a referral tuberculosis laboratory in Guatemala City, Guatemala from 2010 to 2014. Genotyping results were classified according to the international spoligotyping database, SITVIT2. Spoligotype patterns were categorized as clustered or nonclustered depending on their genotype. The proportion of clustering and the index of recent transmission index (RTIn-1) were estimated. In the RTIn-1 method, clustered cases represent recent transmission, whereas nonclustered cases represent reactivation of older tuberculosis infections. As a secondary aim, the potential risk factors associated with clustering in isolates from the subset of participants living with HIV were explored. From 2010 to 2014, a total of 479 study participants were confirmed as culture-positive tuberculosis cases. Among the 400 available isolates, 71 spoligotype patterns were identified. Overall, the most frequent spoligotyping families were Latin American-Mediterranean (LAM) (39%), followed by T (22%) and Haarlem (14%). Out of the 400 isolates, 365 were grouped in 36 clusters (range of cluster size: 2-92). Thus, the proportion of clustering was 91% and the RTIn-1 was 82%. Among PLHIV, pulmonary tuberculosis was associated with clustering (OR = 4.3, 95% CI 1.0-17.7). Our findings suggest high levels of ongoing transmission of M. tuberculosis in Guatemala as revealed by the high proportion of isolates falling into genomic clusters.

Use of a Ferret Model to Test Efficacy and Immunogenicity of Live Attenuated Mycobacterium avium Subspecies paratuberculosis Vaccines

Native hosts for the bacterial agent that causes Johne's disease are ruminants, which include cattle, sheep and goats among others. These large animals are often too costly to be used in testing experimental vaccines. In this chapter, we provide detailed methods to use an inexpensive and more manageable animal host, the ferret, to test efficacy and immunogenicity of live-attenuated Mycobacterium avium subspecies paratuberculosis (MAP) mutant strains prior to consideration as vaccine candidates.

A Role for Mycobacterium tuberculosis Sigma Factor C in Copper Nutritional Immunity

Sigma factor C (SigC) contributes to Mycobacterium tuberculosis virulence in various animal models, but the stress response coordinated by this transcription factor was undefined. The results presented here indicate that SigC prevents copper starvation. Whole genome expression studies demonstrate short-term (4-h) induction of sigC, controlled from a tetracycline-inducible promoter, upregulates ctpB and genes in the nonribosomal peptide synthase (nrp) operon. These genes are expressed at higher levels after 48-h sigC induction, but also elevated are genes encoding copper-responsive regulator RicR and RicR-regulated copper toxicity response operon genes rv0846-rv0850, suggesting prolonged sigC induction results in excessive copper uptake. No growth and global transcriptional differences are observed between a sigC null mutant relative to its parent strain in 7H9 medium. In a copper-deficient medium, however, growth of the sigC deletion strain lags the parent, and 40 genes (including those in the nrp operon) are differentially expressed. Copper supplementation reverses the growth defect and silences most transcriptional differences. Together, these data support SigC as a transcriptional regulator of copper acquisition when the metal is scarce. Attenuation of sigC mutants in severe combined immunodeficient mice is consistent with an inability to overcome innate host defenses that sequester copper ions to deprive invading microbes of this essential micronutrient.

Evaluation of a temperature-restricted, mucosal tuberculosis vaccine in guinea pigs

Tuberculosis (TB) is currently the leading cause of death in humans by a single infectious agent, Mycobacterium tuberculosis. The Bacillus Calmette-Guérin (BCG) vaccine prevents pulmonary TB with variable efficacy, but can cause life-threatening systemic infection in HIV-infected infants. In this study, TBvac85, a derivative of Mycobacterium shottsii expressing M. tuberculosis Antigen 85B, was examined as a safer alternative to BCG. Intranasal vaccination of guinea pigs with TBvac85, a naturally temperature-restricted species, resulted in serum Ag85B-specific IgG antibodies. Delivery of the vaccine by this route also induced protection equivalent to intradermal BCG based on organ bacterial burdens and lung pathology six weeks after aerosol challenge with M. tuberculosis strain Erdman. These results support the potential of TBvac85 as the basis of an effective TB vaccine. Next-generation derivatives expressing multiple M. tuberculosis immunogens are in development.

Effectiveness of WHO's pragmatic screening algorithm for child contacts of tuberculosis cases in resource-constrained settings: a prospective cohort study in Uganda

BACKGROUND

Tuberculosis is a leading cause of global childhood mortality; however, interventions to detect undiagnosed tuberculosis in children are underused. Child contact tracing has been widely recommended but poorly implemented in resource-constrained settings. WHO has proposed a pragmatic screening approach for managing child contacts. We assessed the effectiveness of this screening approach and alternative symptom-based algorithms in identifying secondary tuberculosis in a prospectively followed cohort of Ugandan child contacts.

METHODS

We identified index patients aged at least 18 years with microbiologically confirmed pulmonary tuberculosis at Old Mulago Hospital (Kampala, Uganda) between Oct 1, 1995, and Dec 31, 2008. Households of index patients were visited by fieldworkers within 2 weeks of diagnosis. Coprevalent and incident tuberculosis were assessed in household contacts through clinical, radiographical, and microbiological examinations for 2 years. Disease rates were compared among children younger than 16 years with and without symptoms included in the WHO pragmatic guideline (presence of haemoptysis, fever, chronic cough, weight loss, night sweats, and poor appetite). Symptoms could be of any duration, except cough (>21 days) and fever (>14 days). A modified WHO decision-tree designed to detect high-risk asymptomatic child contacts was also assessed, in which all asymptomatic contacts were classified as high risk (children younger than 3 years or immunocompromised [HIV-infected]) or low risk (aged 3 years or older and immunocompetent [HIV-negative]). We also assessed a more restrictive algorithm (ie, assessing only children with presence of chronic cough and one other tuberculosis-related symptom).

FINDINGS

Of 1718 household child contacts, 126 (7%) had coprevalent tuberculosis and 24 (1%) developed incident tuberculosis, diagnosed over the 2-year study period. Of these 150 cases of tuberculosis, 95 (63%) were microbiologically confirmed with a positive sputum culture. Using the WHO approach, 364 (21%) of 1718 child contacts had at least one tuberculosis-related symptom and 85 (23%) were identified as having coprevalent tuberculosis, 67% of all coprevalent cases detected (diagnostic odds ratio 9·8, 95% CI 6·8-14·5; p<0·0001). 1354 (79%) of 1718 child contacts had no symptoms, of whom 41 (3%) had coprevalent tuberculosis. The WHO approach was effective in contacts younger than 5 years: 70 (33%) of 211 symptomatic contacts had coprevalent disease compared with 23 (6%) of 367 asymptomatic contacts (p<0·0001). This approach was also effective in contacts aged 5 years and older: 15 (10%) of 153 symptomatic contacts had coprevalent disease compared with 18 (2%) of 987 asymptomatic contacts (p<0·0001). More coprevalent disease was detected in child contacts recommended for screening when the study population was restricted by HIV-serostatus (11 [48%] of 23 symptomatic HIV-seropositive child contacts vs two [7%] of 31 asymptomatic HIV-seropositive child contacts) or to only culture-confirmed cases (47 [13%] culture confirmed cases of 364 symptomatic child contacts vs 29 [2%] culture confirmed cases of 1354 asymptomatic child contacts). In the modified algorithm, high-risk asymptomatic child contacts were at increased risk for coprevalent disease versus low-risk asymptomatic contacts (14 [6%] of 224 vs 27 [2%] of 1130; p=0·0021). The presence of tuberculosis infection did not predict incident disease in either symptomatic or asymptomatic child contacts: in symptomatic contacts, eight (5%) of 169 infected contacts and six (5%) of 111 uninfected contacts developed incident tuberculosis (p=0·80). Among asymptomatic contacts, incident tuberculosis occurred in six (<1%) of 795 contacts infected at baseline versus four (<1%) of 518 contacts uninfected at baseline, respectively (p=1·00).

INTERPRETATION

WHO's pragmatic, symptom-based algorithm was an effective case-finding tool, especially in children younger than 5 years. A modified decision-tree identified 6% of asymptomatic child contacts at high risk for subclinical disease. Increasing the feasibility of child-contact tracing using these approaches should be encouraged to decrease tuberculosis-related paediatric mortality in high-burden settings, but this should be partnered with increasing access to microbiological point-of-care testing.

FUNDING

National Institutes of Health, Tuberculosis Research Unit, AIDS International Training and Research Program of the Fogarty International Center, and the Center for AIDS Research.

Detection of anti-HspX antibodies and HspX protein in patient sera for the identification of recent latent infection by Mycobacterium tuberculosis

Mycobacterium tuberculosis is a pathogen causing tuberculosis (TB) a spectrum of disease including acute and asymptomatic latent stages. Identifying and treating latently-infected patients constitutes one of the most important impediments to TB control efforts. Those individuals can remain undiagnosed for decades serving as potential reservoirs for disease reactivation. Tests for the accurate diagnosis of latent infection currently are unavailable. HspX protein (α-crystallin), encoded by Rv2031c gene, is produced in vitro by M. tuberculosis during stationary growth phase and hypoxic or acidic culture conditions. In this study, using standard, and Luminex xMAP® bead capture ELISA, respectively, we report on detection of anti-HspX IgG and IgM antibodies and HspX protein in sera from acute and latent TB patients. For the antibody screen, levels of IgG and IgM antibodies were similar between non-infected and active TB patients; however, individuals classified into the group with latent TB showed higher values of anti-HspX IgM (p = 0.003) compared to active TB patients. Using the bead capture antigen detection assay, HspX protein was detected in sera from 56.5% of putative latent cases (p< 0.050) compared to the background median with an average of 9,900 pg/ml and a range of 1,000 to 36,000 pg/ml. Thus, presence of anti-HspX IgM antibodies and HspX protein in sera may be markers of latent TB.

Lack of intracellular replication of M. tuberculosis and M. bovis BCG caused by delivering bacilli to lysosomes in murine brain microvascular endothelial cells

Invasion and traversal of the blood-brain barrier (BBB) by Mycobacterium tuberculosis cause meningeal tuberculosis (TB) in the central nervous system (CNS). Meningeal TB is a serious, often fatal disease that disproportionately affects young children. The mechanisms involved in CNS invasion by M. tuberculosis bacilli are poorly understood. In this study, we microscopically examined endosomal trafficking and measured survival of M. tuberculosis and M. bovis Bacille Calmette-Guérin (BCG) bacilli in murine brain microvascular endothelial cells (BMECs). The results show that both species internalize but do not replicate in BMECs in the absence of a cytotoxic response. Confocal microscopy indicates that bacilli-containing vacuoles are associated with the early endosomal marker, Rab5, late endosomal marker, Rab7, and lysosomal marker, LAMP2, suggesting that bacilli-containing endosomes mature into endolysosomes in BMECs. Our data also show that a subset of intracellular M. tuberculosis, but not BCG bacilli, escape into the cytoplasm to avoid rapid lysosomal killing. However, the intracellular mycobacteria examined cannot spread cell-to-cell in BMECs. Taken together, these data show that with the exception of the small terminal cytoplasmic population of bacilli, M. tuberculosis does not modulate intracellular trafficking in BMECs as occurs in macrophages and lung epithelial and endothelial cells.

Efficacy of parainfluenza virus 5 (PIV5)-based tuberculosis vaccines in mice

Mycobacterium tuberculosis, the etiological agent of tuberculosis (TB), is an important human pathogen. Bacillus Calmette-Guérin (BCG), a live, attenuated variant of Mycobacterium bovis, is currently the only available TB vaccine despite its low efficacy against the infectious pulmonary form of the disease in adults. Thus, a more-effective TB vaccine is needed. Parainfluenza virus 5 (PIV5), a paramyxovirus, has several characteristics that make it an attractive vaccine vector. It is safe, inexpensive to produce, and has been previously shown to be efficacious as the backbone of vaccines for influenza, rabies, and respiratory syncytial virus. In this work, recombinant PIV5 expressing M. tuberculosis antigens 85A (PIV5-85A) and 85B (PIV5-85B) have been generated and their immunogenicity and protective efficacy evaluated in a mouse aerosol infection model. In a long-term protection study, a single dose of PIV5-85A was found to be most effective in reducing M. tuberculosis colony forming units (CFU) in lungs when compared to unvaccinated, whereas the BCG vaccinated animals had similar numbers of CFUs to unvaccinated animals. BCG-prime followed by a PIV5-85A or PIV5-85B boost produced better outcomes highlighted by close to three-log units lower lung CFUs compared to PBS. The results indicate that PIV5-based M. tuberculosis vaccines are promising candidates for further development.

Rv3351c, a Mycobacterium tuberculosis gene that affects bacterial growth and alveolar epithelial cell viability

Despite the interactions known to occur between various lower respiratory tract pathogens and alveolar epithelial cells (AECs), few reports examine factors influencing the interplay between Mycobacterium tuberculosis bacilli and AECs during infection. Importantly, in vitro studies have demonstrated that the M. tuberculosis hbha and esxA gene products HBHA and ESAT6 directly or indirectly influence AEC survival. In this report, we identify Rv3351c as another M. tuberculosis gene that impacts the fate of both the pathogen and AEC host. Intracellular replication of an Rv3351c mutant in the human AEC type II pneumocyte cell line A549 was markedly reduced relative to the complemented mutant and parent strain. Deletion of Rv3351c diminished the release of lactate dehydrogenase and decreased uptake of trypan blue vital stain by host cells infected with M. tuberculosis bacilli, suggesting attenuated cytotoxic effects. Interestingly, an isogenic hbha mutant displayed reductions in AEC killing similar to those observed for the Rv3351c mutant. This opens the possibility that multiple M. tuberculosis gene products interact with AECs. We also observed that Rv3351c aids intracellular replication and survival of M. tuberculosis in macrophages. This places Rv3351c in the same standing as HBHA and ESAT6, which are important factors in AECs and macrophages. Defining the mechanism(s) by which Rv3351c functions to aid pathogen survival within the host may lead to new drug or vaccine targets.

Infection of A549 human type II epithelial cells with Mycobacterium tuberculosis induces changes in mitochondrial morphology, distribution and mass that are dependent on the early secreted antigen, ESAT-6

Pulmonary infection by Mycobacterium tuberculosis (Mtb) involves the invasion of alveolar epithelial cells (AECs). We used Mitotracker Red(®) to assess changes in mitochondrial morphology/distribution and mass from 6 to 48 h post infection (hpi) by confocal microscopy and flow cytometry in Mtb-infected A549 type II AECs. During early infection there was no effect on mitochondrial morphology, however, by 48 hpi mitochondria appeared fragmented and concentrated around the nucleus. In flow cytometry experiments, the median fluorescence intensity (MFI) decreased by 44% at 48 hpi; double-labelling using antibodies to the integral membrane protein COXIV revealed that these changes were due to a decrease in mitochondrial mass. These changes did not occur with the apathogenic strain, Mycobacterium bovis BCG. ESAT-6 is a virulence factor present in Mtb Erdman but lacking in M. bovis BCG. We performed similar experiments using Mtb Erdman, an ESAT-6 deletion mutant and its complement. MFI decreased at 48 hpi in the parent and complemented strains versus uninfected controls by 52% and 36% respectively; no decrease was detected in the deletion mutant. These results indicate an involvement of ESAT-6 in the perturbation of mitochondria induced by virulent Mtb in AECs and suggest mitophagy may play a role in the infection process.

The role of lipid raft aggregation in the infection of type II pneumocytes by Mycobacterium tuberculosis

Dynamic, cholesterol-dense regions of the plasma membrane, known as lipid rafts (LR), have been observed to develop during and may be directly involved in infection of host cells by various pathogens. This study focuses on LR aggregation induced in alveolar epithelial cells during infection with Mycobacterium tuberculosis (Mtb) bacilli. We report dose- and time-dependent increases in LR aggregation after infection with three different strains at multiplicities of infection of 1, 10 and 100 from 2-24 hr post infection (hpi). Specific strain-dependent variations were noted among H37Rv, HN878 and CDC1551 with H37Rv producing the most significant increase from 15 aggregates per cell (APC) to 27 APC at MOI 100 during the 24 hour infection period. Treatment of epithelial cells with Culture Filtrate Protein, Total Lipids and gamma-irradiated whole cells from each strain failed to induce the level of LR aggregation observed during infection with any of the live strains. However, filtered supernatants from infected epithelial cells did produce comparable LR aggregation, suggesting a secreted mycobacterial product produced during infection of host cells is responsible for LR aggregation. Disruption of lipid raft formation prior to infection indicates that Mtb bacilli utilize LR aggregates for internalization and survival in epithelial cells. Treatment of host cells with the LR-disruption agent Filipin III produced a nearly 22% reduction in viable bacteria for strains H37Rv and HN878, and a 7% reduction for strain CDC1551 after 6 hpi. This study provides evidence for significant mycobacterial-induced changes in the plasma membrane of alveolar epithelial cells and that Mtb strains vary in their ability to facilitate aggregation and utilization of LR.

Dual mechanism for Mycobacterium tuberculosis cytotoxicity on lung epithelial cells

Mycobacterium tuberculosis strains CDC1551 and Erdman were used to assess cytotoxicity in infected A549 human alveolar epithelial cell monolayers. Strain CDC1551 was found to induce qualitatively greater disruption of A549 monolayers than was strain Erdman, although total intracellular and cell-associated bacterial growth rates over the course of the infections were not significantly different. Cell-free culture supernatants from human monocytic cells infected with either of the 2 M. tuberculosis strains produced a cytotoxic effect on A549 cells, correlating with the amount of tumor necrosis factor alpha (TNF-α) released by the infected monocytes. The addition of TNF-α-neutralizing antibodies to the supernatants from infected monocyte cultures did prevent the induction of a cytotoxic effect on A549 cells overlaid with this mixture but did not prevent the death of epithelial cells when added prior to infection with M. tuberculosis bacilli. Thus, these data agree with previous observations that lung epithelial cells infected with M. tuberculosis bacilli are rapidly killed in vitro. In addition, the data indicate that some of the observed epithelial cell killing may be collateral damage; the result of TNF-α released from M. tuberculosis-infected monocytes.

Involvement of the autophagy pathway in trafficking of Mycobacterium tuberculosis bacilli through cultured human type II epithelial cells

Interactions between Mycobacterium tuberculosis bacilli and alveolar macrophages have been extensively characterized, while similar analyses in epithelial cells have not been performed. In this study, we microscopically examined endosomal trafficking of M. tuberculosis strain Erdman in A549 cells, a human type II pneumocyte cell line. Immuno-electron microscopic (IEM) analyses indicate that M. tuberculosis bacilli are internalized to a compartment labelled first with Rab5 and then with Rab7 small GTPase proteins. This suggests that, unlike macrophages, M. tuberculosis bacilli traffic to late endosomes in epithelial cells. However, fusion of lysosomes with the bacteria-containing compartment appears to be inhibited, as illustrated by IEM studies employing LAMP-2 and cathepsin-L antibodies. Examination by transmission electron microscopy and IEM revealed M. tuberculosis-containing compartments surrounded by double membranes and labelled with antibodies against the autophagy marker Lc3, providing evidence for involvement and intersection of the autophagy and endosomal pathways. Interestingly, inhibition of the autophagy pathway using 3-methyladenine improved host cell viability and decreased numbers of viable intracellular bacteria recovered after 72 h post infection. Collectively, these data suggest that trafficking patterns for M. tuberculosis bacilli in alveolar epithelial cells differ from macrophages, and that autophagy is involved this process.

The Afipia toolbox and its use to isolate flagellar mutants

Afipia felis, a Gram-negative alphaproteobacterium, has been implicated as one of the causative agents of cat scratch disease. To identify and begin to examine the virulence traits of this organism, we developed and tested a highly efficient transposon delivery system and a stable plasmid vector expressing green fluorescent protein. The transposome system is based on a Tn5-derived transposon and a phage restriction endonuclease type I inhibitor. Electroporation of this construct produced a library of >2600 mutants, which were screened for flagella biosynthesis mutants using a monoclonal antibody to Afipia flagellin. Insertion loci for two selected mutants were located in the genes for flagellin and flagellin biosynthesis FlhA, confirming the validity of the approach.

CXCL10+ T cells and NK cells assist in the recruitment and activation of CXCR3+ and CXCL11+ leukocytes during Mycobacteria-enhanced colitis

Background

The role of Mycobacteria in the etiology of Crohn's disease (CD) has been a contentious subject for many years. Recently, our laboratory showed that spontaneous colitis in IL-10^-/- mice is driven in part by antigens (Ags) conserved in Mycobacteria. The present study dissects some of the common cellular and molecular mechanism that drive Mycobacteria-mediated and spontaneous colitis in IL-10^-/- mice.

Results

We show that serum from inflammatory bowel disease (IBD) patients contain significantly higher levels of Mycobacterium avium paratuberculosis-specific IgG1 and IgG2 antibodies (Abs), serum amyloid A (SAA) as well as CXCR3 ligands than serum from healthy donors. To study the cellular mechanisms of Mycobacteria-associated colitis, pathogen-free IL-10^-/- mice were given heat-killed or live M. avium paratuberculosis. The numbers of mucosal T cells, neutrophils, NK/NKT cells that expressed TNFα, IFN-γ, and/or CXCL10 were significantly higher in mice that received live Mycobacteria than other groups. The numbers of mucosal CXCR3⁺, CXCL9⁺, CXCL11⁺ and/or IFN-γ⁺ dendritic cells (DCs) were also significantly higher in M. avium paratuberculosis-challenged mice, than compared to control mice.

Conclusion

The present study shows that CD and UC patients mount significant Mycobacteria-specific IgG1 > IgG2 and CXCR3 ligand responses. Several cellular mechanisms that drive spontaneous colitis also mediate Mycobacteria-enhanced colitis in IL-10^-/- mice. Similar to IL-10^-/- mice under conventional housing, we show that Mycobacteria-challenge IL-10^-/- mice housed under otherwise pathogen-free conditions develop colitis that is driven by CXCR3- and CXCR3 ligand-expressing leukocytes, which underscores another important hallmark and molecular mechanism of colitis. Together, the data show that Mycobacteria-dependent host responses, namely CXCL10⁺ T cells and NK cells, assist in the recruitment and activation of CXCR3⁺ and CXCL11⁺ leukocytes to enhance colitis of susceptible hosts.

Influence of Mycobacterium avium subsp. paratuberculosis on colitis development and specific immune responses during disease

The granulomatous and intramural inflammation observed in cases of inflammatory bowel diseases (IBD) and veterinary Johne's disease suggests that Mycobacterium avium subsp. paratuberculosis is a causative agent. However, an incomplete understanding of the immunological steps responsible for the pathologies of IBD makes this conclusion uncertain. Sera from interleukin-10-deficient (IL-10(-/-)) mice with spontaneous colitis displayed significantly higher M. avium subsp. paratuberculosis-specific immunoglobulin G2a antibody responses than did sera from similar mice without disease. Pathogen-free IL-10(-/-) mice received control vehicle or the vehicle containing heat-killed or live M. avium subsp. paratuberculosis. Mucosal CD4(+) T cells from the mice that developed colitis proliferated and secreted higher levels of gamma interferon and tumor necrosis factor alpha after ex vivo stimulation with a Vbeta11(+) T-cell receptor-restricted peptide from the MPT59 antigen (Ag85B) than those secreted from cells from mice before the onset of colitis. The data from this study provide important information regarding the mechanisms of colitis in IL-10(-/-) mice, which are driven in part by Ag85B-specific T cells. The data suggest a plausible mechanism of Ag-specific T-cell responses in colitis driven by potent Ags conserved in Mycobacterium species.

An in vitro model of the leukocyte interactions associated with granuloma formation in Mycobacterium tuberculosis infection

The principal defense of the human host against a Mycobacterium tuberculosis infection is the formation of granulomas, organized collections of activated macrophages, including epithelioid and multinucleated giant cells, surrounded by lymphocytes. This granuloma can sequester and contain the bacteria preventing active disease, and if the granuloma is maintained, these bacteria may remain latent for a person's lifetime. Secretion of a variety of chemoattractant cytokines following phagocytosis of the bacilli by the macrophage is critical not only to the formation of the granuloma but also to its maintenance. To investigate this process of early granuloma formation, we developed an in vitro model composed entirely of human cells. Combining blood lymphocytes and autologous macrophages from healthy purified protein derivative skin test-negative individuals and mycobacteria resulted in the formation of small, rounded aggregate structures. Microscopic examination found macrophage-specific CD68(+) epithelioid macrophages and small round CD3(+) lymphocytes that in complex resembled small granulomas seen in clinical pathology specimens. Acid-fast staining bacteria were observed between and possibly within the cells composing the granulomas. Supernatants from the infected cells collected at 24 and 48 h and 5 and 9 days after infection were analyzed by a multiplexed cytokine bead-based assay using the Luminex 100 and were found to contain interleukin (IL)-6, IL-8, interferon-gamma and tumor necrosis factor-alpha, cytokines known to be involved in human granuloma formation, in quantities from two-fold to 7000-fold higher than supernatants from uninfected control cells. In addition, chemotaxis assays demonstrated that the same supernatants attracted significantly more human peripheral blood mononuclear cells than those of uninfected cells (P<0.001). This model may provide insight into the earliest stages of granuloma formation in those newly infected.

Entry and intracellular replication of Mycobacterium tuberculosis in cultured human microvascular endothelial cells

Establishment of pulmonary Mycobacterium tuberculosis infection requires evasion of host innate defenses. In the lung alveoli, epithelial cells naturally resist uptake by the inhaled bacilli while macrophages patrol the epithelial surface and phagocytose foreign microbes. Alveolar microvascular endothelial cells, however, have not been examined as a potential point of direct interaction with the bacilli. It has been shown with other bacterial and viral lung pathogens that the lung endothelial cells are not only a point of interaction, but a source for intracellular replication and chronic infection by the pathogen. To investigate if endothelial cells are susceptible to M. tuberculosis infection, we examined attachment, internalization, and intracellular replication of M. tuberculosis bacilli in an immortalized human lung microvascular endothelial cell line (HULEC). By 6 h post-infection, 12% of infecting bacilli were associated with the HULEC monolayer cells. This was twice the association observed following a similar infection with cells from a human foreskin microvascular endothelial cell line (HMEC-1). As measured by survival after the addition of a high extracellular concentration of the aminoglycoside amikacin, approximately one-third of the associated bacilli were internalized and unavailable to the drug in both cell lines. Using electron microscopy, large numbers of bacilli were visible in the vacuoles of HULEC cells after 48 h post-infection; the presence of bacterial septa between adjacent mycobacteria suggests intracellular replication. These in vitro findings support the hypothesis that lung endothelial cells have the potential to participate in in vivo lung infections.

Examination of Mycobacterium tuberculosis sigma factor mutants using low-dose aerosol infection of guinea pigs suggests a role for SigC in pathogenesis

Secondary sigma factors in bacteria direct transcription of defence regulons in response to specific stresses. To identify which sigma factors in the human respiratory pathogen Mycobacterium tuberculosis are important for adaptive survival in vivo, defined null mutations were created in individual sigma factor genes. In this study, in vitro growth virulence and guinea pig pathology of M. tuberculosis mutants lacking functional sigma factors (SigC, SigF, or SigM) were compared to the parent strain, H37Rv. None of the mutant strains exhibited a growth deficiency in Middlebrook 7H9 broth, nor were any impaired for intracellular replication in the human monocytic macrophage cell-line THP-1. Following low-dose aerosol infection of guinea pigs, however, differences could be detected. While a SigM mutant resulted in lung and spleen granulomas of comparable composition to those found in H37Rv-infected animals, a SigF mutant was partially attenuated, exhibiting necrotic spleen granulomas and ill-defined lung granulomas. SigC mutants exhibited attenuation in the lung and spleen; notably, necrotic granulomas were absent. These data suggest that while SigF may be important for survival in the lung, SigC is likely a key regulator of pathogenesis and adaptive survival in the lung and spleen. Understanding how SigC mediates survival in the host should prove useful in the development of anti-tuberculosis therapies.

Increased pathology in lungs of mice after infection with an alpha-crystallin mutant of Mycobacterium tuberculosis: changes in cathepsin proteases and certain cytokines

Latency and reactivation are a significant problem that contributes to the incidence, transmission and pathogenesis of tuberculosis. The mechanisms involved in these processes, at the level of both the bacillus and the host, are poorly understood. In Mycobacterium tuberculosis the alpha-crystallin (acr) gene has been linked to latency, because it is highly expressed during hypoxic growth conditions. Deletion of the acr gene in M. tuberculosis H37Rv (Deltaacr strain) was previously shown to reduce the intracellular growth of bacilli in macrophages; however, its impact on pathogenesis in vivo was unknown. This study demonstrated that infection of C57BL6 mice with Deltaacr results in lung bacillary loads 1-2 log units higher in comparison to parental H37Rv. Haematoxylin/eosin staining of lungs revealed exacerbated pathology characterized by extensive obliteration of alveolar air spaces by granulomatous inflammation. RT-PCR analysis and immunostaining of lungs showed that infection with either H37Rv or Deltaacr results in the differential expression of lysosomal cathepsin proteases. A slight increase in the expression of the matrix-degrading acidic-type cathepsins B, D and H was noted in Deltaacr-infected mice and was associated with clusters of macrophages within lung granulomas. Deltaacr-infected mice also showed high serum levels of TNF-alpha, IFN-gamma and G-CSF, suggesting that Acr may play a role in modulating the host response to infection.

Characterization of the twin-arginine translocase secretion system of Mycobacterium smegmatis

The twin-arginine translocation (TAT) system secretes fully folded proteins that contain a twin-arginine motif within their signal sequence across the cytoplasmic membrane in bacteria. Using a green fluorescent protein fused with a TAT signal sequence, we demonstrated that Mycobacterium smegmatis contains a TAT system. By inactivating individual genes, we showed that three genes (tatA, tatB, and tatC) are required for a functional TAT system in M. smegmatis. The tat mutants exhibited a decreased growth rate and altered colony morphology compared to the parent strain. Comparison of the secreted proteins of the deltatatC and parent strain by two-dimensional polyacrylamide gel electrophoresis revealed an alteration in the secretion of at least five proteins, and one of the major TAT-dependent secreted proteins was identified as beta-lactamase (BlaS). The genome of M. smegmatis was analyzed with the TATFIND program, and 49 putative TAT substrates were identified, including the succinate transporter DctP. Because disruption of the TAT secretion system has a direct effect on the physiology of M. smegmatis and homologs of the TAT proteins are also present in the genome of Mycobacterium tuberculosis, the TAT secretion system or its substrates may be good candidates for drug or vaccine development.

Mycobacterium xenopi multiplies within human macrophages and enhances HIV replication in vitro

Mycobacterium xenopi can cause opportunistic infections, particularly in persons infected with human immunodeficiency virus type 1 (HIV-1). The primary focus of this effort was to determine if M. xenopi isolates could survive and grow in human peripheral blood macrophage (MPhi), and if these isolates could promote the replication of HIV-1 in vitro. M. xenopi bacilli survived and replicated 10-fold within 48 h in human MPhi while avirulent Mycobacterium smegmatis, did not grow within the MPhi. M. xenopi bacilli when cultured with peripheral blood mononuclear cells enhanced HIV-1 replication 30- and 50-fold with the macrophage-tropic HIV-1(Ba-L) and 50- and 75-fold with T-cell-tropic strain HIV-1(LAI) by 6 days post-infection when compared to M. smegmatis. The enhanced HIV replication was associated with increased production of TNF-alpha. Partial inhibition of HIV-1 induction was observed using a neutralizing anti-TNF-alpha monoclonal antibody, pentoxifylline, and matrix metalloproteinase (MMP) inhibitor I. Similar mechanisms of pathogenesis among mycobacterial species may help elucidate better treatment approaches in HIV co-infected persons.

Identification of acidic, alkaline, and neutral sphingomyelinase activities in Mycobacterium tuberculosis

BACKGROUND

Sphingomyelinase enzymes are pathogenic factors of several intracellular bacteria species, which have been little studied in Mycobacterium tuberculosis.

MATERIAL/METHODS

Cell free extracts from H37Rv and CDC-1551 M. tuberculosis strains were assayed for sphingomyelinase activity by using [N-methyl-14C]-sphingomyelin as substrate. Double-directional thin-layer chromatography was used to separate the substrate and hydrolysis product. Sphingomyelinase activity was analyzed as a function of incubation time, dose, pH and the presence of MgCl2, CaCl2, ZnSO4, HgCl2, MnCl2, CoCl2 and EDTA (1 or 10 mM).

RESULTS

Mycobacterial preparations hydrolyzed [14C]-sphingomyelin, in time- and dose-dependent manners, producing [14C]-phosphorylcholine as a unique product. The activity of H37Rv neutral sphingomyelinase at pH 7.5 was 2.15 times higher than that of CDC-1551. This activity was inhibited 21-82% by Ca2+, Hg2+ or Zn2+ and EDTA, and stimulated 40-117% by Mn2+ and Mg2+. In addition, preparations from both strains showed two peaks of sphingomyelinase, one at pH 5.5 and the other at pH 3.0. However, these activities were 4-22 times lower than that observed at pH 7.5 for strain H37Rv. Preparations from H37Rv, but not those of CDC-1551, hydrolyzed sphingomyelin at pH 8-9, with a specific activity similar to that of the neutral CDC-1551 enzyme.

CONCLUSIONS

Both strains H37Rv and CDC-1551 of M. tuberculosis have cation-dependent acidic and neutral sphingomyelinase-C enzymes, showing the neutral as the major activity. In addition, H37Rv has an alkaline sphingomyelinase-C. The importance of SMases in M. tuberculosis pathogenesis remains to be elucidated.

Demonstration of spread by Mycobacterium tuberculosis bacilli in A549 epithelial cell monolayers

We developed an in vitro tissue-culture model to analyze the process involved in mycobacterial spread through lung epithelial cell monolayers. A549 cells were infected with low numbers of viable Mycobacterium tuberculosis bacilli expressing the gfp gene. Subsequent addition of a soft agarose overlay prevented the dispersal of the bacilli from the initial points of attachment. By fluorescence microscopy the bacteria were observed to infect and grow within the primary target cells; this was followed by lysis of the infected cells and subsequent infection of adjacent cells. This process repeated itself until an area of clearing (plaque formation) was observed. The addition of amikacin after initial infection did not prevent intracellular growth; however, subsequent plaque formation was not observed. Plaque formation was also observed after infection with Mycobacterium bovis BCG bacilli, but the plaques were smaller than those formed after infection with M. tuberculosis. These observations reinforce the possibility that cell-to-cell spreading of M. tuberculosis bacilli, particularly early in the course of infection within lung macrophages, pneumocytes, and other cells, may be an important component in the infectious process.

Cloning, expression, and functional characterization of the Mycobacterium tuberculosis secA gene

To better understand the protein secretion mechanisms involved in the growth and pathogenesis of Mycobacterium tuberculosis, we examined the secA gene from M. tuberculosis (tbsecA; cosmid sequence accession No. z95121.gb_ba). We generated plasmids containing the full-length tbsecA gene or a fusion containing the 5' sequence from the M. tuberculosis secA gene and the remainder from the Escherichia coli secA gene and evaluated the ability of each construct to complement the defective SecA protein in E. coli MM52ts when grown at the non-permissive temperature. The full-length tbsecA gene was unable to compensate for the temperature-sensitive defect, whereas E. coli MM52ts that has been transformed with plasmid pMF8TB226 containing a chimeric secA gene was able to grow at 42 degrees C. This work confirms that the topography of SecA and its ATP binding sites are highly conserved, whereas its membrane insertion domains are species specific.

Mycobacterium ulcerans cytotoxicity in an adipose cell model

An adipose cell (SW872) model was developed to observe cellular necrosis and apoptosis upon Mycobacterium ulcerans infection and treatment with mycobacterial exudate. Apoptosis was likely due to secreted proteins, while necrosis was likely due to mycolactone. Our data suggest that additional factors in M. ulcerans may be involved in Buruli ulcer pathogenesis.

Increased expression of Borrelia burgdorferi vlsE in response to human endothelial cell membranes

RNA isolated from virulent Borrelia burgdorferi cells incubated with human endothelial or neurological tissue cells was subjected to subtractive hybridization using RNA from the same strain incubated in tissue culture medium alone. This RNA subtractive technique generated specific probes that hybridized to two restriction fragments (8.2 kb and 10 kb respectively) generated by EcoRI digestion of total plasmid DNA. The 10 kb EcoRI fragment localized to lp28-1 and was subsequently identified as the variable membrane protein-like sequence (vls) region, which includes an expression locus (vlsE) and 15 silent cassettes. vlsE encodes a 36 kDa outer surface protein that undergoes antigenic variation during animal infections. Primer extension analysis identified the 5' end of a transcript and a putative promoter for vlsE. Quantitative reverse transcription-polymerase chain reaction (RT-PCR) suggested that the expression of vlsE increased when virulent B. burgdorferi cells were incubated with human tissue cells or purified cell membranes isolated from those same cell lines. A 138 bp region upstream of the vlsE region that was not reported in the genome sequence was sequenced using specific 32P end-labelled primers in a DNA cycle sequencing system at high annealing temperatures. Analysis revealed that it contained a 51 bp inverted repeat, which could form an extremely stable cruciform structure. Southern blots probed with the vlsE promoter/operator region indicated that part or all of this sequence could be found on other B. burgdorferi plasmids.

Differential expression of sigE by Mycobacterium tuberculosis during intracellular growth

The Mycobacterium tuberculosis sigE gene encodes a sigma factor that is a member of the extracytoplasmic function subfamily of sigma factors. Using RT-PCR we demonstrated that sigE is expressed in M. tuberculosis bacilli during growth in human macrophages beginning after 30 min but before 6 h after infection through at least 5 days after infection, but that sigE is not expressed by M. tuberculosis bacteria during growth in Middlebrook 7H9 broth medium. However, sigE expression can be induced by treatment of broth cultures with hydrogen peroxide. Further, sigE is not expressed by M. tuberculosis bacilli during attachment or growth in type II pneumocytes. Using a green fluorescent protein (GFP) reporter gene fused to the sigE promoter, we observed induction of GFP expression following macrophage infection. Western blotting confirmed that sigE protein expression correlated with mRNA expression in induced systems. Analysis of the region of the M. tuberculosis genome encoding sigE suggested it is part of an operon consisting of sigE-orf1-htrA-orf2. The data presented in this report showed that sigE is differentially expressed by M. tuberculosis bacilli in macrophages and might play a role in the pathogenesis of this organism.

A unique Mycobacterium species isolated from an epizootic of striped bass (Morone saxatilis)

We isolated a Mycobacterium sp. resembling Mycobacterium marinum and M. ulcerans from diseased striped bass (Morone saxatilis) during an epizootic of mycobacteriosis in the Chesapeake Bay. This isolate may represent an undescribed Mycobacterium species, based on phenotypic characteristics and comparative 16S rRNA gene sequence.

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.

Changes in the virulence of Mycobacterium avium after passage through embryonated hens' eggs

Eight-day-old embryonated hen's eggs were used as a model to study Mycobacterium avium virulence. Strains isolated from human patients caused 20-90% mortality when eggs were infected by injection of bacterial suspensions into the amniotic sac. Virulence of examined strains subsequently decreased with passage through eggs to between 0 and 40% mortality in four passages. Virulence of the egg-attenuated strains could be restored by passage through human peripheral blood mononuclear cells. The site of infection in the egg was usually the mesodermal layer of the chorioallantoic membrane. A few small granulomas containing acid-fast bacteria were seen in the liver, but not in other organs. Death of chicken embryos may have resulted from destruction of the mesodermal layer of the chorioallantoic membrane with consequent respiratory failure. PBMCs infected with less virulent egg-passaged strains of M. avium produced higher levels of tumor necrosis factor-alpha than did peripheral blood mononuclear cells infected with more virulent nonpassaged strains.

Involvement of matrix metalloproteinases in human immunodeficiency virus type 1-induced replication by clinical Mycobacterium avium isolates

The role of Mycobacterium avium isolates in modulating human immunodeficiency virus type 1 (HIV-1) replication was examined by use of an in vitro, resting T cell system. Two human clinical isolates (serotypes 1 and 4) but not an environmental M. avium isolate (serotype 2) enhanced HIV-1 replication. The M. avium-induced HIV-1 replication was not associated with cell activation or differential cytokine production or utilization. Addition of matrix metalloproteinase (MMP) inhibitors and their in vivo regulators, tissue inhibitors of metalloproteinases-1 and -2, abrogated M. avium-induced HIV-1 replication 80%-95%. The MMP inhibitors did not have any effect on the HIV-1 protease activity, suggesting that they may affect cellular processes. Furthermore, MMP-9 protein was differentially expressed after infection with clinical M. avium isolates and paralleled HIV-1 p24 production. Collectively, these data suggest that M. avium-induced HIV-1 replication is mediated, in part, through the induction of MMP-9.

Observed differences in virulence-associated phenotypes between a human clinical isolate and a veterinary isolate of Mycobacterium avium

Mycobacterium avium, the most common opportunistic pathogen in patients with AIDS, is frequently isolated from a variety of environmental sources, but rarely can these environmental isolates be epidemiologically linked with isolates known to cause human disease. Using a number of in vitro tissue culture assays, we found significant pathogenic differences between a serotype 4 human clinical M. avium isolate and a serotype 2 veterinary isolate. Cell association of the patient strain with a human intestinal cell line was 1.7 times that of the veterinary strain. Growth of this clinical strain in human peripheral blood mononuclear cell-derived macrophages increased from 12-fold higher than that of the veterinary isolate after 2 days to 200-fold higher after 4 days. By the conclusion of each experiment, lysis of all examined host cell types and accumulation of cell debris were observed in infections with the human isolate, but monolayers remained relatively intact in the presence of the animal isolate. The two strains also differed in the ability to stimulate human immunodeficiency virus replication in coinfected host cells, with p24 antigen levels after 6 days threefold higher in the cells coinfected with the clinical strain than in those infected with the veterinary strain. If the genetic differences responsible for the phenotypes observed in these assays can be identified and characterized, it may be possible to determine which M. avium strains in the environment are potential human pathogens.

An in vitro tissue culture bilayer model to examine early events in Mycobacterium tuberculosis infection

A tissue culture bilayer system that mimics some aspects of early alveolar infection by Mycobacterium tuberculosis was developed. This model incorporates human lung epithelial type II pneumocyte (A549) (upper chamber) and endothelial cell (lower chamber) layers separated by a microporous membrane. This construction makes it possible to observe and quantify the passage of bacteria through the two layers, to observe the interaction of the bacteria with the various cell types, and to examine the basic mechanisms of immune cell recruitment to the site of infection. After 10(7) organisms were added to the upper chamber we microscopically observed large numbers of bacteria attached to and within the pneumocytes and we determined by viable-cell counting that a small percentage of the inoculum (0.02 to 0.43%) passed through the bilayer into the lower chamber. When peripheral blood mononuclear cells were added to the lower chamber, microscopic examination indicated a migration of the mononuclear cells through the bilayer to the apical surface, where they were seen associated with the mycobacteria on the pneumocytes. The added complexity of the bilayer system offers an opportunity to define more precisely the roles of the various lung cell types in the pathogenesis of early tuberculosis.

Emergence of a unique group of necrotizing mycobacterial diseases

Although most diseases due to pathogenic mycobacteria are caused by Mycobacterium tuberculosis, several other mycobacterial diseases-caused by M. ulcerans (Buruli ulcer), M. marinum, and M. haemophilum-have begun to emerge. We review the emergence of diseases caused by these three pathogens in the United States and around the world in the last decade. We examine the pathophysiologic similarities of the diseases (all three cause necrotizing skin lesions) and common reservoirs of infection (stagnant or slow-flowing water). Examination of the histologic and pathogenic characteristics of these mycobacteria suggests differences in the modes of transmission and pathogenesis, though no singular mechanism for either characteristic has been definitively described for any of these mycobacteria.

HeLa cells as a model to study the invasiveness and biology of Legionella pneumophila

HeLa cells were established as a model system to study the invasiveness and biology of Legionella pneumophila. In this model, invasion could be distinguished from adherence; virulent strains of L. pneumophila were adherent and invasive, whereas nonvirulent strains were adherent but poorly invasive. Invasion was rapid and did not require de novo bacterial protein synthesis, suggesting that the invasion factor is constitutively expressed by virulent strains. Entry into HeLa cells required actin polymerization and an intact microtubule cytoskeleton and was only moderately inhibited by the presence of 100 mM glucose or galactose. Intracellular replication of virulent L. pneumophila took place in ribosome-studded complex endosomes and led to the formation of free bacteria-laden vesicles presumably released from lysed HeLa cells. These free vesicles (referred to as mature vesicles) were isolated in continuous density gradients of Percoll. The bacteria contained in the isolated mature vesicles had a unique envelope structure and were highly adherent to HeLa cells, characteristics that correlated with a bright red appearance after the Giménez stain (Giménez positive). Plate-grown legionellae and replicating legionellae, harboured in complex endosomes, displayed a typical Gram-negative envelope and stained green after the Giménez stain (Giménez negative). Chronically infected cultures of HeLa cells were also established that may be a useful tool for studying long-term interactions between virulent L. pneumophila and mammalian cells. HeLa cells constitute a valuable model system that offers unique opportunities to study parasite-directed endocytosis, as well as stage specific-parasite interactions.

HeLa cells as a model to study the invasiveness and biology ofLegionella pneumophila

HeLa cells were established as a model system to study the invasiveness and biology of Legionella pneumophila. In this model, invasion could be distinguished from adherence; virulent strains of L. pneumophila were adherent and invasive, whereas nonvirulent strains were adherent but poorly invasive. Invasion was rapid and did not require de novo bacterial protein synthesis, suggesting that the invasion factor is constitutively expressed by virulent strains. Entry into HeLa cells required actin polymerization and an intact microtubule cytoskeleton and was only moderately inhibited by the presence of 100 mM glucose or galactose. Intracellular replication of virulent L. pneumophila took place in ribosome-studded complex endosomes and led to the formation of free bacteria-laden vesicles presumably released from lysed HeLa cells, These free vesicles (referred to as mature vesicles) were isolated in continuous density gradients of Percoll. The bacteria contained in the isolated mature vesicles had a unique envelope structure and were highly adherent to HeLa cells, characteristics that correlated with a bright red appearance after the Giménez stain (Giménez positive). Plate-grown legionellae and replicating legionellae, harboured in complex endosomes, displayed a typical Gram-negative envelope and stained green after the Giménez stain (Giménez negative). Chronically infected cultures of HeLa cells were also established that may be a useful tool for studying long-term interactions between virulent L. pneumophila and mammalian cells. HeLa cells constitute a valuable model system that offers unique opportunities to study parasite-directed endocytosis, as well as stage specific host-parasite interactions.Key words: Legionella pneumophila, HeLa cells, invasion mechanisms, intracellular pathogens.

In search of virulence factors of human bacterial disease

Traditional genetic techniques and a variety of animal and tissue-culture model systems have sustained the study of bacterial virulence mechanisms for several decades. However, the recent application of newly developed molecular and cellular techniques has brought our understanding of bacterial pathogenesis to new heights by permitting the identification and analysis of previously unknown constitutively and differentially expressed virulence-associated factors.