Effect of stimulation of human macrophages on intracellular survival of Mycobacterium bovis Bacillus Calmette-Guerin. Evaluation with a mycobacterial reporter strain

Survival of an epidemic MDR strain of Mycobacterium tuberculosis and its non-prosperous variant within activated macrophages

The fitness of a pathogen results from the interaction of multiple factors favoring either epidemiological success or failure. Herein, we studied the performance of the M strain, a highly successful multidrug resistant Mycobacterium tuberculosis genotype, and its non-prosperous variant, the 410 strain, in activated human monocyte-derived macrophages. Both strains showed comparable ability to induce necrotic cell death and to survive in apoptotic macrophages. Of the various macrophage activation conditions tested, none led to an enhanced control of the outbreak strain. The combination of 1,25(OH)₂ vitaminD3 and IFN-γ favored significantly the control of the non-prosperous 410 strain. These observations indicate that the ability of the M strain to survive within the hostile intracellular milieu is conserved, and the overall fitness cost paid by this genotype would be low. Our results provide additional evidence on bacterial traits that may have contributed to the epidemiological success of the M strain.

Airway delivery of interferon-γ overexpressing macrophages confers resistance to Mycobacterium avium infection in SCID mice

Mycobacterium avium (M. avium) causes significant pulmonary infection, especially in immunocompromised hosts. Alveolar macrophages (AMs) represent the first line of host defense against infection in the lung. Interferon gamma (IFN‐γ) activation of AMs enhances in vitro killing of pathogens such as M. avium. We hypothesized that airway delivery of AMs into the lungs of immunodeficient mice infected with M. avium will inhibit M. avium growth in the lung and that this macrophage function is in part IFN‐γ dependent. In this study, normal BALB/c and BALB/c SCID mice received M. avium intratracheally while on mechanical ventilation. After 30 days, M. avium numbers increased in a concentration‐dependent manner in SCID mice compared with normal BALB/c mice. Airway delivery of IFN‐γ‐activated BALB/c AMs or J774A.1 macrophages overexpressing IFN‐γ into the lungs of SCID mice resulted in a significant decrease in M. avium growth (P < 0.01, both comparisons) and limited dissemination to other organs. In addition, airway delivery of IFN‐γ activated AMs and macrophages overexpressing IFN‐γ increased the levels of IFN‐γ and TNF‐α in SCID mice. A similar protective effect against M. avium infection using J774A.1 macrophages overexpressing IFN‐γ was observed in IFN‐γ knockout mice. These data suggest that administration of IFN‐γ activated AMs or macrophages overexpressing IFN‐γ may partially restore local alveolar host defense against infections like M. avium, even in the presence of ongoing systemic immunosuppression.

Mycobacteria infect different cell types in the human lung and cause species dependent cellular changes in infected cells

Background

Mycobacterial infections remain a significant cause of morbidity and mortality worldwide. Due to limitations of the currently available model systems, there are still comparably large gaps in the knowledge about the pathogenesis of these chronic inflammatory diseases in particular with regard to the human host. Therefore, we aimed to characterize the initial phase of mycobacterial infections utilizing a human ex vivo lung tissue culture model designated STST (Short-Term Stimulation of Tissues).

Methods

Human lung tissues from 65 donors with a size of 0.5–1 cm³ were infected each with two strains of three different mycobacterial species (M. tuberculosis, M. avium, and M. abscessus), respectively. In order to preserve both morphology and nucleic acids, the HOPE® fixation technique was used. The infected tissues were analyzed using histo- and molecular-pathological methods. Immunohistochemistry was applied to identify the infected cell types.

Results

Morphologic comparisons between ex vivo incubated and non-incubated lung specimens revealed no noticeable differences. Viability of ex vivo stimulated tissues demonstrated by TUNEL-assay was acceptable. Serial sections verified sufficient diffusion of the infectious agents deep into the tissues. Infection was confirmed by Ziel Neelsen-staining and PCR to detect mycobacterial DNA. We observed the infection of different cell types, including macrophages, neutrophils, monocytes, and pneumocytes-II, which were critically dependent on the mycobacterial species used. Furthermore, different forms of nuclear alterations (karyopyknosis, karyorrhexis, karyolysis) resulting in cell death were detected in the infected cells, again with characteristic species-dependent differences.

Conclusion

We show the application of a human ex vivo tissue culture model for mycobacterial infections. The immediate primary infection of a set of different cell types and the characteristic morphologic changes observed in these infected human tissues significantly adds to the current understanding of the initial phase of human pulmonary tuberculosis. Further studies are ongoing to elucidate the molecular mechanisms involved in the early onset of mycobacterial infections in the human lung.

Disruption of the SapM locus in Mycobacterium bovis BCG improves its protective efficacy as a vaccine against M. tuberculosis

Mycobacterium bovis bacille Calmette-Guerin (BCG) provides only limited protection against pulmonary tuberculosis. We tested the hypothesis that BCG might have retained immunomodulatory properties from its pathogenic parent that limit its protective immunogenicity. Mutation of the molecules involved in immunomodulation might then improve its vaccine potential. We studied the vaccine potential of BCG mutants deficient in the secreted acid phosphatase, SapM, or in the capping of the immunomodulatory ManLAM cell wall component with α-1,2-oligomannoside. Both systemic and intratracheal challenge of mice with Mycobacterium tuberculosis following vaccination showed that the SapM mutant, compared to the parental BCG vaccine, provided better protection: it led to longer-term survival. Persistence of the SapM-mutated BCG in vivo resembled that of the parental BCG indicating that this mutation will likely not compromise the safety of the BCG vaccine. The SapM mutant BCG vaccine was more effective than the parental vaccine in inducing recruitment and activation of CD11c(+) MHC-II(int) CD40(int) dendritic cells (DCs) to the draining lymph nodes. Thus, SapM acts by inhibiting recruitment of DCs and their activation at the site of vaccination.

Optimisation of bioluminescent reporters for use with mycobacteria

Background

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

Methodology/Principal Findings

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

Conclusions/Significance

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

BCG-infected adherent mononuclear cells release cytokines that regulate group 1 CD1 molecule expression

Increasing evidence is now available showing that CD1-restricted T cell responses against non-peptide mycobacterial antigens could play a role in the immune resistance against tuberculosis. BCG, widely used in anti-tubercular vaccination, shares various constituents with Mycobacterium tuberculosis, but does not provide full protection. In the present study we have investigated the pattern of group 1 CD1 molecule expression in adherent mononuclear cells (AMNC) of human peripheral blood, infected in vitro with BCG. Shortly after exposure to BCG, both BCG-positive and BCG-negative AMNC showed a moderate CD1 expression elicited by BCG-induced release of GM-CSF presumably acting through an autocrine and a paracrine mechanism. This was demonstrated using two-color flow cytometry with green fluorescent BCG and anti-CD1 PE-labeled antibodies. However, high CD1 expression induced by exogenously added GM-CSF in AMNC was reduced if target cells were cocultivated with BCG. Monoclonal antibodies against IL-10 partially restored CD1 expression, thus showing that IL-10, released from infected AMNC, is involved, at least in part, in CD1 negative modulation. Therefore, through a complex cytokine network, including not yet identified factor(s), BCG triggers but does not allow full expression of CD1 on AMNC. It cannot be excluded that this mechanism could play a role in the limited efficiency of BCG vaccination.

Assessment of in vitro immunity to Mycobacterium tuberculosis in a human peripheral blood infection model using a luciferase reporter construct of M. tuberculosis H37Rv

Protective immune responses to tuberculosis in man are primarily cell-mediated and require the interaction of specific T cells, cytokines and activated macrophages. In the present study, Mycobacterium tuberculosis H37Rv labelled with luciferase reporter enzyme was used to analyse the anti-mycobacterial immunity in man using an in vitro whole blood infection model. Peripheral blood samples obtained from M. bovis bacille Calmette-Guérin (BCG)-vaccinated tuberculin-positive healthy volunteers (n = 23) were cultured with M. tuberculosis H37Rv reporter strain. The growth of bacteria in the whole blood cultures was monitored after 48 and 96 h of infection. The results showed that the growth of M. tuberculosis was significantly inhibited after 96 h (P < 0.029) of culture. Among the cytokines studied, interleukin (IL)-10 and IL-12 were not detected at all, whereas low levels of interferon (IFN)-gamma after 96 h (0.4 IU/ml) and tumour necrosis factor (TNF)-alpha after 48 (135 pg/ml) and 96 h (47 pg/ml) of culture were detected in the supernatants of whole blood infected with M. tuberculosis. The magnitude of bacterial growth correlated directly with the concentration of TNF-alpha detected after 48 h (r = 0.722) and 96 h (r = 0.747) of culture (P <or= 0.0001 and P <or= 0.0001, respectively). However, the addition of monoclonal antibodies specific to TNF-alpha and IFN-gamma to the blood cultures did not alter mycobacterial growth indicating the role of other mechanisms/factors in restricting the growth of M. tuberculosis in whole blood cultures.

Repetitive hypoxia increases lipid loading in human macrophages-a potentially atherogenic effect

Obstructive sleep apnea (OSA) is characterized by repetitive episodes of hypoxia and is associated with an increase in cardiovascular disease. We, therefore, investigated the effect of repetitive hypoxia on two key early events in atherogenesis; lipid loading in foam cells and monocyte adhesion to endothelial cells. Human macrophages were loaded with acetylated low-density lipoproteins. During lipid loading, the cells were exposed to 30 min cycles of 2%/21% oxygen or control (room air, 5% CO(2) incubator). Human umbilical vein endothelial cells (HUVECs) were also exposed to 30 min cycles of repetitive hypoxia or control conditions and monocyte adhesion measured. Cell adhesion molecules E-selectin, ICAM-1 and VCAM-1 were measured by ELISA. Repetitive hypoxia increased cholesteryl ester uptake by macrophages (127+/-5% compared to controls; p=0.003). By contrast, monocyte adhesion to HUVECs and cell adhesion molecule expression were unchanged by exposure to repetitive hypoxia, compared to controls (p >0.1). Repetitive hypoxia, at levels relevant to tissues such as the arterial wall, enhances lipid uptake into human macrophages. This may contribute to accelerated atherosclerosis in OSA patients.

Capacity of neutrophils and monocytes from human immunodeficiency virus-infected patients and healthy controls to inhibit growth of Mycobacterium bovis

We compared the differences in growth inhibition of Mycobacterium bovis by monocytes and neutrophils from human immunodeficiency virus (HIV)-infected persons (n = 12; mean CD4 count = 451/mm(3)) and healthy controls (n = 6). Phagocytes from all HIV-infected patients were incubated with or without exogenous granulocyte-macrophate colony-stimulating factor (GMCSF; 500-1000 U/mL). In two of the HIV-infected patients, phagocytes were incubated with or without interleukin (IL)-2 or IL-8 (500-1000 U/mL). Compared with that in HIV-infected patients, the reduction of M. bovis growth at 24 hours was 81% greater among monocytes and 69% greater among neutrophils from healthy controls (P =.03 and.04, respectively). Among HIV-infected patients, we noted greater mycobacterial reduction in monocytes (49%, P =.04) and neutrophils (42%, P =.05) from the early-stage patients (mean CD4 count = 760/mm(3)) compared with that in late-stage patients (mean CD4 count = 172/ mm(3)). Incubation with GM-CSF, IL-2, or IL-8 did not augment mycobactericidal activity. These findings suggest that the capacity of neutrophils and monocytes from HIV-infected patients to inhibit the growth of M. bovis is impaired, and this impairment is more pronounced in later stages of HIV infection.

Alpha/beta interferon impairs the ability of human macrophages to control growth of Mycobacterium bovis BCG

Administration of alpha/beta interferon (IFN-alpha/beta) to mice infected with Mycobacterium tuberculosis has been shown to increase mycobacterial growth. Because IFN-alpha/beta has direct pleiotropic effects on the differentiation and functional activities of macrophages, we evaluated the effect of IFN-alpha/beta on mycobacterial growth in human monocytes/macrophages in vitro. Monocytes cultured at optimal cell density could control the growth of M. bovis BCG, as assessed both by measurement of luciferase activity expressed by a mycobacterial reporter strain and by counting of CFU. In contrast, unrestrained mycobacterial growth was observed when monocytes were treated with alpha interferon (IFN-alpha) 3 days prior to or concomitant with infection. This striking loss of mycobacteriostatic activity was observed with IFN-alpha and IFN-beta and was induced in both freshly isolated monocytes and culture-derived macrophages. Pretreatment of monocytes with IFN-alpha modified cellular morphology and reduced viability following culture, but neither was observed for culture-derived macrophages, indicating that the effects of IFN-alpha on mycobacteriostatic activity and cell differentiation and death could be dissociated. These results are compatible with the possibility that the secretion of IFN-alpha/beta could directly promote mycobacterial growth in patients harboring these organisms.

Killing of Mycobacterium avium subspecies paratuberculosis within macrophages

BACKGROUND

Mycobacterium avium subspecies paratuberculosis (M. paratuberculosis) is a facultative intracellular pathogen that resides within host macrophages during infection of ruminant animals. We examined survival of M. paratuberculosis infections within cultured macrophages to better understand the interplay between bacterium and host.

RESULTS

Serial plating of M. paratuberculosis infected macrophage lysates on Herold's egg yolk medium showed that mycobacterial replication takes place between 0 and 24 hours post-infection. This initial growth phase was followed by a steady decline in viability over the next six days. Antibodies against M. paratuberculosis were affinity purified and used in conjunction with transmission electron microscopy to track the development of intracellular bacilli. Immunogold labeling of infected macrophages with antibody against M. paratuberculosis showed degraded intracellular mycobacteria that were unrecognizable by morphology alone. Conversely, when macrophages were heavily infected with M. paratuberculosis, no degraded forms were observed and macrophages were killed.

CONCLUSIONS

We present a general description of M. paratuberculosis survival within cultured macrophages using transmission electron microscopy and viability counts. The results of this study provides further insight surrounding M. paratuberculosis-macrophage infections and have implications in the pathogenesis of M. paratuberculosis, a pathogen known to persist inside cattle for many years.

Culture at high density improves the ability of human macrophages to control mycobacterial growth

The mechanisms through which granuloma formation helps control mycobacterial infection are poorly understood, but it is possible that the accumulation of macrophages at high density at sites of infection promotes the differentiation of macrophages into cells with improved mycobactericidal activity. To test this possibility, varying numbers of monocytes were cultured in 96-well plates for 3 days, infected with Mycobacterium bovis bacillus Calmette-Guérin, and mycobacterial number was assessed 7 days after infection based on the measurement of luciferase activity expressed by a mycobacterial reporter strain or by counting CFU. Mycobacterial growth was optimal in cultures containing 5 x 10(4) cells/well, but increasing the number of cells to 2 x 10(5) cells/well resulted in complete inhibition of mycobacterial growth. This effect could not be explained by differences in mycobacterial uptake, multiplicity of infection, acidification of the extracellular medium in high density cultures, enhanced NO production, or paracrine stimulation resulting from secretion of cytokines or other proteins. The morphology of cells cultured at high density was strikingly different from that of monocytes cultured at 5 x 10(4) cells/well, including the appearance of numerous giant cells. The bacteriostatic activity of monocyte-derived macrophages was also dependent on cell number, but fewer of these more mature cells were required to control mycobacterial growth. Thus, the ability of human macrophages to control mycobacterial infection in vitro is influenced by the density of cells present, findings that may help explain why the formation of granulomas in vivo appears to be a key event in the control of mycobacterial infections.

The stringent response of Mycobacterium tuberculosis is required for long-term survival

The stringent response utilizes hyperphosphorylated guanine [(p)ppGpp] as a signaling molecule to control bacterial gene expression involved in long-term survival under starvation conditions. In gram-negative bacteria, (p)ppGpp is produced by the activity of the related RelA and SpoT proteins. Mycobacterium tuberculosis contains a single homolog of these proteins (Rel(Mtb)) and responds to nutrient starvation by producing (p)ppGpp. A rel(Mtb) knockout strain was constructed in a virulent strain of M. tuberculosis, H37Rv, by allelic replacement. The rel(Mtb) mutant displayed a significantly slower aerobic growth rate than the wild type in synthetic liquid media, whether rich or minimal. The growth rate of the wild type was equivalent to that of the mutant when citrate or phospholipid was employed as the sole carbon source. These two organisms also showed identical growth rates within a human macrophage-like cell line. These results suggest that the in vivo carbon source does not represent a stressful condition for the bacilli, since it appears to be utilized in a similar Rel(Mtb)-independent manner. In vitro growth in liquid media represents a condition that benefits from Rel(Mtb)-mediated adaptation. Long-term survival of the rel(Mtb) mutant during in vitro starvation or nutrient run out in normal media was significantly impaired compared to that in the wild type. In addition, the mutant was significantly less able to survive extended anaerobic incubation than the wild-type virulent organism. Thus, the Rel(Mtb) protein is required for long-term survival of pathogenic mycobacteria under starvation conditions.