Growth of Mycobacterium tuberculosis in BCG-resistant and -susceptible mice: establishment of latency and reactivation

Tuberculosis vaccination needs to avoid 'decoy' immune reactions

Current search for a new effective vaccine against tuberculosis involves selected antigens, vectors and adjuvants. These are being evaluated usually by their booster inoculation following priming with Bacillus Calmette-Guerin. The purpose of this article is to point out, that despite being attenuated of virulence, priming with BCG may still involve immune mechanisms, which are not favourable for protection against active disease. It is postulated, that the responsible 'decoy' constituents selected during the evolution of pathogenic tubercle bacilli may be involved in the evasion from bactericidal host resistance and stimulate immune responses of a cytokine phenotype, which lead to the transition from latent closed granulomas to reactivation with infectious lung cavities. The decoy mechanisms appear as favourable for most infected subjects but leading in a minority of cases to pathology which can effectively transmit the infection. It is proposed that construction and development of new vaccine candidates could benefit from avoiding decoy-type immune mechanisms.

Association of MBL2 gene polymorphisms with pulmonary tuberculosis susceptibility: trial sequence meta-analysis as evidence

Background

Mannose-binding lectin (MBL) or mannose-binding protein (MBP), encoded by MBL2 gene and secreted by the liver, activates complement system through lectin pathway in innate immunity against the host’s infection. Conflictingly, a number of MBL2 variants, rs1800450 (A>B), rs1800451 (A>C), rs5030737 (A>D), rs7096206 (Y>X), rs11003125 (H>L), and rs7095891 (P>Q) allele, have been found to be associated with compromised serum levels and pulmonary tuberculosis (PTB) susceptibility. The present meta-analysis study was performed to evaluate the potential association of these MBL2 gene variants with PTB susceptibility.

Materials and methods

A quantitative synthesis was performed on PubMed (Medline), EMBASE, and Google Scholar web database searches. A meta-analysis was performed to calculate the pooled odds ratios and 95% CIs for all the genetic models.

Results

A total of 14 eligible studies were included to analyze their pooled data for associations between alleles, genotypes, and minor allele carriers. The statistical analysis revealed the significant reduced PTB risk with homozygous variant genotype of rs1800451 polymorphism (CC vs AA: P=0.043; OR =0.828, 95% CI =0.689–0.994). Contrary to this, the variant allele of rs5030737 polymorphism showed association with increased PTB risk (D vs A: P=0.026; OR =1.563, 95% CI =1.054–2.317). However, the other genetic models of rs1800450 (A>B), rs7096206 (Y>X), and rs11003125 (H>L) MBL2 gene polymorphisms did not divulge any association with PTB susceptibility.

Conclusion

The current meta-analysis concludes that rs1800451 (A>C) and rs5030737 (A>D) polymorphisms of MBL2 gene play a significant role in PTB susceptibility. Further, well-designed epidemiological studies with larger sample size including consideration of environmental factors are warranted for the future.

A Meta-analysis of MBL2 Polymorphisms and Tuberculosis Risk

MBL2 gene encodes mannose-binding lectin, is a member of innate immune system. Earlier studies revealed that MBL2 gene variants, rs1800451, rs1800450, rs5030737, rs7096206, rs11003125 and rs7095891 are associated with impaired serum level and susceptibility to TB, but their results are inconsistent. A meta-analysis was performed by including 22 studies (7095 TB-patients and 7662 controls) and data were analyzed with respect to associations between alleles, genotypes and minor allele carriers to evaluate the potential association between MBL2 polymorphisms and TB risk. Statistically significant results were found only for the homozygous variant genotype (CC vs. AA: p = 0.045; OR = 0.834, 95% CI = 0.699 to 0.996) of rs1800451 and showed reduced risk of TB in overall population. However, other genetic models of rs1800450, rs5030737, rs7096206, rs11003125, rs7095891 and combined rs1800450, rs1800451, rs5030737 polymorphisms of MBL2 gene did not reveal any association with TB risk. Stratified analysis by ethnicity showed decreased risk of TB in African population for rs1800450 and rs1800451. Whereas, no association was observed between other MBL2 polymorphisms and TB risk in all the evaluated ethnic populations. In conclusion, MBL2 rs1800450 and rs1800451 polymorphisms play a protective role in TB infection and reinforce their critical significance as a potential genetic marker for TB resistance.

Structure of the bovine natural resistance associated macrophage protein (NRAMP 1) gene and identification of a novel polymorphism

The NRAMP 1 gene is a major candidate gene influencing the outcome of infections with intracellular pathogens in numerous species. NRAMP 1 is highly conserved in many mammalian species and the NRAMP 1 gene shows considerable conservation in structure between mice and humans. The association of NRAMP 1 gene polymorphisms with disease in cattle has been limited to a single microsatellite located within the 3'-non coding region of the bovine NRAMP 1 gene. In order to facilitate further studies on this important gene, we now report the nearly complete structure of the bovine NRAMP 1 gene, including sizes and positions of 13 introns relative to the bovine NRAMP 1 gene coding sequence and the DNA sequence of intron-exon junctions. Comparison of the bovine, murine and human NRAMP 1 gene structures revealed a high degree of conservation in intron placement, though the lengths of several introns were less-well conserved. In general, the greatest divergence in intron lengths occurred in regions of the NRAMP 1 gene displaying the lowest coding sequence conservation. In addition, mutations near intron-exon junctions could account for 25 of the 75 total amino acid differences between murine and bovine NRAMP 1. Using information gained through this study, it was possible to rapidly identify a novel polymorphism within the bovine NRAMP 1 gene intron X. This polymorphism was shown by direct DNA sequence analysis to consist of insertion of three guanine nucleotides at positions 37,40 and 98 relative to the intron X start point. Initial scans of several cattle breeds suggest that the two intron X alleles identified here are stable and widespread in the Bos taurus population.

Reactivation of latent tuberculosis infection in TNF-deficient mice

TNF-deficient mice are highly susceptible to Mycobacterium tuberculosis H37Rv infection. Here we asked whether TNF is required for postinfectious immunity in aerosol-infected mice. Chemotherapy for 4 wk commencing 2 wk postinfection reduced CFU to undetectable levels. While wild-type mice had a slight rise in CFU, but controlled infection upon cessation of chemotherapy, TNF-deficient mice developed reactivation of infection with high bacterial loads in lungs, spleen, and liver, which was fatal within 13-18 wk. The increased susceptibility of TNF-deficient mice was accompanied by diminished recruitment and activation of T cells and macrophages into the lung, with defective granuloma formation and reduced inducible NO synthase expression. Reduced chemokine production in the lung might explain suboptimal recruitment and activation of T cells and uncontrolled infection. Therefore, despite a massive reduction of the mycobacterial load by chemotherapy, TNF-deficient mice were unable to compensate and mount a protective immune response. In conclusion, endogenous TNF is critical to maintain latent tuberculosis infection, and in its absence no specific immunity is generated.

Reactivation of latent tuberculosis by an inhibitor of inducible nitric oxide synthase in an aerosol murine model

Exposure to Mycobacterium tuberculosis results in clinical tuberculosis only in a small percentage of healthy individuals. In most instances the bacilli are controlled by the immune system and survive in a latent state within granuloma. Immunosuppression, however, may result in reactivation of infection, resulting in clinical disease. Using a low-dose aerosol infection (30 colony-forming units) in mice, we describe a short-duration model for studying spontaneous and drug-induced reactivation of anti-tuberculous drug-treated, latent tuberculosis infection. Although a 4-week treatment with rifampicin and isoniazid reduced the number of bacilli to undetectable levels, the infection spontaneously reactivated following therapy. By contrast, an 8-week treatment period induced a state of latent infection, requiring immunosuppression to reactivate infection. Finally, a 12-week treatment period eliminated the bacilli completely and aminoguanidine did not induce reactivation of infection. In view of the fact that therapy in the selected protocol reduces the mycobacterial load to undetectable levels, the data suggest that an 8-week treatment period is necessary and sufficient to mount protective immunity in mice.

Exposure of BALB/c mice to low doses of Mycobacterium avium increases resistance to a subsequent high-dose infection

BALB/c mice exposed intranasally (i.n.), intradermally (i.d.) or intraperitoneally (i.p.) to low doses of Mycobacterium avium (20 c.f.u. at three different times two weeks apart) showed an increased resistance to a subsequent high-dose (10(5) c.f.u.) infection. I.n.-exposed mice had few mycobacteria in the tissues (>100 c.f.u.) and showed an expansion of CD4(+) T cells associated with overproduction of IL-12 and IFN-gamma, but not IL-4 and IgG antibodies. Parenterally (i.p. and i.d.) exposed animals showed c.f.u. numbers higher than i.n.-exposed mice, together with overproduction of IL-12, IFN-gamma and IL-4 in the case of i.p.-exposed mice, and of IL-12, IFN-gamma and IgG2a and IgG1 antibodies in the case of i.d.-exposed mice. Low-dose exposures were not contained by athymic BALB/c nude mice; however, naive nude mice reconstituted with i.n.-primed CD4(+) T cells of BALB/c mice were protected against high-dose infection, indicating that CD4(+) T cells are essential to control even low-dose infections by M. avium. Overall, these data suggest that continuous i.n. exposure to M. avium doses commonly found in the environment may play a role in determining the natural resistance of normal hosts against this organism.

Immunological and pathological comparative analysis between experimental latent tuberculous infection and progressive pulmonary tuberculosis

Mycobacterium tuberculosis produces latent infection or progressive disease. Indeed, latent infection is more common since it occurs in one-third of the world's population. We showed previously, using human material with latent tuberculosis, that mycobacterial DNA can be detected by in situ PCR in a variety of cell types in histologically-normal lung. We therefore sought to establish an experimental model in which this phenomenon could be studied in detail. We report here the establishment of such a model in C57Bl/6 x DBA/2 F1 hybrid mice by the intratracheal injection of low numbers of virulent mycobacteria (4000). Latent infection was characterized by low and stable bacillary counts without death of animals. Histological and immunological study showed granulomas and small patches of alveolitis, with high expression of tumour necrosis factor alpha (TNFalpha), inducible nitiric oxide synthase (iNOS), interleukin 2 (IL-2) and interferon gamma (IFNgamma). In contrast, the intratracheal instillation of high numbers of bacteria (1 x 106) produced progressive disease. These animals started to die after 2 months of infection, with very high bacillary loads, massive pneumonia, falling expression of TNF-alpha and iNOS, and a mixed Th1/Th2 cytokine pattern. In situ PCR to detect mycobacterial DNA revealed that the most common positive cells in latently-infected mice were alveolar and interstitial macrophages located in tuberculous lesions, but, as in latently-infected human lung, positive signals were also seen in bronchial epithelium, endothelial cells and fibroblasts from histologically-normal areas. Our results suggest that latent tuberculosis is induced and maintained by a type 1 cytokine pattern plus TNFalpha, and that mycobacteria persist intracellularly in lung tissue with and without histological evidence of a local immune response.

Increased cortisol: cortisone ratio in acute pulmonary tuberculosis

To evaluate a possible role for altered cortisol metabolism in mediating the immunoparesis associated with progressive tuberculosis (TB), we have studied the hypothalamic-pituitary-adrenal axis, and the activities of the 11beta-hydroxysteroid dehydrogenases (11-HSDs) that interconvert active cortisol and inactive cortisone. In active pulmonary tuberculosis (PTB), the ratio of cortisol/cortisone metabolites in 24-h urine showed a shift towards active cortisol (ratio, 1.19 +/- 0.1, n = 16 versus 0. 89 +/- 0.05 in cured pulmonary tuberculosis (CTB), n = 13, p < 0. 01; and 0.78 +/- 0.04 healthy volunteers (HV), n = 11, p < 0.005). Conversion of cortisone (administered as 25 mg orally) to cortisol in peripheral plasma was higher in PTB (peak 1,157 +/- 55 nM, n = 14 versus 862 +/- 50 nM in CTB, n = 10, p < 0.005, and 882 +/- 73 nM in HV, n = 10; p < 0.005). Cortisol/cortisone ratio was increased in bronchoalveolar lavage fluid in PTB (7.73 +/- 1.48, mean +/- SE, n = 13) compared with HV (4.05 +/- 0.38, n = 11, p < 0.05) but was not different in plasma (PTB, 3.25 +/- 0.68; HV, 4.01 +/- 0.92). Responses of plasma cortisol to dexamethasone, CRH stimulation, and multidose ACTH stimulation were not different. These data suggest that in pulmonary tuberculosis, central control of glucocorticoid production is normal but that peripheral metabolism, in particular in the lung, is deviated in favor of the active metabolite cortisol. This offers a possible mechanism to explain the immunoparesis observed in progressive pulmonary tuberculosis.

Pulmonary immune responses during primary mycobacterium bovis- Calmette-Guerin bacillus infection in C57Bl/6 mice

Mechanisms of protective immunity to mycobacterial infection in the lung remain poorly defined. In this study, T-cell subset expansion and cytokine expression in bronchoalveolar spaces, lung parenchyma, and mediastinal lymph nodes of mice infected intratracheally with Mycobacterium bovis-Calmette-Guerin bacillus (BCG) were analyzed in parallel with histopathology and bacterial burden. M. bovis-BCG was cleared rapidly from bronchoalveolar spaces without evidence for persistence. In lung parenchyma bacteria grew during the first 4 wk followed by gradual clearance with less than 0.1% of the original inoculum persisting for more than 8 mo. Clearance of M. bovis-BCG from bronchoalveolar lavage was associated with recruitment of both neutrophils and lymphocytes. Lung CD4(+), CD8(+), and gammadelta T-cell receptor-positive T cells expanded maximally by Week 4, and declined by Week 8 to control values despite bacterial persistence. Both CD4(+) and CD8(+) lung T cells produced interferon (IFN)-gamma in response to M. bovis-BCG. Four distinct pathologic states of lung parenchymal infection were noted. Early focal sub-bronchial inflammation with transmigration of cells into airways was followed by diffuse peribronchitis, perivasculitis, and alveolitis with activated macrophages, lymphoblasts, and occasional giant cells. The latter stage corresponded to maximal M. bovis-BCG growth. Resolving infection consisted of small lymphocytes and foamy macrophages, which coincided with decreasing M. bovis-BCG colony-forming units, T-cell infiltration, and IFN-gamma expression. A final quiescent phase consisted of residual lymphoid aggregates and perivasculitis associated with persistent spontaneous IFN-gamma production. Bacterial dissemination to lymph node and spleen occurred by Week 4 and declined in parallel to lung. In contrast to lung, IFN-gamma secretion was detected only late despite early expansion of CD4(+) and CD8(+) T cells. By reverse transcriptase/polymerase chain reaction, IFN-gamma and interleukin (IL)-12 p40 messenger RNA (mRNA) in lung paralleled IFN-gamma protein production. Tumor necrosis factor-alpha, IL-4 and IL-10 mRNA expression was not increased during M. bovis-BCG lung infection. Thus, protective immunity to M. bovis-BCG in the lung evolved differently in air space, lung, and lymph node.

Granuloma formation is required to contain bacillus growth and delay mortality in mice chronically infected with Mycobacterium tuberculosis

Previous studies in this laboratory have shown that mice with a gene disruption to the intracellular adhesion molecule-1 (ICAM-K/O) express normal cell-mediated immunity but cannot mount delayed-type hypersensitivity reactions following Mycobacterium tuberculosis infection. However, even in the absence of any appreciable granuloma formation, these mice control bacterial growth for at least 90 days. While not required to control the infection initially, we hypothesized that granuloma formation was required to control chronic infection, acting by surrounding infected cells to prevent bacterial dissemination. To test this, ICAM-1 knockout mice were infected with a low dose aerosol of M. tuberculosis Erdman and were found to succumb to infection 136+/-30 days later, displaying highly elevated bacterial loads compared to wild-type mice. Lung tissue from ICAM-K/O mice displayed extensive cellular infiltration and widespread tissue necrosis, but no organized granulomatous lesions were evident, whereas the control mice displayed organized compact granulomas. These data demonstrate that while a granulomatous response is not required initially to control M. tuberculosis infection, absence of granulomas during chronic infection leads to increased bacterial growth and host death. Thus these data support the hypothesis that granuloma formation is required to control chronic infection, acting by surrounding and walling off sites of infection to prevent bacterial dissemination and maintain a state of chronic infection.

Consequence of Nramp1 deletion to Mycobacterium tuberculosis infection in mice

129sv mice functionally deleted of the antimicrobial resistance gene, Nramp1, were found to be as resistant as wild-type mice to infection with the virulent H37Rv strain of Mycobacterium tuberculosis, as determined by monitoring bacterial growth in major organs and recording host survival times. Death of infected mice of both types was associated with extensive infection-induced pathology in the lungs but not in other major organs. These findings are in keeping with the view that Nramp1 is of limited importance in resistance to tuberculosis in mice.

Reactivation of tuberculosis is associated with a shift from type 1 to type 2 cytokines

The pattern of cytokines produced by T cells from mice with latent tuberculosis and during reactivation of tuberculosis was determined. A type 1 cytokine pattern was observed in T cells isolated from the lung of mice with latent disease. Reactivation of mycobacterial growth, by activation of the hypothalamic-pituitary-adrenal (HPA) axis, resulted in a shift from a type 1 to a type 2 cytokine pattern in both CD4 and CD8 T cells. Classification of the T cells based on their differential expression of CD45 and CD44 showed that the phenotypically different populations of CD4 and CD8 cells exhibited a type 1 cytokine pattern at latency and that reactivation of latent tuberculosis was associated with a shift in cytokines produced by these populations to a type 2 cytokine response. Control of mycobacterial growth resulted in a return to the type 1 cytokine pattern found during latent disease.

How important is Nramp1 in tuberculosis?

The assumption that the antimicrobial resistance gene Nramp1 is a major determinant of resistance of mice to infection with virulent Mycobacterium tuberculosis can now be challenged on the basis of published evidence to the contrary. It is likely that, with tuberculosis, Nramp1-determined defenses are subordinate to other defenses.

Cytokine production by CD4 and CD8 T cells during the growth of Mycobacterium tuberculosis in mice

Changes in the pattern of cytokines found in CD4 and CD8 T cells during the growth of Mycobacterium tuberculosis that resulted in the establishment of a latent infection were monitored. Subsets of T cells were identified based on their differential expression of CD45 and CD44 which allowed them to be classified as naive, activated or memory. We found that the T cells in the lung produced a predominantly type 1 cytokine response. The appearance of large numbers of Th1 cells coincided with the establishment of latency. In contrast, the predominant response in the mediastinal lymph node and spleen was a Th2-type response.

Cellular reaction to Mycobacterium avium complex (MAC) clinical isolates differing in hemolytic activity and virulence for C57BL/6 mice

In this study we showed that Mycobacterium avium complex (MAC) clinical isolates differed by the expression of hemolytic activity. Two hemolytic MAC strains were less susceptible to the mycobactericidal effect of murine macrophages than two unhemolytic MAC isolates. In vivo, hemolytic MAC bacilli survived in the spleens of infected mice for a longer time than unhemolytic MAC strains. This suggested a role of hemolysins in the virulence of MAC strains. There was no difference in the cytotoxicity of T cells from mice immunized with M. bovis BCG towards macrophages infected in vitro with MAC strains expressing or not expressing hemolytic activity.

Induction of apoptosis in murine macrophages by Mycobacterium tuberculosis is reactive oxygen intermediates-independent

Infection with Mycobacterium tuberculosis induces apoptosis in murine macrophage lines. Resistant macrophages B10R (Bcgr) are more prone to undergo apoptosis than susceptible B10S (Bcgs) macrophages. Apoptosis and inhibition of intracellular growth of the mycobacteria seem to be dependent on the production of nitric oxide, since both can be reverted by aminoguanidine (AMG). Although B10R macrophages produce more superoxide anion than B10S macrophages after infection with M. tuberculosis, reactive oxygen intermediate (ROIs) scavengers did not affect uptake of 3H-uracil incorporation by the mycobacteria nor the induction of apoptosis. These results further suggest that both phenomena are dependent on the production of nitric oxide by the infected macrophages.

Phenotypic changes in T cell populations during the reactivation of tuberculosis in mice

The phenotypic changes of T lymphocytes during the reactivation of latent Mycobacterium tuberculosis infection by activation of the hypothalamic-pituitary-adrenal (HPA) axis was monitored using flow cytometric analysis. Subsets of CD4+ and CD8+ lymphocyte populations from the lung, spleen and draining lymph nodes of infected mice were identified based on their differential expression of the cell surface antigens CD44 and CD45RB. Latent infection was characterized by an accumulation of both naive, activated and memory CD4 and CD8 T lymphocytes in the lung and mediastinal lymph nodes. No changes were observed in the spleen of mice with latent infection when compared with uninfected mice. Immediately following the activation of the HPA axis, a reduction in all CD4+ and CD8+ T cells in the lung and mediastinal lymph nodes was observed. This correlated with the reactivation of mycobacterial growth. The decrease was transient for memory and naive CD4 and CD8 T lymphocyte populations in the lung. However, the number of naive CD4 and CD8 T lymphocyte populations in the mediastinal lymph node following reactivation was less than that found in mice with latent infection. These data provide the first characterization of T lymphocyte populations which may be functionally involved in the immunological response to HPA axis-induced reactivation of M. tuberculosis infection.

Identification of nitric oxide synthase as a protective locus against tuberculosis

Mutagenesis of the host immune system has helped identify response pathways necessary to combat tuberculosis. Several such pathways may function as activators of a common protective gene: inducible nitric oxide synthase (NOS2). Here we provide direct evidence for this gene controlling primary Mycobacterium tuberculosis infection using mice homozygous for a disrupted NOS2 allele. NOS2(-/-) mice proved highly susceptible, resembling wild-type littermates immunosuppressed by high-dose glucocorticoids, and allowed Mycobacterium tuberculosis to replicate faster in the lungs than reported for other gene-deficient hosts. Susceptibility appeared to be independent of the only known naturally inherited antimicrobial locus, NRAMP1. Progression of chronic tuberculosis in wild-type mice was accelerated by specifically inhibiting NOS2 via administration of N6-(1-iminoethyl)-L-lysine. Together these findings identify NOS2 as a critical host gene for tuberculostasis.

Binding of alpha-adrenergic receptors stimulates the anti-mycobacterial activity of murine peritoneal macrophages

The effects of adrenergic stimulation of the anti-mycobacterial activity of peritoneal macrophages was investigated. We found that epinephrine and norepinephrine stimulated macrophages to suppress the growth of Mycobacterium avium. Stimulation was mediated by binding to the alpha 2 adrenergic receptor. The addition of the alpha 2 agonist clonidine to cultures resulted in an inhibition of mycobacterial growth and the effect of epinephrine was blocked by the alpha-antagonist phentolamine. Treatment of the macrophages with propranolol, a beta-antagonist, potentiated the effect of epinephrine. Epinephrine mediates its effect by stimulating the expression of macrophage activation genes.

Differences in the kinetics of T cell accumulations in C3H/HeN (Bcg-resistant) and C57BL/6 (Bcg-susceptible) mice infected with Mycobacterium paratuberculosis

The accumulation of various T cell subsets in Bcg-susceptible (C57BL/6) and- resistant (C3H/HeN) strains of mice were compared following an intraperitoneal infection with Mycobacterium paratuberculosis. Groups of mice from both strains were killed at 3, 5, 10, 15, 30, and 150 days after infection and lymphocytes were harvested from the peritoneal exudate cells (PEC), spleen, intestinal epithelial lymphocytes (IEL), lamina propria lymphocytes (LPL), Peyer's patches, and mesenteric lymph node (MLN) and labelled with monoclonal antibodies to CD3, CD4, CD8, gamma delta TCR, CD25, and CD44 for flow cytometric analysis. Uninfected C3H/HeN mice had higher proportions of CD4+ cells in the spleen, MLN, LPL, IEL, and Peyer's patches, while uninfected C57BL/6 mice had higher proportions of CD8+ and/or gamma delta T cells. Significant increases in accumulation of CD8+ and gamma delta T cells were detected in the peritoneum and other tissues in both strains of mice after infection. Higher CD4/CD8 ratios were observed in most lymphoid tissues of C3H/HeN mice, while increased proportions of CD8+ and/or gamma delta T cells were present in C57BL/6 mice. These results indicate that significant differences in T cell profiles exist between these two strains of mice, both inherently and in response to infection with M. paratuberculosis. Innately lower levels of CD4+ cells and/or higher percentages of CD8+ and gamma delta T cells may play a role in the increased suspectibility of C57BL/6 mice to infection with M. paratuberculosis.