Tumor necrosis factor signaling mediates resistance to mycobacteria by inhibiting bacterial growth and macrophage death

Tuberculous granuloma induction via interaction of a bacterial secreted protein with host epithelium

Granulomas, organized aggregates of immune cells, are a hallmark of tuberculosis and have traditionally been thought to restrict mycobacterial growth. However, analysis of Mycobacterium marinum in zebrafish has shown that the early granuloma facilitates mycobacterial growth; uninfected macrophages are recruited to the granuloma where they are productively infected by M. marinum. Here, we identified the molecular mechanism by which mycobacteria induce granulomas: The bacterial secreted protein 6-kD early secreted antigenic target (ESAT-6), which has long been implicated in virulence, induced matrix metalloproteinase-9 (MMP9) in epithelial cells neighboring infected macrophages. MMP9 enhanced recruitment of macrophages, which contributed to nascent granuloma maturation and bacterial growth. Disruption of MMP9 function attenuated granuloma formation and bacterial growth. Thus, interception of epithelial MMP9 production could hold promise as a host-targeting tuberculosis therapy.

Non-mammalian animal models to study infectious disease: worms or fly fishing?

A major challenge in studying human infectious diseases is to understand in detail the molecular bases, including both pathogen and host-related factors, which contribute to disease development. Non-mammalian models have proven to be of great value for our understanding of disease and have shown conservation in fundamental virulence mechanisms for the infection of evolutionary divergent hosts. In this review we describe recent advances with three major non-mammalian models used for analysis of infectious disease in humans; the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster and the zebrafish Danio rerio.

Identification of the bioactive constituent and its mechanisms of action in mediating the anti-inflammatory effects of black cohosh and related Cimicifuga species on human primary blood macrophages

Cimicifuga species have been used as traditional medicinal herbs to treat inflammation and symptoms associated with menopause in Asia, Europe, and North America. However, the underlying mechanism of their anti-inflammatory effects remains to be investigated. With bioactivity guided purification involving the use of partitioning extraction and high performance liquid chromatography, we isolated one of the key bioactive constituents from the rhizome extracts of Cimicifuga racemosa. By NMR spectroscopy, the molecule was identified to be cimiracemate A (1). This compound (140 muM) suppressed the lipopolysaccharide-induced TNF-alpha production in the blood macrophages by 47 +/- 19% and 58 +/- 30% at LPS concentrations of 1 ng/mL and 10 ng/mL, respectively. The anti-inflammatory activity of compound 1 may be due to its modulation of a signaling mitogen activated protein kinase and transcription factor nuclear factor-kappaB activities. Compound 1 was found in other Cimicifuga species. Our data indicate that compound 1 or its chemical analogues may have the potential to be further developed as a new class of therapeutic agent.

Humoral immunity in tuberculin skin test anergy and its role in high-risk persons exposed to active tuberculosis

The most common test to identify latent tuberculosis is the tuberculin skin test that detects T cell responses of delayed type hypersensitivity type IV. Since it produces false negative reactions in active tuberculosis or in high-risk persons exposed to tuberculosis patients as shown in this report, we studied antibody profiles to explain the anergy of such responses in high-risk individuals without active infection. Our results showed that humoral immunity against tuberculin, regardless of the result of the tuberculin skin test is important for protection from active tuberculosis and that the presence of high antibody titers is a more reliable indicator of infection latency suggesting that latency can be based on the levels of antibodies together with in vitro proliferation of peripheral blood mononuclear cells in the presence of the purified protein derivative. Importantly, anti-tuberculin IgG antibody levels mediate the anergy described herein, which could also prevent reactivation of disease in high-risk individuals with high antibody titers. Such anti-tuberculin IgG antibodies were also found associated with blocking and/or stimulation of in vitro cultures of PBMC with tuberculin. In this regard, future studies need to establish if immune responses to Mycobacterium tuberculosis can generate a broad spectrum of reactions either toward Th1 responses favoring stimulation by cytokines or by antibodies and those toward diminished responses by Th2 cytokines or blocking by antibodies; possibly involving mechanisms of antibody dependent protection from Mtb by different subclasses of IgG.

IL-22 produced by human NK cells inhibits growth of Mycobacterium tuberculosis by enhancing phagolysosomal fusion

We determined whether human NK cells could contribute to immune defenses against Mycobacterium tuberculosis through production of IL-22. CD3(-)CD56(+) NK cells produced IL-22 when exposed to autologous monocytes and gamma-irradiated M. tuberculosis, and this depended on the presence of IL-15 and IL-23, but not IL-12 or IL-18. IL-15-stimulated NK cells expressed 10.6 times more DAP10 mRNA compared with control NK cells, and DAP10 siRNA inhibited IL-15-mediated IL-22 production by NK cells. Soluble factors produced by IL-15-activated NK cells inhibited growth of M. tuberculosis in macrophages, and this effect was reversed by anti-IL-22. Addition of rIL-22 to infected macrophages enhanced phagolysosomal fusion and reduced growth of M. tuberculosis. We conclude that NK cells can contribute to immune defenses against M. tuberculosis through production of IL-22, which inhibits intracellular mycobacterial growth by enhancing phagolysosomal fusion. IL-15 and DAP-10 elicit IL-22 production by NK cells in response to M. tuberculosis.

Biomarkers for tuberculosis disease status and diagnosis

PURPOSE OF REVIEW

Every year, over 8 million people develop tuberculosis and nearly 1.8 million die from it, despite extensive vaccination and drug treatment programmes. It is increasingly recognized that the diagnosis of tuberculosis, which relies heavily on century-old techniques, is one of the weakest links in the chain of tuberculosis control, hampering not just treatment but also the development of new drugs and vaccines. As a result, recent years have seen the initiation of large-scale studies aiming to identify biomarkers of Mycobacterium tuberculosis infection and disease. This review discusses initial results and future prospects for that work.

RECENT FINDINGS

The key finding from recent work has been that no one factor seems able to explain the complex course of Mycobacterium tuberculosis infection. Multifactorial analyses have identified a variety of genes and proteins, mostly involved in bacterial persistence or host responses, that offer promise as biomarkers for different disease stages.

SUMMARY

The challenge now is to validate the suggested biomarkers being described and then reduce them to clinical practice. If this can be done, it offers the possibility of greatly improved clinical management of tuberculosis, allowing segregation of patients and contacts into appropriate treatment regimens.

Transcriptome profiling and functional analyses of the zebrafish embryonic innate immune response to Salmonella infection

Due to the clear separation of innate immunity from adaptive responses, the externally developing zebrafish embryo represents a useful in vivo model for identification of innate host determinants of the response to bacterial infection. Here we performed a time-course transcriptome profiling study and gene ontology analysis of the embryonic innate immune response to infection with two model Salmonella strains that elicit either a lethal infection or an attenuated response. The transcriptional response to infection with both the lethal strain and the avirulent LPS O-Ag mutant strain showed clear conservation with host responses detected in other vertebrate models and human cells, including induction of genes encoding cell surface receptors, signaling intermediates, transcription factors, and inflammatory mediators. Furthermore, our study led to the identification of a large set of novel immune response genes and infection markers, the future functional characterization of which will support vertebrate genome annotation. From the time series and bacterial strain comparisons, matrix metalloproteinase genes, including mmp9, were among the most consistent infection-responsive genes. Purified Salmonella flagellin also strongly induced mmp9 expression. Using knockdown analysis, we showed that this gene was downstream of the zebrafish homologs of the flagellin receptor TLR5 and the adaptor MyD88. Additionally, flagellin-mediated induction of other inflammation markers, including il1b, il8, and cxcl-C1c, was reduced upon Tlr5 knockdown as well as expression of irak3, a putative negative TLR pathway regulator. Finally, we showed that induction of il1b, mmp9, and irak3 requires Myd88-dependent signaling, while ifn1 and il8 were induced Myd88 independently during Salmonella infection.

Cyclic AMP intoxication of macrophages by a Mycobacterium tuberculosis adenylate cyclase

With 8.9 million new cases and 1.7 million deaths per year, tuberculosis is a leading global killer that has not been effectively controlled. The causative agent, Mycobacterium tuberculosis, proliferates within host macrophages where it modifies both its intracellular and local tissue environment, resulting in caseous granulomas with incomplete bacterial sterilization. Although infection by various mycobacterial species produces a cyclic AMP burst within macrophages that influences cell signalling, the underlying mechanism for the cAMP burst remains unclear. Here we show that among the 17 adenylate cyclase genes present in M. tuberculosis, at least one (Rv0386) is required for virulence. Furthermore, we demonstrate that the Rv0386 adenylate cyclase facilitates delivery of bacterial-derived cAMP into the macrophage cytoplasm. Loss of Rv0386 and the intramacrophage cAMP it delivers results in reductions in TNF-alpha production via the protein kinase A and cAMP response-element-binding protein pathway, decreased immunopathology in animal tissues, and diminished bacterial survival. Direct intoxication of host cells by bacterial-derived cAMP may enable M. tuberculosis to modify both its intracellular and tissue environments to facilitate its long-term survival.

Anti-TNF immunotherapy and tuberculosis reactivation: another mechanism revealed

Anti-TNF immunotherapy has revolutionized the treatment of some inflammatory diseases, such as RA. However, a major concern is that patients receiving this therapy have an increased risk of fungal and bacterial infection, particularly of reactivating latent tuberculosis (TB). In this issue of the JCI, in an effort to understand how anti-TNF immunotherapy affects host mechanisms required to control TB, Bruns and colleagues examined the effects of the anti-TNF therapeutic infliximab on Mycobacterium tuberculosis-specific human lymphocytes (see the related article beginning on page 1167). The authors report that a granulysin-expressing CD45RA+ subset of effector memory CD8+ T cells that contributes to the killing of intracellular M. tuberculosis is depleted in vivo by infliximab in patients with RA, and that these cells are susceptible to complement-mediated lysis in the presence of infliximab in vitro. The study provides insight into host defense mechanisms that act to control TB infection and how they are affected during anti-TNF immunotherapy for autoimmune disease.

Mycobacterium marinum lipooligosaccharides are unique caryophyllose-containing cell wall glycolipids that inhibit tumor necrosis factor-alpha secretion in macrophages

Earlier studies have reported a role for lipooligosaccharides (LOSs) in sliding motility, biofilm formation, and infection of host macrophages in Mycobacterium marinum. Although a LOS biosynthetic gene cluster has recently been identified in this species, many structural features of the different LOSs (LOS-I-IV) are still unknown. This clearly hampers assessing the contribution of each LOS in mycobacterial virulence as well as structure-function-based studies of these important cell wall-associated glycolipids. In this study, we have identified an M. marinum isolate, M. marinum 7 (Mma7), which failed to produce LOS-IV but instead accumulated large amounts of LOS-III. Local genomic comparison of the LOS biosynthetic cluster established the presence of a highly disorganized region in Mma7 compared with the standard M strain, characterized by multiple genetic lesions that are likely to be responsible for the defect in LOS-IV production in Mma7. Our results indicate that the glycosyltransferase LosA alone is not sufficient to ensure LOS-IV biosynthesis. The availability of different M. marinum strains allowed us to determine the precise structure of individual LOSs through the combination of mass spectrometric and NMR techniques. In particular, we established the presence of two related 4-C-branched monosaccharides within LOS-II to IV sequences, of which one was never identified before. In addition, we provided evidence that LOSs are capable of inhibiting the secretion of tumor necrosis factor-alpha in lipopolysaccharide-stimulated human macrophages. This unexpected finding suggests that these cell wall-associated glycolipids represent key effectors capable of interfering with the establishment of a pro-inflammatory response.

Who benefits from granulomas, mycobacteria or host?

By investigating host-pathogen interactions in zebrafish using intravital imaging, Davis and Ramakrishnan (2009) provide evidence that aggregates of immune cells known as granulomas, long thought to constrain mycobacterial infection, may instead facilitate its spread.

Pseudomonas aeruginosa Type III secretion system interacts with phagocytes to modulate systemic infection of zebrafish embryos

Pseudomonas aeruginosa is an opportunistic human pathogen that can cause serious infection in those with deficient or impaired phagocytes. We have developed the optically transparent and genetically tractable zebrafish embryo as a model for systemic P. aeruginosa infection. Despite lacking adaptive immunity at this developmental stage, zebrafish embryos were highly resistant to P. aeruginosa infection, but as in humans, phagocyte depletion dramatically increased their susceptibility. The virulence of an attenuated P. aeruginosa strain lacking a functional Type III secretion system was restored upon phagocyte depletion, suggesting that this system influences virulence through its effects on phagocytes. Intravital imaging revealed bacterial interactions with multiple blood cell types. Neutrophils and macrophages rapidly phagocytosed and killed P. aeruginosa, suggesting that both cell types play a role in protection against infection. Intravascular aggregation of erythrocytes and other blood cells with resultant circulatory blockage was observed immediately upon infection, which may be relevant to the pathogenesis of thrombotic complications of human P. aeruginosa infections. The real-time visualization capabilities and genetic tractability of the zebrafish infection model should enable elucidation of molecular and cellular details of P. aeruginosa pathogenesis in conditions associated with neutropenia or impaired phagocyte function.

The role of the granuloma in expansion and dissemination of early tuberculous infection

Granulomas, organized aggregates of immune cells, form in response to persistent stimuli and are hallmarks of tuberculosis. Tuberculous granulomas have long been considered host-protective structures formed to contain infection. However, work in zebrafish infected with Mycobacterium marinum suggests that granulomas contribute to early bacterial growth. Here we use quantitative intravital microscopy to reveal distinct steps of granuloma formation and assess their consequence for infection. Intracellular mycobacteria use the ESX-1/RD1 virulence locus to induce recruitment of new macrophages to, and their rapid movement within, nascent granulomas. This motility enables multiple arriving macrophages to efficiently find and phagocytose infected macrophages undergoing apoptosis, leading to rapid, iterative expansion of infected macrophages and thereby bacterial numbers. The primary granuloma then seeds secondary granulomas via egress of infected macrophages. Our direct observations provide insight into how pathogenic mycobacteria exploit the granuloma during the innate immune phase for local expansion and systemic dissemination.

Illuminating the black box of TNF action in tuberculous granulomas

TNF clearly contributes to immunity to intracellular pathogens, but how it does so is incompletely understood. In this issue of Immunity, Clay et al. (2008) provide unique insights, using intravital microscopy and the zebrafish-embryo model of tuberculosis.