Mycobacterial Metabolic Syndrome: LprG and Rv1410 Regulate Triacylglyceride Levels, Growth Rate and Virulence in Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) mutants lacking rv1411c, which encodes the lipoprotein LprG, and rv1410c, which encodes a putative efflux pump, are dramatically attenuated for growth in mice. Here we show that loss of LprG-Rv1410 in Mtb leads to intracellular triacylglyceride (TAG) accumulation, and overexpression of the locus increases the levels of TAG in the culture medium, demonstrating a role of this locus in TAG transport. LprG binds TAG within a large hydrophobic cleft and is sufficient to transfer TAG from donor to acceptor membranes. Further, LprG-Rv1410 is critical for broadly regulating bacterial growth and metabolism in vitro during carbon restriction and in vivo during infection of mice. The growth defect in mice is due to disrupted bacterial metabolism and occurs independently of key immune regulators. The in vivo essentiality of this locus suggests that this export system and other regulators of metabolism should be considered as targets for novel therapeutics.

Of the estimated 2 billion people worldwide currently infected with Mycobacterium tuberculosis (Mtb), surprisingly few go on to develop active tuberculosis (TB) disease. The vast majority, 95 percent, of infected individuals develop latent TB, remaining infected but without disease. Despite its importance in global health, the question of what determines whether an infected individual will develop active or latent TB remains largely unanswered. Changes in how Mtb grows in response to stressors presented by the host environment likely play an important role in this process. In particular, the manifold ways in which Mtb synthesizes, degrades, and transports lipids dictates its growth in an infected host. Here, we show that lipid transport is an important function of two TB genes known to be required for Mtb’s ability to cause disease in the mouse model of infection. Using a variety of genetic and biochemical techniques, we found that the products of these genes prevent the cytosolic accumulation of a lipid associated with non-growing Mtb under the metabolic conditions it encounters during infection. Our results indicate an important role for the metabolism of Mtb in its ability to orchestrate a productive infection and cause disease.

Advances in T Helper 17 Cell Biology: Pathogenic Role and Potential Therapy in Multiple Sclerosis

The discovery of the T helper (Th) 17 lineage, involved in the protection against fungal and extracellular bacterial infections, has profoundly revolutionized our current understanding of T cell-mediated responses in autoimmune diseases, including multiple sclerosis (MS). Indeed, recent data demonstrate the pathogenic role of Th17 cells in autoimmune disorders. In particular, studies in MS and in its animal model (EAE, experimental autoimmune encephalomyelitis) have revealed a crucial role of Th17 cells in the pathogenesis of autoimmune demyelinating diseases in both mice and humans. Over the past years, several important aspects concerning Th17 cells have been elucidated, such as the factors which promote or inhibit their differentiation and the effector cytokines which mediate their responses. The identification of the features endowing Th17 cells with high pathogenicity in MS is of particular interest, and discoveries in Th17 cell biology and function could lead to the design of new strategies aimed at modulating the immune response in MS. Here, we will discuss recent advances in this field, with particular focus on the mechanisms conferring pathogenicity in MS and their potential modulation.

Versatile myeloid cell subsets contribute to tuberculosis-associated inflammation

Tuberculosis (TB), a chronic bacterial infectious disease caused by Mycobacterium tuberculosis (Mtb), typically affects the lung and causes profound morbidity and mortality rates worldwide. Recent advances in cellular immunology emphasize the complexity of myeloid cell subsets controlling TB inflammation. The specialization of myeloid cell subsets for particular immune processes has tailored their roles in protection and pathology. Among myeloid cells, dendritic cells (DCs) are essential for the induction of adaptive immunity, macrophages predominantly harbor Mtb within TB granulomas and polymorphonuclear neutrophils (PMNs) orchestrate lung damage. However, within each myeloid cell population, diverse phenotypes with unique functions are currently recognized, differentially influencing TB pneumonia and granuloma functionality. More recently, myeloid-derived suppressor cells (MDSCs) have been identified at the site of Mtb infection. Along with PMNs, MDSCs accumulate within the inflamed lung, interact with granuloma-residing cells and contribute to exuberant inflammation. In this review, we discuss the contribution of different myeloid cell subsets to inflammation in TB by highlighting their interactions with Mtb and their role in lung pathology. Uncovering the manifold nature of myeloid cells in TB pathogenesis will inform the development of future immune therapies aimed at tipping the inflammation balance to the benefit of the host.

The microbiome at the pulmonary alveolar niche and its role in Mycobacterium tuberculosis infection

Advances in next generation sequencing (NGS) technology have provided the tools to comprehensively and accurately characterize the microbial community in the respiratory tract in health and disease. The presence of commensal and pathogenic bacteria has been found to have important effects on the lung immune system. Until relatively recently, the lung has received less attention compared to other body sites in terms of microbiome characterization, and its study carries special technological difficulties related to obtaining reliable samples as compared to other body niches. Additionally, the complexity of the alveolar immune system, and its interactions with the lung microbiome, are only just beginning to be understood. Amidst this complexity sits Mycobacterium tuberculosis (Mtb), one of humanity's oldest nemeses and a significant public health concern, with millions of individuals infected with Mtb worldwide. The intricate interactions between Mtb, the lung microbiome, and the alveolar immune system are beginning to be understood, and it is increasingly apparent that improved treatment of Mtb will only come through deep understanding of the interplay between these three forces. In this review, we summarize our current understanding of the lung microbiome, alveolar immunity, and the interaction of each with Mtb.

Association between toll-like receptors 9 (TLR9) gene polymorphism and risk of pulmonary tuberculosis: meta-analysis

BACKGROUND

Previous studies indicated that the single nucleotide polymorphisms (SNPs) in TLR9 gene might be associated with Tuberculosis (TB) risk. However, the results are inconsistent and inconclusive.

METHODS

1745 articles from four databases were involved in our study. A meta-analysis on the associations between the seven polymorphisms and TB risk was carried out by comparison using different genetic models.

RESULTS

In this systematic review 8 studies from seven English articles were analyzed. Our results showed that rs352139 is significantly associated with TB risk (AA vs. AG, OR 0.77, 95% CI 0.65-0.92, P = 0.004). In the ethnic subgroup analysis, Indonesians with AA genotype had a decreased susceptibility while Mexicans with GG allele had an increased risk.

CONCLUSIONS

The meta-analysis indicated that rs352139 polymorphism might be associated with decreased TB risk in Indonesians whereas increased risk in Mexicans. Whether the observed association was due to causal effect needs to be further studied.

Limited Contribution of IL-36 versus IL-1 and TNF Pathways in Host Response to Mycobacterial Infection

IL-36 cytokines are members of the IL-1 family of cytokines that stimulate dendritic cells and T cells leading to enhanced T helper 1 responses in vitro and in vivo; however, their role in host defense has not been fully addressed thus far. The objective of this study was to examine the role of IL-36R signaling in the control of mycobacterial infection, using models of systemic attenuated M. bovis BCG infection and virulent aerogenic M. tuberculosis infection. IL-36γ expression was increased in the lung of M. bovis BCG infected mice. However, IL-36R deficient mice infected with M. bovis BCG showed similar survival and control of the infection as compared to wild-type mice, although their lung pathology and CXCL1 response were transiently different. While highly susceptible TNF-α deficient mice succumbed with overwhelming M. tuberculosis infection, and IL-1RI deficient mice showed intermediate susceptibility, IL-36R-deficient mice controlled the infection, with bacterial burden, lung inflammation and pathology, similar to wild-type controls. Therefore, IL-36R signaling has only limited influence in the control of mycobacterial infection.

Investigation of Immune Biomarkers Using Subcutaneous Model of M. tuberculosis Infection in BALB/c Mice: A Preliminary Report

Evaluation and screening of vaccines against tuberculosis depends on development of proper cost effective disease models along with identification of different immune markers that can be used as surrogate endpoints of protection in preclinical and clinical studies. The objective of the present study was therefore evaluation of subcutaneous model of M.tuberculosis infection along with investigation of different immune biomarkers of tuberculosis infection in BALB/c mice. Groups of mice were infected subcutaneously with two different doses : high (2×10⁶ CFU) and low doses (2×10² CFU) of M.tuberculosis and immune markers including humoral and cellular markers were evaluated 30 days post M.tuberculosis infections. Based on results, we found that high dose of subcutaneous infection produced chronic disease with significant (p<0.001) production of immune markers of infection like IFNγ, heat shock antigens (65, 71) and antibody titres against panel of M.tuberculosis antigens (ESAT-6, CFP-10, Ag85B, 45kDa, GroES, Hsp-16) all of which correlated with high bacterial burden in lungs and spleen. To conclude high dose of subcutaneous infection produces chronic TB infection in mice and can be used as convenient alternative to aerosol models in resource limited settings. Moreover assessment of immune markers namely mycobacterial antigens and antibodies can provide us valuable insights on modulation of immune response post infection. However further investigations along with optimization of study protocols are needed to justify the outcome of present study and establish such markers as surrogate endpoints of vaccine protection in preclinical and clinical studies in future.

Toll-like receptor 2-dependent extracellular signal-regulated kinase signaling in Mycobacterium tuberculosis-infected macrophages drives anti-inflammatory responses and inhibits Th1 polarization of responding T cells

Mycobacterium tuberculosis survives within macrophages and employs immune evasion mechanisms to persist in the host. Protective T helper type 1 (Th1) responses are induced, and the immune response in most individuals is sufficient to restrict M. tuberculosis to latent infection, but most infections are not completely resolved. As T cells and macrophages respond, a balance is established between protective Th1-associated and other proinflammatory cytokines, such as interleukin-12 (IL-12), interferon gamma (IFN-γ), and tumor necrosis factor alpha, and anti-inflammatory cytokines, such as IL-10. The mechanisms by which M. tuberculosis modulates host responses to promote its survival remain unclear. In these studies, we demonstrate that M. tuberculosis induction of IL-10, suppression of IL-12, and inhibition of class II major histocompatibility complex (MHC-II) molecules in infected macrophages are all driven by Toll-like receptor 2 (TLR2)-dependent activation of the extracellular signal-regulated kinases (ERK). Elimination of ERK signaling downstream of TLR2 by pharmacologic inhibition with U0126 or genetic deletion of Tpl2 blocks IL-10 secretion and enhances IL-12 p70 secretion. We demonstrate that M. tuberculosis regulation of these pathways in macrophages affects T cell responses to infected macrophages. Thus, genetic blockade of the ERK pathway in Tpl2(-/-) macrophages enhances Th1 polarization and IFN-γ production by antigen-specific CD4(+) T cells responding to M. tuberculosis infection. These data indicate that M. tuberculosis and its potent TLR2 ligands activate ERK signaling in macrophages to promote anti-inflammatory macrophage responses and blunt Th1 responses against the pathogen.

Interaction of Pattern Recognition Receptors with Mycobacterium Tuberculosis

Tuberculosis (TB) is considered a major worldwide health problem with 10 million new cases diagnosed each year. Our understanding of TB immunology has become greater and more refined since the identification of Mycobacterium tuberculosis (MTB) as an etiologic agent and the recognition of new signaling pathways modulating infection. Understanding the mechanisms through which the cells of the immune system recognize MTB can be an important step in designing novel therapeutic approaches, as well as improving the limited success of current vaccination strategies. A great challenge in chronic disease is to understand the complexities, mechanisms, and consequences of host interactions with pathogens. Innate immune responses along with the involvement of distinct inflammatory mediators and cells play an important role in the host defense against the MTB. Several classes of pattern recognition receptors (PRRs) are involved in the recognition of MTB including Toll-Like Receptors (TLRs), C-type lectin receptors (CLRs) and Nod-like receptors (NLRs) linked to inflammasome activation. Among the TLR family, TLR1, TLR2, TLR4, and TLR9 and their down-stream signaling proteins play critical roles in the initiation of the immune response in the pathogenesis of TB. The inflammasome pathway is associated with the coordinated release of cytokines such as IL-1β and IL-18 which also play a role in the pathogenesis of TB. Understanding the cross-talk between these signaling pathways will impact on the design of novel therapeutic strategies and in the development of vaccines and immunotherapy regimes. Abnormalities in PRR signaling pathways regulated by TB will affect disease pathogenesis and need to be elucidated. In this review we provide an update on PRR signaling during M. tuberculosis infection and indicate how greater knowledge of these pathways may lead to new therapeutic opportunities.

MyD88 signalling in myeloid cells is sufficient to prevent chronic mycobacterial infection

Tuberculosis is a chronic infectious disease caused by Mycobacterium tuberculosis that is responsible for almost 1.5 million deaths per year. Sensing of mycobacteria by the host's immune system relies on different families of receptors present on innate immune cells. Amongst them, several members of the TLR family are involved in the activation of immune cells by mycobacteria, yet the in vivo contribution of individual TLRs to the protective immune response remains controversial. On the contrary, MyD88, the adaptor molecule for most TLRs, plays a non-redundant role in the protection against tuberculosis and mice with a complete germline deletion of MyD88 succumb very early to infection. MyD88 is expressed in both immune and non-immune cells, but it is not clear whether control of mycobacteria requires ubiquitous or cell-type specific MyD88 expression. Therefore, using novel conditional switch-on mouse models, we aimed to investigate the importance of MyD88 signalling in DCs and macrophages for the induction of protective effector mechanisms against mycobacterial infection. We conclude that specific reactivation of MyD88 signalling in CD11c- or lysozyme M-expressing myeloid cells during Mycobacterium bovis Bacille Calmette-Guerin infection is sufficient to restore systemic and local inflammatory cytokine production and to control pathogen burden.

iNKT cell production of GM-CSF controls Mycobacterium tuberculosis

Invariant natural killer T (iNKT) cells are activated during infection, but how they limit microbial growth is unknown in most cases. We investigated how iNKT cells suppress intracellular Mycobacterium tuberculosis (Mtb) replication. When co-cultured with infected macrophages, iNKT cell activation, as measured by CD25 upregulation and IFNγ production, was primarily driven by IL-12 and IL-18. In contrast, iNKT cell control of Mtb growth was CD1d-dependent, and did not require IL-12, IL-18, or IFNγ. This demonstrated that conventional activation markers did not correlate with iNKT cell effector function during Mtb infection. iNKT cell control of Mtb replication was also independent of TNF and cell-mediated cytotoxicity. By dissociating cytokine-driven activation and CD1d-restricted effector function, we uncovered a novel mediator of iNKT cell antimicrobial activity: GM-CSF. iNKT cells produced GM-CSF in vitro and in vivo in a CD1d-dependent manner during Mtb infection, and GM-CSF was both necessary and sufficient to control Mtb growth. Here, we have identified GM-CSF production as a novel iNKT cell antimicrobial effector function and uncovered a potential role for GM-CSF in T cell immunity against Mtb.

Mycobacterium tuberculosis (Mtb) is the cause of tuberculosis, a leading cause of sickness and death worldwide. Although much is known about CD4⁺ and CD8⁺ T cell responses to Mtb, the role of other T cell subsets is poorly understood. Invariant natural killer T (iNKT) cells are innate lymphocytes that express a semi-invariant T cell receptor and recognize lipid antigens presented by CD1d. Although iNKT cells participate in the immune response to many different pathogens, little is known about how iNKT cells directly kill microbes. We previously showed that when co-cultured with Mtb-infected macrophages, iNKT cells inhibit intracellular Mtb replication. Now, we used this model to dissociate the signals that induce iNKT cell activation markers including IFNγ production, from the signals that activate iNKT cell antimicrobial activity. This allowed us to uncover a novel antimicrobial effector function produced by iNKT cells: GM-CSF. GM-CSF is essential for immunity to Mtb, but its role has never been defined. This study is the first report to demonstrate a protective function of GM-CSF production by any T cell subset during Mtb infection. T cell production of GM-CSF should be considered as a potential mechanism of antimicrobial immunity.

Pattern recognition receptors and cytokines in Mycobacterium tuberculosis infection--the double-edged sword?

Tuberculosis, an infectious disease caused by Mycobacterium tuberculosis (Mtb), remains a major cause of human death worldwide. Innate immunity provides host defense against Mtb. Phagocytosis, characterized by recognition of Mtb by macrophages and dendritic cells (DCs), is the first step of the innate immune defense mechanism. The recognition of Mtb is mediated by pattern recognition receptors (PRRs), expressed on innate immune cells, including toll-like receptors (TLRs), complement receptors, nucleotide oligomerization domain like receptors, dendritic cell-specific intercellular adhesion molecule grabbing nonintegrin (DC-SIGN), mannose receptors, CD14 receptors, scavenger receptors, and FCγ receptors. Interaction of mycobacterial ligands with PRRs leads macrophages and DCs to secrete selected cytokines, which in turn induce interferon-γ- (IFNγ-) dominated immunity. IFNγ and other cytokines like tumor necrosis factor-α (TNFα) regulate mycobacterial growth, granuloma formation, and initiation of the adaptive immune response to Mtb and finally provide protection to the host. However, Mtb can evade destruction by antimicrobial defense mechanisms of the innate immune system as some components of the system may promote survival of the bacteria in these cells and facilitate pathogenesis. Thus, although innate immunity components generally play a protective role against Mtb, they may also facilitate Mtb survival. The involvement of selected PRRs and cytokines on these seemingly contradictory roles is discussed.

Relative contribution of IL-1α, IL-1β and TNF to the host response to Mycobacterium tuberculosis and attenuated M. bovis BCG

TNF and IL-1 are major mediators involved in severe inflammatory diseases against which therapeutic neutralizing antibodies are developed. However, both TNF and IL-1 receptor pathways are essential for the control of Mycobacterium tuberculosis infection, and it is critical to assess the respective role of IL-1α, IL-1β, and TNF. Using gene-targeted mice we show that absence of both IL-1α and IL-1β recapitulates the uncontrolled M. tuberculosis infection with increased bacterial burden, exacerbated lung inflammation, high IFNγ, reduced IL-23 p19 and rapid death seen in IL-1R1-deficient mice. However, presence of either IL-1α or IL-1β in single-deficient mice is sufficient to control acute M. tuberculosis infection, with restrained bacterial burden and lung pathology, in conditions where TNF deficient mice succumbed within 4 weeks with overwhelming infection. Systemic infection by attenuated M. bovis BCG was controlled in the absence of functional IL-1 pathway, but not in the absence of TNF. Therefore, although both IL-1α and IL-1β are required for a full host response to virulent M. tuberculosis, the presence of either IL-1α or IL-1β allows some control of acute M. tuberculosis infection, and IL-1 pathway is dispensable for controlling M. bovis BCG acute infection. This is in sharp contrast with TNF, which is essential for host response to both attenuated and virulent mycobacteria and may have implications for anti-inflammatory therapy with IL-1β neutralizing antibodies.

Prominent role for T cell-derived tumour necrosis factor for sustained control of Mycobacterium tuberculosis infection

Tumour Necrosis Factor (TNF) is critical for host control of M. tuberculosis, but the relative contribution of TNF from innate and adaptive immune responses during tuberculosis infection is unclear. Myeloid versus T-cell-derived TNF function in tuberculosis was investigated using cell type-specific TNF deletion. Mice deficient for TNF expression in macrophages/neutrophils displayed early, transient susceptibility to M. tuberculosis but recruited activated, TNF-producing CD4⁺ and CD8⁺ T-cells and controlled chronic infection. Strikingly, deficient TNF expression in T-cells resulted in early control but susceptibility and eventual mortality during chronic infection with increased pulmonary pathology. TNF inactivation in both myeloid and T-cells rendered mice critically susceptible to infection with a phenotype resembling complete TNF deficient mice, indicating that myeloid and T-cells are the primary TNF sources collaborating for host control of tuberculosis. Thus, while TNF from myeloid cells mediates early immune function, T-cell derived TNF is essential to sustain protection during chronic tuberculosis infection.

MyD88 and TRIF synergistic interaction is required for TH1-cell polarization with a synthetic TLR4 agonist adjuvant

Glucopyranosyl lipid adjuvant-stable emulsion (GLA-SE) is a synthetic adjuvant TLR4 agonist that promotes potent poly-functional T(H)1 responses. Different TLR4 agonists may preferentially signal via MyD88 or TIR-domain-containing adapter inducing IFN-beta (TRIF) to exert adjuvant effects; however, the contribution of MyD88 and TRIF signaling to the induction of polyclonal T(H)1 responses by TLR4 agonist adjuvants has not been studied in vivo. To determine whether GLA-SE preferentially signals through MyD88 or TRIF, we evaluated the immune response against a candidate tuberculosis (TB) vaccine Ag following immunization of mice lacking either signaling adapter compared with that of wild-type mice. We find that both MyD88 and TRIF are necessary for GLA-SE to induce a poly-functional T(H)1 immune response characterized by CD4(+) T cells producing IFN-γ, TNF, and IL-2, as well as IgG2c class switching, when paired with the TB vaccine Ag ID93. Accordingly, the protective efficacy of ID93/GLA-SE immunization against aerosolized Mycobacterium tuberculosis was lost when either signaling molecule was ablated. We demonstrate that MyD88 and TRIF must be expressed in the same cell for the in vivo T(H)1-skewing adjuvant activity, indicating that these two signaling pathways cooperate on an intracellular level. Thus engagement of both the MyD88 and TRIF signaling pathways are essential for the effective adjuvant activity of this TLR4 agonist.

Cytokines in the balance of protection and pathology during mycobacterial infections

The outcome of natural infections with pathogenic mycobacteria can range from early asymptomatic clearance through latent infection to clinical disease. Different host and pathogen-specific factors have been implicated in determining the outcome of these infections; however, it is clear that the interaction of mycobacteria with the innate and acquired components of the immune system plays a central role. Specifically, the recognition of mycobacterial components by innate immune cells through different pathogen recognition receptors (PPRs) induces a cytokine response that can promote early control of the infection. In fact, in the majority of individuals that come into contact with mycobacteria, this response is enough to control the infection. Among PRRs, Toll-like receptors (TLRs), Nucleotide Oligomerization Domain (NOD)-like receptors, and C-type lectins have all been implicated in recognition of mycobacteria and in the initiation of the cytokine response. Defining the mechanisms by which distinct mycobacterial components and their receptors stimulate the immune response is an area of intense research.

The immune response in tuberculosis

There are 9 million cases of active tuberculosis reported annually; however, an estimated one-third of the world's population is infected with Mycobacterium tuberculosis and remains asymptomatic. Of these latent individuals, only 5-10% will develop active tuberculosis disease in their lifetime. CD4(+) T cells, as well as the cytokines IL-12, IFN-γ, and TNF, are critical in the control of Mycobacterium tuberculosis infection, but the host factors that determine why some individuals are protected from infection while others go on to develop disease are unclear. Genetic factors of the host and of the pathogen itself may be associated with an increased risk of patients developing active tuberculosis. This review aims to summarize what we know about the immune response in tuberculosis, in human disease, and in a range of experimental models, all of which are essential to advancing our mechanistic knowledge base of the host-pathogen interactions that influence disease outcome.

Unraveling macrophage contributions to bone repair

Macrophages have reemerged to prominence with widened understanding of their pleiotropic contributions to many biologies and pathologies. This includes clear advances in revealing their importance in wound healing. Here we have focused on the current state of knowledge with respect to bone repair, which has received relatively little scientific attention compared with its soft-tissue counterparts. Our detailed characterization of resident tissue macrophages residing in bone-lining tissues (osteomacs), including their pro-anabolic function, exposed a more prominent role for these cells in bone biology than previously anticipated. Recent studies have confirmed the importance of macrophages in early inflammatory processes that establish the healing cascade after bone fracture. Emerging data support that macrophage influence extends into both anabolic and catabolic phases of repair, suggesting that these cells have prolonged and diverse functions during fracture healing. More research is needed to clarify macrophage phase-specific contributions, temporospatial subpopulation variance and macrophage specific-molecular mediators. There is also clear motivation for determining whether macrophage alterations underlie compromised fracture healing. Overall, there is strong justification to pursue strategies targeting macrophages and/or their products for improving normal bone healing and overcoming failed repair.

Mycobacterium tuberculosis Strains Are Differentially Recognized by TLRs with an Impact on the Immune Response

Tuberculosis associates with a wide spectrum of disease outcomes. The Beijing (Bj) lineage of Mycobacterium tuberculosis (Mtb) is suggested to be more virulent than other Mtb lineages and prone to elicit non-protective immune responses. However, highly heterogeneous immune responses were reported upon infection of innate immune cells with Bj strains or stimulation with their glycolipids. Using both in vitro and in vivo mouse models of infection, we here report that the molecular mechanism for this heterogeneity may be related to distinct TLR activations. Among this Mtb lineage, we found strains that preferentially activate TLR2, and others that also activate TLR4. Recognition of Mtb strains by TLR4 resulted in a distinct cytokine profile in vitro and in vivo, with specific production of type I IFN. We also uncover a novel protective role for TLR4 activation in vivo. Thus, our findings contribute to the knowledge of the molecular basis underlying how host innate immune cells handle different Mtb strains, in particular the intricate host-pathogen interaction with strains of the Mtb Bj lineage.

Autophagy as an immune effector against tuberculosis

The now well-accepted innate immunity paradigm that autophagy acts as a cell-autonomous defense against intracellular bacteria has its key origins in studies with Mycobacterium tuberculosis, an important human pathogen and a model microorganism infecting macrophages. A number of different factors have been identified that play into the anti-mycobacterial functions of autophagy, and recent in vivo studies in the mouse model of tuberculosis have uncovered additional anti-inflammatory and tissue-sparing functions of autophagy. Complementing these observations, genome wide association studies indicate a considerable overlap between autophagy, human susceptibility to mycobacterial infections and predisposition loci for inflammatory bowel disease. Finally, recent studies show that autophagy is an important regulator and effector of IL-1 responses, and that autophagy intersects with type I interferon pathology-modulating responses.

Toll like receptor 9 (TLR9) polymorphism G520R in sheep is associated with seropositivity for Small Ruminant Lentivirus

Infectious diseases of sheep are of major economic importance causing direct and indirect losses. Among the major sheep infectious agents are Small Ruminant Lentivirus, Chlamydophila abortus and Mycobacterium avium subsp. paratuberculosis infections, mainly due to their worldwide distribution and economic impact that they cause. Based on the differential susceptibility to infectious diseases between and within breeds and on the recent findings regarding the putative involvement of TLR9 in disease susceptibility, the aim of this study was to evaluate the levels of nucleotide variation of TLR9 and its mediator MyD88 in three sheep flocks originated from different breeds and assess their possible association with seropositivity/seronegativity for different infectious agents. The analysis indicated that the change of G to R at codon 520 of TLR9 polypeptide shows a significant association with Small Ruminant Lentivirus seropositivity. This amino-acid substitution, which can result in polarity change, might influence structure and function of LRR17, interfering with ligand binding and thus could be used in studies investigating susceptibility/resistance to Small Ruminant Lentivirus infections in sheep.

Immune vulnerability of infants to tuberculosis

One of the challenges faced by the infant immune system is learning to distinguish the myriad of foreign but nonthreatening antigens encountered from those expressed by true pathogens. This balance is reflected in the diminished production of proinflammatory cytokines by both innate and adaptive immune cells in the infant. A downside of this bias is that several factors critical for controlling Mycobacterium tuberculosis infection are significantly restricted in infants, including TNF, IL-1, and IL-12. Furthermore, infant T cells are inherently less capable of differentiating into IFN- γ -producing T cells. As a result, infected infants are 5-10 times more likely than adults to develop active tuberculosis (TB) and have higher rates of severe disseminated disease, including miliary TB and meningitis. Infant TB is a fundamentally different disease than TB in immune competent adults. Immunotherapeutics, therefore, should be specifically evaluated in infants before they are routinely employed to treat TB in this age group. Modalities aimed at reducing inflammation, which may be beneficial for adjunctive therapy of some forms of TB in older children and adults, may be of no benefit or even harmful in infants who manifest much less inflammatory disease.

Differentially imprinted innate immunity by mucosal boost vaccination determines antituberculosis immune protective outcomes, independent of T-cell immunity

Homologous and heterologous parenteral prime-mucosal boost immunizations have shown great promise in combating mucosal infections such as tuberculosis and AIDS. However, their immune mechanisms remain poorly defined. In particular, it is still unclear whether T-cell and innate immunity may be independently affected by these immunization modalities and how it impacts immune protective outcome. Using two virus-based tuberculosis vaccines (adenovirus (Ad) and vesicular stomatitis virus (VSV) vectors), we found that while both homologous (Ad/Ad) and heterologous (Ad/VSV) respiratory mucosal boost immunizations elicited similar T-cell responses in the lung, they led to drastically different immune protective outcomes. Compared with Ad-based boosting, VSV-based boosting resulted in poorly enhanced protection against tuberculosis. Such inferior protection was associated with differentially imprinted innate phagocytes, particularly the CD11c(+)CD11b(+/-) cells, in the lung. We identified heightened type 1 interferon (IFN) responses to be the triggering mechanism. Thus, increased IFN-β severely blunted interleukin-12 responses in infected phagocytes, which in turn impaired their nitric oxide production and antimycobacterial activities. Our study reveals that vaccine vectors may differentially imprint innate cells at the mucosal site of immunization, which can impact immune-protective outcome, independent of T-cell immunity, and it is of importance to determine both T-cell and innate cell immunity in vaccine studies.

Heterozygosity at the A625C polymorphic site of the MyD88 gene is associated with Mycobacterium bovis infection in cattle

The study demonstrates that in cattle, animals heterozygous at the MyD88 A625C polymorphic marker have a 5-fold reduced risk for active pulmonary tuberculosis (odds ratio [OR] = 0.19; P = 6 × 10(-12)). The reduced risk, however, does not extend to animals with latent pulmonary tuberculosis (OR = 0.83; P = 0.40). Heterozygosity at the A625C single nucleotide polymorphism is associated with intermediate levels of tumor necrosis factor alpha, gamma interferon, and nitric oxide synthase (NOS). Accordingly, deficiency as well as overexpression of proinflammatory cytokines or NOS favor tuberculosis, while heterozygosity provides the animals with the optimal level of inflammation.

Elimination of intracellularly residing Mycobacterium tuberculosis through targeting of host and bacterial signaling mechanisms

With more than 2 billion latently infected people, TB continues to represent a serious threat to human health. According to the WHO, 1.1 million people died from TB in 2010, which is equal to approximately 3000 deaths per day. The causative agent of the disease, Mycobacterium tuberculosis, is a highly successful pathogen having evolved remarkable strategies to persist within the host. Although normally, upon phagocytosis by macrophages, bacteria are readily eliminated by lysosomes, pathogenic mycobacteria actively prevent destruction within macrophages. The strategies that pathogenic mycobacteria apply range from releasing virulence factors to manipulating host molecules resulting in the modulation of host signal transduction pathways in order to sustain their viability within the infected host. Here, we analyze the current status of how a better understanding of both the bacterial and host factors involved in virulence can be used to develop drugs that may be helpful to curb the TB epidemic.

Experimental and natural infections in MyD88- and IRAK-4-deficient mice and humans

Most Toll-like-receptors (TLRs) and interleukin-1 receptors (IL-1Rs) signal via myeloid differentiation primary response 88 (MyD88) and interleukin-1 receptor-associated kinase 4 (IRAK-4). The combined roles of these two receptor families in the course of experimental infections have been assessed in MyD88- and IRAK-4-deficient mice for almost fifteen years. These animals have been shown to be susceptible to 46 pathogens: 27 bacteria, eight viruses, seven parasites, and four fungi. Humans with inborn MyD88 or IRAK-4 deficiency were first identified in 2003. They suffer from naturally occurring life-threatening infections caused by a small number of bacterial species, although the incidence and severity of these infections decrease with age. Mouse TLR- and IL-1R-dependent immunity mediated by MyD88 and IRAK-4 seems to be vital to combat a wide array of experimentally administered pathogens at most ages. By contrast, human TLR- and IL-1R-dependent immunity mediated by MyD88 and IRAK-4 seems to be effective in the natural setting against only a few bacteria and is most important in infancy and early childhood. The roles of TLRs and IL-1Rs in protective immunity deduced from studies in mutant mice subjected to experimental infections should therefore be reconsidered in the light of findings for natural infections in humans carrying mutations as discussed in this review.

Targeting macrophage activation for the prevention and treatment of Staphylococcus aureus biofilm infections

Biofilm infections often lead to significant morbidity due to their chronicity and recalcitrance to antibiotics. We have demonstrated that methicillin-resistant Staphylococcus aureus (MRSA) biofilms can evade macrophage (MΦ) antibacterial effector mechanisms by skewing MΦs toward an alternatively activated M2 phenotype. To overcome this immune evasion, we have used two complementary approaches. In the first, a proinflammatory milieu was elicited by local administration of classically activated M1 MΦs and in the second by treatment with the C5a receptor (CD88) agonist EP67, which invokes MΦ proinflammatory activity. Early administration of M1-activated MΦs or EP67 significantly attenuated biofilm formation in a mouse model of MRSA catheter-associated infection. Several proinflammatory mediators were significantly elevated in biofilm-infected tissues from MΦ- and EP67-treated animals, revealing effective reprogramming of the biofilm environment to a proinflammatory milieu. A requirement for MΦ proinflammatory activity was demonstrated by the fact that transfer of MyD88-deficient MΦs had minimal impact on biofilm growth. Likewise, neutrophil administration had no effect on biofilm formation. Treatment of established biofilm infections with M1-activated MΦs also significantly reduced catheter-associated biofilm burdens compared with antibiotic treatment. Collectively, these results demonstrate that targeting MΦ proinflammatory activity can overcome the local immune inhibitory environment created during biofilm infections and represents a novel therapeutic strategy.

Mycobacterium tuberculosis controls microRNA-99b (miR-99b) expression in infected murine dendritic cells to modulate host immunity

Mycobacterium tuberculosis resides and replicates within host phagocytes by modulating host microbicidal responses. In addition, it suppresses the production of host protective cytokines to prevent activation of and antigen presentation by M. tuberculosis-infected cells, causing dysregulation of host protective adaptive immune responses. Many cytokines are regulated by microRNAs (miRNAs), a newly discovered class of small noncoding RNAs, which have been implicated in modulating host immune responses in many bacterial and viral diseases. Here, we show that miRNA-99b (miR-99b), an orphan miRNA, plays a key role in the pathogenesis of M. tuberculosis infection. We found that miR-99b expression was highly up-regulated in M. tuberculosis strain H37Rv-infected dendritic cells (DCs) and macrophages. Blockade of miR-99b expression by antagomirs resulted in significantly reduced bacterial growth in DCs. Interestingly, knockdown of miR-99b in DCs significantly up-regulated proinflammatory cytokines such as IL-6, IL-12, and IL-1β. Furthermore, mRNA and membrane-bound protein data indicated that inhibition of miR-99b augments TNF-α and TNFRSF-4 production. Thus, miR-99b targets TNF-α and TNFRSF-4 receptor genes. Treatment of anti-miR-99b-transfected DCs with anti-TNF-α antibody resulted in increased bacterial burden. Thus, our findings unveil a novel host evasion mechanism adopted by M. tuberculosis via miR-99b, which may open up new avenues for designing miRNA-based vaccines and therapies.

Human TOLLIP regulates TLR2 and TLR4 signaling and its polymorphisms are associated with susceptibility to tuberculosis

Tuberculosis, one of the leading causes of death worldwide, stimulates inflammatory responses with beneficial and pathologic consequences. The regulation and nature of an optimal inflammatory response to Mycobacterium tuberculosis remains poorly understood in humans. Insight into mechanisms of negative regulation of the TLR-mediated innate immune response to M. tuberculosis could provide significant breakthroughs in the design of new vaccines and drugs. We hypothesized that TOLLIP and its common variants negatively regulate TLR signaling in human monocytes and are associated with susceptibility to tuberculosis. Using short hairpin RNA knockdown of TOLLIP in peripheral blood human monocytes, we found that TOLLIP suppresses TNF and IL-6 production after stimulation with TLR2 and TLR4 ligands. In contrast, secretion of the anti-inflammatory cytokine IL-10 was induced by TOLLIP. We also discovered two common polymorphisms that are associated with either decreased levels of mRNA expression (rs3750920) or increased IL-6 production (rs5743899) in a sample of 56 healthy volunteers. Furthermore, in a case-population study in Vietnam with 760 cord blood samples and 671 TB case patients, we found that SNPs rs3750920 and rs5743899 were associated with susceptibility to tuberculosis (p = 7.03 × 10(-16) and 6.97 × 10(-7), respectively). These data demonstrate that TOLLIP has an anti-inflammatory effect on TLR signaling in humans and that TOLLIP deficiency is associated with an increased risk of tuberculosis. To our knowledge, these data also show the first associations of TOLLIP polymorphisms with any infectious disease. These data also implicate an unexpected mechanism of negative regulation of TLR signaling in human tuberculosis pathogenesis.

Cellular and humoral mechanisms involved in the control of tuberculosis

Mycobacterium tuberculosis (Mtb) infection is a major international public health problem. One-third of the world's population is thought to have latent tuberculosis, a condition where individuals are infected by the intracellular bacteria without active disease but are at risk for reactivation, if their immune system fails. Here, we discuss the role of nonspecific inflammatory responses mediated by cytokines and chemokines induced by interaction of innate receptors expressed in macrophages and dendritic cells (DCs). We also review current information regarding the importance of several cytokines including IL-17/IL-23 in the development of protective cellular and antibody-mediated protective responses against Mtb and their influence in containment of the infection. Finally, in this paper, emphasis is placed on the mechanisms of failure of Mtb control, including the immune dysregulation induced by the treatment with biological drugs in different autoimmune diseases. Further functional studies, focused on the mechanisms involved in the early host-Mtb interactions and the interplay between host innate and acquired immunity against Mtb, may be helpful to improve the understanding of protective responses in the lung and in the development of novel therapeutic and prophylactic tools in TB.

Revisiting the role of the granuloma in tuberculosis

The granuloma, which is a compact aggregate of immune cells, is the hallmark structure of tuberculosis. It is historically regarded as a host-protective structure that 'walls off' the infecting mycobacteria. This Review discusses surprising new discoveries--from imaging studies coupled with genetic manipulations--that implicate the innate immune mechanisms of the tuberculous granuloma in the expansion and dissemination of infection. It also covers why the granuloma can fail to eradicate infection even after adaptive immunity develops. An understanding of the mechanisms and impact of tuberculous granuloma formation can guide the development of therapies to modulate granuloma formation. Such therapies might be effective for tuberculosis as well as for other granulomatous diseases.

The role of airway epithelial cells in response to mycobacteria infection

Airway epithelial cells (AECs) are part of the frontline defense against infection of pathogens by providing both a physical barrier and immunological function. The role of AECs in the innate and adaptive immune responses, through the production of antimicrobial molecules and proinflammatory factors against a variety of pathogens, has been well established. Tuberculosis (TB), a contagious disease primarily affecting the lungs, is caused by the infection of various strains of mycobacteria. In response to mycobacteria infection, epithelial expression of Toll-like receptors and surfactant proteins plays the most prominent roles in the recognition and binding of the pathogen, as well as the initiation of the immune response. Moreover, the antimicrobial substances, proinflammatory factors secreted by AECs, composed a major part of the innate immune response and mediation of adaptive immunity against the pathogen. Thus, a better understanding of the role and mechanism of AECs in response to mycobacteria will provide insight into the relationship of epithelial cells and lung immunocytes against TB, which may facilitate our understanding of the pathogenesis and immunological mechanism of pulmonary tuberculosis disease.

Regulation of Mycobacterium tuberculosis-dependent HIV-1 transcription reveals a new role for NFAT5 in the toll-like receptor pathway

Tuberculosis (TB) disease in HIV co-infected patients contributes to increased mortality by activating innate and adaptive immune signaling cascades that stimulate HIV-1 replication, leading to an increase in viral load. Here, we demonstrate that silencing of the expression of the transcription factor nuclear factor of activated T cells 5 (NFAT5) by RNA interference (RNAi) inhibits Mycobacterium tuberculosis (MTb)-stimulated HIV-1 replication in co-infected macrophages. We show that NFAT5 gene and protein expression are strongly induced by MTb, which is a Toll-like receptor (TLR) ligand, and that an intact NFAT5 binding site in the viral promoter of R5-tropic HIV-1 subtype B and subtype C molecular clones is required for efficent induction of HIV-1 replication by MTb. Furthermore, silencing by RNAi of key components of the TLR pathway in human monocytes, including the downstream signaling molecules MyD88, IRAK1, and TRAF6, significantly inhibits MTb-induced NFAT5 gene expression. Thus, the innate immune response to MTb infection induces NFAT5 gene and protein expression, and NFAT5 plays a crucial role in MTb regulation of HIV-1 replication via a direct interaction with the viral promoter. These findings also demonstrate a general role for NFAT5 in TLR- and MTb-mediated control of gene expression.

The major cause of AIDS deaths globally has been tuberculosis (TB), which is caused by the bacterium Mycobacterium tuberculosis (MTb). Co-infection with MTb exacerbates human immunodeficiency virus type1 (HIV-1) replication and disease progression via both innate and adaptive host immune responses to MTb infection. In this report, we present evidence that the transcription factor NFAT5 plays a crucial role in MTb-induced HIV-1 replication in human peripheral blood cells and monocytes. We also show that MTb infection itself stimulates NFAT5 gene expression in human monocytes and that its expression involves the TLR signalling pathway and requires the downstream adaptor proteins MyD88, IRAK1, and TRAF6. This identification of a novel role for NFAT5 in TB/HIV-1 co-infection reveals that NFAT5 is a major mediator of TLR-dependent gene expression and thus provides a potential new therapeutic target for treatment of HIV-1 and possibly other diseases.

New Aspects in Immunopathology of Mycobacterium tuberculosis

Our understanding of tuberculosis (TB) pathology and immunology has become extensively deeper and more refined since the identification of Mycobacterium tuberculosis (MTB) as the etiologic agent of disease by Dr. Robert Koch in 1882. A great challenge in chronic disease is to understand the complexities, mechanisms, and consequences of host interactions with pathogens. TB, caused by MTB, is a major health problem in world, with 10 million new cases diagnosed each year. Innate immunity is shown playing an important role in the host defense against the MTB, and the first step in this process is recognition of MTB by cells of the innate immune system. Several classes of pattern recognition receptors (PPRs) are involved in the recognition of MTB, including toll-like receptors (TLRs), C-type lectin receptors (CLRs), and nod-like receptors (NLRs). Among the TLR family, TLR1, TLR2, TLR4, and TLR9 and their down streams, proteins play the most prominent roles in the initiation of the immune response against MTB. Beside of TLRs signaling, recently the activation of inflammasome pathway in the pathogenesis of TB much appreciated. Knowledge about these signaling pathways is crucial for understanding the pathophysiology of TB, on one hand, and for the development of novel strategies of vaccination and treatment such as immunotherapy on the other. Given the critical role of TLRs/inflammasome signaling in innate immunity and initiation of the appropriate adaptive response, the regulation of these pathways is likely to be an important determinant of the clinical outcome of MTB infection. In this review paper we focused on the immune response, which is the recognition of MTB by inflammatory innate immune cells following infection.

Mycobacterium tuberculosis Rv0577, a novel TLR2 agonist, induces maturation of dendritic cells and drives Th1 immune response

Tuberculosis (TB) caused by Mycobacterium tuberculosis constitutes an ongoing threat to global health. An antigen that can induce dendritic cell (DC) maturation and lead to enhanced cellular immunity is crucial to the development of an effective TB vaccine. Here, we investigated the functional roles and the related signaling mechanism of the Rv0577 protein, a M. tuberculosis complex-restricted secreted protein involved in the methylglyoxal detoxification pathway. Rv0577 recognizes Toll-like receptor 2 (TLR2) and functionally induces DC maturation by augmenting the expression of cell surface molecules (CD80, CD86, and MHC class I and II) and proinflammatory cytokine production (TNF-α, IL-1β, IL-6, and IL-12p70) in DCs on MyD88-dependent signaling, mitogen-activated protein kinases, and nuclear factor κB signaling pathways. In addition, Rv0577-treated DCs activated naive T cells, effectively polarized CD4(+) and CD8(+) T cells to secrete IFN-γ and IL-2, and induced T-cell proliferation, indicating that this protein possibly contributes to Th1-polarization of the immune response. More important, unlike LPS, Rv0577-treated DCs specifically induced the proliferation of memory CD4(+)/CD8(+)CD44(high)CD62L(low) T cells in the spleen of M. tuberculosis-infected mice in a TLR2-dependent manner. Taken together, these findings suggest that Rv0577 may regulate innate and adaptive immunity by interacting with TLR2, a finding that could be helpful in the design of new TB vaccines.

The pathology of Mycobacterium tuberculosis infection

Mycobacterium tuberculosis is an old enemy of the human race, with evidence of infection observed as early as 5000 years ago. Although more host-restricted than Mycobacterium bovis, which can infect all warm-blooded vertebrates, M. tuberculosis can infect, and cause morbidity and mortality in, several veterinary species as well. As M. tuberculosis is one of the earliest described bacterial pathogens, the literature describing this organism is vast and overwhelming. This review strives to distill what is currently known about this bacterium and the disease it causes for the veterinary pathologist.

Evaluation of Toll-like receptor and adaptor molecule polymorphisms for susceptibility to tuberculosis in a Colombian population

Immunological studies have supported the idea that innate immunity is critical for the control of Mycobacterium tuberculosis (Mtb) infection in humans. Despite the overwhelming evidence showing the critical role of Toll-like receptors (TLRs) in the in vitro recognition of Mtb, the in vivo significance of individual TLRs has been more difficult to demonstrate consistently. We were interested in examining the role of genes of TLRs and molecules involved in their signalling cascades, and a case-control study was designed to test the association of polymorphisms of these innate immune genes with pulmonary tuberculosis (TB) in a Colombian population. In this study, we did not find an association with TLR2, TLR4, TLR9, MyD88 or MAL/TIRAP polymorphic variants. These findings suggest that those genes are not involved as risk factors for pulmonary TB in our population.

The Mycobacterium tuberculosis SecA2 system subverts phagosome maturation to promote growth in macrophages

The ability of Mycobacterium tuberculosis to grow in macrophages is critical to the virulence of this important pathogen. One way M. tuberculosis is thought to maintain a hospitable niche in macrophages is by arresting the normal process of phagosomes maturing into acidified phagolysosomes. The process of phagosome maturation arrest by M. tuberculosis is not fully understood, and there has remained a need to firmly establish a requirement for phagosome maturation arrest for M. tuberculosis growth in macrophages. Other intracellular pathogens that control the phagosomal environment use specialized protein export systems to deliver effectors of phagosome trafficking to the host cell. In M. tuberculosis, the accessory SecA2 system is a specialized protein export system that is required for intracellular growth in macrophages. In studying the importance of the SecA2 system in macrophages, we discovered that SecA2 is required for phagosome maturation arrest. Shortly after infection, phagosomes containing a ΔsecA2 mutant of M. tuberculosis were more acidified and showed greater association with markers of late endosomes than phagosomes containing wild-type M. tuberculosis. We further showed that inhibitors of phagosome acidification rescued the intracellular growth defect of the ΔsecA2 mutant, which demonstrated that the phagosome maturation arrest defect of the ΔsecA2 mutant is responsible for the intracellular growth defect. This study demonstrates the importance of phagosome maturation arrest for M. tuberculosis growth in macrophages, and it suggests there are effectors of phagosome maturation that are exported into the host environment by the accessory SecA2 system.

Toll-like receptors 2 and 4 regulate the frequency of IFNγ-producing CD4+ T-cells during pulmonary infection with Chlamydia pneumoniae

TLR2 and TLR4 are crucial for recognition of Chlamydia pneumoniae in vivo, since infected TLR2/4 double-deficient mice are unable to control the infection as evidenced by severe loss of body weight and progressive lethal pneumonia. Unexpectedly, these mice display higher pulmonary levels of the protective cytokine IFNγ than wild type mice. We show here, that antigen-specific CD4⁺ T-cells are responsible for the observed IFNγ-secretion in vivo and their frequency is higher in TLR2/4 double-deficient than in wild type mice. The capacity of TLR2/4 double-deficient dendritic cells to re-stimulate CD4⁺ T-cells did not differ from wild type dendritic cells. However, the frequency of CD4⁺CD25⁺Foxp3⁺ T-cells was considerably higher in wild type compared to TLR2/4 double-deficient mice and was inversely related to the number of IFNγ-secreting CD4⁺ effector T-cells. Despite increased IFNγ-levels, at least one IFNγ-mediated response, protective NO-secretion, could not be induced in the absence of TLR2 and 4. In summary, CD4⁺CD25⁺Foxp3⁺ regulatory T-cells fail to expand in the absence of TLR2 and TLR4 during pulmonary infection with C. pneumoniae, which in turn enhances the frequency of CD4⁺IFNγ⁺ effector T-cells. Failure of IFNγ to induce NO in TLR2/4 double-deficient cells represents one possible mechanism why TLR2/4 double-deficient mice are unable to control pneumonia caused by C. pneumoniae and succumb to the infection.

Mycobacterium tuberculosis Hip1 dampens macrophage proinflammatory responses by limiting toll-like receptor 2 activation

Mycobacterium tuberculosis is a highly successful human pathogen that evades host innate immunity by interfering with macrophage functions. In addition to avoiding macrophage microbicidal activities, M. tuberculosis triggers secretion of proinflammatory cytokines and chemokines in macrophages. The levels of proinflammatory cytokines induced by clinical M. tuberculosis isolates are thought to play an important role in determining tuberculosis disease progression and severity, but the mechanisms by which M. tuberculosis modulates the magnitude of inflammatory responses remain unclear. Here we show that M. tuberculosis restricts robust macrophage activation and dampens proinflammatory responses through the cell envelope-associated serine hydrolase Hip1 (hydrolase important for pathogenesis 1). By transcriptionally profiling macrophages infected with either wild-type or hip1 mutant bacteria, we found that the hip1 mutant induced earlier and significantly higher levels of several proinflammatory cytokines and chemokines. We show that increased activation of Toll-like receptor 2 (TLR2)- and MyD88-dependent signaling pathways mediates the enhanced cytokine secretion induced by the hip1 mutant. Thus, Hip1 restricts the onset and magnitude of proinflammatory cytokines by limiting TLR2-dependent activation. We also show that Hip1 dampens TLR2-independent activation of the inflammasome and limits secretion of interleukin-18 (IL-18). Dampening of TLR2 signaling does not require viable M. tuberculosis or phagocytosis but does require Hip1 catalytic activity. We propose that M. tuberculosis restricts proinflammatory responses by masking cell surface interactions between TLR2 agonists on M. tuberculosis and TLR2 on macrophages. This strategy may allow M. tuberculosis to evade early detection by host immunity, delay the onset of adaptive immune responses, and accelerate disease progression.

Transient Receptor Potential Melastatin 2 (TRPM2) ion channel is required for innate immunity against Listeria monocytogenes

The generation of reactive oxygen species (ROS) is inherent to immune responses. ROS are crucially involved in host defense against pathogens by promoting bacterial killing, but also as signaling agents coordinating the production of cytokines. Transient Receptor Potential Melastatin 2 (TRPM2) is a Ca(2+)-permeable channel gated via binding of ADP-ribose, a metabolite formed under conditions of cellular exposure to ROS. Here, we show that TRPM2-deficient mice are extremely susceptible to infection with Listeria monocytogenes (Lm), exhibiting an inefficient innate immune response. In a comparison with IFNγR-deficient mice, TRPM2(-/-) mice shared similar features of uncontrolled bacterial replication and reduced levels of inducible (i)NOS-expressing monocytes, but had intact IFNγ responsiveness. In contrast, we found that levels of cytokines IL-12 and IFNγ were diminished in TRPM2(-/-) mice following Lm infection, which correlated with their reduced innate activation. Moreover, TRPM2(-/-) mice displayed a higher degree of susceptibility than IL-12-unresponsive mice, and supplementation with recombinant IFNγ was sufficient to reverse the unrestrained bacterial growth and ultimately the lethal phenotype of Lm-infected TRPM2(-/-) mice. The severity of listeriosis we observed in TRPM2(-/-) mice has not been reported for any other ion channel. These findings establish an unsuspected role for ADP-ribose and ROS-mediated cation flux for innate immunity, opening up unique possibilities for immunomodulatory intervention through TRPM2.

SHIP-deficient, alternatively activated macrophages protect mice during DSS-induced colitis

mϕ are heterogeneous in their functions, and although it is clear that inflammatory mϕ contribute to inflammation in IBDs, multiple lines of evidence suggest that M2a mϕ may offer protection during intestinal inflammation. In vivo SHIP-deficient mouse mϕ are M2a so SHIP-deficient mice provide a unique genetic model of M2a mϕ. Based on this, this study tested the hypothesis that SHIP-deficient, M2a mϕ protect mice from intestinal inflammation. The objectives were to compare the susceptibility of SHIP+/+ and SHIP-/- littermates with DSS-induced intestinal inflammation and to determine whether protection was mϕ-mediated and whether protection could be transferred to a susceptible host. We have found that SHIP-/- mice are protected during DSS-induced intestinal inflammation. SHIP-/- mice have delayed rectal bleeding and reduced weight loss, disruption of intestinal architecture, and immune cell infiltration during DSS-induced colitis relative to their WT littermates. Using liposome depletion of mϕ, we found that SHIP-/- mouse protection was indeed mϕ-mediated. Finally, we determined that SHIP-/- mϕ-mediated protection could be conferred to susceptible WT mice by adoptive transfer of M2a mϕ derived ex vivo. This study supports our hypothesis by demonstrating that SHIP-deficient, M2a mϕ are protective in this murine model of acute intestinal inflammation. Adoptive transfer of M2a mϕ to patients with IBDs offers a promising, new strategy for treatment that may be particularly useful in patients who are otherwise refractory to conventional therapies.

For better or for worse: the immune response against Mycobacterium tuberculosis balances pathology and protection

Tuberculosis (TB) is a complex disease, and the success of the bacterium as an intracellular pathogen is the outcome of its close and longstanding coevolution with the mammalian host. The dialogue between Mycobacterium tuberculosis and the host is becoming understandable at the molecular, cellular, and tissue level. This has led to the elucidation of the (i) interaction between pattern recognition receptors and pathogen-associated molecular patterns, (ii) cross-talk between immune cells, and (iii) mechanisms underlying granuloma development. Disease as an eventual but not a necessary consequence of infection results from a sensitive balance between protective immunity and destructive pathology. Early events, governed largely by conserved mechanisms of host recognition, impact not only on type and course of adaptive immunity but also on lung parenchymal function. New interpretations of how these responses shape the lung environment and direct granuloma development emphasize that the disease results from pathologic consequences of non-resolving inflammation. We review recent advances in TB research within the context of this ambitious view of TB.

Lipoarabinomannan and related glycoconjugates: structure, biogenesis and role in Mycobacterium tuberculosis physiology and host-pathogen interaction

Approximately one third of the world's population is infected with Mycobacterium tuberculosis, the causative agent of tuberculosis. This bacterium has an unusual lipid-rich cell wall containing a vast repertoire of antigens, providing a hydrophobic impermeable barrier against chemical drugs, thus representing an attractive target for vaccine and drug development. Apart from the mycolyl–arabinogalactan–peptidoglycan complex, mycobacteria possess several immunomodulatory constituents, notably lipomannan and lipoarabinomannan. The availability of whole-genome sequences of M. tuberculosis and related bacilli over the past decade has led to the identification and functional characterization of various enzymes and the potential drug targets involved in the biosynthesis of these glycoconjugates. Both lipomannan and lipoarabinomannan possess highly variable chemical structures, which interact with different receptors of the immune system during host–pathogen interactions, such as Toll-like receptors-2 and C-type lectins. Recently, the availability of mutants defective in the synthesis of these glycoconjugates in mycobacteria and the closely related bacterium, Corynebacterium glutamicum, has paved the way for host–pathogen interaction studies, as well as, providing attenuated strains of mycobacteria for the development of new vaccine candidates. This review provides a comprehensive account of the structure, biosynthesis and immunomodulatory properties of these important glycoconjugates.

Restoration of pattern recognition receptor costimulation to treat chromoblastomycosis, a chronic fungal infection of the skin

Chromoblastomycosis is a chronic skin infection caused by the fungus Fonsecaea pedrosoi. Exploring the reasons underlying the chronic nature of F. pedrosoi infection in a murine model of chromoblastomycosis, we find that chronicity develops due to a lack of pattern recognition receptor (PRR) costimulation. F. pedrosoi was recognized primarily by C-type lectin receptors (CLRs), but not by Toll-like receptors (TLRs), which resulted in the defective induction of proinflammatory cytokines. Inflammatory responses to F. pedrosoi could be reinstated by TLR costimulation, but also required the CLR Mincle and signaling via the Syk/CARD9 pathway. Importantly, exogenously administering TLR ligands helped clear F. pedrosoi infection in vivo. These results demonstrate how a failure in innate recognition can result in chronic infection, highlight the importance of coordinated PRR signaling, and provide proof of the principle that exogenously applied PRR agonists can be used therapeutically.