Tuberculosis: unsealing the apoptotic envelope

Role of PE/PPE proteins of Mycobacterium tuberculosis in triad of host mitochondria, oxidative stress and cell death

The PE and PPE family proteins of Mycobacterium tuberculosis (Mtb) is exclusively found in pathogenic Mycobacterium species, comprising approximately 8-10 % of the Mtb genome. These emerging virulent factors have been observed to play pivotal roles in Mtb pathogenesis and immune evasion through various strategies. These immunogenic proteins are known to modulate the host immune response and cell-death pathways by targeting the powerhouse of the cell, the mitochondria to support Mtb survival. In this article, we are focused on how PE/PPE family proteins target host mitochondria to induce mitochondrial perturbations, modulate the levels of cellular ROS (Reactive oxygen species) and control cell death pathways. We observed that the time of expression of these proteins at different stages of infection is crucial for elucidating their impact on the cell death pathways and eventually on the outcome of infection. This article focuses on understanding the contributions of the PE/PPE proteins by unravelling the triad of host mitochondria, oxidative stress and cell death pathways that facilitate the Mtb persistence. Understanding the role of these proteins in host cellular pathways and the intricate mechanisms paves the way for the development of novel therapeutic strategies to combat TB infections.

Role of eosinophils and apoptosis in PDIMs/PGLs deficient mycobacterium elimination in adult zebrafish

The cell wall lipids phthiocerol dimycocerosates (PDIMs) and its structurally-related compound, phenolic glycolipids (PGLs) are major virulence factors of mycobacterium, as shown by the reduced growth of PDIMs/PGLs deficient mutants in various animal models. PDIMs/PGLs play active roles in modulating host immune responses. However, the cellular and molecular mechanisms of how PDIMs/PGLs deficient mutant was eliminated in vivo are still elusive. Our aim was to investigate what host immune responses have effect on mycobacterium elimination in vivo. Using microarray, we find PDIMs/PGLs modulate divergent host responses, including chemotaxis and focal adhesion's downstream pathway and apoptosis. We examine these two host responses by Diff-Quik stain, coupled with transmission electron microscopy and TUNEL stain respectively. The ultrastructure observation showed that eosinophils appeared in WT-infected zebrafish at day 1, however eosinophils arrived was delayed to day 7 in PDIMs/PGLs-deficient mutant-infected animals. More intriguingly, apoptosis was markedly increased in PDIMs/PGLs-mutant infected zebrafish at day 1 after infection, compared to WT-infected fishes at this time. However, apoptosis trend was fully reversed by day 7, with increased apoptosis were detected in WT-infected zebrafish compared with the PDIMs/PGLs-deficient mutant, especially more apoptosis within the granuloma. This study shows that the anti-apoptotic effects of PDIMs/PGLs and the recruitment of eosinophils in tissue during the early infection in zebrafish might promote bacterium growth in vivo.

Insufficient Generation of Mycobactericidal Mediators and Inadequate Level of Phagosomal Maturation Are Related with Susceptibility to Virulent Mycobacterium tuberculosis Infection in Mouse Macrophages

Tuberculosis is caused by Mycobacterium tuberculosis infection, and it remains major life-threatening infectious diseases worldwide. Although, M. tuberculosis has infected one-third of the present human population, only 5–10% of immunocompetent individuals are genetically susceptible to tuberculosis. All inbred strains of mice are susceptible to tuberculosis; however, some mouse strains are much more susceptible than others. In a previous report, we showed that Th1-mediated immunity was not responsible for the differential susceptibility between mouse models. To examine whether these susceptibility differences between inbred mouse strains are due to the insufficient production of effector molecules in the early stage of innate immunity, we investigated mycobacteriostatic function of bone marrow-derived macrophages (BMDMs) in resistant (BALB/c and C57BL/6) and susceptible strains (DBA/2) that were infected with virulent M. tuberculosis (H37Rv) or attenuated M. tuberculosis (H37Ra). The growth rate of virulent M. tuberculosis in infected cells was significantly higher in DBA/2 BMDMs, whereas the growth of the attenuated strain was similar in the three inbred mouse BMDM strains. In addition, the death rate of M. tuberculosis-infected cells increased with the infectious dose when DBA/2 BMDMs were infected with H37Rv. The intracellular reactive oxygen species level was lower in DBA/2 BMDMs that were infected with virulent M. tuberculosis at an early post-infection time point. The expression levels of phagosomal maturation markers, including early endosomal antigen-1 (EEA1) and lysosome-associated membrane protein-1 (LAMP-1), were significantly decreased in DBA/2 BMDM that were infected with virulent M. tuberculosis, whereas IFNγ-treatment restored the phagosomal maturation activity. The nitric oxide (NO) production levels were also significantly lower in DBA/2 BMDMs that were infected with virulent H37Rv at late post-infection points; however, this was not observed with the attenuated H37Ra strain. Furthermore, IFNγ-treatment rescued the low NO production level and insufficient M. tuberculosis growth control of DBA/2 BMDMs to the same level as of both resistant strains. The secreted TNF-α and IL-10 level were not significantly different between strains. Therefore, our findings suggest that DBA/2 BMDMs may have defects in the phagosomal maturation process and in inflammatory mediator production, as they showed innate immune defects when infected with the virulent, but not attenuated M. tuberculosis strain.

Evasion of Innate and Adaptive Immunity by Mycobacterium tuberculosis

Through thousands of years of reciprocal coevolution, Mycobacterium tuberculosis has become one of humanity's most successful pathogens, acquiring the ability to establish latent or progressive infection and persist even in the presence of a fully functioning immune system. The ability of M. tuberculosis to avoid immune-mediated clearance is likely to reflect a highly evolved and coordinated program of immune evasion strategies that interfere with both innate and adaptive immunity. These include the manipulation of their phagosomal environment within host macrophages, the selective avoidance or engagement of pattern recognition receptors, modulation of host cytokine production, and the manipulation of antigen presentation to prevent or alter the quality of T-cell responses. In this article we review an extensive array of published studies that have begun to unravel the sophisticated program of specific mechanisms that enable M. tuberculosis and other pathogenic mycobacteria to persist and replicate in the face of considerable immunological pressure from their hosts. Unraveling the mechanisms by which M. tuberculosis evades or modulates host immune function is likely to be of major importance for the development of more effective new vaccines and targeted immunotherapy against tuberculosis.

Inhibition of TLR8 mediated signaling promotes BCG induced apoptosis in THP-1 cells

Apoptosis was considered as one of the important host defense mechanisms against mycobacteria infection. In macrophage, the main target cell of Mycobacterium tuberculosis, apoptosis after infection could help kill the bacillus inside and process the antigens for further presentation and proper immune response. Here, we identified a role of TLR8 during the apoptosis induced by Bacillus Calmette Guérin (BCG) infection in THP-1 cells. Knockdown TLR8 further increased the apoptosis induced by BCG infection, and this enhanced apoptosis was caspase-dependent. During this process, Erk1/2, JNK and NFκB pathways were negatively affected and contributed to the enhanced apoptosis.

Current efforts and future prospects in the development of live mycobacteria as vaccines

The development of more effective vaccines against Mycobacterium tuberculosis (Mtb) remains a major goal in the effort to reduce the enormous global burden of disease caused by this pathogen. Whole-cell vaccines based on live mycobacteria with attenuated virulence represent an appealing approach, providing broad antigen exposure and intrinsic adjuvant properties to prime durable immune responses. However, designing vaccine strains with an optimal balance between attenuation and immunogenicity has proven to be extremely challenging. Recent basic and clinical research efforts have broadened our understanding of Mtb pathogenesis and created numerous new vaccine candidates that have been designed to overcome different aspects of immune evasion by Mtb. In this review, we provide an overview of the current efforts to create improved vaccines against tuberculosis based on modifications of live attenuated mycobacteria. In addition, we discuss the use of such vaccine strains as vectors for stimulating protective immunity against other infectious diseases and cancers.

Cytokine and lipid mediator networks in tuberculosis

A major approach for immunologic intervention in tuberculosis involves redirecting the outcome of the host immune response from the induction of disease to pathogen control. Cytokines and lipid mediators known as eicosanoids play key roles in regulating this balance and as such represent important targets for immunologic intervention. While the evidence for cytokine/eicosanoid function derives largely from the investigation of murine and zebrafish experimental infection models, clinical studies have confirmed the existence of many of the same pathways in tuberculosis patients. Here, we summarize new data that reveal important intersections between the cytokine and eicosanoid networks in the host response to mycobacteria and discuss how targeting this crosstalk can promote resistance to lethal Mycobacterium tuberculosis infection. This approach could lead to new host-directed therapies to be used either as an adjunct for improving the efficacy of standard antibiotic treatment or for the management of drug-resistant infections.

Comparative functional genomics and the bovine macrophage response to strains of the mycobacterium genus

Mycobacterial infections are major causes of morbidity and mortality in cattle and are also potential zoonotic agents with implications for human health. Despite the implementation of comprehensive animal surveillance programs, many mycobacterial diseases have remained recalcitrant to eradication in several industrialized countries. Two major mycobacterial pathogens of cattle are Mycobacterium bovis and Mycobacterium avium subspecies paratuberculosis (MAP), the causative agents of bovine tuberculosis (BTB) and Johne's disease (JD), respectively. BTB is a chronic, granulomatous disease of the respiratory tract that is spread via aerosol transmission, while JD is a chronic granulomatous disease of the intestines that is transmitted via the fecal-oral route. Although these diseases exhibit differential tissue tropism and distinct complex etiologies, both M. bovis and MAP infect, reside, and replicate in host macrophages - the key host innate immune cell that encounters mycobacterial pathogens after initial exposure and mediates the subsequent immune response. The persistence of M. bovis and MAP in macrophages relies on a diverse series of immunomodulatory mechanisms, including the inhibition of phagosome maturation and apoptosis, generation of cytokine-induced necrosis enabling dissemination of infection through the host, local pathology, and ultimately shedding of the pathogen. Here, we review the bovine macrophage response to infection with M. bovis and MAP. In particular, we describe how recent advances in functional genomics are shedding light on the host macrophage-pathogen interactions that underlie different mycobacterial diseases. To illustrate this, we present new analyses of previously published bovine macrophage transcriptomics data following in vitro infection with virulent M. bovis, the attenuated vaccine strain M. bovis BCG, and MAP, and discuss our findings with respect to the differing etiologies of BTB and JD.

Antimicrobial responses of teleost phagocytes and innate immune evasion strategies of intracellular bacteria

During infection, macrophage lineage cells eliminate infiltrating pathogens through a battery of antimicrobial responses, where the efficacy of these innate immune responses is pivotal to immunological outcomes. Not surprisingly, many intracellular pathogens have evolved mechanisms to overcome macrophage defenses, using these immune cells as residences and dissemination strategies. With pathogenic infections causing increasing detriments to both aquacultural and wild fish populations, it is imperative to garner greater understanding of fish phagocyte antimicrobial responses and the mechanisms by which aquatic pathogens are able to overcome these teleost macrophage barriers. Insights into the regulation of macrophage immunity of bony fish species will lend to the development of more effective aquacultural prophylaxis as well as broadening our understanding of the evolution of these immune processes. Accordingly, this review focuses on recent advances in the understanding of teleost macrophage antimicrobial responses and the strategies by which intracellular fish pathogens are able to avoid being killed by phagocytes, with a focus on Mycobacterium marinum.

Tumour necrosis factor receptors and apoptosis of alveolar macrophages during early infection with attenuated and virulent Mycobacterium bovis

Apoptosis of macrophages has been reported as an effective host strategy to control the growth of intracellular pathogens, including pathogenic mycobacteria. Tumour necrosis factor-α (TNF-α) plays an important role in the modulation of apoptosis of infected macrophages. It exerts its biological activities via two distinct cell surface receptors, TNFR1 and TNFR2, whose extracellular domain can be released by proteolysis forming soluble TNF receptors (sTNFR1 and sTNFR2). The signalling through TNFR1 initiates the majority of the biological functions of TNF-α, leading to either cell death or survival whereas TNFR2 mediates primarily survival signals. Here, the expression of TNF-α receptors and the apoptosis of alveolar macrophages were investigated during the early phase of infection with attenuated and virulent mycobacteria in mice. A significant increase of apoptosis and high expression of TNFR1 were observed in alveolar macrophages at 3 and 7 days after infection with attenuated Mycobacterium bovis but only on day 7 in infection with the virulent M. bovis. Low surface expression of TNFR1 and increased levels of sTNFR1 on day 3 after infection by the virulent strain were associated with reduced rates of apoptotic macrophages. In addition, a significant reduction in apoptosis of alveolar macrophages was observed in TNFR1(-/-) mice at day 3 after bacillus Calmette-Guérin infection. These results suggest a potential role for TNFR1 in mycobacteria-induced alveolar macrophage apoptosis in vivo. In this scenario, shedding of TNFR1 seems to contribute to the modulation of macrophage apoptosis in a strain-dependent manner.

Role of MHC class Ib molecule, H2-M3 in host immunity against tuberculosis

The MHC class I family comprises both classical (class Ia) and non-classical (class Ib) members. While the prime function of classical MHC class I molecules (MHC class Ia) is to present peptide antigens to pathogen-specific cytotoxic T cells, non-classical MHC-I (MHC class Ib) antigens perform diverse array of functions in both innate and adaptive immunity. Vaccines against intracellular pathogens such as Mycobacterium tuberculosis need to induce strong cellular immune responses. Recent studies have shown that MHC class I molecules play an important role in the protective immune response to M. tuberculosis infection. Both MHC Ia-restricted and MHC class Ib-restricted M. tuberculosis -reactive CD8(+) T cells have been identified in humans and mice, but their relative contributions to immunity is still uncertain. Unlike MHC class Ia-restricted CD8(+) T cells, MHC class Ib-restricted CD8(+) T cells are constitutively activated in naive animals and respond rapidly to infection challenge, hence filling the temporal gap between innate and adaptive immunity. The present review article summarizes the general host immunity against M. tuberculosis infection highlighting the possible role of MHC class Ib molecule, H2-M3 and their ligands (N-formylated peptides) in protection against tuberculosis.

Mycobacterium tuberculosis genes involved in regulation of host cell death

The topic of host cell death response upon Mycobacterium tuberculosis (Mtb) infection has been a controversial one [1]. Recent findings demonstrate that one of the important confounding factors was most likely the fact that while Mtb inhibits host cell apoptosis induction early during the infection it clearly induces a necrotic form of cell death during later infection stages [2, 3]. This bi-phasic intracellular lifestyle in regard to host cell death manipulation is emerging as a common theme shared with other facultative and obligate intracellular bacterial pathogens such as Chlamydia and Legionella [4-6]. Accordingly, the list of discovered bacterial proteins involved in host cell apoptosis inhibition is growing [7, 8]. At the same time it is clearly beneficial for the resistance of the host to overcome the bacterial apoptosis block during the early stage of the infection [9-11]. Hence, host cell components have evolved to recognize intracellular pathogens and mediate host cell apoptosis induction if necessary [12]. There have been several reviews on the various aspects of the host cell death response upon Mtb infection [1, 3, 13-15]. Thus in this chapter I will focus on the pathogen side of the equation and describe the tremendous progress that has been made in the identification and characterization of Mtb genes involved in manipulation of host cell death pathways.

Immunotherapy for TB

Mycobacterium tuberculosis was one of the first human pathogens to be identified as the cause of a specific disease – TB. TB was also one of the first specific diseases for which immunotherapy was attempted. In more than a century since, multiple different immunotherapies have been attempted, alongside vaccination and antibiotic treatment, with varying degrees of success. Despite this, TB remains a major worldwide health problem that causes nearly 2 million deaths annually and has infected an estimated 2 billion people. A major reason for this is that M. tuberculosis is an ancient human pathogen that has evolved complex strategies for persistence in the human host. It has thus been long understood that, to effectively control TB, we will need to address the ability of the pathogen to establish a persistent, latent infection in most infected individuals. This review discusses what is presently known about the interaction of M. tuberculosis with the immune system, and how this knowledge has been used to design immunotherapeutic strategies.

Mycobacterium tuberculosis inhibits neutrophil apoptosis, leading to delayed activation of naive CD4 T cells

Mycobacterium tuberculosis promotes its replication by inhibiting the apoptosis of infected macrophages. A proapoptotic M. tuberculosis mutant lacking nuoG, a subunit of the type I NADH dehydrogenase complex, exhibits attenuated growth in vivo, indicating that this virulence mechanism is essential. We show that M. tuberculosis also suppresses neutrophil apoptosis. Compared to wild-type, the nuoG mutant spread to a larger number of lung phagocytic cells. Consistent with the shorter lifespan of infected neutrophils, infection with the nuoG mutant resulted in fewer bacteria per infected neutrophil, accelerated bacterial acquisition by dendritic cells, earlier trafficking of these dendritic cells to lymph nodes, and faster CD4 T cell priming. Neutrophil depletion abrogated accelerated CD4 T cell priming by the nuoG mutant, suggesting that inhibiting neutrophil apoptosis delays adaptive immunity in tuberculosis. Thus, pathogen modulation of apoptosis is beneficial at multiple levels, and enhancing phagocyte apoptosis promotes CD4 as well as CD8 T cell responses.

YopJ-induced caspase-1 activation in Yersinia-infected macrophages: independent of apoptosis, linked to necrosis, dispensable for innate host defense

Yersinia outer protein J (YopJ) is a type III secretion system (T3SS) effector of pathogenic Yersinia (Yersinia pestis, Yersinia enterocolitica and Yersinia pseudotuberculosis) that is secreted into host cells. YopJ inhibits survival response pathways in macrophages, causing cell death. Allelic variation of YopJ is responsible for differential cytotoxicity in Yersinia strains. YopJ isoforms in Y. enterocolitica O:8 (YopP) and Y. pestis KIM (YopJ^KIM) strains have high cytotoxic activity. In addition, YopJ^KIM-induced macrophage death is associated with caspase-1 activation and interleukin-1β (IL-1β secretion. Here, the mechanism of YopJ^KIM-induced cell death, caspase-1 activation, and IL-1β secretion in primary murine macrophages was examined. Caspase-3/7 activity was low and the caspase-3 substrate poly (ADP-ribose) polymerase (PARP) was not cleaved in Y. pestis KIM5-infected macrophages. In addition, cytotoxicity and IL-1β secretion were not reduced in the presence of a caspase-8 inhibitor, or in B-cell lymphoma 2 (Bcl-2)-associated X protein (Bax)/Bcl-2 homologous antagonist/killer (Bak) knockout macrophages, showing that YopJ^KIM-mediated cell death and caspase-1 activation occur independent of mitochondrial-directed apoptosis. KIM5-infected macrophages released high mobility group protein B1 (HMGB1), a marker of necrosis, and microscopic analysis revealed that necrotic cells contained active caspase-1, indicating that caspase-1 activation is associated with necrosis. Inhibitor studies showed that receptor interacting protein 1 (RIP1) kinase and reactive oxygen species (ROS) were not required for cytotoxicity or IL-β release in KIM5-infected macrophages. IL-1β secretion was reduced in the presence of cathepsin B inhibitors, suggesting that activation of caspase-1 requires cathepsin B activity. Ectopically-expressed YopP caused higher cytotoxicity and secretion of IL-1β in Y. pseudotuberculosis-infected macrophages than YopJ^KIM. Wild-type and congenic caspase 1 knockout C57BL/6 mice were equally susceptible to lethal infection with Y. pseudotuberculosis ectopically expressing YopP. These data suggest that YopJ-induced caspase-1 activation in Yersinia-infected macrophages is a downstream consequence of necrotic cell death and is dispensable for innate host resistance to a strain with enhanced cytotoxicity.

Granuloma transplantation: an approach to study mycobacterium-host interactions

The host-pathogen biology during infection with Mycobacterium tuberculosis is incredibly complex and despite accelerating progress in research, remains poorly understood. Our limited understanding hinders the development of new drugs, next generation vaccines, and novel therapies. The granuloma is the site where mycobacteria are both controlled and allowed to persist, but it remains one of the least studied aspects of the host-pathogen relationship. Here, we review the development, application, potential uses, and limitations of a novel model of granuloma transplantation as a tool to study specific host-pathogen interactions that have been difficult to probe. Application of this new model has already contributed to our understanding of granuloma cell traffic, repopulation, and the relationship between systemic immunity and mycobacteria-containing granulomas. The data collected highlight the dynamic interaction between systemic and local immune processes and support a paradigm that defines the granuloma as a highly dynamic structure. Granuloma transplantation also has special potential as a novel latency model that can contribute to our understanding of host protection factors and bacterial mutants, and serve as a platform for drug testing.

Mycobacterial secretion systems ESX-1 and ESX-5 play distinct roles in host cell death and inflammasome activation

During infection of humans and animals, pathogenic mycobacteria manipulate the host cell causing severe diseases such as tuberculosis and leprosy. To understand the basis of mycobacterial pathogenicity, it is crucial to identify the molecular virulence mechanisms. In this study, we address the contribution of ESX-1 and ESX-5--two homologous type VII secretion systems of mycobacteria that secrete distinct sets of immune modulators--during the macrophage infection cycle. Using wild-type, ESX-1- and ESX-5-deficient mycobacterial strains, we demonstrate that these secretion systems differentially affect subcellular localization and macrophage cell responses. We show that in contrast to ESX-1, the effector proteins secreted by ESX-5 are not required for the translocation of Mycobacterium tuberculosis or Mycobacterium marinum to the cytosol of host cells. However, the M. marinum ESX-5 mutant does not induce inflammasome activation and IL-1β activation. The ESX-5 system also induces a caspase-independent cell death after translocation has taken place. Importantly, by means of inhibitory agents and small interfering RNA experiments, we reveal that cathepsin B is involved in both the induction of cell death and inflammasome activation upon infection with wild-type mycobacteria. These results reveal distinct roles for two different type VII secretion systems during infection and shed light on how virulent mycobacteria manipulate the host cell in various ways to replicate and spread.

Express path analysis identifies a tyrosine kinase Src-centric network regulating divergent host responses to Mycobacterium tuberculosis infection

Global gene expression profiling has emerged as a major tool in understanding complex response patterns of biological systems to perturbations. However, a lack of unbiased analytical approaches has restricted the utility of complex microarray data to gain novel system level insights. Here we report a strategy, express path analysis (EPA), that helps to establish various pathways differentially recruited to achieve specific cellular responses under contrasting environmental conditions in an unbiased manner. The analysis superimposes differentially regulated genes between contrasting environments onto the network of functional protein associations followed by a series of iterative enrichments and network analysis. To test the utility of the approach, we infected THP1 macrophage cells with a virulent Mycobacterium tuberculosis strain (H37Rv) or the attenuated non-virulent strain H37Ra as contrasting perturbations and generated the temporal global expression profiles. EPA of the results provided details of response-specific and time-dependent host molecular network perturbations. Further analysis identified tyrosine kinase Src as the major regulatory hub discriminating the responses between wild-type and attenuated Mtb infection. We were then able to verify this novel role of Src experimentally and show that Src executes its role through regulating two vital antimicrobial processes of the host cells (i.e. autophagy and acidification of phagolysosome). These results bear significant potential for developing novel anti-tuberculosis therapy. We propose that EPA could prove extremely useful in understanding complex cellular responses for a variety of perturbations, including pathogenic infections.

Development of new antituberculous drugs based on bacterial virulence factors interfering with host cytokine networks

The worldwide increase in the prevalence of tuberculosis (TB), especially multidrug-resistant TB and extensively drug-resistant TB, is an important global health concern, and new effective drugs are urgently needed. Information on the genome of Mycobacterium tuberculosis (MTB) and various mycobacterial virulence genes is leading to the identification of genes that code for new drug targets. Mycobacterium tuberculosis (MTB) is resistant to the antimicrobial mechanisms of host macrophages and can survive and replicate in macrophages for long periods, resulting in a persistent infection. Mycobacterial virulence factors suppress macrophage bactericidal functions partly via their downregulatory effects on the host antimicrobial cytokine networks, consisting of proinflammatory, immunopotentiating, and Th1-inducing cytokines. Thus, for the development of unique drugs that exhibit antimycobacterial action through novel mechanisms, it is reasonable to search for targets among bacterial genes encoding virulence factors which interfere with the host cytokine responses protective to mycobacterial pathogens. In this review, we discuss the profiles of cytokine networks related to host resistance to mycobacteria, including the mechanisms of downregulation of host antimycobacterial immunity due to immunosuppressive cytokines, which are occasionally induced in the advanced stages of TB. We also highlight the development of antituberculous drugs based on bacterial virulence factors interfering with the host antimycobacterial cytokine network.

Modulation of cell death by M. tuberculosis as a strategy for pathogen survival

It has been clearly demonstrated that in vitro, virulent M. tuberculosis can favor necrosis over apoptosis in infected macrophages, and this has been suggested as a mechanism for evading the host immune response. We recently reported that an effect consistent with this hypothesis could be observed in cells from the blood of TB patients, and in this paper, we review what is known about evasion strategies employed by M. tuberculosis and in particular consider the possible interaction of the apoptosis-inhibiting effects of M. tuberculosis infection with another factor (IL-4) whose expression is thought to play a role in the failure to control M. tuberculosis infection. It has been noted that IL-4 may exacerbate TNF-α-induced pathology, though the mechanism remains unexplained. Since pathology in TB typically involves inflammatory aggregates around infected cells, where TNF-α plays an important role, we predicted that IL-4 would inhibit the ability of cells to remove M. tuberculosis by apoptosis of infected cells, through the extrinsic pathway, which is activated by TNF-α. Infection of human monocytic cells with mycobacteria in vitro, in the presence of IL-4, appears to promote necrosis over apoptosis in infected cells—a finding consistent with its suggested role as a factor in pathology during M. tuberculosis infection.

Induction of cell death after localization to the host cell mitochondria by the Mycobacterium tuberculosis PE_PGRS33 protein

PE_PGRS33 is the most studied member of the unique PE family of mycobacterial proteins. These proteins are composed of a PE domain (Pro-Glu motif), a linker region and a PGRS domain (polymorphic GC-rich-repetitive sequence). Previous studies have shown that PE_PGRS33 is surface-exposed, constitutively expressed during growth and infection, involved in creating antigenic diversity, and able to induce death in transfected or infected eukaryotic cells. In this study, we showed that PE_PGRS33 co-localizes to the mitochondria of transfected cells, a phenomenon dependent on the linker region and the PGRS domain, but not the PE domain. Using different genetic fusions and chimeras, we also demonstrated a direct correlation between localization to the host mitochondria and the induction of cell death. Finally, although all constructs localizing to the mitochondria did induce apoptosis, only the wild-type PE_PGRS33 with its own PE domain also induced primary necrosis, indicating a potentially important role for the PE domain. Considering the importance of primary necrosis in Mycobacterium tuberculosis dissemination during natural infection, the PE_PGRS33 protein may play a crucial role in the pathogenesis of tuberculosis.

Regulation between survival, persistence, and elimination of intracellular mycobacteria: a nested equilibrium of delicate balances

This review elaborates on recent insights gained on the dynamic aspects of interactions between pathogen and the host intracellular machinery in tuberculosis infection. In addition we will also discuss the utility of adopting a systems biology approach, both for a better understanding of TB biology, and for the development of more effective ways for disease management.

Interactions between bacterial pathogens and mitochondrial cell death pathways

The modulation of host cell death pathways by bacteria has been recognized as a major pathogenicity mechanism. Among other strategies, bacterial pathogens can hijack the cell death machinery of host cells by influencing the signalling pathways that converge on the mitochondria. In particular, many bacterial proteins have evolved to interact in a highly specific manner with host mitochondria, thereby modulating the decision between cell life and death. In this Review, we explore the intimate interactions between bacterial pathogens and mitochondrial cell death pathways.

Tuberculous meningitis

Tuberculous meningitis is a severe form of extrapulmonary tuberculosis. The exact incidence and prevalence are not known. In countries with high burden of pulmonary tuberculosis, the incidence is expected to be proportionately high. Children are much more vulnerable. Human immunodeficiency virus-infected patients have a high incidence of tuberculous meningitis. The hallmark pathological processes are meningeal inflammation, basal exudates, vasculitis and hydrocephalus. Headache, vomiting, meningeal signs, focal deficits, vision loss, cranial nerve palsies and raised intracranial pressure are dominant clinical features. Diagnosis is based on the characteristic clinical picture, neuroimaging abnormalities and cerebrospinal fluid changes (increased protein, low glucose and mononuclear cell pleocytosis). Cerebrospinal fluid smear examination, mycobacterial culture or polymerase chain reaction is mandatory for bacteriological confirmation. The mortality and morbidity of tuberculous meningitis are exceptionally high. Prompt diagnosis and early treatment are crucial. Decision to start antituberculous treatment is often empirical. WHO guidelines recommend a 6 months course of antituberculous treatment; however, other guidelines recommend a prolonged treatment extended to 9 or 12 months. Corticosteroids reduce the number of deaths. Resistance to antituberculous drugs is associated with a high mortality. Patients with hydrocephalus may need ventriculo-peritoneal shunting. Bacillus Calmette-Guérin vaccination protects to some degree against tuberculous meningitis in children.

Physiological consequences of programmed necrosis, an alternative form of cell demise

Cell death occurs spontaneously or in response to external stimuli, and can be largely subdivided into apoptosis and necrosis by the distinct morphological and biochemical features. Unlike apoptosis, necrosis was recognized as the passive and unwanted cell demise committed in a non-regulated and disorganized manner. However, under specific conditions such as caspase intervention, necrosis has been proposed to be regulated in a well-orchestrated way as a backup mechanism of apoptosis. The term programmed necrosis has been coined to describe such an alternative cell death. Recently, at least some regulators governing programmed necrosis have been identified and demonstrated to be interconnected via a wide network of signal pathways by further extensive studies. There is growing evidence that programmed necrosis is not only associated with pathophysiological diseases, but also provides innate immune response to viral infection. Here, we will introduce recent updates on the molecular mechanism and physiological significance of programmed necrosis.

Validation of a medium-throughput method for evaluation of intracellular growth of Mycobacterium tuberculosis

Intracellular pathogens such as Mycobacterium tuberculosis have adapted to a life inside host cells, in which they utilize host nutrients to replicate and spread. Ineffective methods for the evaluation of growth of intracellular pathogens in their true environment pose an obstacle for basic research and drug screening. Here we present the validation of a luminometry-based method for the analysis of intramacrophage growth of M. tuberculosis. The method, which is performed in a medium-throughput format, can easily be adapted for studies of other intracellular pathogens and cell types. The use of host cells in drug-screening assays dedicated to find antimicrobials effective against intracellular pathogens permits the discovery of not only novel antibiotics but also compounds with immunomodulatory and virulence-impairing activities, which may be future alternatives or complements to antibiotics.

How do mycobacteria activate CD8+ T cells?

CD8(+) T cells are activated upon presentation of antigens from the cytosol. Therefore, it was unclear how pathogenic mycobacteria could prime this type of lymphocyte, given that these microbes were thought to remain in phagosomes and, hence, be shielded from the host cytosol. Recently, it was shown that some mycobacteria can enter the cytosol through translocation from phagolysosomes, providing a direct mechanism for CD8(+) T cell priming. However, this mechanism might not apply to other mycobacteria, which do not appear to be able to enter the cytosol. Here, we discuss the different hypotheses to explain the induction of CD8(+) T cell responses in mycobacterial infections.

Precise temporal regulation of roughest is required for correct salivary gland autophagic cell death in Drosophila

The Drosophila roughest (rst) locus encodes an immunoglobulin superfamily transmembrane glycoprotein implicated in a variety of embryonic and postembryonic developmental processes. Here we demonstrate a previously unnoticed role for this gene in the autophagic elimination of larval salivary glands during early pupal stages by showing that overexpression of the Rst protein ectodomain in early pupa leads to persistence of salivary glands up to at least 12 hours after head eversion, although with variable penetrance. The same phenotype is observed in individuals carrying the dominant regulatory allele rst(D), but not in loss of function alleles. Analysis of persistent glands at the ultrastructural level showed that programmed cell death starts at the right time but is arrested at an early stage of the process. Finally we describe the expression pattern and intracellular distribution of Rst in wild type and rst(D) mutants, showing that its downregulation in salivary glands at the beginning of pupal stage is an important factor in the correct implementation of the autophagic program of this tissue in space and time.

Evasion and subversion of antigen presentation by Mycobacterium tuberculosis

Mycobacterium tuberculosis is one of the most successful of human pathogens and has acquired the ability to establish latent or progressive infection and persist even in the presence of a fully functioning immune system. The ability of M. tuberculosis to avoid immune-mediated clearance is likely to reflect a highly evolved and coordinated program of immune evasion strategies, including some that interfere with antigen presentation to prevent or alter the quality of T-cell responses. Here, we review an extensive array of published studies supporting the view that antigen presentation pathways are targeted at many points by pathogenic mycobacteria. These studies show the multiple potential mechanisms by which M. tuberculosis may actively inhibit, subvert or otherwise modulate antigen presentation by major histocompatibility complex class I, class II and CD1 molecules. Unraveling the mechanisms by which M. tuberculosis evades or modulates antigen presentation is of critical importance for the development of more effective new vaccines based on live attenuated mycobacterial strains.

Signal regulatory protein alpha (SIRPalpha) cells in the adaptive response to ESAT-6/CFP-10 protein of tuberculous mycobacteria

BACKGROUND

Early secretory antigenic target-6 (ESAT-6) and culture filtrate protein-10 (CFP-10) are co-secreted proteins of Mycobacterium tuberculosis complex mycobacteria (includes M. bovis, the zoonotic agent of bovine tuberculosis) involved in phagolysosome escape of the bacillus and, potentially, in the efficient induction of granulomas. Upon tuberculosis infection, multi-nucleate giant cells are elicited, likely as a response aimed at containing mycobacteria. In tissue culture models, signal regulatory protein (SIRP)alpha (also referred to as macrophage fusion receptor or CD172a) is essential for multi-nucleate giant cell formation.

METHODOLOGY/PRINCIPAL FINDINGS

In the present study, ESAT-6/CFP-10 complex and SIRPalpha interactions were evaluated with samples obtained from calves experimentally infected with M. bovis. Peripheral blood CD172a(+) (SIRPalpha-expressing) cells from M. bovis-infected calves proliferated upon in vitro stimulation with ESAT-6/CFP-10 (either as a fusion protein or a peptide cocktail), but not with cells from animals receiving M. bovis strains lacking ESAT-6/CFP-10 (i.e, M. bovis BCG or M. bovis DeltaRD1). Sorted CD172a(+) cells from these cultures had a dendritic cell/macrophage morphology, bound fluorescently-tagged rESAT-6:CFP-10, bound and phagocytosed live M. bovis BCG, and co-expressed CD11c, DEC-205, CD44, MHC II, CD80/86 (a subset also co-expressed CD11b or CD8alpha). Intradermal administration of rESAT-6:CFP-10 into tuberculous calves elicited a delayed type hypersensitive response consisting of CD11c(+), CD172a(+), and CD3(+) cells, including CD172a-expressing multi-nucleated giant cells.

CONCLUSIONS/SIGNIFICANCE

These findings demonstrate the ability of ESAT-6/CFP-10 to specifically expand CD172a(+) cells, bind to CD172a(+) cells, and induce multi-nucleated giant cells expressing CD172a.

Interaction of Mycobacterium tuberculosis with the host: consequences for vaccine development

Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), remains a major worldwide health problem that causes more than 2 million deaths annually. In addition, an estimated 2 billion people are latently infected with M. tuberculosis. The bacterium is one of the oldest human pathogens and has evolved complex strategies for survival. Therefore, to be successful in the high endemic regions, any future TB vaccine strategy will have to be tailored in accordance with the resulting complexity of the TB infection and anti-mycobacterial immune response. In this review, we will discuss what is presently known about the interaction of M. tuberculosis with the immune system, and how this knowledge is used in new and more advanced vaccine strategies.

Apoptosis of macrophages during pulmonary Mycobacterium bovis infection: correlation with intracellular bacillary load and cytokine levels

Apoptosis of macrophages infected with pathogenic mycobacteria is an alternative host defence capable of removing the environment supporting bacterial growth. In this work the influence of virulence and bacterial load on apoptosis of alveolar macrophages during the initial phase of infection by Mycobacterium bovis was investigated. BALB/c mice were infected intratracheally with high or low doses of the virulent (ATCC19274) or attenuated (bacillus Calmette-Guérin Moreau) strains of M. bovis. The frequency of macrophage apoptosis, the growth of mycobacteria in macrophages, and the in situ levels of the cytokines tumour necrosis factor-alpha (TNF-alpha), interleukin-10 (IL-10) and IL-12 and of the anti-apoptotic protein Bcl-2 were measured at day 3 and day 7 post-infection. An increase of macrophage apoptosis was observed after infection with both strains but the virulent strain induced less apoptosis than the attenuated strain. On the 3rd day after infection with the virulent strain macrophage apoptosis was reduced in the high-dose group, while on the 7th day post-infection macrophage apoptosis was reduced in the low-dose group. Inhibition of apoptosis was correlated with increased production of IL-10, reduced production of TNF-alpha and increased production of Bcl-2. In addition, the production of IL-12 was reduced at points where the lowest levels of macrophage apoptosis were observed. Our results indicate that virulent mycobacteria are able to modulate macrophage apoptosis to an extent dependent on the intracellular bacterial burden, which benefits its intracellular growth and dissemination to adjacent cells.