Virulent clinical isolates ofMycobacterium tuberculosisgrow rapidly and induce cellular necrosis but minimal apoptosis in murine macrophages

Autophagy restricts Mycobacterium tuberculosis during acute infection in mice

Whether or not autophagy has a role in defence against Mycobacterium tuberculosis infection remains unresolved. Previously, conditional knockdown of the core autophagy component ATG5 in myeloid cells was reported to confer extreme susceptibility to M. tuberculosis in mice, whereas depletion of other autophagy factors had no effect on infection. We show that doubling cre gene dosage to more robustly deplete ATG16L1 or ATG7 resulted in increased M. tuberculosis growth and host susceptibility in mice, although ATG5-depleted mice are more sensitive than ATG16L1- or ATG7-depleted mice. We imaged individual macrophages infected with M. tuberculosis and identified a shift from apoptosis to rapid necrosis in autophagy-depleted cells. This effect was dependent on phagosome permeabilization by M. tuberculosis. We monitored infected cells by electron microscopy, showing that autophagy protects the host macrophage by partially reducing mycobacterial access to the cytosol. We conclude that autophagy has an important role in defence against M. tuberculosis in mammals.

Uptake-independent killing of macrophages by extracellular Mycobacterium tuberculosis aggregates

Mycobacterium tuberculosis (Mtb) infection is initiated by inhalation of bacteria into lung alveoli, where they are phagocytosed by resident macrophages. Intracellular Mtb replication induces the death of the infected macrophages and the release of bacterial aggregates. Here, we show that these aggregates can evade phagocytosis by killing macrophages in a contact-dependent but uptake-independent manner. We use time-lapse fluorescence microscopy to show that contact with extracellular Mtb aggregates triggers macrophage plasma membrane perturbation, cytosolic calcium accumulation, and pyroptotic cell death. These effects depend on the Mtb ESX-1 secretion system, however, this system alone cannot induce calcium accumulation and macrophage death in the absence of the Mtb surface-exposed lipid phthiocerol dimycocerosate. Unexpectedly, we found that blocking ESX-1-mediated secretion of the EsxA/EsxB virulence factors does not eliminate the uptake-independent killing of macrophages and that the 50-kDa isoform of the ESX-1-secreted protein EspB can mediate killing in the absence of EsxA/EsxB secretion. Treatment with an ESX-1 inhibitor reduces uptake-independent killing of macrophages by Mtb aggregates, suggesting that novel therapies targeting this anti-phagocytic mechanism could prevent the propagation of extracellular bacteria within the lung.

In silico identification and synthesis of a multi-drug loaded MOF for treating tuberculosis

Conventional drug delivery systems have been applied to a myriad of active ingredients but may be difficult to tailor for a given drug. Herein, we put forth a new strategy, which designs and selects the drug delivery material by considering the properties of encapsulated drugs (even multiple drugs, simultaneously). Specifically, through an in-silico screening process of 5109 MOFs using grand canonical Monte Carlo simulations, a customized MOF (referred as BIO-MOF-100) was selected and experimentally verified to be biologically stable, and capable of loading 3 anti-Tuberculosis drugs Rifampicin+Isoniazid+Pyrazinamide at 10% + 28% + 23% wt/wt (total > 50% by weight). Notably, the customized BIO-MOF-100 delivery system cleared naturally Pyrazinamide-resistant Bacillus Calmette-Guérin, reduced growth of virulent Erdman infection in macaque macrophages 10-100-fold compared to soluble drugs in vitro and was also significantly reduced Erdman growth in mice. These data suggest that the methodology of identifying-synthesizing materials can be used to generate solutions for challenging applications such as simultaneous delivery of multiple, small hydrophilic and hydrophobic molecules in the same molecular framework.

Immune regulation and emerging roles of noncoding RNAs in Mycobacterium tuberculosis infection

Tuberculosis, caused by Mycobacterium tuberculosis, engenders an onerous burden on public hygiene. Congenital and adaptive immunity in the human body act as robust defenses against the pathogens. However, in coevolution with humans, this microbe has gained multiple lines of mechanisms to circumvent the immune response to sustain its intracellular persistence and long-term survival inside a host. Moreover, emerging evidence has revealed that this stealthy bacterium can alter the expression of demic noncoding RNAs (ncRNAs), leading to dysregulated biological processes subsequently, which may be the rationale behind the pathogenesis of tuberculosis. Meanwhile, the differential accumulation in clinical samples endows them with the capacity to be indicators in the time of tuberculosis suffering. In this article, we reviewed the nearest insights into the impact of ncRNAs during Mycobacterium tuberculosis infection as realized via immune response modulation and their potential as biomarkers for the diagnosis, drug resistance identification, treatment evaluation, and adverse drug reaction prediction of tuberculosis, aiming to inspire novel and precise therapy development to combat this pathogen in the future.

Bacterial Strain-Dependent Dissociation of Cell Recruitment and Cell-to-Cell Spread in Early M. tuberculosis Infection

In the initial stage of respiratory infection, Mycobacterium tuberculosis traverses from alveolar macrophages to phenotypically diverse monocyte-derived phagocytes and neutrophils in the lung parenchyma. Here, we compare the in vivo kinetics of early bacterial growth and cell-to-cell spread of two strains of M. tuberculosis: a lineage 2 strain, 4334, and the widely studied lineage 4 strain H37Rv. Using flow cytometry, live cell sorting of phenotypic subsets, and quantitation of bacteria in cells of the distinct subsets, we found that 4334 induces less leukocyte influx into the lungs but demonstrates earlier population expansion and cell-to-cell spread. The earlier spread of 4334 to recruited cells, including monocyte-derived dendritic cells, is accompanied by earlier and greater magnitude of CD4+ T cell activation. The results provide evidence that strain-specific differences in interactions with lung leukocytes can shape adaptive immune responses in vivo. IMPORTANCE Tuberculosis is a leading infectious disease killer worldwide and is caused by Mycobacterium tuberculosis. After exposure to M. tuberculosis, outcomes range from apparent elimination to active disease. Early innate immune responses may contribute to differences in outcomes, yet it is not known how bacterial strains alter the early dynamics of innate immune and T cell responses. We infected mice with distinct strains of M. tuberculosis and discovered striking differences in innate cellular recruitment, cell-to-cell spread of bacteria in the lungs, and kinetics of initiation of antigen-specific CD4 T cell responses. We also found that M. tuberculosis can spread beyond alveolar macrophages even before a large influx of inflammatory cells. These results provide evidence that distinct strains of M. tuberculosis can exhibit differential kinetics in cell-to-cell spread which is not directly linked to early recruitment of phagocytes but is subsequently linked to adaptive immune responses.

Rapidly Growing Mycobacterium tuberculosis in the Form of Empyema Necessitans: A Case Report

We present a case of rapidly growing disseminated Mycobacterium tuberculosis (MTB) that presented as an empyema necessitans (EN) in a 65-year-old woman with a single right lung transplant admitted for progressive dyspnea. While hospitalized, she had daily fevers and was found to have a right-sided chest wall abscess and pleural effusion. Acid-fast bacilli cultures from the abscess and pleural fluid grew MTB within 4 and 6 days, respectively. Blood cultures later grew MTB as well. Upon initiation of rifampin, isoniazid, pyrazinamide, and ethambutol, she developed hemorrhagic pancreatitis and distributive shock secondary to antituberculosis medications and disseminated MTB. Noteworthy features of this case include the rapid rate of MTB culture growth in less than a week, the development of a likely donor-derived MTB EN, and the clinical challenges of MTB screening and MTB infection management in a solid organ transplant recipient.

A Murine Model of Mycobacterium kansasii Infection Reproducing Necrotic Lung Pathology Reveals Considerable Heterogeneity in Virulence of Clinical Isolates

Among non-tuberculous mycobacteria, Mycobacterium kansasii is one of the most pathogenic, able to cause pulmonary disease indistinguishable from tuberculosis in immunocompetent susceptible adults. The lack of animal models that reproduce human-like lung disease, associated with the necrotic lung pathology, impairs studies of M. kansasii virulence and pathogenicity. In this study, we examined the ability of the C57BL/6 mice, intratracheally infected with highly virulent M. kansasii strains, to produce a chronic infection and necrotic lung pathology. As a first approach, we evaluated ten M. kansasii strains isolated from Brazilian patients with pulmonary disease and the reference strain M. kansasii ATCC 12478 for virulence-associated features in macrophages infected in vitro; five of these strains differing in virulence were selected for in vivo analysis. Highly virulent isolates induced progressive lung disease in mice, forming large encapsulated caseous granulomas in later stages (120–150 days post-infection), while the low-virulent strain was cleared from the lungs by day 40. Two strains demonstrated increased virulence, causing premature death in the infected animals. These data demonstrate that C57BL/6 mice are an excellent candidate to investigate the virulence of M. kansasii isolates. We observed considerable heterogeneity in the virulence profile of these strains, in which the presence of highly virulent strains allowed us to establish a clinically relevant animal model. Comparing public genomic data between Brazilian isolates and isolates from other geographic regions worldwide demonstrated that at least some of the highly pathogenic strains isolated in Brazil display remarkable genomic similarities with the ATCC strain 12478 isolated in the United States 70 years ago (less than 100 SNPs of difference), as well as with some recent European clinical isolates. These data suggest that few pathogenic clones have been widely spread within M. kansasii population around the world.

The Role of microRNAs and Long Non-Coding RNAs in the Regulation of the Immune Response to Mycobacterium tuberculosis Infection

Non-coding RNAs have emerged as critical regulators of the immune response to infection. MicroRNAs (miRNAs) are small non-coding RNAs which regulate host defense mechanisms against viruses, bacteria and fungi. They are involved in the delicate interplay between Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), and its host, which dictates the course of infection. Differential expression of miRNAs upon infection with M. tuberculosis, regulates host signaling pathways linked to inflammation, autophagy, apoptosis and polarization of macrophages. Experimental evidence suggests that virulent M. tuberculosis often utilize host miRNAs to promote pathogenicity by restricting host-mediated antibacterial signaling pathways. At the same time, host- induced miRNAs augment antibacterial processes such as autophagy, to limit bacterial proliferation. Targeting miRNAs is an emerging option for host-directed therapies. Recent studies have explored the role of long non-coding RNA (lncRNAs) in the regulation of the host response to mycobacterial infection. Among other functions, lncRNAs interact with chromatin remodelers to regulate gene expression and also function as miRNA sponges. In this review we attempt to summarize recent literature on how miRNAs and lncRNAs are differentially expressed during the course of M. tuberculosis infection, and how they influence the outcome of infection. We also discuss the potential use of non-coding RNAs as biomarkers of active and latent tuberculosis. Comprehensive understanding of the role of these non-coding RNAs is the first step towards developing RNA-based therapeutics and diagnostic tools for the treatment of TB.

Mycobacterial origin protein Rv0674 localizes into mitochondria, interacts with D-loop and regulates OXPHOS for intracellular persistence of Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) employs diverse strategies to survive inside the host macrophages. In this study, we have identified a conserved hypothetical protein of Mtb; Rv0674, which is present in the mitochondria of the host cell. The genetic knock-out of rv0674 (Mtb-KO) showed increased growth of Mtb. The intracellular infection with recombinant Mycobacterium smegmatis (MSMEG) expressing Rv0674 (MS_Rv0674), established that the protein is involved in promoting the apoptotic cell death of the macrophage. To investigate the mechanism incurred in mitochondria, we observed that the protein physically interacts with the control region (D-loop) of the mitochondrial DNA (LSP and HSP promoters of the loop) of the macrophages and facilitates the increased expression of mRNA in all the complexes of mitochondrial encoded OXPHOS subunits. The changes in OXPHOS levels corroborated with the ATP synthesis, mitochondrial membrane potential and superoxide production. The infection with MS_Rv0674 confirmed the role of this protein in effecting the intracellular infection. The fluorescent and confocal microscopy confirmed that the protein is localized in the mitochondria of infected macrophages and in the cells of BAL of TB patients. Together these findings indicate towards the novel function of the protein which is unlike to the earlier established mechanisms of mycobacterial physiology.

Polyphosphate is an extracellular signal that can facilitate bacterial survival in eukaryotic cells

Polyphosphate is a linear chain of phosphate residues and is present in organisms ranging from bacteria to humans. Pathogens such as Mycobacterium tuberculosis accumulate polyphosphate, and reduced expression of the polyphosphate kinase that synthesizes polyphosphate decreases their survival. How polyphosphate potentiates pathogenicity is poorly understood. Escherichia coli K-12 do not accumulate detectable levels of extracellular polyphosphate and have poor survival after phagocytosis by Dictyostelium discoideum or human macrophages. In contrast, Mycobacterium smegmatis and Mycobacterium tuberculosis accumulate detectable levels of extracellular polyphosphate, and have relatively better survival after phagocytosis by D. discoideum or macrophages. Adding extracellular polyphosphate increased E. coli survival after phagocytosis by D. discoideum and macrophages. Reducing expression of polyphosphate kinase 1 in M. smegmatis reduced extracellular polyphosphate and reduced survival in D. discoideum and macrophages, and this was reversed by the addition of extracellular polyphosphate. Conversely, treatment of D. discoideum and macrophages with recombinant yeast exopolyphosphatase reduced the survival of phagocytosed M. smegmatis or M. tuberculosis D. discoideum cells lacking the putative polyphosphate receptor GrlD had reduced sensitivity to polyphosphate and, compared to wild-type cells, showed increased killing of phagocytosed E. coli and M. smegmatis Polyphosphate inhibited phagosome acidification and lysosome activity in D. discoideum and macrophages and reduced early endosomal markers in macrophages. Together, these results suggest that bacterial polyphosphate potentiates pathogenicity by acting as an extracellular signal that inhibits phagosome maturation.

Mycobacterium tuberculosis exploits host ATM kinase for survival advantage through SecA2 secretome

(Mtb) produces inflections in the host signaling networks to create a favorable milieu for survival. The virulent Mtb strain, Rv caused double strand breaks (DSBs), whereas the non-virulent Ra strain triggered single-stranded DNA generation. The effectors secreted by SecA2 pathway were essential and adequate for the genesis of DSBs. Accumulation of DSBs mediated through Rv activates ATM-Chk2 pathway of DNA damage response (DDR) signaling, resulting in altered cell cycle. Instead of the classical ATM-Chk2 DDR, Mtb gains survival advantage through ATM-Akt signaling cascade. Notably, in vivo infection with Mtb led to sustained DSBs and ATM activation during chronic phase of tuberculosis. Addition of ATM inhibitor enhances isoniazid mediated Mtb clearance in macrophages as well as in murine infection model, suggesting its utility for host directed adjunct therapy. Collectively, data suggests that DSBs inflicted by SecA2 secretome of Mtb provides survival niche through activation of ATM kinase.

Infection resisters: targets of new research for uncovering natural protective immunity against Mycobacterium tuberculosis

“Infection resisters” are broadly defined as individuals who despite significant exposure to Mycobacterium tuberculosis remain persistently unreactive to conventional detection assays, suggesting that they remain uninfected or rapidly clear their infection early on following exposure. In this review, we highlight recent studies that point to underlying host immune mechanisms that could mediate this natural resistance. We also illustrate some additional avenues that are likely to be differently modulated in resisters and possess the potential to be targeted, ranging from early mycobacterial sensing leading up to subsequent killing. Emerging research in this area can be harnessed to provide valuable insights into the development of novel therapeutic and vaccine strategies against M. tuberculosis.

Autophagy in Pulmonary Innate Immunity

Autophagy is a major intracellular digestion system that delivers cytoplasmic components for degradation and recycling. In this capacity, autophagy plays an important role in maintaining cellular homeostasis by mediating the degradation of cellular macromolecules and dysfunctional organelles and regeneration of nutrients for cell growth. Autophagy is important in innate immunity, as it is responsible for the clearance of various pathogens. Deficiency of intracellular autophagy can result in exaggerated activation of the inflammasome. The latter is an innate immune complex that senses diverse pathogen-associated or danger-associated molecular patterns and activates the expression of inflammatory cytokines. In autophagy-deficient cells, accumulation of damaged organelles, misfolded proteins, and reactive oxygen species contribute to inflammasome activation. The lung is continuously exposed to pathogens from the environment, rendering it vulnerable to infection. The lung innate immune cells act as a crucial initial barrier against the continuous threat from pathogens. In this review, we will summarize recent findings on the regulation of autophagy and its inter-action with innate immunity, focusing on the lung.

Deciphering the virulence of Mycobacterium avium subsp. paratuberculosis isolates in animal macrophages using mathematical models

Understanding why pathogenic Mycobacterium avium subsp. paratuberculosis (Map) isolates cause disparate disease outcomes with differing magnitudes of severity is important in designing and implementing new control strategies. We applied a suite of mathematical models: i) general linear, ii) and neurofuzzy logic, to explain how the host of origin of several Map isolates, Map genotype, host, macrophage-based in vitro model and time post-infection contributed to the infection. A logistic growth ordinary differential equation (ODE) model was applied to estimate within macrophage growth rates for the different Map isolates. The models revealed different susceptibilities of bovine and ovine macrophages to Map infection and confirmed distinct virulence profiles for the isolates, judged by their ability to grow within macrophages. Ovine macrophages were able to internalize Map isolates more efficiently than bovine macrophages. While bovine macrophages were able to internalize Map isolates from cattle with more efficiency, ovine macrophages were more efficient in internalizing ovine isolates. Overall, Map isolates from goat and sheep grew minimally within macrophages or did not grow but were able to persist by maintaining its initial population. In contrast, the ability of the bovine isolates and the non-domesticated animal isolates to grow to higher CFU numbers within macrophages suggests that these isolates are more virulent than the sheep and goat isolates, or that these isolates are better adapted to infect domestic ruminants. Overall, our study confirms the different virulence levels for the Map isolates and susceptibility profiles of host macrophages, which is crucial in increasing our understanding of Map infection.

Virulence of Mycobacterium tuberculosis Clinical Isolates Is Associated With Sputum Pre-treatment Bacterial Load, Lineage, Survival in Macrophages, and Cytokine Response

It is uncertain whether differences in Mycobacterium tuberculosis (Mtb) virulence defined in vitro influence clinical tuberculosis pathogenesis, transmission, and mortality. We primarily used a macrophage lysis model to characterize the virulence of Mtb isolates collected from 153 Vietnamese adults with pulmonary tuberculosis. The virulence phenotypes were then investigated for their relationship with sputum bacterial load, bacterial lineages, bacterial growth, and cytokine responses in macrophages. Over 6 days of infection, 34 isolates (22.2%) showed low virulence (< 5% macrophages lysed), 46 isolates (30.1%) showed high virulence (≥90% lysis of macrophages), and 73 isolates (47.7%) were of intermediate virulence (5–90% macrophages lysed). Highly virulent isolates were associated with an increased bacterial load in patients' sputum before anti-tuberculosis therapy (P = 0.02). Isolate-dependent virulence phenotype was consistent in both THP-1 and human monocyte-derived macrophages. High virulence isolates survived better and replicated in macrophages one hundred fold faster than those with low virulence. Macrophages infected with high virulence isolates produced lower concentrations of TNF-α and IL-6 (P = 0.002 and 0.0005, respectively), but higher concentration of IL-1β (P = 5.1 × 10⁻⁵) compared to those infected with low virulence isolates. High virulence was strongly associated with East Asian/Beijing lineage [P = 0.002, Odd ratio (OR) = 4.32, 95% confident intervals (CI) 1.68–11.13]. The association between virulence phenotypes, bacterial growth, and proinflammatory cytokines in macrophages suggest the suppression of certain proinflammatory cytokines (TNF-α and IL-6) but not IL-1β allows better intracellular survival of highly virulent Mtb. This could result in rapid macrophage lysis and higher bacterial load in sputum of patients infected with high virulence isolates, which may contribute to the pathogenesis and success of the Beijing lineage.

Endonuclease G takes part in AIF-mediated caspase-independent apoptosis in Mycobacterium bovis-infected bovine macrophages

Mycobacterium bovis, the causative agent of bovine tuberculosis encodes different virulence mechanisms to survive inside of host cells. One of the possible outcomes in this host-pathogen interaction is cell death. Previous results from our group showed that M. bovis induces a caspase-independent apoptosis in bovine macrophages with the possible participation of apoptosis inducing factor mitochondria associated 1 (AIFM1/AIF), a flavoprotein that functions as a cell-death regulator. However, contribution of other caspase-independent cell death mediators in M. bovis-infected macrophages is not known. In this study, we aimed to further characterize M. bovis-induced apoptosis, addressing Endonuclease G (Endo G) and Poly (ADP-ribose) polymerase 1 (PARP-1). In order to accomplish our objective, we infected bovine macrophages with M. bovis AN5 (MOI 10:1). Analysis of M. bovis-infected nuclear protein extracts by immunoblot, identified a 15- and 43-fold increase in concentration of mitochondrial proteins AIF and Endo G respectively. Interestingly, pretreatment of M. bovis-infected macrophages with cyclosporine A, a mitochondrial permeability transition pore inhibitor, abolished AIF and Endo G nuclear translocation. In addition, it also decreased macrophage DNA fragmentation to baseline and caused a 26.2% increase in bacterial viability. We also demonstrated that PARP-1 protein expression in macrophages did not change during M. bovis infection. Furthermore, pretreatment of M. bovis-infected bovine macrophages with 3-aminobenzamide, a PARP-1 inhibitor, did not change the proportion of macrophage DNA fragmentation. Our results suggest participation of Endo G, but not PARP-1, in M. bovis-induced macrophage apoptosis. To the best of our knowledge this is the first report associating Endo G with caspase-independent apoptosis induced by a member of the Mycobacterium tuberculosis complex.

Variation in the Early Host-Pathogen Interaction of Bovine Macrophages with Divergent Mycobacterium bovis Strains in the United Kingdom

Bovine tuberculosis has been an escalating animal health issue in the United Kingdom since the 1980s, even though control policies have been in place for over 60 years. The importance of the genetics of the etiological agent, Mycobacterium bovis, in the reemergence of the disease has been largely overlooked. We compared the interaction between bovine monocyte-derived macrophages (bMDM) and two M. bovis strains, AF2122/97 and G18, representing distinct genotypes currently circulating in the United Kingdom. These M. bovis strains exhibited differences in survival and growth in bMDM. Although uptake was similar, the number of viable intracellular AF2122/97 organisms increased rapidly, while G18 growth was constrained for the first 24 h. AF2122/97 infection induced a greater transcriptional response by bMDM than G18 infection with respect to the number of differentially expressed genes and the fold changes measured. AF2122/97 infection induced more bMDM cell death, with characteristics of necrosis and apoptosis, more inflammasome activation, and a greater type I interferon response than G18. In conclusion, the two investigated M. bovis strains interact in significantly different ways with the host macrophage. In contrast to the relatively silent infection by G18, AF2122/97 induces greater signaling to attract other immune cells and induces host cell death, which may promote secondary infections of naive macrophages. These differences may affect early events in the host-pathogen interaction, including granuloma development, which could in turn alter the progression of the disease. Therefore, the potential involvement of M. bovis genotypes in the reemergence of bovine tuberculosis in the United Kingdom warrants further investigation.

Mycobacterium tuberculosis: Rewiring host cell signaling to promote infection

The ability of Mycobacterium tuberculosis to cause disease hinges upon successfully thwarting the innate defenses of the macrophage host cell. The pathogen's trump card is its armory of virulence factors that throw normal host cell signaling into disarray. This process of subverting the macrophage begins upon entry into the cell, when M. tuberculosis actively inhibits the fusion of the bacilli‐laden phagosomes with lysosomes. The pathogen then modulates an array of host signal transduction pathways, which dampens the macrophage's host‐protective cytokine response, while simultaneously adapting host cell metabolism to stimulate lipid body accumulation. Mycobacterium tuberculosis also renovates the surface of its innate host cells by altering the expression of key molecules required for full activation of the adaptive immune response. Finally, the pathogen coordinates its exit from the host cell by shifting the balance from the host‐protective apoptotic cell death program toward a lytic form of host cell death. Thus, M. tuberculosis exploits its extensive repertoire of virulence factors in order to orchestrate the infection process to facilitate its growth, dissemination, and entry into latency. This review offers critical insights into the most recent advances in our knowledge of how M. tuberculosis manipulates host cell signaling. An appreciation of such interactions between the pathogen and host is critical for guiding novel therapies and understanding the factors that lead to the development of active disease in only a subset of exposed individuals.

Necroptotic signaling is primed in Mycobacterium tuberculosis-infected macrophages, but its pathophysiological consequence in disease is restricted

Mixed lineage kinase domain-like (MLKL)-dependent necroptosis is thought to be implicated in the death of mycobacteria-infected macrophages, reportedly allowing escape and dissemination of the microorganism. Given the consequent interest in developing inhibitors of necroptosis to treat Mycobacterium tuberculosis (Mtb) infection, we used human pharmacologic and murine genetic models to definitively establish the pathophysiological role of necroptosis in Mtb infection. We observed that Mtb infection of macrophages remodeled the intracellular signaling landscape by upregulating MLKL, TNFR1, and ZBP1, whilst downregulating cIAP1, thereby establishing a strong pro-necroptotic milieu. However, blocking necroptosis either by deleting Mlkl or inhibiting RIPK1 had no effect on the survival of infected human or murine macrophages. Consistent with this, MLKL-deficiency or treatment of humanized mice with the RIPK1 inhibitor Nec-1s did not impact on disease outcomes in vivo, with mice displaying lung histopathology and bacterial burdens indistinguishable from controls. Therefore, although the necroptotic pathway is primed by Mtb infection, macrophage necroptosis is ultimately restricted to mitigate disease pathogenesis. We identified cFLIP upregulation that may promote caspase 8-mediated degradation of CYLD, and other necrosome components, as a possible mechanism abrogating Mtb’s capacity to coopt necroptotic signaling. Variability in the capacity of these mechanisms to interfere with necroptosis may influence disease severity and could explain the heterogeneity of Mtb infection and disease.

Susceptibility of Mycobacterium tuberculosis-infected host cells to phospho-MLKL driven necroptosis is dependent on cell type and presence of TNFα

An important feature of Mycobacterium tuberculosis pathogenesis is the ability to control cell death in infected host cells, including inhibition of apoptosis and stimulation of necrosis. Recently an alternative form of programmed cell death, necroptosis, has been described where necrotic cell death is induced by apoptotic stimuli under conditions where apoptotic execution is inhibited. We show for the first time that M. tuberculosis and TNFα synergise to induce necroptosis in murine fibroblasts via RIPK1-dependent mechanisms and characterized by phosphorylation of Ser345 of the MLKL necroptosis death effector. However, in murine macrophages M. tuberculosis and TNFα induce non-necroptotic cell death that is RIPK1-dependent but independent of MLKL phosphorylation. Instead, M. tuberculosis-infected macrophages undergo RIPK3-dependent cell death which occurs both in the presence and absence of TNFα and involves the production of mitochondrial ROS. Immunocytochemical staining for MLKL phosphorylation further demonstrated the occurrence of necroptosis in vivo in murine M. tuberculosis granulomas. Phosphorylated-MLKL immunoreactivity was observed associated with the cytoplasm and nucleus of fusiform cells in M. tuberculosis lesions but not in proximal macrophages. Thus whereas pMLKL-driven necroptosis does not appear to be a feature of M. tuberculosis-infected macrophage cell death, it may contribute to TNFα-induced cytotoxicity of the lung stroma and therefore contribute to necrotic cavitation and bacterial dissemination.

Intracellular growth of Mycobacterium tuberculosis after macrophage cell death leads to serial killing of host cells

A hallmark of pulmonary tuberculosis is the formation of macrophage-rich granulomas. These may restrict Mycobacterium tuberculosis (Mtb) growth, or progress to central necrosis and cavitation, facilitating pathogen growth. To determine factors leading to Mtb proliferation and host cell death, we used live cell imaging to track Mtb infection outcomes in individual primary human macrophages. Internalization of Mtb aggregates caused macrophage death, and phagocytosis of large aggregates was more cytotoxic than multiple small aggregates containing similar numbers of bacilli. Macrophage death did not result in clearance of Mtb. Rather, it led to accelerated intracellular Mtb growth regardless of prior activation or macrophage type. In contrast, bacillary replication was controlled in live phagocytes. Mtb grew as a clump in dead cells, and macrophages which internalized dead infected cells were very likely to die themselves, leading to a cell death cascade. This demonstrates how pathogen virulence can be achieved through numbers and aggregation states.

DOI:http://dx.doi.org/10.7554/eLife.22028.001

Every year, around two million people worldwide die from tuberculosis, a disease caused by the bacterium Mycobacterium tuberculosis (Mtb). The bacteria generally infect the lungs. In response, the immune system forms structures called granulomas that attempt to control and isolate the infecting pathogens.

Granulomas consist of immune cells known as macrophages, which engulf the M. tuberculosis bacteria and isolate them in a cellular compartment where the bacteria either cannot grow or are killed. However, if a large number of macrophages in a granuloma die, the granuloma’s core liquefies and the structure is coughed up into the airways, from where M. tuberculosis bacteria are transmitted to other people. But how do the bacteria manage to cause the extensive death of the cells that are supposed to control the infection?

By imaging M. tuberculosis in human macrophages using time-lapse microscopy, Mahamed et al. reveal that the bacteria break down macrophage control by serially killing macrophages. M. tuberculosis cells first clump together and ‘gang up’ on a macrophage, which engulfs the clump and dies because the bacteria overwhelm it. This does not kill the bacteria, and they rapidly grow inside the dead macrophage. The dead cell is then cleaned up by another macrophage. However, the increasing number of bacteria inside the dead macrophage means that the new macrophage is even more likely to die than the first one. Hence, the bacteria use dead macrophages as fuel to grow on and as bait to attract the next immune cell.

Overall, Mahamed et al. show that once a clump of M. tuberculosis initiates death of a single macrophage, it may lead to serial killing of other macrophages and a loss of control over the infection. An important next step will be to understand how the initial clump of bacteria is allowed to form.

DOI:http://dx.doi.org/10.7554/eLife.22028.002

Mycobacterium avium MAV2054 protein induces macrophage apoptosis by targeting mitochondria and reduces intracellular bacterial growth

Mycobacterium avium complex induces macrophage apoptosis. However, the M. avium components that inhibit or trigger apoptosis and their regulating mechanisms remain unclear. We recently identified the immunodominant MAV2054 protein by fractionating M. avium culture filtrate protein by multistep chromatography; this protein showed strong immuno-reactivity in M. avium complex pulmonary disease and in patients with tuberculosis. Here, we investigated the biological effects of MAV2054 on murine macrophages. Recombinant MAV2054 induced caspase-dependent macrophage apoptosis. Enhanced reactive oxygen species production and JNK activation were essential for MAV2054-mediated apoptosis and MAV2054-induced interleukin-6, tumour necrosis factor, and monocyte chemoattractant protein-1 production. MAV2054 was targeted to the mitochondrial compartment of macrophages treated with MAV2054 and infected with M. avium. Dissipation of the mitochondrial transmembrane potential (ΔΨ_m) and depletion of cytochrome c also occurred in MAV2054-treated macrophages. Apoptotic response, reactive oxygen species production, and ΔΨ_m collapse were significantly increased in bone marrow-derived macrophages infected with Mycobacterium smegmatis expressing MAV2054, compared to that in M. smegmatis control. Furthermore, MAV2054 expression suppressed intracellular growth of M. smegmatis and increased the survival rate of M. smegmatis-infected mice. Thus, MAV2054 induces apoptosis via a mitochondrial pathway in macrophages, which may be an innate cellular response to limit intracellular M. avium multiplication.

Targeting mitochondria: how intravacuolar bacterial pathogens manipulate mitochondria

Manipulation of host cell function by bacterial pathogens is paramount for successful invasion and creation of a niche conducive to bacterial replication. Mitochondria play a role in many important cellular processes including energy production, cellular calcium homeostasis, lipid metabolism, haeme biosynthesis, immune signalling and apoptosis. The sophisticated integration of host cell processes by the mitochondrion have seen it emerge as a key target during bacterial infection of human host cells. This review highlights the targeting and interaction of this dynamic organelle by intravacuolar bacterial pathogens and the way that the modulation of mitochondrial function might contribute to pathogenesis.

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.

Macrophage takeover and the host-bacilli interplay during tuberculosis

Macrophages are key type of antigen-presenting cells that arbitrate the first line of defense against various intracellular pathogens. Tuberculosis, both pulmonary and extrapulmonary, is an infectious disease of global concern caused by Mycobacterium tuberculosis. The bacillus is a highly successful pathogen and has acquired various strategies to downregulate critical innate-effector immune responses of macrophages, such as phagosome-lysosome fusion, autophagy, induction of cytokines, generation of reactive oxygen and nitrogen species and antigen presentation. In addition, the bacilli also subvert acquired immunity. In this review, we aim to provide an overview of different antimycobacterial immune functions of macrophage and the strategies adopted by the bacilli to manipulate these functions to favor its survival and replication inside the host.

A Role for IFITM Proteins in Restriction of Mycobacterium tuberculosis Infection

The interferon (IFN)-induced transmembrane (IFITM) proteins are critical mediators of the host antiviral response. Here, we expand the role of IFITM proteins to host defense against intracellular bacterial infection by demonstrating that they restrict Mycobacterium tuberculosis (MTb) intracellular growth. Simultaneous knockdown of IFITM1, IFITM2, and IFITM3 by RNAi significantly enhances MTb growth in human monocytic and alveolar/epithelial cells, whereas individual overexpression of each IFITM impairs MTb growth in these cell types. Furthermore, MTb infection, Toll-like receptor 2 and 4 ligands, and several proinflammatory cytokines induce IFITM1–3 gene expression in human myeloid cells. We find that IFITM3 co-localizes with early and, in particular, late MTb phagosomes, and overexpression of IFITM3 enhances endosomal acidification in MTb-infected monocytic cells. These findings provide evidence that the antiviral IFITMs participate in the restriction of mycobacterial growth, and they implicate IFITM-mediated endosomal maturation in its antimycobacterial activity.

Evidence for genes associated with the ability of Mycobacterium avium subsp. hominissuis to escape apoptotic macrophages

Mycobacterium avium subsp. hominissuis (MAH) is an environmental bacteria that infects immunocompromised humans. MAH cases are increasing in incidence, making it crucial to gain knowledge of the pathogenic mechanisms associated with the bacterium. MAH infects macrophages and after several days the infection triggers the phagocyte apoptosis. Many of the intracellular MAH escape the cell undergoing apoptosis leading to infection of neighboring macrophages. We screened a transposon bank of MAH mutants in U937 mononuclear phagocytes for the inability to escape macrophages undergoing apoptosis. Mutations in genes; MAV_2235, MAV_2120, MAV_2410, and MAV_4563 resulted in the inability of the bacteria to exit macrophages upon apoptosis. Complementation of the mutations corrected the phenotype either completely or partially. Testing for the ability of the mutants to survive in macrophages compared to the wild-type bacterium revealed that the mutant clones were not attenuated up to 4 days of infection. Testing in vivo, however, demonstrated that all the MAH clones were attenuated compared with the wild-type MAC 104 in tissues of mice. Although the mechanism associated with the bacterial inability to leave apoptotic macrophages is unknown, the identification of macrophage cytoplasm targets for the MAH proteins suggest that they interfere either with protein degradation machinery or post-translation mechanisms. The identification of tatC as a MAH protein involved in the ability of MAH to leave macrophages, suggests that secreted effector(s) are involved in the process. The study reveals a pathway of escape from macrophages, not shared with Mycobacterium tuberculosis.

Selective enrichment of mycobacterial proteins from infected host macrophages

Upon infection, Mycobacterium tuberculosis (Mtb) deploys specialized secretion machinery to deliver virulent proteins with the capacity to modulate a variety of host-cellular pathways. Studies on the identification of intra-macrophage Mtb proteins, however, are constricted by an inability to selectively enrich these virulent effectors against overwhelming protein content of the host. Here, we introduce an Mtb-selective protein labeling method based on genetic incorporation of azidonorleucine (Anl) through the expression of a mutant methionyl-tRNA synthetase. Exclusive incorporation of Anl, into native Mtb proteins, provided a click handle to pull out low abundant secretory proteins from the lysates of infected cells. Further, temporal secretome profiling, upon infection with strains of varying degree of virulence, revealed the proficiency of virulent Mtb to secrete chaperones. This ability contributed at least partially to the mycobacterial virulence-specific suppression of ER stress in the host macrophage, representing an important facet of mycobacterial virulence. The Anl labeling approach should facilitate new exciting opportunities for imaging and proteomic investigations of differently virulent Mtb isolates to understand determinants of pathogenicity.

Functional drug screening reveals anticonvulsants as enhancers of mTOR-independent autophagic killing of Mycobacterium tuberculosis through inositol depletion

Mycobacterium tuberculosis (MTB) remains a major challenge to global health made worse by the spread of multidrug resistance. We therefore examined whether stimulating intracellular killing of mycobacteria through pharmacological enhancement of macroautophagy might provide a novel therapeutic strategy. Despite the resistance of MTB to killing by basal autophagy, cell-based screening of FDA-approved drugs revealed two anticonvulsants, carbamazepine and valproic acid, that were able to stimulate autophagic killing of intracellular M. tuberculosis within primary human macrophages at concentrations achievable in humans. Using a zebrafish model, we show that carbamazepine can stimulate autophagy in vivo and enhance clearance of M. marinum, while in mice infected with a highly virulent multidrug-resistant MTB strain, carbamazepine treatment reduced bacterial burden, improved lung pathology and stimulated adaptive immunity. We show that carbamazepine induces antimicrobial autophagy through a novel, evolutionarily conserved, mTOR-independent pathway controlled by cellular depletion of myo-inositol. While strain-specific differences in susceptibility to in vivo carbamazepine treatment may exist, autophagy enhancement by repurposed drugs provides an easily implementable potential therapy for the treatment of multidrug-resistant mycobacterial infection.

Bacillary replication and macrophage necrosis are determinants of neutrophil recruitment in tuberculosis

We previously determined that burst size necrosis is the chief mode of mononuclear cell death in the lungs of mice with tuberculosis. The present study explored the link between infection-induced necrosis of mononuclear phagocytes and neutrophil accumulation in the lungs of mice challenged with one of four Mycobacterium tuberculosis strains of increasing virulence (RvΔphoPR mutant, H37Ra, H37Rv and Erdman). At all time points studied, Erdman produced the highest bacterial load and the highest proportion and number of M. tuberculosis-infected neutrophils. These parameters, and the proportion of TUNEL-positive cells, tracked with virulence across all strains tested. Differences in neutrophil infection were not reflected by levels of chemoattractant cytokines in bronchoalveolar lavage fluid, while interferon-γ (reported to suppress neutrophil trafficking to the lung in tuberculosis) was highest in Erdman-infected mice. Treating Erdman-infected mice with ethambutol decreased the proportion of mononuclear phagocytes with high bacterial burden and the ratio of infected neutrophils to infected mononuclear cells in a dose-dependent manner. We propose that faster replicating M. tuberculosis strains cause more necrosis which in turn promotes neutrophil recruitment. Neutrophils infected with M. tuberculosis constitute a biomarker for poorly controlled bacterial replication, infection-induced mononuclear cell death, and increased severity of immune pathology in tuberculosis.

Role of cytokines and other factors involved in the Mycobacterium tuberculosis infection

Mycobacterium tuberculosis (Mtb) is a pathogen that is widely distributed geographically and continues to be a major threat to world health. Bacterial virulence factors, nutritional state, host genetic condition and immune response play an important role in the evolution of the infection. The genetically diverse Mtb strains from different lineages have been shown to induce variable immune system response. The modern and ancient lineages strains induce different cytokines patterns. The immunity to Mtb depends on Th1-cell activity [interferon-γ (IFN-γ), interleukin-12 (IL-12) and tumor necrosis factor-α (TNF-α)]. IL-1β directly kills Mtb in murine and human macrophages. IL-6 is a requirement in host resistance to Mtb infection. IFN-γ, TNF-α, IL-12 and IL-17 are participants in Mycobacterium-induced granuloma formation. Other regulating proteins as IL-27 and IL-10 can prevent extensive immunopathology. CXCL 8 enhances the capacity of the neutrophil to kill Mtb. CXCL13 and CCL19 have been identified as participants in the formation of granuloma and control the Mtb infection. Treg cells are increased in patients with active tuberculosis (TB) but decrease with anti-TB treatment. The increment of these cells causes down- regulation of adaptive immune response facilitating the persistence of the bacterial infection. Predominance of Th2 phenotype cytokines increases the severity of TB. The evolution of the Mtb infection will depend of the cytokines network and of the influence of other factors aforementioned.

Duality of lipid mediators in host response against Mycobacterium tuberculosis: good cop, bad cop

Lipid mediators play an important role in infection- and tissue injury-driven inflammatory responses and in the subsequent inhibition and resolution of the response. Here, we discuss recent findings that substantiate how Mycobacterium tuberculosis promotes its survival in the host by dysregulation of lipid mediator balance. By inhibiting prostaglandin E2 (PGE2) and enhancing lipoxin production, M. tuberculosis induces necrotic death of the macrophage, an environment that favors its growth. These new findings provide opportunities for developing and repurposing therapeutics to modulate lipid mediator balance and enhance M. tuberculosis growth restriction.

Macrophage defense mechanisms against intracellular bacteria

Macrophages and neutrophils play a decisive role in host responses to intracellular bacteria including the agent of tuberculosis (TB), Mycobacterium tuberculosis as they represent the forefront of innate immune defense against bacterial invaders. At the same time, these phagocytes are also primary targets of intracellular bacteria to be abused as host cells. Their efficacy to contain and eliminate intracellular M. tuberculosis decides whether a patient initially becomes infected or not. However, when the infection becomes chronic or even latent (as in the case of TB) despite development of specific immune activation, phagocytes have also important effector functions. Macrophages have evolved a myriad of defense strategies to combat infection with intracellular bacteria such as M. tuberculosis. These include induction of toxic anti-microbial effectors such as nitric oxide and reactive oxygen intermediates, the stimulation of microbe intoxication mechanisms via acidification or metal accumulation in the phagolysosome, the restriction of the microbe's access to essential nutrients such as iron, fatty acids, or amino acids, the production of anti-microbial peptides and cytokines, along with induction of autophagy and efferocytosis to eliminate the pathogen. On the other hand, M. tuberculosis, as a prime example of a well-adapted facultative intracellular bacterium, has learned during evolution to counter-balance the host's immune defense strategies to secure survival or multiplication within this otherwise hostile environment. This review provides an overview of innate immune defense of macrophages directed against intracellular bacteria with a focus on M. tuberculosis. Gaining more insights and knowledge into this complex network of host-pathogen interaction will identify novel target sites of intervention to successfully clear infection at a time of rapidly emerging multi-resistance of M. tuberculosis against conventional antibiotics.

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.

Quantitative Proteomics and Lipidomics Analysis of Endoplasmic Reticulum of Macrophage Infected with Mycobacterium tuberculosis

Even though endoplasmic reticulum (ER) stress associated with mycobacterial infection has been well studied, the molecular basis of ER as a crucial organelle to determine the fate of Mtb is yet to be established. Here, we have studied the ability of Mtb to manipulate the ultrastructural architecture of macrophage ER and found that the ER-phenotypes associated with virulent (H37Rv) and avirulent (H37Ra) strains were different: a rough ER (RER) with the former against a smooth ER (SER) with the later. Further, the functional attributes of these changes were probed by MS-based quantitative proteomics (133 ER proteins) and lipidomics (8 phospholipids). Our omics approaches not only revealed the host pathogen cross-talk but also emphasized how precisely Mtb uses proteins and lipids in combination to give rise to characteristic ER-phenotypes. H37Ra-infected macrophages increased the cytosolic Ca²⁺ levels by attenuating the ATP2A2 protein and simultaneous induction of PC/PE expression to facilitate apoptosis. However, H37Rv inhibited apoptosis and further controlled the expression of EST-1 and AMRP proteins to disturb cholesterol homeostasis resulting in sustained infection. This approach offers the potential to decipher the specific roles of ER in understanding the cell biology of mycobacterial infection with special reference to the impact of host response.

Cell death and autophagy in tuberculosis

Mycobacterium tuberculosis has succeeded in infecting one-third of the human race though inhibition or evasion of innate and adaptive immunity. The pathogen is a facultative intracellular parasite that uses the niche provided by mononuclear phagocytes for its advantage. Complex interactions determine whether the bacillus will or will not be delivered to acidified lysosomes, whether the host phagocyte will survive infection or die, and whether the timing and mode of cell death works to the advantage of the host or the pathogen. Here we discuss cell death and autophagy in TB. These fundamental processes of cell biology feature in all aspects of TB pathogenesis and may be exploited to the treatment or prevention of TB disease.

Differential activation of dendritic cells by Mycobacterium tuberculosis Beijing genotype

Mycobacterium tuberculosis (Mtb) inhibits dendritric cells (DC) function in order to delay T cell response. Furthermore, there is increasing evidence that genetic diversity of Mtb strains can affect their interaction with the immune system. Beijing genotype has attracted attention because of its high prevalence and multi-drug resistance. Although it is known that this genotype is hypervirulent and differentially activates macrophages when compared to other genotypes, little is known about its interaction with DC. In order to address this issue, murine bone marrow derived DC (BMDC) were stimulated with soluble extracts (SE) from BCG, H37Rv, Canetti and Beijing genotypes. We observed that unlike other mycobacteria strains, SE-Beijing was unable to induce maturation of DC as assessed by cell surface MHC-II expression. DC stimulated with SE-Beijing failed to produce IL-12 and TNF-α, but did secrete IL-10. Interestingly, SE-Beijing induced CCR7 and PDL-1 on BMDC, but did not induce the expression of CD86. When BMDC stimulated with SE-Beijing were used to activate CD4+ cells they were unable to induce a Th1 response when compared with less virulent genotypes. These results indicate that Beijing is able to modulate DC activation and function, which may be related to the pathogenesis induced by this genotype.

Cell death paradigms in the pathogenesis of Mycobacterium tuberculosis infection

Cell death or senescence is a fundamental event that helps maintain cellular homeostasis, shapes the growth of organism, and provides protective immunity against invading pathogens. Decreased or increased cell death is detrimental both in infectious and non-infectious diseases. Cell death is executed both by regulated enzymic reactions and non-enzymic sudden collapse. In this brief review we have tried to summarize various cell death modalities and their impact on the pathogenesis of Mycobacterium tuberculosis.

Leishmania donovani prevents oxidative burst-mediated apoptosis of host macrophages through selective induction of suppressors of cytokine signaling (SOCS) proteins

One of the mechanisms for establishment of infection employed by intra-macrophage pathogen-like Leishmania is inhibition of oxidative burst-mediated macrophage apoptosis to protect their niche for survival and replication. We tried to elucidate the underlying mechanism for this by using H2O2 for induction of apoptosis. Leishmania donovani-infected macrophages were much more resistant to H2O2-mediated apoptosis compared with control. Although infected cells were capable of comparable reactive oxygen species production, there was less activation of the downstream cascade consisting of caspase-3 and -7 and cleaved poly(ADP)-ribose polymerase. Suppressors of cytokine signaling (SOCS) 1 and 3 proteins and reactive oxygen species scavenging enzyme thioredoxin, known to be involved in stabilization of protein-tyrosine phosphatases, were found to be induced during infection. Induction of SOCS proteins may be mediated by Egr1, and silencing of Socs1 and -3 either alone or in combination resulted in reduced thioredoxin levels, enhanced activation of caspases, and increased apoptosis of infected macrophages. The induction of protein-tyrosine phosphatases, thioredoxin, SOCS, and Egr1 in L. donovani-infected macrophages was found to be unaffected by H2O2 treatment. SOCS knocked down cells also displayed decreased parasite survival thus marking reduction in disease progression. Taken together, these results suggest that L. donovani may exploit SOCS for subverting macrophage apoptotic machinery toward establishing its replicative niche inside the host.

Different responses of human mononuclear phagocyte populations to Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) infects different populations of macrophages. Alveolar macrophages (AMs) are initially infected, and their response may contribute to controlling Mtb infection and dissemination. However, Mtb infection may disseminate to other tissues, infecting a wide variety of macrophages. Given the difficulty in obtaining AMs, monocyte-derived macrophages (MDMs) are used to model macrophage-mycobacteria interactions in humans. However, the response of other tissue macrophages to Mtb infection has been poorly explored. We have compared MDMs, AMs and splenic human macrophages (SMs) for their in vitro capacity to control Mtb growth, cytokine production, and induction of cell death in response to Mtb H37Rv, and the Colombian isolate UT205, and to the virulence factor ESAT-6. Significant differences in the magnitude of cell death and cytokine production depending mainly on the Mtb strain were observed; however, no major differences in the mycobacteriostatic/mycobacteriocidal activity were detected among the macrophage populations. Infection with the clinical isolate UT205 was associated with an increased cell death with membrane damage, particularly in IFNγ-treated SMs and H37Rv induced a higher production of cytokines compared to UT205. These results are concordant with the interpretation of a differential response to Mtb infection mainly depending upon the strain of Mtb.

Mycobacterium kansasii-induced death of murine macrophages involves endoplasmic reticulum stress responses mediated by reactive oxygen species generation or calpain activation

Although pathogenic mechanisms of tuberculosis have been extensively studied, little is known about the pathogenic mechanisms of Mycobacterium kansasii. In this work the influence of virulence and ER-stress mediated apoptosis of macrophages during two different strains of M. kansasii infection was investigated. We show that M. kansasii infection is associated with ER stress-mediated apoptosis in the murine macrophage cell line RAW 264.7. Infection of RAW 264.7 cells in vitro with apoptosis-inducing a clinical isolate of M. kansasii SM-1 (SM-1) resulted in strong induction of ER stress responses compared with M. kansasii type strain (ATCC 12478)-infected RAW 264.7 cells. Interestingly, inhibition of calpain prevented the induction of CHOP and Bip in ATCC 12478-infected RAW 264.7 cells but not in RAW 264.7 cells infected with SM-1. In contrast, reactive oxygen species (ROS) were significantly increased only in RAW 264.7 cells infected with SM-1. We propose that ROS generation is important for triggering ER stress-mediated apoptosis during SM-1 infection, whereas ATCC 12478-induced, ER stress-mediated apoptosis is associated with calpain activation. Our results demonstrate that the ER stress pathway plays important roles in the pathogenesis of M. kansasii infections, and that different strains of M. kansasii induce different patterns of ER stress-mediated apoptosis.

Insights into the origin, emergence, and current spread of a successful Russian clone of Mycobacterium tuberculosis

Mycobacterium tuberculosis variant Beijing B0/W148 is regarded as a successful clone of M. tuberculosis that is widespread in the former Soviet Union and respective immigrant communities. Understanding the pathobiology and phylogeography of this notorious strain may help to clarify its origin and evolutionary history and the driving forces behind its emergence and current dissemination. I present the first review and analysis of all available data on the subject. In spite of the common perception of the omnipresence of B0/W148 across post-Soviet countries, its geographic distribution shows a peculiar clinal gradient. Its frequency peaks in Siberian Russia and, to a lesser extent, in the European part of the former Soviet Union. In contrast, the frequency of B0/W148 is sharply decreased in the Asian part of the former Soviet Union, and it is absent in autochthonous populations elsewhere in the world. Placing the molecular, clinical, and epidemiological features in a broad historical, demographic, and ecological context, I put forward two interdependent hypotheses. First, B0/W148 likely originated in Siberia, and its primary dispersal was driven by a massive population outflow from Siberia to European Russia in the 1960s to 1980s. Second, a historically recent, phylogenetically demonstrated successful dissemination of the Beijing B0/W148 strain was triggered by the advent and wide use of modern antituberculosis (anti-TB) drugs and was due to the remarkable capacity of this strain to acquire drug resistance. In contrast, there is some indication, but not yet systematic proof, of an enhanced virulence of this strain.

A new unifying theory of the pathogenesis of tuberculosis

It is set in stone that Mycobacterium tuberculosis is a facultative intracellular bacterial parasite. This axiom drives our knowledge of the host response, the way we design vaccines against the organism by generating protective T cells, and to a lesser extent, the way we try to target anti-microbial drugs. The purpose of this article is to commit total heresy. I believe that M. tuberculosis can equally well be regarded as an extracellular pathogen and may in fact spend a large percentage of its human lung "life-cycle" in this environment. It is of course intracellular as well, but this may well be little more than a brief interlude after infection of a new host during which the bacterium must replicate to increase its chances of transmission and physiologically adapt prior to moving back to an extracellular phase. As a result, by focusing almost completely on just the intracellular phase, we may be making serious strategic errors in the way we try to intervene in this pathogenic process. It is my opinion that when a TB bacillus enters the lungs and starts to reside inside an alveolar macrophage, its central driving force is to switch on a process leading to lung necrosis, since it is only by this process that the local lung tissue can be destroyed and the bacillus can be exhaled and transmitted. I present here a new model of the pathogenesis of the disease that attempts to unify the pathogenic process of infection, disease, persistence [rather than latency], and reactivation.

Mycobacterium tuberculosis nucleoside diphosphate kinase inactivates small GTPases leading to evasion of innate immunity

Defining the mechanisms of Mycobacterium tuberculosis (Mtb) persistence in the host macrophage and identifying mycobacterial factors responsible for it are keys to better understand tuberculosis pathogenesis. The emerging picture from ongoing studies of macrophage deactivation by Mtb suggests that ingested bacilli secrete various virulence determinants that alter phagosome biogenesis, leading to arrest of Mtb vacuole interaction with late endosomes and lysosomes. While most studies focused on Mtb interference with various regulators of the endosomal compartment, little attention was paid to mechanisms by which Mtb neutralizes early macrophage responses such as the NADPH oxidase (NOX2) dependent oxidative burst. Here we applied an antisense strategy to knock down Mtb nucleoside diphosphate kinase (Ndk) and obtained a stable mutant (Mtb Ndk-AS) that displayed attenuated intracellular survival along with reduced persistence in the lungs of infected mice. At the molecular level, pull-down experiments showed that Ndk binds to and inactivates the small GTPase Rac1 in the macrophage. This resulted in the exclusion of the Rac1 binding partner p67^phox from phagosomes containing Mtb or Ndk-coated latex beads. Exclusion of p67^phox was associated with a defect of both NOX2 assembly and production of reactive oxygen species (ROS) in response to wild type Mtb. In contrast, Mtb Ndk-AS, which lost the capacity to disrupt Rac1-p67^phox interaction, induced a strong ROS production. Given the established link between NOX2 activation and apoptosis, the proportion of Annexin V positive cells and levels of intracellular active caspase 3 were significantly higher in cells infected with Mtb Ndk-AS compared to wild type Mtb. Thus, knock down of Ndk converted Mtb into a pro-apoptotic mutant strain that has a phenotype of increased susceptibility to intracellular killing and reduced virulence in vivo. Taken together, our in vitro and in vivo data revealed that Ndk contributes significantly to Mtb virulence via attenuation of NADPH oxidase-mediated host innate immunity.

Mycobacterium tuberculosis (Mtb) is a very successful intracellular pathogen that infects lung macrophages. Its resistance to intracellular killing has been linked to the development of pulmonary tuberculosis (TB) in humans. Thus, understanding the mechanism by which Mycobacterium tuberculosis (Mtb) persists in the host is a prerequisite for development of efficient strategies to control TB disease. We have previously shown that Mtb nucleoside diphosphate kinase (Ndk) contributes to phagosome maturation arrest via inactivation of Rab5 and Rab7. In this study, we show that Ndk also targets and inactivates the small GTPase Rac1, an essential component of the macrophage NADPH oxidase (NOX2) complex. Ndk-dependent inactivation of Rac1 was associated with reduced NOX2-mediated production of reactive oxygen species (ROS) and ROS-dependent apoptosis. Conversely, disruption of Ndk expression converted Mtb into a mutant strain that induces strong ROS and apoptosis responses. This phenotype was associated with reduced survival of Ndk mutant in vitro and in vivo. Altogether, our findings demonstrate that Ndk contributes significantly to mycobacterial virulence.

The prostaglandin E2 receptor EP4 is integral to a positive feedback loop for prostaglandin E2 production in human macrophages infected with Mycobacterium tuberculosis

Prostaglandin E2 (PGE2) is an important biological mediator involved in the defense against Mycobacterium tuberculosis (Mtb) infection. Previously, we reported that in macrophages (Mϕs), infection with avirulent Mtb H37Ra resulted in inhibition of necrosis by an inhibitory effect on mitochondrial permeability transition via the PGE2 receptor EP2. However, human Mϕs also express EP4, a PGE2 receptor functionally closely related to EP2 that also couples to stimulatory guanine nucleotide binding protein, but the functional differences between EP2 and EP4 in Mtb-infected Mϕs have been unclear. EP4 antagonist addition to H37Ra-infected Mϕs inhibited the expression of cyclooxygenase 2 (COX2) and microsomal prostaglandin E synthase-1 (mPGES-1), which are involved in PGE2 production. Moreover, H37Ra infection induced PGE2 production through the Toll-like receptor (TLR) 2/p38 mitogen-activated protein kinase (MAPK) signaling pathway. Induction of COX2 and mPGES-1 expression by TLR2 stimulation or Mtb infection was increased after additional stimulation with EP4 agonist. Hence, in Mtb-infected Mϕs, PGE2 production induced by pathogen recognition receptors/p38 MAPK signaling is up-regulated by EP4-triggered signaling to maintain an effective PGE2 concentration.

Nitric oxide not apoptosis mediates differential killing of Mycobacterium bovis in bovine macrophages

To identify the resistance phenotype against Mycobacterium bovis in cattle, we used a bactericidal assay that has been considered a marker of this trait. Three of 24 cows (12.5%) were phenotyped as resistant and 21 as susceptible. Resistance of bovine macrophages (MΦ) to BCG challenge was evaluated for its association with SLC11A1 GT microsatellite polymorphisms within 3'UTR region. Twenty-three cows (95.8%) had a GT13 genotype, reported as resistant, consequently the SLC11A1 polymorphism was not in agreement with our bactericidal assay results. MΦ of cows with resistant or susceptible phenotype were challenged in vitro with virulent M. bovis field strain or BCG, and nitric oxide production, bacterial killing and apoptosis induction were measured in resting and LPS-primed states. M. bovis field strain induced more apoptosis than BCG, although the difference was not significant. Resistant MΦ controlled better the replication of M. bovis (P<0.01), produced more nitric oxide (P<0.05) and were slightly more prone to undergo apoptosis than susceptible cells. LPS pretreatment of MΦ enhanced all the functional parameters analyzed. Inhibition of nitric oxide production with n (G)-monomethyl-L-arginine monoacetate enhanced replication of M. bovis but did not modify apoptosis rates in both resistant and susceptible MΦ. We conclude that nitric oxide production not apoptosis is a major determinant of macrophage resistance to M. bovis infection in cattle and that the influence of SLC11A1 gene 3'UTR polymorphism is not associated with this event.