Postprimary Tuberculosis and Macrophage Necrosis: Is There a Big ConNECtion?

Cellulase Promotes Mycobacterial Biofilm Dispersal in Response to a Decrease in the Bacterial Metabolite Gamma-Aminobutyric Acid

Biofilm dispersal contributes to bacterial spread and disease transmission. However, its exact mechanism, especially that in the pathogen Mycobacterium tuberculosis, is unclear. In this study, the cellulase activity of the M. tuberculosis Rv0062 protein was characterized, and its effect on mycobacterial biofilm dispersal was analyzed by observation of the structure and components of Rv0062-treated biofilm in vitro. Meanwhile, the metabolite factors that induced cellulase-related biofilm dispersal were also explored with metabolome analysis and further validations. The results showed that Rv0062 protein had a cellulase activity with a similar optimum pH (6.0) and lower optimum temperature (30 °C) compared to the cellulases from other bacteria. It promoted mycobacterial biofilm dispersal by hydrolyzing cellulose, the main component of extracellular polymeric substrates of mycobacterial biofilm. A metabolome analysis revealed that 107 metabolites were significantly altered at different stages of M. smegmatis biofilm development. Among them, a decrease in gamma-aminobutyric acid (GABA) promoted cellulase-related biofilm dispersal, and this effect was realized with the down-regulation of the bacterial signal molecule c-di-GMP. All these findings suggested that cellulase promotes mycobacterial biofilm dispersal and that this process is closely associated with biofilm metabolite alterations.

A Rare Case of Post-Primary Tuberculosis Which Was Pathologically Diagnosed as Lipoid Pneumonia

Case Presentation

The patient was a middle-aged housewife who had been using the household spray for a long time, and the main symptoms were cough and sputum production. Chest CT showed lobar ground-glass opacities (GGOs) with small patchy consolidation in the right middle lobe (RML), specifically, lung tissue pathology showed a large number of foamy cells and scattered multinucleated giant cells. The patient received empirical anti-infective treatment, but no clinical improvement was observed. Laboratory tests, including smears and cultures of sputum, blood and bronchoalveolar lavage fluid (BALF), did not provide clear evidence for pathogenic microorganisms. Therefore, the presumptive diagnosis was exogenous LP (ExLP). After 28 days of prednisone treatment, her symptoms improved, but 2 months later, she presented with a worsening cough, and the GGOs had progressed into lobar consolidation. Transbronchial lung biopsy (TBLB) culture showed mycobacterium tuberculosis (MTB), and lung tissue pathology showed granulomatous inflammation. After anti-tuberculosis treatment, the consolidation in the right middle lobe was gradually absorbed, along with a considerable symptom improvement. The final diagnosis of the patient was MTB infection with an endogenous lipoid pneumonia (EnLP)-like presentation.

Conclusion

The current case highlights that the MTB infection should be considered when pathology shows LP accompanied by scattered multinucleated giant cells.

Mycobacterium tuberculosis Exploits Focal Adhesion Kinase to Induce Necrotic Cell Death and Inhibit Reactive Oxygen Species Production

Tuberculosis is a deadly, contagious respiratory disease that is caused by the pathogenic bacterium Mycobacterium tuberculosis (Mtb). Mtb is adept at manipulating and evading host immunity by hijacking alveolar macrophages, the first line of defense against inhaled pathogens, by regulating the mode and timing of host cell death. It is established that Mtb infection actively blocks apoptosis and instead induces necrotic-like modes of cell death to promote disease progression. This survival strategy shields the bacteria from destruction by the immune system and antibiotics while allowing for the spread of bacteria at opportunistic times. As such, it is critical to understand how Mtb interacts with host macrophages to manipulate the mode of cell death. Herein, we demonstrate that Mtb infection triggers a time-dependent reduction in the expression of focal adhesion kinase (FAK) in human macrophages. Using pharmacological perturbations, we show that inhibition of FAK (FAKi) triggers an increase in a necrotic form of cell death during Mtb infection. In contrast, genetic overexpression of FAK (FAK⁺) completely blocked macrophage cell death during Mtb infection. Using specific inhibitors of necrotic cell death, we show that FAK-mediated cell death during Mtb infection occurs in a RIPK1-depedent, and to a lesser extent, RIPK3-MLKL-dependent mechanism. Consistent with these findings, FAKi results in uncontrolled replication of Mtb, whereas FAK⁺ reduces the intracellular survival of Mtb in macrophages. In addition, we demonstrate that enhanced control of intracellular Mtb replication by FAK⁺ macrophages is a result of increased production of antibacterial reactive oxygen species (ROS) as inhibitors of ROS production restored Mtb burden in FAK⁺ macrophages to same levels as in wild-type cells. Collectively, our data establishes FAK as an important host protective response during Mtb infection to block necrotic cell death and induce ROS production, which are required to restrict the survival of Mtb.

Aptamer-Based Diagnostic Systems for the Rapid Screening of TB at the Point-of-Care

The transmission of Tuberculosis (TB) is very rapid and the burden it places on health care systems is felt globally. The effective management and prevention of this disease requires that it is detected early. Current TB diagnostic approaches, such as the culture, sputum smear, skin tuberculin, and molecular tests are time-consuming, and some are unaffordable for low-income countries. Rapid tests for disease biomarker detection are mostly based on immunological assays that use antibodies which are costly to produce, have low sensitivity and stability. Aptamers can replace antibodies in these diagnostic tests for the development of new rapid tests that are more cost effective; more stable at high temperatures and therefore have a better shelf life; do not have batch-to-batch variations, and thus more consistently bind to a specific target with similar or higher specificity and selectivity and are therefore more reliable. Advancements in TB research, in particular the application of proteomics to identify TB specific biomarkers, led to the identification of a number of biomarker proteins, that can be used to develop aptamer-based diagnostic assays able to screen individuals at the point-of-care (POC) more efficiently in resource-limited settings.

Epigenetic reader BRD4 supports mycobacterial pathogenesis by co-modulating host lipophagy and angiogenesis

Mycobacterium tuberculosis (Mtb)-driven lipid accumulation is intricately associated with the progression of tuberculosis (TB) disease. Although several studies elucidating the mechanisms for lipid droplet (LD) biosynthesis exist, we provide evidence for the significance of their regulated turnover via macroautophagy/autophagy during Mtb infection. We demonstrate that Mtb utilizes EGFR (epidermal growth factor receptor) signaling to induce the expression of the histone acetylation reader, BRD4 (bromodomain containing 4). The EGFR-BRD4 axis suppresses lipid-specific autophagy, and hence favors cellular lipid accumulation. Specifically, we found that pharmacological inhibition or knockdown of Egfr or Brd4 enhances autophagic flux and concomitantly decreases cellular LDs that is otherwise maintained at a significant level in chloroquine-treated or Atg5 knocked down autophagy-compromised host cells. In line with the enhanced lipophagy, we found that loss of EGFR or BRD4 function restricts mycobacterial burden that is rescued by external replenishment with oleic acid. We also report that the EGFR-BRD4 axis exerts additional effects by modulating pro-angiogenic gene expression and consequently aberrant angiogenesis during mycobacterial infection. This is important in the context of systemic Mtb dissemination as well as for the efficient delivery of anti-mycobacterial therapeutics to the Mtb-rich core of TB granuloma. Finally, utilizing an in vivo mouse model of TB, we show that pharmacological inhibition of EGFR and BRD4 compromises LD buildup via enhanced lipophagy and normalizes angiogenesis, thereby restricting Mtb burden and rescuing mice from severe TB-like pathology. These findings shed light on the novel roles of BRD4 during Mtb infection, and its possible implication in potentiating anti-TB responses.Abbreviations: ATG5: autophagy related 5; BRDs: bromodomain containing; COL18A1: collagen type XVIII alpha 1 chain; EGFR: epidermal growth factor receptor; EP300: E1A binding protein p300; KDR: kinase insert domain receptor; KLF5: Kruppel like factor 5; LDs: lipid droplets; MAP1LC3B: microtubule associated protein 1 light chain 3 beta; Mtb: Mycobacterium tuberculosis; PECAM1: platelet and endothelial cell adhesion molecule 1; SQSTM1/p62: sequestosome 1; TB: tuberculosis; THBS1: thrombospondin 1; VEGF: vascular endothelial growth factor.

Manipulation of Host Cell Organelles by Intracellular Pathogens

Pathogenic intracellular bacteria, parasites and viruses have evolved sophisticated mechanisms to manipulate mammalian host cells to serve as niches for persistence and proliferation. The intracellular lifestyles of pathogens involve the manipulation of membrane-bound organellar compartments of host cells. In this review, we described how normal structural organization and cellular functions of endosomes, endoplasmic reticulum, Golgi apparatus, mitochondria, or lipid droplets are targeted by microbial virulence mechanisms. We focus on the specific interactions of Salmonella, Legionella pneumophila, Rickettsia rickettsii, Chlamydia spp. and Mycobacterium tuberculosis representing intracellular bacterial pathogens, and of Plasmodium spp. and Toxoplasma gondii representing intracellular parasites. The replication strategies of various viruses, i.e., Influenza A virus, Poliovirus, Brome mosaic virus, Epstein-Barr Virus, Hepatitis C virus, severe acute respiratory syndrome virus (SARS), Dengue virus, Zika virus, and others are presented with focus on the specific manipulation of the organelle compartments. We compare the specific features of intracellular lifestyle and replication cycles, and highlight the communalities in mechanisms of manipulation deployed.

Cavitary tuberculosis: the gateway of disease transmission

Tuberculosis continues to be a major threat to global health. Cavitation is a dangerous consequence of pulmonary tuberculosis associated with poor outcomes, treatment relapse, higher transmission rates, and development of drug resistance. However, in the antibiotic era, cavities are often identified as the most extreme outcome of treatment failure and are one of the least-studied aspects of tuberculosis. We review the epidemiology, clinical features, and concurrent standards of care for individuals with cavitary tuberculosis. We also discuss developments in the understanding of tuberculosis cavities as dynamic physical and biochemical structures that interface the host response with a unique mycobacterial niche to drive tuberculosis-associated morbidity and transmission. Advances in preclinical models and non-invasive imaging can provide valuable insights into the drivers of cavitation. These insights will guide the development of specific pharmacological interventions to prevent cavitation and improve lung function for individuals with tuberculosis.

The stringent response and Mycobacterium tuberculosis pathogenesis

During infection, the host restrains Mycobacterium tuberculosis (Mtb) from proliferating by imposing an arsenal of stresses. Despite this onslaught of attacks, Mtb is able to persist for the lifetime of the host, indicating that this pathogen has substantial molecular mechanisms to resist host-inflicted damage. The stringent response is a conserved global stress response in bacteria that involves the production of the hyperphosphorylated guanine nucleotides ppGpp and pppGpp (collectively called (p)ppGpp). (p)ppGpp then regulates a number of cellular processes to adjust the physiology of the bacteria to promote survival in different environments. Survival in the presence of host-generated stresses is an essential quality of successful pathogens, and the stringent response is critical for the intracellular survival of a number of pathogenic bacteria. In addition, the stringent response has been linked to virulence gene expression, persistence, latency and drug tolerance. In Mtb, (p)ppGpp synthesis is required for survival in low nutrient conditions, long term culture and during chronic infection in animal models, all indicative of a strict requirement for (p)ppGpp during exposure to stresses associated with infection. In this review we discuss (p)ppGpp metabolism and how this functions as a critical regulator of Mtb virulence.

Strategies to Improve Vaccine Efficacy against Tuberculosis by Targeting Innate Immunity

The global tuberculosis epidemic is the most common cause of death after infectious disease worldwide. Increasing numbers of infections with multi- and extensively drug-resistant variants of the Mycobacterium tuberculosis complex, resistant even to newly discovered and last resort antibiotics, highlight the urgent need for an efficient vaccine. The protective efficacy to pulmonary tuberculosis in adults of the only currently available vaccine, M. bovis BCG, is unsatisfactory and geographically diverse. More importantly, recent clinical studies on new vaccine candidates did not prove to be better than BCG, yet. Here, we propose and discuss novel strategies to improve efficacy of existing anti-tuberculosis vaccines. Modulation of innate immune responses upon vaccination already provided promising results in animal models of tuberculosis. For instance, neutrophils have been shown to influence vaccine efficacy, both, positively and negatively, and stimulate specific antibody secretion. Modulating immune regulatory properties after vaccination such as induction of different types of innate immune cell death, myeloid-derived suppressor or regulatory T cells, production of anti-inflammatory cytokines such as IL-10 may have beneficial effects on protection efficacy. Incorporation of lipid antigens presented via CD1 molecules to T cells have been discussed as a way to enhance vaccine efficacy. Finally, concepts of dendritic cell-based immunotherapies or training the innate immune memory may be exploitable for future vaccination strategies against tuberculosis. In this review, we put a spotlight on host immune networks as potential targets to boost protection by old and new tuberculosis vaccines.

Mycobacterial Biofilms: Revisiting Tuberculosis Bacilli in Extracellular Necrotizing Lesions

Under detergent-free in vitro conditions, Mycobacterium tuberculosis, the etiological agent of tuberculosis in humans, spontaneously forms organized multicellular structures called biofilms. Moreover, in vitro biofilms of M. tuberculosis are more persistent against antibiotics than their single-cell planktonic counterparts, thereby raising questions about the occurrence of biofilms in the host tissues and their significance in persistence during chemotherapy of tuberculosis. In this article, we present arguments that extracellular M. tuberculosis in necrotizing lesions likely grows as biofilms.

Mycobacterium tuberculosis replicates within necrotic human macrophages

Mycobacterium tuberculosis triggers macrophage cell death by necrosis, but it is unclear how this affects bacterial replication. Lerner et al. show that this pathogen replicates within necrotic human macrophages before disseminating to other cells upon loss of plasma membrane integrity.

Mycobacterium tuberculosis modulation of macrophage cell death is a well-documented phenomenon, but its role during bacterial replication is less characterized. In this study, we investigate the impact of plasma membrane (PM) integrity on bacterial replication in different functional populations of human primary macrophages. We discovered that IFN-γ enhanced bacterial replication in macrophage colony-stimulating factor–differentiated macrophages more than in granulocyte–macrophage colony-stimulating factor–differentiated macrophages. We show that permissiveness in the different populations of macrophages to bacterial growth is the result of a differential ability to preserve PM integrity. By combining live-cell imaging, correlative light electron microscopy, and single-cell analysis, we found that after infection, a population of macrophages became necrotic, providing a niche for M. tuberculosis replication before escaping into the extracellular milieu. Thus, in addition to bacterial dissemination, necrotic cells provide first a niche for bacterial replication. Our results are relevant to understanding the environment of M. tuberculosis replication in the host.

Analysis of the contribution of MTP and the predicted Flp pilus genes to Mycobacterium tuberculosis pathogenesis

Mycobacterium tuberculosis (Mtb) is one of the world's most successful pathogens. Millions of new cases of tuberculosis occur each year, emphasizing the need for better methods of treatment. The design of novel therapeutics is dependent on our understanding of factors that are essential for pathogenesis. Many bacterial pathogens use pili and other adhesins to mediate pathogenesis. The recently identified Mycobacterium tuberculosis pilus (MTP) and the hypothetical, widely conserved Flp pilus have been speculated to be important for Mtb virulence based on in vitro studies and homology to other pili, respectively. However, the roles for these pili during infection have yet to be tested. We addressed this gap in knowledge and found that neither MTP nor the hypothetical Flp pilus is required for Mtb survival in mouse models of infection, although MTP can contribute to biofilm formation and subsequent isoniazid tolerance. However, differences in mtp expression did affect lesion architecture in infected lungs. Deletion of mtp did not correlate with loss of cell-associated extracellular structures as visualized by transmission electron microscopy in Mtb Erdman and HN878 strains, suggesting that the phenotypes of the mtp mutants were not due to defects in production of extracellular structures. These findings highlight the importance of testing the virulence of adhesion mutants in animal models to assess the contribution of the adhesin to infection. This study also underscores the need for further investigation into additional strategies that Mtb may use to adhere to its host so that we may understand how this pathogen invades, colonizes and disseminates.