NOD2 enhances the innate response of alveolar macrophages to Mycobacterium tuberculosis in humans

A modified BCG with depletion of enzymes associated with peptidoglycan amidation induces enhanced protection against tuberculosis in mice

Mechanisms by which Mycobacterium tuberculosis (Mtb) evades pathogen recognition receptor activation during infection may offer insights for the development of improved tuberculosis (TB) vaccines. Whilst Mtb elicits NOD-2 activation through host recognition of its peptidoglycan-derived muramyl dipeptide (MDP), it masks the endogenous NOD-1 ligand through amidation of glutamate at the second position in peptidoglycan side-chains. As the current BCG vaccine is derived from pathogenic mycobacteria, a similar situation prevails. To alleviate this masking ability and to potentially improve efficacy of the BCG vaccine, we used CRISPRi to inhibit expression of the essential enzyme pair, MurT-GatD, implicated in amidation of peptidoglycan side-chains. We demonstrate that depletion of these enzymes results in reduced growth, cell wall defects, increased susceptibility to antibiotics, altered spatial localization of new peptidoglycan and increased NOD-1 expression in macrophages. In cell culture experiments, training of a human monocyte cell line with this recombinant BCG yielded improved control of Mtb growth. In the murine model of TB infection, we demonstrate that depletion of MurT-GatD in BCG, which is expected to unmask the D-glutamate diaminopimelate (iE-DAP) NOD-1 ligand, yields superior prevention of TB disease compared to the standard BCG vaccine. In vitro and in vivo experiments in this study demonstrate the feasibility of gene regulation platforms such as CRISPRi to alter antigen presentation in BCG in a bespoke manner that tunes immunity towards more effective protection against TB disease.

Role of pattern recognition receptors in sensing Mycobacterium tuberculosis

Mycobacterium tuberculosis is one of the major invasive intracellular pathogens causing most deaths by a single infectious agent. The interaction between host immune cells and this pathogen is the focal point of the disease, Tuberculosis. Host immune cells not only mount the protective action against this pathogen but also serve as the primary niche for growth. Thus, recognition of this pathogen by host immune cells and following signaling cascades are key dictators of the disease state. Immune cells, mainly belonging to myeloid cell lineage, recognize a wide variety of Mycobacterium tuberculosis ligands ranging from carbohydrate and lipids to proteins to nucleic acids by different membrane-bound and soluble pattern recognition receptors. Simultaneous interaction between different host receptors and pathogen ligands leads to immune-inflammatory response as well as contributes to virulence. This review summarizes the contribution of pattern recognition receptors of host immune cells in recognizing Mycobacterium tuberculosis and subsequent initiation of signaling pathways to provide the molecular insight of the specific Mtb ligands interacting with specific PRR, key adaptor molecules of the downstream signaling pathways and the resultant effector functions which will aid in identifying novel drug targets, and developing novel drugs and adjuvants.

A nod to the bond between NOD2 and mycobacteria

Mycobacteria are responsible for several human and animal diseases. NOD2 is a pattern recognition receptor that has an important role in mycobacterial recognition. However, the mechanisms by which mutations in NOD2 alter the course of mycobacterial infection remain unclear. Herein, we aimed to review the totality of studies directly addressing the relationship between NOD2 and mycobacteria as a foundation for moving the field forward. NOD2 was linked to mycobacterial infection at 3 levels: (1) genetic, through association with mycobacterial diseases of humans; (2) chemical, through the distinct NOD2 ligand in the mycobacterial cell wall; and (3) immunologic, through heightened NOD2 signaling caused by the unique modification of the NOD2 ligand. The immune response to mycobacteria is shaped by NOD2 signaling, responsible for NF-κB and MAPK activation, and the production of various immune effectors like cytokines and nitric oxide, with some evidence linking this to bacteriologic control. Absence of NOD2 during mycobacterial infection of mice can be detrimental, but the mechanism remains unknown. Conversely, the success of immunization with mycobacteria has been linked to NOD2 signaling and NOD2 has been targeted as an avenue of immunotherapy for diseases even beyond mycobacteria. The mycobacteria-NOD2 interaction remains an important area of study, which may shed light on immune mechanisms in disease.

The role of NOD-like receptors in innate immunity

The innate immune system in vertebrates and invertebrates relies on conserved receptors and ligands, and pathways that can rapidly initiate the host response against microbial infection and other sources of stress and danger. Research into the family of NOD-like receptors (NLRs) has blossomed over the past two decades, with much being learned about the ligands and conditions that stimulate the NLRs and the outcomes of NLR activation in cells and animals. The NLRs play key roles in diverse functions, ranging from transcription of MHC molecules to initiation of inflammation. Some NLRs are activated directly by their ligands, while other ligands may have indirect effects on the NLRs. New findings in coming years will undoubtedly shed more light on molecular details involved in NLR activation, as well as the physiological and immunological outcomes of NLR ligation.

In-depth systems biological evaluation of bovine alveolar macrophages suggests novel insights into molecular mechanisms underlying Mycobacterium bovis infection

OBJECTIVE

Bovine tuberculosis (bTB) is a chronic respiratory infectious disease of domestic livestock caused by intracellular Mycobacterium bovis infection, which causes ~$3 billion in annual losses to global agriculture. Providing novel tools for bTB managements requires a comprehensive understanding of the molecular regulatory mechanisms underlying the M. bovis infection. Nevertheless, a combination of different bioinformatics and systems biology methods was used in this study in order to clearly understand the molecular regulatory mechanisms of bTB, especially the immunomodulatory mechanisms of M. bovis infection.

METHODS

RNA-seq data were retrieved and processed from 78 (39 non-infected control vs. 39 M. bovis-infected samples) bovine alveolar macrophages (bAMs). Next, weighted gene co-expression network analysis (WGCNA) was performed to identify the co-expression modules in non-infected control bAMs as reference set. The WGCNA module preservation approach was then used to identify non-preserved modules between non-infected controls and M. bovis-infected samples (test set). Additionally, functional enrichment analysis was used to investigate the biological behavior of the non-preserved modules and to identify bTB-specific non-preserved modules. Co-expressed hub genes were identified based on module membership (MM) criteria of WGCNA in the non-preserved modules and then integrated with protein-protein interaction (PPI) networks to identify co-expressed hub genes/transcription factors (TFs) with the highest maximal clique centrality (MCC) score (hub-central genes).

RESULTS

As result, WGCNA analysis led to the identification of 21 modules in the non-infected control bAMs (reference set), among which the topological properties of 14 modules were altered in the M. bovis-infected bAMs (test set). Interestingly, 7 of the 14 non-preserved modules were directly related to the molecular mechanisms underlying the host immune response, immunosuppressive mechanisms of M. bovis, and bTB development. Moreover, among the co-expressed hub genes and TFs of the bTB-specific non-preserved modules, 260 genes/TFs had double centrality in both co-expression and PPI networks and played a crucial role in bAMs-M. bovis interactions. Some of these hub-central genes/TFs, including PSMC4, SRC, BCL2L1, VPS11, MDM2, IRF1, CDKN1A, NLRP3, TLR2, MMP9, ZAP70, LCK, TNF, CCL4, MMP1, CTLA4, ITK, IL6, IL1A, IL1B, CCL20, CD3E, NFKB1, EDN1, STAT1, TIMP1, PTGS2, TNFAIP3, BIRC3, MAPK8, VEGFA, VPS18, ICAM1, TBK1, CTSS, IL10, ACAA1, VPS33B, and HIF1A, had potential targets for inducing immunomodulatory mechanisms by M. bovis to evade the host defense response.

CONCLUSION

The present study provides an in-depth insight into the molecular regulatory mechanisms behind M. bovis infection through biological investigation of the candidate non-preserved modules directly related to bTB development. Furthermore, several hub-central genes/TFs were identified that were significant in determining the fate of M. bovis infection and could be promising targets for developing novel anti-bTB therapies and diagnosis strategies.

Different modalities of host cell death and their impact on Mycobacterium tuberculosis infection

Mycobacterium tuberculosis (Mtb) is the pathogen that causes tuberculosis (TB), a leading infectious disease of humans worldwide. One of the main histopathological hallmarks of TB is the formation of granulomas comprised of elaborately organized aggregates of immune cells containing the pathogen. Dissemination of Mtb from infected cells in the granulomas due to host and mycobacterial factors induces multiple cell death modalities in infected cells. Based on molecular mechanism, morphological characteristics, and signal dependency, there are two main categories of cell death: programmed and nonprogrammed. Programmed cell death (PCD), such as apoptosis and autophagy, is associated with a protective response to Mtb by keeping the bacteria encased within dead macrophages that can be readily phagocytosed by arriving in uninfected or neighboring cells. In contrast, non-PCD necrotic cell death favors the pathogen, resulting in bacterial release into the extracellular environment. Multiple types of cell death in the PCD category, including pyroptosis, necroptosis, ferroptosis, ETosis, parthanatos, and PANoptosis, may be involved in Mtb infection. Since PCD pathways are essential for host immunity to Mtb, therapeutic compounds targeting cell death signaling pathways have been experimentally tested for TB treatment. This review summarizes different modalities of Mtb-mediated host cell deaths, the molecular mechanisms underpinning host cell death during Mtb infection, and its potential implications for host immunity. In addition, targeting host cell death pathways as potential therapeutic and preventive approaches against Mtb infection is also discussed.

NOD1, NOD2, and NLRC5 Receptors in Antiviral and Antimycobacterial Immunity

The innate immune system recognizes pathogen-associated molecular motifs through pattern recognition receptors (PRRs) that induce inflammasome assembly in macrophages and trigger signal transduction pathways, thereby leading to the transcription of inflammatory cytokine genes. Nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) represent a family of cytosolic PRRs involved in the detection of intracellular pathogens such as mycobacteria or viruses. In this review, we discuss the role of NOD1, NOD2, and NLRC5 receptors in regulating antiviral and antimycobacterial immune responses by providing insight into molecular mechanisms as well as their potential health and disease implications.

NOD2-RIP2 signaling alleviates microglial ROS damage and pyroptosis via ULK1-mediated autophagy during Streptococcus pneumonia infection

Meningitis occurs when S. pneumonia invade the blood-brain barrier, provoking inflammatory host response and neurological injury. Nucleotide-binding oligomerization domain 2 (NOD2) has been identified to promote microglial activation and autophagy during pneumococcal meningitis, but the mechanism remains unclear. In the present study, we investigated the passway of NOD2-mediated autophagy activation and the role of autophagy in inflammatory damage of murine microglia and mouse meningitis model. We demonstrated that autophagy was activated during S. pneumonia infection, and NOD2-RIP2 signaling was involved in the process. Treatment of microglia with GSK583, the RIP2 kinase inhibitor resulted in reduced autophagy-related protein and p-ULK1, indicating that RIP2 regulated autophagy in a kinase-dependent manner by phosphorylating ULK1. In addition, microglia with ULK1 knockdown exhibited enhanced production of ROS, leading to IL-1β and IL-18 release and cellular pyroptosis. Similar to the in vitro results, NOD2-RIP2 signaling induced autophagy in the brain in a mouse meningitis model. Moreover, ULK1 or RIP2 silencing significantly increased pyroptosis of brain and induced more inflammatory damage of pneumococcal meningitis mice. Taken together, our study demonstrate that NOD2-RIP2 signaling is involved in the activation of autophagy by promoting ULK1 phosphorylation, which alleviates microglial ROS damage and pyroptosis during S. pneumonia infection.

Pathogenesis of SARS-CoV-2 and Mycobacterium tuberculosis Coinfection

Coronavirus disease-2019 (COVID-19), caused by SARS-CoV-2, is an infectious disease that poses severe threats to global public health and significant economic losses. The COVID-19 global burden is rapidly increasing, with over 246.53 million COVID-19 cases and 49.97 million deaths reported in the WHO 2021 report. People with compromised immunity, such as tuberculosis (TB) patients, are highly exposed to severe COVID-19. Both COVID-19 and TB diseases spread primarily through respiratory droplets from an infected person to a healthy person, which may cause pneumonia and cytokine storms, leading to severe respiratory disorders. The COVID-19-TB coinfection could be fatal, exacerbating the current COVID-19 pandemic apart from cellular immune deficiency, coagulation activation, myocardial infarction, and other organ dysfunction. This study aimed to assess the pathogenesis of SARS-CoV-2-Mycobacterium tuberculosis coinfections. We provide a brief overview of COVID19-TB coinfection and discuss SARS-CoV-2 host cellular receptors and pathogenesis. In addition, we discuss M. tuberculosis host cellular receptors and pathogenesis. Moreover, we highlight the impact of SARS-CoV-2 on TB patients and the pathological pathways that connect SARS-CoV-2 and M. tuberculosis infection. Further, we discuss the impact of BCG vaccination on SARS-CoV-2 cases coinfected with M. tuberculosis, as well as the diagnostic challenges associated with the coinfection.

Surveying the Epigenetic Landscape of Tuberculosis in Alveolar Macrophages

Tuberculosis (TB) remains the leading cause of bacterial disease-related death and is among the top 10 overall causes of death worldwide. The complex nature of this infectious lung disease has proven difficult to treat, and significant research efforts are now evaluating the feasibility of host-directed, adjunctive therapies. An attractive approach in host-directed therapy targets host epigenetics, or gene regulation, to redirect the immune response in a host-beneficial manner. Substantial evidence exists demonstrating that host epigenetics are dysregulated during TB and that epigenetic-based therapies may be highly effective to treat TB. However, the caveat is that much of the knowledge that exists on the modulation of the host epigenome during TB has been gained using in vitro, small-animal, or blood-derived cell models, which do not accurately reflect the pulmonary nature of the disease. In humans, the first and major target cells of Mycobacterium tuberculosis are alveolar macrophages (AM). As such, their response to infection and treatment is clinically relevant and ultimately drives the outcome of disease. In this review, we compare the fundamental differences between AM and circulating monocyte-derived macrophages in the context of TB and summarize the recent advances in elucidating the epigenomes of these cells, including changes to the transcriptome, DNA methylome, and chromatin architecture. We will also discuss trained immunity in AM as a new and emerging field in TB research and provide some perspectives for the translational potential of targeting host epigenetics as an alternative TB therapy.

Trained Immunity: An Overview and the Impact on COVID-19

Effectively treating infectious diseases often requires a multi-step approach to target different components involved in disease pathogenesis. Similarly, the COVID-19 pandemic has become a global health crisis that requires a comprehensive understanding of Severe Acute Respiratory Syndrome Corona Virus 2 (SARS-CoV-2) infection to develop effective therapeutics. One potential strategy to instill greater immune protection against COVID-19 is boosting the innate immune system. This boosting, termed trained immunity, employs immune system modulators to train innate immune cells to produce an enhanced, non-specific immune response upon reactivation following exposure to pathogens, a process that has been studied in the context of in vitro and in vivo clinical studies prior to the COVID-19 pandemic. Evaluation of the underlying pathways that are essential to inducing protective trained immunity will provide insight into identifying potential therapeutic targets that may alleviate the COVID-19 crisis. Here we review multiple immune training agents, including Bacillus Calmette-Guérin (BCG), β-glucan, and lipopolysaccharide (LPS), and the two most popular cell types involved in trained immunity, monocytes and natural killer (NK) cells, and compare the signaling pathways involved in innate immunity. Additionally, we discuss COVID-19 trained immunity clinical trials, emphasizing the potential of trained immunity to fight SARS-CoV-2 infection. Understanding the mechanisms by which training agents activate innate immune cells to reprogram immune responses may prove beneficial in developing preventive and therapeutic targets against COVID-19.

Xenophagy of invasive bacteria is differentially activated and modulated via a TLR-TRAF6-Beclin1 axis in echinoderms

In marine environments, organisms are confronted with numerous microbial challenges, although the differential regulation of xenophagy in response to different pathogenic bacterial species remains relatively unknown. Here, we addressed this issue using Apostichopus japonicus as a model. We identified 39 conserved autophagy-related genes by genome-wide screening, which provided a molecular basis for autophagy regulation in sea cucumbers. Furthermore, xenophagy of two Gram-negative bacteria, Vibrio splendidus and Escherichia coli, but not a Gram-positive bacteria, Micrococcus luteus, was observed in different autophagy assays. Surprisingly, a significantly higher autophagy capacity was found in the E. coli–challenged group than in the V. splendidus–challenged group. To confirm these findings, two different lipopolysaccharides, LPS^{V. splendidus} and LPS^E. coli, were isolated; we found that these LPS species differentially activated coelomocyte xenophagy. To explore the molecular mechanism mediating differential levels of xenophagy, we used an siRNA knockdown assay and confirmed that LPS^{V. splendidus}-mediated xenophagy was dependent on an AjTLR3-mediated pathway, whereas LPS^E. coli-mediated xenophagy was dependent on AjToll. Moreover, the activation of different AjTLRs resulted in AjTRAF6 ubiquitination and subsequent activation of K63-linked ubiquitination of AjBeclin1. Inversely, the LPS^{V. splendidus}-induced AjTLR3 pathway simultaneously activated the expression of AjA20, which reduced the extent of K63-linked ubiquitination of AjBeclin1 and impaired the induction of autophagy; however, this finding was no t evident with LPS^E. coli. Our present results provide the first evidence showing that xenophagy could be differentially induced by different bacterial species to yield differential autophagy levels in echinoderms.

Dual Effect of Low-Molecular-Weight Bioregulators of Bacterial Origin in Experimental Model of Asthma

Asthma is one of the most common noncommunicable diseases, affecting over 200 million people. A large number of drugs control asthma attacks, but there is no effective therapy. Identification of reasons for asthma and preventing this disease is a relevant task. The influence of bacterial components is necessary for the normal development of the immune system and the formation of an adequate immune response to antigens. In the absence of microorganisms or their insufficient exposure, the prerequisites are formed for excessive reactivity to harmless antigens. In the present study, we analyzed cellular and humoral factors in a standard mouse model of OVA-induced asthma modified by 5-fold intraperitoneal injection of bacterial cell wall fragments of glucosaminylmuramyl dipeptide (GMDP) 5 μg/animal or 1 μg lipopolysaccharide (LPS) per animal for 5 days before sensitization by ovalbumin (OVA). Preliminary administration of LPS or GMDP to animals significantly reduced goblet cells as well as the number of neutrophils, lymphocytes, and eosinophils in bronchoalveolar lavage, wherein GMDP corrected neutrophilia to a 2-fold degree, and LPS reduced the severity of eosinophilia by 1.9 times. With OVA administration of GMDP or LPS at the sensitization stage, an increase in the total number of bronchoalveolar lavage cells due to neutrophils, macrophages, lymphocytes, and eosinophils in relation to the group with asthma without GMDP or LPS was observed. The administration of GMDP or LPS to normal mice without asthma for 5 days had no statistically significant effect on the change in the number and population composition of cells in bronchoalveolar lavage in comparison with the control group receiving PBS. As a result of a study in a mouse model of asthma, a dual effect of LPS and GMDP was established: the introduction of LPS or GMDP before sensitization reduces neutrophilia and eosinophilia, while the introduction of LPS or GMDP together with an allergen significantly increases neutrophilia and eosinophilia. The study of the immunoglobulin status shows that in normal-asthma mice, GMDP and LPS slightly increase IgA in bronchoalveolar lavage; at the same time, in the asthma model, injections of GMDP or LPS before sensitization contribute to a significant decrease in IgA (2.6 times and 2.1 times, respectively) in BALF and IgE (2.2 times and 2.0 times, respectively) in blood serum. In an experimental model of asthma, the effect of GMDP and LPS was multidirectional: when they are repeatedly administered before sensitization, the bacterial components significantly reduce the severity of the allergic process, while in the case of a joint injection with an allergen, they increase the influx of macrophages, lymphocytes, and neutrophils into the lungs, which can aggravate the course of pathological process. Thus, the insufficient effect of antigens of a bacterial nature, in particular, with prolonged use of antibiotics can be compensated for by substances based on low-molecular-weight bioregulators of bacterial origin to establish the missing signals for innate immunity receptors, whose constant activation at a certain level is necessary to maintain homeostasis.

Influence of miRNA-30a-5p on Pulmonary Fibrosis in Mice with Streptococcus pneumoniae Infection through Regulation of Autophagy by Beclin-1

The study is aimed at observing the influence of microribonucleic acid- (miRNA-) 30a-50p on the pulmonary fibrosis in mice with Streptococcus pneumoniae infection through the regulation of autophagy by Beclin-1. Specific pathogen-free mice were instilled with Streptococcus pneumoniae through the trachea to establish the pulmonary fibrosis model. Then, they were divided into the miRNA-30a-50p mimics group (mimics group, n = 10) and miRNA-30a-5p inhibitors group (inhibitors group, n = 10), with the control group (n = 10) also set. Pulmonary tissue wet weight/dry weight (W/D) was detected. The content of tumor necrosis factor-α (TNF-α), interleukin- (IL-) 6, and myeloperoxidase (MPO) was determined using enzyme-linked immunosorbent assay (ELISA). Besides, the changes in the pulmonary function index dynamic lung compliance (Cdyn), plateau pressure (Pplat), and peak airway pressure (Ppeak) were monitored, and the gene and protein expression levels were measured via quantitative PCR (qPCR) and Western blotting. The expression level of miRNA-30a-5p was substantially raised in the mimics group (p < 0.05), but extremely low in the inhibitors group (p < 0.05). The mimics group had obviously raised levels of serum aminotransferase (AST), glutamic-pyruvic transaminase (GPT), alkaline phosphatase (ALP), and pulmonary tissue W/D (p < 0.05). Additionally, the expression levels of TNF-α, IL-6, and MPO were notably elevated in the mimics group, while their expression levels showed the opposite conditions in the inhibitors group (p < 0.05). According to the HE staining results, the inhibitors group had arranged orderly cells, while the mimics group exhibited lung injury, pulmonary edema, severe inflammatory response, and alveolar congestion. In the inhibitors group, Cdyn was remarkably elevated, but Pplat and Ppeak declined considerably (p < 0.05). Besides, the inhibitors group exhibited elevated messenger RNA (mRNA) levels of Beclin-1 and LC3, lowered mRNA levels of α-SMA and p62, a raised protein level of Beclin-1, and a markedly decreased protein level of p62 (p < 0.05). Silencing miRNA-30a-5p expression can promote the expression of Beclin-1 to accelerate the occurrence of autophagy, thereby treating pulmonary fibrosis in mice with Streptococcus pneumoniae infection.

Inborn Errors in the LRR Domain of Nod2 and Their Potential Consequences on the Function of the Receptor

The innate immune system plays a critical role in the early detection of pathogens, primarily by relying on pattern-recognition receptor (PRR) signaling molecules. Nucleotide-binding oligomerization domain 2 (NOD2) is a cytoplasmic receptor that recognizes invading molecules and danger signals inside the cells. Recent studies highlight the importance of NOD2′s function in maintaining the homeostasis of human body microbiota and innate immune responses, including induction of proinflammatory cytokines, regulation of autophagy, modulation of endoplasmic reticulum (ER) stress, etc. In addition, there is extensive cross-talk between NOD2 and the Toll-like receptors that are so important in the induction and tuning of adaptive immunity. Polymorphisms of NOD2′s encoding gene are associated with several pathological conditions, highlighting NOD2′s functional importance. In this study, we summarize NOD2′s role in cellular signaling pathways and take a look at the possible consequences of common NOD2 polymorphisms on the structure and function of this receptor.

Next-Generation Antibiotics, Bacteriophage Endolysins, and Nanomaterials for Combating Pathogens

This review presents various strategies to fight causative agents of infectious diseases. Species-specific programmable RNA-containing antibiotics open up new possibilities for creating next-generation of personalized drugs based on microbiome editing and can serve as a new tool for selective elimination of pathogenic bacterial species while keeping intact the rest of microbiota. Another promising approach in combating bacterial infections is genome editing using the CRISPR-Cas systems. Expanding knowledge on the molecular mechanisms of innate immunity has been actively used for developing new antimicrobials. However, obvious risks of using antibiotic adjuvants aimed at activation of the host immune system include development of the autoimmune response with subsequent organ damage. To avoid these risks, it is essential to elucidate action mechanisms of the specific ligands and signal molecules used as components of the hybrid antibiotics. Bacteriophage endolysins are also considered as effective antimicrobials against antibiotic-resistant bacteria, metabolically inactive persisters, and microbial biofilms. Despite significant advances in the design of implants with antibacterial properties, the problem of postoperative infections still remains. Different nanomodifications of the implant surface have been designed to reduce bacterial contamination. Here, we review bactericidal, fungicidal, and immunomodulating properties of compounds used for the implant surface nanomodifications, such as silver, boron nitride nanomaterials, nanofibers, and nanogalvanic materials.

Sensing of mycobacterial arabinogalactan by galectin-9 exacerbates mycobacterial infection

Mycobacterial arabinogalactan (AG) is an essential cell wall component of mycobacteria and a frequent structural and bio-synthetical target for anti-tuberculosis (TB) drug development. Here, we report that mycobacterial AG is recognized by galectin-9 and exacerbates mycobacterial infection. Administration of AG-specific aptamers inhibits cellular infiltration caused by Mycobacterium tuberculosis (Mtb) or Mycobacterium bovis BCG, and moderately increases survival of Mtb-infected mice or Mycobacterium marinum-infected zebrafish. AG interacts with carbohydrate recognition domain (CRD) 2 of galectin-9 with high affinity, and galectin-9 associates with transforming growth factor β-activated kinase 1 (TAK1) via CRD2 to trigger subsequent activation of extracellular signal-regulated kinase (ERK) as well as induction of the expression of matrix metalloproteinases (MMPs). Moreover, deletion of galectin-9 or inhibition of MMPs blocks AG-induced pathological impairments in the lung, and the AG-galectin-9 axis aggravates the process of Mtb infection in mice. These results demonstrate that AG is an important virulence factor of mycobacteria and galectin-9 is a novel receptor for Mtb and other mycobacteria, paving the way for the development of novel effective TB immune modulators.

Active vitamin D3 attenuates the severity of Salmonella colitis in mice by orchestrating innate immunity

INTRODUCTION

Salmonella spp. pose major public health problems worldwide. A better understanding of the pathogenesis of these foodborne pathogens is a prerequisite for the design of improved intervention strategies that could reduce the use of antimicrobial agents and drug-resistant Salmonellosis. Accumulating evidence indicates that vitamin D is involved in regulating innate immunity, and may, therefore, play a key role in human responses to infection. Studies have suggested 1,25-dihydroxyvitamin D3 (1,25D3), the active form of vitamin D, effectively ameliorates colitis. These findings have broad implications for the use of vitamin D compounds in colitis. This study investigated the effect of active vitamin D3 on the severity of Salmonella colitis.

METHODS

A Salmonella colitis model was established with 6-8-week-old male C57BL/6 mice: Streptomycin-pretreated C57BL/6 mice were infected orally with Salmonella enterica serova Typhimurium wild-type strain SL1344 for 48 h. The mice were randomly assigned to control, model, and 1,25(OH)2 D3 -treated groups. After the experiment, the mice were sacrificed, and intestinal, spleen, and liver tissue samples were removed to analyze bacterial colonization, western blot for protein levels, and real-time-polymer chain reaction for messenger RNA (mRNA) expression.

RESULTS

We observed that 1,25D3 reduced the severity of Salmonella colitis in C57BL/6 mice by reducing cecal mIL-1beta, mIL-6, mTNF-alpha, and mIL-8 mRNA expressions, bacterial colonization (CFU/mg tissue) in the liver and spleen, but increased the human β-defensin-2 mRNA and autophagy protein expression, compared to those of the SL1344 infection only.

CONCLUSIONS

Our results document that active vitamin D3 reduced Salmonella colitis by decreasing inflammation, and bacterial translocation via induction of killing and autophagic clearance of pathogenic organisms.

Autophagy Induction as a Host-Directed Therapeutic Strategy against Mycobacterium tuberculosis Infection

Tuberculosis (TB), a bacterialinfectious disease caused by Mycobacterium tuberculosis (M.tb), which causes significant mortality in humans worldwide. Current treatment regimen involve the administration of multiple antibiotics over the course of several months that contributes to patient non-compliance leading to relapse and the development of drug-resistant M.tb (MDR and XDR) strains. Together, these facts highlight the need for the development of shorter TB treatment regimens. Host-directed therapy (HDT) is a new and emerging concept that aims to augment host immune response using drugs/compounds with or without adjunct antibiotics against M.tb infection. Autophagy is a natural catabolic mechanism of the cell that involves delivering the cytosolic constituents to the lysosomes for degradation and recycling the components; thereby maintaining the cellular and energy homoeostasis of a cell. However, over the past decade, an improved understanding of the role of autophagy in immunity has led to autophagy activation by using drugs or agents. This autophagy manipulation may represent a promising host-directed therapeutic strategy for human TB. However, current clinical knowledge on implementing autophagy activation by drugs or agents, as a stand-alone HDT or as an adjunct with antibiotics to treat human TB is insufficient. In recent years, many reports on high-throughput drug screening and measurement of autophagic flux by fluorescence, high-content microscopy, flow cytometry, microplate reader and immunoblotting have been published for the discovery of drugs that modulate autophagy. In this review, we discuss the commonly used chemical screening approaches in mammalian cells for the discovery of autophagy activating drugs against M.tbinfection. We also summarize the various autophagy-activating agents, both pre-clinical candidates and compounds approved for advanced clinical investigation during mycobacterial infection. Finally, we discuss the opportunities and challenges in using autophagy activation as HDT strategy to improve TB outcome and shorten treatment regimen.

Genetics in the Host-Mycobacterium ulcerans interaction

Buruli ulcer is an emerging infectious disease associated with high morbidity and unpredictable outbreaks. It is caused by Mycobacterium ulcerans, a slow-growing pathogen evolutionarily shaped by the acquisition of a plasmid involved in the production of a potent macrolide-like cytotoxin and by genome rearrangements and downsizing. These events culminated in an uncommon infection pattern, whereby M. ulcerans is both able to induce the initiation of the inflammatory cascade and the cell death of its proponents, as well as to survive within the phagosome and in the extracellular milieu. In such extreme conditions, the host is sentenced to rely on a highly orchestrated genetic landscape to be able to control the infection. We here revisit the dynamics of M. ulcerans infection, drawing parallels from other mycobacterioses and integrating the most recent knowledge on its evolution and pathogenicity in its interaction with the host immune response.

The Innate Immune Response to Mycobacterium tuberculosis Infection

Infection with Mycobacterium tuberculosis causes >1.5 million deaths worldwide annually. Innate immune cells are the first to encounter M. tuberculosis, and their response dictates the course of infection. Dendritic cells (DCs) activate the adaptive response and determine its characteristics. Macrophages are responsible both for exerting cell-intrinsic antimicrobial control and for initiating and maintaining inflammation. The inflammatory response to M. tuberculosis infection is a double-edged sword. While cytokines such as TNF-α and IL-1 are important for protection, either excessive or insufficient cytokine production results in progressive disease. Furthermore, neutrophils-cells normally associated with control of bacterial infection-are emerging as key drivers of a hyperinflammatory response that results in host mortality. The roles of other innate cells, including natural killer cells and innate-like T cells, remain enigmatic. Understanding the nuances of both cell-intrinsic control of infection and regulation of inflammation will be crucial for the successful development of host-targeted therapeutics and vaccines.

Innate Immune Pattern Recognition Receptors of Mycobacterium tuberculosis: Nature and Consequences for Pathogenesis of Tuberculosis

Innate immunity against Mycobacterium tuberculosis is a critical early response to prevent the establishment of the infection. Despite recent advances in understanding the host-pathogen dialogue in the early stages of tuberculosis (TB), much has yet to be learnt. The nature and consequences of this dialogue ultimately determine the path of infection: namely, either early clearance of M. tuberculosis, or establishment of M. tuberculosis infection leading to active TB disease and/or latent TB infection. On the frontline in innate immunity are pattern recognition receptors (PRRs), with soluble factors (e.g. collectins and complement) and cell surface factors (e.g. Toll-like receptors and other C-type lectin receptors (Dectin 1/2, Nod-like receptors, DC-SIGN, Mincle, mannose receptor, and MCL) that play a central role in recognising M. tuberculosis and facilitating its clearance. However, in a 'double-edged sword' scenario, these factors can also be involved in enhancement of pathogenesis as well. Furthermore, innate immunity is also a critical bridge in establishing the subsequent adaptive immune response, which is also responsible for granuloma formation that cordons off M. tuberculosis infection, establishing latency and acting as a reservoir for bacterial persistence and dissemination of future disease. This chapter discusses the current understanding of pattern recognition of M. tuberculosis by innate immunity and the role this plays in the pathogenesis and protection against TB.

NOD2/RIG-I Activating Inarigivir Adjuvant Enhances the Efficacy of BCG Vaccine Against Tuberculosis in Mice

Tuberculosis (TB) caused by Mycobacterium tuberculosis (MTB) kills about 1.5 million people each year and the widely used Bacille Calmette-Guérin (BCG) vaccine provides a partial protection against TB in children and adults. Because BCG vaccine evades lysosomal fusion in antigen presenting cells (APCs), leading to an inefficient production of peptides and antigen presentation required to activate CD4 T cells, we sought to boost its efficacy using novel agonists of RIG-I and NOD2 as adjuvants. We recently reported that the dinucleotide SB 9200 (Inarigivir) derived from our small molecule nucleic acid hybrid (SMNH)^® platform, activated RIG-I and NOD2 receptors and exhibited a broad-spectrum antiviral activity against hepatitis B and C, Norovirus, RSV, influenza and parainfluenza. Inarigivir increased the ability of BCG-infected mouse APCs to secrete elevated levels of IL-12, TNF-α, and IFN-β, and Caspase-1 dependent IL-1β cytokine. Inarigivir also increased the ability of macrophages to kill MTB in a Caspase-1-, and autophagy-dependent manner. Furthermore, Inarigivir led to a Capsase-1 and NOD2- dependent increase in the ability of BCG-infected APCs to present an Ag85B-p25 epitope to CD4 T cells in vitro. Consistent with an increase in immunogenicity of adjuvant treated APCs, the Inarigivir-BCG vaccine combination induced robust protection against tuberculosis in a mouse model of MTB infection, decreasing the lung burden of MTB by 1-log10 more than that afforded by BCG vaccine alone. The Inarigivir-BCG combination was also more efficacious than a muramyl-dipeptide-BCG vaccine combination against tuberculosis in mice, generating better memory T cell responses supporting its novel adjuvant potential for the BCG vaccine.

Antimicrobial Peptide against Mycobacterium Tuberculosis That Activates Autophagy Is an Effective Treatment for Tuberculosis

Mycobacterium tuberculosis (MTB) is the principal cause of human tuberculosis (TB), which is a serious health problem worldwide. The development of innovative therapeutic modalities to treat TB is mainly due to the emergence of multi drug resistant (MDR) TB. Autophagy is a cell-host defense process. Previous studies have reported that autophagy-activating agents eliminate intracellular MDR MTB. Thus, combining a direct antibiotic activity against circulating bacteria with autophagy activation to eliminate bacteria residing inside cells could treat MDR TB. We show that the synthetic peptide, IP-1 (KFLNRFWHWLQLKPGQPMY), induced autophagy in HEK293T cells and macrophages at a low dose (10 μM), while increasing the dose (50 μM) induced cell death; IP-1 induced the secretion of TNFα in macrophages and killed Mtb at a dose where macrophages are not killed by IP-1. Moreover, IP-1 showed significant therapeutic activity in a mice model of progressive pulmonary TB. In terms of the mechanism of action, IP-1 sequesters ATP in vitro and inside living cells. Thus, IP-1 is the first antimicrobial peptide that eliminates MDR MTB infection by combining four activities: reducing ATP levels, bactericidal activity, autophagy activation, and TNFα secretion.

Immunological Evaluation for Personalized Interventions in Children with Tuberculosis: Should It Be Routinely Performed?

Childhood tuberculosis (TB) is a significant public health problem and the ninth leading cause of death worldwide. Progression of Mycobacterium tuberculosis infection to active disease depends on mycobacterial virulence, environmental diversity, and host susceptibility and immune response. In children, malnutrition and immaturity of the immune system contribute to an inadequate immune response. Coinfections, though rarely described in TB, might be associated with host immune deficiencies. Here, we describe the immunological evaluation of eight pediatric patients infected with a member of the M. tuberculosis complex, most of them with concomitant pulmonary infections (bacteria, viruses, or fungi). We assessed the functionality of several innate immunity receptors, IL-12 receptor, and IFN-γ receptor, as well as the antioxidant levels (glutathione), which are essential mechanisms for fighting intracellular pathogens such as M. tuberculosis. This study is aimed at developing a thorough immunological evaluation of patients with TB and a coinfection.

THP-1 cells increase TNF-α production upon LPS + soluble human IgG co-stimulation supporting evidence for TLR4 and Fcγ receptors crosstalk

The lipopolysaccharide (LPS) of Gram-negative bacteria is recognized on human monocytes and macrophages by TLR4 and MD2 and induces the production of inflammatory cytokines; the LPS + IgG complexes co-stimulation increases the cytokine production, mediated by the Fc-γRIIa (CD32a). We stimulated human CD14 + monocytes or THP-1 cells with LPS or LPS + soluble human IgG (sIgG) and TNF-α transcription and production, assessed RT-qPCR, ELISA, or flow cytometry, was enhanced by 30% upon LPS + sIgG compared to LPS stimulation. LPS + sIgG co-stimulation affected the NF-κB pathway (p65 phosphorylation and nucleus translocation, and IkB- α degradation). The biochemical inhibition of IRAK 1/4 and Syk kinases suppressed the enhancer effect of LPS + sIgG on TNF- α production, suggesting the involvement of both MyD88 dependent and independent pathways. Our results suggest that during LPS activation, sIgG may participate in a TLR4 - Fc-γR crosstalk.

Natural and trained innate immunity against Mycobacterium tuberculosis

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb) infection, remains a major global health emergency. It is estimated that one third of global population are affected, predominantly with latent granuloma form of the disease. Mtb co-evolved with humans, for its obligatory intra-macrophage phagosome habitat and slow replication, balanced against unique mycobacterial innate immunity, which appears to be highly complex. TB is transmitted via cough aerosol Mtb inhalation. Bovine TB attenuated Bacillus Calmette Guerin (BCG) live vaccine has been in practice for protection of young children from severe disseminated Mtb infection, but not sufficiently for their lungs, as obtained by trials in TB endemic community. To augment BCG vaccine-driven innate and adaptive immunity for neonates and better protection against adult pulmonary TB, a number of BCG pre-vaccination based, subset vaccine candidates have been tested via animal preclinical, followed by safe clinical trials. BCG also enhances innate macrophage trained immunity and memory, through primordial intracellular Toll-like receptors (TLRs) 7 and 9, which recognise distinct mycobacterial molecular pattern signature. This signature is transmitted by TLR signalling via nuclear factor-κB, for activating innate immune transcription and expression of gene profiling in a mycobacterial signature-specific manner. These are epigenetically imprinted in reprogramming of distinct chromatin areas for innate immune memory, to be recalled following lung reinfection. Unique TB innate immunity and its trained memory are considered independent from adaptive immune B and T cells. On the other hand, adaptive immunity is crucial in Mtb containment in granulomatous latency, supported by innate immune cell infiltration. In nearly 5-10 % of susceptible people, latent TB may be activated due to immune evasion by Mtb from intracellular phagosome within macrophage, perpetrating TB. However, BCG and new recombinant BCG vaccines have the capacity, as indicated in pre- and clinical trials, to overcome such Mtb evasion. Various strategies include pro-inflammatory-bactericidal type 1 polarisation (M1) phenotype of the infected macrophage, involving thrombospondin-TLR pathway. Saprophytic M. smegmatis-based recombinant vaccines are also promising candidates against TB. BCG vaccination of neonates/infants in TB endemic countries also reduced their pneumonia caused by various microbes independent of TB immunity. Here, we discuss host immune response against Mtb, its immune evasion strategies, and the important role innate immunity plays in the development of protection against TB.

NOD2 regulates microglial inflammation through the TAK1-NF-κB pathway and autophagy activation in murine pneumococcal meningitis

Streptococcus pneumoniae is responsible for pneumococcal meningitis, with significant mortality and morbidity worldwide. Microglial inflammation plays a vital role in meningitis. The peptidoglycan sensor NOD2 (nucleotide-binding oligomerization domain 2) has been identified to promote microglia activation, but the role in autophagy following pneumococcal meningitis remains unclear. In the present study, we investigated the role of NOD2 in microglial inflammation and autophagy, as well as related signaling pathways, during S. pneumonia infection. NOD2 expression was knocked down by the injection of lentivirus-mediated short-hairpin RNA (shRNA). Our results revealed that NOD2 promotes microglial inflammation by increasing inflammatory mediators. We also showed that the TAK1-NF-κB pathway is involved in this process. In addition, NOD2 increased the expression of autophagy-related proteins and induced autophagosome formation. Rapamycin and 3-MA were utilized to assess the role of autophagy in microglial inflammation induced by S. pneumonia. We demonstrated that autophagy serves as a cellular defense mechanism to reduce inflammatory mediators. Similar to the in vitro results, NOD2 induced inflammation and autophagy in the brain in a mouse meningitis model. Moreover, NOD2 silencing significantly reduced brain edema and improved the neurological function of pneumococcal meningitis mice. Taken together, these data demonstrate that NOD2 promotes microglial inflammation and autophagy in murine pneumococcal meningitis, and the TAK1-NF-κB pathway is involved in microglial activation.

IL-1α and IL-1β promote NOD2-induced immune responses by enhancing MAPK signaling

Both toll-like receptors (TLRs) and nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) induce a tightly regulated inflammatory response at risk of causing tissue damage, depending on the effectiveness of ensuing negative feedback regulatory mechanisms. Cross-regulation between TLRs, NLRs, and cytokine receptors has been observed. However, the cross-regulation between interleukin-1 (IL-1) receptors and NOD2 is not completely understood. In this study, we found that IL-1α/β increased NOD2-induced inflammatory response in human monocytic THP1 cells, peripheral blood mononuclear cells (PBMCs), mouse macrophage RWA264.7 cells and spleen cells, and in an in vivo experiment. IL-1α/β pre-treatment induced the production of CXC chemokines, including growth-regulated oncogene (GRO)-α, GRO-β, and IL-8, and proinflammatory cytokines, including IL-1β, IL-6, and TNFα, which are induced by the activation of NOD2, in a dose- and time-dependent manner. However, pre-treatment with the NOD2 ligand muramyl dipeptide (MDP) did not up-regulate the expression of cytokines induced by IL-1α/β re-treatment. IL-1β treatment increased the expression of A20, which is an important inhibitor of the innate immune response. However, the overexpression of A20 failed to inhibit MDP-induced cytokine production, suggesting that A20 had no effects on the NOD2-induced immune response. In addition, IL-1α/β increased the expression of NOD2 and its downstream adaptor RIP2, and IL-1α/β pre-treatment increased MDP-induced activation of mitogen-activated protein kinases (MAPKs), including ERK, JNK, and P38, which contributed to MDP-induced cytokine production. Based on these results, IL-1α/β promote the NOD2-induced immune responses by enhancing MDP-induced activation of MAPK signaling pathways.

DNA from virulent M. tuberculosis induces TNF-α production and autophagy in M1 polarized macrophages

The macrophage innate immune response is outlined through recognition of the components of Mycobacterium tuberculosis. DNA of M. tuberculosis (MtbDNA) is recognized by macrophages, but the implications of this recognition are poorly characterized. Stimulation of murine macrophages with MtbDNA induces autophagy, a process that promotes elimination of intracellular pathogens. However, it remains unknown whether this or other phenomena also occur in human cells. In this work, we studied the innate response profiles of human macrophages after stimulation with DNA from virulent M. tuberculosis H37Rv. Human monocyte-derived macrophages were polarized into M1 and M2 phenotypes and stimulated with MtbDNA. The plasma membrane markers of the phenotype, production of TNF-α, and induction of autophagy were evaluated. Our results indicate that MtbDNA induced phenotypical changes, the significant production of TNF-α, and autophagy confirmed by the augmented expression of immunity related GTPase M (IRGM) and autophagy related ATG16L1 genes in M1 macrophages, whereas M2 macrophages exhibited limited responses. In addition, MtbDNA activation was TLR-9-dependent. Although TLR-9 expression was similar between M1 and M2 macrophages, only M1 macrophages were fully responsive to MtbDNA. In conclusion, MtbDNA recognition enhanced the antimicrobial mechanisms of M1 macrophages.

Antimicrobial and immunomodulatory activity induced by loperamide in mycobacterial infections

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Loperamide modulates macrophages immune responses towards mycobacteria.

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Loperamide is an immunoregulator of inflammation during mycobacterial infection.

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Loperamide induces immunomodulatory responses and bactericidal mechanisms.

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The activation of opioid receptors by loperamide is involved in its immunomodulatory activity.

Reinforcing the Functionality of Mononuclear Phagocyte System to Control Tuberculosis

The mononuclear phagocyte system (MPS) constitutes dendritic cells, monocytes, and macrophages. This system contributes to various functions that are essential for maintaining homeostasis, activation of innate immunity, and bridging it with the adaptive immunity. Consequently, MPS is highly important in bolstering immunity against the pathogens. However, MPS is the frontline cells in destroying Mycobacterium tuberculosis (Mtb), yet the bacterium prefers to reside in the hostile environment of macrophages. Therefore, it may be very interesting to study the struggle between Mtb and MPS to understand the outcome of the disease. In an event when MPS predominates Mtb, the host remains protected. By contrast, the situation becomes devastating when the pathogen tames and tunes the host MPS, which ultimately culminates into tuberculosis (TB). Hence, it becomes extremely crucial to reinvigorate MPS functionality to overwhelm Mtb and eliminate it. In this article, we discuss the strategies to bolster the function of MPS by exploiting the molecules associated with the innate immunity and highlight the mechanisms involved to overcome the Mtb-induced suppression of host immunity. In future, such approaches may provide an insight to develop immunotherapeutics to treat TB.

Innate immunity in tuberculosis: host defense vs pathogen evasion

The major innate immune cell types involved in tuberculosis (TB) infection are macrophages, dendritic cells (DCs), neutrophils and natural killer (NK) cells. These immune cells recognize the TB-causing pathogen Mycobacterium tuberculosis (Mtb) through various pattern recognition receptors (PRRs), including but not limited to Toll-like receptors (TLRs), Nod-like receptors (NLRs) and C-type lectin receptors (CLRs). Upon infection by Mtb, the host orchestrates multiple signaling cascades via the PRRs to launch a variety of innate immune defense functions such as phagocytosis, autophagy, apoptosis and inflammasome activation. In contrast, Mtb utilizes numerous exquisite strategies to evade or circumvent host innate immunity. Here we discuss recent research on major host innate immune cells, PRR signaling, and the cellular functions involved in Mtb infection, with a specific focus on the host's innate immune defense and Mtb immune evasion. A better understanding of the molecular mechanisms underlying host-pathogen interactions could provide a rational basis for the development of effective anti-TB therapeutics.

Nucleotide-Binding Oligomerization Domain 2 Contributes to Limiting Growth of Mycobacterium abscessus in the Lung of Mice by Regulating Cytokines and Nitric Oxide Production

Mycobacterium abscessus is a prominent cause of pulmonary infection in immunosuppressed patients and those with cystic fibrosis. Nucleotide-binding oligomerization domain (NOD) 2 is a cytosolic receptor which senses a bacterial peptidoglycan component, muramyl dipeptide (MDP). Although nucleotide-binding oligomerization domain 2 (NOD2) contributes to protect host against various microbial infections, it is still unclear whether NOD2 is essential to regulate host immune responses against M. abscessus infection. In this study, we sought to clarify the role of NOD2 and the underlying mechanism in host defense against M. abscessus infection. Mice were infected intranasally with M. abscessus and sacrificed at indicated time points. Bacterial survival, cytokines production, and pathology in the lungs were determined. Bone marrow-derived macrophages were used to clarify cellular mechanism of NOD2-mediated immune response. Bacterial clearance was impaired, and pathology was more severe in the lungs of NOD2-deficient mice compared with the wild-type mice. In macrophages, NOD2-mediated activation of p38 and JNK were required for production of proinflammatory cytokines and nitric oxide (NO) and expression of iNOS in response to M. abscessus. NO was critical for limiting intracellular growth of the pathogen. Intranasal administration of MDP reduced in vivo bacterial replication and thus improved lung pathology in M. abscessus-infected mice. This study offers important new insights into the potential roles of the NOD2 in initiating and potentiating innate immune response against M. abscessus pulmonary infection.

Analysis of host-pathogen modulators of autophagy during Mycobacterium Tuberculosis infection and therapeutic repercussions

Mycobacterium tuberculosis is one of the most deadly human pathogens known today in modern world, responsible for about 1.5 million deaths annually. Development of TB disease occurs only in 1 out of 10 individuals exposed to the pathogen which indicates that the competent host defense mechanisms exist in majority of the hosts to control the infection. In the last decade, autophagy has emerged as a key host immune defense mechanism against intracellular M. tuberculosis infection. Autophagy has been demonstrated not only as an effective antimicrobial mechanism for the clearance of M. tuberculosis, but the process has also been suggested to prevent excessive inflammation to avoid the adverse effects of infection on host. Nevertheless, increasing evidences also show that in order to enhance its intracellular survival, M. tuberculosis has also evolved multiple strategies to compromise the optimal functioning of host autophagic machinery. This review describes an overview of the various host signaling pathways such as pattern recognition receptors, cytokines, nutrient starvation and other cellular stress that have been implicated in induction of autophagy during M. tuberculosis infection. The review also chalk out the complex interplay of several bacterial factors of M. tuberculosis that are known to be involved in compromising autophagy mediated defense of the host. A comprehensive understanding of the interaction of bacterial and host factors at the intersections of autophagic pathways could provide integrative insights for the development of autophagy-based prophylactics and novel therapeutic interventions for TB.

High glucose induces O-GlcNAc glycosylation of the vitamin D receptor (VDR) in THP1 cells and in human macrophages derived from monocytes

Chronic hyperglycemia increases the carbon flux through the hexosamine pathway, allowing the accumulation of UDP-GlcNAc. UDP-GlcNAc is the sugar donor for the enzyme-mediated protein glycosylation event known as OGlcNAcylation. This posttranslational modification targets several transcription factors implicated in glucose toxicity, insulin resistance, and diabetes. Vitamin D plays an important role in glucose homeostasis and insulin secretion through transcriptional mechanisms mediated by its receptor (VDR). Vitamin D deficiency has been associated with higher susceptibility to bacterial diseases in diabetic patients. However, it has not been explored whether VDR is subject to OGlcNAcylation or whether high glucose affects its transcriptional or biological activities. The aim of this study was to evaluate the effect of hyperglycemia on VDR OGlcNAcylation and its effects on vitamin D-mediated transcription. We predicted potential OGlcNAcylation sites using free software. Our results showed that hyperglycemia (30 mM) induces the OGlcNAcylation of VDR in THP1 cells and in human macrophages derived from monocytes (MDM). This condition did not hamper the vitamin D-dependent activation of LL-37 gene expression, and even did not impair the macrophage bactericidal activity. Our study provides new insight into vitamin D receptor posttranslational modification that may have relevance on the physiological responses of long-term hyperglycemia.

Expression and function of NOD-like receptors by human term gestation-associated tissues

INTRODUCTION

Nucleotide-binding oligomerization domain (NOD)-like receptors or NOD-like receptors (NLRs) have been implicated in several disease pathologies associated with inflammation. Since local and systemic inflammation is a hallmark of both term and preterm labour, a role for NLRs at the materno-fetal interface has been postulated.

METHODS

Gene expression and immunolocalisation of NLR family members in human placenta, choriodecidua, and amnion were examined. Tissue explants were used to examine the response to activators of NOD1 (Tri-DAP), NOD2 (MDP) and NLRP3 (nigericin). Cell/tissue-free supernatants were examined for the production of interleukin (IL)-1β, IL-6, IL-8 and IL-10 using specific ELISAs.

RESULTS

Expression of transcripts for NOD1, NOD2, NLRP3, NLRC4, NLRX1, NLRP1 and NAIP and protein expression of NOD1, NOD2 and NLRP3 were a broad feature of all term gestation-associated tissues. Production of cytokines was increased significantly in response to all ligands in placenta and choriodecidua, except for MDP-induced IL-10. Similarly, there was a significant in the amnion except for MDP induced IL-1β and IL-10 response to either agonist. IL-1β production was dependent on caspase-1 regardless of agonist used or tissue examined.

DISCUSSION

Term human gestation-associated tissues express functional NLRs which likely play a role in both sterile and pathogen-driven inflammatory responses at the materno-fetal interface.

Innate Immune Responses to Tuberculosis

Tuberculosis remains one of the greatest threats to human health. The causative bacterium, Mycobacterium tuberculosis, is acquired by the respiratory route. It is exquisitely adapted to humans and is a prototypic intracellular pathogen of macrophages, with alveolar macrophages being the primary conduit of infection and disease. However, M. tuberculosis bacilli interact with and are affected by several soluble and cellular components of the innate immune system which dictate the outcome of primary infection, most commonly a latently infected healthy human host, in whom the bacteria are held in check by the host immune response within the confines of tissue granuloma, the host histopathologic hallmark. Such individuals can develop active TB later in life with impairment in the immune system. In contrast, in a minority of infected individuals, the early host immune response fails to control bacterial growth, and progressive granulomatous disease develops, facilitating spread of the bacilli via infectious aerosols. The molecular details of the M. tuberculosis-host innate immune system interaction continue to be elucidated, particularly those occurring within the lung. However, it is clear that a number of complex processes are involved at the different stages of infection that may benefit either the bacterium or the host. In this article, we describe a contemporary view of the molecular events underlying the interaction between M. tuberculosis and a variety of cellular and soluble components and processes of the innate immune system.

Human Immunology of Tuberculosis

Immunology is a central theme when it comes to tuberculosis (TB). The outcome of human infection with Mycobacterium tuberculosis is dependent on the ability of the immune response to clear or contain the infection. In cases where this fails, the bacterium replicates, disseminates within the host, and elicits a pathologic inflammatory response, and disease ensues. Clinical presentation of TB disease is remarkably heterogeneous, and the disease phenotype is largely dependent on host immune status. Onward transmission of M. tuberculosis to new susceptible hosts is thought to depend on an excessive inflammatory response causing a breakdown of the lung matrix and formation of lung cavities. But this varies in cases of underlying immunological dysfunction: for example, HIV-1 infection is associated with less cavitation, while diabetes mellitus comorbidity is associated with increased cavitation and risk of transmission. In compliance with the central theme of immunology in tuberculosis, we rely on detection of an adaptive immune response, in the form of interferon-gamma release assays or tuberculin skin tests, to diagnose infection with M. tuberculosis. Here we review the immunology of TB in the human host, focusing on cellular and humoral adaptive immunity as well as key features of innate immune responses and the underlying immunological dysfunction which associates with human TB risk factors. Our review is restricted to human immunology, and we highlight distinctions from the immunological dogma originating from animal models of TB, which pervade the field.

Absence of the NOD2 protein renders epithelia more susceptible to barrier dysfunction due to mitochondrial dysfunction

Irregular mitochondria structure and reduced ATP in some patients with IBD suggest that metabolic stress contributes to disease. Loss-of-function mutation in the nucleotide-binding oligomerization domain (NOD)-2 gene is a major susceptibility trait for IBD. Hence, we assessed if loss of NOD2 further impairs the epithelial barrier function instigated by disruption of mitochondrial ATP synthesis via the hydrogen ionophore dinitrophenol (DNP). NOD2 protein (virtually undetectable in epithelia under basal conditions) was increased in T84 (human colon cell line) cells treated with noninvasive Escherichia coli + DNP (16 h). Increased intracellular bacteria in wild-type (WT) and NOD2 knockdown (KD) cells and colonoids from NOD2^-/- mice were mediated by reactive oxygen species (ROS) and the MAPK ERK1/2 pathways as determined by cotreatment with the antioxidant mitoTEMPO and the ERK inhibitor U0126: ROS was upstream of ERK1/2 activation. Despite increased E. coli in DNP-treated NOD2 KD compared with WT cells, there were no differences in the internalization of fluorescent inert beads or dead E. coli particles. This suggests that lack of killing in the NOD2 KD cells was responsible for the increased numbers of viable intracellular bacteria; a conclusion supported by evidence of reduced autophagy in NOD2 KD T84 epithelia. Thus, in a two-hit hypothesis, decreased barrier function due to dysfunctional mitochondrial is amplified by lack of NOD2 in transporting enterocytes: subsequently, greater numbers of bacteria entering the mucosa would be a significant inflammatory threat especially since individuals with NOD2 mutations have compromised macrophage and Paneth cell responses to bacteria.NEW & NOTEWORTHY Increased internalization of bacteria by epithelia with dysfunctional mitochondria (reduced ATP) is potentiated if the cells lack nucleotide-binding oligomerization domain 2 (NOD2), mutations in which are inflammatory bowel disease-susceptibility traits. Uptake of bacteria was dependent on reactive oxygen species and MAP-kinase activity, and the increased viable intracellular bacteria in NOD2^-/- cells likely reflect a reduced ability to recognize and kill bacteria. Thus a significant barrier defect occurs with NOD2 deficiency in conjunction with metabolic stress that could contribute to inflammation.

Role of apoptosis and autophagy in tuberculosis

Tuberculosis (TB) is one of the oldest known human diseases and is transmitted by the bacteria Mycobacterium tuberculosis (Mtb). TB has a rich history with evidence of TB infections dating back to 5,800 bc TB is unique in its ability to remain latent in an individual for decades, with the possibility of later reactivation, causing widespread systemic symptoms. Currently, it is estimated that more than one-third of the world's population (~2 billion people) are infected with Mtb. Prolonged periods of therapy and complexity of treatment regimens, especially in active infection, have led to poor compliance in patients being treated for TB. Therefore, it is vitally important to have a thorough knowledge of the pathophysiology of Mtb to understand the disease progression, as well as to develop novel diagnostic tests and treatments. Alveolar macrophages represent both the primary host cell and the first line of defense against the Mtb infection. Apoptosis and autophagy of macrophages play a vital role in the pathogenesis and also in the host defense against Mtb. This review will outline the role of these two cellular processes in defense against Mtb with particular emphasis on innate immunity and explore developing therapies aimed at altering host responses to the disease.

Molecular basis of mycobacterial survival in macrophages

Macrophages play an essential role in the immune system by ingesting and degrading invading pathogens, initiating an inflammatory response and instructing adaptive immune cells, and resolving inflammation to restore homeostasis. More interesting is the fact that some bacteria have evolved to use macrophages as a natural habitat and tools of spread in the host, e.g., Mycobacterium tuberculosis (Mtb) and some non-tuberculous mycobacteria (NTM). Mtb is considered one of humanity's most successful pathogens and is the causal agent of tuberculosis, while NTMs cause opportunistic infections all of which are of significant public health concern. Here, we describe mechanisms by which intracellular pathogens, with an emphasis on mycobacteria, manipulate macrophage functions to circumvent killing and live inside these cells even under considerable immunological pressure. Such macrophage functions include the selective evasion or engagement of pattern recognition receptors, production of cytokines, reactive oxygen and nitrogen species, phagosome maturation, as well as other killing mechanisms like autophagy and cell death. A clear understanding of host responses elicited by a specific pathogen and strategies employed by the microbe to evade or exploit these is of significant importance for the development of effective vaccines and targeted immunotherapy against persistent intracellular infections like tuberculosis.

Human macrophages chronically exposed to LPS can be reactivated by stimulation with MDP to acquire an antimicrobial phenotype

Macrophages are important in host defense and can differentiate into functionally distinct subsets named classically (M1) or alternatively (M2) activated. In several inflammatory disorders, macrophages become tolerized to prevent deleterious consequences. This tolerization reduces the ability of macrophages to respond to bacterial components (e.g., LPS) maintaining a low level of inflammation but compromising the ability of macrophages to mount an effective immune response during subsequent pathogen encounters. In this study, we aimed to reactivate human monocyte-derived macrophages chronically exposed to LPS by re-stimulation with muramyl dipeptide (MDP). We observed an undefined profile of cell surface marker expression during endotoxin tolerance and absence of TNFα production. Stimulating macrophages chronically exposed to LPS with LPS+MDP restored TNFα, production together with an increased production of IL1, IL6, IFNγ, IL4, IL5 and IL10. These results suggest that macrophages chronically exposed to LPS possess a mixed M1-M2 phenotype with sufficient antimicrobial and homeostatic potential.

Bacterial Pathogens versus Autophagy: Implications for Therapeutic Interventions

Research in recent years has focused significantly on the role of selective macroautophagy in targeting intracellular pathogens for lysosomal degradation, a process termed xenophagy. In this review we evaluate the proposed roles for xenophagy in controlling bacterial infection, highlighting the concept that successful pathogens have evolved ways to subvert or exploit this defense, minimizing the actual effectiveness of xenophagy in innate immunity. Instead, studies in animal models have revealed that autophagy-associated proteins often function outside of xenophagy to influence bacterial pathogenesis. In light of current efforts to manipulate autophagy and the development of host-directed therapies to fight bacterial infections, we also discuss the implications stemming from the complicated relationship that exists between autophagy and bacterial pathogens.

Autophagy in leukocytes and other cells: mechanisms, subsystem organization, selectivity, and links to innate immunity

Autophagy is a fundamental biologic process that fulfills general and specialized roles in cytoplasmic homeostasis. The cell-autonomous antimicrobial functions of autophagy have been established in the macrophage. These cells and other leukocytes continue to be the cells of choice in studying autophagy in immunity and inflammation. This review uses several model examples that will be of interest to leukocyte and cell biologists alike. Furthermore, it comprehensively covers the subsystems in autophagy as they apply to all mammalian cells and incorporates the recent progress in our understanding of how these modules come together-a topic that should be of interest to all readers.

Targeted pulmonary delivery of inducers of host macrophage autophagy as a potential host-directed chemotherapy of tuberculosis

One of the promising host-directed chemotherapeutic interventions in tuberculosis (TB) is based on inducing autophagy as an immune effector. Here we consider the strengths and weaknesses of potential autophagy-based pharmacological intervention. Using the existing drugs that induce autophagy is an option, but it has limitations given the broad role of autophagy in most cells, tissues, and organs. Thus, it may be desirable that the agent being used to modulate autophagy is applied in a targeted manner, e.g. delivered to affected tissues, with infected macrophages being an obvious choice. This review addresses the advantages and disadvantages of delivering drugs to induce autophagy in M. tuberculosis-infected macrophages. One option, already being tested in models, is to design particles for inhalation delivery to lung macrophages. The choice of drugs, drug release kinetics and intracellular residence times, non-target cell exposure and feasibility of use by patients is discussed. We term here this (still experimental) approach, of compartment-targeting, autophagy-based, host-directed therapy as "Track-II antituberculosis chemotherapy."

Immunometabolism within the tuberculosis granuloma: amino acids, hypoxia, and cellular respiration

Tuberculosis (TB) granulomas are compact, organized agglomerations of infected and uninfected macrophages, T cells, neutrophils, and other immune cells. Within the granuloma, several unique metabolic adaptations occur to modify the behavior of immune cells, potentially favoring bacterial persistence balanced with protection against immunopathology. These include the induction of arginase-1 in macrophages to temper nitric oxide (NO) production and block T cell proliferation, inhibition of oxygen-requiring NO production in hypoxic regions, and induction of tryptophan-degrading enzymes that modify T cell proliferation and function. The spatial and time-dependent organization of granulomas further influences immunometabolism, for example through lactate production by activated macrophages, which can induce arginase-1. Although complex, the metabolic changes in and around TB granulomas can be potentially modified by host-directed therapies. While elimination of the TB bacilli is often the goal of any anti-TB therapy, host-directed approaches must also account for the possibility of immunopathologic damage to the lung.