Mycobacterium's arrest of phagosome maturation in macrophages requires Rab5 activity and accessibility to iron

Proteome-Wide Covalent Targeting of Acidic Residues with Tunable N-Aryl Aziridines

Carboxylate side chains in aspartic and glutamic acids play critical roles in protein structure and function due to their polarity and negative charge. These acidic residues, which are abundant in high-value drug targets, represent attractive yet underexplored hotspots for covalent inhibitor discovery. In this study, we introduce N-aryl aziridines as a systematically tunable, chemoselective scaffold for covalent targeting of carboxylates across the proteome. Using a library of N-pyridinium aziridine-based fragments combined with chemoproteomics-enabled target deconvolution, we identified lead hits for aspartates and glutamates in proteins such as mitochondrial carrier homologue 2 (MTCH2), RUN and FYVE domain-containing protein 1 (RUFY1), and delta(24)-sterol reductase (DHCR24). Modular build-and-couple synthetic logic enabled fragment evolution via Ni-catalyzed cross-coupling to access N-aryl aziridines with enhanced affinities for MTCH2 and RUFY1. Notably, N-aryl aziridine 5b selectively modified RUFY1 at E502, disrupting its interactions within the endosomal trafficking network and impairing receptor recycling. This work establishes N-aryl aziridines as versatile carboxylate-targeting covalent inhibitor scaffolds, broadening the scope of covalent ligand discovery.

Targeting Autophagy as a Strategy for Developing New Host-Directed Therapeutics Against Nontuberculous Mycobacteria

Nontuberculous mycobacteria (NTMs) are increasingly being recognized as opportunistic pathogens in clinical practice because of their innate resistance to antimicrobial treatment and the widespread increase in multidrug-resistant strains on a global scale. NTMs pose a tremendous infection management challenge, especially in individuals with pre-existing lung conditions, as well as those who are immunocompromised. NTMs' capability to evade or suppress the immune responses of their host is a key feature that makes them a cause of persistent chronic infection. Autophagy, an essential cellular defense mechanism that delivers and breaks down intracellular materials in lysosomes, protects the host from mycobacterial infection. Initial studies have revealed encouraging therapeutic strategies that augment endogenous autophagic mechanisms or block harmful host responses, thus having the potential to decrease intracellular mycobacterial infection, including that caused by multidrug-resistant strains. This review discusses how NTMs can evade autophagic mechanisms and considers the possibilities of using autophagy-inducing agents to develop novel therapeutic strategies to combat NTM infection.

Microvesicles Derived from Human Bronchial Epithelial Cells Regulate Macrophage Activation During Mycobacterium abscessus Infection

Intercellular communication is important for host immunity in response to bacterial infections. Nontuberculous mycobacterium (NTM), such as Mycobacterium abscessus (M. ab), is a group of environmental bacteria that can cause severe lung infections in individuals with pre-existing lung conditions, including cystic fibrosis (CF) and chronic obstructive pulmonary disease (COPD). There is limited knowledge understanding the interaction between airway epithelial cells and immune cells during NTM infections. In this study, we characterized microvesicles (MVs) released from uninfected and M. ab-infected human bronchial epithelial cells and investigated the effect of these MVs on the activation and polarization of THP-1-derived macrophages in cell culture. Our results indicate that MVs released by M. ab-infected human bronchial epithelial cells stimulated the activation of M2-polarized macrophages in cell culture when compared to MVs released by uninfected cells. Additionally, the proteomic analysis for isolated MVs showed that the proteins involved in the cell adhesion pathway were enriched in MVs from M. ab-infected human bronchial epithelial cells compared to MVs from uninfected cells. Among those, the cell surface protein, intercellular adhesion molecule 1 (ICAM-1), regulated the uptake of MVs released by M. ab-infected human bronchial epithelial cells by recipient macrophages in cell culture. In conclusion, our data suggest that in response to M. ab infection, human airway epithelial cells release MVs to modulate the activation of macrophages, which are key cells for mycobacterial intracellular survival in the host.

Insights into Autophagy in Microbiome Therapeutic Approaches for Drug-Resistant Tuberculosis

Tuberculosis, primarily caused by Mycobacterium tuberculosis, is an airborne lung disease and continues to pose a significant global health threat, resulting in millions of deaths annually. The current treatment for tuberculosis involves a prolonged regimen of antibiotics, which leads to complications such as recurrence, drug resistance, reinfection, and a range of side effects. This scenario underscores the urgent need for novel therapeutic strategies to combat this lethal pathogen. Over the last two decades, microbiome therapeutics have emerged as promising next-generation drug candidates, offering advantages over traditional medications. In 2022, the Food and Drug Administration approved the first microbiome therapeutic for recurrent Clostridium infections, and extensive research is underway on microbiome treatments for various challenging diseases, including metabolic disorders and cancer. Research on microbiomes concerning tuberculosis commenced roughly a decade ago, and the scope of this research has broadened considerably over the last five years, with microbiome therapeutics now viewed as viable options for managing drug-resistant tuberculosis. Nevertheless, the understanding of their mechanisms is still in its infancy. Although autophagy has been extensively studied in other diseases, research into its role in tuberculosis is just beginning, with preliminary developments in progress. Against this backdrop, this comprehensive review begins by succinctly outlining tuberculosis' characteristics and assessing existing treatments' strengths and weaknesses, followed by a detailed examination of microbiome-based therapeutic approaches for drug-resistant tuberculosis. Additionally, this review focuses on establishing a basic understanding of microbiome treatments for tuberculosis, mainly through the lens of autophagy as a mechanism of action. Ultimately, this review aims to contribute to the foundational comprehension of microbiome-based therapies for tuberculosis, thereby setting the stage for the further advancement of microbiome therapeutics for drug-resistant tuberculosis.

Novel fusion protein REA induces robust prime protection against tuberculosis in mice

While many novel candidates for tuberculosis vaccines are presently undergoing pre-clinical or clinical trials, none of them have been able to eliminate infection entirely. In this study, we engineered a potent chimeric protein vaccine candidate, Rv2299cD2D3-ESAT-6-Ag85B (REA), which induced Th1 and Th17 responses via dendritic cell maturation. REA-activated macrophages operated the killing mechanisms of Mycobacterium tuberculosis (MTB), such as phagosomal maturation and phagolysosome fusion, through the (PI3K)-p38 MAPK-Ca2+-NADPH oxidase pathway. Dendritic cells and macrophages activated by REA elicited synergistic anti-mycobacterial responses. Notably, REA-immunized mice suppressed MTB growth to undetectable levels at 16 weeks post-infection, which was supported by gross and pathologic findings and acid-fast staining of the lung tissues, and maintained antigen-specific multifunctional IFN-γ+IL-2+TNF-α CD4+ T and long-lasting T cells producing cytokines in the tissues. Our findings suggest that REA is an outstanding prime prophylactic vaccine candidate against tuberculosis.

Mycobacteriophages: therapeutic approach for mycobacterial infections

Tuberculosis (TB) is a significant global health threat, and cases of infection with non-tuberculous mycobacteria (NTM) causing lung disease (NTM-LD) are rising. Bacteriophages and their gene products have garnered interest as potential therapeutic options for bacterial infections. Here, we have compiled information on bacteriophages and their products that can kill Mycobacterium tuberculosis or NTM. We summarize the mechanisms whereby viable phages can access macrophage-resident bacteria and not elicit immune responses, review methodologies of pharmaceutical product development containing mycobacteriophages and their gene products, mainly lysins, in the context of drug regulatory requirements and we discuss industrially relevant methods for producing pharmaceutical products comprising mycobacteriophages, emphasizing delivery of mycobacteriophages to the lungs. We conclude with an outline of some recent case studies on mycobacteriophage therapy.

Intracellular iron accumulation facilitates mycobacterial infection in old mouse macrophages

Aging has a significant impact on the immune system, leading to a gradual decline in immune function and changes in the body's ability to respond to bacterial infections. Non-tuberculous mycobacteria (NTM), also known as atypical mycobacteria or environmental mycobacteria, are commonly found in soil, water, and various environmental sources. While many NTM species are considered opportunistic pathogens, some can cause significant infections, particularly in individuals with compromised immune systems, such as older individuals. When mycobacteria enter the body, macrophages are among the first immune cells to encounter them and attempt to engulf mycobacteria through a process called phagocytosis. Some NTM species, including Mycobacterium avium (M. avium) can survive and replicate within macrophages. However, little is known about the interaction between NTM and macrophages in older individuals. In this study, we investigated the response of bone marrow-derived macrophage (BMMs) isolated from young (5 months) and old (25 months) mice to M. avium serotype 4, one of the main NTM species in patients with pulmonary NTM diseases. Our results demonstrated that BMMs from old mice have an increased level of intracellular iron and are more susceptible to M. avium serotype 4 infection compared to BMMs from young mice. The whole-cell proteomic analysis indicated a dysregulated expression of iron homeostasis-associated proteins in old BMMs regardless of mycobacterial infection. Deferoxamine, an iron chelator, significantly rescued mycobacterial killing and phagolysosome maturation in BMMs from old mice. Therefore, our data for the first time indicate that an intracellular iron accumulation improves NTM survival within macrophages from old mice and suggest a potential application of iron-chelating drugs as a host-directed therapy for pulmonary NTM infection in older individuals.

Structural basis for specific inhibition of salicylate synthase from Mycobacterium abscessus

Blocking iron uptake and metabolism has been emerging as a promising therapeutic strategy for the development of novel antimicrobial compounds. Like all mycobacteria, M. abscessus (Mab) has evolved several countermeasures to scavenge iron from host carrier proteins, including the production of siderophores, which play a crucial role in these processes. In this study, we solved, for the first time, the crystal structure of Mab-SaS, the first enzyme involved in the biosynthesis of siderophores. Moreover, we screened a small, focused library and identified a compound exhibiting a potent inhibitory effect against Mab-SaS (IC50 ≈ 2 μM). Its binding mode was investigated by means of Induced Fit Docking simulations, performed on the crystal structure presented herein. Furthermore, cytotoxicity data and pharmacokinetic predictions revealed the safety and drug-likeness of this class of compounds. Finally, the crystallographic data were used to optimize the model for future virtual screening campaigns. Taken together, the findings of our study pave the way for the identification of potent Mab-SaS inhibitors, based on both established and unexplored chemotypes.

Fungal mechanisms of intracellular survival: what can we learn from bacterial pathogens?

Fungal infections represent a major, albeit neglected, public health threat with serious medical and economic burdens globally. With unacceptably high mortality rates, invasive fungal pathogens are responsible for millions of deaths each year, with a steadily increasing incidence primarily in immunocompromised individuals. The poor therapeutic options and rise of antifungal drug resistance pose further challenges in controlling these infections. These fungal pathogens have adapted to survive within mammalian hosts and can establish intracellular niches to promote survival within host immune cells. To do that, they have developed diverse methods to circumvent the innate immune system attack. This includes strategies such as altering their morphology, counteracting macrophage antimicrobial action, and metabolic adaptation. This is reminiscent of how bacterial pathogens have adapted to survive within host cells and cause disease. However, relative to the great deal of information available concerning intracellular bacterial pathogenesis, less is known about the mechanisms fungal pathogens employ. Therefore, here we review our current knowledge and recent advances in our understanding of how fungi can evade and persist within host immune cells. This review will focus on the major fungal pathogens, including Cryptococcus neoformans, Candida albicans, and Aspergillus fumigatus, among others. As we discover and understand the strategies used by these fungi, similarities with their bacterial counterparts are becoming apparent, hence we can use the abundant information from bacteria to guide our studies in fungi. By understanding these strategies, new lines of research will open that can improve the treatments of these devastating fungal diseases.

Additive Effects of Cyclic Peptide [R4W4] When Added Alongside Azithromycin and Rifampicin against Mycobacterium avium Infection

Mycobacterium avium (M. avium), a type of nontuberculous mycobacteria (NTM), poses a risk for pulmonary infections and disseminated infections in immunocompromised individuals. Conventional treatment consists of a 12-month regimen of the first-line antibiotics rifampicin and azithromycin. However, the treatment duration and low antibiotic tolerability present challenges in the treatment of M. avium infection. Furthermore, the emergence of multidrug-resistant mycobacterium strains prompts a need for novel treatments against M. avium infection. This study aims to test the efficacy of a novel antimicrobial peptide, cyclic [R4W4], alongside the first-line antibiotics azithromycin and rifampicin in reducing M. avium survival. Colony-forming unit (CFU) counts were assessed after treating M. avium cultures with varying concentrations of cyclic [R4W4] alone or in conjunction with azithromycin or rifampicin 3 h and 4 days post-treatment. M. avium growth was significantly reduced 4 days after cyclic [R4W4] single treatment. Additionally, cyclic [R4W4]-azithromycin and cyclic [R4W4]-rifampicin combination treatments at specific concentrations significantly reduced M. avium survival 3 h and 4 days post-treatment compared with single antibiotic treatment alone. These findings demonstrate cyclic [R4W4] as a potent treatment method against M. avium and provide insight into novel therapeutic approaches against mycobacterium infections.

Stage of infection with Mycobacterium avium subsp. paratuberculosis impacts expression of Rab5, Rab7, and CYP27B1 in macrophages within the ileum of naturally infected cows

Introduction

Macrophages are the preferential target of Mycobacterium avium subsp. paratuberculosis (MAP), the etiologic agent of ruminant paratuberculosis. Uptake of pathogens by intestinal macrophages results in their trafficking through endosomal compartments, ultimately leading to fusion with an acidic lysosome to destroy the pathogen. MAP possesses virulence factors which disrupt these endosomal pathways. Additionally, levels of serum vitamin D₃ have proven relevant to host immunity. Dynamics of endosomal trafficking and vitamin D₃ metabolism have been largely unexplored in bovine paratuberculosis.

Methods

This study aimed to characterize expression of early and late endosomal markers Rab5 and Rab7, respectively, within CD68+ macrophages in frozen mid-ileum sections harvested from cows at different stages of natural paratuberculosis infection. Additionally, factors of vitamin D₃ signaling and metabolism were characterized through expression of vitamin D₃ activating enzyme 1α-hydroxylase (CYP27B1), vitamin D₃ inactivating enzyme 24-hydroxylase (CYP24A1), and vitamin D₃ receptor (VDR) within CD68+ ileal macrophages.

Results and discussion

Cows with clinical paratuberculosis had significantly greater macrophage and MAP burden overall, as well as intracellular MAP. Total expression of Rab5 within macrophages was reduced in clinical cows; however, Rab5 and MAP colocalization was significantly greater in this group. Intracellular Rab7 colocalization with MAP was not detected in subclinical or Johne's Disease negative (JD-) control cows but was present in clinical cows. Additionally, macrophage CYP27B1 expression was significantly reduced in clinical cows. Taken together, the results from this study show disparate patterns of expression for key mediators in intracellular MAP trafficking and vitamin D metabolism for cows at different stages of paratuberculosis.

Targeting Siderophore-Mediated Iron Uptake in M. abscessus: A New Strategy to Limit the Virulence of Non-Tuberculous Mycobacteria

Targeting pathogenic mechanisms, rather than essential processes, represents a very attractive approach for the development of new antimycobacterial drugs. In this context, iron acquisition routes have recently emerged as potentially druggable pathways. However, the importance of siderophore biosynthesis in the virulence and pathogenicity of M. abscessus (Mab) is still poorly understood. In this study, we investigated the Salicylate Synthase (SaS) of Mab as an innovative molecular target for the development of inhibitors of siderophore production. Notably, Mab-SaS does not have any counterpart in human cells, making it an interesting candidate for drug discovery. Starting from the analysis of the binding of a series of furan-based derivatives, previously identified by our group as inhibitors of MbtI from M. tuberculosis (Mtb), we successfully selected the lead compound 1, exhibiting a strong activity against Mab-SaS (IC₅₀ ≈ 5 µM). Computational studies characterized the key interactions between 1 and the enzyme, highlighting the important roles of Y387, G421, and K207, the latter being one of the residues involved in the first step of the catalytic reaction. These results support the hypothesis that 5-phenylfuran-2-carboxylic acids are also a promising class of Mab-SaS inhibitors, paving the way for the optimization and rational design of more potent derivatives.

Synthesis and in vitro analysis of novel gallium tetrakis(4-methoxyphenyl)porphyrin and its long-acting nanoparticle as a potent antimycobacterial agent

Bacterial heme uptake pathways offer a novel target for antimicrobial drug discovery. Recently, gallium (Ga) porphyrin complexes were found to be effective against mycobacterial heme uptake pathways. The goal of the current study is to build on this foundation and develop a new Ga(III) porphyrin and its nanoparticles, formulated by a single emulsion-evaporation technique to inhibit the growth of Mycobacterium avium complex (MAC) with enhanced properties. Gallium 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin chloride (GaMeOTP) was synthesized from 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin and GaCl₃. GaMeOTP showed enhanced antimicrobial activity against MAC104 and some clinical M. avium isolates. The synthesized Ga(III) porphyrin antimicrobial activity resulted in the overproduction of reactive oxygen species. Our study also demonstrated that F127 nanoparticles encapsulating GaMeOTP exhibited a smaller size than GaTP nanoparticles and a better duration of activity in MAC-infected macrophages compared to the free GaMeOTP. The nanoparticles were trafficked to endosomal compartments within MAC-infected macrophages, likely contributing to the antimicrobial activity of the drug.

High Glucose Concentrations Impair the Processing and Presentation of Mycobacterium tuberculosis Antigens In Vitro

Type 2 diabetes is an established risk factor for tuberculosis, but the underlying mechanisms are largely unknown. We established an in vitro model to analyze the effect of high glucose concentrations in antigen processing and presentation in antigen-presenting cells. Human monocyte-derived macrophages (MDMs) were exposed to high (11 mM and 30 mM) and low (5.5 mM) glucose concentrations and infected with Mycobacterium tuberculosis (Mtb). Flow cytometry was used to analyze the effect of high glucose concentrations in histocompatibility complex (MHC) class II molecules (HLA-DR) and co-stimulatory molecules (CD80 and CD86), indispensable for an adequate antigenic presentation and CD4+ T cell activation. HLA-DR and CD86 were significantly decreased by high glucose concentrations compared with low glucose concentrations. Confocal microscopy was used to detect Rab 5 and Lamp-1, proteins involved in the kinetics of antigen processing as early markers, and Rab 7 and cathepsin D as late markers. We observed a delay in the dynamics of the acquisition of Rab 7 and cathepsin D in high glucose concentrations. Moreover, the kinetics of the formation M. tuberculosis peptide-MHC II complexes in MDMs was decreased under high glucose concentrations, reducing their capacity for T cell activation. These findings suggest that high glucose concentrations directly affect antigenic processing, and therefore antigenic presentation.

The key factors contributing to the risk, diagnosis and treatment of non-tuberculous mycobacterial opportunistic infections

The incidence and prevalence of diseases caused by non-tuberculous mycobacteria (NTM) have been steadily increasing worldwide. NTM are environmental saprophytic organisms; however, a few strains are known to produce diseases in humans affecting pulmonary and extra-pulmonary sites. Although the environment is a major source of NTM infection, recent studies have shown that person-to-person dissemination could be an important transmission route for these microorganisms. Structural and functional lung defects and immunodeficiency are major risk factors for acquiring NTM infections. Diagnosis of NTM diseases is very complex owing to the necessity of distinguishing between a true pathogen and an environmental contaminant. Identification at the species level is critical due to differences in the antibiotic susceptibility patterns of various NTM strains. Such identification is mainly achieved by molecular methods; additionally, mass spectrometry (e.g., MALDI-TOF) is useful for NTM species determination. Natural resistance of NTM species to a wide spectrum of antibiotics makes prescribing treatment for NTM diseases very difficult. NTM therapy usually takes more than one year and requires multi-drug regimens, yet the outcome often remains poor. Therefore, alternatives to antibiotic therapy treatment methods is an area under active exploration. NTM infections are an active global health problem imposing the necessity for better diagnostic tools and more effective treatment methods.

Nutritional immunity: the impact of metals on lung immune cells and the airway microbiome during chronic respiratory disease

Nutritional immunity is the sequestration of bioavailable trace metals such as iron, zinc and copper by the host to limit pathogenicity by invading microorganisms. As one of the most conserved activities of the innate immune system, limiting the availability of free trace metals by cells of the immune system serves not only to conceal these vital nutrients from invading bacteria but also operates to tightly regulate host immune cell responses and function. In the setting of chronic lung disease, the regulation of trace metals by the host is often disrupted, leading to the altered availability of these nutrients to commensal and invading opportunistic pathogenic microbes. Similarly, alterations in the uptake, secretion, turnover and redox activity of these vitally important metals has significant repercussions for immune cell function including the response to and resolution of infection. This review will discuss the intricate role of nutritional immunity in host immune cells of the lung and how changes in this fundamental process as a result of chronic lung disease may alter the airway microbiome, disease progression and the response to infection.

Recombinant Rv1654 protein of Mycobacterium tuberculosis induces mitochondria-mediated apoptosis in macrophage

Mycobacterium tuberculosis contains diverse immunologically active components. This study investigated the biological function of a newly identified component, Rv1654, with the potential to induce apoptosis in macrophages. Recombinant Rv1654 induced macrophage apoptosis in a caspase-9/3-dependent manner through the production of reactive oxygen species (ROS) and interaction with Toll-like receptor 4. In addition, Rv1654 induced the production of tumor necrosis factor-α, interleukin-6, and monocyte chemoattractant protein-1 through the mitogen-activated protein kinase pathway. Furthermore, Rv1654-induced c-Jun N-terminal kinase (JNK) activation was inhibited by the ROS scavenger and Rv1654-induced apoptosis was inhibited by the JNK inhibitor. Moreover, it was found that treatment of macrophages with Rv1654 led to the loss of mitochondrial membrane potential, release of cytochrome c into the cytosol, and translocation of Bax into the mitochondria. Finally, Rv1654-mediated apoptosis was inhibited in macrophages transfected with Bax siRNA. These results suggest that Rv1654 induces macrophage apoptosis through a mitochondrial-dependent pathway and ROS-mediated JNK activation.

Calcium Dynamics Regulate Protective Responses and Growth of Staphylococcus aureus in Macrophages

Staphylococcus aureus (S. aureus) is a gram-positive bacteria, which causes various fatal respiratory infections including pneumonia. The emergence of Methicillin-Resistance Staphylococcus aureus (MRSA) demands a thorough understanding of host-pathogen interactions. Here we report the role of calcium in regulating defence responses of S. aureus in macrophages. Regulating calcium fluxes in cells by different routes differentially governs the expression of T cell costimulatory molecule CD80 and Th1 promoting IL-12 receptor. Inhibiting calcium influx from extracellular medium increased expression of IFN-γ and IL-10 while blocking calcium release from the intracellular stores inhibited TGF-β levels. Blocking voltage-gated calcium channels (VGCC) inhibited the expression of multiple cytokines. While VGCC regulated the expression of apoptosis protein Bax, extracellular calcium-regulated the expression of Cytochrome-C. Similarly, VGCC regulated the expression of autophagy initiator Beclin-1. Blocking VGCC or calcium release from intracellular stores promoted phagosome-lysosome fusion, while activating VGCC inhibited phagosomelysosome fusion. Finally, calcium homeostasis regulated intracellular growth of Staphylococcus, although using different mechanisms. While blocking extracellular calcium influx seems to rely on IFN-γ and IL-12Rβ receptor mediated reduction in bacterial survival, blocking either intracellular calcium release or via VGCC route seem to rely on enhanced autophagy mediated reduction of intracellular bacterial survival. These results point to fine-tuning of defence responses by routes of calcium homeostasis.

Immunomodulatory Effects of Recombinant Mycobacterium smegmatis Expressing Antigen-85B Epitopes in Infected J774A.1 Murine Macrophages

Tuberculosis (TB) causes more than 1.5 million deaths each year, remaining a significant global health problem. Mycobacterium smegmatis (M. smegmatis) and Mycobacterium tuberculosis (M. tuberculosis) share features, which support the use of the former use in new generation TB vaccine development. In a previous study, the specific humoral and cellular immunogenicity of a recombinant M. smegmatis strain expressing epitopes from M. tuberculosis Ag85B protein (rMs064), was demonstrated in mice. In the current study, the immunomodulatory capacity of rMs064 was determined in a J774A.1 murine macrophage cell line. To determine the immunomodulatory effect of rMs064 in J774A.1 macrophages, the expression of inducible nitric oxide synthase (iNOS) and production of nitric oxide (NO) was evaluated. The expression of activation surface markers (MHC-II, CD40, CD80 and CD86) and the production of cytokines (IL-1β, TNF-α, IL-12p70 and IL-6) was also determined in rMs064 infected J774A.1 macrophages. Our findings showed the ability of rMs064 to induce substantial increases in macrophage activation markers expression; MHC class II and CD40, compared with M. smegmatis transformed with the empty vector (rMs012) and uninfected cells. rMs064 induced significant increases in IL-12p70 compared to uninfected cells. The expression of iNOS and CD86, and the production of IL-1β, and TNF-α were increased in rMs064 and rMs012, compared to uninfected cells. rMs064 demonstrated its immunomodulatory ability by stimulating the innate immune response, which supports its further evaluation as a TB vaccine candidate.

Heme-Derived Metabolic Signals Dictate Immune Responses

Heme is one of the most abundant molecules in the body acting as the functional core of hemoglobin/myoglobin involved in the O2/CO2 carrying in the blood and tissues, redox enzymes and cytochromes in mitochondria. However, free heme is toxic and therefore its removal is a significant priority for the host. Heme is a well-established danger-associated molecular pattern (DAMP), which binds to toll-like receptor 4 (TLR4) to induce immune responses. Heme-derived metabolites including the bile pigments, biliverdin (BV) and bilirubin (BR), were first identified as toxic drivers of neonatal jaundice in 1800 but have only recently been appreciated as endogenous drivers of multiple signaling pathways involved in protection from oxidative stress and regulators of immune responses. The tissue concentration of heme, BV and BR is tightly controlled. Heme oxygenase-1 (HO-1, encoded by HMOX1) produces BV by heme degradation, while biliverdin reductase-A (BLVR-A) generates BR by the subsequent conversion of BV. BLVR-A is a fascinating protein that possesses a classical protein kinase domain, which is activated in response to BV binding to its enzymatic site and initiates the downstream mitogen-activated protein kinases (MAPK) and phosphatidylinositol 3-kinase (PI3K) pathways. This links BLVR-A activity to cell growth and survival pathways. BLVR-A also contains a bZip DNA binding domain and a nuclear export sequence (NES) and acts as a transcription factor to regulate the expression of immune modulatory genes. Here we will discuss the role of heme-related immune response and the potential for targeting the heme system for therapies directed toward hepatitis and cancer.

Looking beyond Typical Treatments for Atypical Mycobacteria

The genus Mycobacterium comprises not only the deadliest of bacterial pathogens, Mycobacterium tuberculosis, but several other pathogenic species, including M. avium and M. abscessus. The incidence of infections caused by atypical or nontuberculous mycobacteria (NTM) has been steadily increasing, and is associated with a panoply of diseases, including pulmonary, soft-tissue, or disseminated infections. The treatment for NTM disease is particularly challenging, due to its long duration, to variability in bacterial susceptibility profiles, and to the lack of evidence-based guidelines. Treatment usually consists of a combination of at least three drugs taken from months to years, often leading to severe secondary effects and a high chance of relapse. Therefore, new treatment approaches are clearly needed. In this review, we identify the main limitations of current treatments and discuss different alternatives that have been put forward in recent years, with an emphasis on less conventional therapeutics, such as antimicrobial peptides, bacteriophages, iron chelators, or host-directed therapies. We also review new forms of the use of old drugs, including the repurposing of non-antibacterial molecules and the incorporation of antimicrobials into ionic liquids. We aim to stimulate advancements in testing these therapies in relevant models, in order to provide clinicians and patients with useful new tools with which to treat these devastating diseases.

Development of ligand-coated beads to measure macrophage antimicrobial activities

BACKGROUND INFORMATION

After macrophage recognises and phagocytoses the microorganism, their phagosome undergoes a maturation process, which creates a hostile environment for the bacterium. The lumen is acidified, and proteolysis occurs to kill and degrade pathogen for further antigen presentation. It is important to understand the association between the macrophage intracellular activities and the outcome of infection. Different methods have been developed to measure the phagosome dynamics of macrophages, but there are still limitations.

RESULTS

We used Mycobacterium tuberculosis (Mtb) antigens, the causative agent of tuberculosis (TB), as a model of infectious disease. Adopting a fluorescent bead-based assay, we developed beads coated with trehalose 6,6'dimycolate (TDM) from Mtb cell wall and β-glucan from yeast cell wall to measure the macrophage phagosomal activities using a microplate reader. We examined the consistency of the assay using J774 cells and validated it using human monocyte-derived macrophages (hMDM) from healthy volunteers and TB patients. There was a decreased pH and increased proteolysis in the lumen of J774 cells after phagocytosing the ligand-coated beads. J774 macrophage showed no difference in the acidification and proteolysis in response to control IgG beads, TDM and β-glucan beads. hMDM from healthy volunteers or TB patients showed heterogeneity in the intracellular activities when treated with ligand-coated beads.

CONCLUSIONS AND SIGNIFICANCE

The beads coated with specific ligands from Mtb worked well in both macrophage cell line and human primary macrophages, which can be exploited to further study the phagosomal function of macrophage in TB. Our bead model can be applied to different ligands from other pathogens, which could extend the understanding of the associations between macrophage antimicrobial functions and outcomes of infectious diseases and the possible cellular mechanisms involved.

MAV_4644 Interaction with the Host Cathepsin Z Protects Mycobacterium avium subsp. hominissuis from Rapid Macrophage Killing

Mycobacterium avium subspecies hominissuis (MAH) is an opportunistic pathogen that is ubiquitous in the environment and often isolated from faucets and showerheads. MAH mostly infects humans with an underlying disease, such as chronic pulmonary disorder, cystic fibrosis, or individuals that are immunocompromised. In recent years, MAH infections in patients without concurrent disease are increasing in prevalence as well. This pathogen is resistant to many antibiotics due to the impermeability of its envelope and due to the phenotypic resistance established within the host macrophages, making difficult to treat MAH infections. By screening a MAH transposon library for mutants that are susceptible to killing by reactive nitrogen intermediaries, we identified the MAV_4644 (MAV_4644:Tn) gene knockout clone that was also significantly attenuated in growth within the host macrophages. Complementation of the mutant restored the wild-type phenotype. The MAV_4644 gene encodes a dual-function protein with a putative pore-forming function and ADP-ribosyltransferase activity. Protein binding assay suggests that MAV_4644 interacts with the host lysosomal peptidase cathepsin Z (CTSZ), a key regulator of the cell signaling and inflammation. Pathogenic mycobacteria have been shown to suppress the action of many cathepsins to establish their intracellular niche. Our results demonstrate that knocking-down the cathepsin Z in human macrophages rescues the attenuated phenotype of MAV_4644:Tn clone. Although, the purified cathepsin Z by itself does not have any killing effect on MAH, it contributes to bacterial killing in the presence of the nitric oxide (NO). Our data suggest that the cathepsin Z is involved in early macrophage killing of MAH, and the virulence factor MAV_4644 protects the pathogen from this process.

Mycobacterial PknG Targets the Rab7l1 Signaling Pathway To Inhibit Phagosome-Lysosome Fusion

Phagosome maturation is an important innate defense mechanism of macrophages against bacterial infections. The mycobacterial secretory protein kinase G (PknG), a serine/threonine kinase, is known to block phagosome-lysosome (P-L) fusion, and the kinase activity of PknG appears to be crucial for this. However, the detail mechanisms are not well understood. In the current study, we demonstrate that PknG of Mycobacterium sp interacts with the human Rab GTPase protein, Rab7l1, but not with other Rab proteins as well as factors like Rabaptin, Rabex5, PI3K3, Mon1a, Mon1b, early endosome autoantigen 1, and LAMP2 that are known to play crucial roles in P-L fusion. The Rab7l1 protein is shown to play a role in P-L fusion during mycobacterial infection, and its absence promotes survival of bacilli inside macrophages. PknG was found to be translocated to the Golgi complex where it interacted with GDP-bound Rab7l1 and blocked transition of inactive Rab7l1-GDP to active Rab7l1-GTP, resulting in inhibition of recruitment of Rab7l1-GTP to bacilli-containing phagosomes, and these processes are dependent on the kinase activity of PknG. Localization of Rab7l1-GTP to phagosomes was found to be critical for the subsequent recruitment of other phago-lysosomal markers like early endosome autoantigen 1, Rab7, and LAMP2 during infection. Thus, by interfering with the Rab7l1 signaling process, PknG prevents P-L fusion and favors bacterial survival inside human macrophages. This study highlights a novel role of Rab7l1 in the phagosomal maturation process and hints at unique strategies of mycobacteria used to interfere with Rab7l1 function to favor its survival inside human macrophages.

The macrophage heme-heme oxygenase-1 system and its role in inflammation

Heme oxygenase (HO)-1, the inducible isoform of the heme-degrading enzyme HO, plays a critical role in inflammation and iron homeostasis. Regulatory functions of HO-1 are mediated via the catalytic breakdown of heme, which is an iron-containing tetrapyrrole complex with potential pro-oxidant and pro-inflammatory effects. In addition, the HO reaction produces the antioxidant and anti-inflammatory compounds carbon monoxide (CO) and biliverdin, subsequently converted into bilirubin, along with iron, which is reutilized for erythropoiesis. HO-1 is up-regulated by a plethora of stimuli and injuries in most cell types and tissues and provides salutary effects by restoring physiological homeostasis. Notably, HO-1 exhibits critical immuno-modulatory functions in macrophages, which are a major cell population of the mononuclear phagocyte system. Macrophages play key roles as sentinels and regulators of the immune system and HO-1 in these cells appears to be of critical importance for driving resolution of inflammatory responses. In this review, the complex functions and regulatory mechanisms of HO-1 in macrophages will be high-lighted. A particular focus will be the intricate interactions of HO-1 with its substrate heme, which play a contradictory role in distinct physiological and pathophysiological settings. The therapeutic potential of targeted modulation of the macrophage heme-HO-1 system will be discussed in the context of inflammatory disorders.

The Voltage-Dependent Anion Channels (VDAC) of Mycobacterium avium phagosome are associated with bacterial survival and lipid export in macrophages

Mycobacterium avium subsp. hominissuis is associated with infection of immunocompromised individuals as well as patients with chronic lung disease. M. avium infects macrophages and actively interfere with the host killing machinery such as apoptosis and autophagy. Bacteria alter the normal endosomal trafficking, prevent the maturation of phagosomes and modify many signaling pathways inside of the macrophage by secreting effector molecules into the cytoplasm. To investigate whether M. avium needs to attach to the internal surface of the vacuole membrane before releasing efferent molecules, vacuole membrane proteins were purified and binding to the surface molecules present in intracellular bacteria was evaluated. The voltage-dependent anion channels (VDAC) were identified as components of M. avium vacuoles in macrophages. M. avium mmpL4 proteins were found to bind to VDAC-1 protein. The inactivation of VDAC-1 function either by pharmacological means or siRNA lead to significant decrease of M. avium survival. Although, we could not establish a role of VDAC channels in the transport of known secreted M. avium proteins, we demonstrated that the porin channels are associated with the export of bacterial cell wall lipids outside of vacuole. Suppression of the host phagosomal transport systems and the pathogen transporter may serve as therapeutic targets for infectious diseases.

Gallium nanoparticles facilitate phagosome maturation and inhibit growth of virulent Mycobacterium tuberculosis in macrophages

New treatments and novel drugs are required to counter the growing problem of drug-resistant strains of Mycobacterium tuberculosis (M.tb). Our approach against drug resistant M.tb, as well as other intracellular pathogens, is by targeted drug delivery using nanoformulations of drugs already in use, as well as drugs in development. Among the latter are gallium (III) (Ga)-based compounds. In the current work, six different types of Ga and rifampin nanoparticles were prepared in such a way as to enhance targeting of M.tb infected-macrophages. They were then tested for their ability to inhibit growth of a fully pathogenic strain (H37Rv) or a non-pathogenic strain (H37Ra) of M.tb. Encapsulating Ga in folate- or mannose-conjugated block copolymers provided sustained Ga release for 15 days and significantly inhibited M.tb growth in human monocyte-derived macrophages. Nanoformulations with dendrimers encapsulating Ga or rifampin also showed promising anti-tuberculous activity. The nanoparticles co-localized with M.tb containing phagosomes, as measured by detection of mature cathepsin D (34 kDa, lysosomal hydrogenase). They also promoted maturation of the phagosome, which would be expected to increase macrophage-mediated killing of the organism. Delivery of Ga or rifampin in the form of nanoparticles to macrophages offers a promising approach for the development of new therapeutic anti-tuberculous drugs.

Mycobacterium tuberculosis Rv1474c is a TetR-like transcriptional repressor that regulates aconitase, an essential enzyme and RNA-binding protein, in an iron-responsive manner

Mycobacterium tuberculosis (M.tb), tuberculosis (TB) causing bacteria, employs several mechanisms to maintain iron homeostasis which is critical for its survival and pathogenesis. M.tb aconitase (Acn), a [4Fe-4S] cluster-containing essential protein, apart from participating in energy cycle, also binds to predicted iron-responsive RNA elements. In this study, we identified Rv1474c as a regulator of its operonic partner acn and carried out its biochemical and functional characterization. The binding motif for Rv1474c in the upstream region of acn (Rv1475c)-Rv1474c operon was verified by gel-shift assays. Reporter assays in E. coli followed by over-expression studies in mycobacteria, using both wild type and a DNA-binding defective mutant, demonstrated Rv1474c as a Tet-R like repressor of acn. Rv1474c, besides binding tetracycline, could also bind iron which negatively influenced its DNA binding activity. Further, a consistent decrease in the relative transcript levels of acn when M.tb was grown in iron-deficient conditions as compared to either normal or other stress conditions, indicated regulation of acn by Rv1474c in an iron-responsive manner in vivo. The absence of homologs in the human host and its association with indispensable iron homeostasis makes Rv1474c an attractive target for designing novel anti-mycobacterials.

CMTM7 knockdown increases tumorigenicity of human non-small cell lung cancer cells and EGFR-AKT signaling by reducing Rab5 activation

The dysregulation of epidermal growth factor receptor (EGFR) signaling has been well documented to contribute to the progression of non-small cell lung cancer (NSCLC), the leading cause of cancer death in the world. EGF-stimulated EGFR activation induces receptor internalization and degradation, which plays an important role in EGFR signaling. This process is frequently deregulated in cancer cells, leading to enhanced EGFR levels and signaling. Our previous study on CMTM7 is only limited to a brief description of the relationship of overexpressed CMTM7 with EGFR-AKT signaling. The biological functions of endogenous CMTM7 and its molecular mechanism remained unclear. In this study, we show that the stable knockdown of CMTM7 augments the malignant potential of NSCLC cells and enhances EGFR-AKT signaling by decreasing EGFR internalization and degradation. Mechanistically, CMTM7 knockdown reduces the activation of Rab5, a protein known to be required for early endosome fusion. In NSCLC, the loss of CMTM7 would therefore serve to sustain aberrant EGFR-mediated oncogenic signaling. Together, our findings highlight the role of CMTM7 in the regulation of EGFR signaling in tumor cells, revealing CMTM7 as a novel molecule related to Rab5 activation.

The Making and Taking of Lipids: The Role of Bacterial Lipid Synthesis and the Harnessing of Host Lipids in Bacterial Pathogenesis

In order to survive environmental stressors, including those induced by growth in the human host, bacterial pathogens will adjust their membrane physiology accordingly. These physiological changes also include the use of host-derived lipids to alter their own membranes and feed central metabolic pathways. Within the host, the pathogen is exposed to many stressful stimuli. A resulting adaptation is for pathogens to scavenge the host environment for readily available lipid sources. The pathogen takes advantage of these host-derived lipids to increase or decrease the rigidity of their own membranes, to provide themselves with valuable precursors to feed central metabolic pathways, or to impact host signalling and processes. Within, we review the diverse mechanisms that both extracellular and intracellular pathogens employ to alter their own membranes as well as their use of host-derived lipids in membrane synthesis and modification, in order to increase survival and perpetuate disease within the human host. Furthermore, we discuss how pathogen employed mechanistic utilization of host-derived lipids allows for their persistence, survival and potentiation of disease. A more thorough understanding of all of these mechanisms will have direct consequences for the development of new therapeutics, and specifically, therapeutics that target pathogens, while preserving normal flora.

A Mycobacterium avium subsp. paratuberculosis Predicted Serine Protease Is Associated with Acid Stress and Intraphagosomal Survival

The ability to maintain intra-cellular pH is crucial for bacteria and other microbes to survive in diverse environments, particularly those that undergo fluctuations in pH. Mechanisms of acid resistance remain poorly understood in mycobacteria. Although, studies investigating acid stress in M. tuberculosis are gaining traction, few center on Mycobacterium avium subsp. paratuberculosis (MAP), the etiological agent of chronic enteritis in ruminants. We identified a MAP acid stress response network involved in macrophage infection. The central node of this network was MAP0403, a predicted serine protease that shared an 86% amino acid identity with MarP in M. tuberculosis. Previous studies confirmed MarP as a serine protease integral to maintaining intra-bacterial pH and survival in acid in vitro and in vivo. We show that MAP0403 is upregulated in infected macrophages and MAC-T cells that coincided with phagosome acidification. Treatment of mammalian cells with bafilomcyin A1, a potent inhibitor of phagosomal vATPases, diminished MAP0403 transcription. MAP0403 expression was also noted in acidic medium. A surrogate host, M. smegmatis mc(2) 155, was designed to express MAP0403 and when exposed to either macrophages or in vitro acid stress had increased bacterial cell viability, which corresponds to maintenance of intra-bacterial pH in acidic (pH = 5) conditions, compared to the parent strain. These data suggest that MAP0403 may be the equivalent of MarP in MAP. Future studies confirming MAP0403 as a serine protease and exploring its structure and possible substrates are warranted.

Cryptococcus neoformans: Tripping on Acid in the Phagolysosome

Cryptococcus neoformans (Cn) is a basidiomycetous pathogenic yeast that is a frequent cause of meningoencephalitis in immunocompromised individuals. Cn is a facultative intracellular pathogen in mammals, insects and amoeba. Cn infection occurs after inhalation of spores or desiccated cells from the environment. After inhalation Cn localizes to the lungs where it can be phagocytosed by alveolar macrophages. Cn is surrounded by a polysaccharide capsule that helps the fungus survive in vivo by interfering with phagocytosis, quenching free radical bursts and shedding polysaccharides that negatively modulates the immune system. After phagocytosis, Cn resides within the phagosome that matures to become a phagolysosome, a process that results in the acidification of the phagolysosomal lumen. Cn replicates at a higher rate inside macrophages than in the extracellular environment, possibly as a result that the phagosomal pH is near that optimal for growth. Cn increases the phagolysosomal pH and modulates the dynamics of Rab GTPases interaction with the phagolysosome. Chemical manipulation of the phagolysosomal pH with drugs can result in direct and indirect killing of Cn and reduced non-lytic exocytosis. Phagolysosomal membrane damage after Cn infection occurs both in vivo and in vitro, and is required for Cn growth and survival. Macrophage treatment with IFN-γ reduces the phagolysosomal damage and increases intracellular killing of Cn. Studies on mice and humans show that treatment with IFN-γ can improve host control of the disease. However, the mechanism by which Cn mediates phagolysosomal membrane damage remains unknown but likely candidates are phospholipases and mechanical damage from an enlarging capsule. Here we review Cn intracellular interaction with a particular emphasis on phagosomal interactions and develop the notion that the extent of damage of the phagosomal membrane is a key determinant of the outcome of the Cn-macrophage interaction.

Seeing a Mycobacterium-Infected Cell in Nanoscale 3D: Correlative Imaging by Light Microscopy and FIB/SEM Tomography

Mycobacteria pose a threat to the world health today, with pathogenic and opportunistic bacteria causing tuberculosis and non-tuberculous disease in large parts of the population. Much is still unknown about the interplay between bacteria and host during infection and disease, and more research is needed to meet the challenge of drug resistance and inefficient vaccines. This work establishes a reliable and reproducible method for performing correlative imaging of human macrophages infected with mycobacteria at an ultra-high resolution and in 3D. Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM) tomography is applied, together with confocal fluorescence microscopy for localization of appropriately infected cells. The method is based on an Aclar poly(chloro-tri-fluoro)ethylene substrate, micropatterned into an advantageous geometry by a simple thermomoulding process. The platform increases the throughput and quality of FIB/SEM tomography analyses, and was successfully applied to detail the intracellular environment of a whole mycobacterium-infected macrophage in 3D.

Mycobacterium avium MAV_2941 mimics phosphoinositol-3-kinase to interfere with macrophage phagosome maturation

Mycobacterium avium subsp hominissuis (M. avium) is a pathogen that infects and survives in macrophages. Previously, we have identified the M. avium MAV_2941 gene encoding a 73 amino acid protein exported by the oligopeptide transporter OppA to the macrophage cytoplasm. Mutations in MAV_2941 were associated with significant impairment of M. avium growth in THP-1 macrophages. In this study, we investigated the molecular mechanism of MAV_2941 action and demonstrated that MAV_2941 interacts with the vesicle trafficking proteins syntaxin-8 (STX8), adaptor-related protein complex 3 (AP-3) complex subunit beta-1 (AP3B1) and Archain 1 (ARCN1) in mononuclear phagocytic cells. Sequencing analysis revealed that the binding site of MAV_2941 is structurally homologous to the human phosphatidylinositol 3-kinase (PI3K) chiefly in the region recognized by vesicle trafficking proteins. The β3A subunit of AP-3, encoded by AP3B1, is essential for trafficking cargo proteins, including lysosomal-associated membrane protein 1 (LAMP-1), to the phagosome and lysosome-related organelles. Here, we show that while the heat-killed M. avium when ingested by macrophages co-localizes with LAMP-1 protein, transfection of MAV_2941 in macrophages results in significant decrease of LAMP-1 co-localization with the heat-killed M. avium phagosomes. Mutated MAV_2941, where the amino acids homologous to the binding region of PI3K were changed, failed to interact with trafficking proteins. Inactivation of the AP3B1 gene led to alteration in the trafficking of LAMP-1. These results suggest that M. avium MAV_2941 interferes with the protein trafficking within macrophages altering the maturation of phagosome.

Elevated temperature inhibits recruitment of transferrin-positive vesicles and induces iron-deficiency genes expression in Aiptasia pulchella host-harbored Symbiodinium

Coral bleaching is the consequence of disruption of the mutualistic Cnidaria-dinoflagellate association. Elevated seawater temperatures have been proposed as the most likely cause of coral bleaching whose severity is enhanced by a limitation in the bioavailability of iron. Iron is required by numerous organisms including the zooxanthellae residing inside the symbiosome of cnidarian cells. However, the knowledge of how symbiotic zooxanthellae obtain iron from the host cells and how elevated water temperature affects the association is very limited. Since cellular iron acquisition is known to be mediated through transferrin receptor-mediated endocytosis, a vesicular trafficking pathway specifically regulated by Rab4 and Rab5, we set out to examine the roles of these key proteins in the iron acquisition by the symbiotic Symbiodinium. Thus, we hypothesized that the iron recruitments into symbiotic zooxanthellae-housed symbiosomes may be dependent on rab4/rab5-mediated fusion with vesicles containing iron-bound transferrins and will be retarded under elevated temperature. In this study, we cloned a novel monolobal transferrin (ApTF) gene from the tropical sea anemone Aiptasia pulchella and confirmed that the association of ApTF with A. pulchella Rab4 (ApRab4) or A. pulchella Rab5 (ApRab5) vesicles is inhibited by elevated temperature through immunofluorescence analysis. We confirmed the iron-deficient phenomenon by demonstrating the induced overexpression of iron-deficiency-responsive genes, flavodoxin and high-affinity iron permease 1, and reduced intracellular iron concentration in zooxanthellae under desferrioxamine B (iron chelator) and high temperature treatment. In conclusion, our data are consistent with algal iron deficiency being a contributing factor for the thermal stress-induced bleaching of symbiotic cnidarians.

Pma1 is an alkali/alkaline earth metal cation ATPase that preferentially transports Na(+) and K(+) across the Mycobacterium smegmatis plasma membrane

Mycobacterium smegmatis Pma1 is the orthologue of M. tuberculosis P-type ATPase cation transporter CtpF, which is activated under stress conditions, such as hypoxia, starvation and response to antituberculous and toxic substances. The function of Pma1 in the mycobacterial processes across the plasma membrane has not been characterised. In this work, bioinformatic analyses revealed that Pma1 likely contains potential sites for, Na(+), K(+) and Ca(2+) binding and transport. Accordingly, RT-qPCR experiments showed that M. smegmatis pma1 transcription is stimulated by sub-lethal doses of Na(+), K(+) and Ca(2+); in addition, the ATPase activity of plasma membrane vesicles in recombinant Pma1-expressing M. smegmatis cells is stimulated by treatment with these cations. In contrast, M. smegmatis cells homologously expressing Pma1 displayed tolerance to high doses of Na(+) and K(+) but not to Ca(2+) ions. Consistently, the recombinant protein Km embedded in plasma membrane demonstrated that Ca(2+) has more affinity for Pma1 than Na(+) and K(+) ions; furthermore, the estimation of Vmax/Km suggests that Na(+) and K(+) ions are more efficiently translocated than Ca(2+). Thus, these results strongly suggest that Pma1 is a promiscuous alkali/alkaline earth cation ATPase that preferentially transports Na(+) and/or K(+) across the mycobacterial plasma membrane.

Neisseria meningitidis subverts the polarized organization and intracellular trafficking of host cells to cross the epithelial barrier

Translocation of the nasopharyngeal barrier by Neisseria meningitidis occurs via an intracellular microtubule-dependent pathway and represents a crucial step in its pathogenesis. Despite this fact, the interaction of invasive meningococci with host subcellular compartments and the resulting impact on their organization and function have not been investigated. The influence of serogroup B strain MC58 on host cell polarity and intracellular trafficking system was assessed by confocal microscopy visualization of different plasma membrane-associated components (such as E-cadherin, ZO-1 and transferrin receptor) and evaluation of the transferrin uptake and recycling in infected Calu-3 monolayers. Additionally, the association of N. meningitidis with different endosomal compartments was evaluated through the concomitant staining of bacteria and markers specific for Rab11, Rab22a, Rab25 and Rab3 followed by confocal microscopy imaging. Subversion of the host cell architecture and intracellular trafficking system, denoted by mis-targeting of cell plasma membrane components and perturbations of transferrin transport, was shown to occur in response to N. meningitidis infection. Notably, the appearance of all of these events seems to positively correlate with the efficiency of N. meningitidis to cross the epithelial barrier. Our data reveal for the first time that N. meningitidis is able to modulate the host cell architecture and function, which might serve as a strategy of this pathogen for overcoming the nasopharyngeal barrier without affecting the monolayer integrity.

Induction of heme oxygenase-1 contributes to survival of Mycobacterium abscessus in human macrophages-like THP-1 cells

Mycobacterium abscessus (M.abs) is a rapidly growing mycobacterial species that infects macrophages, and is an important pathogen in patients with cystic fibrosis. We studied the early stages of M.abs infection of macrophages, with emphasis on the role of heme-oxygenase-1 (HO-1) in this infection. THP-1 cells were activated using TPA into macrophage-like cells and infected with M.abs for different time points. M.abs infection robustly induced HO-1 expression in the THP-1 cells. Production of HO-1 was p38 MAPK-dependent, as p38 inhibitors suppressed HO-1 induction. Pretreatment with HO-1 inhibitors tin-protoporphyrin (SnPP) significantly inhibited M.abs growth inside macrophages. Furthermore, inhibiting HO-1 using HO-1 siRNA or the HO-1 upstream signaling molecule; Nrf2 using Nrf2 siRNA resulted in similar inhibition of M.abs. In contrast, inducing HO-1 did not increase M.abs intracellular growth above control. Products of HO-1 metabolism of heme are bilirubin, biliverdin, carbon monoxide (CO) and iron. The addition of either bilirubin or biliverdin, but not CO, completely restored the SnPP inhibitory effect and partially that with HO-1 siRNA. To understand the mechanisms, we used Syto-62 labeled M.abs to infect macrophages. Interestingly, HO-1 inhibition promoted M.abs-containing phagosome fusion with lysosomes, which should enhance M.abs killing. M.abs infection enhanced THP-1 ROS production as demonstrated by increased DHE, DCF fluorescence, and EPR signal. HO-1 inhibition further increased ROS production in infected macrophages. Our results indicate that HO-1 induction is important for M.abs growth during the early stages of infection, and that the HO-1 products bilirubin and biliverdin, perhaps through modulation of intracellular ROS levels, may be involved.

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HO-1 induction is important for Mycobacterium abscessus growth inside infected macrophages during the early stages of infection.

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Reducing HO-1 products may enhance the ability of the macrophage to control Mycobacterium abscessus infection.

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HO-1 inhibition increases phagosome–lysosome fusion and thus Mycobacterium abscessus killing.

A structural and functional investigation of a novel protein from Mycobacterium smegmatis implicated in mycobacterial macrophage survivability

The success of pathogenic mycobacterial species is owing in part to their ability to parasitize the generally inhospitable phagosomal environment of host macrophages, utilizing a variety of strategies to avoid their antimycobacterial capabilities and thereby enabling their survival. A recently identified gene target in Mycobacterium smegmatis, highly conserved within Mycobacterium spp. and denoted MSMEG_5817, has been found to be important for bacterial survival within host macrophages. To gain insight into its function, the crystal structure of MSMEG_5817 has been solved to 2.40 Å resolution. The structure reveals a high level of structural homology to the sterol carrier protein (SCP) family, suggesting a potential role of MSMEG_5817 in the binding and transportation of biologically relevant lipids required for bacterial survival. The lipid-binding capacity of MSMEG_5817 was confirmed by ELISA, revealing binding to a number of phospholipids with varying binding specificities compared with Homo sapiens SCP. A potential lipid-binding site was probed by alanine-scanning mutagenesis, revealing structurally relevant residues and a binding mechanism potentially differing from that of the SCPs.

Mycobacterium tuberculosis P-type ATPases: possible targets for drug or vaccine development

Tuberculosis (TB) has been the biggest killer in the human history; currently, Mycobacterium tuberculosis (Mtb) kills nearly 2 million people each year worldwide. The high prevalence of TB obligates the identification of new therapeutic targets and the development of anti-TB vaccines that can control multidrug resistance and latent TB infections. Membrane proteins have recently been suggested as key targets for bacterial viability. Current studies have shown that mycobacteria P-type ATPases may play critical roles in ion homeostasis and in the response of mycobacteria to toxic substances in the intraphagosomal environment. In this review, we bring together the genomic, transcriptomic, and structural aspects of the P-type ATPases that are relevant during active and latent Mtb infections, which can be useful in determining the potential of these ATPases as drug targets and in uncovering their possible roles in the development of new anti-TB attenuated vaccines.

Phagosome maturation in polarized macrophages

Macrophages are capable of assuming distinct, meta-stable, functional phenotypes in response to environmental cues-a process referred to as macrophage polarization. The identity and plasticity of polarized macrophage subsets as well as their functions in the maintenance of homeostasis and the progression of various pathologies have become areas of intense interest. Yet, the mechanisms by which they achieve subset-specific functions at the cellular level remain unclear. It is becoming apparent that phagocytosis and phagosome maturation differ depending on the polarization of macrophages. This minireview summarizes recent progress in this field, highlighting developing trends and discussing the molecular mechanisms that underlie subset-specific functions.

Interleukin 22 inhibits intracellular growth of Mycobacterium tuberculosis by enhancing calgranulin A expression

Previously, we found that interleukin 22 (IL-22) inhibits intracellular growth of Mycobacterium tuberculosis in human monocyte-derived macrophages (MDMs). In the current study, we determined the mechanisms underlying these effects. We found that W7, a phagolysosomal fusion inhibitor, abrogates IL-22-dependent M. tuberculosis growth inhibition in MDMs, suggesting that IL-22 acts through enhanced phagolysosomal fusion. Our microarray analysis indicated that recombinant IL-22 (rIL-22) enhances the expression of an intracellular signaling molecule, calgranulin A. This was confirmed by real-time polymerase chain reaction, Western blot, and confocal microscopy. Calgranulin A small interfering RNA (siRNA) abrogated rIL-22-dependent growth inhibition of M. tuberculosis in MDMs. IL-22 enhanced Rab7 expression and downregulated Rab14 expression of M. tuberculosis-infected MDMs, and these effects were reversed by calgranulin A siRNA. These results suggest that M. tuberculosis growth inhibition by IL-22 depends on calgranulin A and enhanced phagolysosomal fusion, which is associated with increased Rab7 and reduced Rab14 expression.

Iron in intracellular infection: to provide or to deprive?

Due to their chemical versatility, transition metals were incorporated as cofactors for several basic metabolic pathways in living organisms. This same characteristic makes them potentially harmful, since they can be engaged in deleterious reactions like Fenton chemistry. As such, organisms have evolved highly specialized mechanisms to supply their own metal needs while keeping their toxic potential in check. This dual character comes into play in host-pathogen interactions, given that the host can either deprive the pathogen of these key nutrients or exploit them to induce toxicity toward the invading agent. Iron stands as the prototypic example of how a metal can be used to limit the growth of pathogens by nutrient deprivation, a mechanism widely studied in Mycobacterium infections. However, the host can also take advantage of iron-induced toxicity to control pathogen proliferation, as observed in infections caused by Leishmania. Whether we may harness either of the two pathways for therapeutical purposes is still ill-defined. In this review, we discuss how modulation of the host iron availability impacts the course of infections, focusing on those caused by two relevant intracellular pathogens, Mycobacterium and Leishmania.

Drosophila Rab14 mediates phagocytosis in the immune response to Staphylococcus aureus

Drosophila haemocytes are essential for the animal to resist Staphylococcus aureus infections. Phagocytosis is a central component of the haemocyte-mediated immune response. It involves regulated interaction between the phagocytic and the endocytic compartments. RabGTPases are pivotal for the membrane trafficking and fusion events, and thus are often targets of intracellular pathogens that subvert phagocytosis. An in vivo screen identified Rab2 and Rab14 as candidates for proteins regulating phagosome maturation. Since Rab14 is often targeted by intracellular pathogens, an understanding of its function during phagocytosis and the overall immune response can give insight into the pathogenesis of intracellular microbes. We generated a Drosophila Rab14 mutant and characterized the resulting immune defects in animals and specifically in haemocytes in response to an S. aureus infection. Haemocyte based immunofluorescence studies indicate that Rab14 is recruited to the phagosome and like Rab7, a well-characterized regulator of the phagocytic pathway, is essential for progression of phagosome maturation. Rab14 mutant haemocytes show impaired recruitment of Rab7 and of a lysosomal marker onto S. aureus phagosomes. The defect in phagocytosis is associated with higher bacterial load and increased susceptibility to S. aureus in the animal.

Interactions between an M. tuberculosis strain overexpressing mtrA and mononuclear phagocytes

PURPOSE

It was previously shown that the bacterial two-component regulatory signal transduction (2CR) system MtrAB may be associated with the ability of M. tuberculosis (Mtb) to survive in macrophages. In the present work Mtb mutants: Rv-78 with overexpression of mtrA and Rv-129 with elevated level of phosphorylation-defective MtrA were used for further investigation of the potential influence of the MtrAB system on Mtb interaction with human monocytes.

MATERIAL/METHODS

Flow cytometry was used to determine the expression of MHC class II molecules. The expression of genes for inducible nitric oxide synthase (iNOS) and cathepsin G was quantified by RT-PCR. The association of Mtb strains with Rab5 and Rab7 positive vacuoles was investigated applying confocal microscopy. IL-10 and IL-12 secretion by monocytes as well as the Mtb susceptibility to cathepsin G were investigated.

RESULTS

Mutation-carried and wild type Mtb strains inhibited MHC class II expression on monocytes to a similar extent. Monocyte stimulation with mycobacteria led to the increased production of IL-10 but no detectable amounts of IL-12 or NO were observed. Expression of the gene for iNOS was not detected while that for cathepsin G was shown, however its intensity was not associated with MtrA mutation. Mtb mutant strains were more effectively enclosed in phagosomes containing the late endosome marker Rab7 as compared to the control.

CONCLUSIONS

The results may confirm the importance of the MtrAB system in mycobacterial capacity for successful survival in phagocytes, especially in the context of high degree of colocalization of Mtb Rv-78 to mature phagosomes.

Lipid-labeling facilitates a novel magnetic isolation procedure to characterize pathogen-containing phagosomes

Here we describe a novel approach for the isolation and biochemical characterization of pathogen-containing compartments from primary cells: We developed a lipid-based procedure to magnetically label the surface of bacteria and visualized the label by scanning and transmission electron microscopy (SEM, TEM). We performed infection experiments with magnetically labeled Mycobacterium avium, M. tuberculosis and Listeria monocytogenes and isolated magnetic bacteria-containing phagosomes using a strong magnetic field in a novel free-flow system. Magnetic labeling of M. tuberculosis did not affect the virulence characteristics of the bacteria during infection experiments addressing host cell activation, phagosome maturation delay and replication in macrophages in vitro. Biochemical analyses of the magnetic phagosome-containing fractions provided evidence of an enhanced presence of bacterial antigens and a differential distribution of proteins involved in the endocytic pathway over time as well as cytokine-dependent changes in the phagosomal protein composition. The newly developed method represents a useful approach to characterize and compare pathogen-containing compartments, in order to identify microbial and host cell targets for novel anti-infective strategies.

Bacterial pathogens commandeer Rab GTPases to establish intracellular niches

Intracellular bacterial pathogens deploy virulence factors termed effectors to inhibit degradation by host cells and to establish intracellular niches where growth and differentiation take place. Here, we describe mechanisms by which human bacterial pathogens (including Chlamydiae; Coxiella burnetii; Helicobacter pylori; Legionella pneumophila; Listeria monocytogenes; Mycobacteria; Pseudomonas aeruginosa, Salmonella enterica) modulate endocytic and exocytic Rab GTPases in order to thrive in host cells. Host cell Rab GTPases are critical for intracellular transport following pathogen phagocytosis or endocytosis. At the molecular level bacterial effectors hijack Rab protein function to: evade degradation, direct transport to particular intracellular locations and monopolize host vesicles carrying molecules that are needed for a stable niche and/or bacterial growth and differentiation. Bacterial effectors may serve as specific receptors for Rab GTPases or as enzymes that post-translationally modify Rab proteins or endosomal membrane lipids required for Rab function. Emerging data indicate that bacterial effector expression is temporally and spatially regulated and multiple virulence factors may act concertedly to usurp Rab GTPase function, alter signaling and ensure niche establishment and intracellular bacterial growth, making this field an exciting area for further study.

The PtdIns3P-binding protein Phafin 2 mediates epidermal growth factor receptor degradation by promoting endosome fusion

Phosphatidylinositol 3-phosphate (PtdIns3P) orchestrates endosomal cargo transport, fusion and motility by recruiting FYVE or PX domain-containing effector proteins to endosomal membranes. In an attempt to discover novel PtdIns3P effectors involved in the termination of growth factor receptor signalling, we performed an siRNA screen for epidermal growth factor (EGF) degradation, targeting FYVE and PX domain proteins in the human proteome. This screen identified several potential regulators of EGF degradation, including HRS (used as positive control), PX kinase, MTMR4 and Phafin2/PLEKHF2. As Phafin2 has not previously been shown to be required for EGF receptor (EGFR) degradation, we performed further functional studies on this protein. Loss of Phafin2 was found to decrease early endosome size, whereas overexpression of Phafin2 resulted in enlarged endosomes. Moreover, both the EGFR and the fluid-phase marker dextran were retained in abnormally small endosomes in Phafin2-depleted cells. In yeast two-hybrid analysis we identified Phafin2 as a novel interactor of the endosomal-tethering protein EEA1, and Phafin2 colocalized strongly with EEA1 in microdomains of the endosome membrane. Our results suggest that Phafin2 controls receptor trafficking and fluid-phase transport through early endosomes by facilitating endosome fusion in concert with EEA1.