Mycobacterium tuberculosis phagosome maturation arrest: mycobacterial phosphatidylinositol analog phosphatidylinositol mannoside stimulates early endosomal fusion

Understanding Staphylococcus aureus internalisation and induction of antimicrobial tolerance

INTRODUCTION

Staphylococcus aureus, a human commensal, is also one of the most common and serious pathogens for humans. In recent years, its capacity to survive and replicate in phagocytic and non-phagocytic cells has been largely demonstrated. In these intracellular niches, bacteria are shielded from the immune response and antibiotics, turning host cells into long-term infectious reservoirs. Moreover, neutrophils carry intracellular bacteria in the bloodstream, leading to systemic spreading of the disease. Despite the serious threat posed by intracellular S. aureus to human health, the molecular mechanisms behind its intracellular survival and subsequent antibiotic treatment failure remain elusive.

AREA COVERED

We give an overview of the killing mechanisms of phagocytes and of the impressive arsenal of virulence factors, toxins and stress responses deployed by S. aureus as a response. We then discuss the different barriers to antibiotic activity in this intracellular niche and finally describe innovative strategies to target intracellular persisting reservoirs.

EXPERT OPINION

Intracellular niches represent a challenge in terms of diagnostic and treatment. Further research using ad-hoc in-vivo models and single cell approaches are needed to better understand the molecular mechanisms underlying intracellular survival and tolerance to antibiotics in order to identify strategies to eliminate these persistent bacteria.

Host-directed therapy for bacterial infections -Modulation of the phagolysosome pathway

Bacterial infections still impose a significant burden on humanity, even though antimicrobial agents have long since been developed. In addition to individual severe infections, the f fatality rate of sepsis remains high, and the threat of antimicrobial-resistant bacteria grows with time, putting us at inferiority. Although tremendous resources have been devoted to the development of antimicrobial agents, we have yet to recover from the lost ground we have been driven into. Looking back at the evolution of treatment for cancer, which, like infectious diseases, has the similarity that host immunity eliminates the lesion, the development of drugs to eliminate the tumor itself has shifted from a single-minded focus on drug development to the establishment of a treatment strategy in which the de-suppression of host immunity is another pillar of treatment. In infectious diseases, on the other hand, the development of therapies that strengthen and support the immune system has only just begun. Among innate immunity, the first line of defense that bacteria encounter after invading the host, the molecular mechanisms of the phagolysosome pathway, which begins with phagocytosis to fusion with lysosome, have been elucidated in detail. Bacteria have a large number of strategies to escape and survive the pathway. Although the full picture is still unfathomable, the molecular mechanisms have been elucidated for some of them, providing sufficient clues for intervention. In this article, we review the host defense mechanisms and bacterial evasion mechanisms and discuss the possibility of host-directed therapy for bacterial infection by intervening in the phagolysosome pathway.

Shikonin and Juglone Inhibit Mycobacterium tuberculosis Low-Molecular-Weight Protein Tyrosine Phosphatase a (Mt-PTPa)

Low-molecular-weight protein tyrosine phosphatases (LMW-PTPs) are involved in promoting the intracellular survival of Mycobacterium tuberculosis (Mtb), the causative organism of tuberculosis. These PTPs directly alter host signalling pathways to evade the hostile environment of macrophages and avoid host clearance. Among these, protein tyrosine phosphatase A (Mt-PTPa) is implicated in phagosome acidification failure, thereby inhibiting phagosome maturation to promote Mycobacterium tuberculosis (Mtb) survival. In this study, we explored Mt-PTPa as a potential drug target for treating Mtb. We started by screening a library of 502 pure natural compounds against the activities of Mt-PTPa in vitro, with a threshold of 50% inhibition of activity via a <500 µM concentration of the candidate drugs. The initial screen identified epigallocatechin, myricetin, rosmarinic acid, and shikonin as hits. Among these, the naphthoquinone, shikonin (5, 8-dihydroxy-2-[(1R)-1-hydroxy-4-methyl-3-pentenyl]-1,4-naphthoquinone), showed the strongest inhibition (IC50 33 µM). Further tests showed that juglone (5-hydroxy-1,4-naphthalenedione), another naphthoquinone, displayed similar potent inhibition of Mt-PTPa to shikonin. Kinetic analysis of the inhibition patterns suggests a non-competitive inhibition mechanism for both compounds, with inhibitor constants (Ki) of 8.5 µM and 12.5 µM for shikonin and juglone, respectively. Our findings are consistent with earlier studies suggesting that Mt-PTPa is susceptible to specific allosteric modulation via a non-competitive or mixed inhibition mechanism.

Polysaccharides and glycolipids of Mycobacterium tuberculosis and their induced immune responses

Tuberculosis (TB) is a chronic infectious disease mainly caused by Mycobacterium tuberculosis (M. tuberculosis). The structures of polysaccharides and glycolipids at M. tuberculosis cell wall vary among different strains, which affect the physiology and pathogenesis of mycobacteria by activating or inhibiting innate and acquired immunity. Among them, some components such as lipomannan (LM) and lipoarabinomannan (LAM) activate innate immunity by recognizing some kinds of pattern recognition receptors (PRRs) like Toll-like receptors, while other components like mannose-capped lipoarabinomannan (ManLAM) could prevent innate immune responses by inhibiting the secretion of pro-inflammatory cytokines and maturation of phagosomes. In addition, many glycolipids can activate natural killer T (NKT) cells and CD1-restricted T cells to produce interferon-γ (IFN-γ). Furthermore, humoral immunity against cell wall components, such as antibodies against LAM, plays a role in immunity against M. tuberculosis infection. Cell wall polysaccharides and glycolipids of M. tuberculosis have potential applications as antigens and adjuvants for novel TB subunit vaccines.

Listeria InlB Expedites Vacuole Escape and Intracellular Proliferation by Promoting Rab7 Recruitment via Vps34

Rapid phagosomal escape mediated by listeriolysin O (LLO) is a prerequisite for Listeria monocytogenes intracellular replication and pathogenesis. Escape takes place within minutes after internalization from vacuoles that are negative to the early endosomal Rab5 GTPase and positive to the late endosomal Rab7. Using mutant analysis, we found that the listerial invasin InlB was required for optimal intracellular proliferation of L. monocytogenes. Starting from this observation, we determined in HeLa cells that InlB promotes early phagosomal escape and efficient Rab7 acquisition by the Listeria-containing vacuole (LCV). Recruitment of the class III phosphoinositide 3-kinase (PI3K) Vps34 to the LCV and accumulation of its lipid product, phosphatidylinositol 3-phosphate (PI3P), two key endosomal maturation mediators, were also dependent on InlB. Small interfering RNA (siRNA) knockdown experiments showed that Vps34 was required for Rab7 recruitment and early (LLO-mediated) escape and supported InlB-dependent intracellular proliferation. Together, our data indicate that InlB accelerates LCV conversion into an escape-favorable Rab7 late phagosome via subversion of class III PI3K/Vps34 signaling. Our findings uncover a new function for the InlB invasin in Listeria pathogenesis as an intracellular proliferation-promoting virulence factor. IMPORTANCE Avoidance of lysosomal killing by manipulation of the endosomal compartment is a virulence mechanism assumed to be largely restricted to intravacuolar intracellular pathogens. Our findings are important because they show that cytosolic pathogens like L. monocytogenes, which rapidly escape the phagosome after internalization, can also extensively subvert endocytic trafficking as part of their survival strategy. They also clarify that, instead of delaying phagosome maturation (to allow time for LLO-dependent disruption, as currently thought), via InlB L. monocytogenes appears to facilitate the rapid conversion of the phagocytic vacuole into an escape-conducive late phagosome. Our data highlight the multifunctionality of bacterial virulence factors. At the cell surface, the InlB invasin induces receptor-mediated phagocytosis via class I PI3K activation, whereas after internalization it exploits class III PI3K (Vsp34) to promote intracellular survival. Systematically elucidating the mechanisms by which Listeria interferes with PI3K signaling all along the endocytic pathway may lead to novel anti-infective therapies.

Repurposing Inhibitors of Phosphoinositide 3-kinase as Adjuvant Therapeutics for Bacterial Infections

The rise in antimicrobial resistance and the decline in new antibiotics has created a great need for novel approaches to treat drug resistant bacterial infections. Increasing the burden of antimicrobial resistance, bacterial virulence factors allow for survival within the host, where they can evade host killing and antimicrobial therapy within their intracellular niches. Repurposing host directed therapeutics has great potential for adjuvants to allow for more effective bacterial killing by the host and antimicrobials. To this end, phosphoinositide 3-kinase inhibitors are FDA approved for cancer therapy, but also have potential to eliminate intracellular survival of pathogens. This review describes the PI3K pathway and its potential as an adjuvant target to treat bacterial infections more effectively.

Quantitative Spatio-temporal Analysis of Phagosome Maturation in Live Cells

Phagocytosis and phagosome maturation are central processes to the development of the innate and adaptive immune response. Phagosome maturation is a continuous and dynamic process that occurs rapidly. In this chapter we describe fluorescence-based live cell imaging methods for the quantitative and temporal analysis of phagosome maturation of beads and M. tuberculosis as two phagocytic targets. We also describe simple protocols for monitoring phagosome maturation: the use of the acidotropic probe LysoTracker and analyzing the recruitment of EGFP-tagged host proteins by phagosomes.

Quantitative Immunofluorescence to Study Phagosome Maturation and Resolution

Cells such as macrophages and neutrophils can internalize a diverse set of particulate matter, illustrated by bacteria and apoptotic bodies through the process of phagocytosis. These particles are sequestered into phagosomes, which then fuse with early and late endosomes and ultimately with lysosomes to mature into phagolysosomes, through a process known as phagosome maturation. Ultimately, after particle degradation, phagosomes then fragment to reform lysosomes through phagosome resolution. As phagosomes change, they acquire and divest proteins that are associated with the various stages of phagosome maturation and resolution. These changes can be assessed at the single-phagosome level by using immunofluorescence methods. Typically, we use indirect immunofluorescence methods that rely on primary antibodies against specific molecular markers that track phagosome maturation. Commonly, progression of phagosomes into phagolysosomes can be determined by staining cells for Lysosomal-Associated Membrane Protein I (LAMP1) and measuring the fluorescence intensity of LAMP1 around each phagosome by microscopy or flow cytometry. However, this method can be used to detect any molecular marker for which there are compatible antibodies for immunofluorescence.

High content quantitative imaging of Mycobacterium tuberculosis responses to acidic microenvironments within human macrophages

Intracellular pathogens such as Mycobacterium tuberculosis (Mtb) have evolved diverse strategies to counteract macrophage defence mechanisms including phagolysosomal biogenesis. Within macrophages, Mtb initially resides inside membrane-bound phagosomes that interact with lysosomes and become acidified. The ability of Mtb to control and subvert the fusion between phagosomes and lysosomes plays a key role in the pathogenesis of tuberculosis. Therefore, understanding how pathogens interact with the endolysosomal network and cope with intracellular acidification is important to better understand the disease. Here, we describe in detail the use of fluorescence microscopy-based approaches to investigate Mtb responses to acidic environments in cellulo. We report high-content imaging modalities to probe Mtb sensing of external pH or visualise in real-time Mtb intrabacterial pH within infected human macrophages. We discuss various methodologies with step-by-step analyses that enable robust image-based quantifications. Finally, we highlight the advantages and limitations of these different approaches and discuss potential alternatives that can be applied to further investigate Mtb-host cell interactions. These methods can be adapted to study host-pathogen interactions in different biological systems and experimental settings. Altogether, these approaches represent a valuable tool to further broaden our understanding of the cellular and molecular mechanisms underlying intracellular pathogen survival.

Therapeutic Potentials of Immunometabolomic Modulations Induced by Tuberculosis Vaccination

Metabolomics is emerging as a promising tool to understand the effect of immunometabolism for the development of novel host-directed alternative therapies. Immunometabolism can modulate both innate and adaptive immunity in response to pathogens and vaccinations. For instance, infections can affect lipid and amino acid metabolism while vaccines can trigger bile acid and carbohydrate pathways. Metabolomics as a vaccinomics tool, can provide a broader picture of vaccine-induced biochemical changes and pave a path to potentiate the vaccine efficacy. Its integration with other systems biology tools or treatment modes can enhance the cure, response rate, and control over the emergence of drug-resistant strains. Mycobacterium tuberculosis (Mtb) infection can remodel the host metabolism for its survival, while there are many biochemical pathways that the host adjusts to combat the infection. Similarly, the anti-TB vaccine, Bacillus Calmette-Guerin (BCG), was also found to affect the host metabolic pathways thus modulating immune responses. In this review, we highlight the metabolomic schema of the anti-TB vaccine and its therapeutic applications. Rewiring of immune metabolism upon BCG vaccination induces different signaling pathways which lead to epigenetic modifications underlying trained immunity. Metabolic pathways such as glycolysis, central carbon metabolism, and cholesterol synthesis play an important role in these aspects of immunity. Trained immunity and its applications are increasing day by day and it can be used to develop the next generation of vaccines to treat various other infections and orphan diseases. Our goal is to provide fresh insight into this direction and connect various dots to develop a conceptual framework.

Mycobacterium tuberculosis encodes a YhhN family membrane protein with lysoplasmalogenase activity that protects against toxic host lysolipids

The pathogen Mycobacterium tuberculosis (M.tb) resides in human macrophages, wherein it exploits host lipids for survival. However, little is known about the interaction between M.tb and macrophage plasmalogens, a subclass of glycerophospholipids with a vinyl ether bond at the sn-1 position of the glycerol backbone. Lysoplasmalogens, produced from plasmalogens by hydrolysis at the sn-2 carbon by phospholipase A2, are potentially toxic but can be broken down by host lysoplasmalogenase, an integral membrane protein of the YhhN family that hydrolyzes the vinyl ether bond to release a fatty aldehyde and glycerophospho-ethanolamine or glycerophospho-choline. Curiously, M.tb encodes its own YhhN protein (MtbYhhN), despite having no endogenous plasmalogens. To understand the purpose of this protein, the gene for MtbYhhN (Rv1401) was cloned and expressed in Mycobacterium smegmatis (M.smeg). We found the partially purified protein exhibited abundant lysoplasmalogenase activity specific for lysoplasmenylethanolamine or lysoplasmenylcholine (pLPC) (Vmax∼15.5 μmol/min/mg; Km∼83 μM). Based on cell density, we determined that lysoplasmenylethanolamine, pLPC, lysophosphatidylcholine, and lysophosphatidylethanolamine were not toxic to M.smeg cells, but pLPC and LPC were highly toxic to M.smeg spheroplasts, which are cell wall-deficient mycobacterial forms. Importantly, spheroplasts prepared from M.smeg cells overexpressing MtbYhhN were protected from membrane disruption/lysis by pLPC, which was rapidly depleted from the media. Finally, we found that overexpression of full-length MtbYhhN in M.smeg increased its survival within human macrophages by 2.6-fold compared to vector controls. These data support the hypothesis that MtbYhhN protein confers a growth advantage for mycobacteria in macrophages by cleaving toxic host pLPC into potentially energy-producing products.

Therapeutic Role of Antimicrobial Peptides in Diabetes Mellitus

Antimicrobial peptides (AMPs) have recently become widely publicized because they have the potential to function in alternative therapies as “natural” antibiotics, with their main advantage being a broad spectrum of activity. The potential for antimicrobial peptides to treat diabetes mellitus (DM) has been reported. In diabetes mellitus type I (T1D), cathelicidin-related antimicrobial peptide (CRAMP), cathelicidin antimicrobial peptide (CAMP) and mouse-β- defensin 14 (mBD14) are positively affected. Decreased levels of LL-37 and human neutrophil peptide 1-3 (HNP1-3) have been reported in diabetes mellitus type II (T2D) relative to healthy patients. Moreover, AMPs from amphibians and social wasps have antidiabetic effects. In infections occurring in patients with tuberculosis-diabetes or diabetic foot, granulysin, HNP1, HNP2, HNP3, human beta-defensin 2 (HBD2), and cathelicidins are responsible for pathogen clearance. An interesting alternative is also the use of modified M13 bacteriophages containing encapsulated AMPs genes or phagemids.

Photochemically-Mediated Inflammation and Cross-Presentation of Mycobacterium bovis BCG Proteins Stimulates Strong CD4 and CD8 T-Cell Responses in Mice

Conventional vaccines are very efficient in the prevention of bacterial infections caused by extracellular pathogens due to effective stimulation of pathogen-specific antibodies. In contrast, considering that intracellular surveillance by antibodies is not possible, they are typically less effective in preventing or treating infections caused by intracellular pathogens such as Mycobacterium tuberculosis. The objective of the current study was to use so-called photochemical internalization (PCI) to deliver a live bacterial vaccine to the cytosol of antigen-presenting cells (APCs) for the purpose of stimulating major histocompatibility complex (MHC) I-restricted CD8 T-cell responses. For this purpose, Mycobacterium bovis BCG (BCG) was combined with the photosensitiser tetraphenyl chlorine disulfonate (TPCS2a) and injected intradermally into mice. TPCS2a was then activated by illumination of the injection site with light of defined energy. Antigen-specific CD4 and CD8 T-cell responses were monitored in blood, spleen, and lymph nodes at different time points thereafter using flow cytometry, ELISA and ELISPOT. Finally, APCs were infected and PCI-treated in vitro for analysis of their activation of T cells in vitro or in vivo after autologous vaccination of mice. Combination of BCG with PCI induced stronger BCG-specific CD4 and CD8 T-cell responses than treatment with BCG only or with BCG and TPCS2a without light. The overall T-cell responses were multifunctional as characterized by the production of IFN-γ, TNF-α, IL-2 and IL-17. Importantly, PCI induced cross-presentation of BCG proteins for stimulation of antigen-specific CD8 T-cells that were particularly producing IFN-γ and TNF-α. PCI further facilitated antigen presentation by causing up-regulation of MHC and co-stimulatory proteins on the surface of APCs as well as their production of TNF-α and IL-1β in vivo. Furthermore, PCI-based vaccination also caused local inflammation at the site of vaccination, showing strong infiltration of immune cells, which could contribute to the stimulation of antigen-specific immune responses. This study is the first to demonstrate that a live microbial vaccine can be combined with a photochemical compound and light for cross presentation of antigens to CD8 T cells. Moreover, the results revealed that PCI treatment strongly improved the immunogenicity of M. bovis BCG.

The autophagy-resistant Mycobacterium tuberculosis Beijing strain upregulates KatG to evade starvation-induced autophagic restriction

Mycobacterium tuberculosis utilizes several mechanisms to block phagosome–lysosome fusion to evade host cell restriction. However, induction of host cell autophagy by starvation was shown to overcome this block, resulting in enhanced lysosomal delivery to mycobacterial phagosomes and the killing of the M. tuberculosis reference strain H37Rv. Nevertheless, our previous studies found that strains belonging to the M. tuberculosis Beijing genotype can resist starvation-induced autophagic elimination, though the mycobacterial factors involved remain unclear. In this study, we showed that KatG expression is upregulated in the autophagy-resistant M. tuberculosis Beijing strain (BJN) during autophagy induction by the starvation of host macrophages, while such increase was not observed in the H37Rv. KatG depletion using the CRISPR-dCas9 interference system in the BJN resulted in increased lysosomal delivery to its phagosome and decreased its survival upon autophagy induction by starvation. As KatG functions by catabolizing ROS, we determined the source of ROS contributing to the starvation-induced autophagic elimination of mycobacteria. Using siRNA-mediated knockdown, we found that Superoxide dismutase 2, which generates mitochondrial ROS but not NADPH oxidase 2, is important for the starvation-induced lysosomal delivery to mycobacterial phagosomes. Taken together, these findings showed that KatG is vital for the BJN to evade starvation-induced autophagic restriction.

Mycobacterium tuberculosis inhibits the NLRP3 inflammasome activation via its phosphokinase PknF

Mycobacterium tuberculosis (Mtb) has evolved to evade host innate immunity by interfering with macrophage functions. Interleukin-1β (IL-1β) is secreted by macrophages after the activation of the inflammasome complex and is crucial for host defense against Mtb infections. We have previously shown that Mtb is able to inhibit activation of the AIM2 inflammasome and subsequent pyroptosis. Here we show that Mtb is also able to inhibit host cell NLRP3 inflammasome activation and pyroptosis. We identified the serine/threonine kinase PknF as one protein of Mtb involved in the NLRP3 inflammasome inhibition, since the pknF deletion mutant of Mtb induces increased production of IL-1β in bone marrow-derived macrophages (BMDMs). The increased production of IL-1β was dependent on NLRP3, the adaptor protein ASC and the protease caspase-1, as revealed by studies performed in gene-deficient BMDMs. Additionally, infection of BMDMs with the pknF deletion mutant resulted in increased pyroptosis, while the IL-6 production remained unchanged compared to Mtb-infected cells, suggesting that the mutant did not affect the priming step of inflammasome activation. In contrast, the activation step was affected since potassium efflux, chloride efflux and the generation of reactive oxygen species played a significant role in inflammasome activation and subsequent pyroptosis mediated by the Mtb pknF mutant strain. In conclusion, we reveal here that the serine/threonine kinase PknF of Mtb plays an important role in innate immune evasion through inhibition of the NLRP3 inflammasome.

Mycobacterium tuberculosis (Mtb) infections are causing millions of deaths per year and the pathogen is highly adapted to its human host. Host cell phagocytes take up Mtb but the bacterium is capable of manipulating the host cell to enhance its own survival. In the current study we discover a novel pathway of host cell manipulation and innate immune evasion by Mtb. We show that the activation of a host cell defense complex, the inflammasome, is limited after Mtb infection. Most importantly, we identify a bacterial protein, PknF, that is involved in inflammasome inhibition.

Chemical Synthesis of Cell Wall Constituents of Mycobacterium tuberculosis

The pathogen Mycobacterium tuberculosis (Mtb), causing tuberculosis disease, features an extraordinary thick cell envelope, rich in Mtb-specific lipids, glycolipids, and glycans. These cell wall components are often directly involved in host–pathogen interaction and recognition, intracellular survival, and virulence. For decades, these mycobacterial natural products have been of great interest for immunology and synthetic chemistry alike, due to their complex molecular structure and the biological functions arising from it. The synthesis of many of these constituents has been achieved and aided the elucidation of their function by utilizing the synthetic material to study Mtb immunology. This review summarizes the synthetic efforts of a quarter century of total synthesis and highlights how the synthesis layed the foundation for immunological studies as well as drove the field of organic synthesis and catalysis to efficiently access these complex natural products.

Choreographing endo-lysosomal Ca2+ throughout the life of a phagosome

The emergence of endo-lysosomes as ubiquitous Ca²⁺ stores with their unique cohort of channels has resulted in their being implicated in a growing number of processes in an ever-increasing number of cell types. The architectural and regulatory constraints of these acidic Ca²⁺ stores distinguishes them from other larger Ca²⁺ sources such as the ER and influx across the plasma membrane. In view of recent advances in the understanding of the modes of operation, we discuss phagocytosis as a template for how endo-lysosomal Ca²⁺ signals (generated via TPC and TRPML channels) can be integrated in multiple sophisticated ways into biological processes. Phagocytosis illustrates how different endo-lysosomal Ca²⁺ signals drive different phases of a process, and how these can be altered by disease or infection.

Fucosylated lipid nanocarriers loaded with antibiotics efficiently inhibit mycobacterial propagation in human myeloid cells

Antibiotic treatment of tuberculosis (TB) is complex, lengthy, and can be associated with various adverse effects. As a result, patient compliance often is poor, thus further enhancing the risk of selecting multi-drug resistant bacteria. Macrophage mannose receptor (MMR)-positive alveolar macrophages (AM) constitute a niche in which Mycobacterium tuberculosis replicates and survives. Therefore, we encapsulated levofloxacin in lipid nanocarriers functionalized with fucosyl residues that interact with the MMR. Indeed, such nanocarriers preferentially targeted MMR-positive myeloid cells, and in particular, AM. Intracellularly, fucosylated lipid nanocarriers favorably delivered their payload into endosomal compartments, where mycobacteria reside. In an in vitro setting using infected human primary macrophages as well as dendritic cells, the encapsulated antibiotic cleared the pathogen more efficiently than free levofloxacin. In conclusion, our results point towards carbohydrate-functionalized nanocarriers as a promising tool for improving TB treatment by targeted delivery of antibiotics.

Better Together: Current Insights Into Phagosome-Lysosome Fusion

Following phagocytosis, the nascent phagosome undergoes maturation to become a phagolysosome with an acidic, hydrolytic, and often oxidative lumen that can efficiently kill and digest engulfed microbes, cells, and debris. The fusion of phagosomes with lysosomes is a principal driver of phagosomal maturation and is targeted by several adapted intracellular pathogens. Impairment of this process has significant consequences for microbial infection, tissue inflammation, the onset of adaptive immunity, and disease. Given the importance of phagosome-lysosome fusion to phagocyte function and the many virulence factors that target it, it is unsurprising that multiple molecular pathways have evolved to mediate this essential process. While the full range of these pathways has yet to be fully characterized, several pathways involving proteins such as members of the Rab GTPases, tethering factors and SNAREs have been identified. Here, we summarize the current state of knowledge to clarify the ambiguities in the field and construct a more comprehensive phagolysosome formation model. Lastly, we discuss how other cellular pathways help support phagolysosome biogenesis and, consequently, phagocyte function.

Genome scale metabolic model driven strategy to delineate host response to Mycobacterium tuberculosis infection

We analyze high throughput proteomics data reflecting the response of the Mφ-like THP1 cell line to Mycobacterium tuberculosis (M. tuberculosis) infection. M. tuberculosis's engagement with the host's metabolic pathways is a known strategy employed by the pathogen to shift the balance in its favour. Our study revisits this strategy through the integration of the temporal proteomics data in the genome-scale metabolic model (GSMM) giving context-specific GSMMs. THP1 cells were infected with H37Ra, H37Rv, BND433 and JAL2287 strains of M. tuberculosis and the host response was studied at 6, 18, 30 and 42 hours after infection. We have developed a modified flux balance analysis (FBA), which does not use an objective function, to find the fluxes of metabolic reactions in different strains and stages of infection and have revealed different functional modules. Hence, we have established a method of rewiring using GSMMs to explore potential strategies to change the flux state of virulent M. tuberculosis infected macrophages as against their avirulent counterparts. Our methodology gives a correlation between different flux states, the extent of which was interpreted as the extent of rewiring. The accuracy of the results from the proposed methodology was validated with gene knockout experimental data. We found that more than one reaction has to be rewired simultaneously to alter virulent to an avirulent response. The identified modules showed influence across the investigated strains and time points suggesting that these reactions could be therapeutically targeted. This novel methodology is now available for use in other systems.

The adapter protein Myd88 plays an important role in limiting mycobacterial growth in a zebrafish model for tuberculosis

Tuberculosis (TB) is the most prevalent bacterial infectious disease in the world, caused by the pathogen Mycobacterium tuberculosis (Mtb). In this study, we have used Mycobacterium marinum (Mm) infection in zebrafish larvae as an animal model for this disease to study the role of the myeloid differentiation factor 88 (Myd88), the key adapter protein of Toll-like receptors. Previously, Myd88 has been shown to enhance innate immune responses against bacterial infections, and in the present study, we have investigated the effect of Myd88 deficiency on the granuloma morphology and the intracellular distribution of bacteria during Mm infection. Our results show that granulomas formed in the tail fin from myd88 mutant larvae have a more compact structure and contain a reduced number of leukocytes compared to the granulomas observed in wild-type larvae. These morphological differences were associated with an increased bacterial burden in the myd88 mutant. Electron microscopy analysis showed that the majority of Mm in the myd88 mutant are located extracellularly, whereas in the wild type, most bacteria were intracellular. In the myd88 mutant, intracellular bacteria were mainly present in compartments that were not electron-dense, suggesting that these compartments had not undergone fusion with a lysosome. In contrast, approximately half of the intracellular bacteria in wild-type larvae were found in electron-dense compartments. These observations in a zebrafish model for tuberculosis suggest a role for Myd88-dependent signalling in two important phenomena that limit mycobacterial growth in the infected tissue. It reduces the number of leukocytes at the site of infection and the acidification of bacteria-containing compartments inside these cells.

Interaction of TLR4 and TLR8 in the Innate Immune Response against Mycobacterium Tuberculosis

The interaction and crosstalk of Toll-like receptors (TLRs) is an established pathway in which the innate immune system recognises and fights pathogens. In a single nucleotide polymorphisms (SNP) analysis of an Indian cohort, we found evidence for both TLR4-399T and TRL8-1A conveying increased susceptibility towards tuberculosis (TB) in an interdependent manner, even though there is no established TLR4 ligand present in Mycobacterium tuberculosis (Mtb), which is the causative pathogen of TB. Docking studies revealed that TLR4 and TLR8 can build a heterodimer, allowing interaction with TLR8 ligands. The conformational change of TLR4-399T might impair this interaction. With immunoprecipitation and mass spectrometry, we precipitated TLR4 with TLR8-targeted antibodies, indicating heterodimerisation. Confocal microscopy confirmed a high co-localisation frequency of TLR4 and TLR8 that further increased upon TLR8 stimulation. The heterodimerisation of TLR4 and TLR8 led to an induction of IL12p40, NF-κB, and IRF3. TLR4-399T in interaction with TLR8 induced an increased NF-κB response as compared to TLR4-399C, which was potentially caused by an alteration of subsequent immunological pathways involving type I IFNs. In summary, we present evidence that the heterodimerisation of TLR4 and TLR8 at the endosome is involved in Mtb recognition via TLR8 ligands, such as microbial RNA, which induces a Th1 response. These findings may lead to novel targets for therapeutic interventions and vaccine development regarding TB.

Evolution of Drug-Resistant Mycobacterium tuberculosis Strains and Their Adaptation to the Human Lung Environment

In the last two decades, multi (MDR), extensively (XDR), extremely (XXDR) and total (TDR) drug-resistant Mycobacterium tuberculosis (M.tb) strains have emerged as a threat to public health worldwide, stressing the need to develop new tuberculosis (TB) prevention and treatment strategies. It is estimated that in the next 35 years, drug-resistant TB will kill around 75 million people and cost the global economy $16.7 trillion. Indeed, the COVID-19 pandemic alone may contribute with the development of 6.3 million new TB cases due to lack of resources and enforced confinement in TB endemic areas. Evolution of drug-resistant M.tb depends on numerous factors, such as bacterial fitness, strain's genetic background and its capacity to adapt to the surrounding environment, as well as host-specific and environmental factors. Whole-genome transcriptomics and genome-wide association studies in recent years have shed some insights into the complexity of M.tb drug resistance and have provided a better understanding of its underlying molecular mechanisms. In this review, we will discuss M.tb phenotypic and genotypic changes driving resistance, including changes in cell envelope components, as well as recently described intrinsic and extrinsic factors promoting resistance emergence and transmission. We will further explore how drug-resistant M.tb adapts differently than drug-susceptible strains to the lung environment at the cellular level, modulating M.tb-host interactions and disease outcome, and novel next generation sequencing (NGS) strategies to study drug-resistant TB.

Bovine NK-lysin-derived peptides have bactericidal effects against Mycobacterium avium subspecies paratuberculosis

Infection with Mycobacterium avium subspecies paratuberculosis (MAP) is complex, but little is known about the role that natural killer (NK) cells play. In the present study, four bovine NK-lysin peptides were synthesized to evaluate their bactericidal activity against MAP. The results demonstrated that bNK-lysin peptides were directly bactericidal against MAP, with bNK1 and bNK2A being more potent than bNK2B and bNK2C. Mechanistically, transmission electron microscopy revealed that the incubation of MAP with bNK2A resulted in extensive damage to cell membranes and cytosolic content leakage. Furthermore, the addition of bNK2A linked with a cell-penetrating peptide resulted in increased MAP killing in a macrophage model.

Mycobacterium tuberculosis complex and human coadaptation: a two-way street complicating host susceptibility to TB

For centuries, the Mycobacterium tuberculosis complex (MTBC) has infected numerous populations, both human and non-human, causing symptomatic tuberculosis (TB) in some hosts. Research investigating the MTBC and how it has evolved with its host over time is sparse and has not resulted in many significant findings. There are even fewer studies investigating adaptation of the human host susceptibility to TB and these have largely focused on genome-wide association and candidate gene association studies. However, results emanating from these association studies are rarely replicated and appear to be population specific. It is, therefore, necessary to relook at the approach taken to investigate the relationship between the MTBC and the human host. Understanding that the evolution of the pathogen is coupled to the evolution of the host might be the missing link needed to effectively investigate their relationship. We hypothesize that this knowledge will bolster future efforts in combating the disease.

The Interplay of Host Lysosomes and Intracellular Pathogens

Lysosomes are an integral part of the intracellular defense system against microbes. Lysosomal homeostasis in the host is adaptable and responds to conditions such as infection or nutritional deprivation. Pathogens such as Mycobacterium tuberculosis (Mtb) and Salmonella avoid lysosomal targeting by actively manipulating the host vesicular trafficking and reside in a vacuole altered from the default lysosomal trafficking. In this review, the mechanisms by which the respective pathogen containing vacuoles (PCVs) intersect with lysosomal trafficking pathways and maintain their distinctness are discussed. Despite such active inhibition of lysosomal targeting, emerging literature shows that different pathogens or pathogen derived products exhibit a global influence on the host lysosomal system. Pathogen mediated lysosomal enrichment promotes the trafficking of a sub-set of pathogens to lysosomes, indicating heterogeneity in the host-pathogen encounter. This review integrates recent advancements on the global lysosomal alterations upon infections and the host protective role of the lysosomes against these pathogens. The review also briefly discusses the heterogeneity in the lysosomal targeting of these pathogens and the possible mechanisms and consequences.

The Role of Metallic Nanoparticles in Inhibition of Mycobacterium Tuberculosis and Enhances Phagosome Maturation into the Infected Macrophage

This review focuses on the role of gallium (Ga) nanoparticles (NPs) to enhance phagosome maturation into the Mycobacterium tuberculosis-infected macrophage and the role of magnetic iron NPs as nanocarriers of antituberculosis drugs. The literature shows that silver (Ag) and zinc oxide (ZnO) NPs with dimensions less than 10 nm can penetrate directly through the macrophage bilayer membrane. Ag NPs increase the permeability membrane by motiving the aggregation of proteins in the periplasmic space and forming nano-sized pores. ZnO NPs can interact with the membrane of M. tuberculosis, which leads to the formation of surface pores and the release of intracellular nucleotides. The colloidal Ag:ZnO mixture NPs with 1:1 ratio can eliminate M. tuberculosis and shows the lowest cytotoxicity effects on MCF-7 and THP-1 cell lines. Ag/ZnO nanocrystals are not able to kill M. tuberculosis alone ex-vivo. Hence, bimetallic gold (Au)/Ag NPs possessed high efficiency to inhibit M. tuberculosis in an ex-vivo THP-1 infection model. Co-delivery of mixed MeNPs into a polymeric carrier collaborated to selective uptake by macrophages through passive targeting, initial burst release of ions from the encapsulated metallic (Me) NPs, and eventually, reduction of MeNPs toxicity, and plays a pivotal role in increasing the antitubercular activity compared to use alone. In addition, Ga NPs can import drugs to the macrophage, inhibit M. tuberculosis growth, and reduce the inhibition of phagosome maturation. Magnetic encapsulated NPs exhibited good drug release properties and might be suitable as carriers of antituberculosis drugs.

Host Cell Targets of Released Lipid and Secreted Protein Effectors of Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is a very successful pathogen, strictly adapted to humans and the cause of tuberculosis. Its success is associated with its ability to inhibit host cell intrinsic immune responses by using an arsenal of virulence factors of different nature. It has evolved to synthesize a series of complex lipids which form an outer membrane and may also be released to enter host cell membranes. In addition, secreted protein effectors of Mtb are entering the host cell cytosol to interact with host cell proteins. We briefly discuss the current model, involving the ESX-1 type seven secretion system and the Mtb lipid phthiocerol dimycoserosate (PDIM), of how Mtb creates pores in the phagosomal membrane to allow Mtb proteins to access to the host cell cytosol. We provide an exhaustive list of Mtb secreted proteins that have effector functions. They modify (mostly inhibit but sometimes activate) host cell pathways such as: phagosome maturation, cell death, cytokine response, xenophagy, reactive oxygen species (ROS) response via NADPH oxidase 2 (NOX2), nitric oxide (NO) response via NO Synthase 2 (NOS2) and antigen presentation via MHC class I and class II molecules. We discuss the host cell targets for each lipid and protein effector and the importance of the Mtb effector for virulence of the bacterium.

Understanding the Relationship between Glutathione, TGF-β, and Vitamin D in Combating Mycobacterium tuberculosis Infections

Tuberculosis (TB) remains a pervasive global health threat. A significant proportion of the world's population that is affected by latent tuberculosis infection (LTBI) is at risk for reactivation and subsequent transmission to close contacts. Despite sustained efforts in eradication, the rise of multidrug-resistant strains of Mycobacteriumtuberculosis (M. tb) has rendered traditional antibiotic therapy less effective at mitigating the morbidity and mortality of the disease. Management of TB is further complicated by medications with various off-target effects and poor compliance. Immunocompromised patients are the most at-risk in reactivation of a LTBI, due to impairment in effector immune responses. Our laboratory has previously reported that individuals suffering from Type 2 Diabetes Mellitus (T2DM) and HIV exhibited compromised levels of the antioxidant glutathione (GSH). Restoring the levels of GSH resulted in improved control of M. tb infection. The goal of this review is to provide insights on the diverse roles of TGF- β and vitamin D in altering the levels of GSH, granuloma formation, and clearance of M. tb infection. We propose that these pathways represent a potential avenue for future investigation and development of new TB treatment modalities.

Is targeting dysregulation in apoptosis splice variants in Mycobacterium tuberculosis (MTB) host interactions and splicing factors resulting in immune evasion by MTB strategies a possibility?

Mycobacterium tuberculosis (Mtb), is one of the foremost organisms causing mortality in humans, and has been for most of human history. When faced with an infection the human immune system is ordinarily very competent in killing both extracellular and intracellular bacilli. However, Mtb is able to evade the host immune system and is even able to establish a persistent infectious reservoir by "hiding" in the immune cells of the host. While the mechanisms by which the bacteria accomplishes this are not fully understood, it is known that the bacterium can subvert cellular processes in cells such as macrophages that prevent the lysis of the bacteria or the cell undergoing apoptosis. They are also able to interfere with immune cell signalling. One of the greatest effects that Mtb has is too alter the transcriptome of the macrophage. An easy way for the bacterium to accomplish this is to alter the alternative splicing patterns of the host. This can lead to a large change in the population of different protein isoforms, some of which have very different functions when compared to the original protein. At the same time the long history of Mtb infecting humans have led to specific immune reactions that occur in the host immune system in order to fight the infection. Many of these specific reactions involve new isoforms of host defence proteins. In this way the human host can use alternate splicing to create new isoforms of immune- related proteins that are more effective in defending against Mtb.

Mycobacterium tuberculosis (Mtb) lipid mediated lysosomal rewiring in infected macrophages modulates intracellular Mtb trafficking and survival

Intracellular pathogens commonly manipulate the host lysosomal system for their survival. However, whether this pathogen-induced alteration affects the organization and functioning of the lysosomal system itself is not known. Here, using in vitro and in vivo infections and quantitative image analysis, we show that the lysosomal content and activity are globally elevated in Mycobacterium tuberculosis (Mtb)-infected macrophages. We observed that this enhanced lysosomal state is sustained over time and defines an adaptive homeostasis in the infected macrophage. Lysosomal alterations are caused by mycobacterial surface components, notably the cell wall-associated lipid sulfolipid-1 (SL-1), which functions through the mTOR complex 1 (mTORC1)-transcription factor EB (TFEB) axis in the host cells. An Mtb mutant lacking SL-1, MtbΔpks2, shows attenuated lysosomal rewiring compared with the WT Mtb in both in vitro and in vivo infections. Exposing macrophages to purified SL-1 enhanced the trafficking of phagocytic cargo to lysosomes. Correspondingly, MtbΔpks2 exhibited a further reduction in lysosomal delivery compared with the WT. Reduced trafficking of this mutant Mtb strain to lysosomes correlated with enhanced intracellular bacterial survival. Our results reveal that global alteration of the host lysosomal system is a defining feature of Mtb-infected macrophages and suggest that this altered lysosomal state protects host cell integrity and contributes to the containment of the pathogen.

Rab7 controls lipid droplet-phagosome association during mycobacterial infection

Lipid droplets (LDs) are organelles that have multiple roles in inflammatory and infectious diseases. LD act as essential platforms for immunometabolic regulation, including as sites for lipid storage and metabolism, inflammatory lipid mediator production, and signaling pathway compartmentalization. Accumulating evidence indicates that intracellular pathogens may exploit host LDs as source of nutrients and as part of their strategy to promote immune evasion. Notably, numerous studies have demonstrated the interaction between LDs and pathogen-containing phagosomes. However, the mechanism involved in this phenomenon remains elusive. Here, we observed LDs and PLIN2 surrounding M. bovis BCG-containing phagosomes, which included observations of a bacillus cell surrounded by lipid content inside a phagosome and LAM from mycobacteria co-localizing with LDs; these results were suggestive of exchange of contents between these compartments. By using beads coated with M.tb lipids, we demonstrated that LD-phagosome associations are regulated through the mycobacterial cell wall components LAM and PIM. In addition, we demonstrated that Rab7 and RILP, but not Rab5, localizes to LDs of infected macrophages and observed the presence of Rab7 at the site of interaction with an infected phagosome. Moreover, treatment of macrophages with the Rab7 inhibitor CID1067700 significantly inhibited the association between LDs and LAM-coated beads. Altogether, our data demonstrate that LD-phagosome interactions are controlled by mycobacterial cell wall components and Rab7, which enables the exchange of contents between LDs and phagosomes and may represent a fundamental aspect of bacterial pathogenesis and immune evasion.

Calcium Signaling Commands Phagosome Maturation Process

Phagosome-lysosome (P-L) fusion is one of the central immune-effector responses of host. It is known that phagosome maturation process is associated with numerous signaling cascades and among these, important role of calcium (Ca²⁺) signaling has been realized recently. Ca²⁺ plays key roles in actin rearrangement, activation of NADPH oxidase and protein kinase C (PKC). Involvement of Ca²⁺ in these cellular processes directs phagosomal maturation process. Some of the intracellular pathogens have acquired the strategies to modulate Ca²⁺ associated pathways to block P-L fusion process. In this review we have described the mechanism of Ca²⁺ signals that influence P-L fusion by controlling ROS, actin and PKC signaling cascades. We have also discussed the strategies implemented by the intracellular pathogens to manipulate Ca²⁺ signaling to consequently subvert P-L fusion. A detail study of factors associated in manipulating Ca²⁺ signaling may provide new insights for the development of therapeutic tools for more effective treatment options against infectious diseases.

Underestimated Manipulative Roles of Mycobacterium tuberculosis Cell Envelope Glycolipids During Infection

The Mycobacterium tuberculosis cell envelope has been evolving over time to make the bacterium transmissible and adaptable to the human host. In this context, the M. tuberculosis cell envelope contains a peripheral barrier full of lipids, some of them unique, which confer M. tuberculosis with a unique shield against the different host environments that the bacterium will encounter at the different stages of infection. This lipid barrier is mainly composed of glycolipids that can be characterized by three different subsets: trehalose-containing, mannose-containing, and 6-deoxy-pyranose-containing glycolipids. In this review, we explore the roles of these cell envelope glycolipids in M. tuberculosis virulence and pathogenesis, drug resistance, and further, how these glycolipids may dictate the M. tuberculosis cell envelope evolution from ancient to modern strains. Finally, we address how these M. tuberculosis cell envelope glycolipids are impacted by the host lung alveolar environment, their role in vaccination and masking host immunity, and subsequently the impact of these glycolipids in shaping how M. tuberculosis interacts with host cells, manipulating their immune response to favor the establishment of an infection.

The Role of Phosphatidylinositol Mannosides in the Serological Diagnosis of Mycobacterial Infections

Accurate diagnosis of mycobacterial infections, such as bovine tuberculosis and paratuberculosis, remains challenging. Available direct diagnostic tests aimed at detecting the pathogen are highly specific but lack sensitivity, depending on the stage of infection and the prevalence of infection in a population. The sensitivity of indirect diagnostic assays that measure the host immune response to infection is similarly affected by disease characteristics. The choice of antigen used to detect a host response to infection has a critical impact on test sensitivity and specificity. Many indirect tests rely on crude antigen preparations and cell-free extracts, of which the production is poorly standardized. Moreover, these preparations contain ample uncharacterized cross-reactive compounds. To enhance serological test specificity, existing assays depend on the pre-treatment of samples and a relatively high cut-off value, that in turn influences test sensitivity. Research therefore focuses on the identification of more specific, defined antigens to improve diagnostics. In the current study, we extracted phosphatidylinositol mannosides (PIMs) and investigated their potential use in antibody-based tests. Our results demonstrate that specific IgG class antibodies are generated against PIMs in cows, but this is unrelated to tuberculosis or paratuberculosis infection status, making these antigens unsuitable for diagnostic applications. In addition, we demonstrate that PIMs are widely present in crude antigen preparations and in serum pre-absorption buffer. Our results indicate that PIMs are cross-reactive compounds with immunodominant B cell epitopes that could impair serological test specificity.

Mycobacterium tuberculosis infection of host cells in space and time

Tuberculosis (TB) caused by the bacterial pathogen Mycobacterium tuberculosis (Mtb) remains one of the deadliest infectious diseases with over a billion deaths in the past 200 years (Paulson 2013). TB causes more deaths worldwide than any other single infectious agent, with 10.4 million new cases and close to 1.7 million deaths in 2017. The obstacles that make TB hard to treat and eradicate are intrinsically linked to the intracellular lifestyle of Mtb. Mtb needs to replicate within human cells to disseminate to other individuals and cause disease. However, we still do not completely understand how Mtb manages to survive within eukaryotic cells and why some cells are able to eradicate this lethal pathogen. Here, we summarise the current knowledge of the complex host cell-pathogen interactions in TB and review the cellular mechanisms operating at the interface between Mtb and the human host cell, highlighting the technical and methodological challenges to investigating the cell biology of human host cell-Mtb interactions.

The role of low molecular weight thiols in Mycobacterium tuberculosis

Low molecular weight (LMW) thiols are molecules with a functional sulfhydryl group that enable them to detoxify reactive oxygen species, reactive nitrogen species and other free radicals. Their roles range from their ability to modulate the immune system to their ability to prevent damage of biological molecules such as DNA and proteins by protecting against oxidative, nitrosative and acidic stress. LMW thiols are synthesized and found in both eukaryotes and prokaryotes. Due to their beneficial role to both eukaryotes and prokaryotes, their specific functions need to be elucidated, most especially in pathogenic prokaryotes such as Mycobacterium tuberculosis (M.tb), in order to provide a rationale for targeting their biosynthesis for drug development. Ergothioneine (ERG), mycothiol (MSH) and gamma-glutamylcysteine (GGC) are LMW thiols that have been shown to interplay to protect M.tb against cellular stress. Though ERG, MSH and GGC seem to have overlapping functions, studies are gradually revealing their unique physiological roles. Understanding their unique physiological role during the course of tuberculosis (TB) infection, would pave the way for the development of drugs that target their biosynthetic pathway. This review identifies the knowledge gap in the unique physiological roles of LMW thiols and proposes their mechanistic roles based on previous studies. In addition, it gives an update on identified inhibitors of their biosynthetic enzymes.

Enrichment of ATP Binding Proteins Unveils Proteomic Alterations in Human Macrophage Cell Death, Inflammatory Response, and Protein Synthesis after Interaction with Candida albicans

Macrophages are involved in the primary human response to Candida albicans. After pathogen recognition, signaling pathways are activated, leading to the production of cytokines, chemokines, and antimicrobial peptides. ATP binding proteins are crucial for this regulation. Here, a quantitative proteomic and phosphoproteomic approach was carried out for the study of human macrophage ATP-binding proteins after interaction with C. albicans. From a total of 547 nonredundant quantified proteins, 137 were ATP binding proteins and 59 were detected as differentially abundant. From the differentially abundant ATP-binding proteins, 6 were kinases (MAP2K2, SYK, STK3, MAP3K2, NDKA, and SRPK1), most of them involved in signaling pathways. Furthermore, 85 phosphopeptides were quantified. Macrophage proteomic alterations including an increase of protein synthesis with a consistent decrease in proteolysis were observed. Besides, macrophages showed changes in proteins of endosomal trafficking together with mitochondrial proteins, including some involved in the response to oxidative stress. Regarding cell death mechanisms, an increase of antiapoptotic over pro-apoptotic signals is suggested. Furthermore, a high pro-inflammatory response was detected, together with no upregulation of key mi-RNAs involved in the negative feedback of this response. These findings illustrate a strategy to deepen the knowledge of the complex interactions between the host and the clinically important pathogen C. albicans.

Autophagy links antimicrobial activity with antigen presentation in Langerhans cells

DC, through the uptake, processing, and presentation of antigen, are responsible for activation of T cell responses to defend the host against infection, yet it is not known if they can directly kill invading bacteria. Here, we studied in human leprosy, how Langerhans cells (LC), specialized DC, contribute to host defense against bacterial infection. IFN-γ treatment of LC isolated from human epidermis and infected with Mycobacterium leprae (M. leprae) activated an antimicrobial activity, which was dependent on the upregulation of the antimicrobial peptide cathelicidin and induction of autophagy. IFN-γ induction of autophagy promoted fusion of phagosomes containing M. leprae with lysosomes and the delivery of cathelicidin to the intracellular compartment containing the pathogen. Autophagy enhanced the ability of M. leprae-infected LC to present antigen to CD1a-restricted T cells. The frequency of IFN-γ labeling and LC containing both cathelicidin and autophagic vesicles was greater in the self-healing lesions vs. progressive lesions, thus correlating with the effectiveness of host defense against the pathogen. These data indicate that autophagy links the ability of DC to kill and degrade an invading pathogen, ensuring cell survival from the infection while facilitating presentation of microbial antigens to resident T cells.

A paucity of knowledge regarding nontuberculous mycobacterial lipids compared to the tubercle bacillus

All mycobacteria, including nontuberculous mycobacteria (NTM), synthesize an array of lipids including phosphatidylinositol mannosides (PIM), lipomannan (LM), and lipoarabinomannan (LAM). While absent from Mycobacterium tuberculosis (M. tb), glycopeptidolipids (GPL) are critical to the biology of NTM. M. tb and some NTM also synthesize trehalose-containing glycolipids and phenolic glycolipids (PGL), key membrane constituents with essential roles in metabolism. While lipids facilitate immune evasion, they also induce host immunity against tuberculosis. However, much less is known about the significance of NTM-derived PIM, LM, LAM, GPL, trehalose-containing glycolipids, and PGL as virulence factors, warranting further investigation. While culling the scientific literature on NTM lipids, it's evident that such studies were relatively few in number with the overwhelming majority of prior work dedicated to understanding lipids from the saprophyte Mycobacterium smegmatis. The identification and functional analysis of immune reactive NTM-derived lipids remain challenging, but such work is likely to yield a greater understanding of the pathogenesis of NTM lung disease. In this review, we juxtapose the vast literature of what is currently known regarding M. tb lipids to the lesser number of studies for comparable NTM lipids. But because GPL is the most widely recognized NTM lipid, we highlight its role in disease pathogenesis.

Investigating the Role of Everolimus in mTOR Inhibition and Autophagy Promotion as a Potential Host-Directed Therapeutic Target in Mycobacterium tuberculosis Infection

Tuberculosis (TB) is a serious infectious disease caused by the pathogen Mycobacterium tuberculosis (Mtb). The current therapy consists of a combination of antibiotics over the course of four months. Current treatment protocols run into problems due to the growing antibiotic resistance of Mtb and poor compliance to the multi-drug-resistant TB treatment protocol. New treatments are being investigated that target host intracellular processes that could be effective in fighting Mtb infections. Autophagy is an intracellular process that is involved in eliminating cellular debris, as well as intracellular pathogens. Mammalian target of rapamycin (mTOR) is an enzyme involved in inhibiting this pathway. Modulation of mTOR and the autophagy cellular machinery are being investigated as potential therapeutic targets for novel Mtb treatments. In this review, we discuss the background of Mtb pathogenesis, including its interaction with the innate and adaptive immune systems, the mTOR and autophagy pathways, the interaction of Mtb with these pathways, and finally, the drug everolimus, which targets these pathways and is a potential novel therapy for TB treatment.

Endosomal and Phagosomal SNAREs

The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein family is of vital importance for organelle communication. The complexing of cognate SNARE members present in both the donor and target organellar membranes drives the membrane fusion required for intracellular transport. In the endocytic route, SNARE proteins mediate trafficking between endosomes and phagosomes with other endosomes, lysosomes, the Golgi apparatus, the plasma membrane, and the endoplasmic reticulum. The goal of this review is to provide an overview of the SNAREs involved in endosomal and phagosomal trafficking. Of the 38 SNAREs present in humans, 30 have been identified at endosomes and/or phagosomes. Many of these SNAREs are targeted by viruses and intracellular pathogens, which thereby reroute intracellular transport for gaining access to nutrients, preventing their degradation, and avoiding their detection by the immune system. A fascinating picture is emerging of a complex transport network with multiple SNAREs being involved in consecutive trafficking routes.

Tuberculosis and the art of macrophage manipulation

Macrophages are first-line responders against microbes. The success of Mycobacterium tuberculosis (Mtb) rests upon its ability to convert these antimicrobial cells into a permissive cellular niche. This is a remarkable accomplishment, as the antimicrobial arsenal of macrophages is extensive. Normally bacteria are delivered to an acidic, degradative lysosome through one of several trafficking pathways, including LC3-associated phagocytosis (LAP) and autophagy. Once phagocytozed, the bacilli are subjected to reactive oxygen and nitrogen species, and they induce the expression of proinflammatory cytokines, which serve to augment host responses. However, Mtb hijacks these host defense mechanisms, manipulating host cellular trafficking, innate immune responses, and cell death pathways to its benefit. The complex series of measures and countermeasures between host and pathogen ultimately determines the outcome of infection. In this review, we focus on the diverse effectors that Mtb uses in its multipronged effort to subvert the innate immune responses of macrophages. We highlight recent advances in understanding the molecular interface of the Mtb-macrophage interaction.

Heparin-Binding Hemagglutinin Adhesin (HBHA) Is Involved in Intracytosolic Lipid Inclusions Formation in Mycobacteria

The heparin-binding hemagglutinin adhesin (HBHA) is an important virulence factor of Mycobacterium tuberculosis. It is a surface-displayed protein that serves as an adhesin for non-phagocytic cells and is involved in extra-pulmonary dissemination of the tubercle bacillus. It is also an important latency antigen useful for the diagnosis of latently M. tuberculosis-infected individuals. Using fluorescence time-lapse microscopy on mycobacteria that produce HBHA-green fluorescent protein chimera, we show here that HBHA can be found at two different locations and dynamically alternates between the mycobacterial surface and the interior of the cell, where it participates in the formation of intracytosolic lipid inclusions (ILI). Compared to HBHA-producing mycobacteria, HBHA-deficient mutants contain significantly lower amounts of ILI when grown in vitro or within macrophages, and the sizes of their ILI are significantly smaller. Lipid-binding assays indicate that HBHA is able to specifically bind to phosphatidylinositol and in particular to 4,5 di-phosphorylated phosphatidylinositol, but not to neutral lipids, the main constituents of ILI. HBHA derivatives lacking the C-terminal methylated, lysine-rich repeat region fail to bind to these lipids and these derivatives also fail to complement the phenotype of HBHA-deficient mutants. These studies indicate that HBHA is a moonlighting protein that serves several functions depending on its location. When surface exposed, HBHA serves as an adhesin, and when intracellularly localized, it participates in the generation of ILI, possibly as a cargo to transport phospholipids from the plasma membrane to the ILI in the process of being formed.

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.

Getting in and Staying Alive: Role for Coronin 1 in the Survival of Pathogenic Mycobacteria and Naïve T Cells

There are many different pathogenic stimuli that are able to activate the immune system, ranging from microbes that include bacteria, viruses, fungi, and parasites to host-derived triggers such as autoantigens that can induce autoimmunity as well as neoantigens involved in tumorigenesis. One of the key interactions shaping immunity toward these triggers involves the encounter of antigen-processing and -presenting cells such as macrophages and dendritic cells with T cells, resulting in immune responses that are highly selective for the antigenic trigger. Research over the past few years has implicated members of the coronin protein family, in particular coronin 1, in responses against several pathogenic triggers. While coronin 1 was initially described as a host factor allowing the intracellular survival of the pathogen Mycobacterium tuberculosis, subsequent work showed it to be a crucial factor for naïve T cell homeostasis. The activity of coronin 1 in allowing the intracellular survival of pathogenic mycobacteria is relatively well characterized, involving the activation of the Ca²⁺/calcineurin pathway, while coronin 1’s role in modulating naïve T cell homeostasis remains more enigmatic. In this mini review, we discuss the knowledge on the role for coronin 1 in immune cell functioning and provide a number of potential scenarios via which coronin 1 may be able to regulate naïve T cell homeostasis.

Mannose-capped lipoarabinomannan in Mycobacterium tuberculosis pathogenesis

Mannose-capped lipoarabinomannan (ManLAM), present in all members of the Mycobacterium tuberculosis complex and in other pathogenic Mycobacterium spp, is a high molecular mass amphipathic lipoglycan with a defined critical role in mycobacterial survival during infection. In particular, ManLAM is well-characterized for its importance in providing M. tuberculosis a safe portal of entry to phagocytes, regulating the intracellular trafficking network, as well as immune responses of infected host cells. These ManLAM immunological characteristics are thought to be linked to the subtle but unique and well-defined structural characteristics of this molecule, including but not limited to the degree of acylation, the length of the D-mannan and D-arabinan cores, the length of the mannose caps, as well as the presence of other acidic constituents such as succinates, lactates and/or malates, and also the presence of 5-methylthioxylosyl. The impact of all these structural features on ManLAM spatial conformation and biological functions during M. tuberculosis infection is still uncertain. In this review, we dissect the relationship between ManLAM structure and biological function addressing how this relationship determines M. tuberculosis interactions with host cells, and how it aids this exceptional pathogen during the course of infection.

Lysophosphatidylcholine Promotes Phagosome Maturation and Regulates Inflammatory Mediator Production Through the Protein Kinase A-Phosphatidylinositol 3 Kinase-p38 Mitogen-Activated Protein Kinase Signaling Pathway During Mycobacterium tuberculosis Infection in Mouse Macrophages

Tuberculosis is caused by the infectious agent Mycobacterium tuberculosis (Mtb). Mtb has various survival strategies, including blockade of phagosome maturation and inhibition of antigen presentation. Lysophosphatidylcholine (LPC) is a major phospholipid component of oxidized low-density lipoprotein and is involved in various cellular responses, such as activation of second messengers and bactericidal activity in neutrophils. In this study, macrophages were infected with a low infectious dose of Mtb and treated with LPC to investigate the bactericidal activity of LPC against Mtb. In macrophages infected with Mtb strain, H37Ra or H37Rv, LPC suppressed bacterial growth; however, this effect was suppressed in bone marrow-derived macrophages (BMDMs) isolated from G2A (a G protein-coupled receptor involved in some LPC actions) knockout mice. LPC also promoted phagosome maturation via phosphatidylinositol 3 kinase (PI3K)–p38 mitogen-activated protein kinase (MAPK)-mediated reactive oxygen species production and intracellular Ca²⁺ release during Mtb infection. In addition, LPC induced increased levels of intracellular cyclic adenosine monophosphate (cAMP) and phosphorylated glycogen synthase kinase 3 beta (GSK3β) in Mtb-infected macrophages. Protein kinase A (PKA)-induced phosphorylation of GSK3β suppressed activation of NF-κB in LPC-treated macrophages during Mtb infection, leading to decreased secretion of pro-inflammatory cytokines and increased secretion of anti-inflammatory cytokines. These results suggest that LPC can effectively control Mtb growth by promoting phagosome maturation via cAMP-induced activation of the PKA–PI3K–p38 MAPK pathway. Moreover, LPC can regulate excessive production of pro-inflammatory cytokines associated with bacterial infection of macrophages.