Cellubrevin alterations and Mycobacterium tuberculosis phagosome maturation arrest

Macrophage Proteome Analysis at Different Stages of Mycobacterium avium Subspecies paratuberculosis Infection Reveals a Mechanism of Pathogen Dissemination

Johne’s disease is a chronic and usually fatal enteric infection of ruminants caused by Mycobacterium avium subspecies paratuberculosis (MAP) and is responsible for hundreds of millions of dollars in losses for the agricultural industry. Natural infection typically begins with bacterial uptake and translocation through the epithelium of the small intestine, followed by ingestion by tissue macrophages and dissemination via the lymphatic or blood system throughout the body. To gain insights into the host responses and adaptation of MAP within phagocytic cells, we utilized the previously developed cell culture passage model, and mass spectrometric-based quantitative proteomic approach. Using the cell culture system, which mimics an in vivo interaction of MAP with intestinal epithelium and tissue macrophages, bacteria were passed through the bovine epithelial cells and, subsequently, used for macrophage infection (termed indirect infection), while uninfected cells and macrophage infection initiated with the culture grown bacteria (termed direct infection) served as controls. Approximately 3900 proteins were identified across all studied groups. The comparison within the subset of proteins that showed synthesis for more than two-fold in the direct infection over the uninfected control revealed an enrichment for the pro-inflammatory pathways such as the NF-κB and cytokine/chemokine signaling, positive regulation of defense response, cell activation involved in the immune response and adaptive immune system. While these responses were absent in the indirect infection, cellular pathways such as cell cycle, healing, regulation of cell adhesion, ensemble of core extracellular matrix proteins, cell surface integrins and proteins mediating the integrin signaling were remarkably high within the indirect infection. In addition to global analysis of the macrophage proteome, we further validated the proteomics data and confirmed that MAP passage through epithelial cells modulates the expression and signaling of integrins in phagocytes. In this study, we demonstrate that predominant expression of integrins in the indirectly infected macrophages allows phagocytic cells to initiate stronger binding and efficient translocation through the endothelial cells, suggesting the important role of integrins in the spread of MAP infection.

Eukaryotic SNARE VAMP3 Dynamically Interacts with Multiple Chlamydial Inclusion Membrane Proteins

Chlamydia trachomatis, an obligate intracellular pathogen, undergoes a biphasic developmental cycle within a membrane-bound vacuole called the chlamydial inclusion. To facilitate interactions with the host cell, Chlamydia modifies the inclusion membrane with type III secreted proteins, called Incs.

Chlamydia trachomatis, an obligate intracellular pathogen, undergoes a biphasic developmental cycle within a membrane-bound vacuole called the chlamydial inclusion. To facilitate interactions with the host cell, Chlamydia modifies the inclusion membrane with type III secreted proteins, called Incs. As with all chlamydial proteins, Incs are temporally expressed, modifying the chlamydial inclusion during the early and mid-developmental cycle. VAMP3 and VAMP4 are eukaryotic SNARE proteins that mediate membrane fusion and are recruited to the inclusion to facilitate inclusion expansion. Their recruitment requires de novo chlamydial protein synthesis during the mid-developmental cycle. Thus, we hypothesize that VAMP3 and VAMP4 are recruited by Incs. In chlamydia-infected cells, identifying Inc binding partners for SNARE proteins specifically has been elusive. To date, most studies examining chlamydial Inc and eukaryotic proteins have benefitted from stable interacting partners or a robust interaction at a specific time postinfection. While these types of interactions are the predominant class that have been identified, they are likely the exception to chlamydia-host interactions. Therefore, we applied two separate but complementary experimental systems to identify candidate chlamydial Inc binding partners for VAMPs. Based on these results, we created transformed strains of C. trachomatis serovar L2 to inducibly express a candidate Inc-FLAG protein. In chlamydia-infected cells, we found that five Incs temporally and transiently interact with VAMP3. Further, loss of incA or ct813 expression altered VAMP3 localization to the inclusion. For the first time, our studies demonstrate the transient nature of certain host protein-Inc interactions that contribute to the chlamydial developmental cycle.

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.

Comparing the Metabolic Capabilities of Bacteria in the Mycobacterium tuberculosis Complex

Pathogenic mycobacteria are known for their ability to maintain persistent infections in various mammals. The canonical pathogen in this genus is Mycobacterium tuberculosis and this bacterium is particularly successful at surviving and replicating within macrophages. Here, we will highlight the metabolic processes that M. tuberculosis employs during infection in macrophages and compare these findings with what is understood for other pathogens in the M. tuberculosis complex.

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.

Current efforts and future prospects in the development of live mycobacteria as vaccines

The development of more effective vaccines against Mycobacterium tuberculosis (Mtb) remains a major goal in the effort to reduce the enormous global burden of disease caused by this pathogen. Whole-cell vaccines based on live mycobacteria with attenuated virulence represent an appealing approach, providing broad antigen exposure and intrinsic adjuvant properties to prime durable immune responses. However, designing vaccine strains with an optimal balance between attenuation and immunogenicity has proven to be extremely challenging. Recent basic and clinical research efforts have broadened our understanding of Mtb pathogenesis and created numerous new vaccine candidates that have been designed to overcome different aspects of immune evasion by Mtb. In this review, we provide an overview of the current efforts to create improved vaccines against tuberculosis based on modifications of live attenuated mycobacteria. In addition, we discuss the use of such vaccine strains as vectors for stimulating protective immunity against other infectious diseases and cancers.

Role of VAMP3 and VAMP7 in the commitment of Yersinia pseudotuberculosis to LC3-associated pathways involving single- or double-membrane vacuoles

Yersinia pseudotuberculosis can replicate inside macrophages by hijacking autophagy and blocking autophagosome acidification. In bone marrow-derived macrophages, the bacteria are mainly observed inside double-membrane vacuoles positive for LC3, a hallmark of autophagy. Here, we address the question of the membrane traffic during internalization of Yersinia investigating the role of vesicle- associated membrane proteins (VAMPs). First, we show that as in epithelial cells, Yersinia pseudotuberculosis replicates mainly in nonacidic LC3-positive vacuoles. Second, in these cells, we unexpectedly found that VAMP3 localizes preferentially to Yersinia-containing vacuoles (YCVs) with single membranes using correlative light-electron microscopy. Third, we reveal the precise kinetics of VAMP3 and VAMP7 association with YCVs positive for LC3. Fourth, we show that VAMP7 knockdown alters LC3's association with single-and multimembrane-YCVs. Finally, in uninfected epithelial cells stimulated for autophagy, VAMP3 overexpression and knockdown led respectively to a lower and higher number of double-membrane, LC3-positive vesicles. Hence, our results highlight the role that VAMPs play in selection of the pathways leading to generation of ultrastructurally different LC3 compartments and pave the way for determining the full set of docking and fusion proteins involved in Yersinia pseudotuberculosis' intravesicular life cycle.

Class IA phosphatidylinositol 3-kinase p110α regulates phagosome maturation

Of the various phosphatidylinositol 3- kinases (PI3Ks), only the class III enzyme Vps34 has been shown to regulate phagosome maturation. During studies of phagosome maturation in THP-1 cells deficient in class IA PI3K p110α, we discovered that this PI3K isoform is required for vacuole maturation to progress beyond acquisition of Rab7 leading to delivery of lysosomal markers. Bead phagosomes from THP-1 cells acquired p110α and contained PI3P and PI(3,4,5)P3; however, p110α and PI(3,4,5)P3 levels in phagosomes from p110α knockdown cells were decreased. Phagosomes from p110α knock down cells showed normal acquisition of both Rab5 and EEA-1, but were markedly deficient in the lysosomal markers LAMP-1 and LAMP-2, and the lysosomal hydrolase, β-galactosidase. Phagosomes from p110α deficient cells also displayed impaired fusion with Texas Red dextran-loaded lysosomes. Despite lacking lysosomal components, phagosomes from p110α deficient cells recruited normal levels of Rab7, Rab-interacting lysosomal protein (RILP) and homotypic vacuole fusion and protein sorting (HOPs) components Vps41 and Vps16. The latter observations demonstrated that phagosomal Rab7 was active and capable of recruiting effectors involved in membrane fusion. Nevertheless, active Rab7 was not sufficient to bring about the delivery of lysosomal proteins to the maturing vacuole, which is shown for the first time to be dependent on a class I PI3K.

Exclusion of synaptotagmin V at the phagocytic cup by Leishmania donovani lipophosphoglycan results in decreased promastigote internalization

Regulators of membrane fusion play an important role in phagocytosis, as they regulate the focal delivery of endomembrane that is required for optimal internalization of large particles. During internalization of Leishmania promastigotes, the surface glycolipid lipophosphoglycan (LPG) is transferred to the macrophage membrane and modifies its fusogenic properties. In this study, we investigated the impact of LPG on the recruitment of the exocytosis regulator synaptotagmin V (Syt V) at the area of internalization and on the early steps of phagocytosis. Using Leishmania donovani LPG-defective mutants and LPG-coated particles, we established that LPG reduces the phagocytic capacity of macrophages and showed that it causes exclusion of Syt V from the nascent phagosome. Silencing of Syt V inhibited phagocytosis to the same extent as LPG, and these effects were not cumulative, consistent with a Syt V-dependent mechanism for the inhibition of phagocytosis by LPG. Previous work has revealed that LPG-mediated exclusion of Syt V from phagosomes prevents the recruitment of the vacuolar ATPase and acidification. Thus, whereas exclusion of Syt V from phagosomes in the process of formation may be beneficial for the creation of a hospitable intracellular niche, it reduces the phagocytic capacity of macrophages. We propose that the cost associated with a reduced internalization rate may be compensated by increased survival, and could lead to a greater overall parasite fitness.

Viruses, autophagy genes, and Crohn's disease

The etiology of the intestinal disease Crohn's disease involves genetic factors as well as ill-defined environmental agents. Several genetic variants linked to this disease are associated with autophagy, a process that is critical for proper responses to viral infections. While a role for viruses in this disease remains speculative, accumulating evidence indicate that this possibility requires serious consideration. In this review, we will examine the three-way relationship between viruses, autophagy genes, and Crohn's disease and discuss how host-pathogen interactions can mediate complex inflammatory disorders.

Analysis of phagosomal proteomes: from latex-bead to bacterial phagosomes

Phagosomal proteome characterization has contributed significantly to the understanding of host-pathogen interaction and the mechanism of infectious diseases caused by intracellular bacteria. The latex bead-containing phagosome has been widely used as a model system to study phagosomal proteomes at a global level. In contrast, the study of bacteria-containing phagosomes at a similar level has just begun. A number of intracellular microbial species are studied for their proteomes during the invasion of a host, providing insight into their metabolic adaptation in host cells and interaction with host-cell antimicrobial environments. In this review, we attempt to summarize the most recent advancements in the proteomic study of microbial phagosomes, especially those originating from mouse or human cells. We also briefly describe the proteomics of latex bead-containing phagosomes because they are often used as model phagosomes for study. We provide descriptions on major biological and technological components in phagosomal proteome studies. We also discuss the role of phagosomal proteome study in the broader horizon of systems biology and the technological challenges in phagosomal proteome characterization.

Golgi-to-phagosome transport of acid sphingomyelinase and prosaposin is mediated by sortilin

Sortilin, also known as neurotensin receptor 3 (NTR3), is a transmembrane protein with a dual function. It acts as a receptor for neuromediators and growth factors at the plasma membrane, but it has also been implicated in binding and transport of some lysosomal proteins. However, the role of sortilin during phagosome maturation has not been investigated before. Here, we show that in macrophages, sortilin is mainly localized in the Golgi and transported to latex-bead phagosomes (LBPs). Using live-cell imaging and electron microscopy, we found that sortilin is delivered to LBPs in a manner that depends on its cytoplasmic tail. We also show that sortilin participates in the direct delivery of acid sphingomyelinase (ASM) and prosaposin (PS) to the phagosome, bypassing fusion with lysosomal compartments. Further analysis confirmed that ASM and PS are targeted to the phagosome by sortilin in a Brefeldin-A-sensitive pathway. Analysis of primary macrophages isolated from Sort1(-/-) mice indicated that the delivery of ASM and PS, but not pro-cathepsin D, to LBPs was severely impaired. We propose a pathway mediated by sortilin by which selected lysosomal proteins are transported to the phagosome along a Golgi-dependent route during the maturation of phagosomes.

Adjusting to a new home: Mycobacterium tuberculosis gene expression in response to an intracellular lifestyle

Mycobacterium tuberculosis remains the most significant single species of bacteria causing disease in mankind. The ability of M. tuberculosis to survive and replicate within host macrophages is a pivotal step in its pathogenesis. Understanding the microenvironments that M. tuberculosis encounters within the macrophage and the adaptations that the bacterium undergoes to facilitate its survival will lead to insights into possible therapeutic targets for improved treatment of tuberculosis. This is urgently needed with the emergence of multi- and extensively drug resistant strains of M. tuberculosis. Significant advances have been made in understanding the macrophage response on encountering M. tuberculosis. Complementary information is also accumulating regarding the counter responses of M. tuberculosis during the various stages of its interactions with the host. As such, a picture is emerging delineating the gene expression of intracellular M. tuberculosis at different stages of the interaction with macrophages.

Defining macropinocytosis

Macropinocytosis represents a distinct pathway of endocytosis in mammalian cells. This actin-driven endocytic process is not directly co-ordinated by the presence of cargo but can be induced upon activation of growth factor signalling pathways. The capacity to dissect the contribution of macropinocytosis to cellular processes has been hampered by a lack of unique molecular markers and defining features. While aspects of macropinosome formation and maturation are common to those shared by the other endocytic pathways, a number of key differences have recently begun to emerge and will be discussed in this study. It is now well established that macropinocytosis significantly contributes to antigen presentation by the immune system and is exploited by a range of pathogens for cellular invasion and avoidance of immune surveillance.

The CiaR response regulator in group B Streptococcus promotes intracellular survival and resistance to innate immune defenses

Group B Streptococcus (GBS) is major cause of invasive disease in newborn infants and the leading cause of neonatal meningitis. To gain access to the central nervous system (CNS), GBS must not only subvert host defenses in the bloodstream but also invade and survive within brain microvascular endothelial cells (BMEC), the principal cell layer composing the blood-brain barrier (BBB). While several GBS determinants that contribute to the invasion of BMEC have been identified, little is known about the GBS factors that are required for intracellular survival and ultimate disease progression. In this study we sought to identify these factors by screening a random GBS mutant library in an in vitro survival assay. One mutant was identified which contained a disruption in a two-component regulatory system homologous to CiaR/CiaH, which is present in other streptococcal pathogens. Deletion of the putative response regulator, ciaR, in GBS resulted in a significant decrease in intracellular survival within neutrophils, murine macrophages, and human BMEC, which was linked to increased susceptibility to killing by antimicrobial peptides, lysozyme, and reactive oxygen species. Furthermore, competition experiments with mice showed that wild-type GBS had a significant survival advantage over the GBS DeltaciaR mutant in the bloodstream and brain. Microarray analysis comparing gene expression between wild-type and DeltaciaR mutant GBS bacteria revealed several CiaR-regulated genes that may contribute to stress tolerance and the subversion of host defenses by GBS. Our results identify the GBS CiaR response regulator as a crucial factor in GBS intracellular survival and invasive disease pathogenesis.

Mycobacterium tuberculosis prevents inflammasome activation

Mycobacterium tuberculosis (Mtb) parasitizes host macrophages and subverts host innate and adaptive immunity. Several cytokines elicited by Mtb are mediators of mycobacterial clearance or are involved in tuberculosis pathology. Surprisingly, interleukin-1beta (IL-1beta), a major proinflammatory cytokine, has not been implicated in host-Mtb interactions. IL-1beta is activated by processing upon assembly of the inflammasome, a specialized inflammatory caspase-activating protein complex. Here, we show that Mtb prevents inflammasome activation and IL-1beta processing. An Mtb gene, zmp1, which encodes a putative Zn(2+) metalloprotease, is required for this process. Infection of macrophages with zmp1-deleted Mtb triggered activation of the inflammasome, resulting in increased IL-1beta secretion, enhanced maturation of Mtb containing phagosomes, improved mycobacterial clearance by macrophages, and lower bacterial burden in the lungs of aerosol-infected mice. Thus, we uncovered a previously masked role for IL-1beta in the control of Mtb and a mycobacterial system that prevents inflammasome and, therefore, IL-1beta activation.

Pathogenesis of leptospirosis: interaction of Leptospira interrogans with in vitro cultured mammalian cells

Interactions of virulent Leptospira interrogans with murine monocyte-macrophage-like J774A.1 cells and Vero (African green monkey kidney fibroblasts) cells from attachment to internalization were investigated by a series of morphological analysis. Fontana silver staining revealed that only the pathogenic leptospires were able to attach to host cells and the attachment pattern varied depending on cell types that they interacted with. Transmission electron microscopy (TEM) analysis confirmed the formation of the leptospires-containing phagosomes and their colocalization with lysosomes in macrophages were verified by confocal microscopic analysis. Results of F-actin rearrangements examination indicated that virulent leptospires invaded host cells via a microfilament-independent pathway.

Contribution of respiratory burst activity to innate immune function and the effects of disease status and agent on chemiluminescence responses by ruminant phagocytes in vitro

The mechanisms of interaction between phagocytes and different bacteria that help resolve lung infections or contribute to lung pathology are poorly defined. Alveolar phagocytes (resident macrophages and recruited neutrophils) make a major contribution to innate immunity by mounting a respiratory burst that helps kill internalised bacteria. However, this ability may be altered during or after exposure to infection. This review considers the application and limitations of a variety of analytical methods for oxygen-dependent mechanisms of respiratory burst in phagocytes initiated by soluble and particulate activators. Particular reference is given to the study in vitro of phagocytes from healthy and diseased ruminants during either natural infection with Mycobacterium avium paratuberculosis or experimental infection with Pasteurella multocida or Mannheimia haemolytica.

Cholesterol depletion in Mycobacterium avium-infected macrophages overcomes the block in phagosome maturation and leads to the reversible sequestration of viable mycobacteria in phagolysosome-derived autophagic vacuoles

Phagocytic entry of mycobacteria into macrophages requires the presence of cholesterol in the plasma membrane. This suggests that pathogenic mycobacteria may require cholesterol for their subsequent intra-cellular survival in non-maturing phagosomes. Here we report on the effect of cholesterol depletion on pre-existing phagosomes in mouse bone marrow-derived macrophages infected with Mycobacterium avium. Cholesterol depletion with methyl-beta-cyclodextrin resulted in a loosening of the close apposition between the phagosome membrane and the mycobacterial surface, followed by fusion with lysosomes. The resulting phagolysosomes then autonomously executed autophagy, which did not involve the endoplasmic reticulum. After 5 h of depletion, intact mycobacteria had accumulated in large auto-phagolysosomes. Autophagy was specific for phagolysosomes that contained mycobacteria, as it did not involve latex bead-containing phagosomes in infected cells. Upon replenishment of cholesterol, mycobacteria became increasingly aligned to the lysosomal membrane, from where they were individually sequestered in phagosomes with an all-around closely apposed phagosome membrane and which no longer fused with lysosomes. These observations indicate that, cholesterol depletion (i) resulted in phagosome maturation and fusion with lysosomes and (ii) caused mycobacterium-containing phagolysosomes to autonomously undergo autophagy. Furthermore, (iii) mycobacteria were not killed in auto-phagolysosomes, and (iv) cholesterol replenishment enabled mycobacterium to rescue itself from autophagic phagolysosomes to again reside individually in phagosomes which no longer fused with lysosomes.

Francisella tularensis travels a novel, twisted road within macrophages

Francisella tularensis is a highly infectious intracellular bacterium that causes fulminating disease and is a potential bioweapon. Although entry of the bacteria into macrophages is mediated by novel asymmetric, spacious pseudopod loops, the nascent phagosome becomes tight fitting within seconds of formation. Biogenesis of the Francisella-containing phagosome (FCP) is arrested for 2-4h at a unique stage within the endosomal-lysosomal degradation pathway, followed by gradual bacterial escape into the cytosol, where the microbe proliferates. By contrast, other intracellular pathogens either proliferate within an idiosyncratic phagosome or escape within minutes into the cytoplasm to avoid degradation. Thus, trafficking of the FCP defies the dogma of classification of intracellular pathogens into vacuolar or cytosolic. The Francisella pathogenicity island and its transcriptional regulator MglA are essential for arresting biogenesis of the FCP. Despite sophisticated microbial strategies to arrest phagosome biogenesis within quiescent macrophages, trafficking of F. tularensis and other intracellular pathogens within interferon-gamma-activated macrophages is similar, in that the bacterial phagosomes fuse to lysosomes. The potential use of F. tularensis as a bioweapon has generated interest in the study of its molecular pathogenesis to identify targets for therapy, vaccination and rapid diagnosis.

Cell biology of mycobacterium tuberculosis phagosome

Phagocytosis and phagolysosome biogenesis represent fundamental biological processes essential for proper tissue homeostasis, development, elimination of invading microorganisms, and antigen processing and presentation. Phagosome formation triggers a preprogrammed pathway of maturation into the phagolysosome, a process controlled by Ca2+ and the regulators of organellar trafficking centered around the small GTP-binding proteins Rabs and their downstream effectors, including lipid kinases, organellar tethering molecules, and membrane fusion apparatus. Mycobacterium tuberculosis is a potent human pathogen parasitizing macrophages. It interferes with the Rab-controlled membrane trafficking and arrests the maturing phagosome at a stage where no harm can be done to the pathogen while the delivery of nutrients and membrane to the vacuole harboring the microorganism continues. This process, referred to as the M. tuberculosis phagosome maturation arrest or inhibition of phagosome-lysosome fusion, is critical for M. tuberculosis persistence in human populations. It also provides a general model system for dissecting the phagolysosome biogenesis pathways. Here we review the fundamental trafficking processes targeted by M. tuberculosis and the mycobacterial products that interfere with phagosomal maturation.

Microarray analysis reveals induction of lipoprotein genes in mucoid Pseudomonas aeruginosa: implications for inflammation in cystic fibrosis

The main cause of the high morbidity and mortality of cystic fibrosis (CF) is the progressive lung inflammation associated with Pseudomonas aeruginosa colonization. During the course of chronic CF infections, P. aeruginosa undergoes a conversion to a mucoid phenotype. The emergence of mucoid P. aeruginosa in CF is associated with increased inflammation, respiratory decline, and a poor prognosis. Here we show, by the use of microarray analysis, that upon P. aeruginosa conversion to mucoidy, there is a massive and preferential induction of genes encoding bacterial lipoproteins. Bacterial lipoproteins are potent agonists of Toll-like receptor 2 (TLR2) signaling. The expression of TLR2 in human respiratory epithelial cells was ascertained by Western blot analysis. Human respiratory epithelial cells responded in a TLR2-dependent manner to bacterial lipopeptides derived from Pseudomonas lipoproteins induced in mucoid strains. The TLR2 proinflammatory response was further augmented in CF cells. Thus, the excessive inflammation in CF is the result of a global induction in mucoid P. aeruginosa of lipoproteins that act as proinflammatory toxins (here termed lipotoxins) superimposed on the hyperexcitability of CF cells. Blocking the signaling cascade responding to bacterial lipotoxins may provide therapeutic benefits for CF patients.

Acquisition of Hrs, an essential component of phagosomal maturation, is impaired by mycobacteria

Pathogenic mycobacteria survive within macrophages by precluding the fusion of phagosomes with late endosomes or lysosomes. Because the molecular determinants of normal phagolysosome formation are poorly understood, the sites targeted by mycobacteria remain unidentified. We found that Hrs, an adaptor molecule involved in protein sorting, associates with phagosomes prior to their fusion with late endosomes or lysosomes. Recruitment of Hrs required the interaction of its FYVE domain with phagosomal phosphatidylinositol 3-phosphate, but two other attachment sites were additionally involved. Depletion of Hrs by use of small interfering RNA impaired phagosomal maturation, preventing the acquisition of lysobisphosphatidic acid and reducing luminal acidification. As a result, the maturation of phagosomes formed in Hrs-depleted cells was arrested at an early stage, characterized by the acquisition and retention of sorting endosomal markers. This phenotype is strikingly similar to that reported to occur in phagosomes of cells infected by mycobacteria. We therefore tested whether Hrs is recruited to phagosomes containing mycobacteria. Hrs associated readily with phagosomes containing inert particles but poorly with mycobacterial phagosomes. Moreover, Hrs was found more frequently in phagosomes containing avirulent Mycobacterium smegmatis than in phagosomes with the more virulent Mycobacterium marinum. These findings suggest that the inability to recruit Hrs contributes to the arrest of phagosomal maturation induced by pathogenic mycobacteria.

Methionine sulfoxide reductase A (MsrA) deficiency affects the survival of Mycobacterium smegmatis within macrophages

Methionine sulfoxide reductase A (MsrA) is an antioxidant repair enzyme which reduces oxidized methionine to methionine. Since oxidation of methionine in proteins impairs their function, an absence of MsrA leads to abnormalities in different organisms, including alterations in the adherence patterns and in vivo survival of certain pathogenic bacteria. To understand the role of MsrA in intracellular survival of bacteria, we disrupted the gene encoding MsrA in Mycobacterium smegmatis through homologous recombination. The msrA mutant strain of M. smegmatis exhibited significantly reduced intracellular survival in murine J774A.1 macrophages compared to the survival of its wild-type counterpart. Furthermore, immunofluorescence and immunoblotting of phagosomes containing M. smegmatis strains revealed that the phagosomes with the msrA mutant strain acquired both p67(phox) of phagocyte NADPH oxidase and inducible nitric oxide synthase much earlier than the phagosomes with the wild-type strain. In addition, the msrA mutant strain of M. smegmatis was observed to be more sensitive to hydroperoxides than the wild-type strain was in vitro. These results suggest that MsrA plays an important role in both extracellular and intracellular survival of M. smegmatis.

Mycobacteria inhibit nitric oxide synthase recruitment to phagosomes during macrophage infection

Inducible nitric oxide synthase (iNOS) is a cytoplasmic protein responsible for the generation of nitric oxide (NO. ) in macrophages. In this work, we hypothesized that the intracellular localization of iNOS is significant for effective delivery of NO. to phagosomes containing ingested microorganisms. Using immunofluorescence microscopy and Western blot analysis, iNOS was shown to localize in the vicinity of phagosomes containing latex beads in stimulated macrophages. iNOS also localized to phagosomes containing Escherichia coli. The colocalization of iNOS with ingested latex beads was an actin-dependent process, since treatment with the actin microfilament disrupter cytochalasin D prevented iNOS recruitment to latex bead phagosomes. In contrast to E. coli and inert particle phagosomes, mycobacterial phagosomes did not colocalize with iNOS. This study demonstrates that (i). iNOS can be recruited to phagosomes; (ii). this recruitment is dependent on a functional actin cytoskeleton; (iii). certain microorganisms have the ability to prevent or reduce colocalization with iNOS; and (iv). spatial exclusion of iNOS may play a role in Mycobacterium tuberculosis pathogenesis.

The immunological aspects of latency in tuberculosis

A unique feature of Mycobacterium tuberculosis is its ability to exist in the granuloma of an asymptomatic host in a latent state that can subsequently reactivate to cause active disease. The latent state of infection poses a major obstacle to eradicating tuberculosis. In latent tuberculosis, the host immune response is capable of controlling the infection and yet falls short of eradicating the pathogen. That the host immune response contributes to the maintenance of latent tuberculous infection is supported by the observation that certain immunodeficient states, including those associated with the human immunodeficiency virus and tumor necrosis factor neutralization therapy, are associated with increased risks for developing reactivation disease. Latent tuberculosis is the product of a complex set of interactions between M. tuberculosis and the host immune response. The molecular basis for the persistence phenotype of M. Tuberculosis and the pertinent host immune mechanisms that contribute to the maintenance of tuberculous latency are just beginning to be understood. This review discusses the interactions between M. tuberculosis and the macrophage, the primary host cell that the tubercle bacillus parasitizes.

Mycobacterium tuberculosis phagosome maturation arrest: selective targeting of PI3P-dependent membrane trafficking

The ability of Mycobacterium tuberculosis to enter host macrophages, and reside in a phagosome, which does not mature into a phagolysosome, is central to the spread of tuberculosis and the associated pandemic involving billions of people worldwide. Tuberculosis can be viewed as a disease with a significant intracellular trafficking and organellar biogenesis component. Current understanding of the block in M. tuberculosis phagosome maturation also sheds light on fundamental aspects of phagolysosome biogenesis. The maturation block involves interference with the recruitment and function of rabs, rab effectors (phosphatidylinositol 3-kinases and tethering molecules such as EEA1), SNAREs (Syntaxin 6 and cellubrevin) and Ca2+/calmodulin signaling. M. tuberculosis analogs of mammalian phosphatidylinositols interfere with these systems and associated processes.

Tuberculosis toxin blocking phagosome maturation inhibits a novel Ca2+/calmodulin-PI3K hVPS34 cascade

The capacity of Mycobacterium tuberculosis to infect latently over one billion people and cause two million fatalities annually rests with its ability to block phagosomal maturation into the phagolysosome in infected macrophages. Here we describe how M. tuberculosis toxin lipoarabinomannan (LAM) causes phagosome maturation arrest, interfering with a new pathway connecting intracellular signaling and membrane trafficking. LAM from virulent M. tuberculosis, but not from avirulent mycobacteria, blocked cytosolic Ca2+ increase. Ca2+ and calmodulin were required for a newly uncovered Ca2+/calmodulin phosphatidylinositol (PI)3 kinase hVPS34 cascade, essential for production of PI 3 phosphate (PI3P) on liposomes in vitro and on phagosomes in vivo. The interference of the trafficking toxin LAM with the calmodulin-dependent production of PI3P described here ensures long-term M. tuberculosis residence in vacuoles sequestered away from the bactericidal and antigen-processing organelles in infected macrophages.

Cord factor trehalose 6,6'-dimycolate (TDM) mediates trafficking events during mycobacterial infection of murine macrophages

The persistence of tuberculosis within pulmonary granulomatous lesions is a complex phenomenon, with bacterial survival occurring in a focal region of high immune activity. In part, the survival of the organism may be linked to the ability of the surface glycolipid trehalose 6,6'-dimycolate (TDM; cord factor) to inhibit fusion events between phospholipid vesicles inside the host macrophage. At the same time, TDM contributes to macrophage activation and a cascade of events required for initiation and maintenance of granulomatous responses. This allows increased sequestration of organisms and further survival and persistence within host tissues. Bacterial viability, macrophage cytokine and chemokine response, and intracellular trafficking were investigated in Mycobacterium tuberculosis from which TDM had been removed. Removal of surface lipids led to enhanced trafficking of organisms to acidic compartments; reconstitution of delipidated organisms with either pure TDM or the petroleum ether extract containing crude surface lipids restored normal responses. Use of TDM-coated polystyrene beads demonstrated that TDM can mediate intracellular trafficking events, as well as influence macrophage production of pro-inflammatory molecules. Thus, the presence of TDM may be an important determinant for successful infection and survival of M. tuberculosis within macrophages.

Immune evasion by Mycobacterium tuberculosis: living with the enemy

Mycobacterium tuberculosis is successful as a pathogen because of its ability to persist in an immunocompetent host. This bacterium lives within the macrophage, a cell whose function is the elimination of microbes. Recent advances have improved our understanding of how M. tuberculosis evades two major antimicrobial mechanisms of macrophages: phagolysosome fusion and the production of toxic reactive nitrogen intermediates. M. tuberculosis also modulates antigen presentation to prevent the detection of infected macrophages by CD4(+) T cells.

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

Many mycobacteria are intramacrophage pathogens that reside within nonacidified phagosomes that fuse with early endosomes but do not mature to phagolysosomes. The mechanism by which mycobacteria block this maturation process remains elusive. To gain insight into whether fusion with early endosomes is required for mycobacteria-mediated inhibition of phagosome maturation, we investigated how perturbing the GTPase cycles of Rab5 and Rab7, GTPases that regulate early and late endosome fusion, respectively, would affect phagosome maturation. Retroviral transduction of the constitutively activated forms of both GTPases into primary murine macrophages had no effect on Mycobacterium avium retention in an early endosomal compartment. Interestingly, expression of dominant negative Rab5, Rab5(S34N), but not dominant negative Rab7, resulted in a significant increase in colocalization of M. avium with markers of late endosomes/lysosomes and increased mycobacterial killing. This colocalization was specific to mycobacteria since Rab5(S34N) expressing cells showed diminished trafficking of endocytic tracers to lysosomes. We further demonstrated that maturation of M. avium phagosomes was halted in Rab5(S34N) expressing macrophages supplemented with exogenous iron. These findings suggest that fusion with early endosomes is required for mycobacterial retention in early phagosomal compartments and that an inadequate supply of iron is one factor in mycobacteria's inability to prevent the normal maturation process in Rab5(S34N)-expressing macrophages.

Intracellular replication of Mycobacterium marinum within Dictyostelium discoideum: efficient replication in the absence of host coronin

Mycobacterium marinum causes tuberculosis-like disease in fish and amphibians and has been used as a model mycobacterial species because of its rapid growth and less stringent containment requirements relative to other mycobacterial species. We demonstrate here that M. marinum grows within Dictyostelium discoideum cells, allowing the genetic analysis of host factors that may modulate the replication of mycobacterial species. Intracellular growth of M. marinum was shown to mimic the properties previously observed for growth within cultured phagocytes. A defined bacterial mutant defective for growth within phagocytic cells was shown to be similarly defective for growth within D. discoideum. To test the role of host coronin, which was previously hypothesized to positively modulate mycobacterial growth within mouse macrophages, a defined D. discoideum coronin mutant was analyzed. Surprisingly, the absence of coronin resulted in enhanced intracellular replication of M. marinum relative to the control wild-type strain. Consistent with previous observations, some phagosomes showed persistence of coronin about the surface of the compartment, but colocalization of the protein was far from uniform. We conclude that in D. discoideum factors other than coronin support intracellular replication of M. marinum.

Mycobacterium tuberculosis glycosylated phosphatidylinositol causes phagosome maturation arrest

The tubercle bacillus parasitizes macrophages by inhibiting phagosome maturation into the phagolysosome. This phenomenon underlies the tuberculosis pandemic involving 2 billion people. We report here how Mycobacterium tuberculosis causes phagosome maturation arrest. A glycosylated M. tuberculosis phosphatidylinositol [mannose-capped lipoarabinomannan (ManLAM)] interfered with the phagosomal acquisition of the lysosomal cargo and syntaxin 6 from the trans-Golgi network. ManLAM specifically inhibited the pathway dependent on phosphatidylinositol 3-kinase activity and phosphatidylinositol 3-phosphate-binding effectors. These findings identify ManLAM as the M. tuberculosis product responsible for the inhibition of phagosomal maturation.

The Legionella pneumophila LidA protein: a translocated substrate of the Dot/Icm system associated with maintenance of bacterial integrity

Legionella pneumophila establishes a replication vacuole within phagocytes that requires the bacterial Dot/Icm apparatus for its formation. This apparatus is predicted to translocate effectors into host cells. We hypothesized that some translocated proteins also function to maintain the integrity of the Dot/Icm translocator. Mutations that destroy this function are predicted to result in a Dot/Icm complex that poisons the bacterium, resulting in reduced viability. To identify such mutants, strains were isolated (called lid-) that showed reduced viability on bacteriological medium in the presence of an intact Dot/Icm apparatus, but which had high viability in the absence of the translocator. Several such mutants were analysed in detail to identify candidate strains that may have lost the ability to synthesize a translocated substrate of Dot/Icm. Two such strains had mutations in the lidA gene. The LidA protein exhibits properties expected for a translocated substrate of Dot/Icm that is important for maintenance of bacterial cell integrity: it associates with the phagosomal surface, promotes replication vacuole formation, and is important for both efficient intracellular growth and high viability on bacteriological media after introduction of a plasmid that allows high level expression of the dotA gene.

Cutting edge: Mycobacterium tuberculosis blocks Ca2+ signaling and phagosome maturation in human macrophages via specific inhibition of sphingosine kinase

One-third of the world's population is infected with Mycobacterium tuberculosis (Mtb), and three million people die of tuberculosis each year. Following its ingestion by macrophages (MPs), Mtb inhibits the maturation of its phagosome, preventing progression to a bactericidal phagolysosome. Phagocytosis of Mtb is uncoupled from the elevation in MP cytosolic Ca(2+) that normally accompanies microbial ingestion, resulting in inhibition of phagosome-lysosome fusion and increased intracellular viability. This study demonstrates that the mechanism responsible for this failure of Ca(2+)-dependent phagosome maturation involves mycobacterial inhibition of MP sphingosine kinase. Thus, inhibition of sphingosine kinase directly contributes to survival of Mtb within human MPs and represents a novel molecular mechanism of pathogenesis.

Macrophage signalling upon mycobacterial infection: the MAP kinases lead the way

Mycobacteria activate a series of macrophage signalling pathways upon engaging host cell receptors and during the invasion process. These signals initiate a cascade of events leading to the production of immune effector molecules including cytokines, chemokines and reactive nitrogen intermediates. This response by the macrophage is critical for the control of the mycobacterial infection and, not surprisingly, pathogenic mycobacteria have evolved mechanisms to limit this macrophage activation. Recent data has suggested that macrophages infected with pathogenic compared to non-pathogenic mycobacteria are restricted in their activation of the mitogen activated protein kinase (MAPK) pathways. Mitogen activated protein kinase activation in macrophages appears to play an important role in promoting antimycobacterial activity and in the production of various effector molecules following a mycobacterial infection. Therefore, the ability of pathogenic mycobacteria to limit MAPK activity is likely an important virulence mechanism and may be a potential therapeutic target.

Constrained intracellular survival of Mycobacterium tuberculosis in human dendritic cells

Dendritic cells (DCs) are likely to play a key role in immunity against Mycobacterium tuberculosis, but the fate of the bacterium in these cells is still unknown. Here we report that, unlike macrophages (Mphis), human monocyte-derived DCs are not permissive for the growth of virulent M. tuberculosis H37Rv. Mycobacterial vacuoles are neither acidic nor fused with host cell lysosomes in DCs, in a mode similar to that seen in mycobacterial infection of Mphis. However, uptake of the fluid phase marker dextran, and of transferrin, as well as accumulation of the recycling endosome-specific small GTPase Rab11 onto the mycobacterial phagosome, are almost abolished in infected DCs, but not in Mphis. Moreover, communication between mycobacterial phagosomes and the host-cell biosynthetic pathway is impaired, given that <10% of M. tuberculosis vacuoles in DCs stained for the endoplasmic reticulum-specific proteins Grp78/BiP and calnexin. This correlates with the absence of the fusion factor N-ethylmaleimide-sensitive factor onto the vacuolar membrane in this cell type. Trafficking between the vacuoles and the host cell recycling and biosynthetic pathways is strikingly reduced in DCs, which is likely to impair access of intracellular mycobacteria to essential nutrients and may thus explain the absence of mycobacterial growth in this cell type. This unique location of M. tuberculosis in DCs is compatible with their T lymphocyte-stimulating functions, because M. tuberculosis-infected DCs have the ability to specifically induce cytokine production by autologous T lymphocytes from presensitized individuals. DCs have evolved unique subcellular trafficking mechanisms to achieve their Ag-presenting functions when infected by intracellular mycobacteria.

Mycobacterium and the coat of many lipids

Pathogenic Mycobacterium reside inside vacuoles in their host macrophages. These vacuoles fail to fuse with lysosomes yet interact with early endosomes. Glycoconjugates released by the intracellular bacilli traffic through the host cell and are released through exocytosis. These molecules represent both antigens for immune recognition and modulators of immune function. The molecules play key roles in the induction and maintenance of the granuloma, a tissue response that limits bacterial spread yet ensures persistence of the infection.