Single cell measurements of vacuolar rupture caused by intracellular pathogens

Mycobacterium marinum phthiocerol dimycocerosates enhance macrophage phagosomal permeabilization and membrane damage

Phthiocerol dimycocerosates (PDIMs) are a class of mycobacterial lipids that promote virulence in Mycobacterium tuberculosis and Mycobacterium marinum. It has recently been shown that PDIMs work in concert with the M. tuberculosis Type VII secretion system ESX-1 to permeabilize the phagosomal membranes of infected macrophages. As the zebrafish-M. marinum model of infection has revealed the critical role of PDIM at the host-pathogen interface, we set to determine if PDIMs contributed to phagosomal permeabilization in M. marinum. Using an ΔmmpL7 mutant defective in PDIM transport, we find the PDIM-ESX-1 interaction to be conserved in an M. marinum macrophage infection model. However, we find PDIM and ESX-1 mutants differ in their degree of defect, with the PDIM mutant retaining more membrane damaging activity. Using an in vitro hemolysis assay-a common surrogate for cytolytic activity, we find that PDIM and ESX-1 differ in their contributions: the ESX-1 mutant loses hemolytic activity while PDIM retains it. Our observations confirm the involvement of PDIMs in phagosomal permeabilization in M. marinum infection and suggest that PDIM enhances the membrane disrupting activity of pathogenic mycobacteria and indicates that the role they play in damaging phagosomal and red blood cell membranes may differ.

A New ESX-1 Substrate in Mycobacterium marinum That Is Required for Hemolysis but Not Host Cell Lysis

The ESX-1 (ESAT-6 system 1) secretion system plays a conserved role in the virulence of diverse mycobacterial pathogens, including the human pathogen Mycobacterium tuberculosis and M. marinum, an environmental mycobacterial species. The ESX-1 system promotes the secretion of protein virulence factors to the extracytoplasmic environment. The secretion of these proteins triggers the host response by lysing the phagosome during macrophage infection. Using proteomic analyses of the M. marinum secretome in the presence and absence of a functional ESX-1 system, we and others have hypothesized that MMAR_2894, a PE family protein, is a potential ESX-1 substrate in M. marinum We used genetic and quantitative proteomic approaches to determine if MMAR_2894 is secreted by the ESX-1 system, and we defined the requirement of MMAR_2894 for ESX-1-mediated secretion and virulence. We show that MMAR_2894 is secreted by the ESX-1 system in M. marinum and is itself required for the optimal secretion of the known ESX-1 substrates in M. marinum Moreover, we found that MMAR_2894 was differentially required for hemolysis and cytolysis of macrophages, two lytic activities ascribed to the M. marinum ESX-1 system.IMPORTANCE Both Mycobacterium tuberculosis, the cause of human tuberculosis (TB), and Mycobacterium marinum, a pathogen of ectotherms, use the ESX-1 secretion system to cause disease. There are many established similarities between the ESX-1 systems in M. tuberculosis and in M. marinum Yet the two bacteria infect different hosts, hinting at species-specific functions of the ESX-1 system. Our findings demonstrate that MMAR_2894 is a PE protein secreted by the ESX-1 system of M. marinum We show that MMAR_2894 is required for the optimal secretion of mycobacterial proteins required for disease. Because the MMAR_2894 gene is not conserved in M. tuberculosis, our findings demonstrate that MMAR_2894 may contribute to a species-specific function of the ESX-1 system in M. marinum, providing new insight into how the M. marinum and M. tuberculosis systems differ.

Killing from the inside: Intracellular role of T3SS in the fate of Pseudomonas aeruginosa within macrophages revealed by mgtC and oprF mutants

While considered solely an extracellular pathogen, increasing evidence indicates that Pseudomonas aeruginosa encounters intracellular environment in diverse mammalian cell types, including macrophages. In the present study, we have deciphered the intramacrophage fate of wild-type P. aeruginosa PAO1 strain by live and electron microscopy. P. aeruginosa first resided in phagosomal vacuoles and subsequently could be detected in the cytoplasm, indicating phagosomal escape of the pathogen, a finding also supported by vacuolar rupture assay. The intracellular bacteria could eventually induce cell lysis, both in a macrophage cell line and primary human macrophages. Two bacterial factors, MgtC and OprF, recently identified to be important for survival of P. aeruginosa in macrophages, were found to be involved in bacterial escape from the phagosome as well as in cell lysis caused by intracellular bacteria. Strikingly, type III secretion system (T3SS) genes of P. aeruginosa were down-regulated within macrophages in both mgtC and oprF mutants. Concordantly, cyclic di-GMP (c-di-GMP) level was increased in both mutants, providing a clue for negative regulation of T3SS inside macrophages. Consistent with the phenotypes and gene expression pattern of mgtC and oprF mutants, a T3SS mutant (ΔpscN) exhibited defect in phagosomal escape and macrophage lysis driven by internalized bacteria. Importantly, these effects appeared to be largely dependent on the ExoS effector, in contrast with the known T3SS-dependent, but ExoS independent, cytotoxicity caused by extracellular P. aeruginosa towards macrophages. Moreover, this macrophage damage caused by intracellular P. aeruginosa was found to be dependent on GTPase Activating Protein (GAP) domain of ExoS. Hence, our work highlights T3SS and ExoS, whose expression is modulated by MgtC and OprF, as key players in the intramacrophage life of P. aeruginosa which allow internalized bacteria to lyse macrophages.

The ability of professional phagocytes to ingest and kill microorganisms is central to host defense and Pseudomonas aeruginosa has developed mechanisms to avoid being killed by phagocytes. While considered an extracellular pathogen, P. aeruginosa has been reported to be engulfed by macrophages in animal models. Here, we visualized the fate of P. aeruginosa within cultured macrophages, revealing macrophage lysis driven by intracellular P. aeruginosa. Two bacterial factors, MgtC and OprF, recently discovered to be involved in the intramacrophage survival of P. aeruginosa, appeared to play a role in this cytotoxicity caused by intracellular bacteria. We provided evidence that type III secretion system (T3SS) gene expression is lowered intracellularly in mgtC and oprF mutants. We further showed that intramacrophage P. aeruginosa uses its T3SS, specifically the ExoS effector, to promote phagosomal escape and cell lysis. We thus describe a transient intramacrophage stage of P. aeruginosa that could contribute to bacterial dissemination.

Mitochondrial reactive oxygen species regulate the induction of CD8+ T cells by plasmacytoid dendritic cells

Cross-presentation allows exogenous antigen presentation in association with major histocompatibility complex class I molecules, a process crucial for the priming of CD8⁺ T-cell responses against viruses and tumors. By contrast to conventional dendritic cells (cDC), which cross-present antigens in the steady state, plasmacytoid dendritic cells (pDC) acquire this ability only after stimulation by Toll-like receptor (TLR) ligands. The intracellular pathways accounting for this functional difference are still unknown. Here we show that the induction of cross-presentation by pDCs is regulated by mitochondria through a reactive oxygen species (ROS)-dependent mechanism, involving pH alkalization and antigen protection. The reduction of mitochondrial ROS production dramatically decreases the cross-presentation capacity of pDCs, leading to a strong reduction of their capacity to trigger CD8⁺ T-cell responses. Our results demonstrate the importance of mitochondrial metabolism in pDC biology, particularly for the induction of adaptive immune responses.

Quantifying intracellular Mycobacterium tuberculosis: An essential issue for in vitro assays

Many studies about intracellular microorganisms which are important regarding diseases affecting public health have been focused on the recognition of host–pathogen interactions, thereby ascertaining the mechanisms by which the pathogen invades a cell and makes it become its host. Such knowledge enables understanding the immunological response triggered by these interactions for obtaining useful information for developing vaccines and drugs. Quantitative cell infection assay protocols are indispensable regarding studies involving Mycobacterium tuberculosis, which takes the lives of more than 2 million people worldwide every year; however, sometimes these are limited by the pathogen's slow growth. Concerning such limitation, a detailed review is presented here regarding the different methods for quantifying and differentiating an intracellular pathogen, the importance of mycobacteria aggregate dissociation and multiplicity of infection (MOI) in infection assays. The methods’ differences, advantages, and disadvantages are discussed regarding intra and extracellular bacteria (on cell surface) differentiation, current problems are outlined, as are the solutions provided using fluorophores and projections made concerning quantitative infection assays.

LPS targets host guanylate-binding proteins to the bacterial outer membrane for non-canonical inflammasome activation

Pathogenic and commensal Gram-negative bacteria produce and release outer membrane vesicles (OMVs), which present several surface antigens and play an important role for bacterial pathogenesis. OMVs also modulate the host immune system, which makes them attractive as vaccine candidates. At the cellular level, OMVs are internalized by macrophages and deliver lipopolysaccharide (LPS) into the host cytosol, thus activating the caspase-11 non-canonical inflammasome. Here, we show that OMV-induced inflammasome activation requires TLR4-TRIF signaling, the production of type I interferons, and the action of guanylate-binding proteins (GBPs), both in macrophages and in vivo Mechanistically, we find that isoprenylated GBPs associate with the surface of OMVs or with transfected LPS, indicating that the key factor that determines GBP recruitment to the Gram-negative bacterial outer membranes is LPS itself. Our findings provide new insights into the mechanism by which GBPs target foreign surfaces and reveal a novel function for GBPs in controlling the intracellular detection of LPS derived from extracellular bacteria in the form of OMVs, thus extending their function as a hub between cell-autonomous immunity and innate immunity.

Alterations of the Coxiella burnetii Replicative Vacuole Membrane Integrity and Interplay with the Autophagy Pathway

Coxiella burnetii, the etiologic agent of Q fever, is a Gram-negative obligate intracellular bacterium. It has been previously described that both the endocytic and autophagic pathways contribute to the Coxiella replicative vacuole (CRV) generation. Galectins are β-galactoside-binding lectins that accumulate in the cytosol before being secreted via a non-conventional secretory pathway. It has been shown that Galectin-3, -8, -9 monitor bacteria vacuolar rupture and endosomal and lysosomal loss of membrane integrity through binding of host glycans exposed in the cytoplasm after membrane damage. Using microinjection of fluorescence-coupled dextrans, a FRET assay, and galectins distribution, we demonstrate that Coxiella infection actually result in transient phagosomal/CRV membrane damage in a Dot/Icm-dependent manner. We also show the association of different adaptor molecules involved in autophagy and of LC3 to the limiting membrane of the CRV. Moreover, we show that upon autophagy inhibition, the proportion of CRVs labeled with galectins and less acidified increases which is associated with bacteria replication impairment. Based on these observations, we propose that autophagy can facilitate resealing of intracellular damaged membranes.

Mycobacterial ESX-1 secretion system mediates host cell lysis through bacterium contact-dependent gross membrane disruptions

Mycobacterium tuberculosis and Mycobacterium marinum are thought to exert virulence, in part, through their ability to lyse host cell membranes. The type VII secretion system ESX-1 [6-kDa early secretory antigenic target (ESAT-6) secretion system 1] is required for both virulence and host cell membrane lysis. Both activities are attributed to the pore-forming activity of the ESX-1-secreted substrate ESAT-6 because multiple studies have reported that recombinant ESAT-6 lyses eukaryotic membranes. We too find ESX-1 of M. tuberculosis and M. marinum lyses host cell membranes. However, we find that recombinant ESAT-6 does not lyse cell membranes. The lytic activity previously attributed to ESAT-6 is due to residual detergent in the preparations. We report here that ESX-1-dependent cell membrane lysis is contact dependent and accompanied by gross membrane disruptions rather than discrete pores. ESX-1-mediated lysis is also morphologically distinct from the contact-dependent lysis of other bacterial secretion systems. Our findings suggest redirection of research to understand the mechanism of ESX-1-mediated lysis.

Diverted recycling-Shigella subversion of Rabs

Small GTPases of the Rab protein family control intracellular vesicular trafficking to allow their communication and maintenance. It is a common strategy for intracellular bacteria to exploit these pathways to shape their respective niches for survival. The subversion of Rabs for the generation of an intracellular environment favoring the pathogen has been described almost exclusively for intracellular bacteria that reside within bacterial containing vacuoles (BCVs). However, less is known about Rab subversion for bacteria that rupture the BCV to reach the host cytoplasm. Here, we provide recent examples of Rab targeting by both groups of intracellular bacteria with a special focus on Shigella, the causative agent of bacillary dysentery. Shigella recruits Rab11, the hallmark of the perinuclear recycling compartment to in situ formed macropinosomes at the entry foci via the bacterial effector IpgD. This leads to efficient BCV rupture and cytosolic escape. We discuss the concept of diverted recycling through host Rab GTPases that emerges as a novel pathogen strategy.

Identification of a Transcription Factor That Regulates Host Cell Exit and Virulence of Mycobacterium tuberculosis

The interaction of Mycobacterium tuberculosis (Mtb) with host cell death signaling pathways is characterized by an initial anti-apoptotic phase followed by a pro-necrotic phase to allow for host cell exit of the bacteria. The bacterial modulators regulating necrosis induction are poorly understood. Here we describe the identification of a transcriptional repressor, Rv3167c responsible for regulating the escape of Mtb from the phagosome. Increased cytosolic localization of MtbΔRv3167c was accompanied by elevated levels of mitochondrial reactive oxygen species and reduced activation of the protein kinase Akt, and these events were critical for the induction of host cell necrosis and macroautophagy. The increase in necrosis led to an increase in bacterial virulence as reflected in higher bacterial burden and reduced survival of mice infected with MtbΔRv3167c. The regulon of Rv3167c thus contains the bacterial mediators involved in escape from the phagosome and host cell necrosis induction, both of which are crucial steps in the intracellular lifecycle and virulence of Mtb.

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis, is a highly successful human pathogen. Following entry into host phagocytic cells, Mtb resides within a modified phagosomal compartment and inhibits apoptotic host cell death. Recent studies have demonstrated that Mtb eventually translocates from the phagosomal compartment to the cytosol. This event is followed by the induction of necrotic host cell death allowing the bacteria to exit the host cell and infect naive cell populations. Our study adds to this relatively unexplored aspect of Mtb pathogenesis by revealing that the transcriptional repressor Rv3167c of Mtb negatively regulates phagosomal escape and host cell necrosis. We furthermore demonstrate that the increased necrosis induction by the Mtb mutant strain deficient in Rv3167c required elevated reactive oxygen species levels within host cell mitochondria and reduced activation of the protein kinase Akt. In addition, the increased virulence of the Mtb mutant strain observed after aerosol infection of mice strengthens the link between the ability of the bacteria to induce host cell necrosis and virulence. The Mtb genes negatively regulated by Rv3167c are thus potential virulence factors that can be targeted for drug and vaccine development.

Cytosolic Access of Intracellular Bacterial Pathogens: The Shigella Paradigm

Shigella is a Gram-negative bacterial pathogen, which causes bacillary dysentery in humans. A crucial step of Shigella infection is its invasion of epithelial cells. Using a type III secretion system, Shigella injects several bacterial effectors ultimately leading to bacterial internalization within a vacuole. Then, Shigella escapes rapidly from the vacuole, it replicates within the cytosol and spreads from cell-to-cell. The molecular mechanism of vacuolar rupture used by Shigella has been studied in some detail during the recent years and new paradigms are emerging about the underlying molecular events. For decades, bacterial effector proteins were portrayed as main actors inducing vacuolar rupture. This includes the effector/translocators IpaB and IpaC. More recently, this has been challenged and an implication of the host cell in the process of vacuolar rupture has been put forward. This includes the bacterial subversion of host trafficking regulators, such as the Rab GTPase Rab11. The involvement of the host in determining bacterial vacuolar integrity has also been found for other bacterial pathogens, particularly for Salmonella. Here, we will discuss our current view of host factor and pathogen effector implications during Shigella vacuolar rupture and the steps leading to it.

A Dual Microscopy-Based Assay To Assess Listeria monocytogenes Cellular Entry and Vacuolar Escape

Listeria monocytogenes is a Gram-positive bacterium and a facultative intracellular pathogen that invades mammalian cells, disrupts its internalization vacuole, and proliferates in the host cell cytoplasm. Here, we describe a novel image-based microscopy assay that allows discrimination between cellular entry and vacuolar escape, enabling high-content screening to identify factors specifically involved in these two steps. We first generated L. monocytogenes and Listeria innocua strains expressing a β-lactamase covalently attached to the bacterial cell wall. These strains were then incubated with HeLa cells containing the Förster resonance energy transfer (FRET) probe CCF4 in their cytoplasm. The CCF4 probe was cleaved by the bacterial surface β-lactamase only in cells inoculated with L. monocytogenes but not those inoculated with L. innocua, thereby demonstrating bacterial access to the host cytoplasm. Subsequently, we performed differential immunofluorescence staining to distinguish extracellular versus total bacterial populations in samples that were also analyzed by the FRET-based assay. With this two-step analysis, bacterial entry can be distinguished from vacuolar rupture in a single experiment. Our novel approach represents a powerful tool for identifying factors that determine the intracellular niche of L. monocytogenes.

Quantification of Cytosolic vs. Vacuolar Salmonella in Primary Macrophages by Differential Permeabilization

Intracellular bacterial pathogens can replicate in the cytosol or in specialized pathogen-containing vacuoles (PCVs). To reach the cytosol, bacteria like Shigella flexneri and Francisella novicida need to induce the rupture of the phagosome. In contrast, Salmonella typhimurium replicates in a vacuolar compartment, known as Salmonella-containing vacuole (SCV). However certain mutants of Salmonella fail to maintain SCV integrity and are thus released into the cytosol. The percentage of cytosolic vs. vacuolar bacteria on the level of single bacteria can be measured by differential permeabilization, also known as phagosome-protection assay. The approach makes use of the property of detergent digitonin to selectively bind cholesterol. Since the plasma membrane contains more cholesterol than other cellular membranes, digitonin can be used to selectively permeabilize the plasma membrane while leaving intracellular membranes intact. In brief, following infection with the pathogen expressing a fluorescent marker protein (e.g. mCherry among others), the plasma membrane of host cells is permeabilized with a short incubation in digitonin containing buffer. Cells are then washed and incubated with a primary antibody (coupled to a fluorophore of choice) directed against the bacterium of choice (e.g. anti-Salmonella-FITC) and washed again. If unmarked bacteria are used, an additional step can be done, in which all membranes are permeabilized and all bacteria stained with a corresponding antibody. Following the staining, the percentage of vacuolar and cytosolic bacteria can be quantified by FACS or microscopy by counting single or double-positive events. Here we provide experimental details for use of this technique with the bacterium Salmonella typhimurium. The advantage of this assay is that, in contrast to other assay, it provides a quantification on the level of single bacteria, and if analyzed by microscopy provides the exact number of cytosolic and vacuolar bacteria in a given cell.

Retrograde transport is not required for cytosolic translocation of the B-subunit of Shiga toxin

Antigen-presenting cells have the remarkable capacity to transfer exogenous antigens to the cytosol for processing by proteasomes and subsequent presentation on major histocompatibility complex class-I (MHC-I) molecules, a process termed cross-presentation. This is the target of biomedical approaches that aim to trigger a therapeutic immune response. The receptor-binding B-subunit of Shiga toxin (STxB) has been developed as an antigen delivery tool for such immunotherapy applications. In this study, we have analyzed pathways and trafficking factors that are involved in this process. A covalent conjugate between STxB and saporin was generated to quantitatively sample the membrane translocation step to the cytosol in differentiated monocyte-derived THP-1 cells. We have found that retrograde trafficking to the Golgi complex was not required for STxB-saporin translocation to the cytosol or for STxB-dependent antigen cross-presentation. Depletion of endosomal Rab7 inhibited, and lowering membrane cholesterol levels favored STxB-saporin translocation. Interestingly, experiments with reducible and non-reducible linker-arm-STxB conjugates led to the conclusion that after translocation, STxB remains associated with the cytosolic membrane leaflet. In summary, we report new facets of the endosomal escape process bearing relevance to antigen cross-presentation.

Imaging cytosolic translocation of Mycobacteria with two-photon fluorescence resonance energy transfer microscopy

Transition from latency to active tuberculosis requires Mycobacterium tuberculosis (Mtb) to penetrate the phagosomal membrane and translocate to the cytosol of the host macrophage. Quantitative two-photon fluorescence resonance energy transfer (FRET) microscopy is developed to measure cytosolic translocation using Mycobacterium marinum (Mm) as a model organism for Mtb. Macrophages were infected with Mm or non-pathogenic Mycobacterium smegmatis (Ms) as a control, then loaded with a FRET substrate. Once translocation occurs, mycobacterium-bearing β-lactamase cleaves the substrate, resulting in decrease of FRET signal. Quantification of this FRET signal change revealed that Mm, but not Ms, is capable of translocating to the cytosol.