Sensing cytosolic RpsL by macrophages induces lysosomal cell death and termination of bacterial infection

Legionella pneumophila exploits the endo-lysosomal network for phagosome biogenesis by co-opting SUMOylated Rab7

Legionella pneumophila strains harboring wild-type rpsL such as Lp02rpsLWT cannot replicate in mouse bone marrow-derived macrophages (BMDMs) due to induction of extensive lysosome damage and apoptosis. The bacterial factor directly responsible for inducing such cell death and the host factor involved in initiating the signaling cascade that leads to lysosome damage remain unknown. Similarly, host factors that may alleviate cell death induced by these bacterial strains have not yet been investigated. Using a genome-wide CRISPR/Cas9 screening, we identified Hmg20a and Nol9 as host factors important for restricting strain Lp02rpsLWT in BMDMs. Depletion of Hmg20a protects macrophages from infection-induced lysosomal damage and apoptosis, allowing productive bacterial replication. The restriction imposed by Hmg20a was mediated by repressing the expression of several endo-lysosomal proteins, including the small GTPase Rab7. We found that SUMOylated Rab7 is recruited to the bacterial phagosome via SulF, a Dot/Icm effector that harbors a SUMO-interacting motif (SIM). Moreover, overexpression of Rab7 rescues intracellular growth of strain Lp02rpsLWT in BMDMs. Our results establish that L. pneumophila exploits the lysosomal network for the biogenesis of its phagosome in BMDMs.

Legionella pneumophila exploits the endo-lysosomal network for phagosome biogenesis by co-opting SUMOylated Rab7

L. pneumophila strains harboring wild-type rpsL such as Lp02rpsLWT cannot replicate in mouse bone marrow-derived macrophages (BMDMs) due to induction of extensive lysosome damage and apoptosis. The mechanism of this unique infection-induced cell death remains unknown. Using a genome-wide CRISPR/Cas9 screening, we identified Hmg20a and Nol9 as host factors important for restricting strain Lp02rpsLWT in BMDMs. Depletion of Hmg20a protects macrophages from infection-induced lysosomal damage and apoptosis, allowing productive bacterial replication. The restriction imposed by Hmg20a was mediated by repressing the expression of several endo-lysosomal proteins, including the small GTPase Rab7. We found that SUMOylated Rab7 is recruited to the bacterial phagosome via SulF, a Dot/Icm effector that harbors a SUMO-interacting motif (SIM). Moreover, overexpression of Rab7 rescues intracellular growth of strain Lp02rpsLWT in BMDMs. Our results establish that L. pneumophila exploits the lysosomal network for the biogenesis of its phagosome in BMDMs.

Role of mitochondrial outer membrane permeabilization during bacterial infection

Beyond the initial 'powerhouse' view, mitochondria have numerous functions in their mammalian cell and contribute to many physiological processes, and many of these we understand only partially. The control of apoptosis by mitochondria is firmly established. Many questions remain however how this function is embedded into physiology, and how other signaling pathways regulate mitochondrial apoptosis; the interplay of bacteria with the mitochondrial apoptosis pathway is one such example. The outer mitochondrial membrane regulates both import into mitochondria and the release of intermembrane, and in some situations also matrix components from mitochondria, and these mitochondrial components can have signaling function in the cytosol. One function is the induction of apoptotic cell death. An exciting, more recently discovered function is the regulation of inflammation. Mitochondrial molecules, both proteins and nucleic acids, have inflammatory activity when released from mitochondria, an activity whose regulation is intertwined with the activation of apoptotic caspases. Bacterial infection can have more general effects on mitochondrial apoptosis-regulation, through effects on host transcription and other pathways, such as signals controlled by pattern recognition. Some specialized bacteria have products that more specifically regulate signaling to the outer mitochondrial membrane, and to apoptosis; both pro- and anti-apoptotic mechanisms have been reported. Among the intriguing recent findings in this area are signaling contributions of porins and the sub-lethal release of intermembrane constituents. We will here review the literature and place the new developments into the established context of mitochondrial signaling during the contact of bacterial pathogens with human cells.

The Optimization and Biological Significance of a 29-Host-Immune-mRNA Panel for the Diagnosis of Acute Infections and Sepsis

In response to the unmet need for timely accurate diagnosis and prognosis of acute infections and sepsis, host-immune-response-based tests are being developed to help clinicians make more informed decisions including prescribing antimicrobials, ordering additional diagnostics, and assigning level of care. One such test (InSep™, Inflammatix, Inc.) uses a 29-mRNA panel to determine the likelihood of bacterial infection, the separate likelihood of viral infection, and the risk of physiologic decompensation (severity of illness). The test, being implemented in a rapid point-of-care platform with a turnaround time of 30 min, enables accurate and rapid diagnostic use at the point of impact. In this report, we provide details on how the 29-biomarker signature was chosen and optimized, together with its molecular, immunological, and medical significance to better understand the pathophysiological relevance of altered gene expression in disease. We synthesize key results obtained from gene-level functional annotations, geneset-level enrichment analysis, pathway-level analysis, and gene-network-level upstream regulator analysis. Emerging findings are summarized as hallmarks on immune cell interaction, inflammatory mediators, cellular metabolism and homeostasis, immune receptors, intracellular signaling and antiviral response; and converging themes on neutrophil degranulation and activation involved in immune response, interferon, and other signaling pathways.

Comparative Proteomic Analysis Reveals Complex Responses to Bordetella bronchiseptica Infections in the Spleen of Rabbits

Bordetella bronchiseptica (B. bronchiseptica) causes a respiratory disease in rabbits. To determine the proteins of B. bronchiseptica in rabbits related to the disease, differentially accumulated proteins in B. bronchiseptica-infected cells are identified by comparative proteomic analysis. Comparative proteomic analysis detects 5814 proteins and quantifies 4854 of these. Fifty eight upregulated and 38 downregulated proteins are identified in spleen tissue after B. bronchiseptica infection of rabbits (both p < 0.05). The significantly enriched Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways are ribosome, biosynthesis of amino acids, biosynthesis of amino acids, protein export, and carbon metabolism etc. (all p < 0.01). Significantly enriched KEGG pathways include 'ocu03010 ribosome' (a); 'ocu00260 glycine, serine threonine metabolism'. Analyses of control and infected spleen cells detect responses to B. bronchiseptica infection. Many differentially affected proteins are evident, and reflect different biological changes and diverse subcellular localizations between control and infected spleen cells. Infection markedly alters the expressions of proteins linked to the serine protease system, with the 'phagosome,' 'biosynthesis of amino acids,' 'glycine, serine threonine metabolism,' 'intestinal immune network for IgA production', and 'amino sugar and nucleotide sugar metabolism' associated with B. bronchiseptica infection. The result will inform studies of responses to B. bronchiseptica infections in rabbits.

Competitive Cell Death Interactions in Pulmonary Infection: Host Modulation Versus Pathogen Manipulation

In the context of pulmonary infection, both hosts and pathogens have evolved a multitude of mechanisms to regulate the process of host cell death. The host aims to rapidly induce an inflammatory response at the site of infection, promote pathogen clearance, quickly resolve inflammation, and return to tissue homeostasis. The appropriate modulation of cell death in respiratory epithelial cells and pulmonary immune cells is central in the execution of all these processes. Cell death can be either inflammatory or anti-inflammatory depending on regulated cell death (RCD) modality triggered and the infection context. In addition, diverse bacterial pathogens have evolved many means to manipulate host cell death to increase bacterial survival and spread. The multitude of ways that hosts and bacteria engage in a molecular tug of war to modulate cell death dynamics during infection emphasizes its relevance in host responses and pathogen virulence at the host pathogen interface. This narrative review outlines several current lines of research characterizing bacterial pathogen manipulation of host cell death pathways in the lung. We postulate that understanding these interactions and the dynamics of intracellular and extracellular bacteria RCD manipulation, may lead to novel therapeutic approaches for the treatment of intractable respiratory infections.

IFNγ receptor down-regulation facilitates Legionella survival in alveolar macrophages

Legionella pneumophila is an opportunistic human pathogen and causative agent of the acute pneumonia known as Legionnaire's disease. Upon inhalation, the bacteria replicate in alveolar macrophages (AM), within an intracellular vacuole termed the Legionella-containing vacuole. We recently found that, in vivo, IFNγ was required for optimal clearance of intracellular L. pneumophila by monocyte-derived cells (MC), but the cytokine did not appear to influence clearance by AM. Here, we report that during L. pneumophila lung infection, expression of the IFNγ receptor subunit 1 (IFNGR1) is down-regulated in AM and neutrophils, but not MC, offering a possible explanation for why AM are unable to effectively restrict L. pneumophila replication in vivo. To test this, we used mice that constitutively express IFNGR1 in AM and found that prevention of IFNGR1 down-regulation enhanced the ability of AM to restrict L. pneumophila intracellular replication. IFNGR1 down-regulation was independent of the type IV Dot/Icm secretion system of L. pneumophila indicating that bacterial effector proteins were not involved. In contrast to previous work, we found that signaling via type I IFN receptors was not required for IFNGR1 down-regulation in macrophages but rather that MyD88- or Trif- mediated NF-κB activation was required. This work has uncovered an alternative signaling pathway responsible for IFNGR1 down-regulation in macrophages during bacterial infection.

TOLLIP deficiency is associated with increased resistance to Legionella pneumophila pneumonia

Legionella pneumophila (Lp) is a flagellated, intracellular bacterium that can cause Legionnaires' disease (LD). Lp activates multiple innate immune receptors, and TOLLIP dampens MyD88-dependent signaling and may influence susceptibility to LD. We evaluated the effect of TOLLIP on innate immunity, pneumonia severity, and LD susceptibility in mouse lungs and human populations. To accomplish this, we evaluated the effect of TOLLIP on lung-specific Lp control and immune response and associated a common functional TOLLIP variant with Lp-induced innate immune responses and LD susceptibility in humans. After aerosol Lp infection, Tollip^-/- mice demonstrated significantly fewer bacterial colony-forming unit and increased cytokine responses from BAL fluid. Tollip^-/- macrophages also suppressed intracellular Lp replication in a flagellin-independent manner. The presence of a previously characterized, functionally active SNP associated with decreased TOLLIP mRNA transcript in monocytes was associated with increased TNF and IL-6 secretion after Lp stimulation of PBMC ex vivo. This genotype was separately associated with decreased LD susceptibility (309 controls, 88 cases, p = 0.008, OR 0.36, 95% CI 0.16-0.76) in a candidate gene association study. These results suggest that TOLLIP decreases lung-specific TLR responses to increase LD susceptibility in human populations. Better understanding of TOLLIP may lead to novel immunomodulatory therapies.

Viewing Legionella pneumophila Pathogenesis through an Immunological Lens

Legionella pneumophila is the causative agent of the severe pneumonia Legionnaires' disease. L. pneumophila is ubiquitously found in freshwater environments, where it replicates within free-living protozoa. Aerosolization of contaminated water supplies allows the bacteria to be inhaled into the human lung, where L. pneumophila can be phagocytosed by alveolar macrophages and replicate intracellularly. The Dot/Icm type IV secretion system (T4SS) is one of the key virulence factors required for intracellular bacterial replication and subsequent disease. The Dot/Icm apparatus translocates more than 300 effector proteins into the host cell cytosol. These effectors interfere with a variety of cellular processes, thus enabling the bacterium to evade phagosome-lysosome fusion and establish an endoplasmic reticulum-derived Legionella-containing vacuole, which facilitates bacterial replication. In turn, the immune system has evolved numerous strategies to recognize intracellular bacteria such as L. pneumophila, leading to potent inflammatory responses that aid in eliminating infection. This review aims to provide an overview of L. pneumophila pathogenesis in the context of the host immune response.

The virulence of Legionella pneumophila is positively correlated with its ability to stimulate NF-κB activation

AIM

Our work is to study the correlation between the virulence of different Legionella pneumophila in mouse model and its ability to activate NF-κB signaling pathway in vitro.

MATERIALS & METHODS

We measured the abilities of different strains of L. pneumophila to induce the activation of NF-κB signaling pathway in vitro. By using A/J mice, we also detected the virulence of different strains in vivo.

RESULTS & CONCLUSION

We demonstrated that different strains of L. pneumophila induce different levels of activation to NF-κB signaling pathway in vitro. We also found that L. pneumophila strain induced higher NF-κB activation in vitro showed more severe weight losses, higher mortality, more severe lung inflammation and higher levels of serum cytokines production in mice.

C. elegans-based screen identifies lysosome-damaging alkaloids that induce STAT3-dependent lysosomal cell death

Lysosomes are degradation and signaling centers within the cell, and their dysfunction impairs a wide variety of cellular processes. To understand the cellular effect of lysosome damage, we screened natural small-molecule compounds that induce lysosomal abnormality using Caenorhabditis elegans (C. elegans) as a model system. A group of vobasinyl-ibogan type bisindole alkaloids (ervachinines A-D) were identified that caused lysosome enlargement in C. elegans macrophage-like cells. Intriguingly, these compounds triggered cell death in the germ line independently of the canonical apoptosis pathway. In mammalian cells, ervachinines A-D induced lysosomal enlargement and damage, leading to leakage of cathepsin proteases, inhibition of autophagosome degradation and necrotic cell death. Further analysis revealed that this ervachinine-induced lysosome damage and lysosomal cell death depended on STAT3 signaling, but not RIP1 or RIP3 signaling. These findings suggest that lysosome-damaging compounds are promising reagents for dissecting signaling mechanisms underlying lysosome homeostasis and lysosome-related human disorders.

Enteric Virome Sensing-Its Role in Intestinal Homeostasis and Immunity

Pattern recognition receptors (PRRs) sensing commensal microorganisms in the intestine induce tightly controlled tonic signaling in the intestinal mucosa, which is required to maintain intestinal barrier integrity and immune homeostasis. At the same time, PRR signaling pathways rapidly trigger the innate immune defense against invasive pathogens in the intestine. Intestinal epithelial cells and mononuclear phagocytes in the intestine and the gut-associated lymphoid tissues are critically involved in sensing components of the microbiome and regulating immune responses in the intestine to sustain immune tolerance against harmless antigens and to prevent inflammation. These processes have been mostly investigated in the context of the bacterial components of the microbiome so far. The impact of viruses residing in the intestine and the virus sensors, which are activated by these enteric viruses, on intestinal homeostasis and inflammation is just beginning to be unraveled. In this review, we will summarize recent findings indicating an important role of the enteric virome for intestinal homeostasis as well as pathology when the immune system fails to control the enteric virome. We will provide an overview of the virus sensors and signaling pathways, operative in the intestine and the mononuclear phagocyte subsets, which can sense viruses and shape the intestinal immune response. We will discuss how these might interact with resident enteric viruses directly or in context with the bacterial microbiome to affect intestinal homeostasis.

Lysosomal membrane permeabilization as a cell death mechanism in cancer cells

Lysosomes are acidic organelles that contain hydrolytic enzymes that mediate the intracellular degradation of macromolecules. Damage of these organelles often results in lysosomal membrane permeabilization (LMP) and the release into the cytoplasm of the soluble lysosomal contents, which include proteolytic enzymes of the cathepsin family. This, in turn, activates several intracellular cascades that promote a type of regulated cell death, called lysosome-dependent cell death (LDCD). LDCD can be inhibited by pharmacological or genetic blockade of cathepsin activity, or by protecting the lysosomal membrane, thereby stabilizing the organelle. Lysosomal alterations are common in cancer cells and may increase the sensitivity of these cells to agents that promote LMP. In this review, we summarize recent findings supporting the use of LDCD as a means of killing cancer cells.

Legionella: virulence factors and host response

PURPOSE OF REVIEW

Legionella pneumophila is a facultative intracellular pathogen and an important cause of community-acquired and nosocomial pneumonia. This review focuses on the latest literature examining Legionella's virulence strategies and the mammalian host response.

RECENT FINDINGS

Recent studies identify novel virulence strategies used by L. pneumophila and new aspects of the host immune response to this pathogen. Legionella prevents acidification of the phagosome by recruiting Rab1, a host protein. Legionella also blocks a conserved endoplasmic reticulum stress response. To access iron from host stores, L. pneumophila upregulates more regions allowing vacuolar colocalization N. In response to Legionella, the host cell may activate caspase-1, caspase-11 (mice) or caspase-4 (humans). Caspase-3 and apoptosis are activated by a secreted, bacterial effector. Infected cells send signals to their uninfected neighbors, allowing the elaboration of inflammatory cytokines in trans. Antibody subclasses provide robust protection against Legionella.

SUMMARY

L. pneumophila is a significant human pathogen that lives in amoebae in the environment but may opportunistically infect the alveolar macrophage. To maintain its intracellular lifestyle, Legionella extracts essential iron from the cell, blocks inflammatory responses and manipulates trafficking to avoid fusion with the lysosome. The mammalian host has counter strategies, which include the release of proinflammatory cytokines, the activation of caspases and antibody-mediated immunity.

Legionella pneumophila Outer Membrane Vesicles: Isolation and Analysis of Their Pro-inflammatory Potential on Macrophages

Bacteria are able to secrete a variety of molecules via various secretory systems. Besides the secretion of molecules into the extracellular space or directly into another cell, Gram-negative bacteria can also form outer membrane vesicles (OMVs). These membrane vesicles can deliver their cargo over long distances, and the cargo is protected from degradation by proteases and nucleases. Legionella pneumophila (L. pneumophila) is an intracellular, Gram-negative pathogen that causes a severe form of pneumonia. In humans, it infects alveolar macrophages, where it blocks lysosomal degradation and forms a specialized replication vacuole. Moreover, L. pneumophila produces OMVs under various growth conditions. To understand the role of OMVs in the infection process of human macrophages, we set up a protocol to purify bacterial membrane vesicles from liquid culture. The method is based on differential ultracentrifugation. The enriched OMVs were subsequently analyzed with regard to their protein and lipopolysaccharide (LPS) amount and were then used for the treatment of a human monocytic cell line or murine bone marrow-derived macrophages. The pro-inflammatory responses of those cells were analyzed by enzyme-linked immunosorbent assay. Furthermore, alterations in a subsequent infection were analyzed. To this end, the bacterial replication of L. pneumophila in macrophages was studied by colony-forming unit assays. Here, we describe a detailed protocol for the purification of L. pneumophila OMVs from liquid culture by ultracentrifugation and for the downstream analysis of their pro-inflammatory potential on macrophages.

Lysosomal cell death mechanisms in aging

Lysosomes are degradative organelles essential for cell homeostasis that regulate a variety of processes, from calcium signaling and nutrient responses to autophagic degradation of intracellular components. Lysosomal cell death is mediated by the lethal effects of cathepsins, which are released into the cytoplasm following lysosomal damage. This process of lysosomal membrane permeabilization and cathepsin release is observed in several physiopathological conditions and plays a role in tissue remodeling, the immune response to intracellular pathogens and neurodegenerative diseases. Many evidences indicate that aging strongly influences lysosomal activity by altering the physical and chemical properties of these organelles, rendering them more sensitive to stress. In this review we focus on how aging alters lysosomal function and increases cell sensitivity to lysosomal membrane permeabilization and lysosomal cell death, both in physiological conditions and age-related pathologies.

Macrophage cell death in microbial infections

Macrophages can respond to microbial infections with programmed cell death. The major cell death pathways of apoptosis, pyroptosis and necroptosis are tightly regulated to ensure adequate immune reactions to virulent and persistent invaders. Macrophage death eliminates the replicative niche of intracellular pathogens and induces immune attack. Not surprisingly, successful pathogens have evolved strategies to modulate macrophage cell death pathways to enable microbial survival and replication. Uncontrolled macrophage death can also lead to tissue damage, which may augment bacterial dissemination and pathology. In this review, we highlight how pathogens hijack macrophage cell death signals to promote microbial survival and immune evasion.

Cell biology and immunology lessons taught by Legionella pneumophila

Legionella pneumophila is a facultative intracellular pathogen capable of replicating within a broad range of hosts. One unique feature of this pathogen is the cohort of ca. 300 virulence factors (effectors) delivered into host cells via its Dot/Icm type IV secretion system. Study of these proteins has produced novel insights into the mechanisms of host function modulation by pathogens, the regulation of essential processes of eukaryotic cells and of immunosurveillance. In this review, we will briefly discuss the roles of some of these effectors in the creation of a niche permissive for bacterial replication in phagocytes and recent advancements in the dissection of the innate immune detection mechanisms by challenging immune cells with L. pneumophila.

A new way to detect the danger: Lysosomal cell death induced by a bacterial ribosomal protein

The death of immune cells in response to pathogens often dictates the outcome of an infection. In some contexts, pathogens specifically kill immune cells by producing highly potent toxins or by triggering host cell death pathways, thus ensuring successful infections. But for intracellular pathogens and viruses, the death of host cells normally is disastrous for their intracellular life cycle. Our recent experiments with the pathogen Legionella pneumophila revealed that the bacterial ribosomal protein RpsL is able to trigger lysosomal membrane permeabilization (LMP) and the subsequent macrophage cell death. Interestingly, a lysine to arginine mutation at the 88^th residue, which also confers resistance to the antibiotic streptomycin, substantially impaired the cell death inducing activity of RpsL and allowed L. pneumophila to succeed in intracellular replication, suggesting the convergence of resistance mechanisms to innate immunity and antibiotics. The discovery of lysosomal cell death as an immune response to a bacterial ligand has expanded the spectrum of reactions that host cells can mount against bacterial infection; these observations provide a model to study the pathways that lead to the induction of LMP, a currently poorly understood cellular process involved in the development of many diseases.