Guanylate Binding Proteins Regulate Inflammasome Activation in Response to Hyperinjected Yersinia Translocon Components

Yersinia deploys type III-secreted effectors to evade caspase-4 inflammasome activation in human cells

IMPORTANCE

Yersinia are responsible for significant disease burden in humans, ranging from recurrent disease outbreaks (yersiniosis) to pandemics (Yersinia pestis plague). Together with rising antibiotic resistance rates, there is a critical need to better understand Yersinia pathogenesis and host immune mechanisms, as this information will aid in developing improved immunomodulatory therapeutics. Inflammasome responses in human cells are less studied relative to murine models of infection, though recent studies have uncovered key differences in inflammasome responses between mice and humans. Here, we dissect human intestinal epithelial cell and macrophage inflammasome responses to Yersinia pseudotuberculosis. Our findings provide insight into species- and cell type-specific differences in inflammasome responses to Yersinia.

Human GBP1 facilitates the rupture of the Legionella-containing vacuole and inflammasome activation

IMPORTANCE

Inflammasomes are essential for host defense against intracellular bacterial pathogens like Legionella, as they activate caspases, which promote cytokine release and cell death to control infection. In mice, interferon (IFN) signaling promotes inflammasome responses against bacteria by inducing a family of IFN-inducible GTPases known as guanylate-binding proteins (GBPs). Within murine macrophages, IFN promotes the rupture of the Legionella-containing vacuole (LCV), while GBPs are dispensable for this process. Instead, GBPs facilitate the lysis of cytosol-exposed Legionella. In contrast, the functions of IFN and GBPs in human inflammasome responses to Legionella are poorly understood. We show that IFN-γ enhances inflammasome responses to Legionella in human macrophages. Human GBP1 is required for these IFN-γ-driven inflammasome responses. Furthermore, GBP1 co-localizes with Legionella and/or LCVs in a type IV secretion system (T4SS)-dependent manner and promotes damage to the LCV, which leads to increased exposure of the bacteria to the host cell cytosol. Thus, our findings reveal species- and pathogen-specific differences in how GBPs function to promote inflammasome responses.

Live imaging of Yersinia translocon formation and immune recognition in host cells

Yersinia enterocolitica employs a type three secretion system (T3SS) to translocate immunosuppressive effector proteins into host cells. To this end, the T3SS assembles a translocon/pore complex composed of the translocator proteins YopB and YopD in host cell membranes serving as an entry port for the effectors. The translocon is formed in a Yersinia-containing pre-phagosomal compartment that is connected to the extracellular space. As the phagosome matures, the translocon and the membrane damage it causes are recognized by the cell-autonomous immune system. We infected cells in the presence of fluorophore-labeled ALFA-tag-binding nanobodies with a Y. enterocolitica strain expressing YopD labeled with an ALFA-tag. Thereby we could record the integration of YopD into translocons and its intracellular fate in living host cells. YopD was integrated into translocons around 2 min after uptake of the bacteria into a phosphatidylinositol-4,5-bisphosphate enriched pre-phagosomal compartment and remained there for 27 min on average. Damaging of the phagosomal membrane as visualized with recruitment of GFP-tagged galectin-3 occurred in the mean around 14 min after translocon formation. Shortly after recruitment of galectin-3, guanylate-binding protein 1 (GBP-1) was recruited to phagosomes, which was accompanied by a decrease in the signal intensity of translocons, suggesting their degradation or disassembly. In sum, we were able for the first time to film the spatiotemporal dynamics of Yersinia T3SS translocon formation and degradation and its sensing by components of the cell-autonomous immune system.

Reprogramming of Cell Death Pathways by Bacterial Effectors as a Widespread Virulence Strategy

The modulation of programmed cell death (PCD) processes during bacterial infections is an evolving arms race between pathogens and their hosts. The initiation of apoptosis, necroptosis, and pyroptosis pathways are essential to immunity against many intracellular and extracellular bacteria. These cellular self-destructive mechanisms are used by the infected host to restrict and eliminate bacterial pathogens. Without a tight regulatory control, host cell death can become a double-edged sword. Inflammatory PCDs contribute to an effective immune response against pathogens, but unregulated inflammation aggravates the damage caused by bacterial infections. Thus, fine-tuning of these pathways is required to resolve infection while preserving the host immune homeostasis. In turn, bacterial pathogens have evolved secreted virulence factors or effector proteins that manipulate PCD pathways to promote infection. In this review, we discuss the importance of controlled cell death in immunity to bacterial infection. We also detail the mechanisms employed by type 3 secreted bacterial effectors to bypass these pathways and their importance in bacterial pathogenesis.

Toxoplasma-proximal and distal control by GBPs in human macrophages

Human guanylate binding proteins (GBPs) are key players of interferon-gamma (IFNγ)-induced cell intrinsic defense mechanisms targeting intracellular pathogens. In this study, we combine the well-established Toxoplasmagondii infection model with three in vitro macrophage culture systems to delineate the contribution of individual GBP family members to control this apicomplexan parasite. Use of high-throughput imaging assays and genome engineering allowed us to define a role for GBP1, 2 and 5 in parasite infection control. While GBP1 performs a pathogen-proximal, parasiticidal and growth-restricting function through accumulation at the parasitophorous vacuole of intracellular Toxoplasma, GBP2 and GBP5 perform a pathogen-distal, growth-restricting role. We further find that mutants of the GTPase or isoprenylation site of GBP1/2/5 affect their normal function in Toxoplasma control by leading to mis-localization of the proteins.

Role of the Yersinia pseudotuberculosis Virulence Plasmid in Pathogen-Phagocyte Interactions in Mesenteric Lymph Nodes

Yersinia pseudotuberculosis is an Enterobacteriaceae family member that is commonly transmitted by the fecal-oral route to cause infections. From the small intestine, Y. pseudotuberculosis can invade through Peyer's patches and lymph vessels to infect the mesenteric lymph nodes (MLNs). Infection of MLNs by Y. pseudotuberculosis results in the clinical presentation of mesenteric lymphadenitis. MLNs are important for immune responses to intestinal pathogens and microbiota in addition to their clinical relevance to Y. pseudotuberculosis infections. A characteristic of Y. pseudotuberculosis infection in MLNs is the formation of pyogranulomas. Pyogranulomas are composed of neutrophils, inflammatory monocytes, and lymphocytes surrounding extracellular microcolonies of Y. pseudotuberculosis. Key elements of the complex pathogen-host interaction in MLNs have been identified using mouse infection models. Y. pseudotuberculosis requires the virulence plasmid pYV to induce the formation of pyogranulomas in MLNs. The YadA adhesin and the Ysc-Yop type III secretion system (T3SS) are encoded on pYV. YadA mediates bacterial binding to host receptors, which engages the T3SS to preferentially translocate seven Yop effectors into phagocytes. The effectors promote pathogenesis by blocking innate immune defenses such as superoxide production, degranulation, and inflammasome activation, resulting in survival and growth of Y. pseudotuberculosis. On the other hand, certain effectors can trigger immune defenses in phagocytes. For example, YopJ triggers activation of caspase-8 and an apoptotic cell death response in monocytes within pyogranulomas that limits dissemination of Y. pseudotuberculosis from MLNs to the bloodstream. YopE can be processed as an antigen by phagocytes in MLNs, resulting in T and B cell responses to Y. pseudotuberculosis. Immune responses to Y. pseudotuberculosis in MLNs can also be detrimental to the host in the form of chronic lymphadenopathy. This review focuses on interactions between Y. pseudotuberculosis and phagocytes mediated by pYV that concurrently promote pathogenesis and host defense in MLNs. We propose that MLN pyogranulomas are immunological arenas in which opposing pYV-driven forces determine the outcome of infection in favor of the pathogen or host.

Cytosolic detection of phagosomal bacteria-Mechanisms underlying PAMP exodus from the phagosome into the cytosol

The metazoan innate immune system senses bacterial infections by detecting highly conserved bacterial molecules, termed pathogen-associated molecular patterns (PAMPs). PAMPs are detected by a variety of host pattern recognition receptors (PRRs), whose function is to coordinate downstream immune responses. PRR activities are, in part, regulated by their subcellular localizations. Accordingly, professional phagocytes can detect extracellular bacteria and their PAMPs via plasma membrane-oriented PRRs. Conversely, phagocytosed bacteria and their PAMPs are detected by transmembrane PRRs oriented toward the phagosomal lumen. Even though PAMPs are unable to passively diffuse across membranes, phagocytosed bacteria are also detected by PRRs localized within the host cell cytosol. This phenomenon is explained by phagocytosis of bacteria that specialize in phagosomal escape and cytosolic residence. Contrary to this cytosolic lifestyle, most bacteria studied to date spend their entire intracellular lifestyle contained within phagosomes, yet they also stimulate cytosolic PRRs. Herein, we will review our current understanding of how phagosomal PAMPs become accessible to cytosolic PRRs, as well as highlight knowledge gaps that should inspire future investigations.

Genetic targeting of Card19 is linked to disrupted NINJ1 expression, impaired cell lysis, and increased susceptibility to Yersinia infection

Cell death plays a critical role in inflammatory responses. During pyroptosis, inflammatory caspases cleave Gasdermin D (GSDMD) to release an N-terminal fragment that generates plasma membrane pores that mediate cell lysis and IL-1 cytokine release. Terminal cell lysis and IL-1β release following caspase activation can be uncoupled in certain cell types or in response to particular stimuli, a state termed hyperactivation. However, the factors and mechanisms that regulate terminal cell lysis downstream of GSDMD cleavage remain poorly understood. In the course of studies to define regulation of pyroptosis during Yersinia infection, we identified a line of Card19-deficient mice (Card19lxcn) whose macrophages were protected from cell lysis and showed reduced apoptosis and pyroptosis, yet had wild-type levels of caspase activation, IL-1 secretion, and GSDMD cleavage. Unexpectedly, CARD19, a mitochondrial CARD-containing protein, was not directly responsible for this, as an independently-generated CRISPR/Cas9 Card19 knockout mouse line (Card19Null) showed no defect in macrophage cell lysis. Notably, Card19 is located on chromosome 13, immediately adjacent to Ninj1, which was recently found to regulate cell lysis downstream of GSDMD activation. RNA-seq and western blotting revealed that Card19lxcn BMDMs have significantly reduced NINJ1 expression, and reconstitution of Ninj1 in Card19lxcn immortalized BMDMs restored their ability to undergo cell lysis in response to caspase-dependent cell death stimuli. Card19lxcn mice exhibited increased susceptibility to Yersinia infection, whereas independently-generated Card19Null mice did not, demonstrating that cell lysis itself plays a key role in protection against bacterial infection, and that the increased infection susceptibility of Card19lxcn mice is attributable to loss of NINJ1. Our findings identify genetic targeting of Card19 being responsible for off-target effects on the adjacent gene Ninj1, disrupting the ability of macrophages to undergo plasma membrane rupture downstream of gasdermin cleavage and impacting host survival and bacterial control during Yersinia infection.

Human GBP1 Differentially Targets Salmonella and Toxoplasma to License Recognition of Microbial Ligands and Caspase-Mediated Death

Interferon-inducible guanylate-binding proteins (GBPs) promote cell-intrinsic defense through host cell death. GBPs target pathogens and pathogen-containing vacuoles and promote membrane disruption for release of microbial molecules that activate inflammasomes. GBP1 mediates pyroptosis or atypical apoptosis of Salmonella Typhimurium (STm)- or Toxoplasma gondii (Tg)- infected human macrophages, respectively. The pathogen-proximal detection-mechanisms of GBP1 remain poorly understood, as humans lack functional immunity-related GTPases (IRGs) that assist murine Gbps. Here, we establish that GBP1 promotes the lysis of Tg-containing vacuoles and parasite plasma membranes, releasing Tg-DNA. In contrast, we show GBP1 targets cytosolic STm and recruits caspase-4 to the bacterial surface for its activation by lipopolysaccharide (LPS), but does not contribute to bacterial vacuole escape. Caspase-1 cleaves and inactivates GBP1, and a cleavage-deficient GBP1^D192E mutant increases caspase-4-driven pyroptosis due to the absence of feedback inhibition. Our studies elucidate microbe-specific roles of GBP1 in infection detection and its triggering of the assembly of divergent caspase signaling platforms.

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Development of two microscopy assays for microbe/microbe-containing vacuole lysis

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Human GBP1 is essential for the lysis of Toxoplasma gondii vacuoles and parasites

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Caspase-4 recruitment, but not cytosolic escape of Salmonella, is GBP1 dependent

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Caspase-1 cleaves and inactivates GBP1 and suppresses caspase-4-driven pyroptosis

Interferon-induced GTPases orchestrate host cell-autonomous defence against bacterial pathogens

Interferon (IFN)-induced guanosine triphosphate hydrolysing enzymes (GTPases) have been identified as cornerstones of IFN-mediated cell-autonomous defence. Upon IFN stimulation, these GTPases are highly expressed in various host cells, where they orchestrate anti-microbial activities against a diverse range of pathogens such as bacteria, protozoan and viruses. IFN-induced GTPases have been shown to interact with various host pathways and proteins mediating pathogen control via inflammasome activation, destabilising pathogen compartments and membranes, orchestrating destruction via autophagy and the production of reactive oxygen species as well as inhibiting pathogen mobility. In this mini-review, we provide an update on how the IFN-induced GTPases target pathogens and mediate host defence, emphasising findings on protection against bacterial pathogens.

Lipopolysaccharide Recognition in the Crossroads of TLR4 and Caspase-4/11 Mediated Inflammatory Pathways

The innate immune response to lipopolysaccharide is essential for host defense against Gram-negative bacteria. In response to bacterial infection, the TLR4/MD-2 complex that is expressed on the surface of macrophages, monocytes, dendritic, and epithelial cells senses picomolar concentrations of endotoxic LPS and triggers the production of various pro-inflammatory mediators. In addition, LPS from extracellular bacteria which is either endocytosed or transfected into the cytosol of host cells or cytosolic LPS produced by intracellular bacteria is recognized by cytosolic proteases caspase-4/11 and hosts guanylate binding proteins that are involved in the assembly and activation of the NLRP3 inflammasome. All these events result in the initiation of pro-inflammatory signaling cascades directed at bacterial eradication. However, TLR4-mediated signaling and caspase-4/11-induced pyroptosis are largely involved in the pathogenesis of chronic and acute inflammation. Both extra- and intracellular LPS receptors—TLR4/MD-2 complex and caspase-4/11, respectively—are able to directly bind the lipid A motif of LPS. Whereas the structural basis of lipid A recognition by the TLR4 complex is profoundly studied and well understood, the atomic mechanism of LPS/lipid A interaction with caspase-4/11 is largely unknown. Here we describe the LPS-induced TLR4 and caspase-4/11 mediated signaling pathways and their cross-talk and scrutinize specific structural features of the lipid A motif of diverse LPS variants that have been reported to activate caspase-4/11 or to induce caspase-4/11 mediated activation of NLRP3 inflammasome (either upon transfection of LPS in vitro or upon infection of cell cultures with intracellular bacteria or by LPS as a component of the outer membrane vesicles). Generally, inflammatory caspases show rather similar structural requirements as the TLR4/MD-2 complex, so that a “basic” hexaacylated bisphosphorylated lipid A architecture is sufficient for activation. However, caspase-4/11 can sense and respond to much broader variety of lipid A variants compared to the very “narrow” specificity of TLR4/MD-2 complex as far as the number and the length of lipid chains attached at the diglucosamine backbone of lipid A is concerned. Besides, modification of the lipid A phosphate groups with positively charged appendages such as phosphoethanolamine or aminoarabinose could be essential for the interaction of lipid A/LPS with inflammatory caspases and related proteins.

Zebrafish Models to Study Inflammasome-Mediated Regulation of Hematopoiesis

Hematopoiesis is a complex process through which immature bone marrow precursor cells mature into all types of blood cells. Although the association of hematopoietic lineage bias (including anemia and neutrophilia) with chronic inflammatory diseases has long been appreciated, the causes involved are obscure. Recently, cytosolic multiprotein inflammasome complexes were shown to activate inflammatory and immune responses, and directly regulate hematopoiesis in zebrafish models; this was deemed to occur via cleavage and inactivation of the master erythroid transcription factor GATA1. Herein summarized are the zebrafish models that are currently available to study this unappreciated role of inflammasome-mediated regulation of hematopoiesis. Novel putative therapeutic strategies, for the treatment of hematopoietic alterations associated with chronic inflammatory diseases in humans, are also proposed.

Cell-type-specific hypertranslocation of effectors by the Pseudomonas aeruginosa type III secretion system

Many Gram-negative pathogens use a type III secretion system (T3SS) to promote disease by injecting effector proteins into host cells. Common to many T3SSs is that injection of effector proteins is feedback inhibited. The mechanism of feedback inhibition and its role in pathogenesis are unclear. In the case of P. aeruginosa, the effector protein ExoS is central to limiting effector injection. ExoS is bifunctional, with an amino-terminal RhoGAP and a carboxy-terminal ADP-ribosyltransferase domain. We demonstrate that both domains are required to fully feedback inhibit effector injection. The RhoGAP-, but not the ADP-ribosyltransferase domain of the related effector protein ExoT also participates. Feedback inhibition does not involve translocator insertion nor pore-formation. Instead, feedback inhibition is due, in part, to a loss of the activating trigger for effector injection, and likely also decreased translocon stability. Surprisingly, feedback inhibition is abrogated in phagocytic cells. The lack of feedback inhibition in these cells requires phagocytic uptake of the bacteria, but cannot be explained through acidification of the phagosome or calcium limitation. Given that phagocytes are crucial for controlling P. aeruginosa infections, our data suggest that feedback inhibition allows P. aeruginosa to direct its effector arsenal against the cell types most damaging to its survival.

Irgm2 and Gate-16 cooperatively dampen Gram-negative bacteria-induced caspase-11 response

Inflammatory caspase-11 (rodent) and caspases-4/5 (humans) detect the Gram-negative bacterial component LPS within the host cell cytosol, promoting activation of the non-canonical inflammasome. Although non-canonical inflammasome-induced pyroptosis and IL-1-related cytokine release are crucial to mount an efficient immune response against various bacteria, their unrestrained activation drives sepsis. This suggests that cellular components tightly control the threshold level of the non-canonical inflammasome in order to ensure efficient but non-deleterious inflammatory responses. Here, we show that the IFN-inducible protein Irgm2 and the ATG8 family member Gate-16 cooperatively counteract Gram-negative bacteria-induced non-canonical inflammasome activation, both in cultured macrophages and in vivo. Specifically, the Irgm2/Gate-16 axis dampens caspase-11 targeting to intracellular bacteria, which lowers caspase-11-mediated pyroptosis and cytokine release. Deficiency in Irgm2 or Gate16 induces both guanylate binding protein (GBP)-dependent and GBP-independent routes for caspase-11 targeting to intracellular bacteria. Our findings identify molecular effectors that fine-tune bacteria-activated non-canonical inflammasome responses and shed light on the understanding of the immune pathways they control.

The Yersinia Type III Secretion System as a Tool for Studying Cytosolic Innate Immune Surveillance

Microbial pathogens have evolved complex mechanisms to interface with host cells in order to evade host defenses and replicate. However, mammalian innate immune receptors detect the presence of molecules unique to the microbial world or sense the activity of virulence factors, activating antimicrobial and inflammatory pathways. We focus on how studies of the major virulence factor of one group of microbial pathogens, the type III secretion system (T3SS) of human pathogenic Yersinia, have shed light on these important innate immune responses. Yersinia are largely extracellular pathogens, yet they insert T3SS cargo into target host cells that modulate the activity of cytosolic innate immune receptors. This review covers both the host pathways that detect the Yersinia T3SS and the effector proteins used by Yersinia to manipulate innate immune signaling.

Cell-autonomous immunity by IFN-induced GBPs in animals and plants

Inside host cells, guanylate binding proteins (GBPs) rapidly assemble into large antimicrobial defense complexes that combat a wide variety of bacterial pathogens. These massive nanomachines often completely coat targeted microbes where they act as recruitment platforms for downstream effectors capable of direct bactericidal activity. GBP-containing platforms also serve as sensory hubs to activate inflammasome-driven responses in the mammalian cytosol while in plants like Arabidopsis, GBP orthologues may facilitate intranuclear signaling for immunity against invasive phytopathogens. Together, this group of immune GTPases serve as a major defensive repertoire to protect the host cell interior from bacterial colonization across plant and animal kingdoms.

Host inflammasome defense mechanisms and bacterial pathogen evasion strategies

Inflammasomes are a formidable armada of intracellular pattern recognition receptors. They recognize determinants of infection, such as foreign entities or danger signals within the host cell cytosol, rapidly executing innate immune defenses and initiating adaptive immune responses. Although inflammasomes are implicated in many diseases, they are especially critical in host protection against intracellular bacterial pathogens. Given this role, it is not surprising that many pathogens have evolved effective strategies to evade inflammasome activation. In this review, we will provide a brief summary of inflammasome activation during infection with the intent of highlighting recent advances in the field. Additionally, we will review known bacterial evasion strategies and countermeasures that impact pathogenesis.

Inflammasome Regulates Hematopoiesis through Cleavage of the Master Erythroid Transcription Factor GATA1

Chronic inflammatory diseases are associated with altered hematopoiesis that could result in neutrophilia and anemia. Here we report that genetic or chemical manipulation of different inflammasome components altered the differentiation of hematopoietic stem and progenitor cells (HSPC) in zebrafish. Although the inflammasome was dispensable for the emergence of HSPC, it was intrinsically required for their myeloid differentiation. In addition, Gata1 transcript and protein amounts increased in inflammasome-deficient larvae, enforcing erythropoiesis and inhibiting myelopoiesis. This mechanism is evolutionarily conserved, since pharmacological inhibition of the inflammasome altered erythroid differentiation of human erythroleukemic K562 cells. In addition, caspase-1 inhibition rapidly upregulated GATA1 protein in mouse HSPC promoting their erythroid differentiation. Importantly, pharmacological inhibition of the inflammasome rescued zebrafish disease models of neutrophilic inflammation and anemia. These results indicate that the inflammasome plays a major role in the pathogenesis of neutrophilia and anemia of chronic diseases and reveal druggable targets for therapeutic interventions.

Interferon-induced guanylate-binding proteins: Guardians of host defense in health and disease

The role of guanylate-binding proteins in host defense.

Guanylate-binding proteins (GBPs) have recently emerged as central orchestrators of immunity to infection, inflammation, and neoplastic diseases. Within numerous host cell types, these IFN-induced GTPases assemble into large nanomachines that execute distinct host defense activities against a wide variety of microbial pathogens. In addition, GBPs customize inflammasome responses to bacterial infection and sepsis, where they act as critical rheostats to amplify innate immunity and regulate tissue damage. Similar functions are becoming evident for metabolic inflammatory syndromes and cancer, further underscoring the importance of GBPs within infectious as well as altered homeostatic settings. A better understanding of the basic biology of these IFN-induced GTPases could thus benefit clinical approaches to a wide spectrum of important human diseases.

Analysis of Inflammasome Activation in Response to Yersinia Infection by Fluorescence Microscopy Detection of Active Caspase-1 Puncta

The type of cell death triggered by a particular environmental stimulus influences the outcome of infection or inflammatory disease processes. The ability to identify the cell death pathway that is activated in response to infection is essential for understanding the pathogenesis and host response to infection. Activation of the cysteine protease caspase-1 in various inflammasome complexes indicates that cells are undergoing pyroptosis, a regulated, proinflammatory cell death. Inflammasome assembly and caspase activation can be measured by various methods ranging from detection of inflammasome-dependent cell death, cytokine secretion, cleavage of caspase-1, or the formation of "puncta" within the cell that contain inflammasome components, such as caspase-1 or the adapter protein ASC. Here we describe a method for detecting caspase-1 activation on a single cell level in the context of infection by the Gram-negative pathogen Yersinia using immunofluorescence microscopy. We previously used this approach to quantify caspase-1 puncta formation in cells containing Yersinia translocon components (Zwack et al., MBio 6:e02095-14, 2015). This is a modification of methods used previously by Broz et al. (Cell Host Microbe 8:471-483, 2010) and Case and Roy (MBio 2:e00117-11, 2011). By taking a microscopy-based approach that allows us to quantify puncta as well as other cell-biological features of infection (i.e., number of bacteria associated with a particular cell; levels of bacterial effector or translocon proteins in caspase-1 puncta-containing cells; or levels or localization of host cellular proteins), we can better quantify the heterogeneity between cells undergoing pyroptosis and cells that are not under the same infection conditions. These approaches have the potential to generate hypotheses that can enable further mechanistic insight into activation of pyroptosis in response to bacterial infection.

Cytosolic Recognition of Microbes and Pathogens: Inflammasomes in Action

Infection is a dynamic biological process underpinned by a complex interplay between the pathogen and the host. Microbes from all domains of life, including bacteria, viruses, fungi, and protozoan parasites, have the capacity to cause infection. Infection is sensed by the host, which often leads to activation of the inflammasome, a cytosolic macromolecular signaling platform that mediates the release of the proinflammatory cytokines interleukin-1β (IL-1β) and IL-18 and cleavage of the pore-forming protein gasdermin D, leading to pyroptosis. Host-mediated sensing of the infection occurs when pathogens inject or carry pathogen-associated molecular patterns (PAMPs) into the cytoplasm or induce damage that causes cytosolic liberation of danger-associated molecular patterns (DAMPs) in the host cell. Recognition of PAMPs and DAMPs by inflammasome sensors, including NLRP1, NLRP3, NLRC4, NAIP, AIM2, and Pyrin, initiates a cascade of events that culminate in inflammation and cell death. However, pathogens can deploy virulence factors capable of minimizing or evading host detection. This review presents a comprehensive overview of the mechanisms of microbe-induced activation of the inflammasome and the functional consequences of inflammasome activation in infectious diseases. We also explore the microbial strategies used in the evasion of inflammasome sensing at the host-microbe interaction interface.

Gain-of-Function Analysis Reveals Important Virulence Roles for the Yersinia pestis Type III Secretion System Effectors YopJ, YopT, and YpkA

Virulence of Yersinia pestis in mammals requires the type III secretion system, which delivers seven effector proteins into the cytoplasm of host cells to undermine immune responses. All seven of these effectors are conserved across Y. pestis strains, but three, YopJ, YopT, and YpkA, are apparently dispensable for virulence. Some degree of functional redundancy between effector proteins would explain both observations. Here, we use a combinatorial genetic approach to define the minimal subset of effectors required for full virulence in mice following subcutaneous infection. We found that a Y. pestis strain lacking YopJ, YopT, and YpkA is attenuated for virulence in mice and that addition of any one of these effectors to this strain increases lethality significantly. YopJ, YopT, and YpkA likely contribute to virulence via distinct mechanisms. YopJ is uniquely able to cause macrophage cell death in vitro and to suppress accumulation of inflammatory cells to foci of bacterial growth in deep tissue, whereas YopT and YpkA cannot. The synthetic phenotypes that emerge when YopJ, YopT, and YpkA are removed in combination provide evidence that each effector enhances Y. pestis virulence and that YopT and YpkA act through a mechanism distinct from that of YopJ.

Sensing of invading pathogens by GBPs: At the crossroads between cell-autonomous and innate immunity

Guanylate-binding proteins (GBPs) are conserved family of IFN-inducible GTPases that play an important role in the host immunity against bacterial, viral, and protozoan pathogens. GBPs protect the host by associating with intracellular microbes, their vacuolar niche or, in the case of viruses, with their replication complex. This association results in a restriction of the respective pathogen, yet the exact molecular mechanisms of the antimicrobial functions of GBPs are still unclear. Recent work has linked the GBPs with the activation of inflammasomes, multi-protein complexes that assemble upon recognition of pathogen- or host-derived signals and that drive the release of cytokines and host cell death. Here, we will focus on the most recent findings that have started to unravel the manifold restriction mechanism controlled by GBPs in mouse and human cells, and that shed light on the molecular cues that control GBP recruitment to bacterial membranes.

Host-Intrinsic Interferon Status in Infection and Immunity

Most genetic ablations of interferon (IFN) signaling abolish both the experimentally induced IFN response and constitutive IFN, whose effects are well established in autoimmunity but understudied during infection. In host-pathogen interactions, most IFN-mediated responses are attributed to infection-driven IFN. However, IFNs confer their activity by regulating networks of interferon-stimulated genes (ISGs), a process that requires de novo transcription and translation of both IFN and downstream ISGs through feedback of IFN receptor signaling. Due to the temporal requirement for IFN activity, many rapid antimicrobial responses may instead result from pre-established IFN signature stemming from host-intrinsic processes. Addressing the permeating effects of constitutive IFN is therefore needed to accurately describe immunity as host intrinsic or pathogen induced.

Sweet host revenge: Galectins and GBPs join forces at broken membranes

Most bacterial pathogens enter and exit eukaryotic cells during their journey through the vertebrate host. In order to endure inside a eukaryotic cell, bacterial invaders commonly employ bacterial secretion systems to inject host cells with virulence factors that co-opt the host's membrane trafficking systems and thereby establish specialised pathogen-containing vacuoles (PVs) as intracellular niches permissive for microbial growth and survival. To defend against these microbial adversaries hiding inside PVs, host organisms including humans evolved an elaborate cell-intrinsic armoury of antimicrobial weapons that include noxious gases, antimicrobial peptides, degradative enzymes, and pore-forming proteins. This impressive defence machinery needs to be accurately delivered to PVs, in order to fight off vacuole-dwelling pathogens. Here, I discuss recent evidence that the presence of bacterial secretion systems at PVs and the associated destabilisation of PV membranes attract such antimicrobial delivery systems consisting of sugar-binding galectins as well as dynamin-like guanylate-binding proteins (GBPs). I will review recent advances in our understanding of intracellular immune recognition of PVs by galectins and GBPs, discuss how galectins and GBPs control host defence, and highlight important avenues of future research in this exciting area of cell-autonomous immunity.