Caspase-11 activation requires lysis of pathogen-containing vacuoles by IFN-induced GTPases

Salmonella Virulence and Immune Escape

Salmonella genus represents the most common foodborne pathogens causing morbidity, mortality, and burden of disease in all regions of the world. The introduction of antimicrobial agents and Salmonella-specific phages has been considered as an effective intervention strategy to reduce Salmonella contamination. However, data from the United States, European countries, and low- and middle-income countries indicate that Salmonella cases are still a commonly encountered cause of bacterial foodborne diseases globally. The control programs have not been successful and even led to the emergence of some multidrug-resistant Salmonella strains. It is known that the host immune system is able to effectively prevent microbial invasion and eliminate microorganisms. However, Salmonella has evolved mechanisms of resisting host physical barriers and inhibiting subsequent activation of immune response through their virulence factors. There has been a high interest in understanding how Salmonella interacts with the host. Therefore, in the present review, we characterize the functions of Salmonella virulence genes and particularly focus on the mechanisms of immune escape in light of evidence from the emerging mainstream literature.

The NLRP3 inflammasome: molecular activation and regulation to therapeutics

NLRP3 (NOD-, LRR- and pyrin domain-containing protein 3) is an intracellular sensor that detects a broad range of microbial motifs, endogenous danger signals and environmental irritants, resulting in the formation and activation of the NLRP3 inflammasome. Assembly of the NLRP3 inflammasome leads to caspase 1-dependent release of the pro-inflammatory cytokines IL-1β and IL-18, as well as to gasdermin D-mediated pyroptotic cell death. Recent studies have revealed new regulators of the NLRP3 inflammasome, including new interacting or regulatory proteins, metabolic pathways and a regulatory mitochondrial hub. In this Review, we present the molecular, cell biological and biochemical bases of NLRP3 activation and regulation and describe how this mechanistic understanding is leading to potential therapeutics that target the NLRP3 inflammasome.

Interferon inducible GBPs restrict Burkholderia thailandensis motility induced cell-cell fusion

Innate immunity responds to pathogens by producing alarm signals and activating pathways that make host cells inhospitable for pathogen replication. The intracellular bacterium Burkholderia thailandensis invades the cytosol, hijacks host actin, and induces cell fusion to spread to adjacent cells, forming multinucleated giant cells (MNGCs) which promote bacterial replication. We show that type I interferon (IFN) restricts macrophage MNGC formation during B. thailandensis infection. Guanylate-binding proteins (GBPs) expressed downstream of type I IFN were required to restrict MNGC formation through inhibition of bacterial Arp2/3-dependent actin motility during infection. GTPase activity and the CAAX prenylation domain were required for GBP2 recruitment to B. thailandensis, which restricted bacterial actin polymerization required for MNGC formation. Consistent with the effects in in vitro macrophages, Gbp2-/-, Gbp5-/-, GbpChr3-KO mice were more susceptible to intranasal infection with B. thailandensis than wildtype mice. Our findings reveal that IFN and GBPs play a critical role in restricting cell-cell fusion and bacteria-induced pathology during infection.

Advances in the molecular mechanisms of NLRP3 inflammasome activators and inactivators

NLRP3 inflammasome is an intracellular protein complex that initiates cellular injury via assembly of NLRP3, ASC and caspase-1 in response to microbial infection and sterile stressors. The importance of NLRP3 inflammasome in immunity and human diseases has been well documented. Up to now, targeted inhibition of the assembly of NLRP3 inflammasome complex and of its activation was thought to be therapeutic strategy for associated diseases. Recent studies show that a host of molecules such as NIMA-related kinase 7 (Nek7) and DEAD-box helicase 3 X-linked (DDX3X) and a large number of biological mediators including cytokines, microRNAs, nitric oxide, carbon monoxide, nuclear factor erythroid-2 related factor 2 (Nrf2) and cellular autophagy participate in the activation and inactivation of NLRP3 inflammasome. This review summarizes current understanding of the molecular basis of NLRP3 inflammasome activation and inactivation. This knowledge may lead to development of new therapies directed at NLRP3 inflammasome related diseases.

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.

Functional crosstalk between non-canonical caspase-11 and canonical NLRP3 inflammasomes during infection-mediated inflammation

Inflammation is a part of the body's immune response for protection against pathogenic infections and other cellular damages; however, chronic inflammation is a major cause of various diseases. One key step in the inflammatory response is the activation of inflammasomes, intracellular protein complexes comprising pattern recognition receptors and other inflammatory molecules. The role of the NLRP3 inflammasome in inflammatory responses has been extensively investigated; however, the caspase-11 inflammasome has been recently identified and has been classified as a 'non-canonical' inflammasome, and emerging studies have highlighted its role in inflammatory responses. Because the ligands and the mechanisms for the activation of these two inflammasomes are different, studies to date have separately described their roles, although recent studies have reported the functional cooperation between these two inflammasomes during an inflammatory response. This review discusses the studies investigating the functional crosstalk between non-canonical caspase-11 and canonical NLRP3 inflammasomes in the context of inflammatory responses; moreover, it provides insight for the development of novel anti-inflammatory therapeutics to prevent and treat infectious and inflammatory diseases.

IRF5 genetic risk variants drive myeloid-specific IRF5 hyperactivation and presymptomatic SLE

Genetic variants within or near the interferon regulatory factor 5 (IRF5) locus associate with systemic lupus erythematosus (SLE) across ancestral groups. The major IRF5-SLE risk haplotype is common across populations, yet immune functions for the risk haplotype are undefined. We characterized the global immune phenotype of healthy donors homozygous for the major risk and nonrisk haplotypes and identified cell lineage-specific alterations that mimic presymptomatic SLE. Contrary to previous studies in B lymphoblastoid cell lines and SLE immune cells, IRF5 genetic variants had little effect on IRF5 protein levels in healthy donors. Instead, we detected basal IRF5 hyperactivation in the myeloid compartment of risk donors that drives the SLE immune phenotype. Risk donors were anti-nuclear antibody positive with anti-Ro and -MPO specificity, had increased circulating plasma cells and plasmacytoid dendritic cells, and had enhanced spontaneous NETosis. The IRF5-SLE immune phenotype was conserved over time and probed mechanistically by ex vivo coculture, indicating that risk neutrophils are drivers of the global immune phenotype. RNA-Seq of risk neutrophils revealed increased IRF5 transcript expression, IFN pathway enrichment, and decreased expression of ROS pathway genes. Altogether, the data support that individuals carrying the IRF5-SLE risk haplotype are more susceptible to environmental/stochastic influences that trigger chronic immune activation, predisposing to the development of clinical SLE.

The involvement of regulated cell death forms in modulating the bacterial and viral pathogenesis

Apoptosis, necroptosis and pyroptosis represent three distinct types of regulated cell death forms, which play significant roles in response to viral and bacterial infections. Whereas apoptosis is characterized by cell shrinkage, nuclear condensation, bleb formation and retained membrane integrity, necroptosis and pyroptosis exhibit osmotic imbalance driven cytoplasmic swelling and early membrane damage. These three cell death forms exert distinct immune stimulatory potential. The caspase driven apoptotic cell demise is considered in many circumstances as anti-inflammatory, whereas the two lytic cell death modalities can efficiently trigger immune response by releasing damage associated molecular patterns to the extracellular space. The relevance of these cell death modalities in infections can be best demonstrated by the presence of viral proteins that directly interfere with cell death pathways. Conversely, some pathogens hijack the cell death signaling routes to initiate a targeted attack against the immune cells of the host, and extracellular bacteria can benefit from the destruction of intact extracellular barriers upon cell death induction. The complexity and the crosstalk between these cell death modalities reflect a continuous evolutionary race between pathogens and host. This chapter discusses the current advances in the research of cell death signaling with regard to viral and bacterial infections and describes the network of the cell death initiating molecular mechanisms that selectively recognize pathogen associated molecular patterns.

Essential Role of mGBP7 for Survival of Toxoplasma gondii Infection

Guanylate-binding proteins (GBPs) are induced by the inflammatory cytokine interferon gamma (IFN-γ) and have been shown to be important factors in the defense of the intracellular pathogen Toxoplasma gondii. In previous studies, we showed that members of the mouse GBP family, such as mGBP2 and mGBP7, accumulate at the parasitophorous vacuole of T. gondii, which is the replicatory niche of the parasite. In this study, we show that mice deficient in mGBP7 succumb early after infection with T. gondii, showing a complete failure of resistance to the pathogen. On a molecular level, mGBP7 is found directly at the parasite, likely mediating its destruction.

Members of the murine guanylate-binding protein family (mGBP) are induced by interferon gamma (IFN-γ) and have been shown to be important factors in cell-autonomous immunity toward the intracellular pathogen Toxoplasma gondii. Previously, we identified that mGBP2 mediates disruption of the parasitophorous vacuole membrane (PVM) and directly assaults the plasma membrane of the parasite. Here, we show that mGBP7-deficient mice are highly susceptible to T. gondii infection. This is demonstrated by the loss of parasite replication control, pronounced development of ascites, and death of the animals in the acute infection phase. Interestingly, live-cell microscopy revealed that mGBP7 recruitment to the PVM occurs after mGBP2 recruitment, followed by disruption of the PVM and T. gondii integrity and accumulation of mGBP7 inside the parasite. This study defines mGBP7 as a crucial effector protein in resistance to intracellular T. gondii.

Dynamics and Outcome of Macrophage Interaction Between Salmonella Gallinarum, Salmonella Typhimurium, and Salmonella Dublin and Macrophages From Chicken and Cattle

Salmonella Gallinarum only infects avian species, where it causes a severe systemic infection in birds of all ages. It is generally accepted that interaction with phagocytic cells plays an important role in the development of systemic, host-specific Salmonella infections. The current study detailed the interaction of S. Gallinarum with macrophages derived from chicken (HD11) and cattle (Bomac) compared to interaction of the broad host range serovar, Salmonella Typhimurium and the cattle adapted serovar Salmonella Dublin. Results showed a weaker invading ability of S. Gallinarum in both kinds of macrophages, regardless whether the bacteria were opsonized or not before infections. However, opsonization of S. Gallinarum by chicken serum increased its intracellular survival rate in chicken macrophages. No significant induction of nitrogen oxide was observed in the infected HD11 cells within the first 6 h, and levels of reactive oxygen species (ROS) were similar among the three serovars. S. Gallinarum infection was associated with low cell deaths in both chicken and cattle macrophages, whereas S. Dublin only induced a comparable high level of cell death in chicken macrophages, but not in macrophages of its preferred host species (Bomac) compared to host generalist S. Typhimurium. S. Gallinarum-infected HD11 macrophages exhibited low induction of pro-inflammation genes [interleukin (IL)1β, CXCLi1, and CXCLi2] compared to the two other serovars, and contrary to the other serovars, it did not induce significant downregulation of Toll-like receptor (TLR)2, TLR4, and TLR5. In in vivo infection of 1-week-old chicken, a significant upregulation of the TLR4 and TLR5 genes in the spleen was observed in S. Gallinarum-infected chickens, but not in S. Typhimurium-infected chicken at 5 days post-infections. Taken together, results show that S. Gallinarum infection of macrophages was characterized by low uptake and low cytotoxicity, possibly allowing long-term persistence in the intracellular environment, and it caused a low induction of pro-inflammatory responses.

Recent Advances in Lipopolysaccharide Recognition Systems

Lipopolysaccharide (LPS), commonly known as endotoxin, is ubiquitous and the most-studied pathogen-associated molecular pattern. A component of Gram-negative bacteria, extracellular LPS is sensed by our immune system via the toll-like receptor (TLR)-4. Given that TLR4 is membrane bound, it recognizes LPS in the extracellular milieu or within endosomes. Whether additional sensors, if any, play a role in LPS recognition within the cytoplasm remained unknown until recently. The last decade has seen an unprecedented unfolding of TLR4-independent LPS sensing pathways. First, transient receptor potential (TRP) channels have been identified as non-TLR membrane-bound sensors of LPS and, second, caspase-4/5 (and caspase-11 in mice) have been established as the cytoplasmic sensors for LPS. Here in this review, we detail the brief history of LPS discovery, followed by the discovery of TLR4, TRP as the membrane-bound sensor, and our current understanding of caspase-4/5/11 as cytoplasmic sensors.

NLRP3 Inflammasome in the Pathophysiology of Hemorrhagic Stroke: A Review

Hemorrhagic stroke is a devastating disease with high morbidity and mortality. There is still a lack of effective ther-apeutic approach. The recent studies have shown that the innate immune system plays a significant role in hemorrhagic stroke. Microglia, as major components in innate immune system, are activated and then can release cytokines and chemo-kines in response to hemorrhagic stroke, and ultimately led to neuroinflammation and brain injury. The NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome is predominantly released by microglia and is believed as the main contributor of neuroinflammation. Several studies have focused on the role of NLRP3 inflammasome in hemorrhagic stroke-induced brain injury, however, the specific mechanism of NLRP3 activation and regulation remains unclear. This re-view summarized the mechanism of NLRP3 activation and its role in hemorrhagic stroke and discussed the translational sig-nificance.

Autophagy and microbial pathogenesis

Autophagy is a cell biological process that promotes resilience in the face of environmental perturbations. Given that infectious agents represent a major type of environmental threat, it follows that the autophagy pathway is central to the outcome of host-microbe interactions. Detailed molecular studies have revealed intricate ways in which autophagy suppresses or enhances the fitness of infectious agents, particularly intracellular pathogens such as viruses that require the host cell machinery for replication. Findings in animal models have reinforced the importance of these events that occur within individual cells and have extended the role of autophagy to extracellular microbes and immunity at the whole organism level. These functions impact adaptation to bacteria that are part of the gut microbiota, which has implications for the etiology of chronic disorders such as inflammatory bowel disease. Despite major advances in how autophagy regulates inflammatory reactions toward microbes, many challenges remain, including distinguishing autophagy from closely related pathways such as LC3-associated phagocytosis. Here, we review the role of autophagy in microbial pathogenesis at the level of organismal biology. In addition to providing an overview of the prominent function of autophagy proteins in host-microbe interactions, we highlight how observations at the cellular level are informing pathogenesis studies and offer our perspective on the future directions of the field.

Bacterial Endotoxin Activates the Coagulation Cascade through Gasdermin D-Dependent Phosphatidylserine Exposure

Excessive activation of the coagulation system leads to life-threatening disseminated intravascular coagulation (DIC). Here, we examined the mechanisms underlying the activation of coagulation by lipopolysaccharide (LPS), the major cell-wall component of Gram-negative bacteria. We found that caspase-11, a cytosolic LPS receptor, activated the coagulation cascade. Caspase-11 enhanced the activation of tissue factor (TF), an initiator of coagulation, through triggering the formation of gasdermin D (GSDMD) pores and subsequent phosphatidylserine exposure, in a manner independent of cell death. GSDMD pores mediated calcium influx, which induced phosphatidylserine exposure through transmembrane protein 16F, a calcium-dependent phospholipid scramblase. Deletion of Casp11, ablation of Gsdmd, or neutralization of phosphatidylserine or TF prevented LPS-induced DIC. In septic patients, plasma concentrations of interleukin (IL)-1α and IL-1β, biomarkers of GSDMD activation, correlated with phosphatidylserine exposure in peripheral leukocytes and DIC scores. Our findings mechanistically link immune recognition of LPS to coagulation, with implications for the treatment of DIC.

Interaction Differences of the Avian Host-Specific Salmonella enterica Serovar Gallinarum, the Host-Generalist S. Typhimurium, and the Cattle Host-Adapted S. Dublin with Chicken Primary Macrophage

Most Salmonella serovars cause disease in many host species, while a few serovars have evolved to be host specific. Very little is known about the mechanisms that contribute to Salmonella host specificity. We compared the interactions between chicken primary macrophages (CDPM) and host-generalist serovar Salmonella enterica serovar Typhimurium, host-adapted Salmonella enterica serovar Dublin, and avian host-specific Salmonella enterica serovar Gallinarum. S Gallinarum was taken up in lower numbers by CDPM than S Typhimurium and S Dublin; however, a higher survival rate was observed for this serovar. In addition, S Typhimurium and S Dublin caused substantially higher levels of cell death to the CDPM, while significantly higher concentrations of NO were produced by S Gallinarum-infected cells. Global transcriptome analysis performed 2 h postinfection showed that S Gallinarum infection triggered a more comprehensive response in CDPM with 1,114 differentially expressed genes (DEGs) compared to the responses of S Typhimurium (625 DEGs) and S Dublin (656 DEGs). Comparable levels of proinflammation responses were observed in CDPM infected by these three different serovars at the initial infection phase, but a substantially quicker reduction in levels of interleukin-1β (IL-1β), CXCLi1, and CXCLi2 gene expression was detected in the S Gallinarum-infected macrophages than that of two other groups as infections proceeded. KEGG cluster analysis for unique DEGs after S Gallinarum infection showed that the JAK-STAT signaling pathway was top enriched, indicating a specific role for this pathway in response to S Gallinarum infection of CDPM. Together, these findings provide new insights into the interaction between Salmonella and the host and increase our understanding of S Gallinarum host specificity.

Gain-of-function CEBPE mutation causes noncanonical autoinflammatory inflammasomopathy

BACKGROUND

CCAAT enhancer-binding protein epsilon (C/EBPε) is a transcription factor involved in late myeloid lineage differentiation and cellular function. The only previously known disorder linked to C/EBPε is autosomal recessive neutrophil-specific granule deficiency leading to severely impaired neutrophil function and early mortality.

OBJECTIVE

The aim of this study was to molecularly characterize the effects of C/EBPε transcription factor Arg219His mutation identified in a Finnish family with previously genetically uncharacterized autoinflammatory and immunodeficiency syndrome.

METHODS

Genetic analysis, proteomics, genome-wide transcriptional profiling by means of RNA-sequencing, chromatin immunoprecipitation (ChIP) sequencing, and assessment of the inflammasome function of primary macrophages were performed.

RESULTS

Studies revealed a novel mechanism of genome-wide gain-of-function that dysregulated transcription of 464 genes. Mechanisms involved dysregulated noncanonical inflammasome activation caused by decreased association with transcriptional repressors, leading to increased chromatin occupancy and considerable changes in transcriptional activity, including increased expression of NLR family, pyrin domain-containing 3 protein (NLRP3) and constitutively expressed caspase-5 in macrophages.

CONCLUSION

We describe a novel autoinflammatory disease with defective neutrophil function caused by a homozygous Arg219His mutation in the transcription factor C/EBPε. Mutated C/EBPε acts as a regulator of both the inflammasome and interferome, and the Arg219His mutation causes the first human monogenic neomorphic and noncanonical inflammasomopathy/immunodeficiency. The mechanism, including widely dysregulated transcription, is likely not unique for C/EBPε. Similar multiomics approaches should also be used in studying other transcription factor-associated diseases.

Caspases in Cell Death, Inflammation, and Disease

Caspases are an evolutionary conserved family of cysteine proteases that are centrally involved in cell death and inflammation responses. A wealth of foundational insight into the molecular mechanisms that control caspase activation has emerged in recent years. Important advancements include the identification of additional inflammasome platforms and pathways that regulate activation of inflammatory caspases; the discovery of gasdermin D as the effector of pyroptosis and interleukin (IL)-1 and IL-18 secretion; and the existence of substantial crosstalk between inflammatory and apoptotic initiator caspases. A better understanding of the mechanisms regulating caspase activation has supported initial efforts to modulate dysfunctional cell death and inflammation pathways in a suite of communicable, inflammatory, malignant, metabolic, and neurodegenerative diseases. Here, we review current understanding of caspase biology with a prime focus on the inflammatory caspases and outline important topics for future experimentation.

Emerging insights into molecular mechanisms underlying pyroptosis and functions of inflammasomes in diseases

Pyroptosis is a form of necrotic and inflammatory programmed cell death, which could be characterized by cell swelling, pore formation on plasma membranes, and release of proinflammatory cytokines (IL-1β and IL-18). The process of pyroptosis presents as dual effects: protecting multicellular organisms from microbial infection and endogenous dangers; leading to pathological inflammation if overactivated. Two pathways have been found to trigger pyroptosis: caspase-1 mediated inflammasome pathway with the involvement of NLRP1-, NLRP3-, NLRC4-, AIM2-, pyrin-inflammasome (canonical inflammasome pathway) and caspase-4/5/11-mediated inflammasome pathway (noncanonical inflammasome pathway). Gasdermin D (GSDMD) has been proved to be a substrate of inflammatory caspases (caspase-1/4/5/11), and the cleaved N-terminal domain of GSDMD oligomerizes to form cytotoxic pores on the plasma membrane. Here, we mainly reviewed the up to date mechanisms of pyroptosis, and began with the inflammasomes as the activator of caspase-1/caspase-11, 4, and 5. We further discussed these inflammasomes functions in diseases, including infectious diseases, sepsis, inflammatory autoimmune diseases, and neuroinflammatory diseases.

TET2 Regulates the Neuroinflammatory Response in Microglia

Epigenomic mechanisms regulate distinct aspects of the inflammatory response in immune cells. Despite the central role for microglia in neuroinflammation and neurodegeneration, little is known about their epigenomic regulation of the inflammatory response. Here, we show that Ten-eleven translocation 2 (TET2) methylcytosine dioxygenase expression is increased in microglia upon stimulation with various inflammogens through a NF-κB-dependent pathway. We found that TET2 regulates early gene transcriptional changes, leading to early metabolic alterations, as well as a later inflammatory response independently of its enzymatic activity. We further show that TET2 regulates the proinflammatory response in microglia of mice intraperitoneally injected with LPS. We observed that microglia associated with amyloid β plaques expressed TET2 in brain tissue from individuals with Alzheimer's disease (AD) and in 5xFAD mice. Collectively, our findings show that TET2 plays an important role in the microglial inflammatory response and suggest TET2 as a potential target to combat neurodegenerative brain disorders.

Stearoyl Lysophosphatidylcholine Inhibits Endotoxin-Induced Caspase-11 Activation

Stearoyl lysophosphatidylcholine (LPC) exerts protective effect during endotoxemia and in experimental sepsis, but the underlying mechanism is unclear. Here, we demonstrated that stearoyl LPC could block caspase-11-mediated macrophage pyroptosis. In vitro, stearoyl LPC significantly decreased caspase-11 activation and pyroptosis induced by lipopolysaccharide (LPS) plus cholera toxin subunit B independent of the receptor G2A. Stearoyl LPC did not affect LPS uptake by mouse peritoneal macrophages but did significantly inhibit the interaction between LPS and caspase-11. Moreover, stearoyl LPC treatment conferred significant protection against lethal endotoxemia and significantly reduced the release of IL-1α and IL-1β. These findings identify stearoyl LPC as an inhibitor of LPS-mediated caspase-11 activation. This mechanism could explain the protective action of stearoyl LPC in experimental sepsis and endotoxemia.

TNFR2 induced priming of the inflammasome leads to a RIPK1-dependent cell death in the absence of XIAP

The pediatric immune deficiency X-linked proliferative disease-2 (XLP-2) is a unique disease, with patients presenting with either hemophagocytic lymphohistiocytosis (HLH) or intestinal bowel disease (IBD). Interestingly, XLP-2 patients display high levels of IL-18 in the serum even while in stable condition, presumably through spontaneous inflammasome activation. Recent data suggests that LPS stimulation can trigger inflammasome activation through a TNFR2/TNF/TNFR1 mediated loop in xiap^−/− macrophages. Yet, the direct role TNFR2-specific activation plays in the absence of XIAP is unknown. We found TNFR2-specific activation leads to cell death in xiap^−/− myeloid cells, particularly in the absence of the RING domain. RIPK1 kinase activity downstream of TNFR2 resulted in a TNF/TNFR1 cell death, independent of necroptosis. TNFR2-specific activation leads to a similar inflammatory NF-kB driven transcriptional profile as TNFR1 activation with the exception of upregulation of NLRP3 and caspase-11. Activation and upregulation of the canonical inflammasome upon loss of XIAP was mediated by RIPK1 kinase activity and ROS production. While both the inhibition of RIPK1 kinase activity and ROS production reduced cell death, as well as release of IL-1β, the release of IL-18 was not reduced to basal levels. This study supports targeting TNFR2 specifically to reduce IL-18 release in XLP-2 patients and to reduce priming of the inflammasome components.

Galectin-8 in the onset of the immune response and inflammation

Galectins (Gals), a family of mammalian lectins, have emerged as key regulators of the immune response, being implicated in several physiologic and pathologic conditions. Lately, there is increasing data regarding the participation of Galectin-8 (Gal-8) in both the adaptive and innate immune responses, as well as its high expression in inflammatory disorders. Here, we focus on the pro- and anti-inflammatory properties of Gal-8 and discuss the potential use of this lectin in order to shape the immune response, according to the context.

Caspase-11 signaling enhances graft-versus-host disease

Acute graft-versus-host disease (GVHD) remains a major obstacle for the wider usage of allogeneic hematopoietic stem cell transplantation (allo-HSCT), which is an effective therapy for hematopoietic malignancy. Here we show that caspase-11, the cytosolic receptor for bacterial endotoxin (lipopolysaccharide: LPS), enhances GVHD severity. Allo-HSCT markedly increases the LPS-caspase-11 interaction, leading to the cleavage of gasdermin D (GSDMD). Caspase-11 and GSDMD mediate the release of interleukin-1α (IL-1α) in allo-HSCT. Deletion of Caspase-11 or Gsdmd, inhibition of LPS-caspase-11 interaction, or neutralizing IL-1α uniformly reduces intestinal inflammation, tissue damage, donor T cell expansion and mortality in allo-HSCT. Importantly, Caspase-11 deficiency does not decrease the graft-versus-leukemia (GVL) activity, which is essential to prevent cancer relapse. These findings have major implications for allo-HSCT, as pharmacological interference with the caspase-11 signaling might reduce GVHD while preserving GVL activity.

An increasing number of inflammatory pathologies is associated with IL-1 production downstream of caspases 1 and 11. Here the authors show that graft-versus-host-disease (GvHD) is diminished in mice with genetic or pharmacological ablation of caspase-11, and provide mechanistic insights into the signals leading to caspase-11 activation in GvHD.

Guanylate-binding proteins at the crossroad of noncanonical inflammasome activation during bacterial infections

The immune system is armed with a broad range of receptors to detect and initiate the elimination of bacterial pathogens. Inflammasomes are molecular platforms that sense a diverse range of microbial insults to develop appropriate host response. In that context, noncanonical inflammasome arose as a sensor for Gram-negative bacteria-derived LPS leading to the control of infections. This review describes the role of caspase-11/gasdermin-D-dependent immune response against Gram-negative bacteria and presents an overview of guanylate-binding proteins (GBPs) at the interface of noncanonical inflammasome activation. Indeed, caspase-11 acts as a receptor for LPS and this interaction elicits caspase-11 autoproteolysis that is required for its optimal catalytic activity. Gasdermin-D is cleaved by activated caspase-11 generating an N-terminal domain that is inserted into the plasmatic membrane to form pores that induce pyroptosis, a cell death program involved in intracellular bacteria elimination. This mechanism also promotes IL-1β release and potassium efflux that connects caspase-11 to NLRP3 activation. Furthermore, GBPs display many features to allow LPS recognition by caspase-11, initiating the noncanonical inflammasome response prompting the immune system to control bacterial infections. In this review, we discuss the recent findings and nuances related to this mechanism and its biological functions.

Toll-like receptor signalling cross-activates the autophagic pathway to restrict Salmonella Typhimurium growth in macrophages

It has been long recognised that activation of toll-like receptors (TLRs) induces autophagy to restrict intracellular bacterial growth. However, the mechanisms of TLR-induced autophagy are incompletely understood. Salmonella Typhimurium is an intracellular pathogen that causes food poisoning and gastroenteritis in humans. Whether TLR activation contributes to S. Typhimurium-induced autophagy has not been investigated. Here, we report that S. Typhimurium and TLRs shared a common pathway to induce autophagy in macrophages. We first showed that S. Typhimurium-induced autophagy in a RAW264.7 murine macrophage cell line was mediated by the AMP-activated protein kinase (AMPK) through activation of the TGF-β-activated kinase (TAK1), a kinase activated by multiple TLRs. AMPK activation led to increased phosphorylation of Unc-51-like autophagy activating kinase (ULK1) at S317 and S555. ULK1 phosphorylation at these two sites in S. Typhimurium-infected macrophages overrode the inhibitory effect of mTOR on ULK1 activity due to mTOR-mediated ULK1 phosphorylation at S757. Lipopolysaccharide (LPS), flagellin, and CpG oligodeoxynucleotide, which activate TLR4, TLR5, and TLR9, respectively, increased TAK1 and AMPK phosphorylation and induced autophagy in RAW264.7 cells and in bone marrow-derived macrophages. However, LPS was unable to induce TAK1 and AMPK phosphorylation and autophagy in TLR4-deficient macrophages. TAK1 and AMPK-specific inhibitors blocked S. Typhimurium-induced autophagy and xenophagy and increased the bacterial growth in RAW264.7 cells. These observations collectively suggest that activation of the TAK1-AMPK axis through TLRs is essential for S. Typhimurium-induced autophagy and that TLR signalling cross-activates the autophagic pathway to clear intracellular bacteria.

C57BL/6 and 129 inbred mouse strains differ in Gbp2 and Gbp2b expression in response to inflammatory stimuli in vivo

Background: Infections cause the production of inflammatory cytokines such as Interferon gamma (IFNγ). IFNγ in turn prompts the upregulation of a range of host defence proteins including members of the family of guanylate binding proteins (Gbps). In humans and mice alike, GBPs restrict the intracellular replication of invasive microbes and promote inflammation. To study the physiological functions of Gbp family members, the most commonly chosen in vivo models are mice harbouring loss-of-function mutations in either individual Gbp genes or the entire Gbp gene cluster on mouse chromosome 3. Individual Gbp deletion strains differ in their design, as some strains exist on a pure C57BL/6 genetic background, while other strains contain a 129-derived genetic interval encompassing the Gbp gene cluster on an otherwise C57BL/6 genetic background. Methods: To determine whether the presence of 129 alleles of paralogous Gbps could influence the phenotypes of 129-congenic Gbp-deficient strains, we studied the expression of Gbps in both C57BL/6J and 129/Sv mice following in vivo stimulation with adjuvants and after infection with either Toxoplasma gondii or Shigella flexneri. Results: We show that C57BL/6J relative to 129/Sv mice display moderately elevated expression of Gbp2, but more prominently, are also defective for Gbp2b (formerly Gbp1) mRNA induction upon immune priming. Notably, Toxoplasma infections induce robust Gbp2b protein expression in both strains of mice, suggestive of a Toxoplasma-activated mechanism driving Gbp2b protein translation. We further find that the higher expression of Gbp2b mRNA in 129/Sv mice correlates with a gene duplication event at the Gbp2b locus resulting in two copies of the Gbp2b gene on the haploid genome of the 129/Sv strain. Conclusions: Our findings demonstrate functional differences between 129 and C57BL/6 Gbp alleles which need to be considered in the design and interpretation of studies utilizing mouse models, particularly for phenotypes influenced by Gbp2 or Gbp2b expression.

Host Cell Death Responses to Non-typhoidal Salmonella Infection

Salmonella enterica subsp. enterica serovar Typhimurium (S. Typhimurium) is a Gram-negative bacterium with a broad host range that causes non-typhoidal salmonellosis in humans. S. Typhimurium infects epithelial cells and macrophages in the small intestine where it replicates in a specialized intracellular niche called the Salmonella-containing vacuole (SCV) and promotes inflammation of the mucosa to induce typically self-limiting gastroenteritis. Virulence and spread of the bacterium is determined in part by the host individual's ability to limit the infection through innate immune responses at the gastrointestinal mucosa, including programmed cell death. S. Typhimurium however, has evolved a myriad of mechanisms to counteract or exploit host responses through the use of Type III Secretion Systems (T3SS), which allow the translocation of virulence (effector) proteins into the host cell for the benefit of optimal bacterial replication and dissemination. T3SS effectors have been found to interact with apoptotic, necroptotic, and pyroptotic cell death cascades, interfering with both efficient clearance of the bacteria and the recruitment of neutrophils or dendritic cells to the area of infection. The interplay of host inflammation, programmed cell death responses, and bacterial defenses in the context of non-typhoidal Salmonella (NTS) infection is a continuing area of interest within the field, and as such has been reviewed here.

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 NLRP3 Inflammasome: An Overview of Mechanisms of Activation and Regulation

The NLRP3 inflammasome is a critical component of the innate immune system that mediates caspase-1 activation and the secretion of proinflammatory cytokines IL-1β/IL-18 in response to microbial infection and cellular damage. However, the aberrant activation of the NLRP3 inflammasome has been linked with several inflammatory disorders, which include cryopyrin-associated periodic syndromes, Alzheimer's disease, diabetes, and atherosclerosis. The NLRP3 inflammasome is activated by diverse stimuli, and multiple molecular and cellular events, including ionic flux, mitochondrial dysfunction, and the production of reactive oxygen species, and lysosomal damage have been shown to trigger its activation. How NLRP3 responds to those signaling events and initiates the assembly of the NLRP3 inflammasome is not fully understood. In this review, we summarize our current understanding of the mechanisms of NLRP3 inflammasome activation by multiple signaling events, and its regulation by post-translational modifications and interacting partners of NLRP3.

Activation of Pyroptotic Cell Death Pathways in Cancer: An Alternative Therapeutic Approach

Cancer can be considered the result of a series of genetic variations that lead to a normal cell being transformed into a malignant one while avoiding cell death-atypical characteristics of tumor development. Although a large number of genomics and epigenetic alterations have been identified in cells undergoing apoptotic, autophagic or necrotic cell death, the treatment of cancer remains thought-provoking. Pyroptosis is differentiated from other types of programmed cell death and is mainly activated by Caspase-1. To initiate pyroptosis, cells receive specific "death" messages, produce cytokines, swell, burst, and ultimately die. The deficiency of Caspase-1 expression may lead to inflammation-mediated tumor progression. Hence, the molecular mechanisms for the Caspase-1 activation in tumor tissues are yet to be exploited extensively. This review aims to summarise the latest discoveries about pyroptosis and its new exciting role in inducing cancer cell death.

Human GBP1 is a microbe-specific gatekeeper of macrophage apoptosis and pyroptosis

The guanylate binding protein (GBP) family of interferon-inducible GTPases promotes antimicrobial immunity and cell death. During bacterial infection, multiple mouse Gbps, human GBP2, and GBP5 support the activation of caspase-1-containing inflammasome complexes or caspase-4 which trigger pyroptosis. Whether GBPs regulate other forms of cell death is not known. The apicomplexan parasite Toxoplasma gondii causes macrophage death through unidentified mechanisms. Here we report that Toxoplasma-induced death of human macrophages requires GBP1 and its ability to target Toxoplasma parasitophorous vacuoles through its GTPase activity and prenylation. Mechanistically, GBP1 promoted Toxoplasma detection by AIM2, which induced GSDMD-independent, ASC-, and caspase-8-dependent apoptosis. Identical molecular determinants targeted GBP1 to Salmonella-containing vacuoles. GBP1 facilitated caspase-4 recruitment to Salmonella leading to its enhanced activation and pyroptosis. Notably, GBP1 could be bypassed by the delivery of Toxoplasma DNA or bacterial LPS into the cytosol, pointing to its role in liberating microbial molecules. GBP1 thus acts as a gatekeeper of cell death pathways, which respond specifically to infecting microbes. Our findings expand the immune roles of human GBPs in regulating not only pyroptosis, but also apoptosis.

Non-canonical NLRP3 inflammasome activation and IL-1β signaling are necessary to L. amazonensis control mediated by P2X7 receptor and leukotriene B4

Leishmaniasis is a neglected tropical disease affecting millions of individuals worldwide. P2X7 receptor has been linked to the elimination of Leishmania amazonensis. Biological responses evoked by P2X7 receptor activation have been well-documented, including apoptosis, phagocytosis, cytokine release, such as IL-1β. It was demonstrated that NLRP3 inflammasome activation and IL-1β signaling participated in resistance against L. amazonensis. Furthermore, our group has shown that L. amazonensis elimination through P2X7 receptor activation depended on leukotriene B₄ (LTB₄) production and release. Therefore, we investigated whether L. amazonensis elimination by P2X7 receptor and LTB₄ involved NLRP3 inflammasome activation and IL-1β signaling. We showed that macrophages from NLRP3^-/-, ASC^-/-, Casp-1/11^-/-, gp91^{phox-/- ,} and IL-1R^-/- mice treated with ATP or LTB₄ did not decrease parasitic load as was observed in WT mice. When ASC^-/- macrophages were treated with exogenous IL-1β, parasite killing was noted, however, we did not see parasitic load reduction in IL-1R^-/- macrophages. Similarly, macrophages from P2X7 receptor-deficient mice treated with IL-1β also showed decreased parasitic load. In addition, when we infected Casp-11^-/- macrophages, neither ATP nor LTB₄ were able to reduce parasitic load, and Casp-11^-/- mice were more susceptible to L. amazonensis infection than were WT mice. Furthermore, P2X7^-/-L. amazonensis-infected mice locally treated with exogenous LTB₄ showed resistance to infection, characterized by lower parasite load and smaller lesions compared to untreated P2X7^-/- mice. A similar observation was noted when infected P2X7^-/- mice were treated with IL-1β, i.e., lower parasite load and smaller lesions compared to P2X7^-/- mice. These data suggested that L. amazonensis elimination mediated by P2X7 receptor and LTB₄ was dependent on non-canonical NLRP3 inflammasome activation, ROS production, and IL-1β signaling.

Leishmania spp. is a protozoan parasite that infects human and causes several diseases. Leishmania amazonensis causes cutaneous leishmaniasis (CL) and mucocutaneous leishmaniasis (MCL). Leishmania parasites preferentially infect macrophages. In macrophages, several mechanisms have been described as controlling L. amazonensis infection. Here, we showed that P2X7 receptor and LTB₄ eliminated L. amazonensis in macrophages by a pathway dependent on non-canonical NLRP3 inflammasome activation and IL-1β 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.

Caspase-11 promotes renal fibrosis by stimulating IL-1β maturation via activating caspase-1

Caspase-11 is a key upstream modulator for activation of inflammatory response under pathological conditions. In this study, we investigated the roles of caspase-11 in the maturation of interleukin-1β (IL-1β) and development of renal interstitial fibrosis in vivo and in vitro. Mice were subjected to unilateral ureteral obstruction (UUO). The mice were treated with either caspase-11 inhibitor wedelolactone (Wed, 30 mg/kg/day, ig) for 7 days or caspase-11 siRNA (10 nmol/20 g body weight per day, iv) for 14 days. The mice were euthanized on day 14, their renal tissue and blood sample were collected. We found that the obstructed kidney had significantly higher caspase-11 levels and obvious tubular injury and interstitial fibrosis. Treatment with Wed or caspase-11 siRNA significantly mitigated renal fibrosis in UUO mice, evidenced by the improved histological changes. Furthermore, caspase-11 inhibition significantly blunted caspase-1 activation, IL-1β maturation, transforming growth factor-β (TGF-β), fibronectin, and collagen I expressions in the obstructed kidney. Renal tubular epithelial NRK-52E cells were treated in vitro with angiotensin (Ang, 1 μmol/L), which stimulated caspase-11 activation and IL-1β maturation. Treatment with IL-1β (20 ng/ml) significantly increased the expression of TGF-β, fibronectin, and collagen I in the cells. Ang II-induced expression of TGF-β, fibronectin, and collagen I were suppressed by caspase-11 siRNA or Wed. Finally, we revealed using co-immunoprecipitation that caspase-11 was able to interact with caspase-1 in NRK-52E cells. These results suggest that caspase-11 is involved in UUO-induced renal fibrosis. Elevation of caspase-11 in the obstructed kidney promotes renal fibrosis by stimulating caspase-1 activation and IL-1β maturation.

The cleavage of gasdermin D by caspase-11 promotes tubular epithelial cell pyroptosis and urinary IL-18 excretion in acute kidney injury

Inflammation and tubular cell death are the hallmarks of acute kidney injury. However, the precise mechanism underlying these effects has not been fully elucidated. Here we tested whether caspase-11, an inflammatory member of the caspase family, was increased in cisplatin or ischemia-reperfusion-induced acute kidney injury. Caspase-11 knockout mice after cisplatin treatment exhibited attenuated deterioration of renal functional, reduced tubular damage, reduced macrophage and neutrophil infiltration, and decreased urinary IL-18 excretion. Mechanistically, the upregulation of caspase-11 by either cisplatin or ischemia-reperfusion cleaved gasdermin D (GSDMD) into GSDMD-N, which translocated onto the plasma membrane, thus triggering cell pyroptosis and facilitated IL-18 release in primary cultured renal tubular cells. These results were further confirmed in GSDMD knockout mice that cisplatin-induced renal morphological and functional deterioration as well as urinary IL-18 excretion were alleviated. Furthermore, deficiency of GSDMD significantly suppressed cisplatin-induced IL-18 release but not the transcription and maturation level of IL-18 in tubular cells. Thus, our study indicates that caspase-11/GSDMD dependent tubule cell pyroptosis plays a significant role in initiating tubular cell damage, urinary IL-18 excretion and renal functional deterioration in acute kidney injury.

Innate immunity to intracellular LPS

Monitoring of the cytosolic compartment by the innate immune system for pathogen-encoded products or pathogen activities often enables the activation of a subset of caspases. In most cases, the cytosolic surveillance pathways are coupled to activation of caspase-1 via canonical inflammasome complexes. A related set of caspases, caspase-11 in rodents and caspase-4 and caspase-5 in humans, monitors the cytosol for bacterial lipopolysaccharide (LPS). Direct activation of caspase-11, caspase-4 and caspase-5 by intracellular LPS elicits the lytic cell death called 'pyroptosis', which occurs in multiple cell types. The pyroptosis is executed by the pore-forming protein GSDMD, which is activated by cleavage mediated by caspase-11, caspase-4 or caspase-5. In monocytes, formation of GSDMD pores can induce activation of the NLRP3 inflammasome for maturation of the cytokines IL-1β and IL-18. Caspase-11-mediated pyroptosis in response to cytosolic LPS is critical for antibacterial defense and septic shock. Here we review the emerging literature on the sensing of cytosolic LPS and its regulation and pathophysiological functions.

Inner sensors of endotoxin - implications for sepsis research and therapy

Despite great efforts and numerous clinical trials, there is still a major need for effective therapies for sepsis. Neutralization or elimination of bacterial toxins remains a promising approach. The understanding of the interaction of the endotoxin (lipopolysaccharide, LPS) of Gram-negative bacteria with its cellular receptor, namely the CD14/TLR4/MD2 complex, was a major breakthrough. Unfortunately, clinical trials for sepsis on the neutralization of LPS or on the inhibition of TLR4 signaling failed whereas those on LPS removal remain controversial. Recent discoveries of another class of LPS receptors localized within the cytoplasm, namely caspase-11 in mice and caspases-4/5 in humans, have renewed interest in the field. These provide new potential targets for intervention in sepsis pathogenesis. Since cytoplasmic recognition of LPS induces non-canonical inflammasome pathway, a potentially harmful host response, it is conceivable to therapeutically target this mechanism. However, a great deal of care should be used in the translation of research on the non-canonical inflammasome inhibition due to multiple inter-species differences. In this review, we summarize the knowledge on endotoxin sensing in sepsis with special focus on the intracellular sensing. We also highlight the murine versus human differences and discuss potential therapeutic approaches addressing the newly discovered pathways.

Caspase-11, a specific sensor for intracellular lipopolysaccharide recognition, mediates the non-canonical inflammatory pathway of pyroptosis

Pyroptosis, a type of programmed cell death that along with inflammation, is mainly regulated by two main pathways, cysteinyl aspartate specific proteinase (caspase)-1-induced canonical inflammatory pathway and caspase-11-induced non-canonical inflammatory pathway. The non-canonical inflammatory pathway-induced pyroptosis is a unique immune response in response to gram-negative (G⁻) bacteria. It is induced by lipopolysaccharide (LPS) on the surface of G⁻ bacteria. This activates caspase-11 which, in turn, activates a series of downstream proteins eventually forming protein pores on the cell membrane and inducing cell sacrificial processes. Caspase-11 belongs to the caspase family and is an homologous protein of caspase-1. It has the ability to specifically hydrolyze proteins, but it is still unclear how it regulates cell death caused by non-canonical inflammatory pathways. The present study describes a pathway that enables LPS to directly enter the cell and activate caspase-11, and the key role caspase-11 plays in the activation of pyroptosis and inflammation.

Autophagy and Noroviruses

Autophagy is an essential cellular process by which a cell degrades materials within its cytoplasm. Intracellular pathogens like viruses must deal with autophagy, either positively or negatively, for their own survival and replication. For some viruses, autophagy can even play proviral roles, helping their replication or dissemination. For other viruses, including noroviruses, the exact role of autophagy is more complex. This short review seeks to summarize the known interactions between autophagy, autophagy proteins and norovirus, and to address remaining questions relevant to these interactions.

Interferon-λ3 Promotes Epithelial Defense and Barrier Function Against Cryptosporidium parvum Infection

BACKGROUND & AIMS

The epithelial response is critical for intestinal defense against Cryptosporidium, but is poorly understood. To uncover the host strategy for defense against Cryptosporidium, we examined the transcriptional response of intestinal epithelial cells (IECs) to C parvum in experimentally infected piglets by microarray. Up-regulated genes were dominated by targets of interferon (IFN) and IFN-λ3 was up-regulated significantly in infected piglet mucosa. Although IFN-λ has been described as a mediator of epithelial defense against viral pathogens, there is limited knowledge of any role against nonviral pathogens. Accordingly, the aim of the study was to determine the significance of IFN-λ3 to epithelial defense and barrier function during C parvum infection.

METHODS

The significance of C parvum-induced IFN-λ3 expression was determined using an immunoneutralization approach in neonatal C57BL/6 mice. The ability of the intestinal epithelium to up-regulate IFN-λ2/3 expression in response to C parvum infection and the influence of IFN-λ2/3 on epithelial defense against C parvum invasion, intracellular development, and loss of barrier function was examined using polarized monolayers of a nontransformed porcine-derived small intestinal epithelial cell line (IPEC-J2). Specifically, changes in barrier function were quantified by measurement of transepithelial electrical resistance and transepithelial flux studies.

RESULTS

Immunoneutralization of IFN-λ2/3 in C parvum-infected neonatal mice resulted in a significantly increased parasite burden, fecal shedding, and villus blunting with crypt hyperplasia during peak infection. In vitro, C parvum was sufficient to induce autonomous IFN-λ3 and interferon-stimulated gene 15 expression by IECs. Priming of IECs with recombinant human IFN-λ3 promoted cellular defense against C parvum infection and abrogated C parvum-induced loss of barrier function by decreasing paracellular permeability to sodium.

CONCLUSIONS

These studies identify IFN-λ3 as a key epithelial defense mechanism against C parvum infection.

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

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.

Salmonella Single-Cell Metabolism and Stress Responses in Complex Host Tissues

Systemic Salmonella enterica infections are a major cause of mortality worldwide and are becoming increasingly untreatable. Recent single-cell data from a mouse model of typhoid fever show that the host immune system actually eradicates many Salmonella cells, while other Salmonella organisms thrive at the same time in the same tissue, causing lethal disease progression. The surviving Salmonella cells have highly heterogeneous metabolism, growth rates, and exposure to various stresses. Emerging evidence suggests that similarly heterogeneous host-pathogen encounters might be a key feature of many infectious diseases. This heterogeneity offers fascinating opportunities for research and application. If we understand the mechanisms that determine the disparate local outcomes, we might be able to develop entirely novel strategies for infection control by broadening successful host antimicrobial attacks and closing permissive niches in which pathogens can thrive. This review describes suitable technologies, a current working model of heterogeneous host-Salmonella interactions, the impact of diverse Salmonella subsets on antimicrobial chemotherapy, and major open questions and challenges.

Recognition of Intracellular Bacteria by Inflammasomes

Inflammasomes are multiprotein signaling complexes that are assembled by cytosolic sensors upon the detection of infectious or noxious stimuli. These complexes activate inflammatory caspases to induce host cell death and cytokine secretion and are an essential part of antimicrobial host defense. In this review, I discuss how intracellular bacteria are detected by inflammasomes, how the specific sensing mechanism of each inflammasome receptor restricts the ability of bacteria to evade immune recognition, and how host cell death is used to control bacterial replication in vivo.

Differential role of the NLRP3 inflammasome in infection and tumorigenesis

Dysregulated inflammation is one of the hallmarks of cancer initiation and progression. Emerging evidence indicates that inflammasomes play a central role in regulating immune cell functions in various infections and cancer. Inflammasomes are multimeric complexes consisting of nucleotide-binding oligomerization domain (NOD) -like receptors (NLRs). Among the NLRs, NOD1, NOD2 and NLRP3 respond to a variety of endogenous (i.e. damage-associated molecular patterns) and exogenous (i.e. pathogen-associated molecular patterns) stimuli. The NLRP3 inflammasome is associated with the onset and progression of autoinflammatory and autoimmune diseases, including metabolic disorders, multiple sclerosis, inflammatory bowel disease, and cryopyrin-associated periodic fever syndrome. NLRP3 is also associated with a wide variety of infections and tumorigenesis that are closely correlated with chemotherapy response and prognosis. In this review, we explore the rapidly expanding body of research on the expression and functions of NLRP3 in infections and cancers and outline novel inhibitors targeting the NLRP3 inflammasome that could be developed as therapeutic alternatives to current anticancer treatment.

Toll-Like Receptor 4 Signaling Licenses the Cytosolic Transport of Lipopolysaccharide From Bacterial Outer Membrane Vesicles

Outer membrane vesicles (OMVs), released by variety of bacteria, are membrane-enclosed entities enriched in microbial components, toxins, and virulent factors. OMVs could deliver lipopolysaccharide (LPS) into the cytosol of host cells and subsequently activate caspase-11, which critically orchestrates immune responses and mediates septic shock. Although it is known that caspase-11 is activated by intracellular LPS, how OMVs deliver LPS into the cytosol remains largely unknown. Here we show that the activation of toll-like receptor 4 (TLR4), a LPS receptor on the cytoplasmic membrane, licenses macrophages to transport LPS from OMVs into the cytosol through TIR domain-containing adaptor-inducing interferon-β (TRIF). TRIF-mediated cytosolic delivery of LPS from OMVs depends on the production of type 1 interferon and the expression of guanylate-binding proteins (GBPs). Deletion of TRIF or GBPs prevents pyroptosis and lethality induced by OMVs or OMVs-releasing Escherichia coli. Together, these findings provide novel insight into how host coordinates extracellular and intracellular LPS sensing to orchestrate immune responses during gram-negative bacterial infection.

Caspase-4 disaggregates lipopolysaccharide micelles via LPS-CARD interaction

Lipopolysaccharides (LPS) are a major component of the outer membrane of Gram-negative bacteria and are pathogen-associated molecular patterns recognized by the TLR4/MD2 complex that induces an inflammatory response. Recently, the cytosolic receptors caspase-4/-5/-11 that bind LPS inside the cell and trigger inflammasome activation or pyroptosis, have been identified. Despite the important roles of caspase-4 in human immune responses, few studies have investigated its biochemical characteristics and interactions with LPS. Since caspase-4 (C258A) purified from an Escherichia coli host forms aggregates, monomeric proteins including full-length caspase-4, caspase-4 (C258A), and the CARD domain of caspase-4 have been purified from the insect cell system. Here, we report the overexpression and purification of monomeric caspase-4 (C258A) and CARD domain from E. coli and demonstrate that purified caspase-4 (C258A) and CARD domain bind large LPS micelles and disaggregate them to small complexes. As the molar ratio of caspase-4 to LPS increases, the size of the caspase-4/LPS complex decreases. Our results present a new function of caspase-4 and set the stage for structural and biochemical studies, and drug discovery targeting LPS/caspase-4 interactions by establishing a facile purification method to obtain large quantities of purified caspase-4 (C258A) and the CARD domain.