Shigella "Osp"pression of innate immunity.

Interferons are potent antimicrobial effectors and thus an attractive target for pathogen interference. In this issue of Cell, Alphonse et al. reveal that the Shigella effectors OspC1 and OspC3 employ a surprising mechanism to block interferon signaling and attenuate antibacterial responses, thus securing their replicative niche.

Intracellular immune sensing promotes inflammation via gasdermin D-driven release of a lectin alarmin.

Inflammatory caspase sensing of cytosolic lipopolysaccharide (LPS) triggers pyroptosis and the concurrent release of damage-associated molecular patterns (DAMPs). Collectively, DAMPs are key determinants that shape the aftermath of inflammatory cell death. However, the identity and function of the individual DAMPs released are poorly defined. Our proteomics study revealed that cytosolic LPS sensing triggered the release of galectin-1, a ß-galactoside-binding lectin. Galectin-1 release is a common feature of inflammatory cell death, including necroptosis. In vivo studies using galectin-1-deficient mice, recombinant galectin-1 and galectin-1-neutralizing antibody showed that galectin-1 promotes inflammation and plays a detrimental role in LPS-induced lethality. Mechanistically, galectin-1 inhibition of CD45 (Ptprc) underlies its unfavorable role in endotoxin shock. Finally, we found increased galectin-1 in sera from human patients with sepsis. Overall, we uncovered galectin-1 as a bona fide DAMP released as a consequence of cytosolic LPS sensing, identifying a new outcome of inflammatory cell death.

(IR)Factor for NAIP Expression.

IRF8 is a master transcription factor for immune cell development. In this issue, Karki et al. reveal that IRF8 governs the constitutive expression of genes encoding for NAIP proteins that are critical for the innate immune sensing of bacteria.

Bacterial Outer Membrane Vesicles Mediate Cytosolic Localization of LPS and Caspase-11 Activation.

Sensing of lipopolysaccharide (LPS) in the cytosol triggers caspase-11 activation and is central to host defense against Gram-negative bacterial infections and to the pathogenesis of sepsis. Most Gram-negative bacteria that activate caspase-11, however, are not cytosolic, and the mechanism by which LPS from these bacteria gains access to caspase-11 in the cytosol remains elusive. Here, we identify outer membrane vesicles (OMVs) produced by Gram-negative bacteria as a vehicle that delivers LPS into the cytosol triggering caspase-11-dependent effector responses in vitro and in vivo. OMVs are internalized via endocytosis, and LPS is released into the cytosol from early endosomes. The use of hypovesiculating bacterial mutants, compromised in their ability to generate OMVs, reveals the importance of OMVs in mediating the cytosolic localization of LPS. Collectively, these findings demonstrate a critical role for OMVs in enabling the cytosolic entry of LPS and, consequently, caspase-11 activation during Gram-negative bacterial infections.

Gasdermin D Restrains Type I Interferon Response to Cytosolic DNA by Disrupting Ionic Homeostasis.

Inflammasome-activated caspase-1 cleaves gasdermin D to unmask its pore-forming activity, the predominant consequence of which is pyroptosis. Here, we report an additional biological role for gasdermin D in limiting cytosolic DNA surveillance. Cytosolic DNA is sensed by Aim2 and cyclic GMP-AMP synthase (cGAS) leading to inflammasome and type I interferon responses, respectively. We found that gasdermin D activated by the Aim2 inflammasome suppressed cGAS-driven type I interferon response to cytosolic DNA and Francisella novicida in macrophages. Similarly, interferon-ß (IFN-ß) response to F. novicida infection was elevated in gasdermin D-deficient mice. Gasdermin D-mediated negative regulation of IFN-ß occurred in a pyroptosis-, interleukin-1 (IL-1)-, and IL-18-independent manner. Mechanistically, gasdermin D depleted intracellular potassium (K+) via membrane pores, and this K+ efflux was necessary and sufficient to inhibit cGAS-dependent IFN-ß response. Thus, our findings have uncovered an additional interferon regulatory module involving gasdermin D and K+ efflux.

TRIF licenses caspase-11-dependent NLRP3 inflammasome activation by gram-negative bacteria.

Systemic infections with Gram-negative bacteria are characterized by high mortality rates due to the "sepsis syndrome," a widespread and uncontrolled inflammatory response. Though it is well recognized that the immune response during Gram-negative bacterial infection is initiated after the recognition of endotoxin by Toll-like receptor 4, the molecular mechanisms underlying the detrimental inflammatory response during Gram-negative bacteremia remain poorly defined. Here, we identify a TRIF pathway that licenses NLRP3 inflammasome activation by all Gram-negative bacteria. By engaging TRIF, Gram-negative bacteria activate caspase-11. TRIF activates caspase-11 via type I IFN signaling, an event that is both necessary and sufficient for caspase-11 induction and autoactivation. Caspase-11 subsequently synergizes with the assembled NLRP3 inflammasome to regulate caspase-1 activation and leads to caspase-1-independent cell death. These events occur specifically during infection with Gram-negative, but not Gram-positive, bacteria. The identification of TRIF as a regulator of caspase-11 underscores the importance of TLRs as master regulators of inflammasomes during Gram-negative bacterial infection.

Regulation of inflammasome signaling.

Innate immune responses have the ability to both combat infectious microbes and drive pathological inflammation. Inflammasome complexes are a central component of these processes through their regulation of interleukin 1ß (IL-1ß), IL-18 and pyroptosis. Inflammasomes recognize microbial products or endogenous molecules released from damaged or dying cells both through direct binding of ligands and indirect mechanisms. The potential of the IL-1 family of cytokines to cause tissue damage and chronic inflammation emphasizes the importance of regulating inflammasomes. Many regulatory mechanisms have been identified that act as checkpoints for attenuating inflammasome signaling at multiple steps. Here we discuss the various regulatory mechanisms that have evolved to keep inflammasome signaling in check to maintain immunological balance.

The AIM2 inflammasome is essential for host defense against cytosolic bacteria and DNA viruses.

Inflammasomes regulate the activity of caspase-1 and the maturation of interleukin 1beta (IL-1beta) and IL-18. AIM2 has been shown to bind DNA and engage the caspase-1-activating adaptor protein ASC to form a caspase-1-activating inflammasome. Using Aim2-deficient mice, we identify a central role for AIM2 in regulating caspase-1-dependent maturation of IL-1beta and IL-18, as well as pyroptosis, in response to synthetic double-stranded DNA. AIM2 was essential for inflammasome activation in response to Francisella tularensis, vaccinia virus and mouse cytomegalovirus and had a partial role in the sensing of Listeria monocytogenes. Moreover, production of IL-18 and natural killer cell-dependent production of interferon-gamma, events critical in the early control of virus replication, were dependent on AIM2 during mouse cytomegalovirus infection in vivo. Collectively, our observations demonstrate the importance of AIM2 in the sensing of both bacterial and viral pathogens and in triggering innate immunity.

Long Noncoding RNAs in Host-Pathogen Interactions.

Long noncoding RNAs (lncRNAs) are key molecules that regulate gene expression in a variety of organisms. LncRNAs can drive different transcriptional and post-transcriptional events that impact cellular functions. Recent studies have identified many lncRNAs associated with immune cell development and activation; however, an understanding of their functional role in host immunity to infection is just emerging. Here, we provide a detailed and updated review of the functional roles of lncRNAs in regulating mammalian immune responses during host-pathogen interactions, because these functions may be either beneficial or detrimental to the host. With increased mechanistic insight into the roles of lncRNAs, it may be possible to design and/or improve lncRNA-based therapies to treat a variety of infectious and inflammatory diseases.

The NLRP12 inflammasome recognizes Yersinia pestis.

Yersinia pestis, the causative agent of plague, is able to suppress production of inflammatory cytokines IL-18 and IL-1ß, which are generated through caspase-1-activating nucleotide-binding domain and leucine-rich repeat (NLR)-containing inflammasomes. Here, we sought to elucidate the role of NLRs and IL-18 during plague. Lack of IL-18 signaling led to increased susceptibility to Y. pestis, producing tetra-acylated lipid A, and an attenuated strain producing a Y. pseudotuberculosis-like hexa-acylated lipid A. We found that the NLRP12 inflammasome was an important regulator controlling IL-18 and IL-1ß production after Y. pestis infection, and NLRP12-deficient mice were more susceptible to bacterial challenge. NLRP12 also directed interferon-γ production via induction of IL-18, but had minimal effect on signaling to the transcription factor NF-κB. These studies reveal a role for NLRP12 in host resistance against pathogens. Minimizing NLRP12 inflammasome activation may have been a central factor in evolution of the high virulence of Y. pestis.

Actin pedestal formation by enterohemorrhagic Escherichia coli enhances bacterial host cell attachment and concomitant type III translocation.

Attachment of enterohemorrhagic Escherichia coli (EHEC) to intestinal epithelial cells is critical for colonization and is associated with localized actin assembly beneath bound bacteria. The formation of these actin "pedestals" is dependent on the translocation of effectors into mammalian cells via a type III secretion system (T3SS). Tir, an effector required for pedestal formation, localizes in the host cell plasma membrane and promotes attachment of bacteria to mammalian cells by binding to the EHEC outer surface protein Intimin. Actin pedestal formation has been shown to foster intestinal colonization by EHEC in some animal models, but the mechanisms responsible for this remain undefined. Investigation of the role of Tir-mediated actin assembly promoting host cell binding is complicated by other, potentially redundant EHEC-encoded binding pathways, so we utilized cell binding assays that specifically detect binding mediated by Tir-Intimin interaction. We also assessed the role of Tir-mediated actin assembly in two-step assays that temporally segregated initial translocation of Tir from subsequent Tir-Intimin interaction, thereby permitting the distinction of effects on translocation from effects on cell attachment. In these experimental systems, we compromised Tir-mediated actin assembly by chemically inhibiting actin assembly or by infecting mammalian cells with EHEC mutants that translocate Tir but are specifically defective in Tir-mediated pedestal formation. We found that an inability of Tir to promote actin assembly resulted in a significant and striking decrease in bacterial binding mediated by Tir and Intimin. Bacterial mutants defective for pedestal formation translocated type III effectors to mammalian cells with reduced efficiency, but the decrease in translocation could be entirely accounted for by the decrease in host cell attachment.

A bacterial toxin co-opts caspase-3 to disable active gasdermin D and limit macrophage pyroptosis.

During infections, host cells are exposed to pathogen-associated molecular patterns (PAMPs) and virulence factors that stimulate multiple signaling pathways that interact additively, synergistically, or antagonistically. The net effect of such higher-order interactions is a vital determinant of the outcome of host-pathogen interactions. Here, we demonstrate one such complex interplay between bacterial exotoxin- and PAMP-induced innate immune pathways. We show that two caspases activated during enterohemorrhagic Escherichia coli (EHEC) infection by lipopolysaccharide (LPS) and Shiga toxin (Stx) interact in a functionally antagonistic manner; cytosolic LPS-activated caspase-11 cleaves full-length gasdermin D (GSDMD), generating an active pore-forming N-terminal fragment (NT-GSDMD); subsequently, caspase-3 activated by EHEC Stx cleaves the caspase-11-generated NT-GSDMD to render it nonfunctional, thereby inhibiting pyroptosis and interleukin-1ß maturation. Bacteria typically subvert inflammasomes by targeting upstream components such as NLR sensors or full-length GSDMD but not active NT-GSDMD. Thus, our findings uncover a distinct immune evasion strategy where a bacterial toxin disables active NT-GSDMD by co-opting caspase-3.

UBXN9 governs GLUT4-mediated spatial confinement of RIG-I-like receptors and signaling.

The cytoplasmic RIG-I-like receptors (RLRs) recognize viral RNA and initiate innate antiviral immunity. RLR signaling also triggers glycolytic reprogramming through glucose transporters (GLUTs), whose role in antiviral immunity is elusive. Here, we unveil that insulin-responsive GLUT4 inhibits RLR signaling independently of glucose uptake in adipose and muscle tissues. At steady state, GLUT4 is docked at the Golgi matrix by ubiquitin regulatory X domain 9 (UBXN9, TUG). Following RNA virus infection, GLUT4 is released and translocated to the cell surface where it spatially segregates a significant pool of cytosolic RLRs, preventing them from activating IFN-ß responses. UBXN9 deletion prompts constitutive GLUT4 trafficking, sequestration of RLRs, and attenuation of antiviral immunity, whereas GLUT4 deletion heightens RLR signaling. Notably, reduced GLUT4 expression is uniquely associated with human inflammatory myopathies characterized by hyperactive interferon responses. Overall, our results demonstrate a noncanonical UBXN9-GLUT4 axis that controls antiviral immunity via plasma membrane tethering of cytosolic RLRs.

A bacterial autotransporter impairs innate immune responses by targeting the transcription factor TFE3.

Type I interferons (IFNs) are consequential cytokines in antibacterial defense. Whether and how bacterial pathogens inhibit innate immune receptor-driven type I IFN expression remains mostly unknown. By screening a library of enterohemorrhagic Escherichia coli (EHEC) mutants, we uncovered EhaF, an uncharacterized protein, as an inhibitor of innate immune responses including IFNs. Further analyses identified EhaF as a secreted autotransporter-a type of bacterial secretion system with no known innate immune-modulatory function-that translocates into host cell cytosol and inhibit IFN response to EHEC. Mechanistically, EhaF interacts with and inhibits the MiT/TFE family transcription factor TFE3 resulting in impaired TANK phosphorylation and consequently, reduced IRF3 activation and type I IFN expression. Notably, EhaF-mediated innate immune suppression promotes EHEC colonization and pathogenesis in vivo. Overall, this study has uncovered a previously unknown autotransporter-based bacterial strategy that targets a specific transcription factor to subvert innate host defense.

Host extracellular vesicles confer cytosolic access to systemic LPS licensing non-canonical inflammasome sensing and pyroptosis.

Intracellular surveillance for systemic microbial components during homeostasis and infections governs host physiology and immunity. However, a long-standing question is how circulating microbial ligands become accessible to intracellular receptors. Here we show a role for host-derived extracellular vesicles (EVs) in this process; human and murine plasma-derived and cell culture-derived EVs have an intrinsic capacity to bind bacterial lipopolysaccharide (LPS). Remarkably, circulating host EVs capture blood-borne LPS in vivo, and the LPS-laden EVs confer cytosolic access for LPS, triggering non-canonical inflammasome activation of gasdermin D and pyroptosis. Mechanistically, the interaction between the lipid bilayer of EVs and the lipid A of LPS underlies EV capture of LPS, and the intracellular transfer of LPS by EVs is mediated by CD14. Overall, this study demonstrates that EVs capture and escort systemic LPS to the cytosol licensing inflammasome responses, uncovering EVs as a previously unrecognized link between systemic microbial ligands and intracellular surveillance.

Citrobacter rodentium(ϕStx2dact), a murine infection model for enterohemorrhagic Escherichia coli.

The formation of attaching and effacing (A/E) lesions on intestinal epithelium, combined with Shiga toxin production, are hallmarks of enterohemorrhagic Escherichia coli (EHEC) infection that can lead to lethal hemolytic uremic syndrome. Although an animal infection model that fully recapitulates human disease remains elusive, mice orally infected with Citrobacter rodentium(ϕStx2dact), a natural murine pathogen lysogenized with an EHEC-derived Shiga toxin 2-producing bacteriophage, develop intestinal A/E lesions and toxin-dependent systemic disease. This model has facilitated investigation of how: (A) phage gene expression and prophage induction contribute to disease and are potentially triggered by antibiotic treatment; (B) virulence gene expression is altered by microbiota and the colonic metabolomic milieu; and (C) innate immune signaling is affected by Stx. Thus, the model provides a unique tool for accessing diverse aspects of EHEC pathogenesis.

A TLR4-independent critical role for CD14 in intracellular LPS sensing.

Intracellular lipopolysaccharide (LPS) sensing by the noncanonical inflammasome comprising caspase-4 or -11 governs antibacterial host defense. How LPS gains intracellular access in vivo is largely unknown. Here, we show that CD14-an LPS-binding protein with a well-documented role in TLR4 activation-plays a vital role in intracellular LPS sensing in vivo. By generating Cd14-/- and Casp11-/- mice strains on a Tlr4-/- background, we dissociate CD14's known role in TLR4 signaling from its role in caspase-11 activation and show a TLR4-independent role for CD14 in GSDMD activation, pyroptosis, alarmin release, and the lethality driven by cytosolic LPS. Mechanistically, CD14 enables caspase-11 activation by mediating cytosolic localization of LPS in a TLR4-independent manner. Overall, our findings attribute a critical role for CD14 in noncanonical inflammasome sensing of LPS in vivo and establish-together with previous literature-CD14 as an essential proximal component of both TLR4-based extracellular and caspase-11-based intracellular LPS surveillance.

Inflammasome activation and evasion by bacterial pathogens.

Innate immune system plays an essential role in combating infectious diseases by recognizing invading pathogens and activating host defense response. Inflammasomes complexes are a central component of the cytosolic innate immune surveillance and are vital in host defense against bacterial pathogens. Bacterial products or pathogen-induced modifications in the intracellular environment are sensed by the inflammasome receptors that form complexes that serve as a platform for caspase-1-dependent or caspase-11-dependent induction of pyroptosis and secretion of cytokines, IL-1ß and IL-18. However, several pathogenic bacteria have developed strategies to evade inflammasome activation. This review highlights the recent advances in the mechanism of inflammasome activation by bacterial pathogens and some of the bacterial evasion strategies of inflammasome activation.

Differential expression of virulence and stress fitness genes between Escherichia coli O157:H7 strains with clinical or bovine-biased genotypes.

Escherichia coli O157:H7 strains can be classified into different genotypes based on the presence of specific Shiga toxin-encoding bacteriophage insertion sites. Certain O157:H7 genotypes predominate among human clinical cases (clinical genotypes), while others are more frequently found in bovines (bovine-biased genotypes). To determine whether inherent differences in gene expression explain the variation in infectivity of these genotypes, we compared the expression patterns of clinical genotype 1 strains with those of bovine-biased genotype 5 strains using microarrays. Important O157:H7 virulence factors, including locus of enterocyte effacement genes, the enterohemolysin, and several pO157 genes, showed increased expression in the clinical versus bovine-biased genotypes. In contrast, genes essential for acid resistance (e.g., gadA, gadB, and gadC) and stress fitness were upregulated in bovine-biased genotype 5 strains. Increased expression of acid resistance genes was confirmed functionally using a model stomach assay, in which strains of bovine-biased genotype 5 had a 2-fold-higher survival rate than strains of clinical genotype 1. Overall, these results suggest that the increased prevalence of O157:H7 illness caused by clinical genotype 1 strains is due in part to the overexpression of key virulence genes. The bovine-biased genotype 5 strains, however, are more resistant to adverse environmental conditions, a characteristic that likely facilitates O157:H7 colonization of bovines.

Shiga toxin suppresses noncanonical inflammasome responses to cytosolic LPS.

Inflammatory caspase-dependent cytosolic lipopolysaccharide (LPS) sensing is a critical arm of host defense against bacteria. How pathogens overcome this pathway to establish infections is largely unknown. Enterohemorrhagic Escherichia coli (EHEC) is a clinically important human pathogen causing hemorrhagic colitis and hemolytic uremic syndrome. We found that a bacteriophage-encoded virulence factor of EHEC, Shiga toxin (Stx), suppresses caspase-11-mediated activation of the cytosolic LPS sensing pathway. Stx was essential and sufficient to inhibit pyroptosis and interleukin-1 (IL-1) responses elicited specifically by cytosolic LPS. The catalytic activity of Stx was necessary for suppression of inflammasome responses. Stx impairment of inflammasome responses to cytosolic LPS occurs at the level of gasdermin D activation. Stx also suppresses inflammasome responses in vivo after LPS challenge and bacterial infection. Overall, this study assigns a previously undescribed inflammasome-subversive function to a well-known bacterial toxin, Stx, and reveals a new phage protein-based pathogen blockade of cytosolic immune surveillance.