Nonhematopoietic cells control the outcome of infection with Listeria monocytogenes in a nucleotide oligomerization domain 1-dependent manner

Synthesis and validation of click-modified NOD1/2 agonists

NOD1 and NOD2 sense small bacterial peptidoglycan fragments, often called muropeptides, that access the cytosol. These muropeptides include iE-DAP and MDP, the minimal agonists for NOD1 and NOD2, respectively. Here, we synthesized and validated alkyne-modified muropeptides, iE-DAP-Alk and MDP-Alk, for use in click-chemistry reactions. While it has long been known that many cell types respond to extracellular exposure to muropeptides, it is unclear how these innate immune activators access their cytosolic innate immune receptors, NOD1 and NOD2. The subcellular trafficking and transport mechanisms by which muropeptides access these cytosolic innate immune receptors are a major gap in our understanding of these critical host responses. The click-chemistry-enabled agonists developed here will be particularly powerful to decipher the underlying cell biology and biochemistry of NOD1 and NOD2 innate immune sensing.

Discovery of a novel RIPK2 inhibitor for the treatment of inflammatory bowel disease

Nucleotide-binding oligomerization domain-containing protein 1 and 2 (NOD 1/2) are important cytosolic pattern recognition receptors that initiate host immune response. The dysregulation of NOD signaling is highly associated with inflammatory bowel disease (IBD) that needs novel treatment options. Receptor-interacting protein kinase 2 (RIPK2) is a critical mediator of NOD signaling and considered a promising therapeutic target for IBD treatment. However, there are currently no RIPK2 inhibitors available for clinical use. Here, we report the discovery and characterization of Zharp2-1 as a novel and potent RIPK2 inhibitor that effectively blocks RIPK2 kinase function and NOD-mediated NF-κB/MAPK activation in both human and mouse cell lines. Zharp2-1 exhibits significantly superior solubility compared to the non-prodrug form of the advanced RIPK2 inhibitor prodrug GSK2983559. The improved solubility combined with favorable in vitro metabolic stability translated to excellent in vivo pharmacokinetic profiles for Zharp2-1. In addition, Zharp2-1 demonstrates better effects than GSK2983559 in inhibiting the muramyl dipeptide (MDP)-induced production of pro-inflammatory cytokines in human peripheral blood mononuclear cells (PBMCs) and MDP-induced peritonitis in mice. Furthermore, Zharp2-1 markedly reduces Listeria monocytogenes infection-induced cytokines release in both human and mouse cells. Importantly, Zharp2-1 significantly ameliorates DNBS-induced colitis in rats and suppressed pro-inflammatory cytokine release in intestinal specimens from IBD patients. Collectively, our findings indicate that Zharp2-1 is a promising RIPK2 inhibitor with the potential to be further developed for IBD therapy.

The role of NOD-like receptors in innate immunity

The innate immune system in vertebrates and invertebrates relies on conserved receptors and ligands, and pathways that can rapidly initiate the host response against microbial infection and other sources of stress and danger. Research into the family of NOD-like receptors (NLRs) has blossomed over the past two decades, with much being learned about the ligands and conditions that stimulate the NLRs and the outcomes of NLR activation in cells and animals. The NLRs play key roles in diverse functions, ranging from transcription of MHC molecules to initiation of inflammation. Some NLRs are activated directly by their ligands, while other ligands may have indirect effects on the NLRs. New findings in coming years will undoubtedly shed more light on molecular details involved in NLR activation, as well as the physiological and immunological outcomes of NLR ligation.

IFN-γ production by brain-resident cells activates cerebral mRNA expression of a wide spectrum of molecules critical for both innate and T cell-mediated protective immunity to control reactivation of chronic infection with Toxoplasma gondii

We previously demonstrated that brain-resident cells produce IFN-γ in response to reactivation of cerebral infection with Toxoplasma gondii. To obtain an overall landscape view of the effects of IFN-γ from brain-resident cells on the cerebral protective immunity, in the present study we employed NanoString nCounter assay and quantified mRNA levels for 734 genes in myeloid immunity in the brains of T and B cell-deficient, bone marrow chimeric mice with and without IFN-γ production by brain-resident cells in response to reactivation of cerebral T. gondii infection. Our study revealed that IFN-γ produced by brain-resident cells amplified mRNA expression for the molecules to activate the protective innate immunity including 1) chemokines for recruitment of microglia and macrophages (CCL8 and CXCL12) and 2) the molecules for activating those phagocytes (IL-18, TLRs, NOD1, and CD40) for killing tachyzoites. Importantly, IFN-γ produced by brain-resident cells also upregulated cerebral expression of molecules for facilitating the protective T cell immunity, which include the molecules for 1) recruiting effector T cells (CXCL9, CXCL10, and CXCL11), 2) antigen processing (PA28αβ, LMP2, and LMP7), transporting the processed peptides (TAP1 and TAP2), assembling the transported peptides to the MHC class I molecules (Tapasin), and the MHC class I (H2-K1 and H2-D1) and Ib molecules (H2-Q1, H-2Q2, and H2-M3) for presenting antigens to activate the recruited CD8⁺ T cells, 3) MHC class II molecules (H2-Aa, H2-Ab1, H2-Eb1, H2-Ea-ps, H2-DMa, H2-Ob, and CD74) to present antigens for CD4⁺ T cell activation, 4) co-stimulatory molecules (ICOSL) for T cell activation, and 5) cytokines (IL-12, IL-15, and IL-18) facilitating IFN-γ production by NK and T cells. Notably, the present study also revealed that IFN-γ production by brain-resident cells also upregulates cerebral expressions of mRNA for the downregulatory molecules (IL-10, STAT3, SOCS1, CD274 [PD-L1], IL-27, and CD36), which can prevent overly stimulated IFN-γ-mediated pro-inflammatory responses and tissue damages. Thus, the present study uncovered the previously unrecognized the capability of IFN-γ production by brain-resident cells to upregulate expressions of a wide spectrum of molecules for coordinating both innate and T cell-mediated protective immunity with a fine-tuning regulation system to effectively control cerebral infection with T. gondii.

Exosomes derived from bone marrow mesenchymal stem cells protect the injured spinal cord by inhibiting pericyte pyroptosis

Mesenchymal stem cell (MSC) transplantation is a promising treatment strategy for spinal cord injury, but immunological rejection and possible tumor formation limit its application. The therapeutic effects of MSCs mainly depend on their release of soluble paracrine factors. Exosomes are essential for the secretion of these paracrine effectors. Bone marrow mesenchymal stem cell-derived exosomes (BMSC-EXOs) can be substituted for BMSCs in cell transplantation. However, the underlying mechanisms remain unclear. In this study, a rat model of T10 spinal cord injury was established using the impact method. Then, 30 minutes and 1 day after spinal cord injury, the rats were administered 200 μL exosomes via the tail vein (200 μg/mL; approximately 1 × 10⁶ BMSCs). Treatment with BMSC-EXOs greatly reduced neuronal cell death, improved myelin arrangement and reduced myelin loss, increased pericyte/endothelial cell coverage on the vascular wall, decreased blood-spinal cord barrier leakage, reduced caspase 1 expression, inhibited interleukin-1β release, and accelerated locomotor functional recovery in rats with spinal cord injury. In the cell culture experiment, pericytes were treated with interferon-γ and tumor necrosis factor-α. Then, Lipofectamine 3000 was used to deliver lipopolysaccharide into the cells, and the cells were co-incubated with adenosine triphosphate to simulate injury in vitro. Pre-treatment with BMSC-EXOs for 8 hours greatly reduced pericyte pyroptosis and increased pericyte survival rate. These findings suggest that BMSC-EXOs may protect pericytes by inhibiting pyroptosis and by improving blood-spinal cord barrier integrity, thereby promoting the survival of neurons and the extension of nerve fibers, and ultimately improving motor function in rats with spinal cord injury. All protocols were conducted with the approval of the Animal Ethics Committee of Zhengzhou University on March 16, 2019.

NOD1 and NOD2 in inflammatory and infectious diseases

It has been long recognized that NOD1 and NOD2 are critical players in the host immune response, primarily by their sensing bacterial peptidoglycan-conserved motifs. Significant advances have been made from efforts that characterize their upstream activators, assembly of signaling complexes, and activation of downstream signaling pathways. Disruption in NOD1 and NOD2 signaling has also been associated with impaired host defense and resistance to the development of inflammatory diseases. In this review, we will describe how NOD1 and NOD2 sense microbes and cellular stress to regulate host responses that can affect disease pathogenesis and outcomes.

NOD1 and NOD2 Activation by Diverse Stimuli: a Possible Role for Sensing Pathogen-Induced Endoplasmic Reticulum Stress

Prompt recognition of microbes by cells is critical to eliminate invading pathogens. Some cell-associated pattern recognition receptors (PRRs) recognize and respond to microbial ligands. However, others can respond to cellular perturbations, such as damage-associated molecular patterns (DAMPs). Nucleotide oligomerization domains 1 and 2 (NOD1/2) are PRRs that recognize and respond to multiple stimuli of microbial and cellular origin, such as bacterial peptidoglycan, viral infections, parasitic infections, activated Rho GTPases, and endoplasmic reticulum (ER) stress. How NOD1/2 are stimulated by such diverse stimuli is not fully understood but may partly rely on cellular changes during infection that result in ER stress. NOD1/2 are ER stress sensors that facilitate proinflammatory responses for pathogen clearance; thus, NOD1/2 may help mount broad antimicrobial responses through detection of ER stress, which is often induced during a variety of infections. Some pathogens may subvert this response to promote infection through manipulation of NOD1/2 responses to ER stress that lead to apoptosis. Here, we review NOD1/2 stimuli and cellular responses. Furthermore, we discuss pathogen-induced ER stress and how it might potentiate NOD1/2 signaling.

Pathogenic Biohacking: Induction, Modulation and Subversion of Host Transcriptional Responses by Listeria monocytogenes

During infection, the foodborne bacterial pathogen Listeria monocytogenes dynamically influences the gene expression profile of host cells. Infection-induced transcriptional changes are a typical feature of the host-response to bacteria and contribute to the activation of protective genes such as inflammatory cytokines. However, by using specialized virulence factors, bacterial pathogens can target signaling pathways, transcription factors, and epigenetic mechanisms to alter host gene expression, thereby reprogramming the response to infection. Therefore, the transcriptional profile that is established in the host is delicately balanced between antibacterial responses and pathogenesis, where any change in host gene expression might significantly influence the outcome of infection. In this review, we discuss the known transcriptional and epigenetic processes that are engaged during Listeria monocytogenes infection, the virulence factors that can remodel them, and the impact these processes have on the outcome of infection.

Immunity Against Bacterial Infection of the Central Nervous System: An Astrocyte Perspective

Bacterial infection of the central nervous system (CNS) is a severe and life-threatening condition with high mortality, and it may lead to permanent neurological deficits in survivors. Increasing evidence indicates that astrocytes, as the most abundant CNS glial cell population, regulate innate and adaptive immune responses in the CNS under pathological conditions in addition to their role in the maintenance of CNS homeostasis and neuronal function. Following antigen recognition, astrocytes participate in the initiation of innate immune responses, and prompt an adaptive immune response to recruit peripheral immune cells. Investigations have been conducted to understand the immunological role of astrocytes in CNS disease and injury, however, their part in bacterial infections of the CNS has not been fully evaluated. A better understanding will permit the identification of successful therapeutic targets for an improved prognosis and disease outcome.

NOD1 and NOD2: Molecular targets in prevention and treatment of infectious diseases

Nucleotide-binding oligomerization domain (NOD) 1 and NOD2 are pattern-recognition receptors responsible for sensing fragments of bacterial peptidoglycan known as muropeptides. Stimulation of innate immunity by systemic or local administration of NOD1 and NOD2 agonists is an attractive means to prevent and treat infectious diseases. In this review, we discuss novel data concerning structural features of selective and non-selective (dual) NOD1 and NOD2 agonists, main signaling pathways and biological effects induced by NOD1 and NOD2 stimulation, including induction of pro-inflammatory cytokines, type I interferons and antimicrobial peptides, induction of autophagy, alterations of metabolism. We also discuss interactions between NOD1/NOD2 and Toll-like receptor agonists in terms of synergy and cross-tolerance. Finally, we review available animal data on the role of NOD1 and NOD2 in protection against infections, and discuss how these data could be applied in human infectious diseases.

Sensing of Bacterial Cyclic Dinucleotides by the Oxidoreductase RECON Promotes NF-κB Activation and Shapes a Proinflammatory Antibacterial State

Bacterial and host cyclic dinucleotides (cdNs) mediate cytosolic immune responses through the STING signaling pathway, although evidence suggests that alternative pathways exist. We used cdN-conjugated beads to biochemically isolate host receptors for bacterial cdNs, and we identified the oxidoreductase RECON. High-affinity cdN binding inhibited RECON enzyme activity by simultaneously blocking the substrate and cosubstrate sites, as revealed by structural analyses. During bacterial infection of macrophages, RECON antagonized STING activation by acting as a molecular sink for cdNs. Bacterial infection of hepatocytes, which do not express STING, revealed that RECON negatively regulates NF-κB activation. Loss of RECON activity, via genetic ablation or inhibition by cdNs, increased NF-κB activation and reduced bacterial survival, suggesting that cdN inhibition of RECON promotes a proinflammatory, antibacterial state that is distinct from the antiviral state associated with STING activation. Thus, RECON functions as a cytosolic sensor for bacterial cdNs, shaping inflammatory gene activation via its effects on STING and NF-κB.

The Essential Role of Neutrophils during Infection with the Intracellular Bacterial Pathogen Listeria monocytogenes

Neutrophils have historically been characterized as first responder cells vital to host survival because of their ability to contain and eliminate bacterial and fungal pathogens. However, recent studies have shown that neutrophils participate in both protective and detrimental responses to a diverse array of inflammatory and infectious diseases. Although the contribution of neutrophils to extracellular infections has been investigated for decades, their specific role during intracellular bacterial infections has only recently been appreciated. During infection with the Gram-positive intracellular pathogen Listeria monocytogenes, neutrophils are recruited from the bone marrow to sites of infection where they use novel bacterial-sensing pathways leading to phagocytosis and production of bactericidal factors. This review summarizes the requirement of neutrophils during L. monocytogenes infection by examining both neutrophil trafficking and function during primary and secondary infection.

Listeria monocytogenes Inhibits Serotonin Transporter in Human Intestinal Caco-2 Cells

Listeria monocytogenes is a Gram-positive bacterium that can cause a serious infection. Intestinal microorganisms have been demonstrated to contribute to intestinal physiology not only through immunological responses but also by modulating the intestinal serotonergic system. Serotonin (5-HT) is a neuromodulator that is synthesized in the intestinal epithelium and regulates the whole intestinal physiology. The serotonin transporter (SERT), located in enterocytes, controls intestinal 5-HT availability and therefore serotonin's effects. Infections caused by L. monocytogenes are well described as being due to the invasion of intestinal epithelial cells; however, the effect of L. monocytogenes on the intestinal epithelium remains unknown. The main aim of this work, therefore, was to study the effect of L. monocytogenes on SERT. Caco2/TC7 cell line was used as an enterocyte-like in vitro model, and SERT functional and molecular expression assays were performed. Our results demonstrate that living L. monocytogenes inhibits serotonin uptake by reducing SERT expression at the brush border membrane. However, neither inactivated L. monocytogenes nor soluble metabolites were able to affect SERT. The results also demonstrate that L. monocytogenes yields TLR2 and TLR10 transcriptional changes in intestinal epithelial cells and suggest that TLR10 is potentially involved in the inhibitory effect observed on SERT. Therefore, L. monocytogenes, through TLR10-mediated SERT inhibition, may induce increased intestinal serotonin availability and potentially contributing to intestinal physiological changes and the initiation of the inflammatory response.

Susceptibility to quantum dot induced lung inflammation differs widely among the Collaborative Cross founder mouse strains

Quantum dots (QDs) are engineered semiconductor nanoparticles with unique physicochemical properties that make them potentially useful in clinical, research and industrial settings. However, a growing body of evidence indicates that like other engineered nanomaterials, QDs have the potential to be respiratory hazards, especially in the context of the manufacture of QDs and products containing them, as well as exposures to consumers using these products. The overall goal of this study was to investigate the role of mouse strain in determining susceptibility to QD-induced pulmonary inflammation and toxicity. Male mice from 8 genetically diverse inbred strains (the Collaborative Cross founder strains) were exposed to CdSe-ZnS core-shell QDs stabilized with an amphiphilic polymer. QD treatment resulted in significant increases in the percentage of neutrophils and levels of cytokines present in bronchoalveolar lavage fluid (BALF) obtained from NOD/ShiLtJ and NZO/HlLtJ mice relative to their saline (Sal) treated controls. Cadmium measurements in lung tissue indicated strain-dependent differences in disposition of QDs in the lung. Total glutathione levels in lung tissue were significantly correlated with percent neutrophils in BALF as well as with lung tissue Cd levels. Our findings indicate that QD-induced acute lung inflammation is mouse strain dependent, that it is heritable, and that the choice of mouse strain is an important consideration in planning QD toxicity studies. These data also suggest that formal genetic analyses using additional strains or recombinant inbred strains from these mice could be useful for discovering potential QD-induced inflammation susceptibility loci.

Identification of TLR10 as a key mediator of the inflammatory response to Listeria monocytogenes in intestinal epithelial cells and macrophages

Listeria monocytogenes is a Gram-positive bacterium that can cause septicemia and meningitis. TLRs are central receptors of the innate immune system that drive inflammatory responses to invading microbes such as L. monocytogenes. Although intestinal epithelial cells (IECs) represent the initial point of entry used by L. monocytogenes for infection, the innate immune response to L. monocytogenes in these cells has been poorly characterized to date. The aim of this study was to determine which TLRs are involved in mediating the immune response to L. monocytogenes in IECs. We performed an RNA interference screen of TLRs 1-10 in the HT-29 IEC cell line and observed the most significant reduction in chemokine output following silencing of TLR10. This effect was also observed in the macrophage cell line THP-1. The chemokines CCL20, CCL1, and IL-8 were reduced following knockdown of TLR10. Silencing of TLR10 resulted in increased viability of L. monocytogenes in both HT-29 and THP-1 cells. TLR10 was found to be predominantly expressed intracellularly in epithelia, and activation required viable L. monocytogenes. NF-κB activation was seen to require TLR2 in addition to TLR10. Taken together, these data indicate novel roles for TLR10 in sensing pathogenic infection in both the epithelium and macrophages and have identified L. monocytogenes as a source of ligand for the orphan receptor TLR10.

CD40 induces anti-Toxoplasma gondii activity in nonhematopoietic cells dependent on autophagy proteins

Toxoplasma gondii infects both hematopoietic and nonhematopoietic cells and can cause cerebral and ocular toxoplasmosis, as a result of either congenital or postnatally acquired infections. Host protection likely acts at both cellular levels to control the parasite. CD40 is a key factor for protection against cerebral and ocular toxoplasmosis. We determined if CD40 induces anti-T. gondii activity at the level of nonhematopoietic cells. Engagement of CD40 on various endothelial cells including human microvascular brain endothelial cells, human umbilical vein endothelial cells, and a mouse endothelial cell line as well as human and mouse retinal pigment epithelial cells resulted in killing of T. gondii. CD40 stimulation increased expression of the autophagy proteins Beclin 1 and LC3 II, enhanced autophagy flux, and led to recruitment of LC3 around the parasite. The late endosomal/lysosomal marker LAMP-1 accumulated around the parasite in CD40-stimulated cells. This was accompanied by killing of T. gondii dependent on lysosomal enzymes. Accumulation of LAMP-1 and killing of T. gondii were dependent on the autophagy proteins Beclin 1 and Atg7. Together, these studies revealed that CD40 induces toxoplasmacidal activity in various nonhematopoietic cells dependent on proteins of the autophagy machinery.

Commensal and probiotic bacteria influence intestinal barrier function and susceptibility to colitis in Nod1-/-; Nod2-/- mice

BACKGROUND

The intestinal microbiota regulates key host functions. It is unknown whether modulation of the microbiota can affect a genetically determined host phenotype. Polymorphisms in the Nucleotide oligomerization domain (Nod)-like receptor family confer genetic risk for inflammatory bowel disease (IBD). We investigated whether the intestinal microbiota and the probiotic strain Bifidobacterium breve NCC2950 affect intestinal barrier function and responses to intestinal injury in Nod1(-/-); Nod2(-/-) mice.

METHODS

Specific pathogen-free (SPF) Nod1(-/-); Nod2(-/-) mice and mice gnotobiotically derived with altered Schaedler flora (ASF) biota were used. SPF Nod1(+/-); Nod2(+/-) littermates (generated by crossing SPF Nod1(-/-); Nod2(-/-) and germ-free C57BL/6 mice) and ASF Nod1(+/-); Nod2(+/-) mice were used as controls. SPF mice were gavaged daily with 10(9) -CFU B. breve for 14 days before colitis induction. Denaturing gradient gel electrophoresis (DGGE) and real-time polymerase chain reaction (PCR) were used to assess microbiota composition. Intestinal permeability was assessed by in vitro and in vivo techniques. Expressions of epithelial apical junction proteins, mucin, and antimicrobial proteins were assessed by quantitative reverse-transcription PCR (qRT-PCR) and immunofluorescence. Responses to intestinal injury were investigated using an acute experimental model of colitis.

RESULTS

Under SPF conditions, Nod1(-/-); Nod2(-/-) mice had increased paracellular permeability, decreased E-cadherin, and lower colonic antimicrobial RegIII-γ expression compared to Nod1(+/-); Nod2(+/-) littermate controls. These changes were associated with increased susceptibility to colitis. ASF colonization or B. breve supplementation normalized RegIII-γ expression and decreased susceptibility to dextran sodium sulfate (DSS) colitis in Nod1(-/-); Nod2(-/-) mice.

CONCLUSIONS

The intestinal microbiota influences colitis severity in Nod1(-/-); Nod2(-/-) mice. The results suggest that colonization strategies with defined commensals or exogenous specific probiotic therapy may prevent intestinal inflammation in a genetically predisposed host.

Repertoire of Escherichia coli agonists sensed by innate immunity receptors of the bovine udder and mammary epithelial cells

Escherichia coli is a frequent cause of clinical mastitis in dairy cows. It has been shown that a prompt response of the mammary gland after E. coli entry into the lumen of the gland is required to control the infection, which means that the early detection of bacteria is of prime importance. Yet, apart from lipopolysaccharide (LPS), little is known of the bacterial components which are detected by the mammary innate immune system. We investigated the repertoire of potential bacterial agonists sensed by the udder and bovine mammary epithelial cells (bMEC) during E. coli mastitis by using purified or synthetic molecular surrogates of bacterial agonists of identified pattern-recognition receptors (PRRs). The production of CXCL8 and the influx of leucocytes in milk were the readouts of reactivity of stimulated cultured bMEC and challenged udders, respectively. Quantitative PCR revealed that bMEC in culture expressed the nucleotide oligomerization domain receptors NOD1 and NOD2, along with the Toll-like receptors TLR1, TLR2, TLR4, and TLR6, but hardly TLR5. In line with expression data, bMEC proved to react to the cognate agonists C12-iE-DAP (NOD1), Pam3CSK4 (TLR1/2), Pam2CSK4 (TLR2/6), pure LPS (TLR4), but not to flagellin (TLR5). As the udder reactivity to NOD1 and TLR5 agonists has never been reported, we tested whether the mammary gland reacted to intramammary infusion of C12-iE-DAP or flagellin. The udder reacted to C12-iE-DAP, but not to flagellin, in line with the reactivity of bMEC. These results extend our knowledge of the reactivity of the bovine mammary gland to bacterial agonists of the innate immune system, and suggest that E. coli can be recognized by several PRRs including NOD1, but unexpectedly not by TLR5. The way the mammary gland senses E. coli is likely to shape the innate immune response and finally the outcome of E. coli mastitis.

Peptidoglycan: a critical activator of the mammalian immune system during infection and homeostasis

Peptidoglycan is a conserved structural component of the bacterial cell wall with molecular motifs unique to bacteria. The mammalian immune system takes advantage of these properties and has evolved to recognize this microbial associated molecular pattern. Mammals have four secreted peptidoglycan recognition proteins, PGLYRP-1-4, as well as two intracellular sensors of peptidoglycan, Nod1 and Nod2. Recognition of peptidoglycan is important in initiating and shaping the immune response under both homeostatic and infection conditions. During infection, peptidoglycan recognition drives both cell-autonomous and whole-organism defense responses. Here, we examine recent advances in the understanding of how peptidoglycan recognition shapes mammalian immune responses in these diverse contexts.

TLR2 and RIP2 pathways mediate autophagy of Listeria monocytogenes via extracellular signal-regulated kinase (ERK) activation

Listeria monocytogenes is a facultative intracellular pathogen that invades both phagocytic and non-phagocytic cells. Recent studies have shown that L. monocytogenes infection activates the autophagy pathway. However, the innate immune receptors involved and the downstream signaling pathways remain unknown. Here, we show that macrophages deficient in the TLR2 and NOD/RIP2 pathway display defective autophagy induction in response to L. monocytogenes. Inefficient autophagy in Tlr2(-/-) and Nod2(-/-) macrophages led to a defect in bacteria colocalization with the autophagosomal marker GFP-LC3. Consequently, macrophages lacking TLR2 and NOD2 were found to be more susceptible to L. monocytogenes infection, as were the Rip2(-/-) mice. Tlr2(-/-) and Nod2(-/-) cells showed perturbed NF-κB and ERK signaling. However, autophagy against L. monocytogenes was dependent selectively on the ERK pathway. In agreement, wild-type cells treated with a pharmacological inhibitor of ERK or ERK-deficient cells displayed inefficient autophagy activation in response to L. monocytogenes. Accordingly, fewer bacteria were targeted to the autophagosomes and, consequently, higher bacterial growth was observed in cells deficient in the ERK signaling pathway. These findings thus demonstrate that TLR2 and NOD proteins, acting via the downstream ERK pathway, are crucial to autophagy activation and provide a mechanistic link between innate immune receptors and induction of autophagy against cytoplasm-invading microbes, such as L. monocytogenes.

Innate immune recognition and inflammasome activation in listeria monocytogenes infection

Listeria monocytogenes is an intracellular, Gram-positive bacterium that can cause life-threatening illness especially in immunocompromised individuals and newborns. The pathogen propagates within the cytosol of various host cells after escaping from the phagosomal compartment depending on the cytolysin listeriolysin O. While L. monocytogenes can manipulate the endocytic and many host-cell signaling cascades to its advantage, host cells are however capable of detecting Listeria infection at different cellular compartments by expressing innate immune receptors that trigger antibacterial defense pathways. These receptors include the Toll-like receptors, NOD-like receptors (NLRs), and cytosolic DNA sensors. Some NLRs as well as the DNA sensor AIM2 form multiprotein complexes called inflammasomes. Inflammasomes regulate caspase-1-dependent production of the key inflammatory cytokines IL-1β and IL-18 as well as pyroptotic cell death in L. monocytogenes-infected cells. This review describes the current knowledge about innate immune sensing and inflammasome activation in Listeria infection.

NOD1 and NOD2 regulation of pulmonary innate immunity to Legionella pneumophila

The role of nucleotide-binding oligomerization domain-1 (NOD1) and nucleotide-binding oligomerization domain-2 (NOD2), cytoplasmic receptors which detect bacterial cell wall molecules, in pulmonary innate immune responses is poorly understood. We determined that both NOD1 and NOD2 detect heat-killed Legionella and stimulate NF-κb and IFN-β promoter activity using an in vitro luciferase reporter system. We next infected NOD1- and NOD2-deficient animals with aerosolized Legionella pneumophila. At 3 days post infection, Nod1(-/-) mice had impaired bacterial clearance compared to WT controls. In addition, at 4 h and 24 h, Nod1(-/-) mice had impaired neutrophil recruitment to the alveolar space. In contrast, increased lung neutrophils were seen in the Nod2(-/-) animals at 24 h. Analysis of cytokine production at 4 h post infection revealed a significant decrease in proinflammatory cytokines in the Nod1(-/-) animals when compared to WT animals. In contrast, increased 4-h proinflammatory cytokines were seen in the Nod2(-/-) animals. Furthermore, the lungs of both Nod1(-/-) and Nod2(-/-) mice had significantly increased pro-inflammatory cytokine levels at 24 h, suggesting possible suppressive roles for later stages of infection. Together, our data suggest that although both NOD1 and NOD2 can detect Legionella, these receptors modulate the in vivo pulmonary immune response differently.

Involvement of the AIM2, NLRC4, and NLRP3 inflammasomes in caspase-1 activation by Listeria monocytogenes

Infection with Listeria monocytogenes can cause meningitis and septicemia in newborn, elderly, or immunocompromised individuals. Pregnant women are particularly susceptible to Listeria, leading to a potentially fatal infection. Cytosolic Listeria activates the proinflammatory caspase-1 and induces the processing and secretion of interleukins IL-1beta and IL-18 as well as caspase-1-dependent pyroptosis. This study elucidates the role of various inflammasome components of host macrophages in the proinflammatory response to infection with Listeria. Here, we have used macrophages from AIM2-, NLRC4-, NLRP3-, and ASC-deficient mice to demonstrate that AIM2, NLRC4, and NLRP3 inflammasomes as well as the adaptor protein ASC all contribute to activation of caspase-1 in Listeria-infected macrophages. We show that Listeria DNA, which escapes into the cytosol of infected macrophages, triggers AIM2 oligomerization, caspase-1 activation, and pyroptosis. Interestingly, we found that flagellin-deficient Listeria, like Francisella tularensis, is recognized primarily by the AIM2 inflammasome, as no caspase-1 activation or cell death was observed in AIM2-deficient macrophages infected with this Listeria mutant. We further show that prior priming of NLRC4-deficient macrophages with LPS is sufficient for Listeria-induced caspase-1 activation in these macrophages, suggesting that TLR4 signaling is important for activation of the AIM2 and NLRP3 inflammasomes by Listeria in the absence of NLRC4. Taken together, our results indicate that Listeria infection is sensed by multiple inflammasomes that collectively orchestrate a robust caspase-1 activation and proinflammatory response.

Listeria monocytogenes-infected human peripheral blood mononuclear cells produce IL-1beta, depending on listeriolysin O and NLRP3

Different NOD-like receptors, including NLRP1, NLRP3, and NLRC4, as well as the recently identified HIN-200 protein, AIM2, form multiprotein complexes called inflammasomes, which mediate caspase-1-dependent processing of pro-IL-1beta. Listeria monocytogenes is an intracellular pathogen that is actively phagocytosed by monocytes/macrophages and subsequently escapes from the phagosome into the host cell cytosol, depending on its pore-forming toxin listeriolysin O (LLO). In this study, we demonstrate that human PBMCs produced mature IL-1beta when infected with wild-type L. monocytogenes or when treated with purified LLO. L. monocytogenes mutants lacking LLO or expressing a noncytolytic LLO as well as the avirulent Listeria innocua induced strongly impaired IL-1beta production. RNA interference and inhibitor experiments in human PBMCs as well as experiments in Nlrp3 and Rip2 knockout bone marrow-derived macrophages demonstrated that the Listeria-induced IL-1beta release was dependent on ASC, caspase-1, and NLRP3, whereas NOD2, Rip2, NLRP1, NLRP6, NLRP12, NLRC4, and AIM2 appeared to be dispensable. We found that L. monocytogenes-induced IL-1beta production was largely dependent on phagosomal acidification and cathepsin B release, whereas purified LLO activated an IL-1beta production independently of these mechanisms. Our results indicate that L. monocytogenes-infected human PBMCs produced IL-1beta, largely depending on an LLO-mediated phagosomal rupture and cathepsin B release, which is sensed by Nlrp3. In addition, an LLO-dependent but cathepsin B-independent NLRP3 activation might contribute to some extent to the IL-1beta production in L. monocytogenes-infected cells.

Role of Nod1 in mucosal dendritic cells during Salmonella pathogenicity island 1-independent Salmonella enterica serovar Typhimurium infection

Recent advances in immunology have highlighted the critical function of pattern-recognition molecules (PRMs) in generating the innate immune response to effectively target pathogens. Nod1 and Nod2 are intracellular PRMs that detect peptidoglycan motifs from the cell walls of bacteria once they gain access to the cytosol. Salmonella enterica serovar Typhimurium is an enteric intracellular pathogen that causes a severe disease in the mouse model. This pathogen resides within vacuoles inside the cell, but the question of whether cytosolic PRMs such as Nod1 and Nod2 could have an impact on the course of S. Typhimurium infection in vivo has not been addressed. Here, we show that deficiency in the PRM Nod1, but not Nod2, resulted in increased susceptibility toward a mutant strain of S. Typhimurium that targets directly lamina propria dendritic cells (DCs) for its entry into the host. Using this bacterium and bone marrow chimeras, we uncovered a surprising role for Nod1 in myeloid cells controlling bacterial infection at the level of the intestinal lamina propria. Indeed, DCs deficient for Nod1 exhibited impaired clearance of the bacteria, both in vitro and in vivo, leading to increased organ colonization and decreased host survival after oral infection. Taken together, these findings demonstrate a key role for Nod1 in the host response to an enteric bacterial pathogen through the modulation of intestinal lamina propria DCs.