NOD2 signaling contributes to host defense in the lungs against Escherichia coli infection

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.

Natural Bioactive Compounds Targeting NADPH Oxidase Pathway in Cardiovascular Diseases

Cardiovascular disease (CVD) is the leading cause of death worldwide, in both developed and developing countries. According to the WHO report, the morbidity and mortality caused by CVD will continue to rise with the estimation of death going up to 22.2 million in 2030. NADPH oxidase (NOX)-derived reactive oxygen species (ROS) production induces endothelial nitric oxide synthase (eNOS) uncoupling and mitochondrial dysfunction, resulting in sustained oxidative stress and the development of cardiovascular diseases. Seven distinct members of the family have been identified of which four (namely, NOX1, 2, 4 and 5) may have cardiovascular functions. Currently, the treatment and management plan for patients with CVDs mainly depends on the drugs. However, prolonged use of prescribed drugs may cause adverse drug reactions. Therefore, it is crucial to find alternative treatment options with lesser adverse effects. Natural products have been gaining interest as complementary therapy for CVDs over the past decade due to their wide range of medicinal properties, including antioxidants. These might be due to their potent active ingredients, such as flavonoid and phenolic compounds. Numerous natural compounds have been demonstrated to have advantageous effects on cardiovascular disease via NADPH cascade. This review highlights the potential of natural products targeting NOX-derived ROS generation in treating CVDs. Emphasis is put on the activation of the oxidases, including upstream or downstream signalling events.

How Gut Microbiota Are Shaped by Pattern Recognition Receptors in Colitis and Colorectal Cancer

Simple Summary

The pathogenesis of intestinal inflammatory disorders such as colitis and colorectal cancer is complicated and dysregulation of gut microbiota is considered an important contributing factor. Inflammation is often initiated by the activation of pattern recognition receptors. However, the relationship between these innate immune receptors and gut microbiota is not fully understood. Here, we show that pattern recognition receptors not only recognize pathogens and initiate inflammatory signal transduction to induce immune responses, but also influence the composition of intestinal microorganisms, thus affecting the development of intestinal inflammation and cancer through various mechanisms. This suggests that the modification of innate immune receptors and relevant molecules could be therapeutic targets for the treatment of colitis and colorectal cancer by regulating gut microbiota.

Abstract

Disorders of gut microbiota have been closely linked to the occurrence of various intestinal diseases including colitis and colorectal cancer (CRC). Specifically, the production of beneficial bacteria and intestinal metabolites may slow the development of some intestinal diseases. Recently, it has been proposed that pattern recognition receptors (PRRs) not only recognize pathogens and initiate inflammatory signal transduction to induce immune responses but also influence the composition of intestinal microorganisms. However, the mechanisms through which PRRs regulate gut microbiota in the setting of colitis and CRC have rarely been systematically reviewed. Therefore, in this paper, we summarize recent advances in our understanding of how PRRs shape gut microbiota and how this influences the development of colitis and CRC.

Transcriptome Analysis Reveals Altered Expression of Genes Involved in Hypoxia, Inflammation and Immune Regulation in Pdcd10-Depleted Mouse Endothelial Cells

Cerebral cavernous malformations (CCM) are capillary malformations affecting the central nervous system and commonly present with headaches, epilepsy and stroke. Treatment of CCM is symptomatic, and its prevention is limited. CCM are often sporadic but sometimes may be multifocal and/or affect multiple family members. Heterozygous pathogenic variants in PDCD10 cause the rarest and apparently most severe genetic variant of familial CCM. We carried out an RNA-Seq and a Q-PCR validation analysis in Pdcd10-silenced and wild-type mouse endothelial cells in order to better elucidate CCM molecular pathogenesis. Ninety-four differentially expressed genes presented an FDR-corrected p-value < 0.05. A functionally clustered dendrogram showed that differentially expressed genes cluster in cell proliferation, oxidative stress, vascular processes and immune response gene-ontology functions. Among differentially expressed genes, the major cluster fell in signaling related to inflammation and pathogen recognition, including HIF1α and Nos2 signaling and immune regulation. Validation analysis performed on wild-type, Pdcd10-null and Pdcd10-null reconstituted cell lines was consistent with RNA-Seq data. This work confirmed previous mouse transcriptomic data in endothelial cells, which are recognized as a critical tissue for CCM formation and expands the potential molecular signatures of PDCD10-related familial CCM to alterations in inflammation and pathogen recognition pathways.

Neutrophil-Dependent Immunity During Pulmonary Infections and Inflammations

Rapid recruitment of neutrophils to an inflamed site is one of the hallmarks of an effective host defense mechanism. The main pathway through which this happens is by the innate immune response. Neutrophils, which play an important part in innate immune defense, migrate into lungs through the modulation actions of chemokines to execute a variety of pro-inflammatory functions. Despite the importance of chemokines in host immunity, little has been discussed on their roles in host immunity. A holistic understanding of neutrophil recruitment, pattern recognition pathways, the roles of chemokines and the pathophysiological roles of neutrophils in host immunity may allow for new approaches in the treatment of infectious and inflammatory disease of the lung. Herein, this review aims at highlighting some of the developments in lung neutrophil-immunity by focusing on the functions and roles of CXC/CC chemokines and pattern recognition receptors in neutrophil immunity during pulmonary inflammations. The pathophysiological roles of neutrophils in COVID-19 and thromboembolism have also been summarized. We finally summarized various neutrophil biomarkers that can be utilized as prognostic molecules in pulmonary inflammations and discussed various neutrophil-targeted therapies for neutrophil-driven pulmonary inflammatory diseases.

NOD-like receptors mediate inflammatory lung injury during plateau hypoxia exposure

Background

The lung is an important target organ for hypoxia treatment, and hypoxia can induce several diseases in the body.

Methods

We performed transcriptome sequencing for the lungs of rats exposed to plateau hypoxia at 0 day and 28 days. Sequencing libraries were constructed, and enrichment analysis of the differentially expressed genes (DEGs) was implemented using the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG). Subsequently, experimental validation was executed by quantitative real-time PCR (qRT-PCR) and western blot.

Results

The results showed that the nucleotide-binding oligomerization domain (NOD)-like receptor (NLR) signaling pathway that was involved in immunity may play a crucial function in lung injury caused by plateau hypoxia. And the expressions of NOD1, NOD2, IL-1β, TNF-α, IL-6, and IL-18 were higher at 28 days of exposure to plateau hypoxia than that at 0 day. Similarly, CARD9, MYD88, p38 MAPK, and NF-κB p65, which are related to the NF-κB and MAPK signaling pathways, also demonstrated increased expression at 28 days exposure to plateau hypoxia than at 0 day.

Conclusions

Our study suggested that the NF­κBp65 and p38 MAPK signaling pathways may be activated in the lungs of rats during plateau hypoxia. Upregulated expression of NF­κBp65 and p38 MAPK can promote the transcription of downstream inflammatory factors, thereby aggravating the occurrence and development of lung tissue remodeling.

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.

Roles of interleukin-11 during acute bacterial pneumonia

Interleukin-11 (IL-11) is an interleukin-6 (IL-6) family cytokine shown to play a protective role in acute inflammatory settings including systemic infection. In this study we addressed the role of IL-11 in acute bacterial pneumonia using a mouse model of E. coli pneumonia. Compared with other related cytokines, IL-11 protein was maintained at high levels in the lung at baseline, with only mild alterations in whole lung and BALF levels during acute infection. The primary source of IL-11 in the lung was the epithelium, but steady state production was not dependent on the inflammatory transcription factor nuclear factor kappa B in cells of either myeloid or epithelial lineage. Blockade of IL-11 with neutralizing antibodies resulted in a mild but significant decrease in neutrophil recruitment and increase in pulmonary edema during pneumonia, without detectable alterations in bacterial clearance. Exogenous IL-11 administration, however, had no effect at baseline or during infection. Overall, we conclude that maintenance of lung IL-11 concentrations may influence acute pulmonary inflammation during infection, albeit modestly.

NOD2 Expression in Intestinal Epithelial Cells Protects Toward the Development of Inflammation and Associated Carcinogenesis

Nucleotide-binding oligomerization domain 2 (NOD2) is an intracellular pattern recognition receptor that senses bacterial peptidoglycan-conserved motifs in cytosol and stimulates host immune response including epithelial and immune cells. The association of NOD2 mutations with a number of inflammatory pathologies including Crohn's disease (CD), graft-versus-host diseases, or Blau syndrome, highlights its pivotal role in inflammatory response and the associated-carcinogenesis development. Since its identification in 2001 and its association with CD, the role of NOD2 in epithelial cells and immune cells has been investigated extensively but the precise mechanism by which NOD2 mutations lead to CD and the associated carcinogenesis development is largely unknown. In this review, we present and discuss recent developments about the role of NOD2 inside epithelial cells on the control of the inflammatory process and its linked carcinogenesis development.

Protective Role of Rabbit Nucleotide-Binding Oligomerization Domain-2 (NOD2)-Mediated Signaling Pathway in Resistance to Enterohemorrhagic Escherichia coli Infection

Nucleotide-binding oligomerization domain 2 (NOD2), a member of the NOD-like receptors (NLRs) family that is well-known to play a key role in innate immune responses and is involved in innate antibacterial responses. In this study, rabbit NOD2 (rNOD2) was cloned from rabbit kidney (RK) cells. It was distributed in various tissues, and the highest level of rNod2 was detected in spleen. Moreover, the expression of rNod2 was significantly upregulated in the heart, liver, and spleen induced by enterohemorrhagic Escherichia coli (EHEC). Overexpression of rNOD2 induced the expression of pro-inflammatory cytokine, including Il1β, Il6, Ifn-γ, and Tnf, as well as defensins, including Defb124, Defb125, and Defb128 through the nuclear factor (NF)-κB signaling pathway. Furthermore, overexpression of rNOD2 inhibited the growth of EHEC, and knockdown of rNOD2 or inhibition of the NF-κB pathway promoted its replication. In addition, our results suggest that rNOD2 can significantly activate NF-κB signaling and trigger antibacterial defenses to increase the expression of pro-inflammatory cytokine and defensins after stimulation by EHEC. These findings are useful to further understanding the innate immune system of rabbits and providing a new perspective for the prevention of bacterial diseases in rabbits.

Haemophilus parasuis infection activates NOD1/2-RIP2 signaling pathway in PK-15 cells

Haemophilus parasuis, an important swine pathogen, was recently proven able to invade into endothelial or epithelial cell in vitro. NOD1/2 are specialized NLRs that participate in the recognition of pathogens able to invade intracellularly and therefore, we assessed that the contribution of NOD1/2 to inflammation responses during H. parasuis infection. We observed that H. parasuis infection enhanced NOD2 expression and RIP2 phosphorylation in porcine kidney 15 cells. Our results also showed that knock down of NOD1/2 or RIP2 expression respectively significantly decreased H. parasuis-induced NF-κB activity, while the phosphorylation level of p38, JNK or ERK was not changed. Moreover, real-time PCR result showed that NOD1, NOD2 or RIP2 was involved in the expression of CCL4, CCL5 and IL-8. Inhibition of NOD1 and NOD2 significantly reduced CCL5 promoter activity, even in a more effective way compared with inhibition of TLR.

17β-Estradiol and progesterone decrease MDP induced NOD2 expression in bovine mammary epithelial cells

During the periparturient period, many neuroendocrine changes develop in cows. Periparturient hormone fluxes may adversely affect mammary gland immunity and mastitis susceptibility. 17β-Estradiol (E₂) and progesterone (P₄) have been reported to function on immune regulation, and their concentration fluctuates dramatically during the perinatal period. Nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) mediate numerous aspects of innate immunity in humans and experimental animals. This study aimed to explore the effects of E₂ and P₄ on NOD2 expression in bovine mammary epithelial cells (BMECs). BMECs were isolated and purified from bovine mammary tissue and treated with E₂/P₄ and muramyl dipeptide (MDP). After these treatments, the mRNA levels of NOD2, receptor-interacting protein kinase (RIP) 2, interleukin (IL) 1β, IL-6, IL-8 and tumor necrosis factor (TNF) α were assessed by quantitative real-time polymerase chain reaction (qRT-PCR) respectively, and the protein levels of NOD2 were analyzed by western blotting. The results showed that E₂ and P₄ decreased MDP-induced transcriptional expression of NOD2 and the downstream molecules. Moreover, E₂ reduced MDP-induced NOD2 protein expression levels. Our study suggests that down-regulation of NOD2 by E₂ and P₄ may be one of the reasons for mastitis susceptibility in periparturient dairy cows.

Nod2: The intestinal gate keeper

Nucleotide-binding oligomerization domain 2 (NOD2) is an intracellular pattern recognition receptor that senses bacterial peptidoglycan (PGN)-conserved motifs in cytosol and stimulates host immune response. The association of NOD2 mutations with a number of inflammatory pathologies, including Crohn disease (CD), Graft-versus-host disease (GVHD), and Blau syndrome, highlights its pivotal role in host–pathogen interactions and inflammatory response. Stimulation of NOD2 by its ligand (muramyl dipeptide) activates pro-inflammatory pathways such as nuclear factor-κB (NF-κB), mitogen-activated protein kinases (MAPKs), and Caspase-1. A loss of NOD2 function may result in a failure in the control of microbial infection, thereby initiating systemic responses and aberrant inflammation. Because the ligand of Nod2 is conserved in both gram-positive and gram-negative bacteria, NOD2 detects a wide variety of microorganisms. Furthermore, current literature evidences that NOD2 is also able to control viruses’ and parasites’ infections. In this review, we present and discuss recent developments about the role of NOD2 in shaping the gut commensal microbiota and pathogens, including bacteria, viruses, and parasites, and the mechanisms by which Nod2 mutations participate in disease occurrence.

Induction of STAT3-Dependent CXCL5 Expression and Neutrophil Recruitment by Oncostatin-M during Pneumonia

Acute bacterial pneumonia is a significant public health concern worldwide. Understanding the signals coordinating lung innate immunity may foster the development of therapeutics that limit tissue damage and promote host defense. We have previously shown that lung messenger RNA expression of the IL-6 family cytokine oncostatin-M (OSM) is significantly elevated in response to bacterial stimuli. However, its physiological significance during pneumonia is unknown. Here we demonstrate that OSM is rapidly increased in the airspaces of mice after pulmonary infection with Escherichia coli. Neutralization of OSM caused a substantial decrease in airspace neutrophils and macrophages. OSM blockade also caused a marked reduction in lung chemokine (C-X-C motif) ligand (CXCL) 5 expression, whereas other closely related neutrophil chemokines, CXCL1 and CXCL2, were unaffected. Intratracheal administration of recombinant OSM was sufficient to recapitulate the effect on CXCL5 induction, associated with robust activation of the signal transducer and activator of transcription 3 (STAT3) transcription factor. Cell sorting revealed that OSM effects were specific to lung epithelial cells, including a positive feedback loop in which OSM may facilitate expression of its own receptor. Finally, in vitro studies demonstrated that STAT3 was required for maximal OSM-induced CXCL5 expression. These studies demonstrate a novel role for OSM during pneumonia as an important signal to epithelial cells for chemokine induction mediating neutrophil recruitment.

NOD1 and NOD2: signaling, host defense, and inflammatory disease

The nucleotide-binding oligomerization domain (NOD) proteins NOD1 and NOD2, the founding members of the intracellular NOD-like receptor family, sense conserved motifs in bacterial peptidoglycan and induce proinflammatory and antimicrobial responses. Here, we discuss recent developments about the mechanisms by which NOD1 and NOD2 are activated by bacterial ligands, the regulation of their signaling pathways, and their role in host defense and inflammatory disease. Several routes for the entry of peptidoglycan ligands to the host cytosol to trigger activation of NOD1 and NOD2 have been elucidated. Furthermore, genetic screens and biochemical analyses have revealed mechanisms that regulate NOD1 and NOD2 signaling. Finally, recent studies have suggested several mechanisms to account for the link between NOD2 variants and susceptibility to Crohn's disease. Further understanding of NOD1 and NOD2 should provide new insight into the pathogenesis of disease and the development of new strategies to treat inflammatory and infectious disorders.

Collaborative action of Toll-like and NOD-like receptors as modulators of the inflammatory response to pathogenic bacteria

Early sensing of pathogenic bacteria by the host immune system is important to develop effective mechanisms to kill the invader. Microbial recognition, activation of signaling pathways, and effector mechanisms are sequential events that must be highly controlled to successfully eliminate the pathogen. Host recognizes pathogens through pattern-recognition receptors (PRRs) that sense pathogen-associated molecular patterns (PAMPs). Some of these PRRs include Toll-like receptors (TLRs), nucleotide-binding oligomerization domain-like receptors (NLRs), retinoic acid-inducible gene-I- (RIG-I-) like receptors (RLRs), and C-type lectin receptors (CLRs). TLRs and NLRs are PRRs that play a key role in recognition of extracellular and intracellular bacteria and control the inflammatory response. The activation of TLRs and NLRs by their respective ligands activates downstream signaling pathways that converge on activation of transcription factors, such as nuclear factor-kappaB (NF-κB), activator protein-1 (AP-1) or interferon regulatory factors (IRFs), leading to expression of inflammatory cytokines and antimicrobial molecules. The goal of this review is to discuss how the TLRs and NRLs signaling pathways collaborate in a cooperative or synergistic manner to counteract the infectious agents. A deep knowledge of the biochemical events initiated by each of these receptors will undoubtedly have a high impact in the design of more effective strategies to control inflammation.

NADPH oxidases: an overview from structure to innate immunity-associated pathologies

Oxygen-derived free radicals, collectively termed reactive oxygen species (ROS), play important roles in immunity, cell growth, and cell signaling. In excess, however, ROS are lethal to cells, and the overproduction of these molecules leads to a myriad of devastating diseases. The key producers of ROS in many cells are the NOX family of NADPH oxidases, of which there are seven members, with various tissue distributions and activation mechanisms. NADPH oxidase is a multisubunit enzyme comprising membrane and cytosolic components, which actively communicate during the host responses to a wide variety of stimuli, including viral and bacterial infections. This enzymatic complex has been implicated in many functions ranging from host defense to cellular signaling and the regulation of gene expression. NOX deficiency might lead to immunosuppression, while the intracellular accumulation of ROS results in the inhibition of viral propagation and apoptosis. However, excess ROS production causes cellular stress, leading to various lethal diseases, including autoimmune diseases and cancer. During the later stages of injury, NOX promotes tissue repair through the induction of angiogenesis and cell proliferation. Therefore, a complete understanding of the function of NOX is important to direct the role of this enzyme towards host defense and tissue repair or increase resistance to stress in a timely and disease-specific manner.

NOD1 and NOD2 expression and function in very preterm infant mononuclear cells

AIM

To evaluate mononuclear cell expression and function of the cytosolic nucleotide-binding oligomerization domain-containing receptors, NOD1 and NOD2, in very preterm and full-term infants.

METHODS

NOD1 and NOD2 gene and protein expression in very preterm infants, term infants and healthy adult, cord and peripheral blood mononuclear cells (C/PBMC) were quantified using qPCR and flow cytometry. Cytokine responses of purified infant and adult monocytes to NOD1- and NOD2-specific agonists were assessed using a multiplex immunoassay (Bioplex).

RESULTS

NOD1 and NOD2 were expressed by a range of infant and adult mononuclear cell types, including T- and B cells, with highest expression in classical (CD14(++) CD16(-) ) and intermediate (CD14(++) CD16(+) ) monocytes. NOD1 and NOD2 expression levels by monocytes from very preterm infant were similar to those in term infants or adults. Monocyte production of TNFα, IL-6 and IL-1β induced by activation of NOD1 and NOD2 was similar between very preterm infants, term infants and adults.

CONCLUSION

Monocyte expression and function of NOD1 and NOD2 in very preterm infants are intact and comparable/equivalent to term infants and adults. Functional deficiencies in monocyte NOD signalling pathways are unlikely to contribute to the increased susceptibility to bacterial sepsis in preterm infants.

NOD2/RICK-dependent β-defensin 2 regulation is protective for nontypeable Haemophilus influenzae-induced middle ear infection

Middle ear infection, otitis media (OM), is clinically important due to the high incidence in children and its impact on the development of language and motor coordination. Previously, we have demonstrated that the human middle ear epithelial cells up-regulate β-defensin 2, a model innate immune molecule, in response to nontypeable Haemophilus influenzae (NTHi), the most common OM pathogen, via TLR2 signaling. NTHi does internalize into the epithelial cells, but its intracellular trafficking and host responses to the internalized NTHi are poorly understood. Here we aimed to determine a role of cytoplasmic pathogen recognition receptors in NTHi-induced β-defensin 2 regulation and NTHi clearance from the middle ear. Notably, we observed that the internalized NTHi is able to exist freely in the cytoplasm of the human epithelial cells after rupturing the surrounding membrane. The human middle ear epithelial cells inhibited NTHi-induced β-defensin 2 production by NOD2 silencing but augmented it by NOD2 over-expression. NTHi-induced β-defensin 2 up-regulation was attenuated by cytochalasin D, an inhibitor of actin polymerization and was enhanced by α-hemolysin, a pore-forming toxin. NOD2 silencing was found to block α-hemolysin-mediated enhancement of NTHi-induced β-defensin 2 up-regulation. NOD2 deficiency appeared to reduce inflammatory reactions in response to intratympanic inoculation of NTHi and inhibit NTHi clearance from the middle ear. Taken together, our findings suggest that a cytoplasmic release of internalized NTHi is involved in the pathogenesis of NTHi infections, and NOD2-mediated β-defensin 2 regulation contributes to the protection against NTHi-induced otitis media.

The Nod1, Nod2, and Rip2 axis contributes to host immune defense against intracellular Acinetobacter baumannii infection

Acinetobacter baumannii is a major extensively drug-resistant lethal human nosocomial bacterium. However, the host innate immune mechanisms controlling A. baumannii are not well understood. Although viewed as an extracellular pathogen, A. baumannii can also invade and survive intracellularly. However, whether host innate immune pathways sensing intracellular bacteria contribute to immunity against A. baumannii is not known. Here, we provide evidence for the first time that intracellular antibacterial innate immune receptors Nod1 and Nod2, and their adaptor Rip2, play critical roles in the sensing and clearance of A. baumannii by human airway epithelial cells in vitro. A. baumannii infection upregulated Rip2 expression. Silencing of Nod1, Nod2, and Rip2 expression profoundly increased intracellular invasion and prolonged the multiplication and survival of A. baumannii in lung epithelial cells. Notably, the Nod1/2-Rip2 axis did not contribute to the control of A. baumannii infection of human macrophages, indicating that they play cell type-specific roles. The Nod1/2-Rip2 axis was needed for A. baumannii infection-induced activation of NF-κB but not mitogen-activated protein kinases. Moreover, the Nod1/2-Rip2 axis was critical to induce optimal cytokine and chemokine responses to A. baumannii infection. Mechanistic studies showed that the Nod1/2 pathway contributed to the innate control of A. baumannii infection through the production of β-defensin 2 by airway epithelial cells. This study revealed new insights into the immune control of A. baumannii and may contribute to the development of effective immune therapeutics and vaccines against A. baumannii.

The role of NOD2 in murine and human melioidosis

Nucleotide-binding oligomerization domain 2 (NOD2) is a cytosolic pathogen recognition receptor that regulates susceptibility to a variety of infections and chronic diseases. Burkholderia pseudomallei, a facultative intracellular bacterium, causes the tropical infection melioidosis. We hypothesized that NOD2 may participate in host defense in melioidosis. We performed a series of in vitro assays and in vivo experiments and analyzed the association of human genetic variation with infection to delineate the contribution of NOD2 to the host response to B. pseudomallei. We found that transfection with NOD2 mediated NF-κB activation induced by B. pseudomallei stimulation of HEK293 cells. After low-dose inoculation with aerosolized B. pseudomallei, Nod2-deficient mice showed impaired clinical responses and permitted greater bacterial replication in the lung and dissemination to the spleen compared with wild-type mice. IL-6 and KC levels were higher in the lungs of Nod2-deficient mice. In a cohort of 1562 Thai subjects, a common genetic polymorphism in the NOD2 region, rs7194886, was associated with melioidosis, and this effect was most pronounced in women. rs7194886 was not associated with differences in cytokine production induced by whole-blood stimulation with the NOD2 ligand, muramyl dipeptide, or B. pseudomallei. To our knowledge, these findings are the first to characterize the role of NOD2 in host defense in mammalian melioidosis.

NOD-Like Receptors in Lung Diseases

The lung is a particularly vulnerable organ at the interface of the body and the exterior environment. It is constantly exposed to microbes and particles by inhalation. The innate immune system needs to react promptly and adequately to potential dangers posed by these microbes and particles, while at the same time avoiding extensive tissue damage. Nucleotide-binding oligomerization domain-like receptors (NLRs) represent a group of key sensors for microbes and damage in the lung. As such they are important players in various infectious as well as acute and chronic sterile inflammatory diseases, such as pneumonia, chronic obstructive pulmonary disease (COPD), acute lung injury/acute respiratory distress syndrome, pneumoconiosis, and asthma. Activation of most known NLRs leads to the production and release of pro-inflammatory cytokines, and/or to the induction of cell death. We will review NLR functions in the lung during infection and sterile inflammation.

Characterization of NLRP12 during the in vivo host immune response to Klebsiella pneumoniae and Mycobacterium tuberculosis

The majority of nucleotide binding domain leucine rich repeats-containing (NLR) family members has yet to be functionally characterized. Of the described NLRs, most are considered to be proinflammatory and facilitate IL-1β production. However, a newly defined sub-group of NLRs that function as negative regulators of inflammation have been identified based on their abilities to attenuate NF-κB signaling. NLRP12 (Monarch-1) is a prototypical member of this sub-group that negatively regulates both canonical and noncanonical NF-κB signaling in biochemical assays and in colitis and colon cancer models. The role of NLRP12 in infectious diseases has not been extensively studied. Here, we characterized the innate immune response of Nlrp12^−/− mice following airway exposure to LPS, Klebsiella pneumoniae and Mycobacterium tuberculosis. In response to E. coli LPS, Nlrp12^−/− mice showed a slight decrease in IL-1β and increase in IL-6 production, but these levels were not statistically significant. During K. pneumoniae infection, we observed subtle differences in cytokine levels and significantly reduced numbers of monocytes and lymphocytes in Nlrp12^−/− mice. However, the physiological relevance of these findings is unclear as no overt differences in the development of lung disease were observed in the Nlrp12^−/− mice. Likewise, Nlrp12^−/− mice demonstrated pathologies similar to those observed in the wild type mice following M. tuberculosis infection. Together, these data suggest that NLRP12 does not significantly contribute to the in vivo host innate immune response to LPS stimulation, Klebsiella pneumonia infection or Mycobacterium tuberculosis.

Roles of NOD1 (NLRC1) and NOD2 (NLRC2) in innate immunity and inflammatory diseases

NOD1 {nucleotide-binding oligomerization domain 1; NLRC [NOD-LRR (leucine-rich repeat) family with CARD (caspase recruitment domain) 1]} and NOD2 (NLRC2) are among the most prominent members of the NLR (NOD-LRR) family –proteins that contain nucleotide-binding NACHT domains and receptor-like LRR domains. With over 20 members identified in humans, NLRs represent important components of the mammalian innate immune system, serving as intracellular receptors for pathogens and for endogenous molecules elaborated by tissue injury. NOD1 and NOD2 proteins operate as microbial sensors through the recognition of specific PG (peptidoglycan) constituents of bacteria. Upon activation, these NLR family members initiate signal transduction mechanisms that include stimulation of NF-κB (nuclear factor-κB), stress kinases, IRFs (interferon regulatory factors) and autophagy. Hereditary polymorphisms in the genes encoding NOD1 and NOD2 have been associated with an increasing number of chronic inflammatory diseases. In fact, potential roles for NOD1 and NOD2 in inflammatory disorders have been revealed by investigations using a series of animal models. In the present review, we describe recent experimental findings associating NOD1 and NOD2 with various autoimmune and chronic inflammatory disorders, and we discuss prospects for development of novel therapeutics targeting these NLR family proteins.

Lactobacillus priming of the respiratory tract: Heterologous immunity and protection against lethal pneumovirus infection

We showed previously that wild-type mice primed via intranasal inoculation with live or heat-inactivated Lactobacillus species were fully (100%) protected against the lethal sequelae of infection with the virulent pathogen, pneumonia virus of mice (PVM), a response that is associated with diminished expression of proinflammatory cytokines and diminished virus recovery. We show here that 40% of the mice primed with live Lactobacillus survived when PVM challenge was delayed for 5months. This robust and sustained resistance to PVM infection resulting from prior interaction with an otherwise unrelated microbe is a profound example of heterologous immunity. We undertook the present study in order to understand the nature and unique features of this response. We found that intranasal inoculation with L. reuteri elicited rapid, transient neutrophil recruitment in association with proinflammatory mediators (CXCL1, CCL3, CCL2, CXCL10, TNF-alpha and IL-17A) but not Th1 cytokines. IFNγ does not contribute to survival promoted by Lactobacillus-priming. Live L. reuteri detected in lung tissue underwent rapid clearance, and was undetectable at 24h after inoculation. In contrast, L. reuteri peptidoglycan (PGN) and L. reuteri genomic DNA (gDNA) were detected at 24 and 48h after inoculation, respectively. In contrast to live bacteria, intranasal inoculation with isolated L. reuteri gDNA elicited no neutrophil recruitment, had minimal impact on virus recovery and virus-associated production of CCL3, and provided no protection against the negative sequelae of virus infection. Isolated PGN elicited neutrophil recruitment and proinflammatory cytokines but did not promote sustained survival in response to subsequent PVM infection. Overall, further evaluation of the responses leading to Lactobacillus-mediated heterologous immunity may provide insight into novel antiviral preventive modalities.

Down-regulation of NOD1 in neutrophils of periparturient dairy cows

Increased susceptibility of periparturient dairy cows to infectious diseases has been linked to neutrophil dysfunction. Cytoplasmic nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) mediate numerous aspects of innate immunity in humans and experimental animals. The authors hypothesized that neutrophils in periparturient cows have reduced NLR expression, resulting in declined immune responses to NLR activation. In this study, blood neutrophils from periparturient and mid-lactating cows were isolated and assessed for mRNA and protein expression of NOD1 and NOD2. Neutrophil phagocytosis and oxidative burst activity following NLR agonist-induced activation were also investigated. Periparturient cows demonstrated weak-to-absent NOD1 mRNA expression along with a significant reduction in NOD1 protein in their neutrophils. However, no difference in NOD2 protein expression was observed between groups although NOD2 mRNA abundance in periparturient cows was significantly reduced. Phagocytosis and oxidative burst activity measured by production of reactive oxygen species (ROS) were diminished in periparturient cows. NOD1 activation enhanced phagocytosis and ROS production in both isolated cell populations, regardless of the lactation stage. Nevertheless, NOD1-activated neutrophils showed reduced phagocytosis and ROS generation in periparturient cows with respect to mid-lactating subjects. Collectively, our findings suggest that NOD1 down-regulation in neutrophils may be a mechanism involved in periparturient neutrophil dysregulation.