What is new with Nods?

Transcriptomics of early responses to purified Piscine orthoreovirus-1 in Atlantic salmon (Salmo salar L.) red blood cells compared to non-susceptible cell lines

Piscine red blood cells (RBC) are nucleated and have been characterized as mediators of immune responses in addition to their role in gas exchange. Salmonid RBC are major target cells of Piscine orthoreovirus-1 (PRV-1), the etiological agent of heart and skeletal muscle inflammation (HSMI) in farmed Atlantic salmon (Salmo salar). PRV-1 replicates in RBC ex vivo, but no viral amplification has been possible in available A. salmon cell lines. To compare RBC basal transcripts and transcriptional responses to PRV-1 in the early phase of infection with non-susceptible cells, we exposed A. salmon RBC, Atlantic salmon kidney cells (ASK) and Salmon head kidney cells (SHK-1) to PRV-1 for 24 h. The RNA-seq analysis of RBC supported their previous characterization as pluripotent cells, as they expressed a wide repertoire of genes encoding pattern recognition receptors (PRRs), cytokine receptors, and genes implicated in antiviral activities. The comparison of RBC to ASK and SHK-1 revealed immune cell features exclusively expressed in RBC, such as genes involved in chemotactic activity in response to inflammation. Differential expression analysis of RBC exposed to PRV-1 showed 46 significantly induced genes (≥ 2-fold upregulation) linked to the antiviral response pathway, including RNA-specific PRRs and interferon (IFN) response factors. In SHK-1, PRV induced a more potent or faster antiviral response (213 genes induced). ASK cells showed a differential response pattern (12 genes induced, 18 suppressed) less characterized by the dsRNA-induced antiviral pathway. Despite these differences, the RIG-I-like receptor 3 (RLR3) in the family of cytosolic dsRNA receptors was significantly induced in all PRV-1 exposed cells. IFN regulatory factor 1 (IRF1) was significantly induced in RBC only, in contrast to IRF3/IRF7 induced in SHK-1. Differences in IRF expression and activity may potentially affect viral propagation.

The Regulation of Neutrophil Migration in Patients with Sepsis: The Complexity of the Molecular Mechanisms and Their Modulation in Sepsis and the Heterogeneity of Sepsis Patients

Sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. Common causes include gram-negative and gram-positive bacteria as well as fungi. Neutrophils are among the first cells to arrive at an infection site where they function as important effector cells of the innate immune system and as regulators of the host immune response. The regulation of neutrophil migration is therefore important both for the infection-directed host response and for the development of organ dysfunctions in sepsis. Downregulation of CXCR4/CXCL12 stimulates neutrophil migration from the bone marrow. This is followed by transmigration/extravasation across the endothelial cell barrier at the infection site; this process is directed by adhesion molecules and various chemotactic gradients created by chemotactic cytokines, lipid mediators, bacterial peptides, and peptides from damaged cells. These mechanisms of neutrophil migration are modulated by sepsis, leading to reduced neutrophil migration and even reversed migration that contributes to distant organ failure. The sepsis-induced modulation seems to differ between neutrophil subsets. Furthermore, sepsis patients should be regarded as heterogeneous because neutrophil migration will possibly be further modulated by the infecting microorganisms, antimicrobial treatment, patient age/frailty/sex, other diseases (e.g., hematological malignancies and stem cell transplantation), and the metabolic status. The present review describes molecular mechanisms involved in the regulation of neutrophil migration; how these mechanisms are altered during sepsis; and how bacteria/fungi, antimicrobial treatment, and aging/frailty/comorbidity influence the regulation of neutrophil migration.

Study on the Mechanism of NLRP3/IL-1/ NF-κB Signaling Pathway and Macrophage Polarization in the Occurrence and Development of VTE

BACKGROUND

The role of inflammation in venous thromboembolism (VTE) has been the focus of recent research. The NLRP3/IL-1/NF-κB signaling pathway and cytokines such as IL-1, regulated by macrophage polarization, may be the key indicators of a prethrombotic state; however, the mechanisms by which they affect the occurrence of VTE remain unclear.

METHODS

We used neurobiological clamps to stimulate the vein wall to induce vascular endothelial damage to generate a rat model of VTE, applied enzyme-linked immunosorbent assay and real time-polymerase chain reaction technology to identify key proteins (IL1β, Caspase-1, NLRP3, and NF-κB P65), gene mRNA levels and protein expression levels of the NLRP3/IL-1/NF-κB signaling pathway in each group of Sprague Dawley rats, and observed the polarization state of M1 (CD86) and M2 (CD206) macrophages using immunohistochemistry.

RESULTS

A dark red, small thrombus developed in the inferior vena cava immediately after modeling in the model and inhibitor groups. The plasma levels of IL-1 and TNF-α, mRNA expression of key proteins (IL1β, Caspase-1, NLRP3, and NF-κB P65), and expression of key proteins (IL1β, Caspase-1, NLRP3, and NF-κB P65) in VTE model rats were significantly higher than inhibitor, sham operation, and normal control groups (P < 0.05). Six hours after VTE modeling, M₁ type macrophages were more significantly increased than M₂ type macrophages in thrombus tissue (P < 0.05).

CONCLUSIONS

Our analyses demonstrated that the nod-like receptor protein3/Interleukin-1/nuclear factor-κB signaling pathway and macrophage polarization are important in the occurrence and development of VTE and that their target regulation may become a new strategy for VTE prevention and treatment.

NOD1 induces pyroptotic cell death to aggravate liver ischemia-reperfusion injury in mice

Nucleotide-binding oligomerization domain 1 (NOD1) can direct the release of inflammatory factors and influence autophagy and apoptosis in hepatic ischemia-reperfusion injury (IRI) in mice. As pyroptosis is involved in a number of inflammatory reactions, in this report, we investigated the potential for NOD1 to affect pyroptosis. We found that an increased expression of NOD1 during IRI was related to activation of the pyroptotic signaling pathway. With NOD1 activation, cleavage fragments of Caspase-1, gasdermin D (GSDMD), and interleukin (IL)-1β were all increased. Moreover, downregulation of NOD1 expression in AML12 cells exerted an opposite effect. Expression levels of cleaved-Caspase-1 and cleaved-GSDMD decreased after exposure to IRI and the number of cell membrane pores and apoptotic or pyroptotic cells decreased, along with the contents of inflammatory factors and lactate dehydrogenase in the supernatants of AML12 cells. Based on these findings, we conclude that NOD1 aggravates the pyroptotic cell death associated with hepatic ischemia-reperfusion injury in a mouse model via the Caspase-1/GSDMD axis. These findings help to alleviate pyroptotic cell death during liver transplantation or resection, providing new insights into novel protective therapies for liver IRI.

Whole-Genome comparative analysis reveals genetic mechanisms of disease resistance and heat tolerance of tropical Bos indicus cattle breeds

The Bos indicus cattle breeds have been naturally selected over thousands of years for disease resistance and thermo-tolerance. However, a genetic mechanism of these specific inherited characteristics needs to be discovered. Hence, in this study, the whole-genome comparative analysis of Bos indicus cattle breeds of Kangayam, Tharparkar, Sahiwal, Red Sindhi, and Hariana of the Indian subcontinent was conducted. The genetic variants identification analysis revealed a total of 15,58,51,012 SNPs and 1,00,62,805 InDels in the mapped reads across all Bos indicus cattle breeds. The functional annotation of 17,252 genes that comprised both, SNPs and InDels, of high functional impact on proteins, has been carried out. The functional annotation results revealed the pathways that were involved in the innate immune response including toll-like receptors, a retinoic acid-inducible gene I like receptors, NOD-like receptors, Jak-STAT signaling pathways, and the non-synonymous variants in the candidate immune genes. Further, we also identified several pathways involved in heat shock response, hair and skin properties, oxidative stress response, osmotic stress response, thermal sweating, feed intake, metabolism, and the non-synonymous variants in the candidate thermo-tolerant genes. These pathways and genes were directly or indirectly contributing to the disease resistance and thermo-tolerance adaptations of Bos indicus cattle breeds.

[What is confirmed in the treatment of chronic inflammatory bowel diseases]

The prevalence of the chronic inflammatory bowel diseases (CIBD) Crohn's disease (CD) and ulcerative colitis (UC) is on the rise worldwide. In Germany CIBDs are also a significant healthcare problem. The pathogenesis is complex and involves genetic factors, environmental aspects and changes in the immunological constitution. Furthermore, the gut microbiota plays a role in the maintenance of intestinal inflammation. Fortunately, several new drugs, in particular biologicals, have been approved for the treatment of CIBDs. The treatment of UC is mainly based on 5‑aminosalicylic acid formulations, preferably as a topical form for distal colitis and proctitis as well as local budesonide formulations. In the case of extensive spread, high disease activity and refractory disease antibodies (biologicals) are successfully used, similar to CD. In addition to anti-tumor necrosis factor antibodies (infliximab, adalimumab, golimumab), vedolizumab, an anti-integrin antibody and the interleukin 12/23 antibody ustekinumab can be successfully used. The intravenous and also subcutaneous administration of antibodies are increasing in importance and are now available for all forms. Furthermore, the Janus kinase inhibitor tofacitinib is an orally administered option for UC. Clinical scores, endoscopy, ultrasound, laboratory parameters and calprotectin determination in stool are employed to evaluate treatment response (treat to target approach). Ultimately, the long-term goal is mucosal healing. Despite advances in the pharmaceutical treatment, a significant number of patients with CIBD still suffer from treatment refractory courses and need surgery at some time during the disease.

Mapping Transcriptome Data to Protein-Protein Interaction Networks of Inflammatory Bowel Diseases Reveals Disease-Specific Subnetworks

Inflammatory bowel disease (IBD) is the common name for chronic disorders associated with the inflammation of the gastrointestinal tract. IBD is triggered by environmental factors in genetically susceptible individuals and has a significant number of incidences worldwide. Crohn’s disease (CD) and ulcerative colitis (UC) are the two distinct types of IBD. While involvement in ulcerative colitis is limited to the colon, Crohn’s disease may involve the whole gastrointestinal tract. Although these two disorders differ in macroscopic inflammation patterns, they share various molecular pathogenesis, yet the diagnosis can remain unclear, and it is important to reveal their molecular signatures in the network level. Improved molecular understanding may reveal disease type-specific and even individual-specific targets. To this aim, we determine the subnetworks specific to UC and CD by mapping transcriptome data to protein–protein interaction (PPI) networks using two different approaches [KeyPathwayMiner (KPM) and stringApp] and perform the functional enrichment analysis of the resulting disease type-specific subnetworks. TP63 was identified as the hub gene in the UC-specific subnet and p63 tumor protein, being in the same family as p53 and p73, has been studied in literature for the risk associated with colorectal cancer and IBD. APP was identified as the hub gene in the CD-specific subnet, and it has an important role in the pathogenesis of Alzheimer’s disease (AD). This relation suggests that some similar genetic factors may be effective in both AD and CD. Last, in order to understand the biological meaning of these disease-specific subnets, they were functionally enriched. It is important to note that chemokines—special types of cytokines—and antibacterial response are important in UC-specific subnets, whereas cytokines and antimicrobial responses as well as cancer-related pathways are important in CD-specific subnets. Overall, these findings reveal the differences between IBD subtypes at the molecular level and can facilitate diagnosis for UC and CD as well as provide potential molecular targets that are specific to disease subtypes.

The MHC Class-I Transactivator NLRC5: Implications to Cancer Immunology and Potential Applications to Cancer Immunotherapy

The immune system constantly monitors the emergence of cancerous cells and eliminates them. CD8⁺ cytotoxic T lymphocytes (CTLs), which kill tumor cells and provide antitumor immunity, select their targets by recognizing tumor antigenic peptides presented by MHC class-I (MHC-I) molecules. Cancer cells circumvent immune surveillance using diverse strategies. A key mechanism of cancer immune evasion is downregulation of MHC-I and key proteins of the antigen processing and presentation machinery (APM). Even though impaired MHC-I expression in cancers is well-known, reversing the MHC-I defects remains the least advanced area of tumor immunology. The discoveries that NLRC5 is the key transcriptional activator of MHC-I and APM genes, and genetic lesions and epigenetic modifications of NLRC5 are the most common cause of MHC-I defects in cancers, have raised the hopes for restoring MHC-I expression. Here, we provide an overview of cancer immunity mediated by CD8⁺ T cells and the functions of NLRC5 in MHC-I antigen presentation pathways. We describe the impressive advances made in understanding the regulation of NLRC5 expression, the data supporting the antitumor functions of NLRC5 and a few reports that argue for a pro-tumorigenic role. Finally, we explore the possible avenues of exploiting NLRC5 for cancer immunotherapy.

Characterization and expression analysis of Nod-like receptor 3 (NLRC3) against infection with Piscirickettsia salmonis in Atlantic salmon

The nucleotide binding oligomerization domain like receptors, or NOD like receptors (NLRs), are intracellular receptors responsible for recognizing pathogens in vertebrates. Several NLR mammalian models have been characterized and analyzed but few studies have been performed with teleost species. In this study, we analyzed the nucleotide sequence of six mRNA variants of NLRC3 in Atlantic salmon (SsNLRC3), and we deduced the amino acid sequence coding for two different isoforms with a total length of 1135 amino acids and 1093 amino acids. We analyzed the phylogeny of all variants, including a Piscirickettsia salmonis infection in Atlantic salmon. All variants and their expression pattern during infection were analyzed using real-time qPCR. One of the analyzed variants was over-expressed during the early stages of Piscirickettsia salmonis infection, and we were able to identify two different SsNLRC3 isoforms. Lastly, we observed that an alteration in the amino acid sequence of one of the isoforms can directly affect the pathogen recognition function.

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.

A Comprehensive Review and Update on the Pathogenesis of Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a chronic and life-threating inflammatory disease of gastroenteric tissue characterized by episodes of intestinal inflammation. The pathogenesis of IBD is complex. Recent studies have greatly improved our knowledge of the pathophysiology of IBD, leading to great advances in the treatment as well as diagnosis of IBD. In this review, we have systemically reviewed the pathogenesis of IBD and highlighted recent advances in host genetic factors, gut microbiota, and environmental factors and, especially, in abnormal innate and adaptive immune responses and their interactions, which may hold the keys to identify novel predictive or prognostic biomarkers and develop new therapies.

Nucleotide-binding oligomerization domain-containing protein 1 (NOD1) in Asian seabass, Lates calcarifer: Cloning, ontogeny and expression analysis following bacterial infection or ligand stimulation

NOD1 (Nucleotide-binding oligomerization domain-containing protein 1) is one of the most prominent intracellular Nod-like receptors (NLRs), responsible for detecting different microbial components and products arising from tissue injury. Here, we have identified and cloned NOD1 transcript in the Asian seabass, Lates calcarifer (AsNOD1), which consists of 3749 nucleotides and encodes for a predicted putative protein of 900 AA. The AsNOD1 possesses the typical structure of NLR family, consisting of N-terminal CARD domain, centrally located NACHT domain and C-terminal LRRs. The AsNOD1 showed ubiquitous tissue expression in 11 different tissues of healthy animals tested with high levels of expression in hindgut and gill. From the ontogenetic expression profile of AsNOD1, it is quite evident that this gene might follow a maternally-transferred trend in euryhaline teleosts, as it is highly abundant in embryonic developmental stages. The constitutive immunomodulation of AsNOD1 in terms of expression level was clearly evident in the different tissues of Asian seabass-injected either with Vibrio alginolyticus or poly I:C. However, injection with Staphylococcus aureus did not elicit similar immunomodulation except for the up-regulation noticed at few time-points in some tissues. SISK-cell line induced with different ligands such as poly I:C, LPS and PGN also showed up-regulation of AsNOD1 in certain time-points in vitro. Based on the results obtained in the present study, it can be inferred that the AsNOD1 might play an immunoregulatory role upon exposure to different bacterial as well as viral PAMPs and also might be an important component of innate immune element during embryonic and larval development in the euryhaline teleost Asian seabass.

Pathogenesis of Neisseria gonorrhoeae in the female reproductive tract: neutrophilic host response, sustained infection, and clinical sequelae

PURPOSE OF REVIEW

Gonorrhea is a major global health concern, caused by the bacterium Neisseria gonorrhoeae. The main clinical feature of acute gonorrhea is neutrophilic influx that is unable to clear infection. Women of reproductive age are predominantly at risk for serious sequelae of gonorrhea, including pelvic inflammatory disease, ectopic pregnancy, and infertility. This review will highlight how neutrophils are recruited to the female reproductive tract (FRT) in response to N. gonorrhoeae, how N. gonorrhoeae resists killing by neutrophils, and the connection between neutrophilic inflammation and cellular damage.

RECENT FINDINGS

Epithelial cells and immune cells of the FRT recognize and respond to N. gonorrhoeae lipid A and heptose bisphosphate of lipooligosaccharide, porin, lipoproteins, and peptidoglycan fragments. N. gonorrhoeae skews the resulting immune response toward a neutrophilic, Th17-like response. N. gonorrhoeae has multiple, nonredundant mechanisms to survive inside neutrophils and in neutrophil extracellular traps. Infection that ascends to the upper FRT induces the further release of inflammatory cytokines and matrix metalloproteinases, which cause epithelial damage.

SUMMARY

N. gonorrhoeae is remarkable in its ability to recruit neutrophils, yet survive in their midst. New models being developed for FRT infection with N. gonorrhoeae will be useful to reveal the mechanisms underlying these observations.

Emerging Mechanisms of Innate Immunity and Their Translational Potential in Inflammatory Bowel Disease

Activation of the innate immune system through pattern-recognition receptor (PRR) signaling plays a pivotal role in the early induction of host defense following exposure to pathogens. Loss of intestinal innate immune regulation leading aberrant immune responses has been implicated in the pathogenesis of inflammatory bowel disease (IBD). The precise role of PRRs in gut inflammation is not well understood, but considering their role as bacterial sensors and their genetic association with IBD, they likely contribute to dysregulated immune responses to the commensal microbiota. The purpose of this review is to evaluate the emerging functions of PRRs including their functional cross-talk, how they respond to mitochondrial damage, induce mitophagy or autophagy, and influence adaptive immune responses by interacting with the antigen presentation machinery. The review also summarizes some of the recent attempts to harness these pathways for therapeutic approaches in intestinal inflammation.

TLRs/NLRs: Shaping the landscape of host immunity

Innate immune system provides the first line of defense against pathogenic organisms. It has a varied and large collection of molecules known as pattern recognition receptors (PRRs) which can tackle the pathogens promptly and effectively. Toll-like receptors (TLRs) and NOD-like receptors (NLRs) are members of the PRR family that recognize pathogen associated molecular patterns (PAMPs) and play pivotal roles to mediate defense against infections from bacteria, fungi, virus and various other pathogens. In this review, we discuss the critical roles of TLRs and NLRs in the regulation of host immune-effector functions such as cytokine production, phagosome-lysosome fusion, inflammasome activation, autophagy, antigen presentation, and B and T cell immune responses that are known to be essential for mounting a protective immune response against the pathogens. This review may be helpful to design TLRs/NLRs based immunotherapeutics to control various infections and pathophysiological disorders.

Ischemia as a factor affecting innate immune responses in kidney transplantation

PURPOSE OF REVIEW

Ischemic injury inevitably occurs during the procurement of organs for transplantation, and the injury is worsened by inflammation following reperfusion. The purpose of this review is to describe the role of the innate immune system in ischemia-induced renal injury in kidneys procured for transplantation. The key role of pattern recognition receptors in immune responses to ischemia is described. Innate immune receptors are emerging novel targets for the amelioration of ischemic injury of donor kidneys.

RECENT FINDINGS

Several families of pattern recognition receptors are direct mediators of early injurious events during kidney procurement, and also innate and adaptive immune responses after transplantation. The deleterious events associated with the activation of the innate immune system in donor kidneys significantly contribute to short and long-term allograft outcomes.

SUMMARY

Although a number of therapies have been proposed to decrease ischemic donor kidney injury, targeting the innate immune system is an exciting new area that is gaining significant interest in transplantation. As we learn more about how these important receptors are regulated by ischemia, strategies will likely evolve to allow their modulation in ischemic renal injury.

Expression profile of innate immune receptors, NLRs and AIM2, in human colorectal cancer: correlation with cancer stages and inflammasome components

NLRs (nucleotide-binding domain leucine-rich repeat proteins or NOD-like receptors) are regulators of inflammation and immunity. A subgroup of NLRs and the innate immune receptor, AIM2 (absent-in-melanoma 2), can induce the assembly of a large caspase-1 activating complex called the inflammasome. Other NLRs regulate key signaling pathways such as NF-kB and MAPK. Since inflammation is a central component of colorectal cancer (CRC), this work was undertaken to analyze NLR and AIM2 expression in human CRC by combining bioinformatics analysis and experimental verification using clinical tissue samples. Additional experiments analyzed the association of (i) gene expression and cancer staging, and (ii) gene expression among inflammasome components.

Ten public CRC datasets from the Oncomine^® Platform were analyzed. Genes analyzed include NLRP1, NLRP3, NLRP6, NLRP12, NLRC3, NLRC4, NLRC5, NOD1, NOD2 and AIM2. Additionally, forty case-matched cancer samples and adjacent healthy control tissues isolated from a cohort of Chinese CRC patients were profiled.

Three patterns of gene expression in CRC are shown. The expression of NLRC3, a checkpoint of inflammation, and the inflammasome components NLRP1, NLRP3, NLRC4 and AIM2 were reduced in CRC. NOD1 and NOD2 expression was increased in CRC, while NLRC5, NLRP6 and NLRP12 showed little difference compared to controls. Reduced expression of NLRC3 in CRC was verified in all available databases analyzed and confirmed with our patient cohort. Furthermore, the extent of NLRC3 and AIM2 gene reduction was correlated with cancer progression. This report reveals the potential value of NLR and AIM2 genes as biomarkers of CRC and cancer progression.

NOD1 and NOD2 Interact with the Phagosome Cargo in Mast Cells: A Detailed Morphological Evidence

Mast cells (MC) play a key role in triggering the inflammatory process and share some functions with professional phagocytes. It is not clear whether or not the phagocytic process in MC follows the same route and has the same meaning of that of professional phagocytes. Herein we analyze in detail the structure of the phagosome in rat peritoneal mast cells (RPMC). The ultrastructural analysis of the phagosome, containing either model particles or bacteria, reveals that these vacuoles are very tight, and in several areas, their membrane seems to have dissolved. RPMC express NOD1 and NOD2 proteins whose role is to recognize intracellular foreign components and induce the production of pro-inflammatory mediators. Following Escherichia coli ingestion, both these molecules are found on the phagosome membrane and on ingested pathogens, together with phagosome maturation markers. These findings suggest that in RPMC the ingested cargo can, through interruptions of the phagosome membrane, interact directly with NODs, which act as switches in the process of cytokine production.

[The NLRP3 inflammasome: Physiopathology and therapeutic application]

The innate immune system constitutes the first line of host defense against pathogens. "Nonself", such as exogenous particles or pathogens, triggers an inflammatory response. Inflammasomes are molecular platforms activated upon cellular infection or stress that trigger the maturation of proinflammatory cytokines such as IL-1β. Activation of the NLRP3 inflammasome pathway, the most extensively studied, appears to be the corner stone of many inflammatory diseases, including Crohn's disease, rheumatoid arthritis and gout. Cryopyrine-associated periodic syndromes (CAPS) are NLRP3 inflammasome-associated diseases. Canakinumab (Ilaris(®)) is the only drug approved for CAPS treatment in France. Targeted therapy against NLRP3 inflammasome and IL-1β might be the new anti-inflammatory drugs.

Mechanisms and pathways of innate immune activation and regulation in health and cancer

Research on innate immune signaling and regulation has recently focused on pathogen recognition receptors (PRRs) and their signaling pathways. Members of PRRs sense diverse microbial invasions or danger signals, and initiate innate immune signaling pathways, leading to proinflammatory cytokines production, which, in turn, instructs adaptive immune response development. Despite the diverse functions employed by innate immune signaling to respond to a variety of different pathogens, the innate immune response must be tightly regulated. Otherwise, aberrant, uncontrolled immune responses will lead to harmful, or even fatal, consequences. Therefore, it is essential to better discern innate immune signaling and many regulators, controlling various signaling pathways, have been identified. In this review, we focus on the recent advances in our understanding of the activation and regulation of innate immune signaling in the host response to pathogens and cancer.

Identification and biological consequences of the O-GlcNAc modification of the human innate immune receptor, Nod2

Nucleotide-binding oligomerization domain 2 (Nod2) is an intracellular receptor that can sense the bacterial peptidoglycan component, muramyl dipeptide. Upon activation, Nod2 induces the production of various inflammatory molecules such as cytokines and chemokines. Genetic linkage analysis identified and revealed three major mutations in Nod2 that are associated with the development of Crohn's disease. The objective of this study is to further characterize this protein by determining whether Nod2 is posttranslationally modified by O-N-acetylglucosamine (O-GlcNAc). O-GlcNAcylation is one type of posttranslational modification in which the O-GlcNAc transferase transfers GlcNAc from UDP-GlcNAc to selected serine and threonine residues of intracellular proteins. We found that wild-type Nod2 and a Nod2 Crohn's-associated variant are O-GlcNAcylated and this modification affects Nod2's ability to signal via the nuclear factor kappa B pathway.

NLRC5 expression in tumors and its role as a negative prognostic indicator in stage III non-small-cell lung cancer patients

Major histocompatibility complex (MHC) class I molecules have a crucial role in tumor immune evasion; however, the association of MHC class I molecules with outcomes in cancer patients remains controversial. Nucleotide-binding oligomerization-like receptor family caspase recruitment domain-containing 5 (NLRC5) has been reported to be a MHC class I transactivator. However, the expression and function of NLRC5 in cancer remains to be elucidated. The present study aimed to retrospectively examine NLRC5 expression in human tumor tissues and its association with clinical outcomes of non-small-cell lung cancer (NSCLC) stage III patients. The expression of MHC class I and NLRC5 in NSCLC were detected using immunohistochemistry (IHC). The association between their expression levels was assessed using the Pearson's χ² test and their association with survival was assessed using Kaplan-Meier analysis and the log-rank test. In addition, the expression of NLRC5 and MHC class I were examined in 323 cases of seven other types of tumors and their correlations were studied. The results revealed that the expression of NLRC5 was correlated with that of MHC class I in NSCLC patients (P=0.008). MHC class I-positive and nuclear NLRC5-positive NSCLC patients were found to have shorter overall survival (OS) rates (log-rank, P=0.032 and P=0.039, respectively). In addition, in the seven different tumor types, there was a significant correlation between MHC class I and NLRC5 nuclear expression (P<0.001) as well as MHC class I and NLRC5 cytoplasmic expression (P=0.003). In conclusion, NLRC5 was demonstrated to be widely expressed in eight tumor tissues and its expression was correlated with that of MHC class I. Of note, nuclear NLRC5-negative and MHC class I-negative stage III NSCLC patients had improved OS rates compared to those with positive expression. Therefore, NLRC5 and MHC class I may be negative prognostic indicators in NSCLC stage III patients.

Signal transduction by a fungal NOD-like receptor based on propagation of a prion amyloid fold

In the fungus Podospora anserina, the [Het-s] prion induces programmed cell death by activating the HET-S pore-forming protein. The HET-s β-solenoid prion fold serves as a template for converting the HET-S prion-forming domain into the same fold. This conversion, in turn, activates the HET-S pore-forming domain. The gene immediately adjacent to het-S encodes NWD2, a Nod-like receptor (NLR) with an N-terminal motif similar to the elementary repeat unit of the β-solenoid fold. NLRs are immune receptors controlling cell death and host defense processes in animals, plants and fungi. We have proposed that, analogously to [Het-s], NWD2 can activate the HET-S pore-forming protein by converting its prion-forming region into the β-solenoid fold. Here, we analyze the ability of NWD2 to induce formation of the β-solenoid prion fold. We show that artificial NWD2 variants induce formation of the [Het-s] prion, specifically in presence of their cognate ligands. The N-terminal motif is responsible for this prion induction, and mutations predicted to affect the β-solenoid fold abolish templating activity. In vitro, the N-terminal motif assembles into infectious prion amyloids that display a structure resembling the β-solenoid fold. In vivo, the assembled form of the NWD2 N-terminal region activates the HET-S pore-forming protein. This study documenting the role of the β-solenoid fold in fungal NLR function further highlights the general importance of amyloid and prion-like signaling in immunity-related cell fate pathways.

The fungus Podospora anserina uses a prion amyloid fold as a signal transduction device between a Nod-like receptor and a downstream cell death execution protein.

Although amyloids are best known as protein aggregates that are responsible for fatal neurodegenerative diseases, amyloid structures can also fulfill functional roles in cells. In particular, the controlled formation of amyloid structures appears to be involved in different signaling processes in the context of programmed cell death and host defense. The [Het-s] prion of the filamentous fungus Podospora anserina is a model system in which the 3-D structure of the prion form has been solved. The [Het-s] prion works as an activation switch for a second protein termed HET-S. HET-S is a pore-forming protein that is activated when the [Het-s] prion causes its C-terminal domain to adopt an amyloid-like fold. The protein encoded by the gene adjacent to het-S is a Nod-like receptor (NLR) called NWD2. NLRs are immune receptors that control host defense and cell death processes in plants, animals, and fungi. We show that NWD2 can template the formation of the [Het-s] prion fold in a ligand-controlled manner. NWD2 has an N-terminal motif homologous to the HET-S/s prion-forming region; we find that this region is both necessary and sufficient for its prion-inducing activity, and our functional and structural approaches reveal that the N-terminal region of NWD2 adopts a fold closely related to that of the HET-S/s prion. This study illustrates how the controlled formation of a prion amyloid fold can be used in a signaling process whereby a Nod-like receptor protein activates a downstream cell death execution domain.

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.

Nucleotide oligomerization domain 1 ligation suppressed murine allergen-specific T-cell proliferation and airway hyperresponsiveness

The cytosolic nucleotide oligomerization domain (NOD)-like receptors NOD1 and NOD2 are important contributors to the intracellular recognition of pathogens including Chlamydophila pneumoniae, but little is known about their influence on allergen-induced airway inflammation. In BALB/c mice, we observed that infection with C. pneumoniae before systemic sensitization with ovalbumin (OVA) and local OVA airway exposure diminished airway hyperresponsiveness (AHR). Thus, the impact of the NOD1 agonist FK156 and the NOD2 agonist muramyl dipeptide given 6 hours before each sensitization or airway challenge was evaluated regarding AHR, OVA-specific plasma immunoglobulins, bronchoalveolar lavage fluid differentials, and cytokines. Spleen dendritic cells of FK156-treated mice were isolated and cocultured with OVA-specific T cells isolated from DO11.10 mice, and T-cell proliferation was quantified after OVA restimulation. T-cell proliferation was investigated in vivo in lungs and lymph nodes of FK156-treated and OVA-exposed DO11.10 mice. FK156, but not muramyl dipeptide, reduced AHR and pulmonary eosinophilic infiltration if given before OVA sensitization or challenge, whereas T-helper (Th)2 cytokines were not diminished. Dendritic cells from FK156-treated mice evoked less OVA-specific T-cell proliferation as compared with solvent-treated controls. Similarly, antigen-specific T-cell activation in lung tissue was diminished after FK156 treatment. We conclude that NOD1 activation reduced AHR in allergen-induced lung inflammation, which was accompanied by a reduction of allergen-specific T-cell proliferation.

Host-induced bacterial cell wall decomposition mediates pattern-triggered immunity in Arabidopsis

Peptidoglycans (PGNs) are immunogenic bacterial surface patterns that trigger immune activation in metazoans and plants. It is generally unknown how complex bacterial structures such as PGNs are perceived by plant pattern recognition receptors (PRRs) and whether host hydrolytic activities facilitate decomposition of bacterial matrices and generation of soluble PRR ligands. Here we show that Arabidopsis thaliana, upon bacterial infection or exposure to microbial patterns, produces a metazoan lysozyme-like hydrolase (lysozyme 1, LYS1). LYS1 activity releases soluble PGN fragments from insoluble bacterial cell walls and cleavage products are able to trigger responses typically associated with plant immunity. Importantly, LYS1 mutant genotypes exhibit super-susceptibility to bacterial infections similar to that observed on PGN receptor mutants. We propose that plants employ hydrolytic activities for the decomposition of complex bacterial structures, and that soluble pattern generation might aid PRR-mediated immune activation in cell layers adjacent to infection sites.

Nod2 activates NF-kB in CD4+ T cells but its expression is dispensable for T cell-induced colitis

Although the etiology of Crohn's disease (CD) remains elusive this disease is characterized by T cell activation that leads to chronic inflammation and mucosal damage. A potential role for maladaptation between the intestinal microbiota and the mucosal immune response is suggested by the fact that mutations in the pattern recognition receptor Nod2 are associated with higher risks for developing CD. Although Nod2 deletion in CD4⁺ T cells has been shown to impair the induction of colitis in the murine T cell transfer model, the analysis of T cell intrinsic Nod2 function in T cell differentiation and T cell-mediated immunity is inconsistent between several studies. In addition, the role of T cell intrinsic Nod2 in regulatory T cell (Treg) development and function during colitis remain to be analyzed. In this study, we show that Nod2 expression is higher in activated/memory CD4⁺ T cells and its expression was inducible after T cell receptor (TCR) ligation. Nod2 stimulation with muramyl dipeptide (MDP) led to a nuclear accumulation of c-Rel NF-kB subunit. Although functionally active in CD4⁺ T cells, the deletion of Nod2 did not impair the induction and the prevention of colitis in the T cell transfer model. Moreover, Nod2 deletion did not affect the development of Foxp3⁺ Treg cells in the spleen of recipient mice and Nod2 deficient CD4 T cells expressing the OVA specific transgenic TCR were able to differentiate in Foxp3⁺ Treg cells after OVA feeding. In vitro, CD25⁺ Nod2 deficient T cells suppressed T cell proliferation as well as wild type counter parts and T cell stimulation with MDP did not affect the proliferation and the cytokine secretion of T cells. In conclusion, our data indicate that Nod2 is functional in murine CD4⁺ T cells but its expression is dispensable for the T cell regulation of colitis.

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.

The Interplay between NLRs and Autophagy in Immunity and Inflammation

Since they were first described as cytosolic sensors of microbial molecules a decade ago, the Nod-like receptors (NLRs) have been shown to have many different and important roles in various aspects of immune and inflammatory responses, ranging from antimicrobial mechanisms to control of adaptive responses. In this review, we focus on the interplay between NLRs and autophagy, an evolutionarily conserved mechanism that is crucial for homeostasis and has recently been shown to be involved in the protective response against infections. Furthermore, the association between mutations of NLRs as well as proteins that form the autophagic machinery and inflammatory diseases such as Crohn’s disease highlight the importance of these proteins and their interactions in the regulation of inflammation.

IRF-7: an antiviral factor and beyond

This review will summarize main characteristics and functions of IRF-7. IRF-7 and the highly homologous IRF-3 are two members of the IRF family of transcription factors that have emerged as crucial regulators of type I interferon (IFN) in response to pathogenic infections downstream pathogen recognition receptors. IRF-7 is also part of a positive-feedback regulatory loop essential for sustained IFN responses. Thus, tight regulation of its expression and activity is necessary to balance IFN-mediated beneficial effects and unwanted pathological consequences of IFN overproduction. Its role as an antiviral factor independent of IFN expression, and its involvement in other cellular functions beyond antiviral functions, including regulation of oncogenesis and metabolism, underscore its important role in the regulation of cellular homeostasis.

Cyclic-di-GMP and cyclic-di-AMP activate the NLRP3 inflammasome

The cyclic dinucleotides 3'-5'diadenylate (c-diAMP) and 3'-5' diguanylate (c-diGMP) are important bacterial second messengers that have recently been shown to stimulate the secretion of type I Interferons (IFN-Is) through the c-diGMP-binding protein MPYS/STING. Here, we show that physiologically relevant levels of cyclic dinucleotides also stimulate a robust secretion of IL-1β through the NLRP3 inflammasome. Intriguingly, this response is independent of MPYS/STING. Consistent with most NLRP3 inflammasome activators, the response to c-diGMP is dependent on the mobilization of potassium and calcium ions. However, in contrast to other NLRP3 inflammasome activators, this response is not associated with significant changes in mitochondrial potential or the generation of mitochondrial reactive oxygen species. Thus, cyclic dinucleotides activate the NLRP3 inflammasome through a unique pathway that could have evolved to detect pervasive bacterial pathogen-associated molecular patterns associated with intracellular infections.

Activation of NOD receptors by Neisseria gonorrhoeae modulates the innate immune response

NOD1 and NOD2 are members of the NOD-like receptor family of cytosolic pattern recognition receptors that recognize specific fragments of the bacterial cell wall component peptidoglycan. Neisseria species are unique amongst Gram-negative bacteria in that they turn over large amounts of peptidoglycan during growth. We examined the ability of NOD1 and NOD2 to recognize Neisseria gonorrhoeae, and determined the role of NOD-dependent signaling in regulating the immune response to gonococcal infection. Gonococci, as well as conditioned medium from mid-logarithmic phase grown bacteria, were capable of activating both human NOD1 and NOD2, as well as mouse NOD2, leading to the activation of the transcription factor NF-κB and polyubiquitination of the adaptor receptor-interacting serine-threonine kinase 2. We identified a number of cytokines and chemokines that were differentially expressed in wild type versus NOD2-deficient macrophages in response to gonococcal infection. Moreover, NOD2 signaling up-regulated complement pathway components and cytosolic nucleic acid sensors, suggesting a broad impact of NOD activation on innate immunity. Thus, NOD1 and NOD2 are important intracellular regulators of the immune response to infection with N. gonorrhoeae. Given the intracellular lifestyle of this pathogen, we believe these cytosolic receptors may provide a key innate immune defense mechanism for the host during gonococcal infection.

The cutaneous innate immune response in patients with atopic dermatitis

Orchestrating when and how the cutaneous innate immune system should respond to commensal or pathogenic microbes is a critical function of the epithelium. The cutaneous innate immune system is a key determinant of the physical, chemical, microbial, and immunologic barrier functions of the epidermis. A malfunction in this system can lead to an inadequate host response to a pathogen or a persistent inflammatory state. Atopic dermatitis is the most common inflammatory skin disorder and characterized by abnormalities in both skin barrier structures (stratum corneum and tight junctions), a robust T(H)2 response to environmental antigens, defects in innate immunity, and an altered microbiome. Many of these abnormalities may occur as the consequence of epidermal dysfunction. The epidermis directly interfaces with the environment and, not surprisingly, expresses many pattern recognition receptors that make it a key player in cutaneous innate immune responses to skin infections and injury. This review will discuss the role epidermal innate receptors play in regulation of skin barriers and, where possible, discuss the relevance of these findings for patients with atopic dermatitis.

NOD2-mediated suppression of CD55 on neutrophils enhances C5a generation during polymicrobial sepsis

Nucleotide-binding oligomerization domain (NOD) 2 is a cytosolic protein that plays a defensive role in bacterial infection by sensing peptidoglycans. C5a, which has harmful effects in sepsis, interacts with innate proteins. However, whether NOD2 regulates C5a generation during sepsis remains to be determined. To address this issue, cecal ligation & puncture (CLP)-induced sepsis was compared in wild type and Nod2-/- mice. Nod2-/- mice showed lower levels of C5a, IL-10, and IL-1β in serum and peritoneum, but higher survival rate during CLP-induced sepsis compared to wild type mice. Injection of recombinant C5a decreased survival rates of Nod2-/- mice rate during sepsis, whereas it did not alter those in wild type mice. These findings suggest a novel provocative role for NOD2 in sepsis, in contrast to its protective role during bacterial infection. Furthermore, we found that NOD2-mediated IL-10 production by neutrophils enhanced C5a generation by suppressing CD55 expression on neutrophils in IL-1β-dependent and/or IL-1β-independent manners, thereby aggravating CLP-induced sepsis. SB203580, a receptor-interacting protein 2 (RIP2) inhibitor downstream of NOD2, reduced C5a generation by enhancing CD55 expression on neutrophils, resulting in attenuation of polymicrobial sepsis. Therefore, we propose a novel NOD2-mediated complement cascade regulatory pathway in sepsis, which may be a useful therapeutic target.

Innate immunity in donor procurement

PURPOSE OF REVIEW

Ischaemic kidney injury occurs during organ procurement and can lead to delayed graft function or nonviable grafts. The innate immune system is a key trigger of inflammation in renal ischaemia. This review discusses the components of innate immunity known to be involved in renal ischaemic reperfusion injury (IRI). Understanding how inflammatory damage is initiated in renal IRI is important for the development of targeted therapies aimed at preserving the donor organ.

RECENT FINDINGS

Much remains to be determined about the role of innate immune signalling in renal ischaemia/reperfusion injury. Recently, discoveries about complement receptors, Toll-like receptors (TLRs), NOD-like receptors (NLRs) and inflammasomes have opened new avenues of exploration. We are also now learning that macrophages, complement and TLR activation may have additional roles in renal repair following IRI.

SUMMARY

A greater understanding of the mechanisms that contribute to innate immune-mediated renal ischaemic damage will allow for the development of therapeutics targeted to the donor organ. New data suggest that treatment limited to specific receptors on specific cells, or localized to specific regions within the kidney, may provide novel approaches to maximize our use of donor organs, particularly those that may have been discarded due to prolonged preimplantation ischaemia.

NOD2 promotes renal injury by exacerbating inflammation and podocyte insulin resistance in diabetic nephropathy

An increasing number of clinical and animal model studies indicate that activation of the innate immune system and inflammatory mechanisms are important in the pathogenesis of diabetic nephropathy. Nucleotide-binding oligomerization domain containing 2 (NOD2), a member of the NOD-like receptor family, plays an important role in innate immune response. Here we explore the contribution of NOD2 to the pathogenesis of diabetic nephropathy and found that it was upregulated in kidney biopsies from diabetic patients and high-fat diet/streptozotocin-induced diabetic mice. Further, NOD2 deficiency ameliorated renal injury in diabetic mice. In vitro, NOD2 induced proinflammatory response and impaired insulin signaling and insulin-induced glucose uptake in podocytes. Moreover, podocytes treated with high glucose, advanced glycation end-products, tumor necrosis factor-α, or transforming growth factor-β (common detrimental factors in diabetic nephropathy) significantly increased NOD2 expression. NOD2 knockout diabetic mice were protected from the hyperglycemia-induced reduction in nephrin expression. Further, knockdown of NOD2 expression attenuated high glucose-induced nephrin downregulation in vitro, supporting an essential role of NOD2 in mediating hyperglycemia-induced podocyte dysfunction. Thus, NOD2 is one of the critical components of a signal transduction pathway that links renal injury to inflammation and podocyte insulin resistance in diabetic nephropathy.

The effects of NOD activation on adipocyte differentiation

OBJECTIVE

Obesity is associated with chronic inflammation. Toll-like receptors (TLR) and NOD-like receptors (NLR) are two families of pattern recognition receptors that play important roles in immune response and inflammation in adipocytes. It has been reported that TLR4 and TLR2 activation induce proinflammatory changes that impair adipocyte differentiation. However, the effects of activation of NOD1 and NOD2, the two prominent members of NLR, on adipocyte differentiation have not been studied.

DESIGN AND METHODS

3T3-L1 and human adipose-derived stem cells were tested for adipocyte differentiation in the presence or absence of NOD ligand. Adipocyte differentiation was evaluated by the adipocyte markers gene expression and Oil Red O staining for lipid accumulation.

RESULTS

Activation of NOD1, but not NOD2, by a synthetic ligand dose-dependently suppressed 3T3-L1 adipocyte differentiation as revealed by Oil Red O stained cell morphology, lipid accumulation, and attenuated gene expression of adipocyte markers (PPARγ, C/EBPα, SCD, FABP4, Adiponectin). Activation of NOD1, but not NOD2, induced NF-κB activation, which correlated with their abilities to suppress ligand-induced PPARγ transaction. Moreover, the suppressive effect by NOD1 activation was reversed by IκB super-repressor which blocks NF-κB activation. The suppression by NOD1 ligand C12-iEDAP on adipocyte differentiation was reversed by small RNA interference targeting NOD1, demonstrating the specificity of NOD1 activation. In contrast, activation of NOD1 and NOD2 both significantly suppressed adipocyte differentiation of human adipose-derived adult stem cells, demonstrating the species specific effects of NOD activation. In contrast to enhanced leptin mRNA by LPS and TNFα, NOD1 activation suppressed leptin mRNA in adipocytes, suggesting the differential effects of NOD1 activation in adipocytes.

CONCLUSIONS

Overall, our results suggest that NOD1 represents a novel target for adipose inflammation in obesity.

Ubiquitin regulates caspase recruitment domain-mediated signaling by nucleotide-binding oligomerization domain-containing proteins NOD1 and NOD2

NOD1 and NOD2 (nucleotide-binding oligomerization domain-containing proteins) are intracellular pattern recognition receptors that activate inflammation and autophagy. These pathways rely on the caspase recruitment domains (CARDs) within the receptors, which serve as protein interaction platforms that coordinately regulate immune signaling. We show that NOD1 CARD binds ubiquitin (Ub), in addition to directly binding its downstream targets receptor-interacting protein kinase 2 (RIP2) and autophagy-related protein 16-1 (ATG16L1). NMR spectroscopy and structure-guided mutagenesis identified a small hydrophobic surface of NOD1 CARD that binds Ub. In vitro, Ub competes with RIP2 for association with NOD1 CARD. In vivo, we found that the ligand-stimulated activity of NOD1 with a mutant CARD lacking Ub binding but retaining ATG16L1 and RIP2 binding is increased relative to wild-type NOD1. Likewise, point mutations in the tandem NOD2 CARDs at positions analogous to the surface residues defining the Ub interface on NOD1 resulted in loss of Ub binding and increased ligand-stimulated NOD2 signaling. These data suggest that Ub binding provides a negative feedback loop upon NOD-dependent activation of RIP2.

The Legionella pneumophila EnhC protein interferes with immunostimulatory muramyl peptide production to evade innate immunity

Successful pathogens have evolved to evade innate immune recognition of microbial molecules by pattern recognition receptors (PRR), which control microbial growth in host tissues. Upon Legionella pneumophila infection of macrophages, the cytosolic PRR Nod1 recognizes anhydro-disaccharide-tetrapeptide (anhDSTP) generated by soluble lytic transglycosylase (SltL), the predominant bacterial peptidoglycan degrading enzyme, to activate NF-κB-dependent innate immune responses. We show that L. pneumophila periplasmic protein EnhC, which is uniquely required for bacterial replication within macrophages, interferes with SltL to lower anhDSTP production. L. pneumophila mutant strains lacking EnhC (ΔenhC) increase Nod1-dependent NF-κB activation in host cells, while reducing SltL activity in a ΔenhC strain restores intracellular bacterial growth. Further, L. pneumophila ΔenhC is specifically rescued in Nod1- but not Nod2-deficient macrophages, arguing that EnhC facilitates evasion from Nod1 recognition. These results indicate that a bacterial pathogen regulates peptidoglycan degradation to control the production of PRR ligands and evade innate immune recognition.

NLRX1 does not inhibit MAVS-dependent antiviral signalling

NLRX1 is a member of the Nod-like receptor family of intracellular sensors of microbial- and danger-associated molecular patterns. NLRX1 has a N-terminal mitochondrial addressing sequence that localizes the protein to the mitochondrial matrix. Recently, conflicting reports have been presented with regard to the putative implication of NLRX1 as a negative regulator of MAVS-dependent cytosolic antiviral responses. Here, we generated a new NLRX1 knockout mouse strain and observed that bone marrow-derived macrophages and murine embryonic fibroblasts from NLRX1-deficient mice displayed normal antiviral and inflammatory responses following Sendai virus infection. Importantly, wild type and NLRX1-deficient mice exhibited unaltered antiviral and inflammatory gene expression following intranasal challenge with influenza A virus or i.p. injection of Poly (I:C). Together, our results demonstrate that NLRX1 does not participate in the negative regulation of MAVS-dependent antiviral responses.

The innate immune protein Nod2 binds directly to MDP, a bacterial cell wall fragment

Mammalian Nod2 is an intracellular protein that is implicated in the innate immune response to the bacterial cell wall and is associated with the development of Crohn’s disease, Blau syndrome, and gastrointestinal cancers. Nod2 is required for an immune response to muramyl dipeptide (MDP), an immunostimulatory fragment of bacterial cell wall, but it is not known whether MDP binds directly to Nod2. We report the expression and purification of human Nod2 from insect cells. Using novel MDP self-assembled monolayers (SAMs), we provide the first biochemical evidence for a direct, high-affinity interaction between Nod2 and MDP.

TRIM27 negatively regulates NOD2 by ubiquitination and proteasomal degradation

NOD2, the nucleotide-binding domain and leucine-rich repeat containing gene family (NLR) member 2 is involved in mediating antimicrobial responses. Dysfunctional NOD2 activity can lead to severe inflammatory disorders, but the regulation of NOD2 is still poorly understood. Recently, proteins of the tripartite motif (TRIM) protein family have emerged as regulators of innate immune responses by acting as E3 ubiquitin ligases. We identified TRIM27 as a new specific binding partner for NOD2. We show that NOD2 physically interacts with TRIM27 via the nucleotide-binding domain, and that NOD2 activation enhances this interaction. Dependent on functional TRIM27, ectopically expressed NOD2 is ubiquitinated with K48-linked ubiquitin chains followed by proteasomal degradation. Accordingly, TRIM27 affects NOD2-mediated pro-inflammatory responses. NOD2 mutations are linked to susceptibility to Crohn's disease. We found that TRIM27 expression is increased in Crohn's disease patients, underscoring a physiological role of TRIM27 in regulating NOD2 signaling. In HeLa cells, TRIM27 is partially localized in the nucleus. We revealed that ectopically expressed NOD2 can shuttle to the nucleus in a Walker A dependent manner, suggesting that NOD2 and TRIM27 might functionally cooperate in the nucleus.

We conclude that TRIM27 negatively regulates NOD2-mediated signaling by degradation of NOD2 and suggest that TRIM27 could be a new target for therapeutic intervention in NOD2-associated diseases.

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

Bacterial pneumonia remains a significant cause of mortality in the United States. The innate immune response is the first line of defense against invading bacteria. Neutrophil recruitment to the lungs is the first step in a multistep sequence leading to bacterial clearance. Ligand interaction with pattern-recognizing receptors (PRRs) leads to chemokine production, which drives neutrophils to the site of infection. Although we demonstrated that RIP2 is important for host defense in the lungs against Escherichia coli, the individual roles of NOD1 and NOD2 in pulmonary defense have not been addressed. Here, we explored the role of NOD2 in neutrophil-mediated host defense against an extracellular pathogen, E. coli. We found enhanced bacterial burden and reduced neutrophil and cytokine/chemokine levels in the lungs of NOD2⁻/⁻ mice following E. coli infection. Furthermore, we observed reduced activation of NF-κB and mitogen-activated protein kinases (MAPKs) in the lungs of NOD2⁻/⁻ mice upon E. coli challenge. Moreover, NOD2⁻/⁻ neutrophils show impaired intracellular bacterial killing. Using NOD2/RIP2⁻/⁻ mice, we observed bacterial burden and neutrophil accumulation in the lungs similar to those seen with NOD2⁻/⁻ mice. In addition, bone marrow-derived macrophages obtained from NOD2/RIP2⁻/⁻ mice demonstrate a reduction in activation of NF-κB and MAPKs similar to that seen with NOD2⁻/⁻ mice in response to E. coli. These findings unveil a previously unrecognized role of the NOD2-RIP2 axis for host defense against extracellular Gram-negative bacteria. This pathway may represent a novel target for the treatment of lung infection/inflammation.

NLRC5 regulates MHC class I antigen presentation in host defense against intracellular pathogens

NOD-like receptors (NLRs) are a family of intracellular proteins that play critical roles in innate immunity against microbial infection. NLRC5, the largest member of the NLR family, has recently attracted much attention. However, in vitro studies have reported inconsistent results about the roles of NLRC5 in host defense and in regulating immune signaling pathways. The in vivo function of NLRC5 remains unknown. Here, we report that NLRC5 is a critical regulator of host defense against intracellular pathogens in vivo. NLRC5 was specifically required for the expression of genes involved in MHC class I antigen presentation. NLRC5-deficient mice showed a profound defect in the expression of MHC class I genes and a concomitant failure to activate L. monocytogenes-specific CD8(+) T cell responses, including activation, proliferation and cytotoxicity, and the mutant mice were more susceptible to the pathogen infection. NLRP3-mediated inflammasome activation was also partially impaired in NLRC5-deficient mice. However, NLRC5 was dispensable for pathogen-induced expression of NF-κB-dependent pro-inflammatory genes as well as type I interferon genes. Thus, NLRC5 critically regulates MHC class I antigen presentation to control intracellular pathogen infection.