The origin of the synergistic effect of muramyl dipeptide with endotoxin and peptidoglycan

Multifaceted roles and regulation of nucleotide-binding oligomerization domain containing proteins

Nucleotide-binding oligomerization domain-containing proteins, NOD1 and NOD2, are cytosolic receptors that recognize dipeptides and tripeptides derived from the bacterial cell wall component peptidoglycan (PGN). During the past two decades, studies have revealed several roles for NODs beyond detecting PGN fragments, including activation of an innate immune anti-viral response, NOD-mediated autophagy, and ER stress induced inflammation. Recent studies have also clarified the dynamic regulation of NODs at cellular membranes to generate specific and balanced immune responses. This review will describe how NOD1 and NOD2 detect microbes and cellular stress and detail the molecular mechanisms that regulate activation and signaling while highlighting new evidence and the impact on inflammatory disease pathogenesis.

NOD2 in monocytes negatively regulates macrophage development through TNFalpha

Objective

It is believed that intestinal recruitment of monocytes from Crohn's Disease (CD) patients who carry NOD2 risk alleles may repeatedly give rise to recruitment of pathogenic macrophages. We investigated an alternative possibility that NOD2 may rather inhibit their differentiation from intravasating monocytes.

Design

The monocyte fate decision was examined by using germ-free mice, mixed bone marrow chimeras and a culture system yielding macrophages and monocyte-derived dendritic cells (mo-DCs).

Results

We observed a decrease in the frequency of mo-DCs in the colon of Nod2-deficient mice, despite a similar abundance of monocytes. This decrease was independent of the changes in the gut microbiota and dysbiosis caused by Nod2 deficiency. Similarly, the pool of mo-DCs was poorly reconstituted in a Nod2-deficient mixed bone marrow (BM) chimera. The use of pharmacological inhibitors revealed that activation of NOD2 during monocyte-derived cell development, dominantly inhibits mTOR-mediated macrophage differentiation in a TNFα-dependent manner. These observations were supported by the identification of a TNFα-dependent response to muramyl dipeptide (MDP) that is specifically lost when CD14-expressing blood cells bear a frameshift mutation in NOD2.

Conclusion

NOD2 negatively regulates a macrophage developmental program through a feed-forward loop that could be exploited for overcoming resistance to anti-TNF therapy in CD.

Role of Myeloid Tet Methylcytosine Dioxygenase 2 in Pulmonary and Peritoneal Inflammation Induced by Lipopolysaccharide and Peritonitis Induced by Escherichia coli

Tet methylcytosine dioxygenase 2 (Tet2) mediates demethylation of DNA. We here sought to determine the expression and function of Tet2 in macrophages upon exposure to lipopolysaccharide (LPS), and in the host response to LPS induced lung and peritoneal inflammation, and during Escherichia (E.) coli induced peritonitis. LPS induced Tet2 expression in mouse macrophages and human monocytes in vitro, as well as in human alveolar macrophages after bronchial instillation in vivo. Bone marrow-derived macrophages from myeloid Tet2 deficient (Tet2^fl/flLysM^Cre) mice displayed enhanced production of IL-1β, IL-6 and CXCL1 upon stimulation with several Toll-like receptor agonists; similar results were obtained with LPS stimulated alveolar and peritoneal macrophages. Histone deacetylation was involved in the effect of Tet2 on IL-6 production, whilst methylation at the Il6 promoter was not altered by Tet2 deficiency. Tet2^fl/flLysM^Cre mice showed higher IL-6 and TNF levels in bronchoalveolar and peritoneal lavage fluid after intranasal and intraperitoneal LPS administration, respectively, whilst other inflammatory responses were unaltered. E. coli induced stronger production of IL-1β and IL-6 by Tet2 deficient peritoneal macrophages but not in peritoneal lavage fluid of Tet2^fl/flLysM^Cre mice after in vivo intraperitoneal infection. Tet2^fl/flLysM^Cre mice displayed enhanced bacterial growth during E. coli peritonitis, which was associated with a reduced capacity of Tet2^fl/flLysM^Cre peritoneal macrophages to inhibit the growth of E. coli in vitro. Collectively, these data suggest that Tet2 is involved in the regulation of macrophage functions triggered by LPS and during E. coli infection.

Innate control of adaptive immunity and adaptive instruction of innate immunity: bi-directional flow of information

The ability of the innate and adaptive immune systems to communicate with each other is central to protective immune responses and maintenance of host health. Myeloid cells of the innate immune system are able to sense microbial ligands, perturbations in cellular homeostasis, and virulence factors, thereby allowing them to relay distinct pathogen-specific information to naïve T cells in the form of pathogen-derived peptides and a unique cytokine milieu. Once primed, effector T helper cells produce lineage-defining cytokines to help combat the original pathogen, and a subset of these cells persist as memory or effector-memory populations. These memory T cells then play a dual role in host protection by not only responding rapidly to reinfection, but by also directly instructing myeloid cells to express licensing cytokines. This means there is a bi-directional flow of information first from the innate to the adaptive immune system, and then from the adaptive back to innate immune system. Here, we focus on how signals, first from pathogens and then from primed effector and memory T cells, are integrated by myeloid cells and its consequences for protective immunity or systemic inflammation.

Amphiphilic desmuramyl peptides for the rational design of new vaccine adjuvants: Synthesis, in vitro modulation of inflammatory response and molecular docking studies

Nucleotide-binding oligomerization domain 2 (NOD2) is cytosolic surveillance receptor of the innate immune system capable of recognizing the bacterial and viral infections. Muramyl dipeptide (MDP) is the minimal immunoreactive unit of murein. NOD2 perceives MDP as pathogen-associated molecular pattern, thereby triggering an immune response with undesirable side-effects. Beneficial properties of MDP, such as pro-inflammatory characteristics for the rational design of new vaccine adjuvants, can be harnessed by strategically re-designing the molecule. In this work, a new class of amphiphilic desmuramylpeptides (DMPs) were synthesized by replacing the carbohydrate moiety (muramic acid) of the parent molecule with hydrophilic arenes. A lipophilic chain was also introduced at the C-terminus of dipeptide moiety (alanine-isoglutamine), while conserving its L-D configuration. These novel DMPs were found to set off the release of higher levels of tumour necrosis factor alpha (TNF-α) than Murabutide, which is a well-known NOD2 agonist. Molecular docking studies indicate that all these DMPs bind well to NOD2 receptor with similar dock scores (binding energy) through a number of hydrogen bonding and hydrophobic/π interactions with several crucial residues of the receptor. More studies are needed to further assess their immunomodulatory therapeutic potential, as well as the possible involvement of NOD2 activation.

Synthetic mycobacterial molecular patterns partially complete Freund's adjuvant

Complete Freund's adjuvant (CFA) has historically been one of the most useful tools of immunologists. Essentially comprised of dead mycobacteria and mineral oil, we asked ourselves what is special about the mycobacterial part of this adjuvant, and could it be recapitulated synthetically? Here, we demonstrate the essentiality of N-glycolylated peptidoglycan plus trehalose dimycolate (both unique in mycobacteria) for the complete adjuvant effect using knockouts and chemical complementation. A combination of synthetic N-glycolyl muramyl dipeptide and minimal trehalose dimycolate motif GlcC14C18 was able to upregulate dendritic cell effectors, plus induce experimental autoimmunity qualitatively similar but quantitatively milder compared to CFA. This research outlines how to substitute CFA with a consistent, molecularly-defined adjuvant which may inform the design of immunotherapeutic agents and vaccines benefitting from cell-mediated immunity. We also anticipate using synthetic microbe-associated molecular patterns (MAMPs) to study mycobacterial immunity and immunopathogenesis.

Twist1 and Twist2 Induce Human Macrophage Memory upon Chronic Innate Receptor Treatment by HDAC-Mediated Deacetylation of Cytokine Promoters

Intestinal tissues are continuously exposed to microbial products that stimulate pattern-recognition receptors (PRRs). Ongoing PRR stimulation can confer epigenetic changes in macrophages, which can then regulate subsequent immune outcomes and adaptation to the local environment. Mechanisms leading to these changes are incompletely understood. We found that short-term stimulation of the PRR NOD2 in primary human monocyte-derived macrophages resulted in increased H3 and H4 acetylation of cytokine promoters, consistent with the increased cytokine secretion observed. However, with prolonged NOD2 stimulation, both the acetylation and cytokine secretion were dramatically decreased. Chronic NOD2 stimulation upregulated the transcription factors Twist1 and Twist2, which bound to the promoters of the histone deacetylases HDAC1 and HDAC3 and induced HDAC1 and HDAC3 expression. HDAC1 and HDAC3 then mediated histone deacetylation at cytokine promoters and, in turn, cytokine downregulation under these conditions. Similar regulation was observed upon chronic stimulation of multiple PRRs. Consistent with the chronic microbial exposure in the intestinal environment, TWIST1, TWIST2, HDAC1, and HDAC3 were upregulated in human intestinal relative to peripheral macrophages. Importantly, complementing HDAC1 and HDAC3 in Twist1/Twist2-deficient monocyte-derived macrophages restored the reduced histone acetylation on cytokine promoters and the decreased cytokine secretion with chronic NOD2 stimulation. Taken together, we identify mechanisms wherein Twist1 and Twist2 promote chromatin modifications, resulting in macrophage instruction and adaptation to conditions in the intestinal microenvironment.

Role of NOD2 in antiphospholipid antibody-induced and bacterial MDP amplification of trophoblast inflammation

Women with antiphospholipid antibodies (aPL) are at high risk for pregnancy complications, such as preeclampsia. We previously demonstrated that aPL recognizing β₂GPI promote an extravillous trophoblast pro-inflammatory, anti-migratory and anti-angiogenic profile similar to that seen in preeclampsia. Since preeclampsia in the absence of aPL may have an underlying infectious element, women with aPL may be at increased risk for preeclampsia or other adverse outcomes if an infection is present. Our objective was to determine the impact the common bacterial component, muramyl dipeptide (MDP), has on trophoblast responses to aPL. Herein, we report that bacterial MDP amplifies trophoblast IL-1β expression, processing, and secretion in the presence of aPL through activation of NOD2. In the absence of MDP, NOD2 also mediates anti- β₂GPI antibody-induced trophoblast IL-1β and VEGF secretion. Additionally, we report a role for extravillous trophoblast vimentin as a novel danger signal that contributes to the aPL-induced trophoblast IL-1β production. Together our data indicate that NOD2 mediates trophoblast inflammatory and angiogenic responses to aPL alone, and mediates trophoblast inflammation in the presence of bacterial MDP. These findings suggest that a bacterial infection at the maternal-fetal interface may exacerbate the impact aPL have on trophoblast inflammation and, thus, on pregnancy outcome.

Synergistic interactions between NOD receptors and TLRs: Mechanisms and clinical implications

Interactions between pattern recognition receptors (PRRs) shape innate immune responses to particular classes of pathogens. Here, we review interactions between TLRs and nucleotide-binding oligomerization domain 1 and 2 (NOD1 and NOD2) receptors, two major groups of PRRs involved in innate recognition of bacteria. Most of experimental data both in vitro and in vivo suggest that NODs and TLRs synergize with each other at inducing the production of cytokines and antimicrobial peptides. Molecular mechanisms of this synergy remain poorly understood, although several scenarios can be proposed: (i) direct interactions of signaling pathways downstream of NODs and TLRs; (ii) mutual transcriptional regulation of unique components of NOD-dependent and TLR-dependent signaling pathways; and (iii) interactions at the post-transcriptional level. Potential practical implications of NOD-TLR synergy are dual. In sepsis, where synergistic effects probably contribute to excessive proinflammatory cytokine production, blockade of NOD1, and/or NOD2 in addition to TLR4 blockade may be required to achieve therapeutic benefit. On the other hand, synergistic combinations of relatively small doses of NOD and TLR agonists administered before infection could be used to boost innate resistance against bacterial pathogens.

Role and mechanism of the nod-like receptor family pyrin domain-containing 3 inflammasome in oral disease

OBJECTIVE

To summarize evidence and data from experimental studies regarding the role and mechanism of the Nod-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome in the pathogenesis of several representative oral diseases.

MATERIALS AND METHODS

A literature search of PubMed and EBSCO was performed. The literature was searched using a combination of keywords, e.g., NLRP3 inflammasome, inflammation, microorganisms, oral inflammatory diseases, and oral immunological diseases.

RESULTS

The initiation and activation of the NLRP3 inflammasome are associated with the pathogenesis and progression of several representative oral diseases, including periodontitis, oral lichen planus, dental pulp disease, and oral cavity squamous cell carcinoma.

CONCLUSIONS

The NLRP3 inflammasome plays a crucial role in the progression of inflammatory and adaptive immune responses. The possible role of the NLRP3 inflammasome in several oral diseases, including not only periodontitis and pulpitis but also mucosal diseases and oral cavity squamous cell carcinoma, may involve the aberrant regulation of inflammatory and immune responses. Understanding the cellular and molecular biology of the NLRP3 inflammasome is necessary because the NLRP3 inflammasome may be a potential therapeutic target for the treatment and prevention of oral inflammatory and immunological diseases.

Role of Muramyl Dipeptide in Lipopolysaccharide-Mediated Biological Activity and Osteoclast Activity

Lipopolysaccharide (LPS) is an endotoxin and bacterial cell wall component that is capable of inducing inflammation and immunological activity. Muramyl dipeptide (MDP), the minimal essential structural unit responsible for the immunological activity of peptidoglycans, is another inflammation-inducing molecule that is ubiquitously expressed by bacteria. Several studies have shown that inflammation-related biological activities were synergistically induced by interactions between LPS and MDP. MDP synergistically enhances production of proinflammatory cytokines that are induced by LPS exposure. Injection of MDP induces lethal shock in mice challenged with LPS. LPS also induces osteoclast formation and pathological bone resorption; MDP enhances LPS induction of both processes. Furthermore, MDP enhances the LPS-induced receptor activator of NF-κB ligand (RANKL) expression and toll-like receptor 4 (TLR4) expression both in vivo and in vitro. Additionally, MDP enhances LPS-induced mitogen-activated protein kinase (MAPK) signaling in stromal cells. Taken together, these findings suggest that MDP plays an important role in LPS-induced biological activities. This review discusses the role of MDP in LPS-mediated biological activities, primarily in relation to osteoclastogenesis.

Lipid moieties on lipoproteins of commensal and non-commensal staphylococci induce differential immune responses

Lipoproteins (Lpp) of Gram-positive bacteria are major players in alerting our immune system. Here, we show that the TLR2 response induced by commensal species Staphylococcus aureus and Staphylococcus epidermidis is almost ten times lower than that induced by noncommensal Staphylococcus carnosus, and this is at least partially due to their different modifications of the Lpp lipid moieties. The N terminus of the lipid moiety is acylated with a long-chain fatty acid (C17) in S. aureus and S. epidermidis, while it is acylated with a short-chain fatty acid (C2) in S. carnosus. The long-chain N-acylated Lpp, recognized by TLR2-TLR1 receptors, silences innate and adaptive immune responses, while the short-chain N-acetylated Lpp, recognized by TLR2-TLR6 receptors, boosts it.

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

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

Design, synthesis and immunological evaluation of novel amphiphilic desmuramyl peptides

Muramyl dipeptide (MDP) - an essential bacterial cell wall component - is recognized by our immune system as pathogen-associated molecular pattern (PAMP) which results in immune responses with adverse toxic effects. In order to harness the beneficial properties from the pro-inflammatory characteristics of the bacterial cell wall motif, MDP was strategically re-designed while conserving the L-D configurations of the dipeptide moiety. The muramic acid was replaced with a hydrophilic arene and lipophilic chain was introduced at peptide end to give the amphiphilic desmuramyl peptides (DMPs). The novel DMPs were found to modulate the immune response by amplifying the LPS-induced surface glycoprotein (ICAM-1) expression in THP-1 cells without showing significant toxicity. Furthermore, these compounds were able to trigger the secretion of higher levels of pro-inflammatory cytokine (TNF-α) than the well-studied NOD2 agonist, Murabutide.

The Role of the p38-MNK-eIF4E Signaling Axis in TNF Production Downstream of the NOD1 Receptor

Activation of nucleotide-binding oligomerization domain (NOD) 1 and NOD2 by muropeptides triggers a complex transcriptional program in innate immune cells. However, little is known about posttranscriptional regulation of NOD1- and NOD2-dependent responses. When stimulated with a prototypic NOD1 agonist, N-acetylglucosaminyl-N-acetylmuramyl-l-alanyl-d-isoglutamyl-meso-diaminopimelic acid (GM-triDAP), human monocyte-derived macrophages (MDM) produced an order of magnitude more TNF, IL-6, and pro-IL-1β than did monocyte-derived dendritic cells (MDDC), despite similar NOD1 expression, similar cytokine mRNA kinetics, and comparable responses to LPS. TNF production by GM-triDAP-activated MDM was independent of autocrine IL-1. However, GM-triDAP-activated MDM translated TNF mRNA more efficiently than did MDDC. As an underlying mechanism, NOD1 triggering in MDM caused a more potent and long-lasting activation of the signaling axis involving p38 MAPK, MAPK-interacting kinase (MNK), and eukaryotic translation initiation factor 4E, which is a critical regulator of translation. Furthermore, MNK controlled TNF mRNA abundance in MDDC and MDM upon NOD1 triggering. NOD1-dependent responses were more sensitive to MNK inhibition than were TLR4-dependent responses. These results demonstrate the importance of the p38-MNK-eukaryotic translation initiation factor 4E axis in TNF production downstream of NOD1.

Ac2PIM-responsive miR-150 and miR-143 target receptor-interacting protein kinase 2 and transforming growth factor beta-activated kinase 1 to suppress NOD2-induced immunomodulators

Specific and coordinated regulation of innate immune receptor-driven signaling networks often determines the net outcome of the immune responses. Here, we investigated the cross-regulation of toll-like receptor (TLR)2 and nucleotide-binding oligomerization domain (NOD)2 pathways mediated by Ac2PIM, a tetra-acylated form of mycobacterial cell wall component and muramyl dipeptide (MDP), a peptidoglycan derivative respectively. While Ac2PIM treatment of macrophages compromised their ability to induce NOD2-dependent immunomodulators like cyclooxygenase (COX)-2, suppressor of cytokine signaling (SOCS)-3, and matrix metalloproteinase (MMP)-9, no change in the NOD2-responsive NO, TNF-α, VEGF-A, and IL-12 levels was observed. Further, genome-wide microRNA expression profiling identified Ac2PIM-responsive miR-150 and miR-143 to target NOD2 signaling adaptors, RIP2 and TAK1, respectively. Interestingly, Ac2PIM was found to activate the SRC-FAK-PYK2-CREB cascade via TLR2 to recruit CBP/P300 at the promoters of miR-150 and miR-143 and epigenetically induce their expression. Loss-of-function studies utilizing specific miRNA inhibitors establish that Ac2PIM, via the miRNAs, abrogate NOD2-induced PI3K-PKCδ-MAPK pathway to suppress β-catenin-mediated expression of COX-2, SOCS-3, and MMP-9. Our investigation has thus underscored the negative regulatory role of Ac2PIM-TLR2 signaling on NOD2 pathway which could broaden our understanding on vaccine potential or adjuvant utilities of Ac2PIM and/or MDP.

Soluble β-(1,3)-glucans enhance LPS-induced response in the monocyte activation test, but inhibit LPS-mediated febrile response in rabbits: Implications for pyrogenicity tests

In the present study, we aimed to determine the influence of β-(1,3)-d-glucans on the LPS-induced pro-inflammatory cytokine response in the Monocyte Activation Test (MAT) for pyrogens, and on the LPS-induced febrile response in the Rabbit Pyrogen Test (RPT), thus evaluating the resulting effect in the outcome of each test. It was found that β-(1,3)-d-glucans elicited the production of pro-inflammatory cytokines IL-1β, IL-6 and TNF-α, also known as endogenous pyrogens, but not enough to classify them as pyrogenic according to MAT. The same β-(1,3)-d-glucans samples were non-pyrogenic by RPT. However, β-(1,3)-d-glucans significantly enhanced the LPS-induced pro-inflammatory cytokines response in MAT, insomuch that samples containing non-pyrogenic concentrations of LPS become pyrogenic. On the other hand, β-(1,3)-d-glucans had no effect on sub-pyrogenic LPS doses in the RPT, but surprisingly, inhibited the LPS-induced febrile response of pyrogenic LPS concentrations. Thus, while β-(1,3)-d-glucans could mask the LPS pyrogenic activity in the RPT, they exerted an overstimulation of pro-inflammatory cytokines in the MAT. Hence, MAT provides higher safety since it evidences an unwanted biological response, which is not completely controlled and is overlooked by the RPT.

Synergistic effects of NOD1 or NOD2 and TLR4 activation on mouse sickness behavior in relation to immune and brain activity markers

Toll-like receptors (TLRs) and nuclear-binding domain (NOD)-like receptors (NLRs) are sensors of bacterial cell wall components to trigger an immune response. The TLR4 agonist lipopolysaccharide (LPS) is a strong immune activator leading to sickness and depressed mood. NOD agonists are less active but can prime immune cells to augment LPS-induced cytokine production. Since the impact of NOD and TLR co-activation in vivo has been little studied, the effects of the NOD1 agonist FK565 and the NOD2 agonist muramyl dipeptide (MDP), alone and in combination with LPS, on immune activation, brain function and sickness behavior were investigated in male C57BL/6N mice. Intraperitoneal injection of FK565 (0.001 or 0.003mg/kg) or MDP (1 or 3mg/kg) 4h before LPS (0.1 or 0.83mg/kg) significantly aggravated and prolonged the LPS-evoked sickness behavior as deduced from a decrease in locomotion, exploration, food intake and temperature. When given alone, FK565 and MDP had only minor effects. The exacerbation of sickness behavior induced by FK565 or MDP in combination with LPS was paralleled by enhanced plasma protein and cerebral mRNA levels of proinflammatory cytokines (IFN-γ, IL-1β, IL-6, TNF-α) as well as enhanced plasma levels of kynurenine. Immunohistochemical visualization of c-Fos in the brain revealed that NOD2 synergism with TLR4 resulted in increased activation of cerebral nuclei relevant to sickness. These data show that NOD1 or NOD2 synergizes with TLR4 in exacerbating the immune, sickness and brain responses to peripheral immune stimulation. Our findings demonstrate that the known interactions of NLRs and TLRs at the immune cell level extend to interactions affecting brain function and behavior.

Design of bio-molecular interfaces using liquid crystals demonstrating endotoxin interactions with bacterial cell wall components

Liquid crystals offer a promising approach to study and quantify the interactions between different bacterial cell membrane components with endotoxin at an aqueous interface.

Muramyl dipeptide activates human beta defensin 2 and pro-inflammatory mediators through Toll-like receptors and NLRP3 inflammasomes in human dental pulp cells

PURPOSE

The expression levels of intracellular pyrin domain-containing 3 (NLRP3) and microbial pattern-recognition receptors, such as nucleotide-binding oligomerization domain 2 (NOD2), have been reported in human dental pulp cells (HDPCs) and inflamed dental pulp tissue, but the role of NLRP3 and Toll-like receptors (TLRs) in the production of human beta defensin 2 (hBD2) and inflammatory cytokines against invading pathogens remains poorly defined. The aim of this study was to determine whether the NOD2 ligand muramyl dipeptide (MDP) upregulates hBD2 and inflammatory cytokines and whether this response is dependent on TLRs and NLRP inflammasomes in HDPCs.

METHODOLOGY

The effects of MDP on the expression of hBD2, TLRs, inflammasomes, and pro-inflammatory mediators in HDPCs were examined using Western blotting and reverse transcription-polymerase chain reaction. Levels of pro-inflammatory cytokines, such as nitric oxide (NO) and prostaglandin E2 (PGE2), were determined by enzyme-linked immunosorbent assay.

RESULTS

MDP upregulated hBD2, TLR2, and TLR4 mRNAs and protein levels in a dose- and time-dependent manner. TLR2 and TLR4 neutralizing blocking antibodies and NOD2- and hBD2-specific small interfering RNAs (siRNAs) attenuated the MDP-induced production of NO, PGE2, tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and IL-8 and upregulated inducible nitric oxide synthase (iNOS) and cyclooxygenase 2 (COX2) in HDPCs. Additionally, MDP activated inflammasome-related genes, such as NLRP3, caspase 1, apoptotic speck protein containing a caspase recruitment domain, and IL-1β. Furthermore, silencing of the NLRP3 gene using a siRNA significantly decreased the MDP-induced expression of hBD2 and cytokines, such as iNOS-derived NO, COX2, PGE2, TNF-α, IL-6, and IL-8.

CONCLUSION

These results suggest that NOD2 activates the TLR2, TLR4, and NLRP3 inflammasome-signaling pathways in HDPCs to induce the production of multiple inflammatory mediators and antimicrobial peptides, which in turn promote pulp immune defense against microbial challenge.

CLINICAL RELEVANCE

The TLR and NLRP3 inflammasome pathways may represent an important modulatory mechanism of immune defense responses during the progression of pulpitis. Our results suggest that local inhibition of NLRP3 and TLRs may reduce the impact of cytokine-mediated host destructive processes in pulpitis.

Kupffer cells in the liver

Kupffer cells are a critical component of the mononuclear phagocytic system and are central to both the hepatic and systemic response to pathogens. Kupffer cells are reemerging as critical mediators of both liver injury and repair. Kupffer cells exhibit a tremendous plasticity; depending on the local metabolic and immune environment, then can express a range of polarized phenotypes, from the proinflammatory M1 phenotype to the alternative/M2 phenotype. Multiple M2 phenotypes can be distinguished, each involved in the resolution of inflammation and wound healing. Here, we have provided an update on recent research that has contributed to the developing delineation of the contribution of Kupffer cells to different types of liver injury, with an emphasis on alcoholic and nonalcoholic liver diseases. These recent advances in our understanding of Kupffer cell function and regulation will likely provide new insights into the potential for therapeutic manipulation of Kupffer cells to promote the resolution of inflammation and enhance wound healing in liver disease.

CD90(+) Stromal Cells are Non-Professional Innate Immune Effectors of the Human Colonic Mucosa

Immune responses at the intestinal mucosa must allow for host protection whilst simultaneously avoiding inappropriate inflammation. Although much work has focused on the innate immune functionality of hematopoietic immune cells, non-hematopoietic cell populations – including epithelial and stromal cells – are now recognized as playing a key role in innate defense at this site. In this study we examined the innate immune capacity of primary human intestinal stromal cells (iSCs). CD90⁺ iSCs isolated from human colonic mucosa expressed a wide array of innate immune receptors and functionally responded to stimulation with bacterial ligands. iSCs also sensed infection with live Salmonella typhimurium, rapidly expressing IL-1 family cytokines via a RIPK2/p38MAPK-dependent signaling process. In addition to responding to innate immune triggers, primary iSCs exhibited a capacity for bacterial uptake, phagocytosis, and antigen processing, although to a lesser extent than professional APCs. Thus CD90⁺ iSCs represent an abundant population of “non-professional” innate immune effector cells of the human colonic mucosa and likely play an important adjunctive role in host defense and immune regulation at this site.

Macrophages in tuberculosis: friend or foe

Tuberculosis (TB) remains one of the greatest threats to human health. The causative bacterium, Mycobacterium tuberculosis (Mtb), is acquired by the respiratory route. It is exquisitely human adapted and a prototypic intracellular pathogen of macrophages, with alveolar macrophages (AMs) being the primary conduit of infection and disease. The outcome of primary infection is most often a latently infected healthy human host, in whom the bacteria are held in check by the host immune response. Such individuals can develop active TB later in life with impairment in the immune system. In contrast, in a minority of infected individuals, the host immune response fails to control the growth of bacilli, and progressive granulomatous disease develops, facilitating spread of the bacilli via infectious aerosols coughed out into the environment and inhaled by new hosts. The molecular details of the Mtb-macrophage interaction continue to be elucidated. However, it is clear that a number of complex processes are involved at the different stages of infection that may benefit either the bacterium or the host. Macrophages demonstrate tremendous phenotypic heterogeneity and functional plasticity which, depending on the site and stage of infection, facilitate the diverse outcomes. Moreover, host responses vary depending on the specific characteristics of the infecting Mtb strain. In this chapter, we describe a contemporary view of the behavior of AMs and their interaction with various Mtb strains in generating unique immunologic lung-specific responses.

Nucleotide-binding oligomerization domain 2 signaling promotes hyperresponsive macrophages and colitis in IL-10-deficient mice

IL-10 contributes to the maintenance of intestinal homeostasis via the regulation of inflammatory responses to enteric bacteria. Loss of IL-10 signaling results in spontaneous colitis in mice and early onset enterocolitis in humans. Nucleotide-binding oligomerization domain (NOD) 2 is an intracellular receptor of bacterial peptidoglycan products, and, although NOD2 mutations are associated with Crohn's disease, the precise role of NOD2 in the development of intestinal inflammation remains undefined. To determine the role of NOD2 in the development of colitis on the clinically relevant genetic background of IL-10-deficient signaling, we generated mice lacking IL-10 and NOD2 (IL-10(-/-)NOD2(-/-)). Loss of NOD2 in IL-10(-/-) mice resulted in significant amelioration of chronic colitis, indicating that NOD2 signaling promotes the development of intestinal inflammation in IL-10(-/-) mice. Contrary to previous reports investigating immune function in NOD2(-/-) mice, T cell proliferative capacity and IL-2 production were not impaired, and immune polarization toward type 1 immunity was not affected. However, loss of NOD2 in IL-10-deficient macrophages reduced IL-6, TNF-α, and IL-12p40 production in response to bacterial stimulation. Further analysis of the intrinsic macrophage response before the onset of inflammation revealed that, in the absence of IL-10, synergistic signaling between various TLRs and NOD2 resulted in hyperresponsive, proinflammatory macrophages, thus providing the appropriate immune environment for the development of colitis. Data presented in this study demonstrate that NOD2 signaling contributes to intestinal inflammation that arises through loss of IL-10 and provides mechanistic insight into the development of colitis in inflammatory bowel disease patients with impaired IL-10 signaling.

NOD1 and NOD2 Signaling in Infection and Inflammation

Sensing intracellular pathogens is a process mediated by innate immune cells that is crucial for the induction of inflammatory processes and effective adaptive immune responses against pathogenic microbes. NOD-like receptors (NLRs) comprise a family of intracellular pattern recognition receptors that are important for the recognition of damage and microbial-associated molecular patterns. NOD1 and NOD2 are specialized NLRs that participate in the recognition of a subset of pathogenic microorganisms that are able to invade and multiply intracellularly. Once activated, these molecules trigger intracellular signaling pathways that lead to the activation of transcriptional responses culminating in the expression of a subset of inflammatory genes. In this review, we will focus on the role of NOD1 and NOD2 in the recognition and response to intracellular pathogens, including Gram-positive and Gram-negative bacteria, and on their ability to signal in response to non-peptidoglycan-containing pathogens, such as viruses and protozoan parasites.

TLR8 and NOD signaling synergistically induce the production of IL-1β and IL-23 in monocyte-derived DCs and enhance the expression of the feedback inhibitor SOCS2

Pattern recognition receptors (PRRs) like Toll-like receptors (TLRs) and NOD-like receptors (NLRs) are important sensors of microbial products. Although they are referred to as innate immune receptors, they make essential contributions to adaptive immune responses by activating dendritic cells (DCs). Simultaneous activation of DCs via different classes of PRRs provides a powerful tool for inducing strong immune responses. In the present study we investigate the interplay of the NLRs NOD1 and NOD2 and their crosstalk with TLR signaling in terms of DC-activation. We found strong synergistic effects upon treatment with NOD1 and NOD2 ligands combined with the TLR7/8 agonist R848. Simultaneous stimulation of monocyte-derived DCs resulted in highly increased production of IL-1β, IL-23 and SOCS2, a member of the suppressor of cytokine signaling (SOCS) family. Silencing of SOCS2 resulted in enhanced IL-23 expression, indicating that SOCS2 is involved in the regulation of TLR/NOD-dependent cytokine secretion. Finally, we demonstrate that TLR7/8-, NOD1- and NOD2-activated DCs promote CD4+ T cells to release increased amounts of IL-17. These results demonstrate that cooperative activation of DCs with NOD1 and NOD2 agonists and TLR7/8 ligands results in a synergistic release of pro-inflammatory mediators which promote the activation of IL-17-producing T cells.

Fusobacterium nucleatum and Tannerella forsythia induce synergistic alveolar bone loss in a mouse periodontitis model

Tannerella forsythia is strongly associated with chronic periodontitis, an inflammatory disease of the tooth-supporting tissues, leading to tooth loss. Fusobacterium nucleatum, an opportunistic pathogen, is thought to promote dental plaque formation by serving as a bridge bacterium between early- and late-colonizing species of the oral cavity. Previous studies have shown that F. nucleatum species synergize with T. forsythia during biofilm formation and pathogenesis. In the present study, we showed that coinfection of F. nucleatum and T. forsythia is more potent than infection with either species alone in inducing NF-κB activity and proinflammatory cytokine secretion in monocytic cells and primary murine macrophages. Moreover, in a murine model of periodontitis, mixed infection with the two species induces synergistic alveolar bone loss, characterized by bone loss which is greater than the additive alveolar bone losses induced by each species alone. Further, in comparison to the single-species infection, mixed infection caused significantly increased inflammatory cell infiltration in the gingivae and osteoclastic activity in the jaw bones. These data show that F. nucleatum subspecies and T. forsythia synergistically stimulate the host immune response and induce alveolar bone loss in a murine experimental periodontitis model.

Systematic review of membrane components of gram-positive bacteria responsible as pyrogens for inducing human monocyte/macrophage cytokine release

Fifty years after the elucidation of lipopolysaccharides (LPS, endotoxin) as the principal structure of Gram-negative bacteria activating the human immune system, its Gram-positive counterpart is still under debate. Pyrogen tests based on the human monocyte activation have been validated for LPS detection as an alternative to the rabbit test and, increasingly, the limulus amebocyte lysate test. For full replacement, international validations with non-endotoxin pyrogens are in preparation. Following evidence-based medicine approaches, a systematic review of existing evidence as to the structural nature of the Gram-positive pyrogen was undertaken. For the three major constituents suggested, i.e., peptidoglycan, lipoteichoic acids (LTA), and bacterial lipoproteins (LP), the questions to be answered and a search strategy for relevant literature was developed, starting in MedLine. The evaluation was based on the Koch–Dale criteria for a mediator of an effect. A total of 380 articles for peptidoglycan, 391 for LP, and 285 for LTA were retrieved of which 12, 8, and 24, respectively, fulfilled inclusion criteria. The compiled data suggest that for peptidoglycan two Koch–Dale criteria are fulfilled, four for LTA, and two for bacterial LP. In conclusion, based on the best currently available evidence, LTA is the only substance that fulfills all criteria. LTA has been isolated from a large number of bacteria, results in cytokine release patterns inducible also with synthetic LTA. Reduction in bacterial cytokine induction with an inhibitor for LTA was shown. However, this systematic review cannot exclude the possibility that other stimulatory compounds complement or substitute for LTA in being the counterpart to LPS in some Gram-positive bacteria.

Cutting edge: primary innate immune cells respond efficiently to polymeric peptidoglycan, but not to peptidoglycan monomers

The cell wall of bacteria induces proinflammatory cytokines in monocytes and neutrophils in human blood. The nature of the stimulating component of bacterial cell walls is not well understood. We have previously shown polymeric peptidoglycan (PGN) has this activity, and the cytokine response requires PGN internalization and trafficking to lysosomes. In this study, we demonstrate that peptidoglycan monomers such as muramyl dipeptide and soluble peptidoglycan fail to induce robust cytokine production in immune cells, although they activate the nucleotide-binding oligomerization domain proteins in transfected cell models. We further show that lysosomal extracts from immune cells degrade intact peptidoglycan into simpler products and that the lysosomal digestion products activate the nucleotide-binding oligomerization domain proteins. We conclude that naive innate immune cells recognize PGN in its polymeric form rather than monomers such as muramyl dipeptide and require PGN lysosomal hydrolysis to respond. These findings offer new opportunities in the treatment of sepsis, especially sepsis arising from Gram-positive organisms.

Effect of neurotrophic factor, MDP, on rats' nerve regeneration

Our objective was to determine the immune-modulating effects of the neurotrophic factor N-acetylmuramyl-L-alanyl-D-isoglutamine (MDP) on median nerve regeneration in rats. We used male Wistar rats (120-140 days of age, weighing 250-332 g) and compared the results of three different techniques of nerve repair: 1) epineural neurorrhaphy using sutures alone (group S - 10 rats), 2) epineural neurorrhaphy using sutures plus fibrin tissue adhesive (FTA; group SF - 20 rats), and 3) sutures plus FTA, with MDP added to the FTA (group SFM - 20 rats). Functional assessments using the grasp test were performed weekly for 12 weeks to identify recovery of flexor muscle function in the fingers secondary to median nerve regeneration. Histological analysis was also utilized. The total number and diameter of myelinated fibers were determined in each proximal and distal nerve segment. Two indices, reported as percentage, were calculated from these parameters, namely, the regeneration index and the diameter change index. By the 8th week, superiority of group SFM over group S became apparent in the grasping test (P = 0.005). By the 12th week, rats that had received MDP were superior in the grasping test compared to both group S (P < 0.001) and group SF (P = 0.001). Moreover, group SF was better in the grasping test than group S (P = 0.014). However, no significant differences between groups were identified by histological analysis. In the present study, rats that had received MDP obtained better function, in the absence of any significant histological differences.

Divergent responses to peptidoglycans derived from different E. coli serotypes influence inflammatory outcome in trout, Oncorhynchus mykiss, macrophages

Background

Pathogen-associated molecular patterns (PAMPs) are structural components of pathogens such as lipopolysaccharide (LPS) and peptidoglycan (PGN) from bacterial cell walls. PAMP-recognition by the host results in an induction of defence-related genes and often the generation of an inflammatory response. We evaluated both the transcriptomic and inflammatory response in trout (O. mykiss) macrophages in primary cell culture stimulated with DAP-PGN (DAP; meso-diaminopimelic acid, PGN; peptidoglycan) from two strains of Escherichia coli (PGN-K12 and PGN-O111:B4) over time.

Results

Transcript profiling was assessed using function-targeted cDNA microarray hybridisation (n = 36) and results show differential responses to both PGNs that are both time and treatment dependent. Wild type E. coli (K12) generated an increase in transcript number/diversity over time whereas PGN-O111:B4 stimulation resulted in a more specific and intense response. In line with this, Gene Ontology analysis (GO) highlights a specific transcriptomic remodelling for PGN-O111:B4 whereas results obtained for PGN-K12 show a high similarity to a generalised inflammatory priming response where multiple functional classes are related to ribosome biogenesis or cellular metabolism. Prostaglandin release was induced by both PGNs and macrophages were significantly more sensitive to PGN-O111:B4 as suggested from microarray data.

Conclusion

Responses at the level of the transcriptome and the inflammatory outcome (prostaglandin synthesis) highlight the different sensitivity of the macrophage to slight differences (serotype) in peptidoglycan structure. Such divergent responses are likely to involve differential receptor sensitivity to ligands or indeed different receptor types. Such changes in biological response will likely reflect upon pathogenicity of certain serotypes and the development of disease.

Signaling network of dendritic cells in response to pathogens: a community-input supported knowledgebase

Background

Dendritic cells are antigen-presenting cells that play an essential role in linking the innate and adaptive immune systems. Much research has focused on the signaling pathways triggered upon infection of dendritic cells by various pathogens. The high level of activity in the field makes it desirable to have a pathway-based resource to access the information in the literature. Current pathway diagrams lack either comprehensiveness, or an open-access editorial interface. Hence, there is a need for a dependable, expertly curated knowledgebase that integrates this information into a map of signaling networks.

Description

We have built a detailed diagram of the dendritic cell signaling network, with the goal of providing researchers with a valuable resource and a facile method for community input. Network construction has relied on comprehensive review of the literature and regular updates. The diagram includes detailed depictions of pathways activated downstream of different pathogen recognition receptors such as Toll-like receptors, retinoic acid-inducible gene-I-like receptors, C-type lectin receptors and nucleotide-binding oligomerization domain-like receptors. Initially assembled using CellDesigner software, it provides an annotated graphical representation of interactions stored in Systems Biology Mark-up Language. The network, which comprises 249 nodes and 213 edges, has been web-published through the Biological Pathway Publisher software suite. Nodes are annotated with PubMed references and gene-related information, and linked to a public wiki, providing a discussion forum for updates and corrections. To gain more insight into regulatory patterns of dendritic cell signaling, we analyzed the network using graph-theory methods: bifan, feedforward and multi-input convergence motifs were enriched. This emphasis on activating control mechanisms is consonant with a network that subserves persistent and coordinated responses to pathogen detection.

Conclusions

This map represents a navigable aid for presenting a consensus view of the current knowledge on dendritic cell signaling that can be continuously improved through contributions of research community experts. Because the map is available in a machine readable format, it can be edited and may assist researchers in data analysis. Furthermore, the availability of a comprehensive knowledgebase might help further research in this area such as vaccine development. The dendritic cell signaling knowledgebase is accessible at http://tsb.mssm.edu/pathwayPublisher/DC_pathway/DC_pathway_index.html.

Muramyl dipeptide synergizes with Staphylococcus aureus lipoteichoic acid to recruit neutrophils in the mammary gland and to stimulate mammary epithelial cells

Staphylococcus aureus, a major pathogen for the mammary gland of dairy ruminants, elicits the recruitment of neutrophils into milk during mastitis, but the mechanisms are incompletely understood. We investigated the response of the bovine mammary gland to muramyl dipeptide (MDP), an elementary constituent of the bacterial peptidoglycan, alone or in combination with lipoteichoic acid (LTA), another staphylococcal microbial-associated molecular pattern (MAMP). MDP induced a prompt and marked influx of neutrophils in milk, and its combination with LTA elicited a more intense and prolonged influx than the responses to either stimulus alone. The concentrations of several chemoattractants for neutrophils (CXCL1, CXCL2, CXCL3, CXCL8, and C5a) increased in milk after challenge, and the highest increases followed challenge with the combination of MDP and LTA. MDP and LTA were also synergistic in inducing in vitro chemokine production by bovine mammary epithelial cells (bMEpC). Nucleotide-binding oligomerization domain 2 (NOD2), a major sensor of MDP, was expressed (mRNA) in bovine mammary tissue and by bMEpC in culture. The production of interleukin-8 (IL-8) following the stimulation of bMEpC by LTA and MDP was dependent on the activation of NF-κB. LTA-induced IL-8 production did not depend on platelet-activating factor receptor (PAFR), as the PAFR antagonist WEB2086 was without effect. In contrast, bMEpC and mammary tissue are known to express Toll-like receptor 2 (TLR2) and to respond to TLR2 agonists. Although the levels of expression of the inflammatory cytokines tumor necrosis factor alpha (TNF-α) and IL-1β were increased by LTA and MDP at the mRNA level, no protein could be detected in the bMEpC culture supernatant. The level of induction of IL-6 was low at both the mRNA and protein levels. These results indicate that MDP and LTA exert synergistic effects to induce neutrophilic inflammation in the mammary gland. These results also show that bMEpC could contribute to the inflammatory response by recognizing LTA and MDP and secreting chemokines but not proinflammatory cytokines. Overall, this study indicates that the TLR2 and NOD2 pathways could cooperate to trigger an innate immune response to S. aureus mastitis.

Activation of the nucleotide oligomerization domain signaling pathway by the non-bacterially derived xanthone drug 5'6-dimethylxanthenone-4-acetic acid (Vadimezan)

The cytosolic nucleotide-binding oligomerization domain 1 (NOD1)/CARD4 and NOD2/CARD15 proteins are members of NOD-like receptors recognizing specific motifs within peptidoglycans of both Gram-negative and Gram-positive bacteria. NOD1 and NOD2 signal via the downstream adaptor serine/threonine kinase RIP2/CARDIAK/RICK to initiate NF-kappaB activation and the release of inflammatory cytokines/chemokines. In this report, we show that 5,6-dimethylxanthenone-4-acetic acid (DMXAA), a cell-permeable, small molecule that has anti-tumor activity, can also activate NOD1 and NOD2. This was demonstrated: 1) by using human embryonic kidney epithelial (HEK) 293 cells transfected with a NF-kappaB reporter plasmid in combination with NOD1 or NOD2 expression plasmids; 2) by inhibiting DMXAA-induced chemokine (CXCL10) mRNA and protein production in the AB12 mesothelioma cell line using a pharmacological inhibitor of RICK kinase, SB20358; and 3) by using small interfering RNA to knock down NOD2 and lentiviral short hairpin RNA to knock down RICK. These findings expand the potential ligands for the NOD-like receptors, suggesting that other xanthone compounds may act similarly and could be developed as anti-tumor agents. This information also expands our knowledge on the mechanisms of action of the anti-tumor agent DMXAA (currently in clinical trials) and may be important for its biological activity.

Evidence for impaired CARD15 signalling in Crohn's disease without disease linked variants

Background

Sensing of muramyl dipeptide (MDP) is impaired in Crohn's disease (CD) patients with disease-linked variants of the CARD15 (caspase activation and recruitment domain 15) gene. Animal studies suggest that normal CARD15 signalling prevents inflammatory bowel disease, and may be important for disease development in CD. However, only a small fraction of CD patients carry the disease linked CARD15 variants. The aim of this study was thus to investigate if changes could be found in CARD15 signalling in patients without disease associated CARD15 variants.

Methodology/Principal Findings

By mapping the response to MDP in peripheral monocytes obtained from CD patients in remission not receiving immunosuppresives, an impaired response to MDP was found in patients without disease linked CARD15 variants compared to control monocytes. This impairment was accompanied by a decreased activation of IκB kinase α/β (IKKα/β), the initial step in the nuclear factor κB (NFκB) pathway, whereas activation of mitogen-activated protein (MAP)-kinases was unaffected. MDP additionally stimulates the inflammasome which is of importance for processing of cytokines. The inflammasome was constitutively activated in CD, but unresponsive to MDP both in CD and control monocytes.

Conclusions/Significance

These results suggest that inhibited MDP-dependent pathways in CD patients not carrying the disease-associated CARD15 variants might be of importance for the pathogenesis of CD. The results reveal a dysfunctional immune response in CD patients, not able to sense relevant stimuli on the one hand, and on the other hand possessing constitutively active cytokine processing.

MDP-Induced selective tolerance to TLR4 ligands: impairment in NOD2 mutant Crohn's disease patients

BACKGROUND

Pathogen infection is a complex process in which several pathogen-recognition receptor (PRR) pathways are activated to induce proinflammatory mediators. The activation of multiple PRRs suggests an interaction between Toll-like receptors (TLRs) and nucleotide-binding oligomerization domain-like receptor (NOD) signaling pathways.

METHODS

To understand the modulation induced by NOD2 signals on successive responses to pathogen-associated molecular patterns (PAMPs), we examined how muramyl dipeptide (MDP) pretreatment reprograms the MDP+LPS (lipopolysaccharide) response of monocytes from human peripheral blood.

RESULTS

Preexposure to bacterial MDP components induced selective tolerance to a subsequent NOD2+TLR4 stimulation. MDP pretreatment inhibited the production of tumor necrosis factor alpha (TNFalpha) and interleuken 10 (IL10), whereas IL6 and IL8 remained unaffected. MDP-induced tolerance was independent of receptor downregulation but was associated with reduced levels of phosphorylated TAK1 and abrogated phosphorylation of the downstream MAPK.Since Nod2 mutations have been associated with susceptibility to develop Crohn's disease (CD), we compared the MDP-induced tolerance in healthy donors and CD patients with compound heterozygous Nod2 mutations (Mut-Nod2) expressing variant NOD2 proteins. MDP-induced tolerance in Mut-Nod2 patients reduced IL10 but not TNFalpha production. In contrast with healthy donors, a p38-independent TNFalpha production was observed during the kinetics of the MDP+LPS response in Mut-Nod2 patients.

CONCLUSIONS

Our findings suggest that the selective tolerance induced by MDP in healthy donors was related to the modulation of a convergent nub of NOD2 and TLR4 signaling pathways. This MDP-induced tolerance was impaired in Mut-Nod2 CD patients, resulting in a p38-independent TNFalpha production and an imbalance between pro- and antiinflammatory cytokines that could be partly responsible for the pathogenesis of CD.

Use of toll-like receptor assays to detect and identify microbial contaminants in biological products

Toll-like receptor (TLR)-expressing cells, for the first time, detected and identified a microbial contaminant in a product made in Escherichia coli using an old manufacturing process. It was suspected of having a microbial contaminant(s) because, although it tested negative by standard pyrogen assays, it was associated with adverse events in early clinical trials. The assay readout is the induction of NF-kappaB and/or cytokines in response to TLR activation. Four coded samples, labeled A to D, including a sample prepared by the older manufacturing process, were submitted. The cell lines were activated only by samples B and D. Sample D stimulated only Mono-Mac 6 and HEK-human TLR4 (hTLR4) cells and was later identified as lipopolysaccharide. Except for TLR3 cells, sample B stimulated cells bearing the different TLRs (TLRs 2, 4, 5, 7, 8, and 9) and nontransfected HEK293 cells. These data suggested that flagellin was the microbial contaminant, since TLR5, the receptor for flagellin, is known to be expressed constitutively on HEK293 cells. Moreover, purified flagellin from Salmonella enterica serovar Typhimurium behaved like sample B, stimulating HEK293 and HEK-hTLR5 cells but not HEK-hTLR3 cells, and this stimulation by flagellin and sample B was blocked by an anti-hTLR5 neutralizing antibody. Western blots showed bands positive for flagellin and sample B with the molecular sizes expected for the flagellins from S. Typhimurium and E. coli, respectively. Mass spectrometry data were consistent with the presence of flagellin in the manufacturer's sample B. Taken together, these data indicate that the microbial contaminant in sample B was flagellin and may have been associated with adverse events when the recombinant product was administered.

Expression of nucleotide-binding oligomerization domain 2 in normal human dental pulp cells and dental pulp tissues

INTRODUCTION

The nucleotide-binding oligomerization domain (NOD) proteins belong to a distinct family of proteins that are implicated in the intracellular recognition of bacterial components. NOD2 appears to be a sensor of bacterial peptidoglycans because it recognizes a minimal motif present in all peptidoglycans. The interaction of NOD2 with downstream signaling molecules ultimately results in the activation of NF-kappaB and production of inflammatory mediators in innate immunity. As such, NOD2 may play an important role in the detection of bacterial pathogens and the initiation of inflammation within the dental pulp. This study was designed to evaluate the expression of NOD2 in normal human dental pulp cells (HDPCs) and human pulp tissues.

METHODS

Human pulp tissue samples were collected from freshly extracted human wisdom teeth, and HDPCs were prepared from the explants of normal human dental pulp tissues. Nested reverse-transcription polymerase chain reaction (Nested RT-PCR) and Western blotting were performed to detect the expression of NOD2 messenger RNA and protein, respectively. Immunohistochemical staining was used to determine the distribution of NOD2 in the pulp tissues.

RESULTS

The NOD2 messenger RNA and protein were present in normal human dental pulp tissues, with most NOD2 protein expression being localized to odontoblasts and some pulp vascular endothelial cells. In contrast, HDPCs only showed a low level of NOD2 protein expression.

CONCLUSIONS

Our results suggest that NOD2 protein expressed in HDPCs and pulp tissues may play an important role in dental immune defense.

A polymer-type water-soluble peptidoglycan exhibited both Toll-like receptor 2- and NOD2-agonistic activities, resulting in synergistic activation of human monocytic cells

Bacterial peptidoglycan (PGN) has been reported to be sensed by cell-surface Toll-like receptor (TLR)2. On the other hand, intracellular NOD-like receptors recognize PGN partial structures: NOD1 and NOD2 recognize the peptide moiety containing diaminopimelic acid, and the muramyldipeptide (MDP) moiety, respectively. In this study, we examined in human monocytic THP-1 cells the pro-inflammatory cytokine-inducing abilities of PGNs and their fragments enzymatically prepared from Staphylococcus epidermidis ATCC 155: a polymer-type water-soluble PGN possessing an intact glycan chain (SEPS) and a monomer-type PGN (SEPS-M). The water-soluble PGN polymer, SEPS, exhibited considerably stronger activities to induce pro-inflammatory cytokines than parent PGNs and the PGN monomer, SEPS-M. Short interference RNA targeting TLR2 and NOD2 markedly reduced the activities of SEPS. In the same experiments, the activities of PGNs were mainly reduced in TLR2-silenced cells, whereas the activities of SEPS-M as well as a synthetic MDP were markedly reduced in NOD2-silenced cells. Furthermore, the PGNs and a reference PGN from Staphylococcus aureus in combination with MDP synergistically induced interleukin-8 in THP-1 cells. These findings strongly suggested that a polymer-type water-soluble PGN fragment, SEPS, exhibits both TLR2-and NOD2-agonistic activities, which induced the synergistic activation of human monocytic cells.

Increased NOD2-mediated recognition of N-glycolyl muramyl dipeptide

Peptidoglycan-derived muramyl dipeptide (MDP) activates innate immunity via the host sensor NOD2. Although MDP is N-acetylated in most bacteria, mycobacteria and related Actinomycetes convert their MDP to an N-glycolylated form through the action of N-acetyl muramic acid hydroxylase (NamH). We used a combination of bacterial genetics and synthetic chemistry to investigate whether N-glycolylation of MDP alters NOD2-mediated immunity. Upon infecting macrophages with 12 bacteria, tumor necrosis factor (TNF) alpha secretion was NOD2 dependent only with mycobacteria and other Actinomycetes (Nocardia and Rhodococcus). Disruption of namH in Mycobacterium smegmatis obrogated NOD2-mediated TNF secretion, which could be restored upon gene complementation. In mouse macrophages, N-glycolyl MDP was more potent than N-acetyl MDP at activating RIP2, nuclear factor kappaB, c-Jun N-terminal kinase, and proinflammatory cytokine secretion. In mice challenged intraperitoneally with live or killed mycobacteria, NOD2-dependent immune responses depended on the presence of bacterial namH. Finally, N-glycolyl MDP was more efficacious than N-acetyl MDP at inducing ovalbumin-specific T cell immunity in a model of adjuvancy. Our findings indicate that N-glycolyl MDP has a greater NOD2-stimulating activity than N-acetyl MDP, consistent with the historical observation attributing exceptional immunogenic activity to the mycobacterial cell wall.

The NOD2 defect in Blau syndrome does not result in excess interleukin-1 activity

OBJECTIVE

Blau syndrome is a rare, autosomal-dominant, autoinflammatory disorder characterized by granulomatous arthritis, uveitis, and dermatitis. Genetics studies have shown that the disease is caused by single nonsynonymous substitutions in NOD-2, a member of the NOD-like receptor or NACHT-leucine-rich repeat (NLR) family of intracellular proteins. Several NLRs function in the innate immune system as sensors of pathogen components and participate in immune-mediated cellular responses via the caspase 1 inflammasome. Mutations in a gene related to NOD-2, NLRP3, are responsible for excess caspase 1-dependent interleukin-1beta (IL-1beta) in cryopyrinopathies such as Muckle-Wells syndrome. Furthermore, functional studies demonstrate that caspase 1-mediated release of IL-1beta also involves NOD-2. The aim of this study was to test the hypothesis that IL-1beta may mediate the inflammation seen in patients with Blau syndrome.

METHODS

IL-1beta release was measured in peripheral blood mononuclear cells cultured in vitro, obtained from 5 Blau syndrome individuals with a NOD2 (CARD15) mutation.

RESULTS

We observed no evidence for increased IL-1beta production in cells obtained from subjects with Blau syndrome compared with healthy control subjects. Furthermore, we presented 2 cases of Blau syndrome in which recombinant human IL-1 receptor antagonist (anakinra) was ineffective treatment.

CONCLUSION

Taken together, these data suggest that in contrast to related IL-1beta-dependent autoinflammatory cryopyrinopathies, Blau syndrome is not mediated by excess IL-1beta or other IL-1 activity.

Host-microbe symbiosis: the squid-Vibrio association--a naturally occurring, experimental model of animal/bacterial partnerships

Many, if not most, animals have specific symbiotic relationships with bacterial partners. Recent studies suggest that vertebrates create alliances with highly complex consortia of hundreds to thousands of prokaryotic phylotypes. In contrast, invertebrates often have binary associations, i.e., relationships with a population of a single bacterial species. In this chapter, the association between the Hawaiian sepiolid squid Euprymna scolopes and the marine luminous bacterium Vibrio fisheri is highlighted. This symbiosis offers a relatively simple, yet naturally occurring, association that can be experimentally manipulated. Studies of this system are providing insight into the precise mechanisms by which a beneficial animal-bacterial symbiosis can be established and maintained.

Manifold mechanisms of Toll-like receptor-ligand recognition

Toll-like receptors recognize a diverse range of molecules derived from pathogens as well as host cells. As the number and diversity of TLR ligands and host factors increase, more questions are being raised. Here, we review recent advances toward understanding the molecular and cellular mechanisms underlying TLR-mediated direct or indirect recognition of their diverse range of ligands, including lipids, proteins, and nucleic acids. The elucidation of such mechanisms may represent a key for developing novel immunotherapeutics for infectious diseases, allergies, or cancer and to intervene in immunological disorders such as autoimmune diseases.

Polymorphisms in innate immunity genes predispose to bacteremia and death in the medical intensive care unit

OBJECTIVE

Critically ill patients are at risk of sepsis, organ failure, and death. Studying the impact of genetic determinants may improve our understanding of the pathophysiology and allow identification of patients who would benefit from specific treatments. Our aim was to study the influence of single nucleotide polymorphisms in selected genes involved in innate immunity on the development of bacteremia or risk of death in patients admitted to a medical intensive care unit.

DESIGN, SETTING, AND PATIENTS

DNA was available from 774 medical intensive care unit patients. We selected 31 single nucleotide polymorphisms in 14 genes involved in host innate immune defense. Serum levels of MASP2 and chemotactic capacity, phagocytosis, and killing capacity of monocytes at admission were quantified. Univariate Kaplan-Meier estimates with log-rank analysis and multivariate logistic regression were performed. Bootstrap resampling technique and ten-fold cross-validation were used to assess replication stability, prognostic importance of the variables, and repeatability of the final regression model.

MAIN RESULTS

Patients with at least one NOD2 variant were shown to have a reduced phagocytosis by monocytes (p = 0.03) and a higher risk of bacteremia than wild-type patients (p = 0.02). The NOD2/TLR4 combination was associated with bacteremia using survival analyses (time to bacteremia development, log-rank p < 0.0001), univariate regression (p = 0.0003), and multivariate regression analysis (odds ratio [OR] 4.26, 95% confidence interval [CI] 1.85-9.81; p = 0.0006). Similarly, the same combination was associated with hospital mortality using survival analysis (log-rank p = 0.03), univariate regression (p = 0.02), and multivariate regression analysis (OR 2.27, 95% CI 1.09-4.74; p = 0.03). Also variants in the MASP2 gene were significantly associated with hospital mortality (survival analysis log-rank-p = 0.003; univariate regression p = 0.02; multivariate regression analysis OR 2.35, 95% CI 1.38-3.99; p = 0.002).

CONCLUSIONS

Functional polymorphisms in genes involved in innate immunity predispose to severe infections and death, and may become part of a risk model, allowing identification of patients at risk, who could benefit from early introduction of specific preventive or therapeutic interventions.

Critical role of NOD2 in regulating the immune response to Staphylococcus aureus

NOD2 (the nucleotide-binding oligomerization domain containing protein 2) is known to be involved in host recognition of bacteria, although its role in the host response to Staphylococcus aureus infection is unknown. NOD2-deficient (Nod2(-/-)) mice and wild-type (WT) littermate controls were injected intraperitoneally with S. aureus suspension (10(7) bacteria/g of body weight), and their survival was monitored. Cultured bone marrow-derived neutrophils were harvested from Nod2(-/-) and WT mice and tested for cytokine production and phagocytosis. Compared to WT mice, Nod2(-/-) mice were significantly more susceptible to S. aureus infection (median survival of 1.5 days versus >5 days; P = 0.003) and had a significantly higher bacterial tissue burden. Cultured bone marrow-derived neutrophils from Nod2(-/-) and WT mice had similar levels of peritoneal neutrophil recruitment and intracellular killing, but bone marrow-derived neutrophils from Nod2(-/-) mice had significantly reduced ability to internalize fluorescein-labeled S. aureus. Nod2(-/-) mice had significantly higher levels of Th1-derived cytokines in serum (tumor necrosis factor alpha, gamma interferon, and interleukin-2 [IL-2]) compared to WT mice, whereas the levels of Th2-derived cytokines (IL-1beta, IL-4, IL-6, and IL-10) were similar in Nod2(-/-) and WT mice. Thus, mice deficient in NOD2 are more susceptible to S. aureus. Increased susceptibility is due in part to defective neutrophil phagocytosis, elevated serum levels of Th1 cytokines, and a higher bacterial tissue burden.