The innate immune sensor NLRC3 attenuates Toll-like receptor signaling via modification of the signaling adaptor TRAF6 and transcription factor NF-κB.

Several members of the NLR family of sensors activate innate immunity. In contrast, we found here that NLRC3 inhibited Toll-like receptor (TLR)-dependent activation of the transcription factor NF-κB by interacting with the TLR signaling adaptor TRAF6 to attenuate Lys63 (K63)-linked ubiquitination of TRAF6 and activation of NF-κB. We used bioinformatics to predict interactions between NLR and TRAF proteins, including interactions of TRAF with NLRC3. In vivo, macrophage expression of Nlrc3 mRNA was diminished by the administration of lipopolysaccharide (LPS) but was restored when cellular activation subsided. To assess biologic relevance, we generated Nlrc3(-/-) mice. LPS-treated Nlrc3(-/-) macrophages had more K63-ubiquitinated TRAF6, nuclear NF-κB and proinflammatory cytokines. Finally, LPS-treated Nlrc3(-/-) mice had more signs of inflammation. Thus, signaling via NLRC3 and TLR constitutes a negative feedback loop. Furthermore, prevalent NLR-TRAF interactions suggest the formation of a 'TRAFasome' complex.

NLRC3, a member of the NLR family of proteins, is a negative regulator of innate immune signaling induced by the DNA sensor STING.

Stimulator of interferon genes (STING, also named MITA, MYPS, or ERIS) is an intracellular DNA sensor that induces type I interferon through its interaction with TANK-binding kinase 1 (TBK1). Here we found that the nucleotide-binding, leucine-rich-repeat-containing protein, NLRC3, reduced STING-dependent innate immune activation in response to cytosolic DNA, cyclic di-GMP (c-di-GMP), and DNA viruses. NLRC3 associated with both STING and TBK1 and impeded STING-TBK1 interaction and downstream type I interferon production. By using purified recombinant proteins, we found NLRC3 to interact directly with STING. Furthermore, NLRC3 prevented proper trafficking of STING to perinuclear and punctated region, known to be important for its activation. In animals, herpes simplex virus 1 (HSV-1)-infected Nlrc3(-/-) mice exhibited enhanced innate immunity and reduced morbidity and viral load. This demonstrates the intersection of two key pathways of innate immune regulation, NLR and STING, to fine tune host response to intracellular DNA, DNA virus, and c-di-GMP.

Stimulator of interferon genes (STING) is an essential proviral host factor for human rhinovirus species A and C.

Human rhinoviruses (RVs) are positive-strand RNA viruses that cause respiratory tract disease in children and adults. Here we show that the innate immune signaling protein STING is required for efficient replication of members of two distinct RV species, RV-A and RV-C. The host factor activity of STING was identified in a genome-wide RNA interference (RNAi) screen and confirmed in primary human small airway epithelial cells. Replication of RV-A serotypes was strictly dependent on STING, whereas RV-B serotypes were notably less dependent. Subgenomic RV-A and RV-C RNA replicons failed to amplify in the absence of STING, revealing it to be required for a step in RNA replication. STING was expressed on phosphatidylinositol 4-phosphate (PI4P)-enriched membranes and was enriched in RV-A16 compared with RV-B14 replication organelles isolated in isopycnic gradients. The host factor activity of STING was species-specific, as murine STING (mSTING) did not rescue RV-A16 replication in STING-deficient cells. This species specificity mapped primarily to the cytoplasmic, ligand-binding domain of STING. Mouse-adaptive mutations in the RV-A16 2C protein allowed for robust replication in cells expressing mSTING, suggesting a role for 2C in recruiting STING to RV-A replication organelles. Palmitoylation of STING was not required for RV-A16 replication, nor was the C-terminal tail of STING that mediates IRF3 signaling. Despite co-opting STING to promote its replication, interferon signaling in response to STING agonists remained intact in RV-A16 infected cells. These data demonstrate a surprising requirement for a key host mediator of innate immunity to DNA viruses in the life cycle of a small pathogenic RNA virus.

The mitochondrial proteins NLRX1 and TUFM form a complex that regulates type I interferon and autophagy.

The mitochondrial protein MAVS (also known as IPS-1, VISA, and CARDIF) interacts with RIG-I-like receptors (RLRs) to induce type I interferon (IFN-I). NLRX1 is a mitochondrial nucleotide-binding, leucine-rich repeats (NLR)-containing protein that attenuates MAVS-RLR signaling. Using Nlrx1(-/-) cells, we confirmed that NLRX1 attenuated IFN-I production, but additionally promoted autophagy during viral infection. This dual function of NLRX1 paralleled the previously described functions of the autophagy-related proteins Atg5-Atg12, but NLRX1 did not associate with Atg5-Atg12. High-throughput quantitative mass spectrometry and endogenous protein-protein interaction revealed an NLRX1-interacting partner, mitochondrial Tu translation elongation factor (TUFM). TUFM interacted with Atg5-Atg12 and Atg16L1 and has similar functions as NLRX1 by inhibiting RLR-induced IFN-I but promoting autophagy. In the absence of NLRX1, increased IFN-I and decreased autophagy provide an advantage for host defense against vesicular stomatitis virus. This study establishes a link between an NLR protein and the viral-induced autophagic machinery via an intermediary partner, TUFM.

Role of inflammasomes in the pathogenesis of periodontal disease and therapeutics.

Inflammasomes are a group of multimolecular intracellular complexes assembled around several innate immune proteins. Recognition of a diverse range of microbial, stress and damage signals by inflammasomes results in direct activation of caspase-1, which subsequently induces the only known form of secretion of active interleukin-1ß and interleukin-18. Although the importance of interleukin-1ß in the periodontium is not questioned, the impact of inflammasomes in periodontal disease and its potential for therapeutics in periodontology is still in its very early stages. Increasing evidence in preclinical models and human data strongly implicate the involvement of inflammasomes in a number of inflammatory, autoinflammatory and autoimmune disorders. Here we review: (a) the currently known inflammasome functions, (b) clinical/preclinical data supporting inflammasome involvement in the context of periodontal and comorbid diseases and (c) potential therapies targeting inflammasomes. To clarify further the inflammasome involvement in periodontitis, we present analyses of data from a large clinical study (n = 5809) that measured the gingival crevicular fluid-interleukin-1ß and grouped the participants based on current periodontal disease classifications. We review data on 4910 European-Americans that correlate 16 polymorphisms in the interleukin-1B region with high gingival crevicular fluid-interleukin-1ß levels. We show that inflammasome components are increased in diseased periodontal tissues and that the caspase-1 inhibitor, VX-765, inhibits ~50% of alveolar bone loss in experimental periodontitis. The literature review further supports that although patients clinically present with the same phenotype, the disease that develops probably has different underlying biological pathways. The current data indicate that inflammasomes have a role in periodontal disease pathogenesis. Understanding the contribution of different inflammasomes to disease development and distinct patient susceptibility will probably translate into improved, personalized therapies.

Porphyromonas gingivalis mediates inflammasome repression in polymicrobial cultures through a novel mechanism involving reduced endocytosis.

The interleukin (IL)-1ß-processing inflammasome has recently been identified as a target for pathogenic evasion of the inflammatory response by a number of bacteria and viruses. We postulated that the periodontal pathogen, Porphyromonas gingivalis may suppress the inflammasome as a mechanism for its low immunogenicity and pathogenic synergy with other, more highly immunogenic periodontal bacteria. Our results show that P. gingivalis lacks signaling capability for the activation of the inflammasome in mouse macrophages. Furthermore, P. gingivalis can suppress inflammasome activation by another periodontal bacterium, Fusobacterium nucleatum. This repression affects IL-1ß processing, as well as other inflammasome-mediated processes, including IL-18 processing and cell death, in both human and mouse macrophages. F. nucleatum activates IL-1ß processing through the Nlrp3 inflammasome; however, P. gingivalis repression is not mediated through reduced levels of inflammasome components. P. gingivalis can repress Nlrp3 inflammasome activation by Escherichia coli, and by danger-associated molecular patterns and pattern-associated molecular patterns that mediate activation through endocytosis. However, P. gingivalis does not suppress Nlrp3 inflammasome activation by ATP or nigericin. This suggests that P. gingivalis may preferentially suppress endocytic pathways toward inflammasome activation. To directly test whether P. gingivalis infection affects endocytosis, we assessed the uptake of fluorescent particles in the presence or absence of P. gingivalis. Our results show that P. gingivalis limits both the number of cells taking up beads and the number of beads taken up for bead-positive cells. These results provide a novel mechanism of pathogen-mediated inflammasome inhibition through the suppression of endocytosis.

Construction and characterization of a derivative of Neisseria gonorrhoeae strain MS11 devoid of all opa genes.

To better understand the role of Opa in gonococcal infections, we created and characterized a derivative of MS11 (MS11Δopa) that had the coding sequence for all 11 Opa proteins deleted. The MS11Δopa bacterium lost the ability to bind to purified lipooligosaccharide (LOS). While nonpiliated MS11Δopa and nonpiliated Opa-expressing MS11 cells grew at the same rate, nonpiliated MS11Δopa cells rarely formed clumps of more than four bacteria when grown in broth with vigorous shaking. Using flow cytometry analysis, we demonstrated that MS11Δopa produced a homogeneous population of bacteria that failed to bind monoclonal antibody (MAb) 4B12, a MAb specific for Opa. Opa-expressing MS11 cells consisted of two predominant populations, where ∼85% bound MAb 4B12 to a significant level and the other population bound little if any MAb. Approximately 90% of bacteria isolated from a phenotypically Opa-negative colony (a colony that does not refract light) failed to bind MAb 4B12; the remaining 10% bound MAb to various degrees. Piliated MS11Δopa cells formed dispersed microcolonies on ME180 cells which were visually distinct from those of piliated Opa-expressing MS11 cells. When Opa expression was reintroduced into MS11Δopa, the adherence ability of the strain recovered to wild-type levels. These data indicate that Opa contributes to both bacterium-bacterium and bacterium-host cell interactions.

Neisseria gonorrhoeae-induced transactivation of EGFR enhances gonococcal invasion.

Neisseria gonorrhoeae, the causative agent of the sexually transmitted infection gonorrhoea, adheres to and invades into genital epithelial cells. Here, we investigate host components that are used by the bacteria for their entry into epithelial cells. We found that gonococcal microcolony formation on the surface of HEC-1-B cells disrupted the polarized, basolateral distribution of both epidermal growth factor receptor (EGFR) and ErbB2, a related family member, and induced their accumulation under the microcolonies at the apical membrane. Gonococcal infection increased EGFR and ErbB2 phosphorylation. The EGFR kinase inhibitor, AG1478, reduced gonococcal invasion by 80%, but had no effect on adherence or the recruitment of EGFR and ErbB2 to the microcolonies. Gonococcal inoculation upregulated the mRNA levels of several ligands of EGFR. Prevention of EGFR ligand shedding by blocking matrix metalloproteinase activation reduced gonococcal invasion without altering their adherence, while the addition of the EGFR ligand, HB-EGF, was able to restore invasion to 66% of control levels. These data indicate that N. gonorrhoeae modulates the activity and cellular distribution of host EGFR, facilitating their invasion. EGFR activation does not appear to be due to direct gonococcal binding to EGFR, but instead by its transactivation by gonococcal induced increases in EGFR ligands.

Oral biofilm dysbiosis during experimental periodontitis.

OBJECTIVES: We have previously characterized the main osteoimmunological events that occur during ligature periodontitis. This study aims to determine the polymicrobial community shifts that occur during disease development. METHODS: Periodontitis was induced in C57BL/6 mice using the ligature-induced periodontitis model. Healthy oral mucosa swabs and ligatures were collected every 3 days from 0 to 18 days post-ligature placement. Biofilm samples were evaluated by 16SrRNA gene sequencing (Illumina MiSeq) and QIIME. Time-course changes were determined by relative abundance, diversity, and rank analyses (PERMANOVA, Bonferroni-adjusted). RESULTS: Microbial differences between health and periodontal inflammation were observed at all phylogenic levels. An evident microbial community shift occurred in 25 genera during the advancement of "gingivitis" (3-6 days) to periodontitis (9-18 days). From day 0 to 18, dramatic changes were identified in Streptococcus levels, with an overall decrease (54.04%-0.02%) as well an overall increase of Enterococcus and Lactobacillus (23.7%-73.1% and 10.1%-70.2%, respectively). Alpha-diversity decreased to its lowest at 3 days, followed by an increase in diversity as disease advancement. Beta-diversity increased after ligature placement, indicating that bone loss develops in response to a greater microbial variability (p = 0.001). Levels of facultative and strict anaerobic bacteria augmented over the course of disease progression, with a total of eight species significantly different during the 18-day period. CONCLUSION: The data supports that murine gingival inflammation and alveolar bone loss develop in response to microbiome shifts. Bacterial diversity increased during progression to bone loss. These findings further support the utilization of the periodontitis ligature model for microbial shift analysis under different experimental conditions.

Interferon activated gene 204 protects against bone loss in experimental periodontitis.

BACKGROUND: Periodontal destruction can be the result of different known and yet-to-be-discovered biological pathways. Recent human genetic association studies have implicated interferon-gamma inducible protein 16 (IFI16) and absent in melanoma 2 (AIM2) with high periodontal interleukin (IL)-1ß levels and more destructive disease, but mechanistic evidence is lacking. Here, we sought to experimentally validate these observational associations and better understand IFI16 and AIM2's roles in periodontitis. METHODS: Periodontitis was induced in Ifi204-/- (IFI16 murine homolog) and Aim2-/- mice using the ligature model. Chimeric mice were created to identify the main source cells of Ifi204 in the periodontium. IFI16-silenced human endothelial cells were treated with periodontal pathogens in vitro. Periodontal tissues from Ifi204-/- mice were evaluated for alveolar bone (micro-CT), cell inflammatory infiltration (MPO+ staining), Il1b (qRT-PCR), and osteoclast numbers (cathepsin K+ staining). RESULTS: Ifi204-deficient mice> exhibited >20% higher alveolar bone loss than wild-type (WT) (P < 0.05), while no significant difference was found in Aim2-/- mice. Ifi204's effect on bone loss was primarily mediated by a nonbone marrow source and was independent of Aim2. Ifi204-deficient mice had greater neutrophil/macrophage trafficking into gingival tissues regardless of periodontitis development compared to WT. In human endothelial cells, IFI16 decreased the chemokine response to periodontal pathogens. In murine periodontitis, Ifi204 depletion elevated gingival Il1b and increased osteoclast numbers at diseased sites (P < 0.05). CONCLUSIONS: These findings support IFI16's role as a novel regulator of inflammatory cell trafficking to the periodontium that protects against bone loss and offers potential targets for the development of new periodontal disease biomarkers and therapeutics.

Structural characterization of an oligosaccharide made by Neisseria sicca.

Neisseria sicca 4320 expresses two carbohydrate-containing components with sodium dodecyl sulfate-polyacrylamide gel electrophoresis mobilities that resemble those of lipooligosaccharide and lipopolysaccharide. Using matrix-assisted laser desorption ionization--time of flight and electrospray ionization mass spectrometry, we characterized a disaccharide carbohydrate repeating unit expressed by this strain. Gas chromatography identified the sugars composing the unit as rhamnose and N-acetyl-D-glucosamine. Glycosidase digestion confirmed the identity of the nonreducing terminal sugar of the disaccharide and established its beta-anomeric configuration. Mass spectrometry analysis and lectin binding were used to verify the linkages within the disaccharide repeat. The results revealed that the disaccharide repeat is [-4) beta-L-rhamnose (1-3) beta-N-acetyl-D-glucosamine (1-] with an N-acetyl-D-glucosamine nonreducing terminus. This work is the first structural characterization of a molecule that possesses rhamnose in the genus Neisseria.

The NLRP3 inflammasome: molecular activation and regulation to therapeutics.

NLRP3 (NOD-, LRR- and pyrin domain-containing protein 3) is an intracellular sensor that detects a broad range of microbial motifs, endogenous danger signals and environmental irritants, resulting in the formation and activation of the NLRP3 inflammasome. Assembly of the NLRP3 inflammasome leads to caspase 1-dependent release of the pro-inflammatory cytokines IL-1ß and IL-18, as well as to gasdermin D-mediated pyroptotic cell death. Recent studies have revealed new regulators of the NLRP3 inflammasome, including new interacting or regulatory proteins, metabolic pathways and a regulatory mitochondrial hub. In this Review, we present the molecular, cell biological and biochemical bases of NLRP3 activation and regulation and describe how this mechanistic understanding is leading to potential therapeutics that target the NLRP3 inflammasome.

A noncanonical function of cGAMP in inflammasome priming and activation.

Recognition of pathogen-associated molecular patterns and danger-associated molecular patterns by host cells is an important step in innate immune activation. The DNA sensor cyclic guanosine monophosphate-adenosine monophosphate (cGAMP) synthase (cGAS) binds to DNA and produces cGAMP, which in turn binds to stimulator of interferon genes (STING) to activate IFN-I. Here we show that cGAMP has a noncanonical function in inflammasome activation in human and mouse cells. Inflammasome activation requires two signals, both of which are activated by cGAMP. cGAMP alone enhances expression of inflammasome components through IFN-I, providing the priming signal. Additionally, when combined with a priming signal, cGAMP activates the inflammasome through an AIM2, NLRP3, ASC, and caspase-1 dependent process. These two cGAMP-mediated functions, priming and activation, have differential requirements for STING. Temporally, cGAMP induction of IFN-I precedes inflammasome activation, which then occurs when IFN-I is waning. In mice, cGAS/cGAMP amplify both inflammasome and IFN-I to control murine cytomegalovirus. Thus, cGAMP activates the inflammasome in addition to IFN-I, and activation of both is needed to control infection by a DNA virus.

Separation and identification of neisserial lipooligosaccharide oligosaccharides using high-performance anion-exchange chromatography with pulsed amperometric detection.

We determined the optimal conditions for high-performance anion-exchange chromatography with pulsed amperometric detection (HPAE-PAD) of oligosaccharides (OS) released from neisserial lipooligosaccharides (LOS) by mild acid hydrolysis. We efficiently obtained detailed composition, sequence, and linkage information about high Mr LOS. We found that HPAE-PAD can discriminate isobaric (same Mr) molecules of different structure, for example, nLc4 and Gb4, distinguish alpha from beta chain extensions, and determine the number of phosphoethanolamine (PEA) substituents. HPAE-PAD provided quantitative information that could be used to compare the relative abundances of OS. We used HPAE-PAD to identify all of the known LOS alpha chain antennae. When used with antibody-binding profiles and exoglycosidase digestion results, HPAE-PAD can provide nearly complete structures rapidly.

Structural requirements for monoclonal antibody 2-1-L8 recognition of neisserial lipooligosaccharides.

Monoclonal antibodies (MAbs) that bind neisserial lipooligosaccharides (LOS) have been widely used in structural studies of these glycolipids. MAb 2-1-L8 binds LOS with a lactosyl a chain (Gal beta1-4 Glc beta1-4 [Glc-NAc alpha1-2 Hep2 alpha1-3] Hep1 alpha1-KDO) and at least one phosphoethanolamine (PEA) substitution of Hep2, but the requirement for PEA substitution and/or the exact position of this substitution, cyclic or exocyclic, remains unclear. In order to clarify the exact specificity of this MAb, we engineered an isogenic family of lpt mutants that each make LOS with a lactosyl a chain, but that lacked cyclic (-3Hep2), exocyclic (-6Hep2), or any PEA residues. Mass spectrometry showed that mutants that lack either Lpt3 or Lpt6 make small amounts of LOS with two PEA substitutions. Thus, each enzyme is able to phosphoethanolaminylate the alternate site, albeit with low efficiency. LOS made by the mutant that lacked both Lpt3 and Lpt6 was devoid of PEA. LOS made by the deltalpt3 mutant did not bind MAb 2-1-L8 on Western blot analysis, whereas delta pt6 LOS did. Analysis of intact mutants by fluorescence-activated cell sorting confirmed that PEA substitution at 3Hep2, but not at 6Hep2, is needed for optimal binding of MAb 2-1-L8. These data confirm that the MAb 2-1-L8 epitope requires a -3Hep2 cyclic PEA substitution for optimal conformation and that this MAb specifies the PEA-3Hep2 lactosyl LOS structure.

Biochemical analysis of Lpt3, a protein responsible for phosphoethanolamine addition to lipooligosaccharide of pathogenic Neisseria.

The inner core of neisserial lipooligosaccharide (LOS) contains heptose residues that can be decorated by phosphoethanolamine (PEA). PEA modification of heptose II (HepII) can occur at the 3, 6, or 7 position(s). We used a genomic DNA sequence of lpt3, derived from Neisseria meningitidis MC58, to search the genomic sequence of N. gonorrhoeae FA1090 and identified a homolog of lpt3 in N. gonorrhoeae. A PCR amplicon containing lpt3 was amplified from F62DeltaLgtA, cloned, mutagenized, and inserted into the chromosome of N. gonorrhoeae strain F62DeltaLgtA, producing strain F62DeltaLgtAlpt3::Tn5. LOS isolated from this strain lost the ability to bind monoclonal antibody (MAb) 2-1-L8. Complementation of this mutation by genetic removal of the transposon insertion restored MAb 2-1-L8 binding. Mass spectrometry analysis of LOS isolated from the F62DeltaLgtA indicated that this strain contained two PEA modifications on its LOS. F62DeltaLgtAlpt3::Tn5 lacked a PEA modification on its LOS, a finding consistent with the hypothesis that lpt3 encodes a protein mediating PEA addition onto gonococcal LOS. The DNA encoding lpt3 was cloned into an expression vector and Lpt3 was purified. Purified Lpt3 was able to mediate the addition of PEA to LOS isolated from F62DeltaLgtAlpt3::Tn5.

Galectin-3 binds lactosaminylated lipooligosaccharides from Neisseria gonorrhoeae and is selectively expressed by mucosal epithelial cells that are infected.

Galectins are a family of beta-galactoside binding proteins that have been proposed as host receptors for bacteria because beta-galactoside carbohydrates are common in bacterial membrane glycolipid lipooligosaccharides (LOS) and lipopolysaccharides. We investigated the interaction of galectin-3 with gonococcal LOS that make lactosyl (Lc2 or Lac), paraglobosyl (nLc4; LNnT; lacto-N-neotetraose), gangliosyl (IV3GalNAcnLc4), and neolactohexaosyl (nLc6, lactonorhexaosyl) oligosaccharides. All but gangliosyl LOS terminate in beta-galactoside. Galectin-3 had the highest affinity for the nLc6 LOS, which is made by a strain that is highly infectious for the male urethra, but also bound nLc4 LOS and to a Lac LOS. The lacto-N-neotetraose tetrasaccharide was a more potent inhibitor of galectin-3 binding to LOS than either lactose or N-acetyllactosamine. The relative affinity of galectin-3 for gonococci mirrored its affinity for purified LOS. Western blot analysis revealed expression of galectin-3 by human endometrial adenocarcinoma and prostatic epithelial cells that can be invaded by gonococci. Immunohistochemistry of human fallopian tube epithelium showed localized expression of galectin-3 by non-ciliated cells, the specific cell gonococci invade in this tissue. We conclude that because of its location and affinity for gonococcal LOS galectin-3 could play a role in gonococcal infection.