The innate immune system senses infection by detecting evolutionarily conserved molecules essential for microbial survival or abnormal location of molecules. Here we demonstrate the existence of a novel innate detection mechanism, which is induced by fusion between viral envelopes and target cells. Virus-cell fusion specifically stimulated a type I interferon (IFN) response with expression of IFN-stimulated genes (ISGs), in vivo recruitment of leukocytes, and potentiation of Toll-like receptor 7 and 9 signaling. The fusion dependent response was dependent on stimulator of interferon genes (STING) but independent of DNA, RNA and viral capsid. We suggest that membrane fusion is sensed as a danger signal with potential implications for defense against enveloped viruses and various conditions of giant cell formation.

Innate immune responses have the ability to both combat infectious microbes and drive pathological inflammation. Inflammasome complexes are a central component of these processes through their regulation of interleukin 1β (IL-1β), IL-18 and pyroptosis. Inflammasomes recognize microbial products or endogenous molecules released from damaged or dying cells both through direct binding of ligands and indirect mechanisms. The potential of the IL-1 family of cytokines to cause tissue damage and chronic inflammation emphasizes the importance of regulating inflammasomes. Many regulatory mechanisms have been identified that act as checkpoints for attenuating inflammasome signaling at multiple steps. Here we discuss the various regulatory mechanisms that have evolved to keep inflammasome signaling in check to maintain immunological balance.

Background

Cytokines regulated by the inflammasome pathway have been extensively implicated in various age-related immune pathologies. We set out to elucidate the contribution of the nod-like receptor protein 3 (NLRP3) inflammasome pathway to the previously described deficiencies in IL-1β production by macrophages from aged mice. We examined the production of pro-IL-1β and its conversion into IL-1β as two separate steps and compared these cytokine responses in bone marrow derived macrophages from young (6–8 weeks) and aged (18–24 months) C57BL/6 mice.

Findings

Relative to macrophages from young mice, macrophages from aged mice produced less pro-IL-1β after TLR4 stimulation with LPS. However upon activation of the NLRP3 inflammasome with ATP, macrophages from young and aged mice were able to efficiently convert and secrete intracellular pro-cytokines as functional cytokines.

Conclusions

Lower levels of IL-1β production are a result of slower and lower overall production of pro-IL-1β in macrophages from aged mice.

Toll-like receptors (TLRs) are innate sentinels required for clearance of bacterial and fungal infections of the cornea, but their role in viral immunity is currently unknown. We report TLR signaling is expendable in HSV-1 containment as depicted by plaque assays of knockout mice (MyD88−/−, Trif−/− and MyD88−/− Trif−/− DKO) resembling wild type controls. To identify the key sentinel in viral recognition of the cornea, in vivo knockdown of the DNA sensor IFI-16/p204 in corneal epithelium was performed and resulted in a loss of IRF-3 nuclear translocation, interferon-α production, and viral containment. The sensor appears to have a similar function in other HSV clinically-relevant sites such as the vaginal mucosa in which a loss of p204/IFI-16 results in significantly more HSV-2 shedding. Thus, we have identified an IRF-3 dependent, IRF-7 and TLR - independent innate sensor responsible for HSV containment at the site of acute infection.

SUMMARY

Although Toll-like receptor 9 (TLR9) has been implicated in regulating cytokine and type I interferon (IFN) production during malaria in humans and mice, the high AT content of the Plasmodium falciparum genome prompted us to examine the possibility that malarial DNA triggered TLR9-independent DNA sensing pathways. Over 6000 ATTTTTAC (“AT-rich”) motifs are present in the genome of P. falciparum, which we show here potently induce type I IFNs. Parasite DNA, parasitized erythrocytes and oligonucleotides containing the AT-r motif induce type I IFNs via a pathway that did not involve previously described sensors including TLR9, DAI, RNA polymerase-III or IFI16/p204. Rather, AT-rich DNA sensing involved an unknown receptor that coupled to STING, TBK1 and IRF3-IRF7 signaling pathway. Mice lacking both IRF3 and IRF7, the kinase TBK1 or the type I IFN receptor were resistant to otherwise lethal cerebral malaria. Collectively, these observations implicate AT-rich DNA sensing via STING, TBK1 and IRF3-IRF7 in P. falciparum malaria.

Lipopolysaccharide (LPS)-induced production of tumor necrosis factor (TNF)-α requires the recruitment of two pairs of adaptors to the Toll-like receptor 4 cytoplasmic domain. The contribution of one pair – Toll-interleukin-1 receptor domain-containing adaptor inducing interferon-β (TRIF) and TRIF-related adaptor molecule (TRAM) – to TNF-α expression is not well understood. To clarify this issue, we studied TRAM knockout bone marrow-derived macrophages (BMDM). LPS-stimulated TRAM-deficient BMDM had decreased TNF-α protein expression even at times when TNF-α mRNA levels were normal, suggesting impaired translation. Consistent with this idea, knockdown of TRAM in RAW264.7 macrophages decreased translation of a reporter controlled by the TNF-α 3′ untranslated region, while transfection of TRAM in HEK293T cells increased translation of this reporter. Also consistent with a role for TRAM in TNF-α translation, LPS-induced activation of MK2, a kinase involved in this process, was impaired in TRAM-deficient BMDM. TRIF did not increase translation of the TNF-α 3′ untranslated region reporter when expressed in HEK293T cells. However, BMDM that lacked functional TRIF produced reduced levels of TNF-α protein in response to LPS despite normal amounts of the mRNA. Unlike BMDM, LPS-stimulated TRAM-deficient peritoneal macrophages displayed equivalent reductions in TNF-α protein and mRNA. Our results indicate that TRAM- and TRIF-dependent signals have a previously unappreciated, cell type-specific role in regulating TNF-α translation.

Copyright © 2011 S. Karger AG, Basel

Murine Aim2 and Ifi202 genes (encoding for the Aim2 and p202 proteins) are members of the interferon (IFN)-inducible Ifi200-gene family. The Aim2-deficiency in mice activates IFN-signaling and stimulates the expression of lupus susceptibility gene, the Ifi202, located within the Nba2 interval. Given that the deficiency in the expression of the Fcgr2b gene (encoding for the inhibitory FcγRIIB receptor) is associated with increased lupus susceptibility in mice, we investigated whether the Aim2 protein could regulate the expression of Fcgr2b gene. Here we report that Aim2-deficiency in mice suppresses the expression of the FcγRIIB receptor. Interestingly, the Fcgr2b-deficient cells expressed increased levels of the IFN-β, activated IFN-signaling, and expressed reduced levels of the Aim2 protein. Treatment of splenic cells with IFN-α or γ reduced levels of the FcγRIIB mRNA and protein, and also decreased the activity of the FcγRIIB p(−729/+ 585) Luc reporter. Moreover, levels of the FcγRIIB receptor were significantly higher in the Stat1-deficient splenic cells than the wild type cells. Accordingly, increased expression of IFN-β in lupus-prone B6.Nba2-ABC mice, as compared with non lupus-prone B6 or B6.Nba2-C mice, was associated with reduced expression of the FcγRIIB receptor. Notably, overexpression of the p202 protein in cells decreased the expression of the Aim2 gene, activated the IFN-response, and suppressed the expression of the Fcgr2b gene. These observations demonstrate that the expression of Aim2 protein is required to maintain the expression of the Fcgr2b gene and also predict epistatic interactions between the Ifi200-genes and the Fcgr2b gene within the Nba2 interval.

Poxviruses are large DNA viruses that replicate in the cytoplasm of infected cells. Myxoma virus is a rabbit poxvirus that belongs to the Leporipoxvirus genus. It causes a lethal disease called myxomatosis in European rabbits but cannot sustain any detectable infection in nonlagomorphs. Vaccinia virus is a prototypal orthopoxvirus that was used as a vaccine to eradicate smallpox. Myxoma virus is nonpathogenic in mice, whereas systemic infection with vaccinia virus can be lethal even in immunocompetent mice. Plasmacytoid dendritic cells (pDCs) are potent type I interferon (IFN)-producing cells that play important roles in antiviral innate immunity. How poxviruses are sensed by pDCs to induce type I IFN production is not well understood. Here we report that infection of primary murine pDCs with myxoma virus, but not with vaccinia virus, induces IFN-α, IFN-β, tumor necrosis factor (TNF), and interleukin-12p70 (IL-12p70) production. Using pDCs derived from genetic knockout mice, we show that the myxoma virus-induced innate immune response requires the endosomal DNA sensor TLR9 and its adaptor MyD88, transcription factors IRF5 and IRF7, and the type I IFN positive-feedback loop mediated by IFNAR1. It is independent of the cytoplasmic RNA sensing pathway mediated by the mitochondrial adaptor molecule MAVS, the TLR3 adaptor TRIF, or the transcription factor IRF3. Using pharmacological inhibitors, we demonstrate that myxoma virus-induced type I IFN and IL-12p70 production in murine pDCs is also dependent on phosphatidylinositol 3-kinase (PI3K) and Akt. Furthermore, our results reveal that the N-terminal Z-DNA/RNA binding domain of vaccinia virulence factor E3, which is missing in the orthologous M029 protein expressed by myxoma virus, plays an inhibitory role in poxvirus sensing and innate cytokine production by murine pDCs.

DNA viruses are a significant contributor to human morbidity and mortality. The immune system protects against viral infections through coordinated innate and adaptive immune responses. While the antigen-specific adaptive mechanisms have been extensively studied, the critical contributions of innate immunity to anti-viral defenses have only been revealed in the very recent past. Central to these anti-viral defenses is the recognition of viral pathogens by a diverse set of germ-line encoded receptors that survey nearly all cellular compartments for the presence of pathogens. In this review, we discuss the recent advances in the innate immune sensing of DNA viruses and focus on the recognition mechanisms involved.

The type I interferons (IFNs), IFN-α and -β, are key effector molecules of the immune response to viruses. The anti-viral action of IFNs on virus-infected cells and surrounding tissues is mediated by expression of hundreds of IFN-stimulated genes. Viperin (virus inhibitory protein, endoplasmic reticulum-associated, IFN-inducible) is an Interferon stimulated gene (ISG), which is induced by type I, II, and III IFNs or after infection with a broad range of DNA and RNA viruses. Recent evidence indicates that Viperin disrupts lipid rafts to block influenza virus budding and release and interferes with replication of hepatitis C virus by binding to lipid droplets, small organelles involved in lipid homeostasis that are essential for hepatitis C virus replication. Viperin is also induced by nonviral microbial products such as lipopolysaccharide (LPS) and by a wide range of bacteria, suggesting a broader role in innate antimicrobial defenses.

Interleukin (IL)-1β is a cytokine critical to several inflammatory diseases in which pathogenic TH17 responses are implicated. Activation of the NLRP3 inflammasome by microbial and environmental stimuli can enable the caspase-1 dependent processing and secretion of IL-1β. The acute phase protein serum amyloid A (SAA) is highly induced during inflammatory responses, wherein it participates in systemic modulation of innate and adaptive immune responses. Elevated levels of IL-1β, SAA, and IL-17 are present in subjects with severe allergic asthma, yet the mechanistic relationship between these mediators has yet to be identified. Herein, we demonstrate that Saa3 is expressed in the lung of mice exposed to several mixed Th2/Th17-polarizing allergic sensitization regimens. SAA instillation into the lungs elicits robust TLR2-, MyD88-, and IL-1-dependent pulmonary neutrophilic inflammation. Furthermore, SAA drives production of IL-1α, IL-1β, IL-6, IL-23, and PGE2, causes dendritic cell maturation, and requires TLR2, MyD88, and the NLRP3 inflammasome for secretion of IL-1β by dendritic cells and macrophages. CD4+ T cells polyclonally stimulated in the presence of conditioned media from SAA-exposed dendritic cells produced IL-17 and the capacity of polyclonally-stimulated splenocytes to secrete IL-17 is dependent upon IL-1, TLR2, and the NLRP3 inflammasome. Additionally, in a model of allergic airway inflammation, administration of SAA to the lungs functions as an adjuvant to sensitize mice to inhaled ovalbumin, resulting in leukocyte influx after antigen challenge and a predominance of IL-17 production from restimulated splenocytes that is dependent upon IL-1 receptor signaling.

Murine Aim2 and p202 proteins (encoded by the Aim2 and Ifi202 genes) are members of the interferon (IFN)-inducible p200-protein family. Both proteins can sense double-stranded DNA (dsDNA) in the cytoplasm. However, upon sensing dsDNA, only the Aim2 protein through its pyrin domain (PYD) can form an inflammasome to activate caspase-1 and induce cell death. Given that the p202 protein has been predicted to inhibit the activation of caspase-1 by the Aim2 protein and that increased levels of the p202 protein in female mice of certain strains are associated with lupus susceptibility, we compared the expression of Aim2 and Ifi202 genes between Aim2-deficient and age-matched wild type mice. We found that the Aim2-deficiency in immune cells stimulated the expression of Ifi202 gene. The increased levels of the p202 protein in cells were associated with increases in the expression of IFN-β, STAT1, and IFN-inducible genes. Moreover, after knockdown of Aim2 expression in the murine macrophage cell line J774.A1, IFN-β treatment of cells robustly increased STAT1 protein levels (as compared to control cells), increased the activating phosphorylation of STAT1 on Tyr-701, and stimulated the activity of an IFN-responsive reporter. Notably, the expression of Aim2 in non lupus-prone (C57BL/6 and B6.Nba2-C) and lupus-prone B6.Nba2-ABC splenic cells and in a murine macrophage cell line that overexpressed p202 protein was found to be inversely correlated with Ifi202. Collectively, our observations demonstrate an inverse correlation between Aim2 and p202 expressions. We predict that defects in Aim2 expression within immune cells contribute to increased susceptibility to lupus.

Candida sp. are opportunistic fungal pathogens that colonize the skin and oral cavity and, when overgrown under permissive conditions, cause inflammation and disease. Previously, we identified a central role for the NLRP3 inflammasome in regulating IL-1β production and resistance to dissemination from oral infection with Candida albicans. Here we show that mucosal expression of NLRP3 and NLRC4 is induced by Candida infection, and up-regulation of these molecules is impaired in NLRP3 and NLRC4 deficient mice. Additionally, we reveal a role for the NLRC4 inflammasome in anti-fungal defenses. NLRC4 is important for control of mucosal Candida infection and impacts inflammatory cell recruitment to infected tissues, as well as protects against systemic dissemination of infection. Deficiency in either NLRC4 or NLRP3 results in severely attenuated pro-inflammatory and antimicrobial peptide responses in the oral cavity. Using bone marrow chimeric mouse models, we show that, in contrast to NLRP3 which limits the severity of infection when present in either the hematopoietic or stromal compartments, NLRC4 plays an important role in limiting mucosal candidiasis when functioning at the level of the mucosal stroma. Collectively, these studies reveal the tissue specific roles of the NLRP3 and NLRC4 inflammasome in innate immune responses against mucosal Candida infection.

Author Summary

In this manuscript we describe a new role for a group of molecules termed the “inflammasome” that process key immune response proteins including interleukin-1-β. In previous work, we and others have shown that the NLRP3 inflammasome is important in protecting from severe fungal infections. We now show that, in addition to the NLRP3 inflammasome, a different inflammasome containing NLRC4 is also important in protecting against infection with Candida albicans, and appears to be functioning in the mucosal lining of the mouth and intestines, rather than in immune cells. Our research explains a new mechanism of mucosal immunity to fungal infections and has broad implications for developing new treatments against fungal infections, which are a serious cause of illness and death, particularly in immunocompromised persons. Additionally, this research may also lead to new ways to identify those individuals who are at the highest risk for serious fungal infections.

Autophagy, a cellular process for organelle and protein turnover, regulates innate immune responses. We demonstrate that depletion of autophagic proteins microtubule associated protein-1 light chain 3B (LC3B) and Beclin 1 enhances caspase-1 activation and secretion of interleukin-1β and interleukin-18. Autophagic protein depletion promoted accumulation of dysfunctional mitochondria and cytosolic translocation of mitochondrial DNA (mtDNA) in response to lipopolysaccharide (LPS) and ATP in macrophages. Release of mtDNA into the cytosol depended on the NALP3 inflammasome and mitochondrial ROS. Cytosolic mtDNA contributed to IL-1β and IL-18 secretion in response to LPS and ATP. LC3B-deficient mice produced more caspase-1-dependent cytokines in two sepsis models and were susceptible to LPS-induced mortality. Our study suggests that autophagic proteins regulate NALP3-dependent inflammation by preserving mitochondrial integrity.

Chlamydia pneumoniae is a common respiratory pathogen associated with atypical pneumonia, and it has been suggested as a trigger or promoter of several chronic inflammatory conditions, such as asthma and atherosclerosis. The β form of IL-1 (IL-1β) is a proinflammatory cytokine released by many cell types and is an important mediator of inflammation during infection. IL-1β production is a tightly controlled process that includes regulation at multiple levels and typically requires two distinct signals for activation and release. In this study, we investigated the ability of C. pneumoniae to induce IL-1β secretion. We found that C. pneumoniae was unique among the other Chlamydia species tested in its ability to potently induce secretion of mature IL-1β from unprimed bone marrow-derived macrophages during a productive infection. TLR2 was required for induction of pro–IL-1β, whereas the NLRP3/ASC was required for caspase-1 activation and pro–IL-1β cleavage to produce mature IL-1β. Caspase-1 cleavage was independent of endogenous ATP release, but required potassium flux, lysosomal acidification, and cathepsin B release. We further investigated the role of IL-1 in host defense against C. pneumoniae-induced pneumonia using mice deficient in the type I IL-1R. Although the IL-1R−/− mice developed an inflammatory infiltrate, the number of infiltrating neutrophils was lower, whereas there was evidence of increased infiltrating fibroblasts and mesenchymal cells and more lung fibrosis. We conclude that C. pneumoniae directly activates the NLRP3/ASC inflammasome, leading to the release of biologically active IL-1β, and that concurrent IL-1 signaling is required for optimal host defense against acute bacterial pneumonia.

The detection of intracellular microbial DNA is critical to an appropriate innate immune response, however current knowledge on how such DNA is sensed is limited. Here we identify IFI16, a PYHIN protein, as an intracellular DNA sensor mediating interferon-β (IFNβ)-induction. IFI16 directly associated with IFNβ-inducing viral DNA motifs. STING, a critical mediator of IFNβ responses to DNA, was recruited to IFI16 after DNA stimulation. Reduction of expression of IFI16, or its murine ortholog p204, by RNA interference inhibited DNA- and herpes simplex virus (HSV)-1-induced gene induction and IRF3 and NFκB activation. IFI16/p204 is the first PYHIN protein shown to be involved in IFNβ induction, and thus together with AIM2, a PYHIN protein that senses DNA for caspase 1 activation, is part of a new family of innate DNA sensors which we term AIM2-like receptors (ALRs).

The inflammasome pathway functions to regulate caspase-1 activation in response to a broad range of stimuli. Caspase-1 activation is required for the maturation of the pivotal pro-inflammatory cytokines of the pro-IL-1β family. In addition, caspase-1 activation leads to a certain type of cell death known as pyroptosis. Activation of the inflammasome has been shown to play a critical role in the recognition and containment of various microbial pathogens, including the intracellularly replicating Listeria monocytogenes; however, the inflammasome pathways activated during L. monocytogenes infection are only poorly defined. Here, we demonstrate that L. monocytogenes activates both the NLRP3 and the AIM2 inflammasome, with a predominant involvement of the AIM2 inflammasome. In addition, L. monocytogenes-triggered cell death was diminished in the absence of both AIM2 and NLRP3, and is concomitant with increased intracellular replication of L. monocytogenes. Altogether, these data establish a role for DNA sensing through the AIM2 inflammasome in the detection of intracellularly replicating bacteria.

The innate immune response to viral pathogens is critical in order to mobilize protective immunity. Cells of the innate immune system detect viral infection largely through germline-encoded pattern recognition receptors (PRRs) present either on the cell surface or within distinct intracellular compartments. These include the Toll-like receptors (TLRs), the retinoic acid-inducble gene I-like receptors (RLRs), the nucleotide oligomerization domain-like receptors (NLRs, also called NACHT, LRR and PYD domain proteins) and cytosolic DNA sensors. While in certain cases viral proteins are the trigger of these receptors, the predominant viral activators are nucleic acids. The presence of viral sensing PRRs in multiple cellular compartments allows innate cells to recognize and quickly respond to a broad range of viruses, which replicate in different cellular compartments. Here, we review the role of PRRs and associated signaling pathways in detecting viral pathogens in order to evoke production of interferons and cytokines. By highlighting recent progress in these areas, we hope to convey a greater understanding of how viruses activate PRR signaling and how this interaction shapes the anti-viral immune response.

The fibrillar peptide amyloid-β (Aβ) has a chief function in the pathogenesis of Alzheimer’s disease. Interleukin 1β (IL-1β) is a key cytokine in the inflammatory response to Aβ. Insoluble materials such as crystals activate the inflammasome formed by the cytoplasmic receptor NALP3, which results in the release of IL-1β. Here we identify the NALP3 inflammasome as a sensor of Aβ in a process involving the phagocytosis of Aβ and subsequent lysosomal damage and release of cathepsin B. Furthermore, the IL-1β pathway was essential for the microglial synthesis of proinflammatory and neurotoxic factors, and the inflammasome, caspase-1 and IL-1β were critical for the recruitment of microglia to exogenous Aβ in the brain. Our findings suggest that activation of the NALP3 inflammasome is important for inflammation and tissue damage in Alzheimer’s disease.

ABSTRACT

The mucosal epithelium is the initial target for respiratory pathogens of all types. While type I interferon (IFN) signaling is traditionally associated with antiviral immunity, we demonstrate that the extracellular bacterial pathogen Streptococcus pneumoniae activates the type I IFN cascade in airway epithelial and dendritic cells. This response is dependent upon the pore-forming toxin pneumolysin. Pneumococcal DNA activates IFN-β expression through a DAI/STING/TBK1/IRF3 cascade. Tlr4−/−, Myd88−/−, Trif−/−, and Nod2−/− mutant mice had no impairment of type I IFN signaling. Induction of type I IFN signaling contributes to the eradication of pneumococcal carriage, as IFN-α/β receptor null mice had significantly increased nasal colonization with S. pneumoniae compared with that of wild-type mice. These studies suggest that the type I IFN cascade is a central component of the mucosal response to airway bacterial pathogens and is responsive to bacterial pathogen-associated molecular patterns that are capable of accessing intracellular receptors.

IMPORTANCE

The bacterium Streptococcus pneumoniae is a leading cause of bacterial pneumonia, leading to upwards of one million deaths a year worldwide and significant economic burden. Although it is known that antibody is critical for efficient phagocytosis, it is not known how this pathogen is sensed by the mucosal epithelium. We demonstrate that this extracellular pathogen activates mucosal signaling typically activated by viral pathogens via the pneumolysin pore to activate intracellular receptors and the type I interferon (IFN) cascade. Mice lacking the receptor to type I IFNs have a reduced ability to clear S. pneumoniae, suggesting that the type I IFN cascade is central to the mucosal clearance of this important pathogen.

The invasive Argentine ant (Linepithema humile) is established worldwide and displaces native ant species. In northern California, however, the native winter ant (Prenolepis imparis) persists in invaded areas. We found that in aggressive interactions between the two species, P. imparis employs a potent defensive secretion. Field observations were conducted at P. imparis nest sites both in the presence and absence of L. humile. These observations suggested and laboratory assays confirmed that P. imparis workers are more likely to secrete when outnumbered by L. humile. Workers of P. imparis were also more likely to secrete near their nest entrances than when foraging on trees. One-on-one laboratory trials showed that the P. imparis secretion is highly lethal to L. humile, causing 79% mortality. The nonpolar fraction of the secretion was chemically analyzed with gas chromatography/mass spectrometry, and found to be composed of long-chain and cyclic hydrocarbons. Chemical analysis of dissected P. imparis workers showed that the nonpolar fraction is derived from the Dufour's gland. Based on these conclusions, we hypothesize that this chemical defense may help P. imparis to resist displacement by L. humile.

The discovery of the Toll-like receptors (TLRs) and their importance in the regulation of host responses to infection raised attention to the complex interplay between viral gene products and the host innate immune responses in determining the outcome of virus infection. Robust inflammatory cytokine responses are observed in herpes simplex virus (HSV)-infected animals and cells. Our studies have demonstrated that Toll-like receptor 2 (TLR2) activation by HSV results in NF-κB activation with concomitant inflammatory cytokine production and that TLR2 activation plays a critical role in HSV-induced pathology and mortality. Here we demonstrate that the HSV-1 immediate-early ICP0 protein reduces the TLR2-mediated inflammatory response to HSV 1 (HSV-1) infection. Expression of ICP0 alone is sufficient to block TLR2-driven responses to both viral and nonviral ligands at or downstream of the MyD88 adaptor and upstream of p65. ICP0 alone can also reduce the levels of MyD88 and Mal (TIRAP). In HSV-infected cells, the E3 ligase function of ICP0 and cellular proteasomal activity are required for the inhibitory activity. Our results argue for a model in which ICP0 promotes the degradation of TLR adaptor molecules and inhibition of the inflammatory response, much as it inhibits the interferon response by sequestration and degradation of interferon regulatory factor 3 (IRF-3).

Type I interferons (IFNs) play a critical role in the host defense against viruses. Lymphocytic choriomeningitis virus (LCMV) infection induces robust type I IFN production in its natural host, the mouse. However, the mechanisms underlying the induction of type I IFNs in response to LCMV infection have not yet been clearly defined. In the present study, we demonstrate that IRF7 is required for both the early phase (day 1 postinfection) and the late phase (day 2 postinfection) of the type I IFN response to LCMV, and melanoma differentiation-associated gene 5 (MDA5)/mitochondrial antiviral signaling protein (MAVS) signaling is crucial for the late phase of the type I IFN response to LCMV. We further demonstrate that LCMV genomic RNA itself (without other LCMV components) is able to induce type I IFN responses in various cell types by activation of the RNA helicases retinoic acid-inducible gene I (RIG-I) and MDA5. We also show that expression of the LCMV nucleoprotein (NP) inhibits the type I IFN response induced by LCMV RNA and other RIG-I/MDA5 ligands. These virus-host interactions may play important roles in the pathogeneses of LCMV and other human arenavirus diseases.