CD4 T cell-intrinsic STING signaling controls the differentiation and effector functions of TH1 and TH9 cells

Background

While stimulator of interferon genes (STING) activation in innate immune cells of the tumor microenvironment can result in CD8 T cell-dependent antitumor immunity, whether STING signaling affects CD4 T-cell responses remains elusive.

Methods

Here, we tested whether STING activation modulated the effector functions of CD4 T cells in vivo by analyzing tumor-infiltrating CD4 T cells and evaluating the contribution of the CD4 T cell-derived cytokines in the antitumor activity of the STING ligand 2′3′-cyclic guanosine monophosphate-adenosine monophosphate (cGAMP) in two mouse tumor models. We performed ex vivo experiments to assess the impact of STING activation on CD4 T-cell differentiation and investigate the underlying molecular mechanisms. Finally, we tested whether STING activation enhances T_H9 cell antitumor activity against mouse melanoma upon adoptive transfer.

Results

We found that activation of STING signaling cell-intrinsically enhances the differentiation and antitumor functions of T_H1 and T_H9 cells by increasing their respective production of interferon gamma (IFN-γ) and interleukin-9. IRF3 and type I interferon receptors (IFNARs) are required for the STING-driven enhancement of T_H1 cell differentiation. However, STING activation favors T_H9 cell differentiation independently of the IFNARs/IRF3 pathway but through mammalian target of rapamycin (mTOR) signaling, underscoring that STING activation differentially affects the fate of distinct CD4 T-cell subsets. The therapeutic effect of STING activation relies on T_H1 and T_H9-derived cytokines, and STING activation enhances the antitumor activity of T_H9 cells upon adoptive transfer.

Conclusion

Our results reveal the STING signaling pathway as a therapeutic target to boost CD4 T-cell effector functions and antitumor immunity.

In Vivo Matrigel Plug Assay as a Potent Method to Investigate Specific Individual Contribution of Angiogenesis to Blood Flow Recovery in Mice

Neovascularization restores blood flow recovery after ischemia in peripheral arterial disease. The main two components of neovascularization are angiogenesis and arteriogenesis. Both of these processes contribute to functional improvements of blood flow after occlusion. However, discriminating between the specific contribution of each process is difficult. A frequently used model for investigating neovascularization is the murine hind limb ischemia model (HLI). With this model, it is difficult to determine the role of angiogenesis, because usually the timing for the sacrifice of the mice is chosen to be optimal for the analysis of arteriogenesis. More importantly, the occurring angiogenesis in the distal calf muscles is probably affected by the proximally occurring arteriogenesis. Therefore, to understand and subsequently intervene in the process of angiogenesis, a model is needed which investigates angiogenesis without the influence of arteriogenesis. In this study we evaluated the in vivo Matrigel plug assay in genetic deficient mice to investigate angiogenesis. Mice deficient for interferon regulatory factor (IRF)3, IRF7, RadioProtective 105 (RP105), Chemokine CC receptor CCR7, and p300/CBP-associated factor (PCAF) underwent the in vivo Matrigel model. Histological analysis of the Matrigel plugs showed an increased angiogenesis in mice deficient of IRF3, IRF7, and RP105, and a decreased angiogenesis in PCAF deficient mice. Our results also suggest an involvement of CCR7 in angiogenesis. Comparing our results with results of the HLI model found in the literature suggests that the in vivo Matrigel plug assay is superior in evaluating the angiogenic response after ischemia.

The STING-IRF3 pathway is involved in lipotoxic injury of pancreatic β cells in type 2 diabetes

Lipotoxic injury of pancreatic β cells is an important pathological feature in type 2 diabetes mellitus (T2DM). Stimulator of interferon genes (STING) can recognize its own DNA leaked into the cytoplasm from damaged mitochondria or nuclei of the host cell, thus activating its downstream factor interferon regulatory factor 3 (IRF3), causing inflammation and apoptosis. The STING-IRF3 signaling pathway is closely related to glycolipid metabolism, but its relationship with the lipotoxicity of pancreatic β cells has rarely been reported. Here, we investigated the role of the STING-IRF3 signaling pathway in lipotoxicity-induced inflammation, apoptosis, and dysfunction of pancreatic β cells. We examined the activation of STING and IRF3 in islets of db/db mice and identified the role of the STING-IRF3 signaling pathway in palmitic acid (PA)-induced lipotoxic injury of INS-1, a rat insulinoma cell line. STING and phosphorylated IRF3 including downstream interferon-β were upregulated in islets of db/db mice and PA-induced INS-1 cells. Gene silencing of STING or IRF3 ameliorated PA-induced INS-1 cell inflammation and apoptosis, and reversed impaired insulin synthesis. Additionally, PA induced downregulation of the phosphoinositide 3-kinase-AKT signaling pathway, and impaired high glucose-stimulated insulin secretion was reversed after knockdown of STING or IRF3. Our results suggest that activation of the STING-IRF3 pathway triggers inflammation and apoptosis of pancreatic β cells, leading to β-cell damage and dysfunction. Hence, inhibition of this signaling pathway may represent a novel approach for β-cell protection in T2DM.

Mitochondrial Protein PINK1 Positively Regulates RLR Signaling

The serine/threonine kinase phosphatase and tensin homolog (PTEN)-induced putative kinase 1(PINK1) controls mitochondrial quality and plays a vital role in the pathogenesis of early-onset Parkinson's disease. However, whether PINK1 has functions in innate antiviral immunity is largely unknown. Here, we report that viral infection down regulates PINK1 expression in macrophages. PINK1 knockdown results in decreased cytokine production and attenuated IRF3 and NF-κB activation upon viral infection. PINK1 promotes the retinoic-acid-inducible gene I (RIG-I)-like receptors (RLR)-triggered immune responses in a kinase domain-dependent manner. Furthermore, PINK1 associates with TRAF3 via the kinase domain and inhibits Parkin-mediated TRAF3 K48-linked proteasomal degradation. In addition, PINK1 interacts with Yes-associated protein 1 (YAP1) upon viral infection and impairs YAP1/IRF3 complex formation. Collectively, our results demonstrate that PINK1 positively regulates RIG-I triggered innate immune responses by inhibiting TRAF3 degradation and relieving YAP-mediated inhibition of the cellular antiviral response.

Activation of the STING-IRF3 pathway promotes hepatocyte inflammation, apoptosis and induces metabolic disorders in nonalcoholic fatty liver disease

BACKGROUND

Nonalcoholic fatty liver disease (NAFLD) is a common result of obesity and metabolic syndrome. Hepatocyte injury and metabolic disorders are hallmarks of NAFLD. Stimulator of interferon genes (STING) and its downstream factor interferon regulatory factor 3 (IRF3) trigger inflammatory reaction in response to the presence of cytosolic DNA. STING has recently been shown to play an important role in early alcoholic liver disease. However, little is known about the role of STING-IRF3 pathway in hepatocyte injury. Here, we aimed to examine the effect of STING-IRF3 pathway on hepatocyte metabolism, inflammation and apoptosis.

METHODS

We examined the activation of the STING-IRF3 pathway, a high-fat diet (HFD)-induced obese mouse model, and determined the role of this pathway in a free fatty acid (FFA)-induced hepatocyte inflammatory response, injury, and dysfunction in L-O2 human liver cells.

RESULTS

STING and IRF3 were upregulated in livers of HFD-fed mice and in FFA-induced L-O2 cells. Knocking down either STING or IRF3 led to a significant reduction in FFA-induced hepatic inflammation and apoptosis, as evidenced by modulation of the nuclear factor κB (NF-κB) signaling pathway, inflammatory cytokines, and apoptotic signaling. Additionally, STING/IRF3 knockdown enhanced glycogen storage and alleviated lipid accumulation, which were found to be associated with increased expression of hepatic enzymes in glycolysis and lipid catabolism, and attenuated expression of hepatic enzymes in gluconeogenesis and lipid synthesis.

CONCLUSIONS

Our results suggest that the STING-IRF3 pathway promotes hepatocyte injury and dysfunction by inducing inflammation and apoptosis and by disturbing glucose and lipid metabolism. This pathway may be a novel therapeutic target for preventing NAFLD development and progression.

Nuclear Transcription Factors in the Mitochondria: A New Paradigm in Fine-Tuning Mitochondrial Metabolism

Noncanonical functions of several nuclear transcription factors in the mitochondria have been gaining exceptional traction over the years. These transcription factors include nuclear hormone receptors like estrogen, glucocorticoid, and thyroid hormone receptors: p53, IRF3, STAT3, STAT5, CREB, NF-kB, and MEF-2D. Mitochondria-localized nuclear transcription factors regulate mitochondrial processes like apoptosis, respiration and mitochondrial transcription albeit being nuclear in origin and having nuclear functions. Hence, the cell permits these multi-stationed transcription factors to orchestrate and fine-tune cellular metabolism at various levels of operation. Despite their ubiquitous distribution in different subcompartments of mitochondria, their targeting mechanism is poorly understood. Here, we review the current status of mitochondria-localized transcription factors and discuss the possible targeting mechanism besides the functional interplay between these factors.

Interferon regulatory factor 3 is a negative regulator of pathological cardiac hypertrophy

Interferon regulatory factor (IRF) 3, a member of the highly conserved IRF family transcription factors, plays a pivotal role in innate immune response, apoptosis, and oncogenesis. Recent studies have implicated IRF3 in a wide range of host defense. However, whether IRF3 induces defensive responses to hypertrophic stresses such as biomechanical stress and neurohumoral factors remains unclear. Herein, we employed an IRF3-deficient mouse model, cardiac-specific IRF3-overexpression mouse model and isolated cardiomyocytes to investigate the role of IRF3 in cardiac hypertrophy induced by aortic banding (AB) or isoproterenol (ISO). The extent of cardiac hypertrophy was quantitated by echocardiography as well as by pathological and molecular analysis. Our results demonstrate that IRF3 deficiency profoundly exacerbated cardiac hypertrophy, whereas overexpression of IRF3 in the heart significantly blunted pathological cardiac remodeling induced by pressure overload. Similar results were also observed in cultured cardiomyocytes upon the treatment with ISO. Mechanistically, we discovered that IRF3 interacted with ERK2 and thereby inhibited the ERK1/2 signaling. Furthermore, inactivation of ERK1/2 by U0126 offset the IRF3-deficient-mediated hypertrophic response induced by aortic banding. Altogether, these data demonstrate that IRF3 plays a protective role in AB-induced hypertrophic response by inactivating ERK1/2 in the heart. Therefore, IRF3 could be a new target for the prevention and therapy of cardiac hypertrophy and failure.

IL-17 boosts proinflammatory outcome of antiviral response in human cells

Excessive inflammation during bacterial and viral infections is destructive to the host and involves elevated production of proinflammatory cytokines. It is especially deleterious in organs with space constraints such as lung and the CNS. Indeed, a number of viruses that infect lungs, such as avian influenza virus, SARS-associated coronavirus, and respiratory syncytial virus, elicit a very high level of proinflammatory cytokines; however, it is unclear what triggers their production. In this study, we show that IL-17 commonly produced during viral infection specifically augments a proinflammatory response by directly synergizing with antiviral signaling. Costimulation of primary human fibroblasts with IL-17 greatly enhanced respiratory syncytial virus-induced or synthetic dsRNA-based viral mimic polyinosinic:polycytidylic acid-induced expression of proinflammatory genes without affecting expression of IFN-β-stimulated or IFN-stimulated genes. Knockdown of expression of known mediators of the antiviral signaling pathway revealed that the IL-17-poly(I:C) synergy depends on the presence of the transcriptional factors RelA and IFN regulatory factor 3 and IκB kinases. Moreover, this synergy was blocked by an IκB kinase inhibitor, BAY 11-7082. These findings shed light on the molecular mechanisms behind IL-17-dependent immunopathology observed in viral infections.

MyD88 and STING signaling pathways are required for IRF3-mediated IFN-β induction in response to Brucella abortus infection

Type I interferons (IFNs) are cytokines that orchestrate diverse immune responses to viral and bacterial infections. Although typically considered to be most important molecules in response to viruses, type I IFNs are also induced by most, if not all, bacterial pathogens. In this study, we addressed the role of type I IFN signaling during Brucella abortus infection, a facultative intracellular bacterial pathogen that causes abortion in domestic animals and undulant fever in humans. Herein, we have shown that B. abortus induced IFN-β in macrophages and splenocytes. Further, IFN-β induction by Brucella was mediated by IRF3 signaling pathway and activates IFN-stimulated genes via STAT1 phosphorylation. In addition, IFN-β expression induced by Brucella is independent of TLRs and TRIF signaling but MyD88-dependent, a pathway not yet described for Gram-negative bacteria. Furthermore, we have identified Brucella DNA as the major bacterial component to induce IFN-β and our study revealed that this molecule operates through a mechanism dependent on RNA polymerase III to be sensed probably by an unknown receptor via the adaptor molecule STING. Finally, we have demonstrated that IFN-αβR KO mice are more resistant to infection suggesting that type I IFN signaling is detrimental to host control of Brucella. This resistance phenotype is accompanied by increased IFN-γ and NO production by IFN-αβR KO spleen cells and reduced apoptosis.

Induction of Noxa-mediated apoptosis by modified vaccinia virus Ankara depends on viral recognition by cytosolic helicases, leading to IRF-3/IFN-β-dependent induction of pro-apoptotic Noxa

Viral infection is a stimulus for apoptosis, and in order to sustain viral replication many viruses are known to carry genes encoding apoptosis inhibitors. F1L, encoded by the orthopoxvirus modified vaccinia virus Ankara (MVA) has a Bcl-2-like structure. An MVA mutant lacking F1L (MVAΔF1L) induces apoptosis, indicating that MVA infection activates and F1L functions to inhibit the apoptotic pathway. In this study we investigated the events leading to apoptosis upon infection by MVAΔF1L. Apoptosis largely proceeded through the pro-apoptotic Bcl-2 family protein Bak with some contribution from Bax. Of the family of pro-apoptotic BH3-only proteins, only the loss of Noxa provided substantial protection, while the loss of Bim had a minor effect. In mice, MVA preferentially infected macrophages and DCs in vivo. In both cell types wt MVA induced apoptosis albeit more weakly than MVAΔF1L. The loss of Noxa had a significant protective effect in macrophages, DC and primary lymphocytes, and the combined loss of Bim and Noxa provided strong protection. Noxa protein was induced during infection, and the induction of Noxa protein and apoptosis induction required transcription factor IRF3 and type I interferon signalling. We further observed that helicases RIG-I and MDA5 and their signalling adapter MAVS contribute to Noxa induction and apoptosis in response to MVA infection. RNA isolated from MVA-infected cells induced Noxa expression and apoptosis when transfected in the absence of viral infection. We thus here describe a pathway leading from the detection of viral RNA during MVA infection by the cytosolic helicase-pathway, to the up-regulation of Noxa and apoptosis via IRF3 and type I IFN signalling.

Viruses have come up with a diverse set of mechanisms to stop infected cells from committing suicide and hence secure their own propagation. In this study we use the DNA virus Modified Vaccinia virus Ankara, a highly attenuated version Vaccinia Virus, to study how cells detect viral infection and induce apoptosis. Modified Vaccinia virus Ankara is currently in clinical trials for its use in various vaccination protocols. By using a broad array of immortalized and primary cell types we observed that viral infection induced programmed cell death was controlled by proteins predominantly involved in detection of viral RNA, in particular proteins involved in the type 1 interferon response. The novelty of our findings lies on the observation that not only can RNA from DNA viruses be detected and activate the type 1 interferon response to infection, but that these responses can also directly modulate the levels of proteins regulating programmed cell death. Future treatments of infections by viral pathogens could exploit the synergistic ability of the type 1 interferon responses and programmed cell death in order to inhibit viral propagation.

The TRIF/TBK1/IRF-3 activation pathway is the primary inhibitory target of resveratrol, contributing to its broad-spectrum anti-inflammatory effects

Resveratrol, a stilbene type compound identified in wine and fruit juice, has been found to exhibit various pharmacological activities such as anti-oxidative, anti-cancerous, anti-inflammatory and anti-aging effects. Although numerous papers have explored the pharmacology of resveratrol in one particular cellular action, how this compound can have multiple effects simultaneously has not been fully addressed. In this study, therefore, we explored its broad-spectrum inhibitory mechanism using lipopolysaccharide (LPS)-mediated inflammatory responses and reporter gene assays involving overexpression of toll like receptor (TLR) adaptor molecules. Co-transfection of adaptor molecules such as (1) myeloid differentiation primary response gene 88 (MyD88), (2) Toll/4ll-1 Receptor-domain-containing adapter-inducing interferon-beta (TRIF), (3) TRIF-related adaptor molecule (TRAM), or (4) TANK-binding kinase (TBK) 1 strongly enhanced luciferase activity mediated by transcription factors including nuclear factor (NF)-KB, activator protein (AP)-1, and interferon regulatory factor (IRF)-3. Of the adaptor proteins, TRIF and TBK1 but not MyD88 and IKK enhanced luciferase activity mediated by these transcription factors. Resveratrol dose-dependently suppressed LPS-induced NO production in macrophages. It also blocked the increases in levels of mRNA for IFN-1, tumor necrosis factor (TNF)-alpha, and inducible nitric oxide synthase (iNOS) that were induced by LPS. Resveratrol diminished the translocation or activation of IRF-3 at 90min, c-Jun, a subunit of AP-1, and STAT-1 at 120 min, and p50, a subunit of NF-KB, at 60 and 90 min. Resveratrol strongly suppressed the up-regulation of luciferase activity induced by these adaptor molecules with IC50 values of 5 to 65 microM. In particular, higher inhibitory effects of resveratrol were when TRIF or TBK1 were overexpressed following cotransfection of luciferase constructs with IRF-3 binding sequences. Taken together, our data suggest that the suppression of TRIF and TBK1, which mediates transcriptional activation of NF-kappaB, AP-1, and IRF-3, contributes to resveratrol's broad-spectrum inhibitory activity, and that this compound can be further developed as a lead anti-inflammatory compound.

Mouse CD8alpha+ DCs and human BDCA3+ DCs are major producers of IFN-lambda in response to poly IC

Polyinosinic:polycytidylic acid (poly IC), a double-stranded RNA, is an effective adjuvant in vivo. IFN-λs (also termed IL-28/29) are potent immunomodulatory and antiviral cytokines. We demonstrate that poly IC injection in vivo induces large amounts of IFN-λ, which depended on hematopoietic cells and the presence of TLR3 (Toll-like receptor 3), IRF3 (IFN regulatory factor 3), IRF7, IFN-I receptor, Fms-related tyrosine kinase 3 ligand (FL), and IRF8 but not on MyD88 (myeloid differentiation factor 88), Rig-like helicases, or lymphocytes. Upon poly IC injection in vivo, the IFN-λ production by splenocytes segregated with cells phenotypically resembling CD8α(+) conventional dendritic cells (DCs [cDCs]). In vitro experiments revealed that CD8α(+) cDCs were the major producers of IFN-λ in response to poly IC, whereas both CD8α(+) cDCs and plasmacytoid DCs produced large amounts of IFN-λ in response to HSV-1 or parapoxvirus. The nature of the stimulus and the cytokine milieu determined whether CD8α(+) cDCs produced IFN-λ or IL-12p70. Human DCs expressing BDCA3 (CD141), which is considered to be the human counterpart of murine CD8α(+) DCs, also produced large amounts of IFN-λ upon poly IC stimulation. Thus, IFN-λ production in response to poly IC is a novel function of mouse CD8α(+) cDCs and their human equivalents.

Interferon regulatory factor 3-dependent pathways are critical for control of herpes simplex virus type 1 central nervous system infection

The initiation of the immune response at the cellular level relies on specific recognition molecules to rapidly signal viral infection via interferon (IFN) regulatory factor 3 (IRF-3)-dependent pathways. The absence of IRF-3 would be expected to render such pathways inoperative and thereby significantly affect viral infection. Unexpectedly, a previous study found no significant change in herpes simplex virus (HSV) pathogenesis in IRF-3(-/-) mice following intravenous HSV type 1 (HSV-1) challenge (K. Honda, H. Yanai, H. Negishi, M. Asagiri, M. Sato, T. Mizutani, N. Shimada, Y. Ohba, A. Takaoka, N. Yoshida, and T. Taniguchi, Nature 434:772-777, 2005). In contrast, the present study demonstrated that IRF-3(-/-) mice are significantly more susceptible to HSV infection via the corneal and intracranial routes. Following corneal infection with 2 x 10(6) PFU of HSV-1 strain McKrae, 50% of wild-type mice survived, compared to 10% of IRF-3-deficient mice. Significantly increased viral replication and inflammatory cytokine production were observed in brain tissues of IRF-3(-/-) mice compared to control mice, with a concomitant deficit in production of both IFN-beta and IFN-alpha. These data demonstrate a critical role for IRF-3 in control of central nervous system infection following HSV-1 challenge. Furthermore, this work underscores the necessity to evaluate multiple routes of infection and animal models in order to fully determine the role of host resistance factors in pathogenesis.

Pathogen specific, IRF3-dependent signaling and innate resistance to human kidney infection

The mucosal immune system identifies and fights invading pathogens, while allowing non-pathogenic organisms to persist. Mechanisms of pathogen/non-pathogen discrimination are poorly understood, as is the contribution of human genetic variation in disease susceptibility. We describe here a new, IRF3-dependent signaling pathway that is critical for distinguishing pathogens from normal flora at the mucosal barrier. Following uropathogenic E. coli infection, Irf3(-/-) mice showed a pathogen-specific increase in acute mortality, bacterial burden, abscess formation and renal damage compared to wild type mice. TLR4 signaling was initiated after ceramide release from glycosphingolipid receptors, through TRAM, CREB, Fos and Jun phosphorylation and p38 MAPK-dependent mechanisms, resulting in nuclear translocation of IRF3 and activation of IRF3/IFNβ-dependent antibacterial effector mechanisms. This TLR4/IRF3 pathway of pathogen discrimination was activated by ceramide and by P-fimbriated E. coli, which use ceramide-anchored glycosphingolipid receptors. Relevance of this pathway for human disease was supported by polymorphic IRF3 promoter sequences, differing between children with severe, symptomatic kidney infection and children who were asymptomatic bacterial carriers. IRF3 promoter activity was reduced by the disease-associated genotype, consistent with the pathology in Irf3(-/-) mice. Host susceptibility to common infections like UTI may thus be strongly influenced by single gene modifications affecting the innate immune response.

Phosphodiesterase 4B2 gene is an effector of Toll-like receptor signaling in astrocytes

Cyclic AMP is part of an endogenous mechanism that downregulates inflammatory response, and its intracellular concentration is regulated chiefly by cyclic nucleotide phosphodiesterases type 4. The goal of the present study was to determine whether phosphodiesterases 4 are involved in the inflammatory response of astrocytes mediated by Toll-like receptors. Astrocyte cultures established from newborn rat brain were challenged with lipoteichoic acid, a ligand of Toll-like receptor 2, polyinosinic-polycytidylic acid, a ligand of Toll-like receptor 3, or lipopolysaccharide, a ligand of Toll-like receptor 4. After 24 h the expression of genes encoding phosphodiesterase 4A, phosphodiesterase 4B and phosphodiesterase 4D was determined by real time reverse transcription polymerase chain reaction. The challenge of astrocytes with the ligands profoundly up-regulated expression of the phosphodiesterase 4B mRNA, while the phosphodiesterase 4A and 4D mRNA was either unaffected or downregulated. Moreover, Toll-like receptor ligation specifically up-regulated expression of the phosphodiesterase 4B2 transcriptional variant. Thus, polyinosinic-polycytidylic acid, lipopolysaccharide and lipoteichoic acid induced approximately 7-, 5- and 4-fold up-regulation of the message, respectively. Toll-like receptor ligation also led to an over 2-fold increase in the protein level of phosphodiesterase 4B2 as revealed by immunoblot analysis. The inactivation of Rho proteins by pretreatment with toxin B form C. difficile enhanced ligation-induced up-regulation of the phosphodiesterase 4B2 message by 4-9-fold. However, in spite of this increase in the message abundance, there was no increase in the protein level compared to cells challenged with the ligands alone. These results demonstrate that the phosphodiesterase 4B2 gene is an effector of Toll-like receptor signaling in astrocytes, and that its up-regulation at the protein level is controlled by complex mechanisms.

Induction of proinflammatory cytokines in primary human macrophages by influenza A virus (H5N1) is selectively regulated by IFN regulatory factor 3 and p38 MAPK

The hyperinduction of proinflammatory cytokines and chemokines such as TNF-alpha, IFN-beta, and CCL2/MCP-1 in primary human macrophages and respiratory epithelial cells by the highly pathogenic avian influenza H5N1 is believed to contribute to the unusual severity of human H5N1 disease. Here we show that TNF-alpha, IFN-beta, and IFN-lambda1 are the key mediators directly induced by the H5N1 virus in primary human macrophages. In comparison with human influenza (H1N1), the H5N1 virus more strongly activated IFN regulatory factor 3 (IRF3). IRF3 knockdown and p38 kinase inhibition separately and in combination led to a substantial reduction of IFN-beta, IFN-lambda1, and MCP-1 but only to a partial reduction of TNF-alpha. IRF3 translocation was independent of p38 kinase activity, indicating that IRF3 and p38 kinase are distinct pathways leading to cytokine production by H5N1 virus. We conclude that IRF3 and p38 kinase separately and predominantly contribute to H5N1-mediated induction of IFN-beta, IFN-lambda1, and MCP-1 but only partly control TNF-alpha induction. A more precise identification of the differences in the regulation of TNF-alpha and IFN-beta could provide novel targets for the design of therapeutic strategies for severe human H5N1 influenza and also for treating other causes of acute respiratory distress syndrome.

Epstein-Barr virus-encoded small RNA induces IL-10 through RIG-I-mediated IRF-3 signaling

Epstein-Barr virus-encoded small RNA (EBER) is nonpolyadenylated, noncoding RNA, forms stem-loop structure by intermolecular base-pairing giving rise to double-stranded RNA (dsRNA)-like molecule and exists abundantly in EBV-infected cells. EBER induces IL-10 and thus supports the growth of Burkitt's lymphoma (BL) cells. In this study, the mechanism of IL-10 induction by EBER was analysed in the context of dsRNA signaling pathway. Knockdown of retinoic acid-inducible gene I (RIG-I) by small interfering RNA (siRNA), and expression of dominant-negative RIG-I downregulated IL-10 induction in EBER(+) EBV-infected and EBER plasmid-transfected BL cells. Transfection of EBER-expressing plasmid or in vitro synthesized EBER induced IL-10 in RIG-I-expressing cell clones, and activation of IL-10 promoter by EBER was blocked by dominant-negative RIG-I. Blocking of nuclear factor (NF)-kappaB by dominant-negative IkappaB-alpha plasmid did not block IL-10 expression, whereas knockdown of IRF-3 by siRNA resulted in downregulation of IL-10 in EBER(+) BL cells. NF-kappaB is reported to function downstream of RIG-I signaling pathway and is involved in the induction of proinflammatory cytokines. Our results indicate that EBER induces an anti-inflammatory cytokine IL-10 through RIG-I-mediated IRF-3 but not NF-kappaB signaling. These findings suggest a new mechanism of dsRNA signaling pathway that triggers the expression of IL-10, which acts as an autocrine growth factor in BL cells.

Double-stranded RNA induces pancreatic beta-cell apoptosis by activation of the toll-like receptor 3 and interferon regulatory factor 3 pathways

OBJECTIVE

Viral infections contribute to the pathogenesis of type 1 diabetes. Viruses, or viral products such as double-stranded RNA (dsRNA), affect pancreatic beta-cell survival and trigger autoimmunity by unknown mechanisms. We presently investigated the mediators and downstream effectors of dsRNA-induced beta-cell death.

RESEARCH DESIGN AND METHODS

Primary rat beta-cells and islet cells from wild-type, toll-like receptor (TLR) 3, type I interferon receptor (IFNAR1), or interferon regulatory factor (IRF)-3 knockout mice were exposed to external dsRNA (external polyinosinic-polycytidylic acid [PICex]) or were transfected with dsRNA ([PICin]).

RESULTS

TLR3 signaling mediated PICex-induced nuclear factor-kappaB (NF-kappaB) and IRF-3 activation and beta-cell apoptosis. PICin activated NF-kappaB and IRF-3 in a TLR3-independent manner, induced eukaryotic initiation factor 2 alpha phosphorylation, and triggered a massive production of interferon (IFN)-beta. This contributed to beta-cell death, as islet cells from IFNAR1(-/-) or IRF-3(-/-) mice were protected against PICin-induced apoptosis.

CONCLUSIONS

PICex and PICin trigger beta-cell apoptosis via the TLR3 pathway or IRF-3 signaling, respectively. Execution of PICin-mediated apoptosis depends on autocrine effects of type I IFNs.

The full length hepatitis C virus polyprotein and interactions with the interferon-beta signalling pathways in vitro

BACKGROUND

Hepatitis C is a global health problem. The exact mechanisms by which hepatitis C virus (HCV) can evade the host immune system have become controversial. Whether HCV polyproteins modulate IFN signalling pathways or HCV proteins are responsible for such a property is the subject of interest. Therefore, an efficient baculovirus delivery system was developed to introduce the whole genome of HCV1B minus 3’untranslated region (UTR) (HCV1BΔ3’UTR) into hepatoma cells.

METHODS

The whole genome of HCV genotype 1b was developed into hepatoma cells. Also, two replicon constructs were used in this research: a recombinant baculovirus containing the culture adapted sub-genomic replicon (Fk5.1) derived from HCV genotype 1b, and a mutant form containing an inactivating mutation within the non-structural protein 5B (NS5B).

RESULTS

As expected, the baculovirus carrying the FK5.1 replicon induced the production of IFN-beta as judged by the use of an IFN-beta promoter luciferase reporter construct, whereas the GND baculovirus (a control polymerase knock-out replicon) and the full-length 3'UTR deletant failed to induce luciferase expression. The activation of both IFN regulatory factor 3 (IRF3) and nuclear factor kappaB (NFkappaB), two transcription factors induced by dsRNA signalling were examined. Both the wild type and GND-mutant replicon blocked the dsRNA-induced activation of IRF3 and NFkappaB.

CONCLUSION

Inhibition of the transcriptional response to IRF3 and NFkappaB seems to be one of the multiple mechanisms which HCV employs to escape the host immune defense. In contrast, the full length 3'UTR deletant had no significant effect on either transcription factor. These results may be attributed to the function of HCV subgenomic replicons when compared with full length 3'UTR deletant.

Role for MyD88-independent, TRIF pathway in lipid A/TLR4-induced endotoxin tolerance

Repeated exposure to low doses of endotoxin results in progressive hyporesponsiveness to subsequent endotoxin challenge, a phenomenon known as endotoxin tolerance. In spite of its clinical significance in sepsis and characterization of the TLR4 signaling pathway as the principal endotoxin detection mechanism, the molecular determinants that induce tolerance remain obscure. We investigated the role of the TRIF/IFN-beta pathway in TLR4-induced endotoxin tolerance. Lipid A-induced homotolerance was characterized by the down-regulation of MyD88-dependent proinflammatory cytokines TNF-alpha and CCL3, but up-regulation of TRIF-dependent cytokine IFN-beta. This correlated with a molecular phenotype of defective NF-kappaB activation but a functional TRIF-dependent STAT1 signaling. Tolerance-induced suppression of TNF-alpha and CCL3 expression was significantly relieved by TRIF and IFN regulatory factor 3 deficiency, suggesting the involvement of the TRIF pathway in tolerance. Alternatively, selective activation of TRIF by poly(I:C)-induced tolerance to lipid A. Furthermore, pretreatment with rIFN-beta also induced tolerance, whereas addition of IFN-beta-neutralizing Ab during the tolerization partially alleviated tolerance to lipid A but not TLR2-induced endotoxin homo- or heterotolerance. Furthermore, IFNAR1-/- murine embryonal fibroblast and bone-marrow derived macrophages failed to induce tolerance. Together, these observations constitute evidence for a role of the TRIF/IFN-beta pathway in the regulation of lipid A/TLR4-mediated endotoxin homotolerance.

Hepatitis C virus escape from the interferon regulatory factor 3 pathway by a passive and active evasion strategy

Hepatitis C virus (HCV) has been known to replicate with extremely varying efficiencies in different host cells, even within different populations of a single human hepatoma cell line, termed Huh-7. Several reports have implicated the retinoic-acid inducible gene I (RIG-I)/ interferon regulatory factor 3 (IRF-3) pathway of the innate antiviral response with differences in host cell permissiveness to HCV. To investigate the general impact of the IRF-3 response onto HCV replication in cell culture, we generated an ample array of stable Huh-7 cell lines with altered IRF-3 responsiveness. Neither blocking IRF-3 activation in various host cells by expression of dominant negative RIG-I or HCV NS3/4A protease nor reconstitution of RIG-I signaling in Huh7.5, a cell clone known to be defective in this pathway, had any impact on HCV replication. Only by overexpressing constitutively active RIG-I or the signaling adaptor Cardif (also known as interferon-beta promoter stimulator 1, mitochondrial anti-viral signaling protein, or virus-induced signaling adaptor), both leading to a stimulation of the IRF-3 pathway in the absence of inducers, was HCV replication significantly inhibited. We therefore assessed the extent of RIG-I- dependent IRF-3 activation by different species of RNA, including full-length HCV genomes and HCV RNA duplexes, and observed strong induction only in response to double-stranded RNAs.

CONCLUSION

Based on these findings, we propose a refined model of innate immune escape by HCV involving limited initial induction and stringent subsequent control of the IRF-3 response.

IFN regulatory factor family members differentially regulate the expression of type III IFN (IFN-lambda) genes

Virus replication induces the expression of antiviral type I (IFN-alphabeta) and type III (IFN-lambda1-3 or IL-28A/B and IL-29) IFN genes via TLR-dependent and -independent pathways. Although type III IFNs differ genetically from type I IFNs, their similar biological antiviral functions suggest that their expression is regulated in a similar fashion. Structural and functional characterization of the IFN-lambda1 and IFN-lambda3 gene promoters revealed them to be similar to IFN-beta and IFN-alpha genes, respectively. Both of these promoters had functional IFN-stimulated response element and NF-kappaB binding sites. The binding of IFN regulatory factors (IRF) to type III IFN promoter IFN-stimulated response element sites was the most important event regulating the expression of these genes. Ectopic expression of the components of TLR7 (MyD88 plus IRF1/IRF7), TLR3 (Toll/IL-1R domain-containing adapter-inducing factor), or retinoic acid-inducible gene I (RIG-I) signal transduction pathways induced the activation of IFN-lambda1 promoter, whereas the IFN-lambda3 promoter was efficiently activated only by overexpression of MyD88 and IRF7. The ectopic expression of Pin1, a recently identified suppressor for IRF3-dependent antiviral response, decreased the IFN promoter activation induced by any of these three signal transduction pathways, including the MyD88-dependent one. To conclude, the data suggest that the IFN-lambda1 gene is regulated by virus-activated IRF3 and IRF7, thus resembling that of the IFN-beta gene, whereas IFN-lambda2/3 gene expression is mainly controlled by IRF7, thus resembling those of IFN-alpha genes.

Bax-dependent mitochondrial membrane permeabilization enhances IRF3-mediated innate immune response during VSV infection

An effective type I interferon (IFN-alpha/beta) response is critical for the control of many viral infections. Using an oncolytic strain of vesicular stomatitis virus, we have examined the cross-talk between virus-induced apoptosis and initiation of innate immune response. The intrinsic apoptotic cascade, specifically the Bax-Bcl-2-Caspase-9 cascade, was revealed as the primary pathway of VSV-induced apoptosis. Cell death was significantly reduced in BaxBak(-/-) murine embryonic fibroblasts (MEFs) and in human A549 epithelial cells treated with siRNA against Bax. Although inhibition of apoptosis resulted in enhanced virus replication in the BaxBak(-/-) MEFs as compared to wild-type cells, induction of the IFN antiviral response and expression of cytokine genes were attenuated in virus-infected cells. Moreover, Bax but not Bak pro-apoptotic protein was required for IRF-3 phosphorylation and activation, further substantiating a role for the intrinsic mitochondrial pathway in the innate immune response. Therefore, virus-induced apoptosis through a Bax-dependent mitochondrial pathway appears to enhance the full development of the IRF-3 mediated IFN antiviral response.

Early innate immune responses to Sin Nombre hantavirus occur independently of IFN regulatory factor 3, characterized pattern recognition receptors, and viral entry

Sin Nombre virus (SNV) is a highly pathogenic New World virus and etiologic agent of hantavirus cardiopulmonary syndrome. We have previously shown that replication-defective virus particles are able to induce a strong IFN-stimulated gene (ISG) response in human primary cells. RNA viruses often stimulate the innate immune response by interactions between viral nucleic acids, acting as a pathogen-associated molecular pattern, and cellular pattern-recognition receptors (PRRs). Ligand binding to PRRs activates transcription factors which regulate the expression of antiviral genes, and in all systems examined thus far, IFN regulatory factor 3 (IRF3) has been described as an essential intermediate for induction of ISG expression. However, we now describe a model in which IRF3 is dispensable for the induction of ISG transcription in response to viral particles. IRF3-independent ISG transcription in human hepatoma cell lines is initiated early after exposure to SNV virus particles in an entry- and replication-independent fashion. Furthermore, using gene knockdown, we discovered that this activation is independent of the best-characterized RNA- and protein-sensing PRRs including the cytoplasmic caspase recruitment domain-containing RNA helicases and the TLRs. SNV particles engage a heretofore unrecognized PRR, likely located at the cell surface, and engage a novel IRF3-independent pathway that activates the innate immune response.

Retinoic acid-inducible gene-I and interferon-beta promoter stimulator-1 augment proapoptotic responses following mammalian reovirus infection via interferon regulatory factor-3

During viral infection, cells initiate antiviral responses to contain replication and inhibit virus spread. One protective mechanism involves activation of transcription factors interferon regulatory factor-3 (IRF-3) and NF-kappaB, resulting in secretion of the antiviral cytokine, interferon-beta. Another is induction of apoptosis, killing the host cell before virus disseminates. Mammalian reovirus induces both interferon-beta and apoptosis, raising the possibility that both pathways are initiated by a common cellular sensor. We show here that reovirus activates IRF-3 with kinetics that parallel the activation of NF-kappaB, a known mediator of reovirus-induced apoptosis. Activation of IRF-3 requires functional retinoic acid inducible gene-I and interferon-beta promoter stimulator-1, but these intracellular sensors are dispensable for activation of NF-kappaB. Interferon-beta promoter stimulator-1 and IRF-3 are required for efficient apoptosis following reovirus infection, suggesting a common mechanism of antiviral cytokine induction and activation of the cell death response.

Potential of Transfected Muscle Cells to Contribute to DNA Vaccine Immunogenicity

The mechanism(s) by which DNA vaccines trigger the activation of Ag-specific T cells is incompletely understood. A series of in vivo and in vitro experiments indicates plasmid transfection stimulates muscle cells to up-regulate expression of MHC class I and costimulatory molecules and to produce multiple cytokines and chemokines. Transfected muscle cells gain the ability to directly present Ag to CD8 T cells through an IFN-regulatory factor 3-dependent process. These findings suggest that transfected muscle cells at the site of DNA vaccination may contribute to the magnitude and/or duration of the immune response initiated by professional APCs.

Antiviral gene expression in rheumatoid arthritis: role of IKKepsilon and interferon regulatory factor 3

OBJECTIVE

The rheumatoid synovium displays characteristics of Toll-like receptor (TLR) activation and antiviral gene expression, including production of RANTES and interferon-beta (IFNbeta). The mechanism of this activation in rheumatoid synovial tissue is unknown. This study was designed to investigate the role of the IKK-related kinase IKKepsilon and IFN regulatory factor 3 (IRF-3) in the activation of antiviral genes in rheumatoid arthritis (RA).

METHODS

Kinase assay and immunostaining were performed on synovial tissue. Dominant-negative (DN) IKKepsilon adenoviral infection of human fibroblast-like synoviocytes (FLS) was followed by poly(I-C) stimulation and Western blotting. Quantitative polymerase chain reaction was performed on DN IKKepsilon-infected FLS and IKKepsilon(-/-) and IKKepsilon(+/+) mouse FLS.

RESULTS

Western blotting showed that IKKepsilon phosphorylation was significantly greater in RA synovium compared with osteoarthritis synovium. Kinase assay confirmed that IKKepsilon was activated in RA synovium, and immunostaining showed localization of pIKKepsilon to the intimal lining. Western blot analysis demonstrated that activation of IRF-3 was also increased in RA synovium. Poly(I-C), lipopolysaccharide, and tumor necrosis factor alpha (TNFalpha) activated phosphorylation of IKKepsilon and IRF-3 in FLS. DN IKKepsilon inhibited IRF-3 phosphorylation as well as RANTES and IFNbeta protein production in synoviocytes. Antiviral gene expression was also reduced in FLS from IKKepsilon(-/-) mice compared with IKKepsilon(+/+) mice.

CONCLUSION

Antiviral gene expression in RA, especially due to TLR ligands and TNFalpha, is dependent on IKKepsilon and IRF-3, and this pathway plays a key role in the production of type I IFNs and chemokines such as RANTES. These findings indicate that the IKKepsilon pathway may have potential as a therapeutic target in RA.

IFNbeta induction by influenza A virus is mediated by RIG-I which is regulated by the viral NS1 protein

Influenza A virus causes epidemics of respiratory diseases in humans leading to thousands of death annually. One of its major virulence factors, the non-structural protein 1 (NS1), exhibits interferon-antagonistic properties. While epithelial cells of the respiratory tract are the primary targets of influenza virus, the virus-sensing mechanisms in these cells eventually leading to IFNbeta production are incompletely understood. Here we show that infection of epithelial cells with NS1-deficient influenza A virus upregulated expression of two molecules that have been previously implicated in sensing of RNA viruses, the retinoic acid-inducible gene I (RIG-I) and the melanoma differentiation-associated gene 5 (MDA5). Gene silencing and overexpression experiments demonstrated that RIG-I, its adapter interferon-beta promoter stimulator 1 (IPS-1) and interferon-regulated factor 3 (IRF3) were involved in influenza A virus-mediated production of the antiviral IFNbeta. In addition, we showed that the NS1 protein is capable to inhibit the RIG-I-induced signalling, a mechanism which corresponded to the observation that only NS1-deficient but not the wild-type virus induced high-level production of IFNbeta. In conclusion, we demonstrated a critical involvement of RIG-I, IPS-1 and IRF3 in influenza A virus infection of epithelial cells.

Role of IRAK4 and IRF3 in the control of intracellular infection withChlamydia pneumoniae

TLR signal transduction involves a MyD88-mediated pathway, which leads to recruitment of the IL-1 receptor (IL-1R)-associated kinase 4 (IRAK4) and Toll/IL-1R translation initiation region domain-containing adaptor-inducing IFN-beta-mediated pathway, resulting in the activation of IFN regulatory factor (IRF)3. Both pathways can lead to expression of IFN-beta. TLR-dependent and -independent signals converge in the TNF receptor-associated factor 6 (TRAF6) adaptor, which mediates the activation of NF-kappaBeta. Infection of murine bone marrow-derived macrophages (BMM) with Chlamydia pneumoniae induces IFN-alpha/beta- and NF-kappaBeta-dependent expression of IFN-gamma, which in turn, will control bacterial growth. The role of IRAK4 and IRF3 in the regulation of IFN-alpha/beta expression and NF-kappaBeta activation was studied in C. pneumoniae-infected BMM. We found that levels of IFN-alpha, IFN-beta, and IFN-gamma mRNA were reduced in infected IRAK4(-/-) BMM compared with wild-type (WT) controls. BMM also showed an IRAK4-dependent growth control of C. pneumoniae. No increased IRF3 activation was detected in C. pneumoniae-infected BMM. Similar numbers of intracellular bacteria, IFN-alpha, and IFN-gamma mRNA titers were observed in C. pneumoniae-infected IRF3(-/-) BMM. On the contrary, IFN-beta(-/-) BMM showed lower IFN-alpha and IFN-gamma mRNA levels and higher bacterial titers compared with WT controls. C. pneumoniae infection-induced activation of NF-kappaBeta and expression of proinflammatory cytokines were shown to be TRAF6-dependent but did not require IRAK4 or IRF3. Thus, our data indicate that IRAK4, but not IRF3, controls C. pneumoniae-induced IFN-alpha and IFN-gamma secretion and bacterial growth. IRAK4 and IRF3 are redundant for infection-induced NF-kappaB activation, which is regulated by TRAF6.

Viral infections activate types I and III interferon genes through a common mechanism

Viral infections trigger innate immune responses, including the production of type I interferons (IFN-alpha and -beta) and other proinflammatory cytokines. Novel antiviral cytokines IFN-lambda1, IFN-lambda2, and IFN-lambda3 are classified as type III IFNs and have evolved independently of type I IFNs. Type III IFN genes are regulated at the level of transcription and induced by viral infection. Although the regulatory mechanism of type I IFNs is well elucidated, the expression mechanism of IFN-lambdas is not well understood. Here, we analyzed the mechanism by which IFN-lambda gene expression is induced by viral infections. Loss- and gain-of-function experiments revealed the involvement of RIG-I (retinoic acid-inducible gene I), IPS-1, TBK1, and interferon regulatory factor-3, key regulators of the virus-induced activation of type I IFN genes. Consistent with this, a search for the cis-regulatory element of the human ifnlambda1 revealed a cluster of interferon regulatory factor-binding sites and a NF-kappaB-binding site. Functional analysis demonstrated that all of these sites are essential for gene activation by the virus. These results strongly suggest that types I and III IFN genes are regulated by a common mechanism.

Role of interferon regulatory factor 3 in type I interferon responses in rotavirus-infected dendritic cells and fibroblasts

The main pathway for the induction of type I interferons (IFN) by viruses is through the recognition of viral RNA by cytosolic receptors and the subsequent activation of interferon regulatory factor 3 (IRF-3), which drives IFN-alpha/beta transcription. In addition to their role in inducing an antiviral state, type I IFN also play a role in modulating adaptive immune responses, in part via their effects on dendritic cells (DCs). Many viruses have evolved mechanisms to interfere with type I IFN induction, and one recently reported strategy for achieving this is by targeting IRF-3 for degradation, as shown for rotavirus nonstructural protein 1 (NSP1). It was therefore of interest to investigate whether rotavirus-exposed DCs would produce type I IFN and/or mature in response to the virus. Our results demonstrate that IRF-3 was rapidly degraded in rotavirus-infected mouse embryonic fibroblasts (MEFs) and type I IFN was not detected in these cultures. In contrast, rotavirus induced type I IFN production in myeloid DCs (mDCs), resulting in their activation. Type I IFN induction in response to rotavirus was reduced in mDCs from IRF-3(-/-) mice, indicating that IRF-3 was important for mediating the response. Exposure of mDCs to UV-treated rotavirus induced significantly higher type I IFN levels, suggesting that rotavirus-encoded functions also antagonized the response in DCs. However, in contrast to MEFs, this action was not sufficient to completely abrogate type I IFN induction, consistent with a role for DCs as sentinels for virus infection.

Regulation of CXCL-8 (interleukin-8) induction by double-stranded RNA signaling pathways during hepatitis C virus infection

Hepatitis C virus (HCV) infection induces the alpha-chemokine interleukin-8 (CXCL-8), which is regulated at the levels of transcription and mRNA stability. In the current study, CXCL-8 regulation by double-stranded (ds)RNA pathways was analyzed in the context of HCV infection. A constitutively active mutant of the retinoic acid-inducible gene I (RIG-I), RIG-N, activated CXCL-8 transcription. Promoter mutagenesis experiments indicated that NF-kappaB and interferon (IFN)-stimulated response element (ISRE) binding sites were required for the RIG-N induction of CXCL-8 transcription. IFN-beta promoter stimulator 1 (IPS-1) expression also activated CXCL-8 transcription, and mutations of the ISRE and NF-kappaB binding sites reduced and abrogated CXCL-8 transcription, respectively. In the presence of wild-type RIG-I, transfection of JFH-1 RNA or JFH-1 virus infection of Huh7.5.1 cells activated the CXCL-8 promoter. Expression of IFN regulatory factor 3 (IRF-3) stimulated transcription from both full-length and ISRE-driven CXCL-8 promoters. Chromatin immunoprecipitation assays demonstrated that IRF-3 and NF-kappaB bound directly to the CXCL-8 promoter in response to virus infection and dsRNA transfection. RIG-N stabilized CXCL-8 mRNA via the AU-rich element in the 3' untranslated region of CXCL-8 mRNA, leading to an increase in its half-life following tumor necrosis factor alpha induction. The data indicate that HCV infection triggers dsRNA signaling pathways that induce CXCL-8 via transcriptional activation and mRNA stabilization and define a regulatory link between innate antiviral and inflammatory cellular responses to virus infection.

Chlamydia muridarum infection elicits a beta interferon response in murine oviduct epithelial cells dependent on interferon regulatory factor 3 and TRIF

Chlamydia trachomatis is the most common sexually transmitted bacterial infection in the United States. Utilizing cloned murine oviduct epithelial cell lines, we previously identified Toll-like receptor 2 (TLR2) as the principal epithelial pattern recognition receptor (PRR) for infection-triggered release of the acute inflammatory cytokines interleukin-6 and granulocyte-macrophage colony-stimulating factor. The infected oviduct epithelial cell lines also secreted the immunomodulatory cytokine beta interferon (IFN-beta) in a largely MyD88-independent manner. Although TLR3 was the only IFN-beta production-capable TLR expressed by the oviduct cell lines, we were not able to determine whether TLR3 was responsible for IFN-beta production because the epithelial cells were unresponsive to the TLR3 ligand poly(I-C), and small interfering RNA (siRNA) techniques were ineffective at knocking down TLR3 expression. To further investigate the potential role of TLR3 in the infected epithelial cell secretion of IFN-beta, we examined the roles of its downstream signaling molecules TRIF and IFN regulatory factor 3 (IRF-3) using a dominant-negative TRIF molecule and siRNA specific for TRIF and IRF-3. Antagonism of either IRF-3 or TRIF signaling significantly decreased IFN-beta production. These data implicate TLR3, or an unknown PRR utilizing TRIF, as the source of IFN-beta production by Chlamydia-infected oviduct epithelial cells.

A role for IRF3-dependent RXRalpha repression in hepatotoxicity associated with viral infections

Viral infections and antiviral responses have been linked to several metabolic diseases, including Reye's syndrome, which is aspirin-induced hepatotoxicity in the context of a viral infection. We identify an interferon regulatory factor 3 (IRF3)-dependent but type I interferon-independent pathway that strongly inhibits the expression of retinoid X receptor alpha (RXRalpha) and suppresses the induction of its downstream target genes, including those involved in hepatic detoxification. Activation of IRF3 by viral infection in vivo greatly enhances bile acid- and aspirin-induced hepatotoxicity. Our results provide a critical link between the innate immune response and host metabolism, identifying IRF3-mediated down-regulation of RXRalpha as a molecular mechanism for pathogen-associated metabolic diseases.

Distinct Roles for IFN Regulatory Factor (IRF)-3 and IRF-7 in the Activation of Antitumor Properties of Human Macrophages

When properly activated, macrophages can be tumoricidal, thus making them attractive additions to standard cancer therapies. To this end, tolerance and activity of human autologous IFN-gamma-activated macrophages, produced in large scale for clinical use (MAK cells), have been assessed in pilot trials in cancer patients. In the present study, we tested the hypothesis that activation of IFN regulatory factor (IRF)-3 and IRF-7, with subsequent type I IFN production, may be involved in the acquisition of new antitumor functions by macrophages. Adenoviral vectors were generated for the delivery of constitutively active forms of IRF-3 (Ad-IRF-3) or IRF-7 (Ad-IRF-7) into primary human macrophages. Cell death was observed in Ad-IRF-3-transduced macrophages, whereas Ad-IRF-7-transduced macrophages produced type I IFNs and displayed increased expression of genes encoding tumor necrosis factor (TNF)-related apoptosis-inducing ligand, interleukin (IL)-12, IL-15, and CD80, persisting for at least 96 hours. Expression of iNOS, TNF-alpha, FasL, IL-1, and IL-6 genes was unaltered by Ad-IRF-7 transduction. Interestingly, Ad-IRF-3 or Ad-IRF-7 transduction negatively regulated the transcription of protumorigenic genes encoding vascular endothelial growth factor and matrix metalloproteinase-2. Furthermore, Ad-IRF-7-transduced macrophages exerted a cytostatic activity on different cancer cell lines, including SK-BR-3, MCF-7, and COLO-205; the latter cells were shown previously to be insensitive to MAK cells. In conclusion, transduction of active forms of IRF-3 or IRF-7 differentially modulate the apoptotic and antitumor properties of primary macrophages, with active IRF-7 leading to the acquisition of novel antitumor effector functions.

Kaposi sarcoma herpesvirus-encoded interferon regulator factors

The Kaposi sarcoma herpesvirus (KSHV) encodes multiple proteins that disrupt host antiviral responses, including four viral proteins that have homology to the interferon regulatory factor (IRF) family of transcription factors. At least three of the KSHV vIRFs (vIRFs 1-3) alter responses to cellular IRFs and to interferons (IFNs), whereas functional changes resulting from the fourth vIRF (vIRF-4) have not been reported. The vIRFs also affect other important regulatory proteins in the cell, including responses to transforming growth factor beta (TGF-beta) and the tumor suppressor protein p53. This review examines the expression of the vIRFs during the life cycle of KSHV and the functional consequences of their expression.

Negative regulation of intracellular hepatitis C virus replication by interferon regulatory factor 3

BACKGROUND

Interferon regulatory factor (IRF)-3 plays an important role in initiating cellular interferon-stimulated gene-mediated antiviral responses. In the present study, we evaluated the effects of IRF-3 expression and activation on intracellular hepatitis C virus (HCV) replication using an HCV replicon system.

METHODS

An HCV replicon was constructed that expressed a neomycin-selectable chimeric firefly luciferase reporter protein. A small interfering (si) RNA oligonucleotide directed against IRF-3 mRNA was designed and synthesized. A eukaryote expression plasmid vector was constructed that expressed IRF-3 mRNA under control of the cytomegalovirus early promoter/enhancer. To evaluate transcriptional activity of the interferon-stimulated genes, a reporter vector was used that expressed firefly luciferase under control of the interferon-stimulated response element (ISRE).

RESULTS

The baseline expression of IRF-3 did not significantly differ between cells with and without expression of the replicon. Transfection of an IRF-3 expression plasmid into the cells raised the ISRE-luciferase activities. The increase of ISRE activity was significantly more potent in the replicon-expressing cells than in cells without replicon expression. Concomitantly, the overexpression of IRF-3 suppressed HCV replication levels. In contrast, siRNA knockdown of IRF-3 suppressed ISRE activity by 38% +/- 2%. Interestingly, the suppression of IRF-3 resulted in a significant increase of HCV replication, by up to twofold, depending on the IRF-3 suppression levels.

CONCLUSIONS

IRF-3 negatively regulated intracellular HCV replication, and was partially activated in cells that expressed the HCV replicon. Thus, IRF-3 is a key molecule controlling HCV replication through modulation of host interferon gene responses.

SOCS1-negative feedback of STAT1 activation is a key pathway in the dsRNA-induced innate immune response of human keratinocytes

Toll-like receptor (TLR)3 is a receptor for virus-associated double-stranded RNA, and triggers antiviral immune responses during viral infection. Epidermal keratinocytes express TLR3 and provide an innate immune defense against viral infection. Since the intracellular regulatory mechanism is unknown, we hypothesized that the signal transducers and activators of transcription (STAT)-suppressors of cytokine signaling (SOCS) system regulates the innate immune response of keratinocytes. Treatment with polyinosinic-polycytidylic acid (poly(I:C)) resulted in the rapid translocation of IFN regulatory factor (IRF)-3 into the nucleus, followed by phosphorylation of STAT1 and STAT3. The activation of STATs by poly(I:C) probably occurs in an indirect fashion, through poly(I:C)-induced IFN. We infected cells with the dominant-negative forms of STAT1 (STAT1F), STAT3 (STAT3F), and SOCS1 using adenovirus vectors. Infection with STAT1F suppressed the induction of macrophage inflammatory protein (MIP)-1alpha by poly(I:C), whereas STAT3F had a minimal effect, which indicates that STAT1 mediates MIP-1alpha induction. SOCS1, which is a negative feedback regulator of STAT1 signaling, was induced by treatment with poly(I:C). SOCS1 infection inhibited the phosphorylation of STAT1 and significantly reduced poly(I:C)-induced MIP-1alpha production. Furthermore, STAT1-SOCS1 regulated poly(I:C)-induced TLR3 and IRF-7 expression. However, SOCS1 did not affect NF-kappaB signaling. Thus, the STAT1-SOCS1 pathway regulates the innate immune response via TLR3 signaling in epidermal keratinocytes.

Identification of proangiogenic genes and pathways by high-throughput functional genomics: TBK1 and the IRF3 pathway

A genome-wide phenotype screen was used to identify factors and pathways that induce proliferation of human umbilical vein endothelial cells (HUVEC). HUVEC proliferation is a recognized marker for factors that modulate vascularization. Screening "hits" included known proangiogenic factors, such as VEGF, FGF1, and FGF2 and additional factors for which a direct association with angiogenesis was not previously described. These include the kinase TBK1 as well as Toll-like receptor adaptor molecule and IFN regulatory factor 3. All three proteins belong to one signaling pathway that mediates induction of gene expression, including a mixture of secreted factors, which, in concert, mediate proliferative activity toward endothelial cells. TBK1 as the "trigger" of this pathway is induced under hypoxic conditions and expressed at significant levels in many solid tumors. This pattern of expression and the decreased expression of angiogenic factors in cultured cells upon RNA-interference-mediated ablation suggests that TBK1 is important for vascularization and subsequent tumor growth and a target for cancer therapy.

SIKE is an IKK epsilon/TBK1-associated suppressor of TLR3- and virus-triggered IRF-3 activation pathways

Viral infection or TLR3 engagement causes activation of the transcription factors IRF-3 and NF-kappaB, which collaborate to induce transcription of type I IFN genes. IKKepsilon and TBK1 are two IKK-related kinases critically involved in virus- and TLR3-triggered activation of IRF-3. We identified a protein termed SIKE (for Suppressor of IKKepsilon) that interacts with IKKepsilon and TBK1. SIKE is associated with TBK1 under physiological condition and dissociated from TBK1 upon viral infection or TLR3 stimulation. Overexpression of SIKE disrupted the interactions of IKKepsilon or TBK1 with TRIF, RIG-I and IRF-3, components in virus- and TLR3-triggered IRF-3 activation pathways, but did not disrupt the interactions of TRIF with TRAF6 and RIP, components in TLR3-triggered NF-kappaB activation pathway. Consistently, overexpression of SIKE inhibited virus- and TLR3-triggered interferon-stimulated response elements (ISRE) but not NF-kappaB activation. Knockdown of SIKE potentiated virus- and TLR3-triggered ISRE but not NF-kappaB activation. Moreover, overexpression of SIKE inhibited IKKepsilon- and TBK1-mediated antiviral response. These findings suggest that SIKE is a physiological suppressor of IKKepsilon and TBK1 and plays an inhibitory role in virus- and TLR3-triggered IRF-3 but not NF-kappaB activation pathways.

IKK-i signals through IRF3 and NFkappaB to mediate the production of inflammatory cytokines

IKK-i and TBK1 were recently identified as IKK-related kinases that are activated by toll-like receptors TLR3 and TLR4. These kinases were identified as essential components of the virus-activated as well as LPS-MyD88 independent kinase complex that phosphorylates IRF3 and results in the production of cytokines involved in innate immunity. Both IKK-i and TBK1 have also been implicated in the activation of the NFkappaB pathway but the precise mechanism is not clear. Although the literature to date suggests that IKK-i and TBK1 play redundant roles in TLR3 and TLR4 signaling, recent data suggest that there may be subtle differences in the signaling pathways affected by these kinases. We have generated tetracycline-inducible stable cell lines that express a wild type or kinase-inactive mutant form of IKK-i. Our data suggest that expression of IKK-i can activate both NFkappaB and IRF3, leading to the production of several cytokines including interferon beta. IKK-i most likely acts upstream of IKK2 to activate NFkappaB in these cells since expression of the kinase-inactive version of IKK-i did not inhibit TNFalpha mediated production of inflammatory cytokines. The data suggest that IKK-i is not involved in TNF-alpha mediated signaling but instead could likely play a role in activating IKK2 downstream of Toll-like receptor signaling. We also identified STAT1, Tyk2, and JAK1 as secondary mediators of IKK-i signaling as a result of interferon beta production in these cells.