Cytomegalovirus activates interferon immediate-early response gene expression and an interferon regulatory factor 3-containing interferon-stimulated response element-binding complex

Antiviral effects of duck type I and type III interferons against Duck Tembusu virus in vitro and in vivo

Duck Tembusu Virus (DTMUV) is a newly emerging avian flavivirus that causes substantial economic losses to the duck industry in Asia by causing severe egg drop syndrome and fatal encephalitis in domestic ducks. During viral replication, host cells recognize the RNA structures produced by DTMUV, which triggers the production of interferons (IFNs) to inhibit viral replication. However, the function of duck type I and type III IFNs in inhibiting DTMUV infection remains largely unknown. In this study, we expressed and purified recombinant duck IFN-β (duIFN-β) and IFN-λ (duIFN-λ) in Escherichia coli and evaluated their antiviral activity against vesicular stomatitis virus (VSV). Furthermore, we found that both duIFN-β and duIFN-λ activated the ISRE promoter and induced the expression of ZAP, OAS, and RNaseL in duck embryo fibroblasts (DEFs). Notably, duIFN-β showed faster and more potent induction of ISGs in vitro and in vivo compared to duIFN-λ. Moreover, both duIFN-β and duIFN-λ showed high potential to inhibit DTMUV infection in DEFs, with duIFN-β demonstrating better antiviral efficacy than duIFN-λ against DTMUV in ducks. In conclusion, our results revealed that both duIFN-β and duIFN-λ can induce ISGs production and exhibit significant antiviral activity against DTMUV in vitro and in vivo, providing new insights for the development of antiviral therapeutic strategies in ducks.

Regulation of STING activity in DNA sensing by ISG15 modification

Sensing of human immunodeficiency virus type 1 (HIV-1) DNA is mediated by the cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) signaling axis. Signal transduction and regulation of this cascade is achieved by post-translational modifications. Here we show that cGAS-STING-dependent HIV-1 sensing requires interferon-stimulated gene 15 (ISG15). ISG15 deficiency inhibits STING-dependent sensing of HIV-1 and STING agonist-induced antiviral response. Upon external stimuli, STING undergoes ISGylation at residues K224, K236, K289, K347, K338, and K370. Inhibition of STING ISGylation at K289 suppresses STING-mediated type Ⅰ interferon induction by inhibiting its oligomerization. Of note, removal of STING ISGylation alleviates gain-of-function phenotype in STING-associated vasculopathy with onset in infancy (SAVI). Molecular modeling suggests that ISGylation of K289 is an important regulator of oligomerization. Taken together, our data demonstrate that ISGylation at K289 is crucial for STING activation and represents an important regulatory step in DNA sensing of viruses and autoimmune responses.

Interferon-Independent Innate Responses to Cytomegalovirus

The critical role of interferons (IFNs) in mediating the innate immune response to cytomegalovirus (CMV) infection is well established. However, in recent years the functional importance of the IFN-independent antiviral response has become clearer. IFN-independent, IFN regulatory factor 3 (IRF3)-dependent interferon-stimulated gene (ISG) regulation in the context of CMV infection was first documented 20 years ago. Since then several IFN-independent, IRF3-dependent ISGs have been characterized and found to be among the most influential in the innate response to CMV. These include virus inhibitory protein, endoplasmic reticulum-associated IFN-inducible (viperin), ISG15, members of the interferon inducible protein with tetratricopeptide repeats (IFIT) family, interferon-inducible transmembrane (IFITM) proteins and myxovirus resistance proteins A and B (MxA, MxB). IRF3-independent, IFN-independent activation of canonically IFN-dependent signaling pathways has also been documented, such as IFN-independent biphasic activation of signal transducer and activator of transcription 1 (STAT1) during infection of monocytes, differential roles of mitochondrial and peroxisomal mitochondrial antiviral-signaling protein (MAVS), and the ability of human CMV (HCMV) immediate early protein 1 (IE1) protein to reroute IL-6 signaling and activation of STAT1 and its associated ISGs. This review examines the role of identified IFN-independent ISGs in the antiviral response to CMV and describes pathways of IFN-independent innate immune response induction by CMV.

Global landscape of mouse and human cytokine transcriptional regulation

Cytokines are cell-to-cell signaling proteins that play a central role in immune development, pathogen responses, and diseases. Cytokines are highly regulated at the transcriptional level by combinations of transcription factors (TFs) that recruit cofactors and the transcriptional machinery. Here, we mined through three decades of studies to generate a comprehensive database, CytReg, reporting 843 and 647 interactions between TFs and cytokine genes, in human and mouse respectively. By integrating CytReg with other functional datasets, we determined general principles governing the transcriptional regulation of cytokine genes. In particular, we show a correlation between TF connectivity and immune phenotype and disease, we discuss the balance between tissue-specific and pathogen-activated TFs regulating each cytokine gene, and cooperativity and plasticity in cytokine regulation. We also illustrate the use of our database as a blueprint to predict TF-disease associations and identify potential TF-cytokine regulatory axes in autoimmune diseases. Finally, we discuss research biases in cytokine regulation studies, and use CytReg to predict novel interactions based on co-expression and motif analyses which we further validated experimentally. Overall, this resource provides a framework for the rational design of future cytokine gene regulation studies.

Human Cytomegalovirus Utilizes a Nontraditional Signal Transducer and Activator of Transcription 1 Activation Cascade via Signaling through Epidermal Growth Factor Receptor and Integrins To Efficiently Promote the Motility, Differentiation, and Polarization of Infected Monocytes

Human cytomegalovirus (HCMV) infects peripheral blood monocytes and triggers biological changes that promote viral dissemination and persistence. We have shown that HCMV induces a proinflammatory state in infected monocytes, resulting in enhanced monocyte motility and transendothelial migration, prolonged monocyte survival, and differentiation toward a long-lived M1-like macrophage phenotype. Our data indicate that HCMV triggers these changes, in the absence of de novo viral gene expression and replication, through engagement and activation of epidermal growth factor receptor (EGFR) and integrins on the surface of monocytes. We previously identified that HCMV induces the upregulation of multiple proinflammatory gene ontologies, with the interferon-associated gene ontology exhibiting the highest percentage of upregulated genes. However, the function of the HCMV-induced interferon (IFN)-stimulated genes (ISGs) in infected monocytes remained unclear. We now show that HCMV induces the enhanced expression and activation of a key ISG transcriptional regulator, signal transducer and activator of transcription (STAT1), via an IFN-independent but EGFR- and integrin-dependent signaling pathway. Furthermore, we identified a biphasic activation of STAT1 that likely promotes two distinct phases of STAT1-mediated transcriptional activity. Moreover, our data show that STAT1 is required for efficient early HCMV-induced enhanced monocyte motility and later for HCMV-induced monocyte-to-macrophage differentiation and for the regulation of macrophage polarization, suggesting that STAT1 may serve as a molecular convergence point linking the biological changes that occur at early and later times postinfection. Taken together, our results suggest that HCMV reroutes the biphasic activation of a traditionally antiviral gene product through an EGFR- and integrin-dependent pathway in order to help promote the proviral activation and polarization of infected monocytes.IMPORTANCE HCMV promotes multiple functional changes in infected monocytes that are required for viral spread and persistence, including their enhanced motility and differentiation/polarization toward a proinflammatory M1 macrophage. We now show that HCMV utilizes the traditionally IFN-associated gene product, STAT1, to promote these changes. Our data suggest that HCMV utilizes EGFR- and integrin-dependent (but IFN-independent) signaling pathways to induce STAT1 activation, which may allow the virus to specifically dictate the biological activity of STAT1 during infection. Our data indicate that HCMV utilizes two phases of STAT1 activation, which we argue molecularly links the biological changes that occur following initial binding to those that continue to occur days to weeks following infection. Furthermore, our findings may highlight a unique mechanism for how HCMV avoids the antiviral response during infection by hijacking the function of a critical component of the IFN response pathway.

Bacterial Muramyl Dipeptide (MDP) Restricts Human Cytomegalovirus Replication via an IFN-β-Dependent Pathway

We recently reported that induction of NOD2 by human Cytomegalovirus (HCMV) resulted in virus inhibition and upregulation of antiviral and inflammatory cytokines. Here we investigated the effects of muramyl dipeptide (MDP), a bacterial cell wall component that activates NOD2, on HCMV replication and antiviral responses. HCMV infection of human foreskin fibroblasts induced NOD2, the downstream receptor-interacting serine/threonine-protein kinase 2 (RIPK2), resulting in phosphorylation of TANK-binding kinase 1 (TBK1) and interferon regulatory factor 3 (IRF3). MDP treatment following infection at low multiplicity (MOI = 0.1 PFU/cell) inhibited HCMV in a dose-dependent manner and further induced phosphorylation of TBK1, IRF3 and expression of IFN-β. None of these effects of MDP were observed following infection at multiplicity of 1. In infected NOD2 knocked-down cells MDP did not induce IFN-β, irrespective of MOI. Treatment with MDP before infection also inhibited HCMV, an effect augmented with treatment duration. Treatment with an IFN-β receptor blocking antibody or knockdown of IFN-β significantly attenuated the inhibitory effect of MDP on HCMV. MDP treatment before or after infection with herpesvirus 1 did not inhibit its replication. Summarized, NOD2 activation exerts anti-HCMV activities predominantly via IFN-β. Since MDP is a bacterial cell wall component, ongoing microbial exposure may influence HCMV replication.

Characterization of a Novel Human-Specific STING Agonist that Elicits Antiviral Activity Against Emerging Alphaviruses

Pharmacologic stimulation of innate immune processes represents an attractive strategy to achieve multiple therapeutic outcomes including inhibition of virus replication, boosting antitumor immunity, and enhancing vaccine immunogenicity. In light of this we sought to identify small molecules capable of activating the type I interferon (IFN) response by way of the transcription factor IFN regulatory factor 3 (IRF3). A high throughput in vitro screen yielded 4-(2-chloro-6-fluorobenzyl)-N-(furan-2-ylmethyl)-3-oxo-3,4-dihydro-2H-benzo[b][1,4]thiazine-6-carboxamide (referred to herein as G10), which was found to trigger IRF3/IFN-associated transcription in human fibroblasts. Further examination of the cellular response to this molecule revealed expression of multiple IRF3-dependent antiviral effector genes as well as type I and III IFN subtypes. This led to the establishment of a cellular state that prevented replication of emerging Alphavirus species including Chikungunya virus, Venezuelan Equine Encephalitis virus, and Sindbis virus. To define cellular proteins essential to elicitation of the antiviral activity by the compound we employed a reverse genetics approach that utilized genome editing via CRISPR/Cas9 technology. This allowed the identification of IRF3, the IRF3-activating adaptor molecule STING, and the IFN-associated transcription factor STAT1 as required for observed gene induction and antiviral effects. Biochemical analysis indicates that G10 does not bind to STING directly, however. Thus the compound may represent the first synthetic small molecule characterized as an indirect activator of human STING-dependent phenotypes. In vivo stimulation of STING-dependent activity by an unrelated small molecule in a mouse model of Chikungunya virus infection blocked viremia demonstrating that pharmacologic activation of this signaling pathway may represent a feasible strategy for combating emerging Alphaviruses.

pVHL Negatively Regulates Antiviral Signaling by Targeting MAVS for Proteasomal Degradation

The von Hippel-Lindau (VHL) gene is a well-defined tumor suppressor linked to human heredity cancer syndromes. As a component of the VHL-elongin B/C E3 ligase complex, pVHL performs its tumor function by targeting proteins for proteasomal degradation. It is largely unknown whether pVHL functions in antiviral immunity. In this article, we identify that pVHL negatively regulates innate antiviral immunity, which acts mainly by inducing degradation of mitochondrial antiviral-signaling protein (MAVS, also known as Cardif, IPS-1, or VISA). Overexpression of pVHL abrogated the cellular response to viral infection, whereas knockdown of pVHL exerted the opposite effect. pVHL targeted the K420 residue of MAVS to catalyze the formation of K48-linked polyubiquitin chains, leading to proteasomal degradation of MAVS. After viral infection, Mavs levels remained low in wild type zebrafish embryos but became much higher in vhl-deficient (vhl(-/-)) zebrafish embryos. Higher MAVS levels correlated with a greatly exaggerated antiviral response. In this work, we demonstrate that pVHL exhibits a previously unknown role in innate antiviral immunity.

Human three-dimensional engineered neural tissue reveals cellular and molecular events following cytomegalovirus infection

Human cytomegalovirus (HCMV) is the most common cause of congenital infection of the central nervous system (CNS). To overcome the limited access to human neural tissue and stringent species specificity of HCMV, we used engineered neural tissues to: (i) provide a technical advance to mimick features of HCMV infection in a human neural fetal tissue in vitro and (ii) characterize the molecular and cellular phenomenon following HCMV infection in this tissue. Herein, we infected hESC-derived engineered neural tissues (ENTs) whose organization resembles fetal brain. Transcriptome analysis of ENTs demonstrated that HCMV infection displayed features of the infection with the expression of genes involved in lipid metabolism, growth and development, as well as stress and host-response in a time-dependent manner. Immunohistochemical analysis demonstrated that HCMV did not firstly infect neural tubes (i.e. radially organized, proliferating stem cell niches), but rather an adjacent side population of post-mitotic cells expressing nestin, doublecortin, Sox1, musashi and vimentin markers. Importantly, we observe the same tropism in naturally HCMV-infected fetal brain specimens. To the best of our knowledge this system represents the first human brain-like tissue able to provide a more physiologically model for studying HCMV infection.

Abrogation of the interferon response promotes more efficient human cytomegalovirus replication

The effect of abrogating the interferon (IFN) response on human cytomegalovirus (HCMV) replication was investigated using primary human cells engineered to block either the production of or the response to type I IFNs. In IFN-deficient cells, HCMV produced larger plaques and spread and replicated more rapidly than in parental cells. These cells demonstrate the vital role of IFNs in controlling HCMV replication and provide useful tools to investigate the IFN response to HCMV.

The tiers and dimensions of evasion of the type I interferon response by human cytomegalovirus

Human cytomegalovirus (HCMV) is a member of the β-herpesvirus family that invariably occupies hosts for life despite a consistent multi-pronged antiviral immune response that targets the infection. This persistence is enabled by the large viral genome that encodes factors conferring a wide assortment of sophisticated, often redundant phenotypes that disable or otherwise manipulate impactful immune effector processes. The type I interferon system represents a first line of host defense against infecting viruses. The physiological reactions induced by secreted interferon act to effectively block replication of a broad spectrum of virus types, including HCMV. As such, the virus must exhibit counteractive mechanisms to these responses that involve their inhibition, tolerance, or re-purposing. The goal of this review is to describe the impact of the type I interferon system on HCMV replication and to showcase the number and diversity of strategies employed by the virus that allow infection of hosts in the presence of interferon-dependent activity.

HnRNP A1/A2 and SF2/ASF regulate alternative splicing of interferon regulatory factor-3 and affect immunomodulatory functions in human non-small cell lung cancer cells

Heterogeneous nuclear ribonucleoparticule A1/A2 (hnRNP A1/A2) and splicing factor 2/alternative splicing factor (SF2/ASF) are pivotal for precursor messenger RNA (pre-mRNA) splicing. Interferon regulatory factor-3 (IRF-3) plays critical roles in host defense against viral and microbial infection. Truncated IRF-3 proteins resulting from alternative splicing have been identified and characterized as functional antagonists to full-length IRF-3. In this study, we examined the molecular mechanism for splicing regulation of IRF-3 pre-mRNA and first reported the regulatory effect of hnRNP A1/A2 and SF2/ASF on IRF-3 splicing and activation. RNA interference-mediated depletion of hnRNP A1/A2 or SF2/ASF in human non-small cell lung cancer (NSCLC) cells increased exclusion of exons 2 and 3 of IRF-3 gene and reduced expression levels of IRF-3 protein and IRF-3 downstream effector molecules interferon-beta and CXCL10/IP-10. In addition, direct binding of hnRNP A1 and SF2/ASF to specific binding motifs in IRF-3 intron 1 was confirmed by RNA electrophoretic mobility shift assay. Subsequent minigene splicing assay showed that IRF-3 minigenes with mutated hnRNPA 1/A2 or SF2/ASF binding motifs increased exclusion of exons 2 and 3. Moreover, knockdown of hnRNP A1/A2 or SF2/ASF in NSCLC cells reinforced phytohemagglutinin-induced tumor necrosis factor-alpha release by peripheral blood mononuclear cells (PBMC) but suppressed that of interleukin-10 in NSCLC/PBMC co-cultures. Taken together, our results suggest that specific knockdown for hnRNP A1/A2 or SF2/ASF increase exclusion of exons 2 and 3 of IRF-3 pre-mRNA and influence immunomodulatory functions of human NSCLC cells.

DNA-PK is a DNA sensor for IRF-3-dependent innate immunity

Innate immunity is the first immunological defence against pathogens. During virus infection detection of nucleic acids is crucial for the inflammatory response. Here we identify DNA-dependent protein kinase (DNA-PK) as a DNA sensor that activates innate immunity. We show that DNA-PK acts as a pattern recognition receptor, binding cytoplasmic DNA and triggering the transcription of type I interferon (IFN), cytokine and chemokine genes in a manner dependent on IFN regulatory factor 3 (IRF-3), TANK-binding kinase 1 (TBK1) and stimulator of interferon genes (STING). Both cells and mice lacking DNA-PKcs show attenuated cytokine responses to both DNA and DNA viruses but not to RNA or RNA virus infection. DNA-PK has well-established functions in the DNA repair and V(D)J recombination, hence loss of DNA-PK leads to severe combined immunodeficiency (SCID). However, we now define a novel anti-microbial function for DNA-PK, a finding with implications for host defence, vaccine development and autoimmunity.

DOI:http://dx.doi.org/10.7554/eLife.00047.001

For multicellular organisms, the innate immune system is the first immunological defence against infection, rapidly recognizing and responding to the presence of any pathogen. Many different cell types contribute to the innate immunity, including fibroblasts, epithelial cells, dendritic cells and macrophages. Once alerted to injury or infection, these cells release proteins called cytokines, interferons and chemokines into the blood or directly into tissue. These proteins act as messengers and interact with receptors on the surfaces of other cells in the immune system, stimulating them to join the battle against the infection.

Detecting nucleic acids such as DNA is an important part of recognizing pathogens and infectious agents, particularly viruses, and activating the innate immune system. However, while the presence of DNA in the cytoplasm is known to initiate an innate immune response, we do not fully understand how this foreign DNA is sensed, or how the innate immune system is activated once foreign DNA has been detected.

Here Ferguson et al. report that a well-known complex of three proteins, collectively called DNA-dependent protein kinase, is able to activate an innate immune response when it detects foreign DNA. This enzyme, called DNA-PK for short, is best known for its ability to repair broken DNA inside the nucleus. Now Ferguson et al. have found that it is also present at high levels within fibroblasts, cells that are often primary targets of viral infection, and they go on to explain how the detection of DNA by DNA-PK triggers a sequence of events that leads to the innate immune response being activated. These events include the transcription of type I interferon, chemokines and cytokines in a manner that depends on the presence IRF-3, a transcription factor that has a central role in the response of the immune system to viral infection.

By identifying a role for DNA-PK in the cytoplasm as a DNA sensor, the work of Ferguson et al. increases our understanding of innate immunity. It may also, in the future, lead to an improved understanding of autoimmunity, and might also assist in the development of more immunogenic vaccines based on DNA or microbes that contain DNA.

DOI:http://dx.doi.org/10.7554/eLife.00047.002

Phenylmethimazole blocks dsRNA-induced IRF3 nuclear translocation and homodimerization

Previous studies revealed that phenylmethimazole (C10) inhibits IRF3 signaling, preventing dsRNA-induction of type 1 interferon gene expression, production, and downstream signaling. In the present study, we investigated the molecular basis for C10 inhibition of dsRNA-stimulated IRF3 signaling. IRF-3 Trans-AM assays were used to measure C10 effects on dsRNA induction of IRF3 DNA binding. Green fluorescent protein-labeled IRF3 was used to measure C10 effects on dsRNA-induced IRF3 nuclear translocation. Native PAGE, SDS PAGE, and western blotting were used to identify effects of C10 on IRF3 homodimer formation and phosphorylation, respectively. There was a significant impairment of dsRNA-induced IRF3 DNA binding activity in human embryonic kidney and pancreatic cancer cells with C10 treatment. C10 also blocked dsRNA-induced IRF3 nuclear translocation and homodimer formation without blocking serine 396 phosphorylation of IRF3. Together, these results indicate that C10 interferes with IRF3 signaling by blocking dsRNA-induced IRF3 homodimer formation, a prerequisite for nuclear translocation and DNA binding activities.

BST2/Tetherin enhances entry of human cytomegalovirus

Interferon-induced BST2/Tetherin prevents budding of vpu-deficient HIV-1 by tethering mature viral particles to the plasma membrane. BST2 also inhibits release of other enveloped viruses including Ebola virus and Kaposi's sarcoma associated herpesvirus (KSHV), indicating that BST2 is a broadly acting antiviral host protein. Unexpectedly however, recovery of human cytomegalovirus (HCMV) from supernatants of BST2-expressing human fibroblasts was increased rather than decreased. Furthermore, BST2 seemed to enhance viral entry into cells since more virion proteins were released into BST2-expressing cells and subsequent viral gene expression was elevated. A significant increase in viral entry was also observed upon induction of endogenous BST2 during differentiation of the pro-monocytic cell line THP-1. Moreover, treatment of primary human monocytes with siRNA to BST2 reduced HCMV infection, suggesting that BST2 facilitates entry of HCMV into cells expressing high levels of BST2 either constitutively or in response to exogenous stimuli. Since BST2 is present in HCMV particles we propose that HCMV entry is enhanced via a reverse-tethering mechanism with BST2 in the viral envelope interacting with BST2 in the target cell membrane. Our data suggest that HCMV not only counteracts the well-established function of BST2 as inhibitor of viral egress but also employs this anti-viral protein to gain entry into BST2-expressing hematopoietic cells, a process that might play a role in hematogenous dissemination of HCMV.

Identification of DNA-damage DNA-binding protein 1 as a conditional essential factor for cytomegalovirus replication in interferon-γ-stimulated cells

The mouse cytomegaloviral (MCMV) protein pM27 represents an indispensable factor for viral fitness in vivo selectively, antagonizing signal transducer and activator of transcription 2 (STAT2)-mediated interferon signal transduction. We wished to explore by which molecular mechanism pM27 accomplishes this effect. We demonstrate that pM27 is essential and sufficient to curtail the protein half-life of STAT2 molecules. Pharmacologic inhibition of the proteasome restored STAT2 amounts, leading to poly-ubiquitin-conjugated STAT2 forms. PM27 was found in complexes with an essential host ubiquitin ligase complex adaptor protein, DNA-damage DNA-binding protein (DDB) 1. Truncation mutants of pM27 showed a strict correlation between DDB1 interaction and their ability to degrade STAT2. SiRNA-mediated knock-down of DDB1 restored STAT2 in the presence of pM27 and strongly impaired viral replication in interferon conditioned cells, thus phenocopying the growth attenuation of M27-deficient virus. In a constructive process, pM27 recruits DDB1 to exploit ubiquitin ligase complexes catalyzing the obstruction of the STAT2-dependent antiviral state of cells to permit viral replication.

Interaction of the human cytomegalovirus particle with the host cell induces hypoxia-inducible factor 1 alpha

The cellular protein hypoxia-inducible factor 1 alpha (HIF-1α) was induced after infection of human fibroblasts with human cytomegalovirus (HCMV). HCMV irradiated with ultraviolet light (uv-HCMV) also elicited the effect, demonstrating that the response was provoked by interaction of the infecting virion with the cell and that viral gene expression was not required. Although induction of HIF-1α was initiated by an early event, accumulation of the protein was not detected until 9 hours post infection, with levels increasing thereafter. Infection with uv-HCMV resulted in increased abundance of HIF-1α-specific RNA, indicating stimulation of transcription. In addition, greater phosphorylation of the protein kinase Akt was observed, and the activity of this enzyme was required for induction of HIF-1α to occur. HIF-1α controls the expression of many cellular gene products; therefore the findings reveal new ways in which interaction of the HCMV particle with the host cell may cause significant alterations to cellular physiology.

Human cytomegalovirus IE1 protein elicits a type II interferon-like host cell response that depends on activated STAT1 but not interferon-γ

Human cytomegalovirus (hCMV) is a highly prevalent pathogen that, upon primary infection, establishes life-long persistence in all infected individuals. Acute hCMV infections cause a variety of diseases in humans with developmental or acquired immune deficits. In addition, persistent hCMV infection may contribute to various chronic disease conditions even in immunologically normal people. The pathogenesis of hCMV disease has been frequently linked to inflammatory host immune responses triggered by virus-infected cells. Moreover, hCMV infection activates numerous host genes many of which encode pro-inflammatory proteins. However, little is known about the relative contributions of individual viral gene products to these changes in cellular transcription. We systematically analyzed the effects of the hCMV 72-kDa immediate-early 1 (IE1) protein, a major transcriptional activator and antagonist of type I interferon (IFN) signaling, on the human transcriptome. Following expression under conditions closely mimicking the situation during productive infection, IE1 elicits a global type II IFN-like host cell response. This response is dominated by the selective up-regulation of immune stimulatory genes normally controlled by IFN-γ and includes the synthesis and secretion of pro-inflammatory chemokines. IE1-mediated induction of IFN-stimulated genes strictly depends on tyrosine-phosphorylated signal transducer and activator of transcription 1 (STAT1) and correlates with the nuclear accumulation and sequence-specific binding of STAT1 to IFN-γ-responsive promoters. However, neither synthesis nor secretion of IFN-γ or other IFNs seems to be required for the IE1-dependent effects on cellular gene expression. Our results demonstrate that a single hCMV protein can trigger a pro-inflammatory host transcriptional response via an unexpected STAT1-dependent but IFN-independent mechanism and identify IE1 as a candidate determinant of hCMV pathogenicity.

Identification of novel alternative splicing variants of interferon regulatory factor 3

Interferon regulatory factor 3 (IRF-3) plays a crucial role in host defense against viral and microbial infection as well as in cell growth regulation. IRF-3a is the only structurally and functionally characterized IRF-3 splicing variant and has been established to antagonize IRF-3 activity. Here, five novel splicing variants of IRF-3, referred to as IRF-3b, -3c, -3d, -3e, and -3f, were identified and shown to be generated by deletion of exons 2, 3, or 6 or some combination thereof. RT-PCR examination revealed that these novel splicing variants were more frequently expressed in human liver, esophagus, and cervical tumor tissues than in their normal counterparts. Additionally, electrophoretic mobility shift assay and subcellular localization showed only IRF-3 and IRF-3e were capable of binding the PRDI/III element of interferon-beta (IFNβ) promoter in vitro and underwent cytoplasm-to-nucleus translocation following Poly(I:C) stimulation. Coimmunoprecipitation assay revealed that only IRF-3c (3f) of novel splicing variants associated with IRF-3 in vivo. Further luciferase assay showed IRF-3c (3f) and IRF-3e failed to transactivate PRDI/III-containing promoter but appeared to inhibit transactivation potential of IRF-3 to varying degrees. Taken together, our findings suggest novel splicing variants may function as negative modulators of IRF-3 and may be correlated with pathogenesis of human tumors.

The small interferon-induced transmembrane genes and proteins

Interferon-induced transmembrane (IFITM) genes are transcribed in most tissues and are with the exception of IFITM5 interferon inducible. They are involved in early development, cell adhesion, and control of cell growth. Most IFITM genes are activated in response to bacterial and viral infections, and the exact host immune defense mechanisms are still unknown. Elevated gene expression triggered by past or chronic inflammation could prevent spreading of pathogens by limiting host cell proliferation. Accordingly, induction in cells with low basal protein levels is sufficient to drive growth arrest and a senescence-like morphology. On the other hand, loss of IFITM levels in cancer is correlated with pronounced malignancy; thus, these genes are considered as tumor suppressors. However, several cancer cells have deregulated high levels of IFITM transcripts, indicating a tumor progression stage where at least one of the interferon-controlled antiproliferative pathways has been silenced. Phylogenetic analyses of the protein coding genomic sequences suggest a single interferon-inducible gene in the common ancestor of rodents and primates. Biological functions studied so far may have evolved in parallel, and functional characterization of IFITM proteins will provide insight into innate immune defense, cancer development, and other pathways.

Influenza A virus strains that circulate in humans differ in the ability of their NS1 proteins to block the activation of IRF3 and interferon-β transcription

We demonstrate that influenza A virus strains that circulate in humans differ markedly in the ability of their NS1 proteins to block the activation of IRF3 and interferon-β transcription. Strong activation occurs in cells infected with viruses expressing NS1 proteins of seasonal H3N2 and H2N2 viruses, whereas activation is blocked in cells infected with viruses expressing NS1 proteins of some, but not all seasonal H1N1 viruses. The NS1 proteins of the 2009 H1N1 and H5N1 viruses also block these activations. The difference in this NS1 function is mediated largely by the C-terminal region of the effector domain, which contains the only amino acid (K or E at position 196) that covaries with the functional difference. Further, we show that TRIM25 binds the NS1 protein whether or not IRF3 activation is blocked, demonstrating that binding of TRIM25 by the NS1 protein does not necessarily lead to the blocking of IRF3 activation.

Differential relocation and stability of PML-body components during productive human cytomegalovirus infection: detailed characterization by live-cell imaging

In controlling the switch from latency to lytic infection, the immediate early (IE) genes lie at the core of herpesvirus pathogenesis. To image the 72kDa human cytomegalovirus (HCMV) major IE protein (IE1-72K), a recombinant virus encoding IE1 fused with EGFP was constructed. Using this construct, the IE1-EGFP fusion was detected at ND10 (PML-bodies) within 2h post infection (p.i.) and the complete disruption of ND10 imaged through to 6h p.i. HCMV genomes and IE2-86K protein could be detected adjacent to the slowly degrading IE1-72K/ND10 foci. IE1-72K associates with metaphase chromatin, recruiting both PML and STAT2. hDaxx, STAT1 and IE2-86K did not re-locate to metaphase chromatin; the fate of hDaxx is particularly important as this protein contributes to an intrinsic barrier to HCMV infection. While IE1-72K participates in a complex with chromatin, PML, STAT2 and Sp100, IE1-72K releases hDaxx from ND10 yet does not appear to remain associated with it.

Activation of the interferon response by human cytomegalovirus occurs via cytoplasmic double-stranded DNA but not glycoprotein B

In vitro infection of cells with the betaherpesvirus human cytomegalovirus (HCMV) stimulates an innate immune response characterized by phosphorylation of the transcription factor interferon regulatory factor 3 (IRF3) and subsequent expression of IRF3-dependent genes. While previous work suggests that HCMV envelope glycoprotein B is responsible for initiating this reaction, the signaling pathways stimulated by virus infection that lead to IRF3 phosphorylation have largely been uncharacterized. Recently, we identified Z DNA binding protein 1 (ZBP1), a sensor of cytoplasmic DNA, as an essential protein for this response. We now describe a human fibroblast cell line exhibiting a recessive defect that results in the absence of activation of IRF3 following treatment with HCMV but not Sendai virus or double-stranded RNA. In addition, we show that while exposure of these cells to soluble HCMV glycoprotein B is capable of triggering IRF3-dependent gene transcription, transfection of the cells with double-stranded DNA is not. Furthermore, we show that overexpression of ZBP1 in these cells reestablishes their ability to secrete interferon in response to HCMV and that multiple ZBP1 transcriptional variants exist in both wild-type and mutant cells. These results have two major implications for the understanding of innate immune stimulation by HCMV. First, they demonstrate that HCMV glycoprotein B is not the essential molecular pattern that induces an IRF3-dependent innate immune response. Second, IRF3-terminal signaling triggered by HCMV particles closely resembles that which is activated by cytoplasmic double-stranded DNA.

Anti-viral state segregates two molecular phenotypes of pancreatic adenocarcinoma: potential relevance for adenoviral gene therapy

Background

Pancreatic ductal adenocarcinoma (PDAC) remains a leading cause of cancer mortality for which novel gene therapy approaches relying on tumor-tropic adenoviruses are being tested.

Methods

We obtained the global transcriptional profiling of primary PDAC using RNA from eight xenografted primary PDAC, three primary PDAC bulk tissues, three chronic pancreatitis and three normal pancreatic tissues. The Affymetrix GeneChip HG-U133A was used. The results of the expression profiles were validated applying immunohistochemical and western blot analysis on a set of 34 primary PDAC and 10 established PDAC cell lines. Permissivity to viral vectors used for gene therapy, Adenovirus 5 and Adeno-Associated Viruses 5 and 6, was assessed on PDAC cell lines.

Results

The analysis of the expression profiles allowed the identification of two clearly distinguishable phenotypes according to the expression of interferon-stimulated genes. The two phenotypes could be readily recognized by immunohistochemical detection of the Myxovirus-resistance A protein, whose expression reflects the activation of interferon dependent pathways. The two molecular phenotypes discovered in primary carcinomas were also observed among established pancreatic adenocarcinoma cell lines, suggesting that these phenotypes are an intrinsic characteristic of cancer cells independent of their interaction with the host's microenvironment. The two pancreatic cancer phenotypes are characterized by different permissivity to viral vectors used for gene therapy, as cell lines expressing interferon stimulated genes resisted to Adenovirus 5 mediated lysis in vitro. Similar results were observed when cells were transduced with Adeno-Associated Viruses 5 and 6.

Conclusion

Our study identified two molecular phenotypes of pancreatic cancer, characterized by a differential expression of interferon-stimulated genes and easily recognized by the expression of the Myxovirus-resistance A protein. We suggest that the detection of these two phenotypes might help the selection of patients enrolled in virally-mediated gene therapy trials.

Human cytomegalovirus induces the interferon response via the DNA sensor ZBP1

Human cytomegalovirus (HCMV) is a member of the betaherpesvirus family that, unlike other herpesviruses, triggers a strong innate immune response in infected cells that includes transcription of the beta interferon gene via activation of interferon regulatory factor 3 (IRF3). IRF3 activation requires signaling from pattern recognition receptors that is initiated by their interaction with specific pathogen-associated molecules. However, while IRF3-activating pathways are increasingly well characterized, the cellular molecules involved in HCMV-mediated IRF3-dependent beta interferon transcription are virtually unknown. We undertook a systematic examination of new and established IRF3-terminal pathway components to identify those that are essential to HCMV-triggered IRF3 activation. We show here that IRF3 activation induced by HCMV infection involves the newly identified protein STING but, in contrast to infections with other herpesviruses, occurs independently of the adaptor molecule IPS-1. We also show that the protein DDX3 contributes to HCMV-triggered expression of beta interferon. Moreover, we identify Z-DNA binding protein 1 (ZBP1) as being essential for IRF3 activation and interferon beta expression triggered by HCMV, as well as being sufficient to enhance HCMV-stimulated beta interferon transcription and secretion. ZBP1 transcription was also found to be induced following exposure to HCMV in a JAK/STAT-dependent manner, thus perhaps also contributing to a positive feedback signal. Finally, we show that constitutive overexpression of ZBP1 inhibits HCMV replication. ZBP1 was recently identified as a cytosolic pattern recognition receptor of double-stranded DNA, and thus, we propose a model for HCMV-mediated IRF3 activation that involves HCMV-associated DNA as the principal innate immune-activating pathogen-associated molecular pattern.

Characterization of the human IRF-3 promoter and its regulation by the transcription factor E2F1

Interferon regulatory factor 3 (IRF-3), an essential transcriptional regulator of the interferon genes, has been implicated in virus and double-stranded RNA mediated induction of IFN-α, IFN-β and RANTES, in virus-induced apoptosis and in tumor suppression. Promoter plays an important role in the regulation of gene expression, but the characterization of the human IRF-3 promoter has not been systematically analyzed in HEK 293 cells. To characterize the human IRF-3 promoter, we have isolated a genomic clone of the human IRF-3 gene promoter region containing 1,000 nucleotides of the 5'- flanking region. Transient transfection of 5'-deleted promoter-reporter constructs and luciferase assay illustrated the region -149/-93 relative to the transcription start site (TSS) is sufficient for full promoter activity. This region contains HSF, E2F, CdxA and c-Myb transcription factor binding sites. The E2F sites are highly conserved among IRF-3 promoter regions of mouse, rat and human. Therefore, it was suggested that this E2F site may be essential for basal promoter activity. Surprisingly, mutation of this E2F site increased the promoter activity by 2-fold. Furthermore, overexpression of E2F1 reduced the transcription activity by 80%. These results indicated that human IRF-3 gene core promoter was located within the region -149/-93 relative to the TSS. E2F1 transcription factor negatively regulates human IRF-3 gene promoter.

Systemic treatment of xenografts with vaccinia virus GLV-1h68 reveals the immunologic facet of oncolytic therapy

BACKGROUND

GLV-1h68 is an attenuated recombinant vaccinia virus (VACV) that selectively colonizes established human xenografts inducing their complete regression.

RESULTS

Here, we explored xenograft/VACV/host interactions in vivo adopting organism-specific expression arrays and tumor cell/VACV in vitro comparing VACV replication patterns. There were no clear-cut differences in vitro among responding and non-responding tumors, however, tumor rejection was associated in vivo with activation of interferon-stimulated genes (ISGs) and innate immune host's effector functions (IEFs) correlating with VACV colonization of the xenografts. These signatures precisely reproduce those observed in humans during immune-mediated tissue-specific destruction (TSD) that causes tumor or allograft rejection, autoimmunity or clearance of pathogens. We recently defined these common pathways in the "immunologic constant of rejection" hypothesis (ICR).

CONCLUSION

This study provides the first prospective validation of a universal mechanism associated with TSD. Thus, xenograft infection by oncolytic VACV, beyond offering a promising therapy of established cancers, may represent a reliable pre-clinical model to test therapeutic strategies aimed at modulating the central pathways leading to TSD; this information may lead to the identification of principles that could refine the treatment of cancer and chronic infection by immune stimulation or autoimmunity and allograft rejection through immune tolerance.

Differential modification of interferon regulatory factor 3 following virus particle entry

Viral infection elicits the activation of numerous cellular signal transduction pathways, leading to the induction of both innate and adaptive immune responses in the host. In particular, interferon regulatory factor 3 (IRF3) has been shown to be essential for the induction of an antiviral response. Current models suggest that virus replication causes phosphorylation of C-terminal serine and threonine residues on IRF3, leading to its dimerization and translocation to the nucleus, where it activates interferon. Upon entry of replication-deficient Newcastle disease virus (NDV) particles, however, we failed to detect IRF3 dimerization or hyperphosphorylation, despite robust interferon-stimulated gene (ISG) and antiviral state induction and confirmation by small interfering RNA knockdown that IRF3 is essential for this response. To further compare the effects of various viruses and their replication status on IRF3 activation and to determine the minimal posttranslational modification required for IRF3 activation, two-dimensional gel electrophoresis and native polyacrylamide gel electrophoresis were employed. However, we failed to identify a minimal posttranslational modification of IRF3 that correlated with downstream biological activity, and the extent of posttranslational modification observed on IRF3 did not correlate with the degree of subsequent ISG induction. Thus, current techniques used to detect IRF3 activation are insufficient to infer its role in mediating downstream biological response induction and should be utilized with caution.

Feeling manipulated: cytomegalovirus immune manipulation

No one likes to feel like they have been manipulated, but in the case of cytomegalovirus (CMV) immune manipulation, we do not really have much choice. Whether you call it CMV immune modulation, manipulation, or evasion, the bottom line is that CMV alters the immune response in such a way to allow the establishment of latency with lifelong shedding. With millions of years of coevolution within their hosts, CMVs, like other herpesviruses, encode numerous proteins that can broadly influence the magnitude and quality of both innate and adaptive immune responses. These viral proteins include both homologues of host proteins, such as MHC class I or chemokine homologues, and proteins with little similarity to any other known proteins, such as the chemokine binding protein. Although a strong immune response is launched against CMV, these virally encoded proteins can interfere with the host's ability to efficiently recognize and clear virus, while others induce or alter specific immune responses to benefit viral replication or spread within the host. Modulation of host immunity allows survival of both the virus and the host. One way of describing it would be a kind of "mutually assured survival" (as opposed to MAD, Mutually Assured Destruction). Evaluation of this relationship provides important insights into the life cycle of CMV as well as a greater understanding of the complexity of the immune response to pathogens in general.

The adaptor protein MITA links virus-sensing receptors to IRF3 transcription factor activation

Viral infection triggers activation of transcription factors such as NF-kappaB and IRF3, which collaborate to induce type I interferons (IFNs) and elicit innate antiviral response. Here, we identified MITA as a critical mediator of virus-triggered type I IFN signaling by expression cloning. Overexpression of MITA activated IRF3, whereas knockdown of MITA inhibited virus-triggered activation of IRF3, expression of type I IFNs, and cellular antiviral response. MITA was found to localize to the outer membrane of mitochondria and to be associated with VISA, a mitochondrial protein that acts as an adaptor in virus-triggered signaling. MITA also interacted with IRF3 and recruited the kinase TBK1 to the VISA-associated complex. MITA was phosphorylated by TBK1, which is required for MITA-mediated activation of IRF3. Our results suggest that MITA is a critical mediator of virus-triggered IRF3 activation and IFN expression and further demonstrate the importance of certain mitochondrial proteins in innate antiviral immunity.

Crimean-Congo hemorrhagic fever virus delays activation of the innate immune response

As a first line of defence against virus infection, mammalian cells elicit an innate immune response, characterized by secretion of type I interferons and the up-regulation of interferon stimulated genes. Many viruses down-regulate the innate immune responses in order to enhance their virulence. Crimean-Congo hemorrhagic fever virus (CCHFV), a Nairovirus of the family Bunyaviridae is the causative agent of severe hemorrhagic fever in humans with high mortality. Knowledge regarding the innate immune response against CCHFV is most limited. Interestingly, in this study it is shown that replicating CCHFV delays substantially the IFN response, possibly by interfering with the activation pathway of IRF-3. In addition, it is demonstrated that CCHFV replication is almost insensitive to subsequent treatment with interferon-alpha. Once the virus is replicating, virus replication is more or less insensitive to the antiviral effects induced by the interferon. By using an interferon bioassay, it is shown that infected cells secrete interferon relatively late after infection, that is, 48 hr post-infection. In summary, the results suggest the presence of a virulence factor encoded by CCHFV that delays the host defence in order to allow rapid viral spread in the host.

Virus entry and innate immune activation

Human cytomegalovirus (HCMV) exhibits an exceptionally broad cellular tropism as it is capable of infecting most major organ systems and cell types. Definitive proof of an essential role for a cellular molecule that serves as an entry receptor has proven very challenging. It is widely hypothesized that receptor utilization, envelope glycoprotein requirements and entry pathways may all vary according to cell type, which is partially supported by the data. What has clearly emerged in recent years is that virus entry is not going undetected by the host. Robust and rapid induction of innate immune response is intimately associated with entry-related events. Here we review the state of knowledge on HCMV cellular entry mediators confronting the scientific challenges by accruing a definitive data set. We also review the roles of pattern recognition receptors such as Toll-like receptors in activation of specific innate immune response and discuss how entry events are tightly coordinated with innate immune initiation steps.

Human cytomegalovirus regulates bioactive sphingolipids

Sphingolipids are present in membranes of all eukaryotic cells. Bioactive sphingolipids also function as signaling molecules that regulate cellular processes such as proliferation, migration, and apoptosis. Human cytomegalovirus (HCMV) exploits a variety of cellular signaling pathways to promote its own replication. However, whether HCMV modulates lipid signaling pathways is an essentially unexplored area of research in virus-host cell interactions. In this study, we examined the accumulation of the bioactive sphingolipids and the enzymes responsible for the biosynthesis and degradation of these lipids. HCMV infection results in increased accumulation and activity of sphingosine kinase (SphK), the enzyme that generates sphingosine 1-phosphate (S1P) and dihydrosphingosine 1-phosphate (dhS1P). We also utilized a mass spectrometry approach to generate a sphingolipidomic profile of HCMV-infected cells. We show that HCMV infection results in increased levels of dhS1P and ceramide at 24 h, suggesting an enhancement of de novo sphingolipid synthesis. Subsequently dihydrosphingosine and dhS1P decrease at 48 h consistent with attenuation of de novo sphingolipid synthesis. Finally, we present evidence that de novo sphingolipid synthesis and sphingosine kinase activity directly impact virus gene expression and virus growth. Together, these findings demonstrate that host cell sphingolipids are dynamically regulated upon infection with a herpes virus in a manner that impacts virus replication.

Differential initiation of innate immune responses induced by human cytomegalovirus entry into fibroblast cells

Infection of permissive fibroblasts with human CMV (HCMV, AD169) is accompanied by a robust activation of innate immune defense. In this study, we show that inflammatory cytokine (IC) secretion and activation of the type I IFN pathway (alphabeta IFN) are initiated through distinct mechanisms. HCMV is recognized by TLR2 leading to the NF-kappaB activation and IC secretion. However, the IFN response to HCMV is not a TLR2-dependent process, as a dominant negative TLR2 does not affect the antiviral response to infection. Additionally, bafilomycin, an endosomal acidification inhibitor, has no effect on HCMV-induced IFN responses suggesting that IFN signaling is independent of endosomal resident TLRs. By contrast, disruption of lipid rafts by depletion of cellular cholesterol inhibits both HCMV entry as well as IFN responses. Cholesterol depletion had no effect on the induction of ICs by HCMV, illustrating a biological distinction at the cellular level with the initiation of innate immune pathways. Furthermore, HCMV entry inhibitors block IFN responses but not IC signaling. In particular, blocking the interaction of HCMV with beta(1) integrin diminished IFN signaling, suggesting that this virus-cell interaction or subsequent downstream steps in the entry pathway are critical for downstream signal transduction events. These data show that HCMV entry and IFN signaling are coordinated processes that require cholesterol-rich microdomains, whereas IC signaling is activated through outright sensing via TLR2. These findings further highlight the complexity and sophistication of innate immune responses at the earliest points in HCMV infection.

A logic-based diagram of signalling pathways central to macrophage activation

Background

The complex yet flexible cellular response to pathogens is orchestrated by the interaction of multiple signalling and metabolic pathways. The molecular regulation of this response has been studied in great detail but comprehensive and unambiguous diagrams describing these events are generally unavailable. Four key signalling cascades triggered early-on in the innate immune response are the toll-like receptor, interferon, NF-κB and apoptotic pathways, which co-operate to defend cells against a given pathogen. However, these pathways are commonly viewed as separate entities rather than an integrated network of molecular interactions.

Results

Here we describe the construction of a logically represented pathway diagram which attempts to integrate these four pathways central to innate immunity using a modified version of the Edinburgh Pathway Notation. The pathway map is available in a number of electronic formats and editing is supported by yEd graph editor software.

Conclusion

The map presents a powerful visual aid for interpreting the available pathway interaction knowledge and underscores the valuable contribution well constructed pathway diagrams make to communicating large amounts of molecular interaction data. Furthermore, we discuss issues with the limitations and scalability of pathways presented in this fashion, explore options for automated layout of large pathway networks and demonstrate how such maps can aid the interpretation of functional studies.

Interferons and viruses: an interplay between induction, signalling, antiviral responses and virus countermeasures

The interferon (IFN) system is an extremely powerful antiviral response that is capable of controlling most, if not all, virus infections in the absence of adaptive immunity. However, viruses can still replicate and cause disease in vivo, because they have some strategy for at least partially circumventing the IFN response. We reviewed this topic in 2000 [Goodbourn, S., Didcock, L. & Randall, R. E. (2000). J Gen Virol 81, 2341-2364] but, since then, a great deal has been discovered about the molecular mechanisms of the IFN response and how different viruses circumvent it. This information is of fundamental interest, but may also have practical application in the design and manufacture of attenuated virus vaccines and the development of novel antiviral drugs. In the first part of this review, we describe how viruses activate the IFN system, how IFNs induce transcription of their target genes and the mechanism of action of IFN-induced proteins with antiviral action. In the second part, we describe how viruses circumvent the IFN response. Here, we reflect upon possible consequences for both the virus and host of the different strategies that viruses have evolved and discuss whether certain viruses have exploited the IFN response to modulate their life cycle (e.g. to establish and maintain persistent/latent infections), whether perturbation of the IFN response by persistent infections can lead to chronic disease, and the importance of the IFN system as a species barrier to virus infections. Lastly, we briefly describe applied aspects that arise from an increase in our knowledge in this area, including vaccine design and manufacture, the development of novel antiviral drugs and the use of IFN-sensitive oncolytic viruses in the treatment of cancer.

Induction and evasion of the type I interferon response by cytomegaloviruses

Cytomegaloviruses represent supreme pathogens in that they are capable of occupying healthy mammalian hosts for life in the face of constant antiviral immune reactions. The inability of the host to eliminate the virus likely results from numerous counteractive strategies employed to disrupt the immune response. The role of type I interferon in the antiviral response has been well documented although only recently have the pathways of induction of this powerful cytokine been described. Cytomegaloviruses have been shown to both induce and be sensitive to the effects of type I interferon. Yet these viruses also possess numerous and varied phenotypes capable of inhibiting not only interferon induction but also interferon signaling and interferon-induced antiviral processes. The balance between induction and evasion of type I interferon responses by cytomegaloviruses is discussed in this review.

IL-27 Synthesis Induced by TLR Ligation Critically Depends on IFN Regulatory Factor 3

IL-27 is a heterodimeric cytokine composed of EBV-induced gene 3 and p28. Produced by dendritic cells (DCs) in response to TLR ligands, IL-27 recently emerged as a key regulator of inflammatory responses. In this study, we first demonstrate that Toll/IL-1R-containing adaptor inducing IFN-beta and its associated IFN regulatory factor (IRF) 3 transcription factor are critically involved in IL-27p28 expression in mouse DCs stimulated by TLR ligands. We then show that IL-27 serum levels are dramatically reduced in IRF3(-/-) upon LPS injection, indicating a critical role for IRF3 in TLR4-mediated IL-27 production in vivo. We identified an IRF3-binding site within the IL-27p28 promoter region which is required for IL-27p28 gene activation in reporter gene assays. In human DCs, IL-27p28 mRNA was preferentially induced by Toll/IL-1R-containing adaptor inducing IFN-beta-coupled TLR ligands and following CMV infection. Furthermore, chromatin immunoprecipitation studies demonstrate that IRF3 is recruited to the endogenous p28 promoter in TLR4-stimulated human DCs. We conclude that IRF3 activation is a master switch for IL-27 synthesis.

Human cytomegalovirus IE86 attenuates virus- and tumor necrosis factor alpha-induced NFkappaB-dependent gene expression

Human cytomegalovirus (HCMV) infection regulates a number of genes involved in the host antiviral response. We have previously reported that HCMV attenuates the expression of beta interferon (IFN-beta) and a number of proinflammatory chemokines, and this attenuation is mediated by the HCMV immediate-early protein IE86. The present study seeks to identify the mechanism by which IE86 blocks IFN-beta expression. We demonstrate that the induction of IFN-beta during HCMV infection requires the activation of both the IRF-3 and the NFkappaB pathways. Therefore, IE86 may target either pathway to block IFN-beta expression. Our results show that IE86 does not block IRF-3 phosphorylation, dimerization, nuclear translocation, or target gene expression. However, using gel shift analysis, we demonstrate that IE86 efficiently inhibits virus-induced binding of NFkappaB to the IFN-beta promoter, resulting in attenuation of IFN-beta and NFkappaB-dependent gene expression. Furthermore, IE86 expression inhibits tumor necrosis factor alpha-induced NFkappaB DNA binding and target gene expression. Together, these results identify IE86 as a NFkappaB antagonist, which results in the suppression of NFkappaB-dependent cytokine and chemokine gene expression.

The IFN-independent response to virus particle entry provides a first line of antiviral defense that is independent of TLRs and retinoic acid-inducible gene I

The innate immune system responds to pathogen infection by eliciting a nonspecific immune response following the recognition of various pathogen-associated molecular patterns. TLRs and the RNA helicases retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated gene 5 recognize foreign nucleic acid within endosomal and cytoplasmic compartments, respectively, initiating a signaling cascade that involves the induction of type I IFN through the transcription factors IFN regulatory factor (IRF) 3 and NF-kappaB. However, a recent paradigm has emerged in which bacterial DNA and double-stranded B-form DNA trigger type I IFN production through an uncharacterized TLR- and RIG-I-independent pathway. We have previously described a response in primary fibroblasts wherein the entry of diverse RNA- and DNA-enveloped virus particles is sufficient to induce a subset of IFN-stimulated genes and a complete antiviral response in an IRF3-dependent, IFN-independent manner. In this study, we show that the innate immune response to virus particle entry is independent of both TLR and RIG-I pathways, confirming the existence of novel innate immune mechanisms that result in the activation of IRF3. Furthermore, we propose a model of innate antiviral immunity in which exposure to increasing numbers of virus particles elevates the complexity of the cellular response from an intracellular, IFN-independent response to one involving secretion of cytokines and activation of infiltrating immune cells.

Cytomegalovirus induces interferon-stimulated gene expression and is attenuated by interferon in the developing brain

Cytomegalovirus (CMV) is considered the most common infectious agent causing permanent neurological dysfunction in the developing brain. We have previously shown that CMV infects developing brain cells more easily than it infects mature brain cells and that this preference is independent of the host B- and T-cell responses. In the present study, we examined the innate antiviral defenses against mouse (m) and human (h) CMVs in developing and mature brain and brain cells. mCMV infection induced interferon (IFN)-stimulated gene expression by 10- to 100-fold in both glia- and neuron-enriched cultures. Treatment of primary brain cultures with IFN-alpha, -beta, and -gamma or a synthetic RNA, poly(I:C), reduced the number of mCMV-infected cells, both in older cells and in fresh cultures from embryonic mouse brains. When a viral dose that killed almost all unprotected cells was used, IFN-protected cells had a natural appearance, and when they were tested with whole-cell patch clamp recording, they appeared physiologically normal with typical resting membrane potentials and action potentials. mCMV infection increased expression of representative IFN-stimulated genes (IFIT3, OAS, LMP2, TGTP, and USP18) in both neonatal and adult brains to similarly large degrees. The robust upregulation of gene expression in the neonatal brain was associated with a much higher degree of viral replication at this stage of development. In contrast to the case for downstream gene induction, CMV upregulated IFN-alpha/beta expression to a greater degree in the adult brain than in the neonatal brain. Similar to the case with cultured brain cells, IFN treatment of the developing brain in vivo depressed mCMV replication. In parallel work with cultured primary human brain cells, IFN and poly(I:C) treatment reduced hCMV infection and prevented virus-mediated cell death. These results suggest that coupling IFN administration with current treatments may reduce CMV infections in the developing brain.

Kaposi's sarcoma-associated herpesvirus virions inhibit interferon responses induced by envelope glycoprotein gpK8.1

The Kaposi's sarcoma-associated herpesvirus (KSHV) envelope glycoprotein gpK8.1 contributes to cellular attachment through binding cell surface heparan sulfate proteoglycans. By using a soluble recombinant form of gpK8.1, we discovered that a consequence of gpK8.1 interaction with human fibroblasts is the induction of an antiviral response, as characterized by the activation of interferon regulatory factor 3 (IRF-3), production of interferon beta (IFN-beta), and expression of interferon-stimulated antiviral genes. In contrast, neither IFN-beta expression nor a functional antiviral response is observed in cells treated with KSHV virions. The interferon response induced by soluble gpK8.1 can be inhibited by simultaneous treatment with UV-inactivated virions, while the induction of an indicator inflammatory cytokine, interleukin-6, was readily evident in the response to both gpK8.1 and KSHV. In addition, KSHV virions abrogate gpK8.1-mediated activation of IRF-3, an early transcriptional regulator for cellular antiviral responses. Although innate immune responses are initiated during contact between gpK8.1 and cellular receptor(s), these results suggest that the virion contains one or more structural elements that selectively repress an effective antiviral response while allowing cellular responses favorable to the KSHV life cycle.

Cutting edge: apoptosis-regulating signal kinase 1 is required for reactive oxygen species-mediated activation of IFN regulatory factor 3 by lipopolysaccharide

IFN regulatory factor (IRF) 3 participates in the transcriptional induction of IFN-alpha, IFN-beta, and a subset of IFN-stimulated genes (ISGs) as a result of viral infection. In addition, bacterial cell wall components such as LPS activate IRF3 in a p38-dependent manner. In this study we show that IRF3-mediated ISG induction by LPS requires the production of reactive oxygen species (ROS) by the NADPH-dependent oxidase NOX4. Furthermore, we present evidence that LPS-mediated ROS production leads to activation of apoptosis-regulating-signal kinase (ASK) 1, a MAPK kinase kinase family member capable of activating the MAP kinase 6/p38 axis. ASK1 kinase activity proved essential for IRF3-mediated ISG induction by LPS. Thus, our results presented here suggest a novel role for ROS and ASK1 in the innate immune response as signaling intermediates in the IRF3 activation pathway.

Stimulation of interferon-β gene expression by human cytomegalovirus via nuclear factor kappa B and phosphatidylinositol 3-kinase pathway

Infection of human foreskin fibroblast (HFF) cells with human cytomegalovirus (HCMV) induces the secretion of soluble factors including interferon (IFN)-beta that stimulates human leukocyte antigen (HLA) class I expression. In this study, the mechanism of IFN-beta induction by HCMV was investigated. In HCMV-infected HFF cells, IFN-beta secretion increased at 6h post infection (h.p.i.). Reverse transcription polymerase chain reaction (RT-PCR) analysis using ultra violet (UV)-inactivated HCMV indicated that viral gene expression is not necessary for the stimulation of IFN-beta. Stimulation of IFN-beta by HCMV infection was not blocked by cycloheximide, an inhibitor of protein synthesis, further suggesting that the expression of HCMV genes is not required for the stimulation of IFN-beta gene transcription. IFN-beta may be produced from virus-infected cells as an inflammatory response and nuclear factor kappa B (NF-kappaB) plays a central role in inflammatory response. HCMV failed to induce the IFN-beta expression, when the virus-infected cells were treated with pyrrolidine dithiocarbamate (PDTC), an inhibitor of NF-kappaB, or LY294002 and wortmannin, inhibitors of phosphatidylinositol 3-kinase (PI3-K). The result suggests that PI3-K and/or NF-kappaB may be related with the induction pathway of IFN-beta by HCMV.

West Nile virus evades activation of interferon regulatory factor 3 through RIG-I-dependent and -independent pathways without antagonizing host defense signaling

The ability of viruses to control and/or evade the host antiviral response is critical to the establishment of a productive infection. We have previously shown that West Nile virus NY (WNV-NY) delays activation of interferon regulatory factor 3 (IRF-3), a transcription factor critical to the initiation of the antiviral response. Here we demonstrate that the delayed activation of IRF-3 is essential for WNV-NY to achieve maximum virus production. Furthermore, WNV-NY utilizes a unique mechanism to control activation of IRF-3. In contrast to many other viruses that impose a nonspecific block to the IRF-3 pathway, WNV-NY eludes detection by the host cell at early times postinfection. To better understand this process, we assessed the role of the pathogen recognition receptor (PRR) retinoic acid-inducible gene I (RIG-I) in sensing WNV-NY infection. RIG-I null mouse embryo fibroblasts (MEFs) retained the ability to respond to WNV-NY infection; however, the onset of the host response was delayed compared to wild-type (WT) MEFs. This suggests that RIG-I is involved in initially sensing WNV-NY infection, while other PRRs sustain and/or amplify the host response later in infection. The delayed initiation of the host response correlated with an increase in WNV-NY replication in RIG-I null MEFs compared to WT MEFs. Our data suggest that activation of the host response by RIG-I early in infection is important for controlling replication of WNV-NY. Furthermore, pathogenic strains of WNV may have evolved to circumvent stimulation of the host response until after replication is well under way.

A human cytomegalovirus antagonist of type I IFN-dependent signal transducer and activator of transcription signaling

Type I IFNs are crucial components of the innate immune response to viral attack. They are rapidly synthesized and secreted after infection with human cytomegalovirus (CMV) and trigger a signal transduction pathway that involves successive activation and nuclear translocation of signal transducer and activator of transcription 1 (STAT1) and STAT2. The activated STATs, together with the IFN regulatory factor 9 protein, form a trimeric transcription complex (IFN-stimulated gene factor 3) that stimulates expression of numerous IFN-responsive genes, many of which exhibit antiviral activity. Here we demonstrate that the viral 72-kDa IE1 protein (IE1-72kDa) confers partial resistance to the antiviral activity of type I IFNs upon CMV. Accordingly, IFN-responsive transcripts accumulate to substantially increased levels after infection with an IE1-deficient mutant as compared with wild-type virus, and ectopic expression of the viral protein in stably transfected cells is sufficient to block their induction. We further show that IE1-72kDa forms a physical complex with STAT1 and STAT2 in nuclei of infected cells and in vitro and prevents association of STAT1, STAT2, and IFN regulatory factor 9 with promoters of IFN-responsive genes in vivo. Our results indicate that the viral protein blocks an intranuclear step after nuclear translocation and before DNA binding of IFN-stimulated gene factor 3, presumably by interfering with the integrity and/or correct subnuclear localization of the protein complex. This study identifies the CMV IE1-72kDa protein as a viral antagonist of the cellular innate immune response, inhibiting IFN-dependent STAT signaling by means of an unprecedented molecular mechanism.

Human cytomegalovirus immediate-early 2 protein IE86 blocks virus-induced chemokine expression

The effect of human cytomegalovirus (HCMV) gene expression on cytokine (beta interferon) and chemokine (RANTES, MIG, MCP-2, MIP-1alpha, and interleukin-8) expression was examined. We demonstrate that HCMV gene expression is required to suppress the transcriptional induction of these cytokines and that the HCMV immediate-early 2 gene product IE86 can effectively block the expression of cytokines and proinflammatory chemokines during HCMV and Sendai virus infection. Additionally, we present data on viral mutants and ectopic protein expression which demonstrate that pp65, another identified HCMV cytokine antagonist, is not involved in regulating these proinflammatory cytokines. This is the first report to demonstrate that IE86 can act to suppress virus-induced proinflammatory cytokine transcript expression, extending the antiviral properties of this multifunctional viral protein.

Interferon regulatory factor 3 is necessary for induction of antiviral genes during human cytomegalovirus infection

Viral infection activates interferon regulatory factor 3 (IRF3), a cofactor for the induction of interferon-stimulated genes (ISGs). The role of IRF3 in the activation of ISGs by human cytomegalovirus (HCMV) is controversial despite the fact that HCMV has consistently been shown to induce ISGs during infection of fibroblasts. To address the function of IRF3 in HCMV-mediated ISG induction, we monitored ISG expression and global gene expression in HCMV-infected cells in which IRF3 function had been depleted by small interfering RNA or blocked by dominant negative IRF3. A specific reduction of ISG induction was observed, whereas other transcripts were unaffected. We therefore conclude that IRF3 specifically regulates ISG induction during the initial phase of HCMV infection.

Herpesviruses and the innate immune response

Herpesvirus infection leads to the rapid induction of an innate immune response. A central aspect of this host response is the production and secretion of type I interferon. The current model of virus-mediated interferon production includes three stages: sensitization, induction, and amplification. A key mediator of all three stages is the cellular transcription factor interferon regulatory factor 3 (IRF3). Although the precise details of IRF3 activation and interferon production in response to herpesvirus infection are still being elucidated, viral proteins that block components of the interferon pathway, particularly IRF3, have been identified and characterized. In vivo studies have shown that in addition to type I interferon, interleukin-15 (IL-15) and natural killer (NK) cells also play an important role in mediating resistance to herpesvirus infection. Recent investigations have demonstrated a strong association between IRF3, interferon, IL-15, and NK cells. This review will focus on herpesvirus-mediated induction of innate immunity, the central role of the type I interferon response and mechanisms used by herpesviruses to block host antiviral immunity.