Multiple regulatory domains control IRF-7 activity in response to virus infection

Diversity of interferon antagonist activities mediated by NSP1 proteins of different rotavirus strains

Studies involving limited numbers of rotavirus (RV) strains have shown that the viral gene 5 product, NSP1, can antagonize beta interferon (IFN-β) expression by inducing the degradation of IFN-regulatory factors (IRFs) (IRF3, IRF5, and IRF7) or a component of the E3 ubiquitin ligase complex responsible for activating NF-κB (β-transducin repeat-containing protein [β-TrCP]). To gain a broader perspective of NSP1 activities, we examined various RV strains for the ability to inhibit IFN-β expression in human cells. We found that all strains encoding wild-type NSP1 impeded IFN-β expression but not always through IRF3 degradation. To identify other degradation targets involved in suppressing IFN-β expression, we used transient expression vectors to test the abilities of a diverse collection of NSP1 proteins to target IRF3, IRF5, IRF7, and β-TrCP for degradation. The results indicated that human RVs rely predominantly on the NSP1-induced degradation of IRF5 and IRF7 to suppress IFN signaling, whereas NSP1 proteins of animal RVs tended to target IRF3, IRF5, and IRF7, allowing the animal viruses a broader attack on the IFN-β signaling pathway. The results also suggested that the NSP1-induced degradation of β-TrCP is an uncommon mechanism of subverting IFN-β signaling but is one that can be shared with NSP1 proteins that induce IRF degradation. Our analysis reveals that the activities of NSP1 proteins are diverse, with no obvious correlations between degradations of pairs of target proteins. Thus, RVs have evolved functionally distinct approaches for subverting the host antiviral response, a property consistent with the immense sequence variation noted for NSP1 proteins.

The ubiquitin E3 ligase RAUL negatively regulates type i interferon through ubiquitination of the transcription factors IRF7 and IRF3

In the course of combating infectious agents, type I interferon (IFN) needs a timely downregulation mechanism to avoid detrimental overreaction. Here we showed a mechanism for restraining type I IFN responses, which relied on a HECT domain ubiquitin (Ub) E3 ligase, RAUL. RAUL limited type I IFN production by directly catalyzing lysine 48-linked polyubiquitination of both interferon regulatory factor 7 (IRF7) and IRF3 followed by proteasome-dependent degradation. Suppression of RAUL by dominant-negative RAUL or siRNA augmented both basal and virus-induced production of type I IFN, which resulted in reduced viral replication. The Kaposi's sarcoma-associated herpes virus immediate-early lytic cycle trigger protein RTA recruited this mechanism to augment its countermeasures against the host antiviral response. These results unveil a previously unrecognized "brake mechanism" for type I IFN that maintains proper low amounts of type I IFN under physiological conditions and restrains its magnitude when the antiviral response intensifies.

Mechanisms of autoinhibition of IRF-7 and a probable model for inactivation of IRF-7 by Kaposi's sarcoma-associated herpesvirus protein ORF45

IRF-7 is the master regulator of type I interferon-dependent immune responses controlling both innate and adaptive immunity. Given the significance of IRF-7 in the induction of immune responses, many viruses have developed strategies to inhibit its activity to evade or antagonize host antiviral responses. We previously demonstrated that ORF45, a KSHV immediate-early protein as well as a tegument protein of virions, interacts with IRF-7 and inhibits virus-mediated type I interferon induction by blocking IRF-7 phosphorylation and nuclear translocation (Zhu, F. X., King, S. M., Smith, E. J., Levy, D. E., and Yuan, Y. (2002) Proc. Natl. Acad. Sci. U.S.A. 99, 5573-5578). In this report, we sought to reveal the mechanism underlying the ORF45-mediated inactivation of IRF-7. We found that ORF45 interacts with the inhibitory domain of IRF-7. The most striking feature in the IRF-7 inhibitory domain is two α-helices H3 and H4 that contain many hydrophobic residues and two β-sheets located between the helices that are also very hydrophobic. These hydrophobic subdomains mediate intramolecular interactions that keep the molecule in a closed (inactive) form. Mutagenesis studies confirm the contribution of the hydrophobic helices and sheets to the autoinhibition of IRF-7 in the absence of viral signal. The binding of ORF45 to the critical domain of IRF-7 leads to a hypothesis that ORF45 may maintain the IRF-7 molecule in the closed form and prevent it from being activated in response to viral infection.

Differential regulation of human papillomavirus type 8 by interferon regulatory factors 3 and 7

The genus β human papillomavirus (HPV) type 8 is associated with nonmelanoma skin cancer in patients with epidermodysplasia verruciformis, and evidence for its protumorigenic potential in the general population increases. To date, strategies to suppress genus β HPV infections are limited. Interferon regulatory factors IRF-3 and IRF-7 play key roles in the activation of the innate immune response to viral infections. In this study, we show for the first time that both IRF-3 and IRF-7 regulate transcription of a papillomavirus, but with opposing effects. IRF-7, expressed in the suprabasal layers of human epidermis, increased HPV8 late promoter activity via direct binding to viral DNA. UV-B light-induced activation of the HPV8 promoter involved IRF-7 as a downstream effector. In contrast, IRF-3, expressed in all layers of human epidermis, induced strong HPV8 suppression in primary keratinocytes. IRF-3-mediated suppression prevailed over IRF-7-induced HPV8 transcription. Unlike the E6 oncoprotein of the mucosal high-risk HPV16, the HPV8 E6 protein did not bind to IRF-3 and only weakly antagonized its activity. Strong antiviral activity was also observed, when keratinocytes were treated with potent IRF-3 activators, poly(I:C) or RNA bearing 5' phosphates. In conclusion, we show that IRF-3 activation induces a state of cell-autonomous immunity against HPV in primary human keratinocytes. Our study suggests that local application of IRF-3-activating compounds might constitute an attractive novel therapeutic strategy against HPV8-associated diseases, particularly in epidermodysplasia verruciformis patients.

TLR3-mediated IFN-β gene induction is negatively regulated by the TLR adaptor MyD88 adaptor-like

There is limited insight into the mechanisms involved in the counterregulation of TLR. Given the important role of TLR3/TIR domain-containing adaptor-inducing IFN-β (TRIF)-dependent signalling in innate immunity, novel insights into its modulation is of significance in the context of many physiological and pathological processes. Herein, we sought to perform analysis to definitively assign a mechanistic role for MyD88 adaptor-like (Mal), an activator of TLR2/4 signalling, in the negative regulation of TLR3/TRIF signalling. Biochemical and functional analysis demonstrates that Mal negatively regulates TLR3, but not TLR4, mediated IFN-β production. Co-immunoprecipitation experiments demonstrate that Mal associates with IRF7 (IRF, IFN regulatory factor), not IRF3, and Mal specifically blocks IRF7 activation. In doing so, Mal impedes TLR3 ligand-induced IFN-β induction. Interestingly, Mal does not affect the induction of IL-6 and TNF-α upon TLR3 ligand engagement. Together, these data show that the TLR adaptor Mal interacts with IRF7 and, in doing so, impairs IFN-β induction through the positive regulatory domains I-III enhancer element of the IFN-β gene following poly(I:C) stimulation. Our findings offer a new mechanistic insight into TLR3/TRIF signalling through a hitherto unknown mechanism whereby Mal inhibits poly(I:C)-induced IRF7 activation and concomitant IFN-β production. Thus, Mal is essential in restricting TLR3 signalling thereby protecting the host from unwanted immunopathologies associated with excessive IFN-β production.

Gene therapeutic approach for inhibiting hepatitis C virus replication using a recombinant protein that controls interferon expression

The hepatitis C virus (HCV) is a continuing threat to public health. The systemic administration of interferon alpha with ribavirin is the only currently approved treatment. However, this treatment is associated with a wide spectrum of systemic side effects that limits its effectiveness; thus, there is an urgent need for new treatment modalities. In this study, we describe a novel anti-HCV strategy employing a recombinant transcription factor that we have engineered in such a way that NS3/4a viral protease controls its intracellular localization, thereby restoring interferon secretion specifically in cells infected with HCV. Proof-of-concept experiments validated the strategy, showing that the recombinant transcription factor was triggered to stimulate interferon promoter by NS3/4A and remained inactive in cells without NS3/4a. Using an adenovirus-associated viral vector delivery system, we found that the recombinant transcription factor inhibited HCV replication effectively in vitro in cultured cells.

The fight between the teleost fish immune response and aquatic viruses

Teleost fish represent a transition point on the phylogenetic spectrum between invertebrates that depend only on innate immunity and mammals that heavily depend on adaptive immunity. The major mechanisms of the teleost fish innate immune response are suggested to be similar to mammals, although fine details of the process require further studies. Within the innate immune response the type I interferon (IFN) system is an essential innate antiviral component that protects fish from some virus infections. The current progress of cloning and functional characterization of fish antiviral genes is promising in further elucidation of the fish antiviral response. The adaptive immune system of fish utilizes cellular components more or less similar to mammals. Teleost fish produce IgM as a primary antibody response and lack isotype switching to mount virus-specific antibodies during the infection process. Despite this, the development of successful fish rhabdoviral vaccines suggest that vaccination may prove to be an effective way of promoting fish adaptive immune responses to viruses. This paper reviews the bony fish antiviral response with specific discussion on the evolutionary mechanisms that allow aquatic viruses to co-exist with their host. Detailed aspects of the teleost type I IFN system are also addressed.

Interferon regulatory factor-1 acts as a powerful adjuvant in tat DNA based vaccination

Genetic vaccines are safe cost-effective approaches to immunization but DNA immunization is an inefficient process. There is, therefore, a pressing need for adjuvants capable of enhancing the immunogenicity and effectiveness of these vaccines. This is particularly important for diseases for which successful vaccines are still lacking, such as cancer and infectious diseases including HIV-1/AIDS. Here we report an approach to enhance the immunogenicity of DNA vaccines involving the use of transcription factors of the Interferon regulatory factor (IRF) family, specifically IRF-1, IRF-3, and IRF-7 using the tat gene as model antigen. Balb/c mice were immunized by three intramuscular inoculations, using a DNA prime-protein boost protocol, with a DNA encoding tat of HIV-1 and the indicated IRFs and immune responses were compared to those induced by vaccination with tat DNA alone. In vivo administration of plasmid DNA encoding IRF-1, or a mutated version of IRF-1 deleted of the DNA-binding domain, enhanced Tat-specific immune responses and shifted them towards a predominant T helper 1-type immune response with increased IFN-gamma production and cytotoxic T lymphocytes responses. Conversely, the use of IRF-3 or IRF-7 did not affect the tat-induced responses. These findings define IRF-1 and its mutated form as efficacious T helper 1-inducing adjuvants in the context of tat-based vaccination and also providing a new promising candidate for genetic vaccine development.

Foot-and-mouth disease virus leader proteinase inhibits dsRNA-induced type I interferon transcription by decreasing interferon regulatory factor 3/7 in protein levels

The leader proteinase (L(pro)) of foot-and-mouth disease virus (FMDV) has been identified as an interferon-beta (IFN-beta) antagonist that disrupts the integrity of transcription factor nuclear factor kappaB (NF-kappaB). In this study, we showed that the reduction of double stranded RNA (dsRNA)-induced IFN-alpha1/beta expression caused by L(pro) was also associated with a decrease of interferon regulatory factor 3/7 (IRF-3/7) in protein levels, two critical transcription factors for activation of IFN-alpha/beta. Furthermore, overexpression of L(pro) significantly reduced the transcription of multiple IRF-responsive genes including 2',5'-OAS, ISG54, IP-10, and RANTES. Screening L(pro) mutants indicated that the ability to process eIF-4G of L(pro) is not required for suppressing dsRNA-induced activation of the IFN-alpha1/beta promoter and decreasing IRF-3/7 expression. Taken together, our results demonstrate that, in addition to disrupting NF-kappaB, L(pro) also decreases IRF-3/7 expression to suppress dsRNA-induced type I IFN production, suggesting multiple strategies used by FMDV to counteract the immune response to viral infection.

Structural insights into interferon regulatory factor activation

The interferon regulatory factors (IRFs) play important roles in development of the immune system and host defense. Recent crystallographic and biochemical studies have provided insights into the mechanism of activation of IRFs by phosphorylation. The activation of a latent closed conformation of IRF in the cytoplasm is triggered by phosphorylation of Ser/Thr residues in a C-terminal region. Phosphorylation stimulates the C-terminal autoinhibitory domain to attain a highly extended conformation triggering dimerization through extensive contacts to a second subunit. Dimers are then transported into the nucleus and assemble with the coactivator CBP/p300 to activate transcription of type I interferons and other target genes. The advances made in understanding the release of inhibition after IRF dimerization have generated a detailed structural model of how IRFs signaling pathways are activated.

Immune response modeling of interferon beta-pretreated influenza virus-infected human dendritic cells

The pretreatment of human dendritic cells with interferon-beta enhances their immune response to influenza virus infection. We measured the expression levels of several key players in that response over a period of 13 h both during pretreatment and after viral infection. Their activation profiles reflect the presence of both negative and positive feedback loops in interferon induction and interferon signaling pathway. Based on these measurements, we have developed a comprehensive computational model of cellular immune response that elucidates its mechanism and its dynamics in interferon-pretreated dendritic cells, and provides insights into the effects of duration and strength of pretreatment.

Viral interferon regulatory factors

Upon viral infection, the major defensive strategy employed by the host immune system is the activation of the interferon (IFN)-mediated antiviral pathway, which is overseen by IFN regulatory factors (IRFs). In order to complete their life cycles, viruses must find a way to modulate the host IFN-mediated immune response. Kaposi's sarcoma-associated herpesvirus (KSHV), a human tumor-inducing herpesvirus, has developed a unique mechanism for antagonizing cellular IFN-mediated antiviral activity by incorporating viral homolog of the cellular IRFs, called vIRFs, into its genome. Here, we summarize the novel evasion mechanisms by which KSHV, through its vIRFs, circumvents IFN-mediated innate immune responses and deregulates the cell growth control mechanism.

Dual functions of interferon regulatory factors 7C in Epstein-Barr virus-mediated transformation of human B lymphocytes

Epstein-Barr virus (EBV) infection is associated with several human malignancies. Interferon (IFN) regulatory factor 7 (IRF-7) has several splicing variants, and at least the major splicing variant (IRF-7A) has oncogenic potential and is associated with EBV transformation processes. IRF-7C is an alternative splicing variant with only the DNA-binding domain of IRF-7. Whether IRF-7C is present under physiological conditions and its functions in viral transformation are unknown. In this report, we prove the existence of IRF-7C protein and RNA in certain cells under physiological conditions, and find that high levels of IRF-7C are associated with EBV transformation of human primary B cells in vitro as well as EBV type III latency. EBV latent membrane protein 1 (LMP-1) stimulates IRF-7C expression in B lymphocytes. IRF-7C has oncogenic potential in rodent cells and partially restores the growth properties of EBV-transformed cells under a growth-inhibition condition. A tumor array experiment has identified six primary tumor specimens with high levels of IRF-7C protein—all of them are lymphomas. Furthermore, we show that the expression of IRF-7C is apparently closely associated with other IRF-7 splicing variants. IRF-7C inhibits the function of IRF-7 in transcriptional regulation of IFN genes. These data suggest that EBV may use splicing variants of IRF-7 for its transformation process in two strategies: to use oncogenic properties of various IRF-7 splicing variants, but use one of its splicing variants (IRF-7C) to block the IFN-induction function of IRF-7 that is detrimental for viral transformation. The work provides a novel relation of host/virus interactions, and has expanded our knowledge about IRFs in EBV transformation.

Absence of SHIP-1 results in constitutive phosphorylation of tank-binding kinase 1 and enhanced TLR3-dependent IFN-beta production

Autoimmune diseases, such as systemic lupus erythematosus and rheumatoid arthritis, result from a loss of tolerance to self-antigens and immune-mediated injury precipitated by the overproduction of type I IFN and inflammatory cytokines. We have identified the inositol 5' phosphatase SHIP-1 as a negative regulator of TLR3-induced type I IFN production. SHIP-1-deficient macrophages display enhanced TLR-induced IFN-beta production, and overexpression of SHIP-1 negatively regulates the ability of TLR3 and its adaptor, Toll/IL-1 receptor domain-containing adaptor-inducing IFN-beta, to induce IFN-beta promoter activity, indicating that SHIP-1 negatively regulates TLR-induced IFN-beta production. Further dissection of the IFN-beta pathway implicates TANK-binding kinase 1 (TBK1) as the target for SHIP-1. Critically, in the absence of SHIP-1, TBK1 appears to be hyperphosphorylated both in unstimulated cells and following TLR3 stimulation. In addition, TBK1 appears to be constitutively associated with Toll/IL-1 receptor domain-containing adaptor-inducing IFN-beta and TNFR-associated factor 3 in SHIP-1 deficient cells, whereas in wild-type cells this association is inducible following TLR3 stimulation. In support of a role for SHIP-1 in regulating complex formation, confocal microscopy demonstrates that TBK1 distribution in the cell is significantly altered in SHIP-1-deficient cells, with more prominent endosomal staining observed, compared with wild-type controls. Taken together, our results point to SHIP-1 as a critical negative regulator of IFN-beta production downstream of TLR3 through the regulation of TBK1 localization and activity.

The IKK Kinases: Operators of Antiviral Signaling

The ability of a cell to combat an intracellular pathogen requires a mechanism to recognize the threat and elicit a transcriptional response against it. In the context of virus infection, the cell must take measures to inhibit viral replication, meanwhile, convey warning signals to neighboring cells of the imminent threat. This immune response is predominantly mediated by the production of cytokines, notably, interferon beta (IFNβ). IFNβ signaling results in the transcriptional induction of over one hundred antiviral gene products whose timely expression renders infected cells more capable of inhibiting virus replication, while providing the uninfected cells with the reinforcements to generate a less permissive cellular environment. Induction of IFNβ and many aspects of the antiviral response pivot on the function of the IKK and IKK-related kinases. Despite sharing high levels of homology and some degree of functional redundancy, the classic IKK kinases: IKKα and IKKβ, and the IKK-related kinases: TBK1 and IKKɛ, perform distinct roles in regulating the host antiviral defense. These kinases serve as molecular operators in their cooperative ability to integrate incoming cellular cues and act on a range of essential antiviral transcription factors to reshape the cellular transcriptome during infection.

Rotavirus antagonism of the innate immune response

Rotavirus is a primary cause of severe dehydrating gastroenteritis in infants and young children. The virus is sensitive to the antiviral effects triggered by the interferon (IFN)-signaling pathway, an important component of the host cell innate immune response. To counteract these effects, rotavirus encodes a nonstructural protein (NSP1) that induces the degradation of proteins involved in regulating IFN expression, such as members of the IFN regulatory factor (IRF) family. In some instances, NSP1 also subverts IFN expression by causing the degradation of a component of the E3 ubiquitin ligase complex responsible for activating NF-κB. By antagonizing multiple components of the IFN-induction pathway, NSP1 aids viral spread and contributes to rotavirus pathogenesis.

The E3 ubiquitin ligase Triad3A negatively regulates the RIG-I/MAVS signaling pathway by targeting TRAF3 for degradation

The primary role of the innate immune response is to limit the spread of infectious pathogens, with activation of Toll-like receptor (TLR) and RIG-like receptor (RLR) pathways resulting in a pro-inflammatory response required to combat infection. Limiting the activation of these signaling pathways is likewise essential to prevent tissue injury in the host. Triad3A is an E3 ubiquitin ligase that interacts with several components of TLR signaling and modulates TLR activity. In the present study, we demonstrate that Triad3A negatively regulates the RIG-I RNA sensing pathway through Lys⁴⁸-linked, ubiquitin-mediated degradation of the tumor necrosis factor receptor-associated factor 3 (TRAF3) adapter. Triad3A was induced following dsRNA exposure or virus infection and decreased TRAF3 levels in a dose-dependent manner; moreover, Triad3A expression blocked IRF-3 activation by Ser-396 phosphorylation and inhibited the expression of type 1 interferon and antiviral genes. Lys⁴⁸-linked ubiquitination of TRAF3 by Triad3A increased TRAF3 turnover, whereas reduction of Triad3A expression by stable shRNA expression correlated with an increase in TRAF3 protein expression and enhancement of the antiviral response following VSV or Sendai virus infection. Triad3A and TRAF3 physically interacted together, and TRAF3 residues Y440 and Q442—previously shown to be important for association with the MAVS adapter—were also critical for Triad3A. Point mutation of the TRAF-Interacting-Motif (TIM) of Triad3A abrogated its ability to interact with TRAF3 and modulate RIG-I signaling. TRAF3 appears to undergo sequential ubiquitin “immuno-editing” following virus infection that is crucial for regulation of RIG-I-dependent signaling to the antiviral response. Thus, Triad3A represents a versatile E3 ubiquitin ligase that negatively regulates RIG-like receptor signaling by targeting TRAF3 for degradation following RNA virus infection.

RNA virus infection is detected through TLR-dependent and TLR-independent mechanisms. Early viral replicative intermediates are detected by two recently characterized cystolic viral RNA receptors, RIG-I and MDA-5, leading to the production of pro-inflammatory cytokines and type I interferons (IFNs). Dysfunctional responses, either failure to respond or hyper-responsiveness, may lead to both acute and chronic immunodeficiency and inflammatory diseases. Thus, the intensity and duration of RLR signaling must be tightly controlled. One general mechanism by which innate immune receptors and their downstream adapters are regulated involves protein degradation mediated by the ubiquitination pathway. Our study demonstrates that the E3 ubiquitin ligase Triad3A negatively regulates the RIG-I-like receptor pathway by targeting the adapter molecule TRAF3 for proteasomal degradation through Lys⁴⁸-linked ubiquitin-mediated degradation. Thus, Triad3A represents a key molecule involved in the negative regulation of the host antiviral response triggered by RNA virus infection.

Thogoto virus ML protein is a potent inhibitor of the interferon regulatory factor-7 transcription factor

The tick-transmitted orthomyxovirus Thogoto virus (THOV) encodes the ML protein acting as a viral suppressor of the host interferon (IFN) system. Here, we describe that type I IFN is strongly induced in primary mouse embryo fibroblasts as well as plasmacytoid dendritic cells upon infection with a THOV mutant lacking the ML gene. However, wild-type THOV encoding ML suppresses induction of IFN by preventing the activation of members of the IFN regulatory factor (IRF) family. We found that reporter gene expression dependent on IRF3 and IRF7 was strongly inhibited by ML. Further experiments revealed that ML interacts with IRF7 and prevents dimerization of the transcription factor and its association with the coactivator TRAF6. Interestingly, another IRF7 activation step, nuclear translocation, is not affected by ML. Our data elucidate ML protein as a virulence factor with an IRF-specific IFN-antagonistic spectrum.

Differential regulation of human interferon A gene expression by interferon regulatory factors 3 and 7

Differential expression of the human interferon A (IFN-A) gene cluster is modulated following paramyxovirus infection by the relative amounts of active interferon regulatory factor 3 (IRF-3) and IRF-7. IRF-3 expression activates predominantly IFN-A1 and IFN-B, while IRF-7 expression induces multiple IFN-A genes. IFN-A1 gene expression is dependent on three promoter proximal IRF elements (B, C, and D modules, located at positions -98 to -45 relative to the mRNA start site). IRF-3 binds the C module of IFN-A1, while other IFN-A gene promoters are responsive to the binding of IRF-7 to the B and D modules. Maximal expression of IFN-A1 is observed with complete occupancy of the three modules in the presence of IRF-7. Nucleotide substitutions in the C modules of other IFN-A genes disrupt IRF-3-mediated transcription, whereas a G/A substitution in the D modules enhances IRF7-mediated expression. IRF-3 exerts dual effects on IFN-A gene expression, as follows: a synergistic effect with IRF-7 on IFN-A1 expression and an inhibitory effect on other IFN-A gene promoters. Chromatin immunoprecipitation experiments reveal that transient binding of both IRF-3 and IRF-7, accompanied by CBP/p300 recruitment to the endogenous IFN-A gene promoters, is associated with transcriptional activation, whereas a biphasic recruitment of IRF-3 and CBP/p300 represses IFN-A gene expression. This regulatory mechanism contributes to differential expression of IFN-A genes and may be critical for alpha interferon production in different cell types by RIG-I-dependent signals, leading to innate antiviral immune responses.

Transcriptional re-programming of primary macrophages reveals distinct apoptotic and anti-tumoral functions of IRF-3 and IRF-7

The immunoregulatory transcriptional modulators - IFN-regulatory factor (IRF)-3 and IRF-7 - possess similar structural features but distinct gene-regulatory potentials. For example, adenovirus-mediated transduction of the constitutively active form of IRF-3 triggered cell death in primary human MPhi, whereas expression of active IRF-7 induced a strong anti-tumoral activity in vitro. To further characterize target genes involved in these distinct cellular responses, transcriptional profiles of active IRF-3- or IRF-7-transduced primary human MPhi were compared and used to direct further mechanistic studies. The pro-apoptotic BH3-only protein Noxa was identified as a primary IRF-3 target gene and an essential regulator of IRF-3, dsRNA and vesicular stomatitis virus-induced cell death. The critical role of IRF-7 and type I IFN production in increasing the immunostimulatory capacity of MPhi was also evaluated; IRF-7 increased the expression of a broad range of IFN-stimulated genes including immunomodulatory cytokines and genes involved in antigen processing and presentation. Furthermore, active IRF-7 augmented the cross-presentation capacity and tumoricidal activity of MPhi and led to an anti-tumor response against the B16 melanoma model in vivo. Altogether, these data further highlight the respective functions of IRF-3 and IRF-7 to program apoptotic, immune and anti-tumor responses.

Structural and functional studies of an IRF-7-like gene from Atlantic salmon

Interferon regulatory factor 7 (IRF-7) plays a crucial role in virus-induced activation of interferon-alpha/beta transcription in mammals. This work describes a structural and functional homologue of mammalian IRF-7 from Atlantic salmon. The cloned gene encodes a putative protein of 415 amino acids (aa), which groups with mammalian IRF-7 and other fish IRF-7-like proteins in a phylogenetic analysis of vertebrate IRFs. Using an IFN promoter-luciferase assay we showed that salmon IRF-7 gave increased promoter activity after poly I:C stimulation. Transcript levels of IRF-7 were measured by real-time RT-PCR and compared to those of signal transducer and activator of transcription 1 (STAT1), which is important for transcriptional activation of IFN stimulated genes. Recombinant salmon IFN-alpha1 and poly I:C proved to be potent inducers of IRF-7 in Atlantic salmon TO cells, and poly I:C also induced the gene in head kidney and liver of Atlantic salmon. STAT1 was also induced by IFN, but was only weakly induced by poly I:C stimulation in vitro. Differences in transcription kinetics between IRF-7 and STAT1 thus indicate that the genes are regulated through different pathways. Finally, infection of TO cells with infectious salmon anemia virus (ISAV) induced early synthesis of STAT1 mRNA, whereas IRF-7 transcripts were upregulated much later. This indicates that ISAV has mechanisms to antagonize IRF-7 transcription and thus also the IFN system in Atlantic salmon.

Epstein-Barr virus LF2: an antagonist to type I interferon

Upon viral infection, the major defense mounted by the host immune system is activation of the interferon (IFN)-mediated antiviral pathway, which is mediated by IFN regulatory factors (IRFs). In order to complete their life cycle, viruses must modulate host IFN-mediated immune responses. Despite its association with significant human health problems, activities of Epstein-Barr virus (EBV), a human tumor-inducing herpesvirus, to evade host IFN-mediated innate immunity have not been well characterized. To search for EBV genes that block IFN signal transduction, we carried out a screening of EBV open reading frames for their abilities to block IFN-alpha/beta-mediated luciferase expression upon Sendai virus infection. This screening demonstrates that EBV LF2 tegument protein specifically interacts with the central inhibitory association domain of IRF7, and this interaction leads to inhibition of the dimerization of IRF7, which suppresses IFN-alpha production and IFN-mediated immunity. This demonstrates a novel immune evasion mechanism of EBV LF2 in blocking cellular IRF7-mediated innate immunity.

Measles virus V protein is a decoy substrate for IkappaB kinase alpha and prevents Toll-like receptor 7/9-mediated interferon induction

The central role of plasmacytoid dendritic cells (pDC) in activating host immune responses stems from their high capacity to express alpha interferon (IFN-alpha) after stimulation of Toll-like receptors 7 and 9 (TLR7 and -9). This involves the adapter MyD88 and the kinases interleukin-1 receptor-associated kinase 1 (IRAK1), IRAK4, and IkappaB kinase alpha (IKKalpha), which activates IFN regulatory factor 7 (IRF7) and is independent of the canonical kinases TBK1 and IKKepsilon. We have recently shown that the immunosuppressive measles virus (MV) abolishes TLR7/9/MyD88-dependent IFN induction in human pDC (Schlender et al., J. Virol. 79:5507-5515, 2005), but the molecular mechanisms remained elusive. Here, we have reconstituted the pathway in cell lines and identified IKKalpha and IRF7 as specific targets of the MV V protein (MV-V). Binding of MV-V to IKKalpha resulted in phosphorylation of V on the expense of IRF7 phosphorylation by IKKalpha in vitro and in living cells. This corroborates the role of IKKalpha as the kinase phosphorylating IRF7. MV-V in addition bound to IRF7 and to phosphomimetic IRF7 and inhibited IRF7 transcriptional activity. Binding to both IKKalpha and IRF7 required the 68-amino-acid unique C-terminal domain of V. Inhibition of TLR/MyD88-dependent IFN induction by MV-V is unique among paramyxovirus V proteins and should contribute to the unique immunosuppressive phenotype of measles. The mechanisms employed by MV-V inspire strategies to interfere with immunopathological TLR/MyD88 signaling.

MAVS dimer is a crucial signaling component of innate immunity and the target of hepatitis C virus NS3/4A protease

The mitochondrial antiviral signaling (MAVS) protein plays a central role in innate antiviral immunity. Upon recognition of a virus, intracellular receptors of the RIG-I-like helicase family interact with MAVS to trigger a signaling cascade. In this study, we investigate the requirement of the MAVS structure for enabling its signaling by structure-function analyses and resonance energy transfer approaches in live cells. We now report the essential role of the MAVS oligomer in signal transduction and map the transmembrane domain as the main determinant of dimerization. A combination of mutagenesis and computational methods identified a cluster of residues making favorable van der Waals interactions at the MAVS dimer interface. We also correlated the activation of IRF3 and NF-kappaB with MAVS oligomerization rather than its mitochondrial localization. Finally, we demonstrated that MAVS oligomerization is disrupted upon expression of HCV NS3/4A protease, suggesting a mechanism for the loss of antiviral signaling. Altogether, our data suggest that the MAVS oligomer is essential in the formation of a multiprotein membrane-associated signaling complex and enables downstream activation of IRF3 and NF-kappaB in antiviral innate immunity.

IRF7 activation by Epstein-Barr virus latent membrane protein 1 requires localization at activation sites and TRAF6, but not TRAF2 or TRAF3

Epstein-Barr virus (EBV) latent infection membrane protein 1 (LMP1), a constitutively aggregated and activated pseudoreceptor, activates IFN regulatory factor 7 (IRF7) through RIP1. We now report that the LMP1 cytoplasmic carboxyl terminal amino acids 379-386 bound IRF7 and activated IRF7. IRF7 activation required TRAF6 and RIP1, but not TRAF2 or TRAF3. LMP1 Y(384)YD(386), which are required for TRADD and RIP1 binding and for NF-kappaB activation, were not required for IRF7 binding, but were required for IRF7 activation, implicating signaling through TRADD and RIP1 in IRF7 activation. Association with active LMP1 signaling complexes was also critical for IRF7 activation because (i) a dominant-negative IRF7 bound to LMP1, blocked IRF7 association and activation, but did not inhibit LMP1 induced NF-kappaB or TBK1 or Sendai virus-mediated IFN stimulated response element activation; and (ii) two different LMP1 transmembrane domain mutants, which fail to aggregate, each bound IRF7 and prevented LMP1 from binding and activating IRF7 in the same cell, but did not prevent NF-kappaB activation. Thus, efficient IRF7 activation required association with LMP1 CTAR2 in proximity to LMP1 CTAR2 mediated kinase activation sites.

Insights into interferon regulatory factor activation from the crystal structure of dimeric IRF5

Interferon regulatory factors (IRFs) are essential in the innate immune response and other physiological processes. Activation of these proteins in the cytoplasm is triggered by phosphorylation of serine and threonine residues in a C-terminal autoinhibitory region, which stimulates dimerization, transport into the nucleus, assembly with the coactivator CBP/p300 and initiation of transcription. The crystal structure of the transactivation domain of pseudophosphorylated human IRF5 strikingly reveals a dimer in which the bulk of intersubunit interactions involve a highly extended C-terminal region. The corresponding region has previously been shown to block CBP/p300 binding to unphosphorylated IRF3. Mutation of key interface residues supports the observed dimer as the physiologically activated state of IRF5 and IRF3. Thus, phosphorylation is likely to activate IRF5 and other family members by triggering conformational rearrangements that switch the C-terminal segment from an autoinihibitory to a dimerization role.

Molecular characterization of IRF3 and IRF7 in rainbow trout, Oncorhynchus mykiss: functional analysis and transcriptional modulation

Interferon regulatory factors (IRF) 3 and 7 in mammals are known to be crucial in regulating the type I interferon (IFN) response to viral infection as part of transcriptional complexes binding to IRF-binding elements (IRF-Es) and interferon stimulatory response elements (ISREs) within IFN and interferon-stimulated genes (ISGs). Here we report the sequencing and characterization of full-length cDNA homologues of rainbow trout (rt)IRF7 and, for the first time in fish, IRF3. RtIRF3 consists of 2127 bp with a 159 bp 5'-UTR-containing two upstream AUGs and a 573 bp 3'-UTR. RtIRF7 was found to be 2055 bp, with a 102 bp 5'-UTR and a 705 bp 3'-UTR. The open reading frames (ORFs) translate into 464 amino acid and 415 amino acid proteins, respectively, each possessing a putative DNA-binding domain (DBD) containing a tryptophan cluster, which is characteristic of all IRF family members. The presence of putative IRF association domain (IAD)s, serine-rich C terminal domains (poorly conserved in trout IRF3), and phylogenetic analysis places the two genes in the IRF3 subfamily. Both genes were found to be upregulated by poly I:C, type I recombinant rainbow trout (r) IFN (second isoform, type I rIFN), type II rIFN (rIFNgamma), LPS, and rIL-1beta in the trout macrophage cell line, RTS-11. Poly I:C and type I rIFN also induced IRF3 and IRF7 expression in a trout fibroblast cell line (RTG-2). Transient transfection of RTG-2 cells with each IRF fused to GFP revealed a predominant cytoplasmic distribution found most intensely around the nucleus and, to a lesser extent, within cell nuclei. Transient transfection of rtIRF3 in the Mx-1-luciferase reporter cell line, RTG-P1, revealed a modest increase in luciferase activity relative to the vehicle control, which was lost in cells over-expressing a DBD-truncated form of rtIRF3. Both full-length and DBD-truncated forms of rtIRF7 increased reporter activity relative to the control, although to a non-significant extent. Electromobility shift assays (EMSAs) did not reveal a specific interaction between each IRF and the ISRE element found in the Mx-1 promoter, although the Mx-1 ISRE bound specifically to endogenous transcriptional complexes. These data support the premise that rtIRF3 and rtIRF7 are important molecules in the regulation of antiviral responses in fish, with the impact of rIFNgamma on rtIRF3/7 expression implying a role for these IRFs in immune processes other than type I IFN-driven antiviral responses.

TRAF6 and the three C-terminal lysine sites on IRF7 are required for its ubiquitination-mediated activation by the tumor necrosis factor receptor family member latent membrane protein 1

We have recently shown that interferon regulatory factor 7 (IRF7) is activated by Epstein-Barr virus latent membrane protein 1 (LMP1), a member of the tumor necrosis factor receptor (TNFR) superfamily, through receptor-interacting protein-dependent K63-linked ubiquitination (L. E. Huye, S. Ning, M. Kelliher, and J. S. Pagano, Mol. Cell. Biol. 27:2910-2918, 2007). In this study, with the use of small interfering RNA and TNFR-associated factor 6 (TRAF6) knockout cells, we first show that TRAF6 and its E3 ligase activity are required for LMP1-stimulated IRF7 ubiquitination. In Raji cells which are latently infected and express high levels of LMP1 and IRF7 endogenously, expression of a TRAF6 small hairpin RNA construct reduces endogenous ubiquitination and endogenous activity of IRF7. In TRAF6(-/-) mouse embryonic fibroblasts, reconstitution with TRAF6 expression, but not with TRAF6(C70A), which lacks the E3 ligase activity, recovers LMP1's ability to stimulate K63-linked ubiquitination of IRF7. Further, we identify IRF7 as a substrate for TRAF6 E3 ligase and show that IRF7 is ubiquitinated by TRAF6 at multiple sites both in vitro and in vivo. Most important, we determine that the last three C-terminal lysine sites (positions 444, 446, and 452) of human IRF7 variant A are essential for activation of IRF7; these are the first such sites identified. A ubiquitination-deficient mutant of IRF7 with these sites mutated to arginines completely loses transactivational ability in response not only to LMP1 but also to the IRF7 kinase IkappaB kinase epsilon. In addition, we find that K63-linked ubiquitination of IRF7 occurs independently of its C-terminal functional phosphorylation sites. These data support our hypothesis that regulatory ubiquitination of IRF7 is a prerequisite for its phosphorylation. This is the first evidence to imply that ubiquitination is required for phosphorylation and activation of a transcription factor.

Morphine suppresses intracellular interferon-alpha expression in neuronal cells

Interferon alpha (IFN-alpha) not only plays a key role in innate host immunity against infections but also is involved in the cellular functions of the central nervous system (CNS). In this study, we examined the impact of morphine on IFN-alpha expression in human neuronal cells (NT2-N). Similar to human immune cells, NT2-N cells also expressed IFN-alpha at both mRNA and protein levels. IFN-alpha expression in NT2-N cells, however, was inhibited by morphine. Naltrexone antagonized the inhibitory effect of morphine on IFN-alpha expression in NT2-N cells. The specific mu opioid receptor antagonist, Cys2, Tyr3, Arg5, Pen7-amide (CTAP), also blocked the morphine action on intracellular IFN-alpha expression. Investigation of the mechanisms involved in the morphine action showed that although morphine had little effect on the expression of key IFN regulatory factors (IRFs), morphine inhibited IFN-alpha promoter activation and suppressed the expression and phosphorylation of signal transducer and activator of transcription 1 (STAT1) in the neuronal cells. These findings provide direct in vitro evidence that opioids may impair neuronal cell-mediated innate protection in the CNS.

Association of interferon regulatory factor-7 gene polymorphism with liver cirrhosis in chronic hepatitis C patients

BACKGROUND AND AIMS

Interferon (IFN) regulatory factor 7 (IRF-7) has been shown to play an essential role in the transcriptional activation of virus-inducible cellular genes, especially IFN genes. Polymorphisms of the IRF-7 gene may probably affect both the quality and the quantity of IRF-7. We investigated the role of IRF-7 polymorphisms in Japanese patients with chronic hepatitis C virus (HCV) infection.

METHODS

We studied a total of nine polymorphisms of the IRF-7 gene including SNP1047A/G (Lys/Glu) and SNP2157A/G (Gln/Arg) using the Taqman allelic discrimination and sequencing techniques in 406 Japanese patients with chronic HCV infection. We further performed functional analysis of SNP1047 and SNP2157 by transcriptional activation of the IFNA promoter.

RESULTS

We found that SNP1047AG and SNP2157AG genotypes were in complete linkage disequilibrium and were present in a significantly higher proportion in HCV-infected patients with cirrhosis (5.6%) than in those without cirrhosis (1.7%) (P=0.03). Multivariate analysis also revealed that SNP1047 and SNP2157 were independently associated with cirrhosis at an odds ratio of 2.5. Functional analysis revealed that SNP1047G and SNP2157G alleles increased IFNA expression.

CONCLUSION

SNP1047AG and SNP2157AG genotypes were strongly associated with cirrhosis. SNP1047G and SNP2157G alleles might be used as markers of host factors associated with a higher risk of cirrhosis in Japanese patients with chronic HCV infection.

Functional characterization of murine interferon regulatory factor 5 (IRF-5) and its role in the innate antiviral response

Although the role of human IRF-5 in antiviral and inflammatory responses in vitro has been well characterized, much remains to be elucidated about murine IRF-5. Murine IRF-5, unlike the heavily spliced human gene, is primarily expressed as a full-length transcript, with only a single splice variant that was detected in very low levels in the bone marrow of C57BL/6J mice. This bone marrow variant contains a 288-nucleotide deletion from exons 4-6 and exhibits impaired transcriptional activity. The murine IRF-5 can be activated by both TBK1 and MyD88 to form homodimers and bind to and activate transcription of type I interferon and inflammatory cytokine genes. The importance of IRF-5 in the antiviral and inflammatory response in vivo is highlighted by marked reductions in serum levels of type I interferon and tumor necrosis factor alpha (TNFalpha) in Newcastle disease virus-infected Irf5(-)(/)(-) mice. IRF-5 is critical for TLR3-, TLR4-, and TLR9-dependent induction of TNFalpha in CD11c(+) dendritic cells. In contrast, TLR9, but not TLR3/4-mediated induction of type I IFN transcription, is dependent on IRF-5 in these cells. In addition, IRF-5 regulates TNFalpha but not type I interferon gene transcription in Newcastle disease virus-infected peritoneal macrophages. Altogether, these data reveal the cell type-specific importance of IRF-5 in MyD88-mediated antiviral pathways and the widespread role of IRF-5 in the regulation of inflammatory cytokines.

Phosphorylation of IRF-3 on Ser 339 generates a hyperactive form of IRF-3 through regulation of dimerization and CBP association

The IkappaB kinase-related kinases, TBK1 and IKKi, were recently shown to be responsible for the C-terminal phosphorylation of IRF-3. However, the identity of the phosphoacceptor site(s) targeted by these two kinases remains unclear. Using a biological assay based on the IRF-3-mediated production of antiviral cytokines, we demonstrate here that all Ser/Thr clusters of IRF-3 are required for its optimal transactivation capacity. In vitro kinase assays using full-length His-IRF-3 as a substrate combined with mass spectrometry analysis revealed that serine 402 and serine 396 are directly targeted by TBK1. Analysis of Ser/Thr-to-Ala mutants revealed that the S396A mutation, located in cluster II, abolished IRF-3 homodimerization, CBP association, and nuclear accumulation. However, production of antiviral cytokines was still present in IRF-3 S396A-expressing cells. Interestingly, mutation of serine 339, which is involved in IRF-3 stability, also abrogated CBP association and dimerization without affecting gene transactivation as long as serine 396 remained available for phosphorylation. Complementation of IRF-3-knockout mouse embryonic fibroblasts also revealed a compensatory mechanism of serine 339 and serine 396 in the ability of IRF-3 to induce expression of the interferon-stimulated genes ISG56 and ISG54. These data lead us to reconsider the current model of IRF-3 activation. We propose that conventional biochemical assays used to measure IRF-3 activation are not sensitive enough to detect the small fraction of IRF-3 needed to elicit a biological response. Importantly, our study establishes a molecular link between the role of serine 339 in IRF-3 homodimerization, CBP association, and its destabilization.

Convergence of the NF-κB and IRF pathways in the regulation of the innate antiviral response

The type I interferon (IFN) alpha and beta promoters have been a leading paradigm of virus-activated transcriptional regulation for more than two decades, and have contributed substantially to our understanding of virus-inducible gene regulation, the coordinated activities of NF-kappaB and IRF transcription factors, the temporal and spatial recruitment of co-activators to the enhanceosome, and signaling pathways that trigger the innate antiviral response. In 2003, the ISICR Milstein Award was presented to John Hiscott of McGill University and Tom Maniatis of Harvard University for their ongoing research describing the mechanisms of regulation of type 1 interferon genes and specifically for the identification of key signaling kinases involved in phosphorylation of the transcription factors IRF-3 and IRF-7. The specific roles played by IRFs and the IKK-related kinases TBK1 and IKKvarepsilon are now recognized within the broader framework of TLR and RIG-I signaling pathways. This review summarizes the unique features of the IKK-related kinases and offers a summary of recent advances in the regulation of the early host response to virus infection.

Inhibition of interferon regulatory factor 7 (IRF7)-mediated interferon signal transduction by the Kaposi's sarcoma-associated herpesvirus viral IRF homolog vIRF3

Upon viral infection, the major defense mounted by the host immune system is activation of the interferon (IFN)-mediated antiviral pathway that is mediated by IFN regulatory factors (IRFs). In order to complete their life cycle, viruses must modulate the host IFN-mediated immune response. Kaposi's sarcoma-associated herpesvirus (KSHV), a human tumor-inducing herpesvirus, has developed a unique mechanism for antagonizing cellular IFN-mediated antiviral activity by incorporating viral homologs of the cellular IRFs, called vIRFs. Here, we report a novel immune evasion mechanism of KSHV vIRF3 to block cellular IRF7-mediated innate immunity in response to viral infection. KSHV vIRF3 specifically interacts with either the DNA binding domain or the central IRF association domain of IRF7, and this interaction leads to the inhibition of IRF7 DNA binding activity and, therefore, suppression of alpha interferon (IFN-alpha) production and IFN-mediated immunity. Remarkably, the central 40 amino acids of vIRF3, containing the double alpha helix motifs, are sufficient not only for binding to IRF7, but also for inhibiting IRF7 DNA binding activity. Consequently, the expression of the double alpha helix motif-containing peptide effectively suppresses IRF7-mediated IFN-alpha production. This demonstrates a remarkably efficient means of viral avoidance of host antiviral activity.

Triggering the innate antiviral response through IRF-3 activation

Rapid induction of type I interferon (IFN) expression is a central event in the establishment of the innate immune response against viral infection and requires the activation of multiple transcriptional proteins following engagement and signaling through Toll-like receptor-dependent and -independent pathways. The transcription factor interferon regulatory factor-3 (IRF-3) contributes to a first line of defense against viral infection by inducing the production of IFN-beta that in turn amplifies the IFN response and the development of antiviral activity. In murine knock-out models, the absence of IRF-3 and the closely related IRF-7 ablates IFN production and increases viral pathogenesis, thus supporting a pivotal role for IRF-3/IRF-7 in the development of the host antiviral response.

Interferon regulatory factor 7 is activated by a viral oncoprotein through RIP-dependent ubiquitination

As a key mediator of type I interferon (IFN) (IFN-alpha/beta) responses, IFN regulatory factor 7 (IRF7) is essential to host immune defenses. Activation of IRF7 generally requires virus-induced C-terminal phosphorylation, which leads to its nuclear accumulation and activation of target genes. Here we use the Epstein-Barr virus (EBV) oncoprotein LMP1, which activates IRF7, to identify factors involved in IRF7 activation. We demonstrate for the first time that RIP activates IRF7 and that RIP and IRF7 interact under physiological conditions in EBV-positive Burkitt's lymphoma cells. We provide evidence that both RIP and IRF7 are ubiquitinated in these cells and that IRF7 preferentially interacts with ubiquitinated RIP. RIP is required for full activation of IRF7 by LMP1, with LMP1 stimulating the ubiquitination of RIP and its interaction with IRF7. Moreover, LMP1 stimulates RIP-dependent K63-linked ubiquitination of IRF7, which regulates protein function rather than proteasomal degradation of proteins. We suggest that RIP may serve as a general activator of IRF7, responding to and transmitting the signals from various stimuli, and that ubiquitination may be a general mechanism for enhancing the activity of IRF7.

Gene structure and transcription of IRF-1 and IRF-7 in the mandarin fish Siniperca chuatsi

The genes of IRF-1 and IRF-7 have been cloned from the mandarin fish (Siniperca chuatsi). The IRF-1 gene has 4919 nucleotides (nt) and contains 10 exons and 9 introns, with an open reading frame (ORF) of 903nt encoding 301aa. The IRF-7 gene has 6057nt and also contains 10 exons and 9 introns, with an ORF of 1308nt encoding 436aa. The IRF-1 and IRF-7 genes have only one copy each in the genome. The transcription of IRF-1 and IRF-7 in different organs was analyzed by real-time PCR, and both molecules were constitutively expressed. The IRF-1 and IRF-7 mRNAs were abundant in gill, spleen, kidney and pronephros. The temporal transcriptional changes for IRF-1, IRF-7 and Mx were investigated within 48h after poly I: C stimulation in liver, gill, spleen and pronephros. An increased transcription was detected for IRF-1 and IRF-7 12h post-stimulation, being earlier than the transcription of Mx protein; however, IRF-1 and IRF-7 transcription decreased while the Mx protein was stable at 48h post-stimulation.

Anti-HIV state but not apoptosis depends on IFN signature in CD4+ T cells

To gain insights into the molecular mechanisms underlying early host responses to HIV in the CD4(+) T cell target population, we examined gene expression in CD4(+) T cells isolated 24 h after ex vivo HIV infection of lymphocyte aggregate cultures derived from human tonsils. Gene profiling showed a distinct up-regulation of genes related to immune response and response to virus, notably of IFN-stimulated genes (ISGs), irrespective of the coreceptor tropism of the virus. This mostly IFN-alpha-dependent gene signature suggested the involvement of plasmacytoid dendritic cells, a principal component of the antiviral immune response. Indeed, depletion of plasmacytoid dendritic cells before HIV inoculation abrogated transcriptional up-regulation of several ISGs and resulted in increased levels of HIV replication. Treatment with a blocking anti-IFN-alphaR Ab yielded increased HIV replication; conversely, HIV replication was decreased in pDC-depleted cultures treated with IFN-alpha. Among up-regulated ISGs was also TRAIL, indicating a potential role of the IFN signature in apoptosis. However, a blocking anti-TRAIL Ab did not abrogate apoptosis of CD4(+) T cells in CXCR4-tropic HIV-infected cultures, suggesting the involvement of pathways other than TRAIL mediated. We conclude that acute HIV infection of lymphoid tissue results in up-regulation of ISGs in CD4(+) T cells, which induces an anti-HIV state but not apoptosis.

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.

Characterization of the porcine alpha interferon multigene family

The availability of data on the pig genome sequence prompted us to characterize the porcine IFN-alpha (PoIFN-alpha) multigene family. Fourteen functional PoIFN-alpha genes and two PoIFN-alpha pseudogenes were detected in the porcine genome. Multiple sequence alignment revealed a C-terminal deletion of eight residues in six subtypes. A phylogenetic tree of the porcine IFN-alpha gene family defined the evolutionary relationship of the various subtypes. In addition, analysis of the evolutionary rate and the effect of positive selection suggested that the C-terminal deletion is a strategy for preservation in the genome. Eight PoIFN-alpha subtypes were isolated from the porcine liver genome and expressed in BHK-21 cells line. We detected the level of transcription by real-time quantitative RT-PCR analysis. The antiviral activities of the products were determined by WISH cells/Vesicular Stomatitis Virus (VSV) and PK 15 cells/Pseudorabies Virus (PRV) respectively. We found the antiviral activities of intact PoIFN-alpha genes are approximately 2-50 times higher than those of the subtypes with C-terminal deletions in WISH cells and 15-55 times higher in PK 15 cells. There was no obvious difference between the subtypes with and without C-terminal deletion on acid susceptibility.

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.

IRFs: master regulators of signalling by Toll-like receptors and cytosolic pattern-recognition receptors

The interferon-regulatory factor (IRF) family of transcription factors was initially found to be involved in the induction of genes that encode type I interferons. IRFs have now been shown to have functionally diverse roles in the regulation of the immune system. Recently, the crucial involvement of IRFs in innate and adaptive immune responses has been gaining much attention, particularly with the discovery of their role in immunoregulation by Toll-like receptors and other pattern-recognition receptors.

c-Jun N-terminal kinase negatively regulates dsRNA and RSV induction of tumor necrosis factor- alpha transcription in human epithelial cells

Secretion of inflammatory cytokines is the initial step of the immune response to viral infections. This innate immune response is mediated by the expression of a variety of cytokines, exemplified by tumor necrosis factor- alpha (TNF-alpha). The presence of dsRNA during viral infections is a key step in activation of several signaling pathways, including protein kinase R (PKR), toll-like receptor 3 (TLR3), mitogen-activated protein kinase (MAPK), activator protein-1 (AP-1), interferon regulatory factors (IRFs), and NF-kappaB pathways, which are all relevant in the expression of inflammatory cytokines. We previously reported that PKR and p38 MAPK were required for dsRNA and viral induction of inflammatory cytokines in epithelial cells. Here, we report that activation of c-Jun N-terminal kinase (JNK) during dsRNA treatment or respiratory syncytial viral (RSV) infection negatively regulates the induction of TNF-alpha in human epithelial cells. Inhibition of JNK by a pharmacologic inhibitor showed that expression of TNF-alpha increased following both dsRNA treatment and infection with RSV. Importantly, transfection of epithelial cells with a dominant-negative mutant of JNK significantly increased dsRNA induction of TNF-alpha. The mechanism by which JNK inhibition increases TNF-alpha induction appears to be through p38 MAPK activation. Our data show that JNK is a negative regulator of dsRNA and RSV induction of TNF-alpha expression and, thus, may act as a counterbalance to proinflammatory signals generated during viral infections.

Distinct functions of IRF-3 and IRF-7 in IFN-alpha gene regulation and control of anti-tumor activity in primary macrophages

Type I IFN (IFN-alpha/beta) have important biological functions ranging from immune cell development and activation, to tumor cell killing and most importantly inhibition of virus replication. Following viral infection or activation of Toll-like receptors (TLRs) via distinct ligands, IFN-alpha/beta are produced. Two members of the interferon regulatory factor (IRF) family - IRF-3 and IRF-7 - are the major modulators of IFN gene expression. Activation of IRF-3 and IRF-7 by TBK1/IKKvarepsilon mediated phosphorylation promotes IFN gene expression and potentiates the production of IFN responsive genes important to the development of an effective antiviral immune response. IFN treatment can augment anti-tumor properties and they are potentially key players in cancer therapy. For example, adoptive transfer of IFN-gamma-activated macrophages can mediate tumor cell killing via direct cell-cell contact, as well as release of soluble cytotoxic pro-inflammatory molecules. A recent study investigated whether IRF-3 and IRF-7 could mediate the acquisition of new anti-tumor effector functions in macrophages. Adenovirus mediated transduction of the active form of IRF-7 into primary macrophages resulted in the production of type I IFN, upregulation of target genes including TRAIL and increased tumoricidal activity of macrophages; in contrast, the active form of IRF-3 led to induction of cell death. These studies indicate that IRF-7 transduced macrophages may be an attractive candidate for in vivo adoptive therapy of cancer.

Colocalization of interferon regulatory factor 7 (IRF7) with latent membrane protein 1 (LMP1) of Epstein-Barr virus

Interferon regulatory factor 7 (IRF7) is one of the transcriptional factors for the activation of type I Interferon (IFN) genes. It is known that IRF7 and the latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV) are highly expressed in EBV type III latency cells, and LMP1 induces mRNA expression of IRF7. In this study, the expression pattern of endogenous IRF7 was observed in several B cell lines with or without EBV infection by immunofluorescence staining. IRF7 was localized in the cytoplasm of EBV-negative B cells and EBV type I latency B cell lines. However, IRF7 was located both in the cytoplasm and nucleus of EBV type III latency cell lines. In the Jijoye cell (type III latency cell), IRF7 was colocalized with LMP1 in the cytoplasm in a capping configuration, and their interaction was confirmed by co-immunoprecipitation of LMP1 and IRF7. This colocalization was confirmed by co-transfection of IRF7 and LMP1 plasmids in EBV-negative B cells. These results suggest that the IRF7 and LMP1 interact with each other, and this may relate to the mechanism whereby LMP1 exerts functional effects in B-lymphocytes.

The latent membrane protein 1 of Epstein-Barr virus (EBV) primes EBV latency cells for type I interferon production

Epstein-Barr virus (EBV) latency has been associated with a variety of human cancers. Latent membrane protein 1 (LMP-1) is one of the key viral proteins required for transformation of primary B cells in vitro and establishment of EBV latency. We have previously shown that LMP-1 induces the expression of several interferon (IFN)-stimulated genes and has antiviral effect (Zhang, J., Das, S. C., Kotalik, C., Pattnaik, A. K., and Zhang, L. (2004) J. Biol. Chem. 279, 46335-46342). In this report, a novel mechanism related to the antiviral effect of LMP-1 is identified. We show that EBV type III latency cells, in which LMP-1 is expressed, are primed to produce robust levels of endogenous IFNs upon infection of Sendai virus. The priming action is due to the expression of LMP-1 but not EBV nuclear antigen 2 (EBNA-2). The signaling events from the C-terminal activator regions of LMP-1 are essential to prime cells for high IFN production. LMP-1-mediated activation of NF-kappaB is apparently necessary and sufficient for LMP-1-mediated priming effect in DG75 cells, a human B cell line. IFN regulatory factor 7 (IRF-7) that can be activated by LMP-1 is also implicated in the priming action. Taken together, these data strongly suggest that LMP-1 may prime EBV latency cells for IFN production and that the antiviral property of LMP-1 may be an intrinsic part of EBV latency program, which may assist the establishment and/or maintenance of viral latency.

Negative regulation of the retinoic acid-inducible gene I-induced antiviral state by the ubiquitin-editing protein A20

Activation of the interferon regulatory factors (IRFs) 3 and 7 transcription factors is essential for the induction of type I interferon (IFN) and development of the innate antiviral response. Retinoic acid-inducible gene I has been shown to contribute to virus-induced IFN production independent of the Toll-like receptor pathways in response to a variety of RNA viruses and double-stranded RNA. In the present study, we demonstrate that the NF-kappaB-inducible, anti-apoptotic protein A20 efficiently blocks RIG-I-mediated activation of NF-kappaB-, IRF-3-, and IRF-7-dependent promoters but only weakly interferes with TRIF-TLR-3-mediated IFN activation. Expression of A20 completely blocked CARD domain containing DeltaRIG-I-induced IRF-3 Ser-396 phosphorylation, homodimerization, and DNA binding. The level of A20 inhibition was upstream of the TBK1/IKKepsilon kinases that phosphorylate IRF3 and IRF7 and paradoxically, A20 selectively degraded the TRIF protein but not RIG-I. A20 possesses two ubiquitin-editing domains, an N-terminal deubiquitination domain and a C-terminal ubiquitin ligase domain consisting of seven zinc finger domains. Deletion of the N-terminal de-ubiquitination domain had no significant effect on the inhibitory effect of A20, whereas deletion or mutation of zinc finger motif 7 ablated the inhibitory function of A20 on IRF- or NF-kappaB-mediated gene expression. Furthermore, cells stably expressing the active form of RIG-I induced an antiviral state that interfered with replication of vesicular stomatitis virus, an effect that was reversed by stable co-expression of A20. These results suggest that the virus-inducible, NF-kappaB-dependent activation of A20 functions as a negative regulator of RIG-I-mediated induction of the antiviral state.

Morphine withdrawal enhances hepatitis C virus replicon expression

We previously demonstrated that morphine enhances hepatitis C virus (HCV) replication in human hepatic cells. Here we describe the impact of morphine withdrawal (MW), a recurrent event during the course of opioid abuse, on HCV replicon expression in human hepatic cells. MW enhanced both viral RNA and protein expression in HCV replicon cells. Blocking opioid receptors by treatment with naloxone after morphine cessation (precipitated withdrawal, PW) induced greater HCV replicon expression than MW. Investigation of the mechanism responsible for MW- or PW-mediated HCV enhancement showed that both MW and PW inhibited the expression of endogenous interferon-alpha (IFN-alpha) in the hepatic cells. This down-regulation of intracellular IFN-alpha expression was due to the negative impact of MW or PW on IFN-alpha promoter activation and on the expression of IFN regulatory factor 7 (IRF-7), a strong transactivator of the IFN-alpha promoter. In addition, both MW and PW inhibited the anti-HCV ability of recombinant IFN-alpha in the hepatic cells. These in vitro observations support the concept that opioid abuse favors HCV persistence in hepatic cells by suppressing IFN-alpha-mediated intracellular innate immunity and contributes to the development of chronic HCV infection.