Transcriptional regulation of murine IL-33 by TLR and non-TLR agonists

IL-33, a member of the IL-1 family of cytokines, is produced by many cell types, including macrophages, yet its regulation is largely unknown. Treatment of primary murine macrophages with a panel of TLR (e.g., TLR2, TLR3, TLR4, and TLR9) agonists and non-TLR (e.g., MDA5, RIG-I) agonists revealed a pattern of gene and protein expression consistent with a role for IFN regulatory factor-3 (IRF-3) in the expression of IL-33. Accordingly, induction of IL-33 mRNA was attenuated in IRF-3(-/-) macrophages and TBK-1(-/-) mouse embryonic fibroblasts. Despite the fact that all IL-33 agonists were IRF-3 dependent, LPS-induced IL-33 mRNA was fully inducible in IFN-β(-/-) macrophages, indicating that IL-33 is not dependent on IFN-β as an intermediate. Epinephrine and Bordetella pertussis adenylate cyclase toxin (ACT), cAMP-activating agents, activate CREB and greatly synergize with LPS to induce IL-33 mRNA in macrophages. Both LPS-induced and ACT/LPS-enhanced expression of IL-33 mRNA was partially, but significantly, inhibited by the protein kinase A inhibitor H-89 but not by tyrosine kinase or protein kinase C inhibitors. Two IL-33 mRNA species derived from two alternative promoters encode full-length IL-33; however, the shorter "A" species is preferentially induced by all IL-33-inducing agonists except Newcastle disease virus, a RIG-I agonist that induced expression of both "A" and "B" transcripts. Together, these studies greatly extend what is currently known about the regulation of IL-33 induction in macrophages stimulated by bacterial and viral agonists that engage distinct innate immune signaling pathways.

Unique contribution of IRF-5-Ikaros axis to the B-cell IgG2a response

IRF-5 is a transcription factor activated by toll like receptor (TLR)7 and TLR9 during innate immune responses. IRF-5 activates not only Type I IFN, but also inflammatory cytokines. Most importantly, a genetic variation in the IRF-5 gene shows a strong association with autoimmune diseases such as Lupus. Here, we report that IRF5-deficient mice have attenuated IgG2a/c responses to T-cell-dependent and -independent antigens and to polyoma virus infection. This defect is due to the intrinsic deletion of IRF-5 in B cells, as SCID mice reconstituted with Irf5-/- B cells show a decrease in IgG2a/c expression after viral infection compared with mice that received wild-type B cells. Irf5-/-B cells in vitro have diminished TLR and cytokine-induced class switching to IgG2a/c. Addressing the molecular mechanism, we show that IRF-5 regulates IgG2a/c expression by decreasing Ikaros expression; reconstitution of IRF-5 in Irf5-/- B cells downregulates Ikaros levels and increases switching to IgG2a/c. The IRF site in ikzf1 promoter binds IRF-5, IRF-4 and IRF-8. We show that IRF-8 but not IRF-4 activates the ikzf1 promoter, and IRF-5 inhibits the transcriptional activity of IRF-8. Collectively, these results identify the IRF-5-Ikaros axis as a critical modulator of IgG2a/c class switching.

Kaposi sarcoma-associated herpesvirus vIRF-3 protein binds to F-box of Skp2 protein and acts as a regulator of c-Myc protein function and stability

The Kaposi sarcoma-associated herpesvirus (KSHV) has been linked to Kaposi sarcoma, body cavity-based lymphoma, and Castleman disease. vIRF-3 is a KSHV latent gene that is critical for proliferation of KSHV-positive lymphoid cells. Furthermore, vIRF-3 contributes to KSHV-associated pathogenesis by stimulating c-Myc transcription activity. Here we show that vIRF-3 can associate with Skp2, a key component of the SCF(skp2) ubiquitin ligase complex. Skp2 is a transcriptional co-factor for c-Myc that was shown to regulate the stability of c-Myc protein as well as c-Myc-dependent transcription. In this study, we show that vIRF-3 binds to the F-box of Skp2 and recruits it to c-Myc-regulated promoters to activate c-Myc-dependent transcription. Additionally, cells overexpressing vIRF-3 exhibit higher levels of c-Myc ubiquitylation, suggesting that ubiquitylation is necessary for c-Myc-mediated transcription. Moreover, vIRF-3 can stabilize the c-Myc protein by increasing its half-life. Collectively, these results indicate that vIRF-3 can effectively manipulate c-Myc stability and function and thus contribute to c-Myc-induced KSHV-associated lymphomagenesis.

Distinct regulation of murine lupus susceptibility genes by the IRF5/Blimp-1 axis

Genome-wide association studies have identified lupus susceptibility genes such as IRF5 and PRDM1 (encoding for IFN regulatory factor 5 [IRF]5 and Blimp-1) in the human genome. Accordingly, the murine Irf5 and Prdm1 genes have been shown to play a role in lupus susceptibility. However, it remains unclear how IRF5 and Blimp-1 (a transcriptional target of IRF5) contribute to lupus susceptibility. Given that the murine lupus susceptibility locus Nba2 includes the IFN-regulated genes Ifi202 (encoding for the p202 protein), Aim2 (encoding for the Aim2 protein), and Fcgr2b (encoding for the FcγRIIB receptor), we investigated whether the IRF5/Blimp-1 axis could regulate the expression of these genes. We found that an Irf5 deficiency in mice decreased the expression of Blimp-1 and reduced the expression of the Ifi202. However, the deficiency increased the expression of Aim2 and Fcgr2b. Correspondingly, increased expression of IRF5 in cells increased levels of Blimp-1 and p202 protein. Moreover, Blimp-1 expression increased the expression of Ifi202, whereas it reduced the expression of Aim2. Interestingly, an Aim2 deficiency in female mice increased the expression of IRF5. Similarly, the Fcgr2b-deficient mice expressed increased levels of IRF5. Moreover, increased expression of IRF5 and Blimp-1 in lupus-prone C57BL/6.Nba2, New Zealand Black, and C57BL/6.Sle123 female mice (as compared with age-matched C57BL/6 female mice) was associated with increased levels of the p202 protein. Taken together, our observations demonstrate that the IRF5/Blimp-1 axis differentially regulates the expression of Nba2 lupus susceptibility genes, and they suggest an important role for the IRF5/Blimp-1/p202 axis in murine lupus susceptibility.

Antiviral activity of innate immune protein ISG15

The host innate immune response, including the production of type-I IFN, represents the primary line of defense against invading viral pathogens. Of the hundreds of IFN-stimulated genes (ISGs) discovered to date, ISG15 was one of the first identified and shown to encode a ubiquitin-like protein that functions, in part, as a modifier of protein function. Evidence implicating ISG15 as an innate immune protein with broad-spectrum antiviral activity continues to accumulate rapidly. This review will summarize recent findings on the innate antiviral activity of ISG15, with a focus on the interplay between ubiquitination and ISGylation pathways resulting in modulation of RNA virus assembly/budding. Indeed, ubiquitination is known to be proviral for some RNA viruses, whereas the parallel ISGylation pathway is known to be antiviral. A better understanding of the antiviral activities of ISG15 will enhance our fundamental knowledge of host innate responses to viral pathogens and may provide insight useful for the development of novel therapeutic approaches designed to enhance the immune response against such pathogens.

Interferon Targeted Genes in Host Defense

Type I Interferons (IFN) induce the expression of IFN-stimulated genes (ISG). The products of some of these genes have direct antiviral effects, others are involved in immunoregulation or modulate signaling pathways and gene expression, and others yet are mediators of cell growth and death. Their role in autoimmune diseases has been found to be both beneficial and detrimental.

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.

HIV-1 accessory proteins VPR and Vif modulate antiviral response by targeting IRF-3 for degradation

The activation of IRF-3 during the early stages of viral infection is critical for the initiation of the antiviral response; however the activation of IRF-3 in HIV-1 infected cells has not yet been characterized. We demonstrate that the early steps of HIV-1 infection do not lead to the activation and nuclear translocation of IRF-3; instead, the relative levels of IRF-3 protein are decreased due to the ubiquitin-associated proteosome degradation. Addressing the molecular mechanism of this effect we show that the degradation is independent of HIV-1 replication and that virion-associated accessory proteins Vif and Vpr can independently degrade IRF-3. The null mutation of these two genes reduced the capacity of the HIV-1 virus to down modulate IRF-3 levels. The degradation was associated with Vif- and Vpr-mediated ubiquitination of IRF-3 and was independent of the activation of IRF-3. N-terminal lysine residues were shown to play a critical role in the Vif- and Vpr-mediated degradation of IRF-3. These data implicate Vif and Vpr in the disruption of the initial antiviral response and point to the need of HIV-1 to circumvent the antiviral response during the very early phase of replication.

Type I interferon: the ever unfolding story

Since the discovery of interferon 50 years ago, the understanding of the mechanism of the virus-mediated induction of type I IFN and its function has been under intensive investigation. Remarkable progress has been made in recent years both in the identification of cellular receptors detecting the viral infection and in the understanding the signaling pathways resulting in the induction of interferon and interferon-induced genes. In this review of type I interferon, we aim to summarize not only the historical site of the interferon induction and its antiviral function, but also the complexity of the signals that lead to activation of expression of interferon genes and the expanding repertoire of this multifunctional protein.

Stimulation of c-Myc transcriptional activity by vIRF-3 of Kaposi sarcoma-associated herpesvirus

Kaposi sarcoma-associated herpesvirus is associated with two lymphoproliferative disorders, primary effusion lymphoma (PEL) and Castleman disease. In PEL, Kaposi sarcoma-associated herpesvirus is present in a latent form expressing only few viral genes. Among them is a viral homologue of cellular interferon regulatory factors, vIRF-3. To study the role of vIRF-3 in PEL lymphomagenesis, we analyzed the interaction of vIRF-3 with cellular proteins. Using yeast two-hybrid screen, we detected the association between vIRF-3 and c-Myc suppressor, MM-1alpha. The vIRF-3 and MM-1alpha interaction was also demonstrated by glutathione S-transferase pulldown assay and coimmunoprecipitation of endogenous vIRF-3 and MM-1alpha in PEL-derived cell lines. Overexpression of vIRF-3 enhanced the c-Myc-dependent transcription of the gene cdk4. Addressing the molecular mechanism of the vIRF-3-mediated stimulation, we demonstrated that the association between MM-1alpha and c-Myc was inhibited by vIRF-3. Furthermore, the recruitment of vIRF-3 to the cdk4 promoter and the elevated levels of the histone H3 acetylation suggest the direct involvement of vIRF-3 in the activation of c-Myc-mediated transcription. These findings indicate that vIRF-3 can effectively stimulate c-Myc function in PEL cells and consequently contribute to de-regulation of B-cell growth and differentiation.

Viral defense, carcinogenesis and ISG15: novel roles for an old ISG

Recent studies have established that type I interferon modulates expression of large number of cellular genes. While the proteins encoded by some of these genes have a direct antiviral activity, the functions of the majority of the others have not yet been determined. One of the first identified IFN stimulated gene, encodes ubiquitin like protein ISG15 that is also expressed in response to different stress stimuli. Although it was shown that ISG15 functions as protein modifier, it has been only recently that the targets of ISG15 conjugation were identified. Recent studies have also revealed mechanism of ISG15 conjugation and its interaction with the ubiquitin conjugation pathway. This review is focused on the possible role of ISG15 in the antiviral response, regulation of cell growth and carcinogenesis.

Pathogenic viruses: smart manipulators of the interferon system

Vertebrate cells are equipped with specialized receptors that sense the presence of viral nucleic acids and other conserved molecular signatures of infecting viruses. These sensing receptors are collectively called pattern recognition receptors (PRRs) and trigger the production of type I (alpha/beta) interferons (IFNs). IFNs are secreted and establish a local and systemic antiviral state in responsive cells. Viruses, in turn, have evolved multiple strategies to escape the IFN system. They try to avoid PRR activation, inhibit IFN synthesis, bind and inactivate secreted IFN molecules, block IFN-activated signaling, or disturb the action of IFN-induced antiviral proteins. Here, we summarize current knowledge in light of most recent findings on the intricate interactions of viruses with the IFN system.

Beyond double-stranded RNA-type I IFN induction by 3pRNA and other viral nucleic acids

Production of type I IFN is the key response to viral infection. Since the discovery of type I IFNs in 1957, long double-stranded RNA formed during replication of many viruses was thought to be responsible for type I IFN induction, and for decades double-stranded RNA-activated protein kinase (PKR) was thought to be the receptor. Recently, this picture has dramatically changed. It now became evident that not PKR but two members of the Toll-like receptor (TLR) family, TLR7 and TLR9, and two cytosolic helicases, RIG-I and MDA-5, are responsible for the majority of type I IFNs induced upon recognition of viral nucleic acids. In this review, we focus on the molecular mechanisms by which those innate immune receptors detect viral infection. Based on the recent progress in the field, we now know that TLR7, TLR9, and RIG-I do not require long double-stranded RNA for type I IFN induction.

ISG15 enhances the innate antiviral response by inhibition of IRF-3 degradation

The transcription factor, interferon regulatory Factor 3 (IRF-3) plays a critical role in the activation of an antiviral innate immune response. However the transcriptional activity of IRF-3 is tightly regulated by a proteosome mediated degradation. We describe here a novel mechanism by which the activity of IRF-3 is stabilized in infected cells. We have shown that both interferon treatment and NDV infection profoundly increase conjugation of interferon induced ubiquitin- like protein ISG15 to cellular proteins. ISGylated IRF-3 could be detected both in interferon treated and virus-infected cells. ISG15, subverts the ubiquitin mediated degradation of IRF-3 in NDV infected 2fTGH cells and enhances the NDV mediated transactivation of interferonbeta promoter and the translocation of activated IRF-3 to the nucleus. The relative levels of IRF-3 were significantly lower in NDV infected ISG15 null MEF, than in wt MEF. While ISG15 null MEF were more permissive to VSV replication their sensitivity to the antiviral effect of interferon was not modulated. These results reveal that virus mediated subversion of the antiviral response by proteolysis of IRF-3 is counteracted by induction of ISG15 expression and that ISGylation provides a feedback mechanism, which enhances the host innate antiviral response via IRF-3 stabilization.

Innate antiviral response targets HIV-1 release by the induction of ubiquitin-like protein ISG15

The goal of this study was to elucidate the molecular mechanism by which type I IFN inhibits assembly and release of HIV-1 virions. Our study revealed that the IFN-induced ubiquitin-like protein ISG15 mimics the IFN effect and inhibits release of HIV-1 virions without having any effect on the synthesis of HIV-1 proteins in the cells. ISG15 expression specifically inhibited ubiquitination of Gag and Tsg101 and disrupted the interaction of the Gag L domain with Tsg101, but conjugation of ISG15 to Gag or Tsg101 was not detected. The inhibition of Gag-Tsg101 interaction was also detected in HIV-1 infected, IFN-treated cells. Elimination of ISG15 expression by small interfering RNA reversed the IFN-mediated inhibition of HIV-1 replication and release of virions. These results indicated a critical role for ISG15 in the IFN-mediated inhibition of late stages of HIV-1 assembly and release and pointed to a mechanism by which the innate antiviral response targets the cellular endosomal trafficking pathway used by HIV-1 to exit the cell. Identification of ISG15 as the critical component in IFN-mediated inhibition of HIV-1 release advances the understanding of the IFN-mediated inhibition of HIV-1 replication and uncovers a target for the anti HIV-1 therapy.

Interferon research: a brief history

Interferons are the antiviral early inflammatory proteins produced in the cells in response to the infectious agents. The characterization of the interferon genes, their expression, and their function was advanced with the development of novel techniques in molecular and cellular biology. Using genetically modified mice revealed the critical role of the interferons in innate and acquired immune response. The critical steps and discovery that lead to the understanding of the interferon system and its role in the antiviral immune response are summarized in this chapter.

The Interferon Regulatory Factor, IRF5, Is a Central Mediator of Toll-like Receptor 7 Signaling

Interferon regulatory factors (IRFs) are critical components of virus-induced immune activation and type I interferon regulation. IRF3 and IRF7 are activated in response to a variety of viruses or after engagement of Toll-like receptor (TLR) 3 and TLR4 by double-stranded RNA and lipopolysaccharide, respectively. The activation of IRF5, is much more restricted. Here we show that in contrast to IRF3 and IRF7, IRF5 is not a target of the TLR3 signaling pathway but is activated by TLR7 or TLR8 signaling. We also demonstrate that MyD88, interleukin 1 receptor-associated kinase 1, and tumor necrosis factor receptor-associated factor 6 are required for the activation of IRF5 and IRF7 in the TLR7 signaling pathway. Moreover, ectopic expression of IRF5 enabled type I interferon production in response to TLR7 signaling, whereas knockdown of IRF5 by small interfering RNA reduced type I interferon induction in response to the TLR7 ligand, R-848. IRF5 and IRF7, therefore, emerge from these studies as critical mediators of TLR7 signaling.

Two discrete promoters regulate the alternatively spliced human interferon regulatory factor-5 isoforms. Multiple isoforms with distinct cell type-specific expression, localization, regulation, and function

Interferon regulatory factor-5 (IRF-5) is a mediator of virus-induced immune activation and type I interferon (IFN) gene regulation. In human primary plasmacytoid dendritic cells (PDC), IRF-5 is transcribed into four distinct alternatively spliced isoforms (V1, V2, V3, and V4), whereas in human primary peripheral blood mononuclear cells two additional new isoforms (V5 and V6) were identified. The IRF-5 V1, V2, and V3 transcripts have different noncoding first exons and distinct insertion/deletion patterns in exon 6. Here we showed that V1 and V3 have distinct transcription start sites and are regulated by two discrete promoters. The V1 promoter (P-V1) is constitutively active, contains an IRF-E consensus-binding site, and is further stimulated in virus-infected cells by IRF family members. In contrast, endogenous V3 transcripts were up-regulated by type I IFNs, and the V3 promoter (P-V3) contains an IFN-stimulated responsive element-binding site that confers responsiveness to IFN through binding of the ISGF3 complex. In addition to V5 and V6, we have identified three more alternatively spliced IRF-5 isoforms (V7, V8, and V9); V5 and V6 were expressed in peripheral blood mononuclear cells from healthy donors and in immortalized B and T cell malignancies, whereas expression of V7, V8, and V9 transcripts were detected only in human cancers. The results of this study demonstrated the existence of multiple IRF-5 spliced isoforms with distinct cell type-specific expression, cellular localization, differential regulation, and dissimilar functions in virus-mediated type I IFN gene induction.

Global and distinct targets of IRF-5 and IRF-7 during innate response to viral infection

The interferon regulatory factors (IRF) are transcriptional mediators of cellular response to viral invasion that play a critical role in the innate antiviral defense. Two of these factors, IRF-5 and IRF-7, play a critical role in the induction of interferon (IFNA) genes in infected cells; they are expressed constitutively in monocytes, B cells, and precursors of dendritic cells (pDC2) that are high producers of interferon alpha, and their expression can be further stimulated by type I interferon. The goal of the present study was to identify and analyze expression of cellular genes that are modulated by IRF-5 and IRF-7 during the innate response to viral infection. The transcription profiles of infected BJAB cells overexpressing IRF-5 or IRF-7 were determined by using oligonucleotide arrays with probe sets representing about 6800 human genes. This analysis shows that IRF-5 and IRF-7 activate a broad profile of heterologous genes encoding not only antiviral, inflammatory, and pro-apoptotic proteins but also proteins of other functional categories. The number of IRF-5- and IRF-7-modulated genes was significantly higher in infected than in uninfected cells, and the transcription signature was predominantly positive. Although IRF-5 and IRF-7 stimulated a large number of common genes, a distinct functional profile was associated with each of these IRFs. The noted difference was a broad antiviral and early inflammatory transcriptional profile in infected BJAB/IRF-5 cells, whereas the IRF-7-induced transcripts were enriched for the group of mitochondrial genes and genes affecting the DNA structure. Taken together, these data indicate that IRF-5 and IRF-7 act primarily as transcriptional activators and that IRF-5-and IRF-7-induced innate antiviral response results in a broad alteration of the transcriptional profile of cellular genes.

Cutting edge: activation of Toll-like receptor 2 induces a Th2 immune response and promotes experimental asthma

Recognition of microbial components by APCs and their activation through Toll-like receptors (TLR) leads to the induction of adaptive immune responses. In this study, we show that activation of TLR2 by its synthetic ligand Pam3Cys, in contrast to activation of TLR9 by immunostimulatory DNA (ISS-ODN), induces a prominent Th2-biased immune response. Activation of APCs by Pam3Cys resulted in the induction of Th2-associated effector molecules like IL-13, and IL-1beta, GM-CSF and up-regulation of B7RP-1, but low levels of Th1-associated cytokines (IL-12, IFNalpha, IL-18, IL-27). Accordingly, TLR2 ligands aggravated experimental asthma. These data indicate that the type of TLR stimulation during the initial phase of immune activation determines the polarization of the adaptive immune response and may play a role in the initiation of Th2-mediated immune disorders, such as asthma.

Kaposi's sarcoma-associated herpesvirus-encoded vIRF-3 stimulates the transcriptional activity of cellular IRF-3 and IRF-7

Kaposi's sarcoma-associated herpesvirus has been linked to Kaposi's sarcoma, body cavity-based lymphoma, and Castleman's disease. The Kaposi's sarcoma-associated herpesvirus genome contains a cluster of open reading frames encoding proteins (vIRFs) with homology to the cellular transcription factors of the interferon regulatory factor family. vIRF-3, also called LANA2, is a latently expressed nuclear protein. Here we demonstrate that vIRF-3 directly interacts with cellular interferon regulatory factor (IRF) IRF-3, IRF-7, and the transcriptional co-activator CBP/p300. The mapping of the vIRF-3 binding domain revealed that vIRF-3 associates with both IRF-3 and IRF-7 through its C-terminal region. The p300 domain, which interacts with vIRF-3, is distinct from the previously identified IBiD domain, to which both vIRF-1 and IRF-3 bind. Thus, in contrast to vIRF-1, vIRF-3 neither blocks the interaction between IRF-3 and p300 nor inhibits the histone acetylation. Although vIRF-3 is not a DNA-binding protein, it is recruited to the IFNA promoters via its interaction with IRF-3 and IRF-7. The presence of vIRF-3 in the enhanceosome assembled on the IFNA promoters increases binding of IRF-3, IRF-7, and acetylated histone H3 to this promoter region. Consequently, vIRF-3 stimulates the IRF-3- and IRF-7-mediated activation of type I interferon (IFNA and IFNB) genes and the synthesis of biologically active type I interferons in infected B cells. These studies illustrate that vIRF-3 and vIRF-1 have clearly distinct functions. In addition to its co-repressor activity, vIRF-3 can also act as a transcriptional activator on genes controlled by cellular IRF-3 and IRF-7.

Comparative analysis of IRF and IFN-alpha expression in human plasmacytoid and monocyte-derived dendritic cells

Plasmacytoid dendritic cells (PDC) produce high levels of type I IFN upon stimulation with viruses, while monocytes and monocyte-derived dendritic cells (MDDC) produce significantly lower levels. To find what determines the high production of type I IFN in PDC, we examined the relative levels of IRF transcription factors, some of which play critical roles in the induction of IFN. Furthermore, to determine whether the differences could result from expression of distinct IFNA subtypes, the profile of IFNA genes expressed was examined. PDC responded equally well to stimulation with HSV-1 and Sendai virus (SV) by producing high levels of type I IFN, whereas the MDDC and monocyte response to SV were lower, and neither responded well to HSV-1. All three populations constitutively expressed most of the IRF genes. However, real-time RT-PCR demonstrated increased levels of IRF-7 transcripts in PDC compared with monocytes. As determined by intracellular flow cytometry, the PDC constitutively expressed significantly higher levels of IRF-7 protein than the other populations while IRF-3 levels were similar among populations. Analysis of the profile of IFNA genes expressed in virus-stimulated PDC, monocytes and MDDC demonstrated that each population expressed IFNA1 as the major subtype but that the range of the subtypes expressed in PDC was broader, with some donor and stimulus-dependent variability. We conclude that PDC but not MDDC are uniquely preprogrammed to respond rapidly and effectively to a range of viral pathogens with high levels of IFN-alpha production due to the high levels of constitutively expressed IRF-7.

Interferon regulatory factor 5, a novel mediator of cell cycle arrest and cell death

We have previously shown a critical role for IFN regulatory factor 5 (IRF-5) in the innate immune response to virus infection. For the first time, we now show that although IRF-5 is a direct target of p53, its cell cycle regulatory and proapoptotic effects are p53 independent. IRF-5 inhibits both in vitro and in vivo B-cell lymphoma tumor growth in the absence of wild-type p53. The molecular mechanism(s) of IRF-5-mediated growth inhibition is associated with a G(2)-M cell cycle arrest and modulation of growth regulatory and proapoptotic genes, including p21, Bak, DAP kinase 2, and Bax. Taken together, these data indicate that although IRF-5 is a downstream target of p53, its growth inhibitory and proapoptotic effects are independent of p53.

LPS-TLR4 signaling to IRF-3/7 and NF-kappaB involves the toll adapters TRAM and TRIF

Toll–IL-1–resistance (TIR) domain–containing adaptor-inducing IFN-β (TRIF)–related adaptor molecule (TRAM) is the fourth TIR domain–containing adaptor protein to be described that participates in Toll receptor signaling. Like TRIF, TRAM activates interferon regulatory factor (IRF)-3, IRF-7, and NF-κB-dependent signaling pathways. Toll-like receptor (TLR)3 and 4 activate these pathways to induce IFN-α/β, regulated on activation, normal T cell expressed and secreted (RANTES), and γ interferon–inducible protein 10 (IP-10) expression independently of the adaptor protein myeloid differentiation factor 88 (MyD88). Dominant negative and siRNA studies performed here demonstrate that TRIF functions downstream of both the TLR3 (dsRNA) and TLR4 (LPS) signaling pathways, whereas the function of TRAM is restricted to the TLR4 pathway. TRAM interacts with TRIF, MyD88 adaptor–like protein (Mal)/TIRAP, and TLR4 but not with TLR3. These studies suggest that TRIF and TRAM both function in LPS-TLR4 signaling to regulate the MyD88-independent pathway during the innate immune response to LPS.

Molecular basis for the immunostimulatory activity of guanine nucleoside analogs: activation of Toll-like receptor 7

Certain C8-substituted and N7, C8-disubstituted guanine ribonucleosides comprise a class of small molecules with immunostimulatory activity. In a variety of animal models, these agents stimulate both humoral and cellular immune responses. The antiviral actions of these guanosine analogs have been attributed to their ability to induce type I IFNs. However, the molecular mechanisms by which the guanosine analogs potentiate immune responses are not known. Here, we report that several guanosine analogs activate Toll-like receptor 7 (TLR7). 7-Thia-8-oxoguanosine, 7-deazaguanosine, and related guanosine analogs activated mouse immune cells in a manner analogous to known TLR ligands, inducing cytokine production in mouse splenocytes (IL-6 and IL-12, type I and II IFNs), bone marrow-derived macrophages (IL-6 and IL-12), and in human peripheral blood leukocytes (type I IFNs, tumor necrosis factor alpha and IL-12). The guanosine congeners also up-regulated costimulatory molecules and MHC I/II in dendritic cells. Genetic complementation studies in human embryonic kidney 293 cells confirmed that the guanosine analogs activate cells exclusively via TLR7. The stimulation of TLR7 by the guanosine analogs in human cells appears to require endosomal maturation because inhibition of this process with chloroquine significantly reduced the downstream activation of NF-kappaB. However, TLR8 activation by R-848 and TLR2 activation by [S-[2,3-bis(palmitoyloxy)-(2-RS)-propyl]-N-palmitoyl-R-Cys-S-Ser-Lys4-OH, trihydrochloride)] were not inhibited by chloroquine, whereas TLR9 activation by CpG oligodeoxynucleotides was abolished. In summary, we present evidence that guanosine analogs activate immune cells via TLR7 by a pathway that requires endosomal maturation. Thus, the B cell-stimulating and antiviral activities of the guanosine analogs may be explained by their TLR7-activating capacity.

Multiple regulatory domains of IRF-5 control activation, cellular localization, and induction of chemokines that mediate recruitment of T lymphocytes

Transcription factors of the interferon regulatory factor (IRF) family have been identified as critical mediators of early inflammatory gene transcription in infected cells. We recently determined that, besides IRF-3 and IRF-7, IRF-5 serves as a direct transducer of virus-mediated signaling. In contrast to that mediated by the other two IRFs, IRF-5-mediated activation is virus specific. We show that, in addition to Newcastle disease virus (NDV) infection, vesicular stomatitis virus (VSV) and herpes simplex virus type 1 (HSV-1) infection activates IRF-5, leading to the induction of IFNA gene subtypes that are distinct from subtypes induced by NDV. The IRF-5-mediated stimulation of inflammatory genes is not limited to IFNA since in BJAB/IRF-5-expressing cells IRF-5 stimulates transcription of RANTES, macrophage inflammatory protein 1 beta, monocyte chemotactic protein 1, interleukin-8, and I-309 genes in a virus-specific manner. By transient- transfection assay, we identified constitutive-activation (amino acids [aa] 410 to 489) and autoinhibitory (aa 490 to 539) domains in the IRF-5 polypeptide. We identified functional nuclear localization signals (NLS) in the amino and carboxyl termini of IRF-5 and showed that both of these NLS are sufficient for nuclear translocation and retention in infected cells. Furthermore, we demonstrated that serine residues 477 and 480 play critical roles in the response to NDV infection. Mutation of these residues from serine to alanine dramatically decreased phosphorylation and resulted in a substantial loss of IRF-5 transactivation in infected cells. Thus, this study defines the regulatory phosphorylation sites that control the activity of IRF-5 in NDV-infected cells and provides further insight into the structure and function of IRF-5. It also shows that the range of IRF-5 immunoregulatory target genes includes members of the cytokine and chemokine superfamilies.

Regulation of DNA-raised immune responses by cotransfected interferon regulatory factors

Interferon regulatory factor 1 (IRF-1), IRF-3, and IRF-7 have been tested as genetic adjuvants for influenza virus hemagglutinin (HA) and nucleoprotein vaccine DNAs. Cotransfection of HA with IRF-3 and IRF-7 increased CD4 T-cell responses by 2- to 4-fold and CD8 T-cell responses by more than 10-fold. Following intramuscular deliveries of DNA, both CD4 and CD8 T cells were biased towards type 1 immune responses and the production of gamma interferon. Following gene gun bombardments of DNA, both were biased towards type 2 immune responses and the production of interleukin-4. The biases of the T-cell responses towards type 1 or type 2 were stronger for immunizations with IRF-3 as an adjuvant than for immunizations with IRF-7 as an adjuvant. Moderate adjuvant effects for antibody were observed. The isotypes of the antibody responses reflected the method of DNA delivery; intramuscular deliveries of DNA predominantly raised immunoglobulin G2a (IgG2a), whereas gene gun deliveries of DNA predominantly raised IgG1. These biases were enhanced by the codelivered IRFs. Overall, under the conditions of our experiments, IRF-3 had good activity for T cells, IRF-7 had good activity for both antibody and T cells, and IRF-1 had good activity for antibody.

Stimulation of IRF-7 gene expression by tumor necrosis factor alpha: requirement for NFkappa B transcription factor and gene accessibility

Interferon regulatory factor 7 (IRF-7) plays an important role in innate immunity, where, together with IRF-3, it controls the expression of interferon A/B genes as well as chemokine RANTES (regulated on activation normal T cell expressed and secreted). Previously, we characterized human IRF-7 promoter and showed that an interferon-stimulated response element site in the first intron binds interferon-stimulated gene factor 3 (ISGF3) and confers the response to interferon. Here we report the stimulation of IRF-7 expression by 12-O-tetradecanoylphorbol-13-acetate (TPA) and tumor necrosis factor alpha (TNFalpha) in human peripheral blood monocytes. Using promoter analysis in combination with electrophoretic mobility shift assays, we have demonstrated that an NFkappaB site located next to the TATA box, binds p50 and p65 heterodimer and is required for the induction of the IRF-7 gene by TPA and TNFalpha. In addition, we report stimulation of IRF-7 gene expression by topoisomerase II (TOPII) inhibitors. We show by chromatin immunoprecipitation assay that treatment with the TOPII inhibitor etoposide induces association of acetylated histone 3 with the promoter of IRF-7 gene, indicating that TOPII-mediated changes in chromatin structure could be responsible for the induction. This suggests that the IRF-7 gene is localized in the condensed area of the chromosome where it is inaccessible to transcription factors that would promote its constitutive expression.

On the role of IRF in host defense

Transcription factors of the interferon (IFN) regulatory factor (IRF) family have been shown to play an essential role in the regulated expression of type I IFN genes, IFN-stimulated genes (ISG), and other cytokines and chemokines. Three members of the IRF family, IRF-3, IRF-5, and IRF-7, have been identified as acting as direct transducers of virus-mediated signaling. In infected cells, these factors are activated by phosphorylation on the serine residues, transported to the nucleus, where they bind to the promoters of IFNA and IFNB genes and tether histone transacetylases to the transcription complex enhanceosome. IFNB and IFNA subtypes are expressed at different levels in infected cells. The ratio between the relative levels of IRF-3 and IRF-7 was shown to play an essential role in the inducible expression of type I IFN genes, whereas IRF-3 alone is sufficient for expression of the IFNB gene. IRF-5 was identified recently as another inducer of IFNA genes, which has two unique properties: (1) its activation is virus specific, and (2) the profile of IFNA genes induced by IRF-5 is distinct from that induced by IRF-7. Several viruses target functions of IRF to eliminate the early inflammatory response. Kaposi's sarcoma herpesvirus (KSHV) encodes a cluster of four genes with homology to cellular IRF. Three of these vIRF were shown to inhibit induction of IFN genes and ISG in infected cells and function as dominant negative mutants of cellular IRF. The unique properties of previously uncharacterized vIRF-2 and vIRF-3 are discussed.

Monocyte differentiation to macrophage requires interferon regulatory factor 7

Interferon regulatory factors are a growing family of transcription factor that have been implicated in cellular events such as cell-growth regulation, antiviral defense, and development of the immune system. Interferon regulatory factor 7 (IRF-7) is expressed predominantly in lymphoid tissues and has been studied extensively in the context of viral infection and the induction of interferon and cytokine gene expression. In this paper, the involvement of IRF-7 in monocyte differentiation was examined in U937, HL60, and human primary macrophages. We report the induction of IRF-7 expression by 12-O-tetradecanoylphorbol-13-acetate in U937 and HL60 cells and demonstrate that this induction is essential for the monocyte differentiation to macrophages. We show that the monocyte differentiation is inhibited in cells expressing a dominant negative IRF-7 mutant, as evidenced by decreased expression of two macrophage-differentiation markers, CD11b and CD11c, and impaired phagocytic activity. In addition, we demonstrate that overexpression of IRF-7 is sufficient to trigger monocyte differentiation and to induce cell cycle arrest. The identification of IRF-7 as a key regulator in monocyte differentiation suggests a novel function of IRF-7 in innate immunity.

Expression of human and macaque type I IFN transgenes interferes with HSV-1 replication at the transcriptional and translational levels: IFN-beta is more potent than IFN-alpha 2

A study was undertaken to compare the efficacy of plasmid constructs encoding human IFN-alpha 2 and IFN-beta and macaque IFN-beta against herpes simplex virus type 1 in transfected cells. All type I IFN transgenes significantly reduced viral titers in transfected cells by 3 logs. Human IFN-alpha 2-transfected cells produced significantly more IFN (2274 pg/ml) in comparison to IFN-beta-transfected cells (134-165 pg/ml). Viral lytic gene transcript and viral protein levels were lower in IFN-beta- versus IFN-alpha 2-transfected cells, which coincided with elevated PKR and OAS transcript levels and increased total STAT1 and phosphorylated STAT1 (Y701) protein levels in the IFN-beta-transfected cells. Although comparable viral titers were recovered in IFN-alpha 2 and IFN-beta plasmid-transfected cells, IFN-alpha 2 plasmid-transfected cells exhibited significantly more cytopathic effect compared to the IFN-beta transgene-transfected cells. In addition, IFN-alpha 2 transgene-transfected, infected cells displayed a cell cycle profile similar to that of vector-transfected, infected cells, whereas IFN-beta plasmid-transfected cells displayed a profile similar to uninfected control. Collectively, the results indicate that human IFN-beta is superior to IFN-alpha 2 in antagonizing herpes simplex virus type 1 infection.

Recruitment of multiple interferon regulatory factors and histone acetyltransferase to the transcriptionally active interferon a promoters

Type I interferon (IFN) plays a critical role in the innate immunity against viral infection. Expression of IFNA genes in infected cells is cell type-dependent and is regulated at the transcriptional level. The present study is focused on the molecular mechanism underlying the differential expression of human IFNA1 and A2 genes. Two nucleotides, at positions -98 and -81 of IFNA1 and A2 promoter, were pivotal to the differential expression. The DNA pull-down and chromatin precipitation assays have shown that nuclear interferon regulatory factor (IRF)-3 and IRF-7 as well as IRF-1 bind to IFNA1 virus-responsive element (VRE). Interestingly, overexpression of IRF-7 increased the otherwise weak binding of both IRF-3 and IRF-7 to IFNA2 VRE. These data together with the results of two-step chromatin immunoprecipitation strongly suggest that the IRF-3 and IRF-7 bind to IFNA1 promoter as a dimer. Furthermore, binding of IRF-3 and IRF-7 to IFNA VRE is associated with the presence of acetylated histone H3, suggesting that histone acetyltransferase(s) is tethered together with virus-activated IRF-3 and IRF-7 to the IFNA1 promoter. In addition, the constitutively active IRF-3 (5D) and IRF-7 (2D) mutants activate the endogenous IFNA genes in uninfected cells; however, the expression profile of IFNA is not identical to that induced by viral infection.

Interferon-kappa, a novel type I interferon expressed in human keratinocytes

High throughput cDNA sequencing has led to the identification of interferon-kappa, a novel subclass of type I interferon that displays approximately 30% homology to other family members. Interferon-kappa consists of 207 amino acids, including a 27-amino acid signal peptide and a series of cysteines conserved in type I interferons. The gene encoding interferon-kappa is located on the short arm of chromosome 9 adjacent to the type I interferon gene cluster and is selectively expressed in epidermal keratinocytes. Expression of interferon-kappa is significantly enhanced in keratinocytes upon viral infection, upon exposure to double-stranded RNA, or upon treatment with either interferon-gamma or interferon-beta. Administration of interferon-kappa recombinant protein imparts cellular protection against viral infection in a species-specific manner. Interferon-kappa activates the interferon-stimulated response element signaling pathway and a panel of genes similar to those regulated by other type I interferons including anti-viral mediators and transcriptional regulators. An antibody that neutralizes the type I interferon receptor completely blocks interferon-kappa signaling, demonstrating that interferon-kappa utilizes the same receptor as other type I interferons. Interferon-kappa therefore defines a novel subclass of type I interferon that is expressed in keratinocytes and expands the repertoire of known proteins mediating host defense.

Virus-specific activation of a novel interferon regulatory factor, IRF-5, results in the induction of distinct interferon alpha genes

Interferon regulatory factor (IRF) genes encode DNA-binding proteins that are involved in the innate immune response to infection. Two of these proteins, IRF-3 and IRF-7, serve as direct transducers of virus-mediated signaling and play critical roles in the induction of type I interferon genes. We have now shown that another factor, IRF-5, participates in the induction of interferon A (IFNA) and IFNB genes and can replace the requirement for IRF-7 in the induction of IFNA genes. We demonstrate that, despite the functional similarity, IRF-5 possesses unique characteristics and does not have a redundant role. Thus, 1) activation of IRF-5 by phosphorylation is virus-specific, and its in vivo association with the IFNA promoter can be detected only in cells infected with NDV, not Sendai virus, while both viruses activate IRF-3 and IRF-7, and 2) NDV infection of IRF-5-overexpressing cells preferentially induced the IFNA8 subtype, while IFNA1 was primarily induced in IRF-7 expressing cells. These data indicate that multiple signaling pathways induced by infection may be differentially recognized by members of the IRF family and modulate transcription of individual IFNA genes in a virus and cell type-specific manner.

Latently expressed human herpesvirus 8-encoded interferon regulatory factor 2 inhibits double-stranded RNA-activated protein kinase

Human herpesvirus 8 (HHV-8; Kaposi's sarcoma herpesvirus) encodes four open reading frames with homology to cellular proteins of interferon regulatory factor (IRF) family. Three of them, viral IRF-1 (vIRF-1), vIRF-2, and vIRF-3, have been cloned and found, when overexpressed, to down-regulate the transcriptional activity of interferon type I gene promoters in infected cells by interfering with the transactivating activity of cellular IRFs. In this study, we have further characterized vIRF-2 and shown that it is a nuclear protein which is constitutively expressed in HHV-8-positive pleural effusion lymphoma cell lines. Nuclear localization of vIRF-2 was confirmed by in situ detection of ectopically expressed enhanced green fluorescent protein/vIRF-2 fusion protein. We found that the expression of vIRF-2 in HEK293 cells inhibited the antiviral effect of interferon and rescued translation of vesicular stomatitis virus mRNA from interferon-induced translational block. To provide insight into the mechanism of this effect we have demonstrated that vIRF-2 physically interacts with PKR consequently inhibiting autophosphorylation of double-stranded RNA-activated protein kinase (PKR) and blocking phosphorylation of PKR substrates histone 2A and eukaryotic translation initiation factor 2alpha. These results suggest that the latently expressed vIRF-2 has a role in viral mimicry which targets the activity of interferon-induced PKR kinase. By inhibiting the kinase activity of PKR and consequent down-modulation of protein synthesis, HHV-8 has evolved a mechanism by which it can overcome the interferon-mediated antiviral effect. Thus, the anti-interferon functions of vIRF-2 may contribute to the establishment of a chronic or latent infection.

Downregulation of IRF-3 levels by ribozyme modulates the profile of IFNA subtypes expressed in infected human cells

As an early response to viral infection, cells express a number of cellular genes that play a role in innate immunity, including alpha/beta interferons (IFN). IFN-alpha/beta are encoded by a single IFNB gene and multiple, closely related IFNA genes. The induction of these IFN genes in infected cells occurs at the transcriptional level, and two transcription factors of the IRF family, IRF-3 and IRF-7, were shown to play a role in their activation. While the expression of IRF-3 alone was shown to be sufficient for induction of the IFNB gene, induction of all the IFNA subtypes in human cells required the presence of IRF-7. Since IRF-3 is expressed constitutively in all cells examined, the role of IRF-3 in the induction of IFNA genes has not been clarified. Using ribozyme targeted to IRF-3 mRNA, we found that the downregulation of IRF-3 levels in the infected cells inhibited not only the induction of IFNB gene but also the expression of IFNA genes. Furthermore, downmodulation of IRF-3 levels altered the expression profile of IFNA subtypes induced by viral infection. These studies suggest that the ratio between the relative levels of IRF-3 and IRF-7 is a critical determinant for the induction of the individual IFNA subtypes in infected cells.

Analysis of functional domains of interferon regulatory factor 7 and its association with IRF-3

IRF-7 plays an essential role in virus-activated transcription of IFNA genes. To analyze functional domains of IRF-7 we have constructed an amino-terminal deletion mutant of IRF-7 (237-514) which exerted a dominant negative (DN) effect on virus-induced expression of the endogenous Type I IFN genes. Focusing on the molecular mechanism underlying the dominant negative effect of IRF-7 DN, we found that virus-activated transcription of endogenous IFNA genes requires full-length IRF-7 and that Serine 483 and 484 play an essential role. While IRF-7 DN had no effect on virus-stimulated nuclear translocation of IRF-3 and IRF-7, the binding of IRF-7 DN to IRF-3 and IRF-7 was detected by GST pull-down assay as well as by immunoprecipitation in infected cells, indicating that IRF-7 DN targets both IRF-7 and IRF-3. The region by which IRF-7 interacts with IRF-3 was mapped between amino acid 418 and 473. Overexpression of IRF-7 DN in virus-infected 2FTGH cells resulted in an inhibition of IFN synthesis and in a significant reduction of binding of both IRF-3 and IRF-7 to the IFNA1 promoter. Interestingly, the IRF-7 DN-mediated suppression of IFNA gene expression can be negated by overexpression of IRF-3. Altogether these results suggest that the IRF-3/IRF-7 complexes are biologically active and are involved in virus-activated transcription of endogenous IFNA genes.

Regulation of the promoter activity of interferon regulatory factor-7 gene. Activation by interferon snd silencing by hypermethylation

The molecular mechanism by which virus induces expression of the early inflammatory genes has not yet been completely elucidated. Previous studies indicated that the virus-mediated transcription of type I interferon (IFN) genes required activation of two members of IFN regulatory factor (IRF) family, IRF-3 and IRF-7, where the expression of IRF-7 was found to be indispensable for the induction of IFNA genes. To determine the factors that regulate expression of IRF-7 gene, as well as its inducibility by type I IFNs, we have isolated and characterized the promoter and first intron of the human IRF-7 gene. This region shows a presence of two potential interferon-sensitive response elements (ISRE/IRF-E). However, only the ISRE present in the first intron was functional and conferred interferon inducibility in a transient transfection assay. Using a pull-down assay with an oligodeoxynucleotide corresponding to this ISRE immobilized to magnetic beads, we have demonstrated that this ISRE binds ISGF3 complex and IRF-1 from the extract of IFN-treated cells but not from the untreated cells. We have further shown that the previously observed lack of expression of IRF-7 in 2fTGH fibrosarcoma cell line, correlated with hypermethylation of the CpG island in the human IRF-7 promoter. The repression of the promoter activity was relieved by treatment with DNA methyltransferase inhibitor 5-aza-deoxycytidine. In vitro methylation of IRF-7 promoter silenced IRF-7 directed expression of luciferase gene in HeLa cells that express endogenous IRF-7 gene. Whether silencing of IRF-7 by methylation is instrumental for the process of tumorigenesis remains to be determined.

Characterization of a novel human herpesvirus 8-encoded protein, vIRF-3, that shows homology to viral and cellular interferon regulatory factors

The genome of the human herpesvirus 8 (HHV-8) contains a cluster of open reading frames (ORFs) encoding proteins with homology to the cellular transcription factors of the interferon regulatory factor (IRF) family. Two of these homologues, vIRF-1 and vIRF-2, were previously identified and functionally analyzed. In this study, we have characterized a novel gene, designated vIRF-3, encoded within the previously predicted ORF K10.5 and our newly identified ORF K10. 6. Northern blotting of RNA extracted from BCBL-1 cells with a vIRF-3-specific probe and reverse transcription-PCR analyses revealed a single transcript of 2.2 kb with a splice present in the coding region. The vIRF-3 mRNA levels in BCBL-1 cells were increased upon 12-O-tetradecanoylphorbol-13-acetate treatment, with kinetics of expression similar to those of the early immediate genes. The vIRF-3 ORF encodes a 73-kDa protein with homology to cellular IRF-4 and HHV-8-encoded vIRF-2 and K11. In transient transfection assays with the IFNACAT reporter, vIRF-3 functioned as a dominant-negative mutant of both IRF-3 and IRF-7 and inhibited virus-mediated transcriptional activity of the IFNA promoter. Similarly, the overexpression of vIRF-3 in mouse L929 cells resulted in inhibition of virus-mediated synthesis of biologically active interferons. These results suggest that by targeting IRF-3 and IRF-7, which play a critical role in the activation of alpha/beta interferon (IFN) genes, HHV-8 has evolved a mechanism by which it directly subverts the functions of IRFs and down-regulates the induction of the IFN genes that are important components of the innate immunity.

Reconstitution of virus-mediated expression of interferon alpha genes in human fibroblast cells by ectopic interferon regulatory factor-7

Type I interferons constitute an important part of the innate immune response against viral infection. Unlike the expression of interferon (IFN) B gene, the expression of IFNA genes is restricted to the lymphoid cells. Both IFN regulatory factor 3 and 7 (IRF-3 and IRF-7) were suggested to play positive roles in these genes expression. However, their role in the differential expression of individual subtypes of human IFNA genes is unknown. Using various IFNA reporter constructs in transient transfection assay we found that overexpression of IRF-3 in virus infected 2FTGH cells selectively activated IFNA1 VRE, whereas IRF-7 was able to activate IFNA1, A2, and A4. The binding of recombinant IRF-7 and IRF-3 to these VREs correlated with their transcriptional activation. Nuclear proteins from infected and uninfected IRF-7 expressing 2FTGH cells formed multiple DNA-protein complexes with IFNA1 VRE, in which two unique DNA-protein complexes containing IRF-7 were detected. In 2FTGH cells, virus stimulated expression of IFNB gene but none of the IFNA genes. Reconstitution of IRF-7 synthesis in these cells resulted, upon virus infection, in the activation of seven endogenous IFNA genes in which IFNA1 predominated. These studies suggest that IRF-7 is a critical determinant for the induction of IFNA genes in infected cells.