Engagement of the cellular receptor for glycoprotein B of human cytomegalovirus activates the interferon-responsive pathway

The host response to West Nile Virus infection limits viral spread through the activation of the interferon regulatory factor 3 pathway

Recent outbreaks of West Nile Virus (WNV) have been associated with an increase in morbidity and mortality in humans, birds, and many other species. We have initiated studies to define the molecular mechanisms by which a recent pathogenic isolate of WNV evades the host cell innate antiviral response. Biochemical and microarray analyses demonstrated that WNV induced the expression of beta interferon (IFN-beta) and several IFN-stimulated genes late in infection of cultured human cells. The late expression of these antiviral genes was due to the delayed activation of the transcription factor IFN regulatory factor 3 (IRF-3). Despite this host response, WNV was still able to replicate efficiently. The effect of the IRF-3 pathway on WNV replication was assessed by examining virus replication and spread in cultures of wild-type or IRF-3-null mouse embryo fibroblasts. The absence of IRF-3 was marked by a significant increase in plaque size and a sustained production of infectious particles. Although the activation of the IRF-3 pathway was not sufficient to block virus replication, our results suggest that IRF-3 target genes function to constrain WNV infection and limit cell-to-cell virus spread.

Postattachment events associated with viral entry are necessary for induction of interferon-stimulated genes by human cytomegalovirus

Utilizing a human cytomegalovirus-specific fusion inhibitor and an antiglycoprotein H antibody, we studied the role of virion fusion in interferon-stimulated gene (ISG) induction. Our results indicate that ISG induction does not occur when virion-mediated, post-high-affinity attachment events are inhibited by either reagent. Thus, virion-mediated postattachment events, such as fusion, are required for ISG induction.

Activation of the NF-kappaB pathway in human cytomegalovirus-infected cells is necessary for efficient transactivation of the major immediate-early promoter

We previously demonstrated that human cytomegalovirus (HCMV) infection induced the activation of the cellular transcription factor NF-kappaB. Here, we investigate the mechanism for the HCMV-induced NF-kappaB activation and the role that the induced NF-kappaB plays in transactivation of the major immediate-early promoter (MIEP) and production of immediate-early (IE) proteins. Using a dominant-negative inhibitor of NF-kappaB, the IkappaB-superrepressor, we demonstrated that active NF-kappaB is critical for transactivation of the HCMV MIEP. Investigation of the mechanisms of NF-kappaB activation following HCMV infection showed a rapid and sustained decrease in the inhibitors of NF-kappaB, IkappaBalpha and IkappaBbeta. Because the IkappaB kinases (IKKs) regulate the degradation of the IkappaBs, virus-mediated changes in the IKKs were examined next. Using dominant-negative forms of the IKKs, we showed significant decreases in transactivation of the MIEP in the presence of these mutants. In addition, protein levels of members of the IKK complex and IKK kinase activity were upregulated throughout the time course of infection. Lastly, the role NF-kappaB plays in HCMV IE mRNA and protein production during infection was examined. Using aspirin and MG-132, we demonstrated that production of IE protein and mRNA was significantly decreased and delayed in infected cells treated with these drugs. Together, the results of these studies suggest that virus-mediated NF-kappaB activation, through the dysregulation of the IKK complex, plays a primary role in the initiation of the HCMV gene cascade in fibroblasts and may provide new targets for therapeutic intervention.

Structural Similarities in the Cellular Receptors Used by Adenovirus and Reovirus

Adenovirus and reovirus are nonenveloped viruses that engage cell-surface receptors using filamentous attachment proteins with head-and-tail morphology. The coxsackievirus and adenovirus receptor (CAR) and reovirus receptor junctional adhesion molecule 1 (JAM1) are immunoglobulin superfamily members that form homodimers stabilized by ionic and hydrophobic contacts between their N-terminal immunoglobulin-like domains. Both proteins are expressed at regions of cell-cell contact and contain sequences in their cytoplasmic tails that anchor the proteins to the actin cytoskeleton. Like CAR and JAM1, the attachment proteins of adenovirus and reovirus, fiber and sigma1, respectively, also share key structural features. Both fiber and sigma1 have defined regions of flexibility within the tail, which is constructed in part using an unusual triple beta-spiral motif. The head domains of both proteins are formed by an 8-stranded beta-barrel with identical beta-strand connectivity. Strikingly, both adenovirus fiber and reovirus 1 engage their receptors by interacting with sequences that also mediate formation of receptor homodimers. Therefore, while adenovirus and reovirus belong to different virus families and have few overall properties in common, the observed similarities between the receptors and attachment proteins of these viruses suggest a conserved mechanism of attachment and an evolutionary relationship.

Human cytomegalovirus-induced upregulation of intercellular cell adhesion molecule-1 on villous syncytiotrophoblasts

Human cytomegalovirus (HCMV) is secreted apically from villous trophoblasts, thus congenital infection is not likely to occur by basal release across the basement membrane. As an alternative route, we hypothesize that an HCMV-infected villous syncytiotrophoblast (ST) upregulates intercellular adhesion molecule (ICAM)-1, causing blood monocytes to bind to the ST and induce apoptosis. Purified (>99.99%) populations of human villous trophoblasts were differentiated into an ST-like culture, infected with HCMV strain AD169, and assessed for ICAM-1 expression by immunofluorescence. Infection strongly upregulated ICAM-1 24 h after challenge. ICAM-1 was also stimulated by transfection with viral genes IE2-55, IE1-72, and IE2-86, but not by UV-inactivated virus. Infection with a green fluorescent protein recombinant virus allowed infection and ICAM-1 expression to be topographically located. We found that ICAM-1 was expressed on both infected and noninfected cells. Furthermore, antibody to tumor necrosis factor (TNF)alpha and, to a lesser extent, interleukin (IL)1 beta inhibited ICAM-1 upregulation on noninfected cells but not on infected cells. We conclude that HCMV IE proteins stimulate ICAM-1 expression on villous trophoblasts by paracrine release of TNF alpha and IL1 beta, as well as by a direct effect on infected cells.

Early induction of interferon-responsive mRNAs in Creutzfeldt-Jakob disease

Foreign infectious agents typically evoke a host immune response. In scrapie and Creutzfeldt-Jakob disease (CJD), no immune response has been detectable. However, many latent or persistent viruses evade immune recognition but still activate inflammatory pathways. Unique microglial responses in late CJD infection that could be part of a host defense mechanism were previously delineated, although changes secondary to neurodegeneration could not be excluded. Data here show these microglial transcriptional changes are detectable in CJD brain beginning at 30 days after innoculation. In addition, 10 other interferon-sensitive genes were similarly upregulated at very early stages of infection. These responses occurred well before abnormal prion protein (PrP) and clinical signs of CJD were detectable. Further analyses in very pure microglia from CJD brain suggested the CJD agent activated signaling pathways distinct from those induced by amyloidogenic proteins (including abnormal PrP). Although increases in interferon-alpha or -beta transcript levels were not seen in cultures or in whole brain, CJD microglia exhibited a potentiated interferon response when challenged with double-stranded RNA. The induction of interferon-sensitive genes without appreciable interferon synthesis was strikingly similar to that seen in some viral infections. These data suggest the CJD agent is recognized as a foreign virus-like entity. Moreover, the early reactive gene expression profiles described here may be useful in preclinical diagnosis.

Host gene induction and transcriptional reprogramming in Kaposi's sarcoma-associated herpesvirus (KSHV/HHV-8)-infected endothelial, fibroblast, and B cells: insights into modulation events early during infection

Kaposi's sarcoma-associated herpesvirus (KSHV/HHV-8) is etiologically linked to the endothelial tumor Kaposi's sarcoma and with two lymphoproliferatve disorders, primary effusion lymphoma and multicentric Castleman's disease. HHV-8 infects a variety of target cells both in vivo and in vitro, binds to the in vitro target cells via cell surface heparan sulfate, and uses the alpha(3)beta(1) integrin as one of the entry receptors. Within minutes of infection, HHV-8 induced the integrin-mediated signaling pathways and morphological changes in the target cells (S. M. Akula et al., Cell, 108: 407-419, 2002; P. P. Naranatt et al., J. Virol., 77: 1524-1539, 2003). As an initial step toward understanding the role of host genes in HHV-8 infection and pathogenesis, modulation of host cell gene expression immediately after infection was examined. To reflect HHV-8's broad cellular tropism, mRNAs collected at 2 and 4 h after infection of primary human endothelial [human adult dermal microvascular endothelial cells (HMVECd)] and foreskin fibroblast [human foreskin fibroblast (HFF)] cells and human B cell line (BJAB) were analyzed by oligonucleotide array with approximately 22,000 human transcripts. With a criteria of >2-fold gene induction as significant, approximately 1.72% of the genes were differentially expressed, of which, 154 genes were shared by at least two cells and 33 genes shared by all three cells. HHV-8-induced transcriptional profiles in the endothelial and fibroblast cells were closely similar, with substantial differences in the B cells. In contrast to the antiapoptotic regulators induced in HMVECd and HFF cells, proapoptotic regulators were induced in the B cells. A robust increase in the expression of IFN-induced genes suggestive of innate immune response induction was observed in HMVECd and HFF cells, whereas there was a total lack of immunity related protein inductions in B cells. These striking cell type-specific behaviors suggest that HHV-8-induced host cell gene modulation events in B cells may be different compared with the adherent endothelial and fibroblast target cells. Functional clustering of modulated genes identified several host molecules hitherto unknown to HHV-8 infection. These results indicate that early during infection, HHV-8 reprograms the host transcriptional machinery regulating a variety of cellular processes including apoptosis, transcription, cell cycle regulation, signaling, inflammatory response, and angiogenesis, all of which may play important roles in the biology and pathogenesis of HHV-8.

The influenza B virus nonstructural NS1 protein is essential for efficient viral growth and antagonizes beta interferon induction

We analyzed the functions of the influenza B virus nonstructural NS1-B protein, both by utilizing a constructed mutant virus (Delta NS1-B) lacking the NS1 gene and by testing the activities of the protein when expressed in cells. The mutant virus replicated to intermediate levels in 6-day-old embryonated chicken eggs that contain an immature interferon (IFN) system, whereas older eggs did not support viral propagation to a significant extent. The Delta NS1-B virus was a substantially stronger inducer of beta IFN (IFN-beta) transcripts in human lung epithelial cells than the wild type, and furthermore, transiently expressed NS1-B protein efficiently inhibited virus-dependent activation of the IFN-beta promoter. Interestingly, replication of the Delta NS1-B knockout virus was attenuated by more than 4 orders of magnitude in tissue culture cells containing or lacking functional IFN-alpha/beta genes. These findings show that the NS1-B protein functions as a viral IFN antagonist and indicate a further requirement of this protein for efficient viral replication that is unrelated to blocking IFN effects.

Innate cellular response to virus particle entry requires IRF3 but not virus replication

Mammalian cells respond to virus infections by eliciting both innate and adaptive immune responses. One of the most effective innate antiviral responses is the production of alpha/beta interferon and the subsequent induction of interferon-stimulated genes (ISGs), whose products collectively limit virus replication and spread. Following viral infection, interferon is produced in a biphasic fashion that involves a number of transcription factors, including the interferon regulatory factors (IRFs) 1, 3, 7, and 9. In addition, virus infection has been shown to directly induce ISGs in the absence of prior interferon production through the activation of IRF3. This process is believed to require virus replication and results in IRF3 hyperphosphorylation, nuclear localization, and proteasome-mediated degradation. Previously, we and others demonstrated that herpes simplex virus type 1 (HSV-1) induces ISGs and an antiviral response in fibroblasts in the absence of both interferon production and virus replication. In this report, we show that the entry of enveloped virus particles from diverse virus families elicits a similar innate response. This process requires IRF3, but not IRF1, IRF7, or IRF9. Following virus replication, the large DNA viruses HSV-1 and vaccinia virus effectively inhibit ISG mRNA accumulation, whereas the small RNA viruses Newcastle disease virus, Sendai virus, and vesicular stomatitis virus do not. In addition, we found that IRF3 hyperphosphorylation and degradation do not correlate with ISG and antiviral state induction but instead serve as a hallmark of productive virus replication, particularly following a high-multiplicity infection. Collectively, these data suggest that virus entry triggers an innate antiviral response mediated by IRF3 and that subsequent virus replication results in posttranslational modification of IRF3, such as hyperphosphorylation, depending on the nature of the incoming virus.

The herpes simplex virus ICP0 RING finger domain inhibits IRF3- and IRF7-mediated activation of interferon-stimulated genes

Virus infection induces a rapid cellular response in cells characterized by the induction of interferon. While interferon itself does not induce an antiviral response, it activates a number of interferon-stimulated genes that collectively function to inhibit virus replication and spread. Previously, we and others reported that herpes simplex virus type 1 (HSV-1) induces an interferon -independent antiviral response in the absence of virus replication. Here, we report that the HSV-1 proteins ICP0 and vhs function in concert to disable the host antiviral response. In particular, we show that ICP0 blocks interferon regulatory factor IRF3- and IRF7-mediated activation of interferon-stimulated genes and that the RING finger domain of ICP0 is essential for this activity. Furthermore, we demonstrate that HSV-1 modifies the IRF3 pathway in a manner different from that of the small RNA viruses most commonly studied.

Human cytomegalovirus elicits a coordinated cellular antiviral response via envelope glycoprotein B

Previous studies have shown that human cytomegalovirus (CMV) is a potent elicitor of interferon-stimulated gene (ISG) expression. Induction of the interferon pathway does not require replication-competent virus, and envelope glycoprotein B (gB) from CMV is a viral structural component that can directly induce transcription of ISGs. Here we extend these earlier findings by defining the consequences of inducing the interferon pathway. We found that cells respond to CMV or soluble gB by establishing a functional antiviral state within cell types critical in CMV biology, such as fibroblasts and endothelial cells. We have also discovered new insights into the mechanism by which the pathway is initiated. Interferon regulatory factor 3 (IRF3), a key transcriptional regulator of cellular interferon responses, is activated by CMV virions and soluble gB. Thus, IRF3 becomes activated via "outside-in" signal transduction events. This is a novel mechanism of activation of this key transcription factor by viruses. In comparison to soluble gB (gB(1-750)), which comprises the entire ectodomain of gB, a truncation mutant encompassing only the amino-terminal region of gB (gB(1-460)) was markedly less effective at inducing antiviral responses. This indicates that the region of gB from residues 461 to 750 is important for initiation of the antiviral response. In addition, CMV and gB establish an antiviral state in alpha/beta interferon null cells, illustrating that primary induction of ISGs by CMV and gB is sufficient to establish the antiviral response and that interferon secretion is not necessary for the antiviral effect. Taken together, our findings reveal that CMV initiates a coordinated antiviral response through contact between gB and an as-yet-unidentified cell surface receptor(s).

Mechanisms governing maintenance of Cdk1/cyclin B1 kinase activity in cells infected with human cytomegalovirus

Previous work has demonstrated dysregulation of key cell cycle components in human cytomegalovirus (HCMV)-infected human fibroblasts, resulting in cell cycle arrest (F. M. Jault, J.-M. Jault, F. Ruchti, E. A. Fortunato, C. L. Clark, J. Corbeil, D. D. Richman, and D. H. Spector, J. Virol. 69:6697-6704, 1995). The activation of the mitotic kinase Cdk1/cyclin B, which was detected as early as 8 h postinfection (p.i.) and maintained throughout the time course, was particularly interesting. To understand the mechanisms underlying the induction of this kinase activity, we have examined the pathways that regulate the activation of Cdk1/cyclin B1 complexes. The accumulation of the cyclin B1 subunit in HCMV-infected cells is the result of increased synthesis and reduced degradation of the protein. In addition, the catalytic subunit, Cdk1, accumulates in its active form in virus-infected cells. The decreased level of the Tyr15-phosphorylated form of Cdk1 in virus-infected fibroblasts is due in part to the down-regulation of the expression and activity of the Cdk1 inhibitory kinases Myt1 and Wee1. Increased degradation of Wee1 via the proteasome also accounts for its absence at 24 h p.i. At late times, we observed accumulation of the Cdc25 phosphatases that remove the inhibitory phosphates from Cdk1. Interestingly, biochemical fractionation studies revealed that the active form of Cdk1, a fraction of total cyclin B1, and the Cdc25 phosphatases reside predominantly in the cytoplasm of infected cells. Collectively, these data suggest that the maintenance of Cdk1/cyclin B1 activity observed in HCMV-infected cells can be explained by three mechanisms: the accumulation of cyclin B1, the inactivation of negative regulatory pathways for Cdk1, and the accumulation of positive factors that promote Cdk1 activity.

Human cytomegalovirus UL83-coded pp65 virion protein inhibits antiviral gene expression in infected cells

The initial interaction of human cytomegalovirus with fibroblasts triggers, and then partially blocks, an innate immune response pathway that leads to the induction of IFN-responsive genes and proinflammatory chemokines. Infection of fibroblasts with human cytomegalovirus inhibited their ability to respond to exogenous IFN. Consistent with the observation that the block did not depend on de novo viral protein synthesis, ectopic expression of the viral UL83-coded pp65, an abundant virion protein, inhibited IFN signaling. Furthermore, DNA array analysis showed that infection with a pp65-deficient mutant virus caused a much stronger induction of many IFN-response and proinflammatory chemokine RNAs than infection with wild-type virus. The nuclear DNA-binding activities of transcription factors NF-kappaB and IRF1 were induced to a much greater extent after infection with the pp65-deficient mutant than with wild-type virus. IFN-stimulated gene factor 3 DNA-binding was modestly enhanced, whereas IRF3 activity was not affected by mutation of pp65. Together, these results imply that pp65, which is delivered to newly infected cells in the virion, antagonizes a pathway that affects NF-kappaB and IRF1 and prevents the accumulation of mRNAs encoded by numerous cellular antiviral genes.

Specific association of glycoprotein B with lipid rafts during herpes simplex virus entry

Herpes simplex virus (HSV) entry requires the interaction of glycoprotein D (gD) with a cellular receptor such as herpesvirus entry mediator (HVEM or HveA) or nectin-1 (HveC). However, the fusion mechanism is still not understood. Since cholesterol-enriched cell membrane lipid rafts are involved in the entry of other enveloped viruses such as human immunodeficiency virus and Ebola virus, we tested whether HSV entry proceeds similarly. Vero cells and cells expressing either HVEM or nectin-1 were treated with cholesterol-sequestering drugs such as methyl-beta-cyclodextrin or nystatin and then exposed to virus. In all cases, virus entry was inhibited in a dose-dependent manner, and the inhibitory effect was fully reversible by replenishment of cholesterol. To examine the association of HVEM and nectin-1 with lipid rafts, we analyzed whether they partitioned into nonionic detergent-insoluble glycolipid-enriched membranes (DIG). There was no constitutive association of either receptor with DIG. Binding of soluble gD or virus to cells did not result in association of nectin-1 with the raft-containing fractions. However, during infection, a fraction of gB but not gC, gD, or gH associated with DIG. Similarly, when cells were incubated with truncated soluble glycoproteins, soluble gB but not gC was found associated with DIG. Together, these data favor a model in which HSV uses gB to rapidly mobilize lipid rafts that may serve as a platform for entry and cell signaling. It also suggests that gB may interact with a cellular molecule associated with lipid rafts.

Epidermal growth factor receptor is a cellular receptor for human cytomegalovirus

Human cytomegalovirus (HCMV) is a widespread opportunistic herpesvirus that causes severe and fatal diseases in immune-compromised individuals, including organ transplant recipients and individuals with AIDS. It is also a leading cause of virus-associated birth defects and is associated with atherosclerosis and coronary restenosis. HCMV initiates infection and intracellular signalling by binding to its cognate cellular receptors and by activating several signalling pathways including those mediated by mitogen-activated protein kinase, phosphatidylinositol-3-OH kinase, interferons, and G proteins. But a cellular receptor responsible for viral entry and HCMV-induced signalling has yet to be identified. Here we show that HCMV infects cells by interacting with epidermal growth factor receptor (EGFR) and inducing signalling. Transfecting EGFR-negative cells with an EGFR complementary DNA renders non-susceptible cells susceptible to HCMV. Ligand displacement and crosslinking analyses show that HCMV interacts with EGFR through gB, its principal envelope glycoprotein. gB preferentially binds EGFR and EGFR-ErbB3 oligomeric molecules in Chinese hamster ovary cells transfected with erbB family cDNAs. Taken together, these data indicate that EGFR is a necessary component for HCMV-triggered signalling and viral entry.

The human cytomegalovirus

Human cytomegalovirus (HCMV), a betaherpesvirus, represents the major infectious cause of birth defects, as well as an important pathogen for immunocompromised individuals. The viral nucleocapsid containing a linear double-stranded DNA of 230 kb is surrounded by a proteinaceous tegument, which is itself enclosed by a loosely applied lipid bilayer. Expression of the HCMV genome is controlled by a cascade of transcriptional events that leads to the synthesis of three categories of viral proteins designated as immediate-early, early, and late. Clinical manifestations can be seen following primary infection, reinfection, or reactivation. About 10% of infants are infected by the age of 6 months following transmission from their mothers via the placenta, during delivery, or by breastfeeding. HCMV is a significant post-allograft pathogen and contributes to graft loss independently from graft rejection. Histopathologic examination of necropsy tissues demonstrates that the virus enters via the epithelium of the upper alimentary, respiratory, or genitourinary tracts. Hematogenous spreading is typically followed by infection of ductal epithelial cells. Infections are kept under control by the immune system. However, total HCMV clearance is rarely achieved, and the viral genome remains at selected sites in a latent state. Virological and molecular detection of HCMV, as well as serological demonstration of a specific immune response, are used for diagnosis. Treatment of HCMV infections is difficult because there are few options. The presently available drugs produced a significant clinical improvement, but suffer from poor oral bioavailability, low potency, development of resistance in clinical practice, and dose-limiting toxicities.

Human cytomegalovirus activates inflammatory cytokine responses via CD14 and Toll-like receptor 2

Human cytomegalovirus (CMV) is a ubiquitous opportunistic pathogen that causes significant morbidity and mortality in immunocompromised people. An understanding of how CMV induces and circumvents host immunity is of critical importance in efforts to design effective therapeutics. It was recently discovered that mere cell contact by CMV particles leads to profound modulation of cellular gene expression, including induction of inflammatory cytokines and interferon-stimulated genes characteristic of innate immune detection. These findings suggest that a membrane receptor recognizes a CMV envelope protein(s), leading to innate immune activation. Here, we show that the pattern recognition receptors Toll-like receptor 2 (TLR2) and CD14 recognize CMV virions and trigger inflammatory cytokine production. Induction of inflammatory cytokines is mediated via TLR2-dependent activation of NF-kappa B. Since many of the pathological processes associated with CMV disease are facilitated or directly mediated by inflammatory cytokines, identification of the host membrane detection machinery may ultimately lead to improved therapeutics.

The ORF45 protein of Kaposi's sarcoma-associated herpesvirus is associated with purified virions

Kaposi's sarcoma-associated herpesvirus (KSHV) ORF45 is encoded by an immediate-early gene in the KSHV genome. This protein was recently shown to interact with interferon regulatory factor 7 and inhibit virus-mediated alpha/beta interferon induction (Zhu et al., Proc. Natl. Acad. Sci. USA 99:5573-5578, 2002). ORF45 was characterized as a phosphorylated protein, and it is localized in the cytoplasm of infected cells. In this report, we provide evidence that ORF45 is associated with KSHV virions. (i) ORF45 was detected in gradient-purified virions by Western blotting along with known structural proteins of KSHV including gB, K8.1, and major capsid protein. In contrast, ORF50/Rta, K8alpha, and ORF59/PF8 were not detected in the same virion preparation. (ii) ORF45 comigrates with KSHV virions in sucrose gradient ultracentrifugation. (iii) Virion-associated ORF45 was resistant to trypsin digestion but became sensitive after the virions were treated with detergent which destroys the viral envelope. (iv) ORF45 remained associated with tegument-nucleocapsid complex when virion-specific glycoproteins were removed after detergent treatment. (v) An ORF45 protein band was visualized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of extensively purified KSHV virions and identified by mass spectrometry. (vi) By immunoelectron microscopy, virus-like structures were specifically stained by anti-ORF45 antibody. Based on the evidence, we conclude that ORF45 is associated with purified KSHV virions and appears to be a tegument protein. The presence of ORF45 in KSHV virions raised the possibility that this protein may be delivered to host cells at the start of infection and therefore have the opportunity to act at the very early stage of the infection, suggesting an important role of ORF45 in KSHV primary infection.

Cellular antiviral responses against influenza A virus are countered at the posttranscriptional level by the viral NS1A protein via its binding to a cellular protein required for the 3' end processing of cellular pre-mRNAS

The influenza A virus NS1 protein (NS1A protein) binds and inhibits the function of the 30-kDa subunit of CPSF, a cellular factor that is required for the 3'-end processing of cellular pre-mRNAs. Here we generate a recombinant influenza A/Udorn/72 virus that encodes an NS1A protein containing a mutated binding site for the 30-kDa subunit of CPSF. This mutant virus is substantially attenuated, indicating that this binding site in the NS1A protein is required for efficient virus replication. Using this mutant virus, we show that NS1A binding to CPSF mediates the viral posttranscriptional countermeasure against the initial cellular antiviral response--the interferon-alpha/beta (IFN-alpha/beta)-independent activation of the transcription of cellular antiviral genes, which requires the interferon regulatory factor-3 (IRF-3) transcription factor that is activated by virus infection. Whereas the posttranscriptional processing of these cellular antiviral pre-mRNAs is inhibited in cells infected by wild-type influenza A virus, functional antiviral mRNAs are produced in cells infected by the mutant virus. These results establish that the binding of 30-kDa CPSF to the NS1A protein is largely responsible for the posttranscriptional inhibition of the processing of these cellular antiviral pre-mRNAs. Mutation of this binding site in the NS1A protein also affects a second cellular antiviral response: in cells infected by the mutant virus, IFN-beta mRNA is produced earlier and in larger amounts.

The Epstein-Barr virus SM protein induces STAT1 and interferon-stimulated gene expression

Viruses utilize numerous mechanisms to counteract the host's immune response. Interferon production is a major component of the host antiviral response. Many viruses, therefore, produce proteins or RNA molecules that inhibit interferon-induced signal transduction pathways and their associated antiviral effects. Surprisingly, some viruses directly induce expression of interferon-induced genes. SM, an early lytic Epstein-Barr virus (EBV) nuclear protein, was found to specifically increase the expression of several genes (interferon-stimulated genes) that are known to be strongly induced by alpha/beta interferons. SM does not directly stimulate alpha/beta interferon secretion but instead induces STAT1, an intermediate step in the interferon signaling pathway. SM is a posttranscriptional activator of gene expression and increases STAT1 mRNA accumulation, particularly that of the functionally distinct STAT1beta splice variant. SM expression in B lymphocytes is associated with decreased cell proliferation but does not decrease cell viability or induce cell cycle arrest. These results indicate that EBV can specifically induce cellular genes that are normally physiological targets of interferon by inducing components of cytokine signaling pathways. Our findings therefore suggest that some aspects of the interferon response may be positively modulated by infecting viruses.

Structural basis for ubiquitin-like ISG 15 protein binding to the NS1 protein of influenza B virus: a protein-protein interaction function that is not shared by the corresponding N-terminal domain of the NS1 protein of influenza A virus

The N-terminal domains of the NS1 protein of influenza B virus (NS1B protein) and the NS1 protein of influenza A virus (NS1A protein) share one function: binding double-stranded RNA (dsRNA). Here we show that the N-terminal domain of the NS1B protein possesses an additional function that is not shared by its NS1A counterpart: binding the ubiquitin-like ISG15 protein that is induced by influenza B virus infection. Homology modeling predicts that the dimeric six-helical N-terminal domain of the NS1B protein differs from its NS1A protein counterpart in containing large loops between helices 1 and 2 (loops 1 and 1') and between helices 2 and 3 (loops 2 and 2'). Mutagenesis establishes that residues located in loop 1/1' together with residues located in polypeptide segment 94-103 form the ISG15 protein-binding site of NS1B protein. Loop 1/1' is not required for dsRNA binding, which instead requires arginine residues R50, R53, R50', and R53' located in antiparallel helices 1 and 1'. Further, we demonstrate that the binding sites for RNA and protein are independent of each other. In particular, ISG15 and dsRNA can bind simultaneously; the binding of the ISG15 protein does not have a detectable effect on the binding of dsRNA, and vice versa.

Human cytomegalovirus-caused damage to placental trophoblasts mediated by immediate-early gene-induced tumor necrosis factor-alpha

Infection of the fetal epithelium (trophoblast) lining the villous placenta by human cytomegalovirus (HCMV) accompanies placental inflammations and fetal intrauterine growth restriction. However, the consequences of infection on the villous trophoblast have not been explored. We show that HCMV infection of primary immature (cytotrophoblast-like) or mature (syncytiotrophoblast-like) cultures results in loss of half of the cells within 24 hours of virus challenge. Two-color immunofluorescence of HCMV immediate early (IE) gene expression and apoptosis (terminal dUTP nick-end labeling) revealed apoptosis only in uninfected cells. Antibody to tumor necrosis factor (TNF)-alpha completely inhibited infection-induced trophoblast apoptosis and cell loss, as did co-incubation with epidermal growth factor, known to inhibit trophoblast apoptosis. Transfection with HCMV immediate early- (IE)1-72 and IE2-86, but not IE2-55, expression plasmids induced paracrine trophoblast apoptosis inhibitable by epidermal growth factor or antibody to TNF-alpha. These results show that HCMV infection of villous trophoblasts leads to rapid loss of neighboring cells mediated by viral IE protein-induced TNF-alpha secretion. We propose that HCMV infection damages the placental trophoblast barrier by accelerating trophoblast turnover and decreasing its capacity for renewal.

Human influenza viruses activate an interferon-independent transcription of cellular antiviral genes: outcome with influenza A virus is unique

We examine the IFN-alpha/beta-independent activation of cellular transcription that constitutes an early antiviral response of cells against influenza A and B viruses, which cause widespread epidemics in humans. We show that influenza B virus induces the synthesis in human cells of several mature mRNAs encoded by genes containing an IFN-alpha/beta-stimulated response element (ISRE). Consequently, the IFN regulatory factor-3 transcription factor, which is required for the transcription of ISRE-controlled genes, is activated after influenza B virus infection. The production of these cellular mRNAs, some of which encode antiviral proteins, is independent of not only IFN-alpha/beta, but also viral protein synthesis. These mature cellular antiviral mRNAs are not produced after infection with influenza A virus, but IFN regulatory factor-3 is activated and the transcription of the ISRE-controlled p56 gene is induced. Consequently, like other newly synthesized cellular premRNAs in influenza A virus-infected cells, the posttranscriptional processing of premRNAs encoded by ISRE-controlled genes is inhibited. Previous work has established that such posttranscriptional inhibition is mediated by the viral NS1A protein. This unique, global countermeasure against the early, IFN-alpha/beta-independent antiviral response of cells may be an important factor in the pathogenicity of influenza A virus infection.

Viruses and the type I interferon antiviral system: induction and evasion

The type I interferon (IFN) system responds to viral infection and induces an "antiviral state" in cells, providing an important first line of defense against virus infection. Interaction of type I IFNs (IFN alpha and IFN beta) with their receptor induces hundreds of cellular genes. Of the proteins induced by IFN, the antiviral function of only a few is known, and their mechanisms of action are only partly understood. Additionally, although viral-encoded mechanisms that counteract specific components of the type I IFN response have been known for some time, it has recently become clear that many (if not most) viruses encode some form of IFN-antagonist. Understanding the interplay between viral-encoded IFN antagonists and the interferon response will be essential if the therapeutic potential of IFNs is to be fully exploited.

Infection of cells with human cytomegalovirus during S phase results in a blockade to immediate-early gene expression that can be overcome by inhibition of the proteasome

Cells infected with human cytomegalovirus (HCMV) after commencing DNA replication do not initiate viral immediate-early (IE) gene expression and divide before arresting. To determine the nature of this blockade, we examined cells that were infected 24 h after release from G(0) using immunofluorescence, laser scanning cytometry, and fluorescence-activated cell sorting (FACS) analysis. Approximately 40 to 50% of the cells had 2N DNA content, became IE(+) in the first 12 h, and arrested. Most but not all of the cells with >2N DNA content did not express IE antigens until after mitosis. To define the small population of IE(+) cells that gradually accumulated within the S and G(2)/M compartments, cells were pulsed with bromodeoxyuridine (BrdU) just prior to S-phase infection and analyzed at 12 h postinfection for IE gene expression, BrdU positivity, and cell cycle position. Most of the BrdU(+) cells were IE(-) and had progressed into G(2)/M or back to G(1). The majority of the IE(+) cells in S and G(2)/M were BrdU(-). Only a few cells were IE(+) BrdU(+), and they resided in G(2)/M. Multipoint BrdU pulse-labeling revealed that, compared to cells actively synthesizing DNA at the beginning of the infection, a greater percentage of the cells that initiated DNA replication 4 h later could express IE antigens and proceed into S. Synchronization of the cells with aphidicolin also indicated that the blockade to the activation of IE gene expression was established in cells soon after initiation of DNA replication. It appears that a short-lived protein in S-phase cells may be required for IE gene expression, as it is partially restored by treatment with the proteasome inhibitor MG132.

Epstein-Barr virus inhibits the development of dendritic cells by promoting apoptosis of their monocyte precursors in the presence of granulocyte macrophage-colony-stimulating factor and interleukin-4

Epstein-Barr virus (EBV) is a tumorigenic human herpesvirus that persists for life in healthy immunocompetent carriers. The viral strategies that prevent its clearance and allow reactivation in the face of persistent immunity are not well understood. Here we demonstrate that EBV infection of monocytes inhibits their development into dendritic cells (DCs), leading to an abnormal cellular response to granulocyte macrophage-colony-stimulating factor (GM-CSF) and interleukin-4 (IL-4) and to apoptotic death. This proapoptotic activity was not affected by UV inactivation and was neutralized by EBV antibody-positive human sera, indicating that binding of the virus to monocytes is sufficient to alter their response to the cytokines. Experiments with the relevant blocking antibodies or with mutated EBV strains lacking either the EBV envelope glycoprotein gp42 or gp85 demonstrated that interaction of the trimolecular gp25-gp42-gp85 complex with the monocyte membrane is required for the effect. Our data provide the first evidence that EBV can prevent the development of DCs through a mechanism that appears to bypass the requirement for viral gene expression, and they suggest a new strategy for interference with the function of DCs during the initiation and maintenance of virus-specific immune responses.

Complementation of vaccinia virus lacking the double-stranded RNA-binding protein gene E3L by human cytomegalovirus

The cellular response to viral infection often includes activation of pathways that shut off protein synthesis and thereby inhibit viral replication. In order to enable efficient replication, many viruses carry genes such as the E3L gene of vaccinia virus that counteract these host antiviral pathways. Vaccinia virus from which the E3L gene has been deleted (VVDeltaE3L) is highly sensitive to interferon and exhibits a restricted host range, replicating very inefficiently in many cell types, including human fibroblast and U373MG cells. To determine whether human cytomegalovirus (CMV) has a mechanism for preventing translational shutoff, we evaluated the ability of CMV to complement the deficiencies in replication and protein synthesis associated with VVDeltaE3L. CMV, but not UV-inactivated CMV, rescued VVDeltaE3L late gene expression and replication. Thus, complementation of the VVDeltaE3L defect appears to depend on de novo CMV gene expression and is not likely a result of CMV binding to the cell receptor or of a virion structural protein. CMV rescued VVDeltaE3L late gene expression even in the presence of ganciclovir, indicating that CMV late gene expression is not required for complementation of VVDeltaE3L. The striking decrease in overall translation after infection with VVDeltaE3L was prevented by prior infection with CMV. Finally, CMV blocked both the induction of eukaryotic initiation factor 2alpha (eIF2alpha) phosphorylation and activation of RNase L by VVDeltaE3L. These results suggest that CMV has one or more immediate-early or early genes that ensure maintenance of a high protein synthetic capacity during infection by preventing activation of the PKR/eIF2alpha phosphorylation and 2-5A oligoadenylate synthetase/RNase L pathways.

Expression of herpes simplex virus ICP0 inhibits the induction of interferon-stimulated genes by viral infection

The herpes simplex virus type 1 (HSV-1) mutant d109 does not express any of the immediate-early (IE) proteins and persists in cells for a prolonged length of time. As has been shown by Nicholl et al. (J. Gen. Virol. 81:2215-2218, 2000) and Mossman et al. (J. Virol. 75:750-758, 2001) using other mutants defective for IE gene expression, infection with d109 induced the expression of a number of interferon-stimulated genes. Induction of these genes was significantly greater at multiplicities of infection (MOI) of 10 PFU/cell or greater, and the resulting antiviral effect was only seen at MOIs greater than 10 PFU/cell. Using mutants defective for sets of IE genes established that the lack of ICP0 expression was necessary for high levels of interferon-stimulated gene expression in HEL cells. The induction of interferon-stimulated genes by d109 could also be inhibited by infection with an E1-:E3-:E4- adenovirus expressing levels of ICP0 that are comparable to those expressed within the first hour of wild-type virus infection. Lastly, the addition of the proteasome inhibitor MG132 to cells infected with a mutant that expresses ICP0, d106, also resulted in the induction of interferon-stimulated genes. Thus, ICP0 may function through the proteasome very early in HSV infection to inhibit a cellular antiviral response induced by the virion.

Proteolytic processing of human cytomegalovirus glycoprotein B is dispensable for viral growth in culture

Glycoprotein B (gB) of human cytomegalovirus (HCMV), which is considered essential for the viral life cycle, is proteolytically processed during maturation. Since gB homologues of several other herpesviruses remain uncleaved, the relevance of this property of HCMV gB for viral infectivity is unclear. Here we report on the construction of a viral mutant in which the recognition site of gB for the cellular endoprotease furin was destroyed. Because mutagenesis of essential proteins may result in a lethal phenotype, a replication-deficient HCMV gB-null genome encoding enhanced green fluorescent protein was constructed, and complementation by mutant gBs was initially evaluated in transient-cotransfection assays. Cotransfection of plasmids expressing authentic gB or gB with a mutated cleavage site (gB-DeltaFur) led to the formation of green fluorescent miniplaques which were considered to result from one cycle of phenotypic complementation of the gB-null genome. To verify these results, two recombinant HCMV genomes were constructed: HCMV-BAC-DeltaMhdI, with a deletion of hydrophobic domain 1 of gB that appeared to be essential for viral growth in the cotransfection experiments, and HCMV-BACDeltaFur, in which the gB cleavage site was mutated by amino acid substitution. Consistent with the results of the cotransfection assays, only the DeltaFur mutant replicated in human fibroblasts, showing growth kinetics comparable to that of wild-type virus. gB in mutant-infected cells was uncleaved, whereas glycosylation and transport to the cell surface were not impaired. Extracellular mutant virus contained exclusively uncleaved gB, indicating that proteolytic processing of gB is dispensable for viral replication in cell culture.

Viruses and interferons

The interferon system is the first line of defense against viral infection in mammals. This system is designed to block the spread of virus infection in the body, sometimes at the expense of accelerating the death of the infected cells. As expected of potent cytokines, in addition to their antiviral effects, interferons have profound effects on many aspects of cell physiology. All these actions of interferons are mediated by hundreds of interferon-induced proteins that are usually not synthesized in resting cells. Interferons induce their synthesis by activating the Jak-STAT pathways, a paradigm of cell signaling used by many cytokines and growth factors. Surprisingly, some of the same genes can also be induced directly by viruses and double-stranded RNA, a common viral by-product. Some of the interferon-induced proteins have novel biochemical properties and some are inactive as such but can be activated by double-stranded RNA produced during virus infection. Finally, almost all viruses have evolved mechanisms to evade the interferon system by partially blocking interferon synthesis or interferon action. Thus, in nature interferons and viruses maintain an equilibrium that allows regulated viral replication.

Requirement of multiple cis-acting elements in the human cytomegalovirus major immediate-early distal enhancer for viral gene expression and replication

We have shown previously that the human cytomegalovirus (HCMV) major immediate-early (MIE) distal enhancer is needed for MIE promoter-dependent transcription and viral replication at low multiplicities of infection (MOI). To understand how this region works, we constructed and analyzed a series of HCMVs with various distal enhancer mutations. We show that the distal enhancer is composed of at least two parts that function independently to coordinately activate MIE promoter-dependent transcription and viral replication. One such part is contained in a 47-bp segment that has consensus binding sites for CREB/ATF, SP1, and YY1. At low MOI, these working parts likely function in cis to directly activate MIE gene expression, thus allowing viral replication to ensue. Three findings support the view that these working parts are likely cis-acting elements. (i) Deletion of either part of a bisegmented distal enhancer only slightly alters MIE gene transcription and viral replication. (ii) Reversing the distal enhancer's orientation largely preserves MIE gene transcription and viral replication. (iii) Placement of stop codons at -300 or -345 in all reading frames does not impair MIE gene transcription and viral replication. Lastly, we show that these working parts are dispensable at high MOI, partly because of compensatory stimulation of MIE promoter activity and viral replication that is induced by a virion-associated component(s) present at a high viral particle/cell ratio. We conclude that the distal enhancer is a complex multicomponent cis-acting region that is required to augment both MIE promoter-dependent transcription and HCMV replication.

Viperin (cig5), an IFN-inducible antiviral protein directly induced by human cytomegalovirus

Little is known about the mechanism by which IFNs inhibit human cytomegalovirus (HCMV) replication. Indeed, infection of fibroblasts with HCMV initiates the expression of a subset of type I IFN-inducible genes whose role in the infectious process is unclear. We describe here the identification of a cytoplasmic antiviral protein that is induced by IFNs, by HCMV infection, and by the HCMV envelope protein, glycoprotein B (gB). Stable expression of the protein in fibroblasts inhibits productive HCMV infection, down-regulating several HCMV structural proteins (gB, pp28, and pp65) known to be indispensable for viral assembly and maturation. We have named the protein viperin (for virus inhibitory protein, endoplasmic reticulum-associated, interferon-inducible). HCMV infection causes the redistribution of the induced viperin from its normal endoplasmic reticulum association, first to the Golgi apparatus and then to cytoplasmic vacuoles containing gB and pp28. Expression before HCMV infection reduces viperin redistribution from the endoplasmic reticulum to the Golgi apparatus and prevents vacuolar localization, perhaps reflecting the mechanism used by HCMV to evade the antiviral function.

Altered cellular mRNA levels in human cytomegalovirus-infected fibroblasts: viral block to the accumulation of antiviral mRNAs

The effect of human cytomegalovirus (HCMV) infection on cellular mRNA accumulation was analyzed by gene chip technology. During a 48-h time course after infection of human diploid fibroblasts, 1,425 cellular mRNAs were found to be up-regulated or down-regulated by threefold or greater in at least two consecutive time points. Several classes of genes were prominently affected, including interferon response genes, cell cycle regulators, apoptosis regulators, inflammatory pathway genes, and immune regulators. The number of mRNAs that were up-regulated or down-regulated were roughly equal over the complete time course. However, for the first 8 h after infection, the number of up-regulated mRNAs was significantly less than the number of down-regulated mRNAs. By analyzing the mRNA expression profile of cells infected in the presence of cycloheximide, it was found that a minimum of 25 mRNAs were modulated by HCMV in the absence of protein synthesis. These included mRNAs encoded by a small number of interferon-responsive genes, as well as beta interferon itself. Cellular mRNA levels in cytomegalovirus-infected cells were compared to the levels in cells infected with UV-inactivated virus. The inactivated virus caused the up-regulation of a much greater number of mRNAs, many of which encoded proteins with antiviral roles, such as interferon-responsive genes and proinflammatory cytokines. These data argue that one or more newly synthesized viral gene products block the induction of antiviral pathways that are triggered by HCMV binding and entry.

Interferon-stimulated response element (ISRE)-binding protein complex DRAF1 is activated in Sindbis virus (HR)-infected cells

To elucidate the host cell defense mechanisms in response to Sindbis viral infection, we have started to characterize interferon (IFN)-stimulated response element (ISRE)-binding proteins activated in infected cells that are involved in the transcriptional induction of IFN type I-inducible genes. Using electromobility shift assays (EMSA), we detected several protein complexes with a human IFN-stimulated gene 15 (ISG15) ISRE in extracts from virus-infected L929 cells that were absent in extracts from uninfected cells. Comigration with Newcastle disease virus-activated ISRE-binding complexes, ISRE-binding specificity, supershift experiments, and conditions of formation indicate that the complexes activated by Sindbis viral infection in L929 cells correspond to DRAF1 and ISG factor 3 (ISGF3). Transfection of L929 cells with poly rI:rC induced only ISGF3. DRAF1 could be detected in Sindbis virus-infected mouse embryo fibroblasts derived from IFNR type I and type II KO mice. Viral RNA synthesis is required for activation of DRAF1.

Characterization of the signal peptide processing and membrane association of human cytomegalovirus glycoprotein O

Human cytomegalovirus (HCMV) has a structurally complex envelope that contains multiple glycoproteins. These glycoproteins are involved in virus entry, virus maturation, and cell-cell spread of infection. Glycoprotein H (gH), glycoprotein L (gL), and glycoprotein O (gO) associate covalently to form a unique disulfide-bonded tripartite complex. Glycoprotein O was recently discovered, and its basic structure, as well as that of the tripartite complex, remains uncharacterized. Based on hydropathy analysis, we hypothesized that gO could adopt a type II transmembrane orientation. The data presented here, however, reveal that the single hydrophobic domain of gO functions as a cleavable signal peptide that is absent from the mature molecule. Although it lacks a membrane anchor, glycoprotein O is associated with the membranes of HCMV-infected cells. The sophisticated organization of the gH.gL.gO complex reflects the intricate nature of the multicomponent entry and fusion machinery encoded by HCMV.

Human cytomegalovirus binding to heparan sulfate proteoglycans on the cell surface and/or entry stimulates the expression of human leukocyte antigen class I

Human cytomegalovirus (HCMV) is known to down-regulate the expression of human leukocyte antigen (HLA) class I, the process of which involves a subset of virus genes. Infection of human foreskin fibroblast (HFF) cells with UV-inactivated HCMV (UV-HCMV), however, resulted in an increase in HLA class I presentation on the cell surface in the absence of HCMV gene expression. Heparin, which inhibits the interaction of virus particles with cell surface heparan sulfate proteoglycans (HSPGs), blocked the effect of UV-HCMV on HLA class I expression. Pretreatment of cells with heparinase I decreased in a dose-dependent manner the effect of UV-HCMV on HLA class I expression enhancement. Sodium chlorate, which is known to inhibit the sulfation of HSPGs, gave a similar result. Pretreatment of UV-HCMV with trypsin or monoclonal antibody reactive with the envelope glycoprotein gB reduced the increase in HLA class I expression on the HFF cell surface by UV-HCMV. RT-PCR analysis demonstrated that the increase in HLA class I presentation on the HFF cell surface was due to an increase in HLA class I transcription. Thus, binding of HCMV particles to cell surface HSPGs appears to be required for the stimulation of HLA class I expression. It is also possible that virus entry, in addition to binding to HSPGs, may be involved in the stimulation of HLA class I expression, since the UV-HCMV entered the cells and all treatments to block virus binding to HSPGs would necessarily prevent virus entry.

Activation of interferon response factor-3 in human cells infected with herpes simplex virus type 1 or human cytomegalovirus

Activation of cellular interferon-stimulated genes (ISGs) after infection with herpes simplex virus type 1 (HSV-1) or human cytomegalovirus (HCMV) was investigated. The level of ISG54-specific RNA in human fetal lung (HFL) or human foreskin (BJ) fibroblasts increased substantially after infection with either virus in the presence of cycloheximide. HSV-1 particles lacking glycoprotein D or glycoprotein H failed to induce ISG54-specific RNA synthesis, demonstrating that entry of virus particles rather than binding of virions to the cell surface was required for the effect. A DNA-binding complex that recognized an interferon-responsive sequence motif was induced upon infection with HSV-1 or HCMV in the presence of cycloheximide, and the complex was shown to contain the cell proteins interferon response factor 3 (IRF-3) and CREB-binding protein. IRF-3 was modified after infection with HSV-1 or HCMV to a form of lower electrophoretic mobility, consistent with phosphorylation. De novo transcription of viral or cellular genes was not required for the activation of IRF-3, since the effect was not sensitive to inhibition by actinomycin D. Infection of HFL fibroblasts with HSV-1 under conditions in which viral replication proceeded normally resulted in severely reduced levels of the IRF-3-containing complex, defining the activation of IRF-3 as a target for viral interference with ISG induction. In BJ fibroblasts, however, significant activation of IRF-3 was detected even when the viral gene expression program progressed to later stages, demonstrating that the degree of inhibition of the response was dependent on host cell type. As a consequence of IRF-3 activation, endogenous interferon was released from BJ cells and was capable of triggering the appropriate signal transduction pathway in both infected and uninfected cells. Activation of ISG54-specific RNA synthesis was not detected after infection of human U-373MG glioblastoma cells, showing that the induction of the response by infection is cell type dependent.

Nuclear translocation and activation of the transcription factor NFAT is blocked by herpes simplex virus infection

Transcription factors of the NFAT (nuclear factor of activated T cells) family are expressed in most immune system cells and in a range of other cell types. Signaling through NFAT is implicated in the regulation of transcription for the immune response and other processes, including differentiation and apoptosis. NFAT normally resides in the cytoplasm, and a key aspect of the NFAT activation pathway is the regulation of its nuclear import by the Ca(2+)/calmodulin-dependent phosphatase calcineurin. In a cell line stably expressing green fluorescent protein (GFP)-NFAT, this import can be triggered by elevation of intracellular calcium and visualized in live cells. Here we show that the inducible nuclear import of GFP-NFAT is efficiently blocked at early stages of herpes simplex virus (HSV) infection. This is a specific effect, since we observed abundant nuclear accumulation of a test viral protein and no impediment to general nuclear localization signal-dependent nuclear import and retention in infected cells. We show that virus binding at the cell surface is not itself sufficient to inhibit the signaling that induces NFAT nuclear translocation. Since the block occurs following infection in the presence of phosphonoacetic acid but not cycloheximide, we infer that the entry of the virion and early gene transcription are required but the effect is independent of DNA replication or late virus gene expression. A consequence of the block to GFP-NFAT import is a reduction in NFAT-dependent transcriptional activation from the interleukin-2 promoter in infected cells. This HSV-mediated repression of the NFAT pathway may constitute an immune evasion strategy or subversion of other NFAT-dependent cellular processes to promote viral replication.

Post-genomic virology: the impact of bioinformatics, microarrays and proteomics on investigating host and pathogen interactions

Post-genomic research encompasses many diverse aspects of modern science. These include the two broad subject areas of computational biology (bioinformatics) and functional genomics. Laboratory based functional genomics aims to measure and assess either the messenger RNA (mRNA) levels (transcriptome studies) or the protein content (proteome studies) of cells and tissues. All of these methods have been applied recently to the study of host and pathogen interactions for both bacteria and viruses. A basic overview of the technology is given in this review together with approaches to data analysis. The wealth of information produced from even these preliminary studies has shown the generalities, subtleties and specificities of host-pathogen interactions. Such research should ultimately result in new methods for diagnosing and treating infectious diseases.

Role for p53 in gene induction by double-stranded RNA

Cross talk between p53 and interferon-regulated pathways is implicated in the induction of gene expression by biologic and genotoxic stresses. We demonstrate that the interferon-stimulated gene ISG15 is induced by p53 and that p53 is required for optimal gene induction by double-stranded RNA (dsRNA), but not interferon. Interestingly, virus induces ISG15 in the absence of p53, suggesting that virus and dsRNA employ distinct signaling pathways.

The interferon-inducible 204 gene is transcriptionally activated by mouse cytomegalovirus and is required for its replication

Infection of cells with viable or UV-inactivated murine cytomegalovirus (MCMV) increased the IFN-inducible 204 gene at both the mRNA and the protein levels. The activity of a reporter gene driven by the mouse Ifi204 promoter induced following virus infection showed that this increase was due to transcriptional activation. Moreover, FACS analysis of infected mouse embryo fibroblasts (MEF) stably transfected with a p204-dominant-negative mutant (p204dmMEF) revealed that they do not accumulate at the G1/S border in the same way as infected MEF transfected with the empty vector (neoMEF). MCMV DNA synthesis is significantly delayed (144 h in p204dmMEF vs 72 h in neoMEF), due to retarded expression of viral genes, namely, IE1 and DNA polymerase, as shown by Western blot comparison of p204dmMEF and neoMEF extracts. These results demonstrate that MCMV may exploit the Ifi204 gene to regulate the cell cycle and enhance its DNA synthesis.

Human cytomegalovirus up-regulates the phosphatidylinositol 3-kinase (PI3-K) pathway: inhibition of PI3-K activity inhibits viral replication and virus-induced signaling

Infection of quiescent fibroblasts with human cytomegalovirus (HCMV) was found to cause a rapid activation of cellular phosphatidylinositol 3-kinase (PI3-K). Maximum PI3-K activation occurred from 15 to 30 min postinfection. This activation was transient, and by 2 h postinfection (hpi), PI3-K activity had declined to preinfection levels. However, at 4 hpi, a second tier of PI3-K activation was detected, and PI3-K activity remained elevated relative to that of mock-infected cells for the remainder of infection. The cellular kinases Akt and p70S6K and the transcription factor NF-kappaB were activated in a PI3-K-dependent manner at similar times following HCMV infection. Analysis using UV-irradiated virus indicated that no viral protein synthesis was necessary for the first phase of PI3-K activation, but viral protein expression was required for the second tier of PI3-K activation. Treatment of infected fibroblasts with LY294002, a potent and specific inhibitor of PI3-K kinase activity, caused a 4-log decrease in viral titers. LY294002 did not inhibit viral entry, but it did decrease viral immediate-early gene expression. In addition, the protein levels of two viral early genes required for DNA replication, UL84 and UL44, were significantly lower in the presence of LY294002. Furthermore, viral DNA replication was strongly inhibited by LY294002 treatment. This inhibition of viral DNA replication could be reversed by adding back the products of PI3-K activity (PI-3,4-P(2) and PI-3,4,5-P(3)), demonstrating that the effect of LY294002 on the viral life cycle was specifically due to the inhibition of PI3-K activity. These results are the first to suggest that PI3-K mediates HCMV-induced activation of host cell mitogenic pathways. They also provide strong evidence that PI3-K activation is important for initiation of viral DNA replication and completion of the viral lytic life cycle.

Global modulation of cellular transcription by human cytomegalovirus is initiated by viral glycoprotein B

Human cytomegalovirus (HCMV) infection alters the expression of many cellular genes, including IFN-stimulated genes (ISGs) [Zhu, H., Cong, J.-P., Mamtora, G., Gingeras, T. & Shenk, T. (1998) Proc. Natl. Acad. Sci. USA 95, 14470-14475]. By using high-density cDNA microarrays, we show that the HCMV-regulated gene expression profile in fibroblasts does not differ substantially from the response generated by IFN. Furthermore, we identified the specific viral component triggering this response as the envelope glycoprotein B (gB). Cells treated with gB, but not other herpesviral glycoproteins, exhibited the same transcriptional profile as HCMV-infected cells. Thus, the interaction of gB with its as yet unidentified cellular receptor is the principal mechanism by which HCMV alters cellular gene expression early during infection. These findings highlight a pioneering paradigm for the consequences of virus-receptor interactions.

IRF3 and IRF7 phosphorylation in virus-infected cells does not require double-stranded RNA-dependent protein kinase R or Ikappa B kinase but is blocked by Vaccinia virus E3L protein

Induction of interferon-alpha (IFNalpha) gene expression in virus-infected cells requires phosphorylation-induced activation of the transcription factors IRF3 and IRF7. However, the kinase(s) that targets these proteins has not been identified. Using a combined pharmacological and genetic approach, we found that none of the kinases tested was responsible for IRF phosphorylation in cells infected with Newcastle disease virus (NDV). Although the broad-spectrum kinase inhibitor staurosporine potently blocked IRF3 and -7 phosphorylation, inhibitors for protein kinase C, protein kinase A, MEK, SAPK, IKK, and protein kinase R (PKR) were without effect. Both IkappaB kinase and PKR have been implicated in IFN induction, but cells genetically deficient in IkappaB kinase, PKR, or the PKR-related genes PERK, IRE1, or GCN2 retained the ability to phosphorylate IRF7 and induce IFNalpha. Interestingly, PKR mutant cells were defective for response to double-stranded (ds) RNA but not to virus infection, suggesting that dsRNA is not the only activating viral component. Consistent with this notion, protein synthesis was required for IRF7 phosphorylation in virus-infected cells, and the kinetics of phosphorylation and viral protein production were similar. Despite evidence for a lack of involvement of dsRNA and PKR, vaccinia virus E3L protein, a dsRNA-binding protein capable of inhibiting PKR, was an effective IRF3 and -7 phosphorylation inhibitor. These results suggest that a novel cellular protein that is activated by viral products in addition to dsRNA and is sensitive to E3L inhibition is responsible for IRF activation and reveal a novel mechanism for the anti-IFN effect of E3L distinct from its inhibition of PKR.

Molecular pathways in virus-induced cytokine production

Virus infections induce a proinflammatory response including expression of cytokines and chemokines. The subsequent leukocyte recruitment and antiviral effector functions contribute to the first line of defense against viruses. The molecular virus-cell interactions initiating these events have been studied intensively, and it appears that viral surface glycoproteins, double-stranded RNA, and intracellular viral proteins all have the capacity to activate signal transduction pathways leading to the expression of cytokines and chemokines. The signaling pathways activated by viral infections include the major proinflammatory pathways, with the transcription factor NF-kappaB having received special attention. These transcription factors in turn promote the expression of specific inducible host proteins and participate in the expression of some viral genes. Here we review the current knowledge of virus-induced signal transduction by seven human pathogenic viruses and the most widely used experimental models for viral infections. The molecular mechanisms of virus-induced expression of cytokines and chemokines is also analyzed.

The role of MKK1/2 kinase activity in human cytomegalovirus infection

Human cytomegalovirus infection of quiescent fibroblasts was found to induce a bi-phasic activation of mitogen-activated protein kinase (MAPK) kinase 1 and 2 (MKK1/2) and two of their downstream targets, extracellular signal regulated kinase 1 and 2 (ERK1/2), as determined by Western blot analysis using phospho-specific antibodies. Treatment of infected fibroblasts with U0126, a potent and specific inhibitor of MKK1/2 kinase activity, completely blocked ERK1/2 activation following HCMV infection without affecting cell viability. Anti-viral studies demonstrate that in the presence of U0126, viral titres are reduced and viral DNA replication is inhibited. In addition, protein levels of two viral early genes that are required for viral DNA replication, UL44 and UL84, are significantly decreased in the presence of U0126. These results suggest that HCMV-mediated activation of MKK1/2 kinase activity enhances virus infectivity by ensuring timely initiation of viral DNA replication, possibly by regulating early gene expression.