Specific inhibition of interferon signal transduction pathways by adenoviral infection

The repression domain of the E1B 55-kilodalton protein participates in countering interferon-induced inhibition of adenovirus replication

To begin to investigate the mechanism by which the human adenovirus type 5 E1B 55-kDa protein protects against the antiviral effects of type 1 interferon (IFN) (J. S. Chahal, J. Qi, and S. J. Flint, PLoS Pathog. 8:e1002853, 2012 [doi:10.1371/journal.ppat.1002853]), we examined the effects of precise amino acid substitution in this protein on resistance of viral replication to the cytokine. Only substitution of residues 443 to 448 of E1B for alanine (E1B Sub19) specifically impaired production of progeny virus and resulted in a large defect in viral DNA synthesis in IFN-treated normal human fibroblasts. Untreated or IFN-treated cells infected by this mutant virus (AdEasyE1Sub19) contained much higher steady-state concentrations of IFN-inducible GBP1 and IFIT2 mRNAs than did wild-type-infected cells and of the corresponding newly transcribed pre-mRNAs, isolated exploiting 5'-ethynyluridine labeling and click chemistry. These results indicated that the mutations created by substitution of residues 443 to 448 for alanine (Sub19) impair repression of transcription of IFN-inducible genes, by the E1B, 55-kDa protein, consistent with their location in a segment required for repression of p53-dependent transcription. However, when synthesized alone, the E1B 55-kDa protein inhibited expression of the p53-regulated genes BAX and MDM2 but had no impact whatsoever on induction of IFIT2 and GBP1 expression by IFN. These observations correlate repression of transcription of IFN-inducible genes by the E1B 55-kDa protein with protection against inhibition of viral genome replication and indicate that the E1B 55-kDa protein is not sufficient to establish such transcriptional repression.

The human adenovirus type 5 E1B 55 kDa protein obstructs inhibition of viral replication by type I interferon in normal human cells

Vectors derived from human adenovirus type 5, which typically lack the E1A and E1B genes, induce robust innate immune responses that limit their therapeutic efficacy. We reported previously that the E1B 55 kDa protein inhibits expression of a set of cellular genes that is highly enriched for those associated with anti-viral defense and immune responses, and includes many interferon-sensitive genes. The sensitivity of replication of E1B 55 kDa null-mutants to exogenous interferon (IFN) was therefore examined in normal human fibroblasts and respiratory epithelial cells. Yields of the mutants were reduced at least 500-fold, compared to only 5-fold, for wild-type (WT) virus replication. To investigate the mechanistic basis of such inhibition, the accumulation of viral early proteins and genomes was compared by immunoblotting and qPCR, respectively, in WT- and mutant-infected cells in the absence or presence of exogenous IFN. Both the concentration of viral genomes detected during the late phase and the numbers of viral replication centers formed were strongly reduced in IFN-treated cells in the absence of the E1B protein, despite production of similar quantities of viral replication proteins. These defects could not be attributed to degradation of entering viral genomes, induction of apoptosis, or failure to reorganize components of PML nuclear bodies. Nor was assembly of the E1B- and E4 Orf6 protein- E3 ubiquitin ligase required to prevent inhibition of viral replication by IFN. However, by using RT-PCR, the E1B 55 kDa protein was demonstrated to be a potent repressor of expression of IFN-inducible genes in IFN-treated cells. We propose that a primary function of the previously described transcriptional repression activity of the E1B 55 kDa protein is to block expression of IFN- inducible genes, and hence to facilitate formation of viral replication centers and genome replication.

The most frequently used therapeutic vectors for gene transfer or cancer treatment are derived from human adenovirus type 5 (Ad5). We have observed previously that the E1B 55 kDa protein encoded by a gene routinely deleted from these vectors represses expression of numerous cellular genes regulated by interferon (IFN) α and β, which are important components of the innate immune response to viral infection. We therefore compared synthesis of pre-mRNA from IFN-inducible genes, viral yields and early reactions in the infectious cycle in normal human cells exposed to exogenous IFN and infected by wild-type or E1B 55 kDa null-mutant viruses. We report that the E1B 55 kDa protein is a potent repressor of expression of IFN-regulated genes, and protects viral replication against anti-viral actions of IFN by blocking inhibition of formation of viral replication centers and genome replication. These observations provide the first information about the function of the transcription repression activity of E1B during the infectious cycle. Importantly, they also suggest new design considerations for adenoviral vectors that can circumvent induction of innate immune responses, currently a major therapeutic limitation.

Inhibition of IFN-gamma-dependent antiviral airway epithelial defense by cigarette smoke

BACKGROUND

Although individuals exposed to cigarette smoke are more susceptible to respiratory infection, the effects of cigarette smoke on lung defense are incompletely understood. Because airway epithelial cell responses to type II interferon (IFN) are critical in regulation of defense against many respiratory viral infections, we hypothesized that cigarette smoke has inhibitory effects on IFN-gamma-dependent antiviral mechanisms in epithelial cells in the airway.

METHODS

Primary human tracheobronchial epithelial cells were first treated with cigarette smoke extract (CSE) followed by exposure to both CSE and IFN-gamma. Epithelial cell cytotoxicity and IFN-gamma-induced signaling, gene expression, and antiviral effects against respiratory syncytial virus (RSV) were tested without and with CSE exposure.

RESULTS

CSE inhibited IFN-gamma-dependent gene expression in airway epithelial cells, and these effects were not due to cell loss or cytotoxicity. CSE markedly inhibited IFN-gamma-induced Stat1 phosphorylation, indicating that CSE altered type II interferon signal transduction and providing a mechanism for CSE effects. A period of CSE exposure combined with an interval of epithelial cell exposure to both CSE and IFN-gamma was required to inhibit IFN-gamma-induced cell signaling. CSE also decreased the inhibitory effect of IFN-gamma on RSV mRNA and protein expression, confirming effects on viral infection. CSE effects on IFN-gamma-induced Stat1 activation, antiviral protein expression, and inhibition of RSV infection were decreased by glutathione augmentation of epithelial cells using N-acetylcysteine or glutathione monoethyl ester, providing one strategy to alter cigarette smoke effects.

CONCLUSIONS

The results indicate that CSE inhibits the antiviral effects of IFN-gamma, thereby presenting one explanation for increased susceptibility to respiratory viral infection in individuals exposed to cigarette smoke.

Marek's disease virus influences the expression of genes associated with IFN-gamma-inducible MHC class II expression

Chickens infected with Marek's disease virus (MDV) become lifelong carriers regardless of their susceptibility to clinical disease. Therefore various viral immune-evasive mechanisms must play a role in MDV-host interactions. MDV has previously been shown to influence the expression of major histocompatibility complex (MHC) class II molecules. However, little is known about the underlying mechanisms of this phenomenon. In the present study, we studied the effect of MDV infection on the expression of several genes associated with IFN-gamma-inducible MHC class II expression at 4, 7, 14, and 21 days post-infection (dpi). There was a significant (p Collapse

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MESH Headings

• Animals
• Chickens/virology
• Gene Expression Profiling
• Genes, MHC Class II
• Immunologic Surveillance/genetics
• Interferon Regulatory Factor-1/genetics
• Interferon-gamma/immunology
• Mardivirus/immunology
• Marek Disease/genetics
• Marek Disease/immunology
• Nuclear Proteins/genetics
• Poultry Diseases/genetics
• Poultry Diseases/immunology
• Receptors, Interferon/genetics
• STAT1 Transcription Factor/metabolism
• Signal Transduction
• Time Factors
• Trans-Activators/genetics
• Interferon gamma Receptor

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Affiliation(s)

Niroshan Thanthrige-Don Department of Pathobiology, University of Guelph, Guelph, Ontario, Canada
Leah R Read
Mohamed Faizal Abdul-Careem
Hakimeh Mohammadi
Amirul I Mallick
Shayan Sharif

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Myxoma virus selectively disrupts type I interferon signaling in primary human fibroblasts by blocking the activation of the Janus kinase Tyk2

Poxviruses currently are known to disrupt Jak-STAT signal transduction induced by interferon (IFN) through two distinct mechanisms: (1) secreted poxviral IFN decoy receptors that prevent the initiation of IFN signaling from type I or II receptors at the cell surface; and (2) poxviral phosphatase that dephosphorylates STAT1 intracellularly. Here, we report a novel mechanism by which poxviruses can inhibit Jak-STAT signaling in response to type I IFN. Myxoma virus (MV) is a highly species-restricted member of the poxvirus family that infects only rabbits under the natural setting. Interestingly, primary human fibroblasts support a permissive MV infection that is only partially sensitive to the antiviral state induced by type I IFN. In this study we show that when type I IFN is added to primary human fibroblasts following MV infection, the tyrosine phosphorylation of the Janus kinase Tyk2 is specifically blocked, thereby preventing the subsequent activation of downstream STAT1 and STAT2. In stark contrast, type II IFN-induced activation of Jak1, Jak2 and STAT1 remains largely unaffected in MV-infected human fibroblasts. Unlike the de-activation of STAT1 by the poxvirus phosphatase, which is delivered into the cell by the input virions, the Tyk2 inhibition by MV infection requires new viral gene expression. Thus, our study documents a previously unrecognized immune evasion mechanism exploited by a poxvirus to selectively disrupt the type I IFN-Jak-STAT signaling cascade.

Regulation of bacteria-induced intercellular adhesion molecule-1 by CCAAT/enhancer binding proteins

Direct interaction between bacteria and epithelial cells may initiate or amplify the airway response through induction of epithelial defense gene expression by nuclear factor-kappaB (NF-kappaB). However, multiple signaling pathways modify NF-kappaB effects to modulate gene expression. In this study, the effects of CCAAT/enhancer binding protein (C/EBP) family members on induction of the leukocyte adhesion glycoprotein intercellular adhesion molecule-1 (ICAM-1) was examined in primary cultures of human tracheobronchial epithelial cells incubated with nontypeable Haemophilus influenzae. Increased ICAM-1 gene transcription in response to H. influenzae required gene sequences located at -200 to -135 in the 5'-flanking region that contain a C/EBP-binding sequence immediately upstream of the NF-kappaB enhancer site. Constitutive C/EBPbeta was found to have an important role in epithelial cell ICAM-1 regulation, while the adjacent NF-kappaB sequence binds the RelA/p65 and NF-kappaB1/p50 members of the NF-kappaB family to induce ICAM-1 expression in response to H. influenzae. The expression of C/EBP proteins is not regulated by p38 mitogen-activated protein kinase activation, but p38 affects gene transcription by increasing the binding of TATA-binding protein to TATA-box-containing gene sequences. Epithelial cell ICAM-1 expression in response to H. influenzae was decreased by expressing dominant-negative protein or RNA interference against C/EBPbeta, confirming its role in ICAM-1 regulation. Although airway epithelial cells express multiple constitutive and inducible C/EBP family members that bind C/EBP sequences, the results indicate that C/EBPbeta plays a central role in modulation of NF-kappaB-dependent defense gene expression in human airway epithelial cells after exposure to H. influenzae.

Constitutive ERK MAPK activity regulates macrophage ATP production and mitochondrial integrity

A unique feature of human alveolar macrophages is their prolonged survival in the face of a stressful environment. We have shown previously that the ERK MAPK is constitutively active in these cells and is important in prolonging cell survival. This study examines the role of the ERK pathway in maintaining mitochondrial energy production. The data demonstrate that ATP levels in alveolar macrophages depend on intact mitochondria and optimal functioning of the electron transport chain. Significant levels of MEK and ERK localize to the mitochondria and inhibition of ERK activity induces an early and profound depletion in cellular ATP coincident with a loss of mitochondrial transmembrane potential. The effect of ERK suppression on ATP levels was specific, since it did not occur with PI3K/Akt, p38, or JNK suppression. ERK inhibition led to cytosolic release of mitochondrial proteins and caspase activation. Both ERK inhibition and mitochondrial blockers induced loss of plasma membrane permeability and cell death. The cell death induced by ERK inhibition had hallmarks of both apoptotic (caspase activation) and necrotic (ATP loss) cell death. By blocking ERK inhibition-induced reactive oxygen species, caspase activation was prevented, although necrotic pathways continued to induce cell death. This suggests that mitochondrial dysfunction caused by ERK inhibition generates both apoptotic and necrotic cell death-inducing pathways. As a composite, these data demonstrate a novel mitochondrial role for ERK in maintaining mitochondrial membrane potential and ATP production in human alveolar macrophages.

IFN-gamma regulation of ICAM-1 receptors in bronchial epithelial cells: soluble ICAM-1 release inhibits human rhinovirus infection

Background

Intercellular adhesion molecule-1 (ICAM-1) is a critical target-docking molecule on epithelial cells for 90% of human rhinovirus (HRV) serotypes. Two forms of ICAM-1 exist, membranous (mICAM-1) and soluble (sICAM-1), both expressed by bronchial epithelial cells. Interferon-gamma (IFN-γ), a crucial Th-1 immuno-regulatory mediator, can modulate mICAM-1 expression; however its simultaneous effects on mICAM-1: sICAM-1 levels and their consequent outcome on cell infectivity have not been previously explored.

Methods

Primary normal human bronchial epithelial cells were pre-stimulated with IFN-γ (1 ng/ml for 24 h) and subsequently inoculated with HRV-14 or HRV-1b (TCID₅₀ 10 ^2.5). Epithelial surface ICAM-1 expression and soluble ICAM-1 release were measured at the protein and gene level by immunofluorescence and ELISA respectively; mRNA levels were semi-quantified using RT-PCR. Molecular mechanisms regulating ICAM-1 isoform expression and effects on epithelial cell infectivity were explored.

Results

In IFN-γ-biased cells infected with HRV-14, but not HRV-1b, mICAM-1 expression is down-regulated, with simultaneous induction of sICAM-1 release. This differential effect on HRV-14 receptor isoforms appears to be related to a combination of decreased IFN-γ-induced JAK-STAT signalling and proteolytic receptor cleavage of the membranous form in IFN-γ-biased HRV-14 infected cells. The observed changes in relative mICAM-1: sICAM-1 expression levels are associated with reduced HRV-14 viral titres.

Conclusion

These findings support the hypothesis that in epithelial cells conditioned to IFN-γ and subsequently exposed to HRV-14 infection, differential modulation in the ratio of ICAM-1 receptors prevails in favour of an anti-viral milieu, appearing to limit further target cell viral attachment and propagation.

Inhibition of Jak1-dependent signal transduction in airway epithelial cells infected with adenovirus

Adenoviral evolution has generated mechanisms to resist host cell defense systems, but the biochemical basis for evasion of multiple antiviral pathways in the airway by adenoviruses is incompletely understood. We hypothesized that adenoviruses modulate airway epithelial responses to type I interferons by altering the levels and activation of specific Janus family kinase-signal transducer and activator of transcription (JAK-STAT) signaling components. In this study, specific effects of adenovirus type 5 (AdV) on selected JAK-STAT signal transduction pathways were identified in human tracheobronchial epithelial cells, with focus on type I interferon-dependent signaling and gene expression. We found that wild-type AdV infection inhibited IFN-alpha-induced expression of antiviral proteins in epithelial cells by blocking phosphorylation of the Stat1 and Stat2 transcription factors that are required for activation of type I interferon-dependent genes. These effects correlated with AdV-induced down-regulation of expression of the receptor-associated tyrosine kinase Jak1 through a decrease in Jak1 mRNA levels. Phosphorylation of Stat3 in response to IL-6 and oncostatin M was also lost in AdV-infected cells, indicating loss of epithelial cell responses to other cytokines that depend on Jak1. In contrast, IL-4- and IL-13-dependent phosphorylation of Stat6 was not affected during AdV infection, indicating that the virus modulates specific signaling pathways, as these Stat6-activating pathways can function independent of Jak1. Taken together, the results indicate that AdV down-regulates host epithelial cell Jak1 to assure inhibition of the antiviral effects of multiple mediators to subvert airway defense responses and establish a productive infection.

Paracellular permeability restricts airway epithelial responses to selectively allow activation by mediators at the basolateral surface

Respiratory pathogens and toxins often assault the lung from the airway lumen. Airway epithelia may initiate and amplify inflammation in response to these attacks, but under certain conditions confinement of inflammation to the airway lumen may be beneficial to the host. Accordingly, we hypothesized that airway epithelial polarity allows different responses to basolateral vs apical stimuli that may modulate inflammation. Using primary human airway epithelial cells differentiated at an air-liquid interface in culture, we found that responses to several cytokines required basolateral mediator application. In contrast, responses to Haemophilus influenzae occurred after either basolateral or apical interaction with airway epithelia. Experiments focused on IFN-gamma receptor polarity confirmed its predominant basolateral location in cultured airway epithelia as well as in normal human airway tissue. Furthermore, physical and pharmacologic disruption of barrier function in airway epithelia allowed responses to apical application of IFN-gamma and other cytokines. These in vitro studies directly correlated with experiments in mice in which an airway epithelial response to IFN-gamma injected into the airway lumen was seen only after disruption of barrier function. The results indicate that airway epithelia with intact barrier function restrict inflammatory responses by limitation of cell activation through requiring interaction of selected mediators with the basolateral surface. However, loss of barrier integrity allows epithelial responses to these mediators if located in the airway lumen to amplify airway defenses.

Conditioned medium from enterohemorrhagic Escherichia coli-infected T84 cells inhibits signal transducer and activator of transcription 1 activation by gamma interferon

Gamma interferon (IFN-gamma) is a cytokine important to host defense which can signal through signal transducer and activator of transcription 1 (Stat1). Enterohemorrhagic Escherichia coli (EHEC) modulates host cell signal transduction to establish infection, and EHEC serotypes O113:H21 and O157:H7 both inhibit IFN-gamma-induced Stat1 tyrosine phosphorylation in vitro. The aim of this study was to delineate both bacterial and host cell factors involved in the inhibition of Stat1 tyrosine phosphorylation. Human T84 colonic epithelial cells were challenged with direct infection, viable EHEC separated from T84 cells by a filter, sodium orthovanadate, isolated flagellin, bacterial culture supernatants, and conditioned medium treated with proteinase K, trypsin, or heat inactivation. Epithelial cells were then stimulated with IFN-gamma and protein extracts were analyzed by immunoblotting. The data showed that IFN-gamma-inducible Stat1 tyrosine phosphorylation was inhibited when EHEC adhered to T84 cells, but not by bacterial culture supernatants or bacteria separated from the epithelial monolayer. Conditioned medium from T84 cells infected with EHEC O157:H7 suppressed Stat1 activation, and this was not reversed by treatment with proteinases or heat inactivation. Use of pharmacological inhibitors showed that time-dependent bacterial, but not epithelial, protein synthesis was involved. Stat1 inhibition was also independent of bacterial flagellin, host proteasome activity, and protein tyrosine phosphatases. Infection led to altered IFN-gamma receptor domain 1 subcellular distribution and decreased expression in cholesterol-enriched membrane microdomains. Thus, suppression of host cell IFN-gamma signaling by production of a contact-dependent, soluble EHEC factor may represent a novel mechanism for this pathogen to evade the host immune system.

Adenovirus subversion of immune surveillance, apoptotic and growth regulatory pathways: a model for tumorigenesis

The adenovirus system provides a novel model for evaluating the roles of multiple factors involved in tumour progression. In common with other DNA tumour viruses, adenovirus employs a variety of strategies to evade immune surveillance and perturbs cellular apoptotic and growth regulatory pathways to ensure efficient replication of progeny virions. Such subversion of cellular networks is also found in tumour cells. The mechanism behind the avoidance of immune surveillance and the extent of cellular network interference achieved by adenovirus is still being uncovered and is predicted to have ramifications for the design of cancer therapeutics.

Respiratory syncytial virus nonstructural protein 2 specifically inhibits type I interferon signal transduction

Human respiratory syncytial virus (RSV) inhibits type I interferon-induced gene expression by decreasing expression of signal transducer and activator of transcription (Stat)2. To identify the RSV protein that mediates effects on Stat2, airway epithelial cells were infected with vaccinia virus vectors that express single RSV proteins. Expression of RSV nonstructural (NS)2 protein alone was sufficient to decrease Stat2 levels. Furthermore, decreasing RSV NS2 levels using RNA interference in respiratory epithelial cells inhibited the RSV-mediated decrease in Stat2 expression. Airway epithelial cells were also infected with equivalent inoculums of RSV without or with single gene deletions of NS1 or NS2. RSV infection without NS2 expression did not result in decreased Stat2 levels or loss of type I interferon-dependent signaling, indicating that NS2 expression is necessary for RSV effects on Stat2. Taken together, our results indicate that NS2 regulates Stat2 levels during RSV infection, thereby modulating viral effects on interferon-dependent gene expression.

Increased sensitivity to staphylococcal enterotoxin B following adenoviral infection

Staphylococcal enterotoxin B induces toxic shock and is a major virulence factor of staphylococcal diseases. We examined the effects of systemic adenoviral infection on responses to staphylococcal enterotoxin B in a murine model. We found that adenoviral infection markedly increases the severity of liver injury following exposure to staphylococcal enterotoxin B without d-galactosamine sensitization. In adenovirus-infected mice, staphylococcal enterotoxin B triggered a more profound hypothermia and increased apoptosis in the liver. Consistent with these observations, we also found that adenoviral infection primed for an increased production of gamma interferon in vivo and in vitro following stimulation with staphylococcal enterotoxin B. Gamma-interferon-knockout mice did not show increased sensitivity to staphylococcal enterotoxin B following adenoviral infection. These data suggest that a preexisting viral infection primes mice for subsequent staphylococcal enterotoxin B exposure, possibly via a gamma-interferon-mediated mechanism.

Interferons alpha, beta, gamma each inhibit hepatitis C virus replication at the level of internal ribosome entry site-mediated translation

Interferon (IFN)-alpha is the standard therapy for the treatment of chronic hepatitis C, but the mechanisms underlying its antiviral action are not well understood. In this report, we demonstrated that IFN-alpha, -beta and -gamma inhibit replication of the hepatitis C virus (HCV) in a cell culture model at concentrations between 10 and 100 IU/ml. We demonstrated that the antiviral actions each of each these IFNs are targeted to the highly conserved 5' untranslated region of the HCV genome, and that they directly inhibit translation from a chimeric clone between full-length HCV genome and green fluorescent protein (GFP). This effect is not limited to HCV internal ribosome entry site (IRES), since these IFNs also inhibit translation of the encephalomyocardititis virus (EMCV) chimeric mRNA in which GFP is expressed by IRES-dependent mechanisms (pCITE-GFP). These IFNs had minimal effects on the expression of mRNAs from clones in which translation is not IRES dependent. We conclude that IFN-alpha, -beta and -gamma inhibit replication of sub-genomic HCV RNA in a cell culture model by directly inhibiting two internal translation initiation sites of HCV- and EMCV-IRES sequences present in the dicistronic HCV sub-genomic RNA. Results of this in vitro study suggest that selective inhibition of IRES-mediated translation of viral polyprotein is a general mechanism by which IFNs inhibits HCV replication.

Mumps virus V protein antagonizes interferon without the complete degradation of STAT1

Mumps virus (MuV) has been shown to antagonize the antiviral effects of interferon (IFN) through proteasome-mediated complete degradation of STAT1 by using the viral V protein (T. Kubota et al., Biochem. Biophys. Res. Commun. 283:255-259, 2001). However, we found that MuV could inhibit IFN signaling and the generation of a subsequent antiviral state long before the complete degradation of cellular STAT1 in infected cells. In MuV-infected cells, nuclear translocation and phosphorylation of STAT1 and STAT2 tyrosine residue (Y) at 701 and 689, respectively, by IFN-beta were significantly inhibited but the phosphorylation of Jak1 and Tyk2 was not inhibited. The transiently expressed MuV V protein also inhibited IFN-beta-induced Y701-STAT1 and Y689-STAT2 phosphorylation, suggesting that the V protein could block IFN-beta-induced signal transduction without the aid of other viral components. Finally, a substitution of an alanine residue in place of a cysteine residue in the C-terminal V-unique region known to be required for STAT1 degradation and inhibition of anti-IFN signaling resulted in the loss of V protein function to inhibit the Y701-STAT1 and Y689-STAT2 phosphorylation.

Haemophilus influenzae from patients with chronic obstructive pulmonary disease exacerbation induce more inflammation than colonizers

RATIONALE

Airway infection with Haemophilus influenzae causes airway inflammation, and isolation of new strains of this bacteria is associated with increased risk of exacerbations in patients with chronic obstructive pulmonary disease (COPD).

OBJECTIVE

To determine whether strains of H. influenzae associated with exacerbations cause more inflammation than strains that colonize the airways of patients with COPD.

METHODS

Exacerbation strains of H. influenzae were isolated from patients during exacerbation of clinical symptoms with subsequent development of a homologous serum antibody response and were compared with colonization strains that were not associated with symptom worsening or an antibody response. Bacterial strains were compared using an in vivo mouse model of airway infection and in vitro cell culture model of bacterial adherence and defense gene and signaling pathway activation in primary human airway epithelial cells.

RESULTS

H. influenzae associated with exacerbations caused more airway neutrophil recruitment compared with colonization strains in the mouse model of airway bacterial infection. Furthermore, exacerbation strains adhered to epithelial cells in significantly higher numbers and induced more interleukin-8 release after interaction with airway epithelial cells. This effect was likely mediated by increased activation of the nuclear factor-kappaB and p38 mitogen-activated protein kinase signaling pathways.

CONCLUSIONS

The results indicate that H. influenzae strains isolated from patients during COPD exacerbations often induce more airway inflammation and likely have differences in virulence compared with colonizing strains. These findings support the concept that bacteria infecting the airway during COPD exacerbations mediate increased airway inflammation and contribute to decreased airway function.

Activation of the epidermal growth factor receptor by respiratory syncytial virus results in increased inflammation and delayed apoptosis

Respiratory syncytial virus (RSV) preferentially infects lung epithelial cells. Infection by RSV leads to an extended inflammatory response, characterized by the release of interleukin-8 (IL-8). Activation of ERK MAP kinase is required for both RSV-induced inflammation and the extended survival of infected cells. In this study, we analyzed the role of the epidermal growth factor receptor (EGFR) in RSV activation of ERK. We demonstrate for the first time that RSV activates EGFR in lung epithelial cells. Activation of EGFR results in increased ERK activity, contributing to both the inflammatory response (IL-8 release) and prolonging the survival of RSV-infected cells. Inhibition of EGFR with siRNA decreased both ERK activation and IL-8 production after RSV. In analyzing the effect of EGFR activation on survival of RSV-infected cells, we found that EGFR activation by RSV resulted in ERK-dependent alterations in the balance of pro- versus anti-apoptotic Bcl2 proteins. RSV altered the balance between pro- and anti-apoptotic Bcl2 proteins (increased BclxL and decreased BimEL) increasing the relative amount of pro-survival proteins. This occurred in an EGFR-dependent manner. This study supports an important role for EGFR activity in the lifespan and inflammatory potential of RSV-infected epithelial cells.

NF-kappaB activation and sustained IL-8 gene expression in primary cultures of cystic fibrosis airway epithelial cells stimulated with Pseudomonas aeruginosa

The progression of lung disease in cystic fibrosis (CF) is characterized by an exuberant inflammatory response mounted by the respiratory epithelium that is further exacerbated by bacterial infection. Recent studies have demonstrated upregulation of nuclear factor-kappaB (NF-kappaB) in response to infection in genetically modified cell culture models, which is associated with expression of interleukin (IL)-8. Using human airway epithelial cells grown in primary culture, we examined in vitro activation of NF-kappaB in cells isolated from five CF (DeltaF508/DeltaF508) and three non-CF (NCF) patients in response to Pseudomonas aeruginosa. Immunofluorescence, gel-shift, and immunoblot assays demonstrated a rapid translocation of NF-kappaB subunits (p50 and p65) to the nucleus in both CF and NCF cell cultures. However, nuclear extracts from CF cells both before and following P. aeruginosa stimulation revealed elevated NF-kappaB activation compared with NCF cells. Additionally, elevated nuclear levels of the NF-kappaB inhibitor IkappaBalpha were detected in nuclei of CF cells after P. aeruginosa stimulation, but this increase was transient. There was no difference in IL-8 mRNA levels between CF and NCF cells early after stimulation, whereas expression was higher and sustained in CF cells at later times. Our results also demonstrated increased baseline translocation of NF-kappaB to nuclei of primary CF epithelial cell cultures, but intranuclear IkappaBalpha may initially block its effects following P. aeruginosa stimulation. Thus, IL-8 mRNA expression was prolonged after P. aeruginosa stimulation in CF epithelial cells, and this sustained IL-8 expression may contribute to the excessive inflammatory response in CF.

Modulation of airway inflammation and bacterial clearance by epithelial cell ICAM-1

Many cell types in the airway express the adhesive glycoprotein for leukocytes intercellular adhesion molecule-1 (ICAM-1) constitutively and/or in response to inflammatory stimuli. In this study, we identified functions of ICAM-1 on airway epithelial cells in defense against infection with Haemophilus influenzae. Initial experiments using a mouse model of airway infection in which the bacterial inoculum was mixed with agar beads that localize inflammation in airways demonstrated that ICAM-1 expression was required for efficient clearance of H. influenzae. Airway epithelial cell ICAM-1 expression required few or no leukocytes, suggesting that epithelial cells could be activated directly by interaction with bacteria. Specific inhibition of ICAM-1 function on epithelial cells by orotracheal injection of blocking antibodies resulted in decreased leukocyte recruitment and H. influenzae clearance in the airway. Inhibition of endothelial cell ICAM-1 resulted in a similar decrease in leukocyte recruitment but did not affect bacterial clearance, indicating that epithelial cell ICAM-1 had an additional contribution to airway defense independent of effects on leukocyte migration. To assess this possibility, we used an in vitro model of neutrophil phagocytosis of bacteria and observed significantly greater engulfment of bacteria by neutrophils adherent to epithelial cells expressing ICAM-1 compared with nonadherent neutrophils. Furthermore, bacterial phagocytosis and killing by neutrophils after interaction with epithelial cells were decreased when a blocking antibody inhibited ICAM-1 function. The results indicate that epithelial cell ICAM-1 participates in neutrophil recruitment into the airway, but its most important role in clearance of H. influenzae may be assistance with neutrophil-dependent bacterial killing.

Adenovirus vectors activate survival pathways in lung epithelial cells

Airway epithelial cells are often the sites of targeted adenovirus vector delivery. Activation of the host inflammatory response and modulation of signal transduction pathways by adenovirus vectors have been previously documented, including activation of MAP kinases and phosphatidylinositol 3-kinase (PI3-kinase). The effect of activation of these pathways by adenovirus vectors on cell survival has not been examined. Both the PI3-kinase/Akt and ERK/MAP kinase signaling pathways have been linked to cell survival. Akt has been found to play a role in cell survival and apoptosis through its downstream effects on apoptosis-related proteins. Constitutive activation of either PI3-kinase or Akt blocks apoptosis induced by c-Myc, UV radiation, transforming growth factor-β, Fas, and respiratory syncytial virus infection. We examined the effect of adenovirus vector infection on activation of these prosurvival pathways and its downstream consequences. Airway epithelial cells were transduced with replication-deficient adenoviral vectors containing a nonspecific transgene, green fluorescent protein driven by the cytomegalovirus promoter, or an empty vector with no transgene. They were then exposed to the proapoptotic stimulus actinomycin D plus TNF-α, and evidence of apoptosis was evaluated. Compared with the cells treated with actinomycin/TNF alone, the adenovirus vector-infected cells had a 50% reduction in apoptosis. When we examined induction of the prosurvival pathways, ERK and AKT, in the viral vector-infected cells, we found that there was significant activation of both Akt and ERK.

Specific Inhibition of Type I Interferon Signal Transduction by Respiratory Syncytial Virus

Respiratory viruses often express mechanisms to resist host antiviral systems, but the biochemical basis for evasion of interferon effects by respiratory syncytial virus (RSV) is poorly defined. In this study, we identified RSV effects on interferon (IFN)-dependent signal transduction and gene expression in human airway epithelial cells. Initial experiments demonstrated inhibition of antiviral gene expression induced by IFN-alpha and IFN-beta, but not IFN-gamma, in epithelial cells infected with RSV. Selective viral effects on type I IFN-dependent signaling were confirmed when we observed impaired type I, but not type II, IFN-induced activation of the transcription factor Stat1 in RSV-infected cells. RSV infection of airway epithelial cells resulted in decreased Stat2 expression and function with preservation of upstream signaling events, providing a molecular mechanism for viral inhibition of the type I IFN JAK-STAT pathway. Furthermore, nonspecific pharmacologic inhibition of proteasome function in RSV-infected cells restored Stat2 levels and IFN-dependent activation of Stat1. The results indicate that RSV acts on epithelial cells in the airway to directly modulate the type I IFN JAK-STAT pathway, and this effect is likely mediated though proteasome-dependent degradation of Stat2. Decreased antiviral gene expression in RSV-infected airway epithelial cells may allow RSV replication and establishment of a productive viral infection through subversion of IFN-dependent immunity.

Interferon alpha-2b inhibits negative-strand RNA and protein expression from full-length HCV1a infectious clone

We have established a T7-based model system for hepatitis C virus (HCV) 1a strain, which involves the use of a replication-defective adenovirus that carries the gene for T7 RNA polymerase and a transcription plasmid containing full-length HCV cDNA clone. To facilitate high-level expression of HCV, sub-confluent Huh7 cells were first infected with adenovirus containing the gene for the T7 RNA polymerase and then transfected with the transcription plasmid. As a negative control, part of NS5B gene of this clone was deleted which abolishes the HCV RNA-dependent RNA polymerase and prevents replication of viral RNA. This model produces high levels of structural (core, E1, E2) and nonstructural proteins (NS5), which were detected by Western blot analysis and immunofluorescence assay. Negative-strand HCV RNA was detected only in the wild-type clone in the presence of actinomycin D, and no RNA was detected with the NS5B deleted mutant control. As a practical validation of this model, we showed that IFN alpha-2b selectively inhibits negative-strand RNA synthesis by blocking at the level of protein translation. The inhibitory effect of IFN alpha-2b is not due reduction of transcription by T7 polymerase or due to intracellular degradation of HCV RNA. This in vitro model provides an efficient and reliable means of assaying negative-strand RNA, protein processing, and testing the antiviral properties of interferon.

Alternate interferon signaling pathways

More than a half a century ago, interferons (IFN) were identified as antiviral cytokines. Since that discovery, IFN have been in the forefront of basic and clinical cytokine research. The pleiotropic nature of these cytokines continues to engage a large number of investigators to define their actions further. IFN paved the way for discovery of Janus tyrosine kinase (JAK)-signal transducing activators of transcription (STAT) pathways. A number of important tumor suppressive pathways are controlled by IFN. Several infectious pathogens counteract IFN-induced signaling pathways. Recent studies indicate that IFN activate several new protein kinases, including the MAP kinase family, and downstream transcription factors. This review not only details the established IFN signaling paradigms but also provides insights into emerging alternate signaling pathways and mechanisms of pathogen-induced signaling interference.

Sphingosine kinase mediates activation of extracellular signal-related kinase and Akt by respiratory syncytial virus

Respiratory syncytial virus (RSV) preferentially infects lung epithelial cells. Infected cells remain viable well into the infection. This prolonged survival results from RSV-induced activation of pro-survival pathways, including Akt and extracellular signal-related kinase (ERK). Sphingosine 1-phosphate (S1P) is a sphingolipid metabolite with demonstrated links to cell survival. It is enzymatically generated by sequential activation of ceramidase (generation of sphingosine) and sphingosine kinase (generation of S1P). In these studies, we found that RSV stimulated neutral ceramidase and sphingosine kinase activities in lung epithelial cells. The combined effect of activation of these two enzymes would decrease proapoptotic ceramide and increase antiapoptotic S1P. S1P activated Akt and ERK within minutes, and inhibition of sphingosine kinase blocked RSV-induced ERK and Akt activation, leading to accelerated cell death after viral infection. RSV infection does eventually kill infected cells but activation of cell survival pathways significantly delays cell death. The studies are the first evidence linking sphingolipid metabolites to cell survival mechanisms in the context of a viral infection.

Human immunodeficiency virus type 1 infection inhibits granulocyte-macrophage colony-stimulating factor-induced activation of STAT5A in human monocyte-derived macrophages

Human immunodeficiency virus type 1 (HIV-1) infects cells of the monocyte/macrophage lineage. While infection of macrophages by HIV-1 is generally not cytopathic, it does impair macrophage function. In this study, we examined the effect of HIV-1 infection on intracellular signaling in human monocyte-derived macrophages (MDM) stimulated with the growth factor granulocyte-macrophage colony-stimulating factor (GM-CSF). GM-CSF is an important growth factor for cells of both the macrophage and granulocyte lineages and enhances effector functions of these cells via the heterodimeric GM-CSF receptor (GM-CSFR). A major pathway which mediates the effects of GM-CSF on macrophages involves activation of the latent transcription factor STAT5A via a Janus kinase 2 (JAK2)-dependent pathway. We demonstrate that GM-CSF-induced activation of STAT5A is inhibited in MDM after infection in vitro with the laboratory-adapted R5 strain of HIV-1, HIV-1(Ba-L), but not after infection with adenovirus. HIV-1 infection of MDM did not decrease the STAT5A or JAK2 mRNA level or STAT5A protein level or result in increased constitutive activation of STAT5A. Surface expression of either the alpha-chain or common beta(c)-chain of GM-CSFR was also unaffected. We conclude that HIV-1 inhibits GM-CSF activation of STAT5A without affecting expression of the known components of the signaling pathway. These data provide further evidence of disruption of cellular signaling pathways after HIV-1 infection, which may contribute to immune dysfunction and HIV-1 pathogenesis.

Respiratory syncytial virus up-regulates TLR4 and sensitizes airway epithelial cells to endotoxin

Airway epithelial cells are unresponsive to endotoxin (lipopolysaccharide (LPS)) exposure under normal conditions. This study demonstrates that respiratory syncytial virus (RSV) infection results in increased sensitivity to this environmental exposure. Infection with RSV results in increased expression of Toll-like receptor (TLR) 4 mRNA, protein, and increased TLR4 membrane localization. This permits significantly enhanced LPS binding to the epithelial monolayer that is blocked by disruption of the Golgi. The increased TLR4 results in an LPS-induced inflammatory response as demonstrated by increased mitogen-activated protein (MAP) kinase activity, IL-8 production, and tumor necrosis factor alpha production. RSV infection also allowed for tumor necrosis factor alpha production subsequent to TLR4 cross-linking with an immobilized antibody. These data suggest that RSV infection sensitizes airway epithelium to a subsequent environmental exposure (LPS) by altered expression and membrane localization of TLR4. The increased interaction between airway epithelial cells and LPS has the potential to profoundly alter airway inflammation.

Helicobacter pylori infection interferes with epithelial Stat6-mediated interleukin-4 signal transduction independent of cagA, cagE, or VacA

Helicobacter pylori is a bacterial pathogen evolved to chronically colonize the gastric epithelium, evade immune clearance by the host, and cause gastritis, peptic ulcers, and even gastric malignancies in some infected humans. In view of the known ability of this bacterium to manipulate gastric epithelial cell signal transduction cascades, we determined the effects of H. pylori infection on epithelial IL-4-Stat6 signal transduction. HEp-2 and MKN45 epithelial cells were infected with H. pylori strains LC11 or 8823 (type 1; cagA(+)/cagE(+)/VacA(+)), LC20 (type 2; cagA(-), cagE(-), VacA(-)), and cagA, cagE, and vacA isogenic mutants of strain 8823, with some cells receiving subsequent treatment with the Th2 cytokine IL-4, a known Stat6 activator. Immunofluorescence showed a disruption of Stat6-induced nuclear translocation by IL-4 in LC11-infected HEp-2 cells. IL-4-inducible Stat6 DNA binding in HEp-2 and MKN45 cells was abrogated by infection, but MKN45 cell viability was unaffected. A decrease in IL-4-mediated Stat6 tyrosine phosphorylation in nuclear and whole cell lysates was also observed following infection with strains LC11 and LC20, while neither strain altered IL-4 receptor chain alpha or Janus kinase 1 protein expression. Furthermore, parental strain 8823 and its isogenic cagA, cagE, and vacA mutants also suppressed IL-4-induced Stat6 tyrosine phosphorylation to comparable degrees. Thus, H. pylori did not directly activate Stat6, but blocked the IL-4-induced activation of epithelial Stat6. This may represent an evolutionarily conserved strategy to disrupt a Th2 response and evade the host immune system, allowing for successful chronic infection.

Phorbol myristate acetate and Bryostatin 1 rescue IFN-gamma inducibility of MHC class II molecules in LS1034 colorectal carcinoma cell line

BACKGROUND

The expression of major histocompatibility complex class II (MHCII) antigens in both mouse and human tumors is rare, and these antigens are not easily inducible by IFN-gamma (IFNg). Since MHCII may play an important role in the development of host antitumor immune response, we explored the possibility of restoring MHCII inducibility in several IFNg-resistant tumor cell lines using protein kinase C (PKC) agonists phorbol myristate acetate (PMA) or Bryostatin.

RESULTS

Tumor cells were co-cultured with various concentrations of PMA and IFNg for 48 hr. The expression of MHCII antigens and receptors IFNgR1 and IFNgR2 was determined by flow cytometry. We showed that the presence of as little as 0.1 ng/ml of PMA in tissue culture restored the ability of weakly inducible LS1034 colon carcinoma cells to express MHCII in response to IFNg (100 - 10,000 IU/ml) in a dose-dependent manner. Likewise, Bryostatin 1, as low as 10 ng/ml produced a 5-6 fold upregulation of MHCII. The effect of PMA was not observed in two other poorly responding cell lines, MSTO-211H mesothelioma and HepG2 hepatocellular carcinoma, and was abrogated by relatively high concentrations of PKC inhibitors staurosporine (100 nM) and GF 109203X (1,000 nM). Both surface and intracellular staining of all cell lines with antibodies against IFNgR1 and IFNgR2 failed to detect any increase in IFNg receptor expression following incubation with PMA.

CONCLUSION

In this study we showed that IFNg-inducibility of MHCII antigens in weakly inducible LS1034 colorectal carcinoma cell line can be rescued by concomitant incubation with PKC agonists. Bryostatin 1 may be considered for further investigation of IFNg-dependent MHCII induction in resistant tumors in vivo.

Enterohemorrhagic Escherichia coli O157:H7 disrupts Stat1-mediated gamma interferon signal transduction in epithelial cells

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is a clinically important bacterial enteropathogen that manipulates a variety of host cell signal transduction cascades to establish infection. However, the effect of EHEC O157:H7 on Jak/Stat signaling is unknown. To define the effect of EHEC infection on epithelial gamma interferon (IFN-gamma)-Stat1 signaling, human T84 and HEp-2 epithelial cells were infected with EHEC O157:H7 and then stimulated with recombinant human IFN-gamma. Cells were also infected with different EHEC strains, heat-killed EHEC, enteropathogenic E. coli (EPEC) O127:H6, and the commensal strain E. coli HB101. Nuclear and whole-cell protein extracts were prepared and were assayed by an electrophoretic mobility shift assay (EMSA) and by Western blotting, respectively. Cells were also processed for immunofluorescence to detect the subcellular localization of Stat1. The EMSA revealed inducible, but not constitutive, Stat1 activation upon IFN-gamma treatment of both cell lines. The EMSA also showed that 6 h of EHEC O157:H7 infection, but not 30 min of EHEC O157:H7 infection, prevented subsequent Stat1 DNA binding induced by IFN-gamma, whereas infection with EPEC did not. Immunoblotting showed that infection with EHEC, but not infection with EPEC, eliminated IFN-gamma-induced Stat1 tyrosine phosphorylation in both dose- and time-dependent fashions and disrupted inducible protein expression of the Stat1-dependent gene interferon regulatory factor 1. Immunofluorescence revealed that EHEC infection did not prevent nuclear accumulation of Stat1 after IFN-gamma treatment. Also, Stat1 tyrosine phosphorylation was suppressed by different EHEC isolates, including intimin-, type III secretion- and plasmid-deficient strains, but not by HB101 and heat-killed EHEC. These findings indicate the novel disruption of host cell signaling caused by EHEC infection but not by EPEC infection.

Measles virus suppresses interferon-alpha signaling pathway: suppression of Jak1 phosphorylation and association of viral accessory proteins, C and V, with interferon-alpha receptor complex

To establish infections, viruses use various strategies to suppress the host defense mechanism, such as interferon (IFN)-induced antiviral state. We found that cells infected with a wild strain of measles virus (MeV) displayed nearly complete suppression of IFN-alpha-induced antiviral state, but not IFN-gamma-induced state. This phenomenon is due to the suppression of IFN-alpha-inducible gene expression at a transcriptional level. In the IFN-alpha signal transduction pathway, Jak1 phosphorylation induced by IFN-alpha is dramatically suppressed in MeV-infected cells; however, phosphorylation induced by IFN-gamma is not. We performed immunoprecipitation experiments using antibodies against type 1 IFN receptor chain 1 (INFAR1) and antibody against RACK1, which is reported to be a scaffold protein interacting with type I IFN receptor chain 2 and STAT1. These experiments indicated that IFNAR1 forms a complex containing the MeV-accessory proteins C and V, RACK1, and STAT1 in MeV-infected cells but not in uninfected cells. Composition of this complex in the infected cells altered little by IFN-alpha treatment. These results indicate that MeV suppresses the IFN-alpha, but not IFN-gamma, signaling pathway by inhibition of Jak1 phosphorylation. Our data suggest that functional disorder of the type I IFN receptor complex is due to "freezing" of the receptor through its association with the C and/or V proteins of MeV.

Inflammatory response in airway epithelial cells isolated from patients with cystic fibrosis

The concept that inflammatory gene expression is dysregulated in airway epithelial cells from patients with cystic fibrosis (CF) is controversial. To examine this possibility systematically, responses to inflammatory stimuli were compared in CF airway epithelial cell lines without versus with wild-type CF transmembrane conductance regulator (CFTR) complementation and in tracheobronchial epithelial cells from patients with versus without CF. Epithelial cell expression of the leukocyte adhesion glycoprotein intercellular adhesion molecule-1 (ICAM-1) and release of the neutrophil chemoattractant interleukin (IL)-8 were determined under basal conditions or after exposure to stimuli important in CF airway inflammatory responses. We found that uncorrected CF airway epithelial cell lines inconsistently expressed higher ICAM-1 and IL-8 levels. Human CF tracheobronchial epithelial cells in primary culture released moderately increased IL-8 only after exposure to Pseudomonas aeruginosa. In CF cells with higher IL-8 release, transient expression of wild-type CFTR using an adenoviral vector did not specifically affect cytokine levels. The results indicate that there is considerable variability in airway epithelial cell responses to inflammatory stimuli among different individuals and cell models systems. Although increased ICAM-1 and IL-8 expression are observed in some CF airway epithelial cell models, many CF cells do not exhibit significant dysregulation of these important inflammatory genes.

The interferon antiviral response: from viral invasion to evasion

One of the initial responses of an organism to infection by pathogenic viruses is the synthesis of antiviral cytokines such as the type I interferons (interferon-alpha/beta), interleukins, and other proinflammatory cytokines and chemokines. Interferons provide a first line of defence against virus infections by generating an intracellular environment that restricts virus replication and signals the presence of a viral pathogen to the adaptive arm of the immune response. Interferons stimulate cells in the local environment to activate a network of interferon-stimulated genes, which encode proteins that have antiviral, antiproliferative and immunomodulatory activities. The present review focuses on recent reports that describe the activation of multiple signalling pathways following virus infection, new candidate genes that are implicated in the establishment of the antiviral state, and the strategies used by viruses and their specific viral products to antagonize and evade the host antiviral response.