Basal and reovirus-induced beta interferon (IFN-beta) and IFN-beta-stimulated gene expression are cell type specific in the cardiac protective response

Heart-specific NFAT5 knockout suppresses type I interferon signaling and aggravates coxsackievirus-induced myocarditis

Nuclear factor of activated T cells 5 (NFAT5) is an osmosensitive transcription factor that is well-studied in renal but rarely explored in cardiac diseases. Although the association of Coxsackievirus B3 (CVB3) with viral myocarditis is well-established, the role of NFAT5 in this disease remains largely unexplored. Previous research has demonstrated that NFAT5 restricts CVB3 replication yet is susceptible to cleavage by CVB3 proteases. Using an inducible cardiac-specific Nfat5-knockout mouse model, we uncovered that NFAT5-deficiency exacerbates cardiac pathology, worsens cardiac function, elevates viral load, and reduces survival rates. RNA-seq analysis of CVB3-infected mouse hearts revealed the significant impact of NFAT5-deficiency on gene pathways associated with cytokine signaling and inflammation. Subsequent in vitro and in vivo investigation validated the disruption of the cytokine signaling pathway in response to CVB3 infection, evidenced by reduced expression of key cytokines such as interferon β1 (IFNβ1), C-X-C motif chemokine ligand 10 (CXCL10), interleukin 6 (IL6), among others. Furthermore, NFAT5-deficiency hindered the formation of stress granules, leading to a reduction of important stress granule components, including plakophilin-2, a pivotal protein within the intercalated disc, thereby impacting cardiomyocyte structure and function. These findings unveil a novel mechanism by which NFAT5 inhibits CVB3 replication and pathogenesis through the promotion of antiviral type I interferon signaling and the formation of cytoplasmic stress granules, collectively identifying NFAT5 as a new cardio protective protein.

Epoxyeicosatrienoic Acids Prevent Cardiac Dysfunction in Viral Myocarditis via Interferon Type I Signaling

Myocarditis is a challenging inflammatory disease of the heart, and better understanding of its pathogenesis is needed to develop specific drug therapies. Epoxyeicosatrienoic acids (EETs), active molecules synthesized by CYP (cytochrome P450) enzymes from arachidonic acids and hydrolyzed to less active dihydroxyeicosatrienoic acids by sEH (soluble epoxide hydrolase), have been attributed anti-inflammatory activity. Here, we investigated whether EETs have immunomodulatory activity and exert protective effects on coxsackie B3 virus-induced myocarditis. Viral infection altered eicosanoid epoxide and diol levels in both patients with myocarditis and in the murine heart and correlated with the increased expression and activity of sEH after coxsackie B3 virus infection. Administration of a sEH inhibitor prevented coxsackie B3 virus-induced cardiac dysfunction and inflammatory infiltration. Importantly, EET/sEH inhibitor treatment attenuated viral infection or improved viral resistance by activating type I IFN (interferon) signaling. At the molecular level, EETs enhanced the interaction between GSK3β (glycogen synthase kinase-3 beta) and TBK1 (TANK-binding kinase 1) to promote IFN-β production. Our findings revealed that EETs and sEH inhibitors prevent the progress of coxsackie B3 virus-induced myocarditis, particularly by promoting viral resistance by increasing IFN production.

Cell Killing by Reovirus: Mechanisms and Consequences

Infection of host cells by mammalian reovirus in culture or in tissues of infected animals results in cell death. Cell death of infected neurons and myocytes contributes to the pathogenesis of reovirus-induced encephalitis and myocarditis in a newborn mouse model. Thus, reovirus-induced cell death has been used to investigate the basis of viral disease. Depending on the cell type, infection of host cells by reovirus results in one of two forms of cell death-apoptosis and necroptosis. In addition to the obvious differences in how these two forms of cell death are executed, the mechanisms by which reovirus infection initiates and transduces signals that lead to each of these types of cell death are distinct. In this review, we discuss how apoptosis and necroptosis are triggered by events at different stages of infection. We also describe how innate immune recognition of reovirus genomic material and type I interferon signaling pathways connect with the core components of the apoptosis and necroptosis machinery. The impact of different cell death mediators on viral pathogenesis and the potential of reovirus as an oncolytic vector are also outlined.

Spatiotemporal transcriptomics reveals pathogenesis of viral myocarditis

A significant fraction of sudden death in children and young adults is due to viral myocarditis, an inflammatory disease of the heart. In this study, by using integrated single-cell and spatial transcriptomics, we created a high-resolution, spatially resolved transcriptome map of reovirus-induced myocarditis in neonatal mouse hearts. We assayed hearts collected at three timepoints after infection and studied the temporal, spatial and cellular heterogeneity of host-virus interactions. We further assayed the intestine, the primary site of reovirus infection, to establish a full chronology of molecular events that ultimately lead to myocarditis. We found that inflamed endothelial cells recruit cytotoxic T cells and undergo pyroptosis in the myocarditic tissue. Analyses of spatially restricted gene expression in myocarditic regions and the border zone identified immune-mediated cell-type-specific injury and stress responses. Overall, we observed a complex network of cellular phenotypes and spatially restricted cell-cell interactions associated with reovirus-induced myocarditis in neonatal mice.

The M2 Gene Is a Determinant of Reovirus-Induced Myocarditis

Although a broad range of viruses cause myocarditis, the mechanisms that underlie viral myocarditis are poorly understood. Here, we report that the M2 gene is a determinant of reovirus myocarditis. The M2 gene encodes outer capsid protein μ1, which mediates host membrane penetration during reovirus entry. We infected newborn C57BL/6 mice with reovirus strain type 1 Lang (T1L) or a reassortant reovirus in which the M2 gene from strain type 3 Dearing (T3D) was substituted into the T1L genetic background (T1L/T3DM2). T1L was non-lethal in wild-type mice, whereas greater than 90% of mice succumbed to T1L/T3DM2 infection. T1L/T3DM2 produced higher viral loads than T1L at the site of inoculation. In secondary organs, T1L/T3DM2 was detected with more rapid kinetics and reached higher peak titers than T1L. We found that hearts from T1L/T3DM2-infected mice were grossly abnormal, with large lesions indicative of substantial inflammatory infiltrate. Lesions in T1L/T3DM2-infected mice contained necrotic cardiomyocytes with pyknotic debris, and extensive lymphocyte and histiocyte infiltration. In contrast, T1L induced the formation of small purulent lesions in a small subset of animals, consistent with T1L being mildly myocarditic. Finally, more activated caspase-3-positive cells were observed in hearts from animals infected with T1L/T3DM2 compared to T1L. Together, our findings indicate that substitution of the T3D M2 allele into an otherwise T1L genetic background is sufficient to change a non-lethal infection into a lethal infection. Our results further indicate that T3D M2 enhances T1L replication and dissemination in vivo, which potentiates the capacity of reovirus to cause myocarditis. IMPORTANCE Reovirus is a non-enveloped virus with a segmented double-stranded RNA genome that serves as a model for studying viral myocarditis. The mechanisms by which reovirus drives myocarditis development are not fully elucidated. We found that substituting the M2 gene from strain type 3 Dearing (T3D) into an otherwise type 1 Lang (T1L) genetic background (T1L/T3DM2) was sufficient to convert the non-lethal T1L strain into a lethal infection in neonatal C57BL/6 mice. T1L/T3DM2 disseminated more efficiently and reached higher maximum titers than T1L in all organs tested, including the heart. T1L is mildly myocarditic and induced small areas of cardiac inflammation in a subset of mice. In contrast, hearts from mice infected with T1L/T3DM2 contained extensive cardiac inflammatory infiltration and more activated caspase-3-positive cells, which is indicative of apoptosis. Together, our findings identify the reovirus M2 gene as a new determinant of reovirus-induced myocarditis.

Cell type-specific roles of PAR1 in Coxsackievirus B3 infection

Protease-activated receptor 1 (PAR1) is widely expressed in humans and mice, and is activated by a variety of proteases, including thrombin. Recently, we showed that PAR1 contributes to the innate immune response to viral infection. Mice with a global deficiency of PAR1 expressed lower levels of CXCL10 and had increased Coxsackievirus B3 (CVB3)-induced myocarditis compared with control mice. In this study, we determined the effect of cell type-specific deletion of PAR1 in cardiac myocytes (CMs) and cardiac fibroblasts (CFs) on CVB3-induced myocarditis. Mice lacking PAR1 in either CMs or CFs exhibited increased CVB3 genomes, inflammatory infiltrates, macrophages and inflammatory mediators in the heart and increased CVB3-induced myocarditis compared with wild-type controls. Interestingly, PAR1 enhanced poly I:C induction of CXCL10 in rat CFs but not in rat neonatal CMs. Importantly, activation of PAR1 reduced CVB3 replication in murine embryonic fibroblasts and murine embryonic cardiac myocytes. In addition, we showed that PAR1 reduced autophagy in murine embryonic fibroblasts and rat H9c2 cells, which may explain how PAR1 reduces CVB3 replication. These data suggest that PAR1 on CFs protects against CVB3-induced myocarditis by enhancing the anti-viral response whereas PAR1 on both CMs and fibroblasts inhibits viral replication.

Effects of targeting sumoylation processes during latent and induced Epstein-Barr virus infections using the small molecule inhibitor ML-792

As the second leading cause of death in the United States, cancer has a considerable impact on society, and one cellular process that is commonly dysregulated in many cancers is the post-translational modification of proteins by the Small Ubiquitin-like Modifier (SUMO; sumoylation). We documented that sumoylation processes are up-regulated in lymphoma tissues in the presence of Latent Membrane Protein-1 (LMP1), the principal oncoprotein of Epstein-Barr virus (EBV). LMP1-mediated dysregulation of cellular sumoylation processes contributes to oncogenesis, modulates innate immune responses, and aids the maintenance of viral latency. Manipulation of protein sumoylation has been proposed for anti-cancer and anti-viral therapies; however, known inhibitors of sumoylation do not only target sumoylation processes. Recently, a specific and selective small-molecule inhibitor of sumoylation (ML-792) was identified; however, nothing is known about the effect of ML-792 on LMP1-mediated dysregulation of cellular sumoylation or the EBV life-cycle. We hypothesized that ML-792 modulates viral replication and the oncogenic potential of EBV LMP1 by inhibiting protein sumoylation. Results showed that ML-792 inhibited sumoylation processes in multiple EBV-positive B cell lines and EBV-positive nasopharyngeal carcinoma cell lines but not in their EBV-negative counterparts. Focusing on its effect on B cells, ML-792 inhibited B-cell growth and promoted cell death at very low doses. ML-792 also modulated LMP1-induced cell migration and cell adhesion, which suggests the abrogation of the oncogenic potential of LMP1. Finally, while higher concentrations of ML-792 were sufficient to induce low levels EBV spontaneous reactivation, they decreased the production of new infectious virus following an induced reactivation and the infection of new cells, suggesting that ML-792 has anti-viral potential. Together, these findings suggest that ML-792 may be a potential therapeutic drug to treat EBV-associated lymphoid malignancies by targeting oncogenesis and the EBV life-cycle.

The Epstein-Barr Virus Oncoprotein, LMP1, Regulates the Function of SENP2, a SUMO-protease

Epstein-Barr virus (EBV) latent membrane protein-1 (LMP1) activates numerous signal transduction pathways using its C-terminal activating regions. We reported that LMP1 increased global levels of sumoylated proteins, which aided the oncogenic nature of LMP1. Because increased protein sumoylation is detected in numerous cancers, we wanted to elucidate additional mechanisms by which LMP1 modulates the sumoylation machinery. Results indicated that SUMO-protease activity decreased in a LMP1-dependent manner, so we hypothesized that LMP1 inhibits SUMO-protease activity, resulting in reduced de-sumoylation of cellular proteins, which contributes to the detected accumulation of sumoylated proteins in EBV-positive lymphomas. Focusing on SENP2, findings revealed that LMP1 expression corresponded with increased sumoylation of SENP2 at K48 and K447 in a CTAR-dependent manner. Interestingly, independent of LMP1-induced sumoylation of SENP2, LMP1 also decreased SENP2 activity, decreased SENP2 turnover, and altered the localization of SENP2, which led us to investigate if LMP1 regulated the biology of SENP2 by a different post-translational modification, specifically ubiquitination. Data showed that expression of LMP1 inhibited the ubiquitination of SENP2, and inhibition of ubiquitination was sufficient to mimic LMP1-induced changes in SENP2 activity and trafficking. Together, these findings suggest that LMP1 modulates different post-translational modifications of SENP2 in order to modulate its biology and identify a third member of the sumoylation machinery that is manipulated by LMP1 during latent EBV infections, which can affect oncogenesis.

Using glycyrrhizic acid to target sumoylation processes during Epstein-Barr virus latency

Cellular sumoylation processes are proposed targets for anti-viral and anti-cancer therapies. We reported that Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) dysregulates cellular sumoylation processes, contributing to its oncogenic potential in EBV-associated malignancies. Ginkgolic acid and anacardic acid, known inhibitors of sumoylation, inhibit LMP1-induced protein sumoylation; however, both drugs have adverse effects in hosts. Here we test the effects of glycyrrhizic acid, a medicinal botanical extract with anti-inflammatory, anti-carcinogenic, and anti-viral properties, on cellular sumoylation processes. While glycyrrhizic acid is known to inhibit EBV penetration, its affect on cellular sumoylation processes remains to be documented. We hypothesized that glycyrrhizic acid inhibits cellular sumoylation processes and may be a viable treatment for Epstein-Barr virus-associated malignancies. Results showed that glycyrrhizic acid inhibited sumoylation processes (without affecting ubiquitination processes), limited cell growth, and induced apoptosis in multiple cell lines. Similar to ginkgolic acid; glycyrrhizic acid targeted the first step of the sumoylation process and resulted in low levels of spontaneous EBV reactivation. Glycyrrhizic acid did not affect induced reactivation of the virus, but the presence of the extract did reduce the ability of the produced virus to infect additional cells. Therefore, we propose that glycyrrhizic acid may be a potential therapeutic drug to augment the treatment of EBV-associated lymphoid malignancies.

Biphasic and cardiomyocyte-specific IFIT activity protects cardiomyocytes from enteroviral infection

Viral myocarditis is a serious disease, commonly caused by type B coxsackieviruses (CVB). Here we show that innate immune protection against CVB3 myocarditis requires the IFIT (IFN-induced with tetratricopeptide) locus, which acts in a biphasic manner. Using IFIT locus knockout (IFITKO) cardiomyocytes we show that, in the absence of the IFIT locus, viral replication is dramatically increased, indicating that constitutive IFIT expression suppresses CVB replication in this cell type. IFNβ pre-treatment strongly suppresses CVB3 replication in wild type (wt) cardiomyocytes, but not in IFITKO cardiomyocytes, indicating that other interferon-stimulated genes (ISGs) cannot compensate for the loss of IFITs in this cell type. Thus, in isolated wt cardiomyocytes, the anti-CVB3 activity of IFITs is biphasic, being required for protection both before and after T1IFN signaling. These in vitro findings are replicated in vivo. Using novel IFITKO mice we demonstrate accelerated CVB3 replication in pancreas, liver and heart in the hours following infection. This early increase in virus load in IFITKO animals accelerates the induction of other ISGs in several tissues, enhancing virus clearance from some tissues, indicating that–in contrast to cardiomyocytes–other ISGs can offset the loss of IFITs from those cell types. In contrast, CVB3 persists in IFITKO hearts, and myocarditis occurs. Thus, cardiomyocytes have a specific, biphasic, and near-absolute requirement for IFITs to control CVB infection.

Viruses can infect the heart, causing inflammation–termed myocarditis–which is a serious, and sometimes fatal, disease. One way to combat the infection is by stimulating our immune system, encouraging it to fight the virus. However, the treatment that is currently used “revs up” many different parts of our immune system, including some that play little or no role in clearing the virus, and this wide-ranging activation increases the risk of potentially-harmful side effects. We want to identify the parts of the immune system that fight virus infections of the heart, so that we can improve the treatment of viral myocarditis by selectively stimulating only those immune responses, thereby retaining the benefit of treatment (i.e., clearing the virus) while reducing its cost (i.e. lowering the risk of harmful side effects). In this paper, we demonstrate that a family of proteins called IFITs play a role in protecting many tissues against these infections, but are particularly important in heart muscle cells, in which they are indispensable. Thus, IFITs represent a possible target for the treatment of viral myocarditis.

Interferon-λ3 Promotes Epithelial Defense and Barrier Function Against Cryptosporidium parvum Infection

BACKGROUND & AIMS

The epithelial response is critical for intestinal defense against Cryptosporidium, but is poorly understood. To uncover the host strategy for defense against Cryptosporidium, we examined the transcriptional response of intestinal epithelial cells (IECs) to C parvum in experimentally infected piglets by microarray. Up-regulated genes were dominated by targets of interferon (IFN) and IFN-λ3 was up-regulated significantly in infected piglet mucosa. Although IFN-λ has been described as a mediator of epithelial defense against viral pathogens, there is limited knowledge of any role against nonviral pathogens. Accordingly, the aim of the study was to determine the significance of IFN-λ3 to epithelial defense and barrier function during C parvum infection.

METHODS

The significance of C parvum-induced IFN-λ3 expression was determined using an immunoneutralization approach in neonatal C57BL/6 mice. The ability of the intestinal epithelium to up-regulate IFN-λ2/3 expression in response to C parvum infection and the influence of IFN-λ2/3 on epithelial defense against C parvum invasion, intracellular development, and loss of barrier function was examined using polarized monolayers of a nontransformed porcine-derived small intestinal epithelial cell line (IPEC-J2). Specifically, changes in barrier function were quantified by measurement of transepithelial electrical resistance and transepithelial flux studies.

RESULTS

Immunoneutralization of IFN-λ2/3 in C parvum-infected neonatal mice resulted in a significantly increased parasite burden, fecal shedding, and villus blunting with crypt hyperplasia during peak infection. In vitro, C parvum was sufficient to induce autonomous IFN-λ3 and interferon-stimulated gene 15 expression by IECs. Priming of IECs with recombinant human IFN-λ3 promoted cellular defense against C parvum infection and abrogated C parvum-induced loss of barrier function by decreasing paracellular permeability to sodium.

CONCLUSIONS

These studies identify IFN-λ3 as a key epithelial defense mechanism against C parvum infection.

Epstein-Barr Virus Latent Membrane Protein-1 Induces the Expression of SUMO-1 and SUMO-2/3 in LMP1-positive Lymphomas and Cells

Epstein-Barr Virus latent membrane protein-1 (LMP1) interacts with the SUMO-conjugating enzyme Ubc9, which induces protein sumoylation and may contribute to LMP1-mediated oncogenesis. After analyzing human lymphoma tissues and EBV-positive cell lines, we now document a strong correlation between LMP1 and sumo-1/2/3 or SUMO-1/2/3 levels, and show that LMP1-induced sumo expression requires the activation of NF-κB signaling through CTAR1 and CTAR2. Together, these results point to a second mechanism by which LMP1 dysregulates sumoylation processes and adds EBV-associated lymphomas to the list of malignancies associated with increased SUMO expression.

Antiviral Responses and Biological Concequences of Piscine orthoreovirus Infection in Salmonid Erythrocytes

Salmonid red blood cells are the main target cells for Piscine orthoreovirus (PRV). Three genotypes of PRV (PRV-1,2,3) infect Atlantic salmon (Salmo salar), Chinook salmon (Onchorhynchus tshawytscha), Coho salmon (Oncorhynchus kisutch), rainbow trout (Onchorhynchus mykiss) and brown trout (Salmo trutta), and can cause diseases like heart and skeletal muscle inflammation (HSMI), jaundice syndrome, erythrocyte inclusion body syndrome (EIBS) and proliferative darkening syndrome (PDS). Purified PRV administrated to fish has proven the causality for HSMI and EIBS. During the early peak phase of infection, salmonid erythrocytes are the main virus-replicating cells. In this initial phase, cytoplasmic inclusions called "virus factories" can be observed in the erythrocytes, and are the primary sites for the formation of new virus particles. The PRV-infected erythrocytes in Atlantic salmon mount a strong long-lasting innate antiviral response lasting for many weeks after the onset of infection. The antiviral response of Atlantic salmon erythrocytes involves upregulation of potential inhibitors of translation. In accordance with this, PRV-1 protein production in erythrocytes halts while virus RNA can persist for months. Furthermore, PRV infection in Coho salmon and rainbow trout are associated with anemia, and in Atlantic salmon lower hemoglobin levels are observed. Here we summarize and discuss the recently published findings on PRV infection, replication and effects on salmonid erythrocytes, and discuss how PRV can be a useful tool for the study of innate immune responses in erythrocytes, and help reveal novel immune functions of the red blood cells in fish.

25-Hydroxycholesterol Production by the Cholesterol-25-Hydroxylase Interferon-Stimulated Gene Restricts Mammalian Reovirus Infection

Following the initial detection of viral infection, innate immune responses trigger the induction of numerous interferon-stimulated genes (ISGs) to inhibit virus replication and dissemination. One such ISG encodes cholesterol-25-hydroxylase (CH25H), an enzyme that catalyzes the oxidation of cholesterol to form a soluble product, 25-hydroxycholesterol (25HC). Recent studies have found that CH25H is broadly antiviral; it inhibits infection by several viruses. For enveloped viruses, 25HC inhibits membrane fusion, likely by altering membrane characteristics such as hydrophobicity or cholesterol aggregation. However, the mechanisms by which 25HC restricts infection of nonenveloped viruses are unknown. We examined whether 25HC restricts infection by mammalian reovirus. Treatment with 25HC restricted infection by reovirus prototype strains type 1 Lang and type 3 Dearing. In contrast to reovirus virions, 25HC did not restrict infection by reovirus infectious subvirion particles (ISVPs), which can penetrate either directly at the cell surface or in early endosomal membranes. Treatment with 25HC altered trafficking of reovirus particles to late endosomes and delayed the kinetics of reovirus uncoating. These results suggest that 25HC inhibits the efficiency of cellular entry of reovirus virions, which may require specific endosomal membrane dynamics for efficient membrane penetration.IMPORTANCE The innate immune system is crucial for effective responses to viral infection. Type I interferons, central components of innate immunity, induce expression of hundreds of ISGs; however, the mechanisms of action of these antiviral proteins are not well understood. CH25H, encoded by an ISG, represents a significant constituent of these cellular antiviral strategies, as its metabolic product, 25HC, can act in both an autocrine and a paracrine fashion to protect cells from infection and has been shown to limit viral infection in animal models. Further investigation into the mechanism of action of 25HC may inform novel antiviral therapies and influence the use of mammalian reovirus in clinical trials as an oncolytic agent.

Spontaneous activation of a MAVS-dependent antiviral signaling pathway determines high basal interferon-β expression in cardiac myocytes

Viral myocarditis is a leading cause of sudden death in young adults as the limited turnover of cardiac myocytes renders the heart particularly vulnerable to viral damage. Viruses induce an antiviral type I interferon (IFN-α/β) response in essentially all cell types, providing an immediate innate protection. Cardiac myocytes express high basal levels of IFN-β to help pre-arm them against viral infections, however the mechanism underlying this expression remains unclear. Using primary cultures of murine cardiac and skeletal muscle cells, we demonstrate here that the mitochondrial antiviral signaling (MAVS) pathway is spontaneously activated in unstimulated cardiac myocytes but not cardiac fibroblasts or skeletal muscle cells. Results suggest that MAVS association with the mitochondrial-associated ER membranes (MAM) is a determinant of high basal IFN-β expression, and demonstrate that MAVS is essential for spontaneous high basal expression of IFN-β in cardiac myocytes and the heart. Together, results provide the first mechanism for spontaneous high expression of the antiviral cytokine IFN-β in a poorly replenished and essential cell type.

Unphosphorylated ISGF3 drives constitutive expression of interferon-stimulated genes to protect against viral infections

Interferon (IFN)-stimulated genes (ISGs) are antiviral effectors that are induced by IFNs through the formation of a tripartite transcription factor ISGF3, which is composed of IRF9 and phosphorylated forms of STAT1 and STAT2. However, we found that IFN-independent ISG expression was detectable in immortalized cell lines, primary intestinal and liver organoids, and liver tissues. The constitutive expression of ISGs was mediated by the unphosphorylated ISGF3 (U-ISGF3) complex, consisting of IRF9 together with unphosphorylated STAT1 and STAT2. Under homeostatic conditions, STAT1, STAT2, and IRF9 were found in the nucleus. Analysis of a chromatin immunoprecipitation sequencing data set revealed that STAT1 specifically bound to the promoters of ISGs even in the absence of IFNs. Knockdown of STAT1, STAT2, or IRF9 by RNA interference led to the decreased expression of various ISGs in Huh7.5 human liver cells, which was confirmed in mouse embryonic fibroblasts (MEFs) from STAT1^-/-, STAT2^-/-, or IRF9^-/- mice. Furthermore, decreased ISG expression was accompanied by increased replication of hepatitis C virus and hepatitis E virus. Conversely, simultaneous overexpression of all ISGF3 components, but not any single factor, induced the expression of ISGs and inhibited viral replication; however, no phosphorylated STAT1 and STAT2 were detected. A phosphorylation-deficient STAT1 mutant was comparable to the wild-type protein in mediating the IFN-independent expression of ISGs and antiviral activity, suggesting that ISGF3 works in a phosphorylation-independent manner. These data suggest that the U-ISGF3 complex is both necessary and sufficient for constitutive ISG expression and antiviral immunity under homeostatic conditions.

A Cytoplasmic RNA Virus Alters the Function of the Cell Splicing Protein SRSF2

To replicate efficiently, viruses must create favorable cell conditions and overcome cell antiviral responses. We previously reported that the reovirus protein μ2 from strain T1L, but not strain T3D, represses one antiviral response: alpha/beta interferon signaling. We report here that T1L, but not T3D, μ2 localizes to nuclear speckles, where it forms a complex with the mRNA splicing factor SRSF2 and alters its subnuclear localization. Reovirus replicates in cytoplasmic viral factories, and there is no evidence that reovirus genomic or messenger RNAs are spliced, suggesting that T1L μ2 might target splicing of cell RNAs. Indeed, RNA sequencing revealed that reovirus T1L, but not T3D, infection alters the splicing of transcripts for host genes involved in mRNA posttranscriptional modifications. Moreover, depletion of SRSF2 enhanced reovirus replication and cytopathic effect, suggesting that T1L μ2 modulation of splicing benefits the virus. This provides the first report of viral antagonism of the splicing factor SRSF2 and identifies the viral protein that determines strain-specific differences in cell RNA splicing.IMPORTANCE Efficient viral replication requires that the virus create favorable cell conditions. Many viruses accomplish this by repressing specific antiviral responses. We demonstrate here that some mammalian reoviruses, RNA viruses that replicate strictly in the cytoplasm, express a protein variant that localizes to nuclear speckles, where it targets a cell mRNA splicing factor. Infection with a reovirus strain that targets this splicing factor alters splicing of cell mRNAs involved in the maturation of many other cell mRNAs. Depletion of this cell splicing factor enhances reovirus replication and cytopathic effect. Our results provide the first evidence of viral antagonism of this splicing factor and suggest that downstream consequences to the cell are global and benefit the virus.

NF-κB activation is cell type-specific in the heart

Viral myocarditis is common and can progress to cardiac failure. Cardiac cell pro-inflammatory responses are critical for viral clearance, however sustained inflammatory responses contribute to cardiac damage. The transcription factor NF-κB regulates expression of many pro-inflammatory cytokines, but basal and induced activation of NF-κB in different cardiac cell types have not been compared. Here, we used primary cultures of cardiac myocytes and cardiac fibroblasts to identify cardiac cell type-specific events. We show that while viral infection readily stimulates activation of NF-κB in cardiac fibroblasts, cardiac myocytes are largely recalcitrant to activation of NF-κB. Moreover, we show that cardiac myocyte subpopulations differ in their NF-κB subcellular localization and identify the cis-Golgi as a cardiac myocyte-specific host compartment. Together, results indicate that NF-κB-dependent signaling in the heart is cardiac cell type-specific, likely reflecting mechanisms that have evolved to balance responses that can be either protective or damaging to the heart.

Virus-Infected Human Mast Cells Enhance Natural Killer Cell Functions

Mucosal surfaces are protected from infection by both structural and sentinel cells, such as mast cells. The mast cell's role in antiviral responses is poorly understood; however, they selectively recruit natural killer (NK) cells following infection. Here, the ability of virus-infected mast cells to enhance NK cell functions was examined. Cord blood-derived human mast cells infected with reovirus (Reo-CBMC) and subsequent mast cell products were used for the stimulation of human NK cells. NK cells upregulated the CD69 molecule and cytotoxicity-related genes, and demonstrated increased cytotoxic activity in response to Reo-CBMC soluble products. NK cell interferon (IFN)-γ production was also promoted in the presence of interleukin (IL)-18. In vivo, SCID mice injected with Reo-CBMC in a subcutaneous Matrigel model, could recruit and activate murine NK cells, a property not shared by normal human fibroblasts. Soluble products of Reo-CBMC included IL-10, TNF, type I and type III IFNs. Blockade of the type I IFN receptor abrogated NK cell activation. Furthermore, reovirus-infected mast cells expressed multiple IFN-α subtypes not observed in reovirus-infected fibroblasts or epithelial cells. Our data define an important mast cell IFN response, not shared by structural cells, and a subsequent novel mast cell-NK cell immune axis in human antiviral host defense.

LMP1-Induced Sumoylation Influences the Maintenance of Epstein-Barr Virus Latency through KAP1

As a herpesvirus, Epstein-Barr virus (EBV) establishes a latent infection that can periodically undergo reactivation, resulting in lytic replication and the production of new infectious virus. Latent membrane protein-1 (LMP1), the principal viral oncoprotein, is a latency-associated protein implicated in regulating viral reactivation and the maintenance of latency. We recently found that LMP1 hijacks the SUMO-conjugating enzyme Ubc9 via its C-terminal activating region-3 (CTAR3) and induces the sumoylation of cellular proteins. Because protein sumoylation can promote transcriptional repression, we hypothesized that LMP1-induced protein sumoylation induces the repression of EBV lytic promoters and helps maintain the viral genome in its latent state. We now show that with inhibition of LMP1-induced protein sumoylation, the latent state becomes less stable or leakier in EBV-transformed lymphoblastoid cell lines. The cells are also more sensitive to viral reactivation induced by irradiation, which results in the increased production and release of infectious virus, as well as increased susceptibility to ganciclovir treatment. We have identified a target of LMP1-mediated sumoylation that contributes to the maintenance of latency in this context: KRAB-associated protein-1 (KAP1). LMP1 CTAR3-mediated sumoylation regulates the function of KAP1. KAP1 also binds to EBV OriLyt and immediate early promoters in a CTAR3-dependent manner, and inhibition of sumoylation processes abrogates the binding of KAP1 to these promoters. These data provide an additional line of evidence that supports our findings that CTAR3 is a distinct functioning regulatory region of LMP1 and confirm that LMP1-induced sumoylation may help stabilize the maintenance of EBV latency.

IMPORTANCE

Epstein-Barr virus (EBV) latent membrane protein-1 (LMP1) plays an important role in the maintenance of viral latency. Previously, we documented that LMP1 targets cellular proteins to be modified by a ubiquitin-like protein (SUMO). We have now identified a function for this LMP1-induced modification of cellular proteins in the maintenance of EBV latency. Because latently infected cells have to undergo viral reactivation in order to be vulnerable to antiviral drugs, these findings identify a new way to increase the rate of EBV reactivation, which increases cell susceptibility to antiviral therapies.

Roles of PAR1 and PAR2 in viral myocarditis

Viral myocarditis is estimated to cause ~20% of sudden death in people under the age of 40. A variety of viruses have been found to cause myocarditis including coxsackievirus B3 (CVB3). Many studies have been performed with CVB3 because there is a mouse model of CVB3-induced myocarditis. Studies have shown that the TLR3-IFNβ pathway plays a central role in the innate immune response to CVB3 infection. Our laboratory studies the role of protease activated receptors (PAR) in different biological responses including viral infection. We examined the effect of a deficiency in either PAR1 or PAR2 on CVB3-induced myocarditis. Interestingly, we found that PAR1 knockout mice had increased cardiac injury whereas PAR2 knockout mice had decreased cardiac injury. Our studies support the notion that PARs modulate the innate immune response and can have both positive and negative effects on TLR-dependent responses.

Multiple roles of the coagulation protease cascade during virus infection

The coagulation cascade is activated during viral infections. This response may be part of the host defense system to limit spread of the pathogen. However, excessive activation of the coagulation cascade can be deleterious. In fact, inhibition of the tissue factor/factor VIIa complex reduced mortality in a monkey model of Ebola hemorrhagic fever. Other studies showed that incorporation of tissue factor into the envelope of herpes simplex virus increases infection of endothelial cells and mice. Furthermore, binding of factor X to adenovirus serotype 5 enhances infection of hepatocytes but also increases the activation of the innate immune response to the virus. Coagulation proteases activate protease-activated receptors (PARs). Interestingly, we and others found that PAR1 and PAR2 modulate the immune response to viral infection. For instance, PAR1 positively regulates TLR3-dependent expression of the antiviral protein interferon β, whereas PAR2 negatively regulates expression during coxsackievirus group B infection. These studies indicate that the coagulation cascade plays multiple roles during viral infections.

In vivo ablation of type I interferon receptor from cardiomyocytes delays coxsackieviral clearance and accelerates myocardial disease

Acute coxsackievirus B3 (CVB3) infection is one of the most prevalent causes of acute myocarditis, a disease that frequently is identified only after the sudden death of apparently healthy individuals. CVB3 infects cardiomyocytes, but the infection is highly focal, even in the absence of a strong adaptive immune response, suggesting that virus spread within the heart may be tightly constrained by the innate immune system. Type I interferons (T1IFNs) are an obvious candidate, and T1IFN receptor (T1IFNR) knockout mice are highly susceptible to CVB3 infection, succumbing within a few days of challenge. Here, we investigated the role of T1IFNs in the heart using a mouse model in which the T1IFNR gene can be ablated in vivo, specifically in cardiomyocytes. We found that T1IFN signaling into cardiomyocytes contributed substantially to the suppression of viral replication and infectious virus yield in the heart; in the absence of such signaling, virus titers were markedly elevated by day 3 postinfection (p.i.) and remained high at day 12 p.i., a time point at which virus was absent from genetically intact littermates, suggesting that the T1IFN-unresponsive cardiomyocytes may act as a safe haven for the virus. Nevertheless, in these mice the myocardial infection remained highly focal, despite the cardiomyocytes' inability to respond to T1IFN, indicating that other factors, as yet unidentified, are sufficient to prevent the more widespread dissemination of the infection throughout the heart. The absence of T1IFN signaling into cardiomyocytes also was accompanied by a profound acceleration and exacerbation of myocarditis and by a significant increase in mortality.

IMPORTANCE

Acute coxsackievirus B3 (CVB3) infection is one of the most common causes of acute myocarditis, a serious and sometimes fatal disease. To optimize treatment, it is vital that we identify the immune factors that limit virus spread in the heart and other organs. Type I interferons play a key role in controlling many virus infections, but it has been suggested that they may not directly impact CVB3 infection within the heart. Here, using a novel line of transgenic mice, we show that these cytokines signal directly into cardiomyocytes, limiting viral replication, myocarditis, and death.

Interferon regulatory factor 7 functions as a novel negative regulator of pathological cardiac hypertrophy

Cardiac hypertrophy is a complex pathological process that involves multiple factors including inflammation and apoptosis. Interferon regulatory factor 7 (IRF7) is a multifunctional regulator that participates in immune regulation, cell differentiation, apoptosis, and oncogenesis. However, the role of IRF7 in cardiac hypertrophy remains unclear. We performed aortic banding in cardiac-specific IRF7 transgenic mice, IRF7 knockout mice, and the wild-type littermates of these mice. Our results demonstrated that IRF7 was downregulated in aortic banding-induced animal hearts and cardiomyocytes that had been treated with angiotensin II or phenylephrine for 48 hours. Accordingly, heart-specific overexpression of IRF7 significantly attenuated pressure overload-induced cardiac hypertrophy, fibrosis, and dysfunction, whereas loss of IRF7 led to opposite effects. Moreover, IRF7 protected against angiotensin II-induced cardiomyocyte hypertrophy in vitro. Mechanistically, we identified that IRF7-dependent cardioprotection was mediated through IRF7 binding to inhibitor of κB kinase-β, and subsequent nuclear factor-κB inactivation. In fact, blocking nuclear factor-κB signaling with cardiac-specific inhibitors of κBα(S32A/S36A) super-repressor transgene counteracted the adverse effect of IRF7 deficiency. Conversely, activation of nuclear factor-κB signaling via a cardiac-specific conditional inhibitor of κB kinase-β(S177E/S181E) (constitutively active) transgene negated the antihypertrophic effect of IRF7 overexpression. Our data demonstrate that IRF7 acts as a novel negative regulator of pathological cardiac hypertrophy by inhibiting nuclear factor-κB signaling and may constitute a potential therapeutic target for pathological cardiac hypertrophy.

An ITAM in a nonenveloped virus regulates activation of NF-κB, induction of beta interferon, and viral spread

Immunoreceptor tyrosine-based activation motifs (ITAMs) are signaling domains located within the cytoplasmic tails of many transmembrane receptors and associated adaptor proteins that mediate immune cell activation. ITAMs also have been identified in the cytoplasmic tails of some enveloped virus glycoproteins. Here, we identified ITAM sequences in three mammalian reovirus proteins: μ2, σ2, and λ2. We demonstrate for the first time that μ2 is phosphorylated, contains a functional ITAM, and activates NF-κB. Specifically, μ2 and μNS recruit the ITAM-signaling intermediate Syk to cytoplasmic viral factories and this recruitment requires the μ2 ITAM. Moreover, both the μ2 ITAM and Syk are required for maximal μ2 activation of NF-κB. A mutant virus lacking the μ2 ITAM activates NF-κB less efficiently and induces lower levels of the downstream antiviral cytokine beta interferon (IFN-β) than does wild-type virus despite similar replication. Notably, the consequences of these μ2 ITAM effects are cell type specific. In fibroblasts where NF-κB is required for reovirus-induced apoptosis, the μ2 ITAM is advantageous for viral spread and enhances viral fitness. Conversely, in cardiac myocytes where the IFN response is critical for antiviral protection and NF-κB is not required for apoptosis, the μ2 ITAM stimulates cellular defense mechanisms and diminishes viral fitness. Together, these results suggest that the cell type-specific effect of the μ2 ITAM on viral spread reflects the cell type-specific effects of NF-κB and IFN-β. This first demonstration of a functional ITAM in a nonenveloped virus presents a new mechanism for viral ITAM-mediated signaling with likely organ-specific consequences in the host.

PAR-1 contributes to the innate immune response during viral infection

Coagulation is a host defense system that limits the spread of pathogens. Coagulation proteases, such as thrombin, also activate cells by cleaving PARs. In this study, we analyzed the role of PAR-1 in coxsackievirus B3-induced (CVB3-induced) myocarditis and influenza A infection. CVB3-infected Par1(-/-) mice expressed reduced levels of IFN-β and CXCL10 during the early phase of infection compared with Par1(+/+) mice that resulted in higher viral loads and cardiac injury at day 8 after infection. Inhibition of either tissue factor or thrombin in WT mice also significantly increased CVB3 levels in the heart and cardiac injury compared with controls. BM transplantation experiments demonstrated that PAR-1 in nonhematopoietic cells protected mice from CVB3 infection. Transgenic mice overexpressing PAR-1 in cardiomyocytes had reduced CVB3-induced myocarditis. We found that cooperative signaling between PAR-1 and TLR3 in mouse cardiac fibroblasts enhanced activation of p38 and induction of IFN-β and CXCL10 expression. Par1(-/-) mice also had decreased CXCL10 expression and increased viral levels in the lung after influenza A infection compared with Par1(+/+) mice. Our results indicate that the tissue factor/thrombin/PAR-1 pathway enhances IFN-β expression and contributes to the innate immune response during single-stranded RNA viral infection.

Oligodendroglia are limited in type I interferon induction and responsiveness in vivo

Type I interferons (IFNα/β) provide a primary defense against infection. Nevertheless, the dynamics of IFNα/β induction and responsiveness by central nervous system (CNS) resident cells in vivo in response to viral infections are poorly understood. Mice were infected with a neurotropic coronavirus with tropism for oligodendroglia and microglia to probe innate antiviral responses during acute encephalomyelitis. Expression of genes associated with the IFNα/β pathways was monitored in microglia and oligodendroglia purified from naïve and infected mice by fluorescent activated cell sorting. Compared with microglia, oligodendroglia were characterized by low basal expression of mRNA encoding viral RNA sensing pattern recognition receptors (PRRs), IFNα/β receptor chains, interferon sensitive genes (ISG), as well as kinases and transcription factors critical in IFNα/β signaling. Although PRRs and ISGs were upregulated by infection in both cell types, the repertoire and absolute mRNA levels were more limited in oligodendroglia. Furthermore, although oligodendroglia harbored higher levels of viral RNA compared with microglia, Ifnα/β was only induced in microglia. Stimulation with the double stranded RNA analogue poly I:C also failed to induce Ifnα/β in oligodendroglia, and resulted in reduced and delayed induction of ISGs compared with microglia. The limited antiviral response by oligodendroglia was associated with a high threshold for upregulation of Ikkε and Irf7 transcripts, both central to amplifying IFNα/β responses. Overall, these data reveal that oligodendroglia from the adult CNS are poor sensors of viral infection and suggest they require exogenous IFNα/β to establish an antiviral state.

Control of the interferon response in RNAi experiments

The RNA interference (RNAi) and interferons have been an uneasy marriage. Ever since the discovery of RNAi in mammals, the interferon response has been a feared problem. While RNAi became an efficient and widespread method for gene silencing in mammals, numerous studies recognized several obstacles, including undesirable activation of the interferon response, which need to be overcome to achieve a specific and robust RNAi effect. The aim of this text is to provide theoretical and practical information for scientists who want to control interferon response and other adverse effects in their RNAi experiments.

A single-amino-acid polymorphism in reovirus protein μ2 determines repression of interferon signaling and modulates myocarditis

Myocarditis is indicated as the second leading cause of sudden death in young adults. Reovirus induces myocarditis in neonatal mice, providing a tractable model system for investigation of this important disease. Alpha/beta-interferon (IFN-α/β) treatment improves cardiac function and inhibits viral replication in patients with chronic myocarditis, and the host IFN-α/β response is a determinant of reovirus strain-specific differences in induction of myocarditis. Virus-induced IFN-β stimulates a signaling cascade that establishes an antiviral state and further induces IFN-α/β through an amplification loop. Reovirus strain-specific differences in induction of and sensitivity to IFN-α/β are associated with the viral M1, L2, and S2 genes. The reovirus M1 gene-encoded μ2 protein is a strain-specific repressor of IFN-β signaling, providing one possible mechanism for the variation in resistance to IFN and induction of myocarditis between different reovirus strains. We report here that μ2 amino acid 208 determines repression of IFN-β signaling and modulates reovirus induction of IFN-β in cardiac myocytes. Moreover, μ2 amino acid 208 determines reovirus replication, both in initially infected cardiac myocytes and after viral spread, by regulating the IFN-β response. Amino acid 208 of μ2 also influences the cytopathic effect in cardiac myocytes after spread. Finally, μ2 amino acid 208 modulates myocarditis in neonatal mice. Thus, repression of IFN-β signaling mediated by reovirus μ2 amino acid 208 is a determinant of the IFN-β response, viral replication and damage in cardiac myocytes, and myocarditis. These results demonstrate that a single amino acid difference between viruses can dictate virus strain-specific differences in suppression of the host IFN-β response and, consequently, damage to the heart.

Antiviral effects of interferon-β are enhanced in the absence of the translational suppressor 4E-BP1 in myocarditis induced by Coxsackievirus B3

BACKGROUND

Viral myocarditis is most frequently associated with infection by Coxsackievirus B3 (CVB3). Interferon (IFN)-β therapy has been studied and could reduce virally induced tissue damage and improve heart function.

METHODS

In the present study we have investigated the role of translational suppression in the context of an IFN-α/β-mediated antiviral immune response to CVB3 infection. Specifically, we examined the effects of IFN-α/β treatment of CVB3-infected mouse embryonic fibroblast cells and splenocytes lacking eukaryotic initiation factor 4E binding protein-1 (4E-BP1), a suppressor of 5'-capped mRNA translation. Extending these in vitro studies, we examined the effects of CVB3 infection and IFN-β treatment in 4E-BP1(-/-) mice.

RESULTS

Our data show that 4E-BP1(-/-) cells are more -sensitive to the antiviral effects of IFN-α4 and IFN-β treatment than 4E-BP1(+/+) cells when infected with CVB3. Similarly, 4E-BP1(-/-) mice are more sensitive to treatment with IFN-β, exhibiting lower viral titres in heart tissue than 4E-BP1(+/+) mice during the course of infection. Additionally, we demonstrate that treatment with IFN-β reduces inflammatory infiltrates into the hearts of infected mice.

CONCLUSIONS

These data identify 4E-BP1 as a novel drug target to augment responsiveness to IFN-β therapy in CVB3-induced myocarditis.

Epstein-Barr virus latent membrane protein 1 (LMP1) C-terminal-activating region 3 contributes to LMP1-mediated cellular migration via its interaction with Ubc9

Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1), the principal viral oncoprotein and a member of the tumor necrosis factor receptor superfamily, is a constitutively active membrane signaling protein that regulates multiple signal transduction pathways via its C-terminal-activating region 1 (CTAR1) and CTAR2, and also the less-studied CTAR3. Because protein sumoylation among other posttranslational modifications may regulate many signaling pathways induced by LMP1, we investigated whether during EBV latency LMP1 regulates sumoylation processes that control cellular activation and cellular responses. By immunoprecipitation experiments, we show that LMP1 interacts with Ubc9, the single reported SUMO-conjugating enzyme. Requirements for LMP1-Ubc9 interactions include enzymatically active Ubc9: expression of inactive Ubc9 (Ubc9 C93S) inhibited the LMP1-Ubc9 interaction. LMP1 CTAR3, but not CTAR1 and CTAR2, participated in the LMP1-Ubc9 interaction, and amino acid sequences found in CTAR3, including the JAK-interacting motif, contributed to this interaction. Furthermore, LMP1 expression coincided with increased sumoylation of cellular proteins, and disruption of the Ubc9-LMP1 CTAR3 interaction almost completely abrogated LMP1-induced protein sumoylation, suggesting that this interaction promotes the sumoylation of downstream targets. Additional consequences of the disruption of the LMP1 CTAR3-Ubc9 interaction revealed effects on cellular migration, a hallmark of oncogenesis. Together, these data demonstrate that LMP1 CTAR3 does in fact function in intracellular signaling and leads to biological effects. We propose that LMP1, by interaction with Ubc9, modulates sumoylation processes, which regulate signal transduction pathways that affect phenotypic changes associated with oncogenesis.

IFN-alpha expression and antiviral effects are subtype and cell type specific in the cardiac response to viral infection

The interferon-beta (IFN-beta) response is critical for protection against viral myocarditis in several mouse models, and IFN-alpha or -beta treatment is beneficial against human viral myocarditis. The IFN-beta response in cardiac myocytes and cardiac fibroblasts forms an integrated network for organ protection; however, the different IFN-alpha subtypes have not been studied in cardiac cells. We developed a quantitative RT-PCR assay that distinguishes between 13 highly conserved IFN-alpha subtypes and found that reovirus T3D induces five IFN-alpha subtypes in primary cardiac myocyte and fibroblast cultures: IFN-alpha1, -alpha2, -alpha4, -alpha5, and -alpha8/6. Murine IFN-alpha1, -alpha2, -alpha4, or -alpha5 treatment induced IRF7 and ISG56 and inhibited reovirus T3D replication in both cell types. This first investigation of IFN-alpha subtypes in cardiac cells for any virus demonstrates that IFN-alpha is induced in cardiac cells, that it is both subtype and cell type specific, and that it is likely important in the antiviral cardiac response.

Viral myocarditis: diagnosis, aetiology and management

Myocarditis is an inflammatory disease of the cardiac muscle caused by myocardial infiltration of immunocompetent cells following any kind of cardiac injury. Classic myocarditis mainly occurs as a result of the host's immune response against organisms that cause common infectious illnesses, as a manifestation of hypersensitivity or as a toxic reaction to drug therapy. Chronic inflammatory events may survive successful clearance of initial cardiotoxic agents, be triggered or amplified by autoimmunological processes, or develop in the context of systemic diseases. If the underlying infectious or immune-mediated causes of the disease are carefully defined by clinical and biopsy-based tools, specific immunosuppressive and antiviral treatment options may improve the prognosis of patients with acute and chronic disease.

Cardiac cell-specific apoptotic and cytokine responses to reovirus infection: determinants of myocarditic phenotype

BACKGROUND

The pathophysiologic mechanisms underlying viral myocarditis are not well defined. As a result, effective treatments do not exist and viral myocarditis remains a potentially lethal infection of the heart.

METHODS AND RESULTS

We used cultured rat cardiac myocytes and fibroblasts to investigate apoptosis and cytokine production in response to infection by myocarditic vs. non-myocarditic strains of reovirus. Myocarditic reovirus strain 8B and non-myocarditic strain DB188 replicate comparably in each cardiac cell type. However, strain 8B and related myocarditic reoviruses preferentially increase apoptosis of myocytes relative to fibroblasts, whereas DB188 and nonmyocarditic strains preferentially increase fibroblast apoptosis. Infection of cardiac fibroblasts with the nonmyocarditic strain DB188 elicits substantial increases in a panel of cytokines compared to fibroblasts infected with strain 8B or mock-infected controls. Analysis of culture supernatants using cytometric bead arrays revealed that DB188 enhanced release of interleukin (IL)-1beta, IL-4, IL-6, IL-10, IL-12(p70), GRO-KC, tumor necrosis factor-alpha, and MCP-1 relative to 8B or mock-infected controls (all P < .05).

CONCLUSION

We hypothesize that differential cytokine production and cell-specific apoptosis are important determinants of myocarditic potential of reoviral strains. Therapies that target the beneficial effects of cytokines in limiting cytopathic damage may offer an effective and novel treatment approach to viral myocarditis.

Reovirus mu2 protein inhibits interferon signaling through a novel mechanism involving nuclear accumulation of interferon regulatory factor 9

The secreted cytokine alpha/beta interferon (IFN-alpha/beta) binds its receptor to activate the Jak-STAT signal transduction pathway, leading to formation of the heterotrimeric IFN-stimulated gene factor 3 (ISGF3) transcription complex for induction of IFN-stimulated genes (ISGs) and establishment of an antiviral state. Many viruses have evolved countermeasures to inhibit the IFN pathway, thereby subverting the innate antiviral response. Here, we demonstrate that the mildly myocarditic reovirus type 1 Lang (T1L), but not the nonmyocarditic reovirus type 3 Dearing, represses IFN induction of a subset of ISGs and that this repressor function segregates with the T1L M1 gene. Concordantly, the T1L M1 gene product, mu2, dramatically inhibits IFN-beta-induced reporter gene expression. Surprisingly, T1L infection does not degrade components of the ISGF3 complex or interfere with STAT1 or STAT2 nuclear translocation as has been observed for other viruses. Instead, infection with T1L or reassortant or recombinant viruses containing the T1L M1 gene results in accumulation of interferon regulatory factor 9 (IRF9) in the nucleus. This effect has not been previously described for any virus and suggests that mu2 modulates IRF9 interactions with STATs for both ISGF3 function and nuclear export. The M1 gene is a determinant of virus strain-specific differences in the IFN response, which are linked to virus strain-specific differences in induction of murine myocarditis. We find that virus-induced myocarditis is associated with repression of IFN function, providing new insights into the pathophysiology of this disease. Together, these data provide the first report of an increase in IRF9 nuclear accumulation associated with viral subversion of the IFN response and couple virus strain-specific differences in IFN antagonism to the pathogenesis of viral myocarditis.

Novel therapeutic targets in viral myocarditis

Viral myocarditis is an elusive infection of the heart that is currently without an effective or definitive treatment. Viral myocarditis has a complex disease progression that can be divided into early, middle and late phases. Direct cytopathic injury, apoptosis, activation of the innate and adaptive immune system and cardiac remodeling have all been implicated in the pathogenesis of viral myocarditis. Novel treatment approaches are evolving at a rapid pace. The purpose of this review is to provide an update on current research focused on identifying potential treatment options for viral myocarditis.

Basal expression levels of IFNAR and Jak-STAT components are determinants of cell-type-specific differences in cardiac antiviral responses

Viral myocarditis is an important human disease, and reovirus-induced murine myocarditis provides an excellent model system for study. Cardiac myocytes, like neurons in the central nervous system, are not replenished, yet there is no cardiac protective equivalent to the blood-brain barrier. Thus, cardiac myocytes may have evolved a unique antiviral response relative to readily replenished cell types, such as cardiac fibroblasts. Our previous comparisons of these two cell types revealed a conundrum: reovirus T3D induces more beta-interferon (IFN-beta) mRNA in cardiac myocytes, yet there is a greater induction of IFN-stimulated genes (ISGs) in cardiac fibroblasts. Here, we investigated possible underlying molecular determinants. We found that greater basal expression of IFN-beta in cardiac myocytes results in greater basal activated nuclear STAT1 and STAT2 and greater basal ISG mRNA expression and provides greater basal antiviral protection relative to cardiac fibroblasts. Conversely, cardiac fibroblasts express greater basal IFN-alpha/beta receptor 1 (IFNAR1) and greater basal cytoplasmic Jak1, Tyk2, STAT2, and IRF9, leading to a greater increase in reovirus T3D- or IFN-induced nuclear activated STAT1 and STAT2 and greater induction of ISGs for a greater IFN-induced antiviral protection relative to cardiac myocytes. Our results suggest that high basal IFN-beta expression in cardiac myocytes prearms this vulnerable, nonreplenishable cell type, while high basal expression of IFNAR1 and latent Jak-STAT components in adjacent cardiac fibroblasts renders these cells more responsive to IFN and prevents them from inadvertently serving as a reservoir for viral replication and spread to cardiac myocytes. These studies provide the first indication of an integrated network of cell-type-specific innate immune components for organ protection.

Retinoic acid-inducible gene-I and interferon-beta promoter stimulator-1 augment proapoptotic responses following mammalian reovirus infection via interferon regulatory factor-3

During viral infection, cells initiate antiviral responses to contain replication and inhibit virus spread. One protective mechanism involves activation of transcription factors interferon regulatory factor-3 (IRF-3) and NF-kappaB, resulting in secretion of the antiviral cytokine, interferon-beta. Another is induction of apoptosis, killing the host cell before virus disseminates. Mammalian reovirus induces both interferon-beta and apoptosis, raising the possibility that both pathways are initiated by a common cellular sensor. We show here that reovirus activates IRF-3 with kinetics that parallel the activation of NF-kappaB, a known mediator of reovirus-induced apoptosis. Activation of IRF-3 requires functional retinoic acid inducible gene-I and interferon-beta promoter stimulator-1, but these intracellular sensors are dispensable for activation of NF-kappaB. Interferon-beta promoter stimulator-1 and IRF-3 are required for efficient apoptosis following reovirus infection, suggesting a common mechanism of antiviral cytokine induction and activation of the cell death response.

Early production of type I interferon during West Nile virus infection: role for lymphoid tissues in IRF3-independent interferon production

Infection of cells with flaviviruses in vitro is reduced by pretreatment with small amounts of type I interferon (IFN-alpha/beta). Similarly, pretreatment of animals with IFN and experiments using mice defective in IFN signaling have indicated a role for IFN in controlling flavivirus disease in vivo. These data, along with findings that flavivirus-infected cells block IFN signaling, suggest that flavivirus infection can trigger an IFN response. To investigate IFN gene induction by the very first cells infected during in vivo infection with the flavivirus West Nile virus (WNV), we infected mice with high-titer preparations of WNV virus-like particles (VLPs), which initiate viral genome replication in cells but fail to spread. These studies demonstrated a brisk production of IFN in vivo, with peak levels of over 1,000 units/ml detected in sera between 8 and 24 h after inoculation by either the intraperitoneal or footpad route. The IFN response was dependent on genome replication, and WNV genomes and WNV antigen-positive cells were readily detected in the popliteal lymph nodes (pLN) of VLP-inoculated mice. High levels of IFN mRNA transcripts and functional IFN were also produced in VLP-inoculated IFN regulatory factor 3 null (IRF3(-/-)) mice, indicating that IFN production was independent of the IRF3 pathways to IFN gene transcription, consistent with the IFN type produced (predominantly alpha).

Type I interferons produced by hematopoietic cells protect mice against lethal infection by mammalian reovirus

We defined the function of type I interferons (IFNs) in defense against reovirus strain type 1 Lang (T1L), which is a double-stranded RNA virus that infects Peyer's patches (PPs) after peroral inoculation of mice. T1L induced expression of mRNA for IFN-alpha, IFN-beta, and Mx-1 in PPs and caused localized intestinal infection that was cleared in 10 d. In contrast, T1L produced fatal systemic infection in IFNalphaR1 knockout (KO) mice with extensive cell loss in lymphoid tissues and necrosis of the intestinal mucosa. Studies of bone-marrow chimeric mice indicated an essential role for hematopoietic cells in IFN-dependent viral clearance. Dendritic cells (DCs), including conventional DCs (cDCs), were the major source of type I IFNs in PPs of reovirus-infected mice, whereas all cell types expressed the antiviral protein Mx-1. Neither NK cells nor signaling via Toll-like receptor 3 or MyD88 were essential for viral clearance. These data demonstrate a requirement for type I IFNs in the control of an intestinal viral infection and indicate that cDCs are a significant source of type I IFN production in vivo. Therefore, innate immunity in PPs is an essential component of host defense that limits systemic spread of pathogens that infect the intestinal mucosa.

Adenoviral infection induces a multi-faceted innate cellular immune response that is mediated by the toll-like receptor pathway in A549 cells

Adenovirus vectors are known to induce certain genes and impact innate response networks, but a broad understanding of this process and its mechanisms is currently lacking. For this reason, we chose to investigate and characterize Ad innate immunity using homogeneous, primary MEF cells replete with all the elements of the pathogen-sensing Toll-Like Receptor (TLR) pathway. By using an array-based approach to maximally define transcriptome changes induced upon Ad vector infection, we discovered that Ad infection induces a potent gene and transcription factor network response. This response is characterized by significant changes in the expression of genes involved in focal adhesion, tight junction, and RNA regulation, in addition to TLR pathway and other innate sensing genes. Further investigation using human A549 cells knocked down for various TLR pathway adaptors, revealed significant impacts on the Ad initiation of NF-kB and interferon responses, thus confirming TLR involvement in Ad-mediated immunity across diverse species.

Acquired resistance to reoviral oncolysis in Ras-transformed fibrosarcoma cells

Reovirus shows considerable potential as an oncolytic agent for Ras-activated tumors and is currently in clinical trials. Here we ask whether such tumor cell lines can acquire resistance to reoviral oncolysis. We challenged human HT1080 fibrosarcoma cells that carry a Ras mutation by prolonged exposure to reovirus, thereby yielding highly virus-resistant HTR1 cells. These cells are persistently infected with reovirus, exhibit high Ras activity and retain the original Ras gene mutation, showing that resistance to reovirus can be displayed in cells with active Ras. The HTR1 cells also exhibit reduced cellular cathepsin B activity, which normally contributes to viral entry and activation. Persistently infected HTR1 cells were not tumorigenic in vivo, whereas immunologically cured virus-free HTR1 cells were highly tumorigenic. Thus, acquisition of resistance to reovirus may constrain therapeutic strategies. To determine whether reoviral resistance is associated with a general reduction in apoptotic potential, we challenged the HTR1 cells with apoptotic inducers and E1B-defective adenovirus, resulting in significant apoptosis and cell death following both approaches. Therefore, even if resistance to reoviral oncolysis should arise in tumor cells in vivo, other therapeutic strategies may nevertheless remain effective.

Adipokine gene expression in a novel hypothalamic neuronal cell line: resistin-dependent regulation of fasting-induced adipose factor and SOCS-3

Adipokines such as leptin, resistin, and fasting-induced adipose factor (FIAF) are secreted by adipocytes, but their expression is also detectable in the brain and pituitary. The role of central adipokines remains elusive, but we speculate that they may modulate those hypothalamic signaling pathways that control energy homeostasis. Here we describe experiments to test this in which we exploited a novel hypothalamic neuronal cell line (N-1) that expresses a variety of neuropeptides and receptors that are known to be implicated in appetite regulation. Using real-time RT-PCR, we confirmed that N-1 neurons express resistin (rstn) and fiaf, as well as suppressor of cytokine signaling-3 (socs-3), a feedback inhibitor of leptin signaling. Treating N-1 cells with recombinant resistin (200 ng/ml, 30 min) reduced both fiaf (25%, p < 0.005) and socs-3 (29%, p < 0.005) mRNA levels, and similar reductions in fiaf (40%, p < 0.001) and socs-3 (25%, p < 0.001) resulted following the overexpression of resistin. Conversely, when RNA interference (RNAi) was used to reduce endogenous rstn levels (-60%, p < 0.005), fiaf and socs-3 expression was increased (46 and 65% respectively, p < 0.005). A similar reduction in rstn mRNA was achieved using RNAi in differentiated 3T3-L1 adipocytes, and this manipulation also reduced fiaf and socs-3 expression (-53, -21 and -20% respectively, p < 0.005). In contrast, although RNAi successfully reduced fiaf mRNA by 50% (p < 0.001) in N-1 cells and 40% (p < 0.001) in 3T3-L1 cells, there was no effect on rstn or socs-3 mRNA. These data suggest that resistin exerts a novel autocrine/paracrine control over fiaf and socs-3 expression in both 3T3-L1 adipocytes and N-1 neurons. Such a mechanism could be part of the central feedback system that modulates the effects of adipokines, and other adiposity signals, implicated in hypothalamic energy homeostasis. However, it remains to be determined whether these in vitro results can be translated to the control of adipokine expression in brain and adipose tissue.

Genes Induced by Reovirus Infection Have a Distinct Modular Cis-Regulatory Architecture

The availability of complete genomes and global gene expression profiling has greatly facilitated analysis of complex genetic regulatory systems. We describe the use of a bioinformatics strategy for analyzing the cis-regulatory design of genes diferentially regulated during viral infection of a target cell. The large-scale transcriptional activity of human embryonic kidney (HEK293) cells to reovirus (serotype 3 Abney) infection was measured using the Affymetrix HU-95Av2 gene array. Comparing the 2000 base pairs of 5' upstream sequence for the most differentially expressed genes revealed highly preserved sequence regions, which we call "modules". Higher-order patterns of modules, called "super-modules", were significantly over-represented in the 5' upstream regions of transcriptionally responsive genes. These supermodules contain binding sites for multiple transcription factors and tend to define the role of genes in processes associated with reovirus infection. The supermodular design encodes a cis-regulatory logic for transducing upstream signaling for the control of expression of genes involved in similar biological processes. In the case of reovirus infection, these processes recapitulate the integrated response of cells including signal transduction, transcriptional regulation, cell cycle control, and apoptosis. The computational strategies described for analyzing gene expression data to discover cis-regulatory features and associating them with pathological processes represents a novel approach to studying the interaction of a pathogen with its target cells.