A single-amino-acid polymorphism in reovirus protein μ2 determines repression of interferon signaling and modulates myocarditis. J Virol. 2012;86:2302-2311. [PMID: 22156521 DOI: 10.1128/jvi.06236-11]

Infectious factors in myocarditis: a comprehensive review of common and rare pathogens

BACKGROUND

Myocarditis is a significant health threat today, with infectious agents being the most common cause. Accurate diagnosis of the etiology of infectious myocarditis is crucial for effective treatment.

MAIN BODY

Infectious myocarditis can be caused by viruses, prokaryotes, parasites, and fungi. Viral infections are typically the primary cause. However, some rare opportunistic pathogens can also damage heart muscle cells in patients with immunodeficiencies, neoplasms and those who have undergone heart surgery.

CONCLUSIONS

This article reviews research on common and rare pathogens of infectious myocarditis, emphasizing the complexity of its etiology, with the aim of helping clinicians make an accurate diagnosis of infectious myocarditis.

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.

Reovirus μ2 Protein Impairs Translation to Reduce U5 snRNP Protein Levels

Mammalian orthoreovirus (MRV) is a double-stranded RNA virus from the Reoviridae family that infects a large range of mammals, including humans. Recently, studies have shown that MRV alters cellular alternative splicing (AS) during viral infection. The structural protein μ2 appears to be the main determinant of these AS modifications by decreasing the levels of U5 core components EFTUD2, PRPF8, and SNRNP200 during infection. In the present study, we investigated the mechanism by which μ2 exerts this effect on the U5 components. Our results revealed that μ2 has no impact on steady-state mRNA levels, RNA export, and protein stability of these U5 snRNP proteins. However, polysome profiling and metabolic labeling of newly synthesized proteins revealed that μ2 exerts an inhibitory effect on global translation. Moreover, we showed that μ2 mutants unable to accumulate in the nucleus retain most of the ability to reduce PRPF8 protein levels, indicating that the effect of μ2 on U5 snRNP components mainly occurs in the cytoplasm. Finally, co-expression experiments demonstrated that μ2 suppresses the expression of U5 snRNP proteins in a dose-dependent manner, and that the expression of specific U5 snRNP core components have different sensitivities to μ2's presence. Altogether, these results suggest a novel mechanism by which the μ2 protein reduces the levels of U5 core components through translation inhibition, allowing this viral protein to alter cellular AS during infection.

The μ2 and λ1 Proteins of Mammalian Reovirus Modulate Early Events Leading to Induction of the Interferon Signaling Network

It has been previously shown that amino acid polymorphisms in reovirus proteins μ2 and λ1 are associated with differing levels of interferon induction. In the present study, viruses carrying these polymorphisms in either or both proteins, were further studied. The two viral determinants exert a synergistic effect on the control of β-interferon induction at the protein and mRNA level, with a concomitant increase in RIG-I. In contrast, levels of phospho-Stat1 and interferon-stimulated genes are increased in singly substituted viruses but with no further increase when both substitutions were present. This suggests that the viral determinants are acting during initial events of viral recognition. Accordingly, difference between viruses was reduced when infection was performed with partially uncoated virions (ISVPs) and transfection of RNA recovered from early-infected cells recapitulates the differences between viruses harboring the different polymorphisms. Altogether, the data are consistent with a redundant or complementary role of μ2 and λ1, affecting either early disassembly or the nature of the viral RNA in the incoming viral particle. Proteins involved in viral RNA synthesis are thus involved in this likely critical aspect of the ability of different reovirus variants to infect various cell types, and to discriminate between parental and transformed/cancer cells.

Reovirus Efficiently Reassorts Genome Segments during Coinfection and Superinfection

Reassortment, or genome segment exchange, increases diversity among viruses with segmented genomes. Previous studies on the limitations of reassortment have largely focused on parental incompatibilities that restrict generation of viable progeny. However, less is known about whether factors intrinsic to virus replication influence reassortment. Mammalian orthoreovirus (reovirus) encapsidates a segmented, double-stranded RNA (dsRNA) genome, replicates within cytoplasmic factories, and is susceptible to host antiviral responses. We sought to elucidate the influence of infection multiplicity, timing, and compartmentalized replication on reovirus reassortment in the absence of parental incompatibilities. We used an established post-PCR genotyping method to quantify reassortment frequency between wild-type and genetically barcoded type 3 reoviruses. Consistent with published findings, we found that reassortment increased with infection multiplicity until reaching a peak of efficient genome segment exchange during simultaneous coinfection. However, reassortment frequency exhibited a substantial decease with increasing time to superinfection, which strongly correlated with viral transcript abundance. We hypothesized that physical sequestration of viral transcripts within distinct virus factories or superinfection exclusion also could influence reassortment frequency during superinfection. Imaging revealed that transcripts from both wild-type and barcoded viruses frequently co-occupied factories, with superinfection time delays up to 16 h. Additionally, primary infection progressively dampened superinfecting virus transcript levels with greater time delay to superinfection. Thus, in the absence of parental incompatibilities and with short times to superinfection, reovirus reassortment proceeds efficiently and is largely unaffected by compartmentalization of replication and superinfection exclusion. However, reassortment may be limited by superinfection exclusion with greater time delays to superinfection. IMPORTANCE Reassortment, or genome segment exchange between viruses, can generate novel virus genotypes and pandemic virus strains. For viruses to reassort their genome segments, they must replicate within the same physical space by coinfecting the same host cell. Even after entry into the host cell, many viruses with segmented genomes synthesize new virus transcripts and assemble and package their genomes within cytoplasmic replication compartments. Additionally, some viruses can interfere with subsequent infection of the same host or cell. However, spatial and temporal influences on reassortment are only beginning to be explored. We found that infection multiplicity and transcript abundance are important drivers of reassortment during coinfection and superinfection, respectively, for reovirus, which has a segmented, double-stranded RNA genome. We also provide evidence that compartmentalization of transcription and packaging is unlikely to influence reassortment, but the length of time between primary and subsequent reovirus infection can alter reassortment frequency.

Reovirus μ2 protein modulates host cell alternative splicing by reducing protein levels of U5 snRNP core components

Mammalian orthoreovirus (MRV) is a double-stranded RNA virus from the Reoviridae family presenting a promising activity as an oncolytic virus. Recent studies have underlined MRV’s ability to alter cellular alternative splicing (AS) during infection, with a limited understanding of the mechanisms at play. In this study, we investigated how MRV modulates AS. Using a combination of cell biology and reverse genetics experiments, we demonstrated that the M1 gene segment, encoding the μ2 protein, is the primary determinant of MRV’s ability to alter AS, and that the amino acid at position 208 in μ2 is critical to induce these changes. Moreover, we showed that the expression of μ2 by itself is sufficient to trigger AS changes, and its ability to enter the nucleus is not required for all these changes. Moreover, we identified core components of the U5 snRNP (i.e. EFTUD2, PRPF8, and SNRNP200) as interactors of μ2 that are required for MRV modulation of AS. Finally, these U5 snRNP components are reduced at the protein level by both MRV infection and μ2 expression. Our findings identify the reduction of U5 snRNP components levels as a new mechanism by which viruses alter cellular AS.

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.

The multi-functional reovirus σ3 protein is a virulence factor that suppresses stress granule formation and is associated with myocardial injury

The mammalian orthoreovirus double-stranded (ds) RNA-binding protein σ3 is a multifunctional protein that promotes viral protein synthesis and facilitates viral entry and assembly. The dsRNA-binding capacity of σ3 correlates with its capacity to prevent dsRNA-mediated activation of protein kinase R (PKR). However, the effect of σ3 binding to dsRNA during viral infection is largely unknown. To identify functions of σ3 dsRNA-binding activity during reovirus infection, we engineered a panel of thirteen σ3 mutants and screened them for the capacity to bind dsRNA. Six mutants were defective in dsRNA binding, and mutations in these constructs cluster in a putative dsRNA-binding region on the surface of σ3. Two recombinant viruses expressing these σ3 dsRNA-binding mutants, K287T and R296T, display strikingly different phenotypes. In a cell-type dependent manner, K287T, but not R296T, replicates less efficiently than wild-type (WT) virus. In cells in which K287T virus demonstrates a replication deficit, PKR activation occurs and abundant stress granules (SGs) are formed at late times post-infection. In contrast, the R296T virus retains the capacity to suppress activation of PKR and does not mediate formation of SGs at late times post-infection. These findings indicate that σ3 inhibits PKR independently of its capacity to bind dsRNA. In infected mice, K287T produces lower viral titers in the spleen, liver, lungs, and heart relative to WT or R296T. Moreover, mice inoculated with WT or R296T viruses develop myocarditis, whereas those inoculated with K287T do not. Overall, our results indicate that σ3 functions to suppress PKR activation and subsequent SG formation during viral infection and that these functions correlate with virulence in mice.

The σ3 protein of mammalian orthoreoviruses is a double-stranded RNA binding protein that has classically been thought to function by scavenging dsRNA within infected cells and thus prevents activation of cellular sensors of dsRNA such as the kinase PKR. Here we used mutagenesis to identify the region of σ3 responsible for binding dsRNA. Characterization of mutant viruses expressing σ3 proteins incapable of binding dsRNA show that contrary to expectation, dsRNA binding is not required for σ3-mediated inhibition of PKR. We show that one mutant virus (R296T) despite being deficient in dsRNA-binding can inhibit PKR and replicates similar to WT virus. In contrast, another mutant virus (K287T) that bears a σ3 protein that cannot prevent dsRNA-mediated activation of PKR induces stress granules in infected cells and replicates less efficiently than WT virus. In vivo, the K287T mutant is attenuated in its replication and unlike WT virus and the R296T mutant virus does not cause heart disease (myocarditis).

The reovirus μ2 C-terminal loop inversely regulates NTPase and transcription functions versus binding to factory-forming μNS and promotes replication in tumorigenic cells

Wild type reovirus serotype 3 'Dearing PL strain' (T3wt) is being heavily evaluated as an oncolytic and immunotherapeutic treatment for cancers. Mutations that promote reovirus entry into tumor cells were previously reported to enhance oncolysis; herein we aimed to discover mutations that enhance the post-entry steps of reovirus infection in tumor cells. Using directed evolution, we identified that reovirus variant T3v10^M1 exhibited enhanced replication relative to T3wt on a panel of cancer cells. T3v10^M1 contains an alanine-to-valine substitution (A612V) in the core-associated μ2, which was previously found to have NTPase activities in virions and to facilitate virus factory formation by association with μNS. Paradoxically, the A612V mutation in μ2 from T3v10^M1 was discovered to impair NTPase activities and RNA synthesis, leading to five-fold higher probability of abortive infection for T3v10^M1 relative to T3wt. The A612V mutation resides in a previously uncharacterized C-terminal region that juxtaposes the template entry site of the polymerase μ2; our findings thus support an important role for this domain during virus transcription. Despite crippled onset of infection, T3v10^M1 exhibited greater accumulation of viral proteins and progeny during replication, leading to increased overall virus burst size. Both Far-Western and co-immunoprecipitation approaches corroborated that the A612V mutation in μ2 increased association with the non-structural virus protein μNS and enhances burst size. Altogether the data supports that mutations in the C-terminal loop domain of μ2 inversely regulate NTPase and RNA synthesis versus interactions with μNS, but with a net gain of replication in tumorigenic cells.SIGNIFICANCEReovirus is a model system for understanding virus replication but also a clinically relevant virus for cancer therapy. We identified the first mutation that increases reovirus infection in tumorigenic cells by enhancing post-entry stages of reovirus replication. The mutation is in a previously uncharacterized c-terminal region of the M1-derived μ2 protein, which we demonstrated affects multiple functions of μ2; NTPase, RNA synthesis, inhibition of antiviral immune response and association with the virus replication factory-forming μNS protein. These findings promote a mechanistic understanding of viral protein functions. In the future, the benefits of μ2 mutations may be useful for enhancing reovirus potency in tumors.

Reovirus σ3 Protein Limits Interferon Expression and Cell Death Induction

Induction of necroptosis by mammalian reovirus requires both type I interferon (IFN)-signaling and viral replication events that lead to production of progeny genomic double-stranded RNA (dsRNA). The reovirus outer capsid protein μ1 negatively regulates reovirus-induced necroptosis by limiting RNA synthesis. To determine if the outer capsid protein σ3, which interacts with μ1, also functions in regulating necroptosis, we used small interfering RNA (siRNA)-mediated knockdown. Similarly to what was observed in diminishment of μ1 expression, knockdown of newly synthesized σ3 enhances necroptosis. Knockdown of σ3 does not impact reovirus RNA synthesis. Instead, this increase in necroptosis following σ3 knockdown is accompanied by an increase in IFN production. Furthermore, ectopic expression of σ3 is sufficient to block IFN expression following infection. Surprisingly, the capacity of σ3 protein to bind dsRNA does not impact its capacity to diminish production of IFN. Consistent with this, infection with a virus harboring a mutation in the dsRNA binding domain of σ3 does not result in enhanced production of IFN or necroptosis. Together, these data suggest that σ3 limits the production of IFN to control innate immune signaling and necroptosis following infection through a mechanism that is independent of its dsRNA binding capacity.IMPORTANCE We use mammalian reovirus as a model to study how virus infection modulates innate immune signaling and cell death induction. Here, we sought to determine how viral factors regulate these processes. Our work highlights a previously unknown role for the reovirus outer capsid protein σ3 in limiting the induction of a necrotic form of cell death called necroptosis. Induction of cell death by necroptosis requires production of interferon. The σ3 protein limits the induction of necroptosis by preventing excessive production of interferon following infection.

Mammalian orthoreovirus core protein μ2 reorganizes host microtubule-organizing center components

Filamentous mammalian orthoreovirus (MRV) viral factories (VFs) are membrane-less cytosolic inclusions in which virus transcription, replication of dsRNA genome segments, and packaging of virus progeny into newly synthesized virus cores take place. In infected cells, the MRV μ2 protein forms punctae in the enlarged region of the filamentous VFs that are co-localized with γ-tubulin and resistant to nocodazole treatment, and permitted microtubule (MT)-extension, features common to MT-organizing centers (MTOCs). Using a previously established reconstituted VF model, we addressed the functions of MT-components and MTOCs concerning their roles in the formation of filamentous VFs. Indeed, the MTOC markers γ-tubulin and centrin were redistributed within the VF-like structures (VFLS) in a μ2-dependent manner. Moreover, the MT-nucleation centers significantly increased in numbers, and γ-tubulin was pulled-down in a binding assay when co-expressed with histidine-tagged-μ2 and μNS. Thus, μ2, by interaction with γ-tubulin, can modulate MTOCs localization and function according to viral needs.

Bat mammalian orthoreoviruses cause severe pneumonia in mice

Mammalian orthoreovirus (MRV) infections are ubiquitous in mammals. Increasing evidence suggests that some MRVs can cause severe respiratory disease and encephalitis in humans and other animals. Previously, we isolated six bat MRV strains. However, the pathogenicity of these bat viruses remains unclear. In this study, we investigated the host range and pathogenicity of 3 bat MRV strains (WIV2, 3 and 7) which represent three serotypes. Our results showed that all of them can infect cell lines from different mammalian species and displayed different replication efficiency. The BALB/c mice infected by bat MRVs showed clinical symptoms with systematic infection especially in lung and intestines. Obvious tissue damage were found in all infected lungs. One of the strains, WIV7, showed higher replication efficiency in vitro and vivo and more severe pathogenesis in mice. Our results provide new evidence showing potential pathogenicity of bat MRVs in animals and probable risk in humans.

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Bat MRVs show wide cell tropism in vivo and in vitro and have a high replication efficiency in lung and intestines.

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Mice infected by bat MRVs showed clinical illness, but without death.

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The higher replication in brain, lung damage and weak innate immune response may be responsible for severe diseases for WIV7.

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The results indicate the potential pathogenicity of bat MRV to human and animals.

Polymorphisms in the Most Oncolytic Reovirus Strain Confer Enhanced Cell Attachment, Transcription, and Single-Step Replication Kinetics

Reovirus serotype 3 Dearing (T3D) replicates preferentially in transformed cells and is in clinical trials as a cancer therapy. Laboratory strains of T3D, however, exhibit differences in plaque size on cancer cells and differences in oncolytic activity in vivo This study aimed to determine why the most oncolytic T3D reovirus lab strain, the Patrick Lee laboratory strain (T3D^PL), replicates more efficiently in cancer cells than other commonly used laboratory strains, the Kevin Coombs laboratory strain (T3D^KC) and Terence Dermody laboratory (T3D^TD) strain. In single-step growth curves, T3D^PL titers increased at higher rates and produced ∼9-fold higher burst size. Furthermore, the number of reovirus antigen-positive cells increased more rapidly for T3D^PL than for T3D^TD In conclusion, the most oncolytic T3D^PL possesses replication advantages in a single round of infection. Two specific mechanisms for enhanced infection by T3D^PL were identified. First, T3D^PL exhibited higher cell attachment, which was attributed to a higher proportion of virus particles with insufficient (≤3) σ1 cell attachment proteins. Second, T3D^PL transcribed RNA at rates superior to those of the less oncolytic T3D strains, which is attributed to polymorphisms in M1-encoding μ2 protein, as confirmed in an in vitro transcription assay, and which thus demonstrates that T3D^PL has an inherent transcription advantage that is cell type independent. Accordingly, T3D^PL established rapid onset of viral RNA and protein synthesis, leading to more rapid kinetics of progeny virus production, larger virus burst size, and higher levels of cell death. Together, these results emphasize the importance of paying close attention to genomic divergence between virus laboratory strains and, mechanistically, reveal the importance of the rapid onset of infection for reovirus oncolysis.IMPORTANCE Reovirus serotype 3 Dearing (T3D) is in clinical trials for cancer therapy. Recently, it was discovered that highly related laboratory strains of T3D exhibit large differences in their abilities to replicate in cancer cells in vitro, which correlates with oncolytic activity in a murine model of melanoma. The current study reveals two mechanisms for the enhanced efficiency of T3D^PL in cancer cells. Due to polymorphisms in two viral genes, within the first round of reovirus infection, T3D^PL binds to cells more efficiency and more rapidly produces viral RNAs; this increased rate of infection relative to that of the less oncolytic strains gives T3D^PL a strong inherent advantage that culminates in higher virus production, more cell death, and higher virus spread.

How Many Mammalian Reovirus Proteins are involved in the Control of the Interferon Response?

As with most viruses, mammalian reovirus can be recognized and attacked by the host-cell interferon response network. Similarly, many viruses have developed resistance mechanisms to counteract the host-cell response at different points of this response. Reflecting the complexity of the interferon signaling pathways as well as the resulting antiviral response, viruses can-and often have-evolved many determinants to interfere with this innate immune response and allow viral replication. In the last few years, it has been evidenced that mammalian reovirus encodes many different determinants that are involved in regulating the induction of the interferon response or in interfering with the action of interferon-stimulated gene products. In this brief review, we present our current understanding of the different reovirus proteins known to be involved, introduce their postulated modes of action, and raise current questions that may lead to further investigations.

Viral modulation of cellular RNA alternative splicing: A new key player in virus-host interactions?

Upon viral infection, a tug of war is triggered between host cells and viruses to maintain/gain control of vital cellular functions, the result of which will ultimately dictate the fate of the host cell. Among these essential cellular functions, alternative splicing (AS) is an important RNA maturation step that allows exons, or parts of exons, and introns to be retained in mature transcripts, thereby expanding proteome diversity and function. AS is widespread in higher eukaryotes, as it is estimated that nearly all genes in humans are alternatively spliced. Recent evidence has shown that upon infection by numerous viruses, the AS landscape of host‐cells is affected. In this review, we summarize recent advances in our understanding of how virus infection impacts the AS of cellular transcripts. We also present various molecular mechanisms allowing viruses to modulate cellular AS. Finally, the functional consequences of these changes in the RNA splicing signatures during virus–host interactions are discussed.

This article is categorized under:

RNA in Disease and Development > RNA in Disease

RNA Processing > Splicing Regulation/Alternative Splicing

RNA regulatory processes in RNA virus biology

Numerous post‐transcriptional RNA processes play a major role in regulating the quantity, quality and diversity of gene expression in the cell. These include RNA processing events such as capping, splicing, polyadenylation and modification, but also aspects such as RNA localization, decay, translation, and non‐coding RNA‐associated regulation. The interface between the transcripts of RNA viruses and the various RNA regulatory processes in the cell, therefore, has high potential to significantly impact virus gene expression, regulation, cytopathology and pathogenesis. Furthermore, understanding RNA biology from the perspective of an RNA virus can shed considerable light on the broad impact of these post‐transcriptional processes in cell biology. Thus the goal of this article is to provide an overview of the richness of cellular RNA biology and how RNA viruses use, usurp and/or avoid the associated machinery to impact the outcome of infection.

This article is categorized under:

RNA in Disease and Development > RNA in Disease

Piscine orthoreovirus subtype 3 (PRV-3) causes heart inflammation in rainbow trout (Oncorhynchus mykiss)

Piscine orthoreovirus (PRV) mediated diseases have emerged throughout salmonid aquaculture. Three PRV subtypes are currently reported as causative agents of or in association with diseases in different salmonid species. PRV-1 causes heart and skeletal muscle inflammation (HSMI) in Atlantic salmon (Salmo salar) and is associated with jaundice syndrome in farmed chinook salmon (Oncorhynchus tshawytscha). PRV-2 causes erythrocytic inclusion body syndrome (EIBS) in coho salmon in Japan. PRV-3 has recently been associated with a disease in rainbow trout (Oncorhynchus mykiss) characterized by anaemia, heart and red muscle pathology; to jaundice syndrome in coho salmon (Oncorhynchus kisutch). In this study, we conducted a 10-week long experimental infection trial in rainbow trout with purified PRV-3 particles to assess the causal relationship between the virus and development of heart inflammation. The monitoring the PRV-3 load in heart and spleen by RT-qPCR shows a progressive increase of viral RNA to a peak, followed by clearance without a measurable change in haematocrit. The development of characteristic cardiac histopathological findings occurred in the late phase of the trial and was associated with increased expression of CD8+, indicating cytotoxic T cell proliferation. The findings indicate that, under these experimental conditions, PRV-3 infection in rainbow trout act similarly to PRV-1 infection in Atlantic salmon with regards to immunological responses and development of heart pathology, but not in the ability to establish a persistent infection.

A single mutation in the mammalian orthoreovirus S1 gene is responsible for increased interferon sensitivity in a virus mutant selected in Vero cells

In a previous study, a mammalian orthoreovirus mutant was isolated based on its increased ability to infect interferon-defective Vero cells and was referred to as Vero-cells-adapted virus (VeroAV). This virus exhibits reduced ability to resist the antiviral effect of interferon. In the present study, the complete genome sequence of VeroAV was first determined. Reverse genetics was then used to identify a unique mutation on the S1 gene, overlapping the σ1 and σ1 s reading frame, resulting in increased sensitivity to interferon. A virus lacking σ1 s expression consecutive to mutation of its initiation codon was then shown to exhibit a further increase in sensitivity to interferon, supporting the idea that σ1 s is the viral protein responsible. This identification of a new determinant of reovirus sensitivity to interferon gives credentials to the idea that multiple reovirus genes are responsible for the level of interferon induction and susceptibility to the interferon-induced antiviral activities.

Interferon Regulatory Factor 9 Structure and Regulation

Interferon regulatory factor 9 (IRF9) is an integral transcription factor in mediating the type I interferon antiviral response, as part of the interferon-stimulated gene factor 3. However, the role of IRF9 in many important non-communicable diseases has just begun to emerge. The duality of IRF9’s role in conferring protection but at the same time exacerbates diseases is certainly puzzling. The regulation of IRF9 during these conditions is not well understood. The high homology of IRF9 DNA-binding domain to other IRFs, as well as the recently resolved IRF9 IRF-associated domain structure can provide the necessary insights for progressive inroads on understanding the regulatory mechanism of IRF9. This review sought to outline the structural basis of IRF9 that guides its regulation and interaction in antiviral immunity and other diseases.

Reovirus-Induced Apoptosis in the Intestine Limits Establishment of Enteric Infection

Several viruses induce intestinal epithelial cell death during enteric infection. However, it is unclear whether proapoptotic capacity promotes or inhibits replication in this tissue. We infected mice with two reovirus strains that infect the intestine but differ in the capacity to alter immunological tolerance to new food antigen. Infection with reovirus strain T1L, which induces an inflammatory immune response to fed antigen, is prolonged in the intestine, whereas T3D-RV, which does not induce this response, is rapidly cleared from the intestine. Compared with T1L, T3D-RV infection triggered apoptosis of intestinal epithelial cells and subsequent sloughing of dead cells into the intestinal lumen. We conclude that the infection advantage of T1L derives from its capacity to subvert host restriction by epithelial cell apoptosis, providing a possible mechanism by which T1L enhances inflammatory signals during antigen feeding. Using a panel of T1L × T3D-RV reassortant viruses, we identified the viral M1 and M2 gene segments as determinants of reovirus-induced apoptosis in the intestine. Expression of the T1L M1 and M2 genes in a T3D-RV background was sufficient to limit epithelial cell apoptosis and enhance viral infection to levels displayed by T1L. These findings define additional reovirus gene segments required for enteric infection of mice and illuminate the antiviral effect of intestinal epithelial cell apoptosis in limiting enteric viral infection. Viral strain-specific differences in the capacity to infect the intestine may be useful in identifying viruses capable of ameliorating tolerance to fed antigen in autoimmune conditions like celiac disease.IMPORTANCE Acute viral infections are thought to be cleared by the host with few lasting consequences. However, there may be much broader and long-lasting effects of viruses on immune homeostasis. Infection with reovirus, a common, nonpathogenic virus, triggers inflammation against innocuous food antigens, implicating this virus in the development of celiac disease, an autoimmune intestinal disorder triggered by exposure to dietary gluten. Using two reovirus strains that differ in the capacity to abrogate oral tolerance, we found that strain-specific differences in the capacity to replicate in the intestine inversely correlate with the capacity to induce apoptotic death of intestinal epithelial cells, providing a host-mediated process to restrict intestinal infection. This work contributes new knowledge about virus-host interactions in the intestine and establishes a foundation for future studies to define mechanisms by which viruses break oral tolerance in celiac disease.

Multiple proteins differing between laboratory stocks of mammalian orthoreoviruses affect both virus sensitivity to interferon and induction of interferon production during infection

In the course of previous works, it was observed that the virus laboratory stock (T3D^S) differs in sequence from the virus encoded by the ten plasmids currently in use in many laboratories (T3D^K), and derived from a different original virus stock. Seven proteins are affected by these sequence differences. In the present study, replication of T3D^K was shown to be more sensitive to the antiviral effect of interferon. Infection by the T3D^K virus was also shown to induce the production of higher amount of β and α-interferons compared to T3D^S. Two proteins, the μ2 and λ2 proteins, were found to be responsible for increased sensitivity to interferon while both μ2 and λ1 are responsible for increased interferon secretion. Altogether this supports the idea that multiple reovirus proteins are involved in the control of induction of interferon and virus sensitivity to the interferon-induced response. While interrelated, interferon induction and sensitivity can be separated by defined gene combinations. While both μ2 and λ2 were previously suspected of a role in the control of the interferon response, other proteins are also likely involved, as first shown here for λ1. This also further stresses that due caution should be exerted when comparing different virus isolates with different genetic background.

Dissection of mammalian orthoreovirus µ2 reveals a self-associative domain required for binding to microtubules but not to factory matrix protein µNS

Mammalian orthoreovirus protein μ2 is a component of the viral core particle. Its activities include RNA binding and hydrolysis of the γ-phosphate from NTPs and RNA 5´-termini, suggesting roles as a cofactor for the viral RNA-dependent RNA polymerase, λ3, first enzyme in 5´-capping of viral plus-strand RNAs, and/or prohibitory of RNA-5´-triphosphate-activated antiviral signaling. Within infected cells, μ2 also contributes to viral factories, cytoplasmic structures in which genome replication and particle assembly occur. By associating with both microtubules (MTs) and viral factory matrix protein μNS, μ2 can anchor the factories to MTs, the full effects of which remain unknown. In this study, a protease-hypersensitive region allowed μ2 to be dissected into two large fragments corresponding to residues 1-282 and 283-736. Fusions with enhanced green fluorescent protein revealed that these amino- and carboxyl-terminal regions of μ2 associate in cells with either MTs or μNS, respectively. More exhaustive deletion analysis defined μ2 residues 1-325 as the minimal contiguous region that associates with MTs in the absence of the self-associating tag. A region involved in μ2 self-association was mapped to residues 283-325, and self-association involving this region was essential for MT-association as well. Likewise, we mapped that μNS-binding site in μ2 relates to residues 290-453 which is independent of μ2 self-association. These findings suggest that μ2 monomers or oligomers can bind to MTs and μNS, but that self-association involving μ2 residues 283-325 is specifically relevant for MT-association during viral factories formation.

Reovirus inhibits interferon production by sequestering IRF3 into viral factories

Upon viral infection, an arms-race between the cellular intrinsic innate immune system and viral replication is established. To win this race, viruses have established multiple strategies to inhibit the cellular response. Mammalian reovirus (MRV) constitutes a great model to study pathogenesis and life cycle of dsRNA viruses. It replicates in the cytosol of infected cells by forming viral induced-replication compartments, or viral factories. Little is known about the strategy used by MRV to evade the cellular intrinsic immune system. In this study, we unraveled that MRV induces a replication-dependent global reduction in interferon-mediated antiviral immune response. We determined that although MRV leads to the activation and phosphorylation of interferon regulatory factor 3 (IRF3), the nuclear translocation of IRF3 was impaired in infected cells. Additionally, we showed that MRV does not degrade IRF3 but sequesters it in cytoplasmic viral factories. We demonstrate that the viral factory matrix protein μNS is solely responsible for the sequestration of IRF3. This finding highlights novel mechanisms used by MRV to interfere with the intrinsic immune system and places the viral factories as not only a replication compartment but as an active strategy participating in immune evasion.

The Orchestra of Reovirus Cell Entry

Purpose of Review

The ability of viruses to infect host cells is dependent on several factors including the availability of cell-surface receptors, antiviral state of cells, and presence of host factors needed for viral replication. Here, we review findings from in vitro and in vivo studies using mammalian orthoreovirus (reovirus) that have identified an intricate group of molecules and mechanisms used by the virus to attach and enter cells.

Recent Findings

Recent findings provide an improved mechanistic understanding of reovirus cell entry. Of special note is the identification of a cellular mediator of cell entry in neuronal and non-neuronal cells, the effect of cell entry on the outcome of infection and cytopathic effects on the host cell, and an improved understanding of the components that promote viral penetration of cellular membranes.

Summary

A mechanistic understanding of the interplay between host and viral factors has enhanced our view of how viruses usurp cellular processes during infection.

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.

Reovirus infection triggers inflammatory responses to dietary antigens and development of celiac disease

Viral infections have been proposed to elicit pathological processes leading to the initiation of T helper 1 (TH1) immunity against dietary gluten and celiac disease (CeD). To test this hypothesis and gain insights into mechanisms underlying virus-induced loss of tolerance to dietary antigens, we developed a viral infection model that makes use of two reovirus strains that infect the intestine but differ in their immunopathological outcomes. Reovirus is an avirulent pathogen that elicits protective immunity, but we discovered that it can nonetheless disrupt intestinal immune homeostasis at inductive and effector sites of oral tolerance by suppressing peripheral regulatory T cell (pTreg) conversion and promoting TH1 immunity to dietary antigen. Initiation of TH1 immunity to dietary antigen was dependent on interferon regulatory factor 1 and dissociated from suppression of pTreg conversion, which was mediated by type-1 interferon. Last, our study in humans supports a role for infection with reovirus, a seemingly innocuous virus, in triggering the development of CeD.

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.

De novo assembly of Sockeye salmon kidney transcriptomes reveal a limited early response to piscine reovirus with or without infectious hematopoietic necrosis virus superinfection

Background

Piscine reovirus (PRV) has been associated with the serious disease known as Heart and Skeletal Muscle Inflammation (HSMI) in cultured Atlantic salmon Salmo salar in Norway. PRV is also prevalent in wild and farmed salmon without overt disease manifestations, suggesting multifactorial triggers or PRV variant-specific factors are required to initiate disease. In this study, we explore the head kidney transcriptome of Sockeye salmon Oncorhynchus nerka during early PRV infection to identify host responses in the absence of disease in hopes of elucidating mechanisms by which PRV may directly alter host functions and contribute to the development of a disease state. We further investigate the role of PRV as a coinfecting agent following superinfection with infectious hematopoietic necrosis virus (IHNV) – a highly pathogenic rhabdovirus endemic to the west coast of North America.

Results

Challenge of Sockeye salmon with PRV resulted in high quantities of viral transcripts to become present in the blood and kidney of infected fish without manifestations of disease. De novo transcriptome assembly of over 2.3 billion paired RNA-seq reads from the head kidneys of 36 fish identified more than 320,000 putative unigenes, of which less than 20 were suggested to be differentially expressed in response to PRV at either 2 or 3 weeks post challenge by DESeq2 and edgeR analysis. Of these, only one, Ependymin, was confirmed to be differentially expressed by qPCR in an expanded sample set. In contrast, IHNV induced substantial transcriptional changes (differential expression of > 20,000 unigenes) which included transcripts involved in antiviral and inflammatory response pathways. Prior infection with PRV had no significant effect on host responses to superinfecting IHNV, nor did host responses initiated by IHNV exposure influence increasing PRV loads.

Conclusions

PRV does not substantially alter the head kidney transcriptome of Sockeye salmon during early (2 to 3 week) infection and dissemination in a period of significant increasing viral load, nor does the presence of PRV change the host transcriptional response to an IHNV superinfection. Further, concurrent infections of PRV and IHNV do not appear to significantly influence the infectivity or severity of IHNV associated disease, or conversely, PRV load.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3196-y) contains supplementary material, which is available to authorized users.

Engineering Recombinant Reoviruses To Display gp41 Membrane-Proximal External-Region Epitopes from HIV-1

Vaccines to protect against HIV-1, the causative agent of AIDS, are not approved for use. Antibodies that neutralize genetically diverse strains of HIV-1 bind to discrete regions of the envelope glycoproteins, including the gp41 MPER. We engineered recombinant reoviruses that displayed MPER epitopes in attachment protein σ1 (REO-MPER vectors). The REO-MPER vectors replicated with wild-type efficiency, were genetically stable, and retained native antigenicity. However, we did not detect HIV-1-specific immune responses following inoculation of the REO-MPER vectors into small animals. This work provides proof of principle for engineering reovirus to express antigenic epitopes and illustrates the difficulty in eliciting MPER-specific immune responses.

The gp41 membrane-proximal external region (MPER) is a target for broadly neutralizing antibody responses against human immunodeficiency virus type 1 (HIV-1). However, replication-defective virus vaccines currently under evaluation in clinical trials do not efficiently elicit MPER-specific antibodies. Structural modeling suggests that the MPER forms an α-helical coiled coil that is required for function and immunogenicity. To maintain the native MPER conformation, we used reverse genetics to engineer replication-competent reovirus vectors that displayed MPER sequences in the α-helical coiled-coil tail domain of viral attachment protein σ1. Sequences in reovirus strain type 1 Lang (T1L) σ1 were exchanged with sequences encoding HIV-1 strain Ba-L MPER epitope 2F5 or the entire MPER. Individual 2F5 or MPER substitutions were introduced at virion-proximal or virion-distal sites in the σ1 tail. Recombinant reoviruses containing heterologous HIV-1 sequences were viable and produced progeny yields comparable to those with wild-type virus. HIV-1 sequences were retained following 10 serial passages in cell culture, indicating that the substitutions were genetically stable. Recombinant viruses engineered to display the 2F5 epitope or full-length MPER in σ1 were recognized by purified 2F5 antibody. Inoculation of mice with 2F5-containing vectors or rabbits with 2F5- or MPER-containing vectors elicited anti-reovirus antibodies, but HIV-1-specific antibodies were not detected. Together, these findings indicate that heterologous sequences that form α-helices can functionally replace native sequences in the α-helical tail domain of reovirus attachment protein σ1. However, although these vectors retain native antigenicity, they were not immunogenic, illustrating the difficulty of experimentally inducing immune responses to this essential region of HIV-1.

IMPORTANCE Vaccines to protect against HIV-1, the causative agent of AIDS, are not approved for use. Antibodies that neutralize genetically diverse strains of HIV-1 bind to discrete regions of the envelope glycoproteins, including the gp41 MPER. We engineered recombinant reoviruses that displayed MPER epitopes in attachment protein σ1 (REO-MPER vectors). The REO-MPER vectors replicated with wild-type efficiency, were genetically stable, and retained native antigenicity. However, we did not detect HIV-1-specific immune responses following inoculation of the REO-MPER vectors into small animals. This work provides proof of principle for engineering reovirus to express antigenic epitopes and illustrates the difficulty in eliciting MPER-specific immune responses.

Potential for Improving Potency and Specificity of Reovirus Oncolysis with Next-Generation Reovirus Variants

Viruses that specifically replicate in tumor over normal cells offer promising cancer therapies. Oncolytic viruses (OV) not only kill the tumor cells directly; they also promote anti-tumor immunotherapeutic responses. Other major advantages of OVs are that they dose-escalate in tumors and can be genetically engineered to enhance potency and specificity. Unmodified wild type reovirus is a propitious OV currently in phase I–III clinical trials. This review summarizes modifications to reovirus that may improve potency and/or specificity during oncolysis. Classical genetics approaches have revealed reovirus variants with improved adaptation towards tumors or with enhanced ability to establish specific steps of virus replication and cell killing among transformed cells. The recent emergence of a reverse genetics system for reovirus has provided novel strategies to fine-tune reovirus proteins or introduce exogenous genes that could promote oncolytic activity. Over the next decade, these findings are likely to generate better-optimized second-generation reovirus vectors and improve the efficacy of oncolytic reotherapy.

A novel pathogenic Mammalian orthoreovirus from diarrheic pigs and Swine blood meal in the United States

Since May 2013, outbreaks of porcine epidemic diarrhea have devastated the U.S. swine industry, causing immense economic losses. Two different swine enteric coronaviruses (porcine epidemic diarrhea virus and Delta coronavirus) have been isolated from the affected swine population. The disease has been reported from at least 32 states of the United States and other countries, including Mexico, Peru, Dominican Republic, Canada, Columbia, Ecuador, and Ukraine, with repeated outbreaks in previously infected herds. Here we report the isolation and characterization of a novel mammalian orthoreovirus 3 (MRV3) from diarrheic feces of piglets from these outbreaks in three states and ring-dried swine blood meal from multiple sources. MRV3 could not be isolated from healthy or pigs that had recovered from epidemic diarrhea from four states. Several MRV3 isolates were obtained from chloroform-extracted pig feces or blood meal in cell cultures or developing chicken embryos. Biological characterization of two representative isolates revealed trypsin resistance and thermostability at 90°C. NextGen sequencing of ultrapurified viruses indicated a strong homology of the S1 segment to mammalian and bat MRV3. Neonatal piglets experimentally infected with these viruses or a chloroform extract of swine blood meal developed severe diarrhea and acute gastroenteritis with 100% mortality within 3 days postinfection. Therefore, the novel porcine MRV3 may contribute to enteric disease along with other swine enteric viruses. The role of MRV3 in the current outbreaks of porcine epidemic diarrhea in the United States remains to be determined, but the pathogenic nature of the virus warrants further investigations on its epidemiology and prevalence.

IMPORTANCE

Porcine orthoreoviruses causing diarrhea have been reported in China and Korea but not in the United States. We have isolated and characterized two pathogenic reassortant MRV3 isolates from swine fecal samples from porcine epidemic diarrhea outbreaks and ring-dried swine blood meal in the United States. These fecal and blood meal isolates or a chloroform extract of blood meal induced severe diarrhea and mortality in experimentally infected neonatal pigs. Genetic and phylogenetic analyses of two MRV3 isolates revealed that they are identical but differed significantly from nonpathogenic mammalian orthoreoviruses circulating in the United States. The present study provides a platform for immediate development of suitable vaccines and diagnostics to prevent and control porcine orthoreovirus diarrhea.

A single amino acid substitution in the mRNA capping enzyme λ2 of a mammalian orthoreovirus mutant increases interferon sensitivity

In the last few years, the development of a plasmid-based reverse genetics system for mammalian reovirus has allowed the production and characterization of mutant viruses. This could be especially significant in the optimization of reovirus strains for virotherapeutic applications, either as gene vectors or oncolytic viruses. The genome of a mutant virus exhibiting increased sensitivity to interferon was completely sequenced and compared with its parental virus. Viruses corresponding to either the parental or mutant viruses were then rescued by reverse genetics and shown to exhibit the expected phenotypes. Systematic rescue of different viruses harboring either of the four parental genes in a mutant virus backbone, or reciprocally, indicated that a single amino acid substitution in one of λ2 methyltransferase domains is the major determinant of the difference in interferon sensitivity between these two viruses.

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An interferon sensitive reovirus harbors amino acids substitutions in four proteins.

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Wild-type laboratory stock and mutant viruses were reconstructed by reverse genetics.

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Each mutant gene was substituted by its wild-type counterpart and reciprocally.

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Interferon sensitivity was assigned to a substitution in mRNA capping protein λ2.

Generating primary cultures of murine cardiac myocytes and cardiac fibroblasts to study viral myocarditis

Viruses can induce direct damage to cardiac myocytes and cardiac fibroblasts resulting in myocarditis and impaired cardiac function. Cardiac myocytes and cardiac fibroblasts display different capacities to support viral infection and generate a protective antiviral response. This chapter provides detailed protocols for generation and characterization of primary cultures of murine cardiac myocytes and cardiac fibroblasts, offering a powerful tool to probe cell type-specific responses that determine protection against viral myocarditis.

Proinflammatory effects of interferon gamma in mouse adenovirus 1 myocarditis

Adenoviruses are frequent causes of pediatric myocarditis. Little is known about the pathogenesis of adenovirus myocarditis, and the species specificity of human adenoviruses has limited the development of animal models, which is a significant barrier to strategies for prevention or treatment. We have developed a mouse model of myocarditis following mouse adenovirus 1 (MAV-1) infection to study the pathogenic mechanisms of this important cause of pediatric myocarditis. Following intranasal infection of neonatal C57BL/6 mice, we detected viral replication and induction of interferon gamma (IFN-γ) in the hearts of infected mice. MAV-1 caused myocyte necrosis and induced substantial cellular inflammation that was composed predominantly of CD3(+) T lymphocytes. Depletion of IFN-γ during acute infection reduced cardiac inflammation in MAV-1-infected mice without affecting viral replication. We observed decreased contractility during acute infection of neonatal mice, and persistent viral infection in the heart was associated with cardiac remodeling and hypertrophy in adulthood. IFN-γ is a proinflammatory mediator during adenovirus-induced myocarditis, and persistent adenovirus infection may contribute to ongoing cardiac dysfunction.

IMPORTANCE

Studying the pathogenesis of myocarditis caused by different viruses is essential in order to characterize both virus-specific and generalized factors that contribute to disease. Very little is known about the pathogenesis of adenovirus myocarditis, which is a significant impediment to the development of treatment or prevention strategies. We used MAV-1 to establish a mouse model of human adenovirus myocarditis, providing the means to study host and pathogen factors contributing to adenovirus-induced cardiac disease during acute and persistent infection. The MAV-1 model will enable fundamental studies of viral myocarditis, including IFN-γ modulation as a therapeutic strategy.

Systems biology unravels interferon responses to respiratory virus infections

Interferon production is an important defence against viral replication and its activation is an attractive therapeutic target. However, it has long been known that viruses perpetually evolve a multitude of strategies to evade these host immune responses. In recent years there has been an explosion of information on virus-induced alterations of the host immune response that have resulted from data-rich omics technologies. Unravelling how these systems interact and determining the overall outcome of the host response to viral infection will play an important role in future treatment and vaccine development. In this review we focus primarily on the interferon pathway and its regulation as well as mechanisms by which respiratory RNA viruses interfere with its signalling capacity.

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.

Sequence analysis of the genome of piscine orthoreovirus (PRV) associated with heart and skeletal muscle inflammation (HSMI) in Atlantic salmon (Salmo salar)

Piscine orthoreovirus (PRV) is associated with heart- and skeletal muscle inflammation (HSMI) of farmed Atlantic salmon (Salmo salar). We have performed detailed sequence analysis of the PRV genome with focus on putative encoded proteins, compared with prototype strains from mammalian (MRV T3D)- and avian orthoreoviruses (ARV-138), and aquareovirus (GCRV-873). Amino acid identities were low for most gene segments but detailed sequence analysis showed that many protein motifs or key amino acid residues known to be central to protein function are conserved for most PRV proteins. For M-class proteins this included a proline residue in μ2 which, for MRV, has been shown to play a key role in both the formation and structural organization of virus inclusion bodies, and affect interferon-β signaling and induction of myocarditis. Predicted structural similarities in the inner core-forming proteins λ1 and σ2 suggest a conserved core structure. In contrast, low amino acid identities in the predicted PRV surface proteins μ1, σ1 and σ3 suggested differences regarding cellular interactions between the reovirus genera. However, for σ1, amino acid residues central for MRV binding to sialic acids, and cleavage- and myristoylation sites in μ1 required for endosomal membrane penetration during infection are partially or wholly conserved in the homologous PRV proteins. In PRV σ3 the only conserved element found was a zinc finger motif. We provide evidence that the S1 segment encoding σ3 also encodes a 124 aa (p13) protein, which appears to be localized to intracellular Golgi-like structures. The S2 and L2 gene segments are also potentially polycistronic, predicted to encode a 71 aa- (p8) and a 98 aa (p11) protein, respectively. It is concluded that PRV has more properties in common with orthoreoviruses than with aquareoviruses.