Mammalian Reovirus Research Articles

The reovirus μ2 protein, an enigmatic multifunctional protein with numerous secrets yet to be uncovered

Viruses as obligate intracellular parasites are limited by their small genome. They have thus developed various strategies to maximize viral fitness with a limited amount of coding information. Among these strategies is the use of the same viral protein for multiple functions. The μ2 protein of mammalian reovirus is one such example of a multifunctional protein. We will present recent progress in our understanding of some functions and properties of this protein that have been revealed in the last two or three decades, such as its impact on the formation of viral factories or the control of the interferon response. We will also examine the recently established structure of the protein and the most recent data on the protein’s enzymatic activities in the context of viral RNA synthesis. Finally, the impact of μ2 in the regulation of host-cell alternative mRNA splicing will be presented and future avenues of research discussed.

Mammalian reovirus µ1 protein attenuates RIG-I and MDA5-mediated signaling transduction by blocking IRF3 phosphorylation and nuclear translocation

Mammalian reovirus (MRV) is a non-enveloped, gene segmented double-stranded RNA (dsRNA) virus. It is an important zoonotic pathogen that infects many mammals and vertebrates that act as natural hosts and causes respiratory and digestive tract diseases. Studies have reported that RIG-I and MDA5 in the innate immune cytoplasmic RNA-sensing RIG-like receptor (RLR) signaling pathway can recognize dsRNA from MRV and promote antiviral type I interferon (IFN) responses. However, the mechanism by which many MRV-encoded proteins evade the host innate immune response remains unclear. Here, we show that exogenous μ1 protein promoted the proliferation of MRV in vitro, while knockdown of MRV μ1 protein expression by shRNA could impair MRV proliferation. Specifically, μ1 protein inhibited MRV or poly(I:C)-induced IFN-β expression, and attenuated RIG-I/MDA5-mediated signaling axis transduction during MRV infection. Importantly, we found that μ1 protein significantly decreased IFN-β mRNA expression induced by MDA5, RIG-I, MAVS, TBK1, IRF3(5D), and degraded the protein expression of exogenous MDA5, RIG-I, MAVS, TBK1 and IRF3 via the proteasomal and lysosomal pathways. Additionally, we show that μ1 protein can physically interact with MDA5, RIG-I, MAVS, TBK1, and IRF3 and attenuate the RIG-I/MDA5-mediated signaling cascades by blocking the phosphorylation and nuclear translocation of IRF3. In conclusion, our findings reveal that MRV outer capsid protein μ1 is a key factor in antagonizing RLRs signaling cascades and provide new strategies for effective prevention and treatment of MRV infection.

Mammalian Reoviruses: Propagation, Quantification, and Storage.

Mammalian reoviruses are pathogens that cause gastrointestinal and respiratory infections. In humans, the mammalian reoviruses usually cause mild or subclinical disease, and they are ubiquitous, with most people mounting immunity at a young age. Reoviruses are prototypic representations of the Reoviridae family, which contains many highly pathogenic viruses. This article describes techniques for culturing mouse fibroblast L929 cell lines, the preferred cell line in which most mammalian reovirus studies take place. In addition, mammalian reovirus propagation, quantification, purification, and storage are described. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Propagation of mammalian reoviruses in cell culture from virus stocks Alternate Protocol 1: Large-scale propagation (and purification) of mammalian reoviruses in cell culture from virus stocks Basic Protocol 2: Quantification of mammalian reoviruses by plaque assay with neutral red staining Alternate Protocol 2: Quantification of mammalian reoviruses by plaque assay with crystal violet staining Basic Protocol 3: Storage of mammalian reoviruses Support Protocol 1: Growth and maintenance of mouse L929 cells Support Protocol 2: Plating L929 cells.

Oncolytic viruses-modulated immunogenic cell death, apoptosis and autophagy linking to virotherapy and cancer immune response.

Recent reports have revealed that oncolytic viruses (OVs) play a significant role in cancer therapy. The infection of OVs such as oncolytic vaccinia virus (OVV), vesicular stomatitis virus (VSV), parvovirus, mammalian reovirus (MRV), human adenovirus, Newcastle disease virus (NDV), herpes simplex virus (HSV), avian reovirus (ARV), Orf virus (ORFV), inactivated Sendai virus (ISV), enterovirus, and coxsackievirus offer unique opportunities in immunotherapy through diverse and dynamic pathways. This mini-review focuses on the mechanisms of OVs-mediated virotherapy and their effects on immunogenic cell death (ICD), apoptosis, autophagy and regulation of the immune system.

Molecular Characterization and Function of the Nogo-66 Receptor (NgR1) Gene in the Chinese Tree Shrew

Nogo-66 receptor (NgR1) is a glycosylphosphatidylinositol-linked cell surface receptor with high affinity for Nogo-66. The binding of Nogo-66 to NgR1 plays a key role in inhibiting neurite growth, limiting synaptic plasticity and mediating Mammalian Reovirus (MRV) infection. The Chinese tree shrew (Tupaia belangeri chinensis) is, a new and valuable experimental animal that is widely used in biomedical research. Although susceptible to MRV, little is known about tree shrew NgR1 and its role in MRV infection. In this study, we cloned NgR1 form the Chinese tree shrew by RACE technology and analyzed its characteristics, spatial structure and its tissue expression. We also examined the expression pattern of NgR1 in the response of tree shrew primary nerve cells (tNC) to MRV1/TS/2011 infection. Tree shrew NgR1 was found to have a closer relationship to human NgR1 (90.34%) than to mouse NgR1. Similar to the protein structure of human NgR1, the tree shrew NgR1 has the same leucine-rich repeat (LRR) domain structure that is capped by C-terminal and N-terminal cysteine-rich modules. The tree shrew NgR1 mRNAs were predominantly detected in the central nervous system (CNS), and tree shrew NgR1 can mediate infection by MRV1/TS/2011. Taken together, these results help to elucidate the function of NgR1 and provide a basis for using the tree shrew as an animal model for studies of the nervous system and infectious diseases.

In situ structures of polymerase complex of mammalian reovirus illuminate RdRp activation and transcription regulation

Mammalian reovirus (reovirus) is a multilayered, turreted member of Reoviridae characterized by transcription of dsRNA genome within the innermost capsid shell. Here, we present high-resolution in situ structures of reovirus transcriptase complex in an intact double-layered virion, and in the uncoated single-layered core particles in the unloaded, reloaded, pre-elongation, and elongation states, respectively, obtained by cryo-electron microscopy and sub-particle reconstructions. At the template entry of RNA-dependent RNA polymerase (RdRp), the RNA-loading region gets flexible after uncoating resulting in the unloading of terminal genomic RNA and inactivity of transcription. However, upon adding transcriptional substrates, the RNA-loading region is recovered leading the RNAs loaded again. The priming loop in RdRp was found to play a critical role in regulating transcription, which hinders the elongation of transcript in virion and triggers the rearrangement of RdRp C-terminal domain (CTD) during elongation, resulting in splitting of template-transcript hybrid and opening of transcript exit. With the integration of these structures, a transcriptional model of reovirus with five states is proposed. Our structures illuminate the RdRp activation and regulation of the multilayered turreted reovirus.

U5 snRNP Core Proteins Are Key Components of the Defense Response against Viral Infection through Their Roles in Programmed Cell Death and Interferon Induction.

The spliceosome is a massive ribonucleoprotein structure composed of five small nuclear ribonucleoprotein (snRNP) complexes that catalyze the removal of introns from pre-mature RNA during constitutive and alternative splicing. EFTUD2, PRPF8, and SNRNP200 are core components of the U5 snRNP, which is crucial for spliceosome function as it coordinates and performs the last steps of the splicing reaction. Several studies have demonstrated U5 snRNP proteins as targeted during viral infection, with a limited understanding of their involvement in virus-host interactions. In the present study, we deciphered the respective impact of EFTUD2, PRPF8, and SNRNP200 on viral replication using mammalian reovirus as a model. Using a combination of RNA silencing, real-time cell analysis, cell death and viral replication assays, we discovered distinct and partially overlapping novel roles for EFTUD2, PRPF8, and SNRNP200 in cell survival, apoptosis, necroptosis, and the induction of the interferon response pathway. For instance, we demonstrated that EFTUD2 and SNRNP200 are required for both apoptosis and necroptosis, whereas EFTUD2 and PRPF8 are required for optimal interferon response against viral infection. Moreover, we demonstrated that EFTUD2 restricts viral replication, both in a single cycle and multiple cycles of viral replication. Altogether, these results establish U5 snRNP core components as key elements of the cellular antiviral response.

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.

Asymmetric reconstruction of mammalian reovirus reveals interactions among RNA, transcriptional factor \xb52 and capsid proteins

Mammalian reovirus (MRV) is the prototypical member of genus Orthoreovirus of family Reoviridae. However, lacking high-resolution structures of its RNA polymerase cofactor μ2 and infectious particle, limits understanding of molecular interactions among proteins and RNA, and their contributions to virion assembly and RNA transcription. Here, we report the 3.3 Å-resolution asymmetric reconstruction of transcribing MRV and in situ atomic models of its capsid proteins, the asymmetrically attached RNA-dependent RNA polymerase (RdRp) λ3, and RdRp-bound nucleoside triphosphatase μ2 with a unique RNA-binding domain. We reveal molecular interactions among virion proteins and genomic and messenger RNA. Polymerase complexes in three Spinoreovirinae subfamily members are organized with different pseudo-D3d symmetries to engage their highly diversified genomes. The above interactions and those between symmetry-mismatched receptor-binding σ1 trimers and RNA-capping λ2 pentamers balance competing needs of capsid assembly, external protein removal, and allosteric triggering of endogenous RNA transcription, before, during and after infection, respectively.

A Single Point Mutation, Asn16→Lys, Dictates the Temperature-Sensitivity of the Reovirus tsG453 Mutant

Studies of conditionally lethal mutants can help delineate the structure-function relationships of biomolecules. Temperature-sensitive (ts) mammalian reovirus (MRV) mutants were isolated and characterized many years ago. Two of the most well-defined MRV ts mutants are tsC447, which contains mutations in the S2 gene encoding viral core protein σ2, and tsG453, which contains mutations in the S4 gene encoding major outer-capsid protein σ3. Because many MRV ts mutants, including both tsC447 and tsG453, encode multiple amino acid substitutions, the specific amino acid substitutions responsible for the ts phenotype are unknown. We used reverse genetics to recover recombinant reoviruses containing the single amino acid polymorphisms present in ts mutants tsC447 and tsG453 and assessed the recombinant viruses for temperature-sensitivity by efficiency-of-plating assays. Of the three amino acid substitutions in the tsG453 S4 gene, Asn16-Lys was solely responsible for the tsG453 ts phenotype. Additionally, the mutant tsC447 Ala188-Val mutation did not induce a temperature-sensitive phenotype. This study is the first to employ reverse genetics to identify the dominant amino acid substitutions responsible for the tsC447 and tsG453 mutations and relate these substitutions to respective phenotypes. Further studies of other MRV ts mutants are warranted to define the sequence polymorphisms responsible for temperature sensitivity.

Asymmetric profiles of infection and innate immunological responses in human iPS cell-derived small intestinal epithelial-like cell monolayers following infection with mammalian reovirus

The intestinal mucosa plays an important role as an immune barrier due to its continual exposure to invading pathogens, including viruses. It is thus highly important to evaluate virus infection profiles in the intestinal mucosa for prevention of virus infection and development of antivirus medicines; however, only a few enterocyte lines are available as in vitro intestinal models for the evaluation of virus infection. In this study, we evaluated profiles of infection and innate immune responses following infection with a mammalian orthoreovirus (hereafter reovirus), which has often been used as a tractable model for studies of viral pathogenesis, in human iPS cell-derived small intestinal epithelial-like cell (hiPS-SIEC) monolayers and cells of a human colon adenocarcinoma cell line, Caco-2. The levels of reovirus infection were similar between hiPS-SIEC and Caco-2 cell monolayers, which are often used as an intestinal model, after apical and basolateral infection. In hiPS-SIEC monolayers, more efficient replication of the virus genome was observed following basolateral infection than apical infection, while apical infection resulted in higher levels of virus protein expression and progeny virus production than basolateral infection. Reovirus significantly induced innate immune responses, including expression of type I and III interferons (IFNs), in hiPS-SIEC monolayers more efficiently than Caco-2 cells. Higher levels of type I and III interferon (IFN) expression were found in hiPS-SIEC monolayers following apical infection than basolateral infection. These results suggested that hiPS-SIECs are a promising in vitro model for the evaluation of virus infection.

Generation of Genetically RGD σ1-Modified Oncolytic Reovirus That Enhances JAM-A-Independent Infection of Tumor Cells.

Mammalian reovirus (MRV) strain type 3 Dearing (T3D) is a naturally occurring oncolytic virus that has been developed as a potential cancer therapeutic. However, MRV treatment cannot be applied to cancer cells expressing low levels of junctional adhesion molecule A (JAM-A), which is the entry receptor of MRV. In this study, we developed a reverse genetics system for MRV strain T3D-L, which showed high oncolytic potency. To modify the cell tropism of MRV, an arginine-glycine-aspartic acid (RGD) peptide with an affinity to integrin was inserted at the C terminus or loop structures of the viral cell attachment protein σ1. The recombinant RGD σ1-modified viruses induced remarkable cell lysis in human cancer cell lines with marginal JAM-A expression and in JAM-A knockout cancer cell lines generated by a CRISPR/Cas9 system. Pretreatment of cells with anti-integrin antibody decreased cell death caused by the RGD σ1-modified virus, suggesting the infection to the cells was via a specific interaction with integrin αV. By using mouse models, we assessed virulence of the RGD σ1-modified viruses in vivo This system will open new avenues for the use of genetically modified oncolytic MRV for use as a cancer therapy.IMPORTANCE Oncolytic viruses kill tumors without affecting normal cells. A variety of oncolytic viruses are used as cancer therapeutics. Mammalian reovirus (MRV), which belongs to the genus Orthoreovirus, family Reoviridae, is one such natural oncolytic virus. The anticancer effects of MRV are being evaluated in clinical trials. Unlike other oncolytic viruses, MRV has not been genetically modified for use as a cancer therapeutic in clinical trials. Here, we used a reverse genetic approach to introduce an integrin-affinity peptide sequence into the MRV cell attachment protein σ1 to alter the natural tropism of the virus. The recombinant viruses were able to infect cancer cell lines expressing very low levels of the MRV entry receptor, junctional adhesion molecule A (JAM-A), and cause tumor cell death while maintaining its original tropism via JAM-A. This is a novel report of a genetically modified oncolytic MRV by introducing a peptide sequence into σ1.

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.

Reovirus Core Proteins λ1 and σ2 Promote Stability of Disassembly Intermediates and Influence Early Replication Events.

The capsids of mammalian reovirus contain two concentric protein shells, the core and the outer capsid. The outer capsid is composed of μ1-σ3 heterohexamers which surround the core. The core is composed of λ1 decamers held in place by σ2. After entry into the endosome, σ3 is proteolytically degraded and μ1 is cleaved and exposed to form infectious subvirion particles (ISVPs). ISVPs undergo further conformational changes to form ISVP*s, resulting in the release of μ1 peptides, which facilitate the penetration of the endosomal membrane to release transcriptionally active core particles into the cytoplasm. Previous work identified regions or specific residues within reovirus outer capsid proteins that impact the efficiency of cell entry. We examined the functions of the core proteins λ1 and σ2. We generated a reovirus T3D reassortant that carries strain T1L-derived σ2 and λ1 proteins (T3D/T1L L3S2). This virus displays lower ISVP stability and therefore converts to ISVP*s more readily. To identify the molecular basis for lability of T3D/T1L L3S2, we screened for hyperstable mutants of T3D/T1L L3S2 and identified three point mutations in μ1 that stabilize ISVPs. Two of these mutations are located in the C-terminal ϕ region of μ1, which has not previously been implicated in controlling ISVP stability. Independent of compromised ISVP stability, we also found that T3D/T1L L3S2 launches replication more efficiently and produces higher yields in infected cells than T3D. In addition to identifying a new role for the core proteins in disassembly events, these data highlight the possibility that core proteins may influence multiple stages of infection.IMPORTANCE Protein shells of viruses (capsids) have evolved to undergo specific changes to ensure the timely delivery of genetic material to host cells. The 2-layer capsid of reovirus provides a model system to study the interactions between capsid proteins and the changes they undergo during entry. We tested a virus in which the core proteins were derived from a different strain than the outer capsid. In comparison to the parental T3D strain, we found that this mismatched virus was less stable and completed conformational changes required for entry prematurely. Capsid stability was restored by introduction of specific changes to the outer capsid, indicating that an optimal fit between inner and outer shells maintains capsid function. Separate from this property, mismatch between these protein layers also impacted the capacity of the virus to initiate infection and produce progeny. This study reveals new insights into the roles of capsid proteins and their multiple functions during viral replication.

Loss of IKK Subunits Limits NF-κB Signaling in Reovirus-Infected Cells.

Viruses commonly antagonize innate immune pathways that are primarily driven by nuclear factor kappa B (NF-κB), interferon regulatory factor (IRF), and the signal transducer and activator of transcription proteins (STAT) family of transcription factors. Such a strategy allows viruses to evade immune surveillance and maximize their replication. Using an unbiased transcriptome sequencing (RNA-seq)-based approach to measure gene expression induced by transfected viral genomic RNA (vgRNA) and reovirus infection, we discovered that mammalian reovirus inhibits host cell innate immune signaling. We found that, while vgRNA and reovirus infection both induce a similar IRF-dependent gene expression program, gene expression driven by the NF-κB family of transcription factors is lower in infected cells. Potent agonists of NF-κB such as tumor necrosis factor alpha (TNF-α) and vgRNA failed to induce NF-κB-dependent gene expression in infected cells. We demonstrate that NF-κB signaling is blocked due to loss of critical members of the inhibitor of kappa B kinase (IKK) complex, NF-κB essential modifier (NEMO), and IKKβ. The loss of the IKK complex components prevents nuclear translocation and phosphorylation of NF-κB, thereby preventing gene expression. Our study demonstrates that reovirus infection selectively blocks NF-κB, likely to counteract its antiviral effects and promote efficient viral replication.IMPORTANCE Host cells mount a response to curb virus replication in infected cells and prevent spread of virus to neighboring, as yet uninfected, cells. The NF-κB family of proteins is important for the cell to mediate this response. In this study, we show that in cells infected with mammalian reovirus, NF-κB is inactive. Further, we demonstrate that NF-κB is rendered inactive because virus infection results in reduced levels of upstream intermediaries (called IKKs) that are needed for NF-κB function. Based on previous evidence that active NF-κB limits reovirus infection, we conclude that inactivating NF-κB is a viral strategy to produce a cellular environment that is favorable for virus replication.

Forces during cellular uptake of viruses and nanoparticles at the ventral side

Many intracellular pathogens, such as mammalian reovirus, mimic extracellular matrix motifs to specifically interact with the host membrane. Whether and how cell-matrix interactions influence virus particle uptake is unknown, as it is usually studied from the dorsal side. Here we show that the forces exerted at the ventral side of adherent cells during reovirus uptake exceed the binding strength of biotin-neutravidin anchoring viruses to a biofunctionalized substrate. Analysis of virus dissociation kinetics using the Bell model revealed mean forces higher than 30 pN per virus, preferentially applied in the cell periphery where close matrix contacts form. Utilizing 100 nm-sized nanoparticles decorated with integrin adhesion motifs, we demonstrate that the uptake forces scale with the adhesion energy, while actin/myosin inhibitions strongly reduce the uptake frequency, but not uptake kinetics. We hypothesize that particle adhesion and the push by the substrate provide the main driving forces for uptake.

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.

Viral persistence of mammalian reovirus in cell culture: a model of virus-cell coevolution.

Although mammalian reovirus exhibits only limited pathogenicity in humans, it has been, and still remains, instrumental in studies of viral replication and pathogenesis. Generally considered as cytolytic, this virus can sometimes establish long-term persistent infections in tissue culture. In fact, in this context, it constitutes one widely used model to demonstrate coevolution between virus and host cells. Initially limited to the murine L929 fibroblasts model, further studies in different cell types appeared in the last few years. Establishment of viral persistence could also become a preferred approach to isolate new viruses that are better adapted to their applications in virotherapy, for example as oncolytic agents against human or animal cancers. A better understanding of the persistence phenomenon, especially of viral genes involved, is thus essential. The development of new tools, such as reverse genetics, appears very promising to achieve these objectives. Actually, this last approach allows us to establish the biological significance of mutations found on viruses selected during viral persistence.

Viral persistence of mammalian reovirus in cell culture: a model of virus-cell coevolution

Although mammalian reovirus exhibits only limited pathogenicity in humans, it has been, and still remains, instrumental in studies of viral replication and pathogenesis. Generally considered as cytolytic, this virus can sometimes establish long-term persistent infections in tissue culture. In fact, in this context, it constitutes one widely used model to demonstrate coevolution between virus and host cells. Initially limited to the murine L929fibroblasts model, further studies in different cell types appeared in the last few years. Establishment of viral persistence could also become a preferred approach to isolate new viruses that are better adapted to their applications in virotherapy, for example as oncolytic agents against human or animal cancers. A better understanding of the persistence phenomenon, especially of viral genes involved, is thus essential. The development of new tools, such as reverse genetics, appears very promising to achieve these objectives. Actually, this last approach allows us to establish the biological significance of mutations found on viruses selected during viral persistence.

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.