Addition of exogenous polypeptides on the mammalian reovirus outer capsid using reverse genetics

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.

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.

A method for the unbiased quantification of reassortment in segmented viruses

Reassortment of segmented viruses can be an important source of genetic diversity underlying viral evolution and emergence. Methods for the quantification of reassortment have been described but are often cumbersome and best suited for the analysis of reassortment between highly divergent parental strains. While it is useful to understand the potential of divergent parents to reassort, outcomes of such heterologous reassortment are driven by differential selection acting on the progeny and are typically strain specific. To quantify reassortment robustly, a system free of differential selection is needed. We have generated such a system for influenza A virus and for mammalian orthoreovirus by constructing well-matched parental viruses carrying small genetic tags. The method utilizes high-resolution melt technology for the identification of reassortant viruses. Ease of sample preparation and data analysis enables streamlined genotyping of a large number of virus clones. The method described here thereby allows quantification of the efficiency of reassortment and can be applied to diverse segmented viruses.

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.

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.

Characterization of a Replicating Mammalian Orthoreovirus with Tetracysteine-Tagged μNS for Live-Cell Visualization of Viral Factories

Within infected host cells, mammalian orthoreovirus (MRV) forms viral factories (VFs), which are sites of viral transcription, translation, assembly, and replication. The MRV nonstructural protein μNS comprises the structural matrix of VFs and is involved in recruiting other viral proteins to VF structures. Previous attempts have been made to visualize VF dynamics in live cells, but due to current limitations in recovery of replicating reoviruses carrying large fluorescent protein tags, researchers have been unable to directly assess VF dynamics from virus-produced μNS. We set out to develop a method to overcome this obstacle by utilizing the 6-amino-acid (CCPGCC) tetracysteine (TC) tag and FlAsH-EDT2 reagent. The TC tag was introduced into eight sites throughout μNS, and the capacity of the TC-μNS fusion proteins to form virus factory-like (VFL) structures and colocalize with virus proteins was characterized. Insertion of the TC tag interfered with recombinant virus rescue in six of the eight mutants, likely as a result of loss of VF formation or important virus protein interactions. However, two recombinant (r)TC-μNS viruses were rescued and VF formation, colocalization with associating virus proteins, and characterization of virus replication were subsequently examined. Furthermore, the rTC-μNS viruses were utilized to infect cells and examine VF dynamics using live-cell microscopy. These experiments demonstrate active VF movement with fusion events as well as transient interactions between individual VFs and demonstrate the importance of microtubule stability for VF fusion during MRV infection. This work provides important groundwork for future in-depth studies of VF dynamics and host cell interactions.IMPORTANCE MRV has historically been used as a model to study the double-stranded RNA (dsRNA) Reoviridae family, the members of which infect and cause disease in humans, animals, and plants. During infection, MRV forms VFs that play a critical role in virus infection but remain to be fully characterized. To study VFs, researchers have focused on visualizing the nonstructural protein μNS, which forms the VF matrix. This work provides the first evidence of recovery of replicating reoviruses in which VFs can be labeled in live cells via introduction of a TC tag into the μNS open reading frame. Characterization of each recombinant reovirus sheds light on μNS interactions with viral proteins. Moreover, utilizing the TC-labeling FlAsH-EDT2 biarsenical reagent to visualize VFs, evidence is provided of dynamic VF movement and interactions at least partially dependent on intact microtubules.

Exploring Reovirus Plasticity for Improving Its Use as Oncolytic Virus

Reoviruses are non-enveloped viruses with a segmented double stranded RNA genome. In humans, they are not associated with serious disease. Human reoviruses exhibit an inherent preference to replicate in tumor cells, which makes them ideally suited for use in oncolytic virotherapies. Their use as anti-cancer agent has been evaluated in several clinical trials, which revealed that intra-tumoral and systemic delivery of reoviruses are well tolerated. Despite evidence of anti-tumor effects, the efficacy of reovirus in anti-cancer monotherapy needs to be further enhanced. The opportunity to treat both the primary tumor as well as metastases makes systemic delivery a preferred administration route. Several pre-clinical studies have been conducted to address the various hurdles connected to systemic delivery of reoviruses. The majority of those studies have been done in tumor-bearing immune-deficient murine models. This thwarts studies on the impact of the contribution of the immune system to the tumor cell eradication. This review focuses on key aspects of the reovirus/host-cell interactions and the methods that are available to modify the virus to alter these interactions. These aspects are discussed with a focus on improving the reovirus' antitumor efficacy.

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 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.

Replicating reoviruses with a transgene replacing the codons for the head domain of the viral spike

The capacity to modify the reovirus genome facilitates generation of new therapeutic reoviruses. We describe a method for generating replication-competent reoviruses carrying a heterologous transgene. The strategy is based on the expanded-tropism reovirus mutant jin-3, which can infect cells independent of the reovirus receptor junction-adhesion molecule A (JAM-A). Jin-3 harbors a mutation in the S1 segment, resulting in a G196R substitution in the tail of the spike protein σ1. The use of the jin-3 tail-encoding S1 segment allows replacing the codons for the JAM-A-binding head domain by up to 522  nucleotides of foreign sequences, without exceeding the size of the wild-type S1 segment. We inserted the codons for the porcine teschovirus-1 2A element fused with those encoding the fluorescent protein iLOV. Replicating rS1His-2A-iLOV reoviruses were generated by co-transfection of expression plasmids for all reovirus segments. These reoviruses contain the S1His-2A-iLOV segment in the absence of the wild-type S1 segment. Density-gradient centrifugation confirmed the association of the σ1-tail fragment with the capsid. Both JAM-A-positive and -negative cells exposed to the rS1His-2A-iLOV reoviruses exhibited iLOV fluorescence, confirming the jin-3-derived expanded-tropism phenotype. These data demonstrated the feasibility of generating decapitated replication-competent T3D reoviruses carrying a heterologous transgene.

Amino acids substitutions in σ1 and μ1 outer capsid proteins of a Vero cell-adapted mammalian orthoreovirus are required for optimal virus binding and disassembly

In a recent study, the serotype 3 Dearing strain of mammalian orthoreovirus was adapted to Vero cells; cells that exhibit a limited ability to support the early steps of reovirus uncoating and are unable to produce interferon as an antiviral response upon infection. The Vero cell-adapted virus (VeroAV) exhibits amino acids substitutions in both the σ1 and μ1 outer capsid proteins but no changes in the σ3 protein. Accordingly, the virus was shown not to behave as a classical uncoating mutant. In the present study, an increased ability of the virus to bind at the Vero cell surface was observed and is likely associated with an increased ability to bind onto cell-surface sialic acid residues. In addition, the kinetics of μ1 disassembly from the virions appears to be altered. The plasmid-based reverse genetics approach confirmed the importance of σ1 amino acids substitutions in VeroAV's ability to efficiently infect Vero cells, although μ1 co-adaptation appears necessary to optimize viral infection. This approach of combining in vitro selection of reoviruses with reverse genetics to identify pertinent amino acids substitutions appears promising in the context of eventual reovirus modification to increase its potential as an oncolytic virus.

[Orthoreoviruses]

Members of the genus Orthoreovirus in the family Reoviridae are nonenveloped, icosahedral viruses. Their genomes contain 10 segments of double-stranded RNA (dsRNA). The orthoreoviruses are divided into two subgroups, the fusogenic and nonfusogenic reoviruses, based on the ability of the virus to induce cell-to-cell fusion. The fusogenic subgroup consists of the avian reovirus, baboon reovirus, pteropine reovirus, and reptilian reovirus, whereas the nonfusogenic subgroup consists of the prototypical mammalian reovirus (MRV) species. MRVs are highly tractable experimental models for studies of segmented dsRNA virus replication and pathogenesis. Moreover, MRVs can selectively kill tumor cells and have been evaluated as oncolytic agents in clinical trials. This review provides a brief overview of current knowledge on the virological features of MRVs.

Engineering recombinant reoviruses with tandem repeats and a tetravirus 2A-like element for exogenous polypeptide expression

We tested a strategy for engineering recombinant mammalian reoviruses (rMRVs) to express exogenous polypeptides. One important feature is that these rMRVs are designed to propagate autonomously and can therefore be tested in animals as potential vaccine vectors. The strategy has been applied so far to three of the 10 MRV genome segments: S3, M1, and L1. To engineer the modified segments, a 5' or 3' region of the essential, long ORF in each was duplicated, and then exogenous sequences were inserted between the repeats. The inner repeat and exogenous insert were positioned in frame with the native protein-encoding sequences but were separated from them by an in-frame "2A-like" sequence element that specifies a cotranslational "stop/continue" event releasing the exogenous polypeptide from the essential MRV protein. This design preserves a terminal region of the MRV genome segment with essential activities in RNA packaging, assortment, replication, transcription, and/or translation and alters the encoded MRV protein to a limited degree. Recovery of rMRVs with longer inserts was made more efficient by wobble-mutagenizing both the inner repeat and the exogenous insert, which possibly helped via respective reductions in homologous recombination and RNA structure. Immunogenicity of a 300-aa portion of the simian immunodeficiency virus Gag protein expressed in mice by an L1-modified rMRV was confirmed by detection of Gag-specific T-cell responses. The engineering strategy was further used for mapping the minimal 5'-terminal region essential to MRV genome segment S3.

Generation of genetically stable recombinant rotaviruses containing novel genome rearrangements and heterologous sequences by reverse genetics

The rotavirus (RV) genome consists of 11 segments of double-stranded RNA (dsRNA). Typically, each segment contains 5' and 3' untranslated regions (UTRs) that flank an open reading frame (ORF) encoding a single protein. RV variants with segments of atypical size owing to sequence rearrangements have been described. In many cases, the rearrangement originates from a partial head-to-tail sequence duplication that initiates after the stop codon of the ORF, leaving the protein product of the segment unaffected. To probe the limits of the RV genome to accommodate additional genetic sequence, we used reverse genetics to insert duplications (analogous to synthetic rearrangements) and heterologous sequences into the 3' UTR of the segment encoding NSP2 (gene 8). The approach allowed the recovery of recombinant RVs that contained sequence duplications (up to 200 bp) and heterologous sequences, including those for FLAG, the hepatitis C virus E2 epitope, and the internal ribosome entry site of cricket paralysis virus. The recombinant RVs grew to high titer (>10(7) PFU/ml) and remained genetically stable during serial passage. Despite their longer 3' UTRs, rearranged RNAs of recombinant RVs expressed wild-type levels of protein in vivo. Competitive growth experiments indicated that, unlike RV segments with naturally occurring sequence duplications, genetically engineered segments were less efficiently packaged into progeny viruses. Thus, features of naturally occurring rearranged segments, other than their increased length, contribute to their enhanced packaging phenotype. Our results define strategies for developing recombinant RVs as expression vectors, potentially leading to next-generation RV vaccines that induce protection against other infectious agents.

Transient high level mammalian reovirus replication in a bat epithelial cell line occurs without cytopathic effect

Mammalian reoviruses exhibit a large host range and infected cells are generally killed; however, most studies examined only a few cell types and host species, and are probably not representative of all possible interactions between virus and host cell. Many questions thus remain concerning the nature of cellular factors that affect viral replication and cell death. In the present work, it was observed that replication of the classical mammalian reovirus serotype 3 Dearing in a bat epithelial cell line, Tb1.Lu, does not result in cell lysis and is rapidly reduced to very low levels. Prior uncoating of virions by chymotrypsin treatment, to generate infectious subviral particles, increased the initial level of infection but without any significant effect on further viral replication or cell survival. Infected cells remain resistant to virus reinfection and secrete an antiviral factor, most likely interferon, that is protective against the unrelated encephalomyocarditis virus. Although, the transformed status of a cell is believed to promote reovirus replication and viral "oncolysis", resistant Tb1.Lu cells exhibit a classical phenotype of transformed cells by forming colonies in semisolid soft agar medium. Further transduction of Tb.Lu cells with a constitutively active Ras oncogene does not seem to affect cell growth or reovirus effect on these cells. Infected Tb1.Lu cells can produce low-level of infectious virus for a long time without any apparent effect, although these cells are resistant to reinfection. The results suggest that Tb1.Lu cells can mount an unusual antiviral response. Specific properties of bat cells may thus be in part responsible for the ability of the animals to act as reservoirs for viruses in general and for novel reoviruses in particular. Their peculiar resistance to cell lysis also makes Tb1.Lu cells an attractive model to study the cellular and viral factors that determine the ability of reovirus to replicate and destroy infected cells.

Characterization of epitope-tagged foot-and-mouth disease virus

Foot-and-mouth disease (FMD) is a highly contagious and economically devastating disease of cloven-hoofed animals with an almost-worldwide distribution. Conventional FMD vaccines consisting of chemically inactivated viruses have aided in the eradication of FMD from Europe and remain the main tool for control in endemic countries. Although significant steps have been made to improve the quality of vaccines, such as improved methods of antigen concentration and purification, manufacturing processes are technically demanding and expensive. Consequently, there is large variation in the quality of vaccines distributed in FMD-endemic countries compared with those manufactured for emergency use in FMD-free countries. Here, we have used reverse genetics to introduce haemagglutinin (HA) and FLAG tags into the foot-and-mouth disease virus (FMDV) capsid. HA- and FLAG-tagged FMDVs were infectious, with a plaque morphology similar to the non-tagged parental infectious copy virus and the field virus. The tagged viruses utilized integrin-mediated cell entry and retained the tag epitopes over serial passages. In addition, infectious HA- and FLAG-tagged FMDVs were readily purified from small-scale cultures using commercial antibodies. Tagged FMDV offers a feasible alternative to the current methods of vaccine concentration and purification, a potential to develop FMD vaccine conjugates and a unique tool for FMDV research.

Genetics and reverse genetics of rotavirus

Rotavirus is a member of the family Reoviridae, which have genomes consisting of 10-12 double-stranded RNA segments. The functions of proteins encoded by each segment of the rotavirus genome have been studied extensively by several methods including reassortants, temperature-sensitive mutants, isolates with rearranged RNA segments, RNAi analysis, and other procedures. However, as found for most RNA viruses, the technique of reverse genetics is required for precise genotype/phenotype correlation, for the analysis of the role of specific mutation in replication process and pathogenesis, and for the development of vectors and vaccines. In 2006, we presented the first description of a reverse genetics system for rotavirus, although a helper virus and a selection system are required. Since then, two other approaches have been reported for rotavirus reverse genetics, both requiring the presence of a helper virus. A tractable, helper virus-free reverse genetics system for rotavirus has not been developed so far, in contrast to the recent developments of plasmid only-based reverse genetics systems for other members of the Reoviridae.