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

Oscillations in a Spatial Oncolytic Virus Model

Virotherapy treatment is a new and promising target therapy that selectively attacks cancer cells without harming normal cells. Mathematical models of oncolytic viruses have shown predator-prey like oscillatory patterns as result of an underlying Hopf bifurcation. In a spatial context, these oscillations can lead to different spatio-temporal phenomena such as hollow-ring patterns, target patterns, and dispersed patterns. In this paper we continue the systematic analysis of these spatial oscillations and discuss their relevance in the clinical context. We consider a bifurcation analysis of a spatially explicit reaction-diffusion model to find the above mentioned spatio-temporal virus infection patterns. The desired pattern for tumor eradication is the hollow ring pattern and we find exact conditions for its occurrence. Moreover, we derive the minimal speed of travelling invasion waves for the cancer and for the oncolytic virus. Our numerical simulations in 2-D reveal complex spatial interactions of the virus infection and a new phenomenon of a periodic peak splitting. An effect that we cannot explain with our current methods.

Improved oncolytic activity of a reovirus mutant that displays enhanced virus spread due to reduced cell attachment

Wild-type reovirus serotype 3 Dearing (T3wt), a non-pathogenic intestinal virus, has shown promise as a cancer therapy in clinical trials, but it would benefit from an increased potency. Given that T3wt is naturally adapted to the intestinal environment (rather than tumors), we genetically modified reovirus to improve its infectivity in cancer cells. Various reovirus mutants were created, and their oncolytic potency was evaluated in vitro using plaque size as a measure of virus fitness in cancer cells. Notably, Super Virus 5 (SV5), carrying five oncolytic mutations, displayed the largest plaques in breast cancer cells among the mutants tested, indicating the potential for enhancing oncolytic potency through the combination of mutations. Furthermore, in a HER2+ murine breast cancer model, mice treated with SV5 exhibited superior tumor reduction and increased survival compared with those treated with PBS or T3wt. Intriguingly, SV5 did not replicate faster than T3wt in cultured cells but demonstrated a farther spread relative to T3wt, attributed to its reduced attachment to cancer cells. These findings highlight the significance of increased virus spread as a crucial mechanism for improving oncolytic virus activity. Thus, genetic modifications of reovirus hold the potential for augmenting its efficacy in cancer therapy.

Modulation of Reoviral Cytolysis (II): Cellular Stemness

Oncolytic viruses (OVs) are an emerging cancer therapeutic that are intended to act by selectively targeting and lysing cancerous cells and by stimulating anti-tumour immune responses, while leaving normal cells mainly unaffected. Reovirus is a well-studied OV that is undergoing advanced clinical trials and has received FDA approval in selected circumstances. However, the mechanisms governing reoviral selectivity are not well characterised despite many years of effort, including those in our accompanying paper where we characterize pathways that do not consistently modulate reoviral cytolysis. We have earlier shown that reovirus is capable of infecting and lysing both certain types of cancer cells and also cancer stem cells, and here we demonstrate its ability to also infect and kill healthy pluripotent stem cells (PSCs). This led us to hypothesize that pathways responsible for stemness may constitute a novel route for the modulation of reoviral tropism. We find that reovirus is capable of killing both murine and human embryonic and induced pluripotent stem cells. Differentiation of PSCs alters the cells' reoviral-permissive state to a resistant one. In a breast cancer cell line that was resistant to reoviral oncolysis, induction of pluripotency programming rendered the cells permissive to cytolysis. Bioinformatic analysis indicates that expression of the Yamanaka pluripotency factors may be associated with regulating reoviral selectivity. Mechanistic insights from these studies will be useful for the advancement of reoviral oncolytic therapy.

Modulation of Reoviral Cytolysis (I): Combination Therapeutics

Patients with stage IV gastric cancer suffer from dismal outcomes, a challenge especially in many Asian populations and for which new therapeutic options are needed. To explore this issue, we used oncolytic reovirus in combination with currently used chemotherapeutic drugs (irinotecan, paclitaxel, and docetaxel) for the treatment of gastric and other gastrointestinal cancer cells in vitro and in a mouse model. Cell viability in vitro was quantified by WST-1 assays in human cancer cell lines treated with reovirus and/or chemotherapeutic agents. The expression of reovirus protein and caspase activity was determined by flow cytometry. For in vivo studies, athymic mice received intratumoral injections of reovirus in combination with irinotecan or paclitaxel, after which tumor size was monitored. In contrast to expectations, we found that reoviral oncolysis was only poorly correlated with Ras pathway activation. Even so, the combination of reovirus with chemotherapeutic agents showed synergistic cytopathic effects in vitro, plus enhanced reovirus replication and apoptosis. In vivo experiments showed that reovirus alone can reduce tumor size and that the combination of reovirus with chemotherapeutic agents enhances this effect. Thus, we find that oncolytic reovirus therapy is effective against gastric cancer. Moreover, the combination of reovirus and chemotherapeutic agents synergistically enhanced cytotoxicity in human gastric cancer cell lines in vitro and in vivo. Our data support the use of reovirus in combination with chemotherapy in further clinical trials, and highlight the need for better biomarkers for reoviral oncolytic responsiveness.

Cell-based and cell-free immunotherapies for glioblastoma: current status and future directions

Glioblastoma (GBM) is among the most fatal and recurring malignant solid tumors. It arises from the GBM stem cell population. Conventional neurosurgical resection, temozolomide (TMZ)-dependent chemotherapy and radiotherapy have rendered the prognosis of patients unsatisfactory. Radiotherapy and chemotherapy can frequently induce non-specific damage to healthy brain and other tissues, which can be extremely hazardous. There is therefore a pressing need for a more effective treatment strategy for GBM to complement or replace existing treatment options. Cell-based and cell-free immunotherapies are currently being investigated to develop new treatment modalities against cancer. These treatments have the potential to be both selective and successful in minimizing off-target collateral harm in the normal brain. In this review, several aspects of cell-based and cell-free immunotherapies related to GBM will be discussed.

Rational selection of an ideal oncolytic virus to address current limitations in clinical translation

Oncolytic virus therapy (OVT) is a promising modality that leverages the propensity of natural or engineered viruses to selectively replicate in and kill cancer cells. Over the past decade, (pre)clinical studies have focused on the development and testing of adenovirus, herpes simplex virus, and vaccinia virus-based vectors. These studies have identified barriers to success confronting the field. Here, we propose a set of selection criteria or ideal properties of a successful oncolytic virus, which include lack of pathogenicity, low seroprevalence, selectivity (infection and replication), transgene carrying capacity, and genome stability. We use these requirements to analyze the oncolytic virus landscape, and then identify a potentially optimal species for platform development - vesicular stomatitis virus.

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.

A combination therapy of oncolytic viruses and chimeric antigen receptor T cells: a mathematical model proof-of-concept

Combining chimeric antigen receptor T (CAR-T) cells with oncolytic viruses (OVs) has recently emerged as a promising treatment approach in preclinical studies that aim to alleviate some of the barriers faced by CAR-T cell therapy. In this study, we address by means of mathematical modeling the main question of whether a single dose or multiple sequential doses of CAR-T cells during the OVs therapy can have a synergetic effect on tumor reduction. To that end, we propose an ordinary differential equations-based model with virus-induced synergism to investigate potential effects of different regimes that could result in efficacious combination therapy against tumor cell populations. Model simulations show that, while the treatment with a single dose of CAR-T cells is inadequate to eliminate all tumor cells, combining the same dose with a single dose of OVs can successfully eliminate the tumor in the absence of virus-induced synergism. However, in the presence of virus-induced synergism, the same combination therapy fails to eliminate the tumor. Furthermore, it is shown that if the intensity of virus-induced synergy and/or virus oncolytic potency is high, then the induced CAR-T cell response can inhibit virus oncolysis. Additionally, the simulations show a more robust synergistic effect on tumor cell reduction when OVs and CAR-T cells are administered simultaneously compared to the combination treatment where CAR-T cells are administered first or after OV injection. Our findings suggest that the combination therapy of CAR-T cells and OVs seems unlikely to be effective if the virus-induced synergistic effects are included when genetically engineering oncolytic viral vectors.

Genetic Modifications That Expand Oncolytic Virus Potency

Oncolytic viruses (OVs) are a promising type of cancer therapy since they selectively replicate in tumor cells without damaging healthy cells. Many oncolytic viruses have progressed to human clinical trials, however, their performance as monotherapy has not been as successful as expected. Importantly, recent literature suggests that the oncolytic potential of these viruses can be further increased by genetically modifying the viruses. In this review, we describe genetic modifications to OVs that improve their ability to kill tumor cells directly, to dismantle the tumor microenvironment, or to alter tumor cell signaling and enhance anti-tumor immunity. These advances are particularly important to increase virus spread and reduce metastasis, as demonstrated in animal models. Since metastasis is the principal cause of mortality in cancer patients, having OVs designed to target metastases could transform cancer therapy. The genetic alterations reported to date are only the beginning of all possible improvements to OVs. Modifications described here could be combined together, targeting multiple processes, or with other non-viral therapies with potential to provide a strong and lasting anti-tumor response in cancer patients.

Mechanisms of Cell Entry by dsRNA Viruses: Insights for Efficient Delivery of dsRNA and Tools for Improved RNAi-Based Pest Control

While RNAi is often heralded as a promising new strategy for insect pest control, a major obstacle that still remains is the efficient delivery of dsRNA molecules within the cells of the targeted insects. However, it seems overlooked that dsRNA viruses already have developed efficient strategies for transport of dsRNA molecules across tissue barriers and cellular membranes. Besides protecting their dsRNA genomes in a protective shell, dsRNA viruses also display outer capsid layers that incorporate sophisticated mechanisms to disrupt the plasma membrane layer and to translocate core particles (with linear dsRNA genome fragments) within the cytoplasm. Because of the perceived efficiency of the translocation mechanism, it is well worth analyzing in detail the molecular processes that are used to achieve this feat. In this review, the mechanism of cell entry by dsRNA viruses belonging to the Reoviridae family is discussed in detail. Because of the large amount of progress in mammalian versus insect models, the mechanism of infections of reoviruses in mammals (orthoreoviruses, rotaviruses, orbiviruses) will be treated as a point of reference against which infections of reoviruses in insects (orbiviruses in midges, plant viruses in hemipterans, insect-specific cypoviruses in lepidopterans) will be compared. The goal of this discussion is to uncover the basic principles by which dsRNA viruses cross tissue barriers and translocate their cargo to the cellular cytoplasm; such knowledge subsequently can be incorporated into the design of dsRNA virus-based viral-like particles for optimal delivery of RNAi triggers in targeted insect pests.

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.

Oncolytic Reovirus-Mediated Recruitment of Early Innate Immune Responses Reverses Immunotherapy Resistance in Prostate Tumors

Prostate cancers are considered "cold" tumors characterized by minimal T cell infiltrates, absence of a type I interferon (IFN) signature, and the presence of immunosuppressive cells. This non-inflamed phenotype is likely responsible for the lack of sensitivity of prostate cancer patients to immune checkpoint blockade (ICB) therapy. Oncolytic virus therapy can potentially overcome this resistance to immunotherapy in prostate cancers by transforming cold tumors into "hot," immune cell-infiltrated tumors. We investigated whether the combination of intratumoral oncolytic reovirus, followed by targeted blockade of Programmed cell death protein 1 (PD-1) checkpoint inhibition and/or the immunomodulatory CD73/Adenosine system can enhance anti-tumor immunity. Treatment of subcutaneous TRAMP-C2 prostate tumors with combined intratumoral reovirus and anti-PD-1 or anti-CD73 antibody significantly enhanced survival of mice compared with reovirus or either antibody therapy alone. Only combination therapy led to rejection of pre-established tumors and protection from tumor re-challenge. This therapeutic effect was dependent on CD4+ T cells and natural killer (NK) cells. NanoString immune profiling of tumors confirmed that reovirus increased tumor immune cell infiltration and revealed an upregulation of the immune-regulatory receptor, B- and T-lymphocyte attenuator (BTLA). This expression of BTLA on innate antigen-presenting cells (APCs) and its ligand, Herpesvirus entry mediator (HVEM), on T cells from reovirus-infected tumors was in keeping with a role for the HVEM-BTLA pathway in promoting the potent anti-tumor memory response observed.

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.

Past, Present and Future of Oncolytic Reovirus

Oncolytic virotherapy (OVT) has received significant attention in recent years, especially since the approval of talimogene Laherparepvec (T-VEC) in 2015 by the Food and Drug administration (FDA). Mechanistic studies of oncolytic viruses (OVs) have revealed that most, if not all, OVs induce direct oncolysis and stimulate innate and adaptive anti-tumour immunity. With the advancement of tumour modelling, allowing characterisation of the effects of tumour microenvironment (TME) components and identification of the cellular mechanisms required for cell death (both direct oncolysis and anti-tumour immune responses), it is clear that a "one size fits all" approach is not applicable to all OVs, or indeed the same OV across different tumour types and disease locations. This article will provide an unbiased review of oncolytic reovirus (clinically formulated as pelareorep), including the molecular and cellular requirements for reovirus oncolysis and anti-tumour immunity, reports of pre-clinical efficacy and its overall clinical trajectory. Moreover, as it is now abundantly clear that the true potential of all OVs, including reovirus, will only be reached upon the development of synergistic combination strategies, reovirus combination therapeutics will be discussed, including the limitations and challenges that remain to harness the full potential of this promising therapeutic agent.

Chorioallantoic Membrane Tumor Model for Evaluating Oncolytic Viruses

Oncolytic viruses are promising anticancer agents; however, regarding their clinical efficacy, there is still significant scope for improvement. Preclinical in vivo evaluation of oncolytic viruses is mainly based on syngeneic or xenograft tumor models in mice, which is labor-intensive and time-consuming. Currently, a large proportion of developmental work in the research field of oncolytic viruses is directed toward overcoming cellular and noncellular barriers to achieve improved virus delivery to primary tumors and metastases. To evaluate the large number of genetically or chemically modified viruses regarding tumor delivery and biodistribution patterns, it would be valuable to have an in vivo model available that would allow easy screening experiments, that is of higher complexity than monoclonal cell lines, and that could be used as a platform method before confirmatory studies in small and large animals. Based on our data, we believe that the chicken chorioallantoic membrane (CAM) assay is a quick and low-cost high-throughput tumor model system for the in vivo analysis of oncolytic viruses. Here we describe the establishment, careful characterization, and optimization of the CAM model as an in vivo model for the evaluation of oncolytic viruses. We have used human adenovirus type 5 (HAdV-5) as an example for validation but are confident that the model can be used as a test system for replicating viruses of many different virus families. We show that the CAM tumor model enables intratumoral and intravenous virus administration and is a feasible and conclusive model for the analysis of relevant virus-host interactions, biodistribution patterns, and tumor-targeting profiles.

Directed evolution as a tool for the selection of oncolytic RNA viruses with desired phenotypes

Viruses have some characteristics in common with cell-based life. They can evolve and adapt to environmental conditions. Directed evolution can be used by researchers to produce viral strains with desirable phenotypes. Through bioselection, improved strains of oncolytic viruses can be obtained that have better safety profiles, increased specificity for malignant cells, and more efficient spread among tumor cells. It is also possible to select strains capable of killing a broader spectrum of cancer cell variants, so as to achieve a higher frequency of therapeutic responses. This review describes and analyses virus adaptation studies performed with members of four RNA virus families that are used for viral oncolysis: reoviruses, paramyxoviruses, enteroviruses, and rhabdoviruses.

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.

CSV2018: The 2nd Symposium of the Canadian Society for Virology

The 2nd Symposium of the Canadian Society for Virology (CSV2018) was held in June 2018 in Halifax, Nova Scotia, Canada, as a featured event marking the 200th anniversary of Dalhousie University. CSV2018 attracted 175 attendees from across Canada and around the world, more than double the number that attended the first CSV symposium two years earlier. CSV2018 provided a forum to discuss a wide range of topics in virology including human, veterinary, plant, and microbial pathogens. Invited keynote speakers included David Kelvin (Dalhousie University and Shantou University Medical College) who provided a historical perspective on influenza on the 100th anniversary of the 1918 pandemic; Sylvain Moineau (Université Laval) who described CRISPR-Cas systems and anti-CRISPR proteins in warfare between bacteriophages and their host microbes; and Kate O’Brien (then from Johns Hopkins University, now relocated to the World Health Organization where she is Director of Immunization, Vaccines and Biologicals), who discussed the underlying viral etiology for pneumonia in the developing world, and the evidence for respiratory syncytial virus (RSV) as a primary cause. Reflecting a strong commitment of Canadian virologists to science communication, CSV2018 featured the launch of Halifax’s first annual Soapbox Science event to enable public engagement with female scientists, and the live-taping of the 499th episode of the This Week in Virology (TWIV) podcast, hosted by Vincent Racaniello (Columbia University) and science writer Alan Dove. TWIV featured interviews of CSV co-founders Nathalie Grandvaux (Université de Montréal) and Craig McCormick (Dalhousie University), who discussed the origins and objectives of the new society; Ryan Noyce (University of Alberta), who discussed technical and ethical considerations of synthetic virology; and Kate O’Brien, who discussed vaccines and global health. Finally, because CSV seeks to provide a better future for the next generation of Canadian virologists, the symposium featured a large number of oral and poster presentations from trainees and closed with the awarding of presentation prizes to trainees, followed by a tour of the Halifax Citadel National Historic Site and an evening of entertainment at the historic Alexander Keith’s Brewery.

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.

Synthesis and Translation of Viral mRNA in Reovirus-Infected Cells: Progress and Remaining Questions

At the end of my doctoral studies, in 1988, I published a review article on the major steps of transcription and translation during the mammalian reovirus multiplication cycle, a topic that still fascinates me 30 years later. It is in the nature of scientific research to generate further questioning as new knowledge emerges. Our understanding of these fascinating viruses thus remains incomplete but it seemed appropriate at this moment to look back and reflect on our progress and most important questions that still puzzle us. It is also essential of being careful about concepts that seem so well established, but could still be better validated using new approaches. I hope that the few reflections presented here will stimulate discussions and maybe attract new investigators into the field of reovirus research. Many other aspects of the viral multiplication cycle would merit our attention. However, I will essentially limit my discussion to these central aspects of the viral cycle that are transcription of viral genes and their phenotypic expression through the host cell translational machinery. The objective here is not to review every aspect but to put more emphasis on important progress and challenges in the field.

Going (Reo)Viral: Factors Promoting Successful Reoviral Oncolytic Infection

Oncolytic viruses show intriguing potential as cancer therapeutic agents. These viruses are capable of selectively targeting and killing cancerous cells while leaving healthy cells largely unaffected. The use of oncolytic viruses for cancer treatments in selected circumstances has recently been approved by the Food and Drug Administration (FDA) of the US and work is progressing on engineering viral vectors for enhanced selectivity, efficacy and safety. However, a better fundamental understanding of tumour and viral biology is essential for the continued advancement of the oncolytic field. This knowledge will not only help to engineer more potent and effective viruses but may also contribute to the identification of biomarkers that can determine which patients will benefit most from this treatment. A mechanistic understanding of the overlapping activity of viral and standard chemotherapeutics will enable the development of better combinational approaches to improve patient outcomes. In this review, we will examine each of the factors that contribute to productive viral infections in cancerous cells versus healthy cells. Special attention will be paid to reovirus as it is a well-studied virus and the only wild-type virus to have received orphan drug designation by the FDA. Although considerable insight into reoviral biology exists, there remain numerous deficiencies in our understanding of the factors regulating its successful oncolytic infection. Here we will discuss what is known to regulate infection as well as speculate about potential new mechanisms that may enhance successful replication. A joint appreciation of both tumour and viral biology will drive innovation for the next generation of reoviral mediated oncolytic therapy.

Current understanding of reovirus oncolysis mechanisms

Mammalian orthoreovirus (reovirus) is under development as a cancer virotherapy. Clinical trials demonstrate that reovirus-based therapies are safe and tolerated in patients with a wide variety of cancers. Although reovirus monotherapy has proven largely ineffective, reovirus sensitizes cancer cells to existing chemotherapeutic agents and radiation. Clinical trials are underway to test the efficacy of reovirus in combination with chemotherapeutic and radiation regimens and to evaluate the effectiveness of reovirus in conjunction with immunotherapies. Central to the use of reovirus to treat cancer is its capacity to directly kill cancer cells and alter the cellular environment to augment other therapies. Apoptotic cell death is a prominent mechanism of reovirus cancer cell killing. However, reoviruses can also kill cancer cells through nonapoptotic mechanisms. Here, we describe mechanisms of reovirus cancer cell killing, highlight how reovirus is used in combination with existing cancer treatments, and discuss what is known as to how reovirus modulates cancer immunotherapy.

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.

Serotype-Specific Killing of Large Cell Carcinoma Cells by Reovirus

Reovirus is under development as a therapeutic for numerous types of cancer. In contrast to other oncolytic viruses, the safety and efficacy of reovirus have not been improved through genetic manipulation. Here, we tested the oncolytic capacity of recombinant strains (rs) of prototype reovirus laboratory strains T1L and T3D (rsT1L and rsT3D, respectively) in a panel of non-small cell lung cancer (NSCLC) cell lines. We found that rsT1L was markedly more cytolytic than rsT3D in the large cell carcinoma cell lines tested, whereas killing of adenocarcinoma cell lines was comparable between rsT1L and rsT3D. Importantly, non-recombinant T1L and T3D phenocopied the kinetics and magnitude of cell death induced by recombinant strains. We identified gene segments L2, L3, and M1 as viral determinants of strain-specific differences cell killing of the large cell carcinoma cell lines. Together, these results indicate that recombinant reoviruses recapitulate the cell killing properties of non-recombinant, tissue culture-passaged strains. These studies provide a baseline for the use of reverse genetics with the specific objective of engineering more effective reovirus oncolytics. This work raises the possibility that type 1 reoviruses may have the capacity to serve as more effective oncolytics than type 3 reoviruses in some tumor types.

African Swine Fever Virus NP868R Capping Enzyme Promotes Reovirus Rescue during Reverse Genetics by Promoting Reovirus Protein Expression, Virion Assembly, and RNA Incorporation into Infectious Virions

Reoviruses, like many eukaryotic viruses, contain an inverted 7-methylguanosine (m7G) cap linked to the 5' nucleotide of mRNA. The traditional functions of capping are to promote mRNA stability, protein translation, and concealment from cellular proteins that recognize foreign RNA. To address the role of mRNA capping during reovirus replication, we assessed the benefits of adding the African swine fever virus NP868R capping enzyme during reovirus rescue. C3P3, a fusion protein containing T7 RNA polymerase and NP868R, was found to increase protein expression 5- to 10-fold compared to T7 RNA polymerase alone while enhancing reovirus rescue from the current reverse genetics system by 100-fold. Surprisingly, RNA stability was not increased by C3P3, suggesting a direct effect on protein translation. A time course analysis revealed that C3P3 increased protein synthesis within the first 2 days of a reverse genetics transfection. This analysis also revealed that C3P3 enhanced processing of outer capsid μ1 protein to μ1C, a previously described hallmark of reovirus assembly. Finally, to determine the rate of infectious-RNA incorporation into new virions, we developed a new recombinant reovirus S1 gene that expressed the fluorescent protein UnaG. Following transfection of cells with UnaG and infection with wild-type virus, passage of UnaG through progeny was significantly enhanced by C3P3. These data suggest that capping provides nontraditional functions to reovirus, such as promoting assembly and infectious-RNA incorporation.IMPORTANCE Our findings expand our understanding of how viruses utilize capping, suggesting that capping provides nontraditional functions to reovirus, such as promoting assembly and infectious-RNA incorporation, in addition to enhancing protein translation. Beyond providing mechanistic insight into reovirus replication, our findings also show that reovirus reverse genetics rescue is enhanced 100-fold by the NP868R capping enzyme. Since reovirus shows promise as a cancer therapy, efficient reovirus reverse genetics rescue will accelerate production of recombinant reoviruses as candidates to enhance therapeutic potency. NP868R-assisted reovirus rescue will also expedite production of recombinant reovirus for mechanistic insights into reovirus protein function and structure.

Novel High-throughput Approach for Purification of Infectious Virions

Viruses are extensively studied as pathogens and exploited as molecular tools and therapeutic agents. Existing methods to purify viruses such as gradient ultracentrifugation or chromatography have limitations, for example demand for technical expertise or specialized equipment, high time consumption, and restricted capacity. Our laboratory explores mutations in oncolytic reovirus that could improve oncolytic activity, and makes routine use of numerous virus variants, genome reassortants, and reverse engineered mutants. Our research pace was limited by the lack of high-throughput virus purification methods that efficiently remove confounding cellular contaminants such as cytokines and proteases. To overcome this shortcoming, we evaluated a commercially available resin (Capto Core 700) that captures molecules smaller than 700 kDa. Capto. Core 700 chromatography produced virion purity and infectivity indistinguishable from CsCl density gradient ultracentrifugation as determined by electron microscopy, gel electrophoresis analysis and plaque titration. Capto Core 700 resin was then effectively adapted to a rapid in-slurry pull-out approach for high-throughput purification of reovirus and adenovirus. The in-slurry purification approach offered substantially increased virus purity over crude cell lysates, media, or high-spin preparations and would be especially useful for high-throughput virus screening applications where density gradient ultracentrifugation is not feasible.

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.