rVSV(M Delta 51)-M3 is an effective and safe oncolytic virus for cancer therapy

Oncolytic Therapy of Solid Tumors by Modified Vesicular Stomatitis Virus

Vesicular stomatitis virus (VSV) is a promising oncolytic virus for treating solid tumors. We recently engineered a replicating VSV that specifically targets and destroys Her2/neu-expressing cancer cells. This virus was created by eliminating its natural binding site and adding a coding sequence for a single chain antibody to the Her2/neu receptor into its genome. Such an approach can be tailored to target various cellular surface molecules. This mini review will discuss genomic modifications of VSVs and their role in oncolytic therapy and discuss some challenges for moving VSVs to clinical applications.

Repurposing the oncolytic virus VSV∆51M as a COVID-19 vaccine

The coronavirus disease 2019 (COVID-19) pandemic imposes an urgent and continued need for the development of safe and cost-effective vaccines to induce preventive responses for limiting major outbreaks around the world. To combat severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), we repurposed the VSV∆51M oncolytic virus platform to express the spike receptor-binding domain (RBD) antigen. In this study, we report the development and characterization of the VSV∆51M-RBD vaccine. Our findings demonstrate successful expression of the RBD gene by the VSV∆51M-RBD virus, inducing anti-RBD responses without attenuating the virus. Moreover, the VSV∆51M-RBD vaccine exhibited safety, immunogenicity, and the potential to serve as a safe and effective alternative or complementary platform to current COVID-19 vaccines.

Recent advances in oncolytic virus therapy for hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a highly refractory cancer and the fourth leading cause of cancer-related mortality worldwide. Despite the development of a detailed treatment strategy for HCC, the survival rate remains unsatisfactory. Oncolytic virus has been extensively researched as a new cancer therapeutic agent in the treatment of HCC. Researchers have designed a variety of recombinant viruses based on natural oncolytic diseases, which can increase the targeting of oncolytic viruses to HCC and their survival in tumors, as well as kill tumor cells and inhibit the growth of HCC through a variety of mechanisms. The overall efficacy of oncolytic virus therapy is known to be influenced by anti-tumor immunity, toxic killing effect and inhibition of tumor angiogenesis, etc. Therefore, a comprehensive review of the multiple oncolytic mechanisms of oncolytic viruses in HCC has been conducted. So far, a large number of relevant clinical trials are under way or have been completed, and some encouraging results have been obtained. Studies have shown that oncolytic virus combined with other HCC therapies may be a feasible method, including local therapy, chemotherapy, molecular targeted therapy and immunotherapy. In addition, different delivery routes for oncolytic viruses have been studied so far. These studies make oncolytic virus a new and attractive drug for the treatment of HCC.

Oncolytic virus-based hepatocellular carcinoma treatment: Current status, intravenous delivery strategies, and emerging combination therapeutic solutions

Current treatments for advanced hepatocellular carcinoma (HCC) have limited success in improving patients' quality of life and prolonging life expectancy. The clinical need for more efficient and safe therapies has contributed to the exploration of emerging strategies. Recently, there has been increased interest in oncolytic viruses (OVs) as a therapeutic modality for HCC. OVs undergo selective replication in cancerous tissues and kill tumor cells. Strikingly, pexastimogene devacirepvec (Pexa-Vec) was granted an orphan drug status in HCC by the U.S. Food and Drug Administration (FDA) in 2013. Meanwhile, dozens of OVs are being tested in HCC-directed clinical and preclinical trials. In this review, the pathogenesis and current therapies of HCC are outlined. Next, we summarize multiple OVs as single therapeutic agents for the treatment of HCC, which have demonstrated certain efficacy and low toxicity. Emerging carrier cell-, bioengineered cell mimetic- or nonbiological vehicle-mediated OV intravenous delivery systems in HCC therapy are described. In addition, we highlight the combination treatments between oncolytic virotherapy and other modalities. Finally, the clinical challenges and prospects of OV-based biotherapy are discussed, with the aim of continuing to develop a fascinating approach in HCC patients.

SARS-CoV-2 RBD protein enhances the oncolytic activity of the vesicular stomatitis virus

Despite recent advances in the research on oncolytic viruses (OVs), a better understanding of how to enhance their replication is key to improving their therapeutic index. Understanding viral replication is important to improve treatment outcomes based on enhanced viral spreading within the tumor milieu. The VSV-Δ51 oncolytic virus has been widely used as an anticancer agent with a high selectivity profile. In this study, we examined the role of the SARS-CoV-2 spike protein receptor-binding domain (RBD) in enhancing VSV-Δ51 viral production and oncolytic activity. To test this hypothesis, we first generated a novel VSV-Δ51 mutant that encoded the SARS-COV-2 RBD and compared viral spreading and viral yield between VSV-Δ51-RBD and VSV-Δ51 in vitro. Using the viral plaque assay, we demonstrated that the presence of the SARS-CoV-2 RBD in the VSV-Δ51 genome is associated with a significantly larger viral plaque surface area and significantly higher virus titers. Subsequently, using an ATP release-based assay, we demonstrated that the SARS-CoV-2 RBD could enhance VSV-Δ51 oncolytic activity in vitro. This observation was further supported using the B16F10 tumor model. These findings highlighted a novel use of the SARS-CoV-2 RBD as an anticancer agent.

Immunovirotherapy Based on Recombinant Vesicular Stomatitis Virus: Where Are We?

Vesicular stomatitis virus (VSV), a negative-strand RNA virus of the Vesiculovirus genus, has demonstrated encouraging anti-neoplastic activity across multiple human cancer types. VSV is particularly attractive as an oncolytic agent because of its broad tropism, fast replication kinetics, and amenability to genetic manipulations. Furthermore, VSV-induced oncolysis can elicit a potent antitumor cytotoxic T-cell response to viral proteins and tumor-associated antigens, resulting in a long-lasting antitumor effect. Because of this multifaceted immunomodulatory property, VSV was investigated extensively as an immunovirotherapy alone or combined with other anticancer modalities, such as immune checkpoint blockade. Despite these recent opportunities to delineate synergistic and additive antitumor effects with existing anticancer therapies, FDA approval for the use of oncolytic VSV in humans has not yet been granted. This mini-review discusses factors that have prompted the use of VSV as an immunovirotherapy in human cancers and provides insights into future perspectives and research areas to improve VSV-based oncotherapy.

Cutting both ways: the innate immune response to oncolytic virotherapy

Oncolytic viruses (OVs), above and beyond infecting and lysing malignant cells, interact with the immune system in complex ways that have important therapeutic significance. While investigation into these interactions is still in its early stages, important insights have been made over the past two decades that will help improve the clinical efficacy of OV-based management strategies in cancer care moving forward. The inherent immunosuppression that defines the tumor microenvironment can be modified by OV infection, and the subsequent recruitment and activation of innate immune cells, in particular, is central to this. Indeed, neutrophils, macrophages, natural killer cells, and dendritic cells, as well as other populations such as myeloid-derived suppressor cells, are key to the immune escape that allows tumors to survive, but their natural response to infection can be exploited by virotherapy. While stimulation of innate immune cells by OVs can initiate antitumor responses, related antiviral activity can limit virus spread and direct cytopathogenic effects. In this review, we highlight how each innate immune cell population influences this balance of antitumor and antiviral forces during virotherapy, some of the important molecular pathways that have been identified, and specific therapeutic targets that have emerged through this work. We discuss the importance of OV-based combination therapies in optimizing antiviral and antitumor innate immune responses stimulated by virotherapy toward tumor eradication, and how these processes vary depending on the tumor and OV in question. Rather than concentrating on a particular OV species in the review, we present the range of effects that have been documented across OV types to emphasize the context-specific nature of these interactions and how this is important in the design of future OV-based treatment approaches.

Antifibrotic Therapy Augments the Antitumor Effects of Vesicular Stomatitis Virus Via Reprogramming Tumor Microenvironment

Solid tumors are characterized by abundant extracellular matrix originating from cancer-associated fibroblasts (CAFs). High collagen content can trigger the collapse of vascular system in the tumor and form physical barrier that eventually impedes the penetration of drug particles and cytotoxic immune cells. Moreover, CAFs is able to promote the enrichment of tumor-associated macrophages (TAMs) and differentiation of myeloid-derived suppressor cells (MDSCs) that work in concert to develop a highly immunosuppressive tumor microenvironment (TME). In this study, we investigated if halofuginone, an antifibrotic drug, can augment the therapeutic effects of oncolytic vesicular stomatitis virus (VSV). The results revealed that halofuginone significantly disrupts the collagen network in tumors and promotes the distribution of VSV and infiltration of CD8⁺ T cells (p < 0.0001). Combined treatment of VSV and halofuginone also modulates the immunosuppressive TME via deletion of TAM, MDSCs, and regulatory T cells (Tregs). Collectively, the combination therapy remarkably inhibits the tumor growth in multiple murine models and prolongs survival of mice. The results demonstrate the clinical potential of halofuginone in combination with oncolytic virus.

Defective Interfering Genomes and the Full-Length Viral Genome Trigger RIG-I After Infection With Vesicular Stomatitis Virus in a Replication Dependent Manner

Replication competent vesicular stomatitis virus (VSV) is the basis of a vaccine against Ebola and VSV strains are developed as oncolytic viruses. Both functions depend on the ability of VSV to induce adequate amounts of interferon-α/β. It is therefore important to understand how VSV triggers interferon responses. VSV activates innate immunity via retinoic acid-inducible gene I (RIG-I), a sensor for viral RNA. Our results show that VSV needs to replicate for a robust interferon response. Analysis of RIG-I-associated RNA identified a copy-back defective-interfering (DI) genome and full-length viral genomes as main trigger of RIG-I. VSV stocks depleted of DI genomes lost most of their interferon-stimulating activity. The remaining full-length genome and leader-N-read-through sequences, however, still triggered RIG-I. Awareness for DI genomes as trigger of innate immune responses will help to standardize DI genome content and to purposefully deplete or use DI genomes as natural adjuvants in VSV-based therapeutics.

Oncolytic Viruses and Immune Checkpoint Inhibitors: Preclinical Developments to Clinical Trials

Immuno-oncology (IO) has been an active area of oncology research. Following US FDA approval of the first immune checkpoint inhibitor (ICI), ipilimumab (human IgG1 k anti-CTLA-4 monoclonal antibody), in 2011, and of the first oncolytic virus, Imlygic (talimogene laherparepvec), in 2015, there has been renewed interest in IO. In the past decade, ICIs have changed the treatment paradigm for many cancers by enabling better therapeutic control, resuming immune surveillance, suppressing tumor immunosuppression, and restoring antitumor immune function. However, ICI therapies are effective only in a small subset of patients and show limited therapeutic potential due to their inability to demonstrate efficacy in 'cold' or unresponsive tumor microenvironments (TMEs). Relatedly, oncolytic viruses (OVs) have been shown to induce antitumor immune responses, augment the efficacy of existing cancer treatments, and reform unresponsive TME to turn 'cold' tumors 'hot,' increasing their susceptibility to checkpoint blockade immunotherapies. For this reason, OVs serve as ideal complements to ICIs, and multiple preclinical studies and clinical trials are demonstrating their combined therapeutic efficacy. This review will discuss the merits and limitations of OVs and ICIs as monotherapy then progress onto the preclinical rationale and the results of clinical trials of key combination therapies.

Characterization of the Impact of Oncolytic Vesicular Stomatitis Virus on the Trafficking, Phenotype, and Antigen Presentation Potential of Neutrophils and Their Ability to Acquire a Non-Structural Viral Protein

Neutrophils are innate leukocytes that mount a rapid response to invading pathogens and sites of inflammation. Although neutrophils were traditionally considered responders to bacterial infections, recent advances have demonstrated that they are interconnected with both viral infections and cancers. One promising treatment strategy for cancers is to administer an oncolytic virus to activate the immune system and directly lyse cancerous cells. A detailed characterization of how the innate immune system responds to a viral-based therapy is paramount in identifying its systemic effects. This study analyzed how administering the rhabdovirus vesicular stomatitis virus (VSV) intravenously at 1 × 10⁹ PFU acutely influenced neutrophil populations. Bone marrow, blood, lungs, and spleen were acquired three- and 24-h after administration of VSV for analysis of neutrophils by flow cytometry. Infection with VSV caused neutrophils to rapidly egress from the bone marrow and accumulate in the lungs. A dramatic increase in immature neutrophils was observed in the lungs, as was an increase in the antigen presentation potential of these cells within the spleen. Furthermore, the potential for neutrophils to acquire viral transgene-encoded proteins was monitored using a variant of VSV that expressed enhanced green fluorescent protein (GFP). If an in vitro population of splenocytes were exposed to αCD3 and αCD28, a substantial proportion of the neutrophils would become GFP-positive. This suggested that the neutrophils could either acquire more virus-encoded antigens from infected splenocytes or were being directly infected. Five different dosing regimens were tested in mice, and it was determined that a single dose of VSV or two doses of VSV administered at a 24-h interval, resulted in a substantial proportion of neutrophils in the bone marrow becoming GFP-positive. This correlated with a decrease in the number of splenic neutrophils. Two doses administered at intervals longer than 24-h did not have these effects, suggesting that neutrophils became resistant to antigen uptake or direct infection with VSV beyond 24-h of activation. These findings implicated neutrophils as major contributors to oncolytic rhabdoviral therapies. They also provide several clear future directions for research and suggest that neutrophils should be carefully monitored during the development of all oncolytic virus-based treatment regimens.

Oncolytic Virus with Attributes of Vesicular Stomatitis Virus and Measles Virus in Hepatobiliary and Pancreatic Cancers

Recombinant vesicular stomatitis virus (VSV)-fusion and hemagglutinin (FH) was developed by substituting the promiscuous VSV-G glycoprotein (G) gene in the backbone of VSV with genes encoding for the measles virus envelope proteins F and H. Hybrid VSV-FH exhibited a multifaceted mechanism of cancer-cell killing and improved neurotolerability over parental VSV in preclinical studies. In this study, we evaluated VSV-FH in vitro and in vivo in models of hepatobiliary and pancreatic cancers. Our results indicate that high intrahepatic doses of VSV-FH did not result in any significant toxicity and were well tolerated by transgenic mice expressing the measles virus receptor CD46. Furthermore, a single intratumoral treatment with VSV-FH yielded improved survival and complete tumor regressions in a proportion of mice in the Hep3B hepatocellular carcinoma model but not in mice xenografted with BxPC-3 pancreatic cancer cells. Our preliminary findings indicate that VSV-FH can induce potent oncolysis in hepatocellular and pancreatic cancer cell lines with concordant results in vivo in hepatocellular cancer and discordant in pancreatic cancer without the VSV-mediated toxic effects previously observed in laboratory animals. Further study of VSV-FH as an oncolytic virotherapy is warranted in hepatocellular carcinoma and pancreatic cancer to understand broader applicability and mechanisms of sensitivity and resistance.

Modeling oncolytic virus dynamics in the tumor microenvironment using zebrafish

We have adapted a zebrafish (Danio rerio) tumor xenograft model for use in the study of oncolytic virotherapy. Following implantation of mammalian cancer cells into the perivitelline space of developing zebrafish embryos, both local and intravenous oncolytic virus treatments produce a tumor-specific infection with measurable antitumor effects. Tumor cells are injected at 48 h post fertilization, with oncolytic virus treatment then being administered 24 h later to allow for an initial period of tumor development and angiogenesis. Confocal fluorescent imaging is used to quantify dynamics within the tumor environment. The natural translucency of zebrafish at the embryo stage, coupled with the availability of strains with fluorescent immune and endothelial cell reporter lines, gives the model broad potential to allow for real time, in vivo investigation of important events within tumors throughout the course of virotherapy. Zebrafish xenografts offer a system with biologic fidelity to processes in human cancer development that influence oncolytic virus efficacy, and to our knowledge this is the first demonstration of the model's use in the context of virotherapy. Compared with other models, our protocol offers a powerful, inexpensive approach to evaluating novel oncolytic viruses and oncolytic virus-based combination therapies, with potential application to investigating the impacts of virotherapy on immune response, tumor vasculature, and metastatic disease.

Oncolytic virotherapy in hepato-bilio-pancreatic cancer: The key to breaking the log jam?

Traditional therapies have limited efficacy in hepatocellular carcinoma, pancreatic cancer, and biliary tract cancer, especially for advanced and refractory cancers. Through a deeper understanding of antitumor immunity and the tumor microenvironment, novel immunotherapies are becoming available for cancer treatment. Oncolytic virus (OV) therapy is an emerging type of immunotherapy that has demonstrated effective antitumor efficacy in many preclinical studies and clinical studies. Thus, it may represent a potential feasible treatment for hard to treat gastrointestinal (GI) tumors. Here, we summarize the research progress of OV therapy for the treatment of hepato-bilio-pancreatic cancers. In general, most OV therapies exhibits potent, specific oncolysis both in cell lines in vitro and the animal models in vivo. Currently, several clinical trials have suggested that OV therapy may also be effective in patients with refractory hepato-bilio-pancreatic cancer. Multiple strategies such as introducing immunostimulatory genes, modifying virus capsid and combining various other therapeutic modalities have been shown enhanced specific oncolysis and synergistic anti-cancer immune stimulation. Combining OV with other antitumor therapies may become a more effective strategy than using virus alone. Nevertheless, more studies are needed to better understand the mechanisms underlying the therapeutic effects of OV, and to design appropriate dosing and combination strategies.

Residue Mutations in Murine Herpesvirus 68 Immunomodulatory Protein M3 Reveal Specific Modulation of Chemokine Binding

The M3 protein (M3) encoded by murine gammaherpesvirus 68 (MHV-68) is a unique viral immunomodulator with a high-affinity for a broad spectrum of chemokines, key mediators responsible for the migration of immune cells to sites of inflammation. M3 is currently being studied as a very attractive and desirable tool for blocking the chemokine signaling involved in some inflammatory diseases and cancers. In this study, we elucidated the role of M3 residues E70 and T272 in binding to chemokines by examining the effects of the E70A and T272G mutations on the ability of recombinant M3, prepared in Escherichia coli cells, to bind the human chemokines CCL5 and CXCL8. We found that the E70A mutation enhanced binding of M3 to CCL5 two-fold but had little effect on its binding to CXCL8. In contrast, the T272G mutation was found to be important for the thermal stability of M3 and significantly decreased M3's binding to both CCL5 (by about 4×) and CXCL8 (by about 5×). We also constructed in silico models of the wild-type M3–CCL5 and M3–CCL8 complexes and found substantial differences in their physical and chemical properties. M3 models with single mutation E70A and T272G suggested the role of E70 and T272 in binding M3 protein to chemokines. In sum, we have confirmed that site-directed mutagenesis could be an effective tool for modulating the blockade of particular chemokines by M3, as desired in therapeutic treatments for severe inflammatory illnesses arising from chemokine network dysregulation.

R7A mutation in N protein renders temperature sensitive phenotype of VSV by affecting its replication and transcription in vitro

Viral genomic RNA encapsidated by nucleoprotein (N) forms functional template for the transcription and replication of vesicular stomatitis virus (VSV). The crystal structure of the N-RNA complex shows that RNA is tightly sequestered between the two lobes of the N protein. The residue (R7) in N-terminal arm of N is of great importance to the formation of functional N-RNA template. In our study, we found that single amino acid substitution (R7A) resulted in the loss of CAT expression in vitro minigenome system at 37 °C. But the R7A had little effect on CAT expression at 31 °C. Further analysis showed that R7A had great effects on the RNA synthesis and the formation of cytoplasmic inclusions of VSV only at 37 °C not at 31 °C. For the further investigation of the effect of R7A on virus replication, we checked the dominant-negative effect of N_R7A in minigenome system and the single step curve of recombinant virus with R7A mutation in N protein (rVSV_R7A) under 37 °C and 31 °C separately. Our results showed that the mutation of R7A within the N-terminal arm of N affected both replication and transcription and induced VSV to become temperature sensitive.

Oncolytic virotherapy enhances the efficacy of a cancer vaccine by modulating the tumor microenvironment

The efficacy of cancer vaccines has been limited by the immunosuppressive tumor microenvironment, which can be alleviated by immune checkpoint inhibitor (ICI) therapy. Here, we tested if oncolytic viruses (OVs), similar to ICI, can also synergize with cancer vaccines by modulating the tumor microenvironment. VSV‐GP, a chimeric vesicular stomatitis virus (VSV) pseudotyped with the glycoprotein (GP) of the lymphocytic choriomeningitis virus, is a promising new OV candidate. Here, we show that in mouse B16‐OVA melanoma, combination treatment of VSV‐GP with an ovalbumin (OVA) peptide‐loaded dendritic cell (DC) vaccine (DCVacc) significantly enhanced survival over the single agent therapies, although both DCVacc and DCVacc/VSV‐GP treatments induced comparable levels of OVA‐specific CD8 T cell responses. Virus replication was minimal so that direct viral oncolysis in B16‐OVA did not contribute to this synergism. The strong therapeutic effect of the DCVacc/VSV‐GP combination treatment was associated with high numbers of tumor‐infiltrating, highly activated T cells and the relative reduction of regulatory T cells in treated and contra‐lateral nontreated tumors. Accordingly, depletion of CD8 T cells but not natural killer cells abrogated the therapeutic effect of DCVacc/VSV‐GP supporting the crucial role of CD8 T cells. In addition, a drastic increase in several proinflammatory cytokines was observed in VSV‐GP‐treated tumors. Taken together, OVs, similar to ICI, have the potential to markedly increase the efficacy of cancer vaccines by alleviating local immune suppression in the tumor microenvironment.

What's new?

Cancer vaccine efficacy has been limited by the immunosuppressive tumor microenvironment. By inducing cancer cell death with the release of tumor‐related antigens, oncolytic viruses may have an adjuvant effect. Here, the authors show that a combination of the oncolytic rhabdovirus VSV‐GP and a dendritic cell vaccine is highly effective in the treatment of mouse melanoma, most likely because VSV‐GP reprograms the tumor microenvironment to enhance the effectivity of the vaccine‐induced immune response. Oncolytic viruses have the potential to dramatically increase the efficacy of cancer vaccines by alleviating local immune suppression in the tumor microenvironment.

Exploring the role of oncolytic viruses in hepatobiliary cancers

The standard of care for early hepatobiliary cancers (HBC) includes surgical resection. Liver transplantations or locoregional therapies are beneficial in early hepatocellular carcinoma (HCC) under certain circumstances. Systemic treatments have some benefit in advanced HBC, though long-term prognosis remains poor. We evaluated the role of oncolytic viruses in the treatment of HBCs through a systematic literature review. The recombinant vaccinia virus JX-594 improved median survival in patients with local/metastatic HCC more strongly at high dose than at low dose (14.1 vs 6.7 months; p = 0.08) in a Phase II study. A Phase III study with JX-594 and sorafenib in advanced HCC is ongoing. No survival benefit in HCC was seen with two other recombinant adenoviruses (Ad-TK and DL1520). Several preclinical trials using oncolytic viruses in HBC showed promising results, warranting clinical studies.

Oncolytic virotherapy in upper gastrointestinal tract cancers

Upper gastrointestinal tract malignancies are among the most challenging cancers with regard to response to treatment and prognosis. Cancers of the esophagus, stomach, pancreas, liver, and biliary tree have dismal 5-year survival, and very modest improvements in this rate have been made in recent times. Oncolytic viruses are being developed to address these malignancies, with a focus on high safety profiles and low off-target toxicities. Each viral platform has evolved to enhance oncolytic potency and the clinical response to either single-agent viral therapy or combined viral treatment with radiotherapy and chemotherapy. A panel of genomic alterations, chimeric proteins, and pseudotyped capsids are the breakthroughs for vector success. This article revisits developments for each viral platform to each tumor type, in an attempt to achieve maximum tumor selectivity. From the bench to clinical trials, the scope of this review is to highlight the beginnings of translational oncolytic virotherapy research in upper gastrointestinal tract malignancies and provide a bioengineering perspective of the most promising platforms.

Oncotargeting by Vesicular Stomatitis Virus (VSV): Advances in Cancer Therapy

Modern oncotherapy approaches are based on inducing controlled apoptosis in tumor cells. Although a number of apoptosis-induction approaches are available, site-specific delivery of therapeutic agents still remain the biggest hurdle in achieving the desired cancer treatment benefit. Additionally, systemic treatment-induced toxicity remains a major limiting factor in chemotherapy. To specifically address drug-accessibility and chemotherapy side effects, oncolytic virotherapy (OV) has emerged as a novel cancer treatment alternative. In OV, recombinant viruses with higher replication capacity and stronger lytic properties are being considered for tumor cell-targeting and subsequent cell lysing. Successful application of OVs lies in achieving strict tumor-specific tropism called oncotropism, which is contingent upon the biophysical interactions of tumor cell surface receptors with viral receptors and subsequent replication of oncolytic viruses in cancer cells. In this direction, few viral vector platforms have been developed and some of these have entered pre-clinical/clinical trials. Among these, the Vesicular stomatitis virus (VSV)-based platform shows high promise, as it is not pathogenic to humans. Further, modern molecular biology techniques such as reverse genetics tools have favorably advanced this field by creating efficient recombinant VSVs for OV; some have entered into clinical trials. In this review, we discuss the current status of VSV based oncotherapy, challenges, and future perspectives regarding its therapeutic applications in the cancer treatment.

Review: Oncolytic virotherapy, updates and future directions

Oncolytic viruses (OVs) are viral strains that can infect and kill malignant cells while spare their normal counterparts. OVs can access cells through binding to receptors on their surface or through fusion with the plasma membrane and establish a lytic cycle in tumors, while leaving normal tissue essentially unharmed. Multiple viruses have been investigated in humans for the past century. IMLYGIC™ (T-VEC/Talimogene Laherparepvec), a genetically engineered Herpes Simplex Virus, is the first OV approved for use in the United States and the European Union for patients with locally advanced or non-resectable melanoma.

Although OVs have a favorable toxicity profile and are impressively active anticancer agents in vitro and in vivo the majority of OVs have limited clinical efficacy as a single agent. While a virus-induced antitumor immune response can enhance oncolysis, when OVs are used systemically, the antiviral immune response can prevent the virus reaching the tumor tissue and having a therapeutic effect. Intratumoral administration can provide direct access to tumor tissue and be beneficial in reducing side effects.

Immune checkpoint stimulation in tumor tissue has been noted after OV therapy and can be a natural response to viral-induced oncolysis. Also for immune checkpoint inhibition to be effective in treating cancer, an immune response to tumor neoantigens and an inflamed tumor microenvironment are required, both of which treatment with an OV may provide. Therefore, direct and indirect mechanisms of tumor killing provide rationale for clinical trials investigating the combination of OVs other forms of cancer therapy, including immune checkpoint inhibition.

Oncolytic Virus-Based Immunotherapies for Hepatocellular Carcinoma

Hepatocellular carcinoma is highly refractory cancer which is resistant to conventional chemotherapy and radiotherapy, carrying a dismal prognosis. Although many anticancer drugs have been developed for treating HCC, sorafenib is the only effective treatment, but it only prolongs survival duration for about 3 months. Recently, oncolytic virotherapy has shown promising results in treating HCCs and the effects can be more enhanced by adopting immune modulatory molecules. This review discusses the current status of treating HCC and the effective strategy of oncolytic virus-based immunotherapy for the treatment of HCCs.

Oncolytic Vesicular Stomatitis Virus as a Viro-Immunotherapy: Defeating Cancer with a "Hammer" and "Anvil"

Oncolytic viruses have gained much attention in recent years, due, not only to their ability to selectively replicate in and lyse tumor cells, but to their potential to stimulate antitumor immune responses directed against the tumor. Vesicular stomatitis virus (VSV), a negative-strand RNA virus, is under intense development as an oncolytic virus due to a variety of favorable properties, including its rapid replication kinetics, inherent tumor specificity, and its potential to elicit a broad range of immunomodulatory responses to break immune tolerance in the tumor microenvironment. Based on this powerful platform, a multitude of strategies have been applied to further improve the immune-stimulating potential of VSV and synergize these responses with the direct oncolytic effect. These strategies include: 1. modification of endogenous virus genes to stimulate interferon induction; 2. virus-mediated expression of cytokines or immune-stimulatory molecules to enhance anti-tumor immune responses; 3. vaccination approaches to stimulate adaptive immune responses against a tumor antigen; 4. combination with adoptive immune cell therapy for potentially synergistic therapeutic responses. A summary of these approaches will be presented in this review.

Murine Tumor Models for Oncolytic Rhabdo-Virotherapy

The preclinical optimization and validation of novel treatments for cancer therapy requires the use of laboratory animals. Although in vitro experiments using tumor cell lines and ex vivo treatment of patient tumor samples provide a remarkable first-line tool for the initial study of tumoricidal potential, tumor-bearing animals remain the primary option to study delivery, efficacy, and safety of therapies in the context of a complete tumor microenvironment and functional immune system. In this review, we will describe the use of murine tumor models for oncolytic virotherapy using vesicular stomatitis virus. We will discuss studies using immunocompetent and immunodeficient models with respect to toxicity and therapeutic treatments, as well as the various techniques and tools available to study cancer therapy with Rhabdoviruses.

Oncolytic viruses against cancer stem cells: A promising approach for gastrointestinal cancer

Gastrointestinal cancer has been one of the five most commonly diagnosed and leading causes of cancer mortality over the past few decades. Great progress in traditional therapies has been made, which prolonged survival in patients with early cancer, yet tumor relapse and drug resistance still occurred, which is explained by the cancer stem cell (CSC) theory. Oncolytic virotherapy has attracted increasing interest in cancer because of its ability to infect and lyse CSCs. This paper reviews the basic knowledge, CSC markers and therapeutics of gastrointestinal cancer (liver, gastric, colon and pancreatic cancer), as well as research advances and possible molecular mechanisms of various oncolytic viruses against gastrointestinal CSCs. This paper also summarizes the existing obstacles to oncolytic virotherapy and proposes several alternative suggestions to overcome the therapeutic limitations.

Myxoma virus therapy for human embryonal rhabdomyosarcoma in a nude mouse model

Rhabdomyosarcoma (RMS) is a devastating tumor of young people that is difficult to cure. To determine if oncolytic virus therapy can improve outcomes in individuals with RMS, myxoma virus expressing a red fluorescent protein (MYXV-red) was evaluated for antitumoral effects using a murine model of RMS. Fluorescent protein was expressed in four RMS cell lines inoculated with MYXV-red, indicating that these cells were semipermissive to MYXV infection. MYXV-red replication and cytopathic effects were further evaluated using human embryonal RMS (CCL-136) cells. Logarithmic growth of MYXV-red and significant cell death were observed 72 hours after inoculation with MYXV. The oncolytic effects of MYXV-red were then studied in nude mice that were injected subcutaneously with CCL-136 cells to establish RMS xenografts. Once tumors measured 5 mm in diameter, mice were treated with multiple intratumoral injections of MXYV-red or saline. The average final tumor volume and rate of tumor growth were significantly decreased, and median survival time was significantly increased in MYXV-red-treated mice (P-values =0.0416, 0.0037, and 0.0004, respectively). Histologic sections of MYXV-red-treated tumors showed increased inflammation compared to saline-treated tumors (P-value =0.0002). In conclusion, MXYV-red treatment of RMS tumors was successful in individual mice as it resulted in decreased tumor burden in eight of eleven mice with nearly complete tumor remission in five of eleven mice. These data hold promise that MYXV-red treatment may be beneficial for people suffering from RMS. To our knowledge, this is the first report of successful treatment of RMS tumors using an oncolytic poxvirus.

Vesicular stomatitis virus-based vaccines expressing EV71 virus-like particles elicit strong immune responses and protect newborn mice from lethal challenges

Enterovirus 71 (EV71) belonging to the Picornaviridae family is considered the most frequently detected causative agent in hand-foot-and-mouth disease (HFMD) and is a serious threat to public health in the Asia-Pacific region. There are currently no approved vaccines or effective drugs for EV71. In this study, using recombinant vesicular stomatitis virus (rVSV) expressing viral VP1 protein (mVP1) of EV71 as a control, we generated two types of rVSVs that can form EV71 virus-like particles (VLPs). First, we co-infected two rVSVs singly expressing P1 (mP1) and 3CD (m3CD) of EV71. Second, we inserted P1 and 3CD into one VSV backbone to generate an rVSV expressing P1 and 3CD together (mP1-3CD). When P1 and 3CD were expressed in the cells either co-infected with mP1 and m3CD (mP1/m3CD) or infected with mP1-3CD, P1 was cleaved by 3CD and produced VP1, VP3, and VP0 to form VLPs. Furthermore, mice immunized with mP1/m3CD or mP1-3CD showed higher humoral and cellular immunity responses than mice immunized with mVP1. Finally, the rVSVs expressing the EV71 proteins were evaluated in mice to determine their potential to protect against a lethal EV71 virus challenge, and among all the rVSVs, the mP1-3CD was shown to be the most promising vaccine candidate for EV71 protection.

Constrained evolvability of interferon suppression in an RNA virus

Innate immunity responses controlled by interferon (IFN) are believed to constitute a major selective pressure shaping viral evolution. Viruses encode a variety of IFN suppressors, but these are often multifunctional proteins that also play essential roles in other steps of the viral infection cycle, possibly limiting their evolvability. Here, we experimentally evolved a vesicular stomatitis virus (VSV) mutant carrying a defect in the matrix protein (M∆51) that abolishes IFN suppression and that has been previously used in the context of oncolytic virotherapy. Serial transfers of this virus in normal, IFN-secreting cells led to a modest recovery of IFN blocking capacity and to weak increases in viral fitness. Full-genome ultra-deep sequencing and phenotypic analysis of population variants revealed that the anti-IFN function of the matrix protein was not restored, and that the Mdelta51 defect was instead compensated by changes in the viral phosphoprotein. We also show that adaptation to IFN-secreting cells can be driven by the selection of fast-growing viruses with no IFN suppression capacity, and that these population variants can be trans-complemented by other, IFN-suppressing variants. Our results thus suggest that virus-virus interactions and alternative strategies of innate immunity evasion can determine the evolution of IFN suppression in a virus.

N-Myc expression enhances the oncolytic effects of vesicular stomatitis virus in human neuroblastoma cells

N-myc oncogene amplification is associated but not present in all cases of high-risk neuroblastoma (NB). Since oncogene expression could often modulate sensitivity to oncolytic viruses, we wanted to examine if N-myc expression status would determine virotherapy efficacy to high-risk NB. We showed that induction of exogenous N-myc in a non-N-myc-amplified cell line background (TET-21N) increased susceptibility to oncolytic vesicular stomatitis virus (mutant VSVΔM51) and alleviated the type I IFN-induced antiviral state. Cells with basal N-myc, on the other hand, were less susceptible to virus-induced oncolysis and established a robust IFN-mediated antiviral state. The same effects were also observed in NB cell lines with and without N-myc amplification. Microarray analysis showed that N-myc overexpression in TET-21N cells downregulated IFN-stimulated genes (ISGs) with known antiviral functions. Furthermore, virus infection caused significant changes in global gene expression in TET-21N cells overexpressing N-myc. Such changes involved ISGs with various functions. Therefore, the present study showed that augmented susceptibility to VSVΔM51 by N-myc at least involves downregulation of ISGs with antiviral functions and alleviation of the IFN-stimulated antiviral state. Our studies suggest the potential utility of N-myc amplification/overexpression as a predictive biomarker of virotherapy response for high-risk NB using IFN-sensitive oncolytic viruses.

STAT3 inhibition reduces toxicity of oncolytic VSV and provides a potentially synergistic combination therapy for hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a refractory malignancy with a high mortality and increasing worldwide incidence rates, including the United States and central Europe. In this study, we demonstrate that a specific inhibitor of signal transducer and activator of transcription 3 (STAT3), NSC74859, efficiently reduces HCC cell proliferation and can be successfully combined with oncolytic virotherapy using vesicular stomatitis virus (VSV). The potential benefits of this combination treatment are strengthened by the ability of NSC74859 to protect primary hepatocytes and nervous system cells against virus-induced cytotoxicity, with an elevation of the VSV maximum tolerated dose in mice. Hereby we propose a strategy for improving the current regimen for HCC treatment and seek to further explore the molecular mechanisms underlying selective oncolytic specificity of VSV.

Experimental evolution of an oncolytic vesicular stomatitis virus with increased selectivity for p53-deficient cells

Experimental evolution has been used for various biotechnological applications including protein and microbial cell engineering, but less commonly in the field of oncolytic virotherapy. Here, we sought to adapt a rapidly evolving RNA virus to cells deficient for the tumor suppressor gene p53, a hallmark of cancer cells. To achieve this goal, we established four independent evolution lines of the vesicular stomatitis virus (VSV) in p53-knockout mouse embryonic fibroblasts (p53-/- MEFs) under conditions favoring the action of natural selection. We found that some evolved viruses showed increased fitness and cytotoxicity in p53-/- cells but not in isogenic p53+/+ cells, indicating gene-specific adaptation. However, full-length sequencing revealed no obvious or previously described genetic changes associated with oncolytic activity. Half-maximal effective dose (EC50) assays in mouse p53-positive colon cancer (CT26) and p53-deficient breast cancer (4T1) cells indicated that the evolved viruses were more effective against 4T1 cells than the parental virus or a reference oncolytic VSV (MΔ51), but showed no increased efficacy against CT26 cells. In vivo assays using 4T1 syngeneic tumor models showed that one of the evolved lines significantly delayed tumor growth compared to mice treated with the parental virus or untreated controls, and was able to induce transient tumor suppression. Our results show that RNA viruses can be specifically adapted typical cancer features such as p53 inactivation, and illustrate the usefulness of experimental evolution for oncolytic virotherapy.

Type III interferon attenuates a vesicular stomatitis virus-based vaccine vector

Vesicular stomatitis virus (VSV) has been extensively studied as a vaccine vector and oncolytic agent. Nevertheless, safety concerns have limited its widespread use in humans. The type III lambda interferon (IFN-λ) family of cytokines shares common signaling pathways with the IFN-α/β family and thus evokes similar antiviral activities. However, IFN-λ signals through a distinct receptor complex that is expressed in a cell type-specific manner, which restricts its activity to epithelial barriers, particularly those corresponding to the respiratory and gastrointestinal tracts. In this study, we determined how IFN-λ expression from recombinant VSV would influence vector replication, spread, and immunogenicity. We demonstrate that IFN-λ expression severely attenuates VSV in cell culture. In vivo, IFN-λ limits VSV replication in the mouse lung after intranasal administration and reduces virus spread to other organs. Despite this attenuation, however, the vector retains its capacity to induce protective CD8 T cell and antibody responses after a single immunization. These findings demonstrate a novel method of viral vector attenuation that could be used in both vaccine and oncolytic virus applications.

IMPORTANCE

Viruses such as VSV that are used as vaccine vectors can induce protective T cell and antibody responses after a single dose. Additionally, IFN-λ is a potent antiviral agent that has certain advantages for clinical use compared to IFN-α/β, such as fewer patient side effects. Here, we demonstrate that IFN-λ attenuates VSV replication and spread following intranasal virus delivery but does not reduce the ability of VSV to induce potent protective immune responses. These findings demonstrate that the type III IFN family may have widespread applicability for improving the safety and efficacy of viral vaccine and oncolytic vectors.

Sorting Out Pandora's Box: Discerning the Dynamic Roles of Liver Microenvironment in Oncolytic Virus Therapy for Hepatocellular Carcinoma

Oncolytic viral therapies have recently found their way into clinical application for hepatocellular carcinoma (HCC), a disease with limited treatment options and poor prognosis. Adding to the many intrinsic challenges of in vivo oncolytic viral therapy, is the complex microenvironment of the liver, which imposes unique limitations to the successful delivery and propagation of the virus. The normal liver milieu is characterized by an intricate network of hepatocytes and non-parenchymal cells including Kupffer cells, stellate cells, and sinusoidal endothelial cells, which can secrete anti-viral cytokines, provide a platform for non-specific uptake, and form a barrier to efficient viral spread. In addition, natural killer cells are greatly enriched in the liver, contributing to the innate defense against viruses. The situation is further complicated when HCC arises in the setting of underlying hepatitis virus infection and/or hepatic cirrhosis, which occurs in more than 90% of clinical cases. These conditions pose further inhibitory effects on oncolytic virus (OV) therapy due to the presence of chronic inflammation, constitutive cytokine expression, altered hepatic blood flow, and extracellular matrix deposition. In addition, OVs can modulate the hepatic microenvironment, resulting in a complex interplay between virus and host. The immune system undoubtedly plays a substantial role in the outcome of OV therapy, both as an inhibitor of viral replication, and as a potent mechanism of virus-mediated tumor cell killing. This review will discuss the particular challenges of oncolytic viral therapy for HCC, as well as some potential strategies for modulating the immune system and synergizing with the hepatic microenvironment to improve therapeutic outcome.

Preclinical safety and activity of recombinant VSV-IFN-β in an immunocompetent model of squamous cell carcinoma of the head and neck

BACKGROUND

Recombinant vesicular stomatitis virus expressing interferon-β (VSV-IFN-β) has demonstrated antitumor activity in vitro and in vivo. In preparation for clinical testing in human squamous cell carcinoma (SCC) of the head and neck, we conducted preclinical studies of VSV-IFN-β in syngeneic SCC models.

METHODS

In vitro, VSV-IFN-β (expressing rat or mouse interferon [IFN]-β)-induced cytotoxicity and propagated in rat (FAT-7) or mouse (SCC-VII) SCC cells during normoxia and hypoxia. In vivo, intratumoral administration of VSV-rat-IFN-β or VSV-human-IFN-β in FAT-7 bearing or non-tumor bearing immunocompetent rats did not result in acute organ toxicity or death.

RESULTS

VSV-r-IFN-β replicated predominantly in tumors and a dose dependent anti-VSV antibody response was observed. Intratumoral or intravenous administration of VSV-IFN-β resulted in growth delay and improved survival compared with controls.

CONCLUSION

The above data confirm safety and feasibility of VSV-IFN-β administration in immunocompetent animals and support its clinical evaluation in advanced human head and neck cancer.

Oncolytic vesicular stomatitis virus in an immunocompetent model of MUC1-positive or MUC1-null pancreatic ductal adenocarcinoma

Vesicular stomatitis virus (VSV) is a promising oncolytic agent against various malignancies. Here, for the first time, we tested VSV in vitro and in vivo in a clinically relevant, immunocompetent mouse model of pancreatic ductal adenocarcinoma (PDA). Our system allows the study of virotherapy against PDA in the context of overexpression (80% of PDA patients) or no expression of human mucin 1 (MUC1), a major marker for poor prognosis in patients. In vitro, we tested three VSV recombinants, wild-type VSV, VSV-green fluorescent protein (VSV-GFP), and a safe oncolytic VSV-ΔM51-GFP, against five mouse PDA cell lines that either expressed human MUC1 or were MUC1 null. All viruses demonstrated significant oncolytic abilities independent of MUC1 expression, although VSV-ΔM51-GFP was somewhat less effective in two PDA cell lines. In vivo administration of VSV-ΔM51-GFP resulted in significant reduction of tumor growth for tested mouse PDA xenografts (+MUC1 or MUC1 null), and antitumor efficacy was further improved when the virus was combined with the chemotherapeutic drug gemcitabine. The antitumor effect was transient in all tested groups. The developed system can be used to study therapies involving various oncolytic viruses and chemotherapeutics, with the goal of inducing tumor-specific immunity while preventing premature virus clearance.

Understanding and altering cell tropism of vesicular stomatitis virus

Vesicular stomatitis virus (VSV) is a prototypic nonsegmented negative-strand RNA virus. VSV's broad cell tropism makes it a popular model virus for many basic research applications. In addition, a lack of preexisting human immunity against VSV, inherent oncotropism and other features make VSV a widely used platform for vaccine and oncolytic vectors. However, VSV's neurotropism that can result in viral encephalitis in experimental animals needs to be addressed for the use of the virus as a safe vector. Therefore, it is very important to understand the determinants of VSV tropism and develop strategies to alter it. VSV glycoprotein (G) and matrix (M) protein play major roles in its cell tropism. VSV G protein is responsible for VSV broad cell tropism and is often used for pseudotyping other viruses. VSV M affects cell tropism via evasion of antiviral responses, and M mutants can be used to limit cell tropism to cell types defective in interferon signaling. In addition, other VSV proteins and host proteins may function as determinants of VSV cell tropism. Various approaches have been successfully used to alter VSV tropism to benefit basic research and clinically relevant applications.

Myeloid-derived suppressor cells as a vehicle for tumor-specific oncolytic viral therapy

One of the several impediments to effective oncolytic virus therapy of cancer remains a lack of tumor-specific targeting. Myeloid-derived suppressor cells (MDSC) are immature myeloid cells induced by tumor factors in tumor-bearing hosts. The biodistribution kinetics of MDSC and other immune cell types in a murine hepatic colon cancer model was investigated through the use of tracking markers and MRI. MDSCs were superior to other immune cell types in preferential migration to tumors in comparison with other tissues. On the basis of this observation, we engineered a strain of vesicular stomatitis virus (VSV), an oncolytic rhabdovirus that bound MDSCs and used them as a delivery vehicle. Improving VSV-binding efficiency to MDSCs extended the long-term survival of mice bearing metastatic colon tumors compared with systemic administration of wild-type VSV alone. Survival was further extended by multiple injections of the engineered virus without significant toxicity. Notably, direct tumor killing was accentuated by promoting MDSC differentiation towards the classically activated M1-like phenotype. Our results offer a preclinical proof-of-concept for using MDSCs to facilitate and enhance the tumor-killing activity of tumor-targeted oncolytic therapeutics.

Neuroattenuation of vesicular stomatitis virus through picornaviral internal ribosome entry sites

Vesicular stomatitis virus (VSV) is potent and a highly promising agent for the treatment of cancer. However, translation of VSV oncolytic virotherapy into the clinic is being hindered by its inherent neurotoxicity. It has been demonstrated that selected picornaviral internal ribosome entry site (IRES) elements possess restricted activity in neuronal tissues. We therefore sought to determine whether the picornavirus IRES could be engineered into VSV to attenuate its neuropathogenicity. We have used IRES elements from human rhinovirus type 2 (HRV2) and foot-and-mouth disease virus (FMDV) to control the translation of the matrix gene (M), which plays a major role in VSV virulence. In vitro studies revealed slowed growth kinetics of IRES-controlled VSVs in most of the cell lines tested. However, in vivo studies explicitly demonstrated that IRES elements of HRV2 and FMDV severely attenuated the neurovirulence of VSV without perturbing its oncolytic potency.

Resistance of pancreatic cancer cells to oncolytic vesicular stomatitis virus: role of type I interferon signaling

Oncolytic virus (OV) therapy takes advantage of common cancer characteristics, such as defective type I interferon (IFN) signaling, to preferentially infect and kill cancer cells with viruses. Our recent study (Murphy et al., 2012. J. Virol. 86, 3073-87) found human pancreatic ductal adenocarcinoma (PDA) cells were highly heterogeneous in their permissiveness to vesicular stomatitis virus (VSV) and suggested at least some resistant cell lines retained functional type I IFN responses. Here we examine cellular responses to infection by the oncolytic VSV recombinant VSV-ΔM51-GFP by analyzing a panel of 11 human PDA cell lines for expression of 33 genes associated with type I IFN pathways. Although all cell lines sensed infection by VSV-ΔM51-GFP and most activated IFN-α and β expression, only resistant cell lines displayed constitutive high-level expression of the IFN-stimulated antiviral genes MxA and OAS. Inhibition of JAK/STAT signaling decreased levels of MxA and OAS and increased VSV infection, replication and oncolysis, further implicating IFN responses in resistance. Unlike VSV, vaccinia and herpes simplex virus infectivity and killing of PDA cells was independent of the type I IFN signaling profile, possibly because these two viruses are better equipped to evade type I IFN responses. Our study demonstrates heterogeneity in the type I IFN signaling status of PDA cells and suggests MxA and OAS as potential biomarkers for PDA resistance to VSV and other OVs sensitive to type I IFN responses.

Vesicular stomatitis virus as a flexible platform for oncolytic virotherapy against cancer

Oncolytic virus (OV) therapy is an emerging anti-cancer approach that utilizes viruses to preferentially infect and kill cancer cells, while not harming healthy cells. Vesicular stomatitis virus (VSV) is a prototypic non-segmented, negative-strand RNA virus with inherent OV qualities. Antiviral responses induced by type I interferon pathways are believed to be impaired in most cancer cells, making them more susceptible to VSV than normal cells. Several other factors make VSV a promising OV candidate for clinical use, including its well-studied biology, a small, easily manipulated genome, relative independence of a receptor or cell cycle, cytoplasmic replication without risk of host-cell transformation, and lack of pre-existing immunity in humans. Moreover, various VSV-based recombinant viruses have been engineered via reverse genetics to improve oncoselectivity, safety, oncotoxicity and stimulation of tumour-specific immunity. Alternative delivery methods are also being studied to minimize premature immune clearance of VSV. OV treatment as a monotherapy is being explored, although many studies have employed VSV in combination with radiotherapy, chemotherapy or other OVs. Preclinical studies with various cancers have demonstrated that VSV is a promising OV; as a result, a human clinical trial using VSV is currently in progress.

Potent systemic therapy of multiple myeloma utilizing oncolytic vesicular stomatitis virus coding for interferon-β

Multiple myeloma (MM) is an incurable malignancy of plasma secreting B-cells disseminated in the bone marrow. Successful utilization of oncolytic virotherapy for myeloma treatment requires a systemically administered virus that selectively destroys disseminated myeloma cells in an immune-competent host. Vesicular stomatitis virus (VSV) expressing Interferon-β (IFNβ) is a promising new oncolytic agent that exploits tumor-associated defects in innate immune signaling pathways to specifically destroy cancer cells. We demonstrate here that a single, intravenous dose of VSV-IFNβ specifically destroys subcutaneous and disseminated 5TGM1 myeloma in an immune competent myeloma model. VSV-IFN treatment significantly prolonged survival in mice bearing orthotopic myeloma. Viral murine IFNβ expression further delayed myeloma progression and significantly enhanced survival compared to VSV expressing human IFNβ. Evaluation of VSV-IFNβ oncolytic activity in human myeloma cell lines and primary patient samples confirmed myeloma specific oncolytic activity but revealed variable susceptibility to VSV-IFNβ oncolysis. The results indicate that VSV-IFNβ is a potent, safe oncolytic agent that can be systemically administered to effectively target and destroy disseminated myeloma in immune competent mice. IFNβ expression improves cancer specificity and enhances VSV therapeutic efficacy against disseminated myeloma. These data show VSV-IFNβ to be a promising vector for further development as a potential therapy for treatment of Multiple myeloma.

Curative one-shot systemic virotherapy in murine myeloma

Current therapy for multiple myeloma is complex and prolonged. Antimyeloma drugs are combined in induction, consolidation and/or maintenance protocols to destroy bulky disease, then suppress or eradicate residual disease. Oncolytic viruses have the potential to mediate both tumor debulking and residual disease elimination, but this curative paradigm remains unproven. Here we engineered an oncolytic vesicular stomatitis virus to minimize its neurotoxicity, enhance induction of antimyeloma immunity, and facilitate noninvasive monitoring of its intratumoral spread. Using high resolution imaging, autoradiography and immunohistochemistry, we demonstrate that the intravenously administered virus extravasates from tumor blood vessels in immunocompetent myeloma-bearing mice, nucleating multiple intratumoral infectious centers which expand rapidly and necrose at their centers, ultimately coalescing to cause extensive tumor destruction. This oncolytic tumor debulking phase lasts only for 72 hours after virus administration, and is completed before antiviral antibodies become detectable in the bloodstream. Anti-myeloma T cells, cross-primed as the virus-infected cells provoke an antiviral immune response, then eliminate residual uninfected myeloma cells. The study establishes a curative oncolytic paradigm for multiple myeloma where direct tumor debulking and immune eradication of minimal disease are mediated by a single intravenous dose of a single therapeutic agent. Clinical translation is underway.

Vesicular stomatitis virus as an oncolytic agent against pancreatic ductal adenocarcinoma

Vesicular stomatitis virus (VSV) is a promising oncolytic agent against a variety of cancers. However, it has never been tested in any pancreatic cancer model. Pancreatic ductal adenocarcinoma (PDA) is the most common and aggressive form of pancreatic cancer. In this study, the oncolytic potentials of several VSV variants were analyzed in a panel of 13 clinically relevant human PDA cell lines and compared to conditionally replicative adenoviruses (CRAds), Sendai virus and respiratory syncytial virus. VSV variants showed oncolytic abilities superior to those of other viruses, and some cell lines that exhibited resistance to other viruses were successfully killed by VSV. However, PDA cells were highly heterogeneous in their susceptibility to virus-induced oncolysis, and several cell lines were resistant to all tested viruses. Resistant cells showed low levels of very early VSV RNA synthesis, indicating possible defects at initial stages of infection. In addition, unlike permissive PDA cell lines, most of the resistant cell lines were able to both produce and respond to interferon, suggesting that intact type I interferon responses contributed to their resistance phenotype. Four cell lines that varied in their permissiveness to VSV-ΔM51 and CRAd dl1520 were tested in mice, and the in vivo results closely mimicked those in vitro. While our results demonstrate that VSV is a promising oncolytic agent against PDA, further studies are needed to better understand the molecular mechanisms of resistance of some PDAs to oncolytic virotherapy.

Potential of vesicular stomatitis virus as an oncolytic therapy for recurrent and drug-resistant ovarian cancer

In the last decade, we have gained significant understanding of the mechanism by which vesicular stomatitis virus (VSV) specifically kills cancer cells. Dysregulation of translation and defective innate immunity are both thought to contribute to VSV oncolysis. Safety and efficacy are important objectives to consider in evaluating VSV as a therapy for malignant disease. Ongoing efforts may enable VSV virotherapy to be considered in the near future to treat drug-resistant ovarian cancer when other options have been exhausted. In this article, we review the development of VSV as a potential therapeutic approach for recurrent or drug-resistant ovarian cancer.

Replicating viral vectors for cancer therapy: strategies to synergize with host immune responses

Tumour‐specific replicating (oncolytic) viruses are novel anticancer agents, currently under intense investigation in preclinical studies and phase I–III clinical trials. Until recently, most studies have focused on the direct antitumour properties of these viruses. There is now an increasing body of evidence indicating that host immune responses may be critical to the efficacy of oncolytic virotherapy. Although the immune response to oncolytic viruses can rapidly restrict viral replication, thereby limiting the efficacy of therapy, oncolytic virotherapy also has the potential to induce potent antitumoural immune effectors that destroy those cancer cells, which are not directly lysed by virus. In this review, we discuss the role of the immune system in terms of antiviral and antitumoural responses, as well as strategies to evade or promote these responses in favour of improved therapeutic potentials.

Vesicular stomatitis virus as a treatment for colorectal cancer

M protein mutant vesicular stomatitis virus is an attractive candidate oncolytic virus for the treatment of metastatic colorectal cancer due to its ability to kill cancer cells that are defective in their antiviral responses. The oncolytic activity of recombinant wild-type and M protein mutant vesicular stomatitis viruses was determined in RKO, Hct116 and LoVo colorectal cancer cells, as well as in human fibroblast and hepatocyte primary cultures. RKO and Hct116 cells were sensitive to both viruses, whereas LoVo cells were resistant. [(35)S]methionine labeling experiments and viral plaque assays showed that sensitive and resistant colorectal cancer cells supported viral protein and progeny production after infection with either virus. Colorectal cancer cells were pretreated with β-interferon and infected with vesicular stomatitis virus to evaluate the extent to which interferon signaling is downregulated in colorectal cancer cells. Although colorectal cancer cells retained some degree of interferon signaling, this signaling did not negatively impact the oncolytic effects of either virus in sensitive cells. Murine xenografts of RKO cells were effectively treated by intratumoral injections with M protein mutant virus, whereas LoVo xenografts were resistant to treatment with this virus. These results suggest that M protein mutant vesicular stomatitis virus is a good candidate oncolytic virus for the treatment of selected metastatic colorectal cancers.

Vesicular stomatitis virus expressing tumor suppressor p53 is a highly attenuated, potent oncolytic agent

Vesicular stomatitis virus (VSV), a negative-strand RNA rhabdovirus, preferentially replicates in and eradicates transformed versus nontransformed cells and is thus being considered for use as a potential anticancer treatment. The genetic malleability of VSV also affords an opportunity to develop more potent agents that exhibit increased therapeutic activity. The tumor suppressor p53 has been shown to exert potent antitumor properties, which may in part involve stimulating host innate immune responses to malignancies. To evaluate whether VSV expressing p53 exhibited enhanced oncolytic action, the murine p53 (mp53) gene was incorporated into recombinant VSVs with or without a functional viral M gene-encoded protein that could either block (VSV-mp53) or enable [VSV-M(mut)-mp53] host mRNA export following infection of susceptible cells. Our results indicated that VSV-mp53 and VSV-M(mut)-mp53 expressed high levels of functional p53 and retained the ability to lyse transformed versus normal cells. In addition, we observed that VSV-ΔM-mp53 was extremely attenuated in vivo due to p53 activating innate immune genes, such as type I interferon (IFN). Significantly, immunocompetent animals with metastatic mammary adenocarcinoma exhibited increased survival following treatment with a single inoculation of VSV-ΔM-mp53, the mechanisms of which involved enhanced CD49b+ NK and tumor-specific CD8+ T cell responses. Our data indicate that VSV incorporating p53 could provide a safe, effective strategy for the design of VSV oncolytic therapeutics and VSV-based vaccines.

Current good manufacturing practice production of an oncolytic recombinant vesicular stomatitis viral vector for cancer treatment

Vesicular stomatitis virus (VSV) is an oncolytic virus currently being investigated as a promising tool to treat cancer because of its ability to selectively replicate in cancer cells. To enhance the oncolytic property of the nonpathologic laboratory strain of VSV, we generated a recombinant vector [rVSV(MΔ51)-M3] expressing murine gammaherpesvirus M3, a secreted viral chemokine-binding protein that binds to a broad range of mammalian chemokines with high affinity. As previously reported, when rVSV(MΔ51)-M3 was used in an orthotopic model of hepatocellular carcinoma (HCC) in rats, it suppressed inflammatory cell migration to the virus-infected tumor site, which allowed for enhanced intratumoral virus replication leading to increased tumor necrosis and substantially prolonged survival. These encouraging results led to the development of this vector for clinical translation in patients with HCC. However, a scalable current Good Manufacturing Practice (cGMP)-compliant manufacturing process has not been described for this vector. To produce the quantities of high-titer virus required for clinical trials, a process that is amenable to GMP manufacturing and scale-up was developed. We describe here a large-scale (50-liter) vector production process capable of achieving crude titers on the order of 10(9) plaque-forming units (PFU)/ml under cGMP. This process was used to generate a master virus seed stock and a clinical lot of the clinical trial agent under cGMP with an infectious viral titer of approximately 2 × 10(10) PFU/ml (total yield, 1 × 10(13) PFU). The lot has passed all U.S. Food and Drug Administration-mandated release testing and will be used in a phase 1 clinical translational trial in patients with advanced HCC.

Vesicular stomatitis virus-induced immune suppressor cells generate antagonism between intratumoral oncolytic virus and cyclophosphamide

Despite having potent oncolytic activity, in vitro, direct intratumoral injection of oncolytic vesicular stomatitis virus (VSV) into established AE17ova mesothelioma tumors in C57Bl/6 mice had no therapeutic effect. During studies to combine systemic cyclophosphamide (CPA) with VSV to suppress the innate immune reaction against VSV, we observed that CPA alone had highly significant antitumor effects in this model. However, against our expectations, the combination of CPA and VSV consistently reduced therapeutic efficacy compared to CPA alone, despite the fact that the combination increased intratumoral VSV titers. We show here that CPA-mediated therapy against AE17ova tumors was immune-mediated and dependent upon both CD4 T cells and natural killer (NK) cells. However, intratumoral VSV induced a transforming growth factor-β (TGF-β)-dependent suppressive activity, mediated by CD11b(+)GR-1(+) cells that significantly inhibited both antigen-specific T-cell activation, and CPA-activated, NK-dependent killing of AE17ova tumor cells. Overall, our results show that treatment with oncolytic viruses can induce a variety of immune-mediated consequences in vivo with both positive, or negative, effects on antitumor therapy. These underexplored immune consequences of treatment with oncolytic viruses may have significant, and possibly unexpected, impacts on how virotherapy interacts in combination with other agents which modulate antitumor immune effectors.