Model-based rational design of an oncolytic virus with improved therapeutic potential

Cancer therapy with the viral and bacterial pathogens: The past enemies can be considered the present allies

Cancer continues to be one of the leading causes of mortality worldwide despite significant advancements in cancer treatment. Many difficulties have arisen as a result of the detrimental consequences of chemotherapy and radiotherapy as a common cancer therapy, such as drug inability to penetrate deep tumor tissue, and also the drug resistance in tumor cells continues to be a major concern. These obstacles have increased the need for the development of new techniques that are more selective and effective against cancer cells. Bacterial-based therapies and the use of oncolytic viruses can suppress cancer in comparison to other cancer medications. The tumor microenvironment is susceptible to bacterial accumulation and proliferation, which can trigger immune responses against the tumor. Oncolytic viruses (OVs) have also gained considerable attention in recent years because of their potential capability to selectively target and induce apoptosis in cancer cells. This review aims to provide a comprehensive summary of the latest literature on the role of bacteria and viruses in cancer treatment, discusses the limitations and challenges, outlines various strategies, summarizes recent preclinical and clinical trials, and emphasizes the importance of optimizing current strategies for better clinical outcomes.

Emerging Nano-/Biotechnology Drives Oncolytic Virus-Activated and Combined Cancer Immunotherapy

Oncolytic viruses (OVs) as one promising antitumor methods have made important contributions to tumor immunotherapy, which arouse increasing attention. They provide the dual mechanisms including direct killing effect toward tumor cells and immune activation for elevating antitumor responses, which have been proved in many preclinical studies. Especially, natural or genetically modified viruses as clinical immune preparations have emerged as a new promising approach objective to oncology treatment. The approval of talimogene laherparepvec (T-VEC) by the U.S. Food and Drug Administration (FDA) for the therapy of advanced melanoma could be considered as a milestone achievement in the clinical translation of OV. In this review, we first discussed the antitumor mechanisms of OVs with an emphasis on targeting, replication, and propagation. We further outlined the state of the art of current OVs in tumor and underlined the activated biological effects especially including immunity. More significantly, the enhanced immune responses based on OVs were systematically discussed from different perspectives such as combination with immunotherapy, genetic engineering of OVs, integration with nanobiotechnology or nanoparticles, and antiviral response counteraction, where their principles were shed light on. The development of OVs in the clinics was also highlighted to analyze the actuality and concerns of different OV applications in clinical trials. At last, the future perspectives and challenges of OVs as an already widely accepted treatment means were discussed. This review will provide a systematic review and deep insight into OV development and also offer new opportunities and guidance pathways to drive the further clinical translation.

The intratumoral microbiome: Characterization methods and functional impact

Live-pathogenic bacteria, which were identified inside tumors hundreds year ago, are key elements in modern cancer research. As they have a relatively accessible genome, they offer a multitude of metabolic engineering opportunities, useful in several clinical fields. Better understanding of the tumor microenvironment and its associated microbiome would help conceptualize new metabolically engineered species, triggering efficient therapeutic responses against cancer. Unfortunately, given the low microbial biomass nature of tumors, characterizing the tumor microbiome remains a challenge. Tumors have a high host versus bacterial DNA ratio, making it extremely complex to identify tumor-associated bacteria. Nevertheless, with the improvements in next-generation analytic tools, recent studies demonstrated the existence of intratumor bacteria inside defined tumors. It is now proven that each cancer subtype has a unique microbiome, characterized by bacterial communities with specific metabolic functions. This review provides a brief overview of the main approaches used to characterize the tumor microbiome, and of the recently proposed functions of intracellular bacteria identified in oncological entities. The therapeutic aspects of live-pathogenic microbes are also discussed, regarding the tumor microenvironment of each cancer type.

Resistance Mechanisms Influencing Oncolytic Virotherapy, a Systematic Analysis

Resistance to therapy is a frequently observed phenomenon in the treatment of cancer, and as with other cancer therapeutics, therapies based on oncolytic viruses also face the challenges of resistance, such as humoral and cellular antiviral responses, and tumor-associated interferon-mediated resistance. In order to identify additional mechanisms of resistance that may contribute to therapeutic failure, we developed a systematic search strategy for studies published in PubMed. We analyzed 6143 articles on oncolytic virotherapy and found that approximately 8% of these articles use resistance terms in the abstract and/or title. Of these 439 articles, 87 were original research. Most of the findings reported pertain to resistance mediated by tumor-cell-dependent interferon signaling. Yet, mechanisms such as epigenetic modifications, hypoxia-mediated inhibition, APOBEC-mediated resistance, virus entry barriers, and spatiotemporal restriction to viral spread, although not frequently assessed, were demonstrated to play a major role in resistance. Similarly, our results suggest that the stromal compartment consisting of, but not limited to, myeloid cells, fibroblasts, and epithelial cells requires more study in relation to therapy resistance using oncolytic viruses. Thus, our findings emphasize the need to assess the stromal compartment and to identify novel mechanisms that play an important role in conferring resistance to oncolytic virotherapy.

Current strategies to circumvent the antiviral immunity to optimize cancer virotherapy

Cancer virotherapy is a paradigm-shifting treatment modality based on virus-mediated oncolysis and subsequent antitumor immune responses. Clinical trials of currently available virotherapies showed that robust antitumor immunity characterizes the remarkable and long-term responses observed in a subset of patients. These data suggest that future therapies should incorporate strategies to maximize the immunotherapeutic potential of oncolytic viruses. In this review, we highlight the recent evidence that the antiviral immunity of the patients may limit the immunotherapeutic potential of oncolytic viruses and summarize the most relevant approaches to strategically redirect the immune response away from the viruses and toward tumors to heighten the clinical impact of viro-immunotherapy platforms.

Synthetic and systems biology principles in the design of programmable oncolytic virus immunotherapies for glioblastoma

Oncolytic viruses (OVs) are a class of immunotherapeutic agents with promising preclinical results for the treatment of glioblastoma (GBM) but have shown limited success in recent clinical trials. Advanced bioengineering principles from disciplines such as synthetic and systems biology are needed to overcome the current challenges faced in developing effective OV-based immunotherapies for GBMs, including off-target effects and poor clinical responses. Synthetic biology is an emerging field that focuses on the development of synthetic DNA constructs that encode networks of genes and proteins (synthetic genetic circuits) to perform novel functions, whereas systems biology is an analytical framework that enables the study of complex interactions between host pathways and these synthetic genetic circuits. In this review, the authors summarize synthetic and systems biology concepts for developing programmable, logic-based OVs to treat GBMs. Programmable OVs can increase selectivity for tumor cells and enhance the local immunological response using synthetic genetic circuits. The authors discuss key principles for developing programmable OV-based immunotherapies, including how to 1) select an appropriate chassis, a vector that carries a synthetic genetic circuit, and 2) design a synthetic genetic circuit that can be programmed to sense key signals in the GBM microenvironment and trigger release of a therapeutic payload. To illustrate these principles, some original laboratory data are included, highlighting the need for systems biology studies, as well as some preliminary network analyses in preparation for synthetic biology applications. Examples from the literature of state-of-the-art synthetic genetic circuits that can be packaged into leading candidate OV chassis are also surveyed and discussed.

Delivery and Biosafety of Oncolytic Virotherapy

In recent years, oncolytic virotherapy has emerged as a promising anticancer therapy. Oncolytic viruses destroy cancer cells, without damaging normal tissues, through virus self-replication and antitumor immunity responses, showing great potential for cancer treatment. However, the clinical guidelines for administering oncolytic virotherapy remain unclear. Delivery routes for oncolytic virotherapy to patients vary in existing studies, depending on the tumor sites and the objective of studies. Moreover, the biosafety of oncolytic virotherapy, including mainly uncontrolled adverse events and long-term complications, remains a serious concern that needs to be accurately measured. This review provides a comprehensive and detailed overview of the delivery and biosafety of oncolytic virotherapy.

Tumor-targeting oncolytic virus elicits potent immunotherapeutic vaccine responses to tumor antigens

Oncolytic viruses represent a promising therapeutic modality, but they have yet to live up to their therapeutic potential. Safety and efficacy concerns impel us to identify least toxic oncolytic agents that would generate durable and multifaceted anti-tumor immune responses to disrupt the tumors. Here we describe a rational engineered oncolytic herpes virus (OVH) that is a selective killer for targeting tumors, has strong safety records, induces complete regression of tumors in multiple tumor models, and elicits potent antitumor immunity. By far, the potential of OVs in promoting the tumor antigen-specific humoral immune responses remains obscure. In this study, we found that effective treatment by OVH induced immunogenic cell death, which facilitates to elicit humoral immune responses. Depletion experiments revealed that B cells were required for maximal antitumor efficacy of oncolytic immunotherapy. Both serum transfer and antibody treatment experiments revealed that endogenous oncolysis-induced antigen-targeting therapeutic antibodies can lead to systemic tumor regression. Our data demonstrate that tumor-targeting immune modulatory properties confer oncolytic OVH virotherapy as potent immunotherapeutic cancer vaccines that can generate specific and efficacious antitumor humoral responses by eliciting endogenous tumor antigen-targeting therapeutic antibodies in situ, resulting in an efficacious and tumor-specific therapeutic effect.

Developing a Minimally Structured Mathematical Model of Cancer Treatment with Oncolytic Viruses and Dendritic Cell Injections

Mathematical models of biological systems must strike a balance between being sufficiently complex to capture important biological features, while being simple enough that they remain tractable through analysis or simulation. In this work, we rigorously explore how to balance these competing interests when modeling murine melanoma treatment with oncolytic viruses and dendritic cell injections. Previously, we developed a system of six ordinary differential equations containing fourteen parameters that well describes experimental data on the efficacy of these treatments. Here, we explore whether this previously developed model is the minimal model needed to accurately describe the data. Using a variety of techniques, including sensitivity analyses and a parameter sloppiness analysis, we find that our model can be reduced by one variable and three parameters and still give excellent fits to the data. We also argue that our model is not too simple to capture the dynamics of the data, and that the original and minimal models make similar predictions about the efficacy and robustness of protocols not considered in experiments. Reducing the model to its minimal form allows us to increase the tractability of the system in the face of parametric uncertainty.

DNA-PK inhibition synergizes with oncolytic virus M1 by inhibiting antiviral response and potentiating DNA damage

Oncolytic virotherapy is a promising therapeutic strategy that uses replication-competent viruses to selectively destroy malignancies. However, the therapeutic effect of certain oncolytic viruses (OVs) varies among cancer patients. Thus, it is necessary to overcome resistance to OVs through rationally designed combination strategies. Here, through an anticancer drug screening, we show that DNA-dependent protein kinase (DNA-PK) inhibition sensitizes cancer cells to OV M1 and improves therapeutic effects in refractory cancer models in vivo and in patient tumour samples. Infection of M1 virus triggers the transcription of interferons (IFNs) and the activation of the antiviral response, which can be abolished by pretreatment of DNA-PK inhibitor (DNA-PKI), resulting in selectively enhanced replication of OV M1 within malignancies. Furthermore, DNA-PK inhibition promotes the DNA damage response induced by M1 virus, leading to increased tumour cell apoptosis. Together, our study identifies the combination of DNA-PKI and OV M1 as a potential treatment for cancers.

Oncolytic virotherapy is a promising strategy for cancer treatment. In this study, the authors demonstrate that DNA-dependent protein kinase (DNA-PK) inhibition sensitizes cancer cells to M1 virus and improves therapeutic effects in refractory cancer models in vivo and in patient tumour samples.

The importance of imaging strategies for pre-clinical and clinical in vivo distribution of oncolytic viruses

Oncolytic viruses (OVs) are an emergent and unique therapy for cancer patients. Similar to chemo- and radiation therapy, OV can lyse (kill) cancer cell directly. In general, the advantages of OVs over other treatments are primarily: a higher safety profile (as shown by less adverse effects), ability to replicate, transgene(s) delivery, and stimulation of a host’s immune system against cancer. The latter has prompted successful use of OVs with other immunotherapeutic strategies in a synergistic manner. In spite of extended testing in pre-clinical and clinical setting, using biologically derived therapeutics like virus always raises potential concerns about safety (replication at non-intended locations) and bio-availability of the product. Recent advent in in vivo imaging techniques dramatically improves the convenience of use, quality of pictures, and amount of information acquired. Easy assessing of safety/localization of the biotherapeutics like OVs became a new potential weapon in the physician’s arsenal to improve treatment outcome. Given that OVs are typically replicating, in vivo imaging can also track virus replication and persistence as well as precisely mapping tumor tissues presence. This review discusses the importance of imaging in vivo in evaluating OV efficacy, as well as currently available tools and techniques.

Recent advances in vesicular stomatitis virus-based oncolytic virotherapy: a 5-year update

Oncolytic virus (OV) therapy is an anti-cancer approach that uses viruses that preferentially infect, replicate in and kill cancer cells. Vesicular stomatitis virus (VSV, a rhabdovirus) is an OV that is currently being tested in the USA in several phase I clinical trials against different malignancies. Several factors make VSV a promising OV: lack of pre-existing human immunity against VSV, a small and easy to manipulate genome, cytoplasmic replication without risk of host cell transformation, independence of cell cycle and rapid growth to high titres in a broad range of cell lines facilitating large-scale virus production. While significant advances have been made in VSV-based OV therapy, room for improvement remains. Here we review recent studies (published in the last 5 years) that address 'old' and 'new' challenges of VSV-based OV therapy. These studies focused on improving VSV safety, oncoselectivity and oncotoxicity; breaking resistance of some cancers to VSV; preventing premature clearance of VSV; and stimulating tumour-specific immunity. Many of these approaches were based on combining VSV with other therapeutics. This review also discusses another rhabdovirus closely related to VSV, Maraba virus, which is currently being tested in Canada in phase I/II clinical trials.

Insertion of the Type-I IFN Decoy Receptor B18R in a miRNA-Tagged Semliki Forest Virus Improves Oncolytic Capacity but Results in Neurotoxicity

Oncolytic Semliki Forest virus (SFV) has been suggested as a potential candidate for the treatment of glioblastoma and neuroblastoma. However, the oncolytic capacity of SFV is restricted by the anti-viral type-I interferon (IFN) response. The aim of this study was to increase the oncolytic capacity of a microRNA target tagged SFV against glioblastoma by arming it with the Vaccinia-virus-encoded type-I IFN decoy receptor B18R (SFV4B18RmiRT) to neutralize type-I IFN response. Expression of B18R by SFV4B18RmiRT aided neutralization of IFN-β, which was shown by reduced STAT-1 phosphorylation and improved virus spread in plaque assays. B18R expression by SFV4 increased its oncolytic capacity in vitro against murine glioblastoma (CT-2A), regardless of the presence of exogenous IFN-β. Both SFV4B18RmiRT and SFV4miRT treatments controlled tumor growth in mice with syngeneic orthotopic gliomablastoma (CT-2A). However, treatment with SFV4B18RmiRT induced severe neurological symptoms in some mice because of virus replication in the healthy brain. Neither neurotoxicity nor virus replication in the brain was observed when SFV4miRT was administered. In summary, our results indicate that the oncolytic capacity of SFV4 was improved in vitro and in vivo by incorporation of B18R, but neurotoxicity of the virus was increased, possibly due to loss of microRNA targets.

Reovirus FAST Protein Enhances Vesicular Stomatitis Virus Oncolytic Virotherapy in Primary and Metastatic Tumor Models

The reovirus fusion-associated small transmembrane (FAST) proteins are the smallest known viral fusogens (∼100–150 amino acids) and efficiently induce cell-cell fusion and syncytium formation in multiple cell types. Syncytium formation enhances cell-cell virus transmission and may also induce immunogenic cell death, a form of apoptosis that stimulates immune recognition of tumor cells. These properties suggest that FAST proteins might serve to enhance oncolytic virotherapy. The oncolytic activity of recombinant VSVΔM51 (an interferon-sensitive vesicular stomatitis virus [VSV] mutant) encoding the p14 FAST protein (VSV-p14) was compared with a similar construct encoding GFP (VSV-GFP) in cell culture and syngeneic BALB/c tumor models. Compared with VSV-GFP, VSV-p14 exhibited increased oncolytic activity against MCF-7 and 4T1 breast cancer spheroids in culture and reduced primary 4T1 breast tumor growth in vivo. VSV-p14 prolonged survival in both primary and metastatic 4T1 breast cancer models, and in a CT26 metastatic colon cancer model. As with VSV-GFP, VSV-p14 preferentially replicated in vivo in tumors and was cleared rapidly from other sites. Furthermore, VSV-p14 increased the numbers of activated splenic CD4, CD8, natural killer (NK), and natural killer T (NKT) cells, and increased the number of activated CD4 and CD8 cells in tumors. FAST proteins may therefore provide a multi-pronged approach to improving oncolytic virotherapy via syncytium formation and enhanced immune stimulation.

Oncolytic Maraba Virus MG1 as a Treatment for Sarcoma

The poor prognosis of patients with advanced bone and soft-tissue sarcoma has not changed in the past several decades, highlighting the necessity for new therapeutic approaches. Immunotherapies, including oncolytic viral (OV) therapy, have shown great promise in a number of clinical trials for a variety of tumor types. However, the effective application of OV in treating sarcoma still remains to be demonstrated. Although few pre-clinical studies using distinct OVs have been performed and demonstrated therapeutic benefit in sarcoma models, a side-by-side comparison of clinically relevant OV platforms has not been performed. Four clinically relevant OV platforms (Reovirus, Vaccinia virus, Herpes-simplex virus and Rhabdovirus) were screened for their ability to infect and kill human and canine sarcoma cell lines in vitro, and human sarcoma specimens ex vivo. In vivo treatment efficacy was tested in a murine model. The rhabdovirus MG1 demonstrated the highest potency in vitro. Ex vivo, MG1 productively infected more than 80% of human sarcoma tissues tested, and treatment in vivo led to a significant increase in long-lasting cures in sarcoma-bearing mice. Importantly, MG1 treatment induced the generation of memory immune response that provided protection against a subsequent tumor challenge. This study opens the door for the use of MG1-based oncolytic immunotherapy strategies as treatment for sarcoma or as a component of a combined therapy.

Smac mimetics synergize with immune checkpoint inhibitors to promote tumour immunity against glioblastoma

Small-molecule inhibitor of apoptosis (IAP) antagonists, called Smac mimetic compounds (SMCs), sensitize tumours to TNF-α-induced killing while simultaneously blocking TNF-α growth-promoting activities. SMCs also regulate several immunomodulatory properties within immune cells. We report that SMCs synergize with innate immune stimulants and immune checkpoint inhibitor biologics to produce durable cures in mouse models of glioblastoma in which single agent therapy is ineffective. The complementation of activities between these classes of therapeutics is dependent on cytotoxic T-cell activity and is associated with a reduction in immunosuppressive T-cells. Notably, the synergistic effect is dependent on type I IFN and TNF-α signalling. Furthermore, our results implicate an important role for TNF-α-producing cytotoxic T-cells in mediating the anti-cancer effects of immune checkpoint inhibitors when combined with SMCs. Overall, this combinatorial approach could be highly effective in clinical application as it allows for cooperative and complimentary mechanisms in the immune cell-mediated death of cancer cells.

Smac mimetics sensitize cancer cells to the extrinsic cell death pathway and stimulate anti-tumour immunity. In this study, the authors show that Smac mimetics can synergize with immune checkpoint inhibitors to control tumour growth in mouse cancer models, including aggressive CNS tumours, in a cytotoxic CD8⁺ T-cell- and TNFα-dependent manner.

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.

Combination of Paclitaxel and MG1 oncolytic virus as a successful strategy for breast cancer treatment

Background

Breast cancer is the most common malignant disease amongst Western women. The lack of treatment options for patients with chemotherapy-resistant or recurrent cancers is pushing the field toward the rapid development of novel therapies. The use of oncolytic viruses is a promising approach for the treatment of disseminated diseases like breast cancer, with the first candidate recently approved by the Food and Drug Administration for use in patients. In this report, we demonstrate the compatibility of oncolytic virotherapy and chemotherapy using various murine breast cancer models. This one-two punch has been explored in the past by several groups with different viruses and drugs and was shown to be a successful approach. Our strategy is to combine Paclitaxel, one of the most common drugs used to treat patients with breast cancer, and the oncolytic Rhabdovirus Maraba-MG1, a clinical trial candidate in a study currently recruiting patients with late-stage metastatic cancer.

Methods

We used the EMT6, 4 T1 and E0771 murine breast cancer models to evaluate in vitro and in vivo the effects of co-treatment with MG1 and Paclitaxel. Treatment-induced cytotoxicity was assessed and plaque assays, flow cytometry, microscopy and immunocytochemistry analysis were performed to quantify virus production and transgene expression. Orthotopically implanted tumors were measured during and after treatment to evaluate efficacy and Kaplan-Meier survival curves were generated.

Results

Our data demonstrate not only the compatibility of the treatments, but also their synergistic cytopathic activity. With Paclitaxel, EMT6 and 4 T1 tumors demonstrated increased virus production both in vitro and in vivo. Our results also show that Paclitaxel does not impair the safety profile of the virus treatment. Importantly, when combined, MG1 and the drug controlled tumor growth and prolonged survival.

Conclusions

The combination of MG1 and Paclitaxel improved efficacy in all of the breast cancer models we tested and thus is a promising alternative approach for the treatment of patients with refractory breast cancer. Our strategy has potential for rapid translation to the clinic, given the current clinical status of both agents.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-016-0744-y) contains supplementary material, which is available to authorized users.

First-in-class small molecule potentiators of cancer virotherapy

The use of engineered viral strains such as gene therapy vectors and oncolytic viruses (OV) to selectively destroy cancer cells is poised to make a major impact in the clinic and revolutionize cancer therapy. In particular, several studies have shown that OV therapy is safe and well tolerated in humans and can infect a broad range of cancers. Yet in clinical studies OV therapy has highly variable response rates. The heterogeneous nature of tumors is widely accepted to be a major obstacle for OV therapeutics and highlights a need for strategies to improve viral replication efficacy. Here, we describe the development of a new class of small molecules for selectively enhancing OV replication in cancer tissue. Medicinal chemistry studies led to the identification of compounds that enhance multiple OVs and gene therapy vectors. Lead compounds increase OV growth up to 2000-fold in vitro and demonstrate remarkable selectivity for cancer cells over normal tissue ex vivo and in vivo. These small molecules also demonstrate enhanced stability with reduced electrophilicity and are highly tolerated in animals. This pharmacoviral approach expands the scope of OVs to include resistant tumors, further potentiating this transformative therapy. It is easily foreseeable that this approach can be applied to therapeutically enhance other attenuated viral vectors.

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.

CD8(+) T-cell Immune Evasion Enables Oncolytic Virus Immunotherapy

Although counteracting innate defenses allows oncolytic viruses (OVs) to better replicate and spread within tumors, CD8(+) T-cells restrict their capacity to trigger systemic anti-tumor immune responses. Herpes simplex virus-1 (HSV-1) evades CD8(+) T-cells by producing ICP47, which limits immune recognition of infected cells by inhibiting the transporter associated with antigen processing (TAP). Surprisingly, removing ICP47 was assumed to benefit OV immuno-therapy, but the impact of inhibiting TAP remains unknown because human HSV-1 ICP47 is not effective in rodents. Here, we engineer an HSV-1 OV to produce bovine herpesvirus UL49.5, which unlike ICP47, antagonizes rodent and human TAP. Significantly, UL49.5-expressing OVs showed superior efficacy treating bladder and breast cancer in murine models that was dependent upon CD8(+) T-cells. Besides injected subcutaneous tumors, UL49.5-OV reduced untreated, contralateral tumor size and metastases. These findings establish TAP inhibitor-armed OVs that evade CD8(+) T-cells as an immunotherapy strategy to elicit potent local and systemic anti-tumor responses.

Oncolytic Immunotherapy for Treatment of Cancer

Immunotherapy entails the treatment of disease by modulation of the immune system. As detailed in the previous chapters, the different modes of achieving immune modulation are many, including the use of small/large molecules, cellular therapy, and radiation. Oncolytic viruses that can specifically attack, replicate within, and destroy tumors represent one of the most promising classes of agents for cancer immunotherapy (recently termed as oncolytic immunotherapy). The notion of oncolytic immunotherapy is considered as the way in which virus-induced tumor cell death (known as immunogenic cancer cell death (ICD)) allows the immune system to recognize tumor cells and provide long-lasting antitumor immunity. Both immune responses toward the virus and ICD together contribute toward successful antitumor efficacy. What is now becoming increasingly clear is that monotherapies, through any of the modalities detailed in this book, are neither sufficient in eradicating tumors nor in providing long-lasting antitumor immune responses and that combination therapies may deliver enhanced efficacy. After the rise of the genetic engineering era, it has been possible to engineer viruses to harbor combination-like characteristics to enhance their potency in cancer immunotherapy. This chapter provides a historical background on oncolytic virotherapy and its future application in cancer immunotherapy, especially as a combination therapy with other treatment modalities.

What Lies Ahead?

Viral pathogenesis is a field in rapid evolution, reflecting the dynamic development of systems biology and the continuing introduction of new or improved methodologies. Therefore, this final chapter is dedicated to “futurism,” a look at what lies ahead for this field. We have recruited a number of scientists to write short pieces where they are free to speculate on future developments in their respective areas of expertise.

Treatment strategies for combining immunostimulatory oncolytic virus therapeutics with dendritic cell injections

Oncolytic viruses (OVs) are used to treat cancer, as they selectively replicate inside of and lyse tumor cells. The efficacy of this process is limited and new OVs are being designed to mediate tumor cell release of cytokines and co-stimulatory molecules, which attract cytotoxic T cells to target tumor cells, thus increasing the tumor-killing effects of OVs. To further promote treatment efficacy, OVs can be combined with other treatments, such as was done by Huang et al., who showed that combining OV injections with dendritic cell (DC) injections was a more effective treatment than either treatment alone. To further investigate this combination, we built a mathematical model consisting of a system of ordinary differential equations and fit the model to the hierarchical data provided from Huang et al. We used the model to determine the effect of varying doses of OV and DC injections and to test alternative treatment strategies. We found that the DC dose given in Huang et al. was near a bifurcation point and that a slightly larger dose could cause complete eradication of the tumor. Further, the model results suggest that it is more effective to treat a tumor with immunostimulatory oncolytic viruses first and then follow-up with a sequence of DCs than to alternate OV and DC injections. This protocol, which was not considered in the experiments of Huang et al., allows the infection to initially thrive before the immune response is enhanced. Taken together, our work shows how the ordering, temporal spacing, and dosage of OV and DC can be chosen to maximize efficacy and to potentially eliminate tumors altogether.

Quantitative impact of immunomodulation versus oncolysis with cytokine-expressing virus therapeutics

The past century's description of oncolytic virotherapy as a cancer treatment involving specially-engineered viruses that exploit immune deficiencies to selectively lyse cancer cells is no longer adequate. Some of the most promising therapeutic candidates are now being engineered to produce immunostimulatory factors, such as cytokines and co-stimulatory molecules, which, in addition to viral oncolysis, initiate a cytotoxic immune attack against the tumor. This study addresses the combined effects of viral oncolysis and T-cell-mediated oncolysis. We employ a mathematical model of virotherapy that induces release of cytokine IL-12 and co-stimulatory molecule 4-1BB ligand. We found that the model closely matches previously published data, and while viral oncolysis is fundamental in reducing tumor burden, increased stimulation of cytotoxic T cells leads to a short-term reduction in tumor size, but a faster relapse. In addition, we found that combinations of specialist viruses that express either IL-12 or 4-1BBL might initially act more potently against tumors than a generalist virus that simultaneously expresses both, but the advantage is likely not large enough to replace treatment using the generalist virus. Finally, according to our model and its current assumptions, virotherapy appears to be optimizable through targeted design and treatment combinations to substantially improve therapeutic outcomes.

Microtubule disruption synergizes with oncolytic virotherapy by inhibiting interferon translation and potentiating bystander killing

In this study, we show that several microtubule-destabilizing agents used for decades for treatment of cancer and other diseases also sensitize cancer cells to oncolytic rhabdoviruses and improve therapeutic outcomes in resistant murine cancer models. Drug-induced microtubule destabilization leads to superior viral spread in cancer cells by disrupting type I IFN mRNA translation, leading to decreased IFN protein expression and secretion. Furthermore, microtubule-destabilizing agents specifically promote cancer cell death following stimulation by a subset of infection-induced cytokines, thereby increasing viral bystander effects. This study reveals a previously unappreciated role for microtubule structures in the regulation of the innate cellular antiviral response and demonstrates that unexpected combinations of approved chemotherapeutics and biological agents can lead to improved therapeutic outcomes.

8th international conference on oncolytic virus therapeutics

The 8th International Conference on Oncolytic Virus Therapeutics meeting was held from April 10-13, 2014, in Oxford, United Kingdom. It brought together experts in the field of oncolytics from Europe, Asia, Australasia, and the Americas and provided a unique opportunity to hear the latest research findings in oncolytic virotherapy. Presentations of recent work were delivered in an informal and intimate setting afforded by a small group of attendees and an exquisitely focused conference topic. Here we describe the oral presentations and enable the reader to share in the benefits of bringing together experts to share their findings.

Tumor Restrictions to Oncolytic Virus

Oncolytic virotherapy has advanced since the days of its conception but therapeutic efficacy in the clinics does not seem to reach the same level as in animal models. One reason is premature oncolytic virus clearance in humans, which is a reasonable assumption considering the immune-stimulating nature of the oncolytic agents. However, several studies are beginning to reveal layers of restriction to oncolytic virotherapy that are present before an adaptive neutralizing immune response. Some of these barriers are present constitutively halting infection before it even begins, whereas others are raised by minute cues triggered by virus infection. Indeed, we and others have noticed that delivering viruses to tumors may not be the biggest obstacle to successful therapy, but instead the physical make-up of the tumor and its capacity to mount antiviral defenses seem to be the most important efficacy determinants. In this review, we summarize the constitutive and innate barriers to oncolytic virotherapy and discuss strategies to overcome them.

Bacterial-mediated knockdown of tumor resistance to an oncolytic virus enhances therapy

Oncolytic viruses (OVs) and bacteria share the property of tumor-selective replication following systemic administration. In the case of nonpathogenic bacteria, tumor selectivity relates to their ability to grow extracellularly within tumor stroma and is therefore ideally suited to restricting the production of bacterially produced therapeutic agents to tumors. We have previously shown the ability of the type 1 interferon antagonist B18R to enhance the replication and spread of vesicular stomatitis virus (VSV) by overcoming related cellular innate immunity. In this study, we utilized nonpathogenic bacteria (E. coli) expressing B18R to facilitate tumor-specific production of B18R, resulting in a microenvironment depleted of bioactive antiviral cytokine, thus "preconditioning" the tumor to enhance subsequent tumor destruction by the OV. Both in vitro and in vivo infection by VSVΔ51 was greatly enhanced by B18R produced from E. coli. Moreover, a significant increase in therapeutic efficacy resulted from intravenous (i.v.) injection of bacteria to tumor-bearing mice 5 days prior to i.v. VSVΔ51 administration, as evidenced by a significant reduction in tumor growth and increased survival in mice. Our strategy is the first example where two such diverse microorganisms are rationally combined and demonstrates the feasibility of combining complementary microorganisms to improve therapeutic outcome.

Smac mimetics and innate immune stimuli synergize to promote tumor death

Smac mimetic compounds (SMC), a class of drugs that sensitize cells to apoptosis by counteracting the activity of inhibitor of apoptosis (IAP) proteins, have proven safe in Phase I clinical trials in cancer patients. However, because SMCs act by enabling transduction of pro-apoptotic signals, SMC monotherapy may only be efficacious in the subset of patients whose tumors produce large quantities of death-inducing proteins such as inflammatory cytokines. As such, we reasoned that SMCs would synergize with agents that stimulate a potent yet safe “cytokine storm”. Here we show that oncolytic viruses and adjuvants such as poly(I:C) and CpG induce bystander death of cancer cells treated with SMCs that is mediated by interferon beta (IFNβ), tumor necrosis factor alpha (TNFα) and/or TNF-related apoptosis-inducing ligand (TRAIL). This combinatorial treatment resulted in tumor regression and extended survival in two mouse models of cancer. As these and other adjuvants have been proven safe in clinical trials, it may be worthwhile to explore their clinical efficacy in combination with SMCs.

From scourge to cure: tumour-selective viral pathogenesis as a new strategy against cancer

Tumour mutations corrupt cellular pathways, and accumulate to disrupt, dysregulate, and ultimately avoid mechanisms of cellular control. Yet the very changes that tumour cells undergo to secure their own growth success also render them susceptible to viral infection. Enhanced availability of surface receptors, disruption of antiviral sensing, elevated metabolic activity, disengagement of cell cycle controls, hyperactivation of mitogenic pathways, and apoptotic avoidance all render the malignant cell environment highly supportive to viral replication. The therapeutic use of oncolytic viruses (OVs) with a natural tropism for infecting and subsequently lysing tumour cells is a rapidly progressing area of cancer research. While many OVs exhibit an inherent degree of tropism for transformed cells, this can be further promoted through pharmacological interventions and/or the introduction of viral mutations that generate recombinant oncolytic viruses adapted to successfully replicate only in a malignant cellular environment. Such adaptations that augment OV tumour selectivity are already improving the therapeutic outlook for cancer, and there remains tremendous untapped potential for further innovation.

Cell carriers for oncolytic viruses: current challenges and future directions

The optimal route for clinical delivery of oncolytic viruses is thought to be systemic intravenous injection; however, the immune system is armed with several highly efficient mechanisms to remove pathogens from the circulatory system. To overcome the challenges faced in trying to delivery oncolytic viruses specifically to tumors via the bloodstream, carrier cells have been investigated to determine their suitability as delivery vehicles for systemic administration of oncolytic viruses. Cell carriers protect viruses from neutralization, one of the most limiting aspects of oncolytic virus interaction with the immune system. Cell carriers can also possess inherent tumor tropism, thus directing the delivery of the virus more specifically to a tumor. With preclinical studies already demonstrating the success and feasibility of this approach with multiple oncolytic viruses, clinical evaluation of cell-mediated delivery of viruses is on the horizon. Meanwhile, ongoing preclinical studies are aimed at identifying new cellular vehicles for oncolytic viruses and improving current promising cell carrier platforms.

Non-replicating rhabdovirus-derived particles (NRRPs) eradicate acute leukemia by direct cytolysis and induction of antitumor immunity

Rhabdoviruses (RVs) are currently being pursued as anticancer therapeutics for various tumor types, notably leukemia. However, modest virion production and limited spread between noncontiguous circulating leukemic cells requires high-dose administration of RVs, which exceeds the maximum tolerable dose of the live virus. Furthermore, in severely immunosuppressed leukemic patients, the potential for uncontrolled live virus spread may compromise the safety of a live virus approach. We hypothesized that the barriers to oncolytic virotherapy in liquid tumors may be overcome by administration of high-dose non-replicating RVs. We have developed a method to produce unique high-titer bioactive yet non-replicating rhabdovirus-derived particles (NRRPs). This novel biopharmaceutical is multimodal possessing direct cytolytic and immunomodulatory activity against acute leukemia. We demonstrate that NRRP resistance in normal cells is mediated by intact antiviral defences including interferon (IFN). This data was substantiated using murine models of blast crisis. The translational promise of NRRPs was demonstrated in clinical samples obtained from patients with high-burden multidrug-resistant acute myeloid leukemia. This is the first successful attempt to eradicate disseminated cancer using a non-replicating virus-derived agent, representing a paradigm shift in our understanding of oncolytic virus-based therapies and their application toward the treatment of acute leukemia.