Clonal variation in interferon response determines the outcome of oncolytic virotherapy in mouse CT26 colon carcinoma model

Tutorial: design, production and testing of oncolytic viruses for cancer immunotherapy

Oncolytic viruses (OVs) represent a novel class of cancer immunotherapy agents that preferentially infect and kill cancer cells and promote protective antitumor immunity. Furthermore, OVs can be used in combination with established or upcoming immunotherapeutic agents, especially immune checkpoint inhibitors, to efficiently target a wide range of malignancies. The development of OV-based therapy involves three major steps before clinical evaluation: design, production and preclinical testing. OVs can be designed as natural or engineered strains and subsequently selected for their ability to kill a broad spectrum of cancer cells rather than normal, healthy cells. OV selection is further influenced by multiple factors, such as the availability of a specific viral platform, cancer cell permissivity, the need for genetic engineering to render the virus non-pathogenic and/or more effective and logistical considerations around the use of OVs within the laboratory or clinical setting. Selected OVs are then produced and tested for their anticancer potential by using syngeneic, xenograft or humanized preclinical models wherein immunocompromised and immunocompetent setups are used to elucidate their direct oncolytic ability as well as indirect immunotherapeutic potential in vivo. Finally, OVs demonstrating the desired anticancer potential progress toward translation in patients with cancer. This tutorial provides guidelines for the design, production and preclinical testing of OVs, emphasizing considerations specific to OV technology that determine their clinical utility as cancer immunotherapy agents.

Personalizing Oncolytic Immunovirotherapy Approaches

Development of successful cancer therapeutics requires exploration of the differences in genetics, metabolism, and interactions with the immune system among malignant and normal cells. The clinical observation of spontaneous tumor regression following natural infection with microorganism has created the premise of their use as cancer therapeutics. Oncolytic viruses (OVs) originate from viruses with attenuated virulence in humans, well-characterized vaccine strains of known human pathogens, or engineered replication-deficient viral vectors. Their selectivity is based on receptor expression level and post entry restriction factors that favor replication in the tumor, while keeping the normal cells unharmed. Clinical trials have demonstrated a wide range of patient responses to virotherapy, with subgroups of patients significantly benefiting from OV administration. Tumor-specific gene signatures, including antiviral interferon-stimulated gene (ISG) expression profile, have demonstrated a strong correlation with tumor permissiveness to infection. Furthermore, the combination of OVs with immunotherapeutics, including anticancer vaccines and immune checkpoint inhibitors [ICIs, such as anti-PD-1/PD-L1 or anti-CTLA-4 and chimeric antigen receptor (CAR)-T or CAR-NK cells], could synergistically improve the therapeutic outcome. Creating response prediction algorithms represents an important step for the transition to individualized immunovirotherapy approaches in the clinic. Integrative predictors could include tumor mutational burden (TMB), inflammatory gene signature, phenotype of tumor-infiltrating lymphocytes, tumor microenvironment (TME), and immune checkpoint receptor expression on both immune and target cells. Additionally, the gut microbiota has recently been recognized as a systemic immunomodulatory factor and could further be used in the optimization of individualized immunovirotherapy algorithms.

Checkpoint blockade meets gene therapy: Opportunities to improve response and reduce toxicity

Immune checkpoint inhibitors (ICIs) based on monoclonal antibodies represent a breakthrough for the treatment of cancer. However, their efficacy varies among tumor types and patients, and they can lead to adverse effects due to on-target/off-tumor activity, since they are administered systemically at high doses. An alternative and attractive approach for the delivery of ICIs is the use of gene therapy vectors able to express them in vivo. This review focuses on the most recent studies using viral vectors able to express ICIs locally or systemically in preclinical models of cancer. These vectors include non-replicating viruses, oncolytic viruses able to propagate specifically in tumor cells and destroy them, and self-amplifying RNA vectors, armed with different formats of antibodies against immune checkpoints. Non-replicating vectors usually lead to long-term ICI expression, potentially eliminating the need for repeated administration. Vectors with replication capacity, although they have a shorter window of expression, can induce inflammation which enhances the antitumor effect. Finally, these engineered vectors can be used in combination with other immunostimulatory molecules or with CAR-T cells, further boosting the antitumor immune responses.

Repeated dosing improves oncolytic rhabdovirus therapy in mice via interactions with intravascular monocytes

There is debate in the field of oncolytic virus (OV) therapy, whether a single viral dose, or multiple administrations, is better for tumor control. Using intravital microscopy, we describe the fate of vesicular stomatitis virus (VSV) delivered systemically as a first or a second dose. Following primary administration, VSV binds to the endothelium, initiates tumor infection and activates a proinflammatory response. This initial OV dose induces neutrophil migration into the tumor and limits viral replication. OV administered as a second dose fails to infect the tumor and is captured by intravascular monocytes. Despite a lack of direct infection, this second viral dose, in a monocyte-dependent fashion, enhances and sustains infection by the first viral dose, promotes CD8 T cell recruitment, delays tumor growth and improves survival in multi-dosing OV therapy. Thus, repeated VSV dosing engages monocytes to post-condition the tumor microenvironment for improved infection and anticancer T cell responses. Understanding the complex interactions between the subsequent viral doses is crucial for improving the efficiency of OV therapy and virus-based vaccines.

Cellular resistance to an oncolytic virus is driven by chronic activation of innate immunity

The emergence of cellular resistances to oncolytic viruses is an underexplored process that could compromise the efficacy of cancer virotherapy. Here, we isolated and characterized B16 mouse melanoma cells that evolved resistance to an oncolytic vesicular stomatitis virus (VSV-D51). RNA-seq revealed that resistance was associated to broad changes in gene expression, which typically involved chronic upregulation of interferon-stimulated genes. Innate immunity activation was maintained in the absence of the virus or other infection signals, and conferred cross-resistance to wild-type VSV and the unrelated Sindbis virus. Furthermore, we identified differentially expressed genes with no obvious role in antiviral immunity, such as Mnda, Psmb8 and Btn2a2, suggesting novel functions for these genes. Transcriptomic changes associated to VSV resistance were similar among B16 clones and in some clones derived from the mouse colon carcinoma cell line CT26, suggesting that oncolytic virus resistance involves certain conserved mechanisms and is therefore a potentially predictable process.

Oncolytic virotherapy as promising immunotherapy against cancer: mechanisms of resistance to oncolytic viruses

Oncolytic virotherapy has currently emerged as a powerful therapeutic approach in cancer treatment. Although the history of using viruses goes back to the early 20th century, the approval of talimogene laherparepvec (T-VEC) in 2015 increased interest in oncolytic viruses (OVs). OVs are multifaceted biotherapeutic agents because they replicate in and kill tumor cells and augment immune responses by releasing immunostimulatory molecules from lysed cells. Despite promising results, some limitations hinder the efficacy of oncolytic virotherapy. The delivery challenges and the upregulation of checkpoints following oncolytic virotherapy also mediate resistance to OVs by diminishing immune responses. Furthermore, the localization of receptors of viruses in the tight junctions, interferon responses, and the aberrant expression of genes involved in the cell cycle of the virus, including their infection and replication, reduce the efficacy of OVs. In this review, we present different mechanisms of resistance to OVs and strategies to overcome them.

Immuno-Oncolytic Viruses: Emerging Options in the Treatment of Colorectal Cancer

Colorectal cancer is the third most common neoplasm in the world and the third leading cause of cancer-related deaths in the USA. A safer and more effective therapeutic intervention against this malignant carcinoma is called for given the limitations and toxicities associated with the currently available treatment modalities. Immuno-oncolytic or oncolytic virotherapy, the use of viruses to selectively or preferentially kill cancer cells, has emerged as a potential anticancer treatment modality. Oncolytic viruses act as double-edged swords against the tumors through the direct cytolysis of cancer cells and the induction of antitumor immunity. A number of such viruses have been tested against colorectal cancer, in both preclinical and clinical settings, and many have produced promising results. Oncolytic virotherapy has also shown synergistic antitumor efficacy in combination with conventional treatment regimens. In this review, we describe the status of this therapeutic approach against colorectal cancer at both preclinical and clinical levels. Successes with and the challenges of using oncolytic viruses, both as monotherapy and in combination therapy, are also highlighted.

Collateral Lethal Effects of Complementary Oncolytic Viruses

Virus-infected cells release type 1 interferons, which induce an antiviral state in neighboring cells. Naturally occurring viruses are therefore equipped with stealth replication strategies to limit virus sensing and/or with combat strategies to prevent or reverse the antiviral state. Here we show that oncolytic viruses with simple RNA genomes whose spread was suppressed in tumor cells pretreated with interferon were able to replicate efficiently when the cells were coinfected with a poxvirus known to encode a diversity of innate immune combat proteins. In vivo the poxvirus was shown to reverse the intratumoral antiviral state, rescuing RNA virus replication in an otherwise restrictive syngeneic mouse tumor model leading to antitumor efficacy. Pairing of complementary oncolytic viruses is a promising strategy to enhance the antitumor activity of this novel class of anticancer drugs.

Dual-Isotope SPECT Imaging with NIS Reporter Gene and Duramycin to Visualize Tumor Susceptibility to Oncolytic Virus Infection

Noninvasive dual-imaging methods that provide an early readout on tumor permissiveness to virus infection and tumor cell death could be valuable in optimizing development of oncolytic virotherapies. Here, we have used the sodium iodide symporter (NIS) and 125I radiotracer to detect infection and replicative spread of an oncolytic vesicular stomatitis virus (VSV) in VSV-susceptible (MPC-11 tumor) versus VSV-resistant (CT26 tumor) tumors in BALB/c mice. In conjunction, tumor cell death was imaged simultaneously using technetium (99mTc)-duramycin that binds phosphatidylethanolamine in apoptotic and necrotic cells. Dual-isotope single-photon emission computed tomography/computed tomography (SPECT/CT) imaging showed areas of virus infection (NIS and 125I), which overlapped well with areas of tumor cell death (99mTc-duramycin imaging) in susceptible tumors. Multiple infectious foci arose early in MPC-11 tumors, which rapidly expanded throughout the tumor parenchyma over time. There was a dose-dependent increase in numbers of infectious centers and 99mTc-duramycin-positive areas with viral dose. In contrast, NIS or duramycin signals were minimal in VSV-resistant CT26 tumors. Combinatorial use of NIS and 99mTc-duramycin SPECT imaging for simultaneous monitoring of oncolytic virotherapy (OV) spread and the presence or absence of treatment-associated cell death could be useful to guide development of combination treatment strategies to enhance therapeutic outcome.

The lytic activity of VSV-GP treatment dominates the therapeutic effects in a syngeneic model of lung cancer

Background

Oncolytic virotherapy is thought to result in direct virus-induced lytic tumour killing and simultaneous activation of innate and tumour-specific adaptive immune responses. Using a chimeric vesicular stomatitis virus variant VSV-GP, we addressed the direct oncolytic effects and the role of anti-tumour immune induction in the syngeneic mouse lung cancer model LLC1.

Methods

To study a tumour system with limited antiviral effects, we generated interferon receptor-deficient cells (LLC1-IFNAR1^−/−). Therapeutic efficacy of VSV-GP was assessed in vivo in syngeneic C57BL/6 and athymic nude mice bearing subcutaneous tumours. VSV-GP treatment effects were analysed using bioluminescent imaging (BLI), immunohistochemistry, ELISpot, flow cytometry, multiplex ELISA and Nanostring® assays.

Results

Interferon insensitivity correlated with VSV-GP replication and therapeutic outcome. BLI revealed tumour-to-tumour spread of viral progeny in bilateral tumours. Histological and gene expression analysis confirmed widespread and rapid infection and cell killing within the tumour with activation of innate and adaptive immune-response markers. However, treatment outcome was increased in the absence of CD8⁺ T cells and surviving mice showed little protection from tumour re-challenge, indicating limited therapeutic contribution by the activated immune system.

Conclusion

These studies present a case for a predominantly lytic treatment effect of VSV-GP in a syngeneic mouse lung cancer model.

Oncolytic Viruses for Canine Cancer Treatment

Oncolytic virotherapy has been investigated for several decades and is emerging as a plausible biological therapy with several ongoing clinical trials and two viruses are now approved for cancer treatment in humans. The direct cytotoxicity and immune-stimulatory effects make oncolytic viruses an interesting strategy for cancer treatment. In this review, we summarize the results of in vitro and in vivo published studies of oncolytic viruses in different phases of evaluation in dogs, using PubMed and Google scholar as search platforms, without time restrictions (to date). Natural and genetically modified oncolytic viruses were evaluated with some encouraging results. The most studied viruses to date are the reovirus, myxoma virus, and vaccinia, tested mostly in solid tumors such as osteosarcomas, mammary gland tumors, soft tissue sarcomas, and mastocytomas. Although the results are promising, there are issues that need addressing such as ensuring tumor specificity, developing optimal dosing, circumventing preexisting antibodies from previous exposure or the development of antibodies during treatment, and assuring a reasonable safety profile, all of which are required in order to make this approach a successful therapy in dogs.

Engineered Oncolytic Poliovirus PVSRIPO Subverts MDA5-Dependent Innate Immune Responses in Cancer Cells

We are pursuing cancer immunotherapy with a neuro-attenuated recombinant poliovirus, PVSRIPO. PVSRIPO is the live attenuated type 1 (Sabin) poliovirus vaccine carrying a heterologous internal ribosomal entry site (IRES) of human rhinovirus type 2 (HRV2). Intratumoral infusion of PVSRIPO is showing promise in the therapy of recurrent WHO grade IV malignant glioma (glioblastoma), a notoriously treatment-refractory cancer with dismal prognosis. PVSRIPO exhibits profound cytotoxicity in infected neoplastic cells expressing the poliovirus receptor CD155. In addition, it elicits intriguing persistent translation and replication, giving rise to sustained type I interferon (IFN)-dominant proinflammatory stimulation of antigen-presenting cells. A key determinant of the inflammatory footprint generated by neoplastic cell infection and its role in shaping the adaptive response after PVSRIPO tumor infection is the virus's inherent relationship to the host's innate antiviral response. In this report, we define subversion of innate host immunity by PVSRIPO, enabling productive viral translation and cytopathogenicity with extremely low multiplicities of infection in the presence of an active innate antiviral IFN response.IMPORTANCE Engaging innate antiviral responses is considered key for instigating tumor-antigen-specific antitumor immunity with cancer immunotherapy approaches. However, they are a double-edged sword for attempts to enlist viruses in such approaches. In addition to their role in the transition from innate to adaptive immunity, innate antiviral IFN responses may intercept the viral life cycle in cancerous cells, prevent viral cytopathogenicity, and restrict viral spread. This has been shown to reduce overall antitumor efficacy of several proposed oncolytic virus prototypes, presumably by limiting direct cell killing and the ensuing inflammatory profile within the infected tumor. In this report, we outline how an unusual recalcitrance of polioviruses toward innate antiviral responses permits viral cytotoxicity and propagation in neoplastic cells, combined with engaging active innate antiviral IFN responses.

Combining Vascular Normalization with an Oncolytic Virus Enhances Immunotherapy in a Preclinical Model of Advanced-Stage Ovarian Cancer

PURPOSE

Intravenous delivery of oncolytic viruses often leads to tumor vascular shutdown, resulting in decreased tumor perfusion and elevated tumor hypoxia. We hypothesized that using 3TSR to normalize tumor vasculature prior to administration of an oncolytic Newcastle disease virus (NDV) would enhance virus delivery and trafficking of immunologic cell subsets to the tumor core, resulting in systemically enhanced immunotherapy and regression of advanced-stage epithelial ovarian cancer (EOC).

EXPERIMENTAL DESIGN

Using an orthotopic, syngeneic mouse model of advanced-stage EOC, we pretreated mice with 3TSR (4 mg/kg per day) alone or followed by combination with fusogenic NDV(F3aa) (1.0 × 10⁸ plaque-forming units).

RESULTS

Treatment with 3TSR normalized tumor vasculature, enhanced blood perfusion of primary EOC tumors, and induced disease regression. Animals treated with combination therapy had the greatest reduction in primary tumor mass, ascites accumulation, and secondary lesions (50% of mice were completely devoid of peritoneal metastases). Combining 3TSR + NDV(F3aa) led to enhanced trafficking of immunologic cells into the primary tumor core.

CONCLUSIONS

We have shown, for the first time, that NDV, like other oncolytic viruses, is a potent mediator of acute vascular shutdown and that preventing this through vascular normalization can promote regression in a preclinical model of advanced-stage ovarian cancer. This challenges the current focus on induction of intravascular thrombosis as a requisite for successful oncolytic virotherapy.See related commentary by Bykov and Zamarin, p. 1446.

Oncolytic Viruses as Antigen-Agnostic Cancer Vaccines

Selective destruction of neoplastic tissues by oncolytic viruses (OVs) leads to antigen-agnostic boosting of neoantigen-specific cytotoxic T lymphocyte (CTL) responses, making OVs ideal companions for checkpoint blockade therapy. Here we discuss the mechanisms whereby OVs modulate both adjuvanticity and antigenicity of tumor cells. Suppression of antitumor immunity after OV therapy has not been observed, possibly because viral antigen expression diminishes as the antiviral response matures, thereby progressively honing the CTL response to tumor neoantigens. By combining direct in situ tumor destruction with the ability to boost antitumor immunity, OVs also have the potential to be powerful standalone cancer therapies.

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.

The Oncolytic Virus VSV-GP Is Effective against Malignant Melanoma

Previously, we described VSV-GP, a modified version of the vesicular stomatitis virus, as a non-neurotoxic oncolytic virus that is effective for the treatment of malignant glioblastoma and ovarian cancer. Here, we evaluate the therapeutic efficacy of VSV-GP for malignant melanoma. All of the human, mouse, and canine melanoma cell lines that were tested, alongside most primary human melanoma cultures, were infected by VSV-GP and efficiently killed. Additionally, we found that VSV-GP prolonged the survival of mice in both a xenograft and a syngeneic mouse model. However, only a few mice survived with long-term tumor remission. When we analyzed the factors that might limit VSV-GP's efficacy, we found that vector-neutralizing antibodies did not play a role in this context, as even after eight subsequent immunizations and an observation time of 42 weeks, no vector-neutralizing antibodies were induced in VSV-GP immunized mice. In contrast, the type I IFN response might have contributed to the reduced efficacy of the therapy, as both of the cell lines that were used for the mouse models were able to mount a protective IFN response. Nevertheless, early treatment with VSV-GP also reduced the number and size of lung metastases in a syngeneic B16 mouse model. In summary, VSV-GP is a potent candidate for the treatment of malignant melanoma; however, factors limiting the efficacy of the virus need to be further explored.

Multi-modal Potentiation of Oncolytic Virotherapy by Vanadium Compounds

Oncolytic viruses (OV) are an emerging class of anticancer bio-therapeutics that induce antitumor immunity through selective replication in tumor cells. However, the efficacy of OVs as single agents remains limited. We introduce a strategy that boosts the therapeutic efficacy of OVs by combining their activity with immuno-modulating, small molecule protein tyrosine phosphatase inhibitors. We report that vanadium-based phosphatase inhibitors enhance OV infection in vitro and ex vivo, in resistant tumor cell lines. Furthermore, vanadium compounds increase antitumor efficacy in combination with OV in several syngeneic tumor models, leading to systemic and durable responses, even in models otherwise refractory to OV and drug alone. Mechanistically, this involves subverting the antiviral type I IFN response toward a death-inducing and pro-inflammatory type II IFN response, leading to improved OV spread, increased bystander killing of cancer cells, and enhanced antitumor immune stimulation. Overall, we showcase a new ability of vanadium compounds to simultaneously maximize viral oncolysis and systemic anticancer immunity, offering new avenues for the development of improved immunotherapy strategies.

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.

Oncolytic alphavirus SFV-VA7 efficiently eradicates subcutaneous and orthotopic human prostate tumours in mice

Background:

Despite recent therapeutic and diagnostic advances, prostate cancer remains the second leading cause of cancer-related deaths among men in the Western world. Oncolytic viruses that replicate selectively in tumour cells represent a novel treatment candidate for these malignancies.

Methods:

We analysed infectivity of avirulent Semliki Firest virus SFV-VA7 in human prostate cancer cell lines VCaP, LNCaP and 22Rv1 and in nonmalignant prostate epithelial cell line RWPE-1. Therapeutic potency of SFV-VA7 was evaluated in subcutaneous and orthotopic mouse LNCaP xenograft models.

Results:

SFV-VA7 infected and killed the tested human prostate cancer cell lines irrespective of their hormone response status, while the nonmalignant prostate epithelial cell line RWPE-1 proved highly virus resistant. Notably, a single peritoneal dose of SFV-VA7 was sufficient to eradicate all subcutaneous and orthotopic LNCaP tumours.

Conclusions:

Our results indicate that SFV-VA7 is a novel, promising therapeutic virus against prostate cancer warranting further testing in early clinical trials.

Oncolytic Alphaviruses in Cancer Immunotherapy

Oncolytic viruses show specific targeting and killing of tumor cells and therefore provide attractive assets for cancer immunotherapy. In parallel to oncolytic viral vectors based on adenoviruses and herpes simplex viruses, oncolytic RNA viruses and particularly alphaviruses have been evaluated as delivery vehicles. Immunization studies in experimental rodent models for various cancers including glioblastoma, hematologic, hepatocellular, colon, cervix, and lung cancer as well as melanoma have been conducted with naturally occurring oncolytic alphavirus strains such as M1 and Sindbis AR339. Moreover, animals were vaccinated with engineered oncolytic replication-deficient and -competent Semliki Forest virus, Sindbis virus and Venezuelan equine encephalitis virus vectors expressing various antigens. Vaccinations elicited strong antibody responses and resulted in tumor growth inhibition, tumor regression and even complete tumor eradication. Vaccination also led to prolonged survival in several animal models. Furthermore, preclinical evaluation demonstrated both prophylactic and therapeutic efficacy of oncolytic alphavirus administration. Clinical trials in humans have mainly been limited to safety studies so far.

Application of interferon modulators to overcome partial resistance of human ovarian cancers to VSV-GP oncolytic viral therapy

Previously, we described an oncolytic vesicular stomatitis virus variant pseudotyped with the nonneurotropic glycoprotein of the lymphocytic choriomeningitis virus, VSV-GP, which was highly effective in glioblastoma. Here, we tested its potency for the treatment of ovarian cancer, a leading cause of death from gynecological malignancies. Effective oncolytic activity of VSV-GP could be demonstrated in ovarian cancer cell lines and xenografts in mice; however, remission was temporary in most mice. Analysis of the innate immune response revealed that ovarian cancer cell lines were able to respond to and produce type I interferon, inducing an antiviral state upon virus infection. This is in stark contrast to published data for other cancer cell lines, which were mostly found to be interferon incompetent. We showed that in vitro this antiviral state could be reverted by combining VSV-GP with the JAK1/2-inhibitor ruxolitinib. In addition, for the first time, we report the in vivo enhancement of oncolytic virus treatment by ruxolitinib, both in subcutaneous as well as in orthotopic xenograft mouse models, without causing significant additional toxicity. In conclusion, VSV-GP has the potential to be a potent and safe oncolytic virus to treat ovarian cancer, especially when combined with an inhibitor of the interferon response.

Safe and Effective Treatment of Experimental Neuroblastoma and Glioblastoma Using Systemically Delivered Triple MicroRNA-Detargeted Oncolytic Semliki Forest Virus

Background: Glioblastoma multiforme and high-risk neuroblastoma are cancers with poor outcome. Immunotherapy in the form of neurotropic oncolytic viruses is a promising therapeutic approach for these malignancies. Here we evaluate the oncolytic capacity of the neurovirulent and partly IFNβ-resistant Semliki Forest virus (SFV)-4 in glioblastoma multiformes and neuroblastomas. To reduce neurovirulence we constructed SFV4miRT, which is attenuated in normal central nervous system (CNS) cells through insertion of microRNA target sequences for miR124, miR125, miR134.Methods: Oncolytic activity of SFV4miRT was examined in mouse neuroblastoma and glioblastoma multiforme cell lines and in patient-derived human glioblastoma cell cultures (HGCC). In vivo neurovirulence and therapeutic efficacy was evaluated in two syngeneic orthotopic glioma models (CT-2A, GL261) and a syngeneic subcutaneous neuroblastoma model (NXS2). The role of IFNβ in inhibiting therapeutic efficacy was investigated.Results: The introduction of miRNA target sequences reduced neurovirulence of SFV4 in terms of attenuated replication in mouse CNS cells and ability to cause encephalitis when administered intravenously. A single intravenous injection of SFV4miRT prolonged survival and cured four of eight mice (50%) with NXS2 and three of 11 mice (27%) with CT-2A, but not for GL261 tumor-bearing mice. In vivo therapeutic efficacy in different tumor models inversely correlated to secretion of IFNβ by respective cells upon SFV4 infection in vitro Similarly, killing efficacy of HGCC lines inversely correlated to IFNβ response and interferon-α/β receptor-1 expression.Conclusions: SFV4miRT has reduced neurovirulence, while retaining its oncolytic capacity. SFV4miRT is an excellent candidate for treatment of glioblastoma multiforme and neuroblastoma with low IFN-β secretion. Clin Cancer Res; 23(6); 1519-30. ©2016 AACR.

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.

STAT1 and NF-κB Inhibitors Diminish Basal Interferon-Stimulated Gene Expression and Improve the Productive Infection of Oncolytic HSV in MPNST Cells

Interferon-stimulated genes (ISG) encode diverse proteins that mediate intrinsic antiviral resistance in infected cells. Here it was hypothesized that malignant peripheral nerve sheath tumor (MPNST) cells resist the productive infection of oncolytic herpes simplex virus (oHSV) through activation of the JAK/STAT1 pathway and resultant upregulation of ISGs. Multiple human and mouse MPNST cells were used to explore the relationship between STAT1 activation and the productive infection of Δγ134.5 oHSVs. STAT1 activation in response to oHSV infection was found to associate with diminished Δγ134.5 oHSVs replication and spread. Multiday pretreatment, but not cotreatment, with a JAK inhibitor significantly improved viral titer and spread. ISG expression was found to be elevated prior to infection and downregulated when treated with the inhibitor, suggesting that the JAK/STAT1 pathway is active prior to infection. Conversely, upregulation of ISG expression in normally permissive cells significantly decreased oHSV productivity. Finally, a possible link between NF-κB pathway activation and ISG expression was established through the expression of inhibitor of kB (IκB) which decreased basal STAT1 transcription and ISG expression. These results demonstrate that basal ISG expression prior to infection contributes to the resistance of Δγ134.5 oHSVs in MPNST cells.

IMPLICATIONS

Although cancer-associated ISG expression has been previously reported to impart resistance to chemotherapy and radiotherapy, these data show that basal ISG expression also contributes to oncolytic HSV resistance. Mol Cancer Res; 14(5); 482-92. ©2016 AACR.

Big Data Offers Novel Insights for Oncolytic Virus Immunotherapy

Large-scale assays, such as microarrays, next-generation sequencing and various “omics” technologies, have explored multiple aspects of the immune response following virus infection, often from a public health perspective. Yet a lack of similar data exists for monitoring immune engagement during oncolytic virus immunotherapy (OVIT) in the cancer setting. Tracking immune signatures at the tumour site can create a snapshot or longitudinally analyse immune cell activation, infiltration and functionality within global populations or individual cells. Mapping immune changes over the course of oncolytic biotherapy—from initial infection to tumour stabilisation/regression through to long-term cure or escape/relapse—has the potential to generate important therapeutic insights around virus-host interactions. Further, correlating such immune signatures with specific tumour outcomes has significant value for guiding the development of novel oncolytic virus immunotherapy strategies. Here, we provide insights for OVIT from large-scale analyses of immune populations in the infection, vaccination and immunotherapy setting. We analyse several approaches to manipulating immune engagement during OVIT. We further explore immunocentric changes in the tumour tissue following immunotherapy, and compile several immune signatures of therapeutic success. Ultimately, we highlight clinically relevant large-scale approaches with the potential to strengthen future oncolytic strategies to optimally engage the immune system.

To Infection and Beyond: The Multi-Pronged Anti-Cancer Mechanisms of Oncolytic Viruses

Over the past 1–2 decades we have witnessed a resurgence of efforts to therapeutically exploit the attributes of lytic viruses to infect and kill tumor cells while sparing normal cells. We now appreciate that the utility of viruses for treating cancer extends far beyond lytic cell death. Viruses are also capable of eliciting humoral and cellular innate and adaptive immune responses that may be directed not only at virus-infected cells but also at uninfected cancer cells. Here we review our current understanding of this bystander effect, and divide the mechanisms into lytic, cytokine, innate cellular, and adaptive phases. Knowing the key pathways and molecular players during virus infection in the context of the cancer microenvironment will be critical to devise strategies to maximize the therapeutic effects of oncolytic viroimmunotherapy.

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.

MicroRNA-Attenuated Clone of Virulent Semliki Forest Virus Overcomes Antiviral Type I Interferon in Resistant Mouse CT-2A Glioma

Glioblastoma is a terminal disease with no effective treatment currently available. Among the new therapy candidates are oncolytic viruses capable of selectively replicating in cancer cells, causing tumor lysis and inducing adaptive immune responses against the tumor. However, tumor antiviral responses, primarily mediated by type I interferon (IFN-I), remain a key problem that severely restricts viral replication and oncolysis. We show here that the Semliki Forest virus (SFV) strain SFV4, which causes lethal encephalitis in mice, is able to infect and replicate independent of the IFN-I defense in mouse glioblastoma cells and cell lines originating from primary human glioblastoma patient samples. The ability to tolerate IFN-I was retained in SFV4-miRT124 cells, a derivative cell line of strain SFV4 with a restricted capacity to replicate in neurons due to insertion of target sites for neuronal microRNA 124. The IFN-I tolerance was associated with the viral nsp3-nsp4 gene region and distinct from the genetic loci responsible for SFV neurovirulence. In contrast to the naturally attenuated strain SFV A7(74) and its derivatives, SFV4-miRT124 displayed increased oncolytic potency in CT-2A murine astrocytoma cells and in the human glioblastoma cell lines pretreated with IFN-I. Following a single intraperitoneal injection of SFV4-miRT124 into C57BL/6 mice bearing CT-2A orthotopic gliomas, the virus homed to the brain and was amplified in the tumor, resulting in significant tumor growth inhibition and improved survival.

IMPORTANCE

Although progress has been made in development of replicative oncolytic viruses, information regarding their overall therapeutic potency in a clinical setting is still lacking. This could be at least partially dependent on the IFN-I sensitivity of the viruses used. Here, we show that the conditionally replicating SFV4-miRT124 virus shares the IFN-I tolerance of the pathogenic wild-type SFV, thereby allowing efficient targeting of a glioma that is refractory to naturally attenuated therapy vector strains sensitive to IFN-I. This is the first evidence of orthotopic syngeneic mouse glioma eradication following peripheral alphavirus administration. Our findings indicate a clear benefit in harnessing the wild-type virus replicative potency in development of next-generation oncolytic alphaviruses.

Attenuated Semliki Forest virus for cancer treatment in dogs: safety assessment in two laboratory Beagles

Background

Dogs suffer from spontaneous tumors which may be amenable to therapies developed for human cancer patients, and dogs may serve as large-animal cancer models. A non-pathogenic Semliki Forest virus vector VA7-EGFP previously showed promise in targeting human tumor xenografts in mice, but the oncolytic capacity of the virus in canine cancer cells and the safety of the virus in higher mammals such as dogs, are not known. We therefore assessed the oncolytic potency of VA7-EGFP against canine cancer cells by infectivity and viability assays in two dog solid tumor cell lines. Furthermore we performed a 3-week safety study in two adult Beagles which received a single intravenous injection of ~2 × 10⁵ plaque forming units of parental A7(74) strain.

Results

VA7-EGFP was able to replicate in and kill both canine cancer cell lines tested. No adverse events were observed in either of the two virus-injected adult Beagles and no infective virus could be recovered from any of the biological samples collected over the course of the study. Neutralizing antibodies to Semliki Forest virus became detectable in the dogs at 5 days post infection and remained elevated until study termination.

Conclusions

Based on these results, testing of the oncolytic potential of attenuated Semliki Forest virus in canine cancer patients appears feasible.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-015-0498-2) contains supplementary material, which is available to authorized users.