Antiangiogenic arming of an oncolytic vaccinia virus enhances antitumor efficacy in renal cell cancer models

VEGF-Virus Interactions: Pathogenic Mechanisms and Therapeutic Applications

Many types of viruses directly or indirectly target the vascular endothelial growth factor (VEGF) system, which is a central regulator of vasculogenesis and angiogenesis in physiological homeostasis, causing diverse pathologies. Other viruses have been developed into effective therapeutic tools for VEGF modulation in conditions such as cancer and eye diseases. Some viruses may alter the levels of VEGF in the pathogenesis of respiratory syndromes, or they may encode VEGF-like factors, promoting vascular disruption and angiogenesis to enable viruses' systemic spread. Oncogenic viruses may express interactive factors that perturb VEGF's functional levels or downstream signaling, which increases the neovascularization and metastasis of tumors. Furthermore, many viruses are being developed as therapeutic vectors for vascular pathologies in clinical trials. Major examples are those viral vectors that inhibit the role of VEGF in the neovascularization required for cancer progression; this is achieved through the induction of immune responses, by exposing specific peptides that block signaling or by expressing anti-VEGF and anti-VEGF receptor-neutralizing antibodies. Other viruses have been engineered into effective pro- or anti-angiogenesis multitarget vectors for neovascular eye diseases, paving the way for therapies with improved safety and minimal side effects. This article critically reviews the large body of literature on these issues, highlighting those contributions that describe the molecular mechanisms, thus expanding our understanding of the VEGF-virus interactions in disease and therapy. This could facilitate the clinical use of therapeutic virus vectors in precision medicine for the VEGF system.

Enhanced Antitumor Efficacy of Oncolytic Vaccinia Virus Therapy Through Keratin-Mediated Delivery in Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) represents an aggressive subtype characterized by high rates of recurrence and metastasis, necessitating the exploration of alternative treatment strategies. Oncolytic vaccinia virus (OVV) therapy has emerged as a promising approach, selectively infecting and destroying tumor cells. However, its efficacy is often hampered by inadequate viral distribution within the tumor microenvironment. Here, we investigate the potential of keratin (KTN) as a carrier for OVV delivery to enhance viral distribution and antitumor efficacy. In vitro assays revealed that KTN significantly improves OVV stability, leading to increased tumor cell apoptosis and necrosis. Furthermore, KTN effectively inhibits cancer cell migration by suppressing the epithelial-mesenchymal transition (EMT) process and downregulating metastasis-related proteins. These findings are corroborated in a syngeneic TNBC mouse model, where KTN-mediated OVV delivery enhances cytotoxic T cell-mediated antitumor immune responses without compromising the anti-angiogenic effects of the virus. Notably, KTN alone exhibits antitumor effects by suppressing tumor growth and metastasis, underscoring its potential as a standalone therapeutic agent. In conclusion, our study underscores the promise of KTN-mediated OVV delivery as a promising therapeutic strategy for TNBC. By improving viral distribution, suppressing EMT, and enhancing antitumor immunity, this approach holds significant potential for enhancing patient outcomes in TNBC treatment. Further investigation is warranted to explore the broader utility of KTN in various cancer therapy approaches.

Oncolytic viruses and antibodies: are they more successful when delivered separately or when engineered as a single agent?

Oncolytic viruses (OVs) provide the promise of tumor-selective cytotoxicity coupled with amplification of the therapeutic agent (the virus) in situ within the tumor improving its therapeutic index. Despite this promise, however, single agent-treatments have not been as successful as combination therapies, particularly combining with checkpoint inhibitor antibodies. The antibodies may be delivered by two approaches, either encoded within the OV genome to restrict antibody production to sites of active virus infection or alternatively given alongside OVs as separate treatments. Both approaches have shown promising therapeutic outcomes, and this leads to an interesting question of whether one approach is potentially better than the other. In this review, we provide a brief summary of the combination OV-antibody therapies that target tumor cells, tumor microenvironment and immune cells to help define key parameters influencing which approach is superior, thereby improving insight into the rational design of OV treatment strategies.

Recombinant Strains of Oncolytic Vaccinia Virus for Cancer Immunotherapy

Cancer virotherapy is an alternative therapeutic approach based on the viruses that selectively infect and kill tumor cells. Vaccinia virus (VV) is a member of the Poxviridae, a family of enveloped viruses with a large linear double-stranded DNA genome. The proven safety of the VV strains as well as considerable transgene capacity of the viral genome, make VV an excellent platform for creating recombinant oncolytic viruses for cancer therapy. Furthermore, various genetic modifications can increase tumor selectivity and therapeutic efficacy of VV by arming it with the immune-modulatory genes or proapoptotic molecules, boosting the host immune system, and increasing cross-priming recognition of the tumor cells by T-cells or NK cells. In this review, we summarized the data on bioengineering approaches to develop recombinant VV strains for enhanced cancer immunotherapy.

Therapeutic Efficacy of Oncolytic Viruses in Fighting Cancer: Recent Advances and Perspective

Immunotherapy is at the cutting edge of modern cancer treatment. Innovative medicines have been developed with varying degrees of success that target all aspects of tumor biology: tumors, niches, and the immune system. Oncolytic viruses (OVs) are a novel and potentially immunotherapeutic approach for cancer treatment. OVs reproduce exclusively in cancer cells, causing the tumor mass to lyse. OVs can also activate the immune system in addition to their primary activity. Tumors create an immunosuppressive environment by suppressing the immune system’s ability to respond to tumor cells. By injecting OVs into the tumor, the immune system is stimulated, allowing it to generate a robust and long-lasting response against the tumor. The essential biological properties of oncolytic viruses, as well as the underlying mechanisms that enable their usage as prospective anticancer medicines, are outlined in this review. We also discuss the increased efficacy of virotherapy when combined with other cancer medications.

A comprehensive preclinical study supporting clinical trial of oncolytic chimeric poxvirus CF33-hNIS-anti-PD-L1 to treat breast cancer

CF33-hNIS-anti-PD-L1 is an oncolytic chimeric poxvirus encoding two transgenes: human sodium iodide symporter and a single-chain variable fragment against PD-L1. Comprehensive preclinical pharmacology studies encompassing primary and secondary pharmacodynamics and biodistribution and safety studies were performed to support the clinical development of CF33-hNIS-anti-PD-L1. Most of the studies were performed in triple-negative breast cancer (TNBC) models, as the phase I trial is planned for patients with TNBC. Biological functions of virus-encoded transgenes were confirmed, and the virus demonstrated anti-tumor efficacy against TNBC models in mice. In a good laboratory practice (GLP) toxicology study, the virus did not produce any observable adverse effects in mice, suggesting that the doses proposed for the clinical trial should be well tolerated in patients. Furthermore, no neurotoxic effects in mice were seen following intracranial injection of the virus. Also, the risk for horizontal transmission of CF33-hNIS-anti-PD-L1 was assessed in mice, and our results suggest that the virus is unlikely to transmit from infected patients to healthy individuals. Finally, the in-use stability and compatibility of CF33-hNIS-anti-PD-L1 tested under different conditions mimicking the clinical scenarios confirmed the suitability of the virus in clinical settings. The results of these preclinical studies support the use of CF33-hNIS-anti-PD-L1 in a first-in-human trial in patients with TNBC.

Multifaceted applications of genetically modified microorganisms: A biotechnological revolution

BACKGROUND

Genetically modified microorganisms specifically bacteria, viruses, algae and fungi are the novel approaches used in field of healthcare due to more efficacious and targeted delivery in comparison to conventional approaches.

OBJECTIVE

This review article focuses on applications of genetically modified microorganisms such as bacteria, virus, fungi, virus, etc. in treatment of cancer, obesity, and HIV. Gut microbiome is used to cause metabolic disorders but use of genetically-modified bacteria alters the gut microbiota and delivers the therapeutically effective drug in the treatment of obesity.

METHODS

To enhance the activity of different microorganisms for treatment, they are genetically modified by incorporating a fragment into the fungi filaments, integrating a strain into the bacteria, engineer a live-virus with a peptide using methods such as amelioration of NAPE synthesis, silica immobilization, polyadenylation, electrochemical, etc. Results: The development of newer microbial strains using genetic modifications offers higher precision, enhance the molecular multiplicity, prevent the degradation of microbes in atmospheric temperature and reduce the concerned side-effect for therapeutic application. Other side genetically modified microorganisms are used in non-healthcare based sector like generation of electricity, purification of water, bioremediation process etc. Conclusions: The bio-engineered micro-organisms with genetic modification prove the advantage over the treatment of various diseases like cancer, diabetes, malaria, organ regeneration, inflammatory bowel disease, etc. The article provides the insights of various applications of genetically modified microbes in various arena with its implementation for the regulatory approval.

Beyond cancer cells: Targeting the tumor microenvironment with gene therapy and armed oncolytic virus

Cancer gene therapies are usually designed either to express wild-type copies of tumor suppressor genes or to exploit tumor-associated phenotypic changes to endow selective cytotoxicity. However, these approaches become less relevant to cancers that contain many independent mutations, and the situation is made more complex by our increased understanding of clonal evolution of tumors, meaning that different metastases and even regions of the same tumor mass have distinct mutational and phenotypic profiles. In contrast, the relatively genetically stable tumor microenvironment (TME) therefore provides an appealing therapeutic target, particularly since it plays an essential role in promoting cancer growth, immune tolerance, and acquired resistance to many therapies. Recently, a variety of different TME-targeted gene therapy and armed oncolytic strategies have been explored, with particular success observed in strategies targeting the cancer stroma, reducing tumor vasculature, and repolarizing the immunosuppressive microenvironment. Herein, we review the progress of these TME-targeting approaches and try to highlight those showing the greatest promise.

Current Trends in Cancer Immunotherapy

The search for an effective drug to treat oncological diseases, which have become the main scourge of mankind, has generated a lot of methods for studying this affliction. It has also become a serious challenge for scientists and clinicians who have needed to invent new ways of overcoming the problems encountered during treatments, and have also made important discoveries pertaining to fundamental issues relating to the emergence and development of malignant neoplasms. Understanding the basics of the human immune system interactions with tumor cells has enabled new cancer immunotherapy strategies. The initial successes observed in immunotherapy led to new methods of treating cancer and attracted the attention of the scientific and clinical communities due to the prospects of these methods. Nevertheless, there are still many problems that prevent immunotherapy from calling itself an effective drug in the fight against malignant neoplasms. This review examines the current state of affairs for each immunotherapy method, the effectiveness of the strategies under study, as well as possible ways to overcome the problems that have arisen and increase their therapeutic potentials.

Novel Chimeric Poxvirus CF17 Improves Survival in a Murine Model of Intraperitoneal Ovarian Cancer Metastasis

Despite improvements in surgical techniques and chemotherapy, ovarian cancer remains the most lethal gynecologic cancer. Thus, there is an urgent need for more effective therapeutics, particularly for chemo-resistant peritoneal ovarian cancer metastases. Oncolytic virotherapy represents an innovative treatment paradigm; however, for oncolytic viruses tested from the last generation of genetically engineered viruses, the therapeutic benefits have been modest. To overcome these limitations, we generated a chimeric poxvirus, CF17, through the chimerization of nine species of orthopoxviruses. Compared with its parental viruses, CF17 has demonstrated superior oncolytic characteristics. Here, we report the oncolytic potential of CF17 in ovarian cancer. Replication of CF17 and its resulting cytotoxicity were observed at multiplicities of infection (MOIs) as low as 0.001 in human and mouse cancer cell lines in vitro. Furthermore, CF17 exerted potent antitumor effects in a syngeneic mouse model of ovarian cancer at doses as low as 6 × 10⁶ plaque-forming units. Together, these data merit further investigation of the potential use of this novel chimeric poxvirus as an effective treatment for aggressive intraperitoneal ovarian cancer.

Simultaneous Tumor and Stroma Targeting by Oncolytic Viruses

Current cancer therapeutics often insufficiently eradicate malignant cells due to the surrounding dense tumor stroma. This multi-componential tissue consists of mainly cancer-associated fibroblasts, the (compact) extracellular matrix, tumor vasculature, and tumor-associated macrophages, which all exert crucial roles in maintaining a pro-tumoral niche. Their continuous complex interactions with tumor cells promote tumor progression and metastasis, emphasizing the challenges in tumor therapy development. Over the last decade, advances in oncolytic virotherapy have shown that oncolytic viruses (OVs) are a promising multi-faceted therapeutic platform for simultaneous tumor and stroma targeting. In addition to promoting tumor cell oncolysis and systemic anti-tumor immunity, accumulating data suggest that OVs can also directly target stromal components, facilitating OV replication and spread, as well as promoting anti-tumor activity. This review provides a comprehensive overview of the interactions between native and genetically modified OVs and the different targetable tumor stromal components, and outlines strategies to improve stroma targeting by OVs.

Human SNF5 arming of double-deleted vaccinia virus shows oncolytic and cytostatic activity against central nervous system atypical teratoid/rhabdoid tumor cells

Central nervous system (CNS) atypical teratoid/rhabdoid tumor (AT/RT) is a rare, aggressive tumor that most often affects very young children. The common decisive molecular defect in AT/RT has been shown to be a single genetic alteration, i.e., the loss of hSNF5 gene that encodes for a subunit of the SWI/SNF complex that modulates chromatin remodeling activities. As a result, AT/RT cells display unregulated cell proliferation due to the dysfunction of an important epigenetic control. We have previously demonstrated the preclinical efficacy of the oncolytic double-deleted vaccinia virus (VVDD) against AT/RT. Here we report the establishment of a modified VVDD engineered to express wild type hSNF5 gene. We show that this reconstructed vaccinia virus retains comparable infectivity and in vitro cytotoxicity of the parent strain. However, in addition, hSNF5-arming of VVDD results in a decreased cell cycle S phase population and down-regulation of cyclin D1. These findings suggest that hSNF5-arming of VVDD may increase the efficacy in the treatment of AT/RT and validates, as a proof-of-concept, an experimental approach to enhance the effective use of novel modified oncolytic viruses in the treatment of tumors with loss of a tumor suppressor gene function.

Engineering Oncolytic Vaccinia Virus to redirect Macrophages to Tumor Cells

Oncolytic virotherapy has been tested in numerous early phase clinical studies. However, the antitumor activity of oncolytic viruses thus far has been limited. Numerous strategies are being explored to enhance their antitumor activity by activating the adaptive arm of the immune system. We reasoned that it might also be possible to engineer oncolytic viruses to redirect tumor-associated macrophages to tumor cells for therapeutic benefit. We engineered an oncolytic vaccinia virus (VV) to disrupt the CD47/SIRPα interaction by expressing a chimeric molecule that consists of the ectodomain of SIRPα and the Fc domain of IgG4 (SIRPα-Fc-VV). SIRPα-Fc-VV readily replicated in tumor cells and redirected M1 as well as M2 macrophages to tumor cells in vitro. In contrast, control VVs that either encoded YFP (YFP-VV) or SIRPα (SIRPα-VV) did not. In vivo, SIRPα-Fc-VV had greater antitumor activity than YFP-VV and SIRPα-VV in an immune competent osteosarcoma model resulting in a significant survival advantage. Pretreatment with cytoxan further augmented the antitumor activity of SIRPα-Fc-VV. Thus, arming oncolytic viruses with SIRPα-Fc may present a promising strategy to enhance their antitumor activity for the virotherapy of solid tumors.

Novel Oncolytic Virus Armed with Cancer Suicide Gene and Normal Vasculogenic Gene for Improved Anti-Tumor Activity

Here, we developed a novel oncolytic vaccinia virus (NOV) with the dual advantages of cancer selectivity and normal vessel reconstructive activity by replacing the viral thymidine kinase (vTk) and vaccinia growth factor (VGF) genes with genes encoding TNF-related apoptosis-inducing ligand (TRAIL) and angiopoietin 1 (Ang1), respectively. The pan-cancer-specific oncolytic potency of NOV was confirmed in various human and mouse cancer cell lines (colon, liver, pancreas, cholangiocarcinoma, cervical cancer, osteosarcoma, and melanoma). Vaccinia virus (VV) treatment directly induced early apoptosis in tumors within 24 h, and this effect was enhanced with further engineering; VGF and Tk deletion with Ang1 and TRAIL insertion. Meanwhile, treatment with the conventional anti-cancer drug cisplatin did not induce apoptosis. A virus-treated CT26 mouse colon cancer syngeneic model showed attenuated tumor growth, which was in accordance with the results of percent survival measurement, CD8 expression analysis, and TUNEL staining with advanced genetic engineering (vAng1 < vTRAIL < NOV). Taken together, our results indicate that NOV induces cancer tissue apoptosis and anti-tumor immunity and may constitute a highly advantageous therapeutic agent for next-generation solid tumor virotherapy with pan-cancer-specific oncolytic activity and high biosafety.

Oncolytic Viruses for Cancer Therapy: Barriers and Recent Advances

Oncolytic viruses (OVs) are powerful new therapeutic agents in cancer therapy. With the first OV (talimogene laherparepvec [T-vec]) obtaining US Food and Drug Administration approval, interest in OVs has been boosted greatly. Nevertheless, despite extensive research, oncolytic virotherapy has shown limited efficacy against solid tumors. Recent advances in viral retargeting, genetic editing, viral delivery platforms, tracking strategies, OV-based gene therapy, and combination strategies have the potential to broaden the applications of oncolytic virotherapy in oncology. In this review, we present several insights into the limitations and challenges of oncolytic virotherapy, describe the strategies mentioned above, provide a summary of recent preclinical and clinical trials in the field of oncolytic virotherapy, and highlight the need to optimize current strategies to improve clinical outcomes.

A chimeric poxvirus with J2R (thymidine kinase) deletion shows safety and anti-tumor activity in lung cancer models

Oncolytic viruses have shown excellent safety profiles in preclinical and clinical studies; however, in most cases therapeutic benefits have been modest. We have previously reported the generation of a chimeric poxvirus (CF33), with significantly improved oncolytic characteristics, through chimerization among different poxviruses. Here we report the sequence analysis of CF33 and oncolytic potential of a GFP-encoding CF33 virus (CF33-GFP) with a J2R deletion in lung cancer models. Replication of CF33-GFP and the resulting cytotoxicity were higher in cancer cell lines compared to a normal cell line, in vitro. After infection with virus, cancer cells expressed markers for immunogenic cell death in vitro. Furthermore, CF33-GFP was safe and exerted potent anti-tumor effects at a dose as low as 1000 plaque forming units in both virus-injected and un-injected distant tumors in A549 tumor xenograft model in mice. Likewise, in a syngeneic model of lung cancer in mice, the virus showed significant anti-tumor effect and was found to increase tumor infiltration by CD8+ T cells. Collectively, these data warrant further investigation of this novel chimeric poxvirus for its potential use as a cancer bio-therapeutic.

Synergistic suppression effect on tumor growth of acute myeloid leukemia by combining cytarabine with an engineered oncolytic vaccinia virus

BACKGROUND

In consideration of the drug resistance and side effects associated with cytarabine, one of the most effective drugs for the treatment of acute myeloid leukemia (AML), there is a need for safer and effective strategies.

METHODS

In the present investigation, we fabricated a new oncolytic vaccinia virus (oVV-ING4), which expresses the inhibitor of growth family member 4 (ING4) and explored its antitumor activity individually and in combination with cytarabine in AML cells.

RESULTS

The experiments confirmed that oVV can efficiently and specifically infect leukemia cells, and augment the ING4 gene expression. Flow cytometry and western blot demonstrated that oVV-ING4 enhances apoptosis and G2/M phase arrest in AML cells, and causes remarkable cancer cell death. In addition, the synergistic efficiency of oVV-ING4 and cytarabine was investigated in vitro and in vivo; the combination significantly inhibited the survival of leukemia cells in vitro and xenografted KG-1 AML tumor growth in vivo.

CONCLUSION

In brief, oVV-ING4 can increase the sensitivity of leukemia cells to cytarabine and induce cell apoptosis in vitro and in vivo. Thus, oVV-ING4 may be a promising therapeutic candidate for leukemia and in combination with cytarabine represents a potential antitumor therapy.

Advances in the mechanisms of action of cancer-targeting oncolytic viruses

Cancer virotherapy mediated by oncolytic viruses (OV), has emerged as a novel and effective strategy in cancer therapeutics. Preclinical models have demonstrated anticancer activity against numerous types of cancer. Currently, a number of recombinant viruses are in late phase clinical trials, many of which have demonstrated promising results regarding the safety and reliability of the treatments, particularly when combined with standard antineoplastic therapies. In addition to molecular-targeted therapeutics, genetic engineering of the viruses allows functional complementation to chemotherapy or radiotherapy agents. Co-administration of chemotherapy or radiotherapy is imperative for an effective treatment regime. Additionally, these approaches may be used in combination with current treatments to assist in cancer management. The near future may reveal whether this renewed interest in oncological virotherapy will result in meaningful therapeutic effects in patients. The aim of the present review was to highlight how the knowledge of oncolytic viral specificity and cytotoxicity has advanced in recent years, with a view to discuss OV in clinical application and the future directions of this field.

Activating Peripheral Innate Immunity Enables Safe and Effective Oncolytic Virotherapy in the Brain

The oncolytic mutant vesicular stomatitis virus VSVΔ51 achieves robust efficacy in multiple extracranial tumor models. Yet for malignancies of the brain, direct intratumoral infusion of VSVΔ51 causes lethal virus-induced neuropathology. Here, we have developed a novel therapeutic regime that uses peripheral immunization with a single sub-lethal dose of VSVΔ51 to establish an acute anti-viral state that enables the safe intracranial (IC) infusion of an otherwise lethal dose of VSVΔ51 within just 6 hr. Although type I interferons alone appeared insufficient to explain this protective phenotype, serum isolated at early time points from primed animals conferred protection against an IC dose of virus. Adaptive immune populations had minimal contributions. Finally, the therapeutic utility of this novel strategy was demonstrated by peripherally priming and intracranially treating mice bearing aggressive CT2A syngeneic astrocytomas with VSVΔ51. Approximately 25% of animals achieved complete regression of established tumors, with no signs of virus-induced neurological impairment. This approach may harness an early warning system in the brain that has evolved to protect the host against otherwise lethal neurotropic viral infections. We have exploited this protective mechanism to safely and efficaciously treat brain tumors with an otherwise neurotoxic virus, potentially widening the available treatment options for oncolytic virotherapy in the brain.

Genetically Engineered Vaccinia Viruses As Agents for Cancer Treatment, Imaging, and Transgene Delivery

Despite advances in technology, the formidable challenge of treating cancer, especially if advanced, still remains with no significant improvement in survival rates, even with the most common forms of cancer. Oncolytic viral therapies have shown great promise for the treatment of various cancers, with the possible advantages of stronger treatment efficacy compared to conventional therapy due to higher tumor selectivity, and less toxicity. They are able to preferentially and selectively propagate in cancer cells, consequently destroying tumor tissue mainly via cell lysis, while leaving non-cancerous tissues unharmed. Several wild-type and genetically engineered vaccinia virus (VACV) strains have been tested in both preclinical and clinical trials with promising results. Greater understanding and advancements in molecular biology have enabled the generation of genetically engineered oncolytic viruses for safer and more efficacious treatment, including arming VACVs with cytokines and immunostimulatory molecules, anti-angiogenic agents, and enzyme prodrug therapy, in addition to combining VACVs with conventional external and systemic radiotherapy, chemotherapy, immunotherapy, and other virus strains. Furthermore, novel oncolytic vaccinia virus strains have been generated that express reporter genes for the tracking and imaging of viral therapy and monitoring of therapeutic response. Further study is needed to unlock VACVs’ full potential as part of the future of cancer therapy.

Oncolytic virotherapy for pediatric malignancies: future prospects

Pediatric solid tumors remain a major health concern, with nearly 16,000 children diagnosed each year. Of those, ~2,000 succumb to their disease, and survivors often suffer from lifelong disability secondary to toxic effects of current treatments. Countless multimodality treatment regimens are being explored to make advances against this deadly disease. One targeted treatment approach is oncolytic virotherapy. Conditionally replicating viruses can infect tumor cells while leaving normal cells unharmed. Four viruses have been advanced to pediatric clinical trials, including herpes simplex virus-1, Seneca Valley virus, reovirus, and vaccinia virus. In this review, we discuss the mechanism of action of each virus, pediatric preclinical studies conducted to date, past and ongoing pediatric clinical trials, and potential future direction for these novel viral therapeutics.

Expression of DAI by an oncolytic vaccinia virus boosts the immunogenicity of the virus and enhances antitumor immunity

In oncolytic virotherapy, the ability of the virus to activate the immune system is a key attribute with regard to long-term antitumor effects. Vaccinia viruses bear one of the strongest oncolytic activities among all oncolytic viruses. However, its capacity for stimulation of antitumor immunity is not optimal, mainly due to its immunosuppressive nature. To overcome this problem, we developed an oncolytic VV that expresses intracellular pattern recognition receptor DNA-dependent activator of IFN-regulatory factors (DAI) to boost the innate immune system and to activate adaptive immune cells in the tumor. We showed that infection with DAI-expressing VV increases expression of several genes related to important immunological pathways. Treatment with DAI-armed VV resulted in significant reduction in the size of syngeneic melanoma tumors in mice. When the mice were rechallenged with the same tumor, DAI-VV-treated mice completely rejected growth of the new tumor, which indicates immunity established against the tumor. We also showed enhanced control of growth of human melanoma tumors and elevated levels of human T-cells in DAI-VV-treated mice humanized with human peripheral blood mononuclear cells. We conclude that expression of DAI by an oncolytic VV is a promising way to amplify the vaccine potency of an oncolytic vaccinia virus to trigger the innate-and eventually the long-lasting adaptive immunity against cancer.

Features of the Antitumor Effect of Vaccinia Virus Lister Strain

Oncolytic abilities of vaccinia virus (VACV) served as a basis for the development of various recombinants for treating cancer; however, "natural" oncolytic properties of the virus are not examined in detail. Our study was conducted to know how the genetically unmodified L-IVP strain of VACV produces its antitumor effect. Human A431 carcinoma xenografts in nude mice and murine Ehrlich carcinoma in C57Bl mice were used as targets for VACV, which was injected intratumorally. A set of virological methods, immunohistochemistry, light and electron microscopy was used in the study. We found that in mice bearing A431 carcinoma, the L-IVP strain was observed in visceral organs within two weeks, but rapidly disappeared from the blood. The L-IVP strain caused decrease of sizes in both tumors, however, in different ways. Direct cell destruction by replicating virus plays a main role in regression of A431 carcinoma xenografts, while in Ehrlich carcinoma, which poorly supported VACV replication, the virus induced decrease of mitoses by pushing tumor cells into S-phase of cell cycle. Our study showed that genetically unmodified VACV possesses at least two mechanisms of antitumor effect: direct destruction of tumor cells and suppression of mitoses in tumor cells.

Targeting tumor vasculature through oncolytic virotherapy: recent advances

The oncolytic virotherapy field has made significant advances in the last decade, with a rapidly increasing number of early- and late-stage clinical trials, some of them showing safety and promising therapeutic efficacy. Targeting tumor vasculature by oncolytic viruses (OVs) is an attractive strategy that offers several advantages over nontargeted viruses, including improved tumor viral entry, direct antivascular effects, and enhanced antitumor efficacy. Current understanding of the biological mechanisms of tumor neovascularization, novel vascular targets, and mechanisms of resistance has allowed the development of oncolytic viral vectors designed to target tumor neovessels. While some OVs (such as vaccinia and vesicular stomatitis virus) can intrinsically target tumor vasculature and induce vascular disruption, the majority of reported vascular-targeted viruses are the result of genetic manipulation of their viral genomes. Such strategies include transcriptional or transductional endothelial targeting, "armed" viruses able to downregulate angiogenic factors, or to express antiangiogenic molecules. The above strategies have shown preclinical safety and improved antitumor efficacy, either alone, or in combination with standard or targeted agents. This review focuses on the recent efforts toward the development of vascular-targeted OVs for cancer treatment and provides a translational/clinical perspective into the future development of new generation biological agents for human cancers.

The mechanisms of genetically modified vaccinia viruses for the treatment of cancer

The use of oncolytic viruses for the treatment of cancer is an emerging field of cancer research and therapy. Oncolytic viruses are designed to induce tumor specific immunity while replicating selectively within cancer cells to cause lysis of the tumor cells. While there are several forms of oncolytic viruses, the use of vaccinia viruses for oncolysis may be more beneficial than other forms of oncolytic viruses. For example, vaccinia viruses have been shown to exert their anti-tumor effects through genetic engineering strategies which enhance their therapeutic efficacy. This paper will address some of the most common forms of genetically modified vaccinia viruses and will explore the mechanisms whereby they selectively target, enter and destroy cancer cells. Furthermore, this review will highlight how vaccinia viruses activate host immune responses against cancer cells and will address clinical trials evaluating the tumor-directed and killing efficacy of these viruses against solid tumors.

On the potential of oncolytic virotherapy for the treatment of canine cancers

Over 6 million dogs are diagnosed with cancer in the USA each year. Treatment options for many of these patients are limited. It is important that the veterinary and scientific communities begin to explore novel treatment protocols for dogs with cancer. Oncolytic viral therapy is a promising treatment option that may prove to be relatively inexpensive and effective against several types of cancer. The efficacy of oncolytic virus therapies has been clearly demonstrated in murine cancer models, but the positive outcomes observed in mice are not always seen in human cancer patients. These therapies should be thoroughly evaluated in dogs with spontaneously arising cancers to provide needed information about the potential effectiveness of virus treatment for human cancers and to promote the health of our companion animals. This article provides a review of the results of oncolytic virus treatment of canine cancers.

Incomplete but infectious vaccinia virions are produced in the absence of oncolysis in feline SCCF1 cells

Vaccinia virus is a large, enveloped virus of the poxvirus family. It has broad tropism and typically virus replication culminates in accumulation and lytic release of intracellular mature virus (IMV), the most abundant form of infectious virus, as well as release by budding of extracellular enveloped virus (EEV). Vaccinia viruses have been modified to replicate selectively in cancer cells and clinically tested as oncolytic agents. During preclinical screening of relevant cancer targets for a recombinant Western Reserve strain deleted for both copies of the thymidine kinase and vaccinia growth factor genes, we noticed that confluent monolayers of SCCF1 cat squamous carcinoma cells were not destroyed even after prolonged infection. Interestingly, although SCCF1 cells were not killed, they continuously secreted virus into the cell culture supernatant. To investigate this finding further, we performed detailed studies by electron microscopy. Both intracellular and secreted virions showed morphological abnormalities on ultrastructural inspection, suggesting compromised maturation and morphogenesis of vaccinia virus in SCCF1 cells. Our data suggest that SCCF1 cells produce a morphologically abnormal virus which is nevertheless infective, providing new information on the virus-host cell interactions and intracellular biology of vaccinia virus.

Safety and biodistribution of a double-deleted oncolytic vaccinia virus encoding CD40 ligand in laboratory Beagles

We evaluated adverse events, biodistribution and shedding of oncolytic vaccinia virus encoding CD40 ligand in two Beagles, in preparation for a phase 1 trial in canine cancer patients. Dog 1 received one dose of vaccinia virus and was euthanized 24 hours afterwards, while dog 2 received virus four times once weekly and was euthanized 7 days after that. Dogs were monitored for adverse events and underwent a detailed postmortem examination. Blood, saliva, urine, feces, and organs were collected for virus detection. Dog 1 had mild fever and lethargy while dog 2 experienced a possible seizure 5.5 hours after first virus administration. Viral DNA declined quickly in the blood after virus administration in both dogs but was still detectable 1 week later by quantitative polymerase chain reaction. Only samples taken directly after virus infusion contained infectious virus. Small amounts of viral DNA, but no infectious virus, were detected in a few saliva and urine samples. Necropsies did not reveal any relevant pathological changes and virus DNA was detected mainly in the spleen. The dogs in the study did not have cancer, and thus adverse events could be more common and viral load higher in dogs with tumors which allow viral amplification.

Anti-tumour activity of oncolytic Western Reserve vaccinia viruses in canine tumour cell lines, xenografts, and fresh tumour biopsies

Cancer is one of the most common reasons for death in dogs. One promising approach is oncolytic virotherapy. We assessed the oncolytic effect of genetically modified vaccinia viruses in canine cancer cells, in freshly excised tumour biopsies, and in mice harbouring canine tumour xenografts. Tumour transduction efficacy was assessed using virus expressing luciferase or fluorescent marker genes and oncolysis was quantified by a colorimetric cell viability assay. Oncolytic efficacy in vivo was evaluated in a nude mouse xenograft model. Vaccinia virus was shown to infect most tested canine cancer cell lines and primary surgical tumour tissues. Virus infection significantly reduced tumour growth in the xenograft model. Oncolytic vaccinia virus has antitumour effects against canine cancer cells and experimental tumours and is able to replicate in freshly excised patient tumour tissue. Our results suggest that oncolytic vaccinia virus may offer an effective treatment option for otherwise incurable canine tumours.

Soluble vascular endothelial growth factor receptor-1 improves therapeutic efficacy of suicide gene therapy in an angiogenesis-independent manner

Abstract Malignant gliomas (MGs) are highly vascularized, aggressive brain cancers carrying a dismal prognosis. Because of their high vascularity, anti-angiogenic therapy is a potential treatment option. Indeed, the anti-vascular endothelial growth factor (VEGF) antibody bevacizumab has demonstrated promising results in clinical trials. Similarly, adenovirus-medicated Herpes simplex virus thymidine kinase and ganciclovir (AdHSV-tk/GCV) suicide gene therapy has established itself in clinical trials as a potential novel therapeutic strategy for MGs. In this study, we demonstrate the feasibility of combining adenovirus-mediated soluble VEGF receptor-1 anti-angiogenic gene therapy with AdHSV-tk/GCV suicide gene therapy to treat experimental MGs. Our results reveal that, apart from inhibiting angiogenesis, other anti-tumor mechanisms, such as reduction of infiltration by tumor-associated macrophages/microglia, may contribute to the improved therapeutic benefit of combination therapy.

GMCSF-armed vaccinia virus induces an antitumor immune response

Oncolytic Western Reserve strain vaccinia virus selective for epidermal growth factor receptor pathway mutations and tumor-associated hypermetabolism was armed with human granulocyte-macrophage colony-stimulating factor (GMCSF) and a tdTomato fluorophore. As the assessment of immunological responses to human transgenes is challenging in the most commonly used animal models, we used immunocompetent Syrian golden hamsters, known to be sensitive to human GMCSF and semipermissive to vaccinia virus. Efficacy was initially tested in vitro on various human and hamster cell lines and oncolytic potency of transgene-carrying viruses was similar to unarmed virus. The hGMCSF-encoding virus was able to completely eradicate subcutaneous pancreatic tumors in hamsters, and to fully protect the animals from subsequent rechallenge with the same tumor. Induction of specific antitumor immunity was also shown by ex vivo co-culture experiments with hamster splenocytes. In addition, histological examination revealed increased infiltration of neutrophils and macrophages in GMCSF-virus-treated tumors. These findings help clarify the mechanism of action of GMCSF-armed vaccinia viruses undergoing clinical trials.

Oncolytic vaccinia virus demonstrates antiangiogenic effects mediated by targeting of VEGF

Oncolytic vaccinia virus has been shown to induce a profound, rapid and tumor-specific vascular collapse in both preclinical models and clinical studies; however, a complete examination of the kinetics and levels of collapse and revascularization has not been described previously. Contrast-enhanced ultrasound was used to follow tumor perfusion levels in mouse tumor models at times after vaccinia therapy. It was observed that revascularization after viral therapy was dramatically delayed and did not occur until after viral clearance. This indicated that oncolytic vaccinia may possess a previously undescribed antiangiogenic potential that might synergize with the reported anti-vascular effects. Despite a rapid loss of perfusion and widespread hypoxia within the tumor, it was observed that VEGF levels in the tumor were suppressed throughout the period of active viral infection. Although tumor vasculature could eventually reform after the viral therapy was cleared in mouse models, anti-tumor effects could be significantly enhanced through additional combination with anti-VEGF therapies. This was initially examined using a gene therapy approach (Ad-Flk1-Fc) to target VEGF directly, demonstrating that the timing of application of the antiangiogenic therapy was critical. However, it is also known that oncolytic vaccinia sensitizes tumors to tyrosine kinase inhibitors (TKI) in the clinic through an unknown mechanism. It is possible this phenomenon may be mediated through the antiangiogenic effects of the TKIs. This was modeled in mouse tumors using sunitinib in combination with oncolytic vaccinia. It was observed that prevention of angiogenesis mediated by oncolytic vaccinia can be utilized to enhance the TKI therapy.

T-cell engager-armed oncolytic vaccinia virus significantly enhances antitumor therapy

Oncolytic vaccinia virus (VV) therapy has shown promise in preclinical models and in clinical studies. However, complete responses have rarely been observed. This lack of efficacy is most likely due to suboptimal virus spread through the tumor resulting in limited tumor cell destruction. We reasoned that redirecting T cells to the tumor has the potential to improve the antitumor activity of oncolytic VVs. We, therefore, constructed a VV encoding a secretory bispecific T-cell engager consisting of two single- chain variable fragments specific for CD3 and the tumor cell surface antigen EphA2 (EphA2-T-cell engager-armed VV (EphA2-TEA-VV)). In vitro, EphA2-TEA-VV's ability to replicate and induce oncolysis was similar to that of unmodified virus. However, only tumor cells infected with EphA2-TEA-VV induced T-cell activation as judged by the secretion of interferon-γ and interleukin-2. In coculture assays, EphA2-TEA-VV not only killed infected tumor cells, but in the presence of T cells, it also induced bystander killing of noninfected tumor cells. In vivo, EphA2-TEA-VV plus T cells had potent antitumor activity in comparison with control VV plus T cells in a lung cancer xenograft model. Thus, arming oncolytic VVs with T-cell engagers may represent a promising approach to improve oncolytic virus therapy.

Arming viruses in multi-mechanistic oncolytic viral therapy: current research and future developments, with emphasis on poxviruses

The field of oncolytic virology has made great strides in recent years. However, one key finding has been that the use of viral agents that replicate selectively in tumors is usually insufficient to achieve anything beyond small and transient responses. Instead, like most cancer therapies, oncolytic viruses are most effective in combination with other therapies, which is where they have proven therapeutic effects in clinical and preclinical studies. In cases of some of the smaller RNA viruses, effects can only be achieved through combination regimens with chemotherapy, radiotherapy, or targeted conventional therapies. However, larger DNA viruses are able to express one or more transgenes; thus, therapeutic mechanisms can be built into the viral vector itself. The incorporated approaches into arming oncolytic viruses through transgene expression will be the main focus of this review, including use of immune activators, prodrug converting enzymes, anti-angiogenic factors, and targeting of the stroma. This will focus on poxviruses as model systems with large cloning capacities, which have routinely been used as transgene expression vectors in different settings, including vaccine and oncolytic viral therapy.

Tumor vascularization is critical for oncolytic vaccinia virus treatment of peritoneal carcinomatosis

Peritoneal carcinomatosis (PC) represents a significant clinical challenge for which there are few treatment options. Oncolytic viruses are ideal candidates for PC treatment because of their high tumor specificity, excellent safety profile and suitability for peritoneal delivery. Here, we described the use of vvDD-SR-RFP, a recombinant vaccinia virus, in xenograft and syngeneic models of colorectal PC. Colorectal cancer cell lines were highly susceptible to vvDD-SR-RFP replication and cytotoxicity. Intraperitoneal delivery of vvDD-SR-RFP on Day 12 to mice with colorectal carcinomatosis significantly improved survival whereas survival was not improved following virus treatment on Day 8, when tumors were smaller. Immunohistochemistry revealed early tumors had a poorly distributed network of blood vessels and lower proliferation index compared to later tumors. Virus infection was also restricted to tumor rims following Day 8 treatment, whereas it was disseminated in tumors treated on Day 12. Additionally, direct infection of tumor endothelium was observed and virus infection correlated with a loss of endothelial staining and induction of cell death. Our results demonstrate that tumor vasculature has a critical role in virus delivery and tumor response. This will have significant implications in the clinical setting, both in understanding timing of therapies and in designing combination treatment strategies.

Update on oncolytic viral therapy - targeting angiogenesis

Oncolytic viruses (OVs) have the ability to selectively replicate in and lyse cancer cells. Angiogenesis is an essential requirement for tumor growth. Like OVs, the therapeutic effect of many angiogenesis inhibitors has been limited, leading to the development of more effective approaches to combine antiangiogenic therapy with OVs. Angiogenesis can be targeted either directly by OV infection of vascular endothelial cells, or by arming OVs with antiangiogenic transgenes, which are subsequently expressed locally in the tumor microenvironment. In this review, we describe the development and targeting of OVs, the role of angiogenesis in cancer, and the progress made in arming viruses with antiangiogenic transgenes. Future developments required to optimize this approach are addressed.

An infection-enhanced oncolytic adenovirus secreting H. pylori neutrophil-activating protein with therapeutic effects on neuroendocrine tumors

Helicobacter pylori neutrophil-activating protein (HP-NAP) is a major virulence factor involved in H. pylori infection. HP-NAP can mediate antitumor effects by recruiting neutrophils and inducing Th1-type differentiation in the tumor microenvironment. It therefore holds strong potential as a therapeutic gene. Here, we armed a replication-selective, infection-enhanced adenovirus with secretory HP-NAP, Ad5PTDf35-[Δ24-sNAP], and evaluated its therapeutic efficacy against neuroendocrine tumors. We observed that it could specifically infect and eradicate a wide range of tumor cells lines from different origin in vitro. Insertion of secretory HP-NAP did not affect the stability or replicative capacity of the virus and infected tumor cells could efficiently secrete HP-NAP. Intratumoral administration of the virus in nude mice xenografted with neuroendocrine tumors improved median survival. Evidence of biological HP-NAP activity was observed 24 hours after treatment with neutrophil infiltration in tumors and an increase of proinflammatory cytokines such as tumor necrosis factor (TNF)-α and MIP2-α in the systemic circulation. Furthermore, evidence of Th1-type immune polarization was observed as a result of increase in IL-12/23 p40 cytokine concentrations 72 hours postvirus administration. Our observations suggest that HP-NAP can serve as a potent immunomodulator in promoting antitumor immune response in the tumor microenvironment and enhance the therapeutic effect of oncolytic adenovirus.

Mounting a strategic offense: fighting tumor vasculature with oncolytic viruses

Blood supply within a tumor drives progression and ultimately allows for metastasis. Many anticancer therapies target tumor vasculature, but their individual effectiveness is limited because they induce indirect cell death. Agents that disrupt nascent and/or established tumor vasculature while simultaneously killing cancer cells would certainly have a greater impact. Oncolytic virotherapy utilizes attenuated viruses that replicate specifically within a tumor. They induce cytotoxicity through a combination of direct cell lysis, antitumor immune stimulation, and recently identified antitumor vascular effects. This review summarizes the novel preclinical and clinical evidence regarding the antitumor vascular effects of oncolytic viruses, which include infection and lysis of tumor endothelial cells, natural or genetically engineered antiangiogenic properties, and combination therapy with clinically approved antivascular agents.

Obatoclax, saliphenylhalamide, and gemcitabine inhibit influenza a virus infection

Influenza A viruses (IAVs) infect humans and cause significant morbidity and mortality. Different treatment options have been developed; however, these were insufficient during recent IAV outbreaks. Here, we conducted a targeted chemical screen in human nonmalignant cells to validate known and search for novel host-directed antivirals. The screen validated saliphenylhalamide (SaliPhe) and identified two novel anti-IAV agents, obatoclax and gemcitabine. Further experiments demonstrated that Mcl-1 (target of obatoclax) provides a novel host target for IAV treatment. Moreover, we showed that obatoclax and SaliPhe inhibited IAV uptake and gemcitabine suppressed viral RNA transcription and replication. These compounds possess broad spectrum antiviral activity, although their antiviral efficacies were virus-, cell type-, and species-specific. Altogether, our results suggest that phase II obatoclax, investigational SaliPhe, and FDA/EMEA-approved gemcitabine represent potent antiviral agents.

Anti-angiogenic therapy increases intratumoral adenovirus distribution by inducing collagen degradation

Conditionally replicating adenoviruses (CRAd) are a promising class of gene therapy agents that can overcome already known glioblastoma (GBM) resistance mechanisms but have limited distribution upon direct intratumoral (i.t.) injection. Collagen bundles in the extracellular matrix (ECM) play an important role in inhibiting virus distribution. In fact, ECM pre-treatment with collagenases improves virus distributions to tumor cells. Matrix metalloproteinases (MMPs) are an endogenous class of collagenases secreted by tumor cells whose function can be altered by different drugs including anti-angiogenic agents, such as bevacizumab. In this study we hypothesized that up-regulation of MMP activity during antiangiogenic therapy can improve CRAd-S-pk7 distribution in GBM. We find that MMP-2 activity in human U251 GBM xenografts increases (*p=0.03) and collagen IV content decreases (*p=0.01) during vascular endothelial growth factor (VEGF-A) antibody neutralization. After proving that collagen IV inhibits CRAd-S-pk7 distribution in U251 xenografts (Spearman rho= −0.38; **p=0.003), we show that VEGF blocking antibody treatment followed by CRAd-S-pk7 i.t. injection reduces U251 tumor growth more than each individual agent alone (***p<0.0001). Our data proposes a novel approach to improve virus distribution in tumors by relying on the early effects of anti-angiogenic therapy.

Oncolytic virotherapy in veterinary medicine: current status and future prospects for canine patients

Oncolytic viruses refer to those that are able to eliminate malignancies by direct targeting and lysis of cancer cells, leaving non-cancerous tissues unharmed. Several oncolytic viruses including adenovirus strains, canine distemper virus and vaccinia virus strains have been used for canine cancer therapy in preclinical studies. However, in contrast to human studies, clinical trials with oncolytic viruses for canine cancer patients have not been reported. An 'ideal' virus has yet to be identified. This review is focused on the prospective use of oncolytic viruses in the treatment of canine tumors - a knowledge that will undoubtedly contribute to the development of oncolytic viral agents for canine cancer therapy in the future.

Photodynamic therapy augments the efficacy of oncolytic vaccinia virus against primary and metastatic tumours in mice

Background:

Therapies targeted towards the tumour vasculature can be exploited for the purpose of improving the systemic delivery of oncolytic viruses to tumours. Photodynamic therapy (PDT) is a clinically approved treatment for cancer that is known to induce potent effects on tumour vasculature. In this study, we examined the activity of PDT in combination with oncolytic vaccinia virus (OVV) against primary and metastatic tumours in mice.

Methods:

The effect of 2-[1-hexyloxyethyl-]-2-devinyl pyropheophorbide-a (HPPH)-sensitised-PDT on the efficacy of oncolytic virotherapy was investigated against subcutaneously implanted syngeneic murine NXS2 neuroblastoma and human FaDu head and neck squamous cell carcinoma xenografts in nude mice. Treatment efficacy was evaluated by monitoring tumour growth and survival. The effects of combination treatment on vascular function were examined using magnetic resonance imaging (MRI) and immunohistochemistry, whereas viral replication in tumour cells was analysed by a standard plaque assay. Normal tissue phototoxicity following PDT-OV treatment was studied using the mouse foot response assay.

Results:

Combination of PDT with OVV resulted in inhibition of primary and metastatic tumour growth compared with either monotherapy. PDT-induced vascular disruption resulted in higher intratumoural viral titres compared with the untreated tumours. Five days after delivery of OVV, there was a loss of blood flow to the interior of tumour that was associated with infiltration of neutrophils. Administration of OVV did not result in any additional photodynamic damage to normal mouse foot tissue.

Conclusion:

These results provide evidence into the usefulness of PDT as a means of enhancing intratumoural replication and therapeutic efficacy of OV.

Oncolytic adenovirus research and applications

Cancer treatments have improved steadily, but still only few metastatic solid tumors can be cured. Apoptosis-resistant clones frequently develop following standard treatments. Resistance factors are shared between different treatment regimens and, therefore, loss of response can occur rapidly, despite changing the drug, and there is a tendency for crossresistance between modalities. Therefore, new agents with novel mechanisms of action are desperately needed. Oncolytic adenoviruses, featuring cancer-selective cell lysis and spread, constitute an interesting drug platform aimed towards the goals of tumor specificity, and have been engineered in a variety of ways to improve their selectivity and efficacy. They allow rational drug development by the genetic incorporation of targeting mechanisms that can exert their function at different stages of the viral replication cycle. Owing to their immunogenicity, adenoviruses are particularly attractive for immunostimulatory purposes.

Oncolytic vaccinia virus: a theranostic agent for cancer

Vaccinia virus (VACV) is arguably the most successful live biotherapeutic agent because of its critical role in the eradication of smallpox, one of the most deadly diseases in human history. VACV has been exploited as an oncolytic therapeutic agent for cancer since 1922. This virus selectively infects and destroys tumor cells, while sparing normal cells, both in cell cultures and in animal models. A certain degree of therapeutic efficacy also has been demonstrated in patients with different types of cancer. In recent years, several strategies have been successfully developed to further improve its tumor selectivity and antitumor efficacy. Oncolytic VACVs carrying imaging genes represent a new treatment strategy that combines tumor site-specific therapeutics with diagnostics (theranostics).

Impact of tumor microenvironment on oncolytic viral therapy

Interactions between tumor cells and their microenvironment have been shown to play a very significant role in the initiation, progression, and invasiveness of cancer. These tumor-stromal interactions are capable of altering the delivery and effectiveness of therapeutics into the tumor and are also known to influence future resistance and re-growth after treatment. Here we review recent advances in the understanding of the tumor microenvironment and its response to oncolytic viral therapy. The multifaceted environmental response to viral therapy can influence viral infection, replication, and propagation within the tumor. Recent studies have unveiled the complicated temporal changes in the tumor vasculature post-oncolytic virus (OV) treatment, and their impact on tumor biology. Similarly, the secreted extracellular matrix in solid tumors can affect both infection and spread of the therapeutic virus. Together, these complex changes in the tumor microenvironment also modulate the activation of the innate antiviral host immune response, leading to quick and efficient viral clearance. In order to combat these detrimental responses, viruses have been combined with pharmacological adjuvants and "armed" with therapeutic genes in order to suppress the pernicious environmental conditions following therapy. In this review we will discuss the impact of the tumor environment on viral therapy and examine some of the recent literature investigating methods of modulating this environment to enhance oncolysis.

The combination of immunosuppression and carrier cells significantly enhances the efficacy of oncolytic poxvirus in the pre-immunized host

Pre-existing antipoxvirus immunity in cancer patients presents a severe barrier to poxvirus-mediated oncolytic virotherapy. We have explored strategies of immunosuppression (IS) and/or immune evasion for efficient delivery of an oncolytic double-deleted vaccinia virus (vvDD) to tumors in the pre-immunized mice. Transient IS using immunosuppressive drugs, including tacrolimus, mycophenolate mofetil and methylprednisolone sodium succinate, have been used successfully in organ transplantation. This drug cocktail alone did not enhance viral recovery from subcutaneous tumor after systemic viral delivery. Using B-cell knockout mice, we confirmed that the neutralizing antibodies had a significant role in preventing poxvirus infection. Using a MC38 peritoneal carcinomatosis model, we found that the combination of IS and tumor cells as carriers led to the most effective viral delivery, viral replication and viral spread inside the tumor mass. We found that our immunosuppressive drug cocktail facilitated recruitment of tumor-associated macrophages and conversion into an immunosuppressive M2 phenotype (interleukin (IL)-10(hi)/IL-12(low)) in the tumor microenvironment. A combination of IS and carrier cells led to significantly prolonged survival in the tumor model. These results showed the feasibility of treating pre-vaccinated patients with peritoneal carcinomatosis using an oncolytic poxvirus and a combined immune intervention strategy.