Recombinant mumps virus as a cancer therapeutic agent

Insertion of a short non-viral sequence in the 3' noncoding region of the hemagglutinin-neuraminidase increases mumps virus neurovirulence

The mumps virus is a promising candidate as a vaccine vector or oncolytic therapy agent. However, its neurotropic nature, driven by molecular mechanisms that remain unclear, poses a significant obstacle to its development for these applications. This study utilizes recombinant mumps virus carrying the enhanced green fluorescent protein gene (EGFP) and two additional viruses containing, alongside EGFP, unique non-viral, non-coding 84-nucleotide inserts in the 3' non-coding region (NCR) of the hemagglutinin-neuraminidase (HN) gene. We observed a significant increase in neurovirulence for both viruses containing inserts in the 3' NCR of HN. The insert in HN 3' NCR provided a replicative advantage in the brains of newborn rats and rat brain-derived cell cultures. While the viruses were able to infect rat neurons, infection of astrocytes was completely inhibited. Additionally, in infected rat brain and rat brain-derived cell cultures we detected induction of RANTES. We observed no correlation in the extent of neuronal apoptosis or the induction of pro-inflammatory cytokines between viruses with or without the HN 3' NCR insert.

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.

Parainfluenza Virus 5 V Protein Blocks Interferon Gamma-Mediated Upregulation of NK Cell Inhibitory Ligands and Improves NK Cell Killing of Neuroblastoma Cells

Natural killer (NK) cells can be effective immunotherapeutic anti-cancer agents due to their ability to selectively target and kill tumor cells. This activity is modulated by the interaction of NK cell receptors with inhibitory ligands on the surface of target cells. NK cell inhibitory ligands can be upregulated on tumor cell surfaces in response to interferon-gamma (IFN-γ), a cytokine which is produced by activated NK cells. We hypothesized that the resistance of tumor cells to NK cell killing could be overcome by expression of the parainfluenza virus 5 (PIV5) V protein, which has known roles in blocking IFN-γ signaling. This was tested with human PM21-NK cells produced through a previously developed particle-based method which yields superior NK cells for immunotherapeutic applications. Infection of human SK-N-SH neuroblastoma cells with PIV5 blocked IFN-γ-mediated upregulation of three NK cell inhibitory ligands and enhanced in vitro killing of these tumor cells by PM21-NK cells. SK-N-SH cells transduced to constitutively express the V protein alone were resistant to IFN-γ-mediated increases in cell surface expression of NK cell inhibitory ligands. Real-time in vitro cell viability assays demonstrated that V protein expression in SK-N-SH cells was sufficient to increase PM21-NK cell-mediated killing. Toward a potential therapeutic application, transient lentiviral delivery of the V gene also enhanced PM21-NK cell killing in vitro. Our results provide the foundation for novel therapeutic applications of V protein expression in combination with ex vivo NK cell therapy to effectively increase the killing of tumor cells.

Oncolytic viruses against cancer, promising or delusion?

Cancer treatment is one of the most challenging topics in medical sciences. Different methods such as chemotherapy, tumor surgery, and immune checkpoint inhibitors therapy (ICIs) are potential approaches to treating cancer and killing tumor cells, but clinical studies have shown that they have been successful for a limited group of patients. Using viruses as a treatment can be considered as an effective treatment in the field of medicine. This is considered as a potential treatment, especially in comparison to chemotherapy, which has severe side effects related to the immune system. Most oncolytic viruses (OVs) have the potential to multiply in cancer cells, which are more than normal cells in malignant tissue and can induce immune responses. Therefore, tons of efforts and research have been started on the utilization of OVs as a treatment for cancer and have shown promising in treating cancers with less side effects. In this article, we have gathered studies about oncolytic viruses and their effectiveness in cancer treatment.Please confirm if the author names are presented accurately and in the correct sequence (given name, middle name/initial, family name). Author 1 Given name: [Omid Salahi] Last name [Ardekani], Author 2 Given name: [Mohammad Mehdi] Last name [Fazeli], Author 3 Given name: [Nillofar Asadi] Last name [Jemezghani]. Also, kindly confirm the details in the metadata are correct.Confirmed.

Comprehensive assessment on the applications of oncolytic viruses for cancer immunotherapy

The worldwide burden of cancers is increasing at a very high rate, including the aggressive and resistant forms of cancers. Certain levels of breakthrough have been achieved with the conventional treatment methods being used to treat different forms of cancers, but with some limitations. These limitations include hazardous side effects, destruction of non-tumor healthy cells that are rapidly dividing and developing, tumor resistance to anti-cancer drugs, damage to tissues and organs, and so on. However, oncolytic viruses have emerged as a worthwhile immunotherapeutic option for the treatment of different types of cancers. In this treatment approach, oncolytic viruses are being modeled to target cancer cells with optimum cytotoxicity and spare normal cells with optimal safety, without the oncolytic viruses themselves being killed by the host immune defense system. Oncolytic viral infection of the cancer cells are also being genetically manipulated (either by removal or addition of certain genes into the oncolytic virus genome) to make the tumor more visible and available for attack by the host immune cells. Hence, different variants of these viruses are being developed to optimize their antitumor effects. In this review, we examined how grave the burden of cancer is on a global level, particularly in sub-Saharan Africa, major conventional therapeutic approaches to the treatment of cancer and their individual drawbacks. We discussed the mechanisms of action employed by these oncolytic viruses and different viruses that have found their relevance in the fight against various forms of cancers. Some pre-clinical and clinical trials that involve oncolytic viruses in cancer management were reported. This review also examined the toxicity and safety concerns surrounding the adoption of oncolytic viro-immunotherapy for the treatment of cancers and the likely future directions for researchers and general audience who wants updated information.

Repurposing live attenuated trivalent MMR vaccine as cost-effective cancer immunotherapy

It has long been known that oncolytic viruses wield their therapeutic capability by priming an inflammatory state within the tumor and activating the tumor immune microenvironment, resulting in a multifaceted antitumor immune response. Vaccine-derived viruses, such as measles and mumps, have demonstrated promising potential for treating human cancer in animal models and clinical trials. However, the extensive cost of manufacturing current oncolytic viral products makes them far out of reach for most patients. Here by analyzing the impact of intratumoral (IT) administrations of the trivalent live attenuated measles, mumps, and rubella viruses (MMR) vaccine, we unveil the cellular and molecular basis of MMR-induced anti-cancer activity. Strikingly, we found that IT delivery of low doses of MMR correlates with tumor control and improved survival in murine hepatocellular cancer and colorectal cancer models via increased tumor infiltration of CD8+ granzyme B+ T-cells and decreased macrophages. Moreover, our data indicate that MMR activates key cellular effectors of the host's innate and adaptive antitumor immunity, culminating in an immunologically coordinated cancer cell death. These findings warrant further work on the potential for MMR to be repurposed as safe and cost-effective cancer immunotherapy to impact cancer patients globally.

Clinical efficacy evaluation and potential mechanism prediction on Pudilan Xiaoyan oral liquid in treatment of mumps in children based on meta-analysis, network pharmacology, and molecular docking

Background: Mumps is caused by the mumps virus and is characterized by pain and parotid gland swelling. Although its incidence has declined due to vaccines, outbreaks still occur among children. In addition, it can lead to severe complications, so it has a certain perniciousness. Pudilan Xiaoyan oral liquid (PDL), a Chinese patent medicine, commonly treats children with mumps. However, its safety, efficacy, and specific mechanisms lack relevant evaluation and analysis. Therefore, we did a meta-analysis of the randomized controlled trials combined with a network pharmacology analysis to assess the efficacy and safety of PDL in relieving symptoms of mumps in children and investigate its pharmacological mechanisms. Methods: This study systematically searched the China National Knowledge Infrastructure (CNKI), WanFang Data Knowledge Service Platform, VIP Database, Sinomed, Chinese Medical Journal Full-text Database, PubMed, Embase, Cochrane Library, Web of Science, and Google Scholar for the published randomized controlled trials (date up to 3 March 2022; studies in both English and Chinese) comparing PDL and antiviral drug combination treatment to standalone antiviral drug treatment. The primary outcomes in this study were the effective rate and duration of five characteristic symptoms of children's mumps. We assessed the pooled data by using a fix-effect or random-effect model. We illustrated an odds ratio (OR) or standardized mean difference (SMD) with a 95% confidence interval (CI) using the Stata 15 software. In network pharmacology, active components of PDL were collected from the traditional Chinese medicine system pharmacology technology platform and the CNKI studies, while mumps' targets were collected from databases of the Genecards and Online Mendelian Inheritance in Man (OMIM), and then we constructed a "drug-component-target" network and a protein-protein interaction network using Cytoscape 3.9.0 for screening the core components and targets. Next, we ran Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis of intersection targets of PDL and mumps. Finally, molecular docking was performed between core components and targets. Results: Of 70 identified studies, 12 were eligible and included in our analysis (N = 1,307 participants). Compared with the antiviral drug treatments, combination treatment using PDL and antiviral drugs provided higher effective rates (OR = 5.94), shorter symptom durations for fever (SMD = -1.05), headache (SMD = -0.69), parotid gland swelling (SMD = -1.30), parotid gland pain (SMD = -2.53), and loss of appetite (SMD = -0.56) with fewer reported side effects. Of the 113 active components of PDL and 57 mumps' targets, 11 core components like quercetin, isoetin, and seven core targets such as albumin (ALB) and interleukin-6 were obtained. Moreover, the potential pathways identified included cytokine-cytokine receptor interaction and T helper cell 17 (Th17 cell) differentiation. Molecular docking results revealed that most core components and targets could form stable structures. The core components, including isoetin, quercetin, and luteolin, and core targets involving heat shock protein HSP 90-alpha (HSP90AA1), estrogen receptor (ESR1), and ALB showed the best affinities. Conclusion: The combined use of PDL and antiviral drugs could effectively improve the efficacy of mumps among children and rapidly alleviate mumps-related symptoms. This efficacy may be associated with the anti-inflammatory and antiviral mechanisms by which PDL acts using multiple components, multiple targets, and multiple pathways. However, these results should be confirmed by further studies.

Isolation and Characterization of a Discrete Genetically Homogeneous Viral Subpopulation of Mumps Virus RS-12 Strain with Superior Oncolytic Potency Compared to Its Progenitor Virus

Background: Oncolytic virotherapy can serve as a novel therapeutic strategy in oncology. In this study, we aimed to evaluate the oncolytic activity of the mumps virus RS-12 strain after its adaptation to cancer cells via serial passaging. Methods: To adapt the RS-12 strain-based vaccine to cancer cells, it was passaged eight times in the HT1080 cell line and was isolated via two terminal endpoint dilutions. The genetic homogeneity of isolated cancer cell-adapted RS-12 variant was confirmed by direct sequencing of regions, encompassing four known heterogeneous genomic positions. The in vitro cytotoxic effects of viruses was assessed in two different cancer cell lines using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The apoptosis-inducing effects of the cancer cell-adapted variant and its parental virus on cancer cells were quantified by flow cytometry. Results: According to the chromatograms, the RS-12 strain vaccine seed exhibited two peaks at the genomic nucleotide positions 1591, 2417, 3774, and 12977. On the contrary, cancer cell-adapted RS-12, isolated by terminal endpoint serial dilutions, contained no viral subpopulations in these positions. A significant improvement was observed in the oncolytic potency of our cancer cell-adapted variant compared to its parental virus vaccine seed in vitro. Besides, the variant efficiently induced apoptosis in the human fibrosarcoma and adenocarcinoma cell lines. Conclusions: Considering the increased oncolytic potency and apoptosis-inducing capacity of this variant in cancer cells, it can be a promising option for future experiments.

Type I and Type II Interferon Antagonism Strategies Used by Paramyxoviridae: Previous and New Discoveries, in Comparison

Paramyxoviridae is a viral family within the order of Mononegavirales; they are negative single-strand RNA viruses that can cause significant diseases in both humans and animals. In order to replicate, paramyxoviruses–as any other viruses–have to bypass an important protective mechanism developed by the host’s cells: the defensive line driven by interferon. Once the viruses are recognized, the cells start the production of type I and type III interferons, which leads to the activation of hundreds of genes, many of which encode proteins with the specific function to reduce viral replication. Type II interferon is produced by active immune cells through a different signaling pathway, and activates a diverse range of genes with the same objective to block viral replication. As a result of this selective pressure, viruses have evolved different strategies to avoid the defensive function of interferons. The strategies employed by the different viral species to fight the interferon system include a number of sophisticated mechanisms. Here we analyzed the current status of the various strategies used by paramyxoviruses to subvert type I, II, and III interferon responses.

The Multiplicity of Infection of Recombinant Vaccinia Virus Expressing the T7 RNA Polymerase Determines the Rescue Efficiency of Vesicular Stomatitis Virus

Vesicular stomatitis virus (VSV) has a wide range of cell tropism, making it a prototype of studying the negative-strand RNA virus (NSRV), including virus–host interactions and vaccine development. Although VSV rescue systems have been progressively optimized throughout time, the T7-based expression system is the most commonly utilized to rescue VSV. However, it remains a significant barrier for many labs. In our study, we found that rescue VSV’s efficiency is associated with the various multiplicities of infection (MOIs) of recombinant vaccinia virus expressing the T7 RNA polymerase (vTF-7.3). It works at maximum efficiency while the MOI of vTF-7.3 is 5, which is analyzed by quantitative PCR, Western blot, and flow cytometry, compared to the lowest rescue level with MOI of 1. Meanwhile, our data also suggest that purification of vTF-7.3 prior to transfection is a prerequisite for VSV rescue. Overall, our study reveals for the first time a precise correlation between vTF-7.3 and rescue efficiency, which may aid in resolving the uncertainties in the quest to build the VSV reverse genetic system.

Population Variability Generated during Rescue Process and Passaging of Recombinant Mumps Viruses

Recombinant mumps viruses (MuVs) based on established vaccine strains represent attractive vector candidates as they have known track records for high efficacy and the viral genome does not integrate in the host cells. We developed a rescue system based on the consensus sequence of the L-Zagreb vaccine and generated seven different recombinant MuVs by (a) insertion of one or two additional transcription units (ATUs), (b) lengthening of a noncoding region to the extent that the longest noncoding region in MuV genome is created, or (c) replacement of original L-Zagreb sequences with sequences rich in CG and AT dinucleotides. All viruses were successfully rescued and faithfully matched sequences of input plasmids. In primary rescued stocks, low percentages of heterogeneous positions were found (maximum 0.12%) and substitutions were predominantly obtained in minor variants, with maximally four substitutions seen in consensus. ATUs did not accumulate more mutations than the natural MuV genes. Six substitutions characteristic for recombinant viruses generated in our system were defined, as they repetitively occurred during rescue processes. In subsequent passaging of primary rescue stocks in Vero cells, different inconsistencies within quasispecies structures were observed. In order to assure that unwanted mutations did not emerge and accumulate, sub-consensus variability should be closely monitored. As we show for Pro408Leu mutation in L gene and a stop codon in one of ATUs, positively selected variants can rise to frequencies over 85% in only few passages.

Oncolytic parainfluenza virus combines with NK cells to mediate killing of infected and non-infected lung cancer cells within 3D spheroids: role of type I and type III interferon signaling

BACKGROUND

There is intense interest in developing novel oncolytic viruses, which can be used in cancer therapies along with immune cells such as natural killer (NK) cells. We have previously developed a particle-based method for in vitro expansion of highly cytotoxic human NK cells (PM21-NK cells). Here, we have tested the hypothesis that oncolytic parainfluenza virus 5 (P/V virus) can combine with PM21-NK cells for targeted killing of lung cancer cells.

METHODS

PM21-NK cells were assayed for killing of P/V virus-infected A549, H1299 and Calu-1 lung cancer cells in two-dimensional (2D) and three-dimensional (3D) cultures using flow cytometry, luminescence and kinetic imaging-based methods. Blocking antibodies were used to evaluate NK cell activating receptors involved in PM21-NK cell killing of infected target cells. Media transfer experiments tested soluble factors that increase PM21-NK cell killing of both P/V virus-infected and uninfected tumor cells.

RESULTS

In 2D cultures, PM21-NK cells efficiently killed P/V virus-infected cancer cells compared with non-infected cells, through involvement of the viral glycoprotein and NK cell receptors NKp30, NKp46 and NKG2D. In 3D spheroid cultures, P/V virus infection was restricted to the outer layer of the spheroid. However, PM21-NK cells were able to more efficiently kill both the outer layer of infected cells in the spheroid and progressing further to kill the uninfected interior cells. Media transfer experiments demonstrated that P/V virus infection produced both type I and type III interferons, which decreased cell growth, which contributed to a reduction in the overall number of uninfected tumor cells in conjunction with PM21-NK cells. Across five cancer cell lines, the contribution of P/V virus infection on PM21-NK cell killing of target cells correlated with interferon induction.

CONCLUSION

Our data support the potential of combining oncolytic parainfluenza virus with PM21-NK cell adoptive therapy against lung cancer.

Intrinsic Oncolytic Activity of Hoshino Mumps Virus Vaccine Strain Against Human Fibrosarcoma and Cervical Cancer Cell Lines

Background: The use of oncolytic viruses as therapeutic agents is a promising treatment for various human cancers. Several viruses have been extensively examined to achieve tumor cell death. Objectives: This study aimed at evaluating the natural oncolytic activity of mumps Hoshino vaccine strain against two human cancer cell lines, that is, HT1080 fibrosarcoma and HeLa cervical adenocarcinoma cell lines. Methods: The cytolytic activity of the virus was evaluated using an MTT assay. Apoptosis was detected by Annexin-V/propidium iodide (PI) staining and analyzed via flow cytometry. To indicate viral replication in vivo, nude mice with HeLa heterografts were treated with the Hoshino strain of mumps virus. Results: It was found that human fibrosarcoma and cervical cells were more sensitive to the mumps Hoshino strain, even at a very low multiplicity of infection (MOI) compared to normal human diploid cells. The results also showed that the Hoshino strain induced apoptosis in both cancer cells. A preliminary in vivo study revealed the significant suppression of tumor growth in the group treated with the mumps Hoshino strain compared to the control group. Conclusions: The Hoshino vaccine strain of mumps virus showed promising oncolytic activities against human fibrosarcoma and cervical adenocarcinoma cells.

Engineered Bacteriophage T7 as a Potent Anticancer Agent in vivo

Oncolytic viruses (OVs) induce antitumor effect by both direct lysis of target cells and eliciting immunogenic response to the virus and ultimately to the target cells. These viruses are usually natural human pathogens. Bacteriophages are natural pathogens of bacteria that do not infect human and have greater advantages in safety, manipulation, and production over human viruses. We constructed an engineered bacteriophage T7 displaying a peptide, which targets murine melanoma cells and harbors a mammalian expression cassette of the cytokine granulocyte macrophage-colony stimulating factor (GM-CSF) in viral genomic DNA. The engineered phage was successfully transduced to B16F10 melanoma cells both in vitro and in vivo. GM-CSF was expressed from the transduced phage DNA. All mice treated with the phage intravenously survived for 25 days until the end of experiment, while only 40% of those not treated survived. During the 16 days of phage treatment, phage T7 displaying homing peptide and expressing GM-CSF inhibited tumor growth by 72% compared to the untreated control. Serum cytokine levels of IL-1α, TNF-α, and GM-CSF were seen to increase during the treatment. Immunohistochemical analysis of tumor tissue revealed infiltration by macrophages, dendritic cells (DCs), and CD8⁺ T cells. Migration of murine macrophages to bacteriophages was also observed in in vitro transwell assays in both time- and dose-dependent manners. Taken together, the recombinant bacteriophage T7 efficiently inhibited tumor growth by changing the tumor microenvironment and recruiting anti-tumor immune cells.

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.

Attenuated MuV-S79 as vector stably expressing foreign gene

BACKGROUND

To describe mumps virus (MuV) used as a vector to express enhanced green fluorescent protein (EGFP) or red fluorescent protein (RFP) genes.

METHODS

Molecular cloning technique was applied to establish the cDNA clones of recombinant mumps viruses (rMuVs). rMuVs were recovered based on our reverse genetic system of MuV-S79. The properties of rMuVs were determined by growth curve, plaque assay, fluorescent microscopy and determination of fluorescent intensity.

RESULTS

Three recombinant viruses replicated well in Vero cells and similarly as parental rMuV-S79, expressed heterologous genes in high levels, and were genetically stable in at least 15 passages.

CONCLUSION

rMuV-S79 is a promising platform to accommodate foreign genes like marker genes, other antigens and immunomodulators for addressing various diseases.

Turning cold tumours hot: oncolytic virotherapy gets up close and personal with other therapeutics at the 11th Oncolytic Virus Conference

The 11th International Oncolytic Virus Conference (IOVC) was held from April 9-12, 2018 in Oxford, UK. This is part of the high-profile academic-led series of meetings that was started back in 2002 by Steve Russell and John Bell, with most of the previous meetings being held in North America (often in Banff). The conference brought together many of the major players in oncolytic virotherapy from all over the world, addressing all stages of research and development-from aspects of basic science and cellular immunology all the way through to early- and late-phase clinical trials. The meeting welcomed 352 delegates from 24 countries. The top seven delegate countries, namely, the UK, US, Canada, The Netherlands, Germany, Japan and South Korea, contributed 291 delegates while smaller numbers coming from Australia, Austria, Bulgaria, China, Finland, France, Iraq, Ireland, Israel, Italy, Latvia, Malaysia, Poland, Slovenia, Spain, Sweden and Switzerland. Academics comprised about half of the attendees, industry 30% and students 20%. The next IOVC is scheduled to be held on Vancouver Island in autumn 2019. Here we share brief summaries of the oral presentations from invited speakers and proffered papers in the different subtopics presented at IOVC 2018.

Perspectives on immunotherapy via oncolytic viruses

BACKGROUND

With few exceptions, current chemotherapy and radiotherapy protocols only obtain a slightly prolonged survival with severe adverse effects in patients with advanced solid tumors. In particular, most solid malignancies not amenable to radical surgery still carry a dismal prognosis, which unfortunately is also the case for relapsing disease after surgery. Even though targeted therapies obtained good results, clinical experience showed that tumors eventually develop resistance. On the other hand, earlier attempts of cancer immunotherapy failed to show consistent efficacy. More recently, a deeper knowledge of immunosuppression in the tumor microenvironment (TME) allowed the development of effective drugs: in particular, monoclonal antibodies targeting the so-called immune checkpoint molecules yielded striking and lasting effects in some tumors. Unfortunately, these monoclonal antibodies are not effective in a majority of patients and are ineffective in several solid malignancies. Furthermore, due to their mechanism of action, checkpoint inhibitors often elicit autoimmune-like disease.

MAIN BODY

The use of viruses as oncolytic agents (OVs) was considered in the past, while only recently OVs revealed a connection with immunotherapy. However, their antitumoral potential has remained largely unexplored, due to safety concerns and some limitations in the techniques to manipulate viruses. OV research was recently revived by a better knowledge of viral/cancer biology and advances in the methodologies to delete virulence/immune-escape related genes from even complex viral genomes or "to arm" OVs with appropriate transgenes. Recently, the first oncolytic virus, the HSV-1 based Talimogene Laherparepvec (T-VEC), was approved for the treatment of non-resectable melanoma in USA and Europe.

CONCLUSION

OVs have the potential to become powerful agents of cancer immune and gene therapy. Indeed, in addition to their selective killing activity, they can act as versatile gene expression platforms for the delivery of therapeutic genes. This is particularly true for viruses with a large DNA genome, that can be manipulated to address the multiple immunosuppressive features of the TME. This review will focus on the open issues, on the most promising lines of research in the OV field and, more in general, on how OVs could be improved to achieve real clinical breakthroughs in cancers that are usually difficult to treat by immunotherapy.

Trial Watch: Oncolytic viro-immunotherapy of hematologic and solid tumors

Oncolytic viruses selectively target and kill cancer cells in an immunogenic fashion, thus supporting the establishment of therapeutically relevant tumor-specific immune responses. In 2015, the US Food and Drug Administration (FDA) approved the oncolytic herpes simplex virus T-VEC for use in advanced melanoma patients. Since then, a plethora of trials has been initiated to assess the safety and efficacy of multiple oncolytic viruses in patients affected with various malignancies. Here, we summarize recent preclinical and clinical progress in the field of oncolytic virotherapy.

Fusogenic Viruses in Oncolytic Immunotherapy

Oncolytic viruses are under intense development and have earned their place among the novel class of cancer immunotherapeutics that are changing the face of cancer therapy. Their ability to specifically infect and efficiently kill tumor cells, while breaking immune tolerance and mediating immune responses directed against the tumor, make oncolytic viruses highly attractive candidates for immunotherapy. Increasing evidence indicates that a subclass of oncolytic viruses, which encodes for fusion proteins, could outperform non-fusogenic viruses, both in their direct oncolytic potential, as well as their immune-stimulatory properties. Tumor cell infection with these viruses leads to characteristic syncytia formation and cell death due to fusion, as infected cells become fused with neighboring cells, which promotes intratumoral spread of the infection and releases additional immunogenic signals. In this review, we discuss the potential of fusogenic oncolytic viruses as optimal candidates to enhance immunotherapy and initiate broad antitumor responses. We provide an overview of the cytopathic mechanism of syncytia formation through viral-mediated expression of fusion proteins, either endogenous or engineered, and their benefits for cancer therapy. Growing evidence indicates that fusogenicity could be an important feature to consider in the design of optimal oncolytic virus platforms for combinatorial oncolytic immunotherapy.

Combination of Vaccine-Strain Measles and Mumps Viruses Enhances Oncolytic Activity against Human Solid Malignancies

Oncolytic measles and mumps viruses (MeV, MuV) have a potential for anti-cancer treatment. We examined the anti-tumor activity of MeV, MuV, and MeV-MuV combination (MM) against human solid malignancies (HSM). MeV, MuV, and MM targeted and significantly killed various cancer cell lines of HSM but not normal cells. MM demonstrated a greater anti-tumor effect and prolonged survival in a human prostate cancer xenograft tumor model compared to MeV and MuV. MeV, MuV, and MM significantly induced the expression of immunogenic cell death markers and enhanced spleen-infiltrating immune cells. In conclusion, MM combination significantly improves the treatment of human solid malignancies.

Designing and building oncolytic viruses

Oncolytic viruses (OVs) are engineered and/or evolved to propagate selectively in cancerous tissues. They have a dual mechanism of action; direct killing of infected cancer cells cross-primes anticancer immunity to boost the killing of uninfected cancer cells. The goal of the field is to develop OVs that are easily manufactured, efficiently delivered to disseminated sites of cancer growth, undergo rapid intratumoral spread, selectively kill tumor cells, cause no collateral damage and pose no risk of transmission in the population. Here we discuss the many virus engineering strategies that are being pursued to optimize delivery, intratumoral spread and safety of OVs derived from different virus families. With continued progress, OVs have the potential to transform the paradigm of cancer care.