Future directions for the field of oncolytic virotherapy: a perspective on the use of vaccinia virus

Efficient Selection of Recombinant Fluorescent Vaccinia Virus Strains and Rapid Virus Titer Determination by Using a Multi-Well Plate Imaging System

Engineered vaccinia virus (VACV) strains are used extensively as vectors for the development of novel cancer vaccines and cancer therapeutics. In this study, we describe for the first time a high-throughput approach for both fluorescent rVACV generation and rapid viral titer measurement with the multi-well plate imaging system, IncuCyte^®S3. The isolation of a single, well-defined plaque is critical for the generation of novel recombinant vaccinia virus (rVACV) strains. Unfortunately, current methods of rVACV engineering via plaque isolation are time-consuming and laborious. Here, we present a modified fluorescent viral plaque screening and selection strategy that allows one to generally obtain novel fluorescent rVACV strains in six days, with a minimum of just four days. The standard plaque assay requires chemicals for fixing and staining cells. Manual plaque counting based on visual inspection of the cell culture plates is time-consuming. Here, we developed a fluorescence-based plaque assay for quantifying the vaccinia virus that does not require a cell staining step. This approach is less toxic to researchers and is reproducible; it is thus an improvement over the traditional assay. Lastly, plaque counting by virtue of a fluorescence-based image is very convenient, as it can be performed directly on the computer.

A Novel Oncolytic Chimeric Orthopoxvirus Encoding Luciferase Enables Real-Time View of Colorectal Cancer Cell Infection

This study hypothesizes that a novel oncolytic chimeric orthopoxvirus CF33-Fluc is imageable and targets colorectal cancer cells (CRCs). A novel chimeric orthopoxvirus (CF33) was constructed. The thymidine kinase locus was replaced with firefly luciferase (Fluc) to yield a recombinant virus—CF33-Fluc. In vitro cytotoxicity and viral replication assays were performed. In vivo CRC flank xenografts received single doses of intratumoral or intravenous CF33-Fluc. Viral biodistribution was analyzed via luciferase imaging and organ titers. CF33-Fluc infects, replicates in, and kills CRCs in vitro in a dose-dependent manner. CF33 has superior secretion of extracellular-enveloped virus versus all but one parental strain. Rapid tumor regression or stabilization occurred in vivo at a low dose over a short time period, regardless of the viral delivery method in the HCT-116 colorectal tumor xenograft model. Rapid luciferase expression in virus-infected tumor cells was associated with treatment response. CRC death occurs via necroptotic pathways. CF33-Fluc replicates in and kills colorectal cancer cells in vitro and in vivo regardless of delivery method. Expression of luciferase enables real-time tracking of viral replication. Despite the chimerism, CRC death occurs via standard poxvirus-induced mechanisms. Further studies are warranted in immunocompetent models.

Apoptosis-inducing effect of myxoma virus on human neuroglioma cell lines

The purpose of this study was to further evaluate the role of myxoma virus (MYXV) as an oncolytic agent against experimental human gliomas in vitro, and analyze the effect of MYXV on malignant glioma cells at different incubation periods and infected at different multiplicities of infection. Neuroglioma cell lines U251 and A172 were cultured with various infective doses of myxoma virus at different time points (0-3 days) and cellular survival rates were evaluated using an MTT assay. Cell viability and cell death rates were assessed using Annexin V/propidium iodide and applying flow cytometry. Furthermore, the expression levels of phosphorylated AKT (p-AKT) in malignant gliomas were detected by western blot analysis to investigate the possible cell signaling targets in the pathway. MYXV exhibited a dose and time-dependent cytotoxic effect on neuroglioma cells, and there was increased expression of p-AKT in malignant gliomas. The present study confirms that MYXV induces oncolysis of malignant gliomas through regulating the activation of AKT. As such, MYXV is a potential therapeutic agent against human malignant gliomas.

Oncolytic HSV virotherapy in murine sarcomas differentially triggers an antitumor T-cell response in the absence of virus permissivity

Multiple studies have indicated that in addition to direct oncolysis, virotherapy promotes an antitumor cytotoxic T cell response important for efficacy. To study this phenomenon further, we tested three syngeneic murine sarcoma models that displayed varied degrees of permissiveness to oncolytic herpes simplex virus replication and cytotoxicity in vitro, with the most permissive being comparable to some human sarcoma tumor lines. The in vivo antitumor effect ranged from no or modest response to complete tumor regression and protection from tumor rechallenge. The in vitro permissiveness to viral oncolysis was not predictive of the in vivo antitumor effect, as all three tumors showed intact interferon signaling and minimal permissiveness to virus in vivo. Tumor shrinkage was T-cell mediated with a tumor-specific antigen response required for maximal antitumor activity. Further analysis of the innate and adaptive immune microenvironment revealed potential correlates of susceptibility and resistance, including favorable and unfavorable cytokine profiles, differential composition of intratumoral myeloid cells, and baseline differences in tumor cell immunogenicity and tumor-infiltrating T-cell subsets. It is likely that a more complete understanding of the interplay between the immunologic immune microenvironment and virus infection will be necessary to fully leverage the antitumor effects of this therapeutic platform.

A vaccinia virus encoding the human sodium iodide symporter facilitates long-term image monitoring of virotherapy and targeted radiotherapy of pancreatic cancer

To assess therapeutic response and potential toxicity of oncolytic virotherapy, a noninvasive, deep-tissue imaging modality is needed. This study aimed to assess the feasibility, parameters, and determining factors of serial imaging and long-term monitoring of virotherapy and the therapeutic response of pancreatic cancer xenografts treated with a vaccinia virus carrying the human sodium iodide symporter GLV-1h153.

METHODS

Pancreatic cancer xenografts (PANC-1) in nude mice were treated systemically or intratumorally with GLV-1h153 and serially imaged using (124)I PET at 1, 2, 3, and 5 wk after viral injection. Signal intensity was compared with tumor therapeutic response and optical imaging, and tumors were histologically analyzed for morphology and the presence of virus. Autoradiography was performed using technetium-pertechnetate and γ-scintigraphy to assess determining factors for radiouptake in tumors. Finally, the enhanced therapeutic effect of combination therapy with GLV-1h153 and systemic radioiodine was assessed.

RESULTS

GLV-1h153 successfully facilitated serial long-term imaging of virotherapy, with PET signal intensity correlating to tumor response. GLV-1h153 colonization of tumors mediated radioiodine uptake at potentially therapeutic doses. Successful radiouptake required the presence of virus, adequate blood flow, and viable tissue, whereas loss of signal intensity was linked to tumor death and necrosis. Finally, combining systemically administered GLV-1h153 and (131)I led to enhanced tumor kill when compared with virus or (131)I alone (P < 0.01).

CONCLUSION

GLV-1h153 is a promising oncolytic agent for the treatment, long-term imaging, and monitoring of therapeutic response in a xenograft model of pancreatic cancer. GLV-1h153 provided insight into tumor biologic activity and facilitated enhanced tumor kill when combined with systemic targeted radiotherapy. These results warrant further investigation into parameters and potential synergistic effects of combination therapy.

Progress of oncolytic viruses in sarcomas

Oncolytic virotherapy has shown exciting promise for the treatment of many types of solid tumors. Pediatric sarcomas are an aggressive type of pediatric malignancy known to show limited responsiveness to current therapies, leading to unacceptably high morbidity and mortality. Oncolytic viruses have only recently been used for the treatment of this challenging cancer, and results have been encouraging. Five clinical trials are currently open evaluating the use of oncolytic viruses in pediatric malignancies. Advances in genetic engineering of the viruses include improving the ability of the virus to infect tumor cells, engineering the virus with transgenes which improve the virus' ability to kill tumor cells and manipulating the virus to enhance concomitantly administered therapies. Further understanding of the antiviral immune response and a viral induced anti-tumor immune response will permit a maximization of oncolytic virotherapy.

Activation of a helper and not regulatory human CD4+ T cell response by oncolytic H-1 parvovirus

BACKGROUND

H-1 parvovirus (H-1 PV), a rodent autonomous oncolytic parvovirus, has emerged as a novel class of promising anticancer agents, because of its ability to selectively find and destroy malignant cells. However, to probe H-1 PV multimodal antitumor potential one of the major prerequisites is to decipher H-1 PV direct interplay with human immune system, and so prevent any risk of impairment.

METHODOLOGY/PRINCIPAL FINDINGS

Non activated peripheral blood mononuclear cells (PBMCs) are not sensitive to H-1 PV cytotoxic effect. However, the virus impairs both activated PBMC proliferation ability and viability. This effect is related to H-1 PV infection as evidenced by Western blotting detection of H-1 PV main protein NS1. However, TCID50 experiments did not allow newly generated virions to be detected. Moreover, flow cytometry has shown that H-1 PV preferentially targets B lymphocytes. Despite seeming harmful at first sight, H-1 PV seems to affect very few NK cells and CD8+ T lymphocytes and, above all, clearly does not affect human neutrophils and one of the major CD4+ T lymphocyte subpopulation. Very interestingly, flow cytometry analysis and ELISA assays proved that it even activates human CD4+ T cells by increasing activation marker expression (CD69 and CD30) and both effective Th1 and Th2 cytokine secretion (IL-2, IFN-γ and IL-4). In addition, H-1 PV action does not come with any sign of immunosuppressive side effect. Finally, we have shown the efficiency of H-1 PV on xenotransplanted human nasopharyngeal carcinoma, in a SCID mouse model reconstituted with human PBMC.

CONCLUSIONS/SIGNIFICANCE

Our results show for the first time that a wild-type oncolytic virus impairs some immune cell subpopulations while directly activating a Helper CD4+ T cell response. Thus, our data open numerous gripping perspectives of investigation and strongly argue for the use of H-1 PV as an anticancer treatment.

Next-generation oncolytic vaccinia vectors

Oncolytic vaccinia viruses have made some impressive advances over the last 5 years, with a range of -different backbones displaying significant antitumor responses in preclinical models, and some exciting clinical results being reported against liver cancers. Because the virus is capable of rapid spread within the tumor, has evolved to spread relatively undetected within the blood stream, does not integrate into the host cell chromosome, and can infect almost any cell type, it is well-suited to the requirements for a successful oncolytic. In addition, the extensive clinical use of this virus means that contraindications to its use are known, and approved and experimental antivirals are available. Furthermore, because the virus has a large array of virulence genes whose deletion may target different properties of the cancer cell, and a large cloning capacity allowing for insertion of multiple transgenes, the possibilities for further development of novel and next-generation oncolytic vectors are multitude.

Lister vaccine strain of vaccinia virus armed with the endostatin-angiostatin fusion gene: an oncolytic virus superior to dl1520 (ONYX-015) for human head and neck cancer

Oncolytic viral therapy represents a promising strategy for the treatment of head and neck squamous cell carcinoma (HNSCC), with dl1520 (ONYX-015) the most widely used oncolytic adenovirus in clinical trials. This study aimed to determine the effectiveness of the Lister vaccine strain of vaccinia virus as well as a vaccinia virus armed with the endostatin-angiostatin fusion gene (VVhEA) as a novel therapy for HNSCC and to compare them with dl1520. The potency and replication of the Lister strain and VVhEA and the expression and function of the fusion protein were determined in human HNSCC cells in vitro and in vivo. Finally, the efficacy of VVhEA was compared with dl1520 in vivo in a human HNSCC model. The Lister vaccine strain of vaccinia virus was more effective than the adenovirus against all HNSCC cell lines tested in vitro. Although the potency of VVhEA was attenuated in vitro, the expression and function of the endostatin-angiostatin fusion protein was confirmed in HNSCC models both in vitro and in vivo. This novel vaccinia virus (VVhEA) demonstrated superior antitumor potency in vivo compared with both dl1520 and the control vaccinia virus. This study suggests that the Lister strain vaccinia virus armed with an endostatin-angiostatin fusion gene may be a potential therapeutic agent for HNSCC.

Design and testing of novel oncolytic vaccinia strains

Oncolytic or replication-selective viruses have been used as powerful tools for the delivery of therapeutic genes to tumors. Because these vectors are capable of replicating within the tumor, the therapeutic gene is amplified within the target tissue itself, resulting in the spread of the virus both within the tumor, and sometimes also between tumors. Vaccinia virus holds many advantages when serving as the backbone for oncolytic viral strains, including a large cloning capacity (at least 25 kbp) (1); a short life-cycle (2, 3); extensive previous use in humans, with contraindications and adverse reactions well described and antivirals available (4); the potential for systemic (intravenous) delivery to distant tumors; and vaccinia strains have previously demonstrated antitumor benefits in clinical trials (5). Because vaccinia has no known receptor and is capable of infecting almost any cell type, tumor selectivity has to be engineered into vaccinia at steps after infection. We will therefore discuss potential viral virulence genes and metabolic targets that result in tumor-selective vaccinia strains. Because the virus has limited natural requirements for host cell proteins, and, instead, contains a large genome and multiple genes involved in virulence, a large number of possible attenuating gene deletions can result in the production of viral strains reliant on inherent properties of the host cell for replication. The protocols for producing viral gene deletions and constructing viral gene expression vectors have been well established for vaccinia and are summarized briefly in this chapter. Basic assays for testing the tumor selectivity and therapeutic index of new oncolytic constructs in vitro will be covered. In addition, we describe how bioluminescence imaging can be incorporated into preclinical testing of vaccinia gene expression strains to examine the timing, biodistribution, and kinetics of viral gene expression noninvasively after delivery of the viral agents to tumor-bearing mice via different routes.

Oncolytic vaccinia virotherapy of anaplastic thyroid cancer in vivo

CONTEXT

Anaplastic thyroid carcinoma (ATC) is a fatal disease with a median survival of only 6 months. Novel therapies are needed to improve dismal outcomes.

OBJECTIVE

A mutated, replication-competent, vaccinia virus (GLV-1h68) has oncolytic effects on human ATC cell lines in vitro. We assessed the utility of GLV-1h68 in treating anaplastic thyroid cancer in vivo.

DESIGN

Athymic nude mice with xenograft flank tumors of human ATCs (8505C and DRO90-1) were treated with a single intratumoral injection of GLV-1h68 at low dose (5x10(5) plaque-forming unit), high dose (5x10(6) plaque-forming unit), or PBS. Virus-mediated marker gene expression (luciferase, green fluorescent protein, and beta-galactosidase), viral biodistribution, and flank tumor volumes were measured.

RESULTS

Luciferase expression was detected 2 d after injection. Continuous viral replication within tumors was reflected by increasing luciferase activity to d 9. At d 10, tumor viral recovery was increased more than 50-fold as compared with the injected dose, and minimal virus was recovered from the lung, liver, brain, heart, spleen, and kidneys. High-dose virus directly injected into normal tissues was undetectable at d 10. The mean volume of control 8505C tumors increased 50.8-fold by d 45, in contrast to 10.5-fold (low dose) and 2.1-fold (high dose; P=0.028) increases for treated tumors. DRO90-1 tumors also showed significant growth inhibition by high-dose virus. No virus-related toxicity was observed throughout the study.

CONCLUSIONS

GLV-1h68 efficiently infects, expresses transgenes within, and inhibits the growth of ATC in vivo. These promising findings support future clinical trials for patients with ATC.

A phase I trial of Ad.hIFN-beta gene therapy for glioma

Interferon-beta (IFN-beta) is a pleiotropic cytokine with antitumoral activity. In an effort to improve the therapeutic index of IFN-beta by providing local, sustained delivery of IFN-beta to gliomas, the safety and biological activity of a human IFN-beta (hIFN-beta)-expressing adenovirus vector (Ad.hIFN-beta) was evaluated in patients with malignant glioma by stereotactic injection, followed 4-8 days later by surgical removal of tumor with additional injections of Ad.hIFN-beta into the tumor bed. Eleven patients received Ad.hIFN-beta in cohorts of 2 x 10(10), 6 x 10(10), or 2 x 10(11) vector particles (vp). The most common adverse events were considered by the investigator as being unrelated to treatment. One patient, who was enrolled in the cohort with the highest dose levels, experienced dose-limiting, treatment-related Grade 4 confusion following the post-operative injection. Ad.hIFN-beta DNA was detected within the tumor, blood, and nasal swabs in a dose-dependent fashion and hIFN-beta protein was detectable within the tumor. At the highest doses tested, a reproducible increase in tumor cell apoptosis in post-treatment versus pre-treatment biopsies with associated tumor necrosis was observed. Direct Ad.hIFN-beta injection into the tumor and the surrounding normal brain areas after surgical removal was feasible and associated with apoptosis induction.

Anti-tumor activity of ex vivo expanded cytokine-induced killer cells against human hepatocellular carcinoma

Cytokine-induced killer (CIK) cells are ex vivo expanded T cells with natural killer cell phenotypes and functions. In this study, the anti-tumor activity of CIK cells against hepatocellular carcinoma was evaluated in vitro and in vivo. In the presence of anti-CD3 antibody and IL-2 for 14 days, human peripheral blood mononuclear cell population changed to heterogeneous CIK cell population, which comprised 96% CD3(+), 3% CD3( inverted exclamation mark(c))CD56(+), 32% CD3(+)CD56(+), 11% CD4(+), 75% CD8(+), and 30% CD8(+)CD56(+). CIK cells produced significant amounts of IFN-gamma and TNF-alpha; however, produced only slight amounts of IL-2, IL-4, and IL-5. At an effector-target cell ratio of 30:1, CIK cells destroyed 33% of SNU-354 human hepatocellular carcinoma cells, which was determined by the (51)Cr-release assay. In addition, a dose of 1x10(6) CIK cells per mouse inhibited 60% of SNU-354 tumor growth in irradiated nude mice. This study suggests that CIK cells may be used as an adoptive immunotherapy for patients with hepatocellular carcinoma.

The role of cell signaling in poxvirus tropism: the case of the M-T5 host range protein of myxoma virus

Poxviruses demonstrate strict species specificity in vivo that range from narrow to broad, however the fundamental factors that mediate the basis of poxvirus tropism remain poorly understood. It is generally believed that most, if not all, poxviruses can efficiently bind and enter a wide range of mammalian cells and all of the known host anti-viral pathways that block viral replication in nonpremissive cells operate downstream of virus entry. A productive poxvirus infection is heavily dependent upon the production of a vast array of host modulatory products that specifically target and manipulate both extracellular immune response pathways of the host, as well as intracellular signal transduction pathways of the individually infected cells. The unique pathogenesis and host tropism of specific poxviruses can be attributed to the broad diversity of host modulatory proteins they express. Myxoma virus (MV) is a rabbit-specific poxviruses that encodes multiple host range factors, including an ankyrin-repeat protein M-T5, which functions to regulate tropism of MV for rabbit lymphocytes and some human cancer cells. At the molecular level, M-T5 binds and alters at least two distinct cellular proteins: Akt and cullin-1. The direct interaction between M-T5 and Akt was shown to be a key restriction determinant for MV tropism in a spectrum of human cancer cells making MV an excellent oncolytic candidate. Thus, the intricate relationship between viral encoded proteins and components of the host cell signaling networks can have profound impact on poxvirus tropism. The lessons we continue to learn from poxvirus host range factors like M-T5 will provide further insights into the factors that regulate poxvirus tropism and the mechanisms by which poxviruses micromanipulate the signaling pathways of the infected cell.

Induction of an antitumoral immune response by wild-type adeno-associated virus type 2 in an in vivo model of pancreatic carcinoma

We analyzed the immunologic impact of adeno-associated virus type 2 (AAV-2), a small single-stranded parvovirus with tumorsuppressive properties, on DSL6A pancreatic carcinoma in syngeneic rats. Established tumors of animals treated with AAV-2 or mock infected were resected (Ro), and DSL6A cells were rechallenged on the different site. Eleven (92%) of 12 mock-infected animals but only 3 (25%) of 12 AAV-2-treated animals redeveloped tumors. Adeno-associated virus type 2 infection provoked systemic raises in monocytes and neutrophils numbers and in levels of the proinflammatory monocyte chemoattractant protein 1 and interleukin 10. Adeno-associated virus type 2-treated tumors were infiltrated with monocytes, macrophages, natural killer cells, CD4+ T cells, and especially CD8+ T cells. In cytotoxicity assays, AAV-2-infected DSL6A tumor cells were recognized by lymphocytes from AAV-2-treated animals and from controls. Yet, uninfected DSL6A cells were exclusively killed by lymphocytes from AAV-2-treated animals. Additionally, those lymphocytes displayed high natural killer cell activity but failed to attack unrelated tumor targets. Taken together, these results suggest that the antiviral response toward AAV-2 cross-activates the immune system toward simultaneously present tumor disease. This and the known potential to significantly reduce toxic side effects of chemotherapy make nonpathogenic viruses such as AAV-2 as "1-agent combination therapy" to an interesting treatment option of residual tumor disease.

The highly attenuated vaccinia virus strain modified virus Ankara induces apoptosis in melanoma cells and allows bystander dendritic cells to generate a potent anti-tumoral immunity

Vaccinia virus (VV) has been tested as oncolytic virus against malignant melanoma in clinical trials for more than 40 years. Until now, mainly strains comparable to viral strains used for smallpox vaccination have been probed for anti-tumoral therapy. We have shown recently that the wild-type strain Western Reserve (WR) can interfere with crucial functions of monocyte-derived dendritic cells (DCs). Our aim was to examine whether viral immune evasion mechanisms might be responsible for the ineffectiveness of WR-based vaccination strategies and whether the highly attenuated strain modified virus Ankara (MVA) differs from WR with respect to its possible immunostimulatory capacity after intratumoral injection. Using in vitro experiments, we compared the effect of both strains on melanoma cells and on local bystander DCs. We found that both VV-strains infected melanoma cells efficiently and caused disintegration of the actin cytoskeleton, as shown by fluorescence microscopy. In addition, both VV-strains caused apoptotic cell death in melanoma cells after infection. In contrast to MVA, WR underwent a complete viral replication cycle in melanoma cells. Bystander DCs were consecutively infected by newly generated WR virions and lost their capacity to induce allogeneic T cell proliferation. DCs in contact with MVA-infected melanoma cells retained their capacity to induce T cell proliferation. Immature DCs were capable of phagocytosing MVA-infected melanoma cells. Priming of autologous CD8(+) T cells by DCs that had phagocytosed MVA-infected, MelanA positive melanoma cells resulted in the induction of T cell clones specifically reactive against the model antigen MelanA as shown by enzyme-linked immunospot (ELISPOT) analysis. We conclude that the clinical trials with oncolytic wild-type VV failed probably because of suppression of bystander DCs and consecutive suppression of T cell-mediated anti-melanoma immunity. The attenuated VV-strain MVA facilitates the generation of tumour associated antigen (TAA)-specific T cell response as it is oncolytic for melanoma cells, but non-toxic for DC, and should be a promising candidate for intralesional metastatic melanoma therapy.

Systemic armed oncolytic and immunologic therapy for cancer with JX-594, a targeted poxvirus expressing GM-CSF

Targeted oncolytic viruses and immunostimulatory therapeutics are being developed as novel cancer treatment platforms. These approaches can be combined through the expression of immunostimulatory cytokines from targeted viruses, including adenoviruses and herpesviruses. Although intratumoral injection of such viruses has been associated with tumor growth inhibition, eradication of distant metastases was not reported. The major limitations for this approach to date have been (1) inefficient intravenous virus delivery to tumors and (2) the lack of predictive, immunocompetent preclinical models. To overcome these hurdles, we developed JX-594, a targeted, thymidine kinase(-) vaccinia virus expressing human GM-CSF (hGM-CSF), for intravenous (i.v.) delivery. We evaluated two immunocompetent liver tumor models: a rabbit model with reproducible, time-dependent metastases to the lungs and a carcinogen-induced rat liver cancer model. Intravenous JX-594 was well tolerated and had highly significant efficacy, including complete responses, against intrahepatic primary tumors in both models. In addition, whereas lung metastases developed in all control rabbits, none of the i.v. JX-594-treated rabbits developed detectable metastases. Tumor-specific virus replication and gene expression, systemically detectable levels of hGM-CSF, and tumor-infiltrating CTLs were also demonstrated. JX-594 holds promise as an i.v.-delivered, targeted virotherapeutic. These two tumor models hold promise for the optimization of this approach.

Synergistic antitumor effects of immune cell-viral biotherapy

Targeted biological therapies hold tremendous potential for treatment of cancer, yet their use has been limited by constraints on delivery and effective tumor targeting. We combined an immune effector cell population [cytokine-induced killer (CIK) cells] with an oncolytic viral therapy to achieve directed delivery to, and regression of, tumors in both immunodeficient and immunocompetent mouse models. Preinfection of CIK cells with modified vaccinia virus resulted in a prolonged eclipse phase with the virus remaining hidden until interaction with the tumor. Whole-body imaging revealed that the cells retained their ability to traffic to and to infiltrate the tumor effectively before releasing the virus. These results illustrate the potential of combining biotherapeutics for synergistic effects that more effectively treat cancer.

Myxoma virus is a novel oncolytic virus with significant antitumor activity against experimental human gliomas

Myxoma virus, a poxvirus previously considered rabbit specific, can replicate productively in a variety of human tumor cells in culture. The purpose of this study was to determine if there was efficacy or toxicities of this oncolytic virus against experimental models of human malignant gliomas in vitro, in vivo, and ex vivo in malignant glioma specimens. In vitro, the majority of glioma cell lines tested (7 of 8, 87.5%) were fully permissive for myxoma virus replication and killed by infection. In vivo, intracerebral (i.c.) myxoma virus inoculation was well tolerated and produced only minimal focal inflammatory changes at the site of viral inoculation. U87 and U251 orthotopic xenograft models were used to assess myxoma virus efficacy in vivo. A single intratumoral injection of myxoma virus dramatically prolonged median survival compared with treatment with UV-inactivated myxoma virus. Median survival was not reached in myxoma virus-treated groups versus 47.3 days (U87; P = 0.0002) and 50.7 days (U251; P = 0.0027) in UV-inactivated myxoma virus-treated groups. Most myxoma virus-treated animals (12 of 13, 92%) were alive and apparently "cured" when the experiment was finished (>130 days). Interestingly, we found a selective and long-lived myxoma virus infection in gliomas in vivo. This is the first demonstration of the oncolytic activity of myxoma virus in vivo. The nonpathogenic nature of myxoma virus outside of the rabbit host, its capacity to be genetically modified, its ability to produce a long-lived infection in human tumor cells, and the lack of preexisting antibodies in the human population suggest that myxoma virus may be an attractive oncolytic agent against human malignant glioma.

The enhanced tumor selectivity of an oncolytic vaccinia lacking the host range and antiapoptosis genes SPI-1 and SPI-2

The ability of cancer cells to evade apoptosis may permit survival of a recombinant vaccinia lacking antiapoptotic genes in cancer cells compared with normal cells. We have explored the deletion of two vaccinia virus host range/antiapoptosis genes, SPI-1 and SPI-2, for their effects on the viral replication and their ability to induce cell death in infected normal and transformed cells in vitro. Indeed, in three paired normal and transformed cell types, the SPI-1 and SPI-2 gene-deleted virus (vSP) preferentially replicates in transformed cells or p53-null cells when compared with their normal counterparts. This selectivity may be derived from the fact that vSP-infected normal cells died faster than infected cancer cells. A fraction of infected cells died with evidence of necrosis as shown by both flow cytometry and detection of high-mobility group B1 protein released from necrotic cells into the culture supernatant. When administered to animals, vSP retains full ability to replicate in tumor tissues, whereas replication in normal tissues is greatly diminished. In a model of viral pathogenesis, mice treated with vSP survived substantially longer when compared with mice treated with the wild-type virus. The mutant virus vSP displayed significant antitumoral effects in an MC38 s.c. tumor model in both nude (P < 0.001) and immunocompetent mice (P < 0.05). We conclude that this recombinant vaccinia vSP shows promise for oncolytic virus therapy. Given its enhanced tumor selectivity, improved safety profile, and substantial oncolytic effects following systemic delivery in murine models, it should also serve as a useful vector for tumor-directed gene therapy.

Phase I/II trial of intravenous NDV-HUJ oncolytic virus in recurrent glioblastoma multiforme

We undertook a Phase I/II trial in patients with apparent recurrent glioblastoma multiforme (GBM) based on imaging studies to determine the safety and tumor response of repetitive intravenous administration of NDV-HUJ, the oncolytic HUJ strain of Newcastle disease virus. The first part of the study utilized an accelerated intrapatient dose-escalation protocol with one-cycle dosage steps of 0.1, 0.32, 0.93, 5.9, and 11 billion infectious units (BIU) of NDV-HUJ (1 BIU = 1 x 10(9) EID(50) 50% egg infectious dose) followed by three cycles of 55 BIU. Virus was administered by intravenous infusion over 15 min. In the second part, patients received three cycles of 11 BIU. All patients without progressive disease were maintained with two doses of 11 BIU iv weekly. Eleven of the 14 enrolled patients (11-58 years, Karnofsky performance scale 50-90%) received treatment. Toxicity was minimal with Grade I/II constitutional fever being seen in 5 patients. Maximum tolerated dose was not achieved. Anti-NDV hemagglutinin antibodies appeared within 5-29 days. NDV-HUJ was recovered from blood, saliva, and urine samples and one tumor biopsy. One patient achieved a complete response. Intravenous NDV-HUJ is well tolerated. The findings of good tolerability and encouraging responses warrant the continued evaluation of NDV-HUJ in GBM, as well as other cancers.

Parvovirus H-1-induced tumor cell death enhances human immune response in vitro via increased phagocytosis, maturation, and cross-presentation by dendritic cells

Oncotropic and oncolytic viruses have attracted high attention as antitumor agents because they preferentially kill cancer cells in vitro and reduce the incidence of spontaneous, induced, or implanted animal tumors. Some autonomous parvoviruses (H-1, minute virus of mice) and derived recombinant vectors are currently under preclinical evaluation. Still not fully understood, their antitumor properties involve more than just tumor cell killing. Because wild-type parvovirus-mediated tumor cell lysates (TCLs) may trigger antigen-presenting cells (APCs) to augment the host immune repertoire, we analyzed phagocytosis, maturation, and crosspresentation of H-1-induced TCLs by human dendritic cells (DCs). We first established H-1-mediated oncolysis in two HLA-A2(+) and A2(-) variant melanoma cell clones. Monocyte-derived immature DCs phagocytosed H- 1-infected TCLs as well as ultraviolet-induced apoptotic TCLs and better than freeze-thaw-induced necrotic TCLs. Immature DCs incubated with H-1-induced TCLs acquired specific maturation markers comparable to a standard cytokine cocktail. Furthermore, A2(+) DCs pulsed with H-1-infected A2(-) TCLs cross-presented melanoma antigens to specific cytotoxic T lymphocytes (CTLs) and released proinflammatory cytokines. This shows for the first time that tumor cell killing by a wild-type oncolytic virus directly stimulates human APCs and CTLs. Because H-1-infected tumors enhance the immune repertoire, the clinical perspectives of parvoviral vectors are even more promising.

Myxoma virus infection of primary human fibroblasts varies with cellular age and is regulated by host interferon responses

Recent studies have indicated a critical role for interferon (IFN)-mediated antiviral responses in the host range of myxoma virus (MV), a pathogenic poxvirus of rabbits. To investigate the contribution of IFN to MV tropism in nonleporine cells, primary human dermal fibroblasts (HDFs) were tested for permissiveness to MV infection. Low-passage HDFs that underwent fewer than 25 population doublings (PD) were fully permissive for MV infection, supporting productive virus replication and cell-to-cell spread. In contrast, early and late viral gene expression was detectable in high-passage HDF (>75 PD), but MV failed to generate infectious progeny and could not form foci in these cells. Vesicular stomatitis virus (VSV) plaque reduction assays confirmed that constitutive IFN production progressively increased as HDFs were passaged, concurrent with an increase in the expression of transcripts for type I IFN and IFN-responsive genes involved in antiviral responses. These findings correlated with the enhanced sensitivity of higher-passage HDF to inducers of type I IFN responses, such as dsRNA. Furthermore, pretreatment of low-passage HDF with type I IFN abrogated MV spread and replication while treatment of mature HDF with neutralizing antibodies to IFN-beta, but not IFN-alpha, restored the capacity to form foci. These findings emphasize the importance of post-entry events in determining the permissiveness of human cells to MV infection and support a critical role for innate type I IFN responses as key determinants of poxvirus host range and species restriction.

Poxvirus tropism

Despite the success of the WHO-led smallpox eradication programme a quarter of a century ago, there remains considerable fear that variola virus, or other related pathogenic poxviruses such as monkeypox, could re-emerge and spread disease in the human population. Even today, we are still mostly ignorant about why most poxvirus infections of vertebrate hosts show strict species specificity, or how zoonotic poxvirus infections occur when poxviruses occasionally leap into novel host species. Poxvirus tropism at the cellular level seems to be regulated by intracellular events downstream of virus binding and entry, rather than at the level of specific host receptors as is the case for many other viruses. This review summarizes our current understanding of poxvirus tropism and host range, and discusses the prospects of exploiting host-restricted poxvirus vectors for vaccines, gene therapy or tissue-targeted oncolytic viral therapies for the treatment of human cancers.

Poxvirus host range varies markedly ? some viruses, such as variola and molluscum contagiosum virus (both of which are human-specific), exhibit strict species tropism, whereas others such as cowpox virus are able to infect multiple host species.

Members of four of the eight genera of chordopoxviruses can zoonotically infect man. For example, monkeypox virus can cause severe smallpox-like disease in humans that clinically resembles variola virus.

The species tropism that is exhibited by many poxviruses in terms of causing disease is frequently quite different from the range of cultured cells that can be infected by these viruses.

Specific host-cell receptors do not mediate the distinction between cells that are permissive as opposed to non-permissive for poxvirus infection. Rather, restrictive host cells fail to support the full replication cycle of the infecting poxvirus at a point downstream of binding and entry.

A variety of poxviral host-range genes have been identified that contribute to the control of permissive versus non-permissive infection of cultured mammalian cells. The gene products of these host-range genes regulate the ability of the virus to complete its cytoplasmic replication cycle.

The development of host-restricted vaccines, like modified vaccinia Ankara (MVA), that do not replicate in humans but that retain potent immunogenicity, will provide safer platforms for recombinant vaccines.

Another advance has been the development of poxvirus-based oncolytic vectors that replicate preferentially in human tumour cells.