Human immunodeficiency virus type 1-mediated syncytium formation is compatible with adenovirus replication and facilitates efficient dispersion of viral gene products and de novo-synthesized virus particles

The genome position of a therapeutic transgene strongly influences the level of expression in an armed oncolytic human adenovirus vector

Efficacy of oncolytic, conditionally-replicating adenovirus (CRAd) vectors can be enhanced by "arming" the vector with therapeutic transgenes. We examined whether inclusion of an intact early region 3 (E3) and the reptilian reovirus fusogenic p14 fusion-associated small transmembrane (FAST) protein enhanced vector efficacy. The p14 FAST transgene was cloned between the fiber gene and E4 region, with an upstream splice acceptor for replication-dependent expression from the major late promoter. In A549 cells, this vector expressed p14 FAST protein at very low levels, and showed a poor ability to mediate cell-cell fusion, relative to a similar vector encoding p14 FAST within the E3 deletion. Although expression of E3 proteins from the CRAd increased plaque size, poor expression of p14 FAST protein compromised the fusogenic capacity of the vector. Thus, location of a therapeutic transgene within a CRAd can significantly impact expression of the transgene and is an important consideration in vector design.

Fusogenic oncolytic vaccinia virus enhances systemic antitumor immune response by modulating the tumor microenvironment

Oncolytic viruses induce antitumor immunity following direct viral oncolysis. However, their therapeutic effects are limited in distant untreated tumors because their antitumor function depends on indirect antitumor immunity. Here, we generated a novel fusogenic oncolytic vaccinia virus (FUVAC) and compared its antitumor activity with that of its parental non-fusogenic virus. Compared with the parent, FUVAC exerted the cytopathic effect and induced immunogenic cell death in human and murine cancer cells more efficiently. In a bilateral tumor-bearing syngeneic mouse model, FUVAC administration significantly inhibited tumor growth in both treated and untreated tumors. However, its antitumor effects were completely suppressed by CD8⁺ T cell depletion. Notably, FUVAC reduced the number of tumor-associated immune-suppressive cells in treated tumors, but not in untreated tumors. Mice treated with FUVAC before an immune checkpoint inhibitor (ICI) treatment achieved complete response (CR) in both treated and untreated tumors, whereas ICI alone did not show antitumor activity. Mice achieving CR rejected rechallenge with the same tumor cells, suggesting establishment of a long-term tumor-specific immune memory. Thus, FUVAC improves the tumor immune microenvironment and enhances systemic antitumor immunity, suggesting that, alone and in combination with ICI, it is a novel immune modulator for overcoming oncolytic virus-resistant tumors.

Strategies for enhancing intratumoral spread of oncolytic adenoviruses

Oncolytic viruses, effectively replicate viruses within malignant cells to lyse them without affecting normal ones, have recently shown great promise in developing therapeutic options for cancer. Adenoviruses (Ads) are one of the candidates in oncolytic virotheraoy due to its easily manipulated genomic DNA and expression of wide rane of its receptors on the various cancers. Although systematic delivery of oncolytic adenoviruses can target both primary and metastatic tumors, there are some drawbacks in the effective systematic delivery of oncolytic adenoviruses, including pre-existing antibodies and liver tropism. To overcome these limitations, intratumural (IT) administration of oncolytic viruses have been proposed. However, IT injection of Ads leaves much of the tumor mass unaffected and Ads are not able to disperse more in the tumor microenvironment (TME). To this end, various strategies have been developed to enhance the IT spread of oncolytic adenoviruses, such as using extracellular matrix degradation enzymes, junction opening peptides, and fusogenic proteins. In the present paper, we reviewed different oncolytic adenoviruses, their application in the clinical trials, and strategies for enhancing their IT spread.

An Oncolytic Adenovirus Vector Expressing p14 FAST Protein Induces Widespread Syncytium Formation and Reduces Tumor Growth Rate In Vivo

Intratumoral injection of oncolytic viruses provides a direct means of tumor cell destruction for inoperable tumors. Unfortunately, oncolytic vectors based on human adenovirus (HAdV) typically do not spread efficiently throughout the tumor mass, reducing the efficacy of treatment. In this study, we explore the efficacy of a conditionally replicating HAdV vector expressing the p14 Fusion-Associated Small Transmembrane (FAST) protein (CRAdFAST) in both immunocompetent and immunodeficient mouse models of cancer. The p14 FAST protein mediates cell-cell fusion, which may enhance spread of the virus-mediated, tumor cell-killing effect. In the murine 4T1 model of cancer, treatment with CRAdFAST resulted in enhanced cell death compared to vector lacking the p14 FAST gene, but it did not reduce the tumor growth rate in vivo. In the human A549 lung adenocarcinoma model of cancer, CRAdFAST showed significantly improved oncolytic efficacy in vitro and in vivo. In an A549 xenograft tumor model in vivo, CRAdFAST induced tumor cell fusion, which led to the formation of large acellular regions within the tumor and significantly reduced the tumor growth rate compared to control vector. Our results indicate that expression of p14 FAST from an oncolytic HAdV can improve vector efficacy for the treatment of cancer.

The Role of Oncolytic Viruses in the Treatment of Melanoma

PURPOSE OF REVIEW

Oncolytic virotherapy is a new approach to the treatment of cancer and its success in the treatment of melanoma represents a breakthrough in cancer therapeutics. This paper provides a review of the current literature on the use of oncolytic viruses (OVs) in the treatment of melanoma.

RECENT FINDINGS

Talimogene laherparepvec (T-VEC) is the first OV approved for the treatment of melanoma and presents new challenges as it enters the clinical setting. Several other OVs are at various stages of clinical and pre-clinical development for the treatment of melanoma. Reports from phase Ib-III clinical trials combining T-VEC with checkpoint blockade are encouraging and demonstrate potential added benefit of combination immunotherapy. OVs have recently emerged as a standard treatment option for patients with advanced melanoma. Several OVs and therapeutic combinations are in development. Immunooncolytic virotherapy combined with immune checkpoint inhibitors is promising for the treatment of advanced melanoma.

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.

Adenoviral Vectors Armed with Cell Fusion-Inducing Proteins as Anti-Cancer Agents

Cancer is a devastating disease that affects millions of patients every year, and causes an enormous economic burden on the health care system and emotional burden on affected families. The first line of defense against solid tumors is usually extraction of the tumor, when possible, by surgical methods. In cases where solid tumors can not be safely removed, chemotherapy is often the first line of treatment. As metastatic cancers often become vigorously resistant to treatments, the development of novel, more potent and selective anti-cancer strategies is of great importance. Adenovirus (Ad) is the most commonly used virus in cancer clinical trials, however, regardless of the nature of the Ad-based therapeutic, complete responses to treatment remain rare. A number of pre-clinical studies have shown that, for all vector systems, viral spread throughout the tumor mass can be a major limiting factor for complete tumor elimination. By expressing exogenous cell-fusion proteins, many groups have shown improved spread of Ad-based vectors. This review summarizes the research done to examine the potency of Ad vectors expressing fusogenic proteins as anti-cancer therapeutics.

Expression of the fusogenic p14 FAST protein from a replication-defective adenovirus vector does not provide a therapeutic benefit in an immunocompetent mouse model of cancer

When injected directly into a tumor mass, adenovirus (Ad) vectors only transduce cells immediately along the injection tract. Expression of fusogenic proteins from the Ad vector can lead to syncytium formation, which efficiently spreads the therapeutic effect. Fusogenic proteins can also cause cancer cell death directly, and enhance the release of exosome-like particles containing tumor-associated antigens, which boosts the anti-tumor immune response. In this study, we have examined whether delivery of an early region 1 (E1)-deleted, replication-defective Ad vector encoding the reptilian reovirus p14 fusion-associated small transmembrane (FAST) protein can provide therapeutic efficacy in an immunocompetent mouse tumor model. A high multiplicity of infection of AdFAST is required to induce cell fusion in mouse mammary carcinoma 4T1 cells in vitro, and FAST protein expression caused a modest reduction in cell membrane integrity and metabolic activity compared with cells infected with a control vector. Cells expressing FAST protein released significantly higher quantities of exosomes. In immunocompetent Balb/C mice harboring subcutaneous 4T1 tumors, AdFAST did not induce detectable cancer cell fusion, promote tumor regression or prolong mouse survival compared with untreated mice. This study suggests that in the context of the 4T1 model, Ad-mediated FAST protein expression did not elicit a therapeutic effect.

Adenovirus-Mediated Expression of the p14 Fusion-Associated Small Transmembrane Protein Promotes Cancer Cell Fusion and Apoptosis In Vitro but Does Not Provide Therapeutic Efficacy in a Xenograft Mouse Model of Cancer

Adenoviruses (Ads) are used in numerous preclinical and clinical studies for delivery of anti-cancer therapeutic genes. Unfortunately, Ad has a poor ability to distribute throughout a tumor mass after intratumoral injection, and infects cells primarily within the immediate area of the injection tract. Thus, Ad-encoded transgene expression is typically limited to only a small percentage of cells within the tumor. One method to increase the proportion of the tumor impacted by Ad is through expression of fusogenic proteins. Infection of a single cell with an Ad vector encoding a fusogenic protein should lead to syncytium formation with adjacent cells, effectively spreading the effect of Ad and Ad-encoded therapeutic transgenes to a greater percentage of the tumor mass. Moreover, syncytium formation can be cytotoxic, suggesting that such proteins may be effective sole therapeutics. We show that an early region 1 (E1)-deleted Ad expressing reptilian reovirus p14 fusion-associated small transmembrane (FAST) protein caused extensive cell fusion in the replication-permissive 293 cell line and at high multiplicity of infection in non-permissive human lung adenocarcinoma A549 cells in vitro. FAST protein expression in the A549 cancer cell line led to a loss of cellular metabolic activity and membrane integrity, which correlated with induction of apoptosis. However, in an A549 xenograft CD-1 nude mouse cancer model, Ad-mediated FAST gene delivery did not induce detectable cell fusion, reduce tumor burden nor enhance mouse survival compared to controls. Taken together, our results show that, although AdFAST can enhance cancer cell killing in vitro, it is not effective as a sole therapeutic in the A549 tumor model in vivo.

GALV expression enhances the therapeutic efficacy of an oncolytic adenovirus by inducing cell fusion and enhancing virus distribution

The limitations of the current oncolytic adenoviruses for cancer therapy include insufficient potency and poor distribution of the virus throughout the tumor mass. To address these problems, we generated an oncolytic adenovirus expressing the hyperfusogenic form of the gibbon-ape leukemia virus (GALV) envelope glycoprotein under the control of the adenovirus major late promoter. The oncolytic properties of the new fusogenic adenovirus, ICOVIR16, were analyzed both in vitro and in vivo, and compared with that of its non-fusogenic counterpart, ICOVIR15. Our results indicate that GALV expression by ICOVIR16 induced extensive syncytia formation and enhanced tumor cell killing in a variety of tumor cell types. When injected intratumorally or intravenously into mice with large pre-established melanoma or pancreatic tumors, ICOVIR16 rapidly reduced tumor burden, and in some cases, resulted in complete eradication of the tumors. Importantly, GALV expression induced tumor cell fusion in vivo and enhanced the spreading of the virus throughout the tumor. Taken together, these results indicate that GALV expression can improve the antitumoral potency of an oncolytic adenovirus and suggest that ICOVIR16 is a promising candidate for clinical evaluation in patients with cancer.

Active adenoviral vascular penetration by targeted formation of heterocellular endothelial-epithelial syncytia

The endothelium imposes a structural barrier to the extravasation of systemically delivered oncolytic adenovirus (Ad). Here, we introduced a transendothelial route of delivery in order to increase tumor accumulation of virus particles (vp) beyond that resulting from convection-dependent extravasation alone. This was achieved by engineering an Ad encoding a syncytium-forming protein, gibbon ape leukemia virus (GALV) fusogenic membrane glycoprotein (FMG). The expression of GALV was regulated by a hybrid viral enhancer-human promoter construct comprising the human cytomegalovirus (CMV) immediate-early enhancer and the minimal human endothelial receptor tyrosine kinase promoter (“eTie1”). Endothelial cell-selectivity of the resulting Ad-eTie1-GALV vector was demonstrated by measuring GALV mRNA transcript levels. Furthermore, Ad-eTie1-GALV selectively induced fusion between infected endothelial cells and uninfected epithelial cells in vitro and in vivo, allowing transendothelial virus penetration. Heterofusion of infected endothelium to human embryonic kidney 293 (HEK 293) cells, in mixed in vitro cultures or in murine xenograft models, permitted fusion-dependent transactivation of the replication-deficient Ad-eTie1-GALV, due to enabled access to viral E1 proteins derived from the HEK 293 cytoplasm. These data provide evidence to support our proposed use of GALV to promote Ad penetration through tumor-associated vasculature, an approach that may substantially improve the efficiency of systemic delivery of oncolytic viruses to disseminated tumors.

Rapid and sensitive lentivirus vector-based conditional gene expression assay to monitor and quantify cell fusion activity

Cell-to-cell fusion is involved in multiple fundamental biological processes. Prominent examples include osteoclast and giant cell formation, fertilization and skeletal myogenesis which involve macrophage, sperm-egg and myoblast fusion, respectively. Indeed, the importance of cell fusion is underscored by the wide range of homeostatic as well as pathologic processes in which it plays a key role. Therefore, rapid and sensitive systems to trace and measure cell fusion events in various experimental systems are in demand. Here, we introduce a bipartite cell fusion monitoring system based on a genetic switch responsive to the site-specific recombinase FLP. To allow flexible deployment in both dividing as well as non-dividing cell populations, inducer and reporter modules were incorporated in lentivirus vector particles. Moreover, the recombinase-inducible transcription units were designed in such a way as to minimize basal activity and chromosomal position effects in the "off" and "on" states, respectively. The lentivirus vector-based conditional gene expression assay was validated in primary human mesenchymal stem cells and in a differentiation model based on muscle progenitor cells from a Duchenne muscular dystrophy patient using reporter genes compatible with live- and single-cell imaging and with whole population measurements. Using the skeletal muscle cell differentiation model, we showed that the new assay displays low background activity, a 2-log dynamic range, high sensitivity and is amenable to the investigation of cell fusion kinetics. The utility of the bipartite cell fusion monitoring system was underscored by a study on the impact of drug- and RNAi-mediated p38 MAPK inhibition on human myocyte differentiation. Finally, building on the capacity of lentivirus vectors to readily generate transgenic animals the present FLP-inducible system should be adaptable, alone or together with Cre/loxP-based assays, to cell lineage tracing and conditional gene manipulation studies in vivo.

Lac-regulated system for generating adenovirus 5 vaccine vectors expressing cytolytic human immunodeficiency virus 1 genes

Adenovirus (Ad) vectors have been developed as human immunodeficiency-1 (HIV-1) vaccine vectors because they consistently induce immune responses in preclinical animal models and human trials. Strong promoters and codon-optimization are often used to enhance vaccine-induced HIV-1 gene expression and immunogenicity. However, if the transgene is inherently cytotoxic in the cell line used to produce the vector, and is expressed at high levels, it is difficult to rescue a stable Ad HIV-1 vaccine vector. Therefore we hypothesized that generation of Ad vaccine vectors expressing cytotoxic genes, such as HIV-1 env, would be more efficient if expression of the transgene was down-regulated during Ad rescue. To test this hypothesis, a Lac repressor-operator system was applied to regulate expression of reporter luciferase and HIV-1 env transgenes during Ad rescue. The results demonstrate that during Ad rescue, constitutive expression of the Lac repressor in 293 cells reduced transgene expression levels to approximately 5% of that observed in the absence of regulation. Furthermore, Lac-regulation translated into more efficient Ad rescue compared to traditional 293 cells. Importantly, Ad vectors rescued with this system showed high levels of transgene expression when transduced into cells that lack the Lac repressor protein. The Lac-regulated system also facilitated the rescue of modified Ad vectors that have non-native receptor tropism. These tropism-modified Ad vectors infect a broader range of cell types than the unmodified Ad, which could increase their effectiveness as a vaccine vector. Overall, the Lac-regulated system described here (i) is backwards compatible with Ad vector methods that employ bacterial-mediated homologous recombination, (ii) is adaptable for the engineering of tropism-modified Ad vectors, and (iii) does not require co-expression of regulatory genes from the vector or the addition of exogenous chemicals to induce or repress transgene expression. This system therefore could facilitate the development of Ad-based vaccine candidates that otherwise would not be feasible to generate.

Armed replicating adenoviruses for cancer virotherapy

Conditionally replicating adenoviruses (CRAds) have many advantages as agents for cancer virotherapy and have been safely used in human clinical trials. However, replicating adenoviruses have been limited in their ability to eliminate tumors by oncolysis. Thus, the efficacy of these agents must be improved. To this end, CRAds have been engineered to express therapeutic transgenes that exert antitumor effects independent of direct viral oncolysis. These transgenes can be expressed under native gene control elements, in which case placement within the genome determines the expression profile, or they can be controlled by exogenous promoters. The therapeutic transgenes used to arm replicating adenoviruses can be broadly classified into three groups. There are those that mediate killing of the infected cell, those that modulate the tumor microenvironment and those with immunomodulatory functions. Overall, the studies to date in animal models have shown that arming a CRAd with a rationally chosen therapeutic transgene can improve its antitumor efficacy over that of an unarmed CRAd. However, a number of obstacles must be overcome before the full potential of armed CRAds can be realized in the human clinical context. Hence, strategies are being developed to permit intravenous delivery to disseminated cancer cells, overcome the immune response and enable in vivo monitoring of the biodistribution and activity of armed CRAds.

Nuclear survivin abrogates multiple cell cycle checkpoints and enhances viral oncolysis

Survivin (BIRC5) promotes cell division and survival with roles as chromosomal passenger protein and inhibitor of apoptosis protein (IAP). It is overexpressed in many cancers and is associated with resistance to chemotherapy and radiation. Previously, we showed that expression of survivin within the nucleus of HeLa cells accelerates its degradation and blocks apoptosis inhibition without affecting localization during mitosis. Here, we have investigated the effects of survivin on cell cycle control and potential therapeutic consequences using HeLa and IGROV1 cells expressing wild-type and nuclear-targeted survivin. We show that overexpression of survivin, especially within the nucleus, increases control over G(1)-S checkpoint via increased nuclear accumulation of cyclin D and cyclin-dependent kinase 4 and subsequent pRb phosphorylation. We investigated the influence of survivin on the activity of the E1A CR2-deleted oncolytic adenovirus dl922-947, which depends critically on an aberrant G(1)-S checkpoint. Nuclear expression of survivin augments virus-induced S-phase induction and increases viral protein expression and overall viral replication. There is a consequent increase in antitumor activity both in vitro and in vivo. The increased dl922-947 activity is restricted to malignant cells and is not associated with induction of apoptosis, nor does it rely on the role of survivin as an IAP. In addition, we observe the appearance of a large >or=4N population coincident with multiple mitotic defects in dl922-947-infected cells, both of which are significantly increased by nuclear survivin. This indicates that adenoviral activity is facilitated by abrogation of multiple cell cycle checkpoints and can be enhanced by expression of survivin within the nucleus.

Syncytia formation affects the yield and cytotoxicity of an adenovirus expressing a fusogenic glycoprotein at a late stage of replication

Fusogenic membrane glycoproteins (FMGs) may enhance the cytotoxicity of conditionally replicative adenoviruses. However, expression at early stages of infection impairs virus replication. We have inserted the hyperfusogenic form of the gibbon ape leukemia virus (GALV) envelope glycoprotein as a new splice unit of the major late promoter (MLP) to generate a replication-competent adenovirus expressing this protein. At high multiplicity of infection (MOI), this virus replicated efficiently forming clumps of fused cells and showing a faster release. In contrast, at low MOI, infected cells formed syncytia where only one nucleus contained virus DNA, decreasing total virus production but increasing cytotoxicity.

Evaluation of twenty-one human adenovirus types and one infectivity-enhanced adenovirus for the treatment of malignant melanoma

Advanced melanoma is associated with poor prognosis warranting the development of new therapeutics, such as oncolytic adenoviruses for immunovirotherapy. Since this approach critically depends on efficient transduction of targeted tumor cells, we screened a panel of 22 different adenovirus types for their internalization efficiency in melanoma cells. We demonstrated that the virions of Ad35, Ad38, and Ad3 have significantly higher internalization efficiency in melanoma cells than Ad5, so far the only adenovirus type used in clinical trials for melanoma. Therefore, we developed a conditionally replication-competent Ad5-based vector with the Ad35 fiber shaft and knob domains (Ad5/35) and compared its therapeutic efficacy with the homologous vector carrying the native Ad5 fiber. To further enhance virotherapy, we combined the oncolytic adenovirus vectors with intratumoral expression of measles virus fusogenic membrane glycoproteins H and F (MV-H/F) and dacarbazine chemotherapy. In a human melanoma xenograft model, established from a short-term culture of primary melanoma cells, we demonstrated that the Ad5/35-based therapy had a significantly greater anti-neoplastic effect than the homologous Ad5-based therapy. Furthermore, the combination of virotherapy, intratumoral expression of MV-H/F, and chemotherapy was clearly superior to single- or double-agent therapy. In conclusion, Ad35-based vectors are promising for the treatment of melanoma.

Intratumoral expression of respiratory syncytial virus fusion protein in combination with cytokines encoded by adenoviral vectors as in situ tumor vaccine for colorectal cancer

Although cancers can naturally elicit immune responses, immune ignorance is a common observation preventing immune-mediated elimination of tumor cells. We assessed whether intratumoral expression of respiratory syncytial virus fusion (RSV-F) protein, encoded by a replication-defective adenovirus vector (Ad.RSV-F), alone or in combination with local coexpression of cytokines can induce tumor-specific immune responses in a syngeneic murine colon cancer model. We confirmed in vitro by dye colocalization that transduction of murine cells with Ad.RSV-F induces cell-cell fusion. In vivo, we showed in a bilateral syngeneic s.c. colon cancer model in C57BL/6 and BALB/c mice that intratumoral injection of Ad.RSV-F leads to a significant volume reduction not only of the directly vector-treated tumor but also of the contralateral not directly vector-treated tumor. The intratumoral administration of Ad.RSV-F in combination with adenovirus vectors encoding interleukin (IL)-2, IL-12, IL-18, IL-21, or granulocyte macrophage colony-stimulating factor significantly enhanced the antitumor effect on the directly vector-treated tumor and also on the contralateral tumor. The antineoplastic efficacy of this combined treatment was significantly higher than that of the individual treatment components and was associated with the induction of a tumor-specific CTL response and increased infiltration of the tumors by natural killer cells and macrophages. Intratumoral coexpression of RSV-F and IL-21 resulted in the highest tumor growth inhibition and improved survival. Our experimental data indicate that intratumoral expression of RSV-F in combination with cytokines is a promising novel tool for the development of in situ tumor vaccination approaches.

Local and distant immune-mediated control of colon cancer growth with fusogenic membrane glycoproteins in combination with viral oncolysis

We evaluated whether the expression of measles virus fusogenic membrane glycoproteins H and F (MV-FMG), encoded by a herpes simplex virus type 1 (HSV-1) amplicon vector, can serve with or without viral oncolysis (G47Delta) and facultative irinotecan chemotherapy, alone or in combination with the monoclonal epidermal growth factor receptor (EGFR) inhibitory antibody cetuximab, as a platform for inducing tumor-specific immune responses against colon cancer. We demonstrated in vitro that MV-FMG expression in murine cells resulted in cell-cell fusion and synergistically enhanced the cytotoxicity of irinotecan alone or in combination with cetuximab. In a bilateral syngeneic subcutaneous MC38 and Colon26 tumor model in C57BL/6 and BALB/c mice we assessed both the effect on directly vector-treated tumors and the effect on contralateral, not directly vector-treated tumors. We demonstrated that the combination of three treatment components with or without cetuximab resulted in the best volume reduction of both directly vector-treated and not directly vector-treated tumors as well as pronounced infiltration of both tumor types with natural killer cells, macrophages, and T cells. T cells of these animals exhibited strong ex vivo cytotoxic activity against the tumor cells, indicating that the antineoplastic effect on untreated tumors was mediated by an antitumor immune response. Preexisting immunity against HSV-1 or measles virus had no detrimental effect on overall treatment efficacy. Our data indicate that MV-FMG expression in combination with viral oncolysis with or without clinically relevant chemotherapy for colon cancer treatment warrants further investigation.

In situ tumor vaccination with adenovirus vectors encoding measles virus fusogenic membrane proteins and cytokines

AIM: To evaluate whether intratumoral expression of measles virus fusogenic membrane glycoproteins H and F (MV-FMG), encoded by an adenovirus vector Ad.MV-H/F, alone or in combination with local coexpression of cytokines (IL-2, IL-12, IL-18, IL-21 or GM-CSF), can serve as a platform for inducing tumor-specific immune responses in colon cancer.

METHODS: We used confocal laser scanning microscopy and flow cytometry to analyze cell-cell fusion after expression of MV-FMG by dye colocalization. In a syngeneic bilateral subcutaneous MC38 and Colon26 colon cancer model in C57BL/6 and BALB/c mice, we assessed the effect on both the directly vector-treated tumor as well as the contralateral, not directly vector-treated tumor. We assessed the induction of a tumor-specific cytotoxic T lymphocyte (CTL) response with a lactate dehydrogenase (LDH) release assay.

RESULTS: We demonstrated in vitro that transduction of MC38 and Colon26 cells with Ad.MV-H/F resulted in dye colocalization, indicative of cell-cell fusion. In addition, in the syngeneic bilateral tumor model we demonstrated a significant regression of the directly vector-inoculated tumor upon intratumoral expression of MV-FMG alone or in combination with the tested cytokines. We observed the highest anti-neoplastic efficacy with MV-FMG and IL-21 coexpression. The degree of tumor regression of the not directly vector-treated tumor correlated with the anti-neoplastic response of the directly vector-treated tumor. This regression was mediated by a tumor-specific CTL response.

CONCLUSION: Our data indicate that intratumoral expression of measles virus fusogenic membrane glycoproteins is a promising tool both for direct tumor treatment as well as for tumor vaccination approaches that can be further enhanced by cytokine coexpression.

Comparison of herpes simplex virus- and conditionally replicative adenovirus-based vectors for glioblastoma treatment

In this study we compared side-by-side the anti-neoplastic activity of the oncolytic herpes simplex virus-1 (HSV-1) vector G47Delta with that of a conditionally replicative adenoviral vector for the treatment of glioblastoma. We analyzed the transduction efficiency of permanent glioblastoma cell lines and short-term cultures of glioblastoma cells with HSV.Luc and four adenovirus type 5 (Ad5)-based vectors that differed only in their fiber gene (Ad5.Luc, AdlucRGD, and the fiber chimeric vectors Ad5/3.Luc and Ad5/35.Luc). In the tested short-term cultures of glioblastoma cells the vectors Ad5/35.Luc and HSV.Luc had an equal transduction efficiency which was approximately 70% higher than that of Ad5.Luc. In a subcutaneous xenograft glioblastoma model in nude mice we observed a significantly higher local tumor control with the G47Delta vector compared to the conditionally replicative Ad5/35 adenovirus. We confirmed in glioblastoma that the intratumoral expression of measles virus fusogenic membrane glycoproteins (FMG) encoded by replication-defective Ad5/35 or HSV-1 amplicon vectors synergistically enhances chemotherapy with temozolomide. The anti-neoplastic effect was superior when the replication-defective FMG encoding vectors were trans-complemented for replication with the respective oncolytic vector. This approach was necessary due to packaging constraints of adenovirus. At day 100, of 6 treated animals 1 was alive that received the Ad5/35- and 3 that received the HSV-1-based triple therapy. In an intracranial glioblastoma xenograft model we demonstrated the applicability of this strategy. Due to the higher oncolytic efficacy and packaging capacity of the HSV-1 vectors compared to adenovirus, these vectors are promising for the treatment of glioblastoma.

Oncolytic viruses in cancer therapy

Oncolytic virotherapy is a promising form of gene therapy for cancer, employing nature’s own agents to find and destroy malignant cells. The purpose of this review is to provide an introduction to this very topical field of research and to point out some of the current observations, insights and ideas circulating in the literature. We have strived to acknowledge as many different oncolytic viruses as possible to give a broader picture of targeting cancer using viruses. Some of the newest additions to the panel of oncolytic viruses include the avian adenovirus, foamy virus, myxoma virus, yaba-like disease virus, echovirus type 1, bovine herpesvirus 4, Saimiri virus, feline panleukopenia virus, Sendai virus and the non-human coronaviruses. Although promising, virotherapy still faces many obstacles that need to be addressed, including the emergence of virus-resistant tumor cells.

Evaluation of twenty human adenoviral types and one infectivity-enhanced adenovirus for the therapy of soft tissue sarcoma

The clinical course of sarcoma warrants the development of new therapeutic options, such as gene therapy. However, the lack of coxsackievirus-adenovirus receptor (CAR) on sarcoma cells limits the efficacy of adenovirus type 5 (Ad5)-based gene therapy. In this study we evaluated 20 different adenoviral types and 1 Ad5 vector with RGD-containing fiber for their internalization efficiency in sarcoma cells. We demonstrated that adenovirus types 35, 3, 7, 11, 9, and 22 and Ad5lucRGD virions (ranked in descending order) have significantly higher internalization efficiency in the tested sarcoma cells when compared with Ad5. On the basis of these results we developed a conditionally replication-competent adenoviral vector, Ad5Delta24.Ki.COX, and compared its oncolytic efficacy with that of Ad5/35Delta24.Ki.COX, an Ad5-based vector with the Ad35 fiber shaft and knob domains. Because both vectors differed only in the fiber, we were able to assess whether the adenoviral type with the most efficient internalization resulted also in enhanced treatment efficacy. We evaluated the antineoplastic activity of the oncolytic adenoviral vectors alone or in combination with the expression of measles virus fusogenic membrane glycoproteins and/or ifosfamide. The findings of our xenograft model were as follows: animals that received Ad5/35-based therapy had significantly smaller tumors than animals treated with the homologous Ad5-based vectors. In addition, we demonstrated that the combination of virotherapy, intratumoral expression of fusogenic membrane glycoproteins, and ifosfamide was clearly superior compared with treatment with individual components alone or as combinations of two components. In conclusion, Ad35-based vectors are promising for the treatment of sarcoma.

Enhanced killing of pancreatic cancer cells by expression of fusogenic membrane glycoproteins in combination with chemotherapy

Pancreatic cancer has a poor prognosis with an annual mortality rate close to the annual incidence rate. We evaluated whether the expression of measles virus fusogenic membrane glycoproteins (FMG) H and F will enhance chemotherapy. Using Chou-Talalay analysis, we showed in vitro in pancreatic cancer cells that the expression of FMG often synergistically enhances clinically relevant chemotherapy. Furthermore, cell fusion in combination with chemotherapy resulted in strongly enhanced Annexin V binding, an early marker for apoptosis, when compared with single treatment. We showed in an i.p. and s.c. pancreatic xenograft model that the administration of a replication-defective adenoviral vector Ad.H/F encoding tumor-restricted FMG in combination with gemcitabine significantly enhanced treatment outcome when compared with treatment with each compound individually. To improve tumor transduction efficiency, the Ad.H/F vector was also transcomplemented with an oncolytic replication-restricted adenovirus (Ad.COX*MK), resulting in significantly improved treatment efficacy. We assessed treatment efficacy by survival analysis or measuring growth, respectively. In the i.p. model, on day 120, three of eight animals treated with this novel triple therapy consisting of Ad.H/F, gemcitabine, and Ad.COX*MK were alive and tumor free. Treatment with Ad.H/F and Ad.COX*MK resulted in one long-term survivor. In all other treatment groups, there were no long-term survivors. The significantly improved therapeutic outcome of animals receiving the triple therapy was attributed to multiple factors, including most likely improved FMG expression throughout the tumor and enhanced sensitivity of the tumor cells to gemcitabine by adenoviral gene products but also FMG expression. Qualitatively similar results were obtained in a s.c. pancreatic xenograft model.

Construction of doxycyline-dependent mini-HIV-1 variants for the development of a virotherapy against leukemias

T-cell acute lymphoblastic leukemia (T-ALL) is a high-risk type of blood-cell cancer. We describe the improvement of a candidate therapeutic virus for virotherapy of leukemic cells. Virotherapy is based on the exclusive replication of a virus in leukemic cells, leading to the selective removal of these malignant cells. To improve the safety of such a virus, we constructed an HIV-1 variant that replicates exclusively in the presence of the nontoxic effector doxycycline (dox). This was achieved by replacement of the viral TAR-Tat system for transcriptional activation by the Escherichia coli-derived Tet system for inducible gene expression. This HIV-rtTA virus replicates in a strictly dox-dependent manner. In this virus, additional deletions and/or inactivating mutations were introduced in the genes for accessory proteins. These proteins are essential for virus replication in untransformed cells, but dispensable in leukemic T cells. These minimized HIV-rtTA variants contain up to 7 deletions/inactivating mutations (TAR, Tat, vif, vpR, vpU, nef and U3) and replicate efficiently in the leukemic SupT1 T cell line, but do not replicate in normal peripheral blood mononuclear cells. These virus variants are also able to efficiently remove leukemic cells from a mixed culture with untransformed cells. The therapeutic viruses use CD4 and CXCR4 for cell entry and could potentially be used against CXCR4 expressing malignancies such as T-lymphoblastic leukemia/lymphoma, NK leukemia and some myeloid leukemias.

Synergy between expression of fusogenic membrane proteins, chemotherapy and facultative virotherapy in colorectal cancer

Using Chou-Talalay median effect analysis, we demonstrated in permanent and short-term cultures of colorectal cancer cells that the expression of measles virus fusogenic membrane glycoproteins (FMGs) in combination with chemotherapy often causes over most of the cytotoxic dose range synergistic cell killing. In this combined treatment, we observed strongly enhanced annexin V binding and caspase-3/7 activity when compared to single-agent treatment. Furthermore, we showed increased expression of heat-shock protein (Hsp)70 and Hsp90alpha, but not of Hsp60. In a subcutaneous HT-29 colorectal xenograft model, we demonstrated that the administration of a replication-defective adenoviral or herpes simplex virus (HSV) amplicon vector (Ad.H/F or HSV.H/F) encoding tumor-restricted FMG in combination with FOLFOX significantly enhanced treatment outcome when compared to treatment with each compound individually. To increase the fraction of tumor cells expressing the FMG, we trans-complemented the Ad.H/F and HSV.H/F vector with the respective oncolytic replication-restricted adenovirus Ad.COXDeltaMK or HSV-1 G47Delta vector. At the end of the observation period (day 100), eight out of 10 animals that received G47Delta, HSV.H/F and FOLFOX were alive and tumor free. Administration of the analogous adenovirus-based regimen resulted in four out of 10 long-term survivors. We demonstrated that the expression of FMG in combination with chemotherapy can significantly enhance treatment outcome, which is further enhanced by combination with trans-complementing oncolytic vectors.

Construction of a minimal HIV-1 variant that selectively replicates in leukemic derived T-cell lines: towards a new virotherapy approach

T-cell acute lymphoblastic leukemia is a high-risk type of blood-cell cancer. We analyzed the possibility of developing virotherapy for T-cell acute lymphoblastic leukemia. Virotherapy is based on the exclusive replication of a virus in leukemic cells, leading to the selective removal of these malignant cells. We constructed a minimized derivative of HIV-1, a complex lentivirus encoding multiple accessory functions that are essential for virus replication in untransformed cells, but dispensable in leukemic T cells. This mini-HIV virus has five deletions (vif, vpR, vpU, nef, and U3) and replicated in the SupT1 cell line, but did not replicate in normal peripheral blood mononuclear cells. The stripped down mini-HIV variant was also able to efficiently remove leukemic cells from a mixed culture with untransformed control cells. In contrast to wild-type HIV-1, we did not observe bystander killing in mixed culture experiments with the mini-HIV variant. Furthermore, viral escape was not detected in long-term cultures. The mini-HIV variant that uses CD4 and CXCR4 for cell entry could potentially be used against CXCR4-expressing malignancies such as T-lymphoblastic leukemia/lymphoma, natural killer leukemia, and some myeloid leukemias.

AdEasy-based cloning system to generate tropism expanded replicating adenoviruses expressing transgenes late in the viral life cycle

Replicating adenoviral vectors (RAds) hold great promise for the treatment of cancer. Significant therapeutic effects of these vectors do not only rely on tumor targeting but also on efficient release of viral progeny from host cells. Cytotoxic genes expressed late in the adenoviral life cycle can significantly enhance viral release and spreading. Therefore, an adenoviral cloning system that allows easy integration of established tumor targeting techniques together with late expression of transgenes can be a valuable tool for the development of RAds. We expanded the features of the widely used AdEasy adenoviral cloning system toward the production of tropism modified replicating adenoviral vectors that express transgenes late in the viral life cycle. Three vectors (pIRES, pFIBER and pAdEasy-Sce) that facilitate easy manipulation of the adenoviral fiber region were established. Unique BstBI and I-Sce-1 restriction sites facilitate the introduction of retargeting peptides in the fiber HI-loop and of genes of interest in the fiber transcription unit. We validated the system by constructing an E1-positive adenovirus with an RGD motif in the fiber HI-loop and green fluorescent protein (GFP) expressed from the fiber transcription unit (AdDelta24Fiber-rgd-GFP). Additionally, assessment of E1-negative replication-deficient vectors confirmed strict dependence upon E1 expression for the expression of transgenes inserted into the fiber transcription unit. This flexible cloning system allows for straightforward construction of tropism expanded replicating adenoviral vectors that express transgenes late in the adenoviral life cycle.

Enhanced cytotoxicity without internuclear spread of adenovirus upon cell fusion by measles virus glycoproteins

The efficiency of viruses in cancer therapy is enhanced by proteins that mediate the fusion of infected cells with their neighbors. It was reported that replication-competent adenovirus particles can spread between nuclei within fusion-generated syncytia. To assess this conjecture, we generated fusogenic adenoviruses that express a balanced ratio of the F and H glycoproteins of measles virus. The viruses displayed enhanced cytotoxicity but largely unchanged replication efficiencies compared to a nonfusogenic virus. Most notably, the virus genomes did not spread through fusion-generated multinuclear cells. Hence, adenovirus replication in syncytia remains largely restricted to initially transduced nuclei.

Replication-dependent transgene expression from a conditionally replicating adenovirus via alternative splicing to a heterologous splice-acceptor site

BACKGROUND

Oncolytic viruses are promising anticancer agents because they selectively kill cancer cells and multiply within a tumor. Their oncolytic potency might be improved by expressing a therapeutic gene from the virus genome. In this regard, proper kinetics and level of transgene expression are important. In addition, expression of cytotoxic transgene products should be confined to cancer cells. Here, we developed oncolytic adenoviruses that provide transgene expression dependent on viral replication.

METHODS

We constructed an oncolytic adenovirus that expresses luciferase under regulation of the endogenous major late promoter (MLP) via alternative splicing to an inserted splice-acceptor site analogous to that of the adenovirus serotype 40 long fiber gene. Splicing of the luciferase transcript was studied by RT-PCR analysis. Expression was measured in the presence and absence of the flavonoid apigenin, an inhibitor of viral replication.

RESULTS

The inserted splice-acceptor site was properly recognized by the adenoviral splicing machinery. Luciferase expression levels were markedly higher than levels obtained with the cytomegalovirus (CMV) promoter, especially at late stages of infection. Inhibiting adenovirus replication reduced luciferase expression levels dramatically by 4 to 5 logs, whereas expression levels with the CMV-luciferase adenovirus were only moderately affected (2 logs).

CONCLUSIONS

Transgene delivery using the endogenous late gene expression machinery resulted in an expression pattern distinct from expression driven by the conventional CMV promoter. The high expression levels and strict coupling of expression to viral replication should be useful for adequate monitoring of replication and might provide a platform for the design of armed conditionally replicating adenoviruses (CRAds) with enhanced oncolytic potency.

RD114-pseudotyped retroviral vectors kill cancer cells by syncytium formation and enhance the cytotoxic effect of the TK/GCV gene therapy strategy

BACKGROUND

Wild-type RD114 virus is capable of generating syncytia during its replication, and it is believed that cell-free viruses direct the fusion of neighboring cells. The RD114 envelope (Env) that mediates this fusion event is now widely used to pseudotype retroviral and lentiviral vectors in gene therapy. Indeed, vectors pseudotyped with RD114 Env are very efficient to transfer genes into human hematopoietic cells, and they are resistant to human complement inactivation. In this study, we have tested the potential of RD114-pseudotyped vectors produced from the FLYRD18 packaging cell line to induce syncytia.

METHODS

RD114-pseudotyped vectors produced from the FLYRD18 packaging cells were added on tumor cell lines, and the formation of syncytia was assessed by microscopy after cell fixation and methylene blue staining. The kinetics of syncytium formation was analyzed by time-lapse microscopy. Finally, the cytotoxic effect of RD114-pseudotyped vectors was measured by the MTT assay on tumor cells, and in combination with the TK/GCV strategy.

RESULTS

We have found that these vectors were able to mediate cell-to-cell fusion of human tumor cell lines. A few hours after addition of the vector, cells started to aggregate to form syncytia that eventually evolved toward cell death 48 h postinfection. RD114-pseudotyped vectors were very efficient at killing human cancer cells, and they were also able to enhance dramatically the cytotoxic effect of the TK/GCV strategy.

CONCLUSIONS

These findings indicate that RD114-pseudotyped vectors used alone, or in combination with a suicide gene therapy approach, have great potential for the treatment of cancer.

Intratumoral expression of a fusogenic membrane glycoprotein enhances the efficacy of replicating adenovirus therapy

We describe here a novel strategy to enhance the in vivo efficacy of replicating adenovirus therapy, using coinjection of plasmid DNA encoding a fusogenic viral glycoprotein. The combination of fusogenic membrane glycoprotein (FMG)-induced tumor cell fusion and infection with replicating adenovirus effectively treats even large established tumors at doses of plasmid DNA and virus that alone are ineffective. Adenoviral infection appears to increase the transduction of the tumor cells to a modest degree thereby boosting the FMG-mediated component of the therapy. Simultaneously, syncytial formation enhances the therapeutic effects of viral infection by increasing spread of adenoviral particles through the tumor cell population and by increasing titer of virus released from the tumor cells. This effect is due probably to release of intracellular viral particles upon tumor cell death and also to increased levels of E1A protein within syncytia, whose increased metabolic rate is associated with enhanced levels of protein expression. Cotransduction of tumor cells with replicating adenovirus and FMG-expressing vectors could either be combined within single replicating vectors or could be used in strategies using separate administration of two components, both at lower doses than required for either therapy alone.

Effects of adenovirus-mediated SV5 fusogenic glycoprotein expression on tumor cells

BACKGROUND

The fusogenic (F) membrane glycoprotein of the paramyxovirus SV5 allows virus to enter host cells and mediates fusion between neighboring cells, which leads to cell death. F glycoprotein is synthesized as an inactive precursor (F(0)) that is cleaved by cellular protease furine to form the active heterodimer F(1) + F(2). The active protein can induce syncytium formation in the absence of another integral glycoprotein (HN), a property that appears to be unique among paramyxoviruses.

METHODOLOGY

We constructed a non-replicative adenovirus to express SV5 F protein in tumor cells, and its fusion capacity was analyzed by fluorescent and confocal microscopy. Cell viability and bystander effect were compared with the thymidine kinase/ganciclovir suicide gene therapy. The structure of F-expressing cells was studied using electron microscopy.

RESULTS

F glycoprotein expression induced syncytium formation to a maximum at 72 h, after which syncytia progressively lost viability and detached. The cell membrane was disrupted while nuclear structure was preserved. Over-expression of SV5 F protein in tumor cells led to high cytotoxicity comparable with that associated with the thymidine kinase/ganciclovir. A potent bystander killing effect was detected until the ratio of F-transduced to non-transduced cells was 1 : 100.

CONCLUSIONS

These results indicate that the fusogenic glycoprotein of the paramyxovirus SV5 could be used to eliminate tumor cells and may encourage studies aimed at modifying its selectivity and combining its expression with other cytotoxic strategies to improve their efficacy.

Viruses in therapy--royal road or dead end?

The idea of using viruses as gene vehicles to combat disease is tantalizing for the simplicity of its principle, and for the unlimited perspectives that it raises. Yet the initial enthusiasm gave way to deep skepticism, when the complex challenges became apparent. Issues that hampered clinical successes include the specificity and efficiency of gene delivery; the immune response to viral vectors and targeted cells; standardized and affordable production of vectors; and safety for patients and environment. More recently, some obstacles could be mastered through a better understanding of vector-cell-interactions, vector-induced pathogenesis and principles of vector engineering technologies. First clinical successes became apparent, giving raise to a second waive of effort to exploit viruses in gene therapy. Future challenges include the targeting of stem cells, through receptor tropism and the regulation of gene expression; controlled evasion of host defense; combining the beneficial features of several virus vectors; realistic animal models; and clinical protocols for standardized evaluation of safety and efficacy. Monogenetic disorders were initially regarded as principal targets for gene therapy. However, most clinical trials are now addressing cancer or HIV infection. Cancer gene therapy is aiming at the destruction of malignant cells, whereas 'conventional' gene therapy frequently establishes or restores a long-term function in target cells. Therefore, the requirements for viruses to be used against cancer are fundamentally different from conventional vectors. Host cell death, immune response, and spread of replicating viruses can all contribute to oncolytic efficacy. However, limiting these deleterious effects to tumor cells is mandatory for clinical safety. A number of approaches have been taken to improve the specificity and/or efficacy of cancer virotherapy. Recent studies concerning oncolytic adenoviruses exemplify these strategies.