Gene Transfer into Hepatocytes Mediated by Helper Virus-Free HSV/AAV Hybrid Vectors

Best of most possible worlds: Hybrid gene therapy vectors based on parvoviruses and heterologous viruses

Parvoviruses and especially the adeno-associated virus (AAV) species provide an exciting and versatile platform for the rational design or molecular evolution of human gene-therapy vectors, documented by literature from over half a century, hundreds of clinical trials, and the recent commercialization of multiple AAV gene therapeutics. For the last three decades, the power of these vectors has been further potentiated through various types of hybrid vectors created by intra- or inter-genus juxtaposition of viral DNA and protein cis elements or by synergistic complementation of parvoviral features with those of heterologous, prokaryotic, or eukaryotic viruses. Here, we provide an overview of the history and promise of this rapidly expanding field of hybrid parvoviral gene-therapy vectors, starting with early generations of chimeric particles composed of a recombinant AAV genome encapsidated in shells of synthetic AAVs or of adeno-, herpes-, baculo-, or protoparvoviruses. We then dedicate our attention to two newer, highly promising types of hybrid vectors created via (1) pseudotyping of AAV genomes with bocaviral serotypes and capsid mutants or (2) packaging of AAV DNA into, or tethering of entire vector particles to, bacteriophages. Finally, we conclude with an outlook summarizing critical requirements and improvements toward clinical translation of these original concepts.

Autotransplantation in mdx Mice of mdx Myoblasts Genetically Corrected by an HSV-1 Amplicon Vector

Duchenne muscular dystrophy (DMD) is an X-linked recessive disorder, characterized by a lack of dystrophin. To eliminate the need for immunosuppressive drugs, transplantation of genetically modified autologous myoblasts has been proposed as a possible therapy for this myopathy. An HSV-1 amplicon vector (HSVDGN), containing a 17.3-kb full-length MCK-driven mouse dystrophin cDNA, an eGFP gene, and a neomycin resistance gene driven by CMV or SV40 promoters, respectively, was constructed and used to transduce mdx primary myoblasts. The presence of the eGFP and neomycin resistance genes facilitated the evaluation of the initial transduction efficiency and the permanent transduction frequency. At low multiplicities of infection (MOI 1–5), the majority of myoblasts (60–90%) expressed GFP. The GFP-positive mdx myoblasts were sorted by FACS and selected with neomycin (300 μg/ml) for 2 weeks. Up to 2% of initially infected mdx myoblasts stably expressed the three transgenes without further selection at that time. These altered cells were grafted into the tibialis anterior muscles of 18 mdx mice. Some of the mice were immunosuppressed with FK506 due to the anticipation that eGFP and the product of neomycin resistance gene might be immunogenic. One month after transplantation, numerous muscle fibers expressing mouse dystrophin were detected by immunohistochemistry, in both immunosuppressed (10–50%) and nonimmunosuppressed (5–25%) mdx mice. Our results demonstrated the capability of permanently expressing a full-length dystrophin in dystrophic myoblasts with HSV-1 amplicon vector and raised the possibility of an eventual treatment of DMD based on the transplantation of genetically modified autologous myoblasts.

Gene delivery using helper virus-free HSV-1 amplicon vectors

Herpes simplex virus type 1 (HSV-1)-based amplicon vectors contain only a very small percentage of the 152-kbp viral genome. Consequently, replication and packaging of amplicons depend on helper functions that are provided either by replication-defective mutants of HSV-1 or by replication-competent, but packaging-defective, HSV-1 genomes. Sets of cosmids that overlap and represent the entire HSV-1 genome can form, via homologous recombination, circular replication-competent viral genomes, which give rise to infectious virus progeny. However, if the DNA cleavage/packaging signals are deleted, reconstituted virus genomes are not packageable, but still provide all the helper functions required for the packaging of cotransfected amplicon DNA. The resulting stocks of packaged amplicon vectors are essentially free of contaminating helper virus. This unit describes the cotransfection of amplicon and cosmid or bacterial artificial chromosome (BAC) DNA into 2-2 cells by cationic liposome-mediated transfection and the harvesting of packaged vector particles. Support protocols provide methods for preparing cosmid and BAC DNA and determining the titers of amplicon stocks.

Targeted integration of a rAAV vector into the AAVS1 region

Adeno-associated virus (AAV) has been reported to integrate in a site-specific manner into chromosome 19 (a site designated AAVS1), a phenomenon that could be exploited for ex vivo targeted gene therapy. Recent studies employing LM-PCR to determine AAV integration loci; however, have, contrary to previous results with less reliable methods, concluded that the proclivity for AAV integration at AAVS1 is minimal. We tested this conclusion employing LM-PCR protocols designed to avoid bias. Hep G2 cells were infected with rAAV2-GFP and coinfected with wt AAV2 to supply Rep in trans. Sorted cells were cloned and cultured. In 26 clones that retained fluorescence, DNA was extracted and AAV-genomic junctions amplified by two LM-PCR methods. Sequencing was performed without bacterial cloning. Of these 26 clones it was possible to assign a genomic integration site to 14, of which 9 were in the AAVS1 region. In three additional clones, rAAV integration junction were to an integrated wt AAV genome while two were to an rAAV genome. We also show that integration of the AAV-GFP genome can be achieved without cointegration of the AAV genome. Based on the pattern of integrants we propose, for potential use in ex vivo targeted gene therapy, a simplified PCR method to identify clones that have rAAV genomes integrated into AAVS1.

Virion-associated cofactor high-mobility group DNA-binding protein-1 facilitates transposition from the herpes simplex virus/Sleeping Beauty amplicon vector platform

The development of the integration-competent, herpes simplex virus/Sleeping Beauty (HSV/SB) amplicon vector platform has created a means to efficiently and stably deliver therapeutic transcription units (termed "transgenons") to neurons within the mammalian brain. Furthermore, an investigation into the transposition capacity of the HSV/SB vector system revealed that the amplicon genome provides an optimal substrate for the transposition of transgenons at least 12 kb in length [de Silva, S., Mastrangelo, M.A., Lotta, L.T., Jr., Burris, C.A., Federoff, H.J., and Bowers, W.J. ( 2010 ). Gene Ther. 17, 424-431]. These results prompted an investigation into the factors that may contribute toward efficient transposition from the HSV/SB amplicon. One of the cellular cofactors known to play a key role during SB-mediated transposition is the high-mobility group DNA-binding protein-1 (HMGB1). Our present investigation into the role of HMGB1 during amplicon-based transposition revealed that transposition is not strictly dependent on the presence of cellular HMGB1, contrary to what had been previously demonstrated with plasmid-based SB transposition. We have shown for the first time that during amplicon preparation, biologically active HMGB1 derived from the packaging cell line is copackaged into amplicon vector particles. As a result, HSV/SB amplicon virions arrive prearmed with HMGB1 protein at levels sufficient for facilitating SB-mediated transposition in the transduced mammalian cell.

Herpes Virus Amplicon Vectors

Since its emergence onto the gene therapy scene nearly 25 years ago, the replication-defective Herpes Simplex Virus Type-1 (HSV-1) amplicon has gained significance as a versatile gene transfer platform due to its extensive transgene capacity, widespread cellular tropism, minimal immunogenicity, and its amenability to genetic manipulation. Herein, we detail the recent advances made with respect to the design of the HSV amplicon, its numerous in vitro and in vivo applications, and the current impediments this virus-based gene transfer platform faces as it navigates a challenging path towards future clinical testing.

HSV-1-derived amplicon vectors: recent technological improvements and remaining difficulties--a review

Amplicons are defective and non-integrative vectors derived from herpes simplex virus type 1. As the vector genome carries no virus genes, amplicons are both non-toxic for the infected cells and non-pathogenic for the inoculated organisms. In addition, the large transgenic capacity of amplicons, which allow delivery of up to 150 Kbp of foreign DNA, makes these vectors one of the most powerful, interesting and versatile gene delivery platforms. We present here recent technological developments that have significantly improved and extended the use of amplicons, both in cultured cells and in living organisms. In addition, this review also discusses the many difficulties still pending to be solved, in order to achieve stable and physiologically regulated transgene expression.

Overview of gene delivery into cells using HSV-1-based vectors

This overview describes the considerations involved in the preparation and use of herpes simplex virus type 1 (HSV-1) as a vector for gene transfer into neurons. Strategies for gene delivery into neurons, either to study the molecular biology of brain function or for gene therapy, must utilize vectors that persist stably in postmitotic cells and that can be targeted both spatially and temporally in the nervous system in vivo. This unit describes the biology of HSV-1 along with a discussion covering development of amplicon and genomic HSV-1 vectors. Advantages and disadvantages of current HSV-1 vectors are presented, and HSV-1 vectors are compared with other vectors for gene transfer into neurons.

Gene delivery using helper virus-free HSV-1 amplicon vectors

Herpes simplex virus type 1 (HSV-1)-based amplicon vectors contain only 1% of the viral genome. Consequently, replication and packaging of amplicons depend on helper functions provided either by replication-defective mutants of HSV-1 (helper viruses) or by replication-competent, but packaging-defective, HSV-1 genomes. Sets of cosmids that overlap and represent the entire HSV-1 genome can form, via homologous recombination, circular replication-competent viral genomes, which give rise to infectious virus progeny. If the DNA cleavage/packaging signals are deleted, reconstituted virus genomes are not packageable, but still provide all the helper functions required for the packaging of cotransfected amplicon DNA. Resulting stocks of packaged amplicon vectors are free of contaminating helper virus. The basic protocol describes the cotransfection of amplicon and cosmid DNA into 2-2 cells and the harvesting of packaged vector particles. Support protocols describe preparing cosmid DNA and methods for determining the titers of amplicon stocks.

Targeted integration of functional human ATM cDNA into genome mediated by HSV/AAV hybrid amplicon vector

Ataxia-telangiectasia (A-T) is an autosomal recessive disorder characterized by neurodegeneration, immunodeficiency, cancer predisposition, genome instability, and sensitivity to ionizing radiation (IR). We have previously shown that a herpes simplex virus type 1 (HSV-1) amplicon vector carrying the human ataxia-telangiectasia mutated (ATM) complementary DNA (cDNA) is able to correct aspects of the cellular phenotype of human A-T cells in culture, and is also able to transfer the ATM cDNA to the Atm(-/-) mouse cerebellum. In order to achieve stable gene replacement, we have generated an HSV/adeno-associated virus (AAV) hybrid amplicon vector carrying the expression cassettes for the ATM cDNA [(9.2 kilobases (kb)] and enhanced green fluorescent protein (EGFP), flanked by AAV inverted terminal repeats (ITRs). This hybrid vector, in the presence of AAV Rep proteins, mediates site-specific integration into the AAVS1 site on chromosome 19 in human cells and in Atm(-/-) mice carrying that human locus. The functional activity of the vector-derived ATM was confirmed in vitro and in vivo by ATM autophosphorylation at Ser-1981 after IR. This proof-of-principle study establishes the ability of HSV/AAV hybrid amplicon vectors to mediate functional targeted integration of the ATM cDNA into A-T cells in culture and in Atm(-/-) mice in vivo, thus laying a foundation for possible gene therapy approaches in the treatment of A-T patients.

Characterization of a bipartite recombinant adeno-associated viral vector for site-specific integration

Adeno-associated virus type 2 (AAV2) is the only virus known to integrate into a specific locus in the human genome. The locus, AAVS1, is on the q arm of chromosome 19 at position 13.4. AAV is currently a popular vector for human gene therapy. However, current vectors do not contain two important elements needed for site-specific integration, that is, the rep gene or the P5 promoter, although they do integrate with low frequency at random locations in the human genome. We have designed a bipartite vector that does insert the transgene into AAVS1. One component, rAAVSVAV2, contains the rep gene, driven by the simian virus 40 early promoter rather than the P5 promoter. Thus, the integration enhancer element (IEE) within P5, which greatly enhances site-specific integration, has been deleted. The other component, rAAVP5UF11, contains the P5 IEE plus the transgene with associated regulatory elements. We have created clones of transduced HeLa cells, most of which appear to have the transgene inserted in AAVS1. We have not detected any clones that have rep inserted anywhere. With the optimal multiplicity of infection and ratio of rAAVSVAV2 and rAAVP5UF11, the transgene integrated specifically at AAVS1 with high efficiency (>60%). Most importantly, the cloned cell lines with the AAVS1 site-specific integrated green fluorescent protein (GFP) were healthy and stably expressed GFP for 35 passages. An AAV vector that would integrate at a specific site with high frequency could offer significant advantage in the transduction of progenitor cells and stem cells ex vivo and engineered cells could be used for human gene therapy. AAV site-specific integration gene therapy could provide a novel approach for diseases that need long-term gene expression.

HSV-1 amplicon vectors: a promising and versatile tool for gene delivery

Amplicons are defective and non-integrative vectors derived from herpes simplex virus type 1. They carry no virus genes in the vector genome and are, therefore, not toxic to the infected cells or pathogenic for the transduced organisms, making these vectors safe. In addition, the large transgenic capacity of amplicons, which allow delivery of < or = 150 Kbp of foreign DNA, make these vectors one of the most powerful, interesting and versatile gene delivery platforms. Here, the authors present recent technological developments that have significantly improved and extended the use of amplicons, both in cultured cells and in living organisms. In addition, this review illustrates the many possible applications that are presently being developed with amplicons and discuss the many difficulties still pending to be solved in order to achieve stable and physiologically regulated transgenic expression.

An efficient and safe herpes simplex virus type 1 amplicon vector for transcriptionally targeted therapy of human hepatocellular carcinomas

Our previous studies have shown that transgene expression could be targeted to proliferating cells when cell cycle transcriptional regulatory elements were incorporated into herpes simplex virus type 1 (HSV-1) amplicon backbone vectors. In the study reported here, we further demonstrated the transcriptional activation of transgene expression in association with the onset of cellular proliferation using the mouse partial hepatectomy model. Moreover, transcriptional regulation could be rendered specific to human hepatocellular carcinoma (HCC) cells by inserting the chimeric gene Gal4/NF-YA under the regulation of the HCC-specific hybrid promoter. The hybrid promoter, which consists of four copies of the apolipoprotein E (ApoE) enhancer element inserted upstream of the human alpha1-antitrypsin(hAAT) promoter, induced an higher level of transcription than other liver-specific promoters such as alpha-fetoprotein (AFP) and albumin (Alb) promoter. As a consequence, the enhancement of tissue-specific expression in the context of Gal4/NF-YA fusion proteins enabled the monitoring of transgene expression using a bioluminescence imaging system. Furthermore, these vectors have been shown to be non-toxic and exhibited potent infectivity for proliferating primary HCC cells and HCC cell lines. Together, these results demonstrated that the new hybrid vectors could provide options for the design of safe and efficient systemic gene therapeutic strategies for human HCC.

Cytoprotection of beta cells: rational gene transfer strategies

Gene transfer to pancreatic islets may prove useful in preventing islet cell destruction and prolonging islet graft survival after transplantation in patients with type 1 diabetes mellitus (T1DM). Potentially, a host of therapeutically relevant transgenes may be incorporated into an appropriate gene delivery vehicle and used for islet modification. An increasing understanding of the molecular pathogenesis of immune-mediated beta cell death has served to highlight molecules which have become suitable candidates for promoting islet cell survival in the face of oxidative stress. This review aims to give an overview of some conventional gene transfer strategies aimed at promoting islet cell survival in the face of cytokine onslaught. These strategies target three aspects of islet cell physiology: redox status and antioxidant defence, anti-apoptotic gene expression and mediators of cytokine signal transduction pathways.

Efficient site-specific integration of large transgenes by an enhanced herpes simplex virus/adeno-associated virus hybrid amplicon vector

We previously demonstrated that a herpes simplex virus type 1 (HSV-1)/adeno-associated virus (AAV) hybrid amplicon vector constructed by inserting the sequences of regulatory protein (rep) and inverted terminal repeats of AAV into an HSV amplicon vector resulted in the enhanced stability of transgene expression compared to the original HSV-1 amplicon vector. However, problems related to the expression of Rep compromised its therapeutic applications. We report here a new HSV/AAV hybrid amplicon vector system that not only solved problems associated with Rep expression but also markedly improved the stable transduction efficiency of this vector. This new HSV/AAV vector is designed in a way that little or no Rep would be expressed in packaging cells, but it can be expressed in transduced cells if Cre recombinase is provided. Furthermore, Rep expression will be automatically suppressed as a consequence of Rep-mediated integration. Our results showed that the new hybrid amplicon vector yielded titers comparable to those of standard amplicon vectors. When Cre-expressing 293 cells were transduced, a low level of Rep expression was detected, and stable transduction was achieved in approximately 22% of transduced cells; of those cells, approximately 70% transduction was achieved by Rep-mediated site-specific integration. In the majority of the stably transduced cells, Rep expression was no longer observed. Our results also proved that this vector system is capable of efficiently accommodating and site-specifically integrating large transgenes, such as the full-length dystrophin expression cassette. Thus, the new HSV/AAV vector demonstrated unique advantages in safe and effective delivery of long-lasting transgene expression into human cells.

Use of herpes virus amplicon vectors to study brain disorders

There is an enormous initiative to establish the genetic basis for disorders of brain function. Unfortunately, genetic intervention is not accomplished easily in the nervous system. One strategy is to engineer and deliver to neurons specialized viral vectors that carry a gene (or genes) of interest, thereby exploiting the natural ability of viruses to insert genetic material into cells. When delivered to brain cells, these vectors cause infected cells to increase the expression of the genes of interest. The ability to deliver genes into neurons in vitro and in vivo with herpes simplex virus (HSV) amplicon vectors has made it possible to carry out exactly these sorts of experiments. This technology has the potential to offer new insights into the etiology of a wide variety of neuropsychiatric disorders. We describe the use of HSV amplicon vectors to study Alzheimer disease, drug addiction, and depression, and discuss the considerations that enter into the use of these vectors both in vitro and in vivo. The HSV amplicon virus is a user-friendly vector for the delivery of genes into neurons that has come of age for the study of brain function.

Herpes simplex virus amplicon delivery of a hypoxia-inducible soluble vascular endothelial growth factor receptor (sFlk-1) inhibits angiogenesis and tumor growth in pancreatic adenocarcinoma

BACKGROUND

Tumor hypoxia induces vascular endothelial growth factor (VEGF) expression, which stimulates angiogenesis and tumor proliferation. The VEGF signaling pathway is inhibited by soluble VEGF receptors (soluble fetal liver kinase 1; sFlk-1), which bind VEGF and block its interaction with endothelial cells. Herpes simplex virus (HSV) amplicons are replication-incompetent viruses used for gene delivery. We attempted to attenuate angiogenesis and inhibit pancreatic tumor growth through HSV amplicon-mediated expression of sFlk-1 under hypoxic control.

METHODS

A multimerized hypoxia-responsive enhancer (10 x HRE) was cloned upstream of the sFlk-1 gene (10 x HRE/sFlk-1). A novel HSV amplicon expressing 10 x HRE/sFlk-1 was genetically engineered (HSV10 x HRE/sFlk-1).Human pancreatic adenocarcinoma cells (AsPC1) were transduced with HSV10 x HRE/sFlk-1 and incubated in normoxia (21% oxygen) or hypoxia (1% oxygen). Capillary inhibition was evaluated by human umbilical vein endothelial cell assay. Western blot assessed sFlk-1 expression. AsPC1 flank tumor xenografts (n = 24) were transduced with HSV10 x HRE/sFlk-1.

RESULTS

Media from normoxic AsPC1 transduced with HSV10 x HRE/sFlk-1 yielded a 36% reduction in capillary formation versus controls (P < .05), whereas hypoxic AsPC1 yielded a 76% reduction (P < .005). Western blot of AsPC1 transduced with HSV10 x HRE/sFlk-1 demonstrated greater sFlk-1 expression in hypoxia versus normoxia. AsPC1 flank tumors treated with HSV10 x HRE/sFlk-1 exhibited a 59% reduction in volume versus controls (P < .000001).

CONCLUSIONS

HSV amplicon delivery of a hypoxia-inducible soluble VEGF receptor significantly reduces new vessel formation and tumor growth. Tumor hypoxia can thus be used to direct antiangiogenic therapy to pancreatic adenocarcinoma.

Variability in infectivity of primary cell cultures of human brain tumors with HSV-1 amplicon vectors

We investigated the variability in infectivity of cells in primary brain tumor samples from different patients using an HSV-1 amplicon vector. We studied the infectivity of HSV-1 amplicon vectors in tumor samples derived from neurosurgical resections of 20 patients. Cells were infected with a definite amount of HSV-1 amplicon vector HSV-GFP. Transduction efficiency in primary tumor cell cultures was compared to an established human glioma line. Moreover, duration of transgene expression was monitored in different tumor cell types. All primary cell cultures were infectable with HSV-GFP with variable transduction efficiencies ranging between 3.0 and 42.4% from reference human Gli36 Delta EGFR glioma cells. Transduction efficiency was significantly greater in anaplastic gliomas and meningiomas (26.7+/-17.4%) compared to more malignant tumor types (glioblastomas, metastases; 11.2+/-8.5%; P=0.05). To further investigate the possible underlying mechanism of this variability, nectin-1/HevC expression was analyzed and was found to contribute, at least in part, to this variability in infectability. The tumor cells expressed the exogenous gene for 7 to 61 days with significant shorter expression in glioblastomas (18+/-13 d) compared to anaplastic gliomas (42+/-24 d; P<0.05). Interindividual variability of infectivity by HSV-1 virions might explain, at least in part, why some patients enrolled in gene therapy for glioblastoma in the past exhibited a sustained response to HSV-1-based gene- and virus therapy. Infectivity of primary tumor samples from respective patients should be tested to enable the development of efficient and safe herpes vector-based gene and virus therapy for clinical application.

In vivo gene transfer to the rat retina using herpes simplex virus type 1 (HSV-1)-based amplicon vectors

The purpose of our study was to evaluate the transduction profiles of herpes simplex virus type 1 (HSV-1)-based amplicon vectors following subretinal injection in the rat. Two amplicon vectors were tested, pHy-CMVGFP and pHy-RPEGFP, both carrying the green fluorescent protein (GFP) under the control of the cytomegalovirus (CMV) ubiquitous promoter or the RPE65-specific promoter, respectively. For the two amplicon vectors, the GFP reporter gene was efficiently expressed in retinal pigment epithelial (RPE) cells but not in the adjacent photoreceptors. GFP expression was maximum as early as 2 days post-administration but decreased over time to become almost undetectable at 6 weeks postinjection. Super-transduction with a second amplicon vector, pHSVlac, reactivated expression of GFP in approximately 10% of the cells initially transduced at 2 days postinjection of pHy-CMVGFP or pHy-RPEGFP. Reactivation of transgene expression was transient, no GFP signal was detected 8 days after pHSVlac injection. In conclusion, HSV-1 amplicon vectors allow rapid and efficient, but transient, gene transfer in RPE cells following subretinal injection.

Gene transfer into hepatocytes mediated by herpes simplex virus–Epstein-Barr virus hybrid amplicons

Gene transfer into hepatocytes is highly desirable for the long-term goal of replacing deficient proteins and correcting metabolic disorders. Vectors based on herpes simplex virus type-1 (HSV-1) have been demonstrated to mediate efficient gene transfer into hepatocytes both in vitro and in vivo. Large transgene capacity and extrachromosomal persistence make HSV-1/EBV hybrid amplicon vectors an attractive candidate for hepatic gene replacement therapy. To assess liver-directed gene transfer, we constructed (i) a conventional HSV-1 amplicon vector encoding a secreted reporter protein (secreted alkaline phosphatase, SEAP) under the control of the HSV-1 immediate-early 4/5 promoter; (ii) a HSV-1 amplicon encoding SEAP under the control of the artificial CAG promoter (the chicken beta-actin promoter and cytomegalovirus (CMV) immediate-early enhancer); and (iii) a HSV-1/EBV hybrid amplicon, also encoding SEAP under the control of the CAG promoter. While all three vector constructs yielded high SEAP concentrations in vitro and in vivo, use of HSV-1/EBV hybrid amplicon vectors significantly prolonged the duration of gene expression. Using conventional amplicon vectors in cultured hepatocytes, SEAP was detected for two weeks, whereas SEAP was detected for at least six weeks when HSV-1/EBV amplicons were used. Intraparenchymal injection into the liver of SICD mice yielded high (up to 77 ng of SEAP per milliliter serum) and sustained (greater than three weeks) expression of SEAP. Serum transaminases (ALT/AST) were measured at different time points to monitor for hepatocellular damage. While initially elevated four times above baseline, the transaminase levels returned to normal after three to seven days. These results demonstrate the usefulness of HSV-1-based amplicons and SEAP for the evaluation of gene replacement strategies in the liver.

Specific targeted binding of herpes simplex virus type 1 to hepatocytes via the human hepatitis B virus preS1 peptide

To improve the utility of herpes simplex virus type 1 (HSV-1) vectors for gene therapy, the viral envelope needs to be manipulated to achieve cell-specific gene delivery. In this report, we have engineered an HSV-1 mutant virus, KgBpK(-) gC(-), deleted for the glycoprotein C (gC) and the heparan sulfate-binding domain (pK) of gB, in order to express gC:preS1 and gC:preS1 active peptide (preS1ap) fusion molecules. PreS1, and a 27 amino acid active peptide inside preS1 (preS1ap), are supposed to be the molecules that the human hepatitis B virus (HBV) needs to bind specifically to hepatocytes. Biochemical analysis demonstrated that the gC:preS1ap fusion molecule was expressed and incorporated into the envelope of the recombinant HSV-1 virus KgBpK(-)gC:preS1ap. Moreover, KgBpK(-)gC:preS1ap recombinant virus gained a specific binding activity to an hepatoblastoma cell line (HepG2) with a consequent productive infection. In addition, anti-preS1-specific antibodies were shown to neutralize recombinant virus infectivity, and a synthetic preS1ap peptide was able to elute KgBpK(-)gC:preS1ap virus bound on HpeG2 cells. These data provide further evidence that HSV-1 can productively infect cells through a specific binding to a non-HSV-1 receptor. Furthermore, these data strongly support the hypothesis that the HBV preS1ap molecule is an HBV ligand to hepatocytes.

Targeted transgene integration into transgenic mouse fibroblasts carrying the full-length human AAVS1 locus mediated by HSV/AAV rep(+) hybrid amplicon vector

Herpes simplex virus type 1/adeno-associated virus (HSV/AAV) rep(+) hybrid amplicon vectors containing AAV inverted terminal repeats (ITRs) and rep gene sequences can mediate site-specific integration into the human genome. In this study, we have generated and characterized the first transgenic mice that bear the full-length (8.2 kb) human AAVS1 locus. Immortalized mouse embryonic fibroblasts from this mouse line were transduced with the rep(+), rep(-) (containing only ITRs flanking the transgene) hybrid amplicon vectors, and the standard amplicon vector to determine stable integration frequency and the site of integration. Transduction of transgenic fibroblasts resulted in a 10-fold higher stable integration frequency with rep(+) hybrid amplicon vector than with rep(-) or standard amplicon vectors. Southern blot analysis of genomic DNA from transgenic cells stably transduced with the rep(+) hybrid amplicon vector revealed site-specific integration of transgenes at the AAVS1 locus in 50% of clones. Some site-specific and random integration events were limited to the ITR-flanked transgene cassette. In contrast, transduction of transgenic mouse cells with the rep(-) or standard amplicon vectors resulted in random integrations of the entire rep(-) hybrid amplicon or amplicon DNA that were incorporated into the host genome as a concatenate of various sizes. These results demonstrate for the first time that the genome of transgenic mice bearing the human AAVS1 locus serves as a platform for site-specific integration of AAV ITR-flanked transgene cassettes within the hybrid amplicon vector in the presence of Rep.

Herpes simplex virus type-1 infection upregulates cellular promoters and telomerase activity in both tumor and nontumor human cells

Targeted gene expression through viral vectors has been a promising approach for gene therapy. However, the effects of viral gene products expressed from virus vectors on the expression of the host gene are not well known. In the present study, we examined the activities of cellular promoters, including the promoter for genes of human telomerase reverse transcriptase (hTERT), tyrosinase and probasin, in both tumor and normal cells after infection with herpes simplex virus type 1 (HSV-1) vectors. Our results showed that infection with replication-defective HSV-1 vectors significantly upregulated the activity of all three cellular promoters in a nonsequence specific fashion in all cell types tested. Furthermore, viral infection upregulated activities of the hTERT promoter and endogenous telomerase in nontumoral cells. Additional experiments suggested that the viral immediate-early gene product, infected cell protein 0, might be responsible for the deregulation of cellular promoter activity and activation of telomerase. Our study alerts to the potential risk of oncogenesis through deregulation of host gene expression, such as the telomerase by viral vectors in normal cells.

Improved herpes simplex virus type 1 amplicon vectors for proportional coexpression of positron emission tomography marker and therapeutic genes

For the development of efficient and safe gene therapy protocols for clinical application it is desirable to determine the tissue dose of vector-mediated therapeutic gene expression noninvasively in vivo. The herpes simplex virus type 1 thymidine kinase gene (HSV-1-tk) has been shown to function as a marker gene for the direct noninvasive in vivo localization of thymidine kinase (TK) expression by positron emission tomography (PET). Using bicistronic or multicistronic gene-expressing cassettes with tk as the PET marker gene, the quantitative analysis of tk gene expression may indirectly indicate the distribution and the level of expression of linked and proportionally coexpressed genes. Here, we describe the construction and functional evaluation of HSV-1 amplicon vectors mediating proportional coexpression of HSV-1-tk as PET marker gene and the enhanced green fluorescent protein gene (gfp) as proof of principle and cell culture marker gene and the Escherichia coli cytosine deaminase (cd) as therapeutic gene. Several double-/triple-gene constructs expressing HSV-1-tk, gfp, and E. coli cd were engineered based on gene fusion or the use of an internal ribosome entry site (IRES). Functional analysis in cell culture (green fluorescent protein [GFP] fluorescence and sensitivity to the prodrugs ganciclovir [GCV] and 5-fluorocytosine [5-FC]) and Western blots were carried out after infection of proliferating rat 9L gliosarcoma and human Gli36 glioma cells with helper virus-free packaged HSV-1 amplicon vectors. To study the ability of PET to differentiate various levels of tk expression noninvasively in vivo, retrovirally transduced and selected populations of rat F98 and human Gli36dEGFR glioma cells with defined levels of proportionally coexpressed tk and gfp genes were grown as subcutaneous tumors in nude rats and nude mice, and tk imaging by PET was performed. To study HSV-1 amplicon vector-mediated gene coexpression in vivo, HSV-1 amplicon vectors bearing coexpression constructs were injected (4 x 10(7) to 1 x 10(8) transducing units) into subcutaneously growing Gli36dEGFR gliomas in nude animals, and tk imaging was performed 24 hr later. All vector constructs mediated GFP expression and sensitized 9L and Gli36 cells toward GCV- and 5-FC-mediated cell killing in a drug dose-dependent manner, respectively. The levels of gene expression varied depending on the location of the genes within the constructs indicating the influence of the IRES on the level of expression of the second gene. Moreover, functional proportional coexpression of the PET marker gene HSV-1-tk and the linked therapeutic E. coli cd gene was observed. In selected tumor cell populations, subtle IRES-dependent differences of tk gene expression could be noninvasively distinguished by PET with good correlation between quantitative assays for IRES-dependent attenuated GFP and TK expression in culture and in vivo. After infection of subcutaneously growing gliomas with HSV-1 amplicon vectors, various levels of TK expression were found ranging from 0.011-0.062 percentage injected dose per gram (%ID/g). These values were 4.0- to 5.7-fold lower than positive control tumor cells. TK expression could be imaged by PET in vivo even with the tk gene located at the weak position downstream from the IRES. In conclusion, these HSV-1 amplicon vectors carrying HSV-1-tk as PET marker gene and any linked therapeutic gene will serve an indirect noninvasive assessment of the distribution of therapeutic gene expression by PET. Monitoring the correlation between primary transduction and therapeutic efficiency of a given vector is highly desirable for the development of safe and efficient gene therapy and vector application protocols in clinical applications.

Herpes simplex virus type 1/adeno-associated virus hybrid vectors mediate site-specific integration at the adeno-associated virus preintegration site, AAVS1, on human chromosome 19

Herpes simplex virus type 1 (HSV-1)-based amplicon vectors have a large transgene capacity and can efficiently infect many different cell types. One disadvantage of HSV-1 vectors is their instability of transgene expression. By contrast, vectors based on adeno-associated virus (AAV) can either persist in an episomal form or integrate into the host cell genome, thereby supporting long-term gene expression. AAV expresses four rep genes, rep68, -78, -40, and -52. Of those, rep68 or rep78 are sufficient to mediate site-specific integration of the AAV DNA into the host cell genome. The major disadvantage of AAV vectors is the small transgene capacity ( approximately 4.6 kb). In this study, we constructed HSV/AAV hybrid vectors that contained, in addition to the standard HSV-1 amplicon elements, AAV rep68, rep78, both rep68 and -78, or all four rep genes and a reporter gene that was flanked by the AAV inverted terminal repeats (ITRs). Southern blots of Hirt DNA from cells transfected with the hybrid vectors and HSV-1 helper DNA demonstrated that both the AAV elements and the HSV-1 elements were functional in the context of the hybrid vector. All hybrid vectors could be packaged into HSV-1 virions, although those containing rep sequences had lower titers than vectors that did not. Site-specific integration at AAVS1 on human chromosome 19 was directly demonstrated by PCR and sequence analysis of ITR-AAVS1 junctions in hybrid vector-transduced 293 cells. Cell clones that stably expressed the transgene for at least 12 months could easily be isolated without chemical selection. In the majority of these clones, the transgene cassette was integrated at AAVS1, and no sequences outside the ITR cassette, rep in particular, were present as determined by PCR, ITR rescue/replication assays, and Southern analysis. Some of the clones contained random integrations of the transgene cassette alone or together with sequences outside the ITR cassette. These data indicate that the long-term transgene expression observed following transduction with HSV/AAV hybrid vectors is, at least in part, supported by chromosomal integration of the transgene cassette, both randomly and site specifically.

Herpes simplex virus type 1/adeno-associated virus rep(+) hybrid amplicon vector improves the stability of transgene expression in human cells by site-specific integration

Herpes simplex virus type 1 (HSV-1) amplicon vectors are promising gene delivery tools, but their utility in gene therapy has been impeded to some extent by their inability to achieve stable transgene expression. In this study, we examined the possibility of improving transduction stability in cultured human cells via site-specific genomic integration mediated by adeno-associated virus (AAV) Rep and inverted terminal repeats (ITRs). A rep(-) HSV/AAV hybrid amplicon vector was made by inserting a transgene cassette flanked with AAV ITRs into an HSV-1 amplicon backbone, and a rep(+) HSV/AAV hybrid amplicon was made by inserting rep68/78 outside the rep(-) vector 3' AAV ITR sequence. Both vectors also had a pair of loxP sites flanking the ITRs. The resulting hybrid amplicon vectors were successfully packaged and compared to a standard amplicon vector for stable transduction frequency (STF) in human 293 and Gli36 cell lines and primary myoblasts. The rep(+), but not the rep(-), hybrid vector improved STF in all three types of cells; 84% of Gli36 and 40% of 293 stable clones transduced by the rep(+) hybrid vector integrated the transgene into the AAVS1 site. Due to the difficulty in expanding primary myoblasts, we did not assess site-specific integration in these cells. A strategy to attempt further improvement of STF by "deconcatenating" the hybrid amplicon DNA via Cre-loxP recombination was tested, but it did not increase STF. These data demonstrate that introducing the integrating elements of AAV into HSV-1 amplicon vectors can significantly improve their ability to achieve stable gene transduction by conferring the AAV-like capability of site-specific genomic integration in dividing cells.

Generation of stable retrovirus packaging cell lines after transduction with herpes simplex virus hybrid amplicon vectors

BACKGROUND

A number of properties have relegated the use of Moloney murine leukemia virus (Mo-MLV)-based retrovirus vectors primarily to ex vivo protocols. Direct implantation of retrovirus producer cells can bypass some of the limitations, and in situ vector production may result in a large number of gene transfer events. However, the fibroblast nature of most retrovirus packaging cells does not provide for an effective distribution of vector producing foci in vivo, especially in the brain. Effective development of new retrovirus producer cells with enhanced biologic properties may require the testing of a large number of different cell types, and a quick and efficient method to generate them is needed.

METHODS

Moloney murine leukemia virus (Mo-MLV) gag-pol and env genes and retrovirus vector sequences carrying lacZ were cloned into different minimal HSV/AAV hybrid amplicons. Helper virus-free amplicon vectors were used to co-infect glioma cells in culture. Titers and stability of retrovirus vector production were assessed.

RESULTS

Simultaneous infection of two glioma lines, Gli-36 (human) and J3T (dog), with both types of amplicon vectors, generated stable packaging populations that produced retrovirus titers of 0.5-1.2 x 10(5) and 3.1-7.1 x 10(3) tu/ml, respectively. Alternatively, when cells were first infected with retrovirus vectors followed by infection with HyRMOVAmpho amplicon vector, stable retrovirus packaging populations were obtained from Gli-36 and J3T cells producing retrovirus titers comparable to those obtained with a traditional retrovirus packaging cell line, Psi CRIPlacZ.

CONCLUSIONS

This amplicon vector system should facilitate generation of new types of retrovirus producer cells. Conversion of cells with migratory or tumor/tissue homing properties could result in expansion of the spatial distribution or targeting capacity, respectively, of gene delivery by retrovirus vectors in vivo.

Herpes simplex virus type 1 amplicon vector-mediated gene transfer to muscle

Herpes simplex virus type 1 (HSV-1) amplicon vectors were evaluated for feasibility in gene therapy of Duchenne's muscular dystrophy (DMD). An amplicon vector expressing enhanced green fluorescent protein (eGFP) was examined for transduction efficiency and cytotoxicity in cultured muscle cells, and for transduction efficiency, duration of transgene expression, and immunogenicity in tibialis anterior (TA) muscles of neonatal mice. Transduction efficiencies in murine and human myoblasts were 60-90 and 50-60%, respectively, when myoblasts were transduced at multiplicities of infection (MOIs) of 1-5. Similar transduction efficiencies were observed in myotubes of both species. No cytotoxic effects were noticed at an MOI of 10, the highest MOI tested. An amplicon vector, HyMD, containing the full-length mouse dystrophin cDNA and its muscle creatine kinase (MCK) promoter-enhancer, with a total size of 26 kb, was constructed and used to transduce mdx mouse myotubes. The expression of dystrophin in these cells was demonstrated by immunocytochemistry. After injecting 4-6 x 10(5) transduction units (TU) of HSVGN amplicon vectors, 10-50% of myofibers in the injected TA muscles expressed GFP. Although transgene expression was attenuated over time, significant improvement in long-term transgene expression and persistence of vector DNA was achieved, when compared with the first generation of recombinant HSV-1 vectors. Immunohistochemistry showed a modest CD4(+) lymphocyte infiltration in the vicinity of the injection. A gradually developed CD8(+) lymphocyte infiltration was also seen, most likely related to the antigenicity of the transgene product, GFP. We conclude that the HSV-1 amplicon vector is a promising vehicle for gene delivery in DMD. However, new strategies need to be evaluated to increase the stability of transgene expression.

Antitumor effects on human melanoma xenografts of an amplicon vector transducing the herpes thymidine kinase gene followed by ganciclovir

Herpes simplex virus type-1 (HSV-1) has been demonstrated as a potentially useful gene delivery vector for gene therapy due to its high efficiency of in vivo transduction. The helper virus-dependent, HSV- 1 amplicon vectors were developed for easier operation and their larger capacity. In this study, the herpes simplex virus type-1 thymidine kinase (HSVtk) gene was cloned into the pHE700 amplicon vector to make an HE7tk vector and used for in vivo gene delivery. Human melanoma xenografts were established in athymic nude mice. Tumors were injected directly with HE7tk vector alone, HE7tk vector followed by ganciclovir (GCV), or a pHE700 amplicon vector carrying a green fluorescent protein (HE7GFP) gene followed by GCV. Efficient HSVtk transgene expression was found in the tumor 3 days after injection. Animals transduced with HE7tk followed by GCV had minimal tumor growth (P < .01 ). Animals that received either HE7tk vector without GCV or HE7GFP vector with GCV had some reduction in tumor growth compared to animals that were injected with buffer only. These data indicate that replication-defective HSV-1 amplicon vectors can be used effectively to deliver transgenes into solid tumors in vivo.

Gene therapy of pain: emerging strategies and future directions

Gene therapy to alleviate pain could appear surprising and perhaps not appropriate when opioids and other active molecules are available. However, the possibility of introducing a therapeutic protein into some targeted structures, where it would be continuously synthesised and exert its biological effect in the near vicinity of, or inside the cells, might avoid some drawbacks of "classical" drugs. Moreover, the gene-transfer techniques might improve present therapies or lead to novel ones. The recent significant and constant advances in vector systems design suggest that these techniques will be available in the near future for safe application in humans. The first experimental protocols attempting the transfer of opioid precursors genes, leading to their overexpression at the spinal level, demonstrated the feasibility and the potential interest of these approaches. Indeed, overproduction of opioid peptides in primary sensory neurones or spinal cord induced antihyperalgesic effects in various animal models of persistent pain. However, numerous other molecules involved in pain processing or associated with chronic pain have been identified and the gene-based techniques might be particularly adapted for the evaluation of the possible therapeutic interest of these new potential targets.

A novel approach to cancer therapy using an oncolytic herpes virus to package amplicons containing cytokine genes

There are two promising herpes viral-based anticancer strategies: one involves replication-defective viruses to transfer therapeutic transgenes, and the other involves replication-conditional oncolytic viruses, which selectively infect and destroy cancer cells directly. This study examines a novel dual herpesvirus preparation, which combines the immunostimulatory effects of amplicon-mediated IL2 expression with direct viral-induced oncolysis. The oncolytic virus G207 was used as the helper virus to package a herpes simplex virus (HSV)-amplicon vector carrying the gene IL2 (HSV-IL2), yielding a single preparation with two complementary modes of action. In vivo comparison was carried out in a syngeneic squamous cell carcinoma flank tumor model. We directly injected established tumors with HSV-IL2, G207, G207 mixed with HSV-IL2, or G207-packaged HSV-amplicon carrying the IL2 transgene (G207[IL2]). Significant inhibition of tumor growth was seen at 2 weeks in the G207[IL2]-treated tumors relative to controls (0.57+/-0.44 cm(3) versus 39.45+/-5.13 cm(3), P<0.00001), HSV-IL2 (20.97+/-4.60 cm(3)), and the G207 group (7.71+/-2.10 cm(3)). This unique use of a replication-conditional, oncolytic virus to package a replication-incompetent amplicon vector demonstrates impressive efficacy in vitro and in vivo, and avoids the theoretical concerns of recombination with reversion to wild type.

Flow cytometric assessment of transduction efficiency and cytotoxicity of herpes simplex virus type 1-based amplicon vectors

BACKGROUND

In this study, we compared herpes simplex virus type 1 (HSV-1) amplicon vector stocks prepared by transient cotransfection with two different BAC-cloned packaging-defective HSV-1 helper genomes, fHSVDeltapacDelta27 and fHSVDeltapac, with respect to transduction efficiency and cytotoxicity. Both fHSVDeltapacDelta27 and fHSVDeltapac are packaging defective because the pac signals have been deleted; fHSVDeltapacDelta27 contains an additional deletion in the HSV-1 ICP27 gene, which increases the safety of the system.

METHODS

HSV-1 amplicon pHSVGFP under the control of the HSV-1 immediate-early (IE) 4/5 promotor was packaged into virus particles by transient cotransfection with either fHSVDeltapacDelta27 or fHSVDeltapac DNA. Cultures were infected with the two different vector stocks and examined under the fluorescence microscope and analyzed by flow cytometry over a 5-day period to assess transduction efficiency and cytotoxicity.

RESULTS

Both vector stocks, pHSVGFP[fHSVDeltapacDelta27] and pHSVGFP[fHSVDeltapac], efficiently transduced the target cells. Interestingly, the highest mean fluorescence intensities were measured at 1 day after infection, whereas the number of GFP-fluorescent cells reached a peak at day 3 after infection. At day 3 after infection, a slight increase in the number of dead cells was observed in those cultures transduced with high doses of vector stock. Between days 3 and 4 after infection, the number of dead cells increased dramatically in all the cultures, transduced and nontransduced. Only the cultures infected with a high dose of pHSVGFP[fHSVDeltapac] displayed a significant further increase in the number of dead cells between days 4 and 5 postinfection.

CONCLUSIONS

Flow cytometry allowed comparison of transduction efficiency and cytotoxicity mediated by the two different amplicon vector stocks. Cultures infected with pHSVGFP[fHSVDeltapacDelta27] were more viable than those infected with pHSVGFP[fHSVDeltapac](P < 0.05). The practical implications of this study are at the level of vector design. Flow cytometry has proven a fast and reliable approach to assess the quality of potential gene transfer vectors prior to their use in (pre) clinical trials.

Time course of gene expression after plasmid DNA gene transfer to the liver

BACKGROUND

High levels of expression in hepatocytes can be achieved after intraportal delivery of plasmid DNA vectors with up to 10% of all liver cells transfected. CMV promoter-driven expression is very high on Day 1 after injection, but is diminished strongly by Day 2. Expression slowly declines after 1 week. We describe experiments aimed at elucidating the reasons for this rapid decline in transgene expression.

METHODS

Histological methods were used to determine the presence and extent of liver damage and hepatocyte proliferation. Viral and liver-specific promoters were tested to study promoter shut-off, Southern blotting was performed to determine the loss of the pDNA vector over time, and several mouse models were used to study the host immunological response.

RESULTS

pDNA is lost rapidly early after injection, but remains at a relatively stable copy number after Day 4. Southern blotting experiments showed that plasmid DNA could be detected for at least 12 weeks after injection (0.2 copies per genome). The early rapid decline of expression is promoter dependent. A liver-specific albumin promoter resulted in similar levels of expression on Days 1 and 7, suggesting that promoter inactivation may be responsible for the instability of CMV promoter-driven expression. The slow decline in expression levels after 1 week appears to be the result of an immune response directed against the expressed transgene. Expression was much prolonged in immunosuppressed, immunodeficient, or antigen-tolerized mice.

CONCLUSION

The present data suggest that if promoter inactivation can be overcome, intravascular delivery of plasmid DNA could be a highly efficient, simple and non-toxic liver gene therapy approach. Intravascular delivery of pDNA allows for the rapid screening of novel expression vectors in vivo.

Hybrid vector designs to control the delivery, fate and expression of transgenes

One of the greatest challenges to gene therapy is the targetting of gene delivery selectively to the sites of disease and regulation of transgene expression without adverse effects. Ultimately, the successful realization of these goals is dependent upon improvements in vector design. Over the years, viral vector design has progressed from various types of replication-defective viral mutants to replication-conditioned viruses and, more recently, to 'gutted' and hybrid vectors, which have, respectively, eliminated expression of non-relevant or toxic viral genes and incorporated desired elements of different viruses so as to increase the efficacy of gene delivery in vivo. This review will focus on the different viral and cellular elements which have been incorporated into virus vectors to: improve transduction efficiencies; alter the entry specificity of virions; control the fate of transgenes in the host cells; and regulate transgene expression.

Adeno-associated virus-based vectors in gene therapy

Adeno-associated virus (AAV) vectors were shown capable of high efficiency transduction of both dividing and nondividing cells and tissues. AAV-mediated transduction leads to stable, long-term transgene expression in the absence of apparent immune response. These properties and the broad host range of AAV vectors indicate that they constitute a powerful tool for gene therapy purposes. An additional potential benefit of AAV vectors is their ability to integrate site-specifically in the presence of Rep proteins which can be expressed transiently, thus limiting their suspected adverse effects. The major restrictions of AAV as vectors are their limited genetic capacity and strict packaging size constraint of less than 5 kb. Another difficulty is the labor-intensive and expensive procedure for the production and packaging of recombinant AAV vectors. The major benefits and drawbacks of AAV vectors and advances made in the past 3 years are discussed.

Helper virus-free herpes simplex virus type 1 amplicon vectors for granulocyte-macrophage colony-stimulating factor-enhanced vaccination therapy for experimental glioma

Subcutaneous vaccination therapy with glioma cells, which are retrovirally transduced to secrete granulocyte-macrophage colony-stimulating factor (GM-CSF), has previously proven effective in C57BL/6 mice harboring intracerebral GL261 gliomas. However, clinical ex vivo gene therapy for human gliomas would be difficult, as transgene delivery via retroviral vectors occurs only in dividing cells and ex vivo glioma cells have a low growth fraction. To circumvent this problem, a helper virus-free herpes simplex virus type 1 (HSV-1) amplicon vector was used. When primary cultures of human glioblastoma cells were infected with HSV-1 amplicon vectors at an MOI of 1, more than 90% of both dividing and nondividing cells were transduced. When cells were infected with an amplicon vector, HSVGM, bearing the GM-CSF cDNA in the presence of Polybrene, GM-CSF secretion into the medium during the first 24 hr after infection was 1026 ng/10(6) cells, whereas mock-infected cells did not secrete detectable GM-CSF. Subcutaneous vaccination of C57BL/6 mice with 5 x 10(5) irradiated HSVGM-transduced GL261 cells 7 days prior to intracerebral implantation of 10(6) wild-type GL261 cells yielded 60% long-term survivors (>80 days), similar to the 50% long-term survivors obtained by vaccination with retrovirally GM-CSF-transduced GL261 cells. In contrast, animals vaccinated with the same number of nontranduced GL261 cells or with GL261 cells infected with helper virus-free packaged HSV-1 amplicon vectors carrying no transgene showed only 10% long-term survivors. In conclusion, helper virus-free HSV-1 amplicon vectors appear to be effective for cytokine-enhanced vaccination therapy of glioma, with the advantages that both dividing and nondividing tumor cells can be infected, no viral proteins are expressed, and these vectors are safe and compatible with clinical use.

New approaches for imaging in gene therapy

Gene therapy is increasingly used experimentally and clinically to replace defective genes and/or impart new functions to cells and tissues. With the recent advances in vector design, improvements in transgene and prodrug activation strategies, gene therapy has been applied to a wide variety of diseases, tissues and organ systems. It is now clear that our specialty will play a critical role in gene therapy research and its clinical applications. Three aspects of gene therapy are of particular interest to imaging. The first is in delivering genes and vector products by minimally invasive interventional techniques. The second is in quantitating gene and DNA deliveries, for example, by nuclear imaging. Finally, imaging can be used to monitor the levels of transgene expression in vivo. A variety of imaging techniques including PET imaging, nuclear imaging, MR imaging and optical imaging can potentially be used to achieve the latter. This brief introductory overview is intended to summarize current strategies and illustrate the role that radiology will play in this field.

HSV-1-based vectors for gene therapy of neurological diseases and brain tumors: part II. Vector systems and applications

Many properties of HSV-1 are especially suitable for using this virus as a vector to treat diseases affecting the central nervous system (CNS), such as Parkinson's disease or malignant gliomas. These advantageous properties include natural neurotropism, high transduction efficiency, large transgene capacity, and the ability of entering a latent state in neurons. Selective oncolysis in combination with modulation of the immune response mediated by replication-conditional HSV-1 vectors appears to be a highly promising approach in the battle against malignant glioma. Helper virus-free HSV/AAV hybrid amplicon vectors have great promise in mediating long-term gene expression in the PNS and CNS for the treatment of various neurodegenerative disorders or chronic pain. Current research focuses on the design of HSV-1-derived vectors which are targeted to certain cell types and support transcriptionally regulatable transgene expression. Here, we review the recent developments on HSV-1-based vector systems and their applications in experimental and clinical gene therapy protocols.

BAC-VAC, a novel generation of (DNA) vaccines: A bacterial artificial chromosome (BAC) containing a replication-competent, packaging-defective virus genome induces protective immunity against herpes simplex virus 1

This study aimed to exploit bacterial artificial chromosomes (BAC) as large antigen-capacity DNA vaccines (BAC-VAC) against complex pathogens, such as herpes simplex virus 1 (HSV-1). The 152-kbp HSV-1 genome recently has been cloned as an F-plasmid-based BAC in Escherichia coli (fHSV), which can efficiently produce infectious virus progeny upon transfection into mammalian cells. A safe modification of fHSV, fHSVDeltapac, does not give rise to progeny virus because the signals necessary to package DNA into virions have been excluded. However, in mammalian cells fHSVDeltapac DNA can still replicate, express the HSV-1 genes, cause cytotoxic effects, and produce virus-like particles. Because these functions mimic the lytic cycle of the HSV-1 infection, fHSVDeltapac was expected to stimulate the immune system as efficiently as a modified live virus vaccine. To test this hypothesis, mice were immunized with fHSVDeltapac DNA applied intradermally by gold-particle bombardment, and the immune responses were compared with those induced by infection with disabled infectious single cycle HSV-1. Immunization with either fHSVDeltapac or disabled infectious single cycle HSV-1 induced the priming of HSV-1-specific cytotoxic T cells and the production of virus-specific antibodies and conferred protection against intracerebral injection of wild-type HSV-1 at a dose of 200 LD(50). Protection probably was cell-mediated, as transfer of serum from immunized mice did not protect naive animals. We conclude that BAC-VACs per se, or in combination with genetic elements that support replicative amplification of the DNA in the cell nucleus, represent a useful new generation of DNA-based vaccination strategies for many viral and nonviral antigens.