An efficient selection system for packaging herpes simplex virus amplicons

Inhibition of the host translation shutoff response by herpes simplex virus 1 triggers nuclear envelope-derived autophagy

Macroautophagy is a cellular pathway that degrades intracellular pathogens and contributes to antigen presentation. Herpes simplex virus 1 (HSV-1) infection triggers both macroautophagy and an additional form of autophagy that uses the nuclear envelope as a source of membrane. The present study constitutes the first in-depth analysis of nuclear envelope-derived autophagy (NEDA). We established LC3a as a marker that allowed us to distinguish between NEDA and macroautophagy in both immunofluorescence and flow cytometry. NEDA was observed in many different cell types, indicating that it is a general response to HSV-1 infection. This autophagic pathway is known to depend on the viral protein γ34.5, which can inhibit macroautophagy via binding to beclin-1. Using mutant viruses, we were able to show that binding of beclin-1 by γ34.5 had no effect on NEDA, demonstrating that NEDA is regulated differently than macroautophagy. Instead, NEDA was triggered in response to γ34.5 binding to protein phosphatase 1α, an interaction used by the virus to prevent host cells from shutting off protein translation. NEDA was not triggered when late viral protein production was inhibited with acyclovir or hippuristanol, indicating that the accumulation of these proteins might stress infected cells. Interestingly, expression of the late viral protein gH was sufficient to rescue NEDA in the context of infection with a virus that otherwise does not support strong late viral protein expression. We argue that NEDA is a cellular stress response triggered late during HSV-1 infection and might compensate for the viral alteration of the macroautophagic response.

Construction of an oncolytic herpes simplex virus that precisely targets hepatocellular carcinoma cells

Selective replication in tumor cells is a highly desirable feature for oncolytic viruses. Recent studies have shown that microRNAs (miRNAs) play important roles in controlling gene expression, and that certain tissue-specific miRNAs are frequently downregulated in malignant cells. miR-122 is a liver-specific microRNA. It is abundantly expressed in normal hepatocytes but is absent in many hepatocellular carcinoma (HCC) cells. We hypothesized that expression of an essential viral gene by a liver-specific promoter would initially restrict virus replication to cells of hepatic origin and that adding miR-122 complementary sequences to the viral gene would make the transcripts degradable by miR-122 in normal hepatocytes, thus further confining its replication to HCC. We have constructed such an oncolytic herpes simplex virus by linking the essential viral glycoprotein H gene with the liver-specific apolipoprotein E (apoE)-AAT promoter and by adding the miR-122a complimentary sequence to the 3' untranslated region (3'UTR). To further increase the safety of this virus, complementary sequences from miR-124a and let-7 were also engineered into the same 3'UTR. Designated liver-cancer specific oncolytic virus (LCSOV), it was highly selective in killing HCC cells and in shrinking HCC xenografts. We conclude that LCSOV is a highly specific oncolytic virus that can precisely target HCC.

HSV as a vector in vaccine development and gene therapy

The very deep knowledge acquired on the genetics and molecular biology of herpes simplex virus (HSV), major human pathogen whose lifestyle is based on a long-term dual interaction with the infected host characterized by the existence of lytic and latent infections, has allowed the development of potential vectors for several applications in human healthcare. These include delivery and expression of human genes to cells of the nervous system, selective destruction of cancer cells, prophylaxis against infection with HSV or other infectious diseases and targeted infection of specific tissues or organs. Three different classes of vectors can be derived from HSV-1: replication-competent attenuated vectors, replication-incompetent recombinant vectors and defective helper-dependent vectors known as amplicons. This chapter highlights the current knowledge concerning design, construction and recent applications, as well as the potential and current limitations of the three different classes of HSV-1-based vectors.

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.

Viral vector approaches to modify gene expression in the brain

The use of viral vectors as gene transfer tools for the central nervous system has seen a significant growth in the last decade. Improvements in the safety, efficiency and specificity of vectors for clinical applications have proven to be beneficial also for basic neuroscience research. This review will discuss the viral systems currently available to neuroscientists and some of the recent achievements in the study of synaptic function, memory and drug addiction.

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.

Herpes simplex virus-based vectors

Herpes simplex virus (HSV)-based vectors have primarily been developed for neuronal gene delivery, taking advantage of the virus' natural neurotropism. Two types of vector are available: replication defective viruses, whose cytotoxicity has been abolished by deleting viral gene products, and amplicon vectors, which are plasmids packaged into HSV particles with the aid of a helper virus. In this review I discuss how the cytotoxicity of the wild-type virus has been abolished, the progress which has been made toward defining promoter elements capable of directing long-term transgene expression form the latent viral genome and some of the potential clinical uses of these versatile vectors.

Development of new expression vector based on Pseudorabies virus amplicons: application to human insulin expression

Pseudorabies virus (PrV), a herpesvirus from the Alphaherpesvirinae subfamily, is suitable for amplicon vector replication and packaging into virions, with helper virus for trans replication and cleavage-packaging functions. PrV amplicon vectors were developed in a bacterial plasmid construction using PrV ori(s) and pac signals as the required cis elements. Human insulin cDNA was then cloned in the amplicon vector for human proinsulin expression. In the same construction, green fluorescent protein was used as a marker. PrV amplicons may have several advantages over herpes simplex virus type 1 (HSV1) amplicons in human gene therapy because it can infect human cells in vitro and in vivo, it is not pathogenic for primates and there is no pre-existing immunity and risk of recombination with latent PrV as occurs with HSV1.

High-frequency intermolecular homologous recombination during herpes simplex virus-mediated plasmid DNA replication

Homologous recombination is a prominent feature of herpes simplex virus (HSV) type 1 DNA replication. This has been demonstrated and traditionally studied in experimental settings where repeated sequences are present or are being introduced into a single molecule for subsequent genome isomerization. In the present study, we have designed a pair of unique HSV amplicon plasmids to examine in detail intermolecular homologous recombination (IM-HR) between these amplicon plasmids during HSV-mediated DNA replication. Our data show that IM-HR occurred at a very high frequency: up to 60% of the amplicon concatemers retrieved from virion particles underwent intermolecular homologous recombination. Such a high frequency of IM-HR required that both plasmids be replicated by HSV-mediated replication, as IM-HR events were not detected when either one or both plasmids were replicated by simian virus 40-mediated DNA replication, even with the presence of HSV infection. In addition, the majority of the homologous recombination events resulted in sequence replacement or targeted gene repair, while the minority resulted in sequence insertion. These findings imply that frequent intermolecular homologous recombination may contribute directly to HSV genome isomerization. In addition, HSV-mediated amplicon replication may be an attractive model for studying intermolecular homologous recombination mechanisms in general in a mammalian system. In this regard, the knowledge obtained from such a study may facilitate the development of better strategies for targeted gene correction for gene therapy purposes.

A simple selection system for construction of recombinant gD-negative pseudorabies virus as a vaccine vector

We describe a simple, efficient two-step method for construction of glycoprotein D (gD)-negative pseudorabies virus (PrV) carrying transgenes inserted in place of the gD gene. The first step was the use of the thymidine kinase (TK) gene of herpes simplex virus (HSV) for insertional inactivation of the gD gene in a PrV mutant deficient in both TK and glycoprotein E (gE). The gD-negative, HSV-TK-positive mutant could be selected in HAT medium. The second step was substitution of HSV-TK with other genes of interest. The resultant gD/gE/TK-negative mutant was easily isolated by acyclovir selection. The expression of the transgene was detectable in vivo and the antibody responses against both inserted antigens and PrV were induced. The protective efficacy of the gD/gE/TK-negative PrV against lethal PrV challenge was also demonstrated. This PrV mutant carrying immunogenic proteins from unrelated porcine pathogens may be tested as a multivalent vaccine candidate for swine.

Isomerization of a uniquely designed amplicon during herpes simplex virus-mediated replication

Herpes simplex virus (HSV) type 1 DNA isomerization was studied using a uniquely designed amplicon that mimics the viral genomic structure. The results revealed that amplicon concatemers frequently contain adjacent amplicon units with their segments in opposed orientations. These unusual concatemers were generated through homologous recombination, which does not require HSV DNA as the source of homology.

Delivery of herpes simplex virus vectors through liposome formulation

Viral vectors have been widely used as gene delivery vehicles for both experimental and clinical investigations. Although these vectors are capable of achieving high gene transduction efficiency in vitro, one of the major limitations facing the therapeutic viral vectors is that the preexisting host anti-vector immunity can substantially reduce their transduction efficiency in vivo. This is especially of concern when the therapeutic remedy requires repeated systemic administration. Here we report the delivery of herpes simplex virus (HSV) derived vectors through liposome formulation. In these studies, we have prepared HSV vectors in three different forms for liposome formulation: purified viral DNA (obtained from a bacterial artificial chromosome containing an infectious HSV genome), HSV capsids, and intact viral particles. All three forms of HSV were readily transfected into cultured cells and infectious virus was efficiently generated. Furthermore, introduction of HSV vectors as DNA/liposome complexes improved in vivo transduction efficiency, by effectively evading the host anti-HSV immunity during systemic administration. We conclude that viral vectors such as HSV can be systemically delivered through liposome formulation for safe and repeated administration for gene transduction or oncolytic purposes.

Optimization of recombinant adeno-associated virus production using an herpes simplex virus amplicon system

A major limitation of adeno-associated virus (AAV) based vectors for clinical applications to date is the production of high-titer recombinant AAV vector stocks. Despite recent improvements, the amount of recombinant adeno-associated virus vectors (rAAV) particles produced per cell continues to be significantly lower than that of wild-type AAV. In this study, an HSV-based system for rAAV production was used to examine the influence of different parameters including transfection conditions (vector-to-packaging plasmid ratio, amount of total transfected DNA, cell confluency) and multiplicity of infection of herpes helper virus on the resulting titre of rAAV stocks. For herpes helper virus, time-course experiments were carried out to analyse the effect on rAAV yields up to 72 h postinfection and to determine the ideal harvesting time. Taken together, the optimized production scheme consistently yields more than 3x10(3) transducing units per producer cell.

Restoration of photoreceptor ultrastructure and function in retinal degeneration slow mice by gene therapy

The gene Prph2 encodes a photoreceptor-specific membrane glycoprotein, peripherin-2 (also known as peripherin/rds), which is inserted into the rims of photoreceptor outer segment discs in a complex with rom-1 (ref. 2). The complex is necessary for the stabilization of the discs, which are renewed constantly throughout life, and which contain the visual pigments necessary for photon capture. Mutations in Prph2 have been shown to result in a variety of photoreceptor dystrophies, including autosomal dominant retinitis pigmentosa and macular dystrophy. A common feature of these diseases is the loss of photoreceptor function, also seen in the retinal degeneration slow (rds or Prph2 Rd2/Rd2) mouse, which is homozygous for a null mutation in Prph2. It is characterized by a complete failure to develop photoreceptor discs and outer segments, downregulation of rhodopsin and apoptotic loss of photoreceptor cells. The electroretinograms (ERGs) of Prph2Rd2/Rd2 mice have greatly diminished a-wave and b-wave amplitudes, which decline to virtually undetectable concentrations by two months. Subretinal injection of recombinant adeno-associated virus (AAV) encoding a Prph2 transgene results in stable generation of outer segment structures and formation of new stacks of discs containing both perpherin-2 and rhodopsin, which in many cases are morphologically similar to normal outer segments. Moreover, the re-establishment of the structural integrity of the photoreceptor layer also results in electrophysiological correction. These studies demonstrate for the first time that a complex ultrastructural cell defect can be corrected both morphologically and functionally by in vivo gene transfer.

High-titer recombinant adeno-associated virus production from replicating amplicons and herpes vectors deleted for glycoprotein H

Production of high-titer rAAV is essential for in vivo clinical application. One limiting factor may be the failure of existing systems to replicate the packaging genome in such a way that expression of Rep and Cap proteins is coordinately amplified. DISC-HSV (disabled single-cycle virus) is a genetically modified herpes simplex virus (HSV) that by deletion of glycoprotein H (gH) is infectious only if propagated in a complementing cell line. In this study, we have used DISC-HSV as a helper for rAAV replication, and have simulated to some extent the amplication of the rep and cap genomes seen in wtAAV infection by incorporating both these and vector sequences in HSV amplicons. Facilitated production of AAV Rep and Cap proteins translates into a considerably improved recovery of rAAV, which transduces cells of the neuroretina in vivo with high efficiency. The potential for contamination with infectious herpes particles is eliminated by the use of noncomplementing (gH-) cell lines to propagate the virus, and by standard purification methods. The use of DISC-HSV and herpes-derived amplicons for production of rAAV may be a useful strategy for future in vivo studies and for clinical application.

Combined HSV-1 recombinant and amplicon piggyback vectors: replication-competent and defective forms, and therapeutic efficacy for experimental gliomas

BACKGROUND

The versatility of HSV-1 vectors includes large transgene capacity, selective replication of mutants in dividing cells, and availability of recombinant virus (RV) and plasmid-derived (amplicon) vectors, which can be propagated in a co-dependent, 'piggyback', manner.

METHODS

A replication-defective piggyback vector system was generated in which the amplicon carries either of two genes essential for virus replication, IE2 (ICP27) or IE3 (ICP4), as well as lacZ; the RV is deleted in both these genes, and vector stocks are propagated in cells transfected with one of the complementary genes. In the replication-competent system, the amplicon carries the IE2 and lacZ; the RV had a large deletion in the IE2; and stocks are propagated in untransfected cells. Titers over successive passages, recombination between amplicon and RV, and the structural integrity of vector genomes were evaluated. The replication-competent system was tested for therapeutic efficacy in subcutaneous 9L gliosarcoma tumors in nude mice with activation of ganciclovir via the viral HSV-thymidine kinase gene.

RESULTS

Both systems generated high titer amplicon vectors (about 10(7) tu/ml) and amplicon:RV ratios (0.6-3.0). No replication-competent RV was generated in either system. The replication-defective system showed low toxicity and increased packaging efficiency of amplicon vectors, as compared to single mutant RV helper virus. The replication-competent system allowed co-propagation of amplicon and RV; injection into tumors followed by ganciclovir treatment inhibited tumor growth without systemic toxicity.

CONCLUSION

New replication-defective and replication-competent piggyback HSV, vector systems allow gene delivery via amplicon vectors with reduced toxicity and co-propagation of both RV and amplicon vectors in target cells, with effective tumor therapy via focal virus replication and pro-drug activation.

An enhanced packaging system for helper-dependent herpes simplex virus vectors

Helper-dependent herpes simplex virus (HSV) vectors (amplicons) show considerable promise to provide for long-term transduced-gene expression in most cell types. The current packaging system of choice for these vectors involves cotransfection with a set of five overlapping cosmids that encode the full HSV type 1 (HSV-1) helper virus genome from which the packaging (pac) elements have been deleted. Although both the helper virus and the HSV amplicon can replicate, only the latter is packaged into infectious viral particles. Since the titers obtained are too low for practical application, an enhanced second-generation packaging system was developed by modifying both the helper virus and the HSV amplicon vector. The helper virus was reverse engineered by using the original five cosmids to generate a single HSV-bacterial artificial chromosome (BAC) clone in Escherichia coli from which the pac elements were deleted to generate a replication-proficient but packaging-defective HSV-1 genome. The HSV amplicon was modified to contain the simian virus 40 origin of replication, which acts as an HSV-independent replicon to provide for the replicative expansion of the vector. The HSV amplicon is packaged into infectious particles by cotransfection with the HSV-BAC helper virus into the 293T cell line, and the resulting cell lysate is free of detectable helper virus contamination. The combination of both modifications to the original packaging system affords an eightfold increase in the packaged-vector yield.