A Novel Herpes Vector for the High-Efficiency Transduction of Normal and Malignant Human Hematopoietic Cells

Clinical evaluation of cellular immunotherapy in acute myeloid leukaemia

Immunotherapy is currently under active investigation as an adjuvant therapy to improve the overall survival of patients with acute myeloid leukaemia (AML) by eliminating residual leukaemic cells following standard therapy. The graft-versus-leukaemia effect observed following allogeneic haematopoietic stem cell transplantation has already demonstrated the significant role of immune cells in controlling AML, paving the way to further exploitation of this effect in optimized immunotherapy protocols. In this review, we discuss the current state of cellular immunotherapy as adjuvant therapy for AML, with a particular focus on new strategies and recently published results of preclinical and clinical studies. Therapeutic vaccines that are being tested in AML include whole tumour cells as an autologous source of multiple leukaemia-associated antigens (LAA) and autologous dendritic cells loaded with LAA as effective antigen-presenting cells. Furthermore, adoptive transfer of cytotoxic T cells or natural killer cells is under active investigation. Results from phase I and II trials are promising and support further investigation into the potential of cellular immunotherapeutic strategies to prevent or fight relapse in AML patients.

Immunotherapy of acute myeloid leukemia: current approaches

Following standard therapy that consists of chemotherapy with or without stem cell transplantation, both relapsed and refractory disease shorten the survival of acute myeloid leukemia (AML) patients. Therefore, additional treatment options are urgently needed, especially to fight residual AML cells. The identification of leukemia-associated antigens and the observation that administration of allogeneic T cells can mediate a graft-versus-leukemia effect paved the way to the development of active and passive immunotherapy strategies, respectively. The aim of these strategies is the eradication of AML cells by the immune system. In this review, an overview is provided of both active and passive immunotherapy strategies that are under investigation or in use for the treatment of AML. For each strategy, a critical view on the state of the art is given and future perspectives are discussed.

Cancer vaccines: accomplishments and challenges

Advancements in knowledge in diverse fields of science, including genetics, cell biology, molecular biology and biochemistry, have shed light on the origins of cancer and cell intrinsic properties that allow it to grow, invade and metastasize. Many therapies currently in use or under development are based on this knowledge. Advances in immunology, on the other hand, have shed light on how the host responds to these malignant properties of cancer. Based on that knowledge, immunotherapy, in particular vaccines directed at improving the host response against cancer, is being developed as an alternative therapeutic approach. In this review, we address main issues that have driven development of cancer vaccines and the challenges that have been met and/or are anticipated.

Recombinant viral vectors: cancer vaccines

To date cancer vaccines have yet to show efficacy in a phase III trial. However, the clinical benefit seen with monoclonal antibody mediated therapies (e.g., Herceptin) has provided proof of principle that immune responses directed against tumour-associated antigens could have therapeutic potential. The failure of past cancer vaccine trials is likely due to several factors including the inappropriate choice of tumour antigen, use of an unoptimised antigen delivery system or vaccination schedule or selection of the wrong patient group. Any one of these variables could potentially result in the induction of an immune response of insufficient magnitude to deliver clinical benefit. Live recombinant viral vaccines have been used in the development of cancer immunotherapy approaches for the past 10 years. Though such vectors are self-adjuvanted and offer the ability to express multiple tumour-associated antigens (TAAs) along with an array of immune co-factors, arguably, they have yet to demonstrate convincing efficacy in pivotal clinical trials. However, in recent years, more coordinated studies have revealed mechanisms to optimise current vectors and have lead to the development of new advantageous vector systems. In this review, we highlight that live recombinant viral vectors provide a versatile and effective antigen delivery system and describe the optimal properties of an effective viral vector. Additionally, we discuss the advantages and disadvantages of the panel of recombinant viral systems currently available to cancer vaccinologists and how they can work in synergy in heterologous prime boost protocols and with other treatment modalities.

Herpes simplex virus type-1 amplicon vectors for vaccine generation in acute lymphoblastic leukemia

For leukemia vaccine generation, high-efficiency gene transfer is required to express immunomodulatory molecules that stimulate potent antileukemic immune responses. In this context, herpes simplex virus type-1 (HSV-1)-derived vectors have proven to be a promising tool for genetic modification of lymphoblastic leukemia cells. Yet, vector-associated viral protein expression might inadvertently modulate vaccine efficacy facilitating both immune evasion and immune stimulation. To explore the issue of immune-stimulation versus immune-suppression in immature lymphoblastic leukemia cells, two types of HSV-1 amplicon vectors, helper virus-dependent and helper virus-free that express the immunomodulatory molecules CD70 and IL-2, were compared with regard to their vector-associated immunomodulatory potential. We first established that lymphoblastic cell lines and primary acute lymphoblastic leukemia (ALL) cells express HSV receptor genes. Lymphoblastic cell lines were transduced with high efficiency, and in primary ALL cells high gene transfer rates of 47+/-15 and 42+/-14% were obtained with helper virus-dependent and -free HSV-1 amplicon vectors, respectively. The efficacy of the two amplicon vectors to induce antineoplastic responses was assessed in a vaccine setting in mice with pre-existing highly malignant lymphoblastic disease. Treatment of mice with vaccine cells transgenically expressing CD70+IL2 significantly suppressed lymphoblastic cell proliferation and improved survival. Of note, when helper virus-dependent HSV-1 amplicon vectors were used for vaccine preparation, the high immunogenic potential of the vector itself, in the absence of transgenic CD70+IL2 expression, seemed to be sufficient to mediate protection comparable to the antineoplastic response achieved by expression of immunomodulatory molecules. Thus for vaccine generation in B lymphoblastic leukemia, the immunogenic potential of HSV-1 helper virus-dependent amplicon vectors does provide additional benefit to the high transduction efficiency of HSV-1-derived vectors.

Immunotherapeutic potential of DISC-HSV and OX40L in cancer

Several vectors, viral and bacterial, have been developed over the past few years for means of generating an effective antitumor immune response. We have developed and studied a "model for immunotherapy" using a viral vector disabled infectious single cycle-herpes simplex virus (DISC-HSV), which efficiently transduces various tumor cell lines and offers a useful vehicle for the further development of cell-based vaccines. The immunotherapeutic potential of DISC-HSV encoding granulocyte macrophage colony stimulating factor (GM-CSF) was demonstrated in a number of murine carcinoma models, leading to complete regression of well-established tumors in up to 70% of the mice. Moreover, the therapeutic potential of DISC-HSV-GM-CSF was significantly enhanced when used in combination therapy with either OX40L or dendritic cells (DC), even in a poorly immunogenic tumor model. The ability of this vector to accept large gene inserts, its good safety profile, its ability to undergo only a single round of infection, the inherent viral immunostimulatory properties and its ability to infect various tumor cell lines efficiently, make DISC-HSV an ideal candidate vector for immunotherapy. The DISC- CT-26 tumor model was used to investigate the mechanisms associated with immunotherapy induced tumor rejection. Although CTL induction, was positively correlated with regression, MHC class I down regulation and accumulation of immature Gr1+ myeloid cells were shown to be the main immuno-suppressor mechanisms operating against regression and associated with progressive tumor growth. The CTL response was associated with the immuno-dominant AH-1 peptide of the retroviral glycoprotein gp70. This model of immunotherapy has provided an opportunity to dissect further the immunological events associated with tumor-rejection and escape. Since other antigens may be important in initiating tumor rejection, we have investigated the expression of MTA-1, an antigen that appears to be expressed widely in human and murine tumors. The immunogenicity of MTA-1 was studied and its potential as a tumor rejection antigen is under investigation.

CCL3/MIP-1αIs a Potent Immunostimulator When Coexpressed with Interleukin-2 or Granulocyte-Macrophage Colony-Stimulating Factor in a Leukemia/Lymphoma Vaccine

Chemokines orchestrate trafficking of immune effector cells during inflammation. Here we demonstrate that chemokines also serve to potentiate effector cell-mediated antineoplastic immune responses in vaccination strategies. As a critical mediator of inflammation, macrophage inflammatory protein 1alpha (CCL3/MIP-1alpha) attracts and stimulates both antigen-presenting and cytotoxic cells. In the A20 leukemia/lymphoma vaccine model, we explored the efficacy of MIP-1alpha in combination with interleukin-2 (IL-2) or granulocyte-macrophage colony-stimulating factor (GM-CSF). After subcutaneous injection of the MIP-1alpha + IL-2 or MIP-1alpha + GM-CSF combination vaccine, focal but pronounced infiltrates of CD4+ and CD8+ T cells were observed at the vaccination sites. In mice with preestablished leukemia/lymphoma, survival is significantly improved in animals treated with MIP-1alpha + GM-CSF- and MIP-1alpha + IL-2-secreting vaccines. Protection is superior in the MIP-1alpha + GM-CSF group, with the effects of MIP-1alpha and GM-CSF being synergistic. In contrast, suppression of lymphoblast proliferation by single-immunogen vaccines secreting MIP-1alpha, GM-CSF, or IL-2 alone does not translate to improved survival. The systemic protective effects afforded by the MIP-1alpha + IL-2 or MIP-1alpha + GM-CSF combination are mediated by different effector cell populations. In the MIP-1alpha + IL-2 group, antineoplastic defense is mediated by CD8+ T and NK cells, whereas in the MIP-1alpha + GM-CSF group CD4+ T cells are involved in addition to CD8+ cytotoxic T cells, underscoring that T cell help is critical for long-term protection. Thus combination of MIP-1alpha with different cytokines recruits different sets of effector cells into a potent antineoplastic immune response.

In vitro thymidine kinase/ganciclovir-based suicide gene therapy using replication defective herpes simplex virus-1 against leukemic B-cell malignancies (MCL, HCL, B-CLL)

A replication defective herpes simplex virus-1 was evaluated as a therapeutic vector. Mantle cell lymphoma (MCL), hairy cell leukemia (HCL), and B-cell chronic lymphocytic leukemia (B-CLL) were chosen because leukemic cells were collectable from peripheral bloods in these diseases. Cells from six MCL, one HCL, and nine B-CLL were infected in vitro with T0Z.1 at 3 multiplicity of infection (MOI). Herpes simplex virus thymidine kinase (HSV-TK)/ganciclovir (GCV)-mediated suicide gene therapy showed 14.7% of mean tumor killing against leukemic B-cell malignancies. The mean tumor-killing effects were 8.7 and 17.1% in MCL and B-CLL, respectively. The effect against HCL was 29%. The study indicates that herpes simplex virus (HSV)-based gene therapy might be an effective strategy.

Sustained inhibition of experimental neointimal hyperplasia with a genetically modified herpes simplex virus

OBJECTIVE

Reported herein is a potential strategy for sustained smooth muscle cell (SMC) inhibition with a virulence-attenuated herpes simplex virus (HSV). Experiments were conducted in vitro to demonstrate selective SMC cytotoxicity and in vivo to demonstrate reduced neointimal hyperplasia (NIH) in a clinically relevant animal model.

METHODS

In vitro: Cultured human umbilical artery smooth muscle cells (UASMC) and venous endothelial cells (HUVEC) were exposed to varying multiplicities of infection (MOI) of a gamma(1)34.5-deleted HSV-1 virus (R849). Cell survival was assessed at 48 and 72 hours with a colorimetric MTT viability assay. In vivo: New Zealand White rabbit external jugular veins (n = 21) were exposed to R849 (2.5 x 10(6) pfu/mL) or culture medium at 110 to 120 mm Hg for 10 minutes, then fashioned as vein patches on carotid arteries. Carotid arteries were ligated distally to decrease blood flow and stimulate a hyperplastic response (ultra-low shear stress model). After 2, 4, 12, and 24 weeks, patched segments were perfusion-fixed with glutaraldehyde and morphometrically examined for NIH formation.

RESULTS

In vitro: At 48 hours, R849 exhibited preferential cytotoxicity to UASMC compared with HUVEC, with 11% +/- 10% of UASMCs and 49% +/- 8% of HUVECs surviving after infection with MOI = 25 (P <.05). Higher MOI resulted in poor survival of both cell lines. In vivo: Blood flow was similarly reduced in all animals both at surgery (0.9 +/- 0.1 mL/min vs 1.6 +/- 0.3 mL/min) and at harvest (2.7 +/- 0.4 mL/min vs 2.5 +/- 0.5 mL/min). R849-infected patches exhibited markedly less NIH than control patches did at 2 weeks (162 +/- 14 microm vs 49 +/- 6 microm; P <.05), 4 weeks (190 +/- 27 microm vs 67 +/- 8 microm; P <.05), and 12 weeks (233 +/- 18 microm vs 113 +/- 2 microm; P <.05).

CONCLUSION

The virulence-attenuated HSV strain R849 demonstrates selective cytotoxicity for SMC and is capable of sustained inhibition of NIH in an experimental model of vein graft failure.

Immunotherapy with acute leukemia cells modified into antigen-presenting cells: ex vivo culture and gene transfer methods

Adult patients with acute leukemia have, in general, a poor prognosis, with long-term, disease-free survival achieved in only approximately one-third of cases. One of the proposed mechanisms for this poor overall response is the inability of the immune system to detect and eliminate residual malignant leukemia cells, which subsequently serve as a source of leukemic relapse. This review discusses the rationale of immunotherapy for acute leukemia and presents in vitro and in vivo model systems that were devised for pre-B acute lymphocytic leukemia (ALL) and acute myeloid leukemia (AML). New advances in the ex vivo manipulation of acute leukemia cells are presented, which attempt to modify these cells into functional antigen-presenting cells. These cells can then be used as autologous vaccines at the time of minimal residual disease after standard chemotherapy, to stimulate host immune responses against their own leukemia cells. The various approaches toward this aim include incubation of leukemia cells with cytokines or growth factors and gene manipulation of these cells. In particular, ex vivo culture of ALL cells with CD40 ligand, incubation of AML cells with granulocyte-macrophage colony-stimulating factor and interleukin-4 (GM-CSF/IL-4) and lentiviral transduction of ALL and AML cells for expression of immunomodulators (CD80 and GM-CSF) are current approaches under investigation for the development of autologous acute leukemia cell vaccines.

Gene therapy of lymphoma

Gene therapy offers new and promising treatment for patients with hematological malignancies. Tumor cells--lymphoma cells, for example--are possible targets for gene therapy. In general, gene therapeutic approaches require efficient gene transfer into host cells and sufficient transgene expression. Although many methods of gene transfer into mammalian cells exist, most do not allow efficient DNA transfer into primary lymphocytes. In contrast to gene transfer into tumor cells and many other cell types, which can be successfully performed using a variety of methods, the efficient expression of foreign DNA in lymphoma cells presents unique problems and challenges, requiring a careful selection of the mode of gene transfer. In this review, we discuss the current strategies for gene therapy in the treatment of lymphoma. We also summarize the current gene transfer methods for lymphoma cells and efficiency of transgene expression.

Effect of temperature, medium composition, and cell passage on production of herpes-based viral vectors

Our work uses replication-defective genomic herpes simplex virus type-1 (HSV-1)-based vectors to transfer therapeutic genes into cells of the central nervous system and other tissues. Obtaining highly purified high-titer vector stocks is one of the major obstacles remaining in the use of these vectors in gene therapy applications. We have examined the effects of temperature and media conditions on the half-life of HSV-1 vectors. The results reveal that HSV stability is 2.5-fold greater at 33 degrees C than at 37 degrees C and is further stabilized at 4 degrees C. Additionally, a significantly higher half-life was measured for the vector in infection culture conditioned serum medium compared to fresh medium with or without serum. Synchronous infections incubated at 33 degrees C produced 2-fold higher amounts of vector than infected cells incubated at 37 degrees C, but with a lag of 16-24 h. Vector production yielded 3-fold higher titers and remained stable at peak levels for a longer period of time in cultures incubated at 33 degrees C than 37 degrees C. A pronounced negative effect of increased cell passage number on vector yield was observed. Vector production at 33 degrees C yielded similar levels regardless of passage number but was reduced at 37 degrees C as passage number increased. Together, these results contribute to improved methods for high-titer HSV vector production.

Multiple applications for replication-defective herpes simplex virus vectors

Herpes simplex virus (HSV) is a neurotropic DNA virus. The viral genome is large (152 kb), and many genes are dispensable for viral function, allowing insertion of multiple or large transgene expression cassettes. The virus life cycle includes a latent phase, during which the viral genome remains as a stable episomal element within neuronal nuclei for the lifetime of the host, without disturbing normal function. We have exploited these features of HSV to construct a series of nonpathogenic gene therapy vectors that efficiently deliver therapeutic and experimental transgenes to neural and non-neural tissue. Importantly, transgene expression may be sustained long term; reporter gene expression has been demonstrated for over a year in the nervous system. This article discusses the generation of replication-defective HSV vectors and reviews recent studies investigating their use in several animal models of human disease. We have demonstrated correction or prevention of a number of important neurological phenotypes, including neurodegeneration, chronic pain, peripheral neuropathy, and malignancy. In addition, HSV-mediated transduction of non-neurological tissues allows their use as depot sites for synthesis of circulating and locally acting secreted proteins. New applications for this vector system include the genetic modification of stem cell populations; this may become an important means to direct cellular differentiation or deliver therapeutic genes systemically. Replication-defective HSV vectors are an effective and flexible vehicle for the delivery of transgenes to numerous tissues, with multiple applications.

Oncolytic viruses as therapeutic agents

The concept of using viruses as oncolytic agents has a long history. However, relatively new developments are the use of these viruses as gene delivery vehicles and the restriction of viral replication and lysis to tumour cells. The latter is attempted by the use of tumour-specific promoters, which transcriptionally target viral genes involved in replication, or by deletion of viral functions dispensable for replication in tumour cells but essential for productive infection of normal cells. In addition, retargeting of the viral tropism towards tumours by capsid modifications has been examined. Although much progress has been made in developing oncolytic vectors for clinical use, there is still a long way to go to determine which combinations of virus, gene therapy, surgery, radiation, and/or chemotherapy will provide improved therapy for the control and eradication of a variety of human cancers. First controlled clinical trials with an oncolytic adenovirus in combination with chemotherapy have shown encouraging antineoplastic activity. For future vector developments it will be crucial to achieve maximum vector distribution and transgene expression within tumours, to trigger a specific systemic immune effector response against treated and untreated lesions, and to modulate the immune system to avoid immune-mediated inactivation or destruction of the virus. In the context of replication-competent vectors, suicide genes might be used as fail-safe mechanism in the case of a runaway infection.

Autologous antileukemic immune response induced by chronic lymphocytic leukemia B cells expressing the CD40 ligand and interleukin 2 transgenes

Although the B cells of chronic lymphocytic leukemia (B-CLL cells) express both tumor-specific peptides and major histocompatibility complex (MHC) class I antigens, they lack the capacity for costimulatory signaling, contributing to their protection against host antitumor immunity. To stimulate CLL-specific immune responses, we sought to transfer the human CD40 ligand (hCD40L) gene to B-CLL cells, using an adenoviral vector, in order to upregulate costimulating factors on these cells. Because efficient gene transduction with adenoviral vectors requires the expression of virus receptors on target cells, including the coxsackievirus B-adenovirus receptors (CAR) and alpha(v) integrins, we cocultured B-CLL cells with human embryonic lung fibroblasts (MRC-5 line). This exposure led to increased expression of integrin alpha(v)beta3 on B-CLL cells, which correlated with higher transduction rates. Using this novel prestimulation system, we transduced B-CLL cells with the hCD40L gene. The Ad-hCD40L-infected cells had higher expression of B7 molecules and induced activation of autologous T cells in vitro, but these T cells could not recognize parental leukemic cells. By contrast, an admixture of Ad-hCD40L-positive cells and leukemic cells transduced with the human interleukin 2 (IL-2) gene produced greater T cell activation than did either immunostimulator population alone. Importantly, this combination generated autologous T cells capable of specifically recognizing parental B-CLL cells. These findings suggest that the combined use of genetically modified CD40L-expressing B-CLL cells in combination with IL-2-expressing B-CLL cells may induce therapeutically significant leukemia-specific immune responses.

Gene transfer and the treatment of haematological malignancy

Gene therapy offers an additional therapeutic modality for treating haematological malignancy. Because gene therapies could be truly specific for the malignancy, they should ultimately prove both safe and effective. We have far to go before this full potential is realized, but gene transfer strategies are already showing therapeutic promise. Gene transfer may be used to correct the genetic defect in the tumour, to render it more susceptible to conventional therapies, or the normal host cells more resistant, to induce or amplify an antitumour immune response, or simply as a means of tracking the tumour or cells used for treatment. This article describes examples of each approach and discusses future prospects for the field.

Lentiviral vectors for efficient delivery of CD80 and granulocyte-macrophage– colony-stimulating factor in human acute lymphoblastic leukemia and acute myeloid leukemia cells to induce antileukemic immune responses

Cell vaccines engineered to express immunomodulators have shown feasibility in eliminating leukemia in murine models. Vectors for efficient gene delivery to primary human leukemia cells are required to translate this approach to clinical trials. In this study, second-generation lentiviral vectors derived from human immunodeficiency virus 1 were evaluated, with the cytomegalovirus (CMV) promoter driving expression of granulocyte-macrophage–colony-stimulating factor (GM-CSF) and CD80 in separate vectors or in a bicistronic vector. The vectors were pseudotyped with vesicular stomatitis virus G glycoprotein and concentrated to high titers (108-109 infective particles/mL). Human acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), and chronic myeloid leukemia cell lines transduced with the monocistronic pHR-CD80 vector or the bicistronic pHR-GM/CD vector became 75% to 95% CD80 positive (CD80+). More important, transduction of primary human ALL and AML blasts with high-titer lentiviral vectors was consistently successful (40%-95% CD80+). The average amount of GM-CSF secretion by the leukemia cell lines transduced with the pHR-GM-CSF monocistronic vector was 2182.9 pg/106 cells per 24 hours. Secretion was markedly lower with the bicistronic pHR-GM/CD vector (average, 225.7 pg/106 cells per 24 hours). Lower amounts of CMV-driven messenger RNA were detected with the bicistronic vector, which may account for its poor expression of GM-CSF. Primary ALL cells transduced to express CD80 stimulated T-cell proliferation in an autologous mixed lymphocyte reaction. This stimulation was specifically blocked with monoclonal antibodies reactive against CD80 or by recombinant cytotoxic T-lymphocyte antigen 4–immunoglobulin fusion protein. These results show the feasibility of efficiently transducing primary leukemia cells with lentiviral vectors to express immunomodulators to elicit antileukemic immune responses.

Stable marker gene transfer into human bone marrow stromal cells and their progenitors using novel herpesvirus saimiri-based vectors

We have evaluated the ability of new herpesvirus saimiri (HVS)-based vectors to deliver a marker gene green fluorescent protein (GFP) into human bone marrow (BM) stromal cells and their progenitors. Stromal cells expanded from adherent layers of long-term BM cultures (LTC) were susceptible to HVS-based infection in a dose-dependent manner, and the efficiency of 94.8 +/- 2.0% was achieved using single exposure with HVS/EGFP vector at multiplicity of infection (moi) of approximately 50. Colony-forming unit-fibroblast (CFU-F) assay established the ability of HVS-based vectors to infect progenitors for bone marrow stroma fibroblasts and transfer the marker gene over multiple cell divisions in the absence of selective pressure. HVS was not toxic for stromal cells and progenitors and no viral replication was detected upon growth of modified stroma. On the basis these data, we believe that HVS-based constructs can offer a new opportunity for selective gene delivery into bone marrow stromal cells and progenitors. The ability of HVS to infect nondividing cells can be considered advantageous in the development of both ex vivo and in vivo strategies.

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.

Chronic lymphocytic leukemia B cells are highly sensitive to infection by herpes simplex virus-1 via herpesvirus-entry-mediator A

We found that chronic lymphocytic leukemic (CLL) B cells are highly sensitive to infection with vectors derived from replication-defective herpes simplex virus-1 (rdHSV-1). CLL B cells were found to express high levels of herpes virus entry mediator (Hve) A, but not HveC, the other known receptor for HSV-1. An HveA cDNA from CLL cells was found to encode Arg-->Lys and Val-->Iso substitutions at amino acids 17 and 241, respectively. Nevertheless, this cDNA encoded a functional receptor for HSV-1 when transfected into Chinese hamster ovarian (CHO) cells. Antibodies to HveA could block rdHSV-1 infection of CLL cells and HveA-transfected CHO cells with similar efficiencies in vitro. In contrast to B cells of normal donors, CLL B cells were resistant to the cytopathic effects of infection by rdHSV-1 and maintained high-level expression of the transgene for several days in vitro. We propose that this is due to the expression by CLL cells of the anti-apoptotic protein, bcl-2. Consistent with this, we found that transduction of HeLa cells with a retrovirus expression vector encoding bcl-2 rendered HeLa cells resistant to the cytopathic effects of rdHSV-1. HSV-1-derived vectors should be excellent vehicles for gene transfer into CLL B cells, allowing for its potential use in gene therapy for this disease. Gene Therapy (2000) 7, 1210-1216.

Protease-deleted adenovirus vectors and complementing cell lines: potential applications of single-round replication mutants for vaccination and gene therapy

A new kind of versatile adenoviral vector (AdV) has been constructed, one that is completely replication disabled in the absence of Ad-E1 proteins but is capable of a single round of replication when Ad-E1 is present. This was made possible by deletion of the Ad protease gene (PS), which is essential for many steps of the Ad life cycle. The PS-deleted virus can be propagated in 293-derived cell lines engineered to express PS. In these new complementing cells, the PS gene was expressed from a tetracycline-inducible promoter in a dicistronic vector coexpressing the green fluorescent protein (GFP). When induced, the best 293-PS stable clones produced the PS in amounts greater than the level reached after Ad infection. Biological activity was first demonstrated by the ability of 293-PS cells to support the replication of Ad2ts1, a mutant expressing a functionally defective PS. While overexpression of the Ad PS slightly affected cell growth, moderate expression at levels sufficient to fully complement Ad2ts1 was well tolerated in 293 cells. Two PS-deleted mutants, deleted or not deleted for E1/E3, were then generated and characterized. Despite their complete loss of infectivity after a single round of replication in permissive cells, the PS-deleted mutants produced as much viral protein as wildtype Ad. These new vectors should thus be both safer and more efficient for applications in which enhancement of transgene expression is desirable, as in the case of vaccination, in situ therapy for tumors, protein production, or the large-scale production of other viral vectors such as adeno-associated virus (AAV).

gamma-ray irradiation induces B7.1 expression in myeloid leukaemic cells

Expression of B7 molecules provides co-stimulatory signals to T lymphocytes, which prevent the induction of anergy. It has been previously reported that B7.1 gene transfer in a murine leukaemia model induced a potent antileukaemic immunity and that relative expression of B7.1 and B7.2 in human acute myeloid leukaemia (AML) had prognostic significance. As ex vivo engineering of leukaemic cells for immunotherapy protocols would require prior irradiation of these cells before reinjection to the patient, we investigated in murine and leukaemic cell lines and in 20 ex vivo primary cultured acute myeloid leukaemic cells the effect of gamma-irradiation on the expression of B7 molecules. We observed that gamma-irradiation enhanced B7.1 molecule expression in murine leukaemic cell lines and in B7.2 molecules in human HL60 and K562 cell lines. gamma-Irradiation induced B7.1 molecule expression in 90% AML samples but only 21% showed B7.2 molecule expression enhancement. B7.1 expression was increased both at the protein and RNA level in human AML cells but only at the protein level in the DA1-3b murine cell line. Oxidative stress increased B7.1 expression in the murine DA1-3b cell line but human cell lines and AML samples remained unaffected both by heat shock and oxidative stress, suggesting different pathways of B7.1 induction between mouse and human cells. Our data show that B7.1 expression can be induced by ex vivo irradiation of AML cells, indicating that these cells can express co-stimulatory molecules without gene transfer.

Oncolytic viruses as novel anticancer agents: turning one scourge against another

Although the use of viruses as oncolytic agents is an historic concept, the use of genetically modified viruses to selectively target tumour cells is relatively novel and recent. The ability of viruses to efficiently infect and lyse cells, combined with the potential augmentation of this effect by progeny viruses throughout the tumour provide justification for exploitation of these agents in cancer therapy. Before application to humans, though, issues related to tumour cell selectivity, lack of toxicity to normal tissues and the effect of the antiviral immune response, will have to be clarified. The more commonly used oncolytic viruses are based on mutant strains of herpes simplex virus, adenovirus and reovirus. The tumour selectivity of each of these strains is discussed, particularly the complementation of the viral defect by cellular pathways involved in tumourigenesis. The combination of oncolytic viruses with radiation, chemotherapy and gene therapy is also reviewed. Further study of the interaction of viral proteins with cellular pathways involved in cell cycle control will provide the rationale for viral mutants with increased selectivity for tumour cells.

Genetically modified CD34+ cells as cellular vehicles for gene delivery into areas of angiogenesis in a rhesus model

To develop a cellular vehicle able to reach systemically disseminated areas of angiogenesis, we sought to exploit the natural tropism of circulating endothelial progenitor cells (EPCs). Primate CD34+ EPCs were genetically modified with high efficiency and minimal toxicity using a non-replicative herpes virus vector. These EPCs localized in a skin autograft model of angiogenesis in rhesus monkeys, and sustained the expression of a reporter gene for several weeks while circulating in the blood. In animals infused with autologous CD34+ EPCs transduced with a thymidine kinase-encoding herpes virus, skin autografts and subcutaneous Matrigel pellets impregnated with vascular growth factors underwent necrosis or accelerated regression after administration of ganciclovir. Importantly, the whole intervention was perfectly well tolerated. The accessibility, easy manipulation, lack of immunogenicity of the autologous CD34+ cell vehicles, and tropism for areas of angiogenesis render autologous CD34+ circulating endothelial progenitors as ideal candidates for exploration of their use as cellular vehicles when systemic gene delivery to those areas is required. Gene Therapy (2000) 7, 43-52.

Disabled infectious single-cycle herpes simplex virus as an oncolytic vector for immunotherapy of colorectal cancer

New modalities of treatment for colorectal cancer are required to support and improve those currently available. One such approach is immunotherapy by transfer of immunostimulatory genes to tumor cells. Here, we report the use of a herpes simplex virus (HSV) vector that is capable of a single round of infection (disabled infectious single-cycle [DISC]-HSV) as a gene transfer vehicle for colorectal cancer. This vector has potential advantages over other vectors for cancer immunotherapy in that it lyses infected tumor cells. Infection with DISC-HSV inhibited tumor cell growth both in vitro and in vivo. In addition, DISC-HSV-mediated cell killing occurs by both apoptotic and necrotic mechanisms. A range of colorectal tumor cell lines could be rapidly transduced with DISC-HSV/lacZ (14-90% in 4 hr). Both tumor prevention and tumor therapy protocols showed clear antitumor effects with DISC-HSV/mGM-CSF. In the prophylactic approach, an infected/irradiated whole cell vaccine protected up to 80% of mice from rechallenge. In addition, intratumoral injection of established tumors with DISC-HSV/GM-CSF caused rejection in 40% of mice and generated some protection from subsequent rechallenge. In both cases, however, it is clear that a dominant therapeutic effect of the DISC-HSV vector derives from its oncolytic properties, irrespective of the transduced gene. As a prelude to taking these studies forward to human clinical trials, we demonstrate that tumor cells could be successfully grown from freshly obtained human colorectal cancer resections (within 1 week of surgery), were transduced with DISC-HSV/hGM-CSF, and secreted the cytokine. This study provides the preclinical basis for trials of immunotherapy of colorectal cancer using DISC-HSV.

Combination of CD80 and granulocyte-macrophage colony-stimulating factor coexpression by a leukemia cell vaccine: preclinical studies in a murine model recapitulating Philadelphia chromosome-positive acute lymphoblastic leukemia

Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph+ ALL) is a highly aggressive malignancy caused by the bcr-abl translocation oncogene. To explore alternative treatments for Ph+ ALL we tested gene-modified cell vaccines in the BALB/c-derived BM185 leukemia model. We compared the efficacy of BM185 cell vaccine expressing CD80 alone or in combination with IL-2 or GM-CSF. Mice injected with viable BM185 leukemia cells modified to express CD80 and GM-CSF (BM185/CD80+GM-CSF) showed the highest leukemia rejection rates. Cell vaccines consisting of irradiated BM185/CD80+GM-CSF cells administered subcutaneously stimulated a potent cytotoxic T lymphocyte (CTL) response against parental BM185. Histological examination of the vaccination site showed a large concentration of immune cells. Administration of the BM185/CD80+GM-CSF cell vaccine before intravenous challenge with parental cells caused strong inhibition of leukemia development. Vaccination after subcutaneous challenge with BM185 cells caused efficient elimination of leukemia promoting 40-60% long-term survival rates. The immunization efficacy of the BM185/CD80+ GM-CSF cell vaccine was directly correlated with the percentage of cells expressing the transgenes. In all, this preclinical study shows that leukemia cell vaccines coexpressing CD80 and GM-CSF can potentially be explored for immunotherapy in Ph+ ALL patients.

Increased gene transfer in acute myeloid leukemic cells by an adenovirus vector containing a modified fiber protein

Applications of gene transfer in acute myeloid leukemia (AML) blast cells have still not been developed, mostly due to the lack of an efficient vector. Adenoviruses have many advantages as vectors, but remain poorly efficient in cells lacking fiber receptors. A promising strategy is the retargeting of adenoviruses to other cellular receptors. We report the dramatic enhancement of gene transfer efficiency in AML blasts using AdZ.F(pK7), a modified adenovirus containing a heparin/heparan sulfate binding domain incorporated into the fiber protein of the adenovirus. We transduced 25 AML blast samples with efficiency reaching 100% of the cells in most samples. Optimal results were obtained at 8400 physical particles per cell, corresponding to a multiplicity of infection of 100 plaque forming units per cell. Control AdZ.F adenovirus efficiently transduced leukemic cell lines but gave poor results in AML samples. Both addition of soluble heparin and cell treatment with heparinase inhibited AdZ.F(pK7) gene transfer, showing that heparan sulfates are the major receptors mediating AdZ.F(pK7) transduction of AML blasts. Although adenoviruses can infect nondividing cells, we observed that a combination of growth factors (GM-CSF, IL-3, stem cell factor) was required for efficient transduction in order to maintain AML blast cell viability. This study demonstrates that retargeting the adenovirus fiber protein to heparan sulfates can overcome the low efficiency of adenovirus in AML blast cells and may provide a useful tool for gene therapy approaches in AML.

Eradication of Pre-Established Lymphoma Using Herpes Simplex Virus Amplicon Vectors

Herpes simplex virus amplicon vectors expressing RANTES (HSVrantes) and the T-cell costimulatory ligand B7.1 (HSVB7.1) were studied for their ability to elicit a tumor-specific T-cell response in a murine lymphoma model. HSVB7.1- and HSVrantes-transduced EL4 cells expressed high levels of B7.1 and RANTES as analyzed by flow cytometry and enzyme-linked immunosorbent assay, respectively. Inoculation of ex vivo HSVB7.1 transduced cells in syngeneic mice resulted in regression of both transduced cells and nontransduced cells inoculated contralaterally. Direct intratumoral injection of HSVB7.1 and/or HSVrantes alone or in combination into established EL4 tumors led to complete tumor regression in injected tumors as well as in nontransduced contralaterally implanted tumor, whereas control tumors or tumors injected with HSVlac expressing β-galactosidase did not regress. Maximal protection was achieved with combined injection of HSVB7.1 and HSVrantes; mice showing tumor regression were resistant to rechallenge with parental EL4 cells, and tumor cell-specific cytolytic T-cell activity was observed in mice demonstrating regression. HSV amplicon-mediated delivery of immune effector molecules may represent a useful strategy for immunotherapy in the setting of pre-existing tumor.

Eradication of Pre-Established Lymphoma Using Herpes Simplex Virus Amplicon Vectors

Herpes simplex virus amplicon vectors expressing RANTES (HSVrantes) and the T-cell costimulatory ligand B7.1 (HSVB7.1) were studied for their ability to elicit a tumor-specific T-cell response in a murine lymphoma model. HSVB7.1- and HSVrantes-transduced EL4 cells expressed high levels of B7.1 and RANTES as analyzed by flow cytometry and enzyme-linked immunosorbent assay, respectively. Inoculation of ex vivo HSVB7.1 transduced cells in syngeneic mice resulted in regression of both transduced cells and nontransduced cells inoculated contralaterally. Direct intratumoral injection of HSVB7.1 and/or HSVrantes alone or in combination into established EL4 tumors led to complete tumor regression in injected tumors as well as in nontransduced contralaterally implanted tumor, whereas control tumors or tumors injected with HSVlac expressing β-galactosidase did not regress. Maximal protection was achieved with combined injection of HSVB7.1 and HSVrantes; mice showing tumor regression were resistant to rechallenge with parental EL4 cells, and tumor cell-specific cytolytic T-cell activity was observed in mice demonstrating regression. HSV amplicon-mediated delivery of immune effector molecules may represent a useful strategy for immunotherapy in the setting of pre-existing tumor.

Efficient gene delivery into epstein-barr virus (EBV)-transformed human B cells mediated by replication-defective herpes simplex virus-1 (HSV-1): A gene therapy model for EBV-related B cell malignancy

Subgroups of the B cell malignancies are known to be associated with Epstein-Barr virus (EBV) infection, especially in immunocompromised patients. These are fatal and refractory to conventional antineoplastic therapy. B cells are usually post-mitotic cells and even mitogen activated or transformed B cells have shown relative resistance against viral mediated gene transfer. To address this issue, we employed a replication-defective herpes simplex virus-1 (HSV-1) to mediate gene transfer into EBV-transformed B cells. The virus expresses the herpes simplex virus thymidine kinase (HSV-TK) and the E. coli lacZ reporter genes and is designated T0Z.1. We used the lymphoblastoid cell line SWEIG as a model for human EBV-related B cell malignancy. This cell line was established by in vitro EBV infection of primary human peripheral blood mononuclear cells. When SWEIG cells were infected with T0Z.1, X-gal staining revealed lacZ expression in more than 20% cells even at multiplicity of infection (MOI) as low as 1 and the expression persisted for at least one week. Ganciclovir (GCV) administration after T0Z.1 infection effectively decreased the number of the infected tumor cells in a dose-responsive manner. Viral toxicity was analyzed by cell proliferation assay (MTS assay) and found to be little even at 10 MOI infection. Three MOI of the virus yielded maximum antineoplastic effect and more than 50% tumor cells were killed by HSV-TK/GCV. These results suggest the potential utility of replication-defective HSV-1 for the treatment of EBV-related B cell malignancies.

Gene Delivery to Human B-Precursor Acute Lymphoblastic Leukemia Cells

Autologous leukemia cells engineered to express immune-stimulating molecules may be used to elicit antileukemia immune responses. Gene delivery to human B-precursor acute lymphoblastic leukemia (ALL) cells was investigated using the enhanced green fluorescent protein (EGFP) as a reporter gene, measured by flow cytometry. Transfection of the Nalm-6 and Reh B-precursor ALL leukemia cell lines with an expression plasmid was investigated using lipofection, electroporation, and a polycationic compound. Only the liposomal compound Cellfectin showed significant gene transfer (3.9% to 12% for Nalm-6 cells and 3.1% to 5% for Reh cells). Transduction with gibbon-ape leukemia virus pseudotyped Moloney murine leukemia virus (MoMuLV)-based retrovirus vectors was investigated in various settings. Cocultivation of ALL cell lines with packaging cell lines showed the highest transduction efficiency for retroviral gene transfer (40.1% to 87.5% for Nalm-6 cells and 0.3% to 9% for Reh cells), followed by transduction with viral supernatant on the recombinant fibronectin fragment CH-296 (13% to 35.5% for Nalm-6 cells and 0.4% to 6% Reh cells), transduction on human bone marrow stroma monolayers (3.2% to 13.3% for Nalm-6 cells and 0% to 0.2% Reh cells), and in suspension with protamine sulfate (0.7% to 3.1% for Nalm-6 cells and 0% for Reh cells). Transduction of both Nalm-6 and Reh cells with human immunodeficiency virus–type 1 (HIV-1)–based lentiviral vectors pseudotyped with the vesicular stomatitis virus-G envelope produced the best gene transfer efficiency, transducing greater than 90% of both cell lines. Gene delivery into primary human B-precursor ALL cells from patients was then investigated using MoMuLV-based retrovirus vectors and HIV-1–based lentivirus vectors. Both vectors transduced the primary B-precursor ALL cells with high efficiencies. These studies may be applied for investigating gene delivery into primary human B-precursor ALL cells to be used for immunotherapy.

Gene Delivery to Human B-Precursor Acute Lymphoblastic Leukemia Cells

Autologous leukemia cells engineered to express immune-stimulating molecules may be used to elicit antileukemia immune responses. Gene delivery to human B-precursor acute lymphoblastic leukemia (ALL) cells was investigated using the enhanced green fluorescent protein (EGFP) as a reporter gene, measured by flow cytometry. Transfection of the Nalm-6 and Reh B-precursor ALL leukemia cell lines with an expression plasmid was investigated using lipofection, electroporation, and a polycationic compound. Only the liposomal compound Cellfectin showed significant gene transfer (3.9% to 12% for Nalm-6 cells and 3.1% to 5% for Reh cells). Transduction with gibbon-ape leukemia virus pseudotyped Moloney murine leukemia virus (MoMuLV)-based retrovirus vectors was investigated in various settings. Cocultivation of ALL cell lines with packaging cell lines showed the highest transduction efficiency for retroviral gene transfer (40.1% to 87.5% for Nalm-6 cells and 0.3% to 9% for Reh cells), followed by transduction with viral supernatant on the recombinant fibronectin fragment CH-296 (13% to 35.5% for Nalm-6 cells and 0.4% to 6% Reh cells), transduction on human bone marrow stroma monolayers (3.2% to 13.3% for Nalm-6 cells and 0% to 0.2% Reh cells), and in suspension with protamine sulfate (0.7% to 3.1% for Nalm-6 cells and 0% for Reh cells). Transduction of both Nalm-6 and Reh cells with human immunodeficiency virus–type 1 (HIV-1)–based lentiviral vectors pseudotyped with the vesicular stomatitis virus-G envelope produced the best gene transfer efficiency, transducing greater than 90% of both cell lines. Gene delivery into primary human B-precursor ALL cells from patients was then investigated using MoMuLV-based retrovirus vectors and HIV-1–based lentivirus vectors. Both vectors transduced the primary B-precursor ALL cells with high efficiencies. These studies may be applied for investigating gene delivery into primary human B-precursor ALL cells to be used for immunotherapy.

High-efficacy thymidine kinase gene transfer to ovarian cancer cell lines mediated by herpes simplex virus type 1 vector

In this report, a replication-defective herpes simplex virus type 1 (HSV-1) vector has been employed to deliver the Escherichia coli LacZ and HSV thymidine kinase (HSVtk) genes to six human ovarian carcinoma cell lines and the efficacy of gene transfer compared to that of adenoviral vectors in vitro. The transduction efficiency of the LacZ-containing virus TOZ.1 was evaluated qualitatively and quantitatively following infection of the different ovarian cancer cell lines. The therapeutic ability of the HSV-T3 vector, which contains the HSVtk gene, was additionally investigated in comparison to the AdCMVHSVTK. Our results show that HSV-1-mediated gene transfer is quantitatively superior to adenoviral vector in five of the six ovarian cancer cell lines at a 100-fold lower dose in vitro. Our preliminary studies suggest that HSV-1 may be a promising alternative vector for ovarian cancer gene therapy.

Innovative Two-Step Negative Selection of Granulocyte Colony-Stimulating Factor–Mobilized Circulating Progenitor Cells: Adequacy for Autologous and Allogeneic Transplantation

A major obstacle in purifying either autologous or allogeneic hematopoietic stem cells from granulocyte colony-stimulating factor (G-CSF) mobilized circulating progenitor cells (CPC) is represented by the huge cellularity present in each apheretic product. To obtain a significant debulking of unwanted cells from the leukapheresis, we developed a modified protocol of immune rosetting whereby human ABO-Rh– compatible red blood cells (RBCs) are treated with chromium chloride and then coated with murine monoclonal antibodies (MoAbs) against leukocyte antigens. When experiments were performed with leukaphereses obtained from normal donors or from T-cell acute lymphoblastic leukemia (T-ALL) patients, RBCs were coated with murine MoAbs against human mature myeloid cells (CD11b) and T cells (CD6); whereas, in the case of patients with B-precursor ALL, B-cell non-Hodgkin's lymphoma (B-NHL), or multiple myeloma (MM), RBCs were coated with anti-CD11b only. After incubation with CPC, rosetting cells (myeloid precursor cells, granulocytes, monocytes, and T cells) were removed by Ficoll-Hypaque density gradient centrifugation with a blood cell processor apparatus, COBE (Lakewood, CO) 2991. After this step, a significant reduction of the initial cellularity was consistently obtained (range, 72% to 97%), whereas the median absolute recovery of the CD34+ cells was above 85% (range, 64 to 100), with a 10-fold relative enrichment ranging from 3% to 41%. In a second step, CPC can be further purged of contaminating T or B cells by incubation with lymphoid-specific magnetic microbeads (anti-CD2 and -CD7 to remove T cells; anti-CD19 to remove B cells) and elution through a type-D depletion column (composed of ferromagnetic fiber) inserted within a SuperMACS separator device (Miltenyi Biotech, Bergisch-Gladbach, Germany). By this approach, a highly effective (three to four logs) T-cell depletion was achieved in all experiments performed with normal donors or T-ALL patients (median loss of CD3+cells: 99.8% [range 99.2 to 100]) and an equally efficient B-cell depletion was obtained from B-precursor ALL, B-NHL, or MM patients. At the end of the procedure the T- or B-cell depleted fraction retained a high proportion of the initial hematopoietic CD34+ stem cells, with a median recovery above 70% (range 48% to 100%) and an unmodified clonogenic potential. In five patients (two follicular NHL and three ALL) the purified fraction of stem cells was found disease free at the molecular level as assessed by polymerase chain reaction (PCR) analysis of the t(14;18) chromosome translocation or clono-specific DNA sequences of IgH or T-cell receptor γ and δ chain genes. Purified autologous and allogeneic CPCs were transplanted in three and six patients, respectively, who showed a prompt and sustained hematologic engraftment. In conclusion, this method represents a simple and reproducible two-step procedure to obtain a highly efficient purging of T or B cells from G-CSF expanded and mobilized CPCs. This approach might lead to the eradication of the neoplastic clone in the autologous stem cell inoculum as well as for T-cell depletion during allogeneic transplantation.

The Scope of Viral Vectors for the Transduction of Haemopoietic Cells

Over the last five years significant progress has been made towards the transfer of foreign genetic material to eukaryotic cells. The eventual aim to devise novel therapeutic strategies to treat human diseases, in particular solid tumours and monogenic disorders associated with various enzyme deficiency states. The easy accessibility and the ability of haemopoetic stem cells to self replicate and repopulate makes them desirable targets for gene transfer. In theory the introduction of a small number of gene modifed haemopoetic progenitor cells can allow therapy of an individual for life without any further intervention. This approach has been used for the treatment of single gene defects such as ADA deficiency. Furthermore, gene transfer technology has increasingly been exploited for bone marrow and peripheral blood stem cell marking studies, modification of cell sensitivity to cytotoxic drugs and the genetic modification of leukemic cells with the intention of inducing a leukemia specific cytotoxic T cell response. Vector development is of crucial importance for the successful delivery of genes in haemopoetic stem cells and leukemia cells. The objective of this review is to discuss in detail the properties of current vector technology that are pertinent to haemopoietic cell gene transduction.

The use of herpes simplex virus-based vectors for gene delivery to the nervous system

The ability of herpes simplex virus (HSV) to establish a lifelong, latent infection within neurons has led to much interest in the development of HSV-based vectors for neuronal gene delivery. This review discusses the progress made towards the construction of safe, replication-disabled HSV vectors that are capable of directing long-term transgene expression in latently infected neurons. Such vectors are now being investigated in a variety of animal model systems, with a view to developing gene therapy approaches to a number of metabolic and degenerative neurological diseases.