Friendly fire: redirecting herpes simplex virus-1 for therapeutic applications

Gene Therapy Leaves a Vicious Cycle

The human genetic code encrypted in thousands of genes holds the secret for synthesis of proteins that drive all biological processes necessary for normal life and death. Though the genetic ciphering remains unchanged through generations, some genes get disrupted, deleted and or mutated, manifesting diseases, and or disorders. Current treatment options—chemotherapy, protein therapy, radiotherapy, and surgery available for no more than 500 diseases—neither cure nor prevent genetic errors but often cause many side effects. However, gene therapy, colloquially called “living drug,” provides a one-time treatment option by rewriting or fixing errors in the natural genetic ciphering. Since gene therapy is predominantly a viral vector-based medicine, it has met with a fair bit of skepticism from both the science fraternity and patients. Now, thanks to advancements in gene editing and recombinant viral vector development, the interest of clinicians and pharmaceutical industries has been rekindled. With the advent of more than 12 different gene therapy drugs for curing cancer, blindness, immune, and neuronal disorders, this emerging experimental medicine has yet again come in the limelight. The present review article delves into the popular viral vectors used in gene therapy, advances, challenges, and perspectives.

Herpes simplex virus type 1 (HSV-1)-derived recombinant vectors for gene transfer and gene therapy

Herpes simplex virus type 1 (HSV-1 ) is a human pathogen whose lifestyle is based on a long-term dual interaction with the infected host, being able to establish both lytic and latent infections. The virus genome is a 153-kilobase pair (kbp) double-stranded DNA molecule encoding more than 80 genes. The interest of HSV-1 as gene transfer vector stems from its ability to infect many different cell types, both quiescent and proliferating cells, the very high packaging capacity of the virus capsid, the outstanding neurotropic adaptations that this virus has evolved, and the fact that it never integrates into the cellular chromosomes, thus avoiding the risk of insertional mutagenesis. Two types of vectors can be derived from HSV-1, recombinant vectors and amplicon vectors, and different methodologies have been developed to prepare large stocks of each type of vector. This chapter summarizes the approach most commonly used to prepare recombinant HSV-1 vectors through homologous recombination, either in eukaryotic cells or in bacteria.

HSV-1 as a Model for Emerging Gene Delivery Vehicles

The majority of viral vectors currently used possess modest cargo capability (up to 40 kb) being based on retroviruses, lentiviruses, adenoviruses, and adenoassociated viruses. These vectors have made the most rapid transition from laboratory to clinic because their small genomes have simplified their characterization and modification. However, there is now an increasing need both in research and therapy to complement this repertoire with larger capacity vectors able to deliver multiple transgenes or to encode complex regulatory regions, constructs which can easily span more than 100 kb. Herpes Simplex Virus Type I (HSV-1) is a well-characterized human virus which is able to package about 150 kb of DNA, and several vector systems are currently in development for gene transfer applications, particularly in neurons where other systems have low efficiency. However, to reach the same level of versatility and ease of use as that of smaller genome viral vectors, simple systems for high-titer production must be developed. This paper reviews the major HSV-1 vector systems and analyses the common elements which may be most important to manipulate to achieve this goal.

Herpes simplex virus type 1-derived recombinant and amplicon vectors

Herpes simplex virus type 1 (HSV-1) is a human pathogen whose lifestyle is based on a long-term dual interaction with the infected host, being able to establish both lytic and latent infections. The virus genome is a 153 kbp double-stranded DNA molecule encoding more than 80 genes. The interest of HSV-1 as gene transfer vector stems from its ability to infect many different cell types, both quiescent and proliferating cells, the very high packaging capacity of the virus capsid, the outstanding neurotropic adaptations that this virus has evolved, and the fact that it never integrates into the cellular chromosomes, thus avoiding the risk of insertional mutagenesis. Two types of vectors can be derived from HSV-1, recombinant vectors and amplicon vectors, and different methodologies have been developed to prepare large stocks of each type of vector. This chapter summarizes (1) the two approaches most commonly used to prepare recombinant vectors through homologous recombination, either in eukaryotic cells or in bacteria, and (2) the two methodologies currently used to generate helper-free amplicon vectors, either using a bacterial artificial chromosome (BAC)-based approach or a Cre/loxP site-specific recombination strategy.

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.

Engineering oncolytic viruses to exploit tumor specific defects in innate immune signaling pathways

BACKGROUND

The use of oncolytic viruses for treatment of cancer marks a significant alteration in the battle between host and virus. Viruses are confronted by cellular innate immune responses and contain an armamentarium of immunomodulatory proteins that suppress innate immunity. Tumorigenesis can result in impairment of innate immune responses. Viruses engineered to be vulnerable to normal responses may mediate tumor-specific killing with minimal off-target toxicity.

OBJECTIVE

To examine the mechanisms by which mammalian cells respond to viral infections in normal versus cancer cells and how viruses overcome these responses and to illustrate how this knowledge is used to develop physiologically targeted oncolytic viruses.

METHODS

Literature describing studies investigating innate responses to virus infections, cancer-specific molecular defects, immunosuppressive viral products and design of oncolytic viruses is extensively reviewed, and pertinent concepts are distilled and developed.

RESULTS/CONCLUSION

Innate responses to viral infections are complex involving i) viral detection; ii) induction of interferon and other cytokines; and iii) establishment of an antiviral state. Oncolytic viruses are engineered to be susceptible to antiviral responses in normal cells. Cancers can be partially vulnerable to these viruses because they have defective antiviral responses but the antitumor potency of physiologically targeted viruses may be significantly diminished.

Inverse-electron-demand Diels-Alder reaction as a highly efficient chemoselective ligation procedure: synthesis and function of a BioShuttle for temozolomide transport into prostate cancer cells

Hormone-refractory prostate cancer (HRPC), insensitive to most cytostatic interventions, features low response rates and bad prognosis. Studies with HRPC treated with temozolomide (TMZ) showed a poor response and the results were discouraging. Therefore, TMZ has been considered to be ineffective for the treatment of patients with symptomatic and progressive HRPC. A solution to this problem is demonstrated in this study by combining proper solid-phase peptide synthesis and a chemoselective new 'click' chemistry based on the Diels-Alder reaction with 'inverse-electron-demand' (DAR(inv)) for the construction of a highly efficient TMZ-BioShuttle in which TMZ is ligated to transporter and subcellular address molecules. The transport to the targeted nuclei resulted in much higher efficiency and better pharmacological effects. The reformulation of TMZ to TMZ-BioShuttle achieved higher in vitro killing of prostate cancer cells. Accordingly, the potential of TMZ for the treatment of prostate tumors was dramatically enhanced even in a tenfold lower concentration than applied normally. This TMZ-BioShuttle may be well suited for combining chemotherapy with other cytostatic agents or radiation therapy.

Treatment of glioblastoma multiforme cells with temozolomide-BioShuttle ligated by the inverse Diels-Alder ligation chemistry

Recurrent glioblastoma multiforme (GBM), insensitive against most therapeutic interventions, has low response and survival rates. Temozolomide (TMZ) was approved for second-line therapy of recurrent anaplastic astrocytoma. However, TMZ therapy in GBM patients reveals properties such as reduced tolerability and inauspicious hemogram. The solution addressed here concerning GBM therapy consolidates and uses the potential of organic and peptide chemistry with molecular medicine. We enhanced the pharmacologic potency with simultaneous reduction of unwanted adverse reactions of the highly efficient chemotherapeutic TMZ. The TMZ connection to transporter molecules (TMZ-BioShuttle) was investigated, resulting in a much higher pharmacological effect in glioma cell lines and also with reduced dose rate. From this result we can conclude that a suitable chemistry could realize the ligation of pharmacologically active, but sensitive and highly unstable pharmaceutical ingredients without functional deprivation. The TMZ-BioShuttle dramatically enhanced the potential of TMZ for the treatment of brain tumors and is an attractive drug for combination chemotherapy.

Construction of a fully retargeted herpes simplex virus 1 recombinant capable of entering cells solely via human epidermal growth factor receptor 2

A novel frontier in the treatment of tumors that are difficult to treat is oncolytic virotherapy, in which a replication-competent virus selectively infects and destroys tumor cells. Herpes simplex virus (HSV) represents a particularly attractive system. Effective retargeting to tumor-specific receptors has been achieved by insertion in gD of heterologous ligands. Previously, our laboratory generated an HSV retargeted to human epidermal growth factor receptor 2 (HER2), a receptor overexpressed in about one-third of mammary tumors and in some ovarian tumors. HER2 overexpression correlates with increased metastaticity and poor prognosis. Because HER2 has no natural ligand, the inserted ligand was a single-chain antibody to HER2. The objective of this work was to genetically engineer an HSV that selectively targets the HER2-expressing tumor cells and that has lost the ability to enter cells through the natural gD receptors, HVEM and nectin1. Detargeting from nectin1 was attempted by two different strategies, point mutations and insertion of the single-chain antibody at a site in gD different from previously described sites of insertion. We report that point mutations at gD amino acids 34, 215, 222, and 223 failed to generate a nectin1-detargeted HSV. An HSV simultaneously detargeted from nectin1 and HVEM and retargeted to HER2 was successfully engineered by moving the site of single-chain antibody insertion at residue 39, i.e., in front of the nectin1-interacting surface and not lateral to it, and by deleting amino acid residues 6 to 38. The resulting recombinant, R-LM113, entered cells and spread from cell to cell solely via HER2.

Imaging gene delivery with fluorescence microscopy

Gene delivery offers the promise of treatment for a range of human diseases. Although carried out initially with modified viruses, the use of synthetic molecules, including polymers, lipids and peptides, has extended the possibilities greatly for rationally designed vectors tailored to individual gene-delivery applications. Underlying the rational design of gene-delivery vectors is the need to understand the individual steps of the gene-delivery pathway. Using new methods in fluorescence microscopy, it is now possible to isolate individual steps along the gene-delivery pathway to characterize the mechanisms of cellular binding, cellular internalization and nuclear entry. This review describes the advances made in the gene-delivery field with the assistance of fluorescence microscopy. The focus of this review is the use of synthetic gene-delivery vectors, especially polyethylenimine, and the live-cell imaging and single-particle tracking techniques that reveal the intracellular dynamics of the gene-delivery process.

Tumor genotype determines susceptibility to oncolytic herpes simplex virus mutants: strategies for clinical application

Oncolytic Herpes simplex virus -1 (HSV-1) mutants based on deletion of the γ134.5 gene are promising therapies for cancer. Δγ134.5 mutant replication and cytolysis is tumor cell type specific and severely attenuated in normal tissues. The basis for attenuation lies in the activation of the protein kinase R (PKR)-mediated host cellular defense pathway, which inhibits protein synthesis in infected cells. Tumor cells which overexpress MAPK kinase (MEK) activity support robust replication of Δγ134.5 mutants via MEK-mediated inhibition of PKR, resulting in tumor oncolysis. Systemic delivery of γ134.5 mutants may allow selective targeting and destruction of metastases from a broad range of solid human tumors that overexpress MEK. Barriers to systemic HSV-1 oncolytic therapy include innate immunity, adaptive immunity and hepatic adsorption. Immunomodulating agents may overcome innate immunity to HSV-1-based vectors. Preclinical data combined with the pervasiveness of HSV-1 despite widespread immunity suggest that preexisting immunity may not eliminate oncolytic efficacy. In the future, biopsy-determined tumor MEK status may select patients for Δγ134.5 oncolytic therapy.

A herpes simplex virus recombinant that exhibits a single-chain antibody to HER2/neu enters cells through the mammary tumor receptor, independently of the gD receptors

The human epidermal growth factor receptor 2/neuregulin (HER2/neu) receptor is overexpressed in highly malignant mammary and ovarian tumors and correlates with a poor prognosis. It is a target for therapy; humanized monoclonal antibodies to HER2 have led to increased survival of patients with HER2/neu-positive breast cancer. As a first step in the design of an oncolytic herpes simplex virus able to selectively infect HER2/neu-positive cells, we constructed two recombinants, R-LM11 and R-LM11L, that carry a single-chain antibody (scFv) against HER2 inserted at residue 24 of gD. The inserts were 247 or 256 amino acids long, and the size of the gD ectodomain was almost doubled by the insertion. We report the following. R-LM11 and R-LM11L infected derivatives of receptor-negative J or CHO cells that expressed HER2/neu as the sole receptor. Entry was dependent on HER2/neu, since it was inhibited in a dose-dependent manner by monoclonal antibodies to HER2/neu and by a soluble form of the receptor. The scFv insertion in gD disrupted the ability of the virus to enter cells through HVEM but maintained the ability to enter through nectin1. This report provides proof of principle that gD can tolerate fusion to a heterologous protein almost as large as the gD ectodomain itself without loss of profusion activity. Because the number of scFv's to a variety of receptors is continually increasing, this report makes possible the specific targeting of herpes simplex virus to a large collection of cell surface molecules for both oncolytic activity and visualization of tumor cells.

Augmented therapeutic efficacy of an oncolytic herpes simplex virus type 1 mutant expressing ICP34.5 under the transcriptional control of musashi1 promoter in the treatment of malignant glioma

Although second-generation replication-conditional herpes simplex virus type 1 (HSV-1) vectors defective for both ribonucleotide reductase (RR) and the virulence factor gamma(1)34.5 have been proven safe through a number of animal experiments and clinical trials, their therapeutic efficacy was also markedly reduced. To overcome this situation, we concentrated on the use of a tumor-specific promoter in this study, to express ICP34.5 selectively in malignant glioma cells. As a molecular marker for malignant glioma, we focused on the neural RNA-binding protein, Musashi1. On the basis of the results of defective vector dvM345, as reported previously, we created, via homologous recombination, a novel HSV-1 vector termed KeM34.5, which expresses ICP34.5 under the transcriptional control of the musashi1 gene promoter (P/musashi1). Cytotoxicity mediated by KeM34.5 was significantly enhanced in human glioma cell lines (U87MG, U87MG-E6, U251, and T98G), resulting in an approximately 2-log increase in viral yield, compared with its parental vector G207. This virus also showed much higher therapeutic efficacy in the in vivo glioma model, while maintaining the desirable neuroattenuated phenotype. These results suggest that oncolytic HSV-1 expressing ICP34.5 under the transcriptional control of the musashi1 gene promoter could be a promising therapeutic agent for the treatment of malignant glioma.

Enhanced therapeutic efficacy of G207 for the treatment of glioma through Musashi1 promoter retargeting of gamma34.5-mediated virulence

G207 is a conditionally replicating derivative of herpes simplex virus type1 (HSV-1) engineered with deletions of both ICP34.5 loci and a lacZ insertion disabling the ICP6 gene. G207 exhibits an efficient oncolytic activity in vitro and in vivo, yet minimal toxicity in normal tissue, and is now in clinical trial for malignant glioma. According to the results of clinical trials, however, although G207 was proved to be safe, the efficacy was not so impressive. Deletion of the ICP34.5 gene coding for virulence made G207 extremely safe, but it markedly reduced the cytotoxicity mediated by HSV-1. To enhance the therapeutic efficacy of G207 without diminishing its safety, we used a defective vector containing Musashi1 promoter/ICP34.5, with G207 as helper virus. P/musashi1 was functional selectively in human glioma cell lines (U87MG, U251, T98G) in this study and dvM345 showed a much higher therapeutic efficacy both in culture and in the in vivo glioma model, than G207 alone, without diminishing its favorable toxicity profile. These results suggest that transcriptional regulation of ICP34.5 by P/musashi1 can be used to target HSV-1 virulence toward gliomas while maintaining the desirable neuroattenuated phenotype.

Treatment of Schwannomas with an Oncolytic Recombinant Herpes Simplex Virus in Murine Models of Neurofibromatosis Type 2

Gene therapy for schwannomas was evaluated in two mouse models of neurofibromatosis type 2 (NF2): (1) a transgenic model in which mice express a dominant mutant form of merlin and spontaneously develop schwannomas, and (2) a xenograft model in which human schwannoma tissue is implanted subcutaneously into immune- compromised mice. In both models, schwannoma volumes were monitored by magnetic resonance imaging (MRI) and showed strong gadolinium enhancement typical of these tumors in humans. Both types of tumor were positive for the Schwann cell marker S100, and highly infectable with herpes simplex virus (HSV) vectors. Schwannomas were injected with an oncolytic HSV-1 recombinant virus vector, G47Delta, which has deletions in genes for ribonucleotide reductase (ICP6), gamma34.5, and ICP47. In the NF2 transgenic model, schwannomas were reduced by more than half their original size by 10 days after infection. In the case of subcutaneous schwannoma xenografts, reduction in size after infection occurred more slowly, with a mean reduction of onethird by 42 days after treatment. Schwannomas injected with control vehicles continued to grow slowly over time in both schwannoma models. These studies demonstrate the ability of an oncolytic recombinant HSV vector to reduce the volume of schwannoma tumors in NF2 tumor models in mice and extend the possible therapeutic applications of oncolytic vectors for benign tumors to reduce mass while minimizing nerve damage.

Oncolytic HSV exerts direct antiangiogenic activity in ovarian carcinoma

In the present study, we investigated the ability of replication-restricted herpes simplex virus (HSV) 1716 lacking ICP34.5 to infect endothelium and disrupt tumor vasculature. HSV-1716 efficiently infected and killed mouse endothelial cell lines H5V and MS1 cells, as well as human umbilical vein endothelial cells in vitro. Capillary tube formation by endothelial cells was inhibited by HSV-1716 in vitro and in vivo. Following intratumoral administration of oncolytic HSV-1716, HSV-glycoproteins could be detected in CD31-positive tumor vascular endothelium by immunostaining. Viral DNA was recovered from highly purified microdissected tumor vascular endothelium. Furthermore, endothelium of tumors treated with HSV-1716 exhibited expression of tissue factor, a marker of endothelial damage. Importantly, HSV antigen and DNA were also detected in endothelium distant from foci of active tumor infection. After intravascular inoculation of HSV-1716, viral glycoproteins were detected in association to tumor endothelium, but not vascular endothelium of different organs. Purified tumor endothelial cells showed high proliferative capability and were susceptible to HSV-1716 infection and killing ex vivo while endothelium from normal organs was not. We conclude that oncolytic HSV-1716 exerts direct antiangiogenic effects, which may contribute to the overall therapeutic efficacy of the virus.

Reduced immune responses after vaccination with a recombinant herpes simplex virus type 1 vector in the presence of antiviral immunity

Due to the continuous need for new vaccines, viral vaccine vectors have become increasingly attractive. In particular, herpes simplex virus type 1 (HSV-1)-based vectors offer many advantages, such as broad cellular tropism, large DNA-packaging capacity and the induction of pro-inflammatory responses. However, despite promising results obtained with HSV-1-derived vectors, the question of whether pre-existing virus-specific host immunity affects vaccine efficacy remains controversial. For this reason, the influence of pre-existing HSV-1-specific immunity on the immune response induced with a replication-defective, recombinant HSV-1 vaccine was investigated in vivo. It was shown that humoral as well as cellular immune responses against a model antigen encoded by the vaccine were strongly diminished in HSV-1-seropositive mice. This inhibition could be observed in mice infected with wild-type HSV-1 or with a replication-defective vector. Although these data clearly indicate that pre-existing antiviral host immunity impairs the efficacy of HSV-1-derived vaccine vectors, they also show that vaccination under these constraints might still be feasible.

Factors That Limit the Effectiveness of Herpes Simplex Virus Type 1 for Treatment of Oral Cancer in Mice

Although the growth of experimental oral cancers can be inhibited by infection with the herpes simplex virus type 1 (HSV-1), the effect is incomplete. To define factors that might limit the effectiveness of the virus, we examined the roles of the innate immune system and the replication status of the tumor cells. AT-84 tumors were induced in strains of mice that had specific immune defects and were treated with the virus. Explanted tumors and tumor cells in culture were also infected. No differences in viral replication or in the effect of virus on the tumor were seen between mice with a lack of T or B lymphocytes, natural killer cells, phagocytic spleen cells, or complement. The virus did not replicate significantly more in tumors that were maintained as explants. Immediately after recovery of cells from a tumor the proportion of cells in the S phase was around 18%, and replication of virus in those cells was very limited. After 3 weeks in culture, the proportion in S had increased to 50% and both the recovery of virus from the cells and the toxic effect of the virus on the cells had increased significantly. The innate immune system thus seemed to have a minimal effect on replication of HSV-1 when used as an oncolytic virus for oral cancers in mice. Instead, the fraction of cells in the S phase was important. Because human oral cancers, like mouse tumors, have a low fraction of cells in the S phase, it is likely that the in vivo use of HSV-1 as cancer therapy will be limited by the replication of the virus.

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.