Replication, establishment of latency, and induced reactivation of herpes simplex virus gamma 1 34.5 deletion mutants in rodent models

Herpes simplex virus 1 open reading frames O and P are not necessary for establishment of latent infection in mice

Open reading frame (ORF) O and ORF P partially overlap and are located antisense to the gamma(1)34.5 gene within the domain transcribed during latency. In wild-type virus-infected cells, ORF O and ORF P are completely repressed during productive infection by ICP4, the major viral transcriptional activator/repressor. In cells infected with a mutant in which ORF P was derepressed there was a significant delay in the appearance of the viral alpha-regulatory proteins ICP0 and ICP22. The ORF O protein binds to and inhibits ICP4 binding to its cognate DNA site in vitro. These characteristics suggested a role for ORF O and ORF P in the establishment of latency. To test this hypothesis, two recombinant viruses were constructed. In the first, R7538(P-/O-), the ORF P initiator methionine codon, which also serves as the initiator methionine codon for ORF O, was replaced and a diagnostic restriction endonuclease was introduced upstream. In the second, R7543(P-/O-)R, the mutations were repaired to restore the wild-type virus sequences. We report the following. (i) The R7538(P-/O-) mutant failed to express ORF O and ORF P proteins but expressed a wild-type gamma(1)34.5 protein. (ii) R7538(P-/O-) yields were similar to that of the wild type following infection of cell culture or following infection of mice by intracerebral or ocular routes. (iii) R7538(P-/O-) virus reactivated from latency following explanation and cocultivation of murine trigeminal ganglia with Vero cells at a frequency similar to that of the wild type, herpes simplex virus 1(F). (iv) The amount of latent R7538(P-/O-) virus as assayed by quantitative PCR is eightfold less than that of the repair virus. The repaired virus could not be differentiated from the wild-type parent in any of the assays done in this study. We conclude that ORF O and ORF P are not essential for the establishment of latency in mice but may play a role in determining the quantity of latent virus maintained in sensory neurons.

Specific phenotypic restoration of an attenuated virus by knockout of a host resistance gene

To produce disease, viruses must enter the host, multiply locally in host tissues, spread from the site of entry, and overcome or evade host immune responses. At each stage in this infectious process, specific microbial and host genes determine the ultimate virulence of the virus. Genetic approaches have identified many viral genes that play critical roles in virulence and are presumed to target specific components of the host innate and acquired immune response. However, formal proof that a virulence gene targets a specific protein in a host pathway in vivo has not been obtained. Based on cell culture studies, it has been proposed that the herpes simplex virus type 1 gene ICP34.5 (ICP, infected cell protein) enhances neurovirulence by negating antiviral functions of the IFN-inducible double-stranded RNA-dependent protein kinase R or PKR [Chou, J., Chen, J.J., Gross, M. & Roizman, B. (1995) Proc. Natl. Acad. Sci. USA 92, 10516-10520]. Herein, we show that a virus that has been attenuated by deletion of ICP34.5 exhibits wild-type replication and virulence in a host from which the PKR gene has been deleted. We show that restoration of virulence is specific to ICP34.5 and PKR by using additional host and viral mutants. The use of recombinant viruses to infect animals with null mutations in host defense genes provides a formal genetic test for identifying in vivo mechanisms and targets of microbial virulence genes.

Attenuated, replication-competent herpes simplex virus type 1 mutant G207: safety evaluation in mice

Herpes simplex virus type 1 (HSV-1) mutants that are attenuated for neurovirulence are being used for the treatment of cancer. We have examined the safety of G207, a multimutated replication-competent HSV-1 vector, in mice. BALB/c mice inoculated intracerebrally or intracerebroventricularly with 10(7) PFU of G207 survived for over 20 weeks with no apparent symptoms of disease. In contrast, over 80% of animals inoculated intracerebrally with 1.5 x 10(3) PFU of HSV-1 wild-type strain KOS and 50% of animals inoculated intracerebroventricularly with 10(4) PFU of wild-type strain F died within 10 days. Similarly, after intrahepatic inoculation of G207 (3 x 10(7) PFU) all animals survived for over 10 weeks, whereas no animals survived for even 1 week after inoculation with 10(6) PFU of KOS. After intracerebroventricular inoculation, LacZ expression was initially observed in the cells lining the ventricles and subarachnoid space; expression decreased until almost absent within 5 days postinfection, with no apparent loss of ependymal cells. G207 DNA could be detected by PCR in the brains of mice 8 weeks after intracerebral inoculation; however, no infectious virus could be detected after 2 days. As a model for latent HSV in the brain, we used survivors of an intracerebral inoculation of HSV-1 KOS at the 50% lethal dose. Inoculation of a high dose of G207 at the same stereotactic coordinates did not result in reactivation of detectable infectious virus or symptoms of disease. We conclude that G207 is safe at or above doses that were efficacious in mouse tumor studies.

Genetically engineered HSV in the treatment of glioma: a review

Central nervous system malignancies, particularly glioblastoma multiforme, pose significant problems for the development of novel therapeutics. In the absence of advances with standard surgical and chemotherapeutic approaches, the utilisation of genetically engineered viruses, both as direct oncolytic agents as well as for the delivery of foreign proteins, represents a significant advance in the experimental approach to management of patients with these incurable tumours. Among other viruses, HSV offers an opportunity to directly influence the replication of tumour cells within the central nervous system. Because of its propensity to replicate in neuronal tissue as well as its large coding capacity, it provides an experimental model for the development of novel therapeutics. The status of these experimental approaches will be summarised in this review.

HSV-1-based vectors for gene therapy of neurological diseases and brain tumors: part I. HSV-1 structure, replication and pathogenesis

The design of effective gene therapy strategies for brain tumors and other neurological disorders relies on the understanding of genetic and pathophysiological alterations associated with the disease, on the biological characteristics of the target tissue, and on the development of safe vectors and expression systems to achieve efficient, targeted and regulated, therapeutic gene expression. The herpes simplex virus type 1 (HSV-1) virion is one of the most efficient of all current gene transfer vehicles with regard to nuclear gene delivery in central nervous system-derived cells including brain tumors. HSV-1-related research over the past decades has provided excellent insight into the structure and function of this virus, which, in turn, facilitated the design of innovative vector systems. Here, we review aspects of HSV-1 structure, replication and pathogenesis, which are relevant for the engineering of HSV-1-based vectors.

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

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

Attenuated, replication-competent herpes simplex virus type 1 mutant G207: safety evaluation of intracerebral injection in nonhuman primates

This study examined the safety of intracerebral inoculation of G207, an attenuated, replication-competent herpes simplex virus type 1 (HSV-1) recombinant, in nonhuman primates. Sixteen New World owl monkeys (Aotus nancymae [karyotype 1, formerly believed to be A. trivirgatus]), known for their exquisite susceptibility to HSV-1 infection, were evaluated. Thirteen underwent intracerebral inoculation with G207 at doses of 10(7) or 10(9) PFU, two were vehicle inoculated, and one served as an infected wild-type control and received 10(3) PFU of HSV-1 strain F. HSV-1 strain F caused rapid mortality and symptoms consistent with HSV encephalitis, including fever, hemiparesis, meningitis, and hemorrhage in the basal ganglia. One year after G207 inoculation, seven of the animals were alive and exhibited no evidence of clinical complications. Three deaths resulted from nonneurologic causes unrelated to HSV infection, and three animals were sacrificed for histopathologic examination. Two animals were reinoculated with G207 (10(7) PFU) at the same stereotactic coordinates 1 year after the initial G207 inoculation. These animals were alive and healthy 2 years after the second inoculation. Cerebral magnetic resonance imaging studies performed both before and after G207 inoculation failed to reveal radiographic evidence of HSV-related sequelae. Despite the lack of outwardly observable HSV pathology, measurable increases in serum anti-HSV titers were detected. Histopathological examination of multiple organ tissues found no evidence of HSV-induced histopathology or dissemination. We conclude that intracerebral inoculation of up to 10(9) PFU of G207, well above the efficacious dose in mouse tumor studies, is safe and therefore appropriate for human clinical trials.

A novel anti-herpes simplex virus type 1-specific herpes simplex virus type 1 recombinant

A recombinant herpes simplex virus (HSV) capable of inhibiting its own replication as well as the replication of wild-type virus would have greatly increased safety as a general purpose vector for in vivo gene transfer, antitumor therapy, and viral vaccine against HSV infection. By using a tetracycline repressor (tetR)-mediated HSV-1 viral replication switch [Yao and Eriksson (1999). Hum. Gene Ther. 10, 419-427], we have generated a novel anti-HSV-1-specific HSV-1 recombinant (CJ83193) that expresses a trans-dominant negative HSV-1 UL9 origin-binding protein, UL9-C535C. The de novo synthesis of CJ83193 can be suppressed by UL9-C535C by at least 1 x 10(6)-fold in non-tetR-expressing cells, and is subject to tetracycline regulation over a range of four to five orders of magnitude in a tetR-expressing osteosarcoma line. In particular, the UL9-C535C peptides expressed from the CJ83193 genome can inhibit the replication of wild-type HSV-1 by 100- to 200-fold in single-step growth assays. The construction of CJ83193 creates a new general strategy for developing recombinant viral vectors able to function as an intracellular therapy against wild-type viral infections.

Herpes simplex virus vaccines

Herpes simplex virus (HSV) infections are common and produce not only a primary infection, but also latent and recurrent infections. Therefore, the goals of a HSV vaccine are different from other vaccines. In this review, the goals of both prophylactic and therapeutic HSV vaccines are discussed and related to the immunobiology of acute and recurrent HSV infections. Next, the vaccine strategies that have been and are being evaluated for control of HSV disease are described. Current approaches take advantage of many of the modern methodologies for vaccine development. Thus, as we await final evaluations of subunit HSV glycoprotein vaccines, early phase I trails are evaluating newer vaccine approaches including DNA-based vaccines and replication-impaired viruses. The definitive HSV vaccine may await increased knowledge of the optimal viral antigen(s) and routes to induce genital tract immunity.

Herpesvirus vaccines. Development, controversies, and applications

Herpesviruses present difficult challenges in vaccine development because of their ability to evade immune clearance. Data and recommendations regarding the live-attenuated varicella vaccine are discussed. Approaches to developing vaccines to prevent herpes simplex virus (HSV), cytomegalovirus (CMV), and Epstein-Barr virus (EBV)-associated illnesses also are considered.

A herpes simplex virus DNA polymerase mutation that specifically attenuates neurovirulence in mice

Herpes simplex virus can infect the mammalian brain causing lethal encephalitis (neurovirulence). Previously, herpes simplex virus mutants that are attenuated for neurovirulence have exhibited defects in replication in brain and/or blocks to replication in neuronal cells. We investigated the attenuation of neurovirulence of mutant PAAr5, which exhibits resistance to antiviral drugs due to altered viral DNA polymerase. Following intracerebral inoculation of 7-week-old CD1 mice, PAAr5 was 30-fold attenuated for neurovirulence compared to its wild-type parent. A drug-sensitive virus derived by marker rescue with DNA polymerase gene sequences exhibited neurovirulence that was essentially indistinguishable from that of wild-type virus, demonstrating that attenuation was due to a polymerase mutation. PAAr5 replicated in brain similarly to wild-type virus unlike another polymerase mutant, 615.8, that exhibited a similar degree of attenuation. The attenuation of PAAr5 was not associated with altered particle to PFU ratios nor with any obvious reductions in viral antigen expression in neurons, spread, histopathology, or TUNEL staining suggestive of apoptotic cells. Thus PAAr5 differs from other mutants that are attenuated for neurovirulence. Understanding how a polymerase mutation specifically attenuates neurovirulence may shed light on how herpes simplex virus can cause lethal encephalitis.

Treatment of human breast cancer in a brain metastatic model by G207, a replication-competent multimutated herpes simplex virus 1

We investigated the therapeutic efficacy of G207, a replication-competent multimutated herpes simplex virus type 1, for the treatment of human malignant mammary tumors metastatic to the brain. In vitro studies demonstrated that G207 efficiently destroyed three of four human malignant breast cancer cell lines. MDA-MB-435 was most susceptible and MDA-MB-231 was least susceptible to G207. In athymic mice harboring subcutaneous or intracerebral MDA-MB-435 cells, intraneoplastic inoculation of G207 caused growth inhibition and/or prolonged survival. In contrast, G207 had minimal effects on MDA-MB-231 subcutaneous tumor growth or survival in the intracerebral tumor model. The efficacy of G207 therapy in vivo correlated well with the susceptibility of the human cancer cells to G207 in vitro. Histological studies indicate that G207 replication is restricted to tumor cells in vivo and does not occur in the surrounding brain tissue. These results suggest that G207 shows particular promise for use as a novel antineoplastic agent for metastatic brain tumors and that in vitro testing may predict which tumors will be most responsive in vivo.

Inhibition of herpes simplex virus replication by a 2-amino thiazole via interactions with the helicase component of the UL5-UL8-UL52 complex

With the use of a high-throughput biochemical DNA helicase assay as a screen, T157602, a 2-amino thiazole compound, was identified as a specific inhibitor of herpes simplex virus (HSV) DNA replication. T157602 inhibited reversibly the helicase activity of the HSV UL5-UL8-UL52 (UL5/8/52) helicase-primase complex with an IC50 (concentration of compound that yields 50% inhibition) of 5 microM. T157602 inhibited specifically the UL5/8/52 helicase and not several other helicases. The primase activity of the UL5/8/52 complex was also inhibited by T157602 (IC50 = 20 microM). T157602 inhibited HSV growth in a one-step viral growth assay (IC90 = 3 microM), and plaque formation was completely prevented at concentrations of 25 to 50 microM T157602. Vero, human foreskin fibroblast (HFF), and Jurkat cells could be propagated in the presence of T157602 at concentrations exceeding 100 microM with no obvious cytotoxic effects, indicating that the window between antiviral activity and cellular toxicity is at least 33-fold. Seven independently derived T157602-resistant mutant viruses (four HSV type 2 and three HSV type 1) carried single base pair mutations in the UL5 that resulted in single amino acid changes in the UL5 protein. Marker rescue experiments demonstrated that the UL5 gene from T157602-resistant viruses conferred resistance to T157602-sensitive wild-type viruses. Recombinant UL5/8/52 helicase-primase complex purified from baculoviruses expressing mutant UL5 protein showed complete resistance to T157602 in the in vitro helicase assay. T157602 and its analogs represent a novel class of specific and reversible anti-HSV agents eliciting their inhibitory effects on HSV replication by interacting with the UL5 helicase.

The gamma134.5 protein of herpes simplex virus 1 has the structural and functional attributes of a protein phosphatase 1 regulatory subunit and is present in a high molecular weight complex with the enzyme in infected cells

The carboxyl-terminal domain of the gamma134.5 protein of the herpes simplex virus 1 binds to protein phosphatase 1alpha (PP1) and is required to prevent the shut-off of protein synthesis resulting from phosphorylation of the alpha subunit of eIF-2 by the double-stranded RNA-activated protein kinase. The corresponding domain of the conserved GADD34 protein homologous to gamma134.5 functionally substitutes for gamma134.5. This report shows that gamma134.5 and PP1 form a complex in the infected cells, that fractions containing this complex specifically dephosphorylate eIF-2alpha, and that both gamma134.5 and GADD34 proteins contain the amino acid sequence motif common to subunits of PP1 that is required for binding to the PP1 catalytic subunit. An oligopeptide containing this motif competes with gamma134.5 for binding to PP1. Substitution of Val193 and Phe195 in the PP1-binding motif abolished activity. These results suggest that the carboxyl-terminal domain of gamma134.5 protein has the structural and functional attributes of a subunit of PP1 specific for eIF-2alpha, that it has evolved to preclude shut-off of protein synthesis, and that GADD34 may have a similar function.

A virus with a mutation in the ICP4-binding site in the L/ST promoter of herpes simplex virus type 1, but not a virus with a mutation in open reading frame P, exhibits cell-type-specific expression of gamma(1)34.5 transcripts and latency-associated transcripts

The herpes simplex virus type 1 L/S junction-spanning transcripts (L/STs) are a family of multisized transcripts expressed at high levels in cells infected with mutant viruses that (i) do not express ICP4, (ii) specify forms of ICP4 unable to bind to the consensus ICP4 binding site, or (iii) contain mutations in the ICP4 binding site located at the transcriptional start site of the L/STs. By extension, the failure to detect the L/STs in wild-type virus-infected cells is due to the repressive effect of ICP4 bound to its cognate binding site upstream of the L/ST transcription initiation site. ORF-P, the first and largest open reading frame (ORF) encoded by the L/STs, overlaps >90% of the ORF encoding ORF-34.5, a putative neurovirulence factor, which is transcribed from the opposite DNA strand. Viruses with mutations in the overlapping region of ORF-P and ICP34.5 exhibit premature shutoff of infected-cell protein synthesis and are highly attenuated following intracranial inoculation of juvenile mice. To determine whether the premature protein shutoff and neuroattenuated phenotypes of ORF-P ORF-34.5 double mutants are a consequence of alterations in ORF-P, ORF-34.5, or both, viruses containing mutations only in ORF-P or only in the ICP4 binding site in the L/ST promoter were isolated and characterized. Mutant virus L/ST-n38 contains a single-base-pair transition mutation in ORF-P codon 38, resulting in translational termination of the ORF-P protein (OPP). This mutation does not alter the amino acid sequence of ICP34.5. Expression of a truncated form of OPP by mutant virus L/ST-n38 did not result in premature shutoff of infected-cell protein synthesis and produced no other observable phenotype relative to wild-type virus in in vitro tests. Moreover, the 50% lethal dose (LD50) of L/ST-n38 was comparable to that of wild-type virus following intracranial inoculation of 3-week-old mice, as were the latency and reactivation phenotypes of the virus. These properties of L/ST-n38 indicate that the attenuated phenotype of ORF-P ORF-34.5 double mutants is a consequence of mutations that affect the function of ICP34.5 and not the function of OPP. Mutant virus LST-4BS contains four single-base-pair substitutions in the ICP4 binding site in the L/ST promoter that abrogate the binding of ICP4 to this site, leading to high-level expression of the L/STs and OPP. LST-4BS induced premature shutoff of viral and cellular protein synthesis and was slightly growth restricted in cells of neural lineage (SK-N-SH human neuroblastoma cells) but was wild type for these two parameters in cells of nonneural lineage (immortalized primate Vero cells). Of particular interest was the observation that L/ST-4BS exhibited cell-type-specific expression of both the gamma(1)34.5 transcripts and the latency-associated transcripts (LATs). Thus, expression of these transcripts was barely detectable in cells of neural lineage (NB41A3 mouse neuroblastoma cells) but was wild type in Vero cells. In vivo, L/ST-4BS was reactivated from mouse trigeminal ganglia with reduced efficiency and delayed kinetics relative to wild-type virus. L/ST-4BS was completely attenuated for neurovirulence (LD50 > 10(6) PFU) relative to wild-type virus (LD50 < 900 PFU), although the four single-base-pair substitutions lie outside the coding region for the neurovirulence factor, ICP34.5. Collectively, the complex in vitro and in vivo phenotypes of L/ST-4BS can be attributed to (i) disruptions of the ICP4 binding site in the L/ST promoter and subsequent overexpression of the L/STs and OPP; (ii) alterations in ORF-O, which is also mutated in L/ST-4BS; or (iii) alterations in other cryptic genes or cis-acting elements.

Transcription of the derepressed open reading frame P of herpes simplex virus 1 precludes the expression of the antisense gamma(1)34.5 gene and may account for the attenuation of the mutant virus

Open reading frame P (ORF P), located at the 3' terminus of the 8.5-kb DNA sequence transcribed during latency and almost completely antisense to the gamma(1)34.5 gene, is naturally repressed by infected cell protein 4 (ICP4), the major herpes simplex virus 1 regulatory protein. Earlier studies on cells infected with a mutant in which the expression of ORF P is derepressed have shown that (i) the accumulation of the alpha infected cell proteins 0 (ICP0) and 22 (ICP22), the products of spliced mRNAs, is reduced congruent with the binding of ORF P protein to p32, a component of the ASF/SF2 splicing factors, (ii) ORF P protein colocalizes with spliceosomes, (iii) both gamma(1)34.5 mRNA and protein are virtually undetectable, and (iv) the virus is attenuated on intracerebral inoculation in mice. We report the construction and characterization of two recombinant viruses: R7546, in which ORF P transcription was derepressed and the initiator methionine codon was replaced; and R7547, in which both mutations were repaired to the wild-type genotype. The mutations in R7546 do not alter the amino acid sequence of the gamma(1)34.5 gene. We report that (i) the reduction in the accumulation of gamma(1)34.5 mRNA and protein in cells infected with mutant viruses expressing derepressed ORF P genes reflects the effects of antisense transcription of ORF P rather than a function of ORF P protein, (ii) the attenuated phenotype of the viruses carrying derepressed ORF P genes is due largely to the absence of the gamma(1)34.5 protein, and (iii) the reduction in accumulation of ICP0 and ICP22 requires the expression of ORF P protein.

Suppression of the phenotype of gamma(1)34.5- herpes simplex virus 1: failure of activated RNA-dependent protein kinase to shut off protein synthesis is associated with a deletion in the domain of the alpha47 gene

Earlier studies have shown that infection of human cells by herpes simplex virus 1 (HSV-1) results in the activation of RNA-dependent protein kinase (PKR) but that the alpha subunit of eIF-2 is not phosphorylated and that protein synthesis is unaffected. In the absence of the viral gamma(1)34.5 gene, eIF-2alpha is phosphorylated and protein synthesis is prematurely shut off (J. Chou, J. J. Chen, M. Gross, and B. Roizman, Proc. Natl. Acad. Sci. USA 92:10516-10520, 1995). A second recent paper reported the selection of second-site suppressor mutants characterized by near-wild-type protein synthesis in cells infected with gamma(1)34.5- mutants (I. Mohr and Y. Gluzman, EMBO J. 15:4759-4766, 1996). Here, we report the properties of the spontaneous HSV-1 suppressor mutant Sup-1, which is characterized by spontaneous deletion of 503 bp encompassing the domain of the alpha47 gene and junction with the inverted repeats flanking the unique short (U(S)) sequence of the HSV-1 DNA resulting in the juxtaposition of the alpha47 promoter to the coding domain of the U(S)11 gene. This mutant does not exhibit the shutoff of protein synthesis characteristic of the gamma(1)34.5- virus. Specifically, Sup-1 in SK-N-SH human neuroblastoma cells (i) did not exhibit the function of the alpha47 gene characterized by a reduction in the transport of peptides across the endoplasmic reticulum of permealized cells consistent with the absence of alpha47 gene sequences, (ii) accumulated U(S)11 protein at levels analogous to those of the wild-type parent but the protein was made at earlier times after infection, as would be expected from a change in the promoter, and (iii) activated PKR like that of the parent, gamma(1)34.5- virus, but (iv) did not cause premature shutoff of protein synthesis and therefore was similar to the wild-type parent virus rather than the gamma(1)34.5- virus from which it was derived. We conclude that the mechanism by which Sup-1 blocks the shutoff of protein synthesis associated with phosphorylation of eIF-2alpha by the activated PKR is not readily explainable by a secondary mutation characterized by a deletion.

The range and distribution of murine central nervous system cells infected with the gamma(1)34.5- mutant of herpes simplex virus 1

Wild-type herpes simplex virus 1 (HSV-1) multiplies, spreads, and rapidly destroys cells of the murine central nervous system (CNS). In contrast, mutants lacking both copies of the gamma(1)34.5- gene have been shown to be virtually lacking in virulence even after direct inoculation of high-titered virus into the CNS of susceptible mice (J. Chou, E. R. Kern, R. J. Whitley, and B. Roizman, Science 250:1262-1266, 1990). To investigate the host range and distribution of infected cells in the CNS of mice, 4- to 5-week-old mice were inoculated stereotaxically into the caudate/putamen with 3 x 10(5) PFU of the gamma(1)34.5- virus R3616. Four-micrometer-thick sections of mouse brains removed on day 3, 5, or 7 after infection were reacted with a polyclonal antibody directed primarily to structural proteins of the virus and with antibodies specific for neurons, astrocytes, or oligodendrocytes. This report shows the following: (i) most of the tissue damage caused by R3616 was at the site of injection, (ii) the virus spread by retrograde transport from the site of infection to neuronal cell nuclei at distant sites and to ependymal cells by cerebrospinal fluid, (iii) the virus infected neurons, astrocytes, oligodendrocytes, and ependymal cells and hence did not discriminate among CNS cells, (iv) viral replication in some neurons could be deduced from the observation of infected astrocytes and oligodendrocytes at distant sites, and (v) infected cells were being efficiently cleared from the nervous system by day 7 after infection. We conclude that the gamma(1)34.5- attenuation phenotype is reflected in a gross reduction in the ability of the virus to replicate and spread from cell to cell and is not due to a restricted host range. The block in viral replication appears to be a late event in viral replication.

Treatment of experimental subcutaneous human melanoma with a replication-restricted herpes simplex virus mutant

Modified, non-neurovirulent herpes simplex viruses (HSV) have shown promise for the treatment of brain tumors, including intracranial melanoma. In this report, we show that HSV-1716, an HSV-1 mutant lacking both copies of the gene coding-infected cell protein 34.5 (ICP 34.5), can effectively treat experimental subcutaneous human melanoma in mice. In vitro, HSV-1716 replicated in all 26 human melanoma cell lines tested, efficiently lysing the cells. Therapeutic infection of subcutaneous human melanoma nodules with HSV-1716 led to viral replication that was restricted to tumor cells by immunohistochemistry. Moreover, HSV-1716 treatment significantly inhibited progression of preformed subcutaneous human melanoma nodules in SCID mice and caused complete regression of some tumors. This work expands the potential scope of HSV-1-based cancer therapy.

A novel multiply-mutated HSV-1 strain for the treatment of human brain tumors

A promising approach for the therapeutic treatment of brain tumors utilizes replication-competent, neuroattenuated herpes simplex virus-1 (HSV-1) mutants. This approach requires mutation of HSV-1 to eliminate killing of normal, nondividing cells of the brain (e.g., neurons). We have generated a HSV-1 double-mutant, designated 3616UB, by interrupting the uracil DNA glycosylase (UNG) gene in a previously studied ICP34.5 mutant, R3616. The HSV-1-encoded UNG gene is required for efficient HSV-1 replication in nondividing cells, but is dispensable for replication in rapidly dividing cells. The specific function of the HSV-1 ICP34.5 gene is not completely clear, but it is thought to be necessary for viral replication in cells of the nervous system, because, when mutated, the resultant viral strains are fully neuroattenuated. Strain 3616UB did not replicate in primary neuronal cultures in vitro or in mouse brain, but efficiently killed six of six human tumor cell lines within 6 days in vitro and successfully infected and replicated within brain tumor xenografts. The potential safety of 3616UB for human use is enhanced by an unexpected hypersensitivity to the antiherpetic drug ganciclovir. These data suggest that 3616UB may be effective for the treatment of human brain tumors. Intratumoral injection of 3616UB into human medulloblastoma or angiosarcoma xenografts established in severe combined immunodeficient (SCID) mice produced significant growth arrest and some tumor regressions. Strain 3616UB was as effective as R3616 in this therapy study and did not cause any obvious distress in the treated animals. Together, the data show that 3616UB is a very safe alternative to other HSV-1 mutants because the presence of two mutations reduces the possibility of recombinational events in situ that could lead to the generation of virulent viral progeny during 3616UB therapy.

Treatment of spontaneously arising retinoblastoma tumors in transgenic mice with an attenuated herpes simplex virus mutant

The use of viruses to treat tumors has received renewed interest with the availability of genetically defined attenuated mutants. Herpes simplex virus (HSV) type 1 in particular has been shown to be effective for tumors of neuronal origin. However, the model systems used for these studies rely on the use of explanted tumor cells in immunodeficient animals. We have used a recently developed transgenic mouse model, wherein mice spontaneously develop retinoblastomas, to determine if a mutant HSV has a therapeutic effect against an endogenously arising tumor in an immunocompetent host. The injection of 1 x 10(6) PFU of the neuroattenuated HSV-1/HSV-2 recombinant RE6 into the vitreous of transgenic mice resulted in a significant inhibition of tumor growth compared to injection of medium alone (P = 0.0063). Immunohistochemical analysis of viral antigen showed that viral replication was restricted to focal areas of the tumors and the retinal pigment epithelium. Viral growth was not significantly different in the eyes of transgene-positive and transgene-negative mice, suggesting that enhanced replication in tumor cells may not explain the effects. Tumor cells in the treated eyes were significantly less differentiated than those in the untreated eyes (P = 0.04), suggesting that the virus may replicate better in certain cell types in the tumors. Although the injection of RE6 resulted in a difference in tumor size, the treatment did not result in the elimination of tumors in any of the mice improvements in the efficacy of tumor control are needed if this therapy is to be of use.

Development of an HSV-based vector for the treatment of Parkinson's disease

The restricted pattern of neurodegeneration seen in Parkinson's disease, and the identification of trophic factors that prevent toxin-induced degeneration of dopaminergic neurons, has spurred research into potential gene therapy for this disease. Herpes simplex virus (HSV-1) is a neurotrophic virus which naturally establishes latency in neurons. HSV-based vectors have been demonstrated to transfer and transiently express transgenes in neurons in brain in vivo. Recent experiment have shown that deletion of multiple immediate-early HSV genes reduces the potential cytotoxicity of these vectors, and in addition results in altered patterns of transgene expression that may allow for long-term expression required for human gene therapy applications.

The gamma(1)34.5 protein of herpes simplex virus 1 complexes with protein phosphatase 1alpha to dephosphorylate the alpha subunit of the eukaryotic translation initiation factor 2 and preclude the shutoff of protein synthesis by double-stranded RNA-activated protein kinase

In human cells infected with herpes simplex virus 1 the double-stranded RNA-dependent protein kinase (PKR) is activated but phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF-2) and total shutoff of protein synthesis is observed only in cells infected with gamma(1)z34.5- mutants. The carboxyl-terminal 64 aa of gamma(1)34.5 protein are homologous to the corresponding domain of MyD116, the murine growth arrest and DNA damage gene 34 (GADD34) protein and the two domains are functionally interchangeable in infected cells. This report shows that (i) the carboxyl terminus of MyD116 interacts with protein phosphatase 1alpha in yeast, and both MyD116 and gamma(1)34.5 interact with protein phosphatase 1alpha in vitro; (ii) protein synthesis in infected cells is strongly inhibited by okadaic acid, a phosphatase 1 inhibitor; and (iii) the alpha subunit in purified eIF-2 phosphorylated in vitro is specifically dephosphorylated by S10 fractions of wild-type infected cells at a rate 3000 times that of mock-infected cells, whereas the eIF-2alpha-P phosphatase activity of gamma(1)34.5- virus infected cells is lower than that of mock-infected cells. The eIF-2alpha-P phosphatase activities are sensitive to inhibitor 2. In contrast to eIF-2alpha-P phosphatase activity, extracts of mock-infected cells exhibit a 2-fold higher phosphatase activity on [32P]phosphorylase than extracts of infected cells. These results indicate that in infected cells, gamma(1)34.5 interacts with and redirects phosphatase to dephosphorylate eIF-2alpha to enable continued protein synthesis despite the presence of activated PKR. The GADD34 protein may have a similar function in eukaryotic cells. The proposed mechanism for maintenance of protein synthesis in the face of double-stranded RNA accumulation is different from that described for viruses examined to date.

Selective vulnerability of mouse CNS neurons to latent infection with a neuroattenuated herpes simplex virus-1

Herpes simplex viruses that lack ICP34.5 are neuroattenuated and are presently being considered for cancer and gene therapy in the nervous system. Previously, we documented the focal presence of the latency-associated transcripts (LATs) in the hippocampi of immunocompromised mice after intracranial (IC) inoculation of an ICP34.5-deficient virus called strain 1716. To characterize further the biological properties of strain 1716 in the CNS of immunocompetent mice, we determined the extent of viral gene expression in different cell types and regions of the CNS after stereotactic IC inoculation of this virus. At survival times of > 30 d after inoculation, we found that (1) infectious virus was not detectable by titration and immunohistochemical studies; (2) neurons harbored virus as demonstrated by the detection of the LATs by in situ hybridization (ISH); (3) transcripts expressed during the lytic cycle of infection were not detected by ISH; and (4) subsets of neurons were selectively vulnerable to latent infection, depending on the site of inoculation. These results suggest that the absence of ICP34.5 does not abrogate latent infection of the CNS by strain 1716. Additional studies of strain 1716 in the model system described here will facilitate the elucidation of the mechanisms that regulate the selective vulnerability of CNS cells to latent viral infection and lead to the development of ICP34.5 mutant viruses as therapeutic vectors for CNS diseases.

An African swine fever virus virulence-associated gene NL-S with similarity to the herpes simplex virus ICP34.5 gene

We described previously an African swine fever virus (ASFV) open reading frame, 23-NL, in the African isolate Malawi Lil 20/1 whose product shared significant similarity in a carboxyl-terminal domain with those of a mouse myeloid differentiation primary response gene, MyD116, and the herpes simplex virus neurovirulence-associated gene, ICP34.5 (M. D. Sussman, Z. Lu, G. Kutish, C. L. Afonso, P. Roberts, and D. L. Rock, J. Virol. 66:5586-5589, 1992). The similarity of 23-NL to these genes suggested that this gene may function in some aspect of ASFV virulence and/or host range. Sequence analysis of additional pathogenic viral isolates demonstrates that this gene is highly conserved among diverse ASFV isolates and that the gene product exists in either a long (184 amino acids as in 23-NL) or a short form (70 to 72 amino acids in other examined ASFV isolates). The short form of the gene, NL-S, encodes the complete highly conserved, hydrophilic, carboxyl-terminal domain of 56 amino acids common to 23-NL, MyD116, and ICP34.5. Recombinant NL-S gene deletion mutants and their revertants were constructed from the pathogenic ASFV isolate E70 and an E70 monkey cell culture-adapted virus, MS44, to study gene function. Although deletion of NL-S did not affect viral growth in primary swine macrophages or Vero cell cultures in vitro, the null mutant, E70/43, exhibited a marked reduction in pig virulence. In contrast to revertant or parental E70 where mortality was 100%, all E70/43-infected animals survived infection. With the exception of a transient fever response, E70/43-infected animals remained clinically normal and exhibited a 1,000-fold reduction in both mean and maximum viremia titers. All convalescent E70/43-infected animals survived infection when challenged with parental E70 at 30 days postinfection. These data indicate that the highly conserved NL-S gene of ASFV, while nonessential for growth in swine macrophages in vitro, is a significant viral virulence factor and may function as a host range gene.

Immunization with DNA vaccines encoding glycoprotein D or glycoprotein B, alone or in combination, induces protective immunity in animal models of herpes simplex virus-2 disease

DNA vaccines expressing herpes simplex virus type 2 (HSV-2) full-length glycoprotein D (gD), or a truncated form of HSV-2 glycoprotein B (gB) were evaluated for protective efficacy in two experimental models of HSV-2 infection. Intramuscular (i.m.) injection of mice showed that each construction induced neutralizing serum antibodies and protected the mice from lethal HSV-2 infection. Dose-titration studies showed that low doses (< or = 1 microgram) of either DNA construction induced protective immunity, and that a single immunization with the gD construction was effective. The two DNAs were then tested in a low-dosage combination in guinea pigs. Immune sera from DNA-injected animals had antibodies to both gD and gB, and virus neutralizing activity. When challenged by vaginal infection with HSV-2, the DNA-immunized animals were significantly protected from primary genital disease.

The application of genetically engineered herpes simplex viruses to the treatment of experimental brain tumors

Due to lack of effective therapy, primary brain tumors are the focus of intense investigation of novel experimental approaches that use vectors and recombinant viruses. Therapeutic approaches have been both indirect, whereby vectors are used, or direct to allow for direct cell killing by the introduced virus. Genetically engineered herpes simplex viruses are currently being evaluated as an experimental approach to eradicate malignant human gliomas. Initial studies with gamma (1)34.5 mutants, R3616 (from which both copies of the gamma (1)34.5 gene have been deleted) and R4009 (a construct with two stop codons inserted into the gamma (1)34.5 gene), have been assessed. In a syngeneic scid mouse intracranial tumor model, recombinant herpes simplex virus can be experimentally used for the treatment of brain tumors. These viruses and additional engineered viruses were subsequently tested in human glioma cells both in vitro and in vivo. Using a xenogeneic scid mouse intracranial glioma model, R4009 therapy of established tumors significantly prolonged survival. Most importantly, long-term survival was achieved, with histologic evidence that R4009 eradicated intracranial tumors in this model. Furthermore, the opportunity to evaluate gamma (1)34.5 mutants that have enhanced oncolytic activity, e.g., R8309 where the carboxyl terminus of the gamma (1)34.5 gene has been replaced by the murine homologue, MyD116, are considered.

Selective vulnerability of mouse CNS neurons to latent infection with a neuroattenuated herpes simplex virus-1

Herpes simplex viruses that lack ICP34.5 are neuroattenuated and are presently being considered for cancer and gene therapy in the nervous system. Previously, we documented the focal presence of the latency-associated transcripts (LATs) in the hippocampi of immunocompromised mice after intracranial (IC) inoculation of an ICP34.5-deficient virus called strain 1716. To characterize further the biological properties of strain 1716 in the CNS of immunocompetent mice, we determined the extent of viral gene expression in different cell types and regions of the CNS after stereotactic IC inoculation of this virus. At survival times of > 30 d after inoculation, we found that (1) infectious virus was not detectable by titration and immunohistochemical studies; (2) neurons harbored virus as demonstrated by the detection of the LATs by in situ hybridization (ISH); (3) transcripts expressed during the lytic cycle of infection were not detected by ISH; and (4) subsets of neurons were selectively vulnerable to latent infection, depending on the site of inoculation. These results suggest that the absence of ICP34.5 does not abrogate latent infection of the CNS by strain 1716. Additional studies of strain 1716 in the model system described here will facilitate the elucidation of the mechanisms that regulate the selective vulnerability of CNS cells to latent viral infection and lead to the development of ICP34.5 mutant viruses as therapeutic vectors for CNS diseases.

High-dose ocular infection with a herpes simplex virus type 1 ICP34.5 deletion mutant produces no corneal disease or neurovirulence yet results in wild-type levels of spontaneous reactivation

We report here that in the rabbit ocular model of herpes simplex virus type 1 (HSV-1) latency, spontaneous reactivation of the HSV-1 ICP34.5 deletion mutant d34.5 increased significantly in response to increasing infectious doses. At the highest infectious dose of d34.5, the spontaneous reactivation rate was indistinguishable from that of wild-type virus (average spontaneous reactivation rates for d34.5, 0.3 to 1.4% at 2 x 10(5) PFU per eye, 3.4% at 2 x 10(6) PFU per eye, and 6.3 to 11.5% at 1 x 10(8) PFU per eye; average spontaneous reactivation rates for marker-rescued virus, 7.7 to 19.6% at 2 x 10(5) PFU per eye). The percentage of latency-associated transcript (LAT) RNA-positive neurons in sections from trigeminal ganglia (TG) of rabbits latently infected with d34.5 demonstrated a similar dose-response effect as estimated by in situ hybridization (0.05% LAT RNA-positive neurons at 2 x 10(5) PFU per eye and 0.1% LAT RNA-positive neurons at 1 x 10(8) PFU per eye; P = 0.002). In contrast, even at the highest infectious dose (1 x 10(8) PFU per eye), d34.5 was less virulent (23 of 23 survivors) than the normal infectious dose (2 x 10(5) PFU per eye) of marker-rescued virus (14 of 27 survivors; P < 0.0001). In addition, at 1 x 10(8) PFU per eye, d34.5 produced virtually no corneal disease, compared with the production of severe corneal disease by 2 x 10(5) PFU of marker-rescued virus per eye (P < 0.0001). Thus, at increasing infectious doses of d34.5, both spontaneous reactivation and the percentage of neurons expressing LAT appeared to increase, without a corresponding increase in virulence. These results strongly suggest that (i) the phenotypes of neurovirulence and spontaneous reactivation are separable, (ii) the phenotypes of corneal disease and spontaneous reactivation are separable, and (iii) the decreased rate of spontaneous reactivation previously reported for d34.5 (G. C. Perng, R. L. Thompson, N. M. Sawtell, W. E. Taylor, S. M. Slanina, H. Ghiasi, R. Kaiwar, A. B. Nesburn, and S. L. Wechsler, J. Virol. 69:3033-3041, 1995) is at least partially due to a reduced rate of establishing latency.

Phenotypic properties of herpes simplex virus 1 containing a derepressed open reading frame P gene

Open reading frame P (ORF P) maps in the viral DNA sequences transcribed during latency and is located antisense to the gamma 1 34.5 gene. Earlier studies have shown that the expression of ORF P is repressed by an infected cell protein no. 4 binding site straddling the transcription initiation site. We have made monospecific polyclonal antibodies to the protein and constructed a virus, designated ORF P++, in which the infected cell protein no. 4 binding site has been mutagenized, thereby allowing full expression of an unmodified ORF P gene from its natural promoter. We report the following findings. (i) The native protein forms multiple bands on denaturing polyacrylamide gels suggestive of extensive processing and aggregation of the protein; (ii) the protein accumulates in the nucleus in rod-shaped structures perpendicular to the axis of attachment of the infected cell to the solid matrix; (iii) the virus was highly attenuated on inoculation into mice by the intracerebral or ocular route, and virus was not recovered upon explantation of trigeminal ganglia; (iv) although protein synthesis was not prematurely shut off in the human neuroblastoma cell line SK-N-SH, gamma 1 34.5 protein was not detected in immunoblasts. Analyses of electrophoretically separated denatured RNAs indicated that in cells infected with the ORF P++ virus, there was a large increase in the amount of ORF P RNA and a corresponding decrease in the amount of gamma 1 34.5 RNA. We conclude that either the overproduction of ORF P protein blocks the expression of some herpes simplex virus 1 genes or derepression of the transcription of ORF P has a negative effect on the transcription of the antisense gamma 1 34.5 RNA.

The spontaneous reactivation function of the herpes simplex virus type 1 LAT gene resides completely within the first 1.5 kilobases of the 8.3-kilobase primary transcript

The herpes simplex virus type 1 (HSV-1) latency-associated transcript (LAT) gene is essential for efficient spontaneous reactivation of HSV-1 from latency. We report here that although the LAT gene is 8.3 kb in length, the first 1.5 kb of the LAT gene alone is sufficient for wild-type levels of spontaneous reactivation. We began with a LAT deletion mutant of HSV-1 strain McKrae in which the LAT promoter and the first 1.6 kb of the 5' end of the LAT gene had been deleted from both copies of LAT (one in each viral long repeat). As we previously reported, this mutant (dLAT2903) was significantly impaired for spontaneous reactivation (G. C. Perng, E. C. Dunkel, P. A. Geary, S. M. Slanina, H. Ghiasi, R. Kaiwar, A. B. Nesburn, and S. L. Wechsler, J. Virol. 68:8045-8055, 1994). We then inserted the LAT promoter and the first 1.5 kb of the LAT gene into a location in the unique long region of dLAT2903 far removed from the normal location of LAT in the long repeats. This resulted in a virus (LAT15a) whose capacity for transcribing LAT RNA was limited to the first 1.5 kb of the 8.3-kb LAT primary transcript. Rabbits were ocularly infected with this mutant, and spontaneous reactivation was measured in comparison to those of the original LAT-negative mutant and its marker-rescued (wild-type) virus, dLAT2903R. LAT15a had an in vivo spontaneous reactivation rate of 12%, compared with a rate of 11% for the marker-rescued virus and 0% for the LAT-negative virus. Southern analysis confirmed that the spontaneously reactivated LAT15a virus retained the original deletions in both copies of LAT and the 1.5-kb LAT insertion in the unique long region. Thus, insertion of the first 1.5 kb of LAT (and its promoter) at a site distant from the normal LAT location appeared to completely restore in vivo spontaneous reactivation to wild-type levels, despite the remaining inability of the original LAT genes to transcribe any LAT RNA. The function of LAT involved in efficient spontaneous reactivation therefore appeared to map completely within the first 1.5 kb of the LAT gene.

Development of drug targeting based on recombinant expression of the chicken avidin gene

The chemistry required for covalent biotinylation of drugs, radiopharmaceuticals and other ligands is highly developed, and a large number of biotinylated reagents can be readily synthesized. In order to investigate whether expression of avidin cDNA in mammalian cells might be useful as part of a drug targeting strategy, we transiently expressed the avidin gene in two human tumor cell lines (the cervical carcinoma cell line, HeLa, and the liver derived line, Hep G2). Avidin protein as detected by either immunohistochemistry or binding of streptavidin-biotin complexes was present and functional following transient expression. This result indicated that the mechanisms underlying avidin oligomerization which are necessary for proper protein folding are present within mammalian carcinoma cell lines. Next, we generated a producer cell line (derived from psi2) capable of releasing a recombinant retrovirus encoding chicken avidin, and a tumorigenic murine breast cancer cell line (16/C) with stable avidin expression. We show that these cell lines are suitable for conferring functional expression of avidin in vitro. These experiments establish a means by which avidin gene expression can be explored as a mechanism for targeted gene delivery of biotin-derivitized drugs in vitro, and have important implications for utilization of this strategy in vivo.

The carboxyl terminus of the murine MyD116 gene substitutes for the corresponding domain of the gamma(1)34.5 gene of herpes simplex virus to preclude the premature shutoff of total protein synthesis in infected human cells

The herpes simplex virus 1 mutants from which both copies of the gamma(1)34.5 gene had been deleted trigger total shutoff of protein synthesis in human neuroblastoma cells and human foreskin fibroblasts but not in African green monkey (Vero) cells. The carboxyl-terminal 64 amino acids of gamma(1)34.5 are homologous to the corresponding domain of MyD116, a murine myeloid differentiation primary responsive gene. The carboxyl-terminal domain of gamma(1)34.5 is required to preclude the shutoff of protein synthesis (J. Chou and B. Roizman, Proc. Natl. Acad. Sci. USA 91:5247-5251, 1994). We report that in-frame substitution of the carboxyl terminus of gamma(1)34.5 with the corresponding domain of MyD116 in the context of the viral genome restored the ability of gamma(1)34.5 to preclude premature shutoff of protein synthesis in both neuroblastoma cells and in human foreskin fibroblasts. The results suggest that (i) in the course of its evolution, the virus "borrowed" a gene fragment to preclude a cell response to infection and (ii) the carboxyl terminus of MyD116 and its family of genes known as GADD34 may have a similar function(s) in cells stressed by growth arrest, DNA damage, and differentiation and in herpes simplex virus infection.

Association of a M(r) 90,000 phosphoprotein with protein kinase PKR in cells exhibiting enhanced phosphorylation of translation initiation factor eIF-2 alpha and premature shutoff of protein synthesis after infection with gamma 134.5- mutants of herpes simplex virus 1

The protein encoded by the gamma 134.5 gene of herpes simplex virus precludes premature shutoff of protein synthesis in human cells triggered by stress associated with onset of viral DNA synthesis. The carboxyl terminus of the protein is essential for this function. This report indicates that the shutoff of protein synthesis is not due to mRNA degration because mRNA from wild-type or gamma 134.5- virus-infected cells directs protein synthesis. Analyses of the posttranslational modifications of translation initiation factor eIF-2 showed the following: (i) eIF-2 alpha was selectively phosphorylated by a kinase present in ribosome-enriched fraction of cells infected with gamma 134.5- virus. (ii) Endogenous eIF-2 alpha was totally phosphorylated in cells infected with gamma 134.5- virus or a virus lacking the 3' coding domain of the gamma 134.5 gene but was not phosphorylated in mock-infected or wild-type virus-infected cells. (iii) Immune precipitates of the PKR kinase that is responsible for regulation of protein synthesis of some cells by phosphorylation of eIF-2 alpha yielded several phosphorylated polypeptides. Of particular significance were two observations. First, phosphorylation of PKR kinase was elevated in all infected cells relative to the levels in mock-infected cells. Second, the precipitates from lysates of cells infected with gamma 134.5- virus or a virus lacking the 3' coding domain of the gamma 134.5 gene contained an additional labeled phosphoprotein of M(r) 90,000 (p90). This phosphoprotein was present in only trace amounts in the immunoprecipitate from cells infected with wild-type virus or mutants lacking a portion of the 5' domain of gamma 134.5. We conclude that in the absence of gamma 134.5 protein, PKR kinase complexes with the p90 phosphoprotein and shuts off protein synthesis by phosphorylation of the alpha subunit of translation initiation factor eIF-2.

Attenuated multi-mutated herpes simplex virus-1 for the treatment of malignant gliomas

We have created a double mutant of the herpes simplex virus (HSV) type 1 (termed G207) with favourable properties for treating human malignant brain tumours: replication-competence in glioblastoma cells (and other dividing cells), attenuated neurovirulence, temperature sensitivity, ganciclovir hypersensitivity, and the presence of an easily detectable histochemical marker. G207 has deletions at both gamma 34.5 (RL1) loci and a lacZ gene insertion inactivating the ICP6 gene (UL39). G207 kills human glioma cells in monolayer cultures. In nude mice harbouring subcutaneous or intracerebral U-87MG gliomas, intraneoplastic inoculation with G207 causes decreased tumour growth and/or prolonged survival. G207 is avirulent upon intracerebral inoculation of mice and HSV-sensitive non-human primates. These results suggest that G207 should be considered for clinical evaluation in the treatment of glioblastomas.

An avirulent ICP34.5 deletion mutant of herpes simplex virus type 1 is capable of in vivo spontaneous reactivation

The herpes simplex virus type 1 (HSV-1) ICP34.5 gene is a neurovirulence gene in mice. In addition, some ICP34.5 mutants have been reported to have a reduced efficiency of induced reactivation as measured by in vitro explantation of latently infected mouse ganglia. However, since spontaneous reactivation is almost nonexistent in mice, nothing has been reported on the effect of ICP34.5 mutants on spontaneous reactivation in vivo. To examine this, we have deleted both copies of the ICP34.5 neurovirulence gene from a strain of HSV-1 (McKrae) that has a high spontaneous reactivation rate in rabbits and used this mutant to infect rabbit eyes. All rabbits infected with the ICP34.5 mutant virus (d34.5) survived, even at challenge doses greater than 4 x 10(7) PFU per eye. In contrast, a 200-fold-lower challenge dose of 2 x 10(5) PFU per eye was lethal for approximately 50% of rabbits infected with either the wild-type McKrae parental virus or a rescued ICP34.5 mutant in which both copies of the ICP34.5 gene were restored. In mice, the 50% lethal dose of the ICP34.5 mutant was over 10(6) PFU, compared with a value of less than 10 PFU for the rescued virus. The ICP34.5 mutant was restricted for replication in rabbit and mouse eyes and mouse trigeminal ganglia in vivo. The spontaneous reactivation rate in rabbits for the mutant was 1.4% as determined by culturing tear films for the presence of reactivated virus. This was more than 10-fold lower than the spontaneous reactivation rate determined for the rescued virus (19.6%) and was highly significant (P < 0.0001, Fisher exact test). Southern analysis confirmed that the reactivated virus retained both copies of the ICP34.5 deletion. Thus, this report demonstrates that (i) the ICP34.5 gene, known to be a neurovirulence gene in mice, is also important for virulence in rabbits and (ii) in vivo spontaneous reactivation of HSV-1 in the rabbit ocular model, although reduced, can occur in the absence of the ICP34.5 gene.

Two overlapping transcription units which extend across the L-S junction of herpes simplex virus type 1

A region of the herpes simplex virus type 1 genome located upstream of the alpha 0 promoter contains a promoter which regulates transcription in the opposite orientation to that driven by alpha 0. Analyses of mutants from which this promoter, alpha X, was deleted and a mutant in which a fragment that serves as a transcription terminator and polyadenylation signal was inserted upstream of this promoter demonstrate that two distinct transcription units overlap this region of the genome and are transcribed in a direction antisense to the neurovirulence gene gamma (1)34.5. One unit, dependent on the alpha X promoter, is active when cells are infected in the presence of the protein synthesis inhibitor cycloheximide. The second unit, independent of alpha X, is active during the course of productive infection. This transcription unit originates from a promoter upstream of alpha X which is distinct from the latency-associated promoter (LAP). Two polyadenylated transcripts of 0.9 and 4.9 kb accumulate from this region of the genome during productive infection, but no mature transcripts accumulate in infected cells maintained in the presence of cycloheximide. Kinetic analyses demonstrate that the transcripts that accumulate during productive infection fall into the beta class of herpes simplex virus type 1 genes.

Comparison of genetically engineered herpes simplex viruses for the treatment of brain tumors in a scid mouse model of human malignant glioma

Genetically engineered viruses and viral genes inserted into retroviral vectors are increasingly being considered for experimental therapy of brain tumors. A primary target of these viruses and vectors is human gliomas, the most frequently occurring primary human brain tumor. To investigate the potential of genetically engineered herpes simplex viruses (HSVs) in the therapy of these tumors, we compared the attributes of two viruses, a recombinant from which the gamma 1(34.5) gene had been deleted (R3616) and a recombinant in which the gamma 1(34.5) gene had been interrupted by a stop codon (R4009). Previous studies have shown that these recombinants were completely devoid of the ability to multiply in the central nervous system of rodents. To pursue these studies, we developed a scid mouse glioma model. Tumor cell response (survival) for 10(3), 10(4), and 10(5) implanted MT539MG glioma cells was 38, 23, and 15 days, respectively. The results were as follows: (i) both R3616 and R4009 replicate and cause cytolysis in diverse glioma cell lines of murine and human origin in vitro, and (ii) Winn-type assays 10(5) MT539MG cells coinoculated with R3616 or R4009 as compared to saline significantly prolonged survival in a dose-dependent fashion. Mice that received only tumor cells or the wild-type parent strain of the recombinants, HSV-1(F), died within 15 days. Survival was greatest with R4009. These experiments define both a model for screening oncolytic viruses and a genetically engineered virus of significant potential use as an oncolytic agent.

Differential response of human cells to deletions and stop codons in the gamma(1)34.5 gene of herpes simplex virus

Earlier studies have shown that herpes simplex virus mutants lacking the gamma(1)34.5 gene are totally avirulent on intracerebral inoculation of the virus into mice and induce premature shutoff of protein synthesis in human neuroblastoma (SK-N-SH) cells but not in Vero cells. We report the following. (i) Whereas deletion mutant R3616, lacking 1,000 bp of the gamma(1)34.5 gene, caused premature shutoff of protein synthesis in both SK-N-SH and human foreskin fibroblasts (HFF), mutants R4009 and R930 (mutant F), carrying stop codons in all six frames, 27 and 210 codons from the initiation codon of the gamma(1)34.5 genes, respectively, induced shutoff of protein synthesis in SK-N-SH cells but not in HFF. The differences in behavior between the R3616 deletion and R4009 stop codon mutants cannot be attributed to differences in the rate of induction of premature shutoff of protein synthesis and the multiplicity of infection. HFF do not produce detectable truncated gamma(1)34.5 protein or truncated mRNA. (ii) Some clonal lines of SK-N-SH cells carrying a gamma(1)34.5 gene driven by a metallothionein promoter express the gamma(1)34.5 gene constitutively and do not require induction by cadmium to complement the gamma(1)34.5- virus. One clonal cell line complements the gamma(1)34.5- virus only after induction by cadmium. These results are consistent with previous conclusions that the phenotype of premature shutoff of protein synthesis is associated with absence of the gamma(1)34.5 protein and indicate that the amounts of gamma(1)34.5 protein necessary to complement the gamma(1)34.5- viruses are small. We conclude that human cells differ in the manner in which they respond to the presence of stop codons. Shutoff of protein synthesis in HFF infected with the stop codon mutants could have been precluded by small amounts of gamma(1)34.5 protein produced by splicing out of an intron containing the stop codon, downstream initiation of translation, or tRNA suppression of the stop codon.