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

Differentiated Human SH-SY5Y Cells Provide a Reductionist Model of Herpes Simplex Virus 1 Neurotropism

Neuron-virus interactions that occur during herpes simplex virus (HSV) infection are not fully understood. Neurons are the site of lifelong latency and are a crucial target for long-term suppressive therapy or viral clearance. A reproducible neuronal model of human origin would facilitate studies of HSV and other neurotropic viruses. Current neuronal models in the herpesvirus field vary widely and have caveats, including incomplete differentiation, nonhuman origins, or the use of dividing cells that have neuropotential but lack neuronal morphology. In this study, we used a robust approach to differentiate human SH-SY5Y neuroblastoma cells over 2.5 weeks, producing a uniform population of mature human neuronal cells. We demonstrate that terminally differentiated SH-SY5Y cells have neuronal morphology and express proteins with subcellular localization indicative of mature neurons. These neuronal cells are able to support a productive HSV-1 infection, with kinetics and overall titers similar to those seen in undifferentiated SH-SY5Y cells and the related SK-N-SH cell line. However, terminally differentiated, neuronal SH-SY5Y cells release significantly less extracellular HSV-1 by 24 h postinfection (hpi), suggesting a unique neuronal response to viral infection. With this model, we are able to distinguish differences in neuronal spread between two strains of HSV-1. We also show expression of the antiviral protein cyclic GMP-AMP synthase (cGAS) in neuronal SH-SY5Y cells, which is the first demonstration of the presence of this protein in nonepithelial cells. These data provide a model for studying neuron-virus interactions at the single-cell level as well as via bulk biochemistry and will be advantageous for the study of neurotropic viruses in vitroIMPORTANCE Herpes simplex virus (HSV) affects millions of people worldwide, causing painful oral and genital lesions, in addition to a multitude of more severe symptoms such as eye disease, neonatal infection, and, in rare cases, encephalitis. Presently, there is no cure available to treat those infected or prevent future transmission. Due to the ability of HSV to cause a persistent, lifelong infection in the peripheral nervous system, the virus remains within the host for life. To better understand the basis of virus-neuron interactions that allow HSV to persist within the host peripheral nervous system, improved neuronal models are required. Here we describe a cost-effective and scalable human neuronal model system that can be used to study many neurotropic viruses, such as HSV, Zika virus, dengue virus, and rabies virus.

HSV1 latent transcription and non-coding RNA: A critical retrospective

Virologists have invested great effort into understanding how the herpes simplex viruses and their relatives are maintained dormant over the lifespan of their host while maintaining the poise to remobilize on sporadic occasions. Piece by piece, our field has defined the tissues in play (the sensory ganglia), the transcriptional units (the latency-associated transcripts), and the responsive genomic region (the long repeats of the viral genomes). With time, the observed complexity of these features has compounded, and the totality of viral factors regulating latency are less obvious. In this review, we compose a comprehensive picture of the viral genetic elements suspected to be relevant to herpes simplex virus 1 (HSV1) latent transcription by conducting a critical analysis of about three decades of research. We describe these studies, which largely involved mutational analysis of the notable latency-associated transcripts (LATs), and more recently a series of viral miRNAs. We also intend to draw attention to the many other less characterized non-coding RNAs, and perhaps coding RNAs, that may be important for consideration when trying to disentangle the multitude of phenotypes of the many genetic modifications introduced into recombinant HSV1 strains.

Characterization of herpes simplex virus 2 primary microRNA Transcript regulation

In order to understand factors that may influence latency-associated transcription and latency-associated transcript (LAT) phenotypes, we studied the expression of the herpes simplex virus 2 (HSV-2) LAT-associated microRNAs (miRNAs). We mapped the transcription initiation sites of all three primary miRNA transcripts and identified the ICP4-binding sequences at the transcription initiation sites of both HSV-2 LAT (pri-miRNA for miR-I and miR-II, which target ICP34.5, and miR-III, which targets ICP0) and L/ST (a pri-miRNA for miR-I and miR-II) but not at that of the primary miR-H6 (for which the target is unknown). We confirmed activity of the putative HSV-2 L/ST promoter and found that ICP4 trans-activates the L/ST promoter when the ICP4-binding site at its transcription initiation site is mutated, suggesting that ICP4 may play a dual role in regulating transcription of L/ST and, consequently, of miR-I and miR-II. LAT exon 1 (containing LAT enhancer sequences), together with the LAT promoter region, comprises a bidirectional promoter required for the expression of both LAT-encoded miRNAs and miR-H6 in latently infected mouse ganglia. The ability of ICP4 to suppress ICP34.5-targeting miRNAs and to activate lytic viral genes suggests that ICP4 could play a key role in the switch between latency and reactivation.

IMPORTANCE

The HSV-2 LAT and viral miRNAs expressed in the LAT region are the most abundant viral transcripts during HSV latency. The balance between the expression of LAT and LAT-associated miRNAs and the expression of lytic viral transcripts from the opposite strand appears to influence whether individual HSV-infected neurons will be latently or productively infected. The outcome of neuronal infection may thus depend on regulation of gene expression of the corresponding primary miRNAs. In the present study, we characterize promoter sequences responsible for miRNA expression, including identification of the primary miRNA 5' ends and evaluation of ICP4 response. These findings provide further insight into the virus' strategy to tightly control expression of lytic cycle genes (especially the neurovirulence factor, ICP34.5) and suggest a mechanism (via ICP4) for the transition from latency to reactivated productive infection.

Preclinical evaluation of oncolytic δγ(1)34.5 herpes simplex virus expressing interleukin-12 for therapy of breast cancer brain metastases

The metastasis of breast cancer to the brain and central nervous system (CNS) is a problem of increasing importance. As improving treatments continue to extend patient survival, the incidence of CNS metastases from breast cancer is on the rise. New treatments are needed, as current treatments are limited by deleterious side effects and are generally palliative. We have previously described an oncolytic herpes simplex virus (HSV), designated M002, which lacks both copies of the γ₁34.5 neurovirulence gene and carries a murine interleukin 12 (IL-12) expression cassette, and have validated its antitumor efficacy in a variety of preclinical models of primary brain tumors. However, M002 has not been yet evaluated for use against metastatic brain tumors. Here, we demonstrate the following: both human breast cancer and murine mammary carcinoma cells support viral replication and IL-12 expression from M002; M002 replicates in and destroys breast cancer cells from a variety of histological subtypes, including “triple-negative” and HER2 overexpressing; M002 improves survival in an immunocompetent model more effectively than does a non-cytokine control virus. Thus, we conclude from this proof-of-principle study that a γ₁34.5-deleted IL-12 expressing oncolytic HSV may be a potential new therapy for breast cancer brain metastases.

Preclinical evaluation of a genetically engineered herpes simplex virus expressing interleukin-12

Herpes simplex virus 1 (HSV-1) mutants that lack the γ(1)34.5 gene are unable to replicate in the central nervous system but maintain replication competence in dividing cell populations, such as those found in brain tumors. We have previously demonstrated that a γ(1)34.5-deleted HSV-1 expressing murine interleukin-12 (IL-12; M002) prolonged survival of immunocompetent mice in intracranial models of brain tumors. We hypothesized that M002 would be suitable for use in clinical trials for patients with malignant glioma. To test this hypothesis, we (i) compared the efficacy of M002 to three other HSV-1 mutants, R3659, R8306, and G207, in murine models of brain tumors, (ii) examined the safety and biodistribution of M002 in the HSV-1-sensitive primate Aotus nancymae following intracerebral inoculation, and (iii) determined whether murine IL-12 produced by M002 was capable of activating primate lymphocytes. Results are summarized as follows: (i) M002 demonstrated superior antitumor activity in two different murine brain tumor models compared to three other genetically engineered HSV-1 mutants; (ii) no significant clinical or magnetic resonance imaging evidence of toxicity was observed following direct inoculation of M002 into the right frontal lobes of A. nancymae; (iii) there was no histopathologic evidence of disease in A. nancymae 1 month or 5.5 years following direct inoculation; and (iv) murine IL-12 produced by M002 activates A. nancymae lymphocytes in vitro. We conclude that the safety and preclinical efficacy of M002 warrants the advancement of a Δγ(1)34.5 virus expressing IL-12 to phase I clinical trials for patients with recurrent malignant glioma.

HSV-1 gene expression from reactivated ganglia is disordered and concurrent with suppression of latency-associated transcript and miRNAs

In cell cultures, HSV-1 replication is initiated by recruitment by virion protein 16 of transcriptional factors and histone-modifying enzymes to immediate early (α) gene promoters. HSV establishes latent infections characterized by suppression of viral gene expression except for latency-associated transcripts (LATs) and miRNAs. The latent virus reactivates in stressed neurons. A fundamental question is how reactivation initiates in the absence of virion protein 16. We report the following findings in the ganglion explant model. (i) Anti-nerve growth factor antibody accelerated the reactivation of latent virus. Viral mRNAs were detected as early as 9 h after explantation. (ii) After explantation the amounts of viral mRNAs increased whereas amounts of miRNAs and LATs decreased. The decrease in miRNAs and LATs required ongoing protein synthesis, raising the possibility that LAT and miRNAs were degraded by a viral gene product. (iii) The expression of viral genes in explanted ganglia was disordered rather than sequentially ordered as in infected cells in culture. These findings suggest that in reactivating ganglia gene expression is totally derepressed and challenge the current models in that establishment of or exit from latency could not be dependent on the suppression or activation of single or small clusters of viral genes. Finally, miRNAs and LATs reached peak levels 9-11 d after corneal inoculation, thus approximating the pattern of virus replication in these ganglia. These findings suggest that the patterns of accumulation of LATs and miRNAs reflect many different stages in the infection of neurons.

The checkpoints of viral gene expression in productive and latent infection: the role of the HDAC/CoREST/LSD1/REST repressor complex

At the portal of entry into the body, herpes simplex viruses (HSV) vigorously multiply and spread until curtailed by the adaptive immune response. At the same time, HSV invades nerve ending-abutting infected cells and is transported in a retrograde manner to the neuronal nucleus, where it establishes a latent (silent) infection. At intervals, as a consequence of physical or metabolic stress, the virus is activated and transported in an anterograde manner to the body surface. The progression of infection is regulated at four checkpoints. In cell culture or at the portal of entry into the body, HSV uses components of the HDAC1- or HDAC2/CoREST/LSD1/REST repressor complex to activate α genes (checkpoint 1) and then uses an α protein, ICP0, to suppress the same repressor complex from silencing post-α gene expression (checkpoint 2). In neurons destined to harbor latent virus (checkpoint 3), HSV hijacks the same repressor complex to silence itself as a first step in the establishment of the latent state. Suppression of histone deacetylases (HDACs) plays a key role in the reactivation from latency (checkpoint 4). HSV has evolved a strategy of using the same host repressor complex to meet its diverse lifestyle needs.

Disruption of HDAC/CoREST/REST repressor by dnREST reduces genome silencing and increases virulence of herpes simplex virus

In nonneuronal cells, herpes simplex virus 1 overcomes host defenses, replicates, and ultimately kills the infected cell. Among the host defenses suppressed by the virus is a repressor complex whose key components are histone deacetylase (HDAC) 1 or 2, RE-1 silencing transcription factor (REST), corepressor of REST (CoREST), and lysine-specific demethylase (LSD) 1. In neurons innervating cells at the portal of entry into the body, the virus establishes a "latent" infection in which viral DNA is silenced with the exception of a family of genes. The question posed here is whether the virus hijacks this repressor complex to silence itself in neurons during the latent state. To test this hypothesis, we inserted into the wild-type virus genome a wild-type REST [recombinant (R) 111], a dominant-negative REST (dnREST) lacking the N- and C-terminal repressor domains (R112), or an insertion control consisting of tandem repeats of stop codons (R113). The recombinant virus R112 carrying the dnREST replicated better and was more virulent than the wild-type parent or the other recombinant viruses when administered by the corneal or i.p. routes. Moreover, in contrast to other recombinants, corneal route inoculation by R112 recombinant virus resulted in higher DNA copy numbers, higher levels of infectious virus in eye, trigeminal ganglion, or brain, and virtually complete destruction of trigeminal ganglia in mice that may ultimately succumb to infection. These results support an earlier conclusion that the HDAC/CoREST/REST/LSD1 repressor complex is a significant component of the host innate immunity and are consistent with the hypothesis that HSV-1 hijacks the repressor to silence itself during latent infection.

Novel less-abundant viral microRNAs encoded by herpes simplex virus 2 latency-associated transcript and their roles in regulating ICP34.5 and ICP0 mRNAs

We recently identified an acutely and latently expressed viral microRNA (miRNA), miR-I, encoded by herpes simplex virus 2 (HSV-2) latency-associated transcript (LAT) through small RNA cloning and two miRNAs encoded by HSV-1 LAT through prediction. We now report the use of high-throughput sequencing technology to identify two additional relatively less-abundant viral miRNAs, miR-II and miR-III, encoded by HSV-2 LAT exon 2. miR-II includes two miRNAs, miR-II-5p and miR-II-3p, which are processed from the same miRNA precursor. miR-II and miR-III map antisense to the 5' untranslated region of ICP34.5 and to the coding region of ICP0 exon 3, respectively. These novel miRNAs are conserved in different HSV-2 strains, and their presence in infected- and transfected-cell cultures was confirmed by Northern hybridization. All three HSV-2 LAT-encoded miRNAs map to genome locations similar to those of three out of four identified HSV-1 LAT-encoded miRNAs, but the sequences of these miRNAs are not conserved. The expression of LAT-encoded miRNAs is negatively regulated by ICP4, the major viral transactivator. We further show that, similar to miR-I, miR-II is able to efficiently silence the expression of ICP34.5, a key viral neurovirulence factor, and that miR-III is able to silence the expression of ICP0, a key viral transactivator. All these data suggest that LAT sequences likely contribute to HSV latency and reactivation through tight control of these LAT-encoded miRNAs and their viral targets.

Enhancement of Th2 responses to replicative herpes simplex virus type 1 vectors by immunomodulative chemotherapy

Replicating, neuroattenuated gamma(1)34.5-deleted herpes simplex virus (HSV)-vectors are tools for experimental therapy of gliomas and autoimmune diseases. Immunomodulative treatment with Linomide (quinoline-3-carboxamide) has earlier been shown to facilitate some virus infections and reduce autoimmunity. Now we aimed at elucidating the safety of immunomodulatory therapy during infection of mice with HSV vectors. We focused on immunological and virological changes in the nervous system. BALB/c mice were infected intranasally with the HSV-1 recombinant viruses R3616, R3659 and R8306 (with mouse IL-4 transgene) and either treated with Linomide or left untreated as control groups. Treatment with Linomide was started 7 days before infection. Virological analysis consisted of viral culture and PCR for HSV DNA. Cytokine responses were studied with quantitative RT-PCR and EIA. Immunomodulatory treatment did not change the clinical course of infections. The expression of IL-4 and IL-10 in brains increased in Linomide-treated mice, particularly in infection with R8306. The expression of IL-23p19 was decreased in brains in Linomide-treated, vector-infected mice, in comparison with nontreated but virus-infected animals. Immunomodulatory treatment did not increase the viral load in brains in any of the mouse groups infected with R3616, R3659 or R8306. Immunomodulative treatment with Linomide did not compromise the safety of replicating HSV-vectors, not even the one with IL-4 transgene, suggesting that combination of immunomodulation with virotherapy may be beneficial in the treatment of certain diseases of the central nervous system. Further investigations are needed to elucidate the effects of immunomodulatory therapy in order to improve vector survival and efficacy of gene therapy.

Spread and replication of and immune response to gamma134.5-negative herpes simplex virus type 1 vectors in BALB/c mice

We have previously shown that intracranial infection of herpes simplex virus type 1 (HSV-1) vector R8306 expressing interleukin-4 (IL-4) can abolish symptoms of experimental autoimmune encephalomyelitis, which is used as a model for human multiple sclerosis (Broberg et al., Gene Ther. 8:769-777, 2001). The aim of the current study was to search for means other than intracranial injection to deliver HSV-derived vectors to the central nervous system of mice. We also aimed to study the replication efficiency of these vectors in nervous system tissues and to elucidate the effects of the viruses on the immune response. We studied the spread and replication of the following viruses with deletions in neurovirulence gene gamma(1)34.5: R3616, R849 (lacZ transgene), R3659 (alpha-tk), R8306 (murine IL-4 transgene), and R8308 (murine IL-10 transgene). The samples were taken from trigeminal ganglia and brains of BALB/c mice after corneal, intralabial, and intranasal infection, and the viral load was examined by viral culture, HSV DNA PCR, and VP16 reverse transcription (RT)-PCR. The results show that (i) intranasal infection was the most efficient means of spread to the central nervous system (CNS) besides intracranial injection; (ii) the viruses did not grow in the culture from the brain samples, but the viral DNA persisted even until day 21 postinfection; (iii) viral replication, as observed by VP16 mRNA RT-PCR, occurred mainly on days 4 and 7 postinfection in trigeminal ganglia and to a low extent in brain; (iv) R3659, R8306, and R8308 showed reactivation from the trigeminal ganglia in explant cultures; (v) in the brain, the vectors spread to the midbrain more efficiently than to other brain areas; and (vi) the deletions in the R3659 genome significantly limited the ability of this virus to replicate in the nervous system. The immunological studies show that (i) the only recombinant to induce IL-4 mRNA expression in the brain was R8306, the gamma interferon response was very low in the brain for R3659 and R8306, and the IL-23p19 response to R8306 decreased by day 21 postinfection, unlike for the other viruses; (ii) Deltagamma(1)34.5 HSV vectors modulated the subsets of the splenocytes differently depending on the transgene; (iii) R3659 infection of the nervous system induces expression and production of cytokines from the stimulated splenocytes; and (iv) HSV vectors expressing IL-4 or IL-10 induce expression and production of both of the Th2-type cytokines from splenocytes. We conclude that the intranasal route of infection is a possible means of delivery of Deltagamma(1)34.5 HSV vectors to the CNS in addition to intracranial infection, although replication in the CNS remains minimal. The DNA of the HSV vectors is able to reside in the brain for at least 3 weeks. The features of the immune response to the vectors must be considered and may be exploited in gene therapy experiments with these vectors.

Characterization of the novel E3 ubiquitin ligase encoded in exon 3 of herpes simplex virus-1-infected cell protein 0

Infected cell protein 0 (ICP0) of herpes simplex virus-1 is a 775-aa residue multifunctional protein that acts as a promiscuous transactivator linked to the degradation of several proteins. ICP0 is the only protein known which encodes two physically separated E3 ubiquitin (Ub) ligase domains, one, designated herpes virus Ub ligase 1 (HUL-1) located in a domain encoded in exon 3 and one designated herpes virus Ub ligase 2 (HUL-2) associated with the really interesting new gene (RING) finger domain encoded by exon 2. We report the following: (i) ICP0 residues 543-680 are sufficient for HUL-1 E3 activity and necessary determinants are encoded between residues 616 and 680. (ii) In substrate independent in vitro ubiquitylation reactions, a chimeric protein containing the HUL-1 domain promotes the ubiquitylation of itself and the ubiquitin conjugating enzyme (E2) cdc34 and interacts with cdc34. (iii) The mechanism of HUL-1 E3 function does not involve formation of a thioester between the HUL-1 domain and Ub. (iv) Residues 621-625 are essential for in vitro HUL-1 E3 activity and interaction between the HUL-1 domain and cdc34, suggesting that this interaction is required for HUL-1 E3 function.

Herpes simplex virus 1-infected cell protein 0 contains two E3 ubiquitin ligase sites specific for different E2 ubiquitin-conjugating enzymes

Infected cell protein 0 (ICP0) of herpes simplex virus 1, a multifunctional ring finger protein, enhances the expression of genes introduced into cells by infection or transfection, interacts with numerous cellular and viral proteins, and is associated with the degradation of several cellular proteins. Sequences encoded by exon 2 of ICP0 (residues 20-241) bind the UbcH3 (cdc34) ubiquitin-conjugating enzyme, and its carboxy terminus expresses a ubiquitin ligase activity demonstrable by polyubiquitylation of cdc34 in vitro. We report that: (i) The physical interaction of cdc34 and ICP0 leads to its degradation. Thus, substitution of ICP0 aspartate 199 with alanine attenuates the degradation of cdc34 and its binding to the ICP0 ring finger domain. (ii) Substitution of residue 620 reported to abolish the interaction with a ubiquitin-specific protease has no effect on the function of ubiquitin ligase. (iii) ICP0 contains an additional distinct E3 ligase activity specific for the UbcH5a- and UbcH6 E2-conjugating enzymes mapping to the ring finger domain. This is, to our knowledge, the first identification of a viral protein with at least two physically separated E3 ligase activities with different E2 specificities. The results suggest that each activity may target different proteins.

The infected cell protein 0 of herpes simplex virus 1 dynamically interacts with proteasomes, binds and activates the cdc34 E2 ubiquitin-conjugating enzyme, and possesses in vitro E3 ubiquitin ligase activity

The infected cell protein 0 (ICP0) of herpes simplex virus 1, a promiscuous transactivator shown to enhance the expression of genes introduced into cells by infection or transfection, interacts with numerous cellular proteins and has been linked to the disruption of ND10 and degradation of several proteins. ICP0 contains a RING finger domain characteristic of a class of E3 ubiquitin ligases. We report that: (i) in infected cells, ICP0 interacts dynamically with proteasomes and is bound to proteasomes in the presence of the proteasome inhibitor MG132. Also in infected cells, cdc34, a polyubiquitinated E2 ubiquitin-conjugating enzyme, exhibits increased ICP0-dependent dynamic interaction with proteasomes. (ii) In an in vitro substrate-independent ubiquitination system, the RING finger domain encoded by exon 2 of ICP0 binds cdc34, whereas the carboxyl-terminal domain of ICP0 functions as an E3 ligase independent of the RING finger domain. The results indicate that ICP0 can act as a unimolecular E3 ubiquitin ligase and that it promotes ubiquitin-protein ligation and binds the E2 cdc34. It differs from other unimolecular E3 ligases in that the domain containing the RING finger binds E2, whereas the ligase activity maps to a different domain of the protein. The results also suggest that ICP0 shuttles between nucleus and cytoplasm as a function of its dynamic interactions with proteasomes.

Expression of interleukin-4 but not of interleukin-10 from a replicative herpes simplex virus type 1 viral vector precludes experimental allergic encephalomyelitis

We have used interleukin (IL)-4 and -10-producing HSV-1 gamma(1)34.5 deletion viruses in gene therapy of a BALB/c model of experimental allergic encephalomyelitis (EAE), a T cell-mediated demyelinating disease of the central nervous system. It is known that in EAE of mice the Th2-type cytokines are down-regulated and the Th1-type cytokines up-regulated during the onset and relapse of the disease. Therefore, we tested two HSV-1 recombinants expressing the Th2-type cytokines IL-4 and IL-10. The recombinant viruses were injected intracranially (i.c.) in BALB/c mice 6 days after induction of EAE. As control groups we used mice without any infection, mice infected with backbone virus R3659 and mock-infected mice. Weights and symptoms of the mice were recorded daily and the tissue specimens were collected at specific time-points. The results indicate that the intracranial infection with IL-4-producing virus (1) precludes EAE symptoms, (2) protects the spinal cord from massive leukocyte infiltrations and (3) prevents demyelination and axonal loss. The IL-10-expressing virus R8308 did not have a similar favorable effect on the recovery of the mice as did the IL-4 virus R8306.

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.

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.

The second-site mutation in the herpes simplex virus recombinants lacking the gamma134.5 genes precludes shutoff of protein synthesis by blocking the phosphorylation of eIF-2alpha

In cells infected with the herpes simplex virus 1 (HSV-1) recombinant R3616 lacking both copies of the gamma134.5 gene, the double-stranded protein kinase R (PKR) is activated, eIF-2alpha is phosphorylated, and protein synthesis is shut off. Although PKR is also activated in cells infected with the wild-type virus, the product of the gamma134.5 gene, infected-cell protein 34.5 (ICP34.5), binds protein phosphatase 1alpha and redirects it to dephosphorylate eIF-2alpha, thus enabling sustained protein synthesis. Serial passage in human cells of a mutant lacking the gamma134.5 gene yields second-site, compensatory mutants lacking various domains of the alpha47 gene situated next to the US11 gene (I. Mohr and Y. Gluzman, EMBO J. 15:4759-4766, 1996). We report the construction of two recombinant viruses: R5103, lacking the gamma134. 5, US8, -9, -10, and -11, and alpha47 (US12) genes; and R5104, derived from R5103 and carrying a chimeric DNA fragment containing the US10 gene and the promoter of the alpha47 gene fused to the coding domain of the US11 gene. R5104 exhibited a protein synthesis profile similar to that of wild-type virus, whereas protein synthesis was shut off in cells infected with R5103 virus. Studies on the wild-type parent and mutant viruses showed the following: (i) PKR was activated in cells infected with parent or mutant virus but not in mock-infected cells, consistent with earlier studies; (ii) lysates of R3616, R5103, and R5104 virus-infected cells lacked the phosphatase activity specific for eIF-2alpha characteristic of wild-type virus-infected cells; and (iii) lysates of R3616 and R5103, which lacked the second-site compensatory mutation, contained an activity which phosphorylated eIF-2alpha in vitro, whereas lysates of mock-infected cells or cells infected with HSV-1(F) or R5104 did not phosphorylate eIF-2alpha. We conclude that in contrast to wild-type virus-infected cells, which preclude the shutoff of protein synthesis by causing rapid dephosphorylation of eIF-2alpha, in cells infected with gamma134.5(-) virus carrying the compensatory mutation, eIF-2alpha is not phosphorylated. The activity made apparent by the second-site mutation may represent a more ancient mechanism evolved to preclude the shutoff of protein synthesis.

The polyserine tract of herpes simplex virus ICP4 is required for normal viral gene expression and growth in murine trigeminal ganglia

ICP4 of herpes simplex virus (HSV) is essential for productive infection due to its central role in the regulation of HSV transcription. This study identified a region of ICP4 that is not required for viral growth in culture or at the periphery of experimentally inoculated mice but is critical for productive growth in the trigeminal ganglia. This region of ICP4 encompasses amino acids 184 to 198 and contains 13 nearly contiguous serine residues that are highly conserved among the alphaherpesviruses. A mutant in which this region is deleted (DeltaSER) was able to grow on the corneas of mice and be transported back to the trigeminal ganglia. DeltaSER did not grow in the trigeminal ganglia but did express low levels of several immediate-early (ICP4 and ICP27) and early (thymidine kinase [tk] and UL42) genes. It expressed very low levels of the late gC gene and did not appear to replicate DNA. This pattern of gene expression was similar to that observed for a tk mutant, dlsptk. Both DeltaSER and dlsptk expressed higher levels of the latency-associated transcript (LAT) per genome earlier in infected ganglia than did the wild-type virus, KOS. However, infected ganglia from all three viruses accumulated the same level of LAT per genome at 30 days postinfection (during latency). The data suggest that the polyserine tract of ICP4 provides an activity that is required for lytic infection in ganglia to progress to viral DNA synthesis and full lytic gene expression. In the absence of this activity, higher levels of LAT per genome accumulate earlier in infection than with wild-type virus.

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.

The genome sequence of herpes simplex virus type 2

The genomic DNA sequence of herpes simplex virus type 2 (HSV-2) strain HG52 was determined as 154,746 bp with a G+C content of 70.4%. A total of 74 genes encoding distinct proteins was identified; three of these were each present in two copies, within major repeat elements of the genome. The HSV-2 gene set corresponds closely with that of HSV-1, and the HSV-2 sequence prompted several local revisions to the published HSV-1 sequence (D. J. McGeoch, M. A. Dalrymple, A. J. Davison, A. Dolan, M. C. Frame, D. McNab, L. J. Perry, J. E. Scott, and P. Taylor, J. Gen. Virol. 69:1531-1574, 1988). No compelling evidence for the existence of any additional protein-coding genes in HSV-2 was identified.

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.

The product of ORF O located within the domain of herpes simplex virus 1 genome transcribed during latent infection binds to and inhibits in vitro binding of infected cell protein 4 to its cognate DNA site

The partially overlapping ORF P and ORF O are located within the domains of the herpes simplex virus 1 genome transcribed during latency. Earlier studies have shown that ORF P is repressed by infected cell protein 4 (ICP4), the major viral regulatory protein, binding to its cognate site at the transcription initiation site of ORF P. The ORF P protein binds to p32, a component of the ASF/SF2 alternate splicing factors; in cells infected with a recombinant virus in which ORF P was derepressed there was a significant decrease in the expression of products of key regulatory genes containing introns. We report that (i) the expression of ORF O is repressed during productive infection by the same mechanism as that determining the expression of ORF P; (ii) in cells infected at the nonpermissive temperature for ICP4, ORF O protein is made in significantly lower amounts than the ORF P protein; (iii) the results of insertion of a sequence encoding 20 amino acids between the putative initiator methionine codons of ORF O and ORF P suggest that ORF O initiates at the methionine codon of ORF P and that the synthesis of ORF O results from frameshift or editing of its RNA; and (iv) glutathione S-transferase-ORF O fusion protein bound specifically ICP4 and precluded its binding to its cognate site on DNA in vitro. These and earlier results indicate that ORF P and ORF O together have the capacity to reduce the synthesis or block the expression of regulatory proteins essential for viral replication in productive infection.

Experimental investigation of herpes simplex virus latency

The clinical manifestations of herpes simplex virus infection generally involve a mild and localized primary infection followed by asymptomatic (latent) infection interrupted sporadically by periods of recrudescence (reactivation) where virus replication and associated cytopathologic findings are manifest at the site of initial infection. During the latent phase of infection, viral genomes, but not infectious virus itself, can be detected in sensory and autonomic neurons. The process of latent infection and reactivation has been subject to continuing investigation in animal models and, more recently, in cultured cells. The initiation and maintenance of latent infection in neurons are apparently passive phenomena in that no virus gene products need be expressed or are required. Despite this, a single latency-associated transcript (LAT) encoded by DNA encompassing about 6% of the viral genome is expressed during latent infection in a minority of neurons containing viral DNA. This transcript is spliced, and the intron derived from this splicing is stably maintained in the nucleus of neurons expressing it. Reactivation, which can be induced by stress and assayed in several animal models, is facilitated by the expression of LAT. Although the mechanism of action of LAT-mediated facilitation of reactivation is not clear, all available evidence argues against its involving the expression of a protein. Rather, the most consistent models of action involve LAT expression playing a cis-acting role in a very early stage of the reactivation process.

T cell receptor-gamma/delta cells protect mice from herpes simplex virus type 1-induced lethal encephalitis

Increased numbers of T cell receptor (TCR)-gamma/delta cells have been observed in animal models of influenza and sendai virus infections, as well as in patients infected with human immunodeficiency virus and herpes simplex virus type 1 (HSV-1). However, a direct role for TCR-gamma/delta cells in protective immunity for pathogenic viral infection has not been demonstrated. To define the role of TCR-gamma/delta cells in anti-HSV-1 immunity, TCR-alpha-/- mice treated with anti- TCR-gamma/delta monoclonal antibodies or TCR-gamma/delta x TCR-alpha/beta double-deficient mice were infected with HSV-1 by footpad or ocular routes of infection. In both models of HSV-1 infection, TCR-gamma/delta cells limited severe HSV-1-induced epithelial lesions and greatly reduced mortality by preventing the development of lethal viral encephalitis. The observed protection resulted from TCR-gamma/delta cell-mediated arrest of both viral replication and neurovirulence. The demonstration that TCR-gamma/delta cells play an important protective role in murine HSV-1 infections supports their potential contribution to the immune responses in human HSV-1 infection. Thus, this study demonstrates that TCR-gamma/delta cells may play an important regulatory role in human HSV-1 infections.

The herpes simplex virus type 1 2.0-kilobase latency-associated transcript is a stable intron which branches at a guanosine

We have used a minigene construct of the herpes simplex virus type 1 (HSV-1) latency-associated transcript (LAT) gene to analyze its transcripts in transient transfection assays. A 2.8-kb fragment of the approximately 8.5-kb LAT gene encompassing the 2.0-kb LAT was cloned into a eukaryotic expression vector downstream of the cytomegalovirus immediate-early gene promoter. Northern hybridization of RNA isolated from transfected COS-1 cells identified three LAT-specific transcripts, 3.4, 2.0, and 1.4 kb in size. Mapping of these transcripts by Northern hybridization indicated that the 1.4- and 2.0-kb RNAs are nonoverlapping, while the 3.4-kb RNA overlaps both smaller RNAs. Reverse transcription-PCR (RT-PCR) and partial sequencing of the 1.4-kb RNA revealed that this RNA is the spliced exons of the 3.4-kb primary transcript. The 2.0-kb LAT appears to be an intron accumulating after splicing of the minor LAT (mLAT) pre-mRNA. The splice donor and acceptor sites for the 2.0-kb LAT identified in transfected and HSV-1-infected cells are identical. Mapping of the branch point of this intron by RT-PCR in transfected and HSV-1-infected cells, as well as in latently infected murine trigemial ganglia, shows that it is a guanosine. This branch site does not bear homology to consensus mammalian branch site sequences. These data provide evidence that the 2.0-kb LAT is an intron of the mLAT pre-mRNA with a unique branch point.

The function of herpes simplex virus genes: a primer for genetic engineering of novel vectors

Herpes simplex virus vectors are being developed for delivery and expression of human genes to the central nervous system, selective destruction of cancer cells, and as carriers for genes encoding antigens that induce protective immunity against infectious agents. Vectors constructed to meet these objectives must differ from wild-type virus with respect to host range, reactivation from latency, and expression of viral genes. The vectors currently being developed are (i) helper free amplicons, (ii) replication defective viruses, and (iii) genetically engineered replication competent viruses with restricted host range. Whereas the former two types of vectors require stable, continuous cell lines expressing viral genes for their replication, the replication competent viruses will replicate on approved primary human cell strains.

Open reading frame P--a herpes simplex virus gene repressed during productive infection encodes a protein that binds a splicing factor and reduces synthesis of viral proteins made from spliced mRNA

The open reading frame P (ORF P) is located in the domain and on the DNA strand of the herpes simplex virus 1 transcribed during latent infection. ORF P is not expressed in productively infected cells as a consequence of repression by the binding of the major viral regulatory protein to its high-affinity binding site. In cells infected with a mutant virus carrying a derepressed gene, ORF P protein is extensively posttranslationally processed. We report that ORF P interacts with a component of the splicing factor SF2/ASF, pulls down a component of the SM antigens, and colocalizes with splicing factors in nuclei of infected cells. The hypothesis that ORF P protein may act to regulate viral gene expression, particularly in situations such as latently infected sensory neurons in which the major regulatory protein is not expressed, is supported by the evidence that in cells infected with a mutant in which the ORF P gene was derepressed, the products of the regulatory genes alpha 0 and alpha 22 are reduced in amounts early in infection but recover late in infection. The proteins encoded by these genes are made from spliced mRNAs, and the extent of recovery of these proteins late in infection correlates with the extent of accumulation of post-translationally processed forms of ORF P protein.