The RL neurovirulence locus in herpes simplex virus type 2 strain HG52 plays no role in latency

Herpes Simplex Virus 1 γ134.5 Protein Inhibits STING Activation That Restricts Viral Replication

The γ₁34.5 gene of herpes simplex virus 1 (HSV-1) encodes a virulence factor that promotes viral pathogenesis. Although it perturbs TANK-binding kinase 1 (TBK1) in the complex network of innate immune pathways, the underlying mechanism is obscure. Here we report that HSV-1 γ₁34.5 targets stimulator of interferon genes (STING) in the intracellular DNA recognition pathway that regulates TBK1 activation. In virus-infected cells the γ₁34.5 protein associates with and inactivates STING, which leads to downregulation of interferon regulatory factor 3 (IRF3) and IFN responses. Importantly, HSV-1 γ₁34.5 disrupts translocation of STING from the endoplasmic reticulum to Golgi apparatus, a process necessary to prime cellular immunity. Deletion of γ₁34.5 or its amino-terminal domain from HSV-1 abolishes the observed inhibitory activities. Consistently, an HSV mutant that lacks functional γ₁34.5 replicated less efficiently in STING^+/+ than in STING^-/- mouse embryonic fibroblasts. Moreover, reconstituted expression of human STING in the STING^-/- cells activated IRF3 and reduced viral growth. These results suggest that control of the DNA sensing pathway by γ₁34.5 is advantageous to HSV infection.IMPORTANCE Viral inhibition of innate immunity contributes to herpes simplex virus pathogenesis. Although this complex process involves multiple factors, the underlying events remain unclear. We demonstrate that an HSV virulence factor γ₁34.5 precludes the activation of STING, a central adaptor in the intracellular DNA sensing pathway. Upon HSV infection, this viral protein engages with and inactivates STING. Consequently, it compromises host immunity and facilitates HSV replication. These observations uncover an HSV mechanism that is likely to mediate viral virulence.

Inhibition of TANK binding kinase 1 by herpes simplex virus 1 facilitates productive infection

The γ(1)34.5 protein of herpes simplex viruses (HSV) is essential for viral pathogenesis, where it precludes translational arrest mediated by double-stranded-RNA-dependent protein kinase (PKR). Paradoxically, inhibition of PKR alone is not sufficient for HSV to exhibit viral virulence. Here we report that γ(1)34.5 inhibits TANK binding kinase 1 (TBK1) through its amino-terminal sequences, which facilitates viral replication and neuroinvasion. Compared to wild-type virus, the γ(1)34.5 mutant lacking the amino terminus induces stronger antiviral immunity. This parallels a defect of γ(1)34.5 for interacting with TBK1 and reducing phosphorylation of interferon (IFN) regulatory factor 3. This activity is independent of PKR. Although resistant to IFN treatment, the γ(1)34.5 amino-terminal deletion mutant replicates at an intermediate level between replication of wild-type virus and that of the γ(1)34.5 null mutant in TBK1(+/+) cells. However, such impaired viral growth is not observed in TBK1(-/-) cells, indicating that the interaction of γ(1)34.5 with TBK1 dictates HSV infection. Upon corneal infection, this mutant replicates transiently but barely invades the trigeminal ganglia or brain, which is a difference from wild-type virus and the γ(1)34.5 null mutant. Therefore, in addition to PKR, γ(1)34.5 negatively regulates TBK1, which contributes viral replication and spread in vivo.

Construction and characterization of an oncolytic HSV vector containing a fusogenic glycoprotein and prodrug activation for enhanced local tumor control

A large number of oncolytic viral vectors are currently under clinical development for cancer therapy. Herpes simplex virus type 1 (HSV-1) has demonstrated particular promise in this field, showing genetically engineered selective tumor replication and cytotoxicity in a wide variety of tumor types, without damaging healthy tissues. Enhanced activity has been observed when a range of therapeutic genes has been inserted into various oncolytic HSV genomes. Here, we discuss methods used to develop and characterize an oncolytic HSV virus that combines expression of a highly potent prodrug activating gene (yeast cytosine deaminase/uracil phosphoribosyltransferase fusion [Fcy::Fur]) and the fusogenic glycoprotein from gibbon ape leukemia virus (GALV) for enhanced local tumor control.

A conserved domain of herpes simplex virus ICP34.5 regulates protein phosphatase complex in mammalian cells

ICP34.5, encoded by herpes simplex virus 1, is a protein phosphatase 1 (PP1) regulatory subunit that mediates dephosphorylation of the alpha subunit of translation initiation factor 2 (eIF2alpha). However, the mechanism of its action remains poorly understood. Here, we show that amino acid substitutions in the arginine-rich motif have differential effects on ICP34.5 activity. The phenotypes parallel with viral protein synthesis and cytopathic effects in virus infected cells. Besides the consensus PP1 binding motif, the Arg-motif appears to enhance the interaction between ICP34.5 and PP1. These results suggest that concerted action between the PP1 binding domain and the effector domain of ICP34.5 is crucial for eIF2alpha dephosphorylation and viral protein synthesis.

Replication of herpes simplex virus 1 depends on the gamma 134.5 functions that facilitate virus response to interferon and egress in the different stages of productive infection

The ability of the gamma(1)34.5 protein to suppress the PKR response plays a crucial role in herpes simplex virus pathogenesis. In this process, the gamma(1)34.5 protein associates with protein phosphatase 1 to form a large complex that dephosphorylates eIF-2alpha and thereby prevents translation shutoff mediated by PKR. Accordingly, gamma(1)34.5 null mutants are virulent in PKR-knockout mice but not in wild-type mice. However, gamma(1)34.5 deletion mutants, with an extragenic compensatory mutation, inhibit PKR activity but remain avirulent, suggesting that the gamma(1)34.5 protein has additional functions. Here, we show that a substitution of the gamma(1)34.5 gene with the NS1 gene from influenza A virus renders viral resistance to interferon involving PKR. The virus replicates as efficiently as wild-type virus in SK-N-SH and CV-1 cells. However, in mouse 3T6 cells, the virus expressing the NS1 protein grows at an intermediate level between the wild-type virus and the gamma(1)34.5 deletion mutant. This decrease in growth, compared to that of the wild-type virus, is due not to an inhibition of viral protein synthesis but rather to a block in virus release or egress. Virus particles are predominantly present in the nucleus and cytoplasm. Notably, deletions in the amino terminus of the gamma(1)34.5 protein lead to a significant decrease in virus growth in mouse 3T6 cells, which is independent of eIF-2alpha dephosphorylation. In correlation, a series of deletions in the amino-terminal domain impair nuclear as well as cytoplasmic egress. These results indicate that efficient viral replication depends on the gamma(1)34.5 functions required to prevent the PKR response and to facilitate virus egress in the different stages during virus infection.

Dephosphorylation of eIF-2alpha mediated by the gamma(1)34.5 protein of herpes simplex virus type 1 is required for viral response to interferon but is not sufficient for efficient viral replication

The gamma(1)34.5 protein of herpes simplex virus type 1 (HSV-1) functions to block the shutoff of protein synthesis involving double-stranded RNA-dependent protein kinase (PKR). In this process, the gamma(1)34.5 protein recruits cellular protein phosphatase 1 (PP1) to form a high-molecular-weight complex that dephosphorylates eIF-2alpha. Here we show that the gamma(1)34.5 protein is capable of mediating eIF-2alpha dephosphorylation without any other viral proteins. While deletion of amino acids 1 to 52 from the gamma(1)34.5 protein has no effect on eIF-2alpha dephosphorylation, further truncations up to amino acid 146 dramatically reduce the activity of the gamma(1)34.5 protein. An additional truncation up to amino acid 188 is deleterious, indicating that the carboxyl-terminal domain alone is not functional. Like wild-type HSV-1, the gamma(1)34.5 mutant with a truncation of amino acids 1 to 52 is resistant to interferon, and resistance to interferon is coupled to eIF-2alpha dephosphorylation. Intriguingly, this mutant exhibits a similar growth defect seen for the gamma(1)34.5 null mutant in infected cells. Restoration of the wild-type gamma(1)34.5 gene in the recombinant completely reverses the phenotype. These results indicate that eIF-2alpha dephosphorylation mediated by the gamma(1)34.5 protein is required for HSV response to interferon but is not sufficient for viral replication. Additional functions or activities of the gamma(1)34.5 protein contribute to efficient viral infection.

Strain-dependent structural variants of herpes simplex virus type 1 ICP34.5 determine viral plaque size, efficiency of glycoprotein processing, and viral release and neuroinvasive disease potential

The ability of certain strains of herpes simplex virus type 1 (HSV-1) to cause encephalitis or neuroinvasive disease in the mouse upon peripheral infection is dependent on a combination of activities of specific forms of viral proteins. The importance of specific variants of ICP34.5 to neuroinvasive disease potential and its correlation with small-plaque production, inefficient glycoprotein processing, and virus release were suggested by comparison of ICP34.5 from the SP7 virus, originally obtained from the brain of a neonate with disseminated disease, and the tissue culture-passaged progeny of SP7 (SLP5 and SLP10) and the KOS321 virus. SLP5, SLP10, and KOS321 are attenuated and exhibit a large-plaque phenotype, including efficient glycoprotein processing and viral release. We show that expression of the KOS321 ICP34.5 protein in cells infected with SP7 or ICP34.5 deletion mutants promotes large plaque formation and efficient viral glycoprotein processing, while expression of the SP7 ICP34.5 protein decreases efficiency of viral glycoprotein processing. In addition, a recombinant virus, 4hS1, with the SP7 ICP34.5 gene replacing the KOS321-like ICP34.5 gene in the SLP10a background, rescues the small-plaque phenotype and neuroinvasive disease. The major difference in the ICP34.5 gene product is the number of Pro-Ala-Thr repeats in the middle region of the protein, with 18 for SP7 and 3 for KOS321. Strain-dependent differences in the ICP34.5 protein can therefore alter the tissue culture behavior and the virulence of HSV-1.

Signals that dictate nuclear, nucleolar, and cytoplasmic shuttling of the gamma(1)34.5 protein of herpes simplex virus type 1

The gamma(1)34.5 protein of herpes simplex virus type 1 (HSV-1) is required for viral neurovirulence in vivo. In infected cells, this viral protein prevents the shutoff of protein synthesis mediated by double-stranded-RNA-dependent protein kinase PKR. This is accomplished by recruiting protein phosphatase 1 to dephosphorylate the alpha subunit of translation initiation factor eIF-2 (eIF-2 alpha). Moreover, the gamma(1)34.5 protein is implicated in viral egress and interacts with proliferating cell nuclear antigen. In this report, we show that the gamma(1)34.5 protein encoded by HSV-1(F) is distributed in the nucleus, nucleolus, and cytoplasm in transfected or superinfected cells. Deletion analysis revealed that the Arg-rich cluster from amino acids 1 to 16 in the gamma(1)34.5 protein functions as a nucleolar localization signal. The region from amino acids 208 to 236, containing a bipartite basic amino acid cluster, is able to mediate nuclear localization. R(215)A and R(216)A substitutions in the bipartite motif disrupt this activity. Intriguingly, leptomycin B, an inhibitor of nuclear export, blocks the cytoplasmic accumulation of the gamma(1)34.5 protein. L(134)A and L(136)A substitutions in the leucine-rich motif completely excluded the gamma(1)34.5 protein from the cytoplasm. These results suggest that the gamma(1)34.5 protein continuously shuttles between the nucleus, nucleolus, and cytoplasm, which may be a requirement for the different activities of the gamma(1)34.5 protein in virus-infected cells.

A gamma34.5 mutant of herpes simplex 1 causes severe inflammation in the brain

A number of viral vectors are currently being evaluated as potential gene therapy vectors for gene delivery to the brain. As well as evaluating their ability to express a transgene for extended periods of time it is also essential to examine any cytotoxic immune response to such vectors as this may not only limit transgene expression but also cause irreparable harm. This work describes the effect of inoculating a gamma34.5 mutant of herpes simplex type 1 (1716lacZ) into the brain of different strains of rats and mice. Animals were monitored for weight loss and signs of illness, and their brains were evaluated for inflammation, beta-galactosidase expression and recoverable infectious virus. We report that there is (i) a powerful immune response consisting of an early non-specific phase and a later presumably T-cell-mediated phase; (ii) significant weight loss in some animals strains accompanied by severe signs of clinical illness and (iii) transient reporter gene expression in all animal strains examined. To be useful for gene therapy we suggest this virus requires further modification, it should be tested in several animal strains and the dose of virus used may be critical in order to limit damage.

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.

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.

Mutations in the UL53 gene of HSV-1 abolish virus neurovirulence to mice by the intracerebral route of infection

The cell fusion protein, the product of the UL53 gene, is responsible for intracerebral (IC) pathogenicity of HSV-1. Recombinant HSV-1 R15 is apathogenic to mice by the IC route of inoculation, while intratypic recombinants, in which the UL53 gene in R15 was replaced by an analogous sequence from the pathogenic strain R19, regained IC pathogenicity. The nucleotide sequence of the UL53 gene of HSV-1 strains R15 (apathogenic) and R19 (pathogenic) was determined and compared to that of other pathogenic strains. Four mutations were found which are thought to be responsible for the apathogenic phenotype of HSV-1 strain R15. Northern blot hybridization of RNA extracted from BSC-1 cells infected with several HSV-1 strains indicated that all of the virus strains tested expressed equal amounts of UL53 mRNA in infected cell cultures. Demonstration of the expression of UL53 mRNA in brains of mice infected with HSV-1 strains was made possible by the combined use of a rapid method for mRNA extraction (Oligo dT-linked magnetic beads) and a highly sensitive technique for detection of the existence of the UL53-specific mRNA (cDNA synthesis followed by PCR). It was shown that both pathogenic (KOS and P42) and apathogenic (R15) HSV-1 strains expressed the UL53 gene in brains of IC infected mice.