A herpes simplex virus ribonucleotide reductase deletion mutant is defective for productive acute and reactivatable latent infections of mice and for replication in mouse cells

Herpes simplex virus 1 as an oncolytic viral therapy for refractory cancers

The need for efficacious and non-toxic cancer therapies is paramount. Oncolytic viruses (OVs) are showing great promise and are introducing new possibilities in cancer treatment with their ability to selectively infect tumor cells and trigger antitumor immune responses. Herpes Simplex Virus 1 (HSV-1) is a commonly selected OV candidate due to its large genome, relative safety profile, and ability to infect a variety of cell types. Talimogene laherparevec (T-VEC) is an HSV-1-derived OV variant and the first and only OV therapy currently approved for clinical use by the United States Food and Drug Administration (FDA). This review provides a concise description of HSV-1 as an OV candidate and the genomic organization of T-VEC. Furthermore, this review focuses on the advantages and limitations in the use of T-VEC compared to other HSV-1 OV variants currently in clinical trials. In addition, approaches for future directions of HSV-1 OVs as cancer therapy is discussed.

microRNA (miR-KT-635) encoded by Cyprinid herpesvirus 2 regulates the viral replication with targeting to the ORF23

Herpesviruses have been reported to be able to encode and express functional viral microRNAs that target both viral and cellular transcripts. In our previous studies, we found a new miRNA miR-KT-635 encoded by Cyprinid herpesvirus 2, which is predicted to target viral genes and cellular genes involved in innate immune signalling pathway and apoptosis. However, the function and target gene of miR-KT-635 are not proved. In this study, the regulating target gene of miR-KT-635 was proved as the viral gene ORF23 directly, the target point sequence on gene was verified and miR-KT-635 was identified to regulate the expression of ORF23 protein. According to the bioinformatics analysis, the tRNA domain and ribosome domain in the protein sequence of ORF23 were found to share high homology with R2i and P53R2i, which are related to the ribonucleotide reductase small subunit in the host (transform NTP to dNTP). Within expectations, silencing of viral ORF23 or transfecting miR-KT-635 mimics in Carassius auratus gibelio caudal fin cell line (GiCF) could suppress viral propagation significantly.

Identification of interaction domains in the pseudorabies virus ribonucleotide reductase large and small subunits

Alphaherpesviral ribonucleotide reductase (RNR) is composed of large (pUL39, RR1) and small (pUL40, RR2) subunits. This enzyme can catalyze conversion of ribonucleotide to deoxynucleotide diphosphates that are further phosphorylated into deoxynucleotide triphosphate (dNTPs). The dNTPs are substrates for de novo viral DNA synthesis in infected host cells. The enzymatic activity of RNR depends on association between RR1 and RR2. However, the molecular basis underlying alphaherpesviral RNR complex formation is still largely unknown. In the current study, we investigated the pseudorabies virus (PRV) RNR interaction domains in pUL39 and pUL40. The interaction of pUL39 and pUL40 was identified by co-immunoprecipitation (co-IP) and colocalization analyses. Furthermore, the interaction amino acid (aa) domains in pUL39 and pUL40 were mapped using a series of truncated proteins. Consequently, the 90-210 aa in pUL39 was identified to be responsible for the interaction with pUL40. In turn, the 66-152, 218-258 and 280-303 aa in pUL40 could interact with pUL39, respectively. Deletion of 90-210 aa in pUL39 completely abrogated the interaction with pUL40. Deletion of 66-152, 218-258 and 280-303 aa in pUL40 remarkably weakened the interaction with pUL39, whereas a weak interaction could still be observed. Amino acid sequence alignments showed that the interaction domains identified in PRV pUL39/pUL40 were relatively non-conserved among the selected RNR subunits in alphaherpesviruses HSV1, HSV2, HHV3(VZV), BHV1, EHV1 and DEV. However, they were relatively conserved among PRV, HSV1 and HSV2. Collectively, our findings provided some molecular targets for inhibition of pUL39-pUL40 interaction to antagonize viral replication in PRV infected hosts.

Intertwined: SAMHD1 cellular functions, restriction, and viral evasion strategies

SAMHD1 was initially described for its ability to efficiently restrict HIV-1 replication in myeloid cells and resting CD4⁺ T cells. However, a growing body of evidence suggests that SAMHD1-mediated restriction is by far not limited to lentiviruses, but seems to be a general concept that applies to most retroviruses and at least a number of DNA viruses. SAMHD1 anti-viral activity was long believed to be solely due to its ability to deplete cellular dNTPs by enzymatic degradation. However, since its discovery, several new functions have been attributed to SAMHD1. It has been demonstrated to bind nucleic acids, to modulate innate immunity, as well as to participate in the DNA damage response and resolution of stalled replication forks. Consequently, it is likely that SAMHD1-mediated anti-viral activity is not or not exclusively mediated through its dNTPase activity. Therefore, in this review, we summarize current knowledge on SAMHD1 cellular functions and systematically discuss how these functions could contribute to the restriction of a broad range of viruses besides retroviruses: herpesviruses, poxviruses and hepatitis B virus. Furthermore, we aim to highlight different ways how viruses counteract SAMHD1-mediated restriction to bypass the SAMHD1-mediated block to viral infection.

Attenuated Herpes Simplex Virus 1 (HSV-1) Expressing a Mutant Form of ICP6 Stimulates a Strong Immune Response That Protects Mice against HSV-1-Induced Corneal Disease

We previously isolated a herpes simplex virus 1 (HSV-1) mutant, KOS-NA, that carries two nonsynonymous mutations in UL39, resulting in L393P and R950H amino acid substitutions in infected cell protein 6 (ICP6). Our published data studying KOS-NA pathogenesis strongly suggest that one of these ICP6 substitutions expressed from KOS-NA, R950H, severely impaired acute viral replication in the eyes and trigeminal ganglia of mice after inoculation onto the cornea and consequently impaired establishment and reactivation from latency. Because of its significant neuroattenuation, we tested KOS-NA as a potential prophylactic vaccine against HSV-1 in a mouse model of corneal infection. KOS-NA stimulated stronger antibody and T cell responses than a replication-competent ICP0-null mutant and a replication-incompetent ICP8-null mutant optimized for immunogenicity. Immunizations with the ICP0^-, ICP8^-, and KOS-NA viruses all reduced replication of wild-type HSV-1 challenge virus in the corneal epithelium to similar extents. Low immunizing doses of KOS-NA and the ICP8^- virus, but not the ICP0^- virus, protected mice against eyelid disease (blepharitis). Notably, only KOS-NA protected almost completely against corneal disease (keratitis) and greatly reduced latent infection by challenge virus. Thus, vaccination of mice with KOS-NA prior to corneal challenge provides significant protection against HSV-1-mediated disease of the eye, even at a very low immunizing dose. These results suggest that KOS-NA may be the foundation of an effective prophylactic vaccine to prevent or limit HSV-1 ocular diseases.IMPORTANCE HSV-1 is a ubiquitous human pathogen that infects the majority of the world's population. Although most infections are asymptomatic, HSV-1 establishes lifelong latency in infected sensory neurons, from which it can reactivate to cause deadly encephalitis or potentially blinding eye disease. No clinically effective vaccine is available. In this study, we tested the protective potential of a neuroattenuated HSV-1 mutant (KOS-NA) as a vaccine in mice. We compared the effects of immunization with KOS-NA to those of two other attenuated viruses, a replication-competent (ICP0^-) virus and a replication-incompetent (ICP8^-) virus. Our data show that KOS-NA proved superior to the ICP0- and ICP8-null mutants in protecting mice from corneal disease and latent infection. With its significant neuroattenuation, severe impairment in establishing latency, and excellent protective effect, KOS-NA represents a significant discovery in the field of HSV-1 vaccine development.

Herpes Simplex Virus 1 Mutant with Point Mutations in UL39 Is Impaired for Acute Viral Replication in Mice, Establishment of Latency, and Explant-Induced Reactivation

In the process of generating herpes simplex virus 1 (HSV-1) mutations in the viral regulatory gene encoding infected cell protein 0 (ICP0), we isolated a viral mutant, termed KOS-NA, that was severely impaired for acute replication in the eyes and trigeminal ganglia (TG) of mice, defective in establishing a latent infection, and reactivated poorly from explanted TG. To identify the secondary mutation(s) responsible for the impaired phenotypes of this mutant, we sequenced the KOS-NA genome and noted that it contained two nonsynonymous mutations in UL39, which encodes the large subunit of ribonucleotide reductase, ICP6. These mutations resulted in lysine-to-proline (residue 393) and arginine-to-histidine (residue 950) substitutions in ICP6. To determine whether alteration of these amino acids was responsible for the KOS-NA phenotypes in vivo, we recombined the wild-type UL39 gene into the KOS-NA genome and rescued its acute replication phenotypes in mice. To further establish the role of UL39 in KOS-NA's decreased pathogenicity, the UL39 mutations were recombined into HSV-1 (generating UL39^mut), and this mutant virus showed reduced ocular and TG replication in mice comparable to that of KOS-NA. Interestingly, ICP6 protein levels were reduced in KOS-NA-infected cells relative to the wild-type protein. Moreover, we observed that KOS-NA does not counteract caspase 8-induced apoptosis, unlike wild-type strain KOS. Based on alignment studies with other HSV-1 ICP6 homologs, our data suggest that amino acid 950 of ICP6 likely plays an important role in ICP6 accumulation and inhibition of apoptosis, consequently impairing HSV-1 pathogenesis in a mouse model of HSV-1 infection.IMPORTANCE HSV-1 is a major human pathogen that infects ∼80% of the human population and can be life threatening to infected neonates or immunocompromised individuals. Effective therapies for treatment of recurrent HSV-1 infections are limited, which emphasizes a critical need to understand in greater detail the events that modulate HSV-1 replication and pathogenesis. In the current study, we identified a neuroattenuated HSV-1 mutant (i.e., KOS-NA) that contains novel mutations in the UL39 gene, which codes for the large subunit of ribonucleotide reductase (also known as ICP6). This mutant form of ICP6 was responsible for the attenuation of KOS-NA in vivo and resulted in diminished ICP6 protein levels and antiapoptotic effect. Thus, we have determined that subtle alteration of the UL39 gene regulates expression and functions of ICP6 and severely impacts HSV-1 pathogenesis, potentially making KOS-NA a promising vaccine candidate against HSV-1.

Genome sequence variation among isolates of monkey B virus (Macacine alphaherpesvirus 1) from captive macaques

Complete genome sequences of 19 strains of monkey B virus (Macacine alphaherpesvirus 1; BV) isolated from several macaque species were determined. A low level of sequence variation was present among BV isolates from rhesus macaques. Most variation among BV strains isolated from rhesus macaques was located in regions of repetitive or quasi-repetitive sequence. Variation in coding sequences (polypeptides and miRNAs) was minor compared to regions of non-coding sequences. Non-coding sequences in the long and short repeat regions of the genome did however exhibit islands of conserved sequence. Oral and genital isolates from a single monkey were identical in sequence and varied only in the number of iterations of repeat units in several areas of repeats. Sequence variation between BV isolates from different macaque species (different BV genotypes) was much greater and was spread across the entire genome, confirming the existence of different genotypes of BV in different macaque species.

Stability of the Herpesvirus Ribonucleotide Reductase-Inhibiting Nonapeptide YAGAVVNDL in Extracts of HSV-1-Infected Cells

The nonapeptide YAGAVVNDL has previously been shown to specifically inhibit ribonucleotide reductase (RR) induced by herpes simplex virus (HSV) and other herpesviruses. The stability of this peptide has been examined in extracts of HSV-infected cells. It was found to be rapidly modified to yield free tyrosine and the octapeptide AGAWNDL. Modification could be inhibited by several protease inhibitors, suggesting that it arises through cleavage of the amino-terminal tyrosine peptide bond. The implications of these results for the development of a therapeutically useful drug based on the nonapeptide are discussed.

Mutation of UL24 impedes the dissemination of acute herpes simplex virus 1 infection from the cornea to neurons of trigeminal ganglia

Herpes simplex virus 1 (human herpesvirus 1) initially infects epithelial cells of the mucosa and then goes on to infect sensory neurons leading ultimately to a latent infection in trigeminal ganglia (TG). UL24 is a core herpesvirus gene that has been identified as a determinant of pathogenesis in several Alphaherpesvirinae, although the underlying mechanisms are unknown. In a mouse model of ocular infection, a UL24-deficient virus exhibited a reduction in viral titres in tear films of 1 log10, whilst titres in TG are often below the level of detection. Moreover, the efficiency of reactivation from latency was also severely reduced. Herein, we investigated how UL24 contributed to acute infection of TG. Our results comparing the impact of UL24 on viral titres in eye tissue versus in tear films did not reveal a general defect in virus release from the cornea. We also found that the impairment of replication seen in mouse primary embryonic neurons with a UL24-deficient virus was not more severe than that observed in an epithelial cell line. Rather, in situ histological analyses revealed that infection with a UL24-deficient virus led to a significant reduction in the number of acutely infected neurons at 3 days post-infection (p.i.). Moreover, there was a significant reduction in the number of neurons positive for viral DNA at 2 days p.i. for the UL24-deficient virus as compared with that observed for WT or a rescue virus. Our results supported a model whereby UL24 functions in the dissemination of acute infection from the cornea to neurons in TG.

The role of myeloid cell activation and arginine metabolism in the pathogenesis of virus-induced diseases

When an antiviral immune response is generated, a balance must be reached between two opposing pathways: the production of proinflammatory and cytotoxic effectors that drive a robust antiviral immune response to control the infection and regulators that function to limit or blunt an excessive immune response to minimize immune-mediated pathology and repair tissue damage. Myeloid cells, including monocytes and macrophages, play an important role in this balance, particularly through the activities of the arginine-hydrolyzing enzymes nitric oxide synthase 2 (Nos2; iNOS) and arginase 1 (Arg1). Nitric oxide (NO) production by iNOS is an important proinflammatory mediator, whereas Arg1-expressing macrophages contribute to the resolution of inflammation and wound repair. In the context of viral infections, expression of these enzymes can result in a variety of outcomes for the host. NO has direct antiviral properties against some viruses, whereas during other virus infections NO can mediate immunopathology and/or inhibit the antiviral immune response to promote chronic infection. Arg1 activity not only has important wound healing functions but can also inhibit the antiviral immune response during some viral infections. Thus, depending on the specific virus and the tissue(s) involved, the activity of both of these arginine-hydrolyzing enzymes can either exacerbate or limit the severity of virus-induced disease. In this review, we will discuss a variety of viral infections, including HIV, SARS-CoV, LCMV, HCV, RSV, and others, where myeloid cells influence the control and clearance of the virus from the host, as well as the severity and resolution of tissue damage, via the activities of iNOS and/or Arg1. Clearly, monocyte/macrophage activation and arginine metabolism will continue to be important areas of investigation in the context of viral infections.

A single viral gene determines lethal cross-species neurovirulence of baboon herpesvirus HVP2

Alpha-herpesviruses can produce more severe infections in non-natural host species than in their natural host. Isolates of the baboon alpha-herpesvirus Papiine herpesvirus 2 (HVP2) are either very neurovirulent in mice (subtype nv) or non-virulent (subtype ap), but no such difference is evident in the natural baboon host. Comparative genome sequencing was used to identify subtype-specific sequence differences (SSDs) between HVP2nv and HVP2ap isolates. Some genes were identified that despite exhibiting sequence variation among isolates did not have any SSDs, while other genes had comparatively high levels of SSDs. Construction of genomic recombinants between HVP2nv and HVP2ap isolates mapped the mouse neurovirulence determinant to within three genes. Construction of gene-specific recombinants demonstrated that the UL39 ORF is responsible for determining the lethal neurovirulence phenotype of HVP2 in mice. These results demonstrate that differences in a single viral gene can determine the severity of herpesvirus infection in a non-natural host species.

Broadly reactive human CD8 T cells that recognize an epitope conserved between VZV, HSV and EBV

Human herpesviruses are important causes of potentially severe chronic infections for which T cells are believed to be necessary for control. In order to examine the role of virus-specific CD8 T cells against Varicella Zoster Virus (VZV), we generated a comprehensive panel of potential epitopes predicted in silico and screened for T cell responses in healthy VZV seropositive donors. We identified a dominant HLA-A*0201-restricted epitope in the VZV ribonucleotide reductase subunit 2 and used a tetramer to analyze the phenotype and function of epitope-specific CD8 T cells. Interestingly, CD8 T cells responding to this VZV epitope also recognized homologous epitopes, not only in the other α-herpesviruses, HSV-1 and HSV-2, but also the γ-herpesvirus, EBV. Responses against these epitopes did not depend on previous infection with the originating virus, thus indicating the cross-reactive nature of this T cell population. Between individuals, the cells demonstrated marked phenotypic heterogeneity. This was associated with differences in functional capacity related to increased inhibitory receptor expression (including PD-1) along with decreased expression of co-stimulatory molecules that potentially reflected their stimulation history. Vaccination with the live attenuated Zostavax vaccine did not efficiently stimulate a proliferative response in this epitope-specific population. Thus, we identified a human CD8 T cell epitope that is conserved in four clinically important herpesviruses but that was poorly boosted by the current adult VZV vaccine. We discuss the concept of a "pan-herpesvirus" vaccine that this discovery raises and the hurdles that may need to be overcome in order to achieve this.

Roles of conserved residues within the pre-NH2-terminal domain of herpes simplex virus 1 DNA polymerase in replication and latency in mice

The catalytic subunit of the herpes simplex virus 1 DNA polymerase (HSV-1 Pol) is essential for viral DNA synthesis and production of infectious virus in cell culture. While mutations that affect 5'-3' polymerase activity have been evaluated in animal models of HSV-1 infection, mutations that affect other functions of HSV-1 Pol have not. In a previous report, we utilized bacterial artificial chromosome technology to generate defined HSV-1 pol mutants with lesions in the previously uncharacterized pre-NH2-terminal domain. We found that the extreme N-terminal 42 residues (deletion mutant polΔN43) were dispensable for replication in cell culture, while residues 44-49 (alanine-substitution mutant polA6) were required for efficient viral DNA synthesis and production of infectious virus. In this study, we sought to address the importance of these conserved elements in viral replication in a mouse corneal infection model. Mutant virus polΔN43 exhibited no meaningful defect in acute or latent infection despite strong conservation of residues 1-42 with HSV-2 Pol. The polA6 mutation caused a modest defect in replication at the site of inoculation, and was severely impaired for ganglionic replication, even at high inocula that permitted efficient corneal replication. Additionally, the polA6 mutation resulted in reduced latency establishment and subsequent reactivation. Moreover, we found that the polA6 replication defect in cultured cells was exacerbated in resting cells as compared to dividing cells. These results reveal an important role for the conserved motif at residues 44-49 of HSV-1 Pol for ganglionic viral replication.

Phosphorylation of a herpes simplex virus 1 dUTPase by a viral protein kinase, Us3, dictates viral pathogenicity in the central nervous system but not at the periphery

Herpes simplex virus 1 (HSV-1) encodes Us3 protein kinase, which is critical for viral pathogenicity in both mouse peripheral sites (e.g., eyes and vaginas) and in the central nervous systems (CNS) of mice after intracranial and peripheral inoculations, respectively. Whereas some Us3 substrates involved in Us3 pathogenicity in peripheral sites have been reported, those involved in Us3 pathogenicity in the CNS remain to be identified. We recently reported that Us3 phosphorylated HSV-1 dUTPase (vdUTPase) at serine 187 (Ser-187) in infected cells, and this phosphorylation promoted viral replication by regulating optimal enzymatic activity of vdUTPase. In the present study, we show that the replacement of vdUTPase Ser-187 by alanine (S187A) significantly reduced viral replication and virulence in the CNS of mice following intracranial inoculation and that the phosphomimetic substitution at vdUTPase Ser-187 in part restored the wild-type viral replication and virulence. Interestingly, the S187A mutation in vdUTPase had no effect on viral replication and pathogenic effects in the eyes and vaginas of mice after ocular and vaginal inoculation, respectively. Similarly, the enzyme-dead mutation in vdUTPase significantly reduced viral replication and virulence in the CNS of mice after intracranial inoculation, whereas the mutation had no effect on viral replication and pathogenic effects in the eyes and vaginas of mice after ocular and vaginal inoculation, respectively. These observations suggested that vdUTPase was one of the Us3 substrates responsible for Us3 pathogenicity in the CNS and that the CNS-specific virulence of HSV-1 involved strict regulation of vdUTPase activity by Us3 phosphorylation.

IMPORTANCE

Herpes simplex virus 1 (HSV-1) encodes a viral protein kinase Us3 which is critical for pathogenicity both in peripheral sites and in the central nervous systems (CNS) of mice following peripheral and intracranial inoculations, respectively. Whereas some Us3 substrates involved in Us3 pathogenicity in peripheral sites have been reported, those involved in Us3 pathogenicity in the CNS remain to be identified. Here, we report that Us3 phosphorylation of viral dUTPase (vdUTPase) at serine 187 (Ser-187), which has been shown to promote the vdUTPase activity, appears to be critical for viral virulence in the CNS but not for pathogenic effects in peripheral sites. Since HSV proteins critical for viral virulence in the CNS are, in almost all cases, also involved in viral pathogenicity at peripheral sites, this phosphorylation event is a unique report of a specific mechanism involved in HSV-1 virulence in the CNS.

Host factor SAMHD1 restricts DNA viruses in non-dividing myeloid cells

SAMHD1 is a newly identified anti-HIV host factor that has a dNTP triphosphohydrolase activity and depletes intracellular dNTP pools in non-dividing myeloid cells. Since DNA viruses utilize cellular dNTPs, we investigated whether SAMHD1 limits the replication of DNA viruses in non-dividing myeloid target cells. Indeed, two double stranded DNA viruses, vaccinia and herpes simplex virus type 1, are subject to SAMHD1 restriction in non-dividing target cells in a dNTP dependent manner. Using a thymidine kinase deficient strain of vaccinia virus, we demonstrate a greater restriction of viral replication in non-dividing cells expressing SAMHD1. Therefore, this study suggests that SAMHD1 is a potential innate anti-viral player that suppresses the replication of a wide range of DNA viruses, as well as retroviruses, which infect non-dividing myeloid cells.

Various viral pathogens such as HIV-1, herpes simplex virus (HSV) and vaccinia virus infect terminally-differentiated/non-dividing macrophages during the course of viral pathogenesis. Unlike dividing cells, non-dividing cells lack chromosomal DNA replication, do not enter the cell cycle, and harbor very low levels of cellular dNTPs, which are substrates of viral DNA polymerases. A series of recent studies revealed that the host protein SAMHD1 is dNTP triphosphohydrolase, which contributes to the poor dNTP abundance in non-dividing myeloid cells, and restricts proviral DNA synthesis of HIV-1 and other lentiviruses in macrophages, dendritic cells, and resting T cells. In this report, we demonstrate that SAMHD1 also controls the replication of large dsDNA viruses: vaccinia virus and HSV-1, in primary human monocyte-derived macrophages. SAMHD1 suppresses the replication of these DNA viruses to an even greater extent in the absence of viral genes that are involved in dNTP metabolism such as thymidine kinase. Therefore, this study supports that dsDNA viruses evolved to express enzymes necessary to increase the levels of dNTPs as a mechanism to overcome the restriction induced by SAMHD1 in myeloid cells.

Restricting HIV the SAMHD1 way: through nucleotide starvation

HIV replication is limited by cellular restriction factors, such as APOBEC and tetherin, which themselves are counteracted by viral proteins. SAMHD1 was recently identified as a novel HIV restriction factor in myeloid cells, and was shown to be blocked by the lentiviral protein Vpx. SAMHD1 limits viral replication through an original mechanism: it hydrolyses intracellular dNTPs in non-cycling cells, thus decreasing the amount of these key substrates, which are required for viral DNA synthesis. In this Progress article, we describe how SAMHD1 regulates the pool of intracellular nucleotides to control HIV replication and the innate immune response.

Viral delivery for gene therapy against cell movement in cancer

Viral delivery for cancer gene therapy is a promising approach, where traditional radiotherapy or chemotherapy to limit proliferation and movement of cancer cells has met resistance. Based on the new understanding of the biology of the viral vectors, therapeutic viral vectors for cancer gene therapy have been improved for greater safety and efficacy as well as transitioned from being non-replicating to replication-competent. Traditional oncolytic vectors have focused on eliminating tumor growth, while novel vectors simultaneously target epithelial-to-mesenchymal transition (EMT) in cancer cells, which could further prevent and reverse the aggressive tumor progression. In this review, we highlight the illustrative examples of cancer gene therapy in clinical trials as well as preclinical data and include proposals on methods to further enhance the safety and efficacy of oncolytic viral vectors in cancer gene therapy.

In vivo function of the murid herpesvirus-4 ribonucleotide reductase small subunit

The difficulty of eliminating herpesvirus carriage makes host entry a key target for infection control. However, its viral requirements are poorly defined. Murid herpesvirus-4 (MuHV-4) can potentially provide insights into gammaherpesvirus host entry. Upper respiratory tract infection requires the MuHV-4 thymidine kinase (TK) and ribonucleotide reductase large subunit (RNR-L), suggesting a need for increased nucleotide production. However, both TK and RNR-L are likely to be multifunctional. We therefore tested further the importance of nucleotide production by disrupting the MuHV-4 ribonucleotide reductase small subunit (RNR-S). This caused a similar attenuation to RNR-L disruption: despite reduced intra-host spread, invasive inoculations still established infection, whereas a non-invasive upper respiratory tract inoculation did so only at high dose. Histological analysis showed that RNR-S⁻, RNR-L⁻ and TK⁻ viruses all infected cells in the olfactory neuroepithelium but unlike wild-type virus then failed to spread. Thus captured host nucleotide metabolism enzymes, up to now defined mainly as important for alphaherpesvirus reactivation in neurons, also have a key role in gammaherpesvirus host entry. This seemed to reflect a requirement for lytic replication to occur in a terminally differentiated cell before a viable pool of latent genomes could be established.

Generation and characterization of koi herpesvirus recombinants lacking viral enzymes of nucleotide metabolism

Koi herpesvirus (KHV) causes a fatal disease in koi and common carp, but no reliable and genetically characterized vaccines are available up to now. Therefore, we generated KHV recombinants possessing deletions within the viral ribonucleotide reductase (RNR), thymidine kinase (TK), dUTPase, or TK and dUTPase genes, and their corresponding rescuants. All KHV mutants were replication competent in cultured cells. Whereas plaque sizes and titers of RNR-negative KHV were reduced, replication of the other mutants was not affected. Experimental infection of carp indicated attenuation of TK- or dUTPase-deleted KHV, and PCR analysis of tissue samples permitted differentiation of mutant from wild-type virus.

Effect of siRNAs on HSV-1 plaque formation and relative expression levels of RR mRNA

RNA interference (RNAi) is a process by which introduced small interfering RNA (siRNA) can cause the specific degradation of mRNA with identical sequences. The human herpes simplex virus type 1 (HSV-1) RR is composed of two distinct homodimeric subunits encoded by UL39 and UL40, respectively. In this study, we applied siRNAs targeting the UL39 and UL40 genes of HSV-1. We showed that synthetic siRNA silenced effectively and specifically UL39 and UL40 mRNA expression and inhibited HSV-1 replication. Our work offers new possibilities for RNAi as a genetic tool for inhibition of HSV-1 replication.

Exploiting human herpesvirus immune evasion for therapeutic gain: potential and pitfalls

Herpesviruses stand out for their capacity to establish lifelong infections of immunocompetent hosts, generally without causing overt symptoms. Herpesviruses are equipped with sophisticated immune evasion strategies, allowing these viruses to persist for life despite the presence of a strong antiviral immune response. Although viral evasion tactics appear to target virtually any stage of the innate and adaptive host immune response, detailed knowledge is now available on the molecular mechanisms underlying herpesvirus obstruction of MHC class I-restricted antigen presentation to T cells. This opens the way for clinical application. Here, we review and discuss recent efforts to exploit human herpesvirus MHC class I evasion strategies for the rational design of novel strategies for vaccine development, cancer treatment, transplant protection and gene therapy.

Ultrastructural analysis of ICP34.5- herpes simplex virus 1 replication in mouse brain cells in vivo

Replication-competent forms of herpes simplex virus 1 (HSV-1) defective in the viral neurovirulence factor infected cell protein 34.5 (ICP34.5) are under investigation for use in the therapeutic treatment of cancer. In mouse models, intratumoral injection of ICP34.5-defective oncolytic HSVs (oHSVs) has resulted in the infection and lysis of tumor cells, an associated decrease in tumor size, and increased survival times. The ability of these oHSVs to infect and lyse cells is frequently characterized as exclusive to or selective for tumor cells. However, the extent to which ICP34.5-deficient HSV-1 replicates in and may be neurotoxic to normal brain cell types in vivo is poorly understood. Here we report that HSV-1 defective in ICP34.5 expression is capable of establishing a productive infection in at least one normal mouse brain cell type. We show that γ34.5 deletion viruses replicate productively in and induce cellular damage in infected ependymal cells. Further evaluation of the effects of oHSVs on normal brain cells in animal models is needed to enhance our understanding of the risks associated with the use of current and future oHSVs in the brains of clinical trial subjects and to provide information that can be used to create improved oHSVs for future use.

Important role for the murid herpesvirus 4 ribonucleotide reductase large subunit in host colonization via the respiratory tract

Viral enzymes that process small molecules provide potential chemotherapeutic targets. A key constraint-the replicative potential of spontaneous enzyme mutants-has been hard to define with human gammaherpesviruses because of their narrow species tropisms. Here, we disrupted the murid herpesvirus 4 (MuHV-4) ORF61, which encodes its ribonucleotide reductase (RNR) large subunit. Mutant viruses showed delayed in vitro lytic replication, failed to establish infection via the upper respiratory tract, and replicated to only a very limited extent in the lower respiratory tract without reaching lymphoid tissue. RNR could therefore provide a good target for gammaherpesvirus chemotherapy.

Vaccinia virus-encoded ribonucleotide reductase subunits are differentially required for replication and pathogenesis

Ribonucleotide reductases (RRs) are evolutionarily-conserved enzymes that catalyze the rate-limiting step during dNTP synthesis in mammals. RR consists of both large (R1) and small (R2) subunits, which are both required for catalysis by the R1₂R2₂ heterotetrameric complex. Poxviruses also encode RR proteins, but while the Orthopoxviruses infecting humans [e.g. vaccinia (VACV), variola, cowpox, and monkeypox viruses] encode both R1 and R2 subunits, the vast majority of Chordopoxviruses encode only R2 subunits. Using plaque morphology, growth curve, and mouse model studies, we investigated the requirement of VACV R1 (I4) and R2 (F4) subunits for replication and pathogenesis using a panel of mutant viruses in which one or more viral RR genes had been inactivated. Surprisingly, VACV F4, but not I4, was required for efficient replication in culture and virulence in mice. The growth defects of VACV strains lacking F4 could be complemented by genes encoding other Chordopoxvirus R2 subunits, suggesting conservation of function between poxvirus R2 proteins. Expression of F4 proteins encoding a point mutation predicted to inactivate RR activity but still allow for interaction with R1 subunits, caused a dominant negative phenotype in growth experiments in the presence or absence of I4. Co-immunoprecipitation studies showed that F4 (as well as other Chordopoxvirus R2 subunits) form hybrid complexes with cellular R1 subunits. Mutant F4 proteins that are unable to interact with host R1 subunits failed to rescue the replication defect of strains lacking F4, suggesting that F4-host R1 complex formation is critical for VACV replication. Our results suggest that poxvirus R2 subunits form functional complexes with host R1 subunits to provide sufficient dNTPs for viral replication. Our results also suggest that R2-deficient poxviruses may be selective oncolytic agents and our bioinformatic analyses provide insights into how poxvirus nucleotide metabolism proteins may have influenced the base composition of these pathogens.

Efficient genome replication is central to the virulence of all DNA viruses, including poxviruses. To ensure replication efficiency, many of the more virulent poxviruses encode their own nucleotide metabolism machinery, including ribonucleotide reductase (RR) enzymes, which act to provide ample DNA precursors for replication. RR enzymes require both large (R1) and small (R2) subunit proteins for activity. Curiously, some poxviruses only encode R2 subunits. Other poxviruses, such as the smallpox vaccine strain, vaccinia virus (VACV), encode both R1 and R2 subunits. We report here that the R2, but not the R1, subunit of VACV RR is required for efficient replication and virulence. We also provide evidence that several poxvirus R2 proteins form novel complexes with host R1 subunits and this interaction is required for efficient VACV replication in primate cells. Our study explains why some poxviruses only encode R2 subunits and identifies a role for these proteins in poxvirus pathogenesis. Furthermore, we provide evidence that mutant poxviruses unable to generate R2 proteins may become entirely dependent upon host RR activity. This may restrict their replication to cells that over-express RR proteins such as cancer cells, making them potential therapeutics for human malignancies.

Thinking about repairing thinking

The work of Sinden et al. suggests that it may be possible to produce improvement in the “highest” areas of brain function by transplanting brain tissue. What appears to be the limiting factor is not the complexity of the mental process under consideration but the discreteness of the lesion which causes the impairment and the appropriateness and accuracy of placement of the grafted tissue.

Therapeutic uses for neural grafts: Progress slowed but not abandoned

In spite of Stein and Glasier's justifiable conclusion that initial optimism concerning the immediate clinical applicability of neural transplantation was premature, there exists much experimental evidence to support the potential for incorporating this procedure into a therapeutic arsenal in the future. To realize this potential will require continued evolution of our knowledge at multiple levels of the clinical and basic neurosciences.

The structure, operation, and functionality of intracerebral grafts

The concept of structure, operation, and functionality, as they may be understood by clinicians or researchers using neural transplantation techniques, are briefly defined. Following Stein & Glasier, we emphasize that the question of whether an intracerebral graft is really functional should be addressed not only in terms of what such a graft does in a given brain structure, but also in terms of what it does at the level of the organism.

The NGF superfamily of neurotrophins: Potential treatment for Alzheimer's and Parkinson's disease

Stein & Glasier suggest embryonic neural tissue grafts as a potential treatment strategy for Alzheimer's and Parkinson's disease. As an alternative, we suggest that the family of nerve growth factor-related neurotrophins and their trk (tyrosine kinase) receptors underlie cholinergic basal forebrain (CBF) and dopaminergic substantia nigra neuron degeneration in these diseases, respectively. Therefore, treatment approaches for these disorders could utilize neurotrophins.

Some practical and theoretical issues concerning fetal brain tissue grafts as therapy for brain dysfunctions

Grafts of embryonic neural tissue into the brains of adult patients are currently being used to treat Parkinson's disease and are under serious consideration as therapy for a variety of other degenerative and traumatic disorders. This target article evaluates the use of transplants to promote recovery from brain injury and highlights the kinds of questions and problems that must be addressed before this form of therapy is routinely applied. It has been argued that neural transplantation can promote functional recovery through the replacement of damaged nerve cells, the reestablishment of specific nerve pathways lost as a result of injury, the release of specific neurotransmitters, or the production of factors that promote neuronal growth. The latter two mechanisms, which need not rely on anatomical connections to the host brain, are open to examination for nonsurgical, less intrusive therapeutic use. Certain subjective judgments used to select patients who will receive grafts and in assessment of the outcome of graft therapy make it difficult to evaluate the procedure. In addition, little long-term assessment of transplant efficacy and effect has been done in nonhuman primates. Carefully controlled human studies, with multiple testing paradigms, are also needed to establish the efficacy of transplant therapy.

Models of neurological defects and defects in neurological models

The transition from research to patient following advances in transplantation research is likely to be disappointing unless it includes a better understanding of critically relevant characteristics of the neurological disorder and improvements in the animal models, particularly the behavioral features. The appropriateness of the model has less to do with the species than with how the species is used.

Relationship between 31P metabolites and oncolytic viral therapy sensitivity in human colorectal cancer xenografts

Background

Studies using phosphorus magnetic resonance spectroscopy (MRS) have pointed to the significance of phospholipid metabolite alterations as biochemical markers for tumour progression or therapy response.

Methods

Spectroscopic imaging was performed in colorectal flank tumours in nude mice. In vivo tumour doubling times for each cell line were measured. In vivo sensitivity of each tumour line to treatment with G207 and NV1020 oncolytic viruses was assessed. Correlations between viral sensitivity and tumour doubling time and phosphorus MRS were estimated.

Results

For G207 virus, in vitro cytotoxicity tests showed cell viability at multiplicities of infection (ratio of viral particles per tumour cell) of 0·1 on day 6 as follows: C85, less than 1 per cent; HCT8, 1 per cent; LS174T, 9 per cent; HT29, 18 per cent; and C18, 92 per cent. Respective values for NV1020 were 1, 18, 4, 18 and 86 per cent. The phosphoethanolamine to phosphocholine ratio was significantly lower in virus-sensitive than -insensitive cells, and was dependent on tumour doubling time.

Conclusion

Alterations in membrane phospholipid metabolites that relate to proliferation of cancer cells affect the efficacy of oncolytic viral therapy. MRS proved a highly sensitive non-invasive tool for predicting the efficacy of viruses.

Comparison of intravenous versus intraperitoneal administration of oncolytic herpes simplex virus 1 for peritoneal carcinomatosis in mice

hrR3 is an oncolytic herpes simplex virus 1 (HSV-1) mutant that replicates preferentially in tumors compared with normal tissues. Portal venous administration of hrR3 in mice bearing diffuse colorectal carcinoma liver metastases significantly reduces tumor burden and prolongs animal survival. In this study, we compared survival benefit and biodistribution of hrR3 following intravenous (i.v.) administration versus intraperitoneal (i.p.) administration in immunocompetent mice bearing colon carcinoma peritoneal metastases. Mice bearing peritoneal metastases received 1 x 10(8) plaque-forming units hrR3 or mock-infected media every other day for three doses and were randomized to have the viruses administered by either an i.p. or i.v. route. Biodistribution was assessed by PCR amplification of HSV-1-specific sequences from tumor and normal tissues including the small bowel, liver, spleen, kidney, lung, heart and brain. LD(50) for i.p. administration was compared with the LD(50) for i.v. administration. In subsequent experiments, animals were monitored for survival. The frequency of HSV-1 detection in peritoneal tumors was similar in mice randomized to either i.p. or i.v. administration. However, i.p. administration resulted in a more restricted systemic biodistribution, with a reduced frequency of virus detected in the kidney, lung and heart. The LD(50) associated with i.p. administration was higher than that with i.v. administration. Tumor burden was more effectively reduced with i.p. compared with i.v. administration. Median survival following i.p. administration was approximately twice that observed with i.v. administration. I.p. administration of an HSV-1 oncolytic mutant is associated with a more restricted biodistribution, less toxicity and greater efficacy against peritoneal metastases compared with i.v. administration.

Tinkering with a viral ribonucleotide reductase

Ribonucleotide reductase (RNR), a crucial enzyme for nucleotide anabolism, is encoded by all living organisms and by large DNA viruses such as the herpesviruses. Surprisingly, the beta-herpesvirus subfamily RNR R1 subunit homologues are catalytically inactive and their function remained enigmatic for many years. Recent work sheds light on the function of M45, the murine cytomegalovirus R1 homologue; during viral evolution, M45 apparently lost its original RNR activity but gained the ability, via inhibiting RIP1, a cellular adaptor protein, to block cellular signaling pathways involved in innate immunity and inflammation. The discovery of this novel mechanism of viral immune subversion provides further support to the concept of evolutionary tinkering.

Virological pattern in plasma, peripheral blood mononuclear cells and liver tissue and clinical outcome in chronic hepatitis B and C virus coinfection

BACKGROUND

We aim to evaluate in chronic hepatitis B virus-hepatitis C virus (HBV-HCV) coinfection the interplay of these viruses in liver tissue, peripheral blood mononuclear cells (PBMC), and plasma and to analyze the effect on disease course and response to treatment.

METHODS

We enrolled 19 patients with chronic HBV-HCV coinfection, 20 with chronic HCV and 20 with chronic HBV infection at their first liver biopsy, all were naive for antiviral therapy. The patients' plasma, PBMC and liver biopsy samples were tested for HBV DNA and/or HCV RNA by real-time PCR, according to the presence/absence of hepatitis B surface antigen and antibodies against HCV in the serum.

RESULTS

Contemporary presence of HBV DNA and HCV RNA was rare in plasma (5.3% of cases) and PBMC (10.6%), but frequent in liver tissue (52.6%). Of 10 cases circulating only HCV RNA and treated with pegylated interferon (PEG-IFN) plus ribavirin for 12 months, two showed a sustained response, and eight cleared HCV RNA but became HBV-DNA-positive in plasma; these eight had detectable HBV DNA in liver at baseline. One patient, who was plasma HBV-DNA-positive/HCV-RNA-negative at baseline, showed a sustained response after 18 months of PEG-IFN treatment; another, who was plasma HBV-DNA/HCV-RNA-positive at baseline, cleared only HCV RNA during 12 months of PEG-IFN plus ribavirin treatment. Seven cases remained untreated.

CONCLUSION

Despite a reciprocal inhibition in plasma, HBV and HCV frequently coexist in liver tissue, a condition to be taken into consideration when deciding therapy.

Efficacy and safety of the oncolytic herpes simplex virus rRp450 alone and combined with cyclophosphamide

Oncolytic herpes simplex virus (oHSV) mutants are under development as anticancer therapeutics. One such vector, rRp450, is ICP6-deleted and expresses a prodrug enzyme for cyclophosphamide (CPA) (rat CYP2B1). Little is known about rRp450's toxicity profile, especially in combination with CPA. We tested rRp450/CPA for antitumor efficacy in an aggressive human xenograft sarcoma model, measured virus production in primary cells, and tested rRp450/CPA for safety in immunocompetent mice. CPA enhanced the antitumor efficacy of rRp450. Relative to wild-type HSV-1, rRp450 replication was attenuated approximately 10,000-fold in human primary hepatocytes, differentiated primary foreskin keratinocytes, and primary Schwann cells. In vivo, intravenous and intracranial (IC) rRp450 injection at the strength of 10(8) plaque-forming units (pfu) alone or followed 24 hours later by intraperitoneal (IP) CPA was well tolerated and had no significant effect clinically on blood counts or chemistries. By contrast, intravenous KOS was found to be uniformly neurotoxic at 10(5) and fatal at 10(6) pfu, and IC virus was fatal in most mice at 10(4) pfu. Low levels of virus DNA were detected in some organs following intravenous and IC virus injection, but were not significantly altered by CPA. HSV replication was not detected in reactivation studies of isolated organs. Our findings suggest rRp450/CPA is safe and warrants further study as a potential combination in anticancer therapeutics.

Induction of interleukin-6 in human retinal epithelial cells by an attenuated Herpes simplex virus vector requires viral replication and NFkappaB activation

Gene delivery has potential for treating ocular disease and a number of delivery systems have been tested in animal models. However, several viral vectors have been shown to trigger undesirable transient inflammatory responses in the eye. Previously, it was shown that an attenuated Herpes simplex virus vector (hrR3) transduced numerous cell types in the anterior and posterior segments of monkey eyes, but this was accompanied by inflammation. In the retina, retinal pigment epithelial cells were the predominant cell type transduced by hrR3. IL-6 is an important pro-inflammatory cytokine and may play a role in the response to the hrR3 vector. Infection of human ARPE-19 cells with hrR3 resulted in increased IL-6 expression and secretion 3-4h post-infection. In the presence of acyclovir (70 microM) or in cells infected with UV-inactivated hrR3, IL-6 was not up-regulated indicating viral replication was required. Expression of the HSV-1 alpha and beta genes may be necessary but was not sufficient for NF-kappaB activation and IL-6 up-regulation. The translocation of NF-kappaB into the nucleus also occurred between 3 and 4h post-infection, coincident with increased IL-6 expression. Inhibition of NF-kappaB translocation by an Adenovirus vector expressing a dominant negative IkappaB (AdIkappaBam) inhibited IL-6 up-regulation, indicating that NF-kappaB plays a role in increasing IL-6 expression in APRE-19 cells. The hrR3 virus lacks viral ribonucleotide reductase (RR) activity, thus RR is not required for NF-kappaB activation or IL-6 up-regulation in ARPE-19 cells.

[Oncolytic virotherapy using replication-competent herpes simplex viruses]

Oncolytic virotherapy using replication-competent viruses has attracted us as a new modality for cancer treatment. The fundamental concept of oncolytic virotherapy is that the viruses selectively replicate in and lyse tumor cells. Since 1997, numbers of clinical trials have been done in over 500 cancer patients. However, the results of those trials have been disappointing in most cases. We have isolated a spontaneously occurring herpes simplex virus type 1 mutant, designated HF10, which efficiently replicates and induces cell fusion in most transformed cells, but is highly attenuated in mice. HF10 has a number of deletions and insertions in the genome, resulting in the lack of the functional expression of UL43, UL49.5, UL55, UL56 and latency-associated transcripts. We have found that HF10 can be used as an oncolytic virus for treatment of malignant tumors in various animal models. Clinical trials have shown that intratumoral injection of HF10 can induce extensive tumor cell death in patients with recurrent breast cancer and head and neck squamous cell carcinoma without significant adverse effects. HF10 is a promising agent for use in oncolytic virotherapy in non-central nervous system malignancies.

The ribonucleotide reductase domain of the R1 subunit of herpes simplex virus type 2 ribonucleotide reductase is essential for R1 antiapoptotic function

The R1 subunit (ICP10) of herpes simplex virus type 2 (HSV-2) ribonucleotide reductase (RR), which in addition to its C-terminal reductase domain possesses a unique N-terminal domain of about 400 aa, protects cells against apoptosis. As the NH2domain on its own is not antiapoptotic, it has been postulated that both domains of R1 or part(s) of them could be necessary for this function. Here, N- and C-terminal deletions were introduced in HSV-2 R1 to map the domain(s) involved in its antiapoptotic potential. The results showed that, whereas most of the NH2domain including part of the recently described putativeα-crystallin domain is dispensable for antiapoptotic activity, it is the integrity of the structured RR domain that is required for protection. As theα-crystallin domain appears to play an important role in protein folding and oligomerization, the N-terminal boundary of the antiapoptotic domain could not be defined precisely. In addition, this study provided evidence that overexpression of HSV-2 R2 at levels up to 30-fold more than HSV-2 R1 did not decrease protection from tumour necrosis factor alpha, indicating that the R1 surface where R2 binds is not involved in antiapoptotic activity. Importantly, this result suggests that the co-expression of both RR subunits during the lytic cycle should not affect protection from this cytokine.

Determination, by inductively coupled plasma mass spectrometry, of changes in cellular metal content resulting from herpes simplex virus-1 (HSV-1) infection

Metals and metal-containing compounds are known to play important roles in many biological processes, including metabolic and detoxification pathways and the formation and function of proteins. Like all organisms, viruses are expected to contain different metals. These metals, either by themselves or in the form of metalloproteins, may be involved in the virus's ability to infect healthy cells and replicate within them. Identification and speciation of metals in control cells and in cells affected by a virus could be helpful in elucidating infection and replication mechanisms; these might, in turn, be vital to the development of more effective treatments. There has, however, been no extensive investigation of the metals specific viruses contain or affect. The objective of this study was to investigate changes in cellular metal content resulting from herpes simplex virus 1 (HSV-1) infection. Inductively coupled plasma mass spectrometry was used to identify differences between metal concentrations in uninfected and HSV-1-infected mammalian cells. Although it can be assumed that decreases in metal content are a result of cellular response to the virus, increases can be attributed either to cellular response or to the HSV-1 virus itself. Microwave digestion and flow injection methods suitable for small sample volumes were used, and the effects of different virus inactivation procedures were explored. This work is the first step in the identification of metals pertinent to HSV-1 infection and lays the foundation for future studies concentrating on characterization of these metal-associated or containing molecules.

Herpes simplex virus 1 (HSV-1) for cancer treatment

Cancer remains a serious threat to human health, causing over 500 000 deaths each year in US alone, exceeded only by heart diseases. Many new technologies are being developed to fight cancer, among which are gene therapies and oncolytic virotherapies. Herpes simplex virus type 1 (HSV-1) is a neurotropic DNA virus with many favorable properties both as a delivery vector for cancer therapeutic genes and as a backbone for oncolytic viruses. Herpes simplex virus type 1 is highly infectious, so HSV-1 vectors are efficient vehicles for the delivery of exogenous genetic materials to cells. The inherent cytotoxicity of this virus, if harnessed and made to be selective by genetic manipulations, makes this virus a good candidate for developing viral oncolytic approach. Furthermore, its large genome size, ability to infect cells with a high degree of efficiency, and the presence of an inherent replication controlling mechanism, the thymidine kinase gene, add to its potential capabilities. This review briefly summarizes the biology of HSV-1, examines various strategies that have been used to genetically modify the virus, and discusses preclinical as well as clinical results of the HSV-1-derived vectors in cancer treatment.

Versatility of gene therapy vectors through viruses

Several viruses have been engineered for gene therapy applications, and the specific properties of each viral vector have been exploited to target a variety of inherited and acquired diseases. Preclinical and clinical studies demonstrated that viral vectors are highly versatile tools capable of efficient transfer of foreign genetic information into almost all cell types and tissues. Gene therapy applications depend on vector characteristics, such as host range, cell- or tissue-specific targeting, genome integration, efficiency and duration of transgene expression, packaging capacity, and suitability for scale-up production. This review discusses the advances in the development of viral vectors, with particular emphasis on how knowledge of virus biology has been exploited to design a variety of vectors with improved safety characteristics and efficiency, potentially suitable for a large number of gene therapy applications.

Herpes simplex virus-based vectors

Herpes simplex virus (HSV)-based vectors have primarily been developed for neuronal gene delivery, taking advantage of the virus' natural neurotropism. Two types of vector are available: replication defective viruses, whose cytotoxicity has been abolished by deleting viral gene products, and amplicon vectors, which are plasmids packaged into HSV particles with the aid of a helper virus. In this review I discuss how the cytotoxicity of the wild-type virus has been abolished, the progress which has been made toward defining promoter elements capable of directing long-term transgene expression form the latent viral genome and some of the potential clinical uses of these versatile vectors.

Development of prophylactic vaccines for genital and neonatal herpes

Over five decades numerous conventional candidate live attenuated and killed vaccines have failed to prevent genital herpes in clinical trials. However, a vaccine consisting of recombinant glycoprotein D from herpes simplex virus (HSV)-2 and deacylated monophosphoryl lipid A adjuvant has recently shown partial efficacy against clinical disease transmitted from HSV-1 and -2 seronegative women (73-74%). Comparisons between the efficacy of this vaccine and previous failed candidates and their effects on the immune system should help guide development of better vaccines through selection of appropriate HSV proteins, adjuvants or cytokines and newer vaccine vectors, such as DNA vaccines, recombinant viral vaccines and specific HSV mutants.

The dominant-negative herpes simplex virus type 1 (HSV-1) recombinant CJ83193 can serve as an effective vaccine against wild-type HSV-1 infection in mice

By selectively regulating the expression of the trans-dominant-negative mutant polypeptide UL9-C535C, of herpes simplex virus type 1 (HSV-1) origin binding protein UL9 with the tetracycline repressor (tetR)-mediated gene switch, we recently generated a novel replication-defective and anti-HSV-specific HSV-1 recombinant, CJ83193. The UL9-C535C peptides expressed by CJ83193 can function as a potent intracellular therapy against its own replication, as well as the replication of wild-type HSV-1 and HSV-2 in coinfected cells. In this report, we demonstrate that CJ83193 cannot initiate acute productive infection in corneas of infected mice nor can it reactivate from trigeminal ganglia of mice latently infected by CJ83193 in a mouse ocular model. Given that CJ83193 is capable of expressing the viral alpha, beta, and gamma1 genes but little or no gamma2 genes, we tested the vaccine potential of CJ83193 against HSV-1 infection in a mouse ocular model. Our studies showed that immunization with CJ83193 significantly reduced the yields of challenge HSV in the eyes and trigeminal ganglia on days 3, 5, and 7 postchallenge. Like in mice immunized with the wild-type HSV-1 strain KOS, immunization of mice with CJ83193 prevents the development of keratitis and encephalitis induced by corneal challenge with wild-type HSV-1 strain mP. Delayed-type hypersensitivity (DTH) assays demonstrate that CJ83193 can elicit durable cell-mediated immunity at the same level as that of wild-type HSV-1 and is more effective than that induced by d27, an HSV-1 ICP27 deletion mutant. Moreover, mice immunized with CJ83193 developed strong, durable HSV-1-neutralizing antibodies at levels at least twofold higher than those induced by d27. The results presented in this report have shed new light on the development of effective HSV viral vaccines that encode a unique safety mechanism capable of inhibiting the mutant's own replication and that of wild-type virus.

Effect of murine liver cell proliferation on herpes viral behavior: implications for oncolytic viral therapy

Replication-competent herpes simplex oncolytic viruses are promising anticancer agents that partly target increased DNA synthesis in tumor cells. Investigators have proposed that these DNA viruses may be combined with liver resection to enhance killing of liver malignancies. Whether or not the cellular alterations associated with hepatic regeneration affect the efficacy and toxicity of these promising anticancer agents is unknown. This study examined the behavior of two oncolytic viruses, NV1020 and G207, during liver regeneration. When delivered during the peak of liver regeneration, replication and appearance of both G207 and NV1020 in hepatic tissue are enhanced as demonstrated by histochemical staining for the marker gene lac Z, immunohistochemical staining, and quantitative polymerase chain reaction. This increased appearance of virus in liver tissue correlates with increases in cellular ribonucleotide reductase activity and DNA synthesis and is also associated with increased viral binding. However, increased viral presence is transient, and viral detection declines to baseline within 7 days. When these viruses were delivered to animals even as early as 7 days after hepatectomy, there proved to be no measurable viral replication in any organ and no increased morbidity or mortality. In conclusion, the early stages of hepatic regeneration after resection provide an environment suitable for viral replication. Administration of replication-competent herpes simplex virus during the peak of hepatocyte regeneration (24-48 hours) permits viral productivity in tissue that otherwise does not support viral growth. The increase in hepatotoxicity after hepatectomy is short-lived and can be predicted by peak hepatocyte DNA synthesis.

The ribonucleotide reductase R1 homolog of murine cytomegalovirus is not a functional enzyme subunit but is required for pathogenesis

Ribonucleotide reductase (RNR) is the key enzyme in the biosynthesis of deoxyribonucleotides. Alpha- and gammaherpesviruses express a functional enzyme, since they code for both the R1 and the R2 subunits. By contrast, betaherpesviruses contain an open reading frame (ORF) with homology to R1, but an ORF for R2 is absent, suggesting that they do not express a functional RNR. The M45 protein of murine cytomegalovirus (MCMV) exhibits the sequence features of a class Ia RNR R1 subunit but lacks certain amino acid residues believed to be critical for enzymatic function. It starts to be expressed independently upon the onset of viral DNA synthesis at 12 h after infection and accumulates at later times in the cytoplasm of the infected cells. Moreover, it is associated with the virion particle. To investigate direct involvement of the virally encoded R1 subunit in ribonucleotide reduction, recombinant M45 was tested in enzyme activity assays together with cellular R1 and R2. The results indicate that M45 neither is a functional equivalent of an R1 subunit nor affects the activity or the allosteric control of the mouse enzyme. To replicate in quiescent cells, MCMV induces the expression and activity of the cellular RNR. Mutant viruses in which the M45 gene has been inactivated are avirulent in immunodeficient SCID mice and fail to replicate in their target organs. These results suggest that M45 has evolved a new function that is indispensable for virus replication and pathogenesis in vivo.

Herpes simplex virus type 2 vaccines: new ground for optimism?

The development of effective prophylactic and therapeutic vaccines against genital herpes has proven problematic. Difficulties are associated with the complexity of the virus life cycle (latency) and our relatively poor understanding of the mechanism of immune control of primary and recurrent disease. The types of effector cells and the mechanisms responsible for their activation and regulation are particularly important. Studies from my and other laboratories have shown that recurrent disease is prevented by virus-specific T helper 1 (Th1) cytokines (viz., gamma interferon) and activated innate immunity. Th2 cytokines (viz., interleukin-10 [IL-10]) and regulatory (suppressor) T cells downregulate this immune profile, thereby allowing unimpeded replication of reactivated virus and recurrent disease. Accordingly, an effective therapeutic vaccine must induce Th1 immunity and be defective in Th2 cytokine production, at least IL-10. These concepts are consistent with the findings of the most recent clinical trials, which indicate that (i) a herpes simplex virus type 2 (HSV-2) glycoprotein D (gD-2) vaccine formulated with a Th1-inducing adjuvant has prophylactic activity in HSV-2- and HSV-1-seronegative females, an activity attributed to the adjuvant function, and (ii) a growth-defective HSV-2 mutant (ICP10DeltaPK), which is deleted in the Th2-polarizing gene ICP10PK, induces Th1 immunity and has therapeutic activity in both genders. The ICP10DeltaPK vaccine prevents recurrent disease in 44% of treated subjects and reduces the frequency and severity of recurrences in the subjects that are not fully protected. Additional studies to evaluate these vaccines are warranted.