Genetic and biological analyses of a herpes simplex virus intertypic recombinant reduced specifically for neurovirulence

VIRULENCE

Why do parasites harm their hosts? Intuition suggests that parasites should evolve to be benign whenever the host is needed for transmission. Yet a growing theoretical literature offers several models to explain why natural selection may favor virulent parasites over avirulent ones. This perspective first organizes these models into a simple framework and then evaluates the empirical evidence for and against the models. There is relatively scant evidence to support any of the models rigorously, and indeed, there are only a few unequivocal observations of virulence actually evolving in parasite populations. These shortcomings are surmountable, however, and empirical models of host-parasite interactions have been developed for many kinds of pathogens so that the relevant data could be acquired in the near future.

Worldwide circulation of HSV-2 × HSV-1 recombinant strains

Homo sapiens harbor two distinct, medically significant species of simplexviruses, herpes simplex virus (HSV)-1 and HSV-2, with estimated divergence 6-8 million years ago (MYA). Unexpectedly, we found that circulating HSV-2 strains can contain HSV-1 DNA segments in three distinct genes. Using over 150 genital swabs from North and South America and Africa, we detected recombinants worldwide. Common, widely distributed gene UL39 genotypes are parsimoniously explained by an initial >457 basepair (bp) HSV-1 × HSV-2 crossover followed by back-recombination to HSV-2. Blocks of >244 and >539 bp of HSV-1 DNA within genes UL29 and UL30, respectively, have reached near fixation, with a minority of strains retaining sequences we posit as ancestral HSV-2. Our data add to previous in vitro and animal work, implying that in vivo cellular co-infection with HSV-1 and HSV-2 yields viable interspecies recombinants in the natural human host.

Pre-clinical Assessment of C134, a Chimeric Oncolytic Herpes Simplex Virus, in Mice and Non-human Primates

Oncolytic herpes simplex virus (oHSV) type I constructs are investigational anti-neoplastic agents for a variety of malignancies, including malignant glioma. Clinical trials to date have supported the safety of these agents even when directly administered in the CNS. Traditional pre-clinical US Food and Drug Administration (FDA) toxicity studies for these agents have included the use of two species, generally including murine and primate studies. Recently, the FDA has decreased its requirement of non-human primates as an animal model for ethical reasons, especially for established viral systems where there are good alternative model systems. Here we present data demonstrating the safety of C134, a chimeric oHSV construct, in CBA mice as well as in a limited number of the HSV-sensitive non-human primate Aotus nancymaae as a proposed agent for clinical trials. These data, along with the previously conducted clinical trials of oHSV constructs, support the use of the CBA mouse model as sufficient for the pre-clinical toxicity studies of this agent. We summarize our experience with different HSV recombinants and differences between them using multiple assays to assess neurovirulence, as well as our experience with C134 in a limited number of A. nancymaae.

De Novo Herpes Simplex Virus VP16 Expression Gates a Dynamic Programmatic Transition and Sets the Latent/Lytic Balance during Acute Infection in Trigeminal Ganglia

The life long relationship between herpes simplex virus and its host hinges on the ability of the virus to aggressively replicate in epithelial cells at the site of infection and transport into the nervous system through axons innervating the infection site. Interaction between the virus and the sensory neuron represents a pivot point where largely unknown mechanisms lead to a latent or a lytic infection in the neuron. Regulation at this pivot point is critical for balancing two objectives, efficient widespread seeding of the nervous system and host survival. By combining genetic and in vivo in approaches, our studies reveal that the balance between latent and lytic programs is a process occurring early in the trigeminal ganglion. Unexpectedly, activation of the latent program precedes entry into the lytic program by 12 -14hrs. Importantly, at the individual neuronal level, the lytic program begins as a transition out of this acute stage latent program and this escape from the default latent program is regulated by de novo VP16 expression. Our findings support a model in which regulated de novo VP16 expression in the neuron mediates entry into the lytic cycle during the earliest stages of virus infection in vivo. These findings support the hypothesis that the loose association of VP16 with the viral tegument combined with sensory axon length and transport mechanisms serve to limit arrival of virion associated VP16 into neuronal nuclei favoring latency. Further, our findings point to specialized features of the VP16 promoter that control the de novo expression of VP16 in neurons and this regulation is a key component in setting the balance between lytic and latent infections in the nervous system.

Herpes simplex virus remains a significant human pathogen associated with extensive acute and chronic disease in humans worldwide. The virus invades the peripheral and central nervous systems where it replicates but also establishes life-long latent infections in neurons. Two distinct viral transcriptional programs support these distinct lifestyles, but how entry into either the lytic or latent programs is regulated in the neuron is not understood. This process is fundamentally important to a virus with the capacity to be extremely virulent, in balancing two objectives, efficient widespread seeding of the nervous system and host survival. In this report, we provide new insight into this regulation and data that support a novel model in which virus transported into the neuron from the body surface enters the latent program by default. In a subset of these, there is a transition into the lytic cycle, which requires VP16 transactivation and is gated by a region in the VP16 promoter. Thus, HSV takes advantage of the anatomy and axonal transport systems in sensory neurons so that VP16 is left behind and latency is favored, while features of the VP16 promoter insure adequate virus spread in the nervous system and maximized latent infections.

Natural recombinant between equine herpesviruses 1 and 4 in the ICP4 gene

Equine herpesvirus 1 (EHV-1) is a pathogen causing rhinopneumonia in young horses, abortion in mares, and myeloencephalitis in adult horses. Two types, EHV-1 P and EHV-1 B, have recently been dominant among 16 electropherotypes. EHV-1 P and EHV-1 B viruses were compared by long and accurate polymerase chain reaction (LA-PCR) and restriction fragment length polymorphism (RFLP) analysis. Differences in restriction sites were found to be focused in ORF64, which encodes the infected cell protein 4 (ICP4), and downstream of the ICP4 gene. The 3 ' -end and downstream of ICP4 gene of EHV-1 B were found to be replaced by the corresponding region of EHV-4, indicating that EHV-1 B is a naturally occurring recombinant virus between progenitors of EHV-1 P and EHV-4. This is the first report showing a natural interspecies recombinant in alphaherpesviruses.

The role of viral and host genes in corneal infection with herpes simplex virus type 1

Herpes simplex virus infection of the eye is the leading cause of blindness due to infection in the US despite the availability of several antiviral drugs. Studies with animal models have shown that three factors, innate host resistance, the host adaptive immune response, and the strain of virus interact to determine whether an infection is asymptomatic or proceeds to the development of blinding keratitis (HSK). Of these, the role of adaptive immunity has received the most attention. This work has clearly shown that stromal keratitis is an immunopathological disease, most likely due to the induction of a delayed type hypersensitivity response. Substantially less is known about the role of specific host genes in resistance to HSK. The fact that different strains of virus display different disease phenotypes indicates that viral 'virulence' genes are critical. Of the 80 plus HSV genes, few have been formally tested for their role in HSV keratitis. Most studies of virulence genes to date have focused on a single gene or protein and large changes in disease phenotypes are usually measured. Large changes in the ability to cause disease are likely to reduce the fitness of the virus, thus such studies, although useful, do not mimic the natural situation. Viral gene products are known to interact with each other, and with host proteins and these interactions are critical in determining the outcome of infection. In reality, the 'constellation' of genes encoded by each particular strain is critical, and how this constellation of genes works together and with host proteins determines the outcome of an infection. The goal of this review is to discuss the current state of knowledge regarding the role of host and viral genes in HSV keratitis. The roles of specific genes that have been shown to influence keratitis are discussed. Recent data showing that different viral genes cooperate to influence disease severity and confirming that the constellation of genes within a particular strain determines the disease phenotype are also discussed, as are the methods used to test the role of viral genes in virulence. It will become apparent that there is a paucity of information regarding the function of many viral genes in keratitis. Improving our knowledge of the role of viral genes is critical for devising more effective treatments for this disease.

The effect of latency-associated transcript on the herpes simplex virus type 1 latency-reactivation phenotype is mouse strain-dependent

Herpes simplex virus type 1 (HSV-1) latency-associated transcript (LAT) null mutants reactivate poorly in the rabbit ocular model. The situation in mice is less clear. Reports concluding that LAT null mutants reactivate poorly in the mouse explant-induced reactivation (EIR) model are contradicted by a similar number of reports of normal EIR of LAT(-) mutants in mice. To determine if the EIR phenotype might be mouse strain-dependent we infected BALB/c and Swiss Webster mice with LAT(-) or LAT(+) virus and assessed EIR in individual trigeminal ganglia. Compared to LAT(+) virus, LAT(-) virus reactivated poorly in Swiss Webster mice (P<0.05). In contrast, the EIR phenotype of these viruses was similar in BALB/c mice (P>0.1). Thus, LAT appeared to have a much greater impact on the EIR phenotype in Swiss Webster mice than in BALB/c mice. The mouse strain therefore appeared consequential in the HSV-1 EIR phenotype in mice.

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.

Molecular evidence and clinical significance of herpesvirus coinfection in the central nervous system

A total of 60 cerebrospinal fluid (CSF) specimens from patients manifesting symptoms resembling viral central nervous system (CNS) disease were examined for the presence of herpes simplex virus (HSV), human herpesvirus 6 (HHV-6), Epstein-Barr virus (EBV), cytomegalovirus, varicella-zoster virus, Borrelia burgdorferi, and Tropheryma whippelii DNA by PCR. Of 30 specimens which were selected on the basis of HSV DNA positivity, 2 were concomitantly positive for HHV-6 DNA and 1 was positive for EBV DNA. In the three specimens positive for more than one herpesvirus, amplicons generated with virus-specific primer sets hybridized specifically to the corresponding virus-specific probe. Sequence analysis of the two amplified DNA fragments demonstrated that they were derived from distinct herpesviruses. Of 22 patients with clinically diagnosed encephalitis, 2 of 3 patients coinfected with HSV and HHV-6 died, compared to 1 of 19 (5%) patients infected with only HSV. Of 30 CSF specimens that were negative for HSV DNA, EBV DNA was detected in one sample. These data indicated the presence of DNA specific for two distinct herpesviruses in the same CSF specimen, providing molecular evidence that coinfection with this group of viruses may occur in the CNS.

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.

Propranolol suppresses reactivation of herpesvirus

Herpes simplex virus type 1 (HSV-1) reactivates from the nervous system and causes recurrent disease in end organs such as the eye and the lips. We found that the beta-adrenergic receptor blocker, propranolol, reduces HSV-1 reactivation in an animal model. Mice latent for McKrae strain HSV-1 were injected with propranolol or saline once a day for 3 successive days, and subjected to a brief period of hyperthermia on the second day to induce reactivation. Twenty-four hours after the third injection, swabs of the ocular surface and homogenates of the corneas and trigeminal ganglia were analyzed for the presence of infectious virus and viral DNA. Treatment with propranolol significantly decreased the appearance of infectious virus in the tear film, cornea, and trigeminal ganglia (P < 0.05, chi 2-test). The results suggest a possible new pharmacologic approach to suppressing herpesvirus reactivation in the nervous system and thereby preventing recurrent disease.

Herpes simplex virus 1 gamma(1)34.5 gene function, which blocks the host response to infection, maps in the homologous domain of the genes expressed during growth arrest and DNA damage

The gamma(1)34.5 gene of herpes simplex virus is dispensable in some cell lines (e.g., Vero). In others (e.g., human neuroblastoma cell line SK-N-SH), the gamma(1)34.5- deletion mutant triggers a premature total shutoff of all protein synthesis, thereby rendering the cell nonviable and reducing drastically viral yields. The inability to prevent the cellular stress response that causes the infected cell to die may be responsible for the inability of the deletion mutant to multiply and cause pathology in the central nervous system of mice. The gamma(1)34.5 gene consists of an amino-terminal domain, a variable linker sequence consisting of 3 amino acids repeated 5-10 times, and a carboxyl-terminal domain homologous to the corresponding domain of MyD116, a gene expressed in myeloid leukemia cells induced to differentiate by interleukin 6, and growth arrest and DNA damage gene 34 (GADD34), a gene induced by growth arrest and DNA damage. We have constructed several viral mutants from which various domains of the gamma(1)34.5 gene had been deleted or rendered mute by the insertion of a stop codon. Studies on those mutants show that the domain of the gamma(1)34.5 gene necessary to preclude the total shutoff of protein synthesis corresponds to the carboxyl-terminal domain of the gamma(1)34.5 gene homologous to the corresponding coding domain of the MyD116 and GADD34 genes.

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

Previous studies have shown that a gene mapping in the inverted repeats of the L component of herpes simplex virus, type 1 DNA, designated as gamma (1) 34.5, was dispensable for growth in cells in culture but that the deletion mutant (R3616) and a mutant containing a stop codon (R4009) in each copy of the gene were incapable of replicating in the central nervous systems (CNS) of mice. Restoration of the deleted sequences restored the wild type virus phenotype. We report here that the gamma (1) 34.5 mutant viruses (R3616 and R4009) replicated in the vaginal tract of two different strains of mice and guinea pig, although both viruses were shed at lower titer and for fewer days than the wild type and restored viruses. Both R3616 and R4009 failed to replicate or cause significant pathology in the cornea of Balb/C mice or following intranasal inoculation of Swiss Webster mice. Analyses of sensory trigeminal and dorsal root ganglia innervating the site of inoculation indicated that the incidence of establishment of latency or reactivation from latency by R3616 and R4009 viruses was significantly lower than that determined for mice infected with wild type or restored virus. Thus, selective deletion of gamma (1) 34.5 gene abolished the capacity of the virus to spread from peripheral mucosal sites to the CNS or replicate in the CNS, and diminished the capacity of the virus to replicate at mucosal sites and, subsequently, establish latency, or be able to be reactivated ex vivo. The results of our studies may have direct implications for the development of genetically engineered herpes simplex virus vaccines.

Construction of a US3 lacZ insertion mutant of herpes simplex virus type 2 and characterization of its phenotype in vitro and in vivo

We have constructed and characterized a mutant of herpes simplex virus type 2 (HSV-2) which was inserted a modified lacZ gene, placed under the control of HSV-1 beta 8 promotor, into the US3 protein kinase gene. The mutant, L1BR1, could not induce the virus-encoded protein kinase activity, but could replicate in Vero cells as efficiently as the parental virus. When the biological properties of L1BR1 were examined in mice by using four routes (footpad, intraperitoneal, corneal, and intracerebral) of infection, the mutant displayed the route-dependent reduction of virulence; after inoculation by footpad and intraperitoneal routes, the mutant was more than 10,000-fold less virulent than the parental virus, but it exhibited only about a 10-fold decrease in virulence following the corneal and intracerebral infection. In the intraperitoneal inoculation into adult mice, the replication of L1BR1 in the liver and spleen was severely restricted, but in newborn mice the mutant could grow as well as the parental virus in these organs. The adoptive transfer of peritoneal macrophages from adult mice resulted in a marked inhibition in the replication of L1BR1 in the liver and spleen of newborn mice, while the transfer exhibited little or no effect on the production of the wild-type virus in these organs. We also found that the mutant, unlike the parental virus, could not replicate in precultured peritoneal macrophages from adult mice. Taking these observations together, it seems likely that L1BR1 lost the ability to overcome the mononuclear-phagocytic defense system and thereby lost its pathogenicity by intraperitoneal and footpad routes. Furthermore, the mutant was shown to be rescued by a 4.8-kb HindIII/Xbal fragment containing the entire US3 open reading frame. However, we could not rule out the possibility that some of the phenotypes of L1BR1 are due to mutations in the US3-neighboring genes, US2 and US4.

The gamma 1(34.5) gene of herpes simplex virus 1 precludes neuroblastoma cells from triggering total shutoff of protein synthesis characteristic of programed cell death in neuronal cells

The gamma 1(34.5) gene of herpes simplex virus 1 was previously shown to play a role in viral virulence since deletion of the gene reduced by a factor of approximately 100,000 the capacity of the virus to replicate in the central nervous system and cause mortality in the mouse. Here we show that in the human neuroblastoma cell line SK-N-SH of neuronal origin gamma 1(34.5) null mutants expressed early proteins, viral DNA, and mRNA of late genes. However, the onset of viral DNA synthesis triggered complete cessation of incorporation of radioactive precursors into proteins. The mutant and wild-type viruses replicated and could not be differentiated in cell lines or cell strains of nonneuronal origin. The results indicate that in the absence of the gamma 1(34.5) gene the SK-N-SH neuroblastoma cells triggered a response similar to the programed cell death of neuronal cells induced by metabolic stress. The gamma 1(34.5) protein precludes this cell response possibly in order to enable the protein synthesis necessary for viral replication.

Use of single-gene reassortant viruses to study the role of avian influenza A virus genes in attenuation of wild-type human influenza A virus for squirrel monkeys and adult human volunteers

The transfer of six internal RNA segments from the avian influenza A/Mallard/New York/6750/78 (H2N2) virus reproducibly attenuates human influenza A viruses for squirrel monkeys and adult humans. To identify the avian influenza A virus genes that specify the attenuation and host range restriction of avian-human (ah) influenza A reassortant viruses (referred to as ah reassortants), we isolated six single-gene reassortant viruses (SGRs), each having a single internal RNA segment of the influenza A/Mallard/New York/6750/78 virus and seven RNA segments from the human influenza A/Los Angeles/2/87 (H3N2) wild-type virus. To assess the level of attenuation, we compared each SGR with the A/Los Angeles/2/87 wild-type virus and a 6-2 gene ah reassortant (having six internal RNA segments from the avian influenza A virus parent and two genes encoding the hemagglutinin and neuraminidase glycoproteins from the wild-type human influenza A virus) for the ability to replicate in seronegative squirrel monkeys and adult human volunteers. In monkeys and humans, replication of the 6-2 gene ah reassortant was highly restricted. In humans, the NS, M, PB2, and PB1 SGRs each replicated significantly less efficiently (P less than 0.05) than the wild-type human influenza A virus parent, suggesting that each of these genes contributes to the attenuation phenotype. In monkeys, only the NP, PB2, and possibly the M genes contributed to the attenuation phenotype. These discordant observations, particularly with regard to the NP SGR, indicate that not all genetic determinants of attenuation of influenza A viruses for humans can be identified during studies of SGRs conducted with monkeys. The PB2 and M SGRs that were attenuated in humans each exhibited a new phenotype that was not observed for either parental virus. Thus, it was not possible to determine whether avian influenza virus PB2 or M gene itself or a specific constellation of avian and human influenza A virus specified restriction of virus replication in humans.

Mapping neuroinvasiveness of the herpes simplex virus type 1 encephalitis-inducing strain 2762 by the use of monoclonal antibodies

Monoclonal antibodies (MAbs) directed against herpes simplex virus (HSV)-coded glycoproteins gB, gC, gD and gE were employed in a an in vitro model of neuroinvasiveness using sensory neurons from rat dorsal root ganglion (DRG) cells. The neurons were cultured in at two-chamber system allowing infection via the neuritic extensions exclusively. The effects of 30 MAbs on viral replication of the encephalitis-derived HSV-1 strain 2762 and its less neuroinvasive variant 2762p11 were assayed in this model. One MAb reactive with gD gave a nine-fold reduction of the virus yields of both strains. One MAb directed against gB gave an enhanced virus yield of strain 2762, but not of the 2762p11 variant. Another gB-reactive MAb decreased the virus yield of strain 2762p11, but not of 2762 after neuritic infection. The findings indicate that an alteration of gB has occurred during the passage of the strain 2762. Mutants of the same strain were derived by infecting hybridomas producing MAb reactive with gB, gC, gD and gE, respectively. The gB hybridoma mutant showed a significantly lower neuroinvasiveness in the DRG model, and was non-virulent after snout infection of mice. We suggest that the structure of gB of the strain 2762 is of importance for the neuroinvasiveness of this strain.

Mixed ocular infections identify strains of herpes simplex virus for use in genetic studies

Studies on the genetic mechanisms involved in the ocular virulence of herpes simplex virus (HSV) require the careful selection of parental strains. We used the technique of mixed ocular infection in vivo to identify strains of HSV for use in genetic studies. A pair of viruses (OD4 and 994) were identified that cause significantly more severe ocular disease when mixed together and used to infect the eyes of Balb/c mice compared to each strain when used alone. The mixed infection with OD4 and 994 did not result in increased neurovirulence. The technique of mixed ocular infections provides a sensitive screen to identify strains of virus that can act synergistically to cause more severe disease. Marker transfer can then be used to map the genes involved.

Complementary lethal invasion of the central nervous system by nonneuroinvasive herpes simplex virus types 1 and 2

It is well-known that viral thymidine kinase (TK) expression is important for the maximum demonstration of virulence of herpes simplex virus (HSV). In this study, we have investigated interactions of a TK- mutant of virulent HSV type 2 (HSV-2) (syn+) and a nonneuroinvasive HSV-1 (syn) in mice. When the mice were inoculated with each virus alone in their rear footpads, no mice were killed even after infection with high doses of viruses (greater than 10(6) PFU per mouse), whereas 100% of the mice died when inoculated with 10(5) PFU of a 1:1 mixture of HSV-2 TK- mutant and nonneuroinvasive HSV-1. The 1:1 mixture exhibited even more virulence than the parental HSV-2; the mean survival time of coinfected mice was significantly shorter than that of mice inoculated with 10(5) PFU of the virulent HSV-2. We have also examined the genotypes and phenotypes of viruses isolated from the central nervous system of coinfected mice. Of 50 isolates, 7 were judged to be recombinants from their restriction endonuclease cleavage patterns, but all were nonneuroinvasive. In addition, all syn+ viruses (23 clones) tested were found to have TK- phenotypes, indicating that the majority of viruses present in the central nervous system were TK- viruses, since about 90% of viruses detected in spinal cords and brains exhibited syn+ phenotypes. These results strongly suggest that the lethal invasion of the central nervous system by HSV-2 TK- and nonneuroinvasive HSV-1 was the consequence of in vivo complementation between the two viruses.

Attenuating mutations in the E2 glycoprotein gene of Venezuelan equine encephalitis virus: construction of single and multiple mutants in a full-length cDNA clone

Attenuated mutants of Venezuelan equine encephalitis virus (VEE) were isolated by selection for rapid penetration of cultured cells (R. E. Johnston and J. F. Smith, 1988, Virology 162, 437-443). Sequence analysis of these mutants identified candidate attenuating mutations at four loci in the VEE E2 glycoprotein gene: a double mutation at E2 codons 3 and 4, and single substitutions at E2 76, 120, and 209. Each candidate mutation was reproduced in an isogenic recombinant VEE strain using site-directed mutagenesis of a full-length cDNA clone of VEE. Characterization of these molecularly cloned mutant viruses showed that mutation at each of the four loci in the E2 gene was sufficient to confer both the accelerated penetration and attenuation phenotypes. Inoculation of the molecularly cloned viruses into rodent models that differ in their response to VEE suggested that individual mutations affected different aspects of VEE pathogenesis. Full-length clones containing multiple mutations were produced by combining independently attenuating mutations. Molecularly cloned viruses carrying two or three mutations were more attenuated in sensitive animal models than viruses which contained any single mutation alone. However, these highly attenuated strains still retained the ability to induce an immune response sufficient to protect against a high dose challenge with virulent VEE. These results indicate that production of a molecularly cloned live virus vaccine for VEE is feasible.

Seroepidemiology of herpes simplex virus and restriction endonuclease cleavages analysis of herpes simplex virus type 2 in Okinawa

A seroepidemiologic study of herpes simplex virus (HSV) in Okinawa was performed. A total of 423 serum samples were collected from all over Okinawa, and the positivity rate of antibody against HSV was measured using a passive hemagglutination method. The sero-positive rate for HSV in age groups of over 40 years was 100%. Seven HSV type 2 (HSV 2) isolates were obtained in Okinawa, and DNA preparations from Vero cells infected with the isolates were analyzed using five restriction endonucleases: Bam HI, Hind III, Kpn I, Bgl II and Eco RI. Variations in the genomic region were demonstrated in five of the isolates. Such variations have not been reported previously in HSV 2 in mainland Japan. This is the first report of a seroepidemiologic study of HSV and restriction endonuclease cleavage analysis of HSV 2 in Okinawa, is a subtropical island where HSV is endemic.

Mapping of herpes simplex virus-1 neurovirulence to gamma 134.5, a gene nonessential for growth in culture

The gene designated gamma 134.5 maps in the inverted repeats flanking the long unique sequence of herpes simplex virus-1 (HSV-1) DNA, and therefore it is present in two copies per genome. This gene is not essential for viral growth in cell culture. Four recombinant viruses were genetically engineered to test the function of this gene. These were (i) a virus from which both copies of the gene were deleted, (ii) a virus containing a stop codon in both copies of the gene, (iii) a virus containing after the first codon an insert encoding a 16-amino acid epitope known to react with a specific monoclonal antibody, and (iv) a virus in which the deleted sequences were restored. The viruses from which the gene was deleted or which carried stop codons were avirulent on intracerebral inoculation of mice. The virus with the gene tagged by the sequence encoding the epitope was moderately virulent, whereas the restored virus reacquired the phenotype of the parent virus. Significant amounts of virus were recovered only from brains of animals inoculated with virulent viruses. Inasmuch as the product of the gamma 134.5 gene extended the host range of the virus by enabling it to replicate and destroy brain cells, it is a viral neurovirulence factor.

Molecular and biological characterization of a herpes simplex virus type 1 (HSV-1) neuroinvasiveness gene

Pathogenetic studies of herpes simplex virus type 1 (HSV-1) strains ANG and its mouse brain-passaged descendant ANG path revealed no difference in neurovirulence but a significant difference in neuroinvasiveness. Thus, both viruses induced a fatal encephalitis in mice after direct injection into the brain, but only ANG path induced lethal neurologic disease after inoculation on rear footpads. The difference in neuroinvasiveness is not related to the capacity to replicate in mouse neural tissues or mouse cells in general, but is specifically related to virus entry into the peripheral nervous system in the footpad. Marker rescue experiments in which ANG path genes were used to confer neuroinvasiveness on ANG indicated that the gene that codes for glycoprotein D (gD) is responsible for the phenotypic difference. Analyses of the gD genes by dideoxy-sequencing techniques identified a base difference in the coding sequences and predicted that the ANG gD gene codes for alanine (GCC codon) at amino acid position 84 in the open reading frame and the ANG path gD gene codes for glycine (GGC codon) at this site. Using these data, an oligonucleotide probe predicted to be specific for the ANG path gD gene was prepared, and in Southern blot analyses, this probe revealed that neuroinvasiveness-rescued agents had incorporated the base change seen in the ANG path gD gene. We conclude that HSV-1 glycoprotein D functions to effect neuroinvasiveness and we discuss potential mechanisms that may be involved.

Nucleotide sequences responsible for the inability of a herpes simplex virus type 2 strain to grow in human lymphocytes are identical to those responsible for its inability to grow in mouse tissues following ocular infection

A study was undertaken to determine whether genes associated with herpes simplex virus (HSV) neuroinvasiveness in mice influence the growth of HSV in man, the virus's natural host. HSV-2(186), a nonneuroinvasive HSV strain, was found to replicate poorly (less than 3-fold) in cultures of phytohemagglutinin (PHA) stimulated human peripheral blood mononuclear cells (PBMC). In contrast, seven other HSV strains all multiplied 40- to 100-fold. The paucity of HSV-2(186) growth in PBMC was not due to a failure of this strain to grow in primary human cells because high titers (greater than 10(8) PFU/ml) were obtained following infection of human foreskin fibroblasts. The genetic basis for the deficient growth was analyzed by marker rescue experiments. Recombinant HSV-2 strains were generated in marker rescue experiments utilizing HSV-2(186) DNA and plasmids containing a cloned DNA polymerase gene isolated from a neuroinvasive HSV strain possessing the capacity to replicate in human PBMC. Progeny which rescued DNA from the cloned HSV DNA polymerase gene replicated 40- to 100-fold in PHA-stimulated PBMC. Moreover, unlike the HSV-2(186) parent, HSV-2(186) isolates possessing rescued DNA grew well in the eye, trigeminal ganglion, and brain of mice and induced fatal encephalitis. The results indicate that nucleotide sequences responsible for increasing the capacity of HSV-2(186) to grow in PBMC of man are identical to those responsible for increasing the capacity of this strain to grow in mouse tissues and to spread from the eye to the brain.

The herpes simplex virus 1 gene for ICP34.5, which maps in inverted repeats, is conserved in several limited-passage isolates but not in strain 17syn+

In a previous study, it was reported that herpes simplex virus 1 (HSV-1) strain F contains a transcribed open reading frame situated in the inverted repeats of the L component between the terminal a sequence and the open reading frame that encodes the alpha 0 gene (J. Chou and B. Roizman, J. Virol. 57: 629-637, 1986). By means of an antibody to repeats of the trimer Ala-Thr-Pro predicted to be specified by the open reading frame, it was shown that the open reading frame specifies a protein (M. Ackermann, J. Chou, M. Sarmiento, R. A. Lerner, and B. Roizman, J. Virol. 58: 843-850, 1986). This open reading frame is absent from the reported sequence of HSV-1(17)syn+ (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; L. J. Perry and D. J. McGeoch, J. Gen. Virol. 69: 2831-2846, 1988). To define the extent of variability in this open reading frame, we compared the sequences of the ICP34.5-encoding open reading frames of the genomes of three strains characterized by limited passage in cell culture with that of the HSV-1(17)syn+ strain. Furthermore, to establish unambiguously that the antibody to the Ala-Thr-Pro repeats reacts with the product of this open reading frame, we inserted a short sequence that encodes a known epitope in frame at the 5' terminus of the coding domain. Our results indicate that with minor variations, the open reading frame is conserved in the three HSV-1 genomes analyzed but not in HSV-1(17)syn+. Thus, two strains contain an inserted amino acid and one strain, isolated from a case of human encephalitis, lacks a seven-amino-acid sequence. The recombinant virus carrying the foreign epitope expressed a slightly slower-migrating protein which reacted with both the rabbit polyclonal antibody to the Ala-Thr-Pro trimer repeats and the monoclonal antibody to the inserted epitope. The implications of the results are discussed.

Herpes simplex virus neurovirulence and productive infection of neural cells is associated with a function which maps between 0.82 and 0.832 map units on the HSV genome

A herpes simplex virus (HSV) intertypic recombinant (RE6) has been shown to be completely and specifically non-neurovirulent in mice. Direct intracranial inoculation of 10(8) PFU of RE6 does not result in a lethal encephalitis. Neurovirulent recombinant viruses were generated by cotransfection of RE6 DNA with DNA fragments cloned from the pathogenic HSV-1 strain 17 syn+. It was found that a 1.6-kb fragment mapping between 0.82-0.832 m.u. could restore the neurovirulent phenotype. Recombinants which incorporated at least part of this fragment were at least 100,000-fold more neurovirulent than RE6. The recombinants displayed a greatly enhanced capacity to replicate in mouse brain in vivo, but did not display enhanced replication over that of RE6 in cultured mouse cells at 38.5 degrees. Immunohistochemical analysis of infected mouse brain tissue revealed that the permissive host cell range of the recombinants was dramatically altered from that of RE6. While antigen positive cells were extremely rare in mouse brain tissue infected with RE6, the neurovirulent recombinants produced antigens in many cell types including neurons. Thus, wild-type HSV-1 sequences mapping between 0.82-0.832 m.u. can donate a highly neurovirulent phenotype to RE6.

A herpes simplex virus transcript abundant in latently infected neurons is dispensable for establishment of the latent state

We have previously reported that a novel herpes simplex virus RNA transcript partially overlapping the gene encoding ICPO and expressed from the opposite DNA strand is abundant in sensory neurons of mice harboring a latent infection [J.G. Stevens, E.K. Wagner, G.B. Devi-Rao, M.L. Cook, and L.T. Feldman, Science 235, 1056-1059 (1987)]. This finding suggested that this transcript might be involved in establishment, maintenance, or reactivation of latent virus. To determine the function of this latency-associated transcript (LAT), we have examined the latency characteristics of a deletion mutant which is unable to express the LAT gene. Although no viral transcripts could be found in the lumbosacral ganglia of mice surviving rear footpad infection with this deletion virus, a latent infection had been established since infectious virus could be induced and detected after explanation and cocultivation of ganglia with permissive cells in culture. These results indicate that HSV-1 LAT expression is not an absolute requirement for establishment of the latent state.

Pathogenicity of glycoprotein C negative mutants of herpes simplex virus type 1 for the mouse central nervous system

A previous study from our laboratory showed that a mutant of herpes simplex virus type 1 (HSV-1), strain KOS-321, carrying a deletion in the structural gene for glycoprotein C (gC) had reduced pathogenicity for the mouse central nervous system when compared to the wild-type virus (Kümel et al., 1985). In this study, eight additional gC negative (gC-) mutants derived from KOS-321 were shown to vary widely in their ability to induce lethal encephalitis in female DBA/2 mice following intracerebral inoculation. This variation in virulence showed no correlation with thymidine kinase activity. One less virulent gC- strain, gC-39, was further studied to determine whether the neurovirulent phenotype could be restored by rescue of the gC gene using standard marker rescue cotransfection procedures. The resulting progeny contained 2% gC+ recombinant virions and was tested for its ability to cause encephalitis. Although this progeny had increased virulence, it was not attributable to the acquisition of the gC gene since passive immunization of mice with a pool of anti-gC monoclonal antibodies had no effect on the development of encephalitis and only gC- viruses were isolated from diseased brain tissues. In agreement with these findings, individual plaque-purified gC positive (gC+) virus recombinants were shown not to have been restored to the wild-type virus level of neurovirulence. It is concluded that gC is not a virulence determinant in this mouse model of HSV-induced encephalitis and that cotransfection procedures can induce additional mutations that affect viral pathogenesis.

Role of glycoprotein gIII of pseudorabies virus in virulence

Deletion mutants of pseudorabies virus unable to express glycoprotein gIII, gI, or gp63 or double and triple mutants defective in these glycoproteins were constructed, and their virulence for day-old chickens inoculated intracerebrally was determined. Mutants of wild-type pseudorabies virus defective in glycoprotein gIII, gI, or gp63 were only slightly less virulent (at most, fivefold) for chickens than was the wild-type virus. However, mutants defective in both gIII and gI or gIII and gp63 were avirulent for chickens, despite their ability to grow in cell culture in vitro to about the same extent as mutants defective in gIII alone (which were virulent). These results show that gIII plays a role in virulence and does so in conjunction with gI or gp63. The effect of gIII on virulence was also shown when the resident gIII gene of variants of the Bartha vaccine strain (which codes for gIIIB) was replaced with a gIII gene derived from a virulent wild-type strain (which codes for gIIIKa); gIIIKa significantly enhanced the virulence of a variant of the Bartha strain to which partial virulence had been previously restored by marker rescue. Our results show that viral functions that play a role in the virulence of the virus (as measured by intracerebral inoculation of chickens) may act synergistically to affect the expression of virulence and that the ability of the virus to grow in cell culture is not necessarily correlated with virulence.

Partial rescue of herpes simplex virus neurovirulence with a 3.2 kb cloned DNA fragment

A herpes simplex virus (HSV) intertypic recombinant (RE6) has been previously shown to be non-neurovirulent following direct intracranial inoculation of mice. An in vitro recombination/in vivo selection strategy was employed to further characterize the gene or genes responsible for the avirulence of this mutant. It was found that RE6 could be converted to a lethal virus by incorporation of wild-type HSV-1 sequences mapping between 0.698 and 0.721 map units. Restriction endonuclease and Southern blot analysis revealed that viral recombinants which incorporated at least part of the cloned 17syn+ sequences were selected by passage in mouse brains in vivo. The recombinants generated with this fragment were at least 50-fold more neuroviurlent than RE6, as determined by the plaque forming unit to lethal dose 50% assay. Further, they displayed a significant increase in ability to replicate in mouse brain tissue following intracranial inoculation. However, these recombinants did not display a replication advantage over RE6 in cultured mouse cells at 38.5 degrees C. Thus, the defect present within this region of the RE6 genome specifically affects replication in the nervous system. In the accompanying paper we analyze the RNA transcription and DNA sequence in this portion of the RE6 and parental strain genomes.

Comparative analysis of the transcripts mapped in the BamHI DNA fragment B of avirulent HSV-1 HFEM, virulent HSV-1 F, and their intratypic recombinant viruses

HSV-1 HFEM, whose genome harbors a deletion of 4.1 kbp (0.762 to 0.789 map units (mu] is avirulent for mice and tree shrews by the intraperitoneal (i.p.) application route. Insertion of the BamHI DNA fragment B (0.738 to 0.809 mu) and/or the MluI DNA fragment (0.7615 to 0.796 mu) molecularly cloned from virulent HSV-1 F, restored the i.p. pathogenicity to strain HFEM and led to the isolation of virulent intratypic recombinants. In order to determine the RNA transcripts mapped in the BamHI DNA fragment B of the HSV-1 HFEM, HSV-1 F, and their intratypic recombinants R15, R19, R26, and R-Ml-C1, a comparative analysis was performed using Northern blot hybridizations. Two novel RNA transcripts of 3.5 and 1.5 kb were detected which hybridize to the left terminus (0.738 to 0.746 mu) of the BamHI DNA fragment B. The 1.5 kb RNA transcript was missing in the avirulent HSV-1 HFEM. Hybridization with the BssHII DNA fragment F (0.760 to 0.762 mu) led to detection of a 3.5 kb RNA transcript by HSV-1 HFEM which was missing in all other viruses tested. In contrast a 1.5 kb RNA transcript was detectable in all other virus strains with the exception of HSV-1 HFEM. The 3.5 kb transcript hybridized to the right-hand flank of the deleted region in the genome of HSV-1 HFEM (Asp718/SalI DNA fragment; 0.786 to 0.79 mu). The detection of the novel 1.5 kb RNA, which is missing in HSV-1 HFEM, and the appearance of the newly transcribed 3.5 kb RNA in HSV-1 HFEM only, indicates a new open reading frame in this particular region as a consequence of the fusion of the DNA sequences at both ends of the deletion in the genome of HSV-1 HFEM.

Localization of a herpes simplex virus neurovirulence gene dissociated from high-titer virus replication in the brain

Previous studies with the herpes simplex virus type 1 X type 2 intertypic recombinant RS6 suggested that the genomic region from 0.11 to 0.14 map units is involved in neurovirulence (R. T. Javier, R. L. Thompson, and J. G. Stevens, J. Virol. 61:1978-1984, 1987). To study this further, we isolated an RS6-derived herpes simplex virus intertypic recombinant (R13-1) which has a genetic defect within this area. After inoculation into mouse brains, R13-1 was found to be approximately 10,000-fold less neurovirulent than either the wild-type type 1 or type 2 parental virus. However, R13-1 replicated in the mouse brain to titers resembling those of the wild-type parents. Further comparisons with wild-type counterparts indicated that R13-1 expressed equivalent levels of the enzyme thymidine kinase and replicated to intermediate levels in primary mouse embryo fibroblasts maintained at the normal body temperature for mice. Using marker rescue techniques combined with in vivo selection, we found that recombination between unit-length R13-1 DNA and a cloned type 1 DNA fragment spanning the region from 0.11 to 0.14 map units (EcoRI-d, 0.079 to 0.192 map units) generated viruses with a wild-type neurovirulence phenotype. To further refine the genomic region of interest, we performed marker rescue experiments using two EcoRI-d subclones, EcoRI/BamHI dc (0.079 to 0.143 map units) and BamHI/EcoRI and (0.143 to 0.192 map units), representing the left and right halves of the EcoRI d fragment, respectively. In these experiments the EcoRI/BamHI dc clone, but not the BamHI/EcoRI ad clone, yielded recombinant viruses exhibiting wild-type neurovirulence. These results show that at least one herpes simplex virus gene function associated with neurovirulence is located within a 9.1-kilobase region at 0.079 to 0.143 map units of the viral genome. Perhaps more significantly, the results indicate that this neurovirulence property functions independently of high-titer virus replication in the brain.

Evidence that the gene for herpes simplex virus type 1 DNA polymerase accounts for the capacity of an intertypic recombinant to spread from eye to central nervous system

HSV-1(17) replicates 100-fold more efficiently than HSV-2(186) within trigeminal ganglia following ocular infection. In order to identify the nucleotide sequences responsible for the differences in the capacity of the two HSV strains to grow within the peripheral nervous system, an intertypic recombinant was generated by infecting neuroblastoma cells with HSV-2(186) and a HSV strain possessing nucleotide sequences from HSV-1(17). The genome of the intertypic recombinant was composed entirely of HSV-2(186) DNA except for 2.0 kb of HSV-1(17) DNA positioned between m.u. 0.413 and 0.426. Following corneal infection of mice, the intertypic recombinant grew to higher titers in both ocular tissues and trigeminal ganglia than did the HSV-2 parent. Most significantly, the intertypic recombinant could spread into the brain from the trigeminal ganglion and kill the host whereas mice inoculated with the HSV-2(186) parent survived infection. The 2.0 kb of HSV-1(17) DNA inserted into the genome of the intertypic recombinant encodes the 5' terminus of the HSV-1 gene for DNA polymerase. Thus, the results suggest that the difference in the capacity of two HSV strains to replicate within the trigeminal ganglion of its host and to spread into the brain is determined by nucleotide sequences within the gene for DNA polymerase.

Two thymidine kinase deficient herpes simplex viruses exhibit unexpected virulence properties

Herpes simplex virus type 1 (HSV-1) strains ANG and ANG path expressed very low levels of thymidine kinase activity, and their replication in vitro was not significantly inhibited by arabinofuranosyl-thymine. However, after intracranial inoculation of mice, ANG and ANG path demonstrated median lethal doses and brain replication profiles that resembled the fully neurovirulent thymidine kinase positive strain 17 syn+. In short, ANG and ANG path demonstrated the neurovirulence characteristics of standard HSV strains that express high levels of thymidine kinase activity. In an extension of the pathogenesis studies, ANG path resembled 17 syn+ and killed mice after rear footpad inoculation. ANG, by contrast, did not kill after rear footpad inoculation. Thus, ANG path demonstrated neuroinvasiveness while ANG was apparently non-neuroinvasive. The significance of these results as they pertain to genes involved in pathogenesis is discussed.

Host and viral factors that influence viral neurotropism II. Viral genes, host genes, site of entry and route of spread of virus

In the previous article in this series, we focused on how the interaction between viral cell attachment proteins and receptor molecules on the surface of host target cells played a major role in determining the cell and tissue tropism of many neurotropic viruses. In order to complete our review of viral factors that influence the tropism of viruses for the CNS, we will discuss the role of viral genes that function to specifically enhance the replication of viral proteins in certain cells or tissues (“tissue-specific enhancers and promoters”). We will then examine the ways in which host factors, including specific host genes, can influence resistance or susceptibility to certain types of neurotropic viral infections. Finally, we will conclude by reviewing how factors that involve the interaction of the host and the virus, such as the site of the viral entry and its route and method of spread, can influence the distribution of viral infection within the CNS.