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

Recent developments in latency and recombination of Aujeszky's disease (pseudorabies) virus

Latency is a characteristic and fascinating part of the biology of alphaherpesvirinae, including ADV. Tissue explanation, blot hybridization, in situ hybridization and more recently PCR are the experimental methods used to demonstrate that latent infections consistently occur in ganglionic neurons and, at a lower level, in tonsillar and possibly other cells. In vivo reactivation of ADV, resulting in shedding of virulent ADV, has been demonstrated experimentally following administration of high doses of corticosteriods. To determine the influence of vaccination with currently used gene deleted vaccines on field virus latency load, it is essential to use quantitative latency detection methods. We have developed chemiluminescence-based quantitative PCR assays specific for gG and gE, and are currently using these to determine field virus latency loads in tissues of pigs vaccinated with one of several gene deleted vaccines. Recombination between ADV strains has been demonstrated both in vitro and in vivo and has raised concerns about the generation of gene deleted virulent ADV strains. Recent studies in a mouse model have shown that high concentrations of both strains have to be present at the same anatomical site for recombination to take place. This led to the conclusion that ongoing ADV eradication programs, based upon the use of gene deleted vaccines and differential serological testing, are not likely to be threatened by recombination between virulent ADV and gene deleted vaccine strains.

Induction of Fos protein expression in spinal cord neurons by herpes simplex virus infections in the mouse

After the inoculation of a highly attenuated strain of herpes simplex virus (HSV) type 1 into the mouse hind-paw planter skin, a few neurons and glial cells were viral antigen-positive in the dorsal root ganglia (DRGs) and anterior horn of L4 and L5 segments of the lumbar spinal cord. Fos-containing nuclei, however, were not found in the DRGs or the lumbar cord segments. After the inoculation of a highly neuroinvasive strain of HSV type 2, many virus-positive neurons and glial cells were detected in the DRGs, anterior and posterior horns at L4 and L5; Fos-containing nuclei were also observed in spinal neurons except for motoneurons. The results indicate that transneuronal infection of HSV induces Fos expression in spinal neurons.

Immunohistochemical studies on the transneuronal spread of virulent herpes simplex virus type 2 and its US3 protein kinase-deficient mutant after ocular inoculation

The transneuronal spread of a virulent wild-type herpes simplex virus type 2 (HSV-2) and its US3 protein kinase-deficient (US3 PK-) mutant was immunohistochemically studied in mice after inoculations into the cornea, anterior chamber, tongue, and masseter muscle. After corneal inoculation, the wild-type virus was demonstrated in various brain stem areas including the trigeminal tract and nucleus, the reticular formation, and cerebellar nucleus group. Viral antigen-positive neurons were strictly confined to the ipsilateral spinal trigeminal nucleus in mice corneally infected with the US3 PK- mutant. No viral antigens were detected in the central nervous system (CNS) after inoculation with the mutant into the tongue and masseter muscle. However, when mice were immunosuppressed by treatment with cyclophosphamide, both the corneally infected mutant and wild-type virus could invade the CNS. The results suggest that the US3 PK- mutant principally retains the capacity to spread in the CNS.

Growth and cytopathogenicity of herpes simplex virus in a macrophage cell line, RAW264: A good indicator of intraperitoneal pathogenicity

Macrophages are known to play a critical role in host resistance to herpes simplex virus (HSV). In this study, we investigate the interaction between various HSV strains with different virulence and a murine macrophage cell line, RAW264. Highly attenuated strains replicated poorly in RAW264 cells and were cleared from the cultures. For the eleven viruses tested, there was good correlation between intraperitoneal pathogenicity for adult mice and replication in RAW264 cells. It was also shown that interferon alpha/beta was involved in restricted replication of some strains.

Analysis of the relationship between cellular thymidine kinase activity and virulence of thymidine kinase-negative herpes simplex virus types 1 and 2

The virulence of thymidine kinase-negative herpes simplex virus type 1 (HSV-1; VRTK- strain) and type 2 (HSV-2; UWTK- strain) was studied in comparison with that of their parental strains (VR-3 and UW-268, respectively) in an encephalitis model of adult (4-week-old) and newborn (3-day-old) mice. Viral thymidine kinase (TK) activity was essential for the maximum expression of virulence of HSV-1, because the 50% lethal dose (LD50) of VRTK- was 60 times higher than that of VR-3 in the brains of newborn mice expressing high levels of cellular TK activity. However, the UWTK- strain showed that replication of the UWTK- strain was completely supported by cellular TK activity. This difference in the role of viral and cellular TKs for virus growth between HSV-1 and HSV-2 was¿ confirmed with the one-step growth of virus strains in L-M and L-M(TK-) cells.

Genetic recombination of pseudorabies virus: evidence that homologous recombination between insert sequences is less frequent than between autologous sequences

We studied in vivo recombination between a thymidine kinase (TK) negative, glycoprotein E (gE) negative, attenuated strain and a virulent strain of pseudorabies virus (PRV) in pigs. To simplify the detection of recombination we inserted different but overlapping (375 bp) parts of the E1 gene of classical swine fever virus into the gG locus of both virus strains. Recombination between the E1 sequences of these viruses results in reconstitution of the complete E1 coding sequence and expression of the E1 protein. Since E1 is highly immunogenic, we expected to detect in vivo recombination in co-inoculated pigs by the presence of serum antibodies against E1. However, after co-inoculation of pigs with high doses of both virus strains, we were unable to detect antibodies against E1, suggesting that in vivo recombination did not occur or remained below the detection limit. Analysis of individual progeny viruses showed that 13 out of 995 (1.3%) possessed a recombinant TK-negative gE-positive phenotype. In contrast, no E1-positive viruses were detected among 5000 analyzed. This result showed that in vivo recombination between the two virus strains did occur, but was much more frequent between the TK and gE loci than between the E1 sequences. Similar results were obtained in in vitro recombination experiments in which possible growth differences between the various virus strains were excluded. The different recombination frequencies could not be attributed to the difference in distance of the genetic loci since recombination between mutations at a distance of 266 bp in the TK gene occurred as frequent as recombination between the TK and gE genes which are separated by approximately 60 kilobasepairs. These results indicate that some property of the E1 sequence and/or the location of the E1 sequence within the PRV genome affects the frequency of recombination.

Recombinants are isolated at high frequency following in vivo mixed ocular infection with two avirulent herpes simplex virus type 1 strains

Mixed infections with different strains of herpes simplex virus type 1 (HSV-1) may result in more severe disease than infection with either strain alone. This phenomenon is important because it may facilitate the identification of virulence genes through the transfer of virulence determinants between complementing strains, and it may pose a problem in the use of attenuated HSV strains for vaccines and gene delivery vectors. In this study, we have compared the percentage of recombinants present after mixed infection with HSV-1 strains OD4 and 994 in vitro and in vivo. After corneal inoculation, we found that 74% of randomly picked isolates from the trigeminal ganglia were recombinants, compared with 59% from the cornea. Twenty-six percent of randomly picked isolates were recombinant following mixed infection of Vero cells in vitro. Seventeen recombinant strains isolated from the in vivo mixed infections were assayed for ocular virulence, and they were found to exhibit a wide range of virulence phenotypes. The presence of virulent recombinants suggests that recombination plays a role in the increased disease observed in this mixed infection, and the broad range of virulence indicates that there may be multiple genetic factors involved in the increased virulence observed after mixed infection with these two strains. The recombinants were also tested for their ability to grow in NIH 3T3 fibroblasts, and though some correlation was observed between growth in vitro and ability to cause ocular disease, improved growth in murine cells does not sufficiently explain the increased virulence observed in some recombinants.

In vivo recombination of pseudorabies virus strains in mice

We studied in vivo recombination of pseudorabies virus (PRV) by inoculating mice with non-lethal mutants that carry a small deletion or insertion in the thymidine kinase (TK) gene or the ribonucleotide reductase (RR) gene. After co-inoculation of mice with two different mutants, homologous recombination between the viral genomes resulted in the generation of wild-type PRV that was highly lethal for mice. Thus, recombination could easily be assessed by monitoring survival of inoculated animals. Our results demonstrated that recombination was only detectable when high doses of virus were used. Intragenic recombination was more efficient between mutations in the TK gene than between mutations in the RR gene. Efficient intragenic recombination in the TK gene occurred between mutations which were separated by as few as 266 nucleotides. When two mutants were inoculated with an interval of 2 h, recombination still occurred. No recombination could be detected when the viruses were inoculated at the same time but in separate parts of the body. When inoculated separately, none of the mutants tested could be isolated from the brains of mice. Virus could be recovered from the brain, however, after co-inoculation. Surprisingly, of these viruses 36-39% possessed the parental mutant genotype. This observation indicates that complementation enables these mutants to replicate in the brain and suggests that complementation may contribute to pathogenicity of PRV.

The pathogenicity of a US3 protein kinase-deficient mutant of herpes simplex virus type 2 in mice

We have investigated the pathogenicity of a US3 protein kinase-deficient mutant (L1 BR1) of herpes simplex virus type 2 (HSV-2) for 4-week-old ICR mice to define the role of the viral protein kinase in virus-host interaction. When mice were intraperitoneally infected with 10(5)PFU of L1 BR1, the virus disappeared from the peritoneal cavity by 2 days postinfection and failed to induce any significant histopathological changes in the liver and spleen although viral antigens were occasionally detected in the epithelial cells of small bile ducts and small vascular wall. The parental virus (HSV-2 186) and a revertant of the mutant (L1 B-11) both caused severe hepatitis, and viral antigens were clearly detected in the hepatocytes and Kupffer cells in the focal necrotic areas. Both of the virulent viruses, unlike L1 BR1, could produce infectious progeny and cytopathic effects in freshly harvested peritoneal macrophages. The growth of L1 BR1 in peritoneal macrophages was restricted at a stage of or prior to viral DNA synthesis but after the induction of viral DNA polymerase. In addition, the production and/or the spread of mutant in mouse embryo fibroblasts (MEF) was found to be much more effectively suppressed by cocultivation of peritoneal macrophages than that of 186. An almost complete inhibition of L1 BR1-plaque formation was observed at a macrophage-to-MEF ratio of 4:1. These results suggest that the attenuation of L1 BR1 following intraperitoneal infection is primarily due to its high sensitivity to intrinsic and extrinsic inhibition of peritoneal macrophages and that the US3 protein kinase may play a role in viral DNA replication in peritoneal macrophages.

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.