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

Herpesvirus DNA polymerase: Structures, functions, and mechanisms

Herpesviruses comprise a family of DNA viruses that cause a variety of human and veterinary diseases. During productive infection, mammalian, avian, and reptilian herpesviruses replicate their genomes using a set of conserved viral proteins that include a two subunit DNA polymerase. This enzyme is both a model system for family B DNA polymerases and a target for inhibition by antiviral drugs. This chapter reviews the structure, function, and mechanisms of the polymerase of herpes simplex viruses 1 and 2 (HSV), with only occasional mention of polymerases of other herpesviruses such as human cytomegalovirus (HCMV). Antiviral polymerase inhibitors have had the most success against HSV and HCMV. Detailed structural information regarding HSV DNA polymerase is available, as is much functional information regarding the activities of the catalytic subunit (Pol), which include a DNA polymerization activity that can utilize both DNA and RNA primers, a 3'-5' exonuclease activity, and other activities in DNA synthesis and repair and in pathogenesis, including some remaining to be biochemically defined. Similarly, much is known regarding the accessory subunit, which both resembles and differs from sliding clamp processivity factors such as PCNA, and the interactions of this subunit with Pol and DNA. Both subunits contribute to replication fidelity (or lack thereof). The availability of both pharmacologic and genetic tools not only enabled the initial identification of Pol and the pol gene, but has also helped dissect their functions. Nevertheless, important questions remain for this long-studied enzyme, which is still an attractive target for new drug discovery.

Penciclovir and pathogenesis phenotypes of drug-resistant Herpes simplex virus mutants

We compared the penciclovir susceptibilities and pathogenesis phenotypes of mutants of Herpes simplex virus type 1 that are resistant to acyclovir and/or foscarnet. The mutants, which were derived from laboratory strain KOS, included six DNA polymerase mutants, a thymidine kinase negative mutant, a thymidine kinase partial mutant, and a double mutant. Two of four polymerase mutants not previously examined for penciclovir susceptibility exhibited modest resistance to this drug. A thymidine kinase negative mutant exhibited approximately 20-fold resistance while a thymidine kinase partial mutant was penciclovir-sensitive. Following intracerebral inoculation of 7-week old CD1 mice, the mutants ranged from exhibiting near wild-type neurovirulence (thymidine kinase partial) to modest attenuation (e.g. thymidine kinase negative) to more severe attenuation. Following corneal inoculation, three polymerase mutants exhibited modest deficits (relative to those of thymidine kinase negative mutants) in their abilities to replicate acutely in the ganglion and reactivate from latency. For mutant AraA(r)13, the deficit in ganglionic replication was shown to be due to its polymerase mutation by analysis of recombinant viruses derived by marker rescue. These results may have implications for issues of penciclovir action and resistance, for drug resistance in the clinic, and for the interactions of herpes viruses with the peripheral and central nervous systems.

An improved rapid method for purification of herpes simplex virus DNA using cesium trifluoroacetate

A method for purification of herpes simplex virus DNA from cell culture is described which yields highly purified viral DNA within 8 h. The method involves the freezing and thawing of virus-infected cells followed by isopycnic centrifugation of the lysate supernatant in cesium trifluoroacetate. It was found that this method recovered DNA from most of the cell-associated virus particles in such sufficient purity that the DNA was digestible with restriction enzymes and could be used to transfect cells without the need for additional purification steps. Purification of viral DNA from cells that were not subjected to freezing and thawing was less efficient due to the amount of viral DNA that remained cell-associated.

Polygenic control of neuroinvasiveness in California serogroup bunyaviruses

The pathogenesis of the California serogroup bunyaviruses includes both extraneural and intraneural replicative phases that can be separated experimentally. The present study dissects the viral genetic determinants of extraneural replication. We have previously described two attenuated reassortant clones of California serogroup bunyaviruses which exhibit reduced neuroinvasiveness after subcutaneous inoculation into suckling mice. Clone B1-1a bears an attenuated middle RNA segment (neuroinvasiveness phenotype v alpha v), and clone B.5 bears an attenuated large RNA segment (neuroinvasiveness phenotype alpha vv). We prepared reassortant viruses between these two strains and found that the two attenuated gene segments acted independently and additively, since reassortants bearing two attenuated RNA segments were more attenuated than the parental clones. Reassortants bearing no attenuated RNA segments were much more neuroinvasive than either parental clone, indicating that a neuroinvasive strain can be derived from two attenuated clones. Pathogenesis studies demonstrated that after injection of 10(3) PFU, the attenuated reassortant clones did not replicate in peripheral tissue, failed to reach the brain, and did not cause disease. At a dose of 10(6) PFU, attenuated clones failed to replicate to a significant level in peripheral tissue and produced only a minimal passive plasma viremia during the first 24 h but nevertheless reached high titers in the brain and killed mice. Because of this result, we investigated the possibility that neuroinvasion occurs via retrograde axonal transport, by determining whether sciatic nerve sectioning could protect against virus infection after hind leg footpad inoculation. We found that nerve sectioning had no effect on lethality, ruling out this mode of entry and suggesting that passive viremia is likely to be sufficient for invasion of the central nervous system.

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.

Anterograde transport of HSV-1 and HSV-2 in the visual system

The anterograde spread of herpesvirus in the visual system subsequent to retinitis has been observed clinically. We compared the ability of two well-studied Herpes simplex virus (HSV) strains to be transported in the anterograde direction in the hamster visual system: strain McIntyre, representing HSV-1, and strain 186, representing HSV-2. Intravitreal injection of HSV-2 labeled more retinorecipient neurons than did HSV-1, suggesting important type differences in the ability of HSV to infect retinorecipient neurons after intravitreal injection. The most likely explanation for our results is that HSV-2 is more efficiently adsorbed than HSV-1 in the retinal ganglion cells. Our results also suggest that HSV may be useful as an anterograde transneuronal tracer for neuroanatomical studies of the visual system.

Neuroattenuation of an avirulent bunyavirus variant maps to the L RNA segment

The derivation and characterization of a neuroattenuated reassortant clone (RFC 25/B.5) of California serogroup bunyavirus was described previously (M. J. Endres, A. Valsamakis, F. Gonzalez-Scarano, and N. Nathanson, J. Virol. 64:1927-1933, 1990). To map the RNA segment responsible for this attenuation, a panel of reassortants was constructed between the attenuated clone B.5 (genotype TLL) and a virulent clone (B1-1a) of reciprocal genotype (LTT). Parent viruses and clones representing all of the six possible reassortants were examined for neurovirulence by intracerebral injection in adult mice. Reassortants bearing the large RNA segment from the virulent parent were almost as virulent as the virulent parent virus, while reassortants bearing the large RNA segment from the avirulent parent virus exhibited low or intermediate virulence. These results indicate that the large RNA segment is the major determinant of neuroattenuation of clone B.5. In addition to its neuroattenuation, clone B.5 was temperature sensitive and exhibited an altered plaque morphology. These phenotypes also segregated with the large RNA segment. The importance of the large RNA segment (which encodes the viral polymerase) in neurovirulence contrasts with prior studies which indicate that the ability to cause lethal encephalitis after peripheral injection of suckling mice (neuroinvasiveness) is primarily determined by the middle-sized RNA segment, which encodes the viral glycoproteins.

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.

Differences in the capacity of two herpes simplex virus isolates to spread from eye to brain map to 1610 base pairs of DNA found in the gene for DNA polymerase

A intertypic recombinant, designated HSV-R(D1), had previously been generated from non-neuroinvasive HSV-2(186) and neuroinvasive HSV-1(17). Although the recombinant contained less than 2% of the HSV-1 genome, it retained the neuroinvasive phenotype. The nucleotide sequences responsible for the neuroinvasiveness of HSV-R(D1) were previously mapped to a 3.0 kb segment of DNA located within the DNA polymerase gene (mu 0.414 to 0.430) via marker rescue experiments. We have now sequenced this region and compared our results to the published nucleotide sequence of the HSV-1(17) and HSV-2(186) DNA polymerase genes. It was found that the 3.0 kb HSV-R(D1) DNA fragment consisted entirely of HSV-2(186) nucleotide sequences except for the presence of 1610 bp of HSV-1(17) DNA. The 1610 bp of HSV-1 DNA coded for a 536 amino acid (AA) region which were located between AA 254 and 790 of the DNA polymerase enzyme. Comparison of the 536 AA sequence of neuroinvasive HSV-1(17) with the homologous area of the non-neuroinvasive HSV-2(186) DNA polymerase indicated that the two polymerases differed at 56 AA positions. In addition, this area of the HSV-1(17) DNA polymerase was 5 AA acids shorter than the HSV-2(186) DNA polymerase. Specific amino acid changes that might account for the neuroinvasive phenotype of HSV-R(D1) are discussed.

The use of molecular techniques in studying viral pathogenesis in the nervous system

Molecular biological techniques have been used extensively to gain insights into the pathogenesis of a variety of diseases caused by neurotropic viruses. Increasingly sensitive methods for detecting viral nucleic acids and proteins in pathological nervous tissues have clarified the viral aetiology of certain neurological disorders and are now being used to investigate possible viral involvement in others. In addition, a diverse range of molecular techniques has greatly enhanced our understanding of the molecular basis of viral neurotropism and neurovirulence in both humans and experimental animals. This article provides an overview of these various approaches with examples drawn from both clinical neurological disease and animal models.

Determinants of the ability of malignant fibroma virus to induce immune dysfunction and tumor dissemination in vivo

The relationship of virus-induced immunological dysfunction and tumor dissemination was studied using two related tumor-causing leporipoxviruses: malignant fibroma virus (MV) and Shope fibroma virus (SFV). Recombinant viruses, produced by transferring MV's 10.7 kb BamHI C fragment to SFV, replicate in lymphocytes and suppress lymphocyte function in vitro. Those recombinants that replicate in lymphocytes and suppress lymphocyte function in vitro share about 3.5 kb from MV's C fragment. Some recombinants mimic MV in producing immune suppression and disseminated virus infection in vivo. Other recombinants, even some that are highly immunosuppressive in vitro (e.g. R71), only variably induce immune suppression in vivo, and do not cause disseminated disease. A segment of DNA from MV that transfers to Shope fibroma virus almost all of MV's virulence in vivo was identified.

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.

Ocular avirulence of a herpes simplex virus type 1 strain is associated with heightened sensitivity to alpha/beta interferon

BALB/c mice infected on the scarified cornea with herpes simplex virus type 1 strain 35 [HSV-1(35)] rarely developed ocular disease even at challenge doses as high as 10(7) PFU per eye. In contrast, HSV-1(RE) consistently induced stromal keratitis at an inoculum of 2 x 10(4) PFU. The goal of this study was to determine the reason for the difference in virulence between the two HSV strains. Both HSV-1 strains replicated to similar titers in excised corneal "buttons." However, after in vivo infection of the cornea, the growth of strain 35 was evident only during the first 24 h postinfection, whereas the replication of strain RE persisted for at least 4 days. In vitro tests revealed that HSV-1(35) was greater than 10 times more sensitive to alpha/beta interferon (IFN-alpha/beta) than HSV-1(RE). Both strains induced comparable serum levels of IFN after intraperitoneal inoculation. The kinetics of HSV-1(35) clearance from the eye was markedly altered by treatment with rabbit anti-IFN-alpha/beta. Virus titers exceeding 10(4) PFU per eye could be demonstrated 4 to 5 days postinfection in mice given a single inoculation of antiserum 1 h after infection. Furthermore, anti-IFN treatment in 3-week-old mice infected with HSV-1(35) led to the development of clinically apparent corneal disease which subsequently progressed to stromal keratitis in the majority of recipients. These results indicate that the striking difference in the capacity of HSV-1(35) and HSV-1(RE) to induce corneal disease was related to the inherently greater sensitivity of strain 35 to IFN-alpha/beta produced by the host in response to infection.

Neurovirulence of herpes simplex virus types 1 and 2 isolates in diseases of the central nervous system

Herpes simplex virus (HSV) isolates derived from the central nervous system of ten patients with HSV-1-induced encephalitis, one patient with multiple sclerosis, and 14 patients with HSV-2-induced meningitis were investigated for neurovirulence by assaying the LD50 after nose and intracerebral (i.c.) inoculation of mice. HSV-1 encephalitis strains were significantly more virulent after nose inoculation (i.e. neuroinvasive) when compared with HSV-1 isolates from patients with oral lesions only, whereas HSV-2 meningitis strains were significantly more virulent after i.c. inoculation when compared with HSV-2 isolates from patients with genital lesions only. No correlation between high neurovirulence (defined as low LD50 for both routes of infection) and replication in cell cultures of neuronal and non-neuronal cell lines was found, but the weakly neurovirulent HSV-1 strain isolated from a patient with multiple sclerosis gave low replication yields. After nose inoculation, a highly neuroinvasive HSV-1 laboratory reference strain replicated to high titers in nose tissue, the trigeminal ganglia and brainstem, while a strain with low neuroinvasiveness but high i.c. virulence replicated less well in the brainstem. Neuroinvasiveness of the virus strain might be one factor of relevance in the pathogenesis of HSV-1 encephalitis in man.

Characterization of RNA transcripts from herpes simplex virus-1 DNA fragment BamHI-B

Herpes simplex virus type 1 (HSV-1) DNA fragment BamHI-B (0.738-0.809 map units) was recently reported to be associated with the phenotype of intraperitoneal pathogenicity and to encode a latency-associated RNA transcript. Part of this fragment resides within the internal repeat sequence of the long (L) region of the viral genome. In this study, RNA transcripts from BamHI-B were characterized. In addition to immediate-early mRNAs IE-1 and IE-2, eight novel RNA species were found. Three transcripts were mapped in the repeat regions of this fragment and five transcripts in the unique L region of BamHI-B. In addition, transcription activity from these regions was compared in several HSV-1 strains. These included the intraperitoneal virulent F and KOS strains, the avirulent strain HFEM, as well as the HFEM/F intratypic virulent recombinant R-MIC1. Several differences were noted and their possible relevance to virulence is discussed.