Immunohistochemical evaluation of murine HSV-1 keratouveitis

The intraocular inflammatory reaction in a murine model of herpes simplex keratouveitis was studied using immunohistochemical techniques. 5 x 10(5) PFU of HSV-1 (Stewart strain) was applied to the abraded cornea of BALB/c mice. The subsequent development of keratouveitis was documented by sequential examination, conventional histology and immunohistochemical staining of frozen sections with the following monoclonal antibodies: Thy-1, Lyt-1, Lyt-2, Ia, Mac-1 and Lyb-8.2. At one week (in the early stage) there was moderate central corneal edema with inflammatory cell infiltration, consisting of a predominance of polymorphonuclear leukocytes, with the appearance of helper/inducer T-cells(Th/i). At two weeks (in the middle stage), a geographic corneal ulcer developed and the majority of inflammatory cells were still polymorphonuclear leukocytes. However, there was a further increase in the Th/i cell population, with the first appearance of B-cells. At three weeks (in the late stage), a dense stromal corneal scar was observed and there was a decrease in the inflammatory cell population, with the predominant cell being the B-cell. Throughout the evolution of HSV-1 keratouveitis, a mild inflammatory reaction was observed in the uvea. However, the number of inflammatory cells present was too small to allow more definitive characterization. Patchy Ia staining which is noted in the normal choroid increased markedly throughout the keratouveitis.

Herpes simplex virus infections: new concepts in an old disease

This paper reviews recent clinical and laboratory investigations on herpes simplex viruses (HSV). New findings add to our knowledge about factors influencing HSV infection such as site of infection, virus type and they also point to the importance of the immune system for the outcome of infection. It appears that circumstances at the time of primary infection can modify the type of immune response generated and affect the likelihood of later recrudescence. One of these factors is UV light which seems to act via a photoisomer [cis(Z-)urocanic acid] on the immune system.

Coevolution of virulent virus and resistant cells as a mechanism of persistence of herpes simplex virus type 1 in a human T lymphoblastoid cell line

Infection of the lymphoblastoid CEM cell line with herpes simplex virus (HSV) type 1 results in a persistent infection with production of infectious virus. Evidence suggests that the persistent infection was not maintained by interferon or non-interferon-soluble antiviral inhibitors. Treatment of persistently infected cells with anti-HSV serum (termed CEMACR cells) or elevated temperature (39 degrees C) for 14 days (termed CEMTCR cells) resulted in loss of evidence of virus. HSV DNA was not detected in CEMACR or CEMTCR cells by Southern blot or in situ hybridization. The CEMACR or CEMTCR cells, however, were resistant to reinfection with homologous, parental virus (HSV0), but were susceptible to heterologous virus (vesicular stomatitis virus). Resistance to reinfection with HSV was not absolute; CEMACR or CEMTCR cells were less permissive to virus isolated from persistently infected cultures at times early in the course of infection, but were more permissive for HSV isolated at later times. Virus isolated later during persistent infection also displayed progressively increased virulence for the parental CEM cells. These results suggest that persistent infection of a human T lymphoblastoid cell line, CEM, with HSV-1 is maintained by a genetically determined cell-virus equilibrium, in which the resistance of cells and virulence of virus increase during persistence.

Organotropism of latent herpes simplex virus type 1 is correlated to the presence of a 1.5 kb RNA transcript mapped within the BamHI DNA fragment B (0.738 to 0.809 map units)

The transcriptional activity of the DNA sequences within the genome of herpes simplex virus type 1 (HSV-1) at the coordinates 0.760 to 0.762 and their influence in the process of viral latency were investigated. Seven avirulent HSV-1 strains (HFEM, 1752/2, 1752/3, 1752/11, 1469, 1475, 1618), two virulent wild-type HSV-1 strains (F and 17) and three virulent intratypic HSV-1 recombinant viruses (R19, R26, RM1C1) were screened. The virulent HSV-1 strains colonize the ganglia but the avirulent virus strains are only able to persist in the spleen of infected animals (tree shrews). A 1.5 kb RNA transcript was detectable in all virus strains recovered from the ganglia. This RNA transcript hybridised to the HSV-1 DNA sequences at the genome coordinates 0.760 to 0.762 (BssHII DNA fragment F, part of the BamHI DNA fragment B of HSV-1, 0.738 to 0.809 map units (m.u.]. In contrast it was found that the 1.5 kb RNA transcript was missing or its size was changed in cells infected with those HSV-1 strains which were recovered from the spleens of latently infected animals. The state of viral latency of three defined deletion variants of HSV-1 strain 17 (1704, 1705, and 1706) whose genome harbors deletions (2.2 to 5.3 kb) comprising the DNA sequences of the particular region (0.760 to 0.762 m.u.) was investigated. These studies revealed that all three deletion variants could only be recovered from the spleens of latently infected tree shrews.

Ability of herpes simplex virus (HSV) types 1 and 2 to induce clinical disease and establish latency following previous genital infection with the heterologous HSV type

Guinea pigs were infected intravaginally with either herpes simplex virus type 1 (HSV-1) or type 2 (HSV-2). One month postinoculation, animals were inoculated with the heterologous HSV type and observed for clinical disease and shedding of virus. One month after superinfection, animals were killed, and tissues were cocultivated to detect latent virus. Although the severity of clinical disease and the degree of shedding of virus were greatly reduced by prior infection with the heterologous virus type, superinfection did occur in 82%-90% of animals. Nervous system latency with the superinfecting virus was established in 20% of animals superinfected with HSV-2 and 55% superinfected with HSV-1. Of 21 animals tested, 5 had latent infection with both viruses, 6 with the superinfecting virus only, and 6 with the initial virus only. Protection from nervous system latency with the superinfecting virus correlated best with levels of serum neutralizing HSV antibody before superinfection.

Variation in the number of sites of latency of herpes simplex virus in trigeminal ganglia of inbred mice

Following uniocular topical corneal inoculation with herpes simplex virus type 1 (HSV), 176-fold more virus was recovered by 14-day cultivation in vitro from latently infected ipsilateral trigeminal ganglia (TG) of BALB/c mice than from TG of C57BL/6 mice (p = 0.002). Since these quantitative differences may reflect a difference in the number of latently infected cells or a difference in the ability of virus to replicate in secondarily infected cells during cultivation in vitro, experiments were designed to estimate the actual number of sites of latency. Two to four months after topical corneal inoculation, when no active ocular disease was present, minced TG were digested with 2% collagenase. The dissociated cells were placed on monolayers of vero cells, incubated at 31 degrees C, and observed for cytopathic effect (CPE) for 14 days. Ipsilateral TG from BALB/c mice produced five-fold more foci of infection than TG from C57BL/6 mice (p = 0.007). The number of foci of infection was also dependent upon the dose of virus used to infect the eye. Following infection with high doses of HSV, virus was reactivated from contralateral TG, but in lower numbers than from ipsilateral TG. In vitro studies showed that the replication of virus in ganglia from BALB/c mice was 3-8-fold greater than that in ganglia from C57BL/6 mice. These data support the hypothesis that host genetic factors and the number of infectious particles inoculated influence the number of latently infected cells in the trigeminal ganglion after corneal infection with HSV.(ABSTRACT TRUNCATED AT 250 WORDS)

The carboxy-terminal 41 amino acids of herpes simplex virus type 1 glycoprotein B are not essential for production of infectious virus particles

Glycoprotein B (gB) is a virally encoded protein that is found in the envelope of herpes simplex virus type 1 and membranes of cells infected with herpes simplex virus type 1. It is essential for the production of infectious virus particles. An amber mutation was introduced into the gB gene by oligonucleotide-directed mutagenesis at the codon for amino acid 863 of the protein. Virus carrying this mutation should synthesize gB molecules lacking the last 41 amino acids of the cytoplasmic domain. Immunoprecipitation of infected cell extracts demonstrated the synthesis of appropriately truncated gB molecules. Characterization of the mutant virus indicated that the loss of the carboxy-terminal 41 amino acids has little effect on gB function.

Removal of N-linked carbohydrates decreases the infectivity of herpes simplex virus type 1

Purified preparations of herpes simplex virus type 1 Angelotti were digested with the exoglycosidases sialidase, beta-galactosidase, N-acetyl-beta-D-glucosaminidase and alpha-mannosidase, and with the endoglycosidases Endo-H and Endo-F. It was found that treatment of virions with Endo-F specifically decreased viral infectivity by a factor of 10. This reduction in titre was not associated with any measurable differences in virus adsorption, suggesting a role of N-linked complex type oligosaccharide chains in penetration. In contrast, a reduction in titre observed upon digestion of virions with exoglycosidases could be attributed to a proteolytic contamination in these enzyme preparations. Treatment of virions with Endo-H, demonstrated to be free of proteolytic contamination, did not reduce viral infectivity. Analysis of endoglycosidase-digested virions by monospecific antibodies and immunoblotting revealed a susceptibility of all four major glycoproteins (gC, gB, gE and gD) to Endo-F, but only gB was susceptible to Endo-H treatment. In contrast, of all the exoglycosidases used only sialidase was found to be active towards native viral glycoproteins. Upon analysis of endoglycosidase-digested virions we could not find any evidence for proteolysis, degradation or altered protein composition of viral envelopes. In contrast, vigorous inhibition of glycoprotein glycosylation by tunicamycin led to the formation of physically intact virions almost completely lacking all major glycoproteins. These data show that digestion of intact virions with glycosidases allows an analysis of the functional relevance of carbohydrate residues without any obvious alterations in the virion glycoprotein composition.

Ribonucleotide reductase encoded by herpes simplex virus is a determinant of the pathogenicity of the virus in mice and a valid antiviral target

The role of the herpes simplex virus (HSV)-encoded ribonucleotide reductase (RR) in the pathogenicity of the virus has been examined by use of mutants with lesions in either the large or small subunit of the enzyme. The virulence of the mutants in mice was reduced by about 10(6)-fold when compared with that of the parental virus (HSV type 1 strain 17), while the virulence of a revertant of one of the mutants was restored to within about 100-fold of that of the parent virus. These experiments demonstrate that activity of the HSV RR is essential for virus pathogenicity in mice and suggests that the enzyme is a valid target for specific antiviral compounds.

Expression of a truncated viral trans-activator selectively impedes lytic infection by its cognate virus

A virion protein of herpes simplex virus type 1 (HSV-1) specifically and potently activates transcription of the viral immediate early genes. Appropriate function of this protein, termed VP16, depends on an acidic transcriptional activation domain located within the 78 carboxyl-terminal amino acids of the protein. Mutated forms of the protein lacking this acidic domain lose the ability to activate transcription, and can dominantly interfere with the trans-activation function of native VP16 (ref. 1). We have prepared stably transformed mouse L cells that constitutively express a form of VP16 lacking its acidic activating domain. In this report we show that these cells are selectively impaired in their capacity to support the lytic infectious cycle of HSV-1, and that this impairment results from their inability to support immediate early transcription.

In vivo behavior of genetically engineered herpes simplex viruses R7017 and R7020: construction and evaluation in rodents

The herpes simplex virus (HSV) recombinant R7017 was constructed from HSV-1 (strain F) by deleting a portion of the thymidine kinase (tk) gene and by replacing the sequences representing the internal inverted repeats and adjacent genes in the L component with a fragment of the HSV-2 genome encoding the glycoproteins G, D, I, and a portion of E. In addition, the R7020 recombinant contains an HSV-1 DNA fragment encoding the tk gene fused to the alpha 4 gene promoter. The results of studies in mice, guinea pigs, and rabbits were as follows: Both recombinants remained unchanged after nine serial, intracerebral passages in mice; the recombinants could not be differentiated with respect to attenuation in mice injected intracerebrally, in vaginally infected guinea pigs, and in rabbits inoculated on the scarified cornea. Given intradermally or intramuscularly, the recombinants prevented severe infections by virulent challenge viruses, and R7020 established latent infections (at a low frequency) in all species tested, whereas latent R7017 virus was detected in rabbits only.

An acyclovir-resistant strain of herpes simplex virus type 2 which is highly virulent for mice

Herpes simplex virus type 2 (HSV-2), strain YS-4 C-1, isolated by plaque cloning from a clinical isolate was found to be resistant to acyclovir (ACV; acycloguanosine) in vitro. It was sensitive to phosphonoacetic acid and 9-beta-D-arabinofuranosyladenine. Thymidine kinase (TK) activity of YS-4 C-1 was less than 1% of that of other strains from the same clinical source. However, thymidine plaque autoradiography showed that YS-4 C-1 was not completely deficient in TK activity. YS-4 C-1 showed high virulence for mice like other HSV-2 strains which were sensitive to ACV. YS-4 C-1 was able to establish latent infection in mice. Virus isolated from the brain of a mouse died after being inoculated with YS-4 C-1 was also resistant to ACV. ACV was not effective in mice inoculated with YS-4 C-1. This study shows that not all ACV-resistant strains are avirulent for mice.

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.

Pathogenesis of a lethal mixed infection in mice with two nonneuroinvasive herpes simplex virus strains

We previously reported that simultaneous inoculation of mice on abraded rear footpads with two nonneuroinvasive viruses (herpes simplex virus type 1 ANG and KOS) resulted in the deaths of 62% of the animals (R. T. Javier, F. Sedarati, and J. G. Stevens, Science 234:746-748, 1986). In the current study, to better understand the events responsible for the pathogenesis of this virus mixture, we investigated replicative capacity and spread of the virus mixture within specific tissues. We found that, compared with neuroinvasiveness of ANG or KOS alone, neuroinvasiveness of the virus mixture related to significantly increased amounts of the virus within spinal cords and brains of the mice. This finding indicates that ANG and KOS have defects in their capacities to spread and replicate within spinal cords. We also examined whether the increased neuroinvasiveness of the virus mixture related to complementation between viruses in tissues of the nervous system, generation and selection of neuroinvasive recombinants, or both. It was found that, although neuroinvasive recombinant viruses could be detected in the spinal cords of the infected animals, most of the viruses (both recombinants and nonrecombinants) isolated from all tissues tested were nonneuroinvasive (i.e., no mice died as a result of footpad infection with high doses of such plaque-purified isolates). As a result of these findings, we propose that the virulence of the virus mixture is a consequence of the complementation as well as the generation and selection of neuroinvasive recombinants in spinal cords of these mice.

Induction and repression of cellular gene transcription during herpes simplex virus infection are mediated by different viral immediate-early gene products

Nuclear run-off and pulse-labelling techniques have been used to study the changes in transcription rates of a number of cellular genes during infection with Herpes simplex virus. The majority of these genes show a decrease in transcription rate to about 60% of that observed prior to infection. In contrast, a small number of genes are transcriptionally activated during infection. These effects, which occur at a point in infection after the synthesis of viral proteins but prior to the onset of viral DNA synthesis, are mediated by different immediate-early proteins of the virus. Thus we show that, whilst transcriptional activation requires a functional ICP4 protein, repression is dependent upon the presence of another immediate early protein--ICP22.

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.

Monoclonal antibodies define a domain on herpes simplex virus glycoprotein B involved in virus penetration

In an earlier report (S.D. Marlin, S.L. Highlander, T.C. Holland, M. Levine, and J.C. Glorioso, J. Virol. 59: 142-153), we described the production and use of complement-dependent virus-neutralizing monoclonal antibodies (MAbs) and MAb-resistant (mar) mutants to identify five antigenic sites (I to V) on herpes simplex virus type 1 glycoprotein B (gB). In the present study, the mechanism of virus neutralization was determined for a MAb specific for site III (B4), the only site recognized by MAbs which exhibited complement-independent virus-neutralizing ability. This antibody had no detectable effect on virus attachment but neutralized viruses after adsorption to cell monolayers. These findings implied that the mechanism of B4 neutralization involved blocking of virus penetration. The remaining antibodies, which recognized sites I, II, and IV, required active complement for effective neutralization. These were further studied for their ability to impede virus infectivity in the absence of complement. Antibodies to sites I (B1 and B3) and IV (B6) slowed the rate at which viruses penetrated cell surfaces, supporting the conclusion that antibody binding to gB can inhibit penetration by a virus. The data suggest that MAbs can interfere with penetration by a virus by binding to a domain within gB which is involved in this process. In another assay of virus infection, MAb B6 significantly reduced plaque development, indicating that antibody binding to gB expressed on infected-cell surfaces can also interfere with the ability of a virus to spread from cell to cell. In contrast to these results, antibodies to site II (B2 and B5) had no effect on virus infectivity; this suggests that they recognized structures which do not play a direct role in the infectious process. To localize regions of gB involved in these phenomena, antibody-binding sites were operationally mapped by radioimmunoprecipitation of a panel of truncated gB molecules produced in transient-expression assays. Residues critical to recognition by antibodies which affect penetration by a virus (sites I, III, and IV) mapped to a region of the molecule (amino acid residues 241 to 441) which is centrally located within the external domain. Antibodies which had no effect on penetration (site II) recognized sequences distal to this region (residues 596 to 737) near the transmembrane domain. The data suggest that these gB-specific MAbs recognize two major antigenic sites which reside in physically distinct components of the external domain of gB.(ABSTRACT TRUNCATED AT 400 WORDS)

DNA sequence and RNA transcription through a site of recombination in a non-neurovirulent herpes simplex virus intertypic recombinant

RE6 is a herpes simplex type-1 (HSV-1) X herpes simplex type-2 (HSV-2) intertypic recombinant that cannot replicate in the adult mouse nervous system. In the accompanying report, we have shown that HSV-1 sequences between 0.698 and 0.721 map units can restore a partial neurovirulent phenotype to RE6. In this report, we have used comparative DNA sequence analysis of RE6, 17syn+ (HSV-1) and HG52 (HSV-2) to demonstrate that this region contains a site of recombination between HSV-1 and HSV-2 sequences in RE6. High resolution transcription analysis has demonstrated that three readily detected transcripts are present in this region of the genome. In addition, the 5' end of a low abundance 5 kb transcript was also located in the right-hand portion of this region. All the transcripts encoded by HSV-1 and HSV-2 in this region of the genome are expressed by the RE6 recombinant. This and our sequence data suggest that the lack of neurovirulence in RE6 is not due to a simple loss in the expression of a transcript or to a defect in a protein encoded by a gene at the site of recombination.

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.

HSV-1 DNA sequence determining intraperitoneal pathogenicity in mice is required for transcription of viral immediate-early genes in macrophages

The relationship between intraperitoneal (ip) pathogenicity in vivo of herpes simplex virus type 1 (HSV-1) and infection of macrophages (m phi) in vitro was studied. The apathogenic HSV-1 strain HFEM disappeared from the peritoneum of infected mice following ip inoculation, while the pathogenic F strain persisted in the peritoneum and penetrated the mouse nervous system, and eventually the mice died, showing severe neurological signs. When peritoneal m phi were infected in vitro, a direct correlation with pathogenicity in vivo was found with several HSV-1 strains and recombinants. HSV-1 strains (F, KOS, R-M1C1) which were pathogenic for mice by the ip route, induced cytopathic effect (CPE) in m phi in vitro. Strain F transcribed viral immediate-early genes and synthesized viral DNA in m phi that were treated with L-cell conditioned medium (as a source of colony-stimulating factor) prior to infection. Apathogenic HSV-1 strains (HFEM, R-15, R-19) did not cause CPE in m phi. The HFEM strain was already blocked in the transcription of viral alpha genes in the infected m phi, but replicated well in control BSC-1 cells. An intratypic recombinant (R-M1C1), produced by cotransfection of HFEM DNA with a cloned Mlul-Mlul DNA fragment (coordinates 0.7615-0.796) from HSV-1 strain F, that was shown [Becker et al. (1986). Virology 149, 255-259] to have regained partial ip virulence for mice, now transcribed alpha genes, synthesized viral DNA, and induced CPE in m phi. It appears that the viral DNA fragment responsible for ip virulence is involved in tissue-specific recognition of virus by infected m phi, a function necessary for transcription of viral alpha genes.

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.

Neurovirulence of individual plaque stocks of Herpes simplex virus type 2 strain HG 52

The virulence of ten independent plaque stocks of HSV-2 strain HG 52 was determined by intracranial inoculation of 3 week old BALB/c mice. The ten stocks exhibited a range of LD50 which could be grouped into three classes; high (less than 10(3) pfu/mouse), intermediate (10(3)-10(4) pfu/mouse) and low (greater than 10(5) pfu/mouse). Analysis of the ten stocks using six different restriction endonucleases showed no significant variation in the size and distribution of fragments. Isolates from the brains of dead mice had restriction enzyme profiles indistinguishable from the initial infecting virus. The pfu:particle ratios of the virus stocks were comparable. There were no significant differences between the single growth cycle characteristics in vitro of the parental HG 52 and the various plaque purified stocks. In vivo after intracranial inoculation the low virulence stocks grew poorly while those of high virulence grew well. Plaques were picked from the high and low virulence stocks after passage five times in vitro and re-assayed for virulence. In some cases there were differences in the LD50s of passaged plaque isolates from that of parental stock. The restriction enzyme profiles of the passaged plaque isolates remained unchanged. The analysis has clearly demonstrated virulence heterogeneity within a single HSV-2 virus stock.