A herpes simplex type 1 ICPO deletion mutant demonstrates diminished pathogenicity during acute ocular infection in different host animals

We compared a previously characterized herpes simplex type 1 alpha 0 deletion mutant, dlx3.1, which produced no functional ICP0, with its wild-type parental strain, KOS, during acute ocular infection of different host animals. Acute pathogenicity of the viral strains in NZ rabbits, Balb/c and A/J mice was evaluated by keratitis scores, ocular and trigeminal ganglionic viral titers, and host survival. We found that dlx3.1 was significantly less pathogenic than KOS. Host differences proved very important in the evaluation of acute pathogenicity. A species-dependent enhancement of ocular pathogenicity was demonstrated for dlx3.1 following a larger viral inoculum and host immunosuppression. We conclude that alpha 0 gene function appears to play an important role during acute ocular pathogenicity of HSV-1 in animal models. Furthermore, in vivo pathogenicity studies contribute important information in the evaluation of essential viral gene function.

A herpes simplex virus type 1 variant, deleted in the promoter region of the latency-associated transcripts, does not produce any detectable minor RNA species during latency in the mouse trigeminal ganglion

In peripheral sensory ganglia latently infected with herpes simplex virus type 1 (HSV-1) transcription is restricted. A set of viral latency-associated transcripts, the LATs, have been characterized by Northern blotting and in situ hybridization. These transcripts have previously been mapped to a 3 kb region of the viral genome within the repeat long region. However, transcription from adjacent regions of the genome can be detected by in situ hybridization, which cannot be detected by Northern blotting. These RNAs are termed minor LATs or m-LAT. In this study we show that in ganglia latently infected with the HSV-1 variant 1704, which is deleted in one complete copy of the LAT gene and in the promoter and 5' portion of the other copy, m-LATs are not detected by in situ hybridization. Furthermore, the levels of DNA in nervous system tissue latently infected with the parental and the 1704 variant virus are similar. Thus we propose that the sequence elements necessary for initiating transcription or stabilizing m-LATs are within the region deleted in variant 1704 that codes for the promoter and the 5' end of the LATs.

A herpes simplex virus type 1 mutant containing a nontransinducing Vmw65 protein establishes latent infection in vivo in the absence of viral replication and reactivates efficiently from explanted trigeminal ganglia

Vmw65, a herpes simplex virus type 1 (HSV-1) tegument protein, in association with cellular proteins, transactivates viral immediate early genes. In order to examine the role of Vmw65 during acute and latent infection in vivo, a mutant virus (in1814), containing a 12-base-pair insertion in the Vmw65 gene, which lacks the transactivating function of Vmw65 (C. I. Ace, T. A. McKee, J. M. Ryan, J. M. Cameron, and C. M. Preston, J. Virol. 63:2260-2269, 1989) was examined in mice. Following corneal inoculation, the parental virus (17+) and the revertant (1814R) replicated effectively in eyes and trigeminal ganglia with 30 to 60% mortality. At either equal PFU or equal particle numbers, in1814 did not replicate in trigeminal ganglia and none of the infected mice died. Although in1814 did not replicate following corneal inoculation, it established latent infection in trigeminal ganglia. HSV-1 in1814 reactivated at explant as efficiently and rapidly as did 17+ and 1814R. Even low amounts of inoculated in1814 (10(2) PFU) were sufficient to establish latent infection in some animals. Since infectious in1814 was not detected at any time in mouse trigeminal ganglia, in1814 provided a unique opportunity to determine how soon after primary infection latency begins. Latent in1814 infection was detected shortly after virus reached the sensory ganglia, between 24 to 48 h postinfection. Thus, though Vmw65 may be required for lytic infection in vivo, it is dispensable for the establishment of and reactivation from latent infection. These data support the hypotheses that the latent and lytic pathways of HSV-1 are distinct and that latency is established soon after infection without a requirement for viral replication. However, the levels of Vmw65 reaching neuronal nuclei may be a critical determinant of whether HSV-1 forms a lytic or latent infection.

Detection of latent thymidine kinase-deficient herpes simplex virus in trigeminal ganglia of mice using the polymerase chain reaction

Latency of thymidine kinase-negative mutants of herpes simplex virus (TK- HSV) could not be detected by reactivating the virus from the ganglia of infected mice. Because Southern blot hybridization was not sensitive enough to detect viral DNA, positive results obtained by dot blot hybridization were ascertained by the highly specific and sensitive polymerase chain reaction (PCR), which detected both latent TK- HSV type 1 and 2 DNA from the trigeminal ganglia of infected mice.

Experimental reactivation of bovine herpesvirus 1 (BHV-1) by means of corticosteroids in an intranasal rabbit model

Intranasal inoculation of the rabbit was shown to be a viable alternative to eye inoculation as a model to study latency and reactivation of bovine herpesvirus 1 (BHV-1). In four different experiments, separate groups of rabbits were intranasally inoculated with BHV-1. In two experiments some rabbits were inoculated instead with a TK-defective (TK-) mutant strain of BHV-1. The development of a specific antibody response was monitored by both virus neutralization and ELISA assays. Cell-mediated immunity was measured by means of a skin test. Many weeks after virus inoculation the rabbits were treated with corticosteroid. Antibody formation after treatment was markedly different in wild type and in TK- virus inoculated groups. In the former, virus reactivation was suggested by a sudden rise in serum antibody levels with kinetics closely resembling those reported in infected calves following corticosteroid administration, whereas in the case of the TK- group no significant increase in antibody activity was measured. Histopathological changes in trigeminal ganglia also indicated reactivation of virus in the wild-type virus infected animals. Further evidence for reactivation was obtained by virus isolation from nasal swabs after corticosteroid treatment.

Herpes simplex virus glycoprotein D. Protective immunity against murine herpetic keratitis

%$%%protective effect of glycoprotein D (gD) immunization against murine herpetic keratitis was investigated. gD was purified by affinity chromatography using anti-gD monoclonal antibodies. Prior immunization with gD was shown to be effective in protecting mice from both the development of stromal keratitis and the spread of the virus to the central nervous system. The level of serum antibodies for virus neutralization, as well as for complement-dependent cytolysis (CDC), was significantly elevated in gD-immunized animals. Cellular immunity, however, was not detected. These results indicate that two antibody-mediated defense mechanisms--virus neutralization and CDC--were responsible for the protective effect observed in our study.

Herpes simplex virus latent phase transcription facilitates in vivo reactivation

The biological role of latent phase transcripts was studied in a rabbit model of herpes simplex virus type I (HSV-I) ocular reactivation. Virus X10-13, a variant of HSV which does not express latency associated transcripts (LAT), has been previously shown to establish latent infection in mouse sensory nerve ganglia, and LAT(-) virus can be recovered upon explantation and cocultivation of ganglia. In the rabbit, we show here that this virus replicates normally on the cornea and conjunctiva and establishes latent infections in corresponding trigeminal ganglionic neurons. However, X10-13 is not efficiently reactivated after iontophoresis of 0.01% epinephrine into the cornea. In contrast, XC-20, a LAT(+) derivative of X10-13 in which LAT expression had been restored by marker rescue of X10-13 with a cloned HSV-I EcoRI J + K fragment, reactivated at a significantly higher rate. Control experiments indicated that XC-20 and X10-13 established latent infections in an equivalent number of neurons. We conclude that latent phase transcription of HSV facilitates ganglionic reactivation and subsequent ocular shedding of the reactivated virus.

Mapping of low abundance latency-associated RNA in the trigeminal ganglia of mice latently infected with herpes simplex virus type 1

During herpes simplex virus type 1 (HSV-1) latent infection of the mouse trigeminal ganglion there is limited viral gene expression. The latency-associated transcripts (LAT) map approximately to the PstI-MluI fragment within the BamHI B and BamHI E fragments (long repeat regions) of the viral genome. Additional weak hybridization signals have been detected by in situ hybridization that correspond to transcription from HSV-1 DNA fragments adjacent tot he PstI-MluI fragment. We mapped the region encoding this additional transcription. This minor latency-associated RNA (m-LAT) was shown to map to a group of contiguous fragments (approximately 8.3 kb of DNA), which are adjacent to the 3' end of LAT and to a (2.0 kb) fragment adjacent to the 5' end of the LAT. Using single-stranded probes in in situ hybridization experiments, we showed that the KpnI-BamHI and BamHI-SacI regions of m-LAT are transcribed in a rightward direction within the long internal repeat region. This low abundance RNA may be related to the previously described LAT.

Role of the herpes simplex virus 1 internal repeat sequences in pathogenicity

Three independently isolated herpes simplex virus type 1 recombinant viruses containing a deletion of approximately 14 kilobase pairs, representing greater than 95% of the internal repeat DNA sequences, were analyzed for their pathogenicity in mice. The recombinant viruses were found to be avirulent, exhibiting drastically increased LD50 values over wild-type herpes simplex virus 1(F) by intracerebral injection, nonneuroinvasive, unable to spread from the cornea to sensory ganglion, and unable to establish a reactivable latent infection in trigeminal ganglion following either intracerebral inoculation or inoculation of scarified corneas. The potential role of diploid genes in herpes simplex virus pathogenesis in the mouse is discussed.

Cyclic nucleotide modulation of herpes simplex virus latency and reactivation

Clinical observations and experimental studies suggested that the relative proportions of ganglionic neuronal intracellular cyclic adenosine monophosphate (c-AMP) and cyclic guanosine monophosphate (c-GMP) concentrations may influence the state or activity of herpes simplex viral DNA in its relationship with the host cell DNA. We studied the effects of putative modulators of intracellular cyclic nucleotide levels on herpes simplex virus (HSV) reactivation from latency in murine trigeminal ganglion cells. We also investigated the effects of these same mediators on the c-GMP and/or c-AMP concentrations in HSV-latently infected trigeminal ganglion cells and in acyclovir-suppressed, HSV-infected neuroblastoma cells. Cholera toxin and theophylline increased c-AMP levels (2-fold and 5-fold at 1 min and 30 sec, respectively for cholera toxin and 2-fold and 1.5-fold at 1 min and 30 sec for theophylline) and enhanced the rapidity of HSV reactivation from latency (P less than 0.005). Exogenous dibutyryl c-AMP also stimulated viral reactivation (P less than 0.005). Carbamylcholine increased c-GMP levels (7-fold and 6-fold at 15 sec and 30 sec, respectively), produced no significant change in c-AMP levels, and delayed HSV reactivation from latency (P less than 0.005). None of these mediators had a demonstrable effect on HSV replication.

Detection of HSV-1 proteins prior to the appearance of infectious virus in mouse trigeminal ganglia during reactivation of latent infection

Following corneal inoculation of mice HSV-1 produces an acute infection and establishes a latent infection in trigeminal ganglia. The latent virus can be reactivated in vitro by explantation of ganglionic tissue. Viral protein expression was studied in trigeminal ganglia during acute infection of mice and explant reactivation of latent infection. HSV-1 proteins were detectable by immunoprecipitation and immunostaining, in mouse ganglia only from 3-5 days post infection. Although during explant reactivation it has been demonstrated that at 24 h post-explant the trigeminal ganglia are all infectious virus negative (Spivack, O'Boyle II and Fraser (1987) J. Virol. 61, 3288-3291), we have found that three HSV-1 proteins, of 175 kDa, 110 kDa and 90 kDa, are present in latently infected trigeminal ganglia as early as 6-21 h post explantation. Initially, only neuronal cells were positive by immunostaining with anti HSV-1 polyclonal serum for HSV-1 antigens, but at later times HSV-1 antigens were seen in non neuronal cells as well. These proteins may play a role in the initial stages of the reactivation process.

Detection of herpes simplex virus type 1 gene expression in latently and productively infected mouse ganglia using the polymerase chain reaction

The polymerase chain reaction (PCR) was employed to detect herpes simplex virus (HSV) sequences in the DNA, and HSV gene expression in total cell RNA, extracted from cervical and trigeminal ganglia of mice during productive and latent infection with HSV-1, strain SC16. Such gene expression was detected in 1 microgram or less of RNA, the quantity anticipated to be present in one or two cervical ganglia. Within the limits of the primers available, gene expression during latency appeared to be restricted to the latency-associated transcript (LAT). The 195 base portion of the LAT amplified by the PCR was sequenced and found to contain several base changes and deletions with respect to published sequences for different HSV strains. These mutations, within the putative open reading frame 2 of the LAT, formed stop or terminator signals, which suggests that the LAT does not act to establish or maintain latency through translation to a protein. The primers for the LAT also amplified a 300 bp fragment from any murine and some other mammalian RNAs. Apart from the oligonucleotide primers, this fragment did not show any homology with HSV.

A deletion mutant of the latency-associated transcript of herpes simplex virus type 1 reactivates from the latent state with reduced frequency

We have generated and characterized a deletion mutant of herpes simplex virus type-1, dlLAT1.8, which lacks the putative promoter region, transcriptional start site, and 1,015 base pairs of the DNA sequences specifying the latency-associated transcripts (LATs). When tested in a CD-1 mouse ocular model, dlLAT1.8 was replication competent in the eye and in ganglia during acute infection but reactivated from explant cultures of ganglia with reduced efficiency (49%) relative to those of wild-type and marker-rescued viruses (94 and 85%, respectively) despite the fact that levels of mutant viral DNA in ganglia during latent infection were comparable to wild-type levels. The neurovirulence of KOS was not significantly altered by the removal of sequences specifying the LATs, as judged by numbers of animals dying on or before 30 days postinfection. Examination of ganglia latently infected with dlLAT1.8 by in situ hybridization revealed no LAT expression. The genotype of reactivated virus was identical to that of input dlLAT1.8 virus as judged by Southern blot analysis. These studies suggest that although the LATs are not essential for the establishment and reactivation of latency in our model, they may play a role in determining the frequency of reactivation of virus from the latent state.

Detection of pseudorabies virus transcripts in trigeminal ganglia of latently infected swine

Two pseudorabies virus (PRV) poly(A)+ RNAs, 2.0 and 0.95 kilobases, were detected in the trigeminal ganglia of latently infected pigs. These RNAs were partially colinear at the 3' end. They were located in the BamHI J fragment (0.706 to 0.737 map units), 3' of the PRV immediate-early gene. A potential polyadenylation signal was also identified in BamHI-J. The direction of transcription of the latency-associated RNAs was antiparallel (i.e., opposite in polarity) to that of the PRV immediate-early mRNA.

Ocular varicella-zoster virus infection in the guinea pig. A new in vivo model

Corneal intrastromal inoculation of guinea pigs with approximately 10(4) plaque-forming units of live, adapted varicella-zoster virus (VZV) resulted in reproducible, acute, superficial corneal disease in all animals. The culture-positive VZV ocular infection progressed to involve 30% to 40% of the corneal surface in a diffuse punctate keratitis and 10% to 15% of this surface with microdendrites, characteristic of VZV-induced ocular disease. Retrograde dissemination of VZV to the trigeminal ganglia, midbrain, cerebellum, and superior cervical ganglia was demonstrated by whole-cell coculture VZV recovery. Central nervous system VZV dissemination, manifested by transient neurologic symptoms and pneumonitis, was evident in 60% of the animals. Varicella-zoster virus spread to the trigeminal ganglion during acute and early-latent infection was evident by electron microscopy.

Thymidine kinase-negative herpes simplex virus mutants establish latency in mouse trigeminal ganglia but do not reactivate

Herpes simplex virus infection of mammalian hosts involves lytic replication at a primary site, such as the cornea, translocation by axonal transport to sensory ganglia and replication, and latent infection at a secondary site, ganglionic neurons. The virus-encoded thymidine kinase, which is a target for antiviral drugs such as acyclovir, is not essential for lytic replication yet evidently is required at the secondary site for replication and some phase of latent infection. To determine the specific stage in viral pathogenesis at which this enzyme is required, we constructed virus deletion mutants that were acyclovir resistant and exhibited no detectable thymidine kinase activity. After corneal inoculation of mice, the mutants replicated to high titers in the eye but were severely impaired for acute replication in trigeminal ganglia and failed to reactivate from ganglia upon cocultivation with permissive cells. Nevertheless, latency-associated transcripts were expressed in neuronal nuclei of ganglia from mutant-infected mice and superinfection of the ganglia with a second virus rescued the latent mutant virus. Thus, contrary to a widely accepted hypothesis, the thymidine kinase-negative mutants established latent infections, implying that neither thymidine kinase activity nor ganglionic replication is necessary for establishment of latency. Rather, thymidine kinase appears to be necessary for reactivation from latency. These results suggest that acyclovir-resistant viruses could establish latent infections in clinical settings and have implications for the use of genetically engineered herpesviruses to deliver foreign genes to neurons.

Effects of lipofuscin on in situ hybridization in human neuronal tissue

In situ hybridization is a highly sensitive technique for detecting nucleic acid sequences within tissues, and is frequently employed in neurovirology. However, this technique requires many appropriate controls in order to recognize and avoid potential artifactual hybridization. We have encountered abundant reaction to lipofuscin in neurons in human peripheral and central nervous systems, using various DNA probes, which could be misinterpreted as positive signals. This pseudohybridization reaction was resistant to treatment with RNase or DNase and was also present in tissue sections treated with hybridization mixture or nuclear autoradiographic emulsion in the absence of any radioactive probes. Characteristics used to distinguish between authentic in situ hybridization and the reaction to neuronal lipofuscin include cellular localization, color, margins and granular appearance, sensitivity to treatment with nucleases and the effect of exposure time on signal intensity. These guidelines should be used to avoid potential misinterpretation of in situ hybridization results with human tissue.

The isolation of herpes simplex virus from rabbit corneas during latency

Herpes simplex virus type 1 (HSV-1) latency, as operationally defined, is a state in which cell-free infectious virus cannot be demonstrated in tissue at the time of sacrifice, but infectious virus can be isolated from the same tissue after prolonged cultivation. Latent HSV has been routinely detected in sensory ganglia of the infected dermatome. We have isolated HSV-1 (RE) from the corneas of 11% of infected rabbits which harbored virus in a latent state in trigeminal ganglia. HSV-1 (RE) was isolated from 10 of 88 cultures of corneal cells established following collagenase digestion of individual corneas taken from asymptomatic animals 118 days after infection. Virus was recovered only after prolonged primary culture and in some cases serial passage of corneal cells (range 5 to 26 days to initial cytopathic effect, n = 10). Virus was isolated from 68 of 68 trigeminal ganglia from the same rabbits by cocultivation of ganglion pieces with Vero cells (range 9 to 20 days to initial cytopathic effect, n = 68) while no cell-free virus was isolated from ganglia at the time of sacrifice. Virus isolation from corneas during the latent period occurred in a manner independent of prior antiviral or antiviral plus immunosuppressive therapy. Clinical evaluation of the corneas throughout the course of acute disease, stromal disease, and at the time of sacrifice provided no evidence that could be used to predict which corneas would yield virus. These data suggest that HSV-1 can remain in a nonreplicative state characteristic of latency in cells of rabbit corneas for long periods after infection and therapy of herpetic eye disease.

Establishment of latent ganglionic infection with herpes simplex virus via maxillary gingiva and viral re-activation in vivo after trauma

Herpes simplex virus can remain latent for months or years in sensory and automatic ganglia of animals and man, and can be re-activated in vivo by several procedures such as neurectomy, irritation of epithelial surfaces, and administration of immunosuppressive agents. The objective of this study was to determine whether dental stimuli can cause re-activation of the latent herpes simplex virus. Homogenization and explanation of ganglia from mice showed that herpes simplex virus (type 1) traveled from maxillary gingiva to trigeminal ganglia, and remained latent. It was also shown that mice passively immunized with rabbit antibody to herpes simplex virus, following the inoculation of herpes simplex virus by the maxillary gingiva route, developed a latent infection in the trigeminal ganglia. Neutralizing antibody was cleared from the circulation and could not be detected in most of these animals after five weeks. A neutralizing test showed that antibody-negative mice with latent infection were able to produce antibody to re-infection with herpes simplex virus, suggesting that re-activation can be identified by measurement of serum antibody. By use of this mouse model system, it was shown that when maxillary gingiva was traumatized with dry ice, viral re-activation occurred in 58% of these animals, as demonstrated by the appearance of neutralizing antibody. Irradiation by a Stomalaser beam had no effect on the re-activation of latent herpes simplex virus. Our mouse model system may serve as a useful model for obtaining new information on re-activating or inhibitory factors in dentistry.

Neurovirulence of two clonally related herpes simplex virus type 1 strains in a rabbit seizure model

Herpes simplex virus type 1 (HSV-1) strains vary widely with regard to neurovirulence, but their tropism for specific central nervous system structures and their ability to induce seizures are poorly defined. We have used the clonally related +GC and -GC strains of HSV-1 to define the pathophysiological basis of neurovirulence in a rabbit model. Following intranasal inoculation, +GC infection was nearly uniformly fatal while -GC infection was asymptomatic. The +GC infected animals developed electroencephalographic (EEG) abnormalities which preceded severe motor seizures. Tropism of the +GC strain for specific CNS nerve centers and the expression of viral antigens within them correlated with its virulence. Although both viruses invaded and replicated within the brain, +GC replicated to slightly higher titers and expressed more abundant viral antigen than -GC. The relatively less efficient replication of -GC appeared to correlate with its temperature-sensitive phenotype in vitro. Both +GC and -GC antigens were found in cerebral cortical layers IV-VI, and in several central nervous system trigeminal and olfactory system structures. However, +GC spread more completely throughout the brain to involve the amygdala, nucleus accumbens, several brainstem nuclei and the locus ceruleus. The +GC antigens were also found in cerebral cortical layer I of animals that developed seizures. These results indicate that the ability of HSV-1 to induce electrophysiologic brain abnormalities is associated with its ability to replicate within specific brain nerve centers.

Latent HSV-1 infection in mice immunized with a zwitterionic detergent-extracted HSV-1 antigen preparation

A HSV-1 antigen preparation obtained by zwitterionic detergent-extraction of HSV-1 infected Vero cells was investigated for its ability to protect mice against establishment of latent infection in the trigeminal ganglion following HSV-1 challenge by ear scarification. The antigen preparation was shown to induce antibody production and correlation between pre-challenge antibody level and establishment of latency was found. Although immunization did not reduce the overall incidence of ear erythema following challenge the duration of erythema was significantly decreased. Latency in the trigeminal ganglion was also significantly reduced in vaccinated mice and in addition, a correlation was found between the duration of erythema and latency, there being significantly more latent infections in mice with erythema of three or more days duration, than in those with erythema lasting less than three days.

Herpes simplex virus type 1 latency-associated transcripts are evidently not essential for latent infection

The herpes simplex virus type 1 (HSV-1) transcripts that can be detected during latent infection by Northern blot analysis in human and experimental animal sensory ganglia are encoded by diploid genes. To investigate their role in latent infection we studied HSV-1 variant 1704, which has deleted most of the IRL copy of the coding region of these RNAs and has a 1.2-kb deletion that is immediately upstream of the coding region of the TRL copy. During primary infection, 1704 replicated in trigeminal ganglia with kinetics similar to the parent virus (17+) and established latent infection. However, while explant reactivation of latent HSV-1 from trigeminal ganglia was detected in 100% of 17+ infected mice within 7 days, the reactivation of 1704 was significantly delayed, and 31 days elapsed before eight out of nine mice became virus positive. The recognized HSV-1 latency-associated RNAs were not detected during the latent state of 1704 by Northern blot analysis or in situ hybridization, which implies that the 1.2-kb deletion may contain the promoter or other important regulatory elements. The data indicate that detectable levels of these latency-associated transcripts are not required for viral replication, establishment, or maintenance (greater than 6 weeks) of HSV-1 latency in trigeminal ganglia, but suggest a role in reactivation.

Low levels of herpes simplex virus thymidine- thymidylate kinase are not limiting for sensitivity to certain antiviral drugs or for latency in a mouse model

Herpes simplex virus mutant KG111 contains a nonsense mutation at codon 44 of the viral thymidine kinase (tk) gene and produces low amounts of a truncated tk polypeptide. We tested mutant KG111 and related viruses that specify varying amounts of similar truncated tk polypeptides for their sensitivities to antiviral nucleoside analogs at different temperatures using plaque reduction assays. The results of these assays showed that the nonsense mutation confers high resistance to bromovinyldeoxyuridine (BVdU) at any temperature and temperature-dependent resistance to acyclovir (ACV), buciclovir (BCV), ganciclovir (DHPG), and fluoroiodoarabinouracil (FIAU). Above relatively low threshold levels of tk that varied depending on the drug tested, viruses exhibited full sensitivity to ACV, BCV, DHPG, and FIAU at 34 degrees. Below these threshold levels, however, decreases in drug sensitivity were linear with decreases in tk levels, forming the basis of a pharmacological assay for tk gene expression. Studies of thymidine (TdR) anabolism in infected 143 tk-cells showed that when high TdR concentrations were added to the medium, KG111 directed thymidine monophosphate (TMP) formation at rates consonant with the amount of tk polypeptide produced by the mutant. When low concentrations to TdR were added to the medium, however, KG111 directed TMP formation at a rate similar to that directed by wild-type virus, indicating that the truncation of the tk polypeptide had little or no effect on tk activity at 34 degrees. Subsequent anabolism to thymidine diphosphate and thymidine triphosphate was reduced in KG111-infected cells, indicating a defect in TMP kinase activity that explains this mutant's resistance to BVdU. Despite the low levels of tk and TMP kinase activity expressed by KG111, this mutant established reactivatable latent infections as efficiently as wild-type virus in a mouse model.

Detection and preliminary characterization of herpes simplex virus type 1 transcripts in latently infected human trigeminal ganglia

RNA extracted from human trigeminal ganglia was examined for the presence of herpes simplex virus type 1 (HSV-1) transcripts by Northern hybridization (RNA blot) analysis. By using cloned DNA and single-stranded RNA probes, two abundant colinear HSV-1 transcripts (1.85 and 1.35 kilobases) were detected in ganglia from 9 of 17 individuals. These RNAs overlap the 3' end of the transcript for immediate-early gene ICP0 but are transcribed from the opposite strand; thus, they are antisense relative to the ICP0 mRNA. We also report evidence by in situ hybridization that these latently infected ganglia contain HSV-1 RNA homologous to the BamHI SP region of the genome which is transcribed in the same direction as the other latency transcripts. In Vero cells productively infected with laboratory strain HSV-1 KOS and in cultures infected with different low-passage clinical isolates, only the 1.85-kilobase transcript was detected, and there was variation in the size of this larger latency transcript. These novel transcripts may play a role in maintaining HSV-1 latency in human ganglia.