Neurons differentially control expression of a herpes simplex virus type 1 immediate-early promoter in transgenic mice

A 16 bp upstream sequence from the rat tyrosine hydroxylase promoter supports long-term expression from a neurofilament promoter, in a helper virus-free HSV-1 vector system

Helper virus-free Herpes Simplex Virus vector-mediated gene transfer has supported studies on neuronal physiology, and may support specific gene therapies. Long-term, neuron-specific expression is required for many of these applications. A neurofilament heavy gene (NFH) promoter does not support long-term expression. We previously developed a promoter that supports long-term expression by fusing 6.3 kb of upstream sequences from the rat tyrosine hydroxylase (TH) promoter to a NFH promoter, and this promoter has supported physiological studies. The TH promoter fragment contains an enhancer, as it has activity in both orientations and at a distance from the basal promoter. Identifying this enhancer may support further improvements in long-term expression. A previous deletion analysis identified two ~100 bp fragments that each support long-term expression, and are contained within an ~320 bp fragment located ~3 kb from the TH promoter transcription start site. As this analysis used overlapping fragments, the two ~100 bp fragments contained 44 or 23 bp of unique sequence. Here, we used mutagenesis to identify a short sequence that supports long-term expression. We studied a 42 bp sequence, centered on the 23 bp unique sequence. Analysis of the wt sequence, and five mutations containing clustered changes that spanned the sequence, identified two adjacent mutations that do not support long-term expression, which together defined a 16 bp maximum essential sequence. This 16 bp sequence contains a putative E2F-1/DP-1 transcription factor binding site, and this transcription factor is expressed in many brain areas.

Stable transgene expression from HSV amplicon vectors in the brain: potential involvement of immunoregulatory signals

The herpes simplex virus (HSV) amplicon is a plasmid-based, infectious gene delivery system that carries up to 150 kilobase (kb) of exogenous DNA. We previously characterized early host responses and stability of transgene expression in mice systemically injected with HSV amplicon vectors. Transgene expression was readily detected primarily in the liver but rapidly declined to undetectable levels within 2 weeks. Molecular analyses revealed induction of type I interferons (IFN) as the primary response, and early transcriptional silencing of the vector followed IFN's activation of signal transducers and activators of transcription 1 (STAT1). In this study, we investigate vector administration by stereotactic injection into the striatum. In the brain, induction of type I IFN was rather modest, and transgene expression lasted more than 1 year despite dose-dependent inflammation and infiltration of immune cells around injection sites. Further analyses revealed dose-dependent upregulation of immunosuppressive cytokines and molecular markers specific to regulatory T cells in the injected brain regions, which supported the immune-privileged properties of the brain parenchyma. Overall, our findings indicate that the spectrum of host responses can differ significantly depending on target organs and administrative routes, and that HSV amplicon vectors hold great potential for gene therapy of chronic neurological disorders.

Enhanced nigrostriatal neuron-specific, long-term expression by using neural-specific promoters in combination with targeted gene transfer by modified helper virus-free HSV-1 vector particles

Background

Direct gene transfer into neurons has potential for developing gene therapy treatments for specific neurological conditions, and for elucidating neuronal physiology. Due to the complex cellular composition of specific brain areas, neuronal type-specific recombinant gene expression is required for many potential applications of neuronal gene transfer. One approach is to target gene transfer to a specific type of neuron. We developed modified Herpes Simplex Virus (HSV-1) particles that contain chimeric glycoprotein C (gC) – glial cell line-derived neurotrophic factor (GDNF) or brain-derived neurotrophic factor (BDNF) proteins. HSV-1 vector particles containing either gC – GDNF or gC – BDNF target gene transfer to nigrostriatal neurons, which contain specific receptors for GDNF or BDNF. A second approach to achieve neuronal type-specific expression is to use a cell type-specific promoter, and we have used the tyrosine hydroxylase (TH) promoter to restrict expression to catecholaminergic neurons or a modified neurofilament heavy gene promoter to restrict expression to neurons, and both of these promoters support long-term expression from HSV-1 vectors. To both improve nigrostriatal-neuron specific expression, and to establish that targeted gene transfer can be followed by long-term expression, we performed targeted gene transfer with vectors that support long-term, neuronal-specific expression.

Results

Helper virus-free HSV-1 vector packaging was performed using either gC – GDNF or gC – BDNF and vectors that contain either the TH promoter or the modified neurofilament heavy gene promoter. Vector stocks were injected into the midbrain proximal to the substantia nigra, and the rats were sacrificed at either 4 days or 1 month after gene transfer. Immunofluorescent costaining was performed to detect both recombinant gene products and nigrostriatal neurons. The combination of targeted gene transfer with neuronal-specific promoters improved nigrostriatal neuron-specific expression (83 to 93%) compared to either approach alone, and supported long-term (1 month) expression at levels similar to those observed using untargeted gene transfer.

Conclusion

Targeted gene transfer can be used in combination with neuronal-specific promoters to achieve a high level of nigrostriatal neuron-specific expression. Targeted gene transfer can be followed by long-term expression. Nigrostriatal neuron-specific expression may be useful for specific gene therapy approaches to Parkinson's disease or for genetic analyses of nigrostriatal neuron physiology.

Relaxed repression of herpes simplex virus type 1 genomes in Murine trigeminal neurons

The expression of herpes simplex virus (HSV) genomes in the absence of viral regulatory proteins in sensory neurons is poorly understood. Previously, our group reported an HSV immediate early (IE) mutant (d109) unable to express any of the five IE genes and encoding a model human cytomegalovirus immediate early promoter-green fluorescent protein (GFP) transgene. In cultured cells, GFP expressed from this mutant was observed in only a subset of infected cells. The subset exhibited cell type dependence, as the fractions of GFP-expressing cells varied widely among the cell types examined. Herein, we characterize this mutant in murine embryonic trigeminal ganglion (TG) cultures. We found that d109 was nontoxic to neural cultures and persisted in the cultures throughout their life spans. Unlike with some of the cultured cell lines and strains, expression of the GFP transgene was observed in a surprisingly large subset of neurons. However, very few nonneuronal cells expressed GFP. The abilities of ICP0 and an inhibitor of histone deacetylase, trichostatin A (TSA), to activate GFP expression from nonexpressing cells were also compared. The provision of ICP0 by infection with d105 reactivated quiescent genomes in nearly every cell, whereas reactivation by TSA was much more limited and restricted to the previously nonexpressing neurons. Moreover, we found that d109, which does not express ICP0, consistently reactivated HSV type 1 (KOS) in latently infected adult TG cultures. These results suggest that the state of persisting HSV genomes in some TG neurons may be more dynamic and more easily activated than has been observed with nonneuronal cells.

Tissue-specific splicing of the herpes simplex virus type 1 latency-associated transcript (LAT) intron in LAT transgenic mice

To study the regulation of herpes simplex virus type 1 (HSV-1) latency-associated transcript (LAT) expression and processing in the absence of other cis and trans viral functions, a transgenic mouse containing the region encompassing the LAT promoter (LAP1) and the LAT 5' exon through the 2.0-kb intron was created. LAT expression was detectable by reverse transcriptase PCR (RT-PCR) in a number of tissues, including the dorsal root ganglia (DRG), trigeminal ganglia (TG), brain, skin, liver, and kidney. However, when the accumulation of the 2.0-kb LAT intron was analyzed at the cellular level by in situ hybridization, little or no detectable accumulation was observed in the brain, spinal cord, kidney, or foot, although the 2.0-kb LAT intron was detected at high levels (over 90% of neurons) in the DRG and TG. Northern blot analysis detected the stable 2.0-kb LAT intron only in the sensory ganglia. When relative amounts of the spliced and unspliced LAT within the brain, liver, kidney, spinal cord, TG, and DRG were analyzed by real-time RT-PCR, splicing of the 2.0-kb LAT intron was significantly more efficient in the sensory ganglia than in other tissues. Finally, infection of both transgenic mice and nontransgenic littermates with HSV-1 revealed no differences in lytic replication, establishment of latency, or reactivation, suggesting that expression of the LAT transgene in trans has no significant effect on those functions. Taken together, these data indicate that the regulation of expression and processing of LAT RNA within the mouse is highly cell-type specific and occurs in the absence of other viral cis- and trans-acting factors.

Neuronal activity regulates viral replication of herpes simplex virus type 1 in the nervous system

Herpes simplex virus types 1 and 2 (HSV-1, -2) infect and also establish latency in neurons. In the present study, the authors investigated the influence of neuronal activity on the replication of HSV-1. The results showed that the sodium channel blocker tetrodotoxin (TTX) and the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) could significantly increase viral replication in primary neuronal cultures, by two- to fourfold. In contrast, KCl reduced viral production by at least 80% in the same cultures. Inhibitors of GABA(A) receptors completely abolished the effects of GABA. Intravitreously injected TTX in a mouse corneal scarification model enhanced the viral titers > 10-fold in both the trigeminal ganglia and the brain. At 2 h post infection, both TTX and GABA significantly up-regulated the levels of transcription for the viral immediate early (IE) genes ICP0, ICP4, and ICP27, as revealed by real time PCR. These results indicate that the neuronal excitation status may dictate the efficiency of HSV-1 viral replication, probably by regulating the levels of viral IE gene expression. These are the first findings connecting neuronal activity to the molecular mechanisms of HSV replication in the nervous system, which may significantly influence our view of herpesvirus infection and latency.

The herpes simplex virus type 1 early gene (thymidine kinase) promoter is activated in neurons of brain, but not trigeminal ganglia, of transgenic mice in the absence of viral proteins

Latent infection of sensory neurons and reactivation are necessary for maintenance of herpes simplex virus type 1 (HSV-1) in its host population. It has been proposed that the HSV-1 early gene, thymidine kinase (TK), may play an important regulatory role in this process. The authors used reporter transgenic mice to test whether sensory ganglia neurons could activate the HSV-1 TK reporter transgene in the absence of viral proteins. The reporter transgene was activated in subsets of neurons in the brain but was not activated in sensory ganglia neurons following a variety of experimental manipulations. These results do not support a role for TK in regulation of the latent viral genome.

Neuron-specific activation of murine cytomegalovirus early gene e1 promoter in transgenic mice

The brain is the main target in congenital cytomegalovirus (CMV) infection and immunocompromised patients. No definite evidence that a CMV has special affinity for the central nervous system (CNS) has been published. Here, we generated transgenic mice with an e1 promoter/enhancer region connected to the reporter gene lacZ. Surprisingly, expression of the transgene was completely restricted to the CNS in all lines of transgenic mice. The transgene was expressed in subpopulation of neurons in the cerebral cortex, hippocampus, diencephalon, brainstem, cerebellum, and spinal cord in all of the lines. Non-neuronal cells in the CNS were negative for transgene expression. Activation of the transgene was first observed in neurons of mesencephalon in late gestation, and then the number of positive neurons increased in various parts of the brain as development proceeded. Upon infection of the transgenic mouse brains with MCMV, the location of the activated neurons became more extensive, and the number of such neurons increased. These results suggest that there are host factor(s) that directly activate the MCMV early gene promoter in neurons. This neuron-specific activation may be associated with persistent infection in the brain and may be responsible for the neuronal dysfunction and neuronal cell loss caused by CMV infection.

The latency-associated transcript promoter of pseudorabies virus directs neuron-specific expression in trigeminal ganglia of transgenic mice

The latency-associated transcript (LAT) promoter of pseudorabies virus (PRV) is unique among viral promoters in that it remains active in trigeminal ganglia during the latent state. It is not known which the viral or host proteins regulate expression of the PRV LAT gene in latently infected neurons. To determine whether host transcriptional proteins in neurons can regulate the PRV LAT promoter in vivo, three transgenic mouse lines containing the PRV LAT promoter (LAP; LAP1 and LAP2) linked to the chloramphenicol acetyltransferase (CAT) gene were generated. All of the transgenic mouse lines, in the absence of the viral proteins, displayed strong expression of the transgene in trigeminal ganglia in addition to other neuronal tissues such as cerebral cortex, cerebellum, hippocampus and olfactory bulb. Expression of the transgene in neurons of trigeminal ganglia was demonstrated by in situ hybridization. These data provide direct evidence that neuronal transcription factors are sufficient to activate the PRV LAP in vivo and that the promoter is neuron-specific.

Local delivery of CpG oligodeoxynucleotides induces rapid changes in the genital mucosa and inhibits replication, but not entry, of herpes simplex virus type 2

Mucosal surfaces are the entry sites for the vast majority of infectious pathogens and provide the first line of defense against infection. In addition to the epithelial barrier, the innate immune system plays a key role in recognizing and rapidly responding to invading pathogens via innate receptors, such as Toll-like receptors (TLR). Bacterial CpG DNA, a potent activator of innate immunity, is recognized by TLR9. Here, we confirm that local mucosal, but not systemic, delivery of CpG oligodeoxynucleotides (ODN) to the genital tract protects mice from a subsequent lethal vaginal herpes simplex virus type 2 (HSV-2) challenge. Since these effects were so local in action, we examined the genital mucosa. Local delivery of CpG ODN induced rapid proliferation and thickening of the genital epithelium and caused significant recruitment of inflammatory cells to the submucosa. Local CpG ODN treatment also resulted in inhibition of HSV-2 replication but had no effect on HSV-2 entry into the genital mucosa. CpG ODN-induced protection against HSV-2 was not associated with early increases in gamma interferon (IFN-gamma) secretion in the genital tract, and CpG ODN-treated IFN-gamma(-/-) mice were protected from subsequent challenge with a lethal dose of HSV-2. Treatment of human HEK-293 cells transfected with murine TLR9 showed that the antiviral activity of CpG ODN was mediated through TLR9. These studies suggest that local induction of mucosal innate immunity can provide protection against sexually transmitted infections, such as HSV-2 or possibly human immunodeficiency virus, at the mucosal surfaces.

The herpes simplex virus type 1 ICP0 promoter is activated by viral reactivation stimuli in trigeminal ganglia neurons of transgenic mice

Herpes simplex virus type 1 (HSV-1) causes a latent infection in sensory ganglia neurons in humans and in the mouse model. The ability of the virus to latently infect neurons and reactivate is central to the ability of HSV-1 to remain in the human population and spread to new hosts. It is possible that neuronal transcriptional proteins control latency and reactivation by modulating activation of the HSV-1 immediate-early (IE) gene ICP0. We have previously shown that factors in trigeminal ganglia neurons can differentially activate the IE ICP0 promoter and the IE ICP4 promoter in developing trigeminal ganglia neurons of transgenic mice. Ultraviolet (UV) irradiation and hyperthermic stress have been shown to result in HSV-1 reactivation from sensory neurons in the mouse model. Reporter transgenic mice were exposed to UV irradiation or hyperthermia to test whether stimuli that are known to reactivate HSV-1 could activate viral IE promoters in the absence of viral proteins. Measurement of beta-galactosidase activity in trigeminal ganglia from these transgenic mice indicated that the ICP0 promoter activity was significantly increased by both UV irradiation and hyperthermia. The IE genes ICP4 and ICP27 and the late gene gC reporter transgenes failed to be activated in parallel experiments. These results suggest that the ICP0 promoter is a target for activation by host transcription factors in sensory neurons that have undergone damage. It further suggests the possibility that activation of ICP0 gene expression by neuronal transcription factors may be important in reactivation of HSV-1 in neurons.

Neurons differentially activate the herpes simplex virus type 1 immediate-early gene ICP0 and ICP27 promoters in transgenic mice

Herpes simplex virus type 1 (HSV-1) immediate-early (IE) proteins are required for the expression of viral early and late proteins. It has been hypothesized that host neuronal proteins regulate expression of HSV-1 IE genes that in turn control viral latency and reactivation. We investigated the ability of neuronal proteins in vivo to activate HSV-1 IE gene promoters (ICP0 and ICP27) and a late gene promoter (gC). Transgenic mice containing IE (ICP0 and ICP27) and late (gC) gene promoters of HSV-1 fused to the Escherichia coli beta-galactosidase coding sequence were generated. Expression of the ICP0 and ICP27 reporter transgenes was present in anatomically distinct subsets of neurons in the absence of viral proteins. The anatomic locations of beta-galactosidase-positive neurons in the brains of ICP0 and ICP27 reporter transgenic mice were similar and included cerebral cortex, lateral septal nucleus, cingulum, hippocampus, thalamus, amygdala, and vestibular nucleus. Trigeminal ganglion neurons were positive for beta-galactosidase in adult ICP0 and ICP27 reporter transgenic mice. The ICP0 reporter transgene was differentially regulated in trigeminal ganglion neurons depending upon age. beta-galactosidase-labeled cells in trigeminal ganglia and cerebral cortex of ICP0 and ICP27 reporter transgenic mice were confirmed as neurons by double labeling with antineurofilament antibody. Nearly all nonneuronal cells in ICP0 and ICP27 reporter transgenic mice and all neuronal and nonneuronal cells in gC reporter transgenic mice were negative for beta-galactosidase labeling in the absence of HSV-1. We conclude that factors in neurons are able to differentially regulate the HSV-1 IE gene promoters (ICP0 and ICP27) in transgenic mice in the absence of viral proteins. These findings are important for understanding the regulation of the latent and reactivated stages of HSV-1 infection in neurons.

The transgenic ICP4 promoter is activated in Schwann cells in trigeminal ganglia of mice latently infected with herpes simplex virus type 1

Herpes simplex virus type 1 (HSV-1) establishes a latent infection in neurons of sensory ganglia, including those of the trigeminal ganglia. Latent viral infection has been hypothesized to be regulated by restriction of viral immediate-early gene expression in neurons. Numerous in situ hybridization studies in mice and in humans have shown that transcription from the HSV-1 genome in latently infected neurons is limited to the latency-associated transcripts. In other studies, immediate-early gene (ICP4) transcripts have been detected by reverse transcription-PCR (RT-PCR) in homogenates of latently infected trigeminal ganglia of mice. We used reporter transgenic mice containing the HSV-1(F) ICP4 promoter fused to the coding sequence of the beta-galactosidase gene to determine whether neurons in latently infected trigeminal ganglia activated the ICP4 promoter. Mice were inoculated via the corneal route with HSV-1(F). At 5, 11, 23, and 37 days postinfection (dpi), trigeminal ganglia were examined for beta-galactosidase-positive cells. The numbers of beta-galactosidase-positive neurons and nonneuronal cells were similar at 5 dpi. The number of positive neurons decreased at 11 dpi and returned to the level of mock-inoculated transgenic controls at 23 and 37 dpi. The number of positive nonneuronal cells increased at 11 and 23 dpi and remained elevated at 37 dpi. Viral proteins were detected in neurons and nonneuronal cells in acutely infected ganglia, but were not detected in latently infected ganglia. Colabeling experiments confirmed that the transgenic ICP4 promoter was activated in Schwann cells during latent infection. These findings suggest that the cells that express the HSV-1 ICP4 gene in latently infected ganglia are not neurons.

Immunohistochemical analysis of primary sensory neurons latently infected with herpes simplex virus type 1

We characterized the populations of primary sensory neurons that become latently infected with herpes simplex virus (HSV) following peripheral inoculation. Twenty-one days after ocular inoculation with HSV strain KOS, 81% of latency-associated transcript (LAT)-positive trigeminal ganglion (TG) neurons coexpressed SSEA3, 71% coexpressed Trk(A) (the high-affinity nerve growth factor receptor), and 68% coexpressed antigen recognized by monoclonal antibody (MAb) A5; less than 5% coexpressed antigen recognized by MAb KH10. The distribution of LAT-positive, latently infected TG neurons contrasted sharply with (i) the overall distribution of neuronal phenotypes in latently infected TG and (ii) the neuronal distribution of viral antigen in productively infected TG. Similar results were obtained following ocular and footpad inoculation with KOS/62, a LAT deletion mutant in which the LAT promoter is used to drive expression of the Escherichia coli lacZ gene. Thus, although all neuronal populations within primary sensory ganglia appear to be capable of supporting a productive infection with HSV, some neuronal phenotypes are more permissive for establishment of a latent infection with LAT expression than others. Furthermore, expression of HSV LAT does not appear to play a role in this process. These findings indicate that there are marked differences in the outcome of HSV infection among the different neuronal populations in the TG and highlight the key role that the host neuron may play in regulating the repertoire of viral gene expression during the establishment of HSV latent infection.

Diverse stabilities of expression in the rat brain from different cellular promoters in a helper virus-free herpes simplex virus type 1 vector system

Many neural gene transfer studies require both long-term and cell type-specific expression. We have reported a helper virus-free HSV-1 plasmid vector system (Fraefel et al., 1996), and this system supports at least some long-term expression from herpesvirus immediate-early promoters. In this study, we constructed vectors that placed the lacZ reporter gene under the regulation of five different cellular promoters. Vector stocks were microinjected into the midbrain, striatum, or hippocampus; the rats were sacrificed at 4 days to 2 months after gene transfer; and the numbers of X-Gal-positive cells were determined. A 6.8-kb fragment of the rat tyrosine hydroxylase (TH) promoter supported relatively stable expression for up to 2 months and targeted expression to TH-immunoreactive neurons in the substantia nigra pars compacta. The other promoters that were examined were chosen with the goal of obtaining long-term, neuronal-specific expression. At 4 days after gene transfer, a 766-bp fragment of the TH promoter supported expression in cells with neuronal morphology in the midbrain and striatum, consistent with results in transgenic mice. However, expression was absent by 2 weeks. Similarly, at 4 days after gene transfer, a mouse neurofilament heavy subunit promoter supported expression in cells with neuronal morphology in the midbrain, striatum, and hippocampus, but expression was absent by 2 weeks. A rat neuron-specific enolase promoter supported only a low level of expression in cultured neuronal cells, and expression was not detected in the brain. A rat voltage-gated sodium channel promoter supported only a low level of expression in PC12 cells and expression could not be detected in cultured cortical cells. These results demonstrate that different promoters support a wide range of levels and stabilities of expression in this vector system, and the results suggest approaches to improving the stability of long-term expression.

Analysis of bovine herpesvirus 1 transcripts during a primary infection of trigeminal ganglia of cattle

During an infection of nonneuronal cells, bovine herpesvirus 1 (BHV-1) gene expression proceeds in a well-defined cascade. Products of immediate-early (IE) genes are expressed first, and they activate expression of early (E) and late (L) genes. Although the same cascade is assumed to occur during an infection of neurons in trigeminal ganglia (TG) of cattle, no experimental data is available to support this hypothesis. Consequently, we analyzed BHV-1 gene expression in bovine TG at 1, 2, 4, 7, and 15 days postinfection (dpi). Infectious virus was detected in ocular swabs from 1 to 7 dpi but not 15 dpi. By reverse transcription (RT)-PCR, IE (bICP4), E (thymidine kinase, ribonucleotide reductase [RR]), L (glycoprotein C, and alpha trans-inducing factor), and dual-kinetic (bICP0 and bICP22) transcripts were analyzed. When cDNA synthesis was primed with random hexamers, IE and E transcripts were detected at the same time. However, full-length and poly(A)+ (FL&P) RR or bICP22 RNAs were detected before FL&P IE RNAs. Furthermore, FL&P IE transcripts were not detected until viral DNA increased in TG. IE transcripts were detected before E or L RNAs when rabbit kidney cells were infected with a low multiplicity of infection and the same RT-PCR detection method was used. These studies suggested that expression of full-length and polyadenylated IE transcripts in trigeminal ganglia was not efficient compared to that of RR and bICP22 transcripts.

Selective vulnerability of mouse CNS neurons to latent infection with a neuroattenuated herpes simplex virus-1

Herpes simplex viruses that lack ICP34.5 are neuroattenuated and are presently being considered for cancer and gene therapy in the nervous system. Previously, we documented the focal presence of the latency-associated transcripts (LATs) in the hippocampi of immunocompromised mice after intracranial (IC) inoculation of an ICP34.5-deficient virus called strain 1716. To characterize further the biological properties of strain 1716 in the CNS of immunocompetent mice, we determined the extent of viral gene expression in different cell types and regions of the CNS after stereotactic IC inoculation of this virus. At survival times of > 30 d after inoculation, we found that (1) infectious virus was not detectable by titration and immunohistochemical studies; (2) neurons harbored virus as demonstrated by the detection of the LATs by in situ hybridization (ISH); (3) transcripts expressed during the lytic cycle of infection were not detected by ISH; and (4) subsets of neurons were selectively vulnerable to latent infection, depending on the site of inoculation. These results suggest that the absence of ICP34.5 does not abrogate latent infection of the CNS by strain 1716. Additional studies of strain 1716 in the model system described here will facilitate the elucidation of the mechanisms that regulate the selective vulnerability of CNS cells to latent viral infection and lead to the development of ICP34.5 mutant viruses as therapeutic vectors for CNS diseases.

Selective vulnerability of mouse CNS neurons to latent infection with a neuroattenuated herpes simplex virus-1

Herpes simplex viruses that lack ICP34.5 are neuroattenuated and are presently being considered for cancer and gene therapy in the nervous system. Previously, we documented the focal presence of the latency-associated transcripts (LATs) in the hippocampi of immunocompromised mice after intracranial (IC) inoculation of an ICP34.5-deficient virus called strain 1716. To characterize further the biological properties of strain 1716 in the CNS of immunocompetent mice, we determined the extent of viral gene expression in different cell types and regions of the CNS after stereotactic IC inoculation of this virus. At survival times of > 30 d after inoculation, we found that (1) infectious virus was not detectable by titration and immunohistochemical studies; (2) neurons harbored virus as demonstrated by the detection of the LATs by in situ hybridization (ISH); (3) transcripts expressed during the lytic cycle of infection were not detected by ISH; and (4) subsets of neurons were selectively vulnerable to latent infection, depending on the site of inoculation. These results suggest that the absence of ICP34.5 does not abrogate latent infection of the CNS by strain 1716. Additional studies of strain 1716 in the model system described here will facilitate the elucidation of the mechanisms that regulate the selective vulnerability of CNS cells to latent viral infection and lead to the development of ICP34.5 mutant viruses as therapeutic vectors for CNS diseases.