Human corneal cells and other fibroblasts can stimulate the appearance of herpes simplex virus from quiescently infected PC12 cells

In vitro and ex vivo models of microbial keratitis: Present and future

Microbial keratitis (MK) is an infection of the cornea, caused by bacteria, fungi, parasites, or viruses. MK leads to significant morbidity, being the fifth leading cause of blindness worldwide. There is an urgent requirement to better understand pathogenesis in order to develop novel diagnostic and therapeutic approaches to improve patient outcomes. Many in vitro, ex vivo and in vivo MK models have been developed and implemented to meet this aim. Here, we present current in vitro and ex vivo MK model systems, examining their varied design, outputs, reporting standards, and strengths and limitations. Major limitations include their relative simplicity and the perceived inability to study the immune response in these MK models, an aspect widely accepted to play a significant role in MK pathogenesis. Consequently, there remains a dependence on in vivo models to study this aspect of MK. However, looking to the future, we draw from the broader field of corneal disease modelling, which utilises, for example, three-dimensional co-culture models and dynamic environments observed in bioreactors and organ-on-a-chip scenarios. These remain unexplored in MK research, but incorporation of these approaches will offer further advances in the field of MK corneal modelling, in particular with the focus of incorporation of immune components which we anticipate will better recapitulate pathogenesis and yield novel findings, therefore contributing to the enhancement of MK outcomes.

An In Vitro HSV-1 Reactivation Model Containing Quiescently Infected PC12 Cells

Advances in the understanding of the infection and reactivation process of herpes simplex type 1 (HSV-1) are generally gained by monolayer cultures or extensive and cost-intensive animal models. So far, no reliable in vitro skin model exists either to investigate the molecular mechanisms involved in controlling latency and virus reactivation or to test pharmaceuticals. Here we demonstrate the first in vitro HSV-1 reactivation model generated by using the human keratinocyte cell line HaCaT grown on a collagen substrate containing primary human fibroblasts. We integrated the unique feature of a quiescently infected neuronal cell line, the rat pheochromocytoma line PC12, within the dermal layer of the three-dimensional skin equivalent. Transmission electron microscopy, a cell-based TCID₅₀ assay, and polymerase chain reaction analysis were used to verify cell latency. Thereby viral DNA could be detected, whereas extracellular as well as intracellular virus activity could not be found. Further, the infected PC12 cells show no spontaneous reactivation within the in vitro skin equivalent. In order to simulate a physiologically comparable HSV-1 infection, we achieved a specific and pointed reactivation of quiescently HSV-1 infected PC12 cells by UVB irradiation at 1000 mJ/cm².

3D-tissue model for herpes simplex virus-1 infections

Infection with herpes simplex virus type 1 (HSV-1) causes the most common skin disease. Various test systems have been established to recapitulate this cyclical pathway of productive infection, latency, and reactivation. Most studies of latency and reactivation are conducted in animal models. However, the small number of neurons which harbor the viral genome, the complexity of the in vivo setting, and ethical constraints place limits on animal studies. So far, no in vitro model which resembles natural latency exists. Here, we describe the first in vitro HSV-1 infection model based on a human skin equivalent. The 3D infection model is generated using the human keratinocyte cell line HaCaT grown on a collagen substrate containing human primary fibroblasts and in addition a quiescently HSV-1 infected neuronal component.

Direct evidence that HSV DNA damaged by ultraviolet (UV) irradiation can be repaired in a cell type-dependent manner

Infection of permissive cells, in tissue culture, with herpes simplex virus (HSV) has been reported to induce host DNA damage repair responses that are necessary for efficient viral replication. However, direct repair of the damaged viral DNA has not, to our knowledge, been shown. In this report, we detect and determine the amount of damaged HSV-1 DNA, following introduction of experimentally damaged HSV genomes into tissue cultures of permissive Vero, NGF differentiated PC12 cells and primary rat neurons, using a method of detection introduced here. The results show that HSV-1 strain 17 DNA containing UV-induced DNA damage is efficiently repaired, in Vero, but not NGF differentiated PC12 cells. The primary rat neuronal cultures were capable of repairing the damaged viral DNA, but with much less efficiency than did the permissive Vero cells. Moreover, by conducting the experiments with either an inhibitor of the HSV polymerase (phosphonoacetic acid [PAA]) or with a replication defective DNA polymerase mutant virus, HP66, the results suggest that repair can occur in the absence of a functional viral polymerase, although polymerase function seems to enhance the efficiency of the repair, in a replication independent manner. The possible significance of varying cell type mediated repair of viral DNA to viral pathogenesis is discussed.

Evidence that herpes simplex virus DNA derived from quiescently infected cells in vitro, and latently infected cells in vivo, is physically damaged

Using polymerase chain reaction (PCR) and alkaline gel electrophoresis, the authors show that, compared with DNA derived from virions used to establish infection, herpes simplex virus DNA derived from quiescently infected rat pheochromocytoma (PC12) cells in culture accumulates alkaline-labile lesions. That is, compared with equivalent amounts of virion DNA, viral DNA from nerve growth factor-differentiated long-term infected cells in culture is consistently 3 to 10 times more refractory to amplification by PCR. Despite using equal mole amounts of DNA isolated from quiescently infected cells (determined by quantitative Southern blots), DNA from quiescently infected cells could not be detected by PCR under conditions in which the virion-derived DNA was easily detected. Refractoriness to PCR was confirmed by analysis with a ligation-mediated PCR technique. The refractoriness was not the result of genomic circularization. The refractoriness was, however, related to the time that the quiescently infected cells had been maintained in culture. The refractoriness to PCR was taken as an indication that the viral DNA was damaged. This hypothesis was confirmed by showing that viral DNA from quiescently infected PC12 cells accumulated alkaline-labile DNA lesions, as determined by alkaline gel electrophoresis. The phenomenon was not limited to tissue culture, because viral DNA derived from the ganglia of latently infected mice is also 3 to 10 times more refractory to amplification than are equivalent amounts of virion-derived genomes. Taken together, these results represent the first evidence that herpes simplex virus DNA is physically damaged as a function of long-term infection. Implications for viral reactivation and pathogenesis are discussed.

Epigenetic regulation of latent HSV-1 gene expression

Like other alpha-herpesviruses, Herpes Simplex Virus Type 1 (HSV-1) possesses the ability to establish latency in sensory ganglia as a non-integrated, nucleosome-associated episome in the host cell nucleus. Transcription of the genome is limited to the Latency-Associated Transcript (LAT), while the lytic genes are maintained in a transcriptionally repressed state. This partitioning of the genome into areas of active and inactive transcription suggests epigenetic control of HSV-1 latent gene expression. During latency viral transcription is not regulated by DNA methylation but likely by post-translational histone modifications. The LAT region is the only region of the genome enriched in marks indicative of transcriptional permissiveness, specifically dimethyl H3 K4 and acetyl H3 K9, K14, while the lytic genes appear under-enriched in those same marks. In addition, facultative heterochromatin marks, specifically trimethyl H3 K27 and the histone variant macroH2A, are enriched on lytic genes during latency. The distinct epigenetic domains of the LAT and the lytic genes appear to be separated by chromatin insulators. Binding of CTCF, a protein that binds to all known vertebrate insulators, to sites within the HSV-1 genome likely prevents heterochromatic spreading and blocks enhancer activity. When the latent viral genome undergoes stress-induced reactivation, it is possible that CTCF binding and insulator function are abrogated, enabling lytic gene transcription to ensue. In this review we summarize our current understanding of latent HSV-1 epigenetic regulation as it pertains to infections in both the rabbit and mouse models. CTCF insulator function and regulation of histone tail modifications will be discussed. We will also present a current model of how the latent genome is carefully controlled at the epigenetic level and how stress-induced changes to it may trigger reactivation.

Induction of cellular stress overcomes the requirement of herpes simplex virus type 1 for immediate-early protein ICP0 and reactivates expression from quiescent viral genomes

Herpes simplex virus type 1 (HSV-1) mutants impaired in the activities of the structural protein VP16 and the immediate-early (IE) proteins ICP0 and ICP4 establish a quiescent infection in human fibroblasts, with most cells retaining an inactive, repressed viral genome for sustained periods in culture. To date, the quiescent state has been considered stable, since it has been reversed only by provision of herpesviral proteins, such as ICP0, not by alteration of the cell physiological state. We report that the interaction of HSV-1 with human fibroblasts can be altered significantly by transient treatment of cultures with sodium arsenite, an inducer of heat shock and oxidative stress, or gramicidin D, a toxin that selectively permeabilizes cell membranes, prior to infection. These regimens stimulated gene expression from IE-deficient HSV-1 mutants in a promoter sequence-independent manner and also overcame the replication defect of ICP0-null mutants. Reactivation of gene expression from quiescent HSV-1 genomes and the resumption of virus replication were observed following addition of arsenite or gramicidin D to cultures. Both agents induced reorganization of nuclear domain 10 structures, the sites of quiescent genomes, but appeared to do so through different mechanisms. The results demonstrate that the physiological state of the cell is important in determining the outcome of infection with IE-deficient HSV-1 and show novel methods for reactivating quiescent HSV-1 in fibroblasts with a high efficiency.

Efficient quiescent infection of normal human diploid fibroblasts with wild-type herpes simplex virus type 1

Quiescent infection of cultured cells with herpes simplex virus type 1 (HSV-1) provides an important, amenable means of studying the molecular mechanics of a nonproductive state that mimics key aspects of in vivo latency. To date, establishing high-multiplicity nonproductive infection of human cells with wild-type HSV-1 has proven challenging. Here, we describe simple culture conditions that established a cell state in normal human diploid fibroblasts that supported efficient quiescent infection using wild-type virus and exhibited many important properties of the in vivo latent state. Despite the efficient production of immediate early (IE) proteins ICP4 and ICP22, the latter remained unprocessed, and viral late gene products were only transiently and inefficiently produced. This low level of virus activity in cultures was rapidly suppressed as the nonproductive state was established. Entry into quiescence was associated with inefficient production of the viral trans-activating protein ICP0, and the accumulation of enlarged nuclear PML structures normally dispersed during productive infection. Lytic replication was rapidly and efficiently restored by exogenous expression of HSV-1 ICP0. These findings are in agreement with previous models in which quiescence was established with HSV mutants disrupted in their expression of IE gene products that included ICP0 and, importantly, provide a means to study cellular mechanisms that repress wild-type viral functions to prevent productive replication. We discuss this model in relation to existing systems and its potential as a simple tool to study the molecular mechanisms of quiescent infection in human cells using wild-type HSV-1.

Herpes simplex virus type 1 ICP4 deletion mutant virus d120 infection failed to induce apoptosis in nerve growth factor-differentiated PC12 cells

It has been suggested that terminally differentiated neuronal cells and mitotic cells respond differently in many aspects to herpes simplex virus type 1 (HSV-1) infection. The ICP4-deleted, Us3-defective, HSV-1 mutant strain d120 induces classical apoptosis in a variety of mitotic cell lines. Its behavior in postmitotic cells is not known. Here the authors report that mutant d120 virus failed to induce apoptosis in neuronal-like, nerve growth factor (NGF)-differentiated PC12 cells. More strikingly, rather than inducing apoptosis, d120 infection prolonged the life of nondividing NGF-differentiated PC12 cells in the culture flask. The virus genome had a half-life of 30 days. Unlike in other cells, such as Vero, neither wild-type nor d120 infection of NGF-differentiated PC12 cells induced the nuclear factor (NF)-kappa B p65 pathway, which has been associated with virus-induced apoptosis. Thus, the authors demonstrate, for the first time, that a potent apoptosis inducer mutant d120 failed to induce apoptosis in neuronal-like NGF-differentiated PC12 cells, unlike a number of other cell lines studied. The possible mechanisms involved in the failure of d120 to induce apoptosis in neuronal-like NGF-differentiated PC12 cells are discussed.

ICP0 is not required for efficient stress-induced reactivation of herpes simplex virus type 1 from cultured quiescently infected neuronal cells

Viral genes sufficient and required for herpes simplex virus type 1 (HSV-1) reactivation were identified using neuronally differentiated PC12 cells (ND-PC12 cells) in which quiescent infections with wild-type and recombinant strains were established. In this model, the expression of ICP0, VP16, and ICP4 from adenovirus vectors was sufficient to reactivate strains 17+ and KOS. The transactivators induced similar levels of reactivation with KOS; however, 17+ responded more efficiently to ICP0. To identify viral transactivators required for reactivation, we examined quiescently infected PC12 cell cultures (QIF-PC12 cell cultures) established with HSV-1 deletion mutants R7910 (deltaICP0), KD6 (deltaICP4), and in1814, a virus containing an insertion mutation in VP16. Although growth of these mutant viruses was impaired in ND-PC12 cells, R7910 and in1814 reactivated at levels equivalent to or better than their respective parental controls following stress (i.e., heat or forskolin) treatment. After treatment with trichostatin A, in1814 and 17+ reactivated efficiently, whereas the F strain and R7910 reactivated inefficiently. In contrast, KD6 failed to reactivate. In experiments with the recombinant KM100, which contains the in1814 mutation in VP16 and the n212 mutation in ICP0, spontaneous and stress-induced reactivation was observed. However, two strains, V422 and KM110, which lack the acidic activation domain of VP16, did not reactivate above low spontaneous levels after stress. These results demonstrate that in QIF-PC12 cells ICP0 is not required for efficient reactivation of HSV-1, the acidic activation domain of VP16 is essential for stress-induced HSV-1 reactivation, and HSV-1 reactivation is modulated uniquely by different treatment constraints and phenotypes.

Highly efficient regulation of gene expression by tetracycline in a replication-defective herpes simplex viral vector

Employing the tetracycline repressor tetR and the wild-type hCMV major immediate-early promoter, we have developed a highly sensitive tetracycline-inducible transcription switch in mammalian cells (T-REx; Invitrogen, Carlsbad, CA, USA). In view of the previous difficulty in achieving regulatable gene expression in recombinant HSV vector systems, we constructed a T-REx-encoding replication-defective HSV-1 recombinant, QR9TO-lacZ, that encodes two copies of the tetR gene controlled by the HSV-1 immediate-early ICP0 promoter and a reporter, the LacZ gene, under the control of the tetO-bearing hCMV major immediate-early promoter. Infection of cells, such as Vero, PC12, and NGF-differentiated PC12 cells, with QR9TO-lacZ led to 300- to 1000-fold tetracycline-regulated gene expression. Moreover, the expression of the LacZ gene by QR9TO-lacZ can be finely controlled by tetracycline in a dose-dependent fashion. Efficiently regulated gene expression can also be achieved in vivo following intracerebral and footpad inoculations in mice. The demonstrated capability of T-REx for achieving high levels of sensitively regulated gene expression in the context of the HSV-1 genome will significantly expand the utility of HSV-based vector systems for studying gene function in the nervous system and delivering regulated gene expression in therapeutic applications, particularly in the treatment of CNS diseases.

Histone deacetylase inhibitors induce reactivation of herpes simplex virus type 1 in a latency-associated transcript-independent manner in neuronal cells

Histone acetylation is implicated in the regulation of herpes simplex virus type 1 (HSV-1) latency. However, the role of histone acetylation in HSV-1 reactivation is less clear. In this study, the well-established model system, quiescently infected, neuronally differentiated PC12 (QIF-PC12) cells, was used to address the participation of histone acetylation in HSV-1 reactivation. In this model, sodium butyrate and trichostatin A (TSA), two histone deacetylase inhibitors, stimulated production of infectious HSV-1 progeny from a quiescent state. To identify viral genes responsive to TSA, the authors analyzed representative alpha, beta, and gamma viral genes using quantitative real-time polymerase chain reaction. Only the latency-associated transcript (LAT) accumulated in response to TSA treatment, under culture conditions that restricted virus replication and spread. This led the authors to evaluate the importance of LAT expression on TSA-induced reactivation. In QIF-PC12 cells, the LAT deletion mutant virus dLAT2903 reactivated equivalently with its wild-type parental strain (McKrae) after TSA treatment, as well as forskolin and heat stress treatment. Both viruses also reactivated equivalently from latently infected trigeminal ganglia explants from rabbits. In contrast, there was a marked reduction in the recovery of dLAT2903, as compared to wild-type virus, from the eyes of latently infected rabbits following epinephrine iontophoresis. These combined in vitro, ex vivo, and in vivo data suggest that LAT is not required for reactivation from latently infected neuronal cells per se, but may enhance processes that allow for the arrival of virus at, or close to, the site of original inoculation (i.e., recrudescence).

Culture of feline corneal epithelial cells and infection with feline herpesvirus-1 as an investigative tool

OBJECTIVE

To isolate and characterize pure cultures of feline corneal epithelial cells and to assess the extent and nature of feline herpesvirus (FHV)-1 infection in these cells.

SAMPLE POPULATION

Healthy eyes from 23 recently euthanatized cats.

PROCEDURE

Stroma and epithelium of the rostral portion of the cornea were surgically isolated, and epithelial cells were detached from the stroma by enzymatic incubation. Epithelial cells were cultured in hormone-supplemented media. Cells were passaged, and cytokeratin expression was assessed. Cells were then infected with FHV-1, and cytopathic effects were determined.

RESULTS

Cell cultures were readily established from samples obtained from each eye and could be maintained through 6 passages. Cultured cells expressed cytokeratins 3 and 12 but not other cytokeratins. Infection with FHV-1 was rapid and caused widespread cytopathic effects.

CONCLUSIONS AND CLINICAL RELEVANCE

Feline corneal cells cultured in vitro during multiple passages maintain consistent morphologic characteristics and intermediate filament expression. They are susceptible to infection with FHV-1 and may provide a useful in vitro model for investigation of ocular drugs.

Construction of a herpes simplex virus type 1 mutant with only a three-nucleotide change in the branchpoint region of the latency-associated transcript (LAT) and the stability of its two-kilobase LAT intron

Previous studies using a eukaryotic expression system indicated that the unusual stability of the latency-associated transcript (LAT) intron was due to its nonconsensus branchpoint sequence (T.-T Wu, Y.-H. Su, T. M. Block, and J. M. Taylor, Virology, 243:140-149, 1998). The present study investigated the role of the branchpoint sequence in the stability of the intron expressed from the herpes simplex virus type 1 (HSV-1) genome and the role of LAT intron stability in the HSV-1 life cycle. A branchpoint mutant called Sy2 and the corresponding rescued viruses, SyRA and SyRB, were constructed. To preserve the coding sequence of the immediate early gene icp0, which overlaps with the branchpoint region of the 2-kb LAT, a 3-nucleotide mutation into the branchpoint region of the 2-kb LAT was introduced, resulting in a branchpoint that is 85% identical to the consensus intron branchpoint sequence of eukaryotic cells. As anticipated, there was a 90- to 96-fold reduction in 2-kb LAT accumulation following productive infection in tissue culture and latent infection in mice with Sy2, as determined by Northern blot analysis. These results clearly suggest that the accumulation of the 2-kb intron in tissue culture and in vivo is, at least in part, due to the nonconsensus branchpoint sequence of the LAT intron. Interestingly, a failure to accumulate LAT was associated with greater progeny production of Sy2 at a low multiplicity of infection (0.01) in tissue culture, but not in mice. However, the ability of mutant Sy2 to reactivate from trigeminal ganglia (TG) derived from latently infected mice was indistinguishable from that of wild-type virus, as assayed in the mouse TG explant reactivation system.

HSV LAT and neuronal survival

Herpes Simplex Virus (HSV) establishes a latent infection within sensory neurons and periodically reactivates in response to stress. HSV's ability to inhabit neurons for the life of the host involves a number of virally encoded functions that tightly regulate the latency-reactivation cycle, preventing uncontrolled spread of reactivating virus and large-scale death of neurons. The HSV latency-associated transcript (LAT) is a complex transcription unit expressed primarily in neurons containing latent genomes. While mutational analyses indicate LAT is nonessential for viral replication, the 5' exon of LAT greatly enhances reactivation. Several studies have also identified LAT mutations that reduce establishment of latency and enhance virulence. Recently, LAT has also been shown to inhibit cell death through by blocking caspase-8 and caspase-9 pathways. While blocking apoptosis is not essential for either establishment of latency or reactivation, it likely augments these processes and may contribute to HSV's long-term persistence and spread.

Herpes simplex virus-1 and varicella-zoster virus latency in ganglia

Two human alpha-herpesviruses, herpes simplex virus (HSV)-1 and varicella zoster virus (VZV), account for the most frequent and serious neurologic disease caused by any of the eight human herpesviruses. Both HSV-1 and VZV become latent in ganglia. In this review, the authors describe features of latency for these viruses, such as distribution, prevalence, abundance, and configuration of viral DNA in latently infected human ganglia, as well as transcription, translation, and cell type infected. Studies of viral latency in animal models are also discussed. For each virus, remaining questions and future studies to understand the mechanism of latency are discussed with respect to prevention of serious cutaneous, ocular, and neurologic disease produced by virus reactivation.

Stability and circularization of herpes simplex virus type 1 genomes in quiescently infected PC12 cultures

Herpes simplex virus type 1 (HSV-1) DNA has been shown to exist as a linear, double-stranded molecule in the virion and as a non-linear (endless), episomal, nucleosomal form in latently infected trigeminal ganglia. The kinetics of the formation and appearance of endless viral genomes and the stability of linear genomes in neuronal cells are not well understood. Nerve growth factor (NGF)-differentiated PC12 cells can sustain long-term, quiescent infections with HSV-1. In this report, the structure and stability of HSV-1 viral DNA in NGF-differentiated PC12 cells was studied as a function of time following infection using both wild-type and replication-defective virus. Unexpectedly, unencapsidated linear genomes were stable in the nucleus of NGF-differentiated PC12 cells for up to 2-3 weeks following infection, beyond the period at which there is no detectable viral gene expression. However, following infection with wild-type HSV, the majority of quiescent viral genomes were in an endless form after 3-4 weeks. These data suggest that the stability and fate of HSV-1 DNA in non-mitotic neuronal-like cells is different from that in productively infected cells and thus there is a significant cellular role in this process. The relevance to the virus life-cycle in neurones in vivo is discussed.

Herpes simplex virus type 1 infection prevents detachment of nerve growth factor-differentiated PC12 cells in culture

In tissue culture, rat pheochromocytoma (PC12) cells differentiated with nerve growth factor (NGF) cease division, extend neuritic processes and acquire many properties characteristic of neuronal cells. In previous work, we have shown that NGF-differentiated PC12 cells can survive infection with herpes simplex virus type 1 (HSV-1) and maintain the viral genome in a quiescent but reactivatable state. In this study, we report that uninfected NGF-differentiated PC12 cells uniformly and predictably detach from the culture flask substratum after approximately 7 weeks. Although uninfected cells were uniformly lost from the culture by 7 weeks, surprisingly HSV-1-infected cells survived beyond 10 weeks, the time limit of the study. The detachment of uninfected cells was not the result of cell death or apoptosis, as determined by viability assays performed on cells after detachment. Expression of the HSV-1 latency associated transcript (LAT) gene and virus replication was not necessary for the virus to suppress the 'detachment' phenomenon, since NGF-differentiated PC12 cells infected with either wild-type, DNA polymerase mutant or LAT null mutant virus survived in culture for similar lengths of time. Viral gene expression does appear to be necessary for the suppression, however, since cells infected with UV-inactivated virus were lost from culture with kinetics similar to those of uninfected cells. These findings indicate that de novo viral gene synthesis mediates changes to the host NGF-differentiated PC12 cells, which results in prevention of detachment.

Herpes simplex virus type 1 promoter activity during latency establishment, maintenance, and reactivation in primary dorsal root neurons in vitro

A neonatal rat dorsal root ganglion-derived neuronal culture system has been utilized to study herpes simplex virus (HSV) latency establishment, maintenance, and reactivation. We present our initial characterization of viral gene expression in neurons following infection with replication-defective HSV recombinants carrying beta-galactosidase and/or green fluorescent protein reporter genes under the control of lytic cycle- or latency-associated promoters. In this system lytic virus reporter promoter activity was detected in up to 58% of neurons 24 h after infection. Lytic cycle reporter promoters were shut down over time, and long-term survival of neurons harboring latent virus genomes was demonstrated. Latency-associated promoter-driven reporter gene expression was detected in neurons from early times postinfection and was stably maintained in up to 83% of neurons for at least 3 weeks. In latently infected cultures, silent lytic cycle promoters could be activated in up to 53% of neurons by nerve growth factor withdrawal or through inhibition of histone deacetylases by trichostatin A. We conclude that the use of recombinant viruses containing reporter genes, under the regulation of lytic and latency promoter control in neuronal cultures in which latency can be established and reactivation can be induced, is a potentially powerful system in which to study the molecular events that occur during HSV infection of neurons.

Human neuron-committed teratocarcinoma NT2 cell line has abnormal ND10 structures and is poorly infected by herpes simplex virus type 1

Herpes simplex virus type 1 (HSV-1) immediate-early regulatory protein ICP0 stimulates the initiation of lytic infection and reactivation from quiescence in human fibroblast cells. These functions correlate with its ability to localize to and disrupt centromeres and specific subnuclear structures known as ND10, PML nuclear bodies, or promyelocytic oncogenic domains. Since the natural site of herpesvirus latency is in neurons, we investigated the status of ND10 and centromeres in uninfected and infected human cells with neuronal characteristics. We found that NT2 cells, a neuronally committed human teratocarcinoma cell line, have abnormal ND10 characterized by low expression of the major ND10 component PML and no detectable expression of another major ND10 antigen, Sp100. In addition, PML is less extensively modified by the ubiquitin-like protein SUMO-1 in NT2 cells compared to fibroblasts. After treatment with retinoic acid, NT2 cells differentiate into neuron-like hNT cells which express very high levels of both PML and Sp100. Infection of both NT2 and hNT cells by HSV-1 was poor compared to human fibroblasts, and after low-multiplicity infection yields of virus were reduced by 2 to 3 orders of magnitude. ICP0-deficient mutants were also disabled in the neuron-related cell lines, and cells quiescently infected with an ICP0-null virus could be established. These results correlated with less-efficient disruption of ND10 and centromeres induced by ICP0 in NT2 and hNT cells. Furthermore, the ability of ICP0 to activate gene expression in transfection assays in NT2 cells was poor compared to Vero cells. These results suggest that a contributory factor in the reduced HSV-1 replication in the neuron-related cells is inefficient ICP0 function; it is possible that this is pertinent to the establishment of latent infection in neurons in vivo.

Potential role for luman, the cellular homologue of herpes simplex virus VP16 (alpha gene trans-inducing factor), in herpesvirus latency

The cascade of herpes simplex virus (HSV) gene expression that results in viral replication begins with the activation of viral immediate-early (IE) genes by the virion-associated protein VP16. VP16 on its own is inefficient at associating with complexes formed on IE gene promoters and depends upon the cellular factor HCF for its activity. In this respect VP16 mimics the host basic leucine zipper (bZIP) protein Luman, which also requires HCF for activating transcription. Our objective is to explore interactions between Luman and HCF and to determine if they play a role in the biology of herpesviruses. In this report we show that in cultured cells ectopically expressed Luman was retained in the cytoplasm, where it colocalized with Calnexin, a protein normally associated with the endoplasmic reticulum (ER). Retention of Luman in the ER depends on a hydrophobic segment of the protein that probably serves as a transmembrane domain. Deletion of this domain changed the intracellular location of Luman so that most of the mutant protein was in the nucleus of cells. While HCF was present in the nucleus of most cells, in cells expressing Luman it was retained in the cytoplasm where the two proteins colocalized. This cytoplasmic association of Luman and HCF could also be demonstrated in neurons in trigeminal ganglia removed from cattle soon after death. Cells in tissue culture that expressed Luman, but not a mutant form of the protein that fails to bind HCF, were resistant to a productive infection with HSV type 1 (HSV-1). We hypothesize that similar Luman-HCF interactions in sensory neurons in trigeminal ganglia result in the suppression of viral replication and the establishment of latency. Interestingly, Luman could activate the promoters of IE110 and LAT, two genes that are critical for reactivation of HSV-1 from latency. This suggests a role for Luman in the reactivation process as well.