Expression of herpes simplex virus type 2 latency-associated transcript in neurons and nonneurons

Ocular herpes simplex virus: how are latency, reactivation, recurrent disease and therapy interrelated?

Most humans are infected with herpes simplex virus (HSV) type 1 in early childhood and remain latently infected throughout life. While most individuals have mild or no symptoms, some will develop destructive HSV keratitis. Ocular infection with HSV-1 and its associated sequelae account for the majority of corneal blindness in industrialized nations. Neuronal latency in the peripheral ganglia is established when transcription of the viral genome is repressed (silenced) except for the latency-associated transcripts and microRNAs. The functions of latency-associated transcripts have been investigated since 1987. Roles have been suggested relating to reactivation, establishment of latency, neuronal protection, antiapoptosis, apoptosis, virulence and asymptomatic shedding. Here, we review HSV-1 latent infections, reactivation, recurrent disease and antiviral therapies for the ocular HSV diseases.

Cell type specific accumulation of the major latency-associated transcript (LAT) of herpes simplex virus type 2 in LAT transgenic mice

We performed in situ hybridization to determine the cell type specific accumulation of the intron of the latency-associated transcript (LAT) in tissues in HSV-2 LAT transgenic mice in which LAT expression is driven by its native promoter. We identified LAT in multiple cell types in most tissues analyzed from HSV-2 LAT transgenic mice. While weak to moderate signals were seen in brain and spinal cord neurons, epithelial cells, and muscle cells, the strongest signals were detected in neurons from dorsal root and trigeminal ganglia. About 70-86% of neurons in these ganglia were LAT-positive with varying signal intensities, while cells surrounding the neurons were LAT-negative. The frequency of A5 or KH10-positive neurons was similar in LAT-positive and total neurons. These data indicate that HSV-2 LAT promoter activity is not restricted to neurons and that LAT accumulation in ganglionic neurons is likely regulated by cell-specific factors.

The 2.2-kilobase latency-associated transcript of herpes simplex virus type 2 does not modulate viral replication, reactivation, or establishment of latency in transgenic mice

To better understand the mechanisms responsible for the observed effects of deletions in the promoter region of the latency-associated transcript (LAT) gene in impairing herpes simplex virus (HSV) reactivation, we generated mice transgenic for a 5.5-kb HSV type 2 (HSV-2) genomic fragment spanning the major LAT, along with the LAT promoter and flanking regions, in the C57BL/6 background. The mice expressed abundant 2.2-kb major LATs in trigeminal ganglia (TG) and other tissues. The effects of the transgene on HSV-2 infection, latency, and reactivation were assessed. When infected with wild-type (WT) HSV-2 or its LAT promoter deletion (LAT(-)) mutant, primary lung fibroblast lines established from normal C57BL/6 and transgenic mice supported virus growth equally well. The replication of these viruses in the mouse eye and their spread to TG and brains were similar. The quantities of latent viral DNA in TG of transgenic and normal mice, as determined by real-time PCR, were comparable. UV light-induced reactivation of the LAT(-) mutant from transgenic mice (0 to 7%) was no more frequent than that from normal mice (0 to 14%), while WT virus was reactivated from 13 to 54% of normal mice and 22 to 54% of transgenic mice. The cumulative data indicate that, when expressed transgenically, the HSV-2 major LAT cannot influence HSV-2 infection or latency and cannot complement the defect in reactivation of the LAT(-) mutant. These results imply that the phenotype of reduced reactivation associated with the LAT(-) mutant is related to a function encoded in the LAT promoter but not to the major LAT itself.

Latency-associated transcripts of equine herpesvirus type 4 in trigeminal ganglia of naturally infected horses

Equine herpesvirus type 4 (EHV-4) is a major respiratory pathogen of horses. Unlike most other members of the Alphaherpesvirinae, EHV-4 was regarded as non-neurotropic. Here, neural and lymphoid tissues of 17 horses have been analysed post-mortem. EHV-4 DNA was detected in 11 cases (65%) by PCR, exclusively in the trigeminal ganglia. In order to define the transcriptional activity, RNA preparations of 10 EHV-4 DNA-positive ganglia were investigated by nested RT-PCR. EHV-4-specific transcripts derived from genes 63 [herpes simplex virus type 1 (HSV-1) ICPO gene homologue] and 64 (HSV-1 ICP4 gene homologue) were detected in six trigeminal ganglia. In one other case, only gene 64-specific transcripts were present. All of the transcripts proved to be antisense orientated when a strand-specific RT-PCR was applied. Type-specific primers for gene 33 (encoding glycoprotein B) served to detect transcripts of an acute EHV-4-infection, which were found in only one of the six ganglia positive for gene 63- and gene 64-specific transcripts. Overall, these studies clearly demonstrate that EHV-4 is latent in trigeminal ganglia.

Experimental investigation of herpes simplex virus latency

The clinical manifestations of herpes simplex virus infection generally involve a mild and localized primary infection followed by asymptomatic (latent) infection interrupted sporadically by periods of recrudescence (reactivation) where virus replication and associated cytopathologic findings are manifest at the site of initial infection. During the latent phase of infection, viral genomes, but not infectious virus itself, can be detected in sensory and autonomic neurons. The process of latent infection and reactivation has been subject to continuing investigation in animal models and, more recently, in cultured cells. The initiation and maintenance of latent infection in neurons are apparently passive phenomena in that no virus gene products need be expressed or are required. Despite this, a single latency-associated transcript (LAT) encoded by DNA encompassing about 6% of the viral genome is expressed during latent infection in a minority of neurons containing viral DNA. This transcript is spliced, and the intron derived from this splicing is stably maintained in the nucleus of neurons expressing it. Reactivation, which can be induced by stress and assayed in several animal models, is facilitated by the expression of LAT. Although the mechanism of action of LAT-mediated facilitation of reactivation is not clear, all available evidence argues against its involving the expression of a protein. Rather, the most consistent models of action involve LAT expression playing a cis-acting role in a very early stage of the reactivation process.

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.

Quantification of transcripts from the ICP4 and thymidine kinase genes in mouse ganglia latently infected with herpes simplex virus

Herpes simplex virus establishes latency in nervous tissue in which it is maintained for the life of the mammalian host, with occasional reactivation leading to subsequent spread. Latency-associated transcripts are abundant during latency, but viral proteins and productive cycle RNAs have not been detected. Using sensitive, quantitative PCR assays, we have quantified certain viral RNAs specific to productive-cycle genes in mouse ganglia latently infected with herpes simplex virus type 1. Sense-strand RNA specific to the essential immediate-early gene, ICP4, was present in most ganglia in variable amounts relative to the amount of viral DNA, with one to seven molecules of RNA per viral genome in about 20% of ganglia. In contrast, the amount of latency-associated transcripts was much less variable, at an average of 4 x 10(4) molecules per viral genome. The amounts of ICP4-specific RNA were similar at 30 and 60 days postinfection, and at least some of these transcripts initiated within a region consistent with utilization of the ICP4 promoter. RNA specific to the thymidine kinase gene, whose transcription in productive infection is dependent on ICP4, was present in latently infected ganglia at a maximum level of 3.2 x 10(6) molecules per ganglion (500 molecules per viral genome). ICP4-specific and tk-specific RNAs measured from the same samples showed a positive correlation extending over 2 orders of magnitude. We conclude that ICP4-specific RNA is expressed in the absence of detectable reactivation and discuss possible implications of our findings for latent gene expression.

A thymidine kinase deficient HSV-2 strain causes acute keratitis and establishes trigeminal ganglionic latency, but poorly reactivates in vivo

The incidence of herpetic keratitis following intranasal or direct ocular infection with thymidine kinase-negative (TK-) strains of herpes simplex virus (HSV)-2 has not been well studied, and the role of the TK gene in the establishment of latency and virus reactivation is controversial. To determine whether a TK- strain of HSV-2 could establish trigeminal ganglionic latency and be reactivated in vivo to produce recurrent keratitis or nervous system infection, an animal model of acute and recurrent infection was utilized. Rabbits were infected by the intranasal or ocular routes, and latency was reactivated by immunosuppression. Virus shedding in nasal and ocular secretions was monitored, and the eyes were examined for the presence of corneal epithelial lesions during acute and reactivated infections. Central nervous system (CNS) and trigeminal ganglionic tissues were assayed by histologic, virologic, and in situ hybridization techniques. All rabbits intranasally infected shed virus in both ocular and nasal secretions, whereas only 30% of rabbits infected in the eyes shed virus in nasal secretions. Virus was recovered from cocultivation cultures, but not from cell-free homogenates, of trigeminal ganglionic and CNS tissues from animals inoculated by both routes. The incidence of keratitis was much greater after direct ocular inoculation, although both routes of inoculation produced CNS and ganglionic inflammatory lesions. Keratitis healed in 92% of the animals infected by the ocular route by 26 days post infection. Of rabbits initially infected in the eyes and then subjected to drug-induced reactivation, only 30% shed virus, which was limited to a 24 hour period; there was no reappearance of epithelial keratitis, no animal became blind, and none died. In contrast, latently infected control rabbits uniformly reactivated. These studies show that this TK-HSV-2 strain (i) replicates in the eye, (ii) is neuroinvasive but non-neurovirulent following intranasal and direct ocular infection; (iii) sheds in the eye more frequently and for longer periods after ocular than after intranasal inoculation; (iv) induces epithelial keratitis that usually heals spontaneously; (v) establishes latency in trigeminal ganglionic neurons, but no other ganglionic cells; and, (vi) reactivates in a small proportion of animals, but does not produce recurrent ocular lesions following drug-induced immunosuppression. Thus, the TK gene appears directly involved in HSV latency and reactivation in vivo.

Localization of cis-acting sequence requirements in the promoter of the latency-associated transcript of herpes simplex virus type 1 required for cell-type-specific activity

We have previously demonstrated (A. H. Batchelor and P. O'Hare, J. Virol. 64:3269-3279, 1990) the selective activity in human neuroblastoma cells (IMR-32) of a promoter located upstream of the latency-associated transcript of herpes simplex virus type 1. In this work, we provide evidence for the basis of the selective activity of this latency-associated promoter (LAP). Recombinant constructs containing sequences up to -143 (relative to the LAP cap site) linked to the chloramphenicol acetyltransferase gene retain strong activity in HeLa cells but exhibit extremely weak activity in IMR-32 cells. Sequences mapping within the 108 bp upstream of -143 to position -251 enhance LAP activity by over 15-fold, restoring optimal levels of expression in IMR-32 cells, but have little or no effect (1.5-fold) in HeLa cells. This cell-type-specific enhancement of promoter activity took place in two major steps, with sequences between -143 and -158 conferring a four- to fivefold effect and sequences between -177 and -251 conferring a further threefold effect. Furthermore, sequences mapping from -40 to -258 could transfer the ability to be expressed in neuroblastoma cells to the normally inactive immediate-early 110K promoter (IE110K), increasing levels of expression by 35-fold. By comparison, this region had a relatively minor effect (twofold) on the activity of the IE110K promoter in HeLa cells, even though this promoter is open to activation by other mechanisms. However, neither of the overlapping subregions from -40 to -143 or -138 to -258 could confer efficient IMR-32 cell expression on the IE110K promoter, and we present alternative models for multiple element requirements or the requirement for a critical site around -140 which is not retained in either subfragment. We provide consistent evidence for a site around -140 and demonstrate the presence selectively in IMR-32 cells of a DNA-binding factor which binds a probe spanning this region. We propose that this element and the cognate factor (IC-1) may be involved in the selective activity of the LAP in neuroblastoma cells.