Latent Herpes Simplex Virus Infection Research Articles

The role of herpes simplex thymidine kinase expression in neurovirulence and latency in newborn vs. adult mice

Infection by standard thymidine kinase-positive (TK +) and TK − mutant herpes simplex virus (HSV) was performed in order to evaluate the role of HSV TK expression in neurovirulence and in HSV latency. In newborn mice, mortality and trigeminal ganglion (TG) HSV titer correlated (both were high) for TK + and TK − HSV. In adult mice after TK − HSV infection they also correlated (both were low). After TK + infection of adult mice, correlation was not present; mortality was low while HSV titer was moderately high. During the period of HSV latent infection (> 28 days after HSV infection), the number of neurons expressing HSV latency-associated transcript (LAT) was much greater for TK − HSV newborn-inoculated mice (average of 943/ganglion) than adult-inoculated mice (average of 138/ganglion). In addition, total amount of TG LAT was greater in the former than the latter. Reactivation from latency was restricted, however, for both groups. This result supported the important role of HSV TK expression in HSV reactivation, even when the number of LAT-positive neurons was greatly increased. The following conclusions were drawn from the study of TK − HSV in newborn mice: (i) HSV TK expression was of limited importance for neurovirulence and in vivo HSV TG infection (but was of importance in adult mice); (ii) increased in vivo HSV TG infection correlated with increased number of LAT-positive neurons, so that HSV replication and establishment of latency were not completely separable; and (iii) even with greatly increased numbers of latently infected neurons, HSV TK expression was important for reactivation from latency. Results in newborn mice suggested that the role of HSV TK expression in reactivation from latency and in neurovirulence were separable. To further investigate HSV replication in newborn and adult mice, ganglia were infected with HSV in vitro and either maintained in vitro or transplanted beneath the renal capsule of adult recipients. In both of these studies, HSV titers in ganglia were much higher in newborn than adult ganglia. This suggested that in addition to the well-know role of the immune system in HSV neurovirulence in newborn mice, it is likely that HSV replication per se in neural tissue is greater in newborn than adult mice. This may be related to the high level of HSV neurovirulence in newborn mice.

Herpes simplex virus latent infection in the nervous system.

Herpes simplex virus (HSV) establishes a latent infection in the human peripheral nervous system and can cause recurrent disease by reactivation. Intensive effort has been directed in recent years to unveil the molecular, cellular and immune mechanisms, as well as the virus-host interactions associated with latent HSV infection. The aim of this review is to summarize current knowledge regarding the site of latent infection, the molecular phenomena of latency, and the mechanisms of the various stages of HSV-1 latent infection in the nervous system, relating them where possible to the human situation. Specifically, the following biological questions are addressed: (1) How does this lytic virus survive in the nervous system and why can it establish a lifelong latent infection in nerve cells? (2) What advantage is conferred on HSV by establishing latent infection in nervous tissue? (3) What can be gathered from the accumulated knowledge on latency about the pathogenesis of herpes simplex encephalitis?

The gene encoding the VP16-accessory protein HCF (HCFC1) resides in human Xq28 and is highly expressed in fetal tissues and the adult kidney

After herpes simplex virus (HSV) infection, the viral regulatory protein VP16 activates transcription of the HSV immediate-early promoters by directing complex formation with two cellular proteins, the POU-homeo-domaintranscription factor Oct-1 and the host cell factor HCF. The function of HCF in uninfected cells is unknown. Here we show by fluorescence in situ hybridization and somatic cell hybrid analysis that the gene encoding human HCF, HCFC1, maps to the q28 region of the X chromosome. Yeast artificial chromosome and cosmid mapping localizes the HCFC1 gene within 100 kb distal of the renal vasopressin type-2 receptor (V2R) gene and adjacent to the renin-binding protein gene (RENBP). The HCFC1 gene is apparently unique. HCF transcripts and protein are most abundant in fetal and placental tissues and cell lines, suggesting a role in cell proliferation. In adults, HCF protein is abundant in the kidney, but not in the brain, a site of latent HSV infection and where HCF levels may influence progression of HSV infection.

Secondary herpes simplex virus latent infection in transplanted ganglia.

Sensory ganglia latently infected with herpes simplex virus (HSV) were transplanted beneath the renal capsule of syngeneic recipients, and the latent infection remaining was investigated. HSV latency-associated transcript (LAT) expression and reactivation of HSV after explant of transplanted dorsal root ganglia were monitored as markers of latency. Two to four weeks after transplantation, both indicated evidence of HSV latency in transplants. At those times, infectious virus was not detected in direct ganglion homogenates. In addition, viral antigen and infected cell polypeptide 4 RNA were not detected. Taken together, the results suggested that HSV latent infection rather than persistent infection was present in transplants. From these results, two explanations seemed possible: latency was maintained in transplanted neurons, or alternatively, latency developed after transplantation, in neurons not previously latently infected. The latter was considered putative secondary latency and was investigated in three ways. First, evidence of reactivation which might serve as a source for secondary latency was evaluated. Reactivation of HSV in transplants was evident from HSV antigen expression (52% of transplants) and the presence of cell-free virus (38% of transplants) 3 to 5 days after transplantation. Second, putative secondary latency was investigated in recipients immunized with HSV prior to receiving latently infected ganglia. Reactivation was not detected 3 to 5 days after transplantation in immunized recipients, and LAT expression was rare in these recipients after 3 to 4 weeks. Lastly, the possibility of secondary latency was investigated by comparing results obtained with standard HSV and with reactivation-defective thymidine kinase-negative (TK-) HSV. Defective reactivation of TK- HSV was demonstrated by immunohistochemistry and by the inability to isolate infectious virus. Donor dorsal root ganglia latently infected with TK+ HSV showed many LAT-positive neurons 2 or more weeks after transplantation (average, 26 per transplant). However, LAT expression was undetectable or minimal > 2 weeks after transplantation in donor ganglia latently infected with TK- HSV (average, 0.2 per transplant). Impaired reactivation of TK- HSV-infected donor ganglia after transplantation, therefore, was correlated with subsequent limited LAT expression. From these results, the occurrence of secondary latency was concluded for ganglia latently infected with TK+ HSV and transplanted beneath the kidney capsule. In vivo reactivation in this transplant model may provide a more useful means to investigate HSV reactivation than in usual in vitro explant models and may complement other in vivo reactivation models. The occurrence of secondary latency was unique. The inhibition of secondary latency by the immune system may provide an avenue to evaluate immunological control of HSV latency.

Upregulation of class I major histocompatibility complex gene expression in primary sensory neurons, satellite cells, and Schwann cells of mice in response to acute but not latent herpes simplex virus infection in vivo.

Major histocompatibility complex (MHC) deficiency is typical of almost all resident cells in normal neural tissue. However, CD8+ T cells, which recognize antigenic peptides in the context of class I MHC molecules, are known to mediate clearance of herpes simplex virus (HSV) from spinal ganglia of experimentally infected mice, leading to the hypothesis that class I expression in the peripheral nervous system must be upregulated in response to HSV infection. In addressing this hypothesis it is shown, in BALB/c (H-2d) mice, that normally deficient class I transcripts transiently accumulate in peripheral nerve Schwann cells, ganglionic satellite cells, and primary sensory neurons, indicating that in each of these cell types class I expression is regulated at the transcriptional level in vivo. Furthermore, for 3-4 wk after infection, H-2Kd/Dd antigens are expressed by satellite and Schwann cells but not neurons, suggesting additional posttranscriptional regulation of class I synthesis in neurons. Alternatively, the class I RNAs induced in neurons may not be derived from classical class I genes. Factors regulating H-2 class I expression emanate from within infected ganglia, probably from infected neurons themselves. However, induction of class I molecules was not maintained during latency, when viral gene expression in neurons is restricted to a single region within the virus repeats. These data have implications for the long-term survival of cells in HSV-infected neural tissue.

Stress-induced reactivation of latent herpes simplex virus infection in rat lumbar dorsal root ganglia

Clinical reports suggest that stress precipitates recurrent cutaneous Herpes simplex virus (HSV) infection, presumably by reactivating latent infection in sensory ganglia with subsequent centrifugal axonal spread to the skin. As an initial test of this hypothesis, rats with latent HSV, type-1, (HSV-1) infection in lumbar dorsal root ganglia (DRG) were exposed to a well-characterized acute stressor that produced gastric ulcers (U) and elevated plasma corticosterone (CS) concentrations. Stress-induced reactivation of latent HSV infection was suggested by the earlier appearance of cytopathic effect (CPE) in human foreskin fibroblast monolayers co-cultivated with ganglia from stressed rats than from nonstressed ones (4.5 ± 0.2 and 6.4 ± 0.4 [mean ± SEM] days respectively; p<0.001). No CPE was detected in monolayers co-cultivated with ganglia from non-infected rats. These initial results suggest that acute stress reactivates latent HSV-1 ganglionic infection.

Neuronal Control of Herpes Simplex Virus Latency

Herpes simplex virus (HSV) is a common neurotropic virus, and latent infection of sensory ganglion neurons readily occurs in humans and in experimentally infected animals. During HSV latency, infectious virus and viral antigen are not detected, and HSV transcription is limited to specific RNA termed latency-associated transcript (LAT). In the present study, the effect of altered nervous system function on HSV latent infection was investigated in dorsal root ganglia (drg) of experimentally infected mice. Latent infection of lumbar drg was established by footpad inoculation of HSV. During latency, sciatic neurectomy was performed in order to modify the in vivo function of latently infected neurons, and HSV LAT and HSV DNA in drg were investigated. Neurectomy has been used in many neurobiological studies to alter neuronal RNA and protein expression. After neurectomy there was a marked decrease in the number of LAT-positive neurons and in the amount of ganglion LAT. This was determined by in situ and RNA (Northern) blot hybridization. The neurectomy-related decrease of HSV LAT was apparent 9-10 days after neurectomy and was more marked after 21 days. The decrease was noted both in drg latently infected with standard thymidine kinase-positive (TK +) HSV and in ganglia infected with mutant TK - HSV. Since TK - HSV is largely reactivation defective, it is concluded that the neurectomy-induced decrease of LAT was probably not the result of in vivo HSV reactivation. It is acknowledged, however, that abortive reactivation by TK - HSV may occur, and decrease of latency may have resulted from neuronal or other host mechanisms subsequent to this. In order to investigate residual HSV latency, in addition to viral transcription, HSV DNA in drg was evaluated by polymerase chain reaction techniques. Decrease of HSV DNA was noted after neurectomy in drg latently infected with either TK + or TK - HSV. It is suggested that the decrease in LAT expression detected was due to the change in neuronal transcription which is part of the neurectomy-induced axon reaction. Decreased HSV LAT may have led to decreased HSV DNA and latency. The decrease in the molecular markers of HSV latency following neurectomy emphasized the importance of neuronal control mechanisms in the pathogenesis of HSV latent infection.

Molecular biology of herpes simplex virus type 1 latency in the nervous system.

Herpes simplex virus (HSV) is one of the best studied examples of viral ability to remain latent in the human nervous system and to cause recurrent disease by reactivation. Intensive effort was directed in recent years to unveil the molecular viral mechanisms and the virus-host interactions associated with latent HSV infection. The discovery of the state of the latent viral DNA in nervous tissues and of the presence of latency-associated gene expression during latent infection, both differing from the situation during viral replication, provided important clues relevant to the pathogenesis of latent HSV infection. This review summarizes the current state of knowledge on the site of latent infection, the molecular phenomena of latency, and the mechanisms of the various stages of latency: acute infection, establishment and maintenance of latency, and reactivation. This information paved the way to recent trials aiming to use herpes viruses as vectors to deliver genes into the nervous system, an issue that is also addressed in this review.

In vitro reactivation of latent herpes simplex virus by the demethylating compound hexamethylene-bis-acetamide

We have characterized previously a model of herpes simplex virus (HSV) infection of rat dorsal root ganglia (DRG) following cutaneous infection. During acute infection HSV can be isolated from co-cultivated rat ganglia in (mean ± S.E.M) 4.8 ± 0.33 days (d) and from latently infected ganglia in 7.8 ± 0.53 (d) ( P ⩽ 0.0001). We treated co-cultivated rat ganglia from acute and latent infected rats with the demethylating compound hexamethylene- bis-acetamide (HEX) to see what effect, if any, it would have on HSV infection. HEX-treated ganglia from rats with acute infection did not differ significantly from controls in the proportion of rats, skin or ganglia positive for HSV. The mean time to detect virus was not different between treated (3.6 ± 0.38 d) and control (3.1 ± 0.46 d) ( P > 0.05) groups. In latent infected rats there was no difference between treated and controlled groups in the proportions of rats, skin and ganglia positive for HSV. There was a significant difference in the mean time to CPE between the HEX and control groups respectively (4.5 ± 0.72 d vs 8.92 ± 1.42 d, P < 0.01). We conclude that HEX converted latent HSV infection to a productive one.

Effect of ibuprofen on the in vitro and in vivo reactivation of latent HSV-1

Prostaglandins have been suggested to play an important role in the reactivation of latent herpes simplex virus. To further understand the role of prostaglandins in the reactivation process, we investigated the effects of ibuprofen, a nonsteroidal anti-inflammatory drug with prostaglandin synthesis inhibitory activity, on the in vitro and in vivo reactivation of latent type 1 herpes simplex virus in mouse ganglia and rabbits, respectively. Ibuprofen, at a concentration of 50 or 100 μM, did not alter the titer of reactivated virus from explanted ganglia with latent virus, but, at a concentration of 200 or 500 μM, it significantly reduced the reactivated viral titer from the ganglia. Ibuprofen also directly inhibited the replication of herpes simplex virus in trigeminal ganglia and Vero cell monolayers, which indicates that the drug reduced the recovery of reactivated viral titers from explanted ganglia with latent virus by acting on the replication process rather than on the reactivation mechanism in vitro. The systemic administration of ibuprofen failed to demonstrate any significant effect on the ocular shedding of virus after attempted reactivation by 6-hydroxydopamine iontophoresis in rabbits with latent herpes simplex virus infection. This failure in vivo could be due to the short half-life and low concentration of ibuprofen at the site of reactivation and replication of latent virus. Alternatively, in the clinical setting, it is conceivable that ibuprofen may not have an effect on in vivo reactivation of latent herpes.

Elevation of cyclic AMP levels in cell lines derived from latently infectable sensory neurons increases their permissivity for herpes virus infection by activating the viral immediate-early 1 gene promoter

Immortalized cell lines derived from sensory neurons are relatively non-permissive for lytic infection with herpes simplex virus (HSV) and fail to transcribe the viral immediate-early genes following infection. Treatment of these cells with agents which raise the intra-cellular level of cyclic AMP results in increased activity of the IE1 gene which contains a cyclic AMP response element within its promoter and produces a consequent increase in permissivity for HSV infection. The significance of these effects for the regulation of HSV infection of neuronal cells are discussed in the light of the finding that cyclic AMP treatment can reactivate latent HSV infections.

Herpes simplex virus latency in the nervous system--a new model.

Permissive herpes simplex virus (HSV) infection in tissue culture results in host cell destruction. Latent HSV infection in vivo occurs in neurons of peripheral sensory ganglia (PSG) and it therefore can not take place in neurons in which the virus has completed a lytic replication cycle similar to that present in vitro. Our hypothesis, based on experimental data and observations in humans, suggests that establishment of latent infection and reactivation of HSV-1 does not involve neuronal cell loss. Latency is established in neurons in which the virus does not replicate and is determined, in part, by the tissue levels of a herpes transactivating protein (Vmw65) that is a component of the viral tegument. We also suggest that reactivation of latent infection does not involve destruction of neurons and is due to replication of virus at the peripheral mucocutaneous tissues to where virus or viral DNA have been transported from the nervous tissue. Alternatively, reactivation is initiated in the PSG using a replication cycle which does not involve irreversible damage to neurons. This model explains the lack of damage to neurons which continue to serve as permanent reservoirs of latent virus for the entire life of the host.

Latency-associated transcripts in corneas and ganglia of HSV-1 infected rabbits.

Herpes simplex virus (HSV) establishes latent infection in the sensory neuron and possibly in non-neuronal tissue, particularly the cornea. During latency only one region of the HSV genome is transcribed, producing RNAs known as latency associated transcripts (LAT). The gene for LAT overlaps with the HSV gene for the protein ICPO in the downstream regions of both genes. Latency can occur in the absence of LAT. This study reports the detection of ICPO/LAT and thymidine kinase (TK) gene fragments by the polymerase chain reaction in DNA extracted from the corneas and trigeminal ganglia of latently infected rabbits. Both genes were detected in four of four trigeminal ganglia tested and in three of five corneas tested. More importantly, this study reports the first detection of LAT in RNA extracted from 9% of corneas from latently infected rabbits (n = 22) by the polymerase chain reaction. LAT was detected in RNA from 100% of the corresponding trigeminal ganglia (n = 22). Although LAT is not essential for latency, it remains the only known molecular marker for latent HSV infections. Detection of LAT in these rabbit corneas suggests that HSV latency may occur in this non-neuronal tissue and that reactivation from non-neuronal tissue may occur at a low frequency in animals in which HSV latency has been established.

Effect of Herpes simplex virus infection on the trigeminal jaw-opening reflex in guinea pigs

Herpes simplex virus (HSV) infection induces numerous electrophysiological and microscopic changes in neurons in vitro. To investigate the effect of HSV infection on in vivo neuronal activity, we induced an acute, latent and reactivated HSV infection of the trigeminal ganglia of guinea pigs through orofacial HSV inoculation and studied its effect on the trigeminal jaw-opening reflex of anesthetized guinea pigs. During the acute viral infection period both the threshold for elicitation of the reflex, and the latency to the onset of the reflex response were increased. During the latent viral infection in the trigeminal ganglia, the jaw-opening reflexes in the viral infected animals were not different from those of non-infected control animals. However, reactivation of the latent viral infection in these animals resulated in increases in both the threshold and latency of the jaw-opening reflex. These changes were similar to those found in animals with the acute viral infection. These results indicate that acute or reactivated latent HSV infection of the nervous system results in functional changes in the reflex pathways involving the trigeminal gasserian ganglia and beurons harboring infectious HSV-1.

Characterization of acute and latent herpes simplex virus infection of dorsal root ganglia in rats.

The characteristics of HSV type-1 infection following subcutaneous inoculation in the dorsum of one hind paw of Sprague-Dawley rats were studied to determine whether infection in rats might more closely parallel the infection in man than is seen in other animals. The serologic and virologic characteristics of acute and latent ganglion infection conformed to those of human infection. Immunohistochemical studies suggested that sensory ganglion infection arose via centripetal axonal migration of virus as is hypothesized in man. In rat, small type B neuronal cell bodies appeared central to the maintenance of latent infection and reactivation observed during cocultivation of lumbar ganglia. Acute and latent lumbar sensory ganglion infection in rats after subcutaneous hind paw injection of HSV-1 appears to be another suitable model of this infection in man.

Role of Latency-Associated Transcript in Herpes simplex Virus Infection

Herpes simplex virus (HSV)-specified RNA transcript has been detected by in situ hybridization and by RNA blot analyses in latently infected peripheral nerve ganglia in experimentally infected mice and humans. Physical analysis of the nucleotide sequence, approximately 2,500, indicates possible splicing of the transcript into two overlapping open reading frames (ORF) and the presence of potential promoter elements. The latency-associated transcript, probably, is involved in the maintenance of latent herpes simplex virus infection; however, recent findings indicate that latent-phase transcription is involved in some in vivo reactivation.

Role of Herpes simplex Virus Thymidine Kinase Expression in Viral Pathogenesis and Latency

Herpes simplex virus (HSV) thymidine kinase (TK) expression and the HSV TK gene have been evaluated in studies of gene control, as well as in animal and human studies of viral pathogenesis, including HSV latency. In investigations of the biological role of HSV TK, enzyme expression was noted to be important for HSV infection of nonreplicating cells in culture; and, in experimental animal studies, HSV TK was shown to be important for in vivo latent infection of sensory ganglion neurons. Latency in these studies was determined by the ability of HSV to reactivate from sensory ganglion explants. In recent studies, investigators sought to determine whether the role HSV TK expression plays in latency is primarily in the establishment and maintenance of latency or in the reactivation process. Following infection of experimental animals with HSV TK-deficient mutants, the presence of HSV in ganglia was detected in complementation, rescue, and molecular biological studies. Results suggest that HSV TK expression may be important for HSV reactivation from latency. This was supported by in situ hybridization investigations. In the latter studies, HSV latency associated transcript (LAT) was present in ganglion neurons, although reactivation of HSV from such ganglia was defective. LAT-expressing, reactivation-defective infections established by TK mutants of HSV are considered examples of incomplete latency. From the present review, it appears that HSV TK expression, particularly TK expression of HSV-1, is important for the reactivation of latent HSV infection of sensory ganglion neurons, probably because of limited neuronal TK expression and absent replication capacity of these cells.

Acyclovir in the prevention of duodenal ulcer recurrence.

This study tests the hypothesis that reactivation of a latent herpes simplex virus infection may be a cause of recurrent duodenal ulceration. Patients with recently healed duodenal ulcer were entered into a double blind, randomised study of maintenance treatment with the antiviral drug acyclovir (400 mg bid) versus placebo, to determine if suppression of herpes virus infection would influence the natural history of the ulcer disease. One hundred and fifteen patients entered the trial and 76 patients completed it according to the protocol. Endoscopy was performed when ulcer symptoms recurred and at the end of the 25 week trial period. In the acyclovir group the cumulated relapse rate was 63% compared with 56% in the placebo group (NS). This result suggests that reactivation of herpes simplex virus is not a cause of recurrent duodenal ulcer.

The oral cavity as a port of entry for early infections in patients treated with bone marrow transplantation

Before treatment of 181 patients with bone marrow transplantation (BMT) for leukemia, severe aplastic anemia, or metabolic disorders, the oral condition was examined clinically and roentgenologically. Fifty-three patients (29%) had chronic dental infections (osteitis) that need treatment before BMT. In 10 of 181 cases (6%), BMT was postponed because of oral infections. Septicemia during the neutropenic phase was caused by oral microorganisms (alpha streptococci) in 24 of 59 (41%) patients with microbiologically proven septicemia. Septicemia with alpha streptococci was associated with graft-versus-host disease prophylaxis with methotrexate and subsequent increased frequency of oral ulcerations. No difference was observed in the frequency of reactivation of latent herpes simplex virus infection between different graft-versus-host disease prophylaxis regimens. Reactivation was more frequent in patients conditioned with total body irradiation than in patients conditioned without total body irradiation. Antiviral prophylaxis, with subsequent decreased frequency of oral herpes simplex reactivation, appeared to contribute to a low frequency of septicemia with streptococci.

Effects of the immunomodulating agent R837 on acute and latent herpes simplex virus type 2 infections

R837 is an immune modulator with no in vitro activity against herpes simplex virus (HSV). We evaluated topical R837 as therapy for genital HSV type 2 infection using the guinea pig model of this disease. Furthermore, we investigated the effect of R837 therapy on acute and latent neural HSV infections. Therapy initiated 12 h after viral inoculation and given twice a day significantly reduced the acute neural infection so that HSV was recovered from only 1 of 64 neural tissue specimens obtained from R837 recipients compared with 43 of 56 specimens obtained from placebo recipients (P less than 0.0001). R837 initiated 36 h after HSV inoculation and given once a day also significantly reduced the total mean lesion score of the acute disease from 14.1 +/- 4.3 to 2.6 +/- 5.3 (P less than 0.0001) and shortened the period of vaginal HSV shedding from 6.9 +/- 1.7 to 3.2 +/- 1.4 days (P less than 0.001). R837-treated animals also developed fewer HSV recurrences than did controls (2.0 +/- 1.7 versus 5.1 +/- 1.7; P less than 0.0002). Latent HSV was detected in 23 of 24 dorsal root ganglia explant cultures from placebo recipients but in only 2 of 30 cultures from R837-treated animals, and HSV in these 2 cultures was detectable only with the addition of a demethylating agent. Topical R837 exhibited in vivo anti-HSV activity, reducing both acute and latent neural infections as well as acute and recurrent genital disease.