Evidence for herpes simplex viral latency in the human cornea

Diagnosis of Fusarium keratitis in an animal model using the polymerase chain reaction

AIMS/BACKGROUND

The purpose of this study was apply the polymerase chain reaction (PCR) to develop a sensitive, specific, and rapid test to diagnose Fusarium keratitis. Fusarium is the most common cause of fungal corneal infection in some parts of the world. It is often difficult to establish that a keratitis is due to fungal infection.

METHODS

Fusarium solani keratitis was induced in three eyes of three rabbits by injection of a suspension of the fungus into the anterior corneal stroma. In one rabbit the contralateral eye served as a control. From four to 28 days after inoculation, the corneas were scraped for culture, then scraped and swabbed for PCR analysis. The PCR was performed with primers directed against a portion of the Fusarium cutinase gene, and the presence or absence of this amplified target sequence was determined by agarose gel.

RESULTS

The amplified DNA sequence was detected in 25 of 28 samples from the corneas infected with Fusarium, for a sensitivity of 89%. Only three of the 14 samples from these eyes with Fusarium keratitis were positive by culture, for a sensitivity of 21%. Seven of eight control samples were negative by the PCR based test, for a specificity of 88%.

CONCLUSION

This PCR based test holds promise of being an effective method of diagnosing Fusarium keratitis as well as Fusarium infections at other sites.

Herpes simplex virus genome replication and transcription during induced reactivation in the rabbit eye

PCR analysis of herpes simplex virus (HSV) genome replication and productive-cycle transcription was used to examine the role of the cornea in the latency-associated transcript (LAT)-mediated reactivation of HSV type 1 (HSV-1) in the rabbit eye model. The reduced relative reactivation frequency of 17 delta Pst (a LAT- virus) compared to those of wild-type and LAT+ rescuants correlated with reduced levels of viral DNA and transcription in the cornea following epinephrine induction. The timing of virus appearance in the cornea was most consistent with tissue peripheral to the cornea itself mediating a LAT-sensitive step in the reactivation process. Specific results include the following. (i) While viral DNA was found in the corneas of rabbits latently infected with either the LAT+ or LAT- virus prior to and during the first 16 to 24 h following induction, more was found in animals infected with the LAT+ virus. (ii) A significant increase in levels of viral DNA occurred 20 to 168 h following induction. (iii) The average relative amount of viral DNA was lower at all time points following reactivation of animals infected with the LAT- virus. (iv) Expression of productive-cycle transcripts could be detected in corneas of some rabbits latently infected with either the LAT+ or LAT- virus, and the amount recovered and the timing of appearance differed during the reactivation of rabbits latently infected with the LAT+ or LAT- virus. (v) Despite the reduced recoveries of LAT- virus DNA and productive-cycle transcripts in reactivating corneas in vivo compared to those of their LAT+ counterparts, such differences were not detected in cultured keratinocytes or in experiments in which relatively high titers of virus were superinfected into the eyes of latently infected rabbits. (vi) A number of LAT(+)-virus-infected rabbits expressed LAT in corneas isolated from uninduced rabbits. When seen, its amount was significantly higher than that of a productive-cycle (VP5) transcript.

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.

Low rate shedding of HSV-1 DNA, but not of infectious virus from human donor corneae into culture media

Fluid samples derived from 451 organ cultured corneae were tested for the presence of HSV-1 DNA after electroseparation and amplification for fragments of the glycoprotein D- and thymidine kinase-encoding genes. Of the culture media, 134 were processed immediately after withdrawal (Group 1); 100 were stored at ambient temperature for 6 to 60 weeks (Group 2); 90 were stored at -8 degrees C for 4 to 9 weeks (Group 3); and 127 were stored at -20 degrees C for 2 to 30 weeks (Group 4). The degradation of human DNA (marker gene, betaglobin) under these different storage conditions and of human and HSV-1 DNA as a sequential function of time at ambient temperature was gauged by the loss of a detectable signal for the respective component. Endothelial cell density within each of the corneal discs was determined before and after organ culture. In 7/451 culture fluid samples, HSV-1 DNA corresponding to either the glycoprotein D- or thymidine kinase-encoding genes was detected. In culture fluid samples derived from Group 2 at ambient temperature, for 6 to 60 weeks) and 3 (at -8 degrees C, for 4 to 9 weeks), complete degradation precluded the detection of human DNA, and hence probably also of HSV-1 DNA; only at -20 degrees C did DNA remain stable for protracted periods of time. Even so, HSV-1 DNA was detected in only 2% of those media in which no degradation was to be expected; additionally, there existed no correlation between its presence in culture fluid samples and the loss of endothelial cells or cytopathic changes. DNA can be extracted successfully and concentrated twenty-fold from high-volume samples by electroseparation. When shed into culture fluid, it is remarkably prone to a time and temperature dependent degradation, which may lead to false negative results. It is concluded that there is no infectious virus to be expected in the specimens; the occurrence of HSV-1 DNA in donor corneae would not appear to be an important factor influencing their biological quality during the period of organ culture.

Newly acquired herpes simplex virus keratitis after penetrating keratoplasty

BACKGROUND

After penetrating keratoplasty for reasons unrelated to herpes simplex virus (HSV) keratitis, any nonspecific epithelial defect may still be caused by HSV. The purpose of this study is to determine the incidence of newly acquired herpetic keratitis and to assess contributing factors.

METHODS

The authors retrospectively studied the results of 2398 penetrating keratoplasties performed between 1980 and 1995. Three typical case histories are discussed.

RESULTS

Of 2112 patients in whom the primary diagnosis was not related to HSV keratitis, 18 presented with epithelial herpetic keratitis in their corneal graft. The incidence of newly acquired herpetic keratitis after penetrating keratoplasty was 1.2 per 1000 person-years. In most cases, the infection occurred in the first 2 years after the transplantation. Most often, well-known reactivating stimuli could have caused the HSV infection.

CONCLUSIONS

Herpes simplex virus keratitis may develop after penetrating keratoplasty even without a clinical history of HSV in the host. Thus, HSV should be considered in the differential diagnosis of a postpenetrating keratoplasty epithelial defect. The high incidence of this infection in the first 2 years after such surgery suggests a causal relation between corneal transplantation and the HSV infection.

Detection of herpes simplex virus DNA in donor cornea culture medium by polymerase chain reaction

AIMS/BACKGROUND

Herpes simplex virus (HSV) may establish latent infection in the cornea and therefore be transmissible by corneal transplantation. Monitoring of donor cornea culture medium was evaluated for HSV infection.

METHODS

HSV was sought using virus isolation in cell culture, and its DNA was amplified to detectable levels using the polymerase chain reaction (PCR).

RESULTS

Virus isolation in cell culture was negative on neat, cell pellet, and cell free supernatant prepared from the spent culture media of 80 corneas. Three cell pellets (3.8%) were positive for HSV DNA. The PCR positive culture negative results might have reflected latent rather than active HSV infection of the cornea. Post transplant follow up of the three recipients of corneas with HSV PCR positive organ culture media revealed no evidence of HSV induced eye disease or primary graft failure.

CONCLUSION

Screening of corneal culture medium for HSV by virus culture or for HSV DNA by PCR could not be recommended.

Herpes simplex virus DNA in normal corneas: persistence without viral shedding from ganglia

Herpes simplex virus type 1 (HSV-1) DNA has been shown to persist in the cornea not only after inoculation of experimental animals but also in surgical samples from patients with herpes keratitis. The further observation of corneal HSV-1 DNA in subjects without known HSV eye disease prompted the present study of the presence and distribution of HSV-1 in eye bank corneas. Prior to DNA extraction, the corneas were trephined, separating the central and peripheral cornea. With polymerase chain reaction (PCR) for HSV-1 thymidine kinase (TK) and glycoprotein D (gD) gene sequences, we found HSV-1 in 10 of 24 eye bank corneas, from the 4 mm wide corneal rim in 8 eyes and from the 8 mm diameter central cornea in 2 eyes. In 9 subjects, both eyes were assayed, and HSV-1 was detected in 6 subjects. In only one subject was HSV-1 detected in both eyes and in only one subject was HSV-1 detected in the central and peripheral cornea of the same eye. The biological role of HSV-1 DNA corneal sequences is unknown. To investigate this, a rabbit animal model was established by transplantation of corneas containing viral DNA sequences in HSV-1 naive recipients. Followed for 5 months, there was no evidence of sheeding of HSV-1 in the tear film or seroconversion of the recipient rabbits. At the end of this time, HSV-1 DNA was detected in the corneal graft at a similar intensity to the PCR signal from the donor rims.(ABSTRACT TRUNCATED AT 250 WORDS)

Recurrence and rejection rates following corneal transplantation for herpes simplex keratitis

Seventy-two penetrating primary keratoplasties for herpes simplex keratitis performed from 1970 to 1993 were reviewed. Postoperative follow-up periods ranged from 6 months to 16 years with 2 years median. Epithelial herpetic recurrence occurred in 25% of the transplants the first year and in 44% during the first 2 years. Primary allograft rejection was observed in 29% of the grafts the first year and in 46% during the first 2 years. Preoperative inflammation was a risk factor for preservation of a clear cornea (p < 0.10). The degree of corneal vascularization prior to operation did not influence the survival of the grafts. The overall one-year survival rate of a clear graft was 84% and the 2-year survival rate was 67%. Prophylactic antiviral treatment is discussed.

Examination of non-involved skin, previously involved skin, and peripheral blood for herpes simplex virus DNA in patients with recurrent herpes-associated erythema multiforme

The association between infection with HSV and the subsequent development of erythema multiforme is well established, although the role that the virus plays in the pathogenesis of this disorder is not known. HSV DNA has been detected in cutaneous lesions of herpes-associated erythema multiforme (HAEM), and it has been suggested that the tissue damage seen in these lesions is virus-specific. In the current, prospective study, we examined biopsies of lesional, non-involved, and previously involved but healed skin, in addition to specimens of peripheral blood, from patients with HAEM, for HSV DNA by using the polymerase chain reaction. HSV DNA was detected in lesional skin of 10 of 11 patients compared to 2 of 11 non-involved skin biopsies obtained at the same time. HSV was present in 4 of 6 blood specimens obtained during the acute episode. Five patients returned 3 months after the acute episode resolved for biopsies of previously involved skin. HSV was detected in 4 of these 5 biopsies. Thus, the presence of HSV DNA in the skin of patients with HAEM appears to be predominantly in areas of clinical involvement; the virus remains in those cutaneous sites for up to 3 months without evidence of clinical disease; and HSV DNA may be detected in the peripheral blood cells during acute HAEM. Based on these findings, we suggest that the virus plays a role in lesion development, that the skin may function as a site of viral persistence, and that hematogenous spread of viral DNA may be an important factor in the development of HAEM.

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.

Latent infection with the MS strain of herpes simplex virus type 2 in the mouse following intracerebral inoculation

Intracerebral inoculation of the MS strain of herpes simplex virus type 2 (HSV-2) into mice causes an acute encephalitis associated with multifocal demyelination and necrotizing retinitis. We have studied the distribution of latent virus in mice that had recovered from the acute encephalitis. Four weeks or longer after inoculation, HSV-2 could be recovered from the trigeminal ganglia of all mice examined by co-culture of explants in roller tubes. The virus could not be recovered from explants of retina or brain stem. HSV-2 latency associated transcript (LAT) was readily detected in the trigeminal ganglia by reverse transcriptase-PCR more than 4 months after inoculation. LAT was also demonstrated in the brain but this required nested PCR for consistent detection. Both LAT and ICP0 mRNA were detected in brain tissue during the acute encephalitis but, unlike LAT, ICP0 mRNA could not be amplified from the trigeminal ganglia or brain beyond 4 weeks after inoculation of the virus. In situ hybridisation with a double-stranded DNA probe to the ICP0/LAT overlap region of HSV-2 revealed signal in trigeminal ganglion neurons and occasional cells in the brain stem. These findings indicate that HSV-2 introduced by intracerebral inoculation becomes latent in the trigeminal ganglia and that transcription of LAT also persists within the brain.

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.

Prophylactic chemotherapy with acyclovir for recurrent herpes simplex labialis

The medical literature was reviewed and 11 clinical trials of prophylactic topical or peroral acyclovir for the suppression of recurrent herpes simplex labialis were identified. The results of these trials showed that prophylactic topical acyclovir was mostly ineffective, but that prophylactic peroral acyclovir, in doses ranging from 400 to 1,000 mg/day, reduced the frequency of herpes labialis during treatment by 50-78%. The reduction in the frequency of episodes of herpes labialis with acyclovir prophylaxis is less than the suppressive effect that has been reported for herpes genitalis (50-78% vs. 80-90%). In trials of prophylactic acyclovir for herpes labialis induced by experimental ultraviolet radiation, 26% of induced lesions developed within 48 hours of radiation exposure ("immediate" lesions) and, in contrast to "delayed" lesions that developed 2-7 days post-irradiation, were not suppressed by the antiviral compound. It is proposed that these treatment-unresponsive immediate lesions have an atypical pathogenesis, possibly involving latency of herpes simplex virus in the labial epithelium, and that these may be responsible for the apparent difference between herpes labialis and genitalis in the degree of benefit from prophylactic acyclovir therapy.

Effects of sodium butyrate on the expression of sodium channels by neuronal cell lines derived from the rat CNS

We have studied the effects of sodium butyrate on cell morphology and the expression of mRNAs encoding voltage-gated sodium channels in five neuronal cell lines, B35, B50, B65, B103 and B104, all derived from the rat CNS. The cells were grown in medium supplemented with 2.5 mM sodium-n-butyrate and examined daily by phase-contrast microscopy. Sodium butyrate caused slowing of cell division and the formation of longer and more highly branched cytoplasmic processes than were present in untreated cells. Expression of sodium channel mRNA was analysed by PCR with primers that allow the transcripts encoding the different types of sodium channel to be distinguished according to the lengths of the PCR products. The identity of the PCR products was confirmed by restriction enzyme digestion. Southern blotting and hybridization with internal radiolabelled probes. Prior to sodium butyrate treatment, expression of sodium channel mRNA was largely restricted to B50 and B104 cells: B50 cells showed expression of rat brain types I and II sodium channel and B104 cells expressed rat brain type III sodium channel. After treatment for 5 days with sodium butyrate, sodium channel mRNA was detected in all five cell lines. In addition to type I and type II sodium channel, B50 cells expressed rat brain type III sodium channel. These three types of sodium channel were also expressed by B35, B65 and B103 cells. Even after butyrate treatment, B104 cells expressed only type III sodium channel. The treatment also induced expression of rat skeletal muscle SkM1 sodium channel in B35 cells but only trace amounts in the other neuronal cell lines.

Reactivation of latent herpes simplex virus by excimer laser photokeratectomy

We tested whether excimer laser photorefractive and phototherapeutic keratectomy may reactivate latent herpes simplex and cause recurrent keratitis in mice. Two of ten latently infected mice that were treated with ten excimer laser pulses to the corneal epithelium shed herpes simplex virus type 1, as did four of ten mice that were treated with 50 excimer laser pulses. Ocular shedding of herpes simplex virus was detected in four of ten mice that were treated with ethylenediamine-tetraacetic acid (EDTA) scraping of the corneal epithelium without laser keratectomy, and in six of ten mice on which combined EDTA-facilitated epithelial removal was performed followed by the application of ten excimer laser pulses. In both EDTA-treated groups, viral shedding was prolonged and 18 of 20 mice developed marked corneal opacification or neovascularization, or both. Corneal photoablation with the excimer laser may induce reactivation of latent herpes simplex virus, even in mice with clear and smooth-appearing corneas, and should be considered in the differential diagnosis of humans with persistent corneal epithelial defects after refractive or therapeutic excimer procedures.

Non-traumatic acquisition of herpes simplex virus infection through the eye

Primary ocular herpes is usually seen as a follicular conjunctivitis and blepharitis, with or without involvement of the cornea. It is unknown, however, to what extent asymptomatic and/or subclinical primary disease occurs, and whether primary ocular herpes follows direct droplet spread to the eye. Previous models of murine ocular herpes have used trauma (scarification) to introduce virus into the cornea, producing disease which results in significant corneal scarring. To mimic a likely route of infection in humans, a droplet containing virus was placed on the mouse eye and clinical disease recorded. At least 1 month after inoculation, serum was assayed for neutralising antibodies and the cornea, iris, and trigeminal ganglion were investigated for evidence of herpes simplex virus type 1, by cocultivation and the polymerase chain reaction. Some animals showed a severe ulcerative blepharitis with little to no involvement of the cornea, while disease was undetectable in others. The development of disease depended on the dose and strain of virus and age of the animal, with older mice appearing more resistant. Virus was isolated from the trigeminal ganglion of younger animals inoculated with higher doses of virus, after 21 days in culture, suggesting that latency had been established. Neutralising antibodies were present in most mice irrespective of the presence of recognisable clinical disease. Using primers for the thymidine kinase and glycoprotein C regions of the viral genome, herpes simplex virus type 1 DNA was found in the cornea, iris, and trigeminal ganglion of most animals and showed a good correlation with the presence of neutralising antibodies. It would thus appear that herpes simplex virus type 1 is able to accede into the cornea, iris, and trigeminal ganglion following nontraumatic application of virus onto the mouse eye. This model mimics primary ocular disease in humans and may be useful for studies on recurrent disease and the spread of ocular herpes.

Biology and molecular aspects of herpes simplex and varicella-zoster virus infections

The herpes simplex and varicella-zoster viruses are members of the subfamily alpha herpesviruses with specific properties of the virion and with the capacity to establish latent infections in humans. The genome of each of these viruses has been determined with an estimate of the number of genes and proteins encoded. The biology and molecular events of the herpes simplex virus productive and latent infection have been detailed with the use of both in vitro and in vivo model systems. The neuron is the site of latency in the ganglia with a limited transcription of genes expressed during the latent period. The specific molecular regulation of latency and reactivation are not well established. There are co-cultivation, electron microscopy, and biochemical studies that support the concept of corneal latency, although this has not been proven conclusively. Details about the varicella-zoster virus biology and molecular events are not as well advanced since animal models have been lacking. The biology of the productive infection (varicella) is different from herpes simplex virus infection since the portal of entry is the respiratory system. Data support the concept of the maintenance of latency within satellite cells in the ganglia rather than within neurons. There are multiple genes expressed during this latency. These features may explain the different clinical presentations and course of reactivation (zoster) compared with herpes simplex virus reactivation.

Immunofluorescent staining and corneal sensitivity in patients suspected of having herpes simplex keratitis

We examined immunofluorescent staining and corneal sensitivity in 25 control subjects (25 eyes) with normal corneas, six patients (eight eyes) with possible herpes simplex keratitis, and 44 patients (48 eyes) with corneal lesions (recurrent erosion, superficial punctate keratitis, marginal ulcer, and follicular keratoconjunctivitis) in whom herpes simplex keratitis was not suspected. On immunofluorescent staining, all 25 control subjects had negative reactions, all eight eyes suspected of having herpes simplex keratitis had positive reactions, and 11 (23%) of the 48 eyes not suspected of having herpes simplex keratitis had positive reactions; the remaining 37 eyes had negative reactions. Of the 11 eyes not suspected of having herpes simplex keratitis but that had positive reactions on immunofluorescent staining, nine had recurrent erosions and the remaining two eyes had superficial punctate keratitis. Of the eight eyes with possible herpes simplex keratitis, seven (88%) had decreased corneal sensitivity. Of the 11 eyes not suspected of having herpes simplex keratitis but that had positive reactions on immunofluorescent staining, eight (73%) had decreased corneal sensitivity. Of the 37 eyes not suspected of having herpes simplex keratitis that had negative reactions on immunofluorescent staining, 11 (30%) had decreased corneal sensitivity.

Detection of viral DNA within skin of healed recurrent herpes simplex infection and erythema multiforme lesions

The polymerase chain reaction (PCR) was used to detect HSV DNA in genomic DNA extracted from skin biopsies obtained from healed skin of five patients with hyperpigmented macules following recurrent cutaneous HSV infections and from eight patients with HSV-associated erythema multiforme (EM). A 92-bp HSV-1 DNA fragment was found in all the skin biopsies from the site of recurrent HSV infection and in five of eight (62%) biopsies from the EM patients. Virus DNA was not found in tissues distant from the site of HSV recurrence or from a patient without a history of HSV infection. These findings confirm the presence of HSV in healed skin from the site of recurrent HSV disease and are consistent with the concept that HSV is involved in EM pathogenesis.