Herpes simplex virus, type 1 invasion of the rabbit and mouse nervous systems revealed by in situ hybridization

Noncytotoxic lytic granule-mediated CD8+ T cell inhibition of HSV-1 reactivation from neuronal latency

Reactivation of herpes simplex virus type 1 (HSV-1) from neuronal latency is a common and potentially devastating cause of disease worldwide. CD8+ T cells can completely inhibit HSV reactivation in mice, with interferon-gamma affording a portion of this protection. We found that CD8+ T cell lytic granules are also required for the maintenance of neuronal latency both in vivo and in ex vivo ganglia cultures and that their directed release to the junction with neurons in latently infected ganglia did not induce neuronal apoptosis. Here, we describe a nonlethal mechanism of viral inactivation in which the lytic granule component, granzyme B, degrades the HSV-1 immediate early protein, ICP4, which is essential for further viral gene expression.

CD8 T cells and latent herpes simplex virus type 1: keeping the peace in sensory ganglia

Herpes simplex virus type 1 (HSV-1) infections represent a significant worldwide heath problem. The lack of an effective therapy to curtail reactivation of HSV-1 from a state of neuronal latency has lead to significant morbidity and mortality. Effective therapies to prevent reactivation must likely elicit a protective CD8 T-cell response that could act to prevent reactivation from sensory neurons prior to release of infectious virus at the periphery. This review focuses on the present understanding of how CD8 T cells maintain HSV-1 latency and how this knowledge could facilitate the generation of more effective therapeutic modalities.

Human herpesvirus latency

Herpesviruses are among the most successful human pathogens. In healthy individuals, primary infection is most often inapparent. After primary infection, the virus becomes latent in ganglia or blood mononuclear cells. Three major subfamilies of herpesviruses have been identified based on similar growth characteristics, genomic structure, and tissue predilection. Each herpesvirus has evolved its own unique ecological niche within the host that allows the maintenance of latency over the life of the individual (e.g. the adaptation to specific cell types in establishing latent infection and the mechanisms, including expression of different sets of genes, by which the virus remains latent). Neurotropic alphaherpesviruses become latent in dorsal root ganglia and reactivate to produce epidermal ulceration, either localized (herpes simplex types 1 and 2) or spread over several dermatomes (varicalla-zoster virus). Human cytomegalovirus, the prototype betaherpesvirus, establishes latency in bone marrow-derived myeloid progenitor cells. Reactivation of latent virus is especially serious in transplant recipients and AIDS patients. Lymphotropic gammaherpesviruses (Epstein-Barr virus) reside latent in resting B cells and reactivate to produce various neurologic complications. This review highlights the alphaherpesvirus, specifically herpes simplex virus type 1 and varicella-zoster virus, and describes the characteristics of latent infection.

Herpes simplex virus as a transneuronal tracer

Determining the connections of neural systems is critical for determining how they function. In this review, we focus on the use of HSV-1 and HSV-2 as transneuronal tracers. Using HSV to examine neural circuits is technically simple. HSV is injected into the area of interest, and after several days, the animals are perfused and processed for immunohistochemistry with antibodies to HSV proteins. Variables which influence HSV infection include species of host, age of host, titre of virus, strain of virus and phenotype of infected cell. The choice of strain of HSV is critically important. Several strains of HSV-1 and HSV-2 have been utilized for purposes of transneuronal tract-tracing. HSV has been used successfully to study neuronal circuitry in a variety of different neuroanatomical systems including the somatosensory, olfactory, visual, motor, autonomic and limbic systems.

An improved rapid method for purification of herpes simplex virus DNA using cesium trifluoroacetate

A method for purification of herpes simplex virus DNA from cell culture is described which yields highly purified viral DNA within 8 h. The method involves the freezing and thawing of virus-infected cells followed by isopycnic centrifugation of the lysate supernatant in cesium trifluoroacetate. It was found that this method recovered DNA from most of the cell-associated virus particles in such sufficient purity that the DNA was digestible with restriction enzymes and could be used to transfect cells without the need for additional purification steps. Purification of viral DNA from cells that were not subjected to freezing and thawing was less efficient due to the amount of viral DNA that remained cell-associated.

Cloning and restriction endonuclease mapping of herpes simplex virus type-1 strains H129 and +GC

EcoRI fragments of herpes simplex virus I (HSV-1) strains H129 and +GC were cloned and the EcoRI and BglII restriction enzyme sites were mapped. Comparison of these enzyme sites with the sequence of HSV-1 strain 17syn+ demonstrated that all EcoRI sites were identical. For H129, the BglII sites were also found to match strain 17syn+ BglII sites. With one exception, the BglII sites in strain +GC also aligned with the strain 17syn+ sequence. The one exception was a missing BglII site from strain +GC located between bases 25,149 and 25,154 in the EcoRI D fragment within the viral deoxyribonuclease gene (UL12). The BglII site represents the first difference to be mapped within HSV-1 strains H129 and +GC which have unique pathobiological properties in animal models of acute and reactivated infections.

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.

Nervous system inflammatory lesions and viral nucleic acids in rabbits with herpes simplex virus encephalitis-induced rotational behaviour

Rabbits with herpes simplex virus (HSV) encephalitis induced by corneal virus challenge exhibit rotational behaviour linked with altered brain dopamine functions. The neuropathology and the distribution of the HSV-specific nucleic acids were studied, using probes for the viral trans-inducing factor alpha TIF and for the latency-associated transcript LAT-1 RNA to detect productive and latent infections, respectively. The rotational behaviour began 4 days after inoculation, and at that time the inflammatory process was observed only in the brain stem and the productive infection, revealed by in situ hybridisation, was seen in the trigeminal entry and nuclei. No HSV-specific nucleic acids or neural destruction were observed in the regions of the serotoninergic raphe or dopaminergic substantia nigra. At 8 days after inoculation, when the rotational behaviour was beginning to attenuate, the inflammatory lesions spread into the hemispheres, involving particularly the ventral parts of the limbic system including the olfactory system. In no cases were HSV-specific nucleic acids detected in the olfactory system. The inflammation in the limbic system was also detectable in animals without inflammatory lesions in the olfactory bulbs or tracts, suggesting that the infection had spread from the brain stem. The present study shows that in this model the altered neurotransmitter functions observed previously, appearing as rotational behaviour, occur without productive infection or necrosis, suggesting specific interaction of HSV with monoaminergic neurons. Additionally, the results suggest that HSV could reach the limbic system via ascending serotoninergic projections from the raphe neurons.

Herpes simplex virus type 1 strain KOS-63 does not cause acute or recurrent ocular disease and does not reactivate ganglionic latency in vivo

The virological, clinical, and histopathological manifestations of acute and experimentally reactivated infections of eyes and trigeminal ganglia have been studied following intranasal infection of rabbits with herpes simplex virus type 1 (strain KOS-63). All animals shed virus in nasal secretions, but only three shed virus in tear film during the first 12 days of infection. No animal developed clinical or histological evidence of corneal or retinal ocular disease at any time after infection. KOS-63 established trigeminal ganglionic latency; viral RNA, restricted to neuronal nuclei, was detected by in situ hybridization, and virus was recovered from co-cultivation cultures of nervous tissue, but not from cell-free homogenates. Reactivation of latent trigeminal ganglionic infection was attempted by intravenous administration of cyclophosphamide, followed by dexamethasone 24 h later. Injection of the drugs failed to reactivate KOS-63 latency; no animal shed virus in nasal or ocular secretions, and no animal developed gross or microscopic corneal lesions. In addition, viral antigens were not detected by immunofluorescence microscopy in ganglia from rabbits subjected to the drug protocol, and virus was only recovered from ganglia by in vitro co-cultivation reactivation techniques. The failure of KOS-63 to reactivate was not due to an inherent failure of populate and infect the ganglion, because the virus did not reactivate from ganglia that contained many latently infected cells. These studies demonstrate that, although KOS-63 is neuroinvasive and capable of establishing latency, it is virtually nonvirulent for the eye, and cannot be reactivated by a systemic immunosuppressive trigger known to reactivate other HSV-1 strains.

Targets of herpes simplex virus type 1 infection in a mouse corneal model

In animal models, spread of herpes simplex virus type 1 (HSV-1) from epithelial replication sites to the peripheral and central nervous system is known from analysis of individually dissected tissues. To examine virus spread in undissociated tissues, corneas of adult mice were inoculated with HSV-1. After 1 to 13 days groups of mice were perfused with formalin, and decalcified blocks of head and neck were embedded in paraffin. At intervals, serial sections were screened for HSV antigen. On days 1 and 2, viral antigen was restricted to cornea and conjunctiva but by days 3 and 4 was also seen in autonomic ganglia and the trigeminal system. On day 6, HSV antigen reached its maximum extent; infected sites included the trigeminal complex (ganglion, root, peripheral ophthalmic and maxillary branches and spinal nucleus and tract), ethmoid sinus and olfactory bulb, visual system, and autonomic ganglia (ciliary, pterygopalatine and superior cervical). Antigen progressively diminished on days 8 and 10, and was not detected on day 13. This method demonstrates a broader range of infected tissues and suggests a more complex pattern of HSV spread than has been previously recognized. Virus appears to reach the intracranial compartment by four different neural routes. When effects of higher and lower corneal inoculation doses were compared, a lower dose resulted in lower peak HSV titers in trigeminal ganglion and brain stem and later virus appearance in these tissues. Thus, dose may influence the kinetics of HSV spread from the peripheral inoculation site to the CNS.

Cellular neuroimmunologic responses to ocular herpes simplex virus infection

Infectious herpes simplex virus type 1 (HSV-1) was cultivated from the trigeminal ganglion between days 3 and 21 after ocular infection. T lymphocytes were first seen 9 days after infection and were present in ganglia collected 21 days after corneal infection. Neuron cell bodies expressing cytoplasmic HSV-1 antigens were present in the ganglion by 6 days after infection and could be found up to 21 days after infection although the frequency was low and decreased with time. Neuron cell bodies containing nuclear viral DNA were seen with the same frequency as cells expressing cytoplasmic viral antigens. T lymphocytes were seen surrounding neuron cell bodies some of which contained either cytoplasmic HSV-1 antigens or nuclear HSV-1 DNA.

Herpes simplex virus type I infection of the CNS induces major histocompatibility complex antigen expression on rat microglia

Rats were infected with herpes simplex virus type I (HSV-1) by corneal scarification. The spread of virus in the brain, the infiltration of leucocytes into infected areas, and the expression of major histocompatibility complex (MHC) glycoproteins by brain cells were assessed as a function of time by immunohistochemistry. Virus moved along neuronal pathways, achieving widespread distribution in the brain by days 8-10 when the illness appeared most severe. Granulocytes, T-lymphocytes, and monocytes infiltrated the tissue matrix at sites of infection. Microglial cells were induced to express MHC class I and class II glycoproteins. Reactive microglia near the sites of infection most vigorously expressed such glycoproteins. At the peak of the infection they were detectable on microglia throughout the brain, including areas apparently separated from active infection. Evidence of viral antigens, as well as microglial MHC expression, had largely disappeared by day 30. Neurons, astrocytes, and oligodendroglial cells failed to express MHC antigens.

Spread of herpes simplex virus type 1 in the central nervous system during experimentally reactivated encephalitis

Because many of the features of reactivated herpes simplex virus type 1 (HSV-1) central nervous systems (CNS) infections in vivo are incompletely understood, we used an animal model to study the development of the morphological, ultrastructural, radiological and immunological changes which occurred during acute and experimentally reactivated diseases. Rabbits were intranasally inoculated with HSV-1, and their latent trigeminal ganglionic and CNS infections were reactivated by intravenous injection of cyclophosphamide and dexamethasone. Technetium brain scans were performed to localize areas of blood-brain barrier breakdown, and cerebrospinal fluid (CSF) was analysed for IgG content by radial immunodiffusion assays. Nervous system tissues were studied by in situ hybridization and by immunofluorescent, light and electron microscopic techniques. Diffuse uptake of technetium was observed as HSV-1 spread transsynaptically into the brain during the acute phase of infection, and viral antigens and nucleic acids were detected in both the CNS olfactory and trigeminal systems. During latency, viral RNA was detected in the nuclei of neurons within the CNS olfactory cerebral and entorhinal cortices, indicating that HSV-1 became latent within the same CNS structures that were involved during the acute phase of infection. Following drug-induced reactivation, the brain scans revealed a more focal breakdown of the blood-brain barrier, and both neurons and neuronal processes in the entorhinal and olfactory cortices contained viral nucleic acids which correlated with the ultrastructural presence of HSV-1 virions. During the reactivated phase of infection a marked increase in the CSF IgG index occurred without an increase in the CSF: serum albumen ratio indicating a prompt intrathecal response in infected rabbits as compared to controls. To some extent, the CSF IgG index reflected the degree of histopathological damage.