Pathogenesis and Immunology of Herpesvirus Infections of the Nervous System

Application of a systemic herpes simplex virus type 1 infection in the rat as a tool for sunscreen photoimmunoprotection studies

The application of a novel model for sunscreen photoimmunoprotection studies was assessed using a systemic infection of rats with herpes simplex virus type 1 (HSV-1). Rats were irradiated daily with 1 minimal erythemal/oedematous dose of UVB for 7 consecutive days on their shaved backs with or without application of a broad-spectrum sunscreen (containing TiO2) with a sun protection factor of 10. Subsequently, rats were infected intranasally with HSV. UV exposure prior to HSV infection induced increased severity and incidence of clinical signs of disease, suppression of cellular immune responses as assessed by delayed type hypersensitivity and increased viral load in the brain. The sunscreen provided protection against all these UV-induced effects. We conclude that this novel model is a promising way of testing the immunoprotective qualities of sunscreens, based on the response to a common infectious agent of human subjects.

Gamma interferon expression during acute and latent nervous system infection by herpes simplex virus type 1

This study was initiated to evaluate a role for gamma interferon (IFN-gamma) in herpes simplex virus type 1 (HSV-1) infection. At the acute stage of infection in mice, HSV-1 replication in trigeminal ganglia and brain stem tissue was modestly but consistently enhanced in mice from which IFN-gamma was by ablated monoclonal antibody treatment and in mice genetically lacking the IFN-gamma receptor (Rgko mice). As determined by reverse transcriptase PCR, IFN-gamma and tumor necrosis factor alpha transcripts were present in trigeminal ganglia during both acute and latent HSV-1 infection. CD4+ and CD8+ T cells were detected initially in trigeminal ganglia at day 5 after HSV-1 inoculation, and these cells persisted for 6 months into latency. The T cells were focused around morphologically normal neurons that showed no signs of active infection, but many of which expressed HSV-1 latency-associated transcripts. Secreted IFN-gamma was present up to 6 months into latency in areas of the T-cell infiltration. By 9 months into latency, both the T-cell infiltrate and IFN-gamma expression had cleared, although there remained a slight increase in macrophage levels in trigeminal ganglia. In HSV-1-infected brain stem tissue, T cells and IFN-gamma expression were present at 1 month but were gone by 6 months after infection. Our hypothesis is that the persistence of T cells and the sustained IFN-gamma expression occur in response to an HSV-1 antigen(s) in the nervous system. This hypothesis is consistent with a new model of HSV-1 latency which suggests that limited HSV-1 antigen expression occurs during latency (M. Kosz-Vnenchak, J. Jacobson, D.M. Coen, and D.M. Knipe, J. Virol. 67:5383-5393, 1993). We speculate that prolonged secretion of IFN-gamma during latency may modulate a reactivated HSV-1 infection.

Experimental herpes simplex virus type 1 (HSV-1) infection of the spinal cord and dorsal root ganglia

Host factors determining the outcome of herpes simplex virus type 1 (HSV-1) infection within neurons are poorly understood. This paper aims to identify regional differences in the behaviour of HSV-1 within the nervous system as an approach to investigating the role of the host environment in determining the outcome of infection. We describe a mouse model of HSV infection focused on motor neurons of the spinal cord, resulting from intramuscular injection (i.m.) and compare this with the behaviour of virus within sensory neurons following scarification of virus on to skin. Viral antigen was detectable immunohistochemically by 2 days in both models and disappeared by 9-11 days. The time course of acute infection was reflected in the i.m. group by quantitative plaque assay for virus. Inflammation and cell destruction occurred in both models, but clinical features and histological destruction were greater in the group infected via the intramuscular route. In the sensory ganglia, a latent state from which virus could be reactivated by explanation, was established with LATS expression detectable in many neurons at 35 days post-infection (p.i.), but not in non-neuronal cells. Expression of latency associated transcript (LATS) was detected in motor neutrons in spinal cords at 35 days p.i. providing evidence for establishment of a LATS-positive latent state at this site, and continued to be detectable up to 6 months post-infection. In addition, LATS was detected in white matter at late times, suggesting a non-neuronal site of latency. In contrast to the behaviour in sensory ganglia, induced reactivation from spinal cords, by explanation and nerve section, was a very rare event. We have shown that a LATS-positive latent state can be established within motor neurons of the CNS, but that there are regional differences in the biology and outcome of infection between the CNS and peripheral nervous system. We propose that this may be a useful model to study reproducibly, the behaviour of HSV-1 in a CNS environment and, by comparison with sensory ganglion infection, to explore host factors which may underlie these regional differences. The relevance of this model for using HSV-1 as a therapeutic vector for motor neurons is also discussed.