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Although herpes simplex virus type 1 (HSV-1) is known to reside latently in trigeminal ganglia between episodes of reactivation, the mechanisms involved in restricting the virus to this state are not understood. Using in situ nucleic acid hybridization methods, we show that there is HSV-encoded RNA in ganglion cells from 10 of 12 seropositive and zero of three seronegative patients studied at autopsy. Transcripts mapping to the region encoding the immediate-early polypeptide ICPO and the latency-associated transcript (LAT) were detected in the nuclei of these neurons. No other region of the HSV-1 genome was found to be expressed. When carefully defined probes were used to identify the transcripts, RNA corresponding to the LAT and, rarely, to ICPO was found. These results suggest that HSV-1 latency is an active process and that the LAT may be involved in regulating viral genetic expression.

Thirteen temperature-sensitive (ts) mutants of HSV-1 were analysed for their capacity to establish latent infections in the brains of mice. Eleven of the mutants could be classified as latency-positive or -negative; two could not be assigned to either group. Leakiness of mutants in the brain and differences in particle/infectivity ratios were found not to play a role in the results. Ts+ revertants of selected latency-negative mutants regained the capacity to establish latent infections, indicating that it was the ts lesion in these agents which was involved in latency. Ultrastructural studies of neuroblastoma cells infected with various mutants and maintained at the restrictive temperature showed that no absolute correlations could be made between capacity to establish latent infection and synthesis of various morphologically identifiable virus products. Finally, from a comparison of latency characteristics with previously established polypeptide phenotypes of mutants it was concluded that one immediate early and one or more later virus functions are necessary for establishment and/or maintenance of the latent state.

In attempts to reactivate latent herpes simplex virus, we instilled Diplococcus pneumoniae intratracheally into mice harboring latent infections in sacrosciatic spinal ganglia. All mice developed a severe pneumonia within 24 h and were given penicillin therapy. Representative mice that survived the penumonia were sacrificed at daily intervals, and appropriate tissues were examined for evidence of viral reactivation. Herpes simplex virus was reactivated in the ganglia and appeared to travel both proximally and distally in associated nerve trunks. Clinically apparent disease due to the virus was not detected in any mice.

The spectrum of tissues harbouring latent herpes simplex virus following intravenous inoculation of mice was defined by in vitro co-cultivation techniques. The virus could be detected in central and peripheral nervous systems (including adrenal medulla), but could not be found in any non-neural tissues. Spinal ganglia were the organs most commonly involved. The relationship of these findings to the natural history of herpetic infections is discussed.

The cell type(s) harboring latent herpes simplex virus in the spinal ganglia of mice was investigated. Taken together, results from immunofluorescence, electron microscopic, autoradiographic, and in situ nucleic acid hybridization methods suggest strongly that, in mice, latent virus is maintained in neurons.

The pathogenesis of acute herpetic infection in the nervous system has been studied following rear footpad inoculation of mice. Viral assays performed on appropriate tissues at various time intervals indicated that the infection progressed sequentially from peripheral to the central nervous system, with infectious virus reaching the sacrosciatic spinal ganglia in 20 to 24 hr. The infection also progressed to ganglia in mice given high levels of anti-viral antibody. Immunofluorescent techniques demonstrated that both neurons and supporting cells produced virus-specific antigens. By electron microscopy, neurons were found to produce morphologically complete virions, but supporting cells replicated principally nucleocapsids. These results are discussed in the context of possible mechanisms by which herpes simplex virus might travel in nerve trunks. They are considered to offer strong support for centripetal transport in axons.