Efficient reactivation of latent herpes simplex virus from mouse central nervous system tissues

For decades, numerous ex vivo studies have documented that latent herpes simplex virus (HSV) reactivates efficiently from ganglia, but rarely from the central nervous systems (CNS), of mice when assayed by mincing tissues before explant culture, despite the presence of viral genomes in both sites. Here we show that 88% of mouse brain stems reactivated latent virus when they were dissociated into cell suspensions before ex vivo explant culture. The efficient reactivation of HSV from the mouse CNS was demonstrated with more than one viral strain, viral serotype, and mouse strain, further indicating that the CNS can be an authentic latency site for HSV with the potential to cause recurrent disease.     

Nerve growth factor deprivation results in the reactivation of latent herpes simplex virus in vitro.

Primary sympathetic neuronal cultures were maintained for up to 5 weeks after inoculation with herpes simplex virus (HSV) without evidence of viral infection. Upon deprivation of nerve growth factor, the cultures produced infectious HSV, indicating that the cultures harbored latent HSV. This study demonstrates a function of nerve growth factor in the maintenance of HSV latency.     

Radiation enhanced reactivation of herpes simplex virus: effect of caffeine.

Ultaviolet enhanced (Weigle) reactivation of UV-irradiated herpes simplex virus in UV-irradiated CV-1 monkey kidney cell monolayers was decreased by caffeine. X-ray enhanced reactivation of UV-irradiated virus in X-irradiated monolayers (X-ray reactivation) and UV- or X-ray-inactivated capacity of the cells to support unirradiated virus plaque formation were unaffected by caffeine. The results suggest that a caffeine-sensitive process is necessary for the expression of Weigle reactivation for herpes virus. Since cafeine did not significantly affect X-ray reactivation, different mechanisms may be responsible for the expression of Weigle reactivation and X-ray reactivation.     

Involvement of cellular death signals in the reactivation of herpes simplex virus type 1 and lambda bacteriophage from a latent state

Herpes simplex virus (HSV) 1 has adapted to the human host through two modes of infection, the acute-transient infection that may cause diseases (such as encephalitis) and the latent state, which is a source for recurrent infection and disease. While much information has been gathered on the cellular and molecular concomitants of establishment and maintenance of HSV-1 latent state, the biological basis of viral reactivation is still unclear. Despite their obvious differences, HSV-1 and the bacterial temperate virus, the bacteriophage lambda, shares four distinct features that may help understand the viral latency phenomenon: (i) two modes of life cycle and a decision point to choose either latency (HSV-1) and lysogeny (bacteriophage lambda), or active replication, that results in cell destruction, (ii) establishment of lysogeny/latency of the respective virus is associated with protection from cell death, (iii) immunity/resistance to super-infection, (iv) agents that trigger mammalian and bacterial cell death also induce reactivation of both HSV-1 and lambda bacteriophage. Thus, despite their differences, these two viruses might display analogous mechanism(s) of reactivation. Based on clinical and experimental data, we propose in this hypothesis that while HSV-1 latency, like bacteriophage lambda lysogeny, is associated with protection from cell death and restriction to super-infection, viral reactivation from the latent state is triggered by exogenous stress signals that interfere with cellular viability and may eventually lead to cell death.     

Protease-deficient herpes simplex virus protects mice from lethal herpesvirus infection.

Null mutants and attenuated mutants of herpes simplex virus (HSV) have been shown to induce immunity against challenge from wild-type virus. Null viruses with a defect in late gene products would be expected to express more viral genes than viruses with defects in essential early gene products and thus induce a better immune response. Herpesviruses encode a late gene product (serine protease) that is autocatalytic and cleaves the capsid assembly protein during viral replication. To determine whether a virus with a mutation in this gene could induce immunity, we constructed a recombinant virus containing the gusA reporter gene in the protease domain of the HSV type 1 UL26 open reading frame (ORF). Consistent with previous results (M. Gao, L. Matusick-Kumar, W. Hurlburt, S. F. DiTusa, W. W. Newcomb, J. C. Brown, P. J. McCann, I. Deckman, and R. J. Colonno, J. Virol. 68:3702-3712, 1994), recombinant virus could be isolated only from helper cell lines expressing the product of the UL26 ORF. Mice inoculated with the recombinant virus were unaffected by doses of virus that were lethal to mice infected with wild-type virus. Mice which were previously inoculated with the recombinant virus were also protected by a subsequent challenge with wild-type virus in a dose-dependent manner. These results indicate that recombinant viruses lacking the protease gene are avirulent but render protection from subsequent challenge.     

Factors affecting recovery of latent herpes simplex virus from human trigeminal ganglia

The rate of recovery of herpes simplex virus (HSV) from human trigeminal ganglia explant monolayers is affected by two factors: (1) time elapsed from the death of an individual to the establishment of in vitro culture of ganglia and (2) surface area onto which ganglia are explanted. Spontaneous reactivation of HSV from human trigeminal ganglia can be maximized when ganglia are obtained within 12 h of death and explanted into surface area of 250 cm2. Viruses isolated by explantation of human trigeminal ganglia were found to be uniformly HSV type 1 by restriction enzyme analysis.     

Gamma interferon impedes the establishment of herpes simplex virus type 1 latent infection but has no impact on its maintenance or reactivation in mice

Murine models of gamma interferon (IFN-gamma) deficiency demonstrate the role of this cytokine in attenuating acute herpes simplex virus (HSV) disease; however, the effect of IFN-gamma on the establishment and maintenance of neuronal latency and viral reactivation is not known. Using the IFN-gamma knockout (GKO) model of IFN-gamma deficiency and sensitive quantitative PCR methods, we show that IFN-gamma significantly reduces the ganglion content of latent HSV-1 in BALB/c mice, which in turn delays viral time to reactivation following UV irradiation. Similar effects were not seen in the C57BL/6 strain. These results indicate that IFN-gamma significantly attenuates latent HSV infection in the mouse model of ocular infection but has no impact on the maintenance of latency or virus reactivation.     

Expression of the herpes simplex virus 1 alpha transinducing factor (VP16) does not induce reactivation of latent virus or prevent the establishment of latency in mice.

A feature of the cascade regulation of herpes simplex virus 1 gene expression in productive infection is that the first genes to be expressed, the alpha genes, are transactivated by a structural component of the virion designated as the alpha transinducing factor (alpha TIF). In this study, we have tested the hypothesis that latent infection of sensory neurons results from the failure of alpha TIF, a tegument protein, to be transported from the nerve endings to the nucleus of the sensory neuron. Two viruses were constructed. The first recombinant virus (R6003) contained a second copy of the alpha TIF gene placed under the control of a metallothionein promoter. The second recombinant virus (R6004) is identical to R6003 except for the presence of a stop codon inserted at amino acid 70 of the second alpha TIF gene. The metallothionein promoter inserted into the viral genome was shown to be expressed, and alpha TIF mRNA was detected by in situ hybridization of sections of trigeminal ganglia of mice infected with R6003, both untreated and those given cadmium injections. In all experiments, there were no significant differences in the recovery of latent virus from mice infected with R6003 or R6004, whether injected with cadmium or not. Cadmium administration at the time of infection and at intervals thereafter did not preclude establishment of latency. In another series of experiments, transgenic mice expressing the metallothionein-driven alpha TIF did not differ from nontransgenic siblings with respect to the incidence of latent virus in trigeminal ganglia. We conclude that the absence of alpha TIF cannot alone account for the establishment of latency.