Trigeminal ganglion DNA from mice latently infected with herpes simplex virus type 1 was analysed by restriction enzyme digestion, agarose gel electrophoresis and blot hybridization to 32P-labelled viral DNA. Viral DNA from parental virions and from virions obtained as a consequence of reactivation by ganglion neurectomy were similarly analysed. The BamHI restriction fragments of parental and reactivated virions were almost indistinguishable from each other, and several of the larger BamHI fragments of viral DNA were also found in latently infected ganglion at unaltered sizes. In contrast, fractionation of EcoRI fragments of latently infected ganglion DNA by reverse phase column (RPC-5) chromatography, followed by gel electrophoresis and blot hybridization to a viral DNA probe from the S component terminal repetition, revealed the presence of several terminal fragments at discrete sizes ranging from 1 kb to 15 kb, quite unlike the 5.7 kb terminal EcoRI K fragment of virion-derived DNA. These results indicate that structural changes occur in the viral genome concomitantly with the establishment of latency, such as may result from extensive gene rearrangement or integration into cellular DNA.

It has been amply documented that herpes simplex virus (HSV) persists in sensory ganglia of the peripheral nervous system (PNS). In contrast, HSV latency in the central nervous system (CNS) has not been well characterized. Corneal inoculation of virus results in a productive viral infection in the CNS during the first week after inoculation, indicating that the virus can progress from the PNS to the CNS. During latency, HSV has been found by co-cultivation of CNS tissue in only a very small fraction of inoculated mice. We have used here molecular hybridization techniques to analyse the fate of viruses that reach the CNS by anatomical pathways. We show that 6 days after corneal inoculation of HSV-1 a productive viral infection was present in brain tissue as well as in peripheral ganglia in at least 90%F of the inoculated mice. The mortality during this acute phase was only 2%. In the survivors, latent HSV could be recovered by explantation from 95% of the trigeminal ganglia, but only 5% of the brain tissue explants of the same mice yielded infectious virus. However, HSV DNA sequences were detected in the brains of 30% of mice which harboured latent HSV in their trigeminal ganglia. These results suggest that viruses that progress from the PNS into the CNS are not eliminated, but are capable of establishing a latent infection in the CNS that cannot be reactivated by explantation techniques.

The role of antiviral antibody in controlling the acute and latent phases of herpes simplex virus (HSV) infection in sensory ganglia of mice was studied in vitro and in vivo. Organ cultures of ganglia inoculated in vitro with HSV produced infectious virus for at least 3 weeks. In the presence of antiviral antibody, the titre of virus was markedly reduced, but the infection was not eliminated. Similarly, passive administration of antibody to HSV-infected immunodeficient (nude) mice reduced the virus titre but did not eliminate the acute phase of the ganglionic infection. Suppression of the cell-mediated immune response in latently infected immunocompetent mice by treatment with cyclophosphamide and/or X-irradiation resulted in reactivation of HSV in up to 70% of the animals. Reactivation was demonstrated by recovering infectious virus in cell-free homogenates of ganglia and eye globes and by finding virus antigens in ganglia by immunofluorescent staining. Reactivation occurred both in vitro and in vivo in the presence of high concentrations of neutralizing antibody. It is concluded that antibody alone is not sufficient to eliminate the acute phase of the ganglionic infection and that cytotoxic agents known to suppress the host's cellular immune response can reactivate virus in the presence of neutralizing antibody.