The Transgenic ICP4 Promoter Is Activated in Schwann Cells in Trigeminal Ganglia of Mice Latently Infected with Herpes Simplex Virus Type 1

Herpes simplex virus type 1 (HSV-1) establishes a latent infection in peripheral sensory ganglionic neurons in humans and in the mouse model. Latent infection in ganglionic neurons is the means by which HSV-1 remains in infected humans for years. Periodic reactivation of latent infection results in spread of the virus to naive hosts. The mechanism of regulation of the latent viral genome in neurons has been intensively investigated in mouse models. Many studies utilizing in situ hybridization in mice and in humans have shown that transcription from the latent HSV-1 genome in neurons is limited to RNA that maps to the gene coding for latency-associated transcripts (LATs) (reviewed in references 17, 50, and 59). In contrast, lytic viral infection is characterized by an ordered cascade of viral gene expression: immediate-early (IE) genes are expressed first followed by early and late genes (43). Based on these findings, it has been hypothesized that latent infection of sensory neurons requires the absence of viral IE gene expression (reviewed in references 18 and 20). According to this hypothesis, reactivation of HSV-1 from latency is dependent upon activation or loss of repression of viral IE gene expression in neurons (20, 41, 43). IE genes would then activate the lytic cycle of viral gene expression, leading to the production of infectious virus. Some studies utilizing reverse transcription-PCR (RT-PCR) have demonstrated the presence of viral IE (ICP4) transcripts in homogenates of trigeminal ganglia from mice latently infected with HSV-1 (25, 26). These studies have raised the question of whether HSV-1 IE genes are transcribed in neurons during latent infection. Trigeminal ganglia are comprised of multiple cell types, including Schwann cells, satellite cells, and different types of neurons (31, 45, 53). In order to understand the significance of the finding of ICP4 mRNA in latently infected sensory ganglia, it is important to know whether the ICP4 transcription originates from neurons. Presently available techniques such as in situ hybridization are not sensitive enough to localize the ICP4 transcripts to specific cells. It is possible to examine by very sensitive methods which specific cells activate the HSV-1 ICP4 promoter in latently infected trigeminal ganglia. Activation of the ICP4 promoter in neurons would be required for production of ICP4 transcripts in latently infected neurons. We used reporter transgenic mice containing the HSV-1(F) ICP4 promoter fused to the β-galactosidase coding sequence to assay for HSV-1 ICP4 promoter activation in specific cells in latently infected trigeminal ganglia. Promoter transgenic mice were inoculated by the intracorneal route with HSV-1(F) to determine whether neurons in trigeminal ganglia activated the ICP4 promoter during latency. Trigeminal ganglia were assayed for β-galactosidase-positive cells at 5, 11, 23, and 37 days postinoculation (dpi). Moderate numbers of neurons and nonneuronal cells in trigeminal ganglia were positive for β-galactosidase at 5 dpi. The number of positive neurons decreased at 11 dpi, and no positive neurons were detected at 23 and 37 dpi. In contrast, the number of positive nonneuronal cells increased at 11 dpi, and large numbers remained positive at 23 and 37 dpi. The β-galactosidase-positive nonneuronal cells were morphologically and immunohistochemically determined to be Schwann cells. These studies showed that the ICP4 promoter was activated in Schwann cells in latently infected trigeminal ganglia. This suggests that Schwann cells may be the source of the ICP4 transcripts detected in previous studies of latently infected trigeminal ganglia (25, 26).