Cell Type Dependent Effects on HSV Transcription

Herpes Simplex Virus type 1 (HSV-1) infects both epithelial cells and neurons during the course of its lifecycle. Transcription of viral genes relies heavily on the cellular transcription machinery and chromatin remodeling complexes. Differences in the expression of transcription factors and in the epigenetic landscape in these two cell types could affect the ability of the virus to transcribe its genes and to reactivate from latency. To determine how cell type affects viral gene expression, RNA sequencing was used to quantify differences in viral transcription from wild type and mutant viruses in neurons and MRC5 cells. To ascertain if differences in the cellular epigenetic landscape affect the ability of the virus to reactivate, an in vitro model of latency was used in which viral genomes persist in a quiescent state and acquire chromatin modifications similar to in vivo latency. Adenovirus vectors were used to deliver the viral activators ICP0 and ICP4 to quiescently infected cells to determine if they could induce transcription from quiescent genomes. RNA sequencing and ChIP were used to quantify viral transcription and the binding of ICP4 to the genome. From these studies, it was concluded that i) viral gene expression from a mutant virus devoid of immediate early gene expression was highly repressed and dysregulated, ii) expression from this virus was greater in neurons than in MRC5 cells, iii) ICP4 was unable to bind to or induce expression from quiescent genomes in MRC5 cells in the absence of ICP0, iv) ICP0 removes repressive chromatin, enabling ICP4 to function on quiescent genomes in MRC5 cells, and v) ICP4 can bind to and induce transcription from quiescent genomes in neurons in the absence of ICP0. The results of these studies suggest that in the absence of immediate early gene expression, transcription from the genome is highly dysregulated and repressed. However, the degree of repression differs between cell types. Neurons were more permissive to transcription, suggesting that they are less able to repress the viral genome. Furthermore, the viral transcription factor ICP4 was unable to access repressed genomes in MRC5 cells due to the presence of chromatin on the genome. However, ICP4 was able to access some regions of repressed genomes in neurons, suggesting that the form of chromatin on these genomes is less repressive and is not uniform throughout the genome. Together, these data imply that neurons are less efficient in repressing the viral genomes, and this may allow for a finer balance of latency vs. reactivation based on the expression of viral activators and the latency associated transcript