ICP0 is not required for efficient stress-induced reactivation of herpes simplex virus type 1 from cultured quiescently infected neuronal cells

Viral genes sufficient and required for herpes simplex virus type 1 (HSV-1) reactivation were identified using neuronally differentiated PC12 cells (ND-PC12 cells) in which quiescent infections with wild-type and recombinant strains were established. In this model, the expression of ICP0, VP16, and ICP4 from adenovirus vectors was sufficient to reactivate strains 17⁺ and KOS. The transactivators induced similar levels of reactivation with KOS; however, 17⁺ responded more efficiently to ICP0. To identify viral transactivators required for reactivation, we examined quiescently infected PC12 cell cultures (QIF-PC12 cell cultures) established with HSV-1 deletion mutants R7910 (ΔICP0), KD6 (ΔICP4), and in1814, a virus containing an insertion mutation in VP16. Although growth of these mutant viruses was impaired in ND-PC12 cells, R7910 and in1814 reactivated at levels equivalent to or better than their respective parental controls following stress (i.e., heat or forskolin) treatment. After treatment with trichostatin A, in1814 and 17⁺ reactivated efficiently, whereas the F strain and R7910 reactivated inefficiently. In contrast, KD6 failed to reactivate. In experiments with the recombinant KM100, which contains the in1814 mutation in VP16 and the n212 mutation in ICP0, spontaneous and stress-induced reactivation was observed. However, two strains, V422 and KM110, which lack the acidic activation domain of VP16, did not reactivate above low spontaneous levels after stress. These results demonstrate that in QIF-PC12 cells ICP0 is not required for efficient reactivation of HSV-1, the acidic activation domain of VP16 is essential for stress-induced HSV-1 reactivation, and HSV-1 reactivation is modulated uniquely by different treatment constraints and phenotypes.