Heat stress activates production of herpes simplex virus type 1 from quiescently infected neurally differentiated PC12 cells.

We have previously described a novel in vitro model of a non-productive herpes simplex virus type 1 (HSV-1) infection in neurally differentiated (Nd)-PC12 cells that allows for inducible virus replication upon forskolin treatment. In this study, we further characterized the quiescent state of infection and examined the ability of heat stress (HS) to induce virus from this non-productive state. These studies demonstrated that (i) the quiescent state is characterized by the absence of cell-associated virus, capsids, and viral antigens; (ii) HS (43 degrees C, 3 h) efficiently activated virus from quiescently infected Nd-PC12 (QIF-PC12) cells; (iii) the rate of virus production was significantly greater following HS than forskolin treatment, and the rates of both were dependent on MOI; (iv) forskolin and HS appeared to affect pathways of viral activation from a quiescent state as they did not enhance viral growth in Nd-PC12 cells; (v) viral alpha4 gene and host HSP72 gene transcription were rapidly induced in QIF-PC12 as soon as 3 h post-HS initiation; (vi) induction of the viral alpha27 gene followed that of representative beta and gamma genes, U(L)30 and U(L)18, respectively, and (vii) HS induced asynchronous HSV-1 replication from QIF-PC12 cells with 1:400 to 1:22000 positive foci detected as rapid as 24 h post-induction when established at MOIs of 30 and 3, respectively. These findings provide evidence that alpha4 may be involved in the switch from quiescence to productive infection. Furthermore, this model has the potential to advance our understanding of how HS initiates the HSV-1 productive cycle from a cryptic viral genome.