Human corneal cells and other fibroblasts can stimulate the appearance of herpes simplex virus from quiescently infected PC12 cells.

A two-cell system for the stimulation of herpes simplex virus type 1 (HSV-1) from an in vitro model of long-term (quiescent) infection is described. Rat pheochromocytoma (PC12) cells differentiated with nerve growth factor were infected with HSV-1 strain 17. Little, if any, cytotoxicity was observed, and a quiescent infection was established. The long-term infection was characterized by the absence of all detectable virus in the culture medium and little, if any, detectable early or late viral-gene expression as determined by reverse transcriptase PCR analysis. The presence of HSV-1 DNA was determined by PCR analysis. This showed that approximately 180 viral genomes were present in limiting dilutions where as few as 16 cells were examined. The viral DNA was infectious, since cocultivation with human corneal fibroblasts (HCF) or human corneal epithelial cells (HCE) resulted in recovery of virus from most, if not all, clusters of PC12 cells. Following cocultivation, viral antigens appeared first on PC12 cells and then on neighboring inducing cells, as determined by immunofluorescent staining, demonstrating that de novo viral protein synthesis first occurred in the long-term-infected PC12 cells. Interestingly, the ability to induce HSV varied among the cell lines tested. For example, monkey kidney CV-1 cells and human hepatoblastoma HepG2 cells, but not mouse neuroblastoma cells or undifferentiated PC12 cells, mediated stimulation. This work thus shows that (i) quiescent HSV infections can be maintained in PC12 cells in vitro, (ii) HSV can be induced from cells which do not accumulate significant levels of latency-associated transcripts, and (iii) the activation of HSV gene expression can be induced via neighboring cells. The ability of adjacent cells to stimulate HSV gene expression in neuron-like cells represents a novel area of study. The mechanism(s) whereby HCF, HCE, and HepG2 and CV-1 cells communicate with PC12 cells and stimulate viral replication, as well as how this system compares with other in vitro models of long-term infection, is discussed.