Persistent replication of herpes simplex virus type 1 in JOK-1 cells

Characterization of a potent refractory state and persistence of herpes simplex virus 1 in cell culture

Herpes simplex virus (HSV) normally undergoes productive cytocidal infection in culture and is thought of as relatively resistant to innate immune responses such as interferon. We previously described an unusual pattern of infection in culture in MDBK cells, which after initial productive infection, surprisingly resulted in progressive suppression of replication and cell recovery. The dominance of the refractory state was due to the inability to suppress interferon production and subsequent paracrine signaling. Here, using a wild-type HSV-1 strain expressing green fluorescent protein (GFP)-VP16, we analyze aspects of long-term HSV persistence resulting from this oscillating refractory state. We show that the gradual suppression of GFP-VP16 expression correlated with a biphasic pattern of accumulation of viral DNA and extracellular virus titers. We quantify virus maintenance in a minor subpopulation of cells during subculture, show the reemergence of virus by infectious center assay, and demonstrate that this required intracellular events over a 24- to 48-h time course. We also demonstrate that conditioned medium (cMed) from infected cells induced a profound shutoff of HSV gene expression at the transcriptional level. Finally, we demonstrate that this suppression was extremely rapid, requiring only 1 h of treatment to essentially abolish HSV immediate-early expression, and surprisingly persisted for almost 2 days after removal of the cMed. These combined effects underpin the oscillating effect both in plaque progression, where infection spreads but is overwhelmed by the accumulation of inhibitory components, enabling cell recovery, and virus maintenance in a subpopulation of cells. These results may be relevant to consider in studies of HSV latency in different animal models.

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.

Transcriptional and translational analyses of the UL2 gene of equine herpesvirus 1: a homolog of UL55 of herpes simplex virus type 1 that is maintained in the genome of defective interfering particles

Defective interfering particles (DIPs) of equine herpesvirus 1 (EHV-1; Kentucky A strain) mediate persistent infection. DNA sequences at the L terminus, which contain the UL2 gene (homolog of UL55 of herpes simplex virus type 1 and open reading frame 3 of varicella-zoster virus) of standard EHV-1, have been shown to be highly conserved in all clones of the EHV-1 DIP genome. The UL2 mRNA was characterized by S1 nuclease analyses, which mapped the 5' and 3' termini of the 0.9-kb early UL2 mRNA to approximately 26 and 16 nucleotides downstream of a TTTAAA box and polyadenylation signal, respectively. The UL2 open reading frame, present within both the EHV-1 standard and DIP genomes, was inserted into the transcription expression vector pGEM-3Z to yield constructs pGEML2 and pDIL2, respectively. After in vitro transcription and translation, both constructs yielded a comigrating 23-kDa protein, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Polyclonal antiserum was raised against the UL2 protein by injecting rabbits with a TrpE/UL2 fusion protein expressed from plasmid pATH23L2 in Escherichia coli. The UL2-specific antiserum reacted in Western immunoblot and immunoprecipitation analyses with a 23-kDa polypeptide synthesized in cells infected with standard EHV-1 or DIP-enriched virus. These data also indicated that the UL2 polypeptide was more abundant in DIP-infected cells than in standard EHV-1-infected cells. Results from time course and pulse-chase analyses suggested that the UL2 polypeptide has a rapid turnover rate in DIP-infected cells.

The octamer-binding protein Oct-2 represses HSV immediate-early genes in cell lines derived from latently infectable sensory neurons

Transcription of herpes simplex virus (HSV) immediate-early (IE) genes does not occur in sensory neurons latently infected with the virus or following infection of neuronal cell lines. In neuronal cell lines this inability results from the weak activity of the viral IE promoters, which is caused by a neuron-specific repressor factor that binds specifically to the TAATGARAT motif in these promoters and to related octamer elements. Cells expressing this repressor contain an additional octamer-binding protein that is absent from permissive cells. We identify this factor as the lymphocyte- and neuron-specific octamer-binding protein Oct-2 and show that Oct-2 mRNA is also present in dorsal root ganglion neurons, the natural site of HSV latency in vivo. Moreover, artificially elevated expression of Oct-2 can repress the IE promoter. The potential role of Oct-2 in the initiation and maintenance of in vivo latent infection with HSV is discussed.