Although human cytomegalovirus (HCMV) predominantly infects epithelial cells in vivo, the majority of studies of HCMV gene expression and replication have been conducted using non-epithelial cell lines in part because of the absence of a good experimental system using epithelial cells. To address the nature of epithelial cell infection, we investigated the susceptibility of an epithelial cell line (K-1034) established from the retinal pigment epithelium to HCMV infection. This cell line exhibited high susceptibility to HCMV, as evidenced by detection of one of the immediate early antigens, IE2, in the nuclei of more than 80% of K-1034 cells at 24h following inoculation at a multiplicity of infection of 3 plaque forming units per cell. However, the yield after one-step growth of HCMV in K-1034 cells was about twenty-fold less than that in human embryonic lung fibroblast cells. Cytopathic effect (CPE) on K-1034 cells was not prominent in medium supplemented with 10% fetal bovine serum and viral late antigens were detected in less than 5% of K-1034 cells. Interestingly, infected cells expressing late antigens and exhibiting CPE were markedly increased in serum-free medium, even though the yield of infectious HCMV and viral genome copy numbers were almost the same in the different serum concentrations, due to viral instability in the absence of serum. Thus, the progression of late antigens expression and the induction of CPE in infected epithelial cells is influenced by physiological conditions, and are negatively regulated by some serum factor.
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