Differences in cell type-specific blocks to immediate early gene expression and DNA replication of human, simian and murine cytomegalovirus

We have previously described blocks to the viral lytic cycle at two different levels in cytomegalovirus (CMV)-infected non-permissive cells. BALB/c-3T3 cells express only the predominant immediate early (IE) nuclear phosphoproteins (IE68 or IE94) of human CMV (HCMV) or simian CMV (SCMV) and do not replicate the input viral genomes. However, in human teratocarcinoma stem cells and 293 cells, expression of the HCMV IE68 gene (but not the SCMV IE94 gene) is blocked at the transcriptional level. Here we report the results of an extensive comparison of the level of permissiveness for HCMV, SCMV and murine CMV (MCMV) in a variety of additional cell types of human, monkey and mouse origin. We also describe a subtle change in the tryptic peptide pattern of the IE68 polypeptide produced in BALB/c-3T3 cells compared to permissive human foreskin fibroblasts. Neither the IE68 nor IE94 proteins could be detected by biochemical labelling procedures in infected mouse Ltk^- or F9 teratocarcinoma stem cells, although IE94 was synthesized after retinoic acid-induced differentiation of the F9 cells. Synthesis of [³⁵S]methionine-labelled IE94 protein, but not that of HCMV IE68, was detected in infected Vero cells and in human peripheral blood leukocyte cultures. The failure to synthesize detectable IE68 protein in infected Vero cells appeared to be unrelated to a lack of entry of viral DNA and to a lack of appropriate trnascription factors. Indeed, immunofluorescence assays showed that the IE68 antigen was expressed efficiently in DNA-transfected Vero cells and in a small fraction of infected Vero cells. Overall, two clear host range trends emerged. First, whilst all three viruses showed a tendency for repression of IE expression in transformed cell lines, the effect was severe for HCMV and only minimal for SCMV. Secondly, progression of infection to the viral DNA synthesis level in non-transformed fibroblast cell types occurred in a much wider range of host species cell types for SCMV and MCMV than for HCMV.