Immunofluorescent detection of bovine papillomavirus E4 antigen in the cytoplasm of cells permissive in vitro for viral DNA amplification

The E4 gene of several human papillomavirus types is expressed in association with vegetative viral DNA synthesis in differentiated epidermal cells. To develop reagents to study expression of the bovine papillomavirus type 1 (BPV-1) E4 gene in warts and in virus-transformed cell lines, rabbit polyclonal antiserum was raised to the BPV-1 E4 antigen produced as a fusion polypeptide in Escherichia coli. By immunoblotting analysis of productively infected bovine fibropapilloma tissue, E4-related proteins of 16K, 21K, 30K and 42K were detected. In some but not all C127 cell lines transformed by BPV-1 or by a replication-competent BPV-1 deletion mutant, cytoplasmic E4 antigen with a predominantly perinuclear localization was detected by immunofluorescence analysis in a subpopulation of cells in stationary-phase cultures. The E4-expressing cells were identified by their grossly enlarged size to represent the same cell subpopulation shown earlier to support BPV-1 DNA amplification. The observation of synthesis of the E4 protein in association with viral DNA amplification in this system provides further evidence that there is a switch in viral early region gene expression in a subpopulation of division-arrested cells, which may accurately reflect events occurring during the vegetative phase of BPV-1 replication in terminally differentiated cells in vivo.