Occurrence and role of an early antigen and evidence for transforming ability of porcine circovirus.

By means of indirect immunofluorescence assay (IFA) using natural swine immune serum and hyperimmune serum from rabbits infected with porcine circovirus (PCV), a PCV antigen was detected present prior to the onset of viral and cellular DNA synthesis in nucleoli of cells of synchronized and growth stimulated infected PS cell cultures grown for more than 12 h in the presence of hydroxyurea. The number of cells containing specifically fluorescing nucleoli increased with increasing time of growth in the presence of hydroxyurea. The concomitant increase in the number of cells containing virus structural (VS) antigen in the nuclei and the increase in the amount of replicative (RF) DNA and accompanying 5 S DNA after release from the hydroxyurea block suggest that EA is involved in induction of PCV DNA replication. Primary pig kidney cell cultures persistently infected with PCV survived mock-infected control cultures for 16 passages. They had lost contact inhibition and formed cell colonies in soft agar at a ratio of 0.1 to 0.4%. Cell lines derived from agar colonies showed properties of transformed cells e.g. low requirement for serum growth factors, ability to overgrow a continuous cell layer, anchorage independence of growth. In transformed cells stimulated to growth and grown in the presence of hydroxyurea, non-structural viral antigen visible by IFA in nucleoli and VS antigen located in the cytoplasm were expressed. Contrary to virus bound nuclear VS antigen in productive infection, accumulation of cytoplasmatic VS antigen was independent of DNA synthesis and caused cell destruction, thus limiting growth of cell layers and colonies in soft agar.