Persistent BK papovavirus infection of transformed human fetal brain cells. I. Episomal viral DNA in cloned lines deficient in T-antigen expression

Abstract

After infection of permissive human fetal brain cells by BK human papovavirus (BKV), most cells were killed by the virus but rare survivors were recovered after frequent medium changes. These surviving cells grew and formed visible colonies after 5-6 wk and were thereafter established as permanent cell lines. These cells, designated as BK-HFB cells, were persistently infected and shed BKV. Morphologically they were small, polygonal cells and had transformed growth properties. Their plating efficiency on solid substrates or in semisolid medium was high, and they were tumorigenic in athymic nude mice. Cloning experiments in medium containing BKV antiserum revealed that BKV did not persist in the cultures in a simple carrier state. All cloned cell lines were initially T[tumor]-antigen negative and virus-free. Every clone began to release BKV and again became persistently infected within 3 wk after removal of BKV antiserum. After rigorous antibody treatment, 4 of 7 clones still released virus spontaneously upon removal of antiserum; 3 clones remained virus-free and are apparently cured. Although these cloned cell lines are T- and V[viral]-antigen negative when grown in antiserum-containing medium, they retain free or episomal BKV genomes; integrated viral DNA was not detected in any of the clones. These free genomes are indistinguishable from prototype BKV DNA are are found in much larger amounts in virus-shedding cell lines.