Ultrastructural aspects of BK virus uptake and replication in human fibroblasts.

After exposure of human embryonic fibroblasts to BK virus, virus particles adsorbed to the plasma membrane were engulfed by pinocytosis or captured by vesicles, possibly originating from the endoplasmic reticulum, within 2 h after infection. Most of the virus particles were then transported into lysosomes or into the nucleus, while a small amount of virus was found free in the cytoplasm. Virus particles entered the nucleus between 2 and 12 h after infection, were still detectable in the nucleus at 24 h after infectionand becamse morphologically undiscernible at 30 h after infection, suggesting that a nuclear uncoating mechanism was active between 24 and 30 h after infection. Virus progeny started to appear in the nucleus of infected cells at 4 days after infection, but not until 7 to 8 days after infection did the virus escape into the cytoplasm and cell degeneration became evident. The possible explanations for the long replicative cycle of BK virus are discussed.