Display of complete life cycle of human papillomavirus type 16 in cultured placental trophoblasts

Abstract

Human papillomavirus (HPV) infection is threefold more prevalent in spontaneous abortion specimens compared to elective abortions, preferentially targeting the placental trophoblasts in these specimens. Here, by using infectious center and Southern blot analysis, we demonstrate that the transfected HPV-16 genome de novo replicates in 3A trophoblasts in culture. Peak DNA replication occurred 9-24 days posttransfection, showing classic DNA forms I, II, and III and an 8-kb monomer band upon DpnI/BamHI digestion. Reverse transcription-polymerase chain reaction (RT-PCR) analysis of mRNA expression revealed that E6 and E2 were significantly expressed by day 9, coinciding with HPV-16 DNA replication. However, significant L1 expression was delayed until day 18. L1 protein expression on day 18, but not day 9, was also confirmed by Western blot analysis. The production of HPV-16 virions was demonstrated by three techniques: The appearance of HPV-16 infectious units coinciding with L1 expression, the neutralization of these infectious units with known neutralizing anti-HPV-16 antibodies, and the appearance of spliced E1∧E4 and E6∧E7 transcripts (RT-PCR) in normal keratinocyte rafts infected with these trophoblast-produced HPV-16 infectious units. These data suggest that HPV-16 is carrying out its complete life cycle in trophoblasts. Previously, HPVs were known to productively replicate only in differentiating keratinocytes of skin. These findings expand HPV biology, support the hypothesis of a possible link between HPV and some spontaneous abortions, and present a new technology for studying HPV. © 2001 Academic Press.