Papilloma induced by the cottontail rabbit papillomavirus (CRPV) progress at a high frequency to cancers. This property makes the CRPV system unique to study the role of papillomaviruses in cancer development. In contrast to bovine papillomavirus type 1, no convenient in vitro transformation system for CRPV has been available. Here, we describe two in vitro systems that we have developed. Transformation of NIH 3T3 cells is greatly facilitated by deletions in the CRPV late region. Specifically, a 300-bp segment located in the 5′ half of open reading frame L2 inhibits transformation of NIH 3T3 cells by CRPV DNA. In a second system involving the natural host cell of CRPV, rabbit skin epithelial cells, transformation is a two-stage phenomenon leading first to cells with an increased growth potential which subsequently become morphologically transformed. Transformation of rabbit skin epithelial cells does not require deletions in the L2 ORF. However, late region sequences located in the 3' half of ORF L1 are needed, as they are for papilloma induction in rabbits.
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