Induction of human papillomavirus type 18 late gene expression and genomic amplification in organotypic cultures from transfected DNA templates

The genetic analysis of human papillomavirus (HPV) functions during the vegetative viral life cycle is dependent upon the ability to generate human keratinocyte cell lines which maintain episomal copies of transfected viral genomes. We have previously demonstrated that lipofection of normal human foreskin keratinocytes with recircularized cloned HPV-31 genomic sequences resulted in a high frequency of cell lines which maintained viral genomes as extrachromosomal elements (M. G. Frattini, H. Lim, and L. A. Laimins, Proc. Natl. Acad. Sci. USA 93:3062-3067, 1996). Following the growth of these cell lines in organotypic (raft) cultures, the differentiation-dependent expression of viral late genes, the amplification of viral genomes, and virion biosynthesis were observed. In the present study, we demonstrate that these methodologies are not restricted to HPV-31 but are applicable to other HPV types, including the oncogenic HPV-18. HPV-18 genomes were purified from bacterial vector sequences, religated, and transfected into normal human foreskin keratinocytes together with a neomycin-selectable marker. Following drug selection, resistant cells were expanded and examined for the state of the viral DNA. All cell lines examined were found to contain approximately 100 to 200 episomal copies of HPV-18 DNA per cell. Growth of these cell lines in raft cultures resulted in the differentiation-dependent expression of the E1 E4 and L1 capsid genes. In addition, viral genome amplification was observed in suprabasal cells following DNA in situ hybridization analysis of differentiated raft cultures. The induction of these late viral functions has previously been shown to be directly associated with differentiation- dependent virion biosynthesis. Our studies indicate the ability to perform a detailed genetic analysis of the various phases of the viral life cycle, including control of the differentiation-dependent late viral functions, using a second oncogenic HPV type.