A highly efficient system to produce infectious human papillomavirus: Elucidation of natural virus-host interactions

A simple, efficient system has been developed to produce high titers of infectious human papillomavirus type 18 (HPV-18) in organotypic raft cultures of primary human keratinocytes (PHKs). Molecular characterization elucidated key early and late events in the reproductive program. The system obviates the need for immortalized cells and allows the analyses of mutant HPV genomes not previously possible. An E6 deletion mutant incapable of causing p53 degradation is defective in viral DNA amplification and capsid protein production. The high levels of p53 protein which accumulated in numerous cells did not lead to apoptosis over a prolonged duration. Time course and metabolic labeling experiments revealed novel interactions with the host. Notably, post-mitotic, differentiated cells are induced by HPV E7 expression to reenter S phase, whereupon host chromosomes replicate, but HPV DNA does not amplify until the cells have progressed to and are arrested in G(2) phase. Here, we present data that strongly suggest that the abundant cytoplasmic viral E1;E4 protein is not responsible for this G(2) arrest, as described in the literature upon ectopic expression in cell lines. We provide additional insights into the viral life cycle and contrast them to conclusions derived from experiments in cell lines.