Production of human papillomavirus type 16 virions in a keratinocyte cell line

During negative regulation of the human papillomavirus-16 E6 promoter, the viral E2 protein can displace Sp1 from a proximal promoter element

The principal early promoter of human papillomaviruses (HPVs), designated P97 in the case of HPV-16, contains four characteristically aligned cis-responsive elements, namely one binding site for Sp1, two for the viral E2 proteins, and the TATA box. The Sp1 binding site is needed to mediate activation of P97 by the remote epithelial-specific enhancer, and the two E2 binding sites contribute to a negative feedback-loop of viral gene expression. The Sp1 consensus motif and the TATA-box distal E2 binding site are spaced in all genital papillomaviruses by a single nucleotide. We show here that at physiological concentrations, the binding of E2 proteins and Sp1 are mutually exclusive events, since a bandshift analysis with nuclear extracts from ID13, a mouse cell line transformed by BPV-1, showed only the E2 or the Sp1 bandshift, but no complex indicative of the concomitant binding of both factors. Increasing concentrations of in vitro translated E2 protein compete efficiently with the Sp1 factor for binding to an oligonucleotide containing both binding sites. Interference between Sp1 and E2 protein binding is apparently relevant for P97 repression in vivo, since a mutational analysis revealed that both E2 binding sites are necessary for negative transcriptional regulation: Alone, neither the distal site, where E2 protein can induce Sp1 displacement, nor the proximal site, where E2 protein interferes with formation and function of the pre-initiation complex, have a significant effect, but two functional E2 binding sites lead to repression of P97.

Expression of vaccinia recombinant HPV 16 L1 and L2 ORF proteins in epithelial cells is sufficient for assembly of HPV virion-like particles

A recombinant vaccinia virus termed pLC201VV was designed to coexpress the L1 and L2 late genes of human papillomavirus type 16 (HPV16). Synthesis of the L1 and L2 proteins occurred in cells infected with pLC201VV, and 40-nm virus-like particles with a density of 1.31 g/ml were produced in the nuclei of cells synthesizing both L1 and L2, but not in cells synthesizing either protein alone. Virus-like particles were partially purified from infected cells by sucrose gradient sedimentation and shown to consist of capsomeres similar to HPV and contain glycosylated L1 viral capsid protein. The production of HPV-like particles using recombinant vaccinia virus should be useful for biochemical studies and could provide a safe source of material for the development of a vaccine.

Amplification of human papillomavirus genomes in vitro is dependent on epithelial differentiation

Human papillomaviruses (HPVs) infect squamous epithelium and establish their genomes as episomes in proliferating basal cells. As infected cells differentiate, the viral DNA is amplified to high copy number and infectious virus is produced. Viral production has not yet been observed in vitro due to the inability of standard culture methods to duplicate most stages of epithelial differentiation. In this study, we have examined a cell line derived from a low-grade cervical lesion and found that it contained episomal copies of an HPV-31 subtype, HPV-31b, at approximately 50 copies per cell. When allowed to stratify at the air-liquid interface of in vitro raft cultures, this cell line differentiates in a manner which histologically resembles a low-grade cervical lesion in vivo. We have observed the amplification of HPV-31b genomes in distinct foci in the upper portion of the in vitro-stratified epithelium similar to that found in productive HPV infections in vivo. Although transcripts from the late region of HPV-31b were also detected specifically in stratified raft cultures, no capsid protein was found. These studies duplicate one important aspect of a productive HPV infection in vitro, the differentiation-dependent amplification of papillomavirus genomes.