The search for a culture system for papillomavirus

A rapid DNA extraction method suitable for human papillomavirus detection

Infection with oncogenic human papillomavirus (HPV) genotypes is necessary for the development of cervical cancer. Testing for HPV DNA from liquid based cervical samples can be used as an adjunct to traditional cytological screening. In addition there are ongoing viral load, genotyping, and prevalence studies. Therefore, a sensitive DNA extraction method is needed to maximize the efficiency of HPV DNA detection. The XytXtract Tissue kit is a DNA extraction kit that is rapid and so could be useful for HPV testing, particularly in screening protocols. This study was undertaken to determine the suitability of this method for HPV detection. DNA extraction from HeLa and Caski cell lines containing HPV 18 and 16 respectively together with DNA from five liquid based cervical samples were used in a HPV PCR assay. DNA was also extracted using the QIAamp DNA mini kit (Qiagen, Hilden, Germany) as a comparison. DNA extracts were serially diluted and assayed. HPV DNA was successfully detected in cell lines and cervical samples using the XytXtract Tissue kit. In addition, the XytXtract method was found to be more sensitive than the QIAmp method as determined by a dilution series of the extracted DNA. While the XytXtract method is a closed, the QIAamp method uses a spin column with possible loss of DNA through DNA binding competition of the matrix, which could impact on the final extraction efficiency. The XytXtract is a cheap, rapid and efficient method for extracting HPV DNA from both cell lines and liquid based cervical samples.

Identification of a novel human gammapapillomavirus species

By using random PCR amplification, shotgun sequencing and sequence similarity searches, we analysed nucleic acids present in cell cultures inoculated with samples from unexplained cases of encephalitis. We identified a divergent human papillomavirus (HPV) sequence originating from a rectal swab. The full genome was amplified by inverse PCR and sequenced. The prototype of the sixth gammapapillomavirus species, HPV116, was not found in the patient's cerebrospinal fluid or respiratory secretions, nor in culture supernatants from other unexplained cases of encephalitis, indicating that its identification in an encephalitis patient was accidental.

Papillomaviruses infect cells via a clathrin-dependent pathway

In this study we have examined the pathway by which papillomaviruses infect cells, using bovine papillomavirus (BPV) virions and mouse C127 cells as the model system. By confocal microscopy, the entry of BPV virions, BPV virus-like particles (VLPs), and HPV16 VLPs were very similar. In dually exposed cells, HPV-16 VLPs and BPV virions colocalized intracellularly. BPV VLPs colocalized with AP-2, a clathrin adapter molecular and a marker of the clathrin-dependent endocytic pathway; and also with transferrin receptor, a marker of early endosomes; and Lamp-2, a marker of late endosomes and lysosomes. BPV infection was detected within 12 h of virion cell-surface binding, as measured by an RT-PCR assay. Infection was prevented by several pharmacologic inhibitors, including chlorpromazine, which blocks clathrin-dependent endocytosis and the lysosomotropic agent, bafilomycin A. By contrast, two inhibitors of caveolae-dependent uptake, filipin and nystatin, did not prevent BPV infection. We conclude that papillomaviruses infect cells via clathrin-dependent receptor-mediated endocytosis. Surprisingly, the kinetics of internalization were unusually slow for this mechanism, with the t(1/2) of entry of BPV-1 being approximately 4 h versus 5-15 min for a typical ligand.

Expression of the major capsid protein of human papillomavirus type 16 in Escherichia coli

Major capsid proteins (MCPs) of various papillomaviruses have recently been expressed in heterologous cells as soluble and functional polypeptides. The host cells for producing these proteins have so far been of eukaryotic origin; however, E. coli has potential utility a host, with advantages over eukaryotic cells such as relatively simple culture requirements and greater ease of mutation of expressed sequences. We studied the expression by E. coli of the MCP of human papillomavirus type 16 (HPV16) using the gene derived from the 'prototype' HPV16 genome. Using expression vector pTrc99A, the protein was produced in full-length unfused form at levels of 3-4% of cell protein. Soluble polypeptide was detected, albeit at low levels. The level of solubility was not increased by growing cells at low temperature and slowing the rate of protein synthesis. The soluble protein was degraded at its carboxy terminus by an outer membrane protease of E. coli, OmpT, giving rise to two slightly shortened protein species of 52K and 56K in addition to the full-length 57K polypeptide. Since the MCP of prototype HPV16 is known to be prone to excessive aggregation compared with other papillomaviral MCPs, the recovery of soluble polypeptide indicates that E. coli is worth consideration as an alternative host to eukaryotes for producing these proteins.

Papillomavirus capsid binding and uptake by cells from different tissues and species

The inability of papillomaviruses (PV) to replicate in tissue culture cells has hampered the study of the PV life cycle. We investigated virus-cell interactions by the following two methods: (i) using purified bovine PV virions or human PV type 11 (HPV type 11) virus-like particles (VLP) to test the binding to eukaryotic cells and (ii) using different VLP-reporter plasmid complexes of HPV6b, HPV11 L1 or HPV11 L1/L2, and HPV16 L1 or HPV16 L1/L2 to study uptake of particles into different cell lines. Our studies showed that PV capsids bind to a broad range of cells in culture in a dose-dependent manner. Binding of PV capsids to cells can be blocked by pretreating the cells with the protease trypsin. Penetration of PV into cells was monitored by using complexes in which the purified PV capsids were physically linked to DNA containing the gene for beta-galactosidase driven by the human cytomegalovirus promoter. Expression of beta-galactosidase occurred in < 1% of the cells, and the efficiency of PV receptor-mediated gene delivery was greatly enhanced (up to 10 to 20% positive cells) by the use of a replication-defective adenovirus which promotes endosomal lysis. The data generated by this approach further confirmed the results obtained from the binding assays, showing that PV enter a wide range of cells and that these cells have all functions required for the uptake of PV. Binding and uptake of PV particles can be blocked by PV-specific antisera, and different PV particles compete for particle uptake. Our results suggest that the PV receptor is a conserved cell surface molecule(s) used by different PV and that the tropism of infection by different PV is controlled by events downstream of the initial binding and uptake.

Towards vaccines against human papillomavirus type-16 genital infections

Human papillomavirus type-16 (HPV-16) is strongly associated with cervical carcinoma and cervical intraepithelial neoplasia. It may soon be possible to develop prophylactic vaccines designed to induce neutralizing antibodies to HPV-16 virions in genital secretions and therapeutic vaccines to induce cytotoxic T-cell responses against HPV-16 early proteins in cervical intraepithelial neoplasia and cervical cancers. Although significant advances have been achieved, problems remain before such vaccines can be used routinely.

An in vitro system for studying the initial stages of cottontail rabbit papillomavirus infection

Cottontail rabbit papillomavirus (CRPV) infection of an established cottontail epidermal cell line (Sf1Ep) resulted in the production of CRPV-specific transcripts without concomitant morphological transformation. The most abundant transcripts corresponded in size to those of the E6 and E7 open reading frames (ORFs), which are also among the commonest in domestic and cottontail rabbit papillomas. CRPV RNA production was both time- and dose-dependent, with RNA production diminishing with decreasing viral dose and increasing culture passage. Infected cultures contained episomal CRPV DNA, which did not appreciably change in abundance with time but was significantly reduced with culture passage. All features of in vitro infection, especially RNA production, were inhibited by CRPV-neutralizing, but not HPV-11-neutralizing, monoclonal antibodies. Much of this inhibition could be attributed to a blockage of viral penetration, as indicated by the reduction of CRPV DNA within virus-neutralized cultures. The results indicate that, although CRPV infection of Sf1Ep cells was abortive, it serves as a useful model for analysis of early infection events.

Induction of bovine papillomavirus E2 gene expression and early region transcription by cell growth arrest: correlation with viral DNA amplification and evidence for differential promoter induction

The bovine papillomavirus type 1 (BPV-1) genome replicates as a latent plasmid in mouse C127 cells transformed in vitro by the virus. However, we have recently shown that BPV-1 DNA amplification can be induced in a subpopulation of cells under culture conditions which suppress cell proliferation, a finding which led us to hypothesize that expression of a viral replication factor was regulated by cell growth stage. In this report, we describe the detection in these cells of abundant BPV-1 nuclear E2 antigen by immunofluorescence analysis. Expression of E2 antigen in fibropapilloma tissue was similarly localized to nonproliferating epidermal cells of the lower spinous layers--the natural site of induction of vegetative viral DNA replication. Immunoprecipitation analysis showed that the previously characterized 48-kilodalton (transactivator) and 31-kilodalton (repressor) E2 proteins were both induced in growth-arrested cell cultures. In parallel with E2 antigen synthesis under conditions of serum-deprivation in vitro, we observed a significant increase in levels of BPV-1 early region mRNAs. Furthermore, we present evidence for preferential induction of the P2443 promoter, in addition to specific induction of the P7940 promoter in response to serum deprivation. These observations indicate a central role for E2 transcription factors in the induction of viral DNA amplification in division-arrested cells in vitro and in vivo and suggest that this process is associated with a qualitative switch in the expression of viral early region genes.

Evaluation of varicella-zoster antiviral drugs by a nucleic acid hybridization assay

A simple nucleic acid hybridization assay was developed for the evaluation of antiviral compounds with activity against varicella-zoster virus. Antiviral-induced reduction in varicella-zoster virus DNA, as measured by hybridization with 32P-labeled probes, correlated with drug-induced inhibition of viral cytopathic effect in cell culture. Three compounds were shown to be more potent than acyclovir at inhibiting varicella-zoster virus in vitro.

Expression of lectin binding in cutaneous papillomas of animals

A group of spontaneously occurring animal papillomas which were negative or positive for papillomavirus group-specific antigen were examined with a battery of biotinylated lectins including Con A, WGA, succinylated-WGA, PNA and UEA-I. Canine papillomas, equine papillomas, white-tailed deer fibromas, mule deer fibromas, and bovine fibropapillomas were examined. Each lectin had a specific staining pattern. No obvious differences in staining patterns between normal skin, viral antigen-positive and -negative neoplasms were identified. This may be due to the well-differentiated and organized nature of these tumours.

A novel spontaneous mutation of the bovine papillomavirus-1 genome

A cell clone (cl.2) having an atypical transformed morphology was isolated from a murine C127 cell culture experimentally infected with a bovine papillomavirus type 1 (BPV-1) virion preparation. cl.2 cells exhibited minimal transformed characteristics and contained multiple copies of a BPV-1 plasmid with a molecular size slightly less than that of the wild type viral genome. A simple deletion of 277 bp was mapped to the distal portion of the viral 69% transforming fragment where the early gene region merges with the late gene region. None of the recognized early open reading frames were affected by the deletion but sequences including the common early gene mRNA polyadenylation (poly(A] signal and several base pairs of the "distal" enhancer element were deleted. Transfection of C127 cultures with low molecular weight (Hirt) DNA prepared from cl.2 cells led to the appearance of transformed cell foci, and Southern blotting analysis of a cl.2 Hirt DNA-transformed cell line confirmed that the deletion did not destroy the ability to replicate as a high copy plasmid. Removal of the natural early poly(A) signal did not obligate use of the alternative natural viral poly(A) signal located towards the end of the late region. Instead, a new major early mRNA polyadenylation site was mapped close to the unique BamHI recognition sequence at the distal end of the transforming region. Our results underline previous observations that there is a block to the production of stable mRNAs from the late gene region in BPV-1-transformed C127 cells, yet this is not necessarily explained by premature termination of transcription within this region.

Study of the E2 gene product of the cottontail rabbit papillomavirus reveals a common mechanism of transactivation among papillomaviruses

The long control region (LCR) of the cottontail rabbit papillomavirus (CRPV) harbors a transcriptional promoter which can be transactivated, as reflected by cat gene expression, by cotransfection with plasmids which express the intact E2 open reading frame of CRPV, human papillomavirus type 18 (HPV18), and bovine papillomavirus type 1 (BPV1). The E2 protein of CRPV can also transactivate the LCRs of BPV1, HPV1, and HPV18 inserted in front of the cat gene in enhancer or promoter configuration. Competition experiments in vivo and binding studies with CRPV E2 protein synthesized in vitro suggest that the different E2 proteins transactivate transcription by a common mechanism involving binding to the same ACCG-CGGT target sequence. The C-terminal part of the protein is necessary for its DNA-binding function. Analysis of the transactivation data and of the LCR sequences of these four viruses suggests that the two cutaneous viruses (CRPV and BPV1) present a similar pattern of promoter regulation but that the activity of the promoters of genital human viruses is less dependent on E2 regulation and is at least partially regulated by cellular factors.

Messenger RNAs from the E1 region of bovine papillomavirus type 1 detected in virus-infected bovine cells

Bovine papillomavirus type 1 DNA replicated to a high copy number in virus-infected bovine fibroblasts. Infected bovine cells were therefore used as a source of RNA for Northern blotting analysis to search for viral transcripts hybridizing to the E1 gene region, implicated in viral DNA replication. Cytoplasmic polyadenylated RNA preparations contained at least five different E1-region transcripts, ranging from 1200 to approximately 4500 nucleotides in length. All of these species contained sequence information from the 5'-end of the E1 open reading frame, but only the largest species included sequences from its central portion. The latter transcript is a candidate mRNA for a stimulatory replication factor (R) previously mapped by genetic experiments (1).

Two Shope papillomavirus-associated VX2 carcinoma cell lines with different levels of keratinocyte differentiation and transplantability

Two cell lines, named VX2T and VX2R, were isolated from the transplantable VX2 carcinoma, a wholly anaplastic tumor established from a carcinoma induced by the Shope cottontail rabbit papillomavirus (CRPV) (J.G. Kidd and P. Rous, J. Exp. Med. 71:813-838, 1940). The CRPV genome was found to be maintained and transcribed in both cell lines, as in the VX2 carcinoma. The VX2T cells retained the tumor-producing capacity in the rabbit and the low expression of epidermal keratinocyte differentiation of the VX2 tumor cells. The VX2R cells, although tumorigenic for nude mice, were no longer serially transplantable in the rabbit and expressed differentiated functions of keratinocytes. These data indicate that the anaplastic characteristic and the transplantability of VX2 carcinoma cells to immune competent allogenic hosts may be lost without any detectable modification of the physical state and transcription of the CRPV genome.