Epithelial differentiation fails to support replication of cloned human papillomavirus type 16 DNA in transfected keratinocytes

Human Papillomavirus 31 Tyrosine 102 Regulates Interaction with E2 Binding Partners and Episomal Maintenance

Several serine and threonine residues of the papillomavirus early E2 protein have been found to be phosphorylated. In contrast, only one E2 tyrosine phosphorylation site in BPV-1 (tyrosine 102) and one in HPV-16/31 (tyrosine 138) site have been characterized. Between BPV-1 and HPV-31 E2, 8 of the 11 tyrosines are conserved in the N-terminal domain, suggesting that phosphorylation of tyrosines has an essential role in E2 biology. In this study, we examine the effect of Y102 phosphorylation on HPV-31 E2 biology. Y102 proteins mutated either to the potential phospho-mimetic glutamic acid (Y102E) or to the nonphosphorylated homologue phenylalanine (Y102F) remain nuclear; however, Y102E is more associated with the nuclear matrix fraction. This is consistent with the inability of Y102E to bind TopBP1. Both BPV-1 and HPV-31 Y102E are similar in that neither binds the C terminus of Brd4, but in all other aspects the mutant behaves differently between the two families of papillomaviruses. BPV-1 Y102E was unable to bind E1 and did not replicate in a transient in vitro assay, while HPV-31 Y102E binds E1 and was able to replicate, albeit at lower levels than wild type. To examine the effect of E2 mutations under more native-like infection conditions, a neomycin-selectable marker was inserted into L1/L2 of the HPV-31 genome, creating HPV-31neo. This genome was maintained in every cell line tested for at least 50 days posttransfection/infection. Y102E in both transfection and infection conditions was unable to maintain high episome copy numbers in epithelial cell lines.IMPORTANCE Posttranslational modifications by phosphorylation can change protein activities, binding partners, or localization. Tyrosine 102 is conserved between delta papillomavirus BPV-1 and alpha papillomavirus HPV-31 E2. We characterized mutations of HPV-31 E2 for interactions with relevant cellular binding partners and replication in the context of the viral genome.

Induction of productive human papillomavirus type 11 life cycle in epithelial cells grown in organotypic raft cultures

The study of the human papillomavirus (HPV) life cycle was hampered for more than 50 years by the lack of a conventional cell culture system for propagating HPV. Considerable progress has been made in the production of several HPV types using either organotypic rafts or human epithelial xenografts in immunocompromised mice. In this study, we demonstrated episomal maintenance of HPV-11 DNA in N-Tert cells. HPV-11 episomal DNA containing cell populations grown in raft culture showed induction of the productive viral life cycle. HPV-11 DNA amplification and viral capsid antigen synthesis were detected in differentiated layers of epithelia. The viruses generated were able to infect keratinocytes in vitro, which indicate that viruses generated were infectious. The demonstration of the productive HPV-11 life cycle in raft culture from cloned HPV-11 DNA will facilitate genetic analyses of viral gene functions that was not possible using the human xenograft athymic mouse model.

Production of infectious human papillomavirus independently of viral replication and epithelial cell differentiation

Papillomaviruses are small DNA viruses that are associated with benign and malignant epithelial lesions, including >95% of cervical cancers and approximately 20% of head and neck cancers. Because papillomavirus replication and virion production are tied to epithelial cell differentiation, infectious papillomavirus virion production has been limited to cumbersome organotypic cultures and mouse xenografts. Consequent difficulties in obtaining useful amounts of wild-type or mutant human papillomavirus (HPV) virions have greatly limited studies on many aspects of papillomavirus biology. To overcome these limitations, we developed a system to encapsidate the full-length papillomaviral genome into infectious virions, independently of viral DNA replication and epithelial differentiation. This transient-transfection-based system produces >1,000 times more infectious virus per cell culture dish than the much more labor-intensive organotypic culture. Furthermore, we show that this method allows the facile generation of infectious particles containing wild-type, mutant, or chimeric papillomaviral genomes, overcoming barriers to studying many facets of replication, host interactions, and vaccine and drug development, which has been limited by the insufficient availability of infectious virions.

Propagation, infection, and neutralization of authentic HPV16 virus

Despite the prevalence of HPV16 in invasive cervical cancers, an in vitro system capable of producing infectious HPV16 is lacking. The organotypic (raft) culture system has allowed for the study of the entire differentiation-dependent life cycle of human papillomaviruses (HPVs). However, the use of this system with the prototype HPV16-containing cell line, W12, has failed to yield infectious virus. Our laboratory has introduced clinically derived HPV16(114/B) genomic DNA into primary keratinocytes, where it subsequently recircularized and maintained episomally at 50-100 copies per cell. Virion morphogenesis occurred after epithelial stratification and differentiation in raft culture. HPV16 virions were isolated that were able to infect keratinocytes in vitro. Infection was neutralized by monoclonal antibodies raised against HPV16 but not by monoclonal antibodies known to neutralize other HPV types.

Establishment of the human papillomavirus type 16 (HPV-16) life cycle in an immortalized human foreskin keratinocyte cell line

The study of human papillomaviruses (HPVs) in cell culture has been hindered because of the difficulty in recreating the three-dimensional structure of the epithelium on which the virus depends to complete its life cycle. Additionally, the study of genetic mutations in the HPV genome and its effects on the viral life cycle are difficult using the current method of transfecting molecularly cloned HPV genomes into early-passage human foreskin keratinocytes (HFKs) because of the limited life span of these cells. Unless the HPV genome transfected into the early-passage HFK extends the life span of the cell, analysis of stable transfectants becomes difficult. In this study, we have used BC-1-Ep/SL cells, an immortalized human foreskin keratinocyte cell line, to recreate the HPV-16 life cycle. This cell line exhibits many characteristics of the early-passage HFKs including the ability to stratify and terminally differentiate in an organotypic raft culture system. Because of their similarity to early-passage HFKs, these cells were tested for their ability to support the HPV-16 life cycle. The BC-1-Ep/SL cells could stably maintain two HPV genotypes, HPV-16 and HPV-31b, episomally. Additionally, when the BC-1-Ep/SL cell line was stably transfected with HPV-16 and cultured using the organotypic raft culture system (rafts), it sustained the HPV-16 life cycle. Evidence for the productive stage of the HPV-16 life cycle was provided by: DNA in situ hybridization demonstrating HPV-16 DNA amplification in the suprabasal layers of the rafts, immunohistochemical staining for L1 showing the presence of capsid protein in the suprabasal layers of the rafts, and electron microscopy indicating the presence of virus like particles (VLPs) in nuclei from cells in the differentiated layers of the rafts.

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 [symbol: see text] 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.

In vitro synthesis of oncogenic human papillomaviruses requires episomal genomes for differentiation-dependent late expression

Human papillomavirus (HPV) types 16, 18, 31, and 51 are the etiologic agents of many anogenital cancers including those of the cervix. These "high risk" HPVs specifically target genital squamous epithelia, and their lytic life cycle is closely linked to epithelial differentiation. We have developed a genetic assay for HPV functions during pathogenesis using recircularized cloned HPV 31 genomes that were transfected together with a drug resistance marker into monolayer cultures of normal human foreskin keratinocytes, the natural host cell. After drug selection, cell lines were isolated that stably maintained HPV 31 DNA as episomes and underwent terminal differentiation when grown in organotypic raft cultures. In differentiated rafts, the expression of late viral genes, amplification of viral DNA, and production of viral particles were detected in suprabasal cells. This demonstrated the ability to synthesize HPV 31 virions from transfected DNA templates and allowed an examination of HPV functions during the vegetative viral life cycle. We then used this system to investigate whether an episomal genome was required for the induction of late viral gene expression. When an HPV 31 genome (31E1*) containing a missense mutation in the E1 open reading frame was transfected into normal human keratinocytes, the mutant viral sequences were found to integrate into the host cell chromosomal DNA with both early and late regions intact. While high levels of early viral gene transcription were observed, no late gene expression was detected in rafts of cell lines containing the mutant viral genome despite evidence of terminal differentiation. Therefore, the induction of late viral gene expression required that the viral genomes be maintained as extrachromosomal elements, and terminal differentiation alone was not sufficient. These studies provide the basis for a detailed examination of HPV functions during viral pathogenesis.