BPV1 E2 protein enhances packaging of full-length plasmid DNA in BPV1 pseudovirions

Novel Production of Bovine Papillomavirus Pseudovirions in Tobacco Plants

Vaccine efficacy requires the production of neutralising antibodies which offer protection against the native virus. The current gold standard for determining the presence of neutralising antibodies is the pseudovirion-based neutralisation assay (PBNA). PBNAs utilise pseudovirions (PsVs), structures which mimic native virus capsids, but contain non-viral nucleic material. PsVs are currently produced in expensive cell culture systems, which limits their production, yet plant expression systems may offer cheaper, safer alternatives. Our aim was to determine whether plants could be used for the production of functional PsVs of bovine papillomavirus 1 (BPV1), an important causative agent of economically damaging bovine papillomas in cattle and equine sarcoids in horses and wild equids. BPV1 capsid proteins, L1 and L2, and a self-replicating reporter plasmid were transiently expressed in Nicotiana benthamiana to produce virus-like particles (VLPs) and PsVs. Strategies to enhance particle yields were investigated and optimised protocols were established. The PsVs' ability to infect mammalian cells and express their encapsidated reporter genes in vitro was confirmed, and their functionality as reagents in PBNAs was demonstrated through their neutralisation by several different antibodies. This is the first report of BPV PsVs expressed in plants and demonstrates the potential for the development of therapeutic veterinary vaccines in planta.

Association of Human Papillomavirus 16 E2 with Rad50-Interacting Protein 1 Enhances Viral DNA Replication

Rad50-interacting protein 1 (Rint1) associates with the DNA damage response protein Rad50 during the transition from the S phase to the G₂/M phase and functions in radiation-induced G₂ checkpoint control. It has also been demonstrated that Rint1 is essential in vesicle trafficking from the Golgi apparatus to the endoplasmic reticulum (ER) through an interaction with Zeste-White 10 (ZW10). We have isolated a novel interaction between Rint1 and the human papillomavirus 16 (HPV16) transcription and replication factor E2. E2 binds to Rint1 within its ZW10 interaction domain, and we show that in the absence of E2, Rint1 is localized to the ER and associates with ZW10. E2 expression results in a disruption of the Rint1-ZW10 interaction and an accumulation of nuclear Rint1, coincident with a significant reduction in vesicle movement from the ER to the Golgi apparatus. Interestingly, nuclear Rint1 and members of the Mre11/Rad50/Nbs1 (MRN) complex were found in distinct E2 nuclear foci, which peaked during mid-S phase, indicating that the recruitment of Rint1 to E2 foci within the nucleus may also result in the recruitment of this DNA damage-sensing protein complex. We show that exogenous Rint1 expression enhances E2-dependent virus replication. Conversely, the overexpression of a truncated Rint1 protein that retains the E2 binding domain but not the Rad50 binding domain acts as a dominant negative inhibitor of E2-dependent HPV replication. Put together, these experiments demonstrate that the interaction between Rint1 and E2 has an important function in HPV replication.

IMPORTANCE HPV infections are an important driver of many epithelial cancers, including those within the anogenital and oropharyngeal tracts. The HPV life cycle is tightly regulated and intimately linked to the differentiation of the epithelial cells that it infects. HPV replication factories formed in the nucleus are locations where viral DNA is copied to support virus persistence and amplification of infection. The recruitment of specific cellular protein complexes to these factories aids efficient and controlled viral replication. We have identified a novel HPV-host interaction that functions in the cellular response to DNA damage and cell cycle control. We show that the HPV E2 protein targets Rad50-interacting protein 1 (Rint1) to facilitate virus genome replication. These findings add to our understanding of how HPV replicates and the host cell pathways that are targeted by HPV to support virus replication. Understanding these pathways will allow further research into novel inhibitors of HPV genome replication.

Comparison of human papillomavirus type 16 replication in tonsil and foreskin epithelia

Human papillomavirus (HPV) is well recognized as a causative agent for anogenital and oropharyngeal cancers, however, the biology of HPV infection at different mucosal locations, specifically the oral cavity, is not well understood. Importantly, it has yet to be determined if oral tissues are permissive for HPV infection and replication. We investigated for the first time the titers, infectivity, and maturation of HPV16 in oral epithelial versus genital epithelial tissue. We show that infectious HPV16 virions can be produced in oral tissue. This demonstrates, for the first time, that infectious virus could be spread via the oral cavity. HPV16 derived from oral tissue utilize a tissue-spanning redox gradient that facilitates the maturation of virions over time. Maturation is manifested by virion stability and increased susceptibility to neutralization with anti-HPV16 L1 antibodies. However, susceptibility to neutralization by anti-HPV16 L2 specific antibodies decreases during the maturation of HPV16 virions in oral tissue.

The papillomavirus E2 proteins

The papillomavirus E2 proteins are pivotal to the viral life cycle and have well characterized functions in transcriptional regulation, initiation of DNA replication and partitioning the viral genome. The E2 proteins also function in vegetative DNA replication, post-transcriptional processes and possibly packaging. This review describes structural and functional aspects of the E2 proteins and their binding sites on the viral genome. It is intended to be a reference guide to this viral protein.

The HPV E2-Host Protein-Protein Interactions: A Complex Hijacking of the Cellular Network

Over 100 genotypes of human papillomaviruses (HPVs) have been identified as being responsible for unapparent infections or for lesions ranging from benign skin or genital warts to cancer. The pathogenesis of HPV results from complex relationships between viral and host factors, driven in particular by the interplay between the host proteome and the early viral proteins. The E2 protein regulates the transcription, the replication as well as the mitotic segregation of the viral genome through the recruitment of host cell factors to the HPV regulatory region. It is thereby a pivotal factor for the productive viral life cycle and for viral persistence, a major risk factor for cancer development. In addition, the E2 proteins have been shown to engage numerous interactions through which they play important roles in modulating the host cell. Such E2 activities are probably contributing to create cell conditions appropriate for the successive stages of the viral life cycle, and some of these activities have been demonstrated only for the oncogenic high-risk HPV. The recent mapping of E2-host protein-protein interactions with 12 genotypes representative of HPV diversity has shed some light on the large complexity of the host cell hijacking and on its diversity according to viral genotypes. This article reviews the functions of E2 as they emerge from the E2/host proteome interplay, taking into account the large-scale comparative interactomic study.

Large scale genotype comparison of human papillomavirus E2-host interaction networks provides new insights for e2 molecular functions

Human Papillomaviruses (HPV) cause widespread infections in humans, resulting in latent infections or diseases ranging from benign hyperplasia to cancers. HPV-induced pathologies result from complex interplays between viral proteins and the host proteome. Given the major public health concern due to HPV-associated cancers, most studies have focused on the early proteins expressed by HPV genotypes with high oncogenic potential (designated high-risk HPV or HR-HPV). To advance the global understanding of HPV pathogenesis, we mapped the virus/host interaction networks of the E2 regulatory protein from 12 genotypes representative of the range of HPV pathogenicity. Large-scale identification of E2-interaction partners was performed by yeast two-hybrid screenings of a HaCaT cDNA library. Based on a high-confidence scoring scheme, a subset of these partners was then validated for pair-wise interaction in mammalian cells with the whole range of the 12 E2 proteins, allowing a comparative interaction analysis. Hierarchical clustering of E2-host interaction profiles mostly recapitulated HPV phylogeny and provides clues to the involvement of E2 in HPV infection. A set of cellular proteins could thus be identified discriminating, among the mucosal HPV, E2 proteins of HR-HPV 16 or 18 from the non-oncogenic genital HPV. The study of the interaction networks revealed a preferential hijacking of highly connected cellular proteins and the targeting of several functional families. These include transcription regulation, regulation of apoptosis, RNA processing, ubiquitination and intracellular trafficking. The present work provides an overview of E2 biological functions across multiple HPV genotypes.

Over 100 types of human papillomaviruses are responsible for widespread infections in humans. They cause a wide range of pathologies, ranging from inapparent infections to benign lesions, hyperplasia or cancers. Such heterogeneity results from variable interplay among viral and host cell proteins. Aiming to identify specific features that distinguish different pathological genotypes, we mapped the virus-host interaction networks of the regulatory E2 proteins from a set of 12 genotypes representative of HPV diversity. The E2-host interaction profiles recapitulate HPV phylogeny, thus providing a valuable framework for understanding the role of E2 in HPV infection of different pathological traits. The E2 proteins tend to bind to highly connected cellular proteins, indicating a profound effect on the host cell. These interactions predominantly impact on a subset of cellular processes, like transcriptional regulation, apoptosis, RNA metabolism, ubiquitination or intracellular transport. This work improves the global understanding of HPV-associated pathologies, and provides a framework to select interactions that can be used as targets for the development of new therapeutics.

Enhancing efficacy and mucosa-tropic distribution of an oral HIV-PsV DNA vaccine in animal models

A strategy combined the oral delivery route and bovine papillomavirus (BPV) pseudovirus (PsV)-based human immunodeficiency virus (HIV) DNA vaccine, which has been proven to enhance the mucosal immunization compared with the systemic immunization and in general does not induce effective mucosal immune responses. In this study, the immune responses against the BPV expressing HIV gp41 epitopes (ELDKWA, NWFDIT) after oral administration in Cynomolgus monkeys (Macaca fascicularis) were assessed, and the biodistribution of plasmid DNA encapsulated in the papillomavirus-like particles (VLPs) were evaluated in murine models. Results showed that oral immunization with the HIV-PsV DNA vaccine in monkey generated p24 and gp41 epitopes-specific serum IgG. Importantly, these induced antibodies had been shown to neutralize HIV-1 primary strain. In addition, the advantage of VLPs as vehicles delivering genes had been first revealed in biodistribution results. Therefore, orally administered HIV-PsV DNA vaccine was well-tolerated, enhanced the mucosa targeting property of the plasmid DNA, and reduced the nontargeting distribution, which indicate that it would reduce stress associated with systemic vaccination.

Evolution of human papillomavirus carcinogenicity

Members of the Alphapapillomavirus genus are the causative agent for virtually all cases of cervical cancer. However, strains (commonly referred to as types) within this genus span the entire range of pathogenicity from highly carcinogenic (e.g., HPV16, odds ratio = 281.9, responsible for 50% of all cervical cancers), moderately carcinogenic (e.g., HPV31) to not carcinogenic (e.g., HPV71). The persistent expression of the viral oncoproteins (E6 and E7) from HPV16 has been shown to be necessary and sufficient to transform primary human keratinocytes in vitro. A plethora of functions have been described for both oncoproteins, and through functional comparisons between HPV16 and HPV6, a subset of these functions have been suggested to be oncogenic. However, extrapolating functional differences from these comparisons is unlikely to tease apart the fine details. In this review, we argue that a thorough understanding of the molecular mechanisms differentiating oncogenic from nononcogenic types should be obtained by performing functional assays in an evolutionary and epidemiological framework. We continue by interpreting some recent results using this paradigm and end by suggesting directions for future inquiries.

A novel interaction between the human papillomavirus type 16 E2 and E1--E4 proteins leads to stabilization of E2

The E4 (also called E1--E4) and E2 proteins of human papillomavirus type 16 are thought to be expressed within the same cells of a lesion, and their open reading frames overlap, suggesting that they may have a functional relationship. We have examined the effect of co-expression of these two proteins and found that each enhances the level of the other. We also identified the N-terminus of E2 as the first example of a viral protein that directly binds the HPV16 E1--E4 protein. This appears to result in the E2 becoming less soluble and promotes its relocation from the nucleus to the cytoplasm. In addition, the turnover of the E2 protein is decreased in the presence of E1--E4. All this raises the possibility that E1--E4 acts to influence E2 activity by varying the amount of available E2 in the cell.

Biology of papillomavirus replication in infected epithelium

Human papillomaviruses complete their life cycle in differentiating epithelial cells that would not normally be competent for either cellular or viral DNA replication. To overcome this, papillomaviruses encode two groups of proteins that work together in the upper epithelial layers to amplify viral genomes. The E6 and E7 proteins play a critical role in driving differentiating epithelial cells that have left the basal layer, back into the cell cycle, in order to produce a replication-competent environment that can be used by the virus for genome amplification. Papillomavirus replication is heavily dependent on cellular replication proteins, but in addition needs the viral E1 and E2 proteins, which act to unwind viral DNA around the origin of replication, and to recruit essential cellular proteins to the replication site. Recent work using mutant viral genomes has suggested that two other viral proteins, E4 and E5, contribute to efficient replication in the upper epithelial layers, although the mechanisms by which they do this have not yet been clearly established. Genome amplification in the upper epithelial layers differs from maintenance replication in the basal layer, where viral genome replication appears coupled to that of the cellular genome. The onset of genome amplification during differentiation is thought to be triggered at least in part by an increase in E1 and E2 levels, and possibly also by a change in the relative levels of the two proteins. The role of E6 and E7 in basal cell replication is, however, uncertain and there is even some question as to the exact requirement for E1. Although similarities in papillomavirus lifecycle organization and protein function suggest a common mechanism by which viral DNA replication is regulated, differences in the site of infection and transmission route appear to manifest themselves as differences in the timing and extent of genome amplification. Understanding the patterns of protein expression seen during natural infection will be important in fully understanding how these differences arise.

Intrabody strategies for the treatment of human papillomavirus-associated disease

Human papillomaviruses (HPVs) are associated with a variety of epithelial lesions, including benign genital warts and cervical intraepithelial neoplasia. Both cause significant morbidity in the general population, with cervical intraepithelial neoplasia progressing to cervical cancer in a subset of women who cannot resolve their infection. At present, there are no antiviral agents for the treatment of genital HPV infections, with many lesions requiring surgical intervention. Although other approaches are available for the treatment of genital warts, HPV infection cannot usually be cured and lesion recurrence is often a problem. A growing understanding of the molecular biology of HPV infection has identified several viral protein functions that may serve as drug targets. Among these are the HPV E1 and E2 proteins, which are necessary for viral genome replication and partitioning, and the E6 and E7 proteins, which are necessary for cell proliferation and apoptotic inhibition. With the exception of E1, these proteins lack enzymatic activity and achieve their effects by interacting with cellular proteins. Protein-protein interactions are in general quite difficult to inhibit using conventional small molecule drugs, but are amenable to inhibition using intracellular antibodies or intrabodies, which bind the viral proteins and sterically inhibit their association with cellular partners. The lack of homology between viral and cellular proteins, and the fact that HPV infections can be treated topically, makes them particularly well suited to the intrabody approach. This review covers the various strategies that are being considered for the treatment of HPV infections and the different intrabody formats that have been used to inhibit HPV function in model systems. The clinical utility of the approach is considered alongside the general difficulties of using protein molecules as intracellular therapeutics.

Disassembly and reassembly of yeast‐derived recombinant human papillomavirus virus‐like particles (HPV VLPs)

The human papillomavirus (HPV) virus-like particles (VLPs) produced by recombinant expression systems are promising candidate vaccine antigens for prevention of cervical cancers as well as genital warts. However, expression of HPV type 6, 11, and 16 L1 proteins in Saccharomyces cerevisiae yielded irregularly shaped, broadly distributed VLPs smaller in size (30-50 nm) than expected (60 nm). In this study, we demonstrate that these HPV VLPs can be disassembled into the constituent capsomers (L1 pentamers) by incubation at low ionic strength and elevated pH in the presence of relatively low concentration of reducing agents. Following the removal of reducing agents, lowering of pH and increasing of ionic strength, the capsomers spontaneously reassembled into homogenous, 60-nm VLPs characterized by significantly enhanced structural stability and improved immunogenicity. In order to achieve quantitative recovery of HPV VLPs, the disassembly/reassembly process was further optimized by use of high ionic strength (>0.5 M sodium chloride) to prevent aggregation of VLPs. The reassembled VLPs possess an architectural structure very similar to that of the natural HPV virus particles. This development illustrates how the natural, in vivo mechanisms facilitating cell entry and virus replication can be utilized to achieve an optimal, in vitro assembly state of yeast-expressed HPV VLPs.

Molecular biology of human papillomavirus infection and cervical cancer

HPVs (human papillomaviruses) infect epithelial cells and cause a variety of lesions ranging from common warts/verrucas to cervical neoplasia and cancer. Over 100 different HPV types have been identified so far, with a subset of these being classified as high risk. High-risk HPV DNA is found in almost all cervical cancers (>99.7%), with HPV16 being the most prevalent type in both low-grade disease and cervical neoplasia. Productive infection by high-risk HPV types is manifest as cervical flat warts or condyloma that shed infectious virions from their surface. Viral genomes are maintained as episomes in the basal layer, with viral gene expression being tightly controlled as the infected cells move towards the epithelial surface. The pattern of viral gene expression in low-grade cervical lesions resembles that seen in productive warts caused by other HPV types. High-grade neoplasia represents an abortive infection in which viral gene expression becomes deregulated, and the normal life cycle of the virus cannot be completed. Most cervical cancers arise within the cervical transformation zone at the squamous/columnar junction, and it has been suggested that this is a site where productive infection may be inefficiently supported. The high-risk E6 and E7 proteins drive cell proliferation through their association with PDZ domain proteins and Rb (retinoblastoma), and contribute to neoplastic progression, whereas E6-mediated p53 degradation prevents the normal repair of chance mutations in the cellular genome. Cancers usually arise in individuals who fail to resolve their infection and who retain oncogene expression for years or decades. In most individuals, immune regression eventually leads to clearance of the virus, or to its maintenance in a latent or asymptomatic state in the basal cells.

Papillomavirus virus-like particles as vehicles for the delivery of epitopes or genes

Papillomaviruses (PVs) are simple double-strand DNA viruses whose virion shells are T = 7 icosahedrons and composed of major capsid protein L1 and minor capsid protein L2.L1 alone or together with L2 can self-assemble into virus-like particles (VLPs) when expressed in eukaryotic or prokaryotic expression systems. Although the VLPs lack the virus genome DNA, their morphological and immunological characteristics are very similar to those of nature papillomaviruses. PV VLP vaccination can induce high titers of neutralizing antibodies and can effectively protect animals or humans from PV infection. Moreover, PV VLPs have been good candidates for vehicles to deliver epitopes or genes to target cells. They are widely used in the fields of vaccine development, neutralizing antibody detection, basic virologic research on papillomaviruses, and human papillomavirus (HPV) screening. Besides the structural biology and immunological basis for PV VLPs used as vehicles to deliver epitopes or genes, this review details the latest findings on chimeric papillomavirus VLPs and papillomavirus pseudoviruses, which are two important forms of PV VLPs used to transfer epitopes or genes.

Designing nonviral vectors for efficient gene transfer and long-term gene expression

Although the genetic therapy of human diseases has been conceptually possible for many years we still lack a vector system that allows safe and reproducible genetic modification of eukaryotic cells and ensures faithful long-term expression of transgenes. There is increasing agreement that vectors that are based exclusively on chromosomal elements, which replicate autonomously in human cells, could fulfill these criteria. The rational construction of such vectors is still hindered by our limited knowledge of the factors that regulate chromatin function in eukaryotic cells. This review sets out to summarize how our current knowledge of nuclear organization can be applied to the design of extrachromosomal gene expression vectors that can be used for human gene therapy. Within the past years a number of episomal nonviral constructs have been designed and their replication strategies, expression of transgenes, mitotic stability, and delivery strategies and the mechanisms required for their stable establishment will be discussed. To date, these nonviral vectors have not been used in clinical trials. Even so, many compelling arguments can be developed to support the view that nonviral vector systems will play a major role in future gene therapy protocols.

Replication and encapsidation of papillomaviruses in Saccharomyces cerevisiae

Improvements in methodologies to recapitulate and study particular biological functions of the papillomavirus life cycle have led to great advances in our knowledge of these viruses. Described in this chapter are techniques that allow low-copy and high-copy replication of full-length human papillomavirus (HPV) genomes, as well as assembly of virus-like particles, in Saccharomyces cerevisiae (yeast). This system has several distinct advantages that make it an attractive complement to the well-established raft-culturing system. First, yeast are inexpensive, rapid, and simple to culture in the lab. Second, they provide an ever-widening array of genetic tools to analyze HPV functions--most recently notable, the yeast open reading frame (ORF)-deletion library. Third, yeast provide a potentially high-efficiency means to produce large quantities of infectious virus in a short time frame. Fourth, assembly of HPV virus in yeast allows encapsidation of mutant genomes, since previous studies have shown that no viral ORF is required for replication of full-length HPV in yeast.

Interaction of tSNARE syntaxin 18 with the papillomavirus minor capsid protein mediates infection

The papillomavirus capsid mediates binding to the cell surface and passage of the virion to the perinuclear region during infection. To better understand how the virus traffics across the cell, we sought to identify cellular proteins that bind to the minor capsid protein L2. We have identified syntaxin 18 as a protein that interacts with bovine papillomavirus type 1 (BPV1) L2. Syntaxin 18 is a target membrane-associated soluble N-ethylmaleimide-sensitive factor-attachment protein receptor (tSNARE) that resides in the endoplasmic reticulum (ER). The ectopic expression of FLAG-tagged syntaxin 18, which disrupts ER trafficking, blocked BPV1 pseudovirion infection. Furthermore, the expression of FLAG-syntaxin 18 prevented the passage of BPV1 pseudovirions to the perinuclear region that is consistent with the ER. Genetic studies identified a highly conserved L2 domain, DKILK, comprising residues 40 to 44 that mediated BPV1 trafficking through the ER during infection via an interaction with the tSNARE syntaxin 18. Mutations within the DKILK motif of L2 that did not significantly impact virion morphogenesis or binding at the cell surface prevented the L2 interaction with syntaxin 18 and disrupted BPV1 infection.

Efficient intracellular assembly of papillomaviral vectors

Although the papillomavirus structural proteins, L1 and L2, can spontaneously coassemble to form virus-like particles, currently available methods for production of L1/L2 particles capable of transducing reporter plasmids into mammalian cells are technically demanding and relatively low-yield. In this report, we describe a simple 293 cell transfection method for efficient intracellular production of papillomaviral-based gene transfer vectors carrying reporter plasmids. Using bovine papillomavirus type 1 (BPV1) and human papillomavirus type 16 as model papillomaviruses, we have developed a system for producing papillomaviral vector stocks with titers of several billion transducing units per milliliter. Production of these vectors requires both L1 and L2, and transduction can be prevented by papillomavirus-neutralizing antibodies. The stocks can be purified by an iodixanol (OptiPrep) gradient centrifugation procedure that is substantially more effective than standard cesium chloride gradient purification. Although earlier data had suggested a potential role for the viral early protein E2, we found that E2 protein expression did not enhance the intracellular production of BPV1 vectors. It was also possible to encapsidate reporter plasmids devoid of BPV1 DNA sequences. BPV1 vector production efficiency was significantly influenced by the size of the target plasmid being packaged. Use of 6-kb target plasmids resulted in BPV1 vector yields that were higher than those with target plasmids closer to the native 7.9-kb size of papillomavirus genomes. The results suggest that the intracellular assembly of papillomavirus structural proteins around heterologous reporter plasmids is surprisingly promiscuous and may be driven primarily by a size discrimination mechanism.

Saccharomyces cerevisiae is permissive for replication of bovine papillomavirus type 1

We recently demonstrated that Saccharomyces cerevisiae protoplasts can take up bovine papillomavirus type 1 (BPV1) virions and that viral episomal DNA is replicated after uptake. Here we demonstrate that BPV virus-like particles are assembled in infected S. cerevisiae cultures from newly synthesized capsid proteins and also package newly synthesized DNA, including full-length and truncated viral DNA and S. cerevisiae-derived DNA. Virus particles prepared in S. cerevisiae are able to convey packaged DNA to Cos1 cells and to transform C127 cells. Infectivity was blocked by antisera to BPV1 L1 but not antisera to BPV1 E4. We conclude that S. cerevisiae is permissive for the replication of BPV1 virus.

Animal models of papillomavirus pathogenesis

Tumorigenesis due to papillomavirus (PV) infection was first demonstrated in rabbits and cattle early last century. Despite the evidence obtained in animals, the role of viruses in human cancer was dismissed as irrelevant. It took a paradigm shift in the late 1970s for some viruses to be recognised as 'tumour viruses' in humans, and in 1995, more than 60 years after Rous's first demonstration of CRPV oncogenicity, WHO officially declared that 'HPV-16 and HPV-18 are carcinogenic to humans'. Experimental studies with animal PVs have been a determining factor in this decision. Animal PVs have been studied both as agents of disease in animals and as models of human PV infection. In addition to the study of PV infection in whole animals, in vitro studies with animal PV proteins have contributed greatly to the understanding of the mechanisms of cell transformation. Animal PVs cause distressing diseases in both farm and companion animals, such as teat papillomatosis in cattle, equine sarcoids and canine oral papillomatosis and there is an urgent need to understand the pathogenesis of these problematic infections. Persistent and florid teat papillomatosis in cows can lead to mastitis, prevent the suckling of calves and make milking impossible; heavily affected animals are culled and so occasionally are whole herds. Equine sarcoids are often recurrent and untreatable and lead to loss of valuable animals. Canine oral papillomatosis can be very extensive and persistent and lead to great distress. Thus the continuing research in the biology of animal PVs is amply justified. BPVs and CRPV have been for many years the model systems with which to study the biology of HPV. Induction of papillomas and their neoplastic progression has been experimentally demonstrated and reproduced in cattle and rabbits, and virus-cofactor interactions have been elucidated in these systems. With the advancements in molecular and cell culture techniques, the direct study of HPV has become less problematic and understandably research efforts have shifted in focus from animal to human PVs. However, there are still areas in which studies on animal PVs will continue to provide answers to questions pertaining to virus biology. One of these questions is the involvement of HPV in oesophageal and bladder cancer in humans as is the case for BPV in cattle. Another is the site of viral latency. Lymphocytes have been proposed as a site of latency for both BPV and HPV but only experiments performed in animals could clarify this point. Animal PVs have been instrumental in the development of vaccines as cattle, rabbit and more recently dog all provide the opportunity to study vaccination in the natural host. Several anti-HPV vaccines, both prophylactic and therapeutic, based on those developed in animals, are now in clinical trials with encouraging results. In vitro studies with two animal PV early proteins, the transcriptional regulator E2 and the oncoprotein E5, among others, have contributed to the elucidation of viral gene control and cell transformation. BPV E2 was the first viral product to be identified as a transcriptional regulator; more recently, its association with mitotic chromosomes has been suggested as a mechanism for the partition of viral genomes between daughter cells, and its L2-mediated localisation in the sub-nuclear compartments PODs is believed to favour viral DNA encapsidation. E5 is the major transforming protein of several BPVs. Many of the function of E5 proteins have been first established for BPV E5 and later validated for HPV E5. E5 interacts with 16k ductin/subunit c and this interaction is deemed responsible for the down-regulation of gap junction intercellular communication and the inhibition of acidification of endomembranes. E5 activates growth factor receptors and numerous kinases, including cdks, and down-regulates expression of MHC class I. Thus E5 would help the establishment of viral infection by promoting both cell proliferation and immune evasion. Despite the extensive studies on vaccination in animals, E5 has not been tried inE5 has not been tried in animal models as a possible anti-papillomavirus vaccine. A recent study has reported that vaccination of mice with HPV-16 E5 in a recombinant adenovirus reduced the growth of tumours induced by E5-expressing cells. Perhaps this is an instance in which work on animal PVs should follow HPV and the potential for E5 vaccination should be validated in naturally occurring animal models.

L1 interaction domains of papillomavirus l2 necessary for viral genome encapsidation

BPHE-1 cells, which harbor 50 to 200 viral episomes, encapsidate viral genome and generate infectious bovine papillomavirus type 1 (BPV1) upon coexpression of capsid proteins L1 and L2 of BPV1, but not coexpression of BPV1 L1 and human papillomavirus type 16 (HPV16) L2. BPV1 L2 bound in vitro via its C-terminal 85 residues to purified L1 capsomers, but not with intact L1 virus-like particles in vitro. However, when the efficiency of BPV1 L1 coimmunoprecipitation with a series of BPV1 L2 deletion mutants was examined in vivo, the results suggested that residues 129 to 246 and 384 to 460 contain independent L1 interaction domains. An L2 mutant lacking the C-terminal L1 interaction domain was impaired for encapsidation of the viral genome. Coexpression of BPV1 L1 and a chimeric L2 protein composed of HPV16 L2 residues 1 to 98 fused to BPV1 L2 residues 99 to 469 generated infectious virions. However, inefficient encapsidation was seen when L1 was coexpressed with either BPV1 L2 with residues 91 to 246 deleted or with BPV1 L2 with residues 1 to 225 replaced with HPV16 L2. Impaired genome encapsidation did not correlate closely with impairment of the L2 proteins either to localize to promyelocytic leukemia oncogenic domains (PODs) or to induce localization of L1 or E2 to PODs. We conclude that the L1-binding domain located near the C terminus of L2 may bind L1 prior to completion of capsid assembly, and that both L1-binding domains of L2 are required for efficient encapsidation of the viral genome.