Infectious virus replication in papillomas induced by molecularly cloned cottontail rabbit papillomavirus DNA

Modeling HPV-Associated Disease and Cancer Using the Cottontail Rabbit Papillomavirus

Approximately 5% of all human cancers are attributable to human papillomavirus (HPV) infections. HPV-associated diseases and cancers remain a substantial public health and economic burden worldwide despite the availability of prophylactic HPV vaccines. Current diagnosis and treatments for HPV-associated diseases and cancers are predominantly based on cell/tissue morphological examination and/or testing for the presence of high-risk HPV types. There is a lack of robust targets/markers to improve the accuracy of diagnosis and treatments. Several naturally occurring animal papillomavirus models have been established as surrogates to study HPV pathogenesis. Among them, the Cottontail rabbit papillomavirus (CRPV) model has become known as the gold standard. This model has played a pivotal role in the successful development of vaccines now available to prevent HPV infections. Over the past eighty years, the CRPV model has been widely applied to study HPV carcinogenesis. Taking advantage of a large panel of functional mutant CRPV genomes with distinct, reproducible, and predictable phenotypes, we have gained a deeper understanding of viral–host interaction during tumor progression. In recent years, the application of genome-wide RNA-seq analysis to the CRPV model has allowed us to learn and validate changes that parallel those reported in HPV-associated cancers. In addition, we have established a selection of gene-modified rabbit lines to facilitate mechanistic studies and the development of novel therapeutic strategies. In the current review, we summarize some significant findings that have advanced our understanding of HPV pathogenesis and highlight the implication of the development of novel gene-modified rabbits to future mechanistic studies.

Rabbit Models for Studying Human Infectious Diseases

Using an appropriate animal model is crucial for mimicking human disease conditions, and various facets including genetics, anatomy, and pathophysiology should be considered before selecting a model. Rabbits (Oryctolagus cuniculus) are well known for their wide use in production of antibodies, eye research, atherosclerosis and other cardiovascular diseases. However, a systematic description of the rabbit as primary experimental models for the study of various human infectious diseases is unavailable. This review focuses on the human infectious diseases for which rabbits are considered a classic or highly appropriate model, including AIDS (caused by HIV1), adult T-cell leukemia-lymphoma (human T-lymphotropic virus type 1), papilloma or carcinoma (human papillomavirus) , herpetic stromal keratitis (herpes simplex virus type 1), tuberculosis (Mycobacterium tuberculosis), and syphilis (Treponema pallidum). In addition, particular aspects of the husbandry and care of rabbits used in studies of human infectious diseases are described.

Vaccine generated immunity targets an HPV16 E7 HLA-A2.1-restricted CD8(+) T cell epitope relocated to an early gene or a late gene of the cottontail rabbit papillomavirus (CRPV) genome in HLA-A2.1 transgenic rabbits

The newly established HLA-A2.1 transgenic rabbit model has proven useful for testing the immunogenicity of well known and computer-predicted A2-restricted epitopes. In the current study we compared the protective immunity induced to a preferred HPV16 E7 A2-restricted epitope that has been relocated to positions within the CRPV E7 gene and the CRPV L2 gene. Epitope expression from both the E7 protein and the L2 protein resulted in increased protection against viral DNA challenge of the HLA-A2.1 transgenic rabbits as compared to control-vaccinated rabbit groups. These data indicate that proteins expressed at both early and late time points during a natural papillomavirus infection can be targeted by epitope-specific immunity and indicate this immunity is increased to early rather than late expressed proteins of papillomaviruses. This study also highlights the broad utility of the HLAA2.1 transgenic rabbit model for testing numerous immunological factors involved in vaccine generated protective immunity.

Cottontail rabbit papillomavirus in Langerhans cells in Sylvilagus spp

A wildlife sanctuary presented an adult female cottontail rabbit (Sylvilagus spp.), age unknown, to the Colorado State University Pathology service for postmortem examination. Gross examination revealed numerous pigmented wartlike lesions arising from the skin of the head surrounding the ears, eyes, nares, mouth, and dorsum. Masses were firm, friable, and easily detached from the underlying skin. Differential diagnoses included Cottontail rabbit papillomavirus, Rabbit fibroma virus, and Myxoma virus. Histological examination revealed multiple papillary masses lined by stratified squamous epithelial cells with central cores of fibrovascular connective tissue and parakeratotic hyperkeratosis. Cells of the Stratum spinosum were frequently swollen with abundant perinuclear, cytoplasmic, clearing, and occasional intranuclear basophilic, glassy, spherical inclusions up to 3 microm in diameter. The lesions were consistent with Cottontail rabbit papillomavirus infection. Papilloma virus antigens were identified by immunohistochemistry. In addition, papillomavirus particles were identified by transmission electron microscopy within Langerhans cells of the epidermis, suggesting a unique mechanism for systemic dissemination of the virus. The present case report highlights the finding of viral particles within the Langerhans cells and suggests a novel mechanism of pathogenesis.

Papillomavirus DNA complementation in vivo

Recent phylogenic studies indicate that DNA recombination could have occurred in ancient papillomavirus types. However, no experimental data are available to demonstrate this event because of the lack of human papillomavirus infection models. We have used the cottontail rabbit papillomavirus (CRPV)/rabbit model to study pathogenesis and immunogenicity of different mutant genomes in vivo. Although the domestic rabbit is not a natural host for CRPV infection, it is possible to initiate infection with naked CRPV DNA cloned into a plasmid and monitor papilloma outgrowth on these animals. Taking advantage of a large panel of mutants based on a CRPV strain (Hershey CRPV), we tested the hypothesis that two non-viable mutant genomes could induce papillomas by either recombination or complementation. We found that co-infection with a dysfunctional mutant with an E2 transactivation domain mutation and another mutant with an E7 ATG knock out generated papillomas in rabbits. DNA extracted from these papillomas contained genotypes from both parental genomes. Three additional pairs of dysfunctional mutants also showed similar results. Individual wild type genes were also shown to rescue the function of corresponding dysfunctional mutants. Therefore, we suggest that complementation occurred between these two non-viable mutant PV genomes in vivo.

Issues associated with residual cell-substrate DNA in viral vaccines

The presence of some residual cellular DNA derived from the production-cell substrate in viral vaccines is inevitable. Whether this DNA represents a safety concern, particularly if the cell substrate is derived from a tumor or is tumorigenic, is unknown. DNA has two biological activities that need to be considered. First, DNA can be oncogenic; second, DNA can be infectious. As part of our studies to assess the risk of residual cell-substrate DNA in viral vaccines, we have established assays that can quantify the biological activities of DNA. From data obtained using these assays, we have estimated the risk of an oncogenic or an infectious event from DNA. Because these estimates were derived from the most sensitive assays identified so far, they likely represent worst-case estimates. In addition, methods that inactivate the biological activities of DNA can be assessed and estimations of risk reduction by these treatments can be made. In this paper, we discuss our approaches to address potential safety issues associated with residual cellular DNA from neoplastic cell substrates in viral vaccines, summarize the development of assays to quantify the oncogenic and infectivity activities of DNA, and discuss methods to reduce the biological activities of DNA.

Detection of L1, infectious virions and anti-L1 antibody in domestic rabbits infected with cottontail rabbit papillomavirus

Shope papillomavirus or cottontail rabbit papillomavirus (CRPV) is one of the first small DNA tumour viruses to be characterized. Although the natural host for CRPV is the cottontail rabbit (Sylvilagus floridanus), CRPV can infect domestic laboratory rabbits (Oryctolagus cuniculus) and induce tumour outgrowth and cancer development. In previous studies, investigators attempted to passage CRPV in domestic rabbits, but achieved very limited success, leading to the suggestion that CRPV infection in domestic rabbits was abortive. The persistence of specific anti-L1 antibody in sera from rabbits infected with either virus or viral DNA led us to revisit the questions as to whether L1 and infectious CRPV can be produced in domestic rabbit tissues. We detected various levels of L1 protein in most papillomas from CRPV-infected rabbits using recently developed monoclonal antibodies. Sensitive in vitro infectivity assays additionally confirmed that extracts from these papillomas were infectious. These studies demonstrated that the CRPV/New Zealand White rabbit model could be used as an in vivo model to study natural virus infection and viral life cycle of CRPV and not be limited to studies on abortive infections.

Vaccination against cutaneous and mucosal papillomavirus in cattle

Viruses are responsible for approximately 15% of human cancer worldwide. Human papillomavirus and hepatitis B virus are the recognized agents of cervical and liver cancer, respectively, which together constitute 80% of all virally induced cancers. If measures could be found to bring viral infection under control, a great proportion of human cancer would be greatly reduced. Experimental vaccines are being developed against papillomavirus. In principle two different types of vaccine can be envisaged: prophylactic vaccines that would elicit virus-neutralizing antibodies and would prevent infection and therapeutic vaccines that would induce regression of established lesions before progression to malignancy took place. The research on vaccines against human papillomavirus is hampered by the difficulties encountered in growing the virus in tissue culture and by the unacceptable nature of experimentation in humans. Effective vaccines, both natural and genetically engineered, have been developed against bovine papillomavirus and cottontail rabbit papillomavirus. The success obtained with the animal models supports the optimistic prediction that in the relatively near future vaccines will be available against the most problematic or potentially dangerous forms of papillomatosis in humans.

Establishment of a cottontail rabbit papillomavirus/HLA-A2.1 transgenic rabbit model

Three transgenic rabbit lines that express a well-characterized human major histocompatibility complex class I (MHC-I) gene (HLA-A2.1) have been established. All three lines carry the HLA-A2.1 heavy chain and are able to pass the transgene to their offspring with both the outbred and the inbred EIII/JC genetic background. HLA-A2.1 colocalizes exclusively with rabbit MHC-I on the cell surfaces. These HLA-A2.1 transgenic rabbits demonstrated infection patterns similar to those found after cottontail rabbit papillomavirus (CRPV) challenge when compared with results in normal rabbits, although higher regression rates were found in HLA-A2.1 transgenic rabbits. Because the CRPV genome can accommodate significant modifications, the CRPV/HLA-A2.1 rabbit model has the potential to be used to screen HLA-A2.1-restricted immunogenic epitopes from human papillomaviruses in the context of in vivo papillomavirus infection.

An HLA-A2.1-transgenic rabbit model to study immunity to papillomavirus infection

We have established several HLA-A2.1-transgenic rabbit lines to provide a host to study CD8(+) T cell responses during virus infections. HLA-A2.1 protein expression was detected on cell surfaces within various organ tissues. Continuous cultured cells from these transgenic rabbits were capable of presenting both endogenous and exogenous HLA-A2.1-restricted epitopes to an HLA-A2.1-restricted epitope-specific CTL clone. A DNA vaccine containing an HLA-A2.1-restricted human papillomavirus type 16 E7 epitope (amino acid residues 82-90) stimulated epitope-specific CTLs in both PBLs and spleen cells of transgenic rabbits. In addition, vaccinated transgenic rabbits were protected against infection with a mutant cottontail rabbit papillomavirus DNA containing an embedded human papillomavirus type 16 E7/82-90 epitope. Complete protection was achieved using a multivalent epitope DNA vaccine based on epitope selection from cottontail rabbit papillomavirus E1 using MHC class I epitope prediction software. HLA-A2.1-transgenic rabbits will be an important preclinical animal model system to study virus-host interactions and to assess specific targets for immunotherapy.

Impact of genetic changes to the CRPV genome and their application to the study of pathogenesis in vivo

The cottontail rabbit papillomavirus (CRPV)/rabbit model has been used to study oncogenicity and immunogenicity of different antigens from the papillomavirus genome and has therefore served as a preclinical model for the development of preventive and therapeutic vaccines against papillomavirus infections. One unique property of the CRPV model is that infection can be initiated using viral DNA. This property allows for the functional testing of viral mutants in vivo. We have introduced point mutations, insertions and deletions into all of the different coding and non-coding regions of the CRPV genome and have tested their infectivity in this model. We found that the majority of the mutant genomes retained viability and could induce papillomas in domestic rabbits. These data indicated that the CRPV genome is tolerant of many modifications without compromising its ability to initiate skin papillomas. In combination with our recently established HLA-A2.1 transgenic rabbit model, this plasticity allows us to extend the utility of the CRPV/rabbit model to the screening of HLA-A2.1 restricted epitopes from other human viral and tumor antigens.

Human papillomaviruses and cervical cancer

Carcinoma of the uterine cervix, a leading cause of cancer death in women worldwide, is initiated by infection with high-risk types of human papillomaviruses (HPVs). This review summarizes laboratory studies over the past 20 years that have elucidated the major features of the HPV life cycle, identified the functions of the viral proteins, and clarified the consequences of HPV infection for their host cells. This information has allowed the development of various strategies to prevent or treat infections, including prophylactic vaccination with virus-like particles, therapeutic vaccination against viral proteins expressed in cancer cells, and antiviral approaches to inhibit virus replication, spread, or pathogenesis. These strategies have the potential to cause a dramatic reduction in the incidence of cervical carcinoma and serve as the prototype for comprehensive efforts to combat virus-induced tumors.

The human papillomavirus type 11 E1/\E4 protein is not essential for viral genome amplification

An abundant human papillomavirus (HPV) protein E1/\E4 is expressed late in the virus life cycle in the terminally differentiated layers of epithelia. The expression of E1/\E4 usually coincides with the onset of viral DNA amplification. However, the function of E1/\E4 in viral life cycle is not completely understood. To examine the role of E1/\E4 in the virus life cycle, we introduced a single nucleotide change in the HPV-11 genome to result in a truncation of E1/\E4 protein without affecting the E2 amino acid sequence. This mutated HPV-11 genome was introduced into a human foreskin keratinocyte cell line immortalized by the catalytic subunit of human telomerase, deficient in p16(INK4a) expression, and previously shown to support the HPV-11 life cycle when grown in organotypic raft culture. We have demonstrated that E1/\E4 is dispensable for HPV-11 viral DNA amplification in the late stages of the viral life cycle.

Life cycle heterogeneity in animal models of human papillomavirus-associated disease

Animal papillomaviruses are widely used as models to study papillomavirus infection in humans despite differences in genome organization and tissue tropism. Here, we have investigated the extent to which animal models of papillomavirus infection resemble human disease by comparing the life cycles of 10 different papillomavirus types. Three phases in the life cycles of all viruses were apparent using antibodies that distinguish between early events, the onset of viral genome amplification, and the expression of capsid proteins. The initiation of these phases follows a highly ordered pattern that appears important for the production of virus particles. The viruses examined included canine oral papillomavirus, rabbit oral papillomavirus (ROPV), cottontail rabbit papillomavirus (CRPV), bovine papillomavirus type 1, and human papillomavirus types 1, 2, 11, and 16. Each papillomavirus type showed a distinctive gene expression pattern that could be explained in part by differences in tissue tropism, transmission route, and persistence. As the timing of life cycle events affects the accessibility of viral antigens to the immune system, the ideal model system should resemble human mucosal infection if vaccine design is to be effective. Of the model systems examined here, only ROPV had a tissue tropism and a life cycle organization that resembled those of the human mucosal types. ROPV appears most appropriate for studies of the life cycles of mucosal papillomavirus types and for the development of prophylactic vaccines. The persistence of abortive infections caused by CRPV offers advantages for the development of therapeutic vaccines.

High and low levels of cottontail rabbit papillomavirus E2 protein generate opposite effects on gene expression

The papillomavirus E2 protein plays an important role in viral transcriptional regulation and replication. We chose to study the cottontail rabbit papillomavirus (CRPV) E2 protein as a transcriptional regulator because of the availability of an animal model for papilloma formation, which may be relevant for human papillomavirus (HPV) infection and replication. We studied the effect of expression levels of E2 on the long control region, which contains transcriptional promoter and enhancer elements, and synthetic E2-dependent artificial promoters in which the E2 was the dominant factor in the transcriptional activation. These experiments indicated that high levels of E2 were inhibitory and low levels were stimulatory for transactivation. In addition, we showed that the complex formed between CRPV E2 and the cognate binding site was less stable than the complex formed between HPV E2 and the same cognate binding site. Furthermore, we showed that CRPV E2 binding to its transcriptional regulatory sequence was stabilized by other proteins such as E1, which produced increments in transcriptional activation of E2-dependent genes. The data may be used to define conditions in which the rabbit model can be used for the screening of drugs which are inhibitory to the HPV and CRPV replication and gene expression.

Gene transfer by biolistic process

Gene transfer into somatic tissues is a tool for both the study of gene function in the basic science laboratory and for gene therapy and genetic immunization in the clinic. Biolistic processes can be used to deliver both viral and nonviral vectors into somatic tissues. This review discusses the advantages and disadvantages of three biolistic processes: jet injection, microparticle bombardment, and needle and syringe injection. Jet injection and needle and syringe injection can be used to deliver both viral and nonviral vectors. Both jet injection and microparticle bombardment can be used to target a broad range of tissues. Needle and syringe injection has been most widely used in muscle tissue. The choice of which biolistic process to use is dependent on the specific application.

Rapid, efficient, large-scale purification of unfused, non-denatured E7 protein of cottontail rabbit papillomavirus

Cottontail rabbit papillomavirus (CRPV) E7 protein is one of the :high risk' papillomavirus E7 oncoproteins that are partially insoluble in aqueous solution. An Escherichia coli expression system was used for purification of CRPV E7 protein in quantities sufficient for immunologic studies and structural analysis. A glutathione S-transferase (GST)-CRPV E7 fusion protein was solubilized in the presence of non-ionic and ionic detergents, and isolated on an affinity column of glutathione Sepharose beads. The CRPV E7 portion was cleaved from the column with thrombin at a thrombin cleavage site between the fused partners. Thrombin was removed subsequently by adsorption to benzamidine. This method is rapid, requiring just one week, and efficient, yielding 3 mg of pure CRPV E7 protein per liter of bacterial culture. It produced a protein product that was about 95% pure. High-titered polyclonal antisera generated to the product recognized CRPV E7 but not GST. Purified CRPV E7 protein exhibited the ability to bind pRB, making it the first unfused, non-denatured CRPV E7 product reported to do so. This attribute could facilitate structure-function studies of CRPV E7-pRB interactions.

High efficiency induction of papillomas in vivo using recombinant cottontail rabbit papillomavirus DNA

Plasmids containing cottontail rabbit papillomavirus (CRPV) DNA can induce papillomas in vivo, but efficiency has been low. The aim of the present investigation was to explore some of the technical variables involved in inoculation of rabbits with recombinant CRPV DNA in attempts to improve both yield and consistency of papilloma induction. It was found that induction of epidermal hyperplasia, with either a mixture of turpentine and acetone or phorbol esters, produced a marked increase in papilloma yield. An additional powerful factor was the use of very vigorous, cutaneous scarification, sufficient to penetrate the papillary dermis and produce bleeding. When used in combination, papilloma yields were consistent and often reached 90-100% of inoculated sites. A number of other variables which did not consistently affect papilloma yield were tested. These included bleb and puncture injections, plasmid dose, vector type, occlusive dressings, lipofection reagent, carrier DNA, and different methods for plasmid DNA extraction and purification. It is concluded that the most important variables in improving papilloma yields were prior induction of epidermal hyperplasia and vigorous cutaneous scarification.

Gene transfer by jet injection into differentiated tissues of living animals and in organ culture

Jet injection can be used to introduce genes into the cells of differentiated tissues of living animals and organ cultures. When a solution of plasmid DNA is jet injected into a selected tissue or organ, cells lying in or near the path of the jet injection are transfected with the DNA and the introduced gene(s) are expressed. Since there is minimal morbidity from each jet injection, multiple injections can be performed at the same or nearby sites. Both mRNA and protein expression from transfected genes can be quantitated using standard methods. In addition, the technique is an efficient means of DNA immunization. Methodology for using jet injection to transfer plasmid DNA into the cells of skin, fat, mammary gland, and muscle are described.

Papilloma formation in human foreskin xenografts after inoculation of human papillomavirus type 16 DNA

A mouse model of high-risk human papillomavirus infection was developed in which human papillomavirus (HPV) type 16 DNA was inoculated into human foreskin grafted to the skin of severe combined immunodeficient (scid) mice. Grafted skin contained human epidermis and dermis and, like normal human skin, expressed involucrin in differentiating keratinocytes. HPV type 16 DNA, attached to gold particles, was delivered directly into human epidermal cells and induced exophytic papilloma with histologic features of papillomavirus infection, including koilocytosis and expression of papillomavirus capsid antigen. This model should be useful for determining in vivo the functions of viral genes and for developing strategies to prevent and treat HPV-associated disease. It may also be of value in developing animal models of other human skin diseases.

Gene transfer into mammalian cells by jet injection

Jet injection of DNA in solution is a technique that can be used to transfer DNA into tissues of living animals, where the introduced genes are expressed. The muscle, fat, skin, and mammary tissue of mice, and the fat, skin, and mammary tissue of sheep can be transfected with DNA. A jet injector, such as the Ped-o-jet (Stirn Industries, Dayton, NJ), is used to form a jet from 100 to 300 microliters of a DNA solution. This jet has sufficient force to travel into and through tissues of adult and juvenile animals. The introduced DNA is found in cells surrounding the path of the jet. When jet injection is performed through the surface of intact skin, underlying muscle, mammary, and fat, cells up to 2 cm distant from the point of injection are transfected with DNA. In this study, we demonstrate that the efficiency of DNA transfer is dependent upon the force of injection. Jet injection is an alternative to needle injection, lipofection, and particle bombardment for the introduction of "naked" DNA into the tissues of animals. This technique has potential for the introduction of genes into living organisms for genetic vaccination and gene therapy.

Papilloma formation by cottontail rabbit papillomavirus requires E1 and E2 regulatory genes in addition to E6 and E7 transforming genes

The present study used the cottontail rabbit papillomavirus DNA-rabbit system to evaluate whether the regulatory genes E1 and E2 and the transforming gene E6 are required for papilloma formation. Frameshift mutations were generated in the individual genes in the context of a full-length cottontail rabbit papillomavirus genome, and the mutant DNAs were intradermally inoculated into domestic rabbits. None of the mutants induced papillomas. Marker rescue experiments confirmed that the defects were due to mutations that we deliberately introduced. Marker rescue also confirmed our previous report that the upstream region of E7 around position 9 was critical for papilloma induction. These results demonstrate that the E1 and E2 regulatory genes as well as the E6 and E7 transforming genes are each required for papilloma formation. Each gene may provide molecular targets for therapeutic intervention.