Infection of mucosal epithelium by papillomaviruses is responsible for the induction of genital and oral warts and plays a critical role in the development of human cervical and oropharyngeal cancer. We have employed a canine model to develop a systemic vaccine that completely protects against experimentally-induced oral mucosal papillomas. The major capsid protein, Li, of canine oral papillomavirus (COPV) was expressed in Sf9 insect cells in native conformation. LI protein, which self-assembled into virus-like particles, was purified on CsCl gradients and injected intradermally into the foot pad of beagles. Vaccinated animals developed circulating antibodies against COPV and became completely resistant to experimental challenge with COPV. Successful immunization was strictly dependent upon native Li protein conformation and LI type. Partial protection was achieved with as little as 0.125 ng of LI protein, and adjuvants appeared useful for prolonging the host immune response. Serum immunoglobulins passively transferred from COPV LI-immunized beagles to naive beagles conferred protection from experimental infection with COPV. Our results indicate the feasibility of developing a human vaccine to prevent mucosal papillomas, which can progress to malignancy.Papillomaviruses are small (55 nm), nonenveloped DNA viruses that induce epidermal and mucosal papillomas in humans and animals (1, 2). Canine, bovine, rabbit, and some human papillomas can progress to the malignant state (3-7). In humans, the development of cervical carcinoma is closely associated with genital mucosal infection by a small subset of human papillomaviruses (HPVs), including 8). In regions where mass cancer screening is inadequate (e.g., Southeast Asia and South America), cervical cancer represents the leading cause of death by cancer in women. Although the United States has fewer annual deaths (4000-6000) from cervical cancer, the medical health care costs are enormous for screening and treating early HPVrelated lesions. The development of an effective prophylatic HPV vaccine could potentially reduce the occurrence of genital warts, cervical dysplasia, and neoplasia by an estimated 90%.Currently there are no vaccines to prevent disease caused by HPVs. These viruses possess certain properties which make vaccine development difficult. First, HPVs are highly species specific, making it impossible to use animals for the direct evaluation of a vaccines' efficacy. Second, there is no reliable in vivo or in vitro source of intact papillomaviruses. Mucosal lesions caused by and HPV-18 yield small quantities of infectious virus, and papillomaviruses cannot be propagated efficiently in vitro.Infection of animals with species-specific papillomaviruses offers the best opportunity for evaluating vaccines. We recently described the use of canine oral papillomavirus (COPV)The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 sole...

Most papillomaviruses (PVs) are oncogenic. There are at least 100 different human PVs and 65 nonhuman vertebrate hosts, including wild rodents, which have species-specific PV infections. Florid papillomatosis arose in a colony of NMRI-Foxn1 nu / Foxn1 nu (nude) mice at the Advanced Centre for Treatment Research and Education in Cancer in India. Lesions appeared at the mucocutaneous junctions of the nose and mouth. Histologically, lesions were classical papillomas with epidermal hyperplasia on thin fibrovascular stalks in a verrucous pattern. Koilocytotic cells were observed in the stratum granulosum of the papillomatous lesions. Immunohistochemically, these abnormal cells were positive for PV group-specific antigens. With transmission electron microscopy, virus particles were observed in crystalline intranuclear inclusions within keratinocytes. The presence of a mouse PV, designated MusPV, was confirmed by amplification of PV DNA with degenerative primers specific for PVs. This report is the first of a PV and its related disease in laboratory mice.

Abstract. Papillomaviruses (PVs) are highly species-and site-specific pathogens of stratified squamous epithelium. Although PV infections in the various Felidae are rarely reported, we identified productive infections in six cat species. PV-induced proliferative skin or mucous membrane lesions were confirmed by immunohistochemical screening for papillomavirus-specific capsid antigens. Seven monoclonal antibodies, each of which reacts with an immunodominant antigenic determinant of the bovine papillomavirus L1 gene product, revealed that feline PV capsid epitopes were conserved to various degrees. This battery of monoclonal antibodies established differential expression patterns among cutaneous and oral PVs of snow leopards and domestic cats, suggesting that they represent distinct viruses. Clinically, the lesions in all species and anatomic sites were locally extensive and frequently multiple. Histologically, the areas of epidermal hyperplasia were flat with a similarity to benign tumors induced by cutaneotropic, carcinogenic PVs in immunosuppressed human patients. Limited restriction endonuclease analyses of viral genomic DNA confirmed the variability among three viral genomes recovered from available frozen tissue. Because most previous PV isolates have been species specific, these studies suggest that at least eight different cat papillomaviruses infect the oral cavity (tentative designa-

SUMMARY Activation of the RB1 pathway triggers the cell-cycle arrest that mediates cell-cell contact inhibition. Accordingly, mutation of all three RB1 family members leads to loss of contact inhibition and outgrowth of fibroblasts into spheres where cell-cell contacts predominate. We present evidence that such outgrowth triggers reprogramming to generate cells with properties of cancer stem cells. Fibroblasts with only a single RB1 mutation remain contact inhibited; however, if this contact inhibition is bypassed by forcing the RB1−/− cells to form spheres in suspension, cells with properties of cancer stem cells are also generated. These cells not only form tumors in nude mice but also generate differentiated cells. We propose that contact inhibition imposed by the RB1 pathway performs an unexpected tumor suppressor function by preventing cell outgrowth into structures where cells with properties of cancer stem cells can be generated from differentiated somatic cells in advancing cancers.