Papillomatosis then and now.

To celebrate 50 years of Veterinary Pathology, reviews have been invited that revisit papers published in volume 1, which remain relevant today. These reviews on papillomatosis and brucellosis underscore the remarkable scientific progress in the life of the journal. As a response to the first review, an exciting comparison of bovine and human papillomatosis by John Munday, I have been asked by Matti Kiupel to describe research on bovine papillomavirus in the 1960s. The forces that generated our journal in that decade were driving an explosion in cancer research—medical science had finally grasped the idea that viruses cause cancer in animals. In the early 20th century, responses to the seminal discoveries of transmissible viral tumors ranged from disinterest to disbelief: avian leukemia (Ellermann and Bang, Royal Veterinary School, Copenhagen, 1908) and the Rous sarcoma (Rous, Rockefeller Institute, 1911). In the 1930s, the first mammalian DNA tumor virus, rabbit papillomavirus (Shope, Rockefeller Institute 1933); the first cancer-associated herpesvirus, causing renal adenocarcinomas in leopard frogs (Lucke, University of Pennsylvania); and the first transmissible mammary tumor viruses from mice (Bittner, the Jackson Laboratory) were ignored or generated an inappropriate lack of attention. But in the 1960s, views changed as these oncogenic viruses were identified in tumor tissue by electron microscopy and could be propagated in vitro and in ovo. Then, in 1964, Jarrett’s publication of the viral nature of leukemia in cats was the coup de grâce for nonbelievers. Feline leukemia virus, Rous sarcoma virus, the murine viruses, and papillomaviruses soon became universal models for viral oncogenesis (noteworthy: Rous was awarded the Nobel Prize in 1966, 55 years after his discovery). Carl Olson had published work in 1951 showing that bovine papillomavirus (BPV) caused sarcoids in horses. Joining his group at Wisconsin in 1961, I was assigned to examine the pathogenesis of BPV in skin and to begin a long-term study on the combined effects of BPV and bracken fern toxin on bladder cancer in cattle. We viewed BPV then as a single genetic entity that acted only locally. Munday’s review makes it clear that, in cattle, papillomas are caused by 12 BPV types, that 83% of papillomas contain 3 or more BPV types, that BPV-infected lymphocytes circulate in blood, and that fibropapillomas are caused by the unique delta-BPVs 1 and 2. In the 1960s, we found, in skin, that BPV-induced mesenchymal reactions preceded epithelial change. Fibroblastic responses were intense, but there was no evidence of virus using fluorescent antibodies or electron microscopy—the new techniques of the day. More surprising was that BPV given subcutaneously, intracerebrally, or next to cartilage in hamsters produced fibromas, meningiomas, or chondromas, respectively. When we inoculated papillomaviruses (PVs) into skin, the response in basal keratinocytes proved to be biphasic (Fig. 1). The first was keratinocyte hyperplasia, a massive increase in the number and size of keratin-filled squamous cells. The second, virus production, developed at the junction of the stratum corneum and was characterized by cell swelling, keratinolysis, keratohyalin granule retention, chromatinolysis, nucleolar degeneration, and virions that first appeared near perinucleolar chromatin. Virion-producing cells were limited to spinous-corneum junctions, appeared late, and made up less than 20% of keratinocytes in mature papillomas. From the Munday review, we begin to understand how these events develop based on how PV genes control the processes of keratinocyte hyperplasia and virus production. At the time that basal keratinocytes are infected, PV DNA replication occurs, forming low-copy, autonomously replicating plasmids that initiate the hyperplasia pathway. The state of nuclear chromatin is critical. Early E2 proteins tether the infecting PV genome to fragile sites in host chromatin. Association with active chromatin ensures that PVs remain transcriptionally active and are not shunted to repressed heterochromatin. Once tethered to chromatin, to ensure efficient replication, E1 and E2 proteins hijack host DNA damage repair proteins. Moving into the spinous layers, late viral genes are expressed that initiate PV DNA amplification. Keratinocyte differentiation triggers production of PV proteins that prevent basal cells from leaving the cell cycle by causing them to reenter the S-phase and to avoid programmed cell death. Protein E6, which binds to and degrades p53 to prevent apoptosis, also binds to LPDZ domains of other proteins that may be involved in keratinocyte proliferation.