Potential of a BPV 1 L 1 VLP vaccine to prevent BPV 1-or BPV 2-induced pseudo-sarcoid formation and safety and immunogenicity of EcPV 2 L 1 VLPs in horse

We have previously shown that immunization of horses with bovine papillomavirus type 1 (BPV1) L1 virus-like particles (VLPs) is safe and highly immunogenic and that BPV1 and bovine papillomavirus type 2 (BPV2) are closely related serotypes. Here we evaluated the protective potential of a BPV1 L1 VLP vaccine against experimental BPV1 and BPV2 challenge and studied the safety and immunogenicity of a bivalent equine papillomavirus type 2 (EcPV2)/BPV1 L1 VLP vaccine. Fourteen healthy horses were immunized with BPV1 L1 VLPs (100 μg per injection) plus adjuvant on days 0 and 28, while seven remained unvaccinated. On day 42, all 21 horses were challenged intradermally at 10 sites of the neck with 10 BPV1 virions per injection. In analogy, 14 horses immunized twice with EcPV2 plus BPV1 L1 VLPs (50 μg each) and seven control animals were challenged with 10 BPV2 virions per injection. Immunization with BPV1 L1 VLPs alone induced a robust antibody response (day 42 median titre: 12 800), and BPV1-inoculated skin remained unchanged in 13/14 vaccinated horses. Immunization with the bivalent vaccine was safe, resulted in lower median day 42 antibody titres of 400 for BPV1 and 1600 for EcPV2 and conferred significant yet incomplete cross-protection from BPV2-induced tumour formation, with 11/14 horses developing small, short-lived papules. Control horses developed pseudo-sarcoids at all inoculation sites. The monovalent BPV1 L1 VLP vaccine proved highly effective in protecting horses from BPV1-induced pseudo-sarcoid formation. Incomplete protection from BPV2-induced tumour development conferred by the bivalent vaccine is due to the poorer immune response by immune interference or lower cross-neutralization titres to heterologous BPV2 virions. INTRODUCTION Papillomaviruses (PVs) are a family of genetically heterogeneous viruses, which adhere to common biological principles. All PVs consist of an icosahedral capsid composed of 72 L1 protein pentamers termed capsomeres and an illdefined number of L2 protein monomers [1]. The capsid harbours a circular dsDNA genome of up to 8 kbp in length, which contains ORFs for early (E) regulatory and transforming proteins, the late (L) viral capsid proteins L1 and L2 and a non-coding long control region regulating viral replication and transcription [2]. Infection by PVs usually causes benign papillomas or warts. However, persistent infection with oncogenic types is associated with the development of epithelial cancer in a wide range of animal species and in humans, with the best-known example being human cervical cancer [3]. PVs are usually species specific and are thought to have a strict tropism for keratinocytes and/or fibroblasts [2]. Bovine papillomavirus types 1 and 2 (BPV1, BPV2) are an exception to this rule in that they not only infect bovines but also other ungulate species such as tapirs, giraffes, antelopes or water buffaloes [4–8]. Importantly, BPV1/2 can also infect equids, e.g. horses, donkeys, zebras or mules [4]. In cattle, BPV1/2 infection commonly leads to the Received 12 October 2016; Accepted 2 December 2016 Author affiliations: Research Group Oncology, Equine Clinic, University of Veterinary Medicine, Vienna, Austria; Large Animal Surgery and Orthopaedics, Equine Clinic, University of Veterinary Medicine, Vienna, Austria; Laboratory of Viral Oncology, Division of Immunology, Allergy and Infectious Disease, Department of Dermatology, Medical University of Vienna, Vienna, Austria; Institute of Protein Biochemistry, National Research Council of Italy, Naples, Italy; Department of Veterinary Medicine and Animal Productions, University of Naples Federico II, Naples, Italy; Bioinformatics and Biostatistics Platform, Department of Biomedical Sciences, University of Veterinary Medicine, Vienna, Austria. *Correspondence: Sabine Brandt, sabine.brandt@vetmeduni.ac. at