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.