Evaluation of a prototype sub-unit vaccine against equine arteritis virus comprising the entire ectodomain of the virus large envelope glycoprotein (GL):: induction of virus-neutralizing antibody and assessment of protection in ponies

An Escherichia coli-expressed recombinant protein (6hisG(L)ecto) comprising the entire ectodomain (aa 18-122) of equine arteritis virus (EAV) glycoprotein G(L), the immunodominant viral antigen, induced higher neutralizing antibody titres than other G(L)-derived polypeptides when compared in an immunization study in ponies. The potential of the recombinant G(L) ectodomain to act as a subunit vaccine against EAV was evaluated further in three groups of four ponies vaccinated with doses of 35, 70 or 140 mug of protein. All vaccinated animals developed a virus-neutralizing antibody (VNAb) response with peak titres 1-2 weeks after the administration of a booster on week 5 (VNAb titres of 1.8-3.1), 13 (VNAb titres of 1.4-2.9) or 53 (VNAb titres of 1.2-2.3). Vaccinated and unvaccinated control ponies were infected with EAV at different times post-vaccination to obtain information about the degree of protection relative to the levels of pre-challenge VNAb. Vaccination conferred varying levels of protection, as indicated by reduced or absent pyrexia, viraemia and virus excretion from the nasopharynx. The degree of protection correlated well with the levels of pre-challenge VNAb and, in particular, with levels of virus excretion. These results provide the first evidence that a sub-unit vaccine protects horses against EAV. The use of the subunit vaccine in combination with a differential diagnostic test based on other EAV antigens would enable serological discrimination between naturally infected and vaccinated equines.