Development of a candidate VP2-based subunit vaccine for African horsesickness virus serotype 5

Abstract

African horse sickness (AHS) is a lethal disease of horses. The aetiological agent is African horsesickness virus (AHSV) (genus Orbivirus; family: Reoviridae). Immunity to all nine serotypes is needed for full protection. The AHSV virion is composed of seven structural proteins and 10 dsRNA genes. The outer capsid protein, VP2, determines the serotype and elicits protective neutralising antibodies (NAbs). The existing polyvalent attenuated vaccine has some drawbacks. The most important ones are the exclusion of serotypes 5 and 9. Also, vaccinated and naturally infected horses cannot be differentiated. This impedes the international movement of horses. Recombinant su.bunit vaccines should solve both problems. Previously, it was established that baculovirus-expressed AHSV VP2 induces NAbs and partial protection. The main aim of this investigation was to improve the level of protection and develop a prototype VP2-based AHSV -5 vaccine to supplement the current attenuated vaccine and pave the way for developing a VP2-based subunit vaccine that incorporates all nine AHSV serotypes. Preliminary challenge experiments in horses indicated that AHSV -5 rVP2 provides incomplete protection against a lethal AHSV-5 challenge. Probable causes and solutions for this problem had to be sought. Analysis of the baculovirus-expressed AHSV-5 VP2 showed that most (ca. 90 %) is aggregated and that only the soluble part (ca. 10 %) elicits NAbs in guinea pigs and full protection in horses. Preliminary small-scale production studies indicated that solubility of the AHSV-5 rVP2 could be improved considerably by optimising in vitro infection conditions. A significant finding was that the safety and efficacy of soluble AHSV -5 rVP2 is determined by the adjuvant used. Saponin-based adjuvants rendered the best results, albeit with dose-related side effects in horses. Saponin Q was tolerated best. These results for the first time implicated AHSV VP2 in stimulating a protective cell-mediated immunity. Based on these results, it is recommended that a candidate AHSV-5 VP2-based subunit vaccine, consisting of 50 µg rVP2 and 3.0 mg Saponin Q for the primary immunisation followed by a 50 µµg rVP2 and 0.6 mg Saponin Q booster, be formulated for the purpose of field trials. The knowledge generated during this study, combined with the recent cloning of the VP2 genes of all nine AHSV serotypes, provides a route for the development of a complete recombinant vaccine that will offer protection against all nine AHSV serotypes and could well free the restraint on the import and export of horses to and from South Africa.