Antiserum from mice vaccinated with modified vaccinia Ankara virus expressing African horse sickness virus (AHSV) VP2 provides protection when it is administered 48 h before, or 48 h after challenge

• Passive immunisation studies were conducted in IFNAR −/− mice using splenocytes and antiserum from donors. • Donors were immunised with modified vaccinia Ankara (MVA) expressing African horse sickness virus (AHSV) VP2. • After AHSV challenge, splenocyte recipients were poorly protected against clinical signs and not protected against viraemia. • After AHSV challenge, antiserum recipients were highly protected against viraemia and clinical signs. • MVA-VP2 vaccination immunity is strongly associated with neutralising antibodies, indicating potential for sero-therapy. Previous studies show that a recombinant modified vaccinia Ankara (MVA) virus expressing VP2 of AHSV serotype 4 (MVA-VP2) induced virus neutralising antibodies in horses and protected interferon alpha receptor gene knock-out mice (IFNAR −/−) against challenge. Follow up experiments indicated that passive transfer of antiserum, from MVA-VP2 immune donors to recipient mice 1 h before challenge, conferred complete clinical protection and significantly reduced viraemia. These studies have been extended to determine the protective effect of MVA-VP2 vaccine-induced antiserum, when administered 48 h before, or 48 h after challenge. In addition, passive transfer of splenocytes was undertaken to assess if they confer any degree of immunity to immunologically naïve recipient mice. Thus, antisera and splenocytes were collected from groups of mice that had been vaccinated with MVA-VP2, or wild type MVA (MVA-wt), for passive immunisation of recipient mice. The latter were subsequently challenged with AHSV-4 (together with appropriate vaccinated or unvaccinated control animals) and protection was assessed by comparing clinical signs, lethality and viraemia between treated and control groups. All antiserum recipients showed high protection against disease (100% survival rates even in mice that were immunised 48 h after challenge) and statistically significant reduction or viraemia in comparison with the control groups. The mouse group receiving splenocytes from MVA-VP2 vaccinates, showed only a 40% survival rate, with a small reduction in viraemia, compared to those mice that had received splenocytes from MVA-wt vaccinates. These results confirm the primarily humoral nature of protective immunity conferred by MVA-VP2 vaccination and show the potential of administering MVA-VP2 specific antiserum as an emergency treatment for AHSV.