AFRICAN HORSESICKNESS VIRUS VP7 SUBUNIT VACCINE PROTECTS MICE AGAINSTA LETHAL, HETEROLOGOUS SEROTYPE CHALLENGE

An established mouse model was used to evaluate the effectiveness of t he major outer core protein of African horsesickness virus (AHSV), VP7 , as a subunit vaccine. Adult female BALB/c mice were immunized with V P7 crystals purified from BHK cells infected with AHSV serotype 9 (AHS V-9), using three inoculations in Freund's adjuvant. Eighty to one hun dred per cent of the immunized mice were protected against a heterolog ous challenge with a known lethal dose of AHSV-7. The protected immuni zed mice did not develop any clinical signs characteristic of virulent AHSV infection in this model during the study. In contrast, 80-100% m ortality was observed in the non-immunized mice that received the same challenge virus. Subsequent studies indicated that a single inoculati on of 1.5 mu g purified AHSV VP7 in Freund's complete adjuvant was suf ficient to protect at least 90% of mice from AHSV-7 challenge. If the antigen was presented in the absence of Freund's complete adjuvant, 70 % of the mice were still protected by one inoculation of VP7 crystals. Titres of circulating antibody against AHSV VP7, determined by compet itive ELISA, did not appear to correlate with protection and passive a ntibody transfer from immunized BALB/c mice failed to protect syngenei c recipients from AHSV-7 challenge. Therefore, the observed protection is unlikely to be due to an antibody-mediated immune response. The nu mber of viraemic mice and the duration of viraemia post-challenge was significantly reduced in vaccinated mice compared to nonvaccinated con trols. However, the levels of viraemia were similar.