Induction of humoral neutralising and haemagglutination-inhibiting antibody by the spike protein of avian infectious bronchitis virus

The spike (S), membrane (M) and nucleocapsid (N) proteins of avian infectious bronchitis virus strain M41 (IBV-M41) were separated by sucrose gradient sedimentation after dissociation of the virus by non-ionic detergent. Groups of four chickens were inoculated intramuscularly with 20 microg of S, M or N in Freund's complete adjuvant and at 4 and 7 weeks later with 20 microg, of S, M or N protein in Freund's incomplete adjuvant. Chickens were bled at 4, 7, 10 and 13 weeks after the first vaccination and the sera analysed for serum virus neutralising (SN) and haemagglutination-inhibiting (HAI) antibody. Sera from all four chickens inoculated with S contained SN and HAI antibody. Maximum titres, in the range 5 to 9 log(2), were attained after one, two or three injections in one, two and one chickens respectively. The SN and HAI titres rose in parallel. Neither M nor N induced detectable SN or HAI antibody. None of the chickens resisted challenge to the respiratory tract at 6 weeks after the final vaccination. Sera from chickens which had been infected twice with live IBV-M41 immunoprecipitated more radiolabelled S than N protein and little or no M. Similar results were obtained with sera from guinea pigs which had been inoculated intramuscularly with inactivated virus.