Loss of haemagglutination activity of an infectious bronchitis virus after attenuation

Avian infectious bronchitis virus (IBV) has been reported to acquire haemagglutination (HA) activity after treatment with neuraminidase, which depends on the IBV S1 protein. The purpose of this study was to test the IBV HA activity and its relationship to the S1 sequences. A wild strain of IBV, 2575/98 possessed HA activity after neuraminidase treatment. On the contrary, its attenuated strain through 75 chicken embryo passages did not. The nucleotide sequence differences in S1 genes before and after attenuation were C166A and G280T, resulting in amino acid changes of P56T and A94S. The S1 and S genes of the wild strain were cloned and expressed in a baculovirus (B) expression system. To test the relationship between HA activity and the S sequence changes after attenuation, the two nucleotide residues were mutated. The HA activity of those recombinant baculoviruses (rBs) was tested. The results showed that rB containing S gene from wild strain possesses the HA activity, that containing S gene with C166A mutation shows partial HA but that containing G280T mutation shows no any HA activity. Thus, a Taiwan IBV strain 2575/98 loses its HA activity after attenuation and this loss might be related to amino acid changes of P56T and A94S in the IBV S protein.