Replicase 1a gene plays a critical role in pathogenesis of avian coronavirus infectious bronchitis virus

Avian coronavirus infectious bronchitis virus (IBV) is an important pathogen threatening poultry production worldwide. Here, two recombinant IBVs (rYN-1a-aYN and rYN-1b-aYN) were generated in which ORF1a or ORF1b of the virulent YN genome were replaced by the corresponding regions from the attenuated strain aYN. The pathogenicity and virulence of rIBVs were evaluated in ovo and in vivo. The results revealed that mutations in the ORF1a gene during passage in embryonated eggs caused the decreased pathogenicity of virulent IBV YN strain, proven by determination of virus replication in ECEs and CEK cells, the observation of clinical signs, gross lesions, microscopic lesions, tracheal ciliary activity and virus distribution in chickens following exposure to rIBVs. However, mutations in ORF1b had no obvious effect on virus replication in both ECEs and CEK cells, or pathogenicity in chickens. Our findings demonstrate that the replicase la gene of avian coronavirus IBV is a determinant of pathogenicity.