Attenuation mechanism of virulent infectious bronchitis virus strain with QX genotype by continuous passage in chicken embryos

The virulent isolate SDZB0808 of QX-type infectious bronchitis virus (IBV) was continuously passaged in chicken embryos for 110 generations. The safety and immune efficacy of the 110th generation of IBVs (P110) were evaluated. Damage was not found in the appearance of the 3-day-old specific-pathogen-free (SPF) chicks immunized with 104.5 EID50 (median embryo infective dose) of P110 by intranasal and ocular administration. At 14 d after the vaccination with 104.5 EID50 of P110, all the 3-day-old SPF chicks were immune from the attack of the homologous virulent strain SDZB0808 and the heterologous virulent strain SDIB821/2012. The whole genome sequencing of SDZB0808 of different generations (P1-P110) indicated that the replicase 1a sequences of P60-P110 all lost a length of 30 bp in the same region. Specific primers were designed according to the differences in the genomes of P1-P110. SYBR Green I real-time quantitative PCR was adopted to analyze the proportion of the viruses with 30 bp deletion in P60, P100, and P110. Results showed that with the passage in chicken embryos, the proportion of the viruses with 30 bp deletion gradually increased. Almost 100% of the viruses in the P110 had 30 bp deletion in the replicase 1a sequence. Therefore, the attenuation of IBV's virulence may be the outcome of directional screening in the chicken embryos. This work confirmed the high safety and immune efficacy of P110 in SPF chickens. Thus, P110 can serve as an attenuated IBV vaccine candidate.