Emergence of Novel QX-Type IBV in China: Molecular Insights into Unique S Protein Cleavage Sites, Antigenic Drift, and Enhanced Pathogenicity (2025)

Avian infectious bronchitis virus (IBV) is a major pathogen impacting the global poultry industry. The QX genotype (GI-19 lineage) of IBV has rapidly spread worldwide and is now the dominant genotype in Asia and Europe. In this study, three QX-type field strains (JS/773, JS/774, and SD/783) were isolated from diseased chicken flocks in eastern China, which had been vaccinated with IBV live attenuated vaccines (H120 or QXL87) between December 2024 and January 2025. Notably, the JS/773 strain showed an H536P mutation at the S protein cleavage site, marking the first identification of a PRRRR cleavage motif in this lineage and highlighting the diversity of cleavage sites among QX-type strains. Recombination analysis showed that these isolates are recombinant variants from vaccine strains 4/91, H120, and QXL87, as well as circulating field strains, with recombination occurring in the ORF1ab and ORF5ab regions. Pathogenicity testing in SPF chickens showed that all isolates caused significant tissue damage and varying mortality rates. Cross-neutralization assays revealed substantial antigenic differences between the isolates and the H120 vaccine strain, and with reduced antigenic relatedness to the QXL87 strain. Seven amino acid mutations occurred in the isolates S1 subunit neutralizing epitope region, altering protein conformation and potentially contributing to antigenic variation and immune evasion. In conclusion, the genetic traits and pathogenicity of these isolates highlight the evolving QX-type strains in China, that offers new insights into the molecular evolution of QX-type IBV antigenicity and highlights the need for vaccines based on locally circulating strains for effective control.