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

<b>禽传染性支气管炎病毒 （IBV） 是影响全球家禽业的主要病原体。IBV的QX基因型（GI-19谱系）已迅速传播到全球，目前是亚洲和欧洲的优势基因型。本研究从2024年12月—2025年1月接种IBV减毒活疫苗（H120或QXL87）的华东病鸡群中分离出3株QX型田间菌株（JS/773、JS/774和SD/783）。值得注意的是，JS/773菌株在S蛋白切割位点显示出H536P突变，标志着该谱系中首次鉴定出PRRRR切割基序，并凸显了QX型菌株中切割位点的多样性。重组分析表明，这些分离株是来自疫苗株4/91、H120和QXL87的重组变体，以及循环田间毒株，重组发生在ORF1ab和ORF5ab区域。SPF鸡的致病性检测表明，所有分离株都造成了严重的组织损伤和不同的死亡率。交叉中和测定显示分离株与 H120 疫苗株之间的抗原存在显着差异，并且与 QXL87 毒株的抗原相关性降低。分离株 S1 亚基中和表位区域发生了 7 个氨基酸突变，改变了蛋白质构象，并可能导致抗原变异和免疫逃逸。总之，这些分离株的遗传性状和致病性凸显了中国不断进化的QX型毒株，这为QX型IBV抗原性的分子进化提供了新的见解，并强调了基于本地传播毒株的疫苗进行有效控制的必要性。</b>

Avian Infectious Bronchitis Virus (IBV) is a major pathogen affecting the global poultry industry. The QX genotype (GI-19 lineage) of IBV has spread rapidly worldwide and currently predominates in Asia and Europe. In this study, three QX-type field isolates (JS/773, JS/774 and SD/783) were isolated from diseased chicken flocks in East China that were vaccinated with IBV attenuated live vaccines (H120 or QXL87) between December 2024 and January 2025. Notably, the JS/773 isolate harbors an H536P mutation at the S protein cleavage site, marking the first identification of the PRRRR cleavage motif in this lineage and highlighting the diversity of cleavage sites among QX-type strains. Recombination analysis revealed that these isolates are recombinant variants derived from vaccine strains 4/91, H120, QXL87 and circulating field strains, with recombination events occurring in the ORF1ab and ORF5ab regions. Pathogenicity tests in Specific Pathogen Free (SPF) chickens showed that all isolates caused severe tissue damage and varying mortality rates. Cross-neutralization assays demonstrated significant antigenic differences between the isolates and the H120 vaccine strain, as well as reduced antigenic correlation with the QXL87 strain. Seven amino acid mutations were identified in the neutralizing epitope region of the isolates' S1 subunit, which altered protein conformation and potentially contributed to antigenic variation and immune escape. Collectively, the genetic traits and pathogenicity of these isolates highlight the evolving QX-type strains in China, providing new insights into the molecular evolution of QX-type IBV antigenicity and emphasizing the necessity of implementing effective control measures using vaccines based on locally circulating strains.