The V577S mutation in the S2 subunit of QX-type IBV attenuates viral adaptation to chicken oviduct epithelial cells and pathogenicity in laying hens.

BACKGROUND: The cell and tissue tropism of infectious bronchitis virus (IBV) varies among strains and genotypes, with the S2 subunit of the spike protein serving as a key determinant. METHODS AND RESULTS: In this study, we compared S2 sequences of IBV strains with distinct adaptation to chicken oviduct epithelial cells (COEC) and identified seven potential sites associated with COEC adaptation. Using the highly COEC-adapted GI-19 genotype strain zy30 as a backbone, we generated a series of S2 single-point mutants via reverse genetics. Viral proliferation in COEC and pathogenicity in laying hens were evaluated. Results showed that the V577S mutation significantly reduced zy30 adaptation in COEC and attenuated viral replication and histopathological lesions in the oviduct. Transcriptome sequencing and bioinformatics analysis of COEC samples infected with the V577S mutant strain and the zy30 strain showed that differentially expressed genes were significantly enriched in the endocytosis pathway, Toll-like receptor signaling pathway, and NOD-like receptor signaling pathway, suggesting that this mutation may be associated with altered endocytosis and host immune responses. CONCLUSIONS: This study identifies the V577 site in the S2 subunit as a determinant of IBV COEC tropism and provides transcriptomic evidence that the V577S mutation may influence viral adaptability through pathways potentially involving endocytosis and host innate immunity, providing a theoretical basis for in-depth understanding of the mechanism underlying IBV cell tropism and rational design of attenuated vaccine strains. However, further functional studies are required to establish the causal mechanisms underlying these observations.