Identification and fine mapping of a novel linear B-cell epitope within the S1-CTD of the QX-type infectious...

Infectious bronchitis virus (IBV) causes severe economic losses to the poultry industry. The IBV S1-CTD mediates receptor binding and serves as a key target for antibody recognition. In this study, the S1-CTD protein of the QX-type IBV QXL87 strain was expressed in HEK293F cells. Four monoclonal antibodies (10H5, 11D11, 12H7, and 13H1) were generated via hybridoma technology and characterized by indirect ELISA, Western blotting, and immunofluorescence assay; variable-region sequencing showed that the four mAbs are clonally related and recognize the same antigenic region. Using overlapping peptide scanning, a novel linear B-cell epitope (²⁸³AQPNSGGVNTFHLYQ²⁹⁷) was identified within the S1-CTD. Alanine scanning mutagenesis revealed that residues ²⁸⁶N, ²⁹³F, ²⁹⁵L and ²⁹⁶Y are critical for antibody recognition. Additionally, residue ²⁹²T was identified as an ancillary contact site that contributes to the binding of a subset of the mAbs. Computational structural modeling suggested that this epitope is located within a predicted surface-exposed flexible loop of the S1-CTD, a conformation that may be favorable for antibody binding. Sequence analysis revealed that, among the representative genotypes circulating in China examined in this study, the critical residues of this epitope are conserved only in GI-19 and GI-13, with variations observed in the other genotypes; the critical residues were also fully conserved in the North American GI-9 (Arkansas) strains included for comparison. This study provides the first fine mapping of a linear B-cell epitope within the S1-CTD of QX-type IBV, and identifies a candidate target that may inform the future development of genotype-specific serological detection tools for IBV.