Expression of spike and hemagglutinin-esterase proteins is necessary to recover infectious recombinant bovine coronavirus

Bovine coronavirus (BCoV) is a significant pathogen in cattle, and its virological analysis has been hampered by the difficulty in isolating and culturing field isolates with high titers. Here, we successfully generated recombinant BCoV using an infectious bacterial artificial chromosome DNA clone co-expressing the BCoV spike (S) and hemagglutinin-esterase (HE) proteins. We also investigated the role of trypsin in BCoV culture using a recombinant virus expressing the ZsGreen reporter gene (Rec-BCoV-Kakegawa-ZsGreen). We found that an optimized concentration of 2.5 µg/mL significantly enhanced viral titers, reaching 2 × 10⁴ TCID₅₀/mL. To explore the functional significance of the ORF2 protein, we engineered a recombinant virus in which the ORF2 gene was replaced with ZsGreen. While Rec-BCoV-Kakegawa-ZsGreen exhibited growth kinetics comparable to those of the parental BCoV Kakegawa strain during early infection, peak titers were lower, suggesting a possible role for the ORF2 protein in the later stages of the viral replication cycle. Additionally, we determined the role of S and HE protein expression in the recovery of recombinant BCoV from infectious DNA using the ZsGreen virus. In HE or S protein alone, the signal of the reporter protein ZsGreen in transfected cells was stronger than that of infectious DNA alone, but no infectious virus particles were recovered in subsequent steps. However, infectious viral particles were successfully produced only when both HE and S were present. This indicates that the addition of S and HE is necessary to produce recombinant BCoV, and the present method provides important insights into the replication mechanism and pathogenicity of BCoV.IMPORTANCEIn this study, we generated a recombinant BCoV (Rec-BCoV-Kakegawa-WT) using an infectious bacterial artificial chromosome DNA clone and confirmed that the HE protein enhanced viral release. We also identified that an optimal trypsin concentration (2.5 µg/mL) improves viral titers. Additionally, we developed a reporter virus with a ZsGreen insertion, suggesting that the ORF2 protein may play a role in late-stage viral replication. This study contributes to the optimization of BCoV culture conditions and advances vaccine development.