Monoclonal antibodies target conserved S1 ^B epitopes across deltacoronaviruses to inhibit porcine deltacoronavirus infection

Porcine deltacoronavirus (PDCoV) is an emerging enteric pathogen that causes severe diarrhea, vomiting, and dehydration in piglets, and a recent human infection report raises concerns about its zoonotic potential. The S1 subunit of the PDCoV-spike protein is the primary target of neutralizing antibodies and is critical for viral attachment and entry. In this study, we generated and characterized eight monoclonal antibodies (mAbs) against S1, two targeting the S1 ^A domain, five targeting the S1 ^B domain, and one binding the S1 ^C domain. Using a genetically engineered luciferase reporter virus, we demonstrated that all five S1 ^B -targeting mAbs neutralize viral infection by disrupting S1-aminopeptidase N receptor engagement. These S1 ^B mAbs cross-react with S1 ^B proteins from multiple human- and avian-origin deltacoronaviruses, suggesting the recognition of conserved antigenic regions. Importantly, we found that our PDCoV strain caused embryo lethality in embryonated chicken eggs (ECEs). Using this ECE lethal model, we performed a preliminary in vivo evaluation of the five S1 ^B -specific neutralizing mAbs. All five mAbs improved embryo survival, with 3E9 providing complete protection against PDCoV-induced embryonic lethality. Collectively, these novel mAbs represent valuable tools for diagnosis and therapeutic development. Moreover, the luciferase-reporter virus and the ECE model establish robust platforms for future evaluation of neutralizing antibodies against PDCoV.