Deletion of the N-terminal 400 amino acids of DPV pUL36 attenuated virulence and conferred effective protection against virulent challenge.

Duck plague virus (DPV) causes a highly contagious disease in ducks, resulting in substantial economic losses to the poultry industry. The large tegument protein pUL36 is conserved among herpesviruses and plays critical roles in viral replication and pathogenesis. However, the function of its unique N-terminal region in DPV remains unclear. In this study, a recombinant DPV mutant (Δ400) lacking the N-terminal 1-400 amino acids of pUL36 was generated using a BAC-based reverse genetics system, and its biological characteristics were evaluated in vitro and in vivo. Deletion of the N-terminal region did not impair viral replication in duck embryo fibroblasts, and viral growth kinetics of the mutant Δ400 were comparable to those of the wild-type virus. However, in vivo infection of 14-day-old ducklings with the Δ400 mutant resulted in no mortality, only mild clinical symptoms, and reduced viral loads (P < 0.01) and pathological lesions in major target organs. These findings indicated that the mutant was markedly attenuated. Importantly, immunization with the mutant Δ400 provided complete protection against lethal challenge with virulent DPV and induced robust neutralizing antibody responses that were higher than those elicited by a commercially available vaccine (P < 0.0001). No adverse clinical reactions or organ damage were observed in vaccinated ducks. Overall, these findings indicate that the N-terminal region of DPV pUL36 contributes to viral virulence in vivo but is dispensable for viral replication in vitro. The mutant Δ400 represents a safe, immunogenic, and promising live-attenuated vaccine candidate for the prevention and control of duck plague.