Oral Immunization with Recombinant Lactococcus lactis Expressing VP3 and VP3-IFNα Fusion Protein Protects Ducklings Against Novel Goose Parvovirus Infection

The novel goose parvovirus (NGPV), an emerging variant of goose parvovirus, is the causative agent of short beak and dwarfism syndrome (SBDS) in ducks, posing a substantial threat to the global waterfowl industry. There is an urgent need for safe and convenient vaccines that can effectively induce mucosal immunity. This study aimed to develop and evaluate the immunogenicity and protective efficacy of an oral vaccine based on recombinant Lactococcus lactis ( L. lactis ) expressing the major capsid protein VP3 of NGPV, with or without fusion to duck interferon-alpha (IFNα). Codon-optimized genes encoding NGPV VP3 and a VP3-IFNα fusion were cloned into the nisin-inducible pNZ8149 vector and expressed in L. lactis NZ3900. One-day-old ducklings were orally immunized three times with PBS, empty-vector L. lactis (L.l-Empty), recombinant L. lactis expressing VP3 (L.l-VP3), or the VP3-IFNα fusion (L.l-VP3-IFNα). Humoral (serum IgG), mucosal (intestinal sIgA), and cellular (serum IFN-γ, IL-10) immune responses were assessed. Lymphocyte proliferation was evaluated using a CCK-8 assay. Immunized ducklings were subsequently challenged with a virulent NGPV strain (NMG21), and protection was evaluated based on viral shedding (qPCR), mortality, and intestinal histopathology. The results demonstrated that both recombinant strains successfully expressed the target proteins. Oral immunization with L.l-VP3 and L.l-VP3-IFNα significantly induced VP3-specific serum IgG, intestinal sIgA, and elevated levels of IFN-γ and IL-10. Notably, the L.l-VP3-IFNα group elicited stronger sIgA and cytokine responses. Following the challenge, both vaccinated groups exhibited significantly reduced viral shedding and were protected from severe intestinal damage, with the L.l-VP3-IFNα group showing the lowest viral load. Therefore, recombinant L. lactis expressing NGPV VP3, particularly when fused with IFNα, serves as an effective oral vaccine candidate, inducing comprehensive systemic and mucosal immunity and providing significant protection against NGPV challenge. This study established a foundation for developing live bacterial vaccines against waterfowl parvoviruses.