Oral Immunogenicity of Recombinant Lactococcus lactis Expressing Porcine Epidemic Diarrhea Virus S1 and S1-IFN-γ Fusion Protein in Mice

Porcine epidemic diarrhea virus (PEDV) remains endemic in many swine-producing regions and causes recurrent outbreaks, with morbidity approaching 100% in susceptible herds and mortality up to 80–100% in neonatal piglets. Current injectable vaccines induce systemic immunity but are less effective at eliciting mucosal protection at the intestinal portal of PEDV entry. This study aimed to develop an oral vaccine based on recombinant Lactococcus lactis (L. lactis) expressing the PEDV spike S1 subunit alone or as an S1-porcine interferon-γ (IFN-γ) fusion protein. The recombinant strains were constructed and orally administered to BALB/c mice in three immunization cycles. Serum PEDV-specific immunoglobulin G (IgG), neutralizing antibodies, intestinal secretory immunoglobulin A (sIgA), lymphocyte proliferation, T-cell subsets, and serum cytokines were measured. Both recombinant strains induced PEDV-specific serum IgG, neutralizing antibodies, and intestinal sIgA. The S1-IFN-γ fusion strain elicited stronger mucosal and cellular immune responses than the S1-only strain, as shown by enhanced lymphocyte proliferation, increased CD4+ and CD8+ T-cell proportions, and elevated levels of T helper 1 (Th1) and T helper 2 (Th2) cytokines, including IFN-γ, interleukin-2 (IL-2), IL-4, and IL-10. These results demonstrate that oral administration of recombinant L. lactis expressing PEDV S1 induces PEDV-specific humoral, mucosal, and cellular immune responses in mice, and that fusion with porcine IFN-γ enhances the magnitude of these responses. However, the specific adjuvant contribution of porcine IFN-γ and protective efficacy against PEDV challenge remain to be determined. These findings provide a foundation for further evaluation of this platform in the natural host (piglets) under virulent PEDV challenge.