Engineered Low-Endotoxin Bacterial Biomimetic Vesicles for Enhanced Oral Dual-Antigen Subunit Vaccine Delivery

Subunit vaccines are promising for disease prevention because of their safety and cost-effectiveness. However, their efficacy is limited by low immunogenicity and gastrointestinal degradation after oral administration. To address this issue, low-endotoxin Salmonella choleraesuis strain SC-L3 was engineered via lipid A modification to generate bacterial biomimetic vesicles (BBVs) with reduced endotoxin activity. BBVs were functionalized using ClyA-embedded SpyCatcher and Streptococcus protein G for dual antigen coupling, and further coated with chitosan oligosaccharides (COS) to enhance mucosal penetration and gastrointestinal stability. Using mCherry as a model antigen, we obtained optimized mCherry-CSS-BBV@COS that showed high antigen protection rates (83% and 63% in simulated gastric and intestinal fluids, respectively), capacity for lysosomal escape and effective stimulation of M1 macrophage polarization in vitro. Oral administration of mCherry-CSS-BBV@COS elicited robust systemic IgG and mucosal sIgA responses in mice. Furthermore, dual-antigen BBV conjugates (GDH-gD-Fc-CSS-BBV@COS) co-delivering Streptococcus suis glutamate dehydrogenase and pseudorabies virus gD-Fc induced antigen-specific humoral, mucosal and cellular immunity, conferring complete protection against lethal challenges with the respective pathogens. In summary, we generated a versatile, low-endotoxin BBV platform for oral combination subunit vaccines, offering a novel strategy for protection against viral and bacterial infections.