Oral Food-Derived Proanthocyanidins as Immunostimulants: Enhanced Vaccine Immunogenicity via Host Immune Modulation in a Murine CSFV Model

Traditional injectable vaccine adjuvants are limited by local granulomatous reactions, chronic inflammation, and reduced compliance, creating an urgent need for safe, host-directed strategies to enhance vaccine immunogenicity. Proanthocyanidins (PACs)—natural polyphenols abundant in grapes, cocoa, and tea—are food-derived bioactive compounds with antioxidant, anti-inflammatory, and immunomodulatory activities, positioning them as attractive candidates for oral immunostimulants. Using classical swine fever virus (CSFV), a Pestivirus within the Flaviviridae that shares conserved genomic and immunological features with human pathogens such as hepatitis C and dengue virus, we systematically evaluated oral PACs (15, 30, and 60 mg/kg/d) combined with attenuated live or subunit CSFV vaccines in mice. PACs were well tolerated across the full dose range, with no impact on body weight or major organs, while selectively increasing the splenic index—suggesting spleen-targeted immune activation without systemic inflammation. Compared with vaccine alone, the moderate dose (30 mg/kg/d) produced the strongest immunostimulation, particularly with the subunit vaccine: peak antigen-specific IgG titer reached 1:409,600, a 4-fold increase versus 1:102,400 for subunit alone and exceeding the 3.4-fold gain observed with the attenuated vaccine. IgG2a and IgG2b rose by 199.4% and 269.1%, respectively, alongside coordinated elevations of Th1 (IL-2 +160.6%, IFN-γ +91.9%) and Th2 (IL-4, IL-10) cytokines, enhanced B- and T-lymphocyte proliferation, and expansion of CD4+ and CD8+ subsets, establishing a balanced and durable Th1/Th2 response. These findings provide preclinical evidence that food-derived PACs can serve as safe oral immunostimulants for low-immunogenicity subunit vaccines, supporting their further development in human and veterinary vaccinology.