Formulation and Evaluation of a Self-Emulsifying Emulsion as an Effective Adjuvant for Inactivated Foot-and-Mouth Disease Vaccine in Mice

Background/Objectives: Foot-and-mouth disease (FMD) is an acute and febrile infectious disease that spreads rapidly among cloven-hoofed animals. Vaccination is one of the most effective methods for controlling FMD. However, commercially available FMD vaccines often have limitations in terms of stability, safety, and efficacy. Methods: In this study, a novel self-emulsifying oil-in-water (O/W) emulsion (SEEX2-17) was developed, characterized, and evaluated as an adjuvant against the inactivated FMD vaccine in mice. Results: The data revealed that SEEX2-17 was a homogeneous nanoemulsion with a small particle size (~25 nm) and excellent stability (for at least 2 months at 37 °C). Toxicity evaluation of SEEX2-17 showed that it had good safety and tolerability in mice. Moreover, the SEEX2-17-adjuvanted inactivated FMD vaccine induced significantly higher titers of FMDV-specific IgG antibodies when administered to mice subcutaneously and prolonged the duration of high-titer antibodies compared with the commercial ISA206-adjuvanted vaccine or antigen alone, respectively, within 12 weeks after immunization (p < 0.05). Furthermore, the vaccine adjuvanted with SEEX2-17 elicited significantly higher levels of IgG1 and IgG2a antibodies, as well as higher concentrations of cytokines (IFN-γ, IL-2, IL-4, and IL-6) than the antigen alone, indicating that the SEEX2-17-adjuvanted vaccine stimulated vigorous humoral and cellular immunity in mice. Moreover, SEEX2-17 promoted the maturation and activation of dendritic cells (DCs) in the lymph nodes as well as the proliferation of lymphocytes in the spleen. Conclusions: Collectively, SEEX2-17 could trigger enhanced immunity and provide a promising and easy-to-use adjuvant platform for inactivated FMD vaccines with favorable stability and safety.