Development and Immunogenicity of a Five-Antigen Strangles Vaccine Based on Equine Ferritin Nanoparticles in Mice

As a naturally self-assembling protein nanocarrier, ferritin enables multivalent antigen display and functions as an intrinsic adjuvant to enhance vaccine-induced immune responses. Streptococcus equi subsp. equi (S. equi) is the causative agent of equine strangles, an acute and highly contagious respiratory disease responsible for substantial economic losses worldwide. However, currently available vaccines often show suboptimal immunogenicity and limited protective efficacy. In this study, we developed a recombinant equine ferritin (rHF)-based nanoparticle vaccine, rSE5Mix, presenting five core protective antigens (EQ8, EQ5, CNE, IdeE, EAG). The fusion proteins efficiently assembled into uniform nanoparticles. Immunization of BALB/c mice elicited rapid, high-titer antigen-specific IgG antibodies and a balanced Th1/Th2 immune response without additional adjuvants. Following lethal challenge with S. equi, rSE5Mix-immunized mice showed 100% survival and markedly milder clinical signs. Histopathological analysis demonstrated significantly alleviated organ damage, and bacterial loads in major tissues were reduced to nearly undetectable levels. Importantly, compared with the octavalent tandem vaccine rSE8, rSE5Mix induced faster elevation of partial antigen-specific antibodies, especially for EQ8 and CNE. Their antibody titers were comparable at later stages. This study developed a safe and effective ferritin nanoparticle vaccine candidate against equine strangles and verified that equine ferritin is a promising candidate delivery platform for veterinary bacterial vaccines.