Outer membrane vesicles from Fusobacterium necrophorum elicit protective immunity in mice and robust immune responses in sheep

Fusobacterium necrophorum is a major opportunistic pathogen of ruminants that causes foot rot and liver abscesses and contributes to substantial economic losses. Current vaccines provide limited protection. Here, we prepared and evaluated outer membrane vesicles (OMVs)-based vaccine candidate derived from F. necrophorum. OMVs were purified by density-gradient centrifugation and were characterized for morphology, composition, and immunological activity. The purified OMVs showed intact vesicle structure and a uniform particle size distribution and contained virulence-associated antigens and pathogen-associated molecular patterns. OMVs were rapidly taken up by dendritic cells and promoted dendritic cell maturation, as indicated by activation of PI3K/AKT signaling and increased expression of MHC I, MHC II, CD80, and CD86. In vivo imaging showed persistent fluorescence at the injection site and signal accumulation in draining lymph nodes and the spleen. In mice, OMVs immunization induced high titers of antigen-specific IgG and elicited a Th1-biased cellular immune response. In a lethal challenge, OMVs immunization achieved 100% survival, significantly reduced liver bacterial loads, and alleviated inflammatory and necrotic liver lesions. In sheep, OMVs immunization also induced progressively increased antigen-specific IgG and a predominantly Th1 type cellular immune response, supporting strong immunogenicity in a natural host. Together, these results support F. necrophorum OMVs as a promising vaccine candidate containing multiple native antigens for the prevention and control of F. necrophorum - associated diseases in ruminants.