Outer Membrane Vesicles Derived from Yak Isolates of Pasteurella multocida Exhibit Promising Vaccine Potential

Pasteurella multocida (P. multocida) is a significant pathogenic bacterium that causes serious disease and death in the yaks of the Tibetan Plateau, and the existing inactivated vaccines are limited by low protection and reactogenicity. Outer membrane vesicles (OMVs) derived from a yak-origin serogroup B P. multocida isolate were evaluated as a potential vaccine candidate in the present study. The purified OMVs were characterized by transmission electron microscopy and nanoparticle tracking analysis, which demonstrated the presence of typical bilayer vesicles ranging from 20 to 300 nm in diameter. Proteomic profiling revealed 1213 proteins, with many of them being immunologically relevant outer membrane-associated proteins like OmpA, OmpH, Omp16, OmpW, TbpA and PlpP. The functional enrichment analysis showed that these proteins were linked to translation, membrane structure, transport, metabolism, and pathways of adaptation of bacteria. In vitro OMVs were effectively taken up by RAW264.7 macrophages and stimulated robust expression of inflammatory mediators, such as TNF-α, IL-1β, IL-6, iNOS and IL-10, which is indicative of strong innate immunostimulatory capacity. OMV immunization induced significant antigen specific humoral responses in mice and yaks in vivo. In mice, intramuscular immunization was effective in giving full protection against P. multocida challenge but not intranasal immunization. Histopathology also indicated less tissue damage in vaccinated animals, especially in the lung and liver. These findings, taken together, prove that yak-derived P. multocida OMVs have high immunogenicity and protection capabilities, which show their potential as a next-generation vaccine platform to tackle P. multocida infection.