Tea saponin-soybean oil submicron emulsion promotes durable protective immunity against Pasteurella multocida

Inactivated Pasteurella multocida (Pm) vaccines are safe and practical for veterinary use, but their protection is often limited by weak cellular immune activation and insufficient durability of antibody responses. This study investigated whether a tea saponin-soybean oil submicron emulsion could improve the durability, breadth and protective efficacy of immune responses induced by an inactivated P. multocida vaccine.

Inactivated P. multocida antigen was formulated with tea saponin and soybean oil to generate TS-Pm. The formulation was characterized by particle size, polydispersity, surface charge, morphology and storage stability. Macrophage antigen uptake, cytokine secretion and activation-associated surface markers were examined in vitro. In mice, we evaluated antibody persistence, B-cell differentiation, splenic cellular immunity, pulmonary CD11b⁺ myeloid phenotypes, systemic tolerability, protection after lethal challenge and draining lymph-node proteomic profiles.

TS-Pm formed a stable oil-in-water submicron emulsion of approximately 300 nm with low polydispersity, negative surface charge and preserved morphology after 6 months at 4 °C. In RAW264.7 macrophages, TS-Pm increased antigen uptake, pro-inflammatory cytokine production and CD80, CD86 and MHC-II expression. Compared with alum-adjuvanted vaccine, TS-Pm maintained higher Pm-specific IgG responses, increased both IgG1 and IgG2a, and promoted plasmablast, plasma cell and germinal-center-associated B-cell responses. TS-Pm also enhanced antigen-responsive splenic proliferation, cytokine-producing CD4⁺ and CD8⁺ T-cell subsets, and activation-associated phenotypes in pulmonary CD11b⁺ myeloid cells. After lethal challenge, TS-Pm improved survival to 90%, compared with 60% for Alum-Pm, and reduced pulmonary bacterial burden and lung pathology without evidence of aggravated systemic injury. Draining lymph-node proteomics showed changes associated with antigen processing and presentation, NF-κB-related signaling and immune-cell trafficking.

Tea saponin-soybean oil submicron emulsion improved the durability, breadth and protective efficacy of immune responses induced by an inactivated P. multocida vaccine. These findings identify TS-Pm as a practical plant-derived adjuvant formulation for inactivated bacterial vaccines requiring sustained humoral and cellular immunity.