Adjuvant screening of an inactivated Pasteurella multocida vaccine in mice and evaluation of its immunogenicity in sheep.

This study systematically evaluated the adjuvant efficacy of Rehydragel^® LV alum (AL) and Freund's adjuvant in enhancing the immunogenicity of an inactivated Pasteurella multocida vaccine in both murine and ovine models. The mice were immunized with vaccines adjuvanted with either AL and Freund's adjuvant, followed by assessments of humoral and cellular immune responses, organ bacterial burdens, histopathology, and post-challenge survival. Sheep-natural hosts-were immunized with the PM + AL vaccine to validate translational efficacy. The results demonstrated that the PM + AL vaccine elicited significantly higher IgG titers (P < 0.01) and Th2-skewed immune response in mice, characterized by a marked increase in IL-4 and moderate induction of IFN-γ, compared to the Freund's adjuvant. Immunization with the PM + AL vaccine resulted in 100% survival and complete bacterial clearance in organs post-challenge, whereas PM + Freund's vaccine resulted in only 40% survival. Histopathology revealed minimal inflammation in PM + AL vaccinated mice. In sheep, PM + AL vaccination elicited robust IgG responses by day 14, complete protection following challenge, elevated lymphocyte stimulation indices (P < 0.01), and increased serum bactericidal activity, without any observable clinical symptoms. By contrast, sheep in the control group exhibited signs of depression and coughing. These findings underscore the importance of validating adjuvant efficacy in target species and highlight the potential of AL as a promising adjuvant for P. multocida vaccines in sheep.