[Immunization efficacy and safety of Brucella 104M against aerosol challenge in BALB/c mice]

Objective: To evaluate the protective efficacy and safety of Brucella 104M against aerosol challenge in BALB/c mice and characterize its immunological effects. Methods: Female mice of 6-8 weeks old were immunized with Brucella abortus strain 104M by intratracheal aerosol delivery or intranasal instillation or subcutaneous injection route. Six mice of each group were sacrificed at 4, 8, 16, 24 weeks after immunization. At each time point, the clinical manifestations of mice were investigated, the serum, spleen and lung samples of mice were collected, body weight, spleen weight, bacteria loads in spleens, the anti-Brucella antibodies titers in serum and the cytokines concentrations of IFN-γ, IL-18 in serum or lung homogenate of the mice were detected. Twenty two weeks after immunization, all the mice were challenged with Brucella A19 through intratracheal aerosol delivery. Results: Compared with the control group, neither abnormal clinical symptoms nor significant changes in body weight were found in 104M immunization groups, at each time point when immunized through either nose dropping route, subcutaneous injection or aerosol routes; and the spleen weight of immunization groups were lower than control group after challenge (P<0.05): *M1 (0.26±0.16)gP<0.05): *M1(4.49±0.13)log(10) (Colony-Forming Units) CFU/gBrucella 104M specific antibodies were detected at week 4, and the peak was reached at week 8 of immunization groups. At each time point, the serum antibody titers IgG 2a was higher than that of IgG 1; before challenge, the cytokines concentrations of IFN-γ and IL-18 in all immunization groups were significantly higher than that in control group (P<0.05), after challenge, cytokines concentrations of IFN-γ and IL-18 in the serum were lower in all immunization groups than in control group (P<0.05). At each time point, IFN-γ, IL-1βand IL-18 concentrations in lung homogenate were higher in immunization groups than in control group (P<0.05). There were no significant differences among the three immunization routes in all sample detection indexes (P<0.05). Conclusions: Intratracheal aerosol delivery is a safe and effective immunization route in BALB/c mice. Both humoral immunity and cellular immunity were stimulated by Brucella 104M, showing obvious protective efficacy against aerosol challenge in BALB/c mice. No significant weight loss was detected. However, the colonizing of Brucella 104M in the spleen of mice was too long (20 weeks), and slighter spleen swelling of the mice were detected, which illustrated that the attenuated strain 104M has residual virulence as a vaccine strain in mice.