Combined immunization with inactivated vaccine reduces the dose of live B. abortus A19 vaccine

Background: Brucella spp. is an important zoonotic pathogen responsible for brucellosis in humans and animals. Brucella abortus A19 strain is a widespread vaccine in China. However, it has a drawback of residual virulence in animals and humans.

Methods: In this study, the BALB/c mice were inoculated with either 100 μL PBS(control group, C group), 10⁹ CFU/mL inactivated B. abortus A19 strain (I group), 10⁵ CFU/mL (low-dose group, L group) 10⁶ CFU/mL live B. abortus A19 strain (high-dose group, H group), or 10⁵ CFU/mL live B. abortus A19 strain combined with 10⁹ CFU/mL inactivated B. abortus A19 strain (LI group). Mice were challenged with B. abortus strain 2308 at 7 week post vaccination. Subsequently, the immune and protective efficacy of the vaccines were evaluated by measuring splenic bacterial burden, spleen weight, serum IgG, interferon-gamma (IFN-γ), interleukin-4 (IL-4) percentage of CD4 + and CD8 + T cells of mice via bacterial isolation, weighing, ELISA and flow cytometry, respectively.

Results: The splenic bacterial burden and spleen weight of the mice in group LI were mostly equivalent to the mice of group H. Moreover, Brucella-specific serum IgG, IFN-γ, IL-4, and the percentage of CD4⁺ and CD8⁺ T cells of the LI group mice were similar to those of the H group. In the subsequent challenge test, both vaccines conferred protective immunity to wild-type (WT) 2308 strain. In addition, the levels of IL-4 and IFN-γ, CD4⁺ and CD8⁺ T cells in these mice were similar to those of the mice in the H group.

Conclusions: Combined immunization with low dose live vaccine and inactivated vaccine allowed to reduce the live B. abortus A19 vaccine, dose with an equivalent protection of the high-dose live vaccine.