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.

Brucellosis is an important zoonotic disease that causes great economic losses. Vaccine immunisation is the main strategy for the prevention and control of brucellosis. Although live attenuated vaccines play important roles in the prevention of this disease, they also have several limitations, such as residual virulence and difficulty in the differentiation of immunisation and infection. We developed and evaluated a new bacterial ghost vaccine of Brucella abortus A19 by a new double inactivation method. The results showed that the bacterial ghost vaccine of Brucella represents a more safe and efficient vaccine for brucellosis. We further characterised the antigenic components and signatures of the vaccine candidate A19BG. Here, we utilised a mass spectrometry-based label-free relative quantitative proteomics approach to investigate the global proteomics changes in A19BGs compared to its parental A19. The proteomic analysis identified 2014 proteins, 1116 of which were differentially expressed compared with those in A19. The common immunological proteins of OMPs (Bcsp31, Omp25, Omp10, Omp19, Omp28, and Omp2a), HSPs (DnaK, GroS, and GroL), and SodC were enriched in the proteome of A19BG. By protein micro array-based antibody profiling, significant differences were observed between A19BG and A19 immune response, and a number of signature immunogenic proteins were identified. Two of these proteins, the BMEII0032 and BMEI0892 proteins were significantly different (P < 0.01) in distinguishing between A19 and A19BG immune sera and were identified as differential diagnostic antigens for the A19BG vaccine candidate. In conclusion, using comparative proteomics and antibody profiling, protein components and signature antigens were identified for the ghost vaccine candidate A19BG, which are valuable for further developing the vaccine and its monitoring assays.