Brucella, a genus of bacteria that causes
brucellosis, infects and threatens domestic animals, and humans in endemic areas. Presently, some live
attenuated vaccines of Brucella are used to immunize livestock; however, these
vaccines are pathogenic to humans, can provoke abortion when administered to pregnant livestock, and induce
antibodies in vaccinated livestock that affect the diagnosis of field
infection. It is, therefore, very important for improving the safety and immune protection effects of
Brucella vaccine. Currently,
recombinant protein-based
subunit vaccines are considered promising safe and effective alternatives against
brucellosis. Here, we separately expressed the recombinant Omp10-Omp28-L7/L12
proteins of Brucella using eukaryotic and prokaryotic expression systems, which were then used as immunogens for evaluating their immune responses. Taishan Pinus massoniana pollen
polysaccharides (TPPPS), an already verified natural adjuvant, was utilized to evaluate the immune conditioning effect on the
recombinant proteins. Antibody levels, spleen lymphocyte proliferation, percentages of CD4+ and CD8+ T cells, and
cytokine secretion in mice were examined after three successive immunizations. The protective effects against Brucella challenge were also evaluated in mice, and used a live
vaccine as a positive control. The results indicated that the immune responses of the recombinant Omp10-Omp28-L7/L12
protein groups were significantly higher than those of the PBS control group. The recombinant Omp10-Omp28-L7/L12
protein expressed in Pichia pastoris (P. pastoris) exhibited a slightly higher expression level and immunogenicity than that expressed in Escherichia coli (E. coli), and the Omp10-Omp28-L7/L12 (P. pastoris) + TPPPS group provided the most pronounced immune effect. The protective results showed that the recombinant Omp10-Omp28-L7/L12
proteins expressed in the two expression systems had significantly better protective effects against Brucella melitensis challenge compared with the negative control, and the addition of TPPPS adjuvant could significantly improve the protective effects of
subunit vaccines. However, we also noticed that all of the evaluated
subunit vaccines induced less protection than the B. melitensis M5 live
vaccine. These results indicate that the combination of recombinant Omp10-Omp28-L7/L12
antigen and TPPPS adjuvant shows potential as an effective
brucellosis subunit vaccine, and P. pastoris is a preferred expression system to prepare this recombinant subunit
antigen.