DNA vaccine and DNA prime-protein boost with the virB9 and virB12 genes induced low level of protection against Brucella abortus infection in mice
VIRB proteins from Brucella spp. constitute the type IV Secretion System (T4SS), a key virulence factor that mediates the intracellular survival of these bacteria. We investigated the immunogenicity and protection of proteins produced by the virB9 and virB12 genes in the DNA vaccine and DNA prime-protein boost strategies. Groups of 10 mice were vaccinated with pcDNAvirB9, pcDNAvirB12, pcDNAvirB9+rVIRB9 or pcDNAvirB12+rVIRB12. The latter two groups were vaccinated with the proteins rVIRB9 and rVIRB12, respectively, during the third immunization. Three weeks after the last immunization, six animals from each group were challenged intraperitoneally with B. abortus strain S2308, and the efficacy of the vaccines was calculated as the log10 of protection by subtracting the mean log CFU of the vaccinated group from the mean log CFU of the negative control group (injected with sterile saline). Most of the vaccinated mice produced total IgG and the subclasses IgG1 and IgG2a against the respective protein, except for the mice vaccinated with pcDNAvirB12. Cytokines IFN-γ and IL-10 were produced, but without a significant difference between the vaccinated and negative control groups. The vaccines did not induce significant levels of protection, in contrast to the immunization obtained with the S19 vaccine strain (Log10, 1.48). In conclusion, the virB9 and virB12 genes of B. abortus, using DNA vaccine and DNA prime-protein boost strategies, were able to induce both humoral and cellular immune responses, but not enough to induce significant protection in the immunized mice. However, given the response in this system, further investigations using the virB9 and virB12 genes of Brucella spp., together with different immune modulators, are warranted. An effort should be made to direct and enhance the immune response, in order to identify a combination that stimulates a better immune response and, consequently, a better level of protection.