Improved Immunogenicity of a Vaccination Regimen Combining a DNA Vaccine Encoding Brucella melitensis Outer Membrane Protein 31 (Omp31) and Recombinant Omp31 Boosting

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Abstract
In the present study, we report an attempt to improve the immunogenicity of the Omp31 antigen by a DNA prime-protein boost immunization regimen. We immunized BALB/c mice with an Omp31 DNA vaccine (pCIOmp31) followed by boosting with recombinant Omp31 (rOmp31) in incomplete Freund's adjuvant and characterized the resulting immune responses and the protective efficacy against Brucella ovis and B. melitensis infection. Immunoglobulin G1 (IgG1) and IgG2a titers were higher in sera from pCIOmp31/rOmp31-immunized mice than in sera from mice immunized with pCIOmp31 or rOmp31 alone. Splenocytes from pCIOmp31/rOmp31-immunized mice produced significantly higher levels of gamma interferon than did those from mice given rOmp31 alone. In contrast, interleukin 2 (IL-2) production levels were comparable between the two groups of immunized mice. Cells from all immunized mice produced undetectable levels of IL-4. Notably, rOmp31 stimulated IL-10 production in the pCIOmp31/rOmp31-immunized group but not in the pCIOmp31- or rOmp31-immunized group. Although the prime-boost regimen induced specific cytotoxic responses, these responses could not reach the levels achieved by the pCIOmp31 immunization. In conclusion, pCIOmp31 priming followed by rOmp31 boosting led to moderately improved protection against a challenge with B. ovis or B. melitensis .

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