Evaluation of immunogenicity and protective activity in BALB/c mice of the 25-kDa major outer-membrane protein of Brucella melitensis (Omp25) expressed in Escherichia coli.

The antibody response specific to the 25-kDa major outer-membrane protein (Omp25) of Brucella melitensis expressed in Escherichia coli was assessed in BALB/c mice. Groups of mice were immunised and boosted either with sonicated E. coli carrying plasmid pAC2533-E. coli (pAC2533)-expressing the gene coding for Omp25 (omp25 gene) of B. melitensis, or with E. coli carrying plasmid pUC19-E. coli (pUC19). One control group received saline. The evolution of antibody responses was investigated by indirect ELISA with whole rough (R) B. melitensis H38 cells as antigen. Serum antibody titres of mice immunised with E. coli (pAC2533) were appreciably higher than those of mice immunised with E. coli (pUC19). The specificity to Omp25 of murine antibodies induced by E. coli (pAC2533) was demonstrated by SDS-PAGE and immunoblotting of five B. melitensis strains. Binding of antibody in E. coli (pAC2533) immune sera to the surface of B. melitensis strains differing in their smooth lipopolysaccharide (S-LPS) expression was also studied by whole-cell ELISA and by flow cytometry. Antibody reactivity to R and smooth-rough (S-R) was much stronger than that to smooth (S) B. melitensis strains, indicating a much better accessibility of Omp25 to antibody on strains lacking or expressing less O-polysaccharide on their surface. The antibodies to Omp25 were predominantly of IgG2a isotype. The capacity of E. coli (pAC2533) to induce protective immune responses against four challenge strains of B. melitensis was further evaluated in mice. Significant reductions in splenic infections, in comparison with mice immunised with E. coli (pUC19) and unimmunised (saline injection) mice, were observed in R B. melitensis B115, S-R B. melitensis EP and S B. melitensis H38 infected mice. Protection against S B. melitensis 16M was not significant. The data from the present study, together with previous results, suggest that humoral immunity against probably conformational, well-exposed epitopes of the Omp25 could contribute to protective mechanisms against B. melitensis infection in mice.