Minor nucleotide substitutions in the omp31 gene of Brucella ovis result in antigenic differences in the major outer membrane protein that it encodescompared to those of the other Brucella species

The gene coding for the major outer membrane protein Omp31 was sequenced in five Brucella species and their biovars. Although the omp31 genes appeared to be highly conserved in the genus Brucella, nine nucleotide substitution s were detected in the gene of Brucella ovis compared to that of Brucella m elitensis. As shown by differential binding properties of monoclonal antibo dies (MAbs) to the two Brucella species, these nucleotide substitutions res ult in different antigenic properties of Omp31. The antigenic differences w ere also evidenced when sera from B. ovis-infected rams were tested by West ern blotting with the recombinant B. melitensis or B. ovis Omp31 proteins. Twelve available sera reacted with recombinant B. ovis Omp31, but only four of them reacted with recombinant B. melitensis Omp31. These results valida te previous evidence for the potential of Omp31 as a diagnostic antigen for B. ovis infection in rams and demonstrate that B. ovis Omp31, instead of B . melitensis Omp31, should be used to evaluate this point. The antigenic di fferences between the A melitensis and B. ovis Omp31 proteins should also b e taken into account when Omp31 is evaluated as a candidate for the develop ment of subcellular vaccines against B. ovis infection. No reactivity again st recombinant B. melitensis Omp31 was detected, by Western blotting, with sera from B. melitensis-infected sheep. Accordingly, Omp31 does not seem to be a good diagnostic antigen for B. melitensis infections in sheep. Two im munodominant regions were identified on the B. ovis Omp31 protein by using recombinant DNA techniques and specific MAbs. Sera from B. ovis-infected ra ms that reacted with the recombinant protein were tested by Western blottin g against one of these immunodominant regions shown to be exposed at the ba cterial surface. Only 4 of the 12 sera reacted, but with strong intensity.