Overexpression of protective antigen as a novel approach to enhance vaccine efficacy of Brucella abortus strain RB51

Brucella abortus strain RB51 is an attenuated rough strain that is currentl y being used as the official live vaccine for bovine brucellosis in the Uni ted States and several other countries. We reasoned that overexpression of a protective antigen(s) of B. abortus in strain RB51 should enhance its vac cine efficacy. To test this hypothesis, we overexpressed Cu/Zn superoxide d ismutase (SOD) protein of B. abortus in strain RB51. This was accomplished by transforming strain RB51 with a broad-host-range plasmid, pBBR1MCS, cont aining the sodC gene along with its promoter. Strain RB51 overexpressing SO D (RB51SOD) was tested in BALB/c mice for its ability to protect against ch allenge infection with virulent strain 2308. Mice vaccinated with RB51SOD, but not RB51, developed antibodies and cell-mediated immune responses to Cu /Zn SOD, Strain RB51SOD vaccinated mice dcl:eloped significantly (P < 0.05) more resistance to challenge than those vaccinated with strain RB51 alone. The presence of the plasmid alone in strain RB51 did not alter its vaccine efficacy. Also, overexpression of SOD did not alter the attenuation charac teristic of strain RB51.