A19Δbpe275 emerges as a safer live attenuated vaccine candidate

Brucellosis remains a major global challenge to both animal production and public health. Brucella abortus A19, are limited in their application due to residual virulence and interference with serodiagnosis. The intracellular survival and immune evasion of Brucella critically depend on effector proteins delivered by the Type IV Secretion System (T4SS), yet the functions of many of these effectors remain poorly defined. In this study, we constructed a markerless deletion mutant A19Δbpe275, and comprehensively evaluated its phenotype, virulence, and vaccine potential. The A19Δbpe275 mutant retained smooth lipopolysaccharide (LPS) structure, in vitro growth kinetics, and genetic stability, but exhibited significantly impaired long-term intracellular survival in macrophages. In murine infection models, A19Δbpe275 displayed markedly attenuated virulence, characterized by consistently lower splenic bacterial loads, milder histopathological lesions, and accelerated clearance. Immunologically, infection with A19Δbpe275 was associated with a robust and sustained immune profile characterized by elevated Th1-associated cytokines. A19Δbpe275 conferred comparable protective efficacy an improved safety profile in the non-pregnant murine model against challenge with the virulent B. abortus 2308 and demonstrated cross-protection against B. melitensis 16 M. Collectively, by achieving an optimal balance between attenuated virulence and preserved immunogenicity, A19Δbpe275 emerges as a promising candidate for next-generation live attenuated brucellosis vaccines.