Characterization of outer membrane vesicles from Brucella melitensis and protection induced in mice.

The outer membrane vesicles (OMVs) from smooth B. melitensis 16 M and a derived rough mutant, VTRM1 strain, were purified and characterized with respect to protein content and induction of immune responses in mice. Proteomic analysis showed 29 proteins present in OMVs from B. melitensis 16 M; some of them are well-known Brucella immunogens such as SOD, GroES, Omp31, Omp25, Omp19, bp26, and Omp16. OMVs from a rough VTRM1 induced significantly higher expression of IL-12, TNF alpha, and IFN gamma genes in bone marrow dendritic cells than OMVs from smooth strain 16 M. Relative to saline control group, mice immunized intramuscularly with rough and smooth OMVs were protected from challenge with virulent strain B. melitensis 16 M just as well as the group immunized with live strain B. melitensis Rev1 (P < 0.005). Additionally, the levels of serum IgG2a increased in mice vaccinated with OMVs from rough strain VTRM1 consistent with the induction of cell-mediated immunity.