Immunogenic properties of Brucella melitensis cell-wall fractions in BALB/c mice

The immunogenicity of several Brucella melitensis cell-wall (CW) fractions was tested in BALB/c mice. These CW fractions were smooth lipopolysaccharide (S-LPS) fraction from smooth (S) B. melitensis strain 16M, sodium dodecyl sulphate-insoluble (SDS-I) CW fraction from B. melitensis strain 16M (S) undigested or digested with pepsin, and SDS-I CW fraction from rough (R) B. melitensis strain H38. The B. melitensis SDS-I CW fraction contained two major outer-membrane proteins (OMPs) of 25-27 kDa and 31-34 kDa, peptidoglycan (PG) and a small quantity (1.5%) of LPS. One month after immunisation, mice were challenged with virulent B. melitensis strain H38 (S) and Brucella spleen counts were recorded on days 28 and 49 after challenge. Before challenge, as measured by ELISA, the highest antibody responses to S-LPS were observed in mice immunised with SDS-I CW fraction from B. melitensis strain 16M (S), whether digested with pepsin or undigested. All immunised mice, except those immunised with the SDS-I CW fraction from the R strain, showed higher IgG1 than IgG2a antibody responses to S-LPS (IgG1:IgG2a ratio 3.64-7.71). Antibody responses to the 25-27-kDa OMP were very low, with the highest responses in the mice immunised with the SDS-I CW fraction from the R strain. These results indicated that, in BALB/c mice, these CW fractions probably induced Th2-dependent more than Th1-dependent antibody responses.(ABSTRACT TRUNCATED AT 250 WORDS)