Surface exposure of outer membrane protein and lipopolysaccharide epitopes in Brucella species studied by enzyme-linked immunosorbent assay and flow cytometry.

Seven surface-exposed outer membrane proteins (OMPs) in Brucella supp. have been previously described (A. Cloeckaert, P. de Wergifosse, G. Dubray, and J. N. Limet, Infect. Immun. 58:3980-3987, 1990). OMPs were shown to be more accessible to monoclonal antibodies (MAbs) on rough (R) Brucella melitensis and B. abortus strains than to MAbs on their smooth (S) counterparts. In this work, we have extended this study to representatives of the main Brucella species, using MAbs specific for OMPs and S and R lipopolysaccharides (S-LPS and R-LPS). Enzyme-linked immunosorbent assay (ELISA), flow cytometry, and immunoelectron microscopy showed important differences between strains in the binding of OMP- and R-LPS-specific MAbs which were in part related to the particular expression of S-LPS, irrespective of the species. Results indicated that both the amount and the length of O polysaccharide on S-LPS greatly influenced the accessibility of OMP and R-LPS epitopes to MAbs. S-R B. melitensis EP and S B. suis 40, for instance, which express O-polysaccharide chains in small amounts and with short mean length, respectively, bound a greater number of OMP- and R-LPS-specific MAbs than the other S Brucella strains. The major 31- to 34-kDa OMP was the most exposed OMP on S strains of B. melitensis and B. suis. In most cases, flow cytometry results agreed with those of ELISA and supplied additional data, such as the homogeneity or heterogeneity of OMP expression at the strain level. However, there were some discordances between flow cytometry and ELISA results concerning the surface exposure of the 25- to 27-kDa and 31- to 34-kDa OMPs on S strains and that of minor OMPs in vaccine strain B. melitensis Rev.1. Immunoelectron microscopy confirmed the poor accessibility of OMPs to MAbs on the surface of S Brucella strains. The naturally R pathogenic species B. ovis and B. canis bound the majority of OMP-specific MAbs as well as the R-LPS-specific MAbs. Therefore, the conserved OMP and R-LPS epitopes could play a role as targets of protective antibody-mediated immunity in infections caused by naturally R B. ovis and B. canis.