Immunity Conferred Upon Mice by Anti-LPS Monoclonal Antibodies in Murine Brucellosis

An immune serum infused into mice prior to intravenous virulent Brucella abortus challenge may reduce initial colonization of the spleen on day 7 post-challenge and increase bacteriolysis in the spleen and liver, as evidenced on day 21. Three monoclonal antibodies (IgG1, IgG3 and IgG2a) directed toward the lipopolysaccharide-A epitope (LPS-A) of B. abortus were studied in this model and compared with three previously studied polyclonal immune sera. The three monoclonals restricted spleen infection on day 7 post-challenge and spleen and liver infections on day 21. These early and late effects were similar to those obtained with the two polyclonal sera of high anti-LPS-A titres, namely, serum from mice either infected by B. abortus or vaccinated with a cell-wall B. abortus fraction. In contrast, serum from mice vaccinated with LPS-M from B. melitensis, which had a high anti-LPS-M but a low anti-LPS-A titre, had lower activity, evidenced at day 7 only. Hence, immune serum protection was mediated by antibodies directed toward the LPS-dominant epitope of the challenge strain.