Babesia bovis: isolation of a protective antigen by using monoclonal antibodies.

Three distinct monoclonal antibody-producing hybridomas have been produced against a partly purified protective fraction of Babesia bovis. All three stain the parasite or infected erythrocytes or both in precise and different manners when fluorescent-antibody techniques are used. The relevant antigens for each monoclonal antibody were isolated by immunoadsorption, their native molecular weights being 1.3 X 10(6), 180 X 10(3), and 44 X 10(3). Each antigen reacted in serological assays with homologous and heterologous bovine antisera to B. bovis. Susceptible splenectomized calves were immunized twice, 4 weeks apart, with the respective antigens and were challenged with virulent homologous organisms 2 weeks later. Strong protective immunity was induced by the antigen with a molecular weight of 44 X 10(3), but no significant protection was induced by either of the other two antigens.