Topological analysis of antigenic determinants on a variant surface glycoprotein of Trypanosoma brucei.

Nine monoclonal antibodies specific for the variant surface glycoprotein (VSG) of Trypanosoma brucei MITat 1.6 have been produced and characterised as part of a coordinated project to define the three dimensional structure and immunological profile of this and other VSG molecules. Competition radioimmunoassays using all combinations of these antibodies identified five distinct antigenic determinants, showing varying degrees of overlap, on the MITat 1.6 VSG molecule. A map has been constructed of the determinants which have been identified. Immunofluorescent staining of living trypanosomes in suspension revealed that only one of the five determinants is exposed on the surface of intact trypanosomes, the other four being accessible to antibody only after the surface coat has been disrupted or released into solution. Immunoblots, to detect binding of the antibodies to MITat 1.6 VSG transferred from sodium dodecyl sulphate gels to nitrocellulose paper, showed binding of all the monoclonal antibodies, except those recognising the surface of the living trypanosome, to VSG in this form. This suggests the surface determinant may be more conformationally iabile than the others identified. The significance of the existence, and apparent predominance of variant specific antigenic determinants which are cryptic in the intact surface coat is discussed.