Trypanosoma congolense: Surface glycoproteins of two early bloodstream variants: III. Immunochemical characterization

A radioimmunoassay (RIA) for the variant-specific glycoproteins (VSG-1 and VSG-2) of two sequentially appearing variants of Trypanosoma congolense has been devised. When the isoelectrically focused VSG-1 components (VSG-1a, VSG-1b, and VSG-1c) are used as inhibitors of the VSG-1-anti-VSG-1 interaction, the RIA inhibition curves resemble each other, although minor differences in the high-affinity region of the curves can be detected. The heterologous antigen (VSG-2) does not inhibit the VSG-1-anti-VSG-1 interaction except at very high concentrations, indicating there is little cross-reactivity between highly purified VSG-1 and VSG-2. Nevertheless, heterologous antiserum, directed against VSG-2, will inhibit the VSG-1 -anti-VSG-1 interaction, and this property is shared to a significant degree by rabbit antiserum directed against an unrelated antigen. We have interpreted these findings as suggesting that: (1) there may be a constant region common to both VSG proteins, and (2) the constant region of the immunoglobulin molecule may also bind VSG proteins. Preliminary experiments show that the VSG-1 molecule augments binding of the Clq component of complement to the Fc region of immunoglobulin G.