Trypanosoma brucei variable surface antigen is released by degenerating parasites but not by actively dividing parasites.

Surface antigen biosynthesis and fate in monomorphic and pleomorphic Trypanosoma brucei was examined to assess how slender and stumpy form T. brucei parasites present their variant specific glycoprotein (VSG) to the host immune system. Monomorphic and pleomorphic T. brucei did not release recently synthesized VSG in vitro. Slender form T. brucei, either from monomorphic or pleomorphic populations, did not release VSG in vivo. Detection of free VSG in plasma from irradiated mice infected with pleomorphic parasites correlated with the appearance of stumpy form parasites and possibly arose as a result of degeneration of those parasites. The in vivo released VSG was found to react well with some but not all antibodies directed against VSG determinants. Monoclonal and monospecific antibodies which react with VSG on living trypanosomes did not react with the released VSG whereas VSG-specific monoclonal antibodies which do not react with the surface of living T. brucei did react with the released VSG. It was unclear whether released VSG had lost a conformational determinant expressed on trypanosome-attached VSG or whether antibodies which react strongly with VSG on living trypanosomes are of such low avidity that they fail to bind released VSG. The results suggest that trypanosome-attached VSG is more important for stimulation of protective humoral responses than released VSG. The requirements for stimulation of protective anti-VSG responses are reported elsewhere (Sendashonga & Black 1982).