Crossreacting determinants in variant-specific surface antigens of African trypanosomes.

Abstract

A number of variant-specific surface antigens (VSSA) were purified from clones of Trypanosoma brucei and T. congolense and tested for immunological crossreactivity. Anti-VSSA sera [rabbit] were clone-specific when tested by indirect immunofluorescence of living trypanosomes, but they were not clone-specific when tested by radioimmunoassay with purified 125I-labeled VSSA. In this double-antibody radioimmunoassay, every VSSA tested was precipitated by the homologous and all heterologous anti-VSSA sera. Any unlabeled VSSA inhibited the heterologous precipitation reactions by 100%. The homologous precipitation reactions were effectively inhibited only by unlabeled homologous VSSA. Crossreactions between different VSSA molecules were also revealed by microcomplement fixation tests. There are variable determinants specific to individual VSSA and crossreacting determinants in all VSSA tested, including those isolated from different species of trypanosomes. These results contrast with previous studies which failed to find evidence for immunological crossreactivity between different VSSA molecules. The crossreacting determinants in VSSA may represent common structural regions. The existence of such regions may have considerable implication for the development of theories and experiments concerning the mechanism of antigenic variation in African trypanosomes.