African trypanosomes express an immunogenic protein with a repeating epitope of 24 amino acids

Infection with intracellular protozoan parasites such as Plasmodium, Leishmania and Trypanosoma cruzi induces a strong antibody response against proteins containing tandem repeats, suggesting that these repetitive epitopes may camouflage vulnerable parasite antigens from a 'protective' immune response. We tested this theory by immunoscreening a cDNA expression library of African trypanosomes, extracellular parasites that evade their hosts' immune response by antigenic variation, and found that the most frequently detected trypanosome protein contains more than 40 tandem copies of a 24-amino acid repeat with a consensus sequence of A-M-E-D-E-L-D-S-L-R-A-L-N-E-Q-Y-E-A-L-Q-R-T-N-A (net charge = -4). This protein is encoded on an mRNA of more than 20 kb and has slight sequence similarities with cytoskeletal, intermediate filament proteins in other organisms. Thus, protozoan proteins with tandemly repeating epitopes do not exist solely to divert the humoral immune response; they have other specific physiological functions for the parasites and affect the overall parasite-host interaction in unknown and perhaps different ways.