Failure of immunization with trypanosome endocytotic vesicle membrane proteins to provide nonvariant immunoprotection against Trypanosoma brucei.

Purified trypanosome endocytotic vesicles were subjected to Triton X-114 phase separation to obtain a fraction enriched in putative parasite receptors for adsorptive endocytosis. Rabbits immunized with this material produced antibodies that recognized many parasite proteins, including nonvarying epitopes on the parasite's endocytotic surface, the flagellar pocket membrane, as well as on membranes of endosomes and lysosome-like structures. These antibodies were unable to stimulate in vitro complement-mediated lysis of trypanosomes, and in an in vitro test of parasite growth inhibition they actually marginally enhanced parasite proliferation. No effect was observed on the parasite prepatent period or parasitemia in mice injected with antibody purified from the rabbit antisera, but their survival with the infection was significantly shortened. Finally, little difference was detected in parasitologic or hematologic parameters between immunized and control rabbits upon challenge with T. brucei infection.