COMPARISON OF THE EFFECTS OF IMMUNE KILLING MECHANISMS ON TRYPANOSOMA-BRUCEI PARASITES OF SLENDER AND STUMPY MORPHOLOGY

Trypanosoma brucei slender forms predominate over stumpy forms as the parasite population grows but at the peak of a parasitaemic wave and d uring remission of infection stumpy forms predominate. To determine wh ether this change in predominance might be caused by selective killing of slender forms, the fates of slender and stumpy form trypanosomes i n two in vitro assays of immune-mediated killing were compared. Parasi te populations in which > 90% of cells were of slender morphology were observed to be killed by antibody-dependent complement-mediated lysis approximately five times faster than populations in which < 15% of ce lls were slender and most were of intermediate or stumpy morphology. Q uantification of the relationship between the proportion of slender fo rms in the population and the rate of lysis indicated that slender for ms were killed approximately 7.3 times faster than other forms. In an opsonization assay, no differences were observed between slender and s tumpy forms in the extent to which they attached to macrophages in an antibody-dependent manner. These results suggest that the change in pr oportions of slender and stumpy forms at the peak of a parasitaemic wa ve results from slender forms being more susceptible to complement-med iated killing as the antibody response develops.