Developmentally Regulated Sensitivity of Trypanosoma brucei brucei to the Cytotoxic Effects of Human High-Density Lipoprotein

The bloodstream developmental stages of the protozoan parasite Trypanosoma brucei brucei are lysed by normal human serum. The cytotoxic factor is a minor sub-class of human high-density lipoprotein termed trypanosome lyric factor (TLF). T. b. brucei rapidly develops resistance to TLF when incubated at 26°C under conditions which allow differentiation to the procyclic, insect midgut developmental stage. This in vitro differentiation system allowed us to correlate loss of TLF sensitivity with other parameters of differentiation to the procyclic form. The onset of resistance to TLF occurs within 2 hr after shifting bloodstream forms to differentiation conditions. TLF resistance is correlated with a rapid but transient decrease in protein synthesis by the parasite, is acquired prior to cell division at 26°C, and precedes the loss of variant surface glycoprotein. Tn addition, we found binding and uptake of TLF by established procyclic trypanosomes was reduced approximately fivefold relative to bloodstream trypanosomes and the TLF binding observed in procyclics was nonspecific. No TLF was bound to the procyclic flagellar pocket membrane and the procyclics failed to endocytose any of the surface-bound TLF. In contrast, bloodstream forms bind TLF via a flagellar pocket-localized protein and bound TLF is taken up;by endocytosis. These findings suggest that resistance of procyclic T. b. brucei to TLF-mediated lysis is due to a reduction in the endocytosis of TLF by this developmental stage of the parasite.