An investigation into the mechanism of trypanosome lysis by human serum factors

Abstract

African trypanosomes are the causative agents of sleeping sickness in humans and of Nagana in cattle. The infectivity of African trypanosome species for humans appears to be defined by their susceptibility to two lytic factors in human serum; trypanosome lytic factor (TLF)1, a subclass of human high density lipoprotein (HDL) and TLF2, a high molecular weight protein complex. Available evidence indicates that following receptor mediated uptake, TLF is targeted to the lysosome where the low pH triggers a TLF-dependant peroxidase activity resulting in the formation of reactive oxygen radicals with consequent lipid peroxidation and destruction of the lysosomal membrane. Nearly all previous work on the mechanism of parasite lysis has been performed using TLF1. In this study, we directly test the hypothesis that TLF1 and TLF2 kill Trypanosoma brucei by a mechanism involving oxidative stress. We found no evidence for lipid peroxidation in trypanosomes exposed to high concentrations of trypanolytic HDL (impure TLF1), although lipid peroxidation was detected in parasites exposed to low concentrations of low molecular weight peroxides. Neither HDL, TLF1 nor TLF2 generated detectable levels of intracellular reactive oxygen intermediates. Various antioxidants also had no effect on TLF1 or TLF2-mediated lysis, although the antioxidants catalase and superoxide dismutase were effective at inhibiting peroxide generation and parasite lysis in control systems. Various metal chelating agents and protease inhibitors were also tested without effect. These data provide strong evidence against a peroxidative mechanism being involved in TLF-mediated lysis.