The resistance to human plasma of Trypanosoma brucei, T. rhodesiense and T. gambiense: III. Clones of two plasma-resistant strains

Natural immunity to human African trypanosomiasis: trypanosome lytic factors and the blood incubation infectivity test

This review focuses on the epidemiology of human African trypanosomiasis: why it occurs in humans, the current methods of surveillance, and the drugs available to treat it. Emphasis is placed on the identification of human-infective trypanosomes by the blood incubation infectivity test. This test distinguishes between trypanosomes that are non-infective for humans and those that are potentially infective. Currently the test requires incubation of parasites with human serum before injection into mice; any surviving parasites are considered human-infective. The factors in serum that kill all non-human-infective parasites are known as trypanosome lytic factors. The paper details the biochemistry of these factors and recommends standardization of the test based on current knowledge. This test can be used to screen animals with trypanosomiasis, in order to evaluate their role during endemic and epidemic human African trypanosomiasis.

Human serum-sensitive Trypanosoma brucei rhodesiense: a comparison with serologically identical human serum-resistant clones

Trypanosoma brucei rhodesiense clones, which are susceptible to lysis by normal human serum, were isolated from 3 different human serum-resistant clones originally derived from strain ETat 1.10. Serologically, these pairs of serum-sensitive and serum-resistant clones displayed the same variant surface glycoprotein (VSG) on their surface. Acquisition of human serum sensitivity correlated with susceptibility to lysis by human high density lipoprotein, a trypanocidal factor in normal human serum. Analysis of these paired populations by two-dimensional gel electrophoresis of whole trypanosomes and various subcellular fractions failed to reveal any differences in mobility of VSG and other proteins. Northern blot analysis of mRNAs from serum-sensitive and serum-resistant clones showed no differences when probed with a previously described resistance-specific probe. In addition, the ethanolamine membrane transport system and the overall membrane lipid fluidity did not reveal any detectable biochemical or biophysical differences in membrane properties. If resistance to lysis is indeed mediated by membrane changes at the enzymatic or structural level, the data presented suggest that the gene product(s) responsible for this change in human serum sensitivity may be present in very small quantities.

High-density lipoprotein-mediated lysis of trypanosomes

Nearly 90 years after the discovery that certain African trypanosornes were killed by normal human serum, we still do not understand how this innate trypanocidal factor works. Biochemical studies have provided us with an unlikely candidate: human high-density lipoprotein (HDL). This trypanosome lytic factor (TLF) from human serum is important since its activity restricts the host range of Trypanosoma brucei brucei, and the expression of this natural killing factor in cattle would represent a novel approach to the control of bovine tryponosomiasis. Here, Steve Hajduk, Kristin Hager and Jeffrey Esko discuss evidence for the TLF being a minor subclass of serum HDL and propose a mechanism for lysis based on the binding, endocytosis and lysosomal targeting of TLF.