Human infectivity trait in Trypanosoma brucei: stability, heritability and relationship to sra expression

Abstract

Email: m.turner@bio.gla.ac.ukT SUMMARY Some Trypanosoma brucei lines infect humans whereas others do not because the parasites are lysed by human serum. We have developed a robust, quantitative in vitro assay based on differential uptake of fluorescent dyes by live and dead trypanosomes to quantify the extent and kinetics of killing by human serum. This method has been used to discriminate between three classes of human serum resistance; sensitive, resistant and intermediate. TREU 927/4, the parasite used for the T. brucei genome project, is intermediate. The phenotype is expressed in both bloodstream and metacyclic forms, is stably expressed during chronic infections and on cyclical transmission through tsetse flies. Trypanosomes of intermediate phenotype are distinguished from sensitive populations of cells by the slower rate of lysis and by the potential to become fully resistant to killing by human serum as a result of selection or long-term serial passaging in mice and to pass on full resistance phenotype to its progeny in a genetic cross. The sra gene has been shown previously to determine human serum resistance in T. brucei but screening for the presence and expression of this gene indicated that it is not responsible for the human serum resistance phenotype in the trypanosome lines that we have examined, indicating an alternative mechanism for HSR exists in these stocks. Examination of the inheritance of the phenotype in F1 hybrids for both bloodstream and metacyclic stages from two genetic crosses demonstrated that the phenotype is co-inherited in both life cycle stages in a manner consistent with being a Mendelian trait, determined by only one or a few genes.

Role of expression site switching in the development of resistance to human Trypanosome Lytic Factor-1 in Trypanosoma brucei brucei

“…Several mechanisms have been described to account for T. b. rhodesiense and T. b. gambiense resistance to TLF-1 [17–19]. Most T. b. rhodesiense isolates express the SRA gene and that is sufficient to confer resistance to TLF-1 and normal human serum [23]. Found primarily within endosomal vesicles in T. b. rhodesiense , SRA co-localizes with TLF-1 following endocytosis and the two molecules traffic together to the lysosome [12].…”

Blood incubation infectivity test (BIIT): usefulness as compared to the SRAPCR in the identification of <i>Trypanosoma brucei rhodesiense</i> in animal reservoirs

“…In addition, there is a T.b. brucei reference strain (TREU927/4) that was recently observed as partially resistant to human serum and yet the SRA gene is not detectable by PCR (Radwanska et al, 2002;Turner, 2004;Vanhamme, 2004). A number of genes with homology to SRA have been identifiable which are unable to confer resistance to human serum (Turner, 2004;Vanhamme, 2004), the ease to apply SRA-PCR and short time taken to define the risk of the animal reservoir makes it become a technique of choice.…”

In Vitro Generation of Human High-Density-Lipoprotein-Resistant Trypanosoma brucei brucei

“…Although compelling evidence supports the role of SRA in human infectivity by T. b. rhodesiense, several investigators have reported the occurrence of human serum-resistant lines of African trypanosomes that lack the SRA gene (2,47,66,67). To investigate the mechanism of SRA-independent resistance, we generated human serum-resistant lines of T. b. brucei 427 (MiTat 1.2) by repeated in vitro exposure to human HDLs.…”

“…Less is known about ESAG1, but it is predicted to be a transmembrane cell surface protein in bloodstream trypanosomes (44). While the function of each of these proteins is unknown, it is interesting to note that another human serum-resistant line of T. b. brucei (TREU 927; GuTat 10.1) also lacks these genes in the active expression site (66,67).…”