Expression and localization of serum resistance associated protein in Trypanosoma brucei rhodesiense

The trypanosome lytic factor (TLF) is a primate specific innate defense mechanism that restricts the host range of African trypanosomes. Trypanosoma brucei rhodesiense, the causative agent of the acute form of human sleeping sickness, is resistant to the cytolytic action of TLF. By differential display PCR we have identified a gene in T. b. rhodesiense that is preferentially expressed in cell lines resistant to TLF. The protein sequence predicted from the gene shows homology to the trypanosome variable surface glycoprotein (VSG) gene family and in particular, to the previously reported human serum resistance associated gene (SRA). The amount of SRA mRNA is over 1000-fold higher in TLF resistant cells relative to TLF sensitive trypanosomes. Treatment of TLF sensitive trypanosomes with increasing concentrations of TLF in mice results in the selection of parasites that have reverted back to the TLF resistant phenotype. These trypanosomes also showed high levels of SRA mRNA. Antibodies against recombinant SRA react with a 59 kDa protein on western blots of total cell protein from TLF resistant trypanosomes but not TLF sensitive cells. Indirect immunofluorescence revealed that SRA is a cell surface protein present only in TLF resistant trypanosomes. These results suggest that TLF resistance in human sleeping sickness trypanosomes is a consequence of the selective, high level expression of a cell surface molecule(s). In addition, these studies support the role of TLF as a major factor in human serum mediated killing of susceptible trypanosomes.