Studies on the expression of the major cell surface molecules of insect forms of Trypanosoma congolense, a major parasite of cattle in Africa




Loveless, Bianca C.

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African trypanosomes are protozoan parasites that cause African trypanosomiasis, diseases that affect humans and their livestock. Not only has trypanosomiasis had an overwhelming effect on the development of tropical Africa in the past, but it also constitutes one of the most significant present economic problems of the continent. Trypanosomes alternate between a mammalian host and a tsetse vector using a complex life cycle. In the mammalian host the trypanosomes live as bloodstream forms (BSFs) that are so proficient at antigenic variation, and thus host immune system evasion, that no suitable vaccine candidates have yet been identified. In contrast, the lifecycle stages that exist in the tsetse vector do not undergo antigenic variation. This potentially makes the vector-occupying trypanosomes much better targets for control if strategies can be devised to disrupt their lifecycle in the vector or to interfere with their transmission to mammalian hosts. The primary impediment to developing strategies for disruption of trypanosome life cycles in tsetse is a lack of understanding of the molecular basis of trypanosome-tsetse interactions. Although several major surface molecules have been identified on insect form trypanosomes, these have not been well studied due to a lack of appropriate antibody probes and to the difficulty in obtaining sufficient quantities of the different parasite life cycle stages required for such molecular studies. My thesis research was focused on developing and using monoclonal antibody probes for analysis of expression of major surface molecules of Trypanosoma congolense, a serious pathogen of cattle in Africa. I used this species of trypanosome since in addition to being a socioeconomically important parasite, all four of its major life cycle stages can be grown in vitro in amounts sufficient for immunochemical analysis. I successfully derived and characterized monoclonal antibodies that were useful for detecting the three major surface proteins of T. congolense insect forms: glutamic acid/alanine rich protein (GARP), the T. congolense heptapeptide repeat protein (TcHRP) and congolense epimastogote specific protein (CESP). Selected monoclonal antibody probes were then employed for expression analysis of these molecules throughout the parasite life cycle using in vitro grown trypanosomes and parasites taken directly from infected tsetse. In addition, I determined the peptide epitopes for two of my GARP-specific monoclonal antibodies and in collaboration with Dr. Martin Boulanger and Jeremy Mason was able to localize the epitopes on a high resolution three-dimensional structure obtained by X-ray crystallography. This allowed us to derive a model that describes the orientation of GARP in the trypanosome surface membrane and explains the possible structure-function relationships involved in replacement of the bloodstream form variant surface glycoprotein (VSG) by GARP as trypanosomes differentiate in the tsetse vector after a bloodmeal.



Trypanosomes, Sleeping sickness, nagana, Africa, tropical diseases, tsetse