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Title: The dynamics and nature of the hierarchy of VSG expression during Trypanosoma brucei infection
Author: Morrison, Liam
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 2004
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Trypanosoma brucei is a tsetse fly-transmitted kinetoplastid protozoan that parasitises a wide range of mammals in sub-Saharan Africa. The chronic infections typical of trypanosomes have a profile characterised by intermittent parasitaemic peaks, that become smaller and further apart. A key component that facilitates the survival of the parasite within the bloodstream of the host, and the generation of chronic infections, is antigenic variation. This process involves the inherent switching of the surface-expressed variant surface glycoprotein (VSG), and the new variable antigen types (VATs) in order to preempt the host immune response. There are approximately 1,000 genes in the VSG repertoire, and the observation that certain VATs appear at similar times within infections, has led to the conclusion that the expression of VSGs is semi-predictable and semiordered. This hierarchical system allows optimal use of the VSG repertoire, and is highly likely to be a significant factor in generating chronicity. It has been established that VSGs encoded by subtelomeric genes tend to appear early in infection, whilst those encoded by chromosomal internal genes tend to appear later. The first aim of this thesis was to examine the timing of expression of a subset of VSGs within replicate infections in mice and cattle, and to link the timing of expression to the genetic locus of the silent, donor, VSG gene. The VSGs examined represented all VSG locus types, incorporating 1 bloodstream expression site (BBS) gene (the transcription site for VSG genes in the bloodstream stage), 2 metacyclic expression site (MBS) genes, 2 minichromosomal genes, and 2 chromosomal internal genes. The infections were initiated with pleomorphic trypanosomes that switch at a high rate, and are a close-to-field, 'wild type' strain. The infections in mice confirmed that there was a statistically significant difference in the timing of onset of VAT-specific immune responses across replicate batches of infections with two separate analyses; one by comparing the average time of onset of the immune response (General Linear Model [GLM]; F6,106=7.49, p < 0.0001), and the second by ranking the onset of the immune response by sequence of appearance (GLM; Fe,112=8.03, p<0.0001). The appearance of VSGs within a restricted period of time was confirmed by directly analysing the parasite population in cattle, using FSG-specific reverse transcriptase polymerase chain reaction (RT-PCR). These findings allow further dissection of the hierarchical expression of VSGs, and provide statistically significant confirmation of the existence of semi-ordered expression in high-switching pleomorphic trypanosomes for the first time. A mathematical model was formulated (with the assistance of Dr. K. Lythgoe, University of Edinburgh) in order to simulate the dynamics of trypanosome infections. The model incorporated and built upon successful aspects of previous studies, and included measured biological parameters that are known to affect in vivo parasite kinetics. Manipulations were undertaken to investigate the effect, in silico, of differential VSG switching rates. The effect of varying the intrinsic rate of growth of the immune response was also analysed. The simulations indicated areas for potential further experimental studies. In particular, results suggested that the rate of growth of the immune response may be extremely important in shaping the profile, and duration, of an infection. The outcome of modelling whereby VSG switching was manipulated, suggested that there are distinct subsets of VSGs, which have differing probabilities in switching to each other. The significance of genomic position of the silent, donor, VSG, and also sequence homology between the donor VSG and the expressed VSG were investigated. The proportional influence of genomic locus and homology-driven switching was suggested to be the most important aspect to be elucidated, with respect to further resolving the hierarchical switching of VSGs.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available