Many parasites of mammals avoid elimination by varying their exposed antigens. African trypanosomes—deadly parasites of humans and livestock in tropical Africa—possess a comprehensive system of antigenic variation (AV). Trypanosoma brucei undergo frequent, stochastic changes to their variant surface glycoprotein (VSG) coats, and therefore a developing immune response will be only partially effective against the trypanosome population as some trypanosomes will have already switched to a different VSG coat. The source of VSG variability is an archive of ~2000, mostly pseudogenic, silent VSG genes, of which only one is expressed. VSG genes can also be segmentally recombined: ‘mosaic’ VSGs, constructed from more than one silent VSG donor, allow both the reparation of pseudogenes and potentially generation of additional VSG variability. The aim of this research was to investigate the patterns of segmental VSG gene conversion in T. brucei, and assess its contribution to AV. Multiple, longitudinal samples were taken from chronic infections to follow the course of AV, and VSG cDNA sequences were analysed, building a detailed portrait of VSG expression across infection. VSG variability during an infection was extensive, and segmental gene conversion was found to be a frequent occurrence from approximately week three. Two main patterns were found: (i) expressed VSGs readily acquired a 3’ end different from their silent copy, a pattern that probably represents the 3’ boundary of gene conversion occurring within the coding sequence; (ii) expressed VSGs often appeared in sets of related ‘mosaics’, whereby more than one donor gene had contributed to the putative epitope-encoding part of the VSG. To test whether varying donor contributions represents an additional source of antigenic variability available to trypanosomes, a set of five mosaic VSGs retrieved from a single infection was expressed in non-switching trypanosomes and used to raise antibody responses. Indirect immuno-fluorescence, complement-mediated lysis, and agglutination assays using both polyclonal and monoclonal antibodies showed that although 4/5 mosaics were cross-reactive, one variant was completely antigenically distinct. Segmental gene conversion was therefore found to be both prominent in chronic African trypanosome antigenic variation, and capable of bringing antigenic novelty to an infection, with important consequences for the dynamics of AV, and the nature of selection pressure on the silent VSG archive.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:564095 |
| Date | January 2012 |
| Creators | Hall, James Peter John |
| Publisher | University of Glasgow |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://theses.gla.ac.uk/3796/ |
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