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Abstract

Trypanosoma brucei is an extracellular pathogen, that causes human and animal African trypanosomiasis. It actively evades the host ́s immune response, in a process termed antigenic variation, by continuously changing its dense coat of Variant Surface Glycoproteins (VSGs). Structurally, VSGs are surface proteins, connected to the plasma membrane via a glycosylphosphatidylinositol (GPI) anchor, and consisting of a smaller C-terminal domain (CTD) attached to a larger N-terminal domain (NTD) by an unstructured region (linker). The CTD is considered to be inaccessible to antibodies, because of the coat ́s dense packaging. On the NTD, post-translation modifications (PTMs) have been shown to alter the immune response, as observed by the O-glycosylation on VSG3. Here, I focused initially on the plasma cell antibody repertoires elicited during infection with the double-, single-or non-glycosylated VSG3. I showed that these infections induce a similar response in mice infected with the same strain and elicit repertoires that are directed towards immunodominant epitopes on the surface of the VSGs. I also found that minor alterations within these epitopes, elicit distinct repertoires, reducing cross-reactivity amongst VSGs and facilitating prolonged immune evasion. On a second step, I also explored the structural differences and elicited repertoires of mosaic VSGs, created by swapping the CTDs of different variants. I found that mosaic and parental VSGs can be antigenically distinct, leading to differential binding by monoclonal antibodies. Their repertoires can be diverse and have heavy and light chain genes that are also present in the repertoires of the parental VSGs, but they form new and distinct pairs in the mosaics. As VSG mosaic formation is most commonly seen in the swapping of CTDs, where the same NTD can be found with different CTDs, these observations could imply that the antigenicity of the VSG protein is indirectly impacted by the CTD, despite the CTD itself being protected from antibody exposure. Overall, my findings suggest that VSGs elicit a stereotyped immune response, focused on a restricted set of immunodominant epitopes and that swapping of the CTD can change this response, further increasing VSG diversity, limiting cross-reactivity and facilitating long-term infection in the host.