Contributions of Bim and Nur77 to multiple mechanisms of T cell tolerance following high affinity antigen encounter Open Access
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University of Alberta
- Author or creator
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Baldwin, Troy (Medical Microbiology and Immunology)
- Examining committee member and department
Lesage, Sylvie (Immunology-Oncology, University of Montreal)
Ingham, Rob (Medical Microbiology and Immunology)
Ostergaard, Hanne (Medical Microbiology and Immunology)
Elahi, Shokrollah (Dentistry)
Department of Medical Microbiology and Immunology
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Doctor of Philosophy
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Prevention of autoimmune disease requires the elimination or inactivation of T cells that are highly reactive to self-peptides. During T cell development in the thymus, self-reactive thymocytes undergo negative selection upon receiving a high affinity T cell receptor (TCR) signal following encounter with self-antigens. Negative selection can occur in the cortex against ubiquitous antigens (UbA) and in the medulla against tissue-restricted antigens (TRA). While negative selection encompasses several mechanisms, most self-reactive thymocytes are removed from the repertoire by clonal deletion. Bim and Nur77 are two proteins upregulated during negative selection that are thought to be key mediators of clonal deletion due to their established pro-apoptotic functions. We therefore sought to characterize the contributions of Bim and Nur77 to UbA and TRA-mediated clonal deletion in vivo using the well established HYcd4 and OT-I Rip-mOva TCR transgenic mouse models, respectively. Despite being a major mediator of thymocyte apoptosis, we found that Bim is ultimately not required for UbA-mediated clonal deletion in the HYcd4 model. Nur77 was also dispensable for UbA-mediated deletion and could not compensate for Bim deficiency in inducing caspase-3 activation. In fact, our data suggests that the apoptotic function of Nur77 may be inhibited in the presence of TCR signalling. However, deficiency in both Bim and Nur77 tended to result in a greater number of antigen specific thymocytes, which appeared to be due to thymocyte-intrinsic and extrinsic factors. In contrast to UbA-mediated clonal deletion, Nur77 deficiency was sufficient to modestly impair TRA-mediated deletion in the OT-I Rip-mOva model, and Bim deficiency completely abrogated deletion. Furthermore, our data suggests a novel role for Nur77 in anergy induction in this model. These results reveal differences in the molecular mechanisms of negative selection against UbA vs. TRA. In addition, we examined negative selection in a diverse T cell repertoire by examining Bim and Nur77 deficiency in unmanipulated mice. In polyclonal mice lacking Bim and Nur77, we found an accumulation of thymocytes that had experienced high affinity TCR signalling, suggesting that clonal deletion was impaired. This was accompanied by increases in regulatory T cells and anergic phenotype T cells, two alternative fates that can occur upon high affinity antigen encounter. Nevertheless, polyclonal Bim-/-Nur77-/- mice tended to develop a phenotype suggestive of excessive inflammation, whereas this was never observed in Bim or Nur77 single knockout mice. Similarly, only OT-I Bim-/-Nur77-/- Rip-mOva chimeras developed autoimmune diabetes. These results indicate that deficiency in two key molecules involved in negative selection greatly increases the risk of autoimmune disease. Collectively, these studies shed light on the multiple mechanisms that work in concert to maintain self-tolerance.
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- Citation for previous publication
Hu, Q., A. Sader, J. C. Parkman, and T. A. Baldwin. 2009. Bim-mediated apoptosis is not necessary for thymic negative selection to ubiquitous self-antigens. J Immunol 183: 7761-7767.Hu, Q. and T. A. Baldwin. 2015. Differential roles for Bim and Nur77 in thymocyte clonal deletion induced by ubiquitous self-antigen. J Immunol (in press).
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