Characterizing tolerance in CD8 T cells in the absence of Bim-mediated clonal deletion to tissue restricted antigens

• Author / Creator

  Suen, Alexander YW

• T cells are part of the adaptive immune system that eliminate pathogen-infected cells, but do not harm healthy cells. Precursor T cells, termed thymocytes, develop in the thymus where they randomly generate and express unique antigen specific T cell receptors (TCR) during the CD4+CD8+ double positive (DP) stage of development. Due to the random nature of generation, the specificity of the TCR needs to be screened by a selection process controlled by TCR affinity for self-peptide presented in major histocompatability complex (MHC) proteins. DP thymocytes that express TCRs with low or moderate affinity with self-peptide-MHC are positively selected. These cells then migrate from the cortex of the thymus to the medulla to mature into CD4 single positive (SP) or CD8SP thymocytes. DP thymocytes that express potentially harmful TCRs that have high affinity for self-peptide-MHC are negatively selected. The exact mechanisms and pathways that mediate this binary fate decision remain unclear. One protein that may be involved in this fate decision is the pro-apoptotic Bcl-2 family protein Bim. Bim plays an important role in negative selection by inducing cell death in thymocytes that recognize self-antigen with high affinity. However, in the case of ubiquitous self-antigens (UbA), self-antigens expressed by all cells in the body, it has been shown that Bim is not required for the elimination of self-reactive thymocytes, suggesting the existence of non apoptotic negative selection mechanisms. However, the role of Bim in negative selection to tissue-restricted antigens (TRA), self-antigen expressed only in certain tissues (insulin for example), is unclear, as negative selection to TRA occurs in a developmentally, temporally and physically distinct manner than negative selection to UbA. I generated bone marrow (BM) chimeras by transplanting Bim-deficient OT-I TCR transgenic BM, a MHC class I restricted TCR specific for chicken ovalbumin (OVA), into mice expressing membrane bound OVA as a TRA under control of the rat insulin promoter (RIPmOVA). In these mice, I demonstrated that Bim is required for clonal deletion to TRA. While Bim-deficient OT-I thymocytes are not deleted in RIPmOVA chimeras, they are rendered anergic and are unable to proliferate OVA-pulsed splenocytes or cause diabetes. This anergic state persists in the periphery with OT-I Bim-deficient T cells being unable to proliferate or produce cytokines in response to stimulation with OVA-pulsed splenocytes. While proliferation can be rescued by stimulation with plate-bound αCD3/αCD28 antibodies, cytokine production cannot. Persistent antigen encounter is required to maintain this anergic state, as both proliferation and cytokine production is rescued in chimeras where OVA is expressed only in the thymus. Taken together, these data demonstrate that Bim is required for clonal deletion to TRA and highlights the fact that the immune system has “fail-safe” mechanisms in place to control auto-reactive T cells and limit their potential for autoimmunity. These data also provide some insight into methods of breaking immunological tolerance as might be desirable in adoptive T cell cancer immunotherapies.

• Subjects / Keywords

  • T cell development
  • T cell
  • Clonal Deletion
  • Central Tolerance
  • Bim
  • Anergy
  • Negative Selection
  • Thymocyte

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3ZC7S288

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.