Equal recognition of self by mediators and suppressors while poor recognition leads to ignorance. Open Access

Hood, Jennifer Dianna (2015)

Permanent URL: https://etd.library.emory.edu/concern/etds/jh343s762?locale=en
Published

Abstract

The strength of interaction of the T cell receptor (TCR) with peptide presented in the context of major histocompatibility complex (pMHC) is thought to influence the fate of T cells. For thymocyte development, TCR must bind self pMHC at a basal level to institute MHC restriction while any cell with extremely high affinity for self needs to be eliminated during negative selection to limit autoimmunity. Differential processing of ubiquitous antigens and induction of anergy is thought to prevent recognition of these selecting ligands in the periphery. High affinity interactions potentially drive thymocytes to become Foxp3-expressing regulatory CD4+ T cells (Tregs). Once in the periphery, affinity influences the ability of cells to functionally respond to antigen, where in a polyclonal setting high affinity cells could out-compete lower affinity TCRs to dominate. We investigated T cell affinity for 3 different self antigens. Using a mouse model of autoimmunity against myelin, the affinity profiles of polyclonal self-specific CD4+ conventional T cells (Tconvs) and Tregs were found to overlap and span a similar range. Both were dominated by lower affinity cells, but at the site of autoimmunity, there was skewing towards higher affinity cells in the Tconvs, which is contrary to what would be expected if Tregs are to be enriched for higher affinity self-specific cells. In a second system, determination of TCR affinity for a known endogenous positive selecting ligand revealed that it is lower than cognate agonist as expected. Lastly, a diabetogenic antigen (ChgA) was identified with its lower affinity leading to a lack of functional response to this self antigen. Moreover, analysis of TCR affinities for monoclonal transgenic T cells demonstrated that affinity can be dynamic changing with developmental or activation state. We found that DP thymocytes have the highest affinity followed by activated cells and then na ï ve peripheral cells regardless of antigen examined. Together these data demonstrate that there are important factors altering the effective affinity which is consistent with the differential sensitivity to stimulation of the various cell states. These results also highlight the potential importance of matched antigen specificities and similar affinities for Teffs and Tregs during autoimmunity.

Table of Contents

Chapter 1: Introduction.......................................................................................... 1-25

Figure 1.1 The instructive model of thymocyte selection...................................... 26

Figure 1.2 Two-step model of thymocyte development........................................ 27

Table 1.1 Known endogenous positive selecting ligands for the indicated TCR transgenic mice. 28

Figure 1.3 EAE disease course.............................................................................. 29

Chapter 2: Regulatory and T effector cells have overlapping low to high ranges in TCR affinities for self during demyelinating disease............................................................................... 30-53

Table 2.1 Two-sided Fisher's Exact p values for the data presented in Figure 2.6. 54

Figure 2.1 Enrichment of MOG-specific Foxp3+ Tregs in the CNS during EAE. 55-56

Figure 2.2 Representative FACS plots showing the staining of MOG:IAb tetramers and CD44 for indicated cell types for the indicated organs isolated from mice at peak of EAE....................... 57

Figure 2.3 Production of cytokines by Tconv and Treg cells during EAE............ 58

Figure 2.4 Tregs and Tconvs have overlapping ranges of affinity for MOG but unique distributions in the CNS................................................................................................................................ 59

Figure 2.5 CNS Tconv cells are normally distributed unlike CNS Tregs and cells in periphery. 60

Figure 2.6 CNS Tregs are of thymic origin...................................................... 61-62

Figure 2.7 Helios vs. Foxp3 expression for CD4+ T cells..................................... 63

Chapter 3 Affinity and lack of functional response suggests that Chromogranin A could be a selecting ligand for BDC2.5 cells.................................................................................................... 64-84

Table 3.1. Summary of ChgA29-42 affinity data.................................................. 85

Figure 3.1 BDC2.5 Rg and Tg cells lack functional responsiveness to ChgA29-42. 86-87

Figure 3.2 ChgA29-42 is recognized by two different diabetogenic TCRs.... 88-89

Figure 3.3 BDC2.5 transgenic T cells exhibit different affinities and TCR densities depending on the developmental state............................................................................................... 90

Figure 3.4 BDC2.5 Tg cells possess a higher affinity for a mimotope compared to cognate self antigen. 91

Figure 3.5 Drop in ChgA29-42 reactivity of naïve splenocytes as the expression of an endogenous TCR alpha chain increases as cells mature............................................................................... 92

Figure 3.6 Endogenous positive selecting ligands are lower affinity compared to agonist peptides. 93

Chapter 4: Discussion......................................................................................... 94-104 References......................................................................................................... 105-120


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