Dieter Naeher

A constant affinity threshold for T cell tolerance

T cell tolerance depends on the T cell receptors affinity for peptide/major histocompatibility complex (MHC) ligand; this critical parameter determines whether a thymocyte will be included (positive selection) or excluded (negative selection) from the T cell repertoire. A quantitative analysis of ligand binding was performed using an experimental system permitting receptor–coreceptor interactions on live cells under physiological conditions. Using three transgenic mouse strains expressing distinct class I MHC–restricted T cell receptors, we determined the affinity that defines the threshold for negative selection. The affinity threshold for self-tolerance appears to be a constant for cytotoxic T lymphocytes.

A Role for the α-Chain Connecting Peptide Motif in Mediating TCR-CD8 Cooperation

To generate peripheral T cells that are both self-MHC restricted and self-MHC tolerant, thymocytes are subjected to positive and negative selection. How the TCR discriminates between positive and negative selection ligands is not well understood, although there is substantial evidence that the CD4 and CD8 coreceptors play an important role in this cell fate decision. We have previously identified an evolutionarily conserved motif in the TCR, the α-chain connecting peptide motif (α-CPM), which allows the TCR to deliver positive selection signals. Thymocytes expressing α-CPM-deficient receptors do not undergo positive selection, whereas their negative selection is not impaired. In this work we studied the ligand binding and receptor function of α-CPM-deficient TCRs by generating T cell hybridomas expressing wild-type or α-CPM-deficient forms of the T1 TCR. This Kd-restricted TCR is specific for a photoreactive derivative of the Plasmodium berghei circumsporozoite peptide252–260 IASA-YIPSAEK(ABA)I and is therefore amenable to TCR photoaffinity labeling. The experiments presented in this work show that α-CPM-deficient TCRs fail to cooperate with CD8 to enhance ligand binding and functional responses.

The T Cell Receptor’s α-Chain Connecting Peptide Motif Promotes Close Approximation of the CD8 Coreceptor Allowing Efficient Signal Initiation

The CD8 coreceptor contributes to the recognition of peptide-MHC (pMHC) ligands by stabilizing the TCR-pMHC interaction and enabling efficient signaling initiation. It is unclear though, which structural elements of the TCR ensure a productive association of the coreceptor. The α-chain connecting peptide motif (α-CPM) is a highly conserved sequence of eight amino acids in the membrane proximal region of the TCR α-chain. TCRs lacking the α-CPM respond poorly to low-affinity pMHC ligands and are unable to induce positive thymic selection. In this study we show that CD8 participation in ligand binding is compromised in T lineage cells expressing mutant α-CPM TCRs, leading to a slight reduction in apparent affinity; however, this by itself does not explain the thymic selection defect. By fluorescence resonance energy transfer microscopy, we found that TCR-CD8 association was compromised for TCRs lacking the α-CPM. Although high-affinity (negative-selecting) pMHC ligands showed reduced TCR-CD8 interaction, low-affinity (positive-selecting) ligands completely failed to induce molecular approximation of the TCR and its coreceptor. Therefore, the α-CPM of a TCR is an important element in mediating CD8 approximation and signal initiation.

Optimal T-cell receptor affinity for inducing autoimmunity

Significance The adaptive immune system has the potential to generate a self-reactive response, which can eventually lead to an autoimmune disease. To avoid this outcome, T lymphocytes with high-affinity, self-reactive antigen receptors are blocked from entering the mature T-cell pool (negative selection). Given this mechanism for removing dangerous high-affinity T cells, we wondered whether autoimmunity is more likely to be caused by chronic stimulation of low-affinity T cells or by stimulation of a few high-affinity T cells that escaped negative selection. In this paper, we show that T cells with an affinity just above the selection threshold can bypass negative selection and have the highest potential to cause an experimental autoimmune disease. T-cell receptor affinity for self-antigen has an important role in establishing self-tolerance. Three transgenic mouse strains expressing antigens of variable affinity for the OVA transgenic-I T-cell receptor were generated to address how TCR affinity affects the efficiency of negative selection, the ability to prime an autoimmune response, and the elimination of the relevant target cell. Mice expressing antigens with an affinity just above the negative selection threshold exhibited the highest risk of developing experimental autoimmune diabetes. The data demonstrate that close to the affinity threshold for negative selection, sufficient numbers of self-reactive T cells escape deletion and create an increased risk for the development of autoimmunity.