TCR-pMHC Binding Research Articles

The receptor deformation model of TCR triggering

Through T cell receptors (TCRs), T cells can detect and respond to very small numbers of foreign peptides among a huge number of self-peptides presented by major histocompatibility complexes (pMHCs) on the surface of antigen-presenting cells (APCs). How T cells achieve such remarkable sensitivity and specificity through pMHC-TCR binding is an intensively pursued issue in immunology today; the key question is how pMHC-TCR binding initiates, or triggers, a signal from TCRs. Multiple competing models have been proposed, none of which fully explains the sensitivity and specificity of TCR triggering. What has been omitted from existing theories is that the pMHC-TCR interaction at the T cell/APC interface must be under constant mechanical stress, due to the dynamic nature of cell-cell interaction. Taking this condition into consideration, we propose the receptor deformation model of TCR triggering. In this model, TCR signaling is initiated by conformational changes of the TCR/CD3 complex, induced by a pulling force originating from the cytoskeleton and transmitted through pMHC-TCR binding interactions with enough strength to resist rupture. By introducing mechanical force into a model of T cell signal initiation, the receptor deformation model provides potential mechanistic solutions to the sensitivity and specificity of TCR triggering.

Multiple thermodynamic mechanisms contribute to the binding of pepMHC ligands by wild type and high-affinity TCRs (93.16)

Abstract To better understand the mechanisms that govern TCR-pMHC binding and the role that different regions of the TCR play in affinity and antigen specificity, we have studied the TCR from mouse CTL clone 2C, which recognizes QL9/Ld as a strong agonist. High affinity mutations of the 2C TCR were previously generated in various CDRs (1β, 2α, 3α and 3β) by in vitro directed evolution. Using a single-chain version of the TCR and a stabilized Ldprotein, we performed isothermal titration calorimetry (ITC) on these mutants, which bind with high affinity to QL9/Ld. Unexpectedly, our results showed that the wild-type interaction was favorable entropically. In contrast, all high affinity mutants except one (2C-1β) bound in an enthalpically driven manner. In addition, we measured the binding and kinetics of these mutants for both QL9/Ld and position 5 peptide variant Y5/Ld using SPR. While other mutants had higher affinity for the Y5 peptide variant than for QL9, the 2C-1β mutant had a lower affinity for Y5. Thus it appears that multiple thermodynamic mechanisms are used to achieve high affinity in this system. In addition, some high-affinity TCR mutants could achieve high affinity while exhibiting enhanced fine specificity. Supported by grants from the NIH.

Altered Peptide Ligands Induce Delayed CD8-T Cell Receptor Interaction—a Role for CD8 in Distinguishing Antigen Quality

How T cells translate T cell receptor (TCR) recognition of almost identical pMHC ligands into distinct biological responses has remained enigmatic. Although differences in affinity or off rate are important, they offer at best an incomplete explanation. By using Förster resonance energy transfer (FRET), we have visualized the ligand-induced interaction between OT-I TCR and CD8. We found that both recruitment of TCR to the immunological synapse and the TCR-CD8 interaction induced by weak agonists (positive-selecting ligands) was delayed but not necessarily weaker than strong agonists (negative selectors). A delayed and perhaps longer lasting CD8-TCR interaction results in delayed phospho-ERK recruitment to the synapse. The kinetics of the TCR-CD8 interaction can reconcile previously anomalous data, where biological activity did not correlate with TCR-pMHC binding kinetics for certain ligands. Our findings indicate that the T cell translates antigen recognition into T cell responses by differential recruitment of CD8 to the TCR.

T cell receptor binding kinetics required for T cell activation depend on the density of cognate ligand on the antigen-presenting cell

CD8(+) T cells recognize peptides of eight to nine amino acid residues long in the context of MHC class I molecules on the surface of antigen-presenting cells (APCs). This recognition event is highly sensitive, as evidenced by the fact that T cells can be activated by cognate peptide/MHC complex (pMHC) at extremely low densities (1-50 molecules). High sensitivity is particularly valuable for detection of antigens at low density, such as those derived from tumor cells and intracellular pathogens, which can down-modulate cognate pMHCs from the surface of APCs to evade recognition by the adaptive immune system. T cell activation is only triggered in response to interactions between the T cell receptor (TCR) and the pMHC ligand that reach a specific half-life threshold. However, interactions with excessively long half-lives result in impaired T cell activation. Thus, efficient T cell activation by pMHC on the surface of APCs requires an optimal dwell time of TCR-pMHC interaction. Here, we show that, although this is a requirement at low cognate pMHC density on the APC surface, at high epitope density there is no impairment of T cell activation by extended TCR-pMHC dwell times. This observation was predicted by mathematical simulations for T cell activation by pMHC at different densities and supported by experiments performed on APCs selected for varied expression of cognate pMHC. According to these results, effective T cell activation depends on a complex interplay between inherent TCR-pMHC binding kinetics and the epitope density on the APC.