Daniel S. Lyons

A TCR Binds to Antagonist Ligands with Lower Affinities and Faster Dissociation Rates Than to Agonists

T lymphocyte activation is mediated by the interaction of specific TCR with antigenic peptides bound to MHC molecules. Single amino acid substitutions are often capable of changing the effect of a peptide from stimulatory to antagonistic. Using surface plasmon resonance, we have analyzed the interaction between a complex consisting of variants of the MCC peptide bound to a mouse class II MHC (Ek) and a specific TCR. Using both an improved direct binding method as well as a novel inhibition assay, we show that the affinities of three different antagonist peptide-Ek complexes are approximately 10-50 times lower than that of the wildtype MCC-Ek complex for the TCR, largely due to an increased off-rate. These results suggest that the biological effects of peptide antagonists and partial agonists may be largely based on kinetic parameters.

Two-step binding mechanism for T-cell receptor recognition of peptide-MHC

T cells probe a diverse milieu of peptides presented by molecules of the major histocompatibility complex (MHC) by using the T-cell receptor (TCR) to scan these ligands with high sensitivity and specificity. Here we describe a physical basis for this scanning process by studying the residues involved in both the initial association and the stable binding of TCR to peptide–MHC, using the well-characterized TCR and peptide–MHC pair of 2B4 and MCC-IEk (moth cytochrome c, residues 88–103). We show that MHC contacts dictate the initial association, guiding TCR docking in a way that is mainly independent of the peptide. Subsequently, MCC-IEk peptide contacts dominate stabilization, imparting specificity and influencing T-cell activation by modulating the duration of binding. This functional subdivision of the peptide–MHC ligand suggests that a two-step process for TCR recognition facilitates the efficient scanning of diverse peptide–MHC complexes on the surface of cells and also makes TCRs inherently crossreactive towards different peptides bound by the same MHC.

Ligand-specific oligomerization of T-cell receptor molecules

T cells initiate many immune responses through the interaction of their T-cell antigen receptors (TCR) with antigenic peptides bound to major histocompatibility complex (MHC) molecules. This interaction sends a biochemical signal into the T cell by a mechanism that is not clearly understood. We have used quasi-elastic light scattering (QELS) to show that, in the presence of MHC molecules bound to a full agonist peptide, TCR/peptide–MHC complexes oligomerize in solution to form supramolecular structures at concentrations near the dissociation constant of the binding reaction. The size of the oligomers is concentration dependent and is calculated to contain two to six ternary complexes for the concentrations tested here. This effect is specific as neither molecule forms oligomers by itself, nor were oligomers observed unless the correct peptide was bound to the MHC. These results provide direct evidence for models of T-cell signalling based on the specific assembly of multiple TCR/peptide-MHC complexes in which the degree of assembly determines the extent and qualitative nature of the transduced signal. They may also explain how T cells maintain sensitivity to antigens present in only low abundance on the antigen-presenting cell.