The Binding Energetics of a T-Cell Receptor Show a Bias Toward the Conserved Antigen Presentation Molecule, HLA

Abstract

T-cell receptors (TCRs) are heterodimeric receptors on the surface of T-cells with Complementarity Determining Region (CDR) loops similar to immunoglobulins. Their ligands are peptides presented by Major-Histocompatiblity Complexes (MHCs) on the surface of most nucleated cells. The TCR binds to an MHC presenting an antigenic peptide with a dramatically stronger affinity than MHCs presenting “self” peptides despite the fact that the majority of the interface is conserved between the two. To assess the energetic contributions of different portions of the TCR-pMHC interface, we quantify the contributions to binding of the side-chain contacts between the residues at the interface through alanine double-mutant cycles. The interaction energy between those residues is defined as native free energy change minus the free energy changes of the two single mutants, plus the free energy change of the double-mutant; ddGoint = dGo(Xwt-Ywt) - dGo(Xz->a-Ywt) - dGo(Xwt-Yz->a) + dGo(Xz->a-Yz->a). Our results for the A6 TCR and the tax9 peptide show that contrary to expectations, the contacts between the central CDR3 loops and the peptide do not have a unique energetic importance, but CDR3α has a “hot-spot” ∼3 kcal/mol interaction with the MHC, HLA-A2. This interaction motif between a positively charged residue in the α1 helix of the MHC and a negatively charged residue in the CDR3α loop appears to be commonly utilized based on a comparison of TCR-pMHC x-ray crystal structures. Additionally, CDR1α and β both make significant hydrogen bonds to the peptide. These data show that the energetic basis for T-cell recognition is not parsed into recognition of the peptide by CDR3 and the MHC by CDR 1 and 2 but rather that TCRs bind a composite interface.