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An important early molecular recognition event that triggers T cell mediated immune responses is the interaction of a T cell receptor (TCR) on the surface of the T cell with a heterodimeric complex displayed on the surface of pathogen-infected cells. This heterodimeric complex consists of a peptide, 8-10 amino acids in length, bound to the highly polymorphic major histocompatibility complex (MHC). A TCR binds to this heterodimeric complex (peptide-MHC) with sufficient affinity only if interactions between the TCR and certain key regions located on both the MHC and the bound peptide are favorable. On the MHC, these key regions or “hotspots” are restricted to just a few amino acids. The molecular mechanisms by which mutations of these MHC “hotspot” residues influence TCR/peptide-MHC binding are not well understood.Molecular Dynamics simulations coupled with free energy calculations based on the inverse form of the Potential Distribution Theorem were carried out to evaluate the effect of single-amino acid mutations of the MHC “hotspot” residues on the binding affinity of the A6 TCR to the HLA-A2 MHC complexed with the Tax peptide of the Type I T lymphotropic virus. In agreement with experimental observations, this analysis reveals a strong influence of the MHC “hotspot” residue mutations on TCR/peptide-MHC binding affinity. Also, the changes in TCR binding affinities resulting from the MHC “hotspot” mutations are compared to those resulting from mutations of key amino acids in the bound peptide of the peptide-MHC complex in order to provide a quantitative comparison of the relative contributions of the peptide and the MHC to the TCR binding affinity. These comparisons permit a detailed thermodynamic analysis of the effect of mutations on TCR molecular recognition of peptide-MHCs. |
