Single MHC mutation eliminates enthalpy associated with T cell receptor binding.

Area of Science:

• Immunology
• Structural Biology
• Biochemistry

Background:

• T cell receptor (TCR) recognition of peptide-MHC (pMHC) complexes is central to adaptive immunity.
• The interaction involves a large interface, with atypical binding orientations observed.
• Understanding the molecular basis of TCR-pMHC interaction is crucial for immunology and drug development.

Purpose of the Study:

• To investigate the impact of specific major histocompatibility complex (MHC) mutations on T cell receptor (TCR) recognition.
• To determine the structural and energetic basis of altered TCR-pMHC binding affinity.
• To re-evaluate the roles of different TCR loops in MHC restriction and peptide interaction.

Main Methods:

• Crystallography to determine co-crystal structures of AHIII TCR with wild-type and mutant HLA-A2.
• Isothermal titration calorimetry (ITC) to measure binding constants and thermodynamic parameters.
• Analysis of hydrogen bonding and conformational changes at the TCR-pMHC interface.

Main Results:

• The K66A mutation in HLA-A2 significantly reduced AHIII TCR binding affinity and T cell response.
• This mutation did not alter the conformation of the presented peptide.
• A conformational change in the TCR's complementarity-determining region 3 (CDR3) loop was observed, leading to a loss of hydrogen bonds and reduced binding enthalpy.

Conclusions:

• TCR CDR3 loop conformation can be altered by MHC mutations independent of peptide conformation.
• The findings challenge the model where TCR CDR1 and CDR2 loops mediate MHC restriction and CDR3 solely interacts with the peptide.
• TCR-pMHC binding affinity may correlate with the magnitude of the T cell response.