Positive selection of an H2-M3 restricted T cell receptor

Area of Science:

• Immunology
• Molecular Biology
• T cell biology

Background:

• Positive selection of thymocytes ensures T cell antigen receptors (TCRs) recognize self-major histocompatibility complex (MHC) molecules.
• The precise role of MHC-bound peptides in this crucial selection process remains debated.
• Major histocompatibility complex class Ib (MHC Ib) molecules present peptides distinct from classical MHC class I/II molecules.

Purpose of the Study:

• To identify the specific peptide ligand responsible for positive selection mediated by the MHC class Ib molecule H2-M3.
• To investigate the characteristics of this peptide ligand and its implications for T cell development and peripheral tolerance.
• To explore the impact of epitope density on the promiscuity of positive selection.

Main Methods:

• Generation of a TCR transgenic mouse model with a TCR specific for the H2-M3 molecule.
• Characterization of H2-M3 binding peptides using the established transgenic system.
• Identification of a specific peptide derived from NADH dehydrogenase subunit 1 (ND1) as the physiological ligand.

Main Results:

• H2-M3 was confirmed as the positively selecting MHC molecule.
• A unique ND1-derived peptide was identified as the physiological ligand for positive selection.
• This peptide ligand exhibits no obvious sequence homology to its cognate peptide and is ubiquitously expressed.
• Despite ubiquitous expression, the ND1 peptide does not impair peripheral T cell function.
• High epitope densities were observed to induce promiscuous positive selection.

Conclusions:

• The study elucidates a specific peptide ligand (ND1-derived) for positive selection mediated by H2-M3, resolving a long-standing controversy regarding peptide involvement.
• The findings highlight the specificity of T cell selection, even with ubiquitously expressed ligands, and suggest mechanisms for maintaining peripheral tolerance.
• The observation that high epitope densities lead to promiscuous selection provides new insights into the regulation of T cell development and potential autoimmunity.