γδ T Cell Receptors Recognize the Non-classical Major Histocompatibility Complex (MHC) Molecule T22 via Conserved Anchor Residues in a MHC Peptide-like Fashion

The molecular mechanisms by which gamma delta T cells recognize ligand remain a mystery. The non-classical MHC molecule T22 represents the best characterized ligand for murine gamma delta T cells, with a motif (W ... EGYEL) present in the gamma delta T cell receptor complementary-determining region 3 delta (CDR3 delta) loop mediating gamma delta T cell recognition of this molecule. Produced through V(D)J recombination, this loop is quite diverse, with different numbers and chemical types of amino acids between Trp and EGYEL, which have unknown functional consequences for T22 recognition. We have investigated the biophysical and structural effects of CDR3 delta loop diversity, revealing a range of affinities for T22 but a common thermodynamic pattern. Mutagenesis of these CDR3 delta loops defines the key anchor residues involved in T22 recognition as W ... EGYEL, similar to those found for the G8 CDR3 delta loop, and demonstrates that spacer residues modulate but are not required for T22 recognition. Comparison of the location of these residues in the T22 interface reveals a striking similarity to peptide anchor residues in classically presented MHC peptides, with the key Trp residue of the CDR3 delta motif completing the deficient peptide-binding groove of T22. This suggests that gamma delta T cell recognition of T22 utilizes the conserved ligand-presenting nature of the MHC fold.