Alloreactive T Cell Recognition of MHC Class I Molecules the T Cell Receptor Interacts with Limited Regions of the MHC Class I Long α Helices

T cells recognize MHC-bound peptide, suggesting that the TCR contacts surface MHC residues adjacent to bound peptide, but the extent of MHC contact is not known. T cells also may recognize peptide-induced conformational changes, and alloreactive T cells may recognize surface MHC structures in addition to or independent of bound peptide. Alloreactive T cells are not intentionally biased to recognize particular MHC-bound peptides and should reveal general constraints for TCR binding. To map TCR binding sites, we tested 60 HLA-B7 site-specific mutations with 12 alloreactive CTL clones that express different TCRs. The alloreactive CTL clones recognize solvent-accessible residues that cluster between positions 62 to 80 and 150 to 170. Thus, TCRs contact largely overlapping MHC structures in the α₁ and α₂ domain long α helices. The dimensions and location of this site are consistent with recently reported crystallographic studies of two TCR/peptide-MHC class I complexes. In contrast to TCR, Abs recognize multiple discrete epitopes that encircle the peptide binding groove and potentially encompass the entire surface of the MHC molecule. Our data suggest that TCRs dock with a common discrete MHC site and that recent crystallographic models are likely to be generally applicable to T cell recognition of peptide-MHC class I complexes.