A RESIDUE IN THE CENTER OF PEPTIDE QL9 AFFECTS BINDING TO BOTH L(D) AND THE T-CELL RECEPTOR

The TCR from the alloreactive clone 2C recognizes p2C (LSPFPFDL)/L(d) and QL9 (QLSPFPFDL)/L(d) complexes with affinities of 2 x 10(6) and 10 (7) M(-1). Recently, it was proposed that the Phe at position 4 of p2C is critical for recognition by the 2C TCR. To further characterize th e role of this peptide position in binding to L(d) and in recognition by the 2C TCR, we changed the corresponding peptide position in QL9 (p osition 5) tea other amino acids. Binding affinities of these peptides for L(d) and of the peptide/L(d) complexes for a soluble single chain TCR were determined. Unexpectedly, it was shown that this peptide pos ition has a significant effect on L(d) binding (100-fold), with positi vely charged and hydrophobic residues having a beneficial effect and n egatively charged residues having a detrimental effect. Measurements o f the binding affinities of these peptide/L(d) complexes for the 2C TC R showed that at 4 degrees C only a Tyr substitution at this position retained high affinity for the TCR. However, significant differences i n TCR binding were observed among QL9 peptide variants al 4 degrees C compared with that at 37 degrees C. The influence of this peptide posi tion on both L(d) binding and TCR binding may suggest that the 2C TCR recognizes an L(d) conformational determinant that is altered by inter actions with the residue at position 5 of QL9. A strong correlation wa s also observed between peptide-L(d) affinity and the ability of pepti des to sensitize L(d) target cells for lysis by CTL 2C. The results ap e considered in view of recent models on the relationship between T ce ll activity and TCR-peptide-MHC binding properties.