Role of hapten-anchoring peptides in defining hapten-epitopes for MHC-restricted cytotoxic T cells. Cross-reactive TNP-determinants on different peptides.

Synthetic hapten-peptide conjugates selectively modify cell-bound MHC class I molecules in a haplotype-specific way. We investigated the contribution of the carrier peptides to the structural specificity of T cell-antigenic TNP epitopes, using different H-2Kb-binding TNP-peptides and a collection of TNP/Kb-specific CTL clones. Adjustment of peptide sequences to the proposed Kb-specific "motif" (octamers with F or Y and L in positions 5 and 8, respectively) enhanced Kb-binding and antigenicity by many orders of magnitude. Moreover, several clones reacted to peptides, containing the "motif" and TNP-lysine in position 4 but were otherwise unrelated by sequence. TNP in other positions was not recognized by these cells, but other CTL reacted to TNP in position 7. This points to the positioning of hapten determinants within the MHC binding groove as a major role of the anchoring peptide. However, determination of the limiting amounts of TNP peptides that elicit antigenicity or inhibit other Kb-restricted CTL reactions revealed that TCR also recognize variations in the sequences of carrier peptides. This contribution is low for TNP in position 4 but high in position 7, indicating lysine in position 4 as a particularly dominant and cross-reactive hapten-anchoring site in Kb-associated peptides. This implies that cell modification with lysine-reactive TNP reagents results in immunodominant, highly repetitive TNP epitopes, which may explain the strong antigenicity and the allergenic properties of TNP, as well as the restricted TCR repertoire directed against this hapten. Our data further recommend hapten peptides for general studies of TCR-Ag interactions because in contrast to pure protein Ag, hapten epitopes tolerate substantial structural variations in the MHC-anchoring peptide, and can be located by hapten-specific antibodies.