Lack of strict correlation of functional sensitization with the apparent affinity of MHC/peptide complexes for the TCR

We describe a comprehensive analysis of the effect of acidity of TCR- MHC/peptide interaction on activation of the alloreactive 2C CTL clone, which recognizes H-2L(d) plus an octamer peptide (p2Ca). In this study, monosubstituted variants of p2Ca were used and assessed for binding to purified H-2L(d), binding of H-2L(d)/peptide complexes to sTCR, and ability to activate 2C cells to two independent effector functions. Among the >20 variants analyzed, functional activity of most peptides that bound the MHC well correlated with the strength of interaction of MHC/peptide complexes with sTCR. However, with some variants, a clear discordance between the apparent TCR-MHC/peptide affinity and biologic function was observed, demonstrating that the former cannot always be gauged by the latter. In the case of L4 peptide (phenylalanine at position 4 substituted with leucine), peptide/MHC complexes showed no detectable binding to sTCR, indicating a 10- fold or greater decrease in affinity. Nevertheless, this peptide sensitized target cells for lysis at a level equivalent to the parental peptide. A clearer understanding was revealed by studying the extent to which activation by variant peptides was dependent on CD8. Our data indicate that resistance to anti-CD8 mAb blocking correlates with strong binding affinity between sTCR and MHC/peptide complexes. These data suggest that, for the activation of CTL function, the absolute level of intrinsic affinity of TCR for MHC/peptide ligand is not a single critical determinant, but rather, that activation is governed by the compound influence of several factors, which ensures a minimum threshold of intracellular triggering is reached to elicit the response.