Class I MHC molecules bind
peptides in the endoplasmic reticulum and present them at the cell surface to circulating CD8+ T cells for analysis. We have examined binding of
peptides and stabilization of
HLA-Aw68 class I molecules using synthetic
peptide variants of an influenza virus
nucleoprotein peptide, NP91-99 (KTGGPIYKR). We have demonstrated that insertion of increasing numbers of alanines in the center of the
peptide (between P and I), to increase a natural bulging out of the
peptide-binding cleft, results in a large decrease in thermal stability. Although there is a great decrease in the t 1/2 of the MHC/
peptide complex for NP-1A compared with NP91-99, a T cell line, stimulated by NP91-99, recognizes NP-1A efficiently.
Peptide variants with three or more alanines do not show saturable binding to
HLA-Aw68 and also are not recognized by the T cell line. Thermal studies show that
polyalanine peptides with minimal anchors and nearly all TCR contact residues exchanged stabilized
HLA-Aw68 to high temperatures. Additionally, some of these
polyalanine peptides are recognized by T cell lines generated against NP91-99. Analysis of the
peptide sequences show that the stabilization effects are not due to the hydrophobicity of the
peptide. These data suggest that the strength of binding of
peptides to
HLA-Aw68 is not only dictated by the presence of anchor residues but also by the lack of unfavorable contacts between the
peptide ligand and class I MHC-binding cleft.