High-affinity
ligands of non-peptidic nature, binding to the class I major histocompatibility complex
protein HLA B*2705 whose expression is strongly linked to the pathogenesis of the
autoimmune disease ankylosing spondylitis, should give way to a selective
immunotherapy by blocking or antagonising the interaction with autoreactive T cell clones. Here we present experimental data on the binding of modified
peptides, designed to optimally bind to
HLA-B*2705 by filling a hydrophobic binding pocket (pocket D) with nonencoded
aromatic amino acids. Three
peptides with altered side chains (alpha-naphthylalanine, betanaphthylalanine and
homophenylalanine) in position 3 were synthesised. The thermal denaturation profiles of the HLA
protein in complex with the modified
peptides, monitored by circular dichroism spectroscopy, showed a significant shift towards higher melting temperatures with respect to the parent
T cell epitope. The proposed binding mode of the nonnatural
peptides was checked by site-directed mutagenesis of the pocket D, hypothesised to accommodate the large hydrophobic side chains. Reducing the size and depth of the pocket by mutating Leu 156 into Trp only affects the binding of the non-natural
ligands, thus providing experimental evidence that the nonnatural
peptide amino acids bind as predicted to the host MHC
protein.