N-terminal elongation of a peptide determinant beyond the first primary anchor improves binding to H-2 I-A(d) and HLA-DR1 by backbone-dependent and aromatic side chain-dependent interactions, respectively

Abstract

The IgG2ab heavy chain allopeptide determinant y2ab 436-451 (Kabat numbering) presented by the major histocompatibility complex (MHC) class II molecule I-A(d) is recognized by T cells which cross-react with a corneal self antigen and with the UL6 protein of the herpes simplex virus which induce autoimmune keratitis, and is the target of Th1 clones that suppress IgG2ab production in vivo. In the y2ab peptide/I-A(d) complex, tyrosine438 is the first primary anchor (P1) and residues 440-445 encompass the T cell receptor contact residues. Aminoterminal elongation of y2ab 437-451 by a single residue (P-2) augmented the I-A(d) binding capacity 10-fold and the antigenicity 55-195-fold. This was a function of the peptide main chain, since non-conservative substitutions were accepted. The y2ab peptide also bound HLA-DR1, and amino-terminal extension by a single aromatic amino acid at P-3 augmented binding 15-fold. The interaction between HLA-DR1 and P-3 specifically required an aromatic peptide side chain, and computer simulations indicated that the aromatic ring at P-3 engaged conserved HLA-DR1 phenylalanine residues at the edge of the peptide binding groove. Thus, these data demonstrate that residues amino terminal to P1 may substantially increase peptide affinity for MHC class II by main chain-dependent as well as side chain-dependent interactions, and imply that the HLA-DR1 motif should be extended to include an aromatic amino acid at P-3.