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

The IgG2a(b) heavy chain allopeptide determinant gamma2a(b) 436-451 (Kabat numbering) presented by the major histocompatibility complex (MHC) class II molecule I-Ad 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 IgG2a(b) production in vivo. In the gamma2a(b) peptide/l-Ad complex, tyrosine438 is the first primary anchor (P1) and residues 440-445 encompass the T cell receptor contact residues. Amino-terminal elongation of gamma2a(b) 437-451 by a single residue (P-2) augmented the I-Ad 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 gamma2a(b) 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.