A novel first primary anchor extends the MHC class II I-Ad binding motif to encompass nine amino acids.

The MHC class II molecule I-Ad has been reported to bind peptides containing a motif of six consecutive amino acids. We demonstrate that binding of the murine IgG2ab heavy chain allopeptide gamma 2ab 435-451 (Kabat numbering) to I-Ad is strongly enhanced by a novel first primary anchor (P1) three residues N-terminal to this hexamer. This is based on flow cytometric assessment of the I-Ad binding capacity of gamma 2ab peptide analogues, their antigenicity for I-Ad-restricted T cell clones and molecular modelling. The P1 pocket is broadly specific since allphatic, aromatic, acidic, the basic histidine and small polar side chains all allowed good binding. By contrast, asparagine, arginine and glycine reduced the binding capacity 10-, 16- and > 100-fold respectively. Truncation or glycine substitution at P1 decreased antigenicity by a factor > 1000. Nevertheless, I-Ad-restricted T cells are not completely dependent on this anchor since high concentrations of a peptide with glycine-substituted P1 elicited maximal responses. Additional anchoring side chains are found at P4, P6 and P9. The autologous IgG2aa heavy chain shares prominent epitopic residues with gamma 2ab 435-451 at P3, P5 and P8. However, the lysine of gamma 2aa at P9 impairs binding to I-Ad, which may explain why the gamma 2ab allopeptide-reactive T cells escaped negative selection. The data rationalize our observation (Bartnes, K. and Hannestad, K. 1997. Eur. J. Immunol. 27:1124) that these T cells recognize a syngeneic B cell lymphoma, provided its presentation of intrinsic gamma 2aa is enhanced by surface IgG2aa ligation.