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From * the Walter and Eliza Hall Institute of Medical Research, Royal Melbourne Hospital, Parkville
3050, Australia; and The class II major histocompatibility complex molecule I-Ag7 is strongly linked to the development of spontaneous insulin-dependent diabetes mellitus (IDDM) in non obese diabetic mice
and to the induction of experimental allergic encephalomyelitis in Biozzi AB/H mice. Structurally,
it resembles the HLA-DQ molecules associated with human IDDM, in having a non-Asp residue at position 57 in its
Roche Milano Ricerche, 1-20132, Milano, Italy
chain. To identify the requirements for peptide binding to I-Ag7 and
thereby potentially pathogenic T cell epitopes, we analyzed a known I-Ag7-restricted T cell
epitope, hen egg white lysozyme (HEL) amino acids 9-27. NH2- and COOH-terminal truncations demonstrated that the minimal epitope for activation of the T cell hybridoma 2D12.1 was M12-R21 and the minimum sequence for direct binding to purified I-Ag7 M12-Y20/
K13-R21. Alanine (A) scanning revealed two primary anchors for binding at relative positions
(p) 6 (L) and 9 (Y) in the HEL epitope. The critical role of both anchors was demonstrated by incorporating L and Y in poly(A) backbones at the same relative positions as in the HEL
epitope. Well-tolerated, weakly tolerated, and nontolerated residues were identified by analyzing the binding of peptides containing multiple substitutions at individual positions. Optimally,
p6 was a large, hydrophobic residue (L, I, V, M), whereas p9 was aromatic and hydrophobic (Y
or F) or positively charged (K, R). Specific residues were not tolerated at these and some other
positions. A motif for binding to I-Ag7 deduced from analysis of the model HEL epitope was
present in 27/30 (90%) of peptides reported to be I-Ag7-restricted T cell epitopes or eluted
from I-Ag7. Scanning a set of overlapping peptides encompassing human proinsulin revealed
the motif in 6/6 good binders (sensitivity = 100%) and 4/13 weak or non-binders (specificity = 70%). This motif should facilitate identification of autoantigenic epitopes relevant to the pathogenesis and immunotherapy of IDDM.
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