Autoreactive CD4+ T cells are involved in the pathogenesis of many autoimmune diseases, but the antigens that stimulate their responses have been difficult to identify and in most cases are not well defined. In the nonobese diabetic (NOD) mouse model of type 1 diabetes (T1D), we have identified a peptide WE14 from chromogranin A (ChgA) as the antigen for highly diabetogenic CD4+ T cell clones. Truncation and extension analysis showed that WE14 binds to the NOD mouse MHCII molecule, I-Ag7, in an atypical manner, occupying only the C-terminal half of the I-Ag7 peptide-binding groove. This finding extends the list of T cell antigens in T1D and supports the idea that autoreactive T cells respond to unusually presented self-peptides.

A peptide derived from the insulin B chain contains a major epitope for diabetogenic CD4 + T cells in the NOD mouse model of type 1 diabetes (T1D). This peptide can fill the binding groove of the NOD MHCII molecule, IA g7 , in a number of ways or "registers." We show here that a diverse set of NOD anti-insulin T cells all recognize this peptide bound in the same register. Surprisingly, this register results in the poorest binding of peptide to IA g7 . The poor binding is due to an incompatibility between the p9 amino acid of the peptide and the unique IA g7 p9 pocket polymorphisms that are strongly associated with susceptibility to T1D. Our findings suggest that the association of autoimmunity with particular MHCII alleles may be do to poorer, rather than more favorable, binding of the critical self-epitopes, allowing T-cell escape from thymic deletion.T he development of type 1 diabetes (T1D) in NOD mice and humans is associated with certain alleles of MHCII (1-3). These alleles present sets of peptides distinct from those presented by other MHCII alleles (4, 5), thus their association with T1D may be because they are better than other MHCII alleles at presentation of peptides derived from pancreatic islet β-cell proteins (6). However, tests of this and other hypotheses about the role of MHCII in T1D require a precise identification of the peptides recognized by pathogenic CD4 + T cells.Peptides bind in an extended conformation to a groove of MHCII. The binding involves a nonamer of the peptide (p), in which the side chains of "anchor" amino acids at positions p1, p4, p6, and p9 interact with corresponding pockets in the groove (7). Polymorphic MHCII residues lining the binding groove influence the preference of each pocket for particular peptide side chains, providing each MHCII allele with a unique preferred peptidebinding motif that dictates the position or "register" of the peptide in the groove. However, the specificities of these pockets are not absolute, and so a peptide may bind in more than one register, with the hierarchy of registers determined by how well the pockets accept the anchor residues. The register of the peptide affects which of its side chains are pointing out of the MHCII groove and hence interaction with T cell receptors. Thus individual peptide-responsive T cells will recognize the peptide bound to MHCII in only one of the possible registers.The NOD mouse has a single MHCII molecule, IA g7 , which is essential for the development of disease (8). Polymorphisms in the IA g7 β-chain (9) shape the p9 binding pocket (10, 11) and contribute to a side chain preference distinct from that of other IA alleles (6). In particular, an Asp-to-Ser alteration at IA g7 β57 disrupts a conserved salt bridge to an Arg at α76. This allows the positively charged Arg to interact with the peptide amino acid side chain in the p9 pocket (11) and confers a unique preference for binding peptide registers with acidic residues at this position (6).The insulin B-chain peptide (B:12-23) is recognized by diabetogenic ...