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N-terminal additions to the WE14 peptide of chromogranin A create strong autoantigen agonists in type 1 diabetes

Significance Type 1 diabetes is an autoimmune disease in which the insulin-producing beta cells within the islets of Langerhans of the pancreas are destroyed by T cell-mediated immune attack. The peptide epitopes derived from islet proteins that are targeted by CD4+ T cells have been difficult to determine. We show in the nonobese diabetic (NOD) mouse model of the disease that a peptide (WE14) derived from chromogranin A is likely posttranslationally modified to create a target epitope. We hypothesize that the modification is caused by transpeptidation in which other peptides are fused to the N terminus of WE14. We propose that, in autoimmunity, new epitopes created in the target organs can be attacked by T cells that are normally nonreactive to natural self-antigens. Chromogranin A (ChgA) is an autoantigen for CD4+ T cells in the nonobese diabetic (NOD) mouse model of type 1 diabetes (T1D). The natural ChgA-processed peptide, WE14, is a weak agonist for the prototypical T cell, BDC-2.5, and other ChgA-specific T-cell clones. Mimotope peptides with much higher activity share a C-terminal motif, WXRM(D/E), that is predicted to lie in the p5 to p9 position in the mouse MHC class II, IAg7 binding groove. This motif is also present in WE14 (WSRMD), but at its N terminus. Therefore, to place the WE14 motif into the same position as seen in the mimotopes, we added the amino acids RLGL to its N terminus. Like the other mimotopes, RLGL-WE14, is much more potent than WE14 in T-cell stimulation and activates a diverse population of CD4+ T cells, which also respond to WE14 as well as islets from WT, but not ChgA−/− mice. The crystal structure of the IAg7–RLGL–WE14 complex confirmed the predicted placement of the peptide within the IAg7 groove. Fluorescent IAg7–RLGL–WE14 tetramers bind to ChgA-specific T-cell clones and easily detect ChgA-specific T cells in the pancreas and pancreatic lymph nodes of NOD mice. The prediction that many different N-terminal amino acid extensions to the WXRM(D/E) motif are sufficient to greatly improve T-cell stimulation leads us to propose that such a posttranslational modification may occur uniquely in the pancreas or pancreatic lymph nodes, perhaps via the mechanism of transpeptidation. This modification could account for the escape of these T cells from thymic negative selection.