An altered peptide of Chromogranin A is a potent antigen for the diabetogenic T cell clone BDC-2.5 (44.21)

Abstract

Abstract Pathogenesis of type 1 diabetes is mediated by autoreactive T cells directed toward antigens in islet beta cells. We have recently identified Chromogranin A (ChgA) as the antigen target for three diabetogenic CD4+ T cell clones that were derived from the non-obese diabetic mouse, including the clone BDC-2.5. These clones respond to WE14, a natural cleavage product of ChgA, but only at very high peptide concentrations, suggesting that WE14 may not be the natural T cell ligand. Our question in this study was to determine whether a post-translational modification (PTM) could be involved in the formation of the natural ligand for the T cell clones. Although WE14 is expressed in beta cells, we were not able to identify the complete peptide sequence in purified antigenic fractions from beta cell tumors. Mass spectrometric analysis revealed the presence of an amino acid sequence corresponding only to the C-terminal region of WE14. Glutamine, which is in the N-terminal region of WE14, is a potential target for the enzyme transglutaminase. We treated the peptide WE14 with transglutaminase and demonstrated that this treatment significantly increased the antigenicity. Transglutaminases are known to deamidate or covalently crosslink proteins and we are currently investigating whether those modifications can be found in the natural antigen from beta cells. If transglutaminase is required for the generation of autoantigenic T cell epitopes, it could be an attractive therapeutic target.