Type 1 diabetes is a chronic condition caused by the autoimmune destruction of insulin-producing pancreatic β-cells. However, the identity of the neoantigens that contribute to disease and how they are produced in the pancreas remains unclear. In this preprint (not peer reviewed), Srivastava et al. report a single-residue transformation of insulin, driven by a stressed microenvironment, that increases its autoreactivity.

The authors used an innovative β-cell degranulation assay coupled with immunopeptidomics to probe HLA-II-bound neoepitopes in human peripheral blood mononuclear cells (PBMCs) of non-diabetic and diabetic individuals after a mixed-meal tolerance test. They identified a previously unknown modification of insulin (C19S) bound to HLA-II in all groups. Further experiments using mouse and human islets showed that C19S-modified insulin localizes to the crinosomes of pancreatic β-cells, and occurs after inflammatory cytokine stimulation or endoplasmic recitulum (ER) stress. Peptidomic analyses of pancreatic crinosomes confirmed that Cys-to-Ser transformations of insulin are increased under these conditions. Coculture experiments using dispersed islets from non-diabetic or pre-diabetic NOD mice with C19S-reactive CD4+ T cell hybridomas showed that T cell proliferation is markedly upregulated at the pre-diabetic stage. Further ex vivo peptide rechallenge experiments of pancreatic cells showed that the C19S peptide induced a greater interferon-γ response than native insulin.