The antigenic landscape of autoimmune diabetes reflects a failure to preserve self-tolerance, yet how novel neoantigens emerge in humans remains incompletely understood. Here we designed an immunopeptidomics-based approach to probe HLA-II-bound, islet-derived neoepitopes in patients with type 1 diabetes. We uncovered a Cys→Ser transformation, conserved between mice and humans, that reshapes autoreactivity to insulin at the single-residue level. This transformation, which we call C19S, arises from oxidative remodeling of insulin in stressed pancreatic islets and also occurs in cytokine-activated antigen-presenting cells, contributing to a feed-forward loop of neoepitope formation and presentation. Despite involving just one amino acid, C19S is recognized by HLA-DQ8-restricted, register-specific CD4+ T cells that expand at diabetes onset. These neoepitope-specific CD4+ T cells lack regulatory potential but acquire a poised central memory phenotype that persists throughout disease progression. These findings reveal a distinct, microenvironment-driven route of neoantigen formation that fuels sustained autoreactivity in diabetes.
© 2025. The Author(s).