Hybrid Insulin Peptides as Antigens and Tolerogens for Pathogenic T Cells in Autoimmune Diabetes

In this review, we cover the discovery of hybrid insulin peptides (HIPs) as antigens for CD4 T cells involved in pathogenesis and regulation of autoimmune diabetes. HIPs represent a unique posttranslational modification in autoimmunity and consist of peptide sequences from two beta-cell proteins, one being proinsulin, covalently joined to form new nongenomic peptides. Using the nonobese diabetic (NOD) mouse model, we showed that HIPs are target antigens for a panel of diabetogenic CD4 T-cell clones. The prototype clone of this panel is BDC-2.5, and the first HIP identified was the peptide ligand for BDC-2.5, the 2.5HIP, consisting of an insulin C-peptide fragment combined with a natural cleavage product of chromogranin A. T cells with different TCRs, all specific for the 2.5HIP, were shown to be a dominant population among the T cells infiltrating the islets of NOD mice. T cells reactive to HIPs are significantly elevated in the PBMC of newly diagnosed patients with type 1 diabetes (T1D) and in at-risk subjects, an important finding from a clinical standpoint. When coupled to biodegradable nanoparticles (NPs), HIPs can serve as epitopes to induce antigen-specific tolerance. 2.5HIP NPs not only prevent transfer of disease by BDC-2.5 T cells but also prolong islet graft survival in diabetic NOD mice. Investigation of the mechanisms underlying 2.5HIP NP-induced tolerance revealed that protection occurs through an IL-10-dependent process in which regulatory T cells in the graft tissue are increased, limiting dendritic cell licensing and the subsequent terminal differentiation of both CD4 and CD8 islet-specific T cells.