How C-terminal additions to insulin B-chain fragments create superagonists for T cells in mouse and human type 1 diabetes

In type 1 diabetes, mouse and human CD4+ T cells recognize mutated and chimeric superagonist insulin B-chain epitopes similarly. Superagonist diabetogenic peptides Type 1 diabetes is an autoimmune disease triggered, in part, by activation of CD4+ T cells specific for insulin-derived autoantigens. Wang et al. prepared ternary complexes of diabetogenic T cell receptors attached to ligands formed by binding of insulin B-chain peptides to mouse or human MHC class II proteins known to be associated with genetic susceptibility to diabetes. Structural and functional analyses revealed that fusion peptides featuring covalent linkage of the C terminus of an insulin B-chain peptide to specific fragments of insulin C-peptide were highly potent ligands, enabling avid binding of both mouse and human TCRs. These findings point to transpeptidation reactions in lysosomes as a potential source of superagonist chimeric insulin peptides with an important driver role in autoimmune diabetes. In type 1 diabetes (T1D), proinsulin is a major autoantigen and the insulin B:9-23 peptide contains epitopes for CD4+ T cells in both mice and humans. This peptide requires carboxyl-terminal mutations for uniform binding in the proper position within the mouse IAg7 or human DQ8 major histocompatibility complex (MHC) class II (MHCII) peptide grooves and for strong CD4+ T cell stimulation. Here, we present crystal structures showing how these mutations control CD4+ T cell receptor (TCR) binding to these MHCII-peptide complexes. Our data reveal stricking similarities between mouse and human CD4+ TCRs in their interactions with these ligands. We also show how fusions between fragments of B:9-23 and of proinsulin C-peptide create chimeric peptides with activities as strong or stronger than the mutated insulin peptides. We propose transpeptidation in the lysosome as a mechanism that could accomplish these fusions in vivo, similar to the creation of fused peptide epitopes for MHCI presentation shown to occur by transpeptidation in the proteasome. Were this mechanism limited to the pancreas and absent in the thymus, it could provide an explanation for how diabetogenic T cells escape negative selection during development but find their modified target antigens in the pancreas to cause T1D.