A structural basis of T cell cross-reactivity to native and spliced self-antigens presented by HLA-DQ8.

Type 1 diabetes (T1D) is a T cell-mediated autoimmune disease that has a strong HLA association, where a number of self-epitopes have been implicated in disease pathogenesis. Human pancreatic islet-infiltrating CD4 T cell clones not only respond to proinsulin C-peptide (PI GQVELGGGPGAGSLQ) but also cross-react with a hybrid insulin peptide (HIP; PI-IAPP GQVELGGG-NAVEVLK) presented by HLA-DQ8. How T cell receptors recognize self-peptide and cross-react to HIPs is unclear. We investigated the cross-reactivity of the CD4 T cell clones reactive to native PI epitope and multiple HIPs fused at the same N-terminus (PI) to the degradation products of two highly expressed pancreatic islet proteins, neuropeptide Y (NPY) and amyloid polypeptide (IAPP and IAPP). We observed that five out of the seven selected SKW3 T cell lines expressing TCRs isolated from CD4 T cells of people with T1D responded to multiple HIPs. Despite shared TRAV26-1-TRBV5-1 gene usage in some T cells, these clones cross-reacted to varying degrees with the PI and HIP epitopes. Crystal structures of two TRAV26-1-TRBV5-1 T cell receptors (TCRs) in complex with PI and HIPs bound to HLA-DQ8 revealed that the two TCRs had distinct mechanisms responsible for their differential recognition of the PI and HIP epitopes. Alanine scanning mutagenesis of the PI and HIPs determined that the P2, P7, and P8 residues in these epitopes were key determinants of TCR specificity. Accordingly, we provide a molecular basis for cross-reactivity towards native insulin and HIP epitopes presented by HLA-DQ8.