Recognition of Posttranslationally Modified GAD65 Epitopes in Subjects With Type 1 Diabetes

Abstract

Posttranslational modification (PTM) of self-proteins has been shown to elicit clinically relevant immune responses in rheumatoid arthritis and celiac disease. Accumulating evidence suggests that recognition of modified self-proteins may also be important in type 1 diabetes. Our objective was to identify posttranslationally modified GAD65 peptides, which are recognized by subjects with type 1 diabetes, and to assess their disease relevance. We show that citrullination and transglutamination of peptides can enhance their binding to DRB1*04:01, a diabetes-susceptible HLA allele. These and corresponding modifications to amino acids at T-cell contact positions modulated the recognition of multiple GAD65 peptides by self-reactive T cells. Using class II tetramers, we verified that memory T cells specific for these modified epitopes were detectable directly ex vivo in the peripheral blood of subjects with type 1 diabetes at significantly higher frequencies than healthy controls. Furthermore, T cells that recognize these modified epitopes were either less responsive or nonresponsive to their unmodified counterparts. Our findings suggest that PTM contributes to the progression of autoimmune diabetes by eliciting T-cell responses to new epitope specificities that are present primarily in the periphery, thereby circumventing tolerance mechanisms.

Autoreactive T cells in type 1 diabetes

“…A PTM epitope exists in the insulin A chain (A1-A13): T cell recognition requires oxidized cysteine residues at A6 and A7, with the formation of a vicinal disulfide bond between them (60). Other forms of PTM include citrullination (163) and transglutamination, which enhance GAD65 peptide binding to HLA-DRB1*04:01 and modulate recognition by modifying amino acids at TCR contact positions (164). Memory T cells reacting with these PTM epitopes were detected in blood at a higher frequency in T1D patients than controls.…”

Endoplasmic reticulum stress and the unfolded protein response in pancreatic islet inflammation

“…1) (Rondas et al 2015). The presence of citrullinated proteins correlates with activation of the immune system in autoimmune diseases (Blass et al 2001, Shoda et al 2011 and increased response to citrullinated GAD65 peptides is observed in T1D patients (McGinty et al 2014). Another evidence that ER stress response may influence β-cell antigen presentation is delayed appearance of autoantibodies observed in NOD mice knockout for Chop (Satoh et al 2011).…”

NETosis-associated serum biomarkers are reduced in type 1 diabetes in association with neutrophil count

“…In anti-neutrophil cytoplasmic antibody (ANCA)-positive vasculitis and SLE, there is direct evidence for production of autoantibodies targeting NETassociated proteins [13], while anti-citrullinated protein autoantibodies (ACPA), which are linked closely to and predictive of RA, may occur in response to the posttranslationally modified protein content in NETs [14]. Immune responses against antigens that are posttranslationally modified, including by citrullination, have also been reported in T1D [15,16]. Whether these autoantigen modifications have any dependence on NETosis or are unrelated events occurring in beta cells or antigenpresenting cells is not known.…”