Investigating the potential impact of post translational modification of auto-antigens by tissue transglutaminase on humoral islet autoimmunity in type 1 diabetes

BACKGROUND: Post-translational modification (PTM) of antigens plays a role in the pathogenesis of many autoimmune disorders. In coeliac disease (CD), tissue transglutaminase (tTG) deamidates gliadin peptides to activate the immune response against the gut endomysium. CD is six times more prevalent in type 1 diabetes (T1D) patients than in the general population. HYPOTHESIS: tTG also modifies auto-antigens implicated in the pathogenesis of T1D, leading to an autoimmune response to pancreatic β-cells. METHODS: tTG PTM was investigated in the following auto-antigens, which had been previously shown to have high importance in the development of T1D: glutamic acid decarboxylase isoform 65 (GAD65), full length islet antigen (IA-2), intracellular portion of IA-2 (IA-2ic), and both isoforms of zinc transporter 8 (ZnT8W and ZnT8R), on antibody binding. Radiolabelled antigen was incubated with tTG for 20 h at 37 °C in 100 mM Denver buffer, 3.33 nM CaCl(2), at pH 7.3. Antibody binding in 20 mixed samples from the Bart’s-Oxford (BOX) cohort was measured by radiobinding assay. RESULTS: Results varied between serum samples. Generally, tTG treatment of ZnT8W, ZnT8R and IA-2ic showed no significant change in antigen: autoantibody binding, while increases in binding were observed with tTG-treated GAD65 and full length IA-2. CONCLUSION: In the case of GAD65, full length IA-2, the strength of antibody: antigen binding increased after incubation with tTG. However, the exact tTG-modification events that occurred requires further elucidation.