Autoantibodies against the glial glutamate transporter GLT1/EAAT2 in Type 1 diabetes mellitus-Clues to novel immunological and non-immunological therapies

Islet cell surface autoantibodies were previously found in subjects with type 1 diabetes mellitus (T1DM), but their target antigens and pathogenic mechanisms remain elusive. The glutamate transporter solute carrier family 1, member 2 (GLT1/EAAT2) is expressed on the membrane of pancreatic 13-cells and physiologically controls extracellular glutamate concentrations thus preventing glutamate-induced 13-cell death. We hypothesized that GLT1 could be an immunological target in T1DM and that autoantibodies against GLT1 could be pathogenic. Immunoprecipitation and ELISA experiments showed that sera from T1DM subjects recognized GLT1 expressed in brain, pancreatic islets, and GLT1-transfected COS7-cell extracts. We validated these findings in two cohorts of T1DM patients by quantitative immunofluorescence assays. Analysis of the combined data sets indicated the presence of autoantibodies against GLT1 in 32 of the 87 (37%) T1DM subjects and in none of healthy controls (n = 64) (p < 0.0001). Exposure of pancreatic beta TC3 cells and human islets to purified IgGs from anti-GLT1 positive sera supplemented with complement resulted in plasma membrane ruffling, cell lysis and death. The cytotoxic effect was prevented when sera were depleted from IgGs. Furthermore, in the absence of complement, 6 out of 16 (37%) anti-GLT1 positive sera markedly reduced GLT1 transport activity in beta TC3 cells by inducing GLT1 internalization, also resulting in beta-cell death. In conclusion, we provide evidence that GLT1 is a novel T1DM autoantigen and that anti-GLT1 autoantibodies cause beta-cell death through complement-dependent and independent mechanisms. GLT1 seems an attractive novel therapeutic target for the prevention of beta-cell death in individuals with diabetes and prediabetes.

Automated summary

Islet cell surface autoantibodies have been found in subjects with type 1 diabetes mellitus (T1DM), but the specific antigens and mechanisms involved are still unknown. This study explored the potential immunological target of the glutamate transporter solute carrier family 1, member 2 (GLT1/EAAT2) in T1DM and the pathogenicity of autoantibodies against GLT1. The results showed the presence of autoantibodies against GLT1 in a significant percentage of T1DM patients and demonstrated that these autoantibodies caused beta-cell death through complement-dependent and independent mechanisms. GLT1 could be a promising therapeutic target for the prevention of beta-cell death in individuals with diabetes and prediabetes.