Improved specificity of glutamate decarboxylase 65 autoantibody measurement using luciferase-based immunoprecipitation system (LIPS) assays

Autoantibodies to glutamate decarboxylase (GADA) are widely used in the prediction and classification of type 1 diabetes. GADA radiobinding assays (RBAs) using N-terminally truncated antigens offer improved specificity but radioisotopes limit the high-throughput potential for population screening. Luciferase-based Immunoprecipitation System (LIPS) assays are sensitive and specific alternatives to RBAs with the potential to improve risk stratification. The performance of assays using the Luciferase (Nluc-) conjugated GAD₆₅ constructs, Nluc-GAD65(96-585) and full length Nluc-GAD65(1-585) were evaluated in 434 well-characterised sera from recent-onset type 1 diabetes patients and first-degree relatives. Non-radioactive, high-throughput LIPS assays are quicker and require less serum than RBAs. Of 171 relatives previously tested single autoantibody positive for autoantibodies to full-length GAD₆₅ by RBA but had not progressed to diabetes, fewer retested positive by LIPS using either truncated (n=72) or full-length (n=111) antigen. The Nluc-GAD65(96-585) truncation demonstrated the highest specificity in LIPS assays overall but in contrast to RBA, N-terminus truncations did not result in a significant increase in disease-specificity compared with the full-length antigen. This suggests that binding of non-specific antibodies is affected by the conformational changes resulting from addition of the Nluc antigen. Nluc-GAD65(96-585) LIPS assays offer low blood volume, high specificity GADA tests for screening and diagnostics.