Affinity purification of serum-derived anti-IA-2 autoantibodies in type 1 diabetes using a novel MBP-IA-2 fusion protein

Autoantibodies targeting epitopes contained within the intracellular domain (IC) of the protein phosphatase-like islet antigen 2 (IA-2) are a common marker of autoimmune type 1 diabetes (T1D), however the isolation of genuine, serum derived anti-IA-2 autoantibodies has proven challenging due to a lack of suitable bioassays. In the current study, an ELISA format was developed for affinity purification of human anti-IA-2ic autoantibodies utilizing a fusion protein (FP) incorporating maltose binding protein and the full-length IA-2IC domain. Using a T1D patient cohort validated for anti-IA-2ic autoantibodies by commercial ELISA, we demonstrate the MBP-IA-2ic FP ELISA detects serum anti-IA-2IC autoantibodies from 3 of 9 IA-2 positive patients. Further to this, a multi-plate MBP-IA-2ic FP ELISA protocol specifically affinity purifies IgG enriched for anti-IA-2ic autoantibodies. Interestingly, serum derived autoantibodies immobilised on the MBP-IA-2ic FP ELISA demonstrate increased Kappa light chain usage when compared to the respective total IgG derived from donor patients, suggesting a clonally restricted repertoire of anti-IA-2ic autoantigen specific B plasma cells is responsible for autoantibodies detect by the MBP-IA-2ic FP ELISA. This study is the first to demonstrate the generation of specific, genuine human derived anti-IA-2ic autoantibodies, thereby facilitating further investigation into the origin and functional significance of IA-2 autoantibodies in T1D.