Differential Contributions of IgM and IgG Autoantibodies to Serologic IA2 Reactivity in Type 1 Diabetes

Autoantibodies targeting islet antigen 2 (IA2) are critical diagnostic and prognostic markers for type 1 diabetes (T1D). Standard clinical assays do not differentiate between IgG and IgM isotypes, yet these antibodies have distinct roles in the T1D autoimmunity. We therefore adapted electrochemiluminescence (ECL) assays to separately detect IgG and IgM antibodies against the IA2 intracellular domain (AA601-979). Assay specificity was confirmed by indirect immunofluorescence, which showed autoantibody binding to IA2-overexpressing cells. Plasma samples were analyzed from two independent cohorts: organ donors of the Human Pancreas Analysis Program (HPAP, n = 69) and children from a Janssen–Breakthrough T1D-funded study (n = 65). Diabetics had significantly higher levels of IA2 IgG (p < 0.001) but not IgM (p > 0.05) compared with controls. Notably, IgM and IgG IA2 antibody levels were not correlated. However, IgM modulates IgG detection: IgM depletion increased detected IgG levels to IA2 in some donors, and sera from donors with high IA2-specific IgM levels reduced monoclonal IgG anti-IA2 antibody binding to IA2. Purified IgM from healthy individuals also suppressed monoclonal IgG binding. These findings support distinct, non-redundant roles for IA2-specific IgG and IgM in T1D serology. Isotype-specific autoantibody analysis may improve risk stratification and monitoring of T1D individuals receiving immunomodulatory therapies.