Presence of Insulin Autoantibodies as Regular Feature of Nondiabetic Repertoire of Immunity

With an ultrasensitive noncompetitive enzyme-linked immunosorbent assay (ELISA), we tested the hypothesis that the presence of insulin autoantibodies in nondiabetic individuals is a normal event. Plasma and peripheral blood mononuclear cells were obtained from 50 nondiabetic whites for determination of insulin autoantibodies by ELISA and radioimmunoassay (anti-insulin IgG [AI-IgG] and ¹²⁵I-labeled insulin bound [%]), islet cell antibodies, anti-nuclear antibodies and rheumatoid factor, and HLA class II–type antigens (DR, DRw, and DQ). The range of ¹²⁵I-insulin binding was significantly < was seen in pretreatment sera from individuals with diabetes (from −0.4 to 0.4% vs. −0.8 to 7.7%, respectively, P = 0.001). Eighty-eight percent of these nondiabetic individuals had significant levels of AI-IgG with preferential binding to human insulin. The geometric mean of AI-IgG concentrations in individuals with significant levels was 180 pM. Binding to human insulin was seen in 88%, to pork insulin in 42%, and to beef insulin in 24% of individuals (P < 0.001 overall; P < 0.05 where more bound to pork than beef insulin). Binding of AI-IgG to human insulin–coated plates was substantially inhibited by preincubation with human insulin (median inhibition 57.6%) with little if any inhibition by glucagon, C-peptide, albumin, or IgG. Four individuals had highly specific human AI-IgG as shown by immunoaffinity studies. AI-IgGs were significantly higher in individuals with the HLA haplotype DR4,DRw53,DQ3 and lower in individuals with DR5,DRw52,DQ1 (P = 0.03 for both). Nondiabetic individuals regularly have elevated levels of human insulin–specific AI-IgG in their plasmas, the magnitude of which may be related to HLA class II–type antigens.