DIAGNOSTIC SENSITIVITY OF IMMUNODOMINANT EPITOPES OF GLUTAMIC-ACID DECARBOXYLASE (GAD65) AUTOANTIBODIES IN CHILDHOOD IDDM

The prevalence and titre of epitope-specific autoantibodies to glutami c acid decarboxylase (GAD65) in 155 insulin-dependent diabetic (IDDM) and 9 GAD65 antibody (Ab)-positive healthy children were determined us ing four GAD65/67 chimaeric molecules which discriminate among the N-t erminal (N), middle (M) and C-terminal (C) epitopes of GAD65. Radiolig and binding assays for IgG Ab used immunoprecipitation of in vitro tra nslated S-35-GAD. We found autoantibodies to GAD65 in 116 of 155 (75 % ), to GAD67 in 19 of 155 (12 %) (p < 0.0001) and to the GAD65-N-67 chi maera in 25 of 155 (16 %) (p < 0.0001) IDDM sera. GAD67Ab were found a lmost exclusively (17 of 19, 89 %) in GAD65Ab-positive sera and the le vels of GAD67Ab correlated with those of GAD65Ab (r(2)=0.5913; p=0.009 ). GAD65Ab directed to GAD65-M were found in 104 of 155 (67 %), to GAD 65-C in 104 of 155 (67 %) and to GAD65-M + C in 116 of 155 (75 %) of I DDM sera, and indicated reactivity to at least two distinct epitopes. Among the nine GAD65Ab-positive healthy children, two (22 %) were also positive with GAD67, nine (100 %) with GAD65-M + C, seven (78 %) with GAD65-M, eight (89 %) with GAD65-C and two (22 %) with GAD65-N-67. Ti tres of GAD65Ab (p = 0.007), GAD65-C-Ab (p = 0.002) and GAD65-C + M-Ab (p = 0.003), but not of GAD65-M-Ab (p = 0.101) were significantly hig her in IDDM than in healthy children. We conclude that GAD65Ab in IDDM and healthy children are directed to middle and C-terminal epitopes, and propose that levels of antibodies specifically directed to the car boxy-terminal end of GAD65 may distinguish IDDM from healthy children.