Journal of Autoimmunity (1995) 8, 901-914 Reactivity of Islet Cell Antibodies (ICA) to Recombinant Glutamic Acid Decarboxylase (GAD) Laura Guazzarotti, Charles Thivolet*, Isabelle Tardivel, Annie Chevalier, European Prediabetes Study Group and Jean-Claude Care1 INSERM U342 and University Paris V, Department of Pediatric Endocrinology, H6pital Saint Vincent de Paul, Paris; *INSERM U197, Lyon, France (Received 27January 1995 and accepted 21 June 1995) To study the heterogeneity of islet cell antibodies (ICA), recombinant rat and human GAD65 expressed as bacterial fusion proteins were used to inhibit ICA reactivity in sera f~om recent onset type 1 diabetic children and ICA-positive first degree relatives of diabetic patients. Rat GAD65 was expressed as a fusion protein in the expression vector RSET and inhibited ICA (GAD+ICA) in 26% of 23 recent onset patients, 29% of 14 ICA positive first degree relatives (FDR) who progressed to diabetes (prediabetics) and 50% of 20 FDR who did not progress to diabetes 18 months to 5 years (31 + 14 months) after collection of the sample. GAD+ICA were inversely associated with the presence of insulin auto- antibodies (IAA) (P=0.006). GAD antibodies (GAD-Ab) were also detected by irnmunoprecipitation of in vitro transcribed and translated [3sS] methionine-labelled human GAD65. GAD-Ab were present in 83% of recent onset patients, 86% of prediabetics and 95% of the relatives who did not progress to diabetes. The level of GAD-Ab was higher in the presence of GAD+ICA (1.39+0.57 vs 0.79±0.6 index units; P--0.001). ICA levels were higher in GAD-Ab negative than in GAD-Ab positive sera (377 ± 256 vs 195 ± 231 JDFU; P=0.03). Our results confirm that a recombinant GAD65 fusion protein can be used to detect ICA heterogeneity. However, neither inhibition of ICA with recombinant GAD, nor direct detection of GAD-Ab improved the prediction of progression to clinical diabetes in ICA positive FDR. © 1995 Academic Press Limited Corresponding author: Dr Jean-Claude Carel, INSERM U342, H6pital Saint Vincent de Paul, 82 av Denfert Rochereau, 75014 Paris, France. 901 0896-8411/95/060901 + 14 $12.00/0 © 1995 Academic Press Limited