Protein tyrosine phosphatase-like protein IA2-antibodies plus glutamic acid decarboxylase 65 antibodies (GADA) indicates autoimmunity as frequently as islet cell antibodies assay in children with recently diagnosed diabetes mellitus

Abstract

Islet cell antibodies (ICA), the classical autoimmunity marker for insulin-dependent diabetes mellitus (IDDM), are detected in approximately 85% of children with recently diagnosed diabetes. Because the ICA assay is semiquantitative and difficult to standardize, alternative assays are needed. When glutamic acid decarboxylase 65 (GAD 65) was discovered as a major islet antigen, the measurement of antibodies to GAD 65 (GADA) was considered a good alternative to ICA. Recently, however, we showed that 1 in 3 ICA-positive diabetic patients do not have GADA. Now, antibodies against the protein tyrosine phosphatase-like protein IA2 (IA2-ab) have been detected in IDDM. To find out whether measurements of IA2-ab combined with those of GADA could detect autoimmunity to the same extent as ICA, we have measured all three kinds of antibodies (using radioligand binding assays for IA2-ab and GADA) in 100 recently diagnosed diabetic and 100 control children: ICA were found in 87, IA2-ab in 69, and GADA in 66 of the 100 diabetic patients, whereas in the 100 control children ICA were found in 2, IA2-ab in 1, and GADA in 3. Among the 87 ICA-positive patients, 45 (52%) had both IA2-ab and GADA, 21 (24%) had only IA2-ab, and 16 (18%) had only GADA, whereas 5 (6%) lacked both IA2-ab and GADA. Among the 13 ICA-negative patients, 1 (8%) had both IA2-ab and GADA, 2 (15%) had only IA2-ab, and 4 (31%) had only GADA. Thus, 6 of the 100 patients had neither ICA, IA2-ab, nor GADA. Combining the IA2-ab and GADA assays gave positive results for autoimmunity in 89 of the 100 patients, compared with 87 by the ICA assay. The combination of the IA2-ab and GADA assays appears to be an effective alternative to the ICA assay.