Type 1.5 diabetes (1) is characterized by rapid loss of β-cell function, failure of oral agents, and acquirement of insulin requirement (2,3). These patients have islet cell antibodies (ICAs), GAD65 autoantibodies (GAD65Abs) (4,5,6), or both, indicating an underlying autoimmune pathogenesis. Therefore, the question of whether type 1.5 diabetes represents a separate clinical disease or a slowly progressive form of type 1 diabetes has been raised (7,8). The aim of the present study was to investigate whether GAD65Ab epitopes in type 1.5 diabetic patients differ from those found in type 1 diabetic patients and other GAD65Ab-positive phenotypes.

Type 1.5 diabetic patients (n = 34) were identified as GAD65Ab-positive type 2 diabetic patients as part of a screening program in the greater Seattle area. The patients were classified with type 2 diabetes according to the 1997 ADA criteria (9) and had to meet all of the following criteria: 1) age ≥30 years at diagnosis of diabetes, 2) no history of ketonuria or ketoacidosis, and 3) not requiring insulin treatment at diagnosis. All patients had been diagnosed with diabetes within 12 months of blood sampling. The GAD65Ab epitope pattern was compared with the following three groups of GAD65Ab-positive subjects described in detail elsewhere (10,11), all of whom were reanalyzed for the present investigation: type 1 diabetic patients (n = 200), first-degree relatives (n = 41), and healthy subjects (n = 28). The studies were approved by the Ethics Committee of the Karolinska Institute, Stockholm, Sweden; the Umeå University, Sweden; and the University of Washington Human Subjects Committee. All individuals gave their informed consent to participate in the study.

The GAD65Ab epitope pattern in the four groups of GAD65Ab-positive subjects was analyzed by a previously described radioimmunoassay (12,13) using recombinant 35S-GAD65/67 fusion proteins. Human GAD65, rat GAD67, and fusion GAD cDNA molecules N-, M+C, M, and C used in the present study were described previously (11,14,15).

The NH2-terminus of GAD65 was recognized by 20% (7 of 34) of the type 1.5 diabetic patients compared with 5% (10 of 200) in type 1 diabetic patients (P = 0.03). No significant difference in binding was observed compared with healthy individuals (11%, 4 of 41) and first-degree relatives (12%, 3 of 28). These data suggested that GAD65Ab reacting with the NH2-terminal epitope were common in these patients. As previously reported (11), the highest frequency of samples with GAD65Ab binding only to the M-, C-, and/or M+C fusion protein was found in type 1 diabetic patients (90%), whereas first-degree relatives (67%; P = 0.0016), healthy individuals (75%; P = 0.02), and type 1.5 diabetic patients (65%; P = 0.0003) showed significantly lower frequencies of GAD65Ab specific to those parts of GAD65.

We conclude that GAD65Ab binding to the NH2-terminus of GAD65 or to GAD67, as observed in our group of GAD65Ab-positive type 1.5 diabetic patients, is rarely found in early-onset type 1 diabetic patients. This difference in the binding pattern of GAD65Ab of type 1.5 diabetic patients compared with that of type 1 diabetic patients supports the notion that the disease process may differ between these two types of patients.

This work was supported by the Medical Research Service of the Department of Veterans Affairs, the National Institutes of Health (grants DK26190, DK53004, and DK17047), and the Juvenile Diabetes Foundation International, including a fellowship for C.S.H.

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Address correspondence to Dr. Christiane S. Hampe, Department of Medicine, University of Washington, Seattle, WA 98195. E-mail: champe@u.washington.edu.