Diabetes is classified clinically into two types: type 1 and type 2 diabetes. Type 1 diabetes is an autoimmune diabetes, whereas, in contrast, type 2 diabetes is nonautoimmune. However, there is a group of phenotypic adult type 2 diabetic patients (ϳ10%) who have islet autoantibodies similar to type 1 diabetes. These patients are said to have latent autoimmune diabetes in adults (LADA) or type 1.5 diabetes. T-cells reacting with islet proteins have been demonstrated in type 1 and type 1.5 diabetic patients. In contrast, classic autoantibody-negative type 2 diabetic patients are also negative for T-cell responses to islet proteins. Therefore, we questioned whether type 1 and type 1.5 diabetes are similar or different autoimmune diseases. We have investigated the immunological and metabolic differences between type 1, type 1.5, and classic type 2 diabetic patients. We have identified autoantibody differences, differences in islet proteins recognized by T-cells, and differences in insulin resistance. We have also identified a small group of patients who have T-cells responsive to islet proteins but who are autoantibody negative. These patients appear to be similar to type 1.5 patients in having decreased stimulated C-peptide values. These immunological differences between type 1 and type 1.5 diabetes suggest at least partially distinct disease processes. Diabetes 54 (Suppl. 2):S62-S67, 2005 T he finding nearly simultaneously by two groups in 1974 (1,2) that islet cell antibodies (ICAs) were common in the sera of patients with type 1 diabetes (type 1 diabetes) provided strong evidence that the ␤-cell lesion of type 1 diabetes was autoimmune in nature. Shortly thereafter, it was published that ϳ11% of patients with type 2 diabetes were also positive for ICAs and that this ICA ϩ subset of type 2 diabetic patients tended to fail sulfonylurea therapy and needed insulin treatment earlier than ICA Ϫ type 2 diabetic patients (3). Many other groups have also identified a subset of phenotypic type 2 diabetic patients who are positive for the antibodies commonly found in type 1 diabetes; this subset has been said to have type 1.5 diabetes, latent autoimmune diabetes in adults (LADA), slowly progressive IDDM, latent type 1 diabetes, youth-onset diabetes of maturity, latent-onset type 1 diabetes, and antibody-positive non-insulin-dependent diabetes (4 -6) ( Table 1). Although the different names have caused some confusion, the finding of this subset of phenotypic type 2 diabetic patients by many different investigators rather than just one or two groups confirms their existence as an important subset of phenotypic type 2 diabetic patients.Probably the greatest area of confusion involves the distinction of LADA from type 1 diabetes occurring in individuals over the age of 30 -35 years. Epidemiologic evidence suggests that type 1 diabetes peaks around puberty and again around age 40 years (7), and Nerup and colleagues (8) have suggested that the incidence of type 1 diabetes is approximately equivalent below and above age 20 years...

Latent autoimmune diabetes in adults or type 1.5 diabetes is considered to be a T-cell-mediated autoimmune disease. However, identification of patients is based commonly on autoantibody (Ab) detection. To determine whether measuring T-cell reactivity to islet proteins compared with measuring Abs improves detection of autoimmune diabetes and how ␤-cell function correlates with T-cell reactivity compared with Ab positivity, we assessed the T-cell proliferative responses and Ab responses (islet cell autoantibodies, insulin autoantibodies, insulinoma-associated protein-2 autoantibodies, and GAD Abs) to islet proteins of 36 phenotypic type 2 diabetic patients. To be considered Ab

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