T-cell mediated autoimmunity to the insulinoma-associated protein 2 islet tyrosine phosphatase in type 1 diabetes mellitus

The target molecules of the T-cell response in type I diabetes, despite the ir pathogenic importance, remain largely uncharacterized, especially in hum ans, Interestingly molecules such as insulin and glutamic acid decarboxylas e (GAD) have been shown to be a target not only of autoantibodies, but also of autoreactive T-lymphocytes both in man and in the non-obese diabetic (N OD) mouse. In the present study we aimed to determine the existence of a sp ecific T-cell response towards the insulinoma-associated protein 2 (IA-2) i slet tyrosine phosphatase, a recently identified autoantigen which is the t arget of autoantibodies strongly associated with diabetes development. Huma n recombinant IA-2 produced in Escherichia coli. was tested for its reactiv ity with peripheral blood lymphocytes obtained from 16 newly diagnosed type I diabetic patients and from 25 normal controls, 15 of whom were HLA-DR-ma tched. A T-cell proliferation assay was performed in triplicate employing f reshly isolated cells in the absence or in the presence of the antigen to b e tested (at two different concentrations: 2 mu g/ml and 10 mu g/ml). A spe cific T-cell proliferation (defined as a stimulation index (S.I.) greater t han or equal to 3) was observed against IA-2 used at a concentration of 10 mu g/ml (but not of 2 mu g/ml) in 8/16 diabetic patients, in 1/15 HLA-DR-ma tched control subjects (P < 0.01 by Fisher exact test) and in 0/10 of the r emaining normal individuals. A statistically significant difference (P < 0. 003 by Mann-Whitney U test) was also observed in S.I. values between patien ts (3.1 +/- 1.4) and HLA-DR-matched controls (1.7 +/- 0.54) employing IA-2 at a concentration of 10 mu g/ml. However, when IA-2 was used at a concentr ation of 2 mu g/ml, the difference in S.I. between patients (1.65 +/- 0.8) and controls (1.0 +/- 0.3) did not reach statistical significance. In concl usion, these data show the presence of a specific, dose-dependent T-lymphoc yte response against the IA-2 islet tyrosine phosphatase at the onset of ty pe I diabetes. Consequently, this molecule appears to be a target not only at the B-lymphocyte but also at the T-lymphocyte level, reinforcing the pot ential pathogenic role of this autoantigen in the islet destructive process .