T cell proliferative responses to glutamic acid decarboxylase-65 in IDDM are negatively associated with HLA DR3/4

Based on studies in spontaneously non-obese diabetic (NOD) mice, it has been suggested that the Mr 65.000 isoform of glutamic acid decarboxylase (GAD(65)) is of major importance in the pathogenesis of insulin-dependent diabetes mellitus (IDDM). In humans, antibodies to GAD(65) are present before and at onset of the disease and in vitro T cell reactivity to GAD has also been reported. To further characterize the T cell recognition of GAD(65), we incubated peripheral blood mononuclear cells from 45 newly diagnosed IDDM patients with purified recombinant human islet GAD(65) and correlated the proliferative response with HLA DR haplotype and the presence of GAD(65) autoantibodies. Fifty healthy individuals were studied as controls. Of the patients, 49% showed proliferative responses to GAD(65) in contrast to only 4% of the controls. T cell proliferation to GAD(65) was significantly more frequent in patients not being HLA DR3/4 heterozygous(19/29, 66%) as compared to HLA DR3/4 heterozygous patients (3/16, 19%) (p < 0.01). The difference was most pronounced in females with 64% (9/14) of the HLA non-DR3/4 patients being positive compared to none (0/6) of the HLA DR3/4 patients (p < 0.05). The overall frequency of GAD(65) autoantibodies was 71% (32/45) with a similar distribution between patients with HLA DR3/4 (10/16, 63%) and HLA non-DR 3/4 (22/29, 76%). There was no correlation between levels of the T and B cell responses to GAD(65) (r = 0.24). In conclusion, we find a proliferative T cell response to GAD(65) in approximately 50% of recent onset IDDM patients and unexpectedly find the majority of responders to be HLA non-DR3/4 heterozygous patients. No difference was observed in B cell responsiveness between the two HLA groups.