T Cell Proliferative Responses to Glutamic Acid Decarboxylase-65 in IDDM are Negatively Associated with HLA DR3/4

Abstract

Based on studies in spontaneously non-obese diabetic (NOD) mice, it has been suggested that the Mr 65.000 isoform of glutamic acid decarboxylase (GAD₆₅) is of major importance in the pathogenesis of insulin-dependent diabetes mellitus (IDDM). In humans, antibodies to GAD₆₅ 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 GAD₆₅, we incubated peripheral blood mononuclear cells from 45 newly diagnosed IDDM patients with purified recombinant human islet GAD₆₅ and correlated the proliferative response with HLA DR haplotype and the presence of GAD₆₅ autoantibodies. Fifty healthy individuals were studied as controls. Of the patients, 49% showed proliferative responses to GAD₆₅ in contrast to only 4% of the controls. T cell proliferation to GAD₆₅ 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 GAD₆₅ 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 GAD₆₅ (r = 0.24). In conclusion, we find a proliferative T cell response to GAD₆₅ 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.

Key Words:

• GAD₆₅
• T cell proliferation
• GAD autoantibodies
• HLA DR
• Sf9 cells
• baculovirus