Comparative study of GAD65-specific CD4+ T cells in healthy and type 1 diabetic subjects

Glutamic acid decarboxylase 65 (GAD65) is a putative autoantigen associated with the pathogenesis of type 1 diabetes (T1D). The prevalence of autoreactive CD4+ T cells towards the immunodominant GAD65555-567 epitope in DR4 healthy and T1D subjects was investigated with class II tetramers. A slightly higher percentage of diabetic subjects had GAD65555-567 tetramer-positive T cells upon GAD65555-567 peptide stimulation on the total CD4+ T-cell populations compared to healthy subjects. In contrast, three quarters of subjects in both groups had tetramer-positive T cells resulting from stimulation of the CD4+CD25+ regulatory T-cell depleted CD4+ T cells. The frequencies and TCR Vβ gene usages of GAD65555-567 T cells were also similar in both groups. Experiments demonstrated that GAD65555-567-reactive T cells in healthy and diabetic subjects had different CD45RA phenotypes. For the healthy group, GAD65555-567-reactive T cells were generally found in the CD45RA+ naïve T-cell pool while GAD65555-567-reactive T cells from T1D subjects were present in both CD45RA+ naïve and CD45RA− memory T-cell pools. These findings suggested that there is no difference in thymic selection of DR4 restricted GAD-reactive T cells amongst healthy and T1D individuals but GAD65555-567-reactive T cells have been preferentially activated in diabetic patients.