GAD65 autoantibodies (GAD65Ab) are highly prevalent in type 1 diabetes, but their functional role in the pathogenesis of the disease and their relationship to T-cell reactivity to GAD65 is still unclear. We tested the hypothesis that GAD65Ab modulate presentation of GAD65 to T-cells. T-cell hybridoma T33. 1, which recognizes the GAD65 274-286 epitope in the context of HLA-DRB 1* 0401, was incubated with antigen-presenting cells exposed to recombinant human GAD65 alone or complexed with GAD65Ab'or GAD65Ab-sera. Stimulation of the T33. 1 hybridoma was greatly enhanced by multiple GAD65Ab+ sera. The enhancement effect was most prominent with sera from patients with high GAD65 autoantibody levels. Sera from GAD65Ab-subjects had no effect. The correlation between T-cell stimulation and GAD65Ab levels was not absolute, suggesting that other variables such as autoantibody recognition of different regions of GAD65 and variable effects on processing of the 274-286 epitope may contribute. Uptake of antibody-complexed GAD65 was Fc receptor (FcR)-mediated because the enhancement of presentation was inhibited by monoclonal antibodies against FcR. Our results support the hypothesis that GAD65Ab modulate presentation of GAD65 to T-cells. Increased antigen uptake and heterogeneity in the autoantibody specificity may provide a mechanism for antibody-facilitated T-cell response influencing the progression of type 1 diabetes.