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Type 1 diabetes (T1D) is caused by a deficiency of insulin as a result of an autoimmune destruction of the pancreatic ² -cells. A possibility to preserve remaining ² -cells in children with newly diagnosed T1D is of great importance since sustained ² -cell function is recognized to result in reduced end-organ complications. Glutamic acid decarboxylase 65 (GAD65) is one of the major antigens targeted by self-reactive T cells in T1D, and immunomodulation with GAD65 formulated in aluminum (GAD-alum) has been considered both in prevention and treatment of T1D. Results from a Phase II trial have shown clinical effect of subcutaneous injections with GAD-alum, this was unfortunately not fully confirmed in the following larger Phase III trial which therefore was closed after 15 months. The general aim of this thesis was to study the immunomodulatory effect of GAD-alum-treatment in children with T1D participating in the Phase II and Phase III trials. We hypothesized that treatment with GAD-alum contributes to the preservation of residual insulin secretion through deviation of the GAD65-specific immune response from a destructive to a protective process, accompanied by a shift from T helper (Th) 1 towards a predominant Th2 profile. In the Phase II trial, GAD-alum-treated patients responded with an early GAD65-specific Th2 skewed cytokine secretion, with highest IL-5 and IL-13 secretion in clinical responders. Also, the CCR4/CCR5 ratio indicating balance between Th2/Tc2 and Th1/Tc1 responses, increased in treated patients. The recall response to GAD65 was characterized by a wide range of cytokines, but the relative contribution of each cytokine suggests a shift towards a more pronounced Th2-associated profile over time. Induction of a CD4+ cell subset upon GAD65-stimulation 4 years after treatment, suggesting clonal expansion of the memory T-cell compartment upon antigen re-challenge, was seen in parallel to a persistent GAD65-specific cytokine response. Finally, even if the phase III trial failed to reach the primary endpoint at 15 months, a subgroup analysis showed that the treatment had an effect on preservation of residual insulin secretion, but the effect was not seen until after 30 months. Taken together, these results suggest that GAD-alum treatment might exert its effect through induction of an early Th2 skewed immune response which tends to deviate away from a destructive Th1/Tc1 response upon GAD65 re-challenge, and generation of GAD65-specific memory T cells that produce cytokines and exert effector responses which may be important for regulating GAD65 immunity. Continued research to better understand how immunomodulation with autoantigen modifies T-cell responses and also which patients are suitable for treatment, is crucial for optimizing future intervention trials using ² -cell antigens.