• Insulin production (C-peptide secretion) [ Time Frame: 2 years of follow up ] [ Designated as safety issue: No ]
  

• HbA1c [ Time Frame: 2 years of follow up ] [ Designated as safety issue: No ]
  
• Number and severity of hypoglycemic events [ Time Frame: 2 years of follow up ] [ Designated as safety issue: No ]
  
• Immunologic outcomes [ Time Frame: 2 years of follow up ] [ Designated as safety issue: No ]
  

Type 1 diabetes mellitus (T1DM) is a T-cell mediated autoimmune disease in which insulin-producing beta cells are completely or near completely destroyed resulting in life-long dependence on exogenous insulin.

CTLA4-Ig (Abatacept) inhibits a crucial stimulatory pathway in the activation of T cells. By this mechanism, the drug is thought to arrest or slow the T cell mediated autoimmune destruction of beta-cells and preserve their function. At the time of clinical onset of T1DM, a significant amount of insulin producing beta cells are destroyed, but as many as 10-20% are still capable of insulin production. By using CTLA4-Ig close to the onset of T1DM, we hope to arrest or slow down the autoimmune destruction of these beta-cells and extend the endogenous insulin production. CTLA4-Ig regulates T cell function but does not deplete T cells. Therefore, its safety profile appears to be better than other immunosuppressive agents.

Eligible participants will be randomized to the experimental or control groups. The experimental group will receive intravenous infusions of CTLA-4 Ig. The first infusion will occur at the time of randomization, followed by another infusion 2 and 4 weeks later. Subsequent infusions will be given monthly for two years during the treatment phase of the study. There is a total of 27 infusions during the treatment phase of the study.

Participants in the control group will receive intravenous infusions of placebo according to the same schedule outlined above.

Both groups will receive standard intensive diabetes treatment with insulin and dietary management.

All participants randomized into the study will be seen at study site monthly for 24 months and then every 6 months for up to an additional 2 years. Participants will undergo assessments of their insulin production, immunologic status, overall health and well being and diabetes care.