A method of treating, preventing, or delaying the progression of Type 1 diabetes mellitus by administering an effective amount of a fusion protein composition comprising a T-cell co-stimulation antagonist and a portion of an immunoglobulin molecule and an effective amount of a Type 1 diabetes autoantigen. The method includes, for example, administering a cytotoxic T-lymphocyte-associated antigen 4 (CTLA4) molecule and a Type 1 diabetes autoantigen. Pharmaceutical compositions are also provided herewith.
