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.
