JDM is a chronic systemic autoimmune disease with a predominance of muscle and skin inflammation of unknown etiology and varying prognosis. Children with JDM unresponsive to corticosteroids or other immunosuppressive medications face poor clinical and functional outcome and suffer various sequelae of the disease. Abatacept is a fully human soluble recombinant protein consisting of the cytotoxic T cell Lymphocyte Antigen-4 (CTLA4) fused with Fc region of human IgG1 that has been modified to prevent complement fixation and antibody-dependent cellular cytotoxicity. Abatacept binds specifically to the CD80 (B7-1) and CD86 (B7-2) molecules, those expressed on antigen-presenting cells (APCs). Upon engagement of CTLA4 to CD80 or CD86, the resultant inhibition of signal transduction inhibits T cell activation.. The rationale for use of Abatacept in the therapy of JDM includes the expression of CTLA4, CD28, CD86, and CD40 on inflammatory cells of muscle biopsies of patients with DM, as well as CTLA4 and CD28 on muscle cells.