Abatacept in Juvenile Dermatomyositis

Study participation will consist of a screening visit and 5 protocol visits over six months (week 0, week 6, week 12, week 18, and week 24) for each subject, and phone follow-up (week 2, week 4, week 8, week 10, week 14, week 16, week 20, and week 22). At Visit 1, which will be treatment initiation, eligible subjects will be instructed on the use and side effects of subcutaneous abatacept and will be started on the study drug (abatacept 125 mg SQ weekly for subjects with body weight ≥ 50 KG or abatacept 87.5mg SQ for subjects with body weight \< 50 KG).