Abatacept in Juvenile Dermatomyositis

Inclusion Criteria: 1. Adults with definite or probable JDM and pediatric patients 7 years of age and older with definite or probable JDM. 2. Body weight of at least 25 kg (or at least 55 lbs) and age ≥ 7 years of age. 3. Patients must reside within the United States or Canada. 4. Refractory myositis, as defined by the intolerance to or an inadequate response to corticosteroids plus an adequate regimen of at least one other immunosuppressive agent, including azathioprine, methotrexate, IVIG, mycophenolate mofetil, cyclophosphamide, tacrolimus or cyclosporine A, or a biologic therapy. The definition of intolerance is: side effects that require discontinuation of the medication or an underlying condition that precludes the further use of the medication. 5. At least moderately active disease as documented by: * MD global VAS with a minimum value of 2.5 cm on a 10 cm scale AND * At least 2 other abnormal core set measures listed below: 6. Therapy with prednisone or another glucocorticoid is required, unless there is documented intolerance in the medical record or a medical condition that contraindicates further use of prednisone. The prednisone dose must be stable for at least 4 weeks prior to the screening visit. 7. Background therapy with at least 1 non-corticosteroid immunosuppressive agent is required at a stable dose for at least 6 weeks prior to the screening visit unless there is documentation that the patient is intolerant, which is defined as side effects that require discontinuation of the medication(s) or an underlying condition that precludes the further use of the IS medication. 8. If an immunosuppressive agent was discontinued prior to the screening visit then there must be a: * 4 week washout for prednisone or methotrexate * 8 week washout for any other IS agent * For discontinuation of biologic therapies, a washout of 5 terminal half lives 9. If on hydroxychloroquine or colchicine, the dose should be stable for 6 weeks prior to Visit 1. 10. If on statin or fibric acid derivative agents, the dose should be stable for 6 weeks prior to Visit 1. 11. Ability of patient or parent to complete self-report questionnaires. 12. Men and women of reproductive potential must agree to use a reliable method of birth control during the 24 week duration of the trial described in the reproductive risks section of this protocol (section 4.3). They must also agree to use a reliable method of birth control for 100 days after the last dose of study drug is administered. 13. Patients must agree to forgo immunization with a live vaccine during the course of the study or within 3 months after discontinuation. 14. Patients must have a letter from the referring rheumatologist or specialist supervising the care of the JDM, agreeing to the patient's participation in the study and to continuing to provide care for the patient, including emergency care during the trial. Core Set Measures: * An MMT-8 score that is no greater than 125/150 * Patient/Parent Global VAS with a minimum value of 2.0 cm on a 10cm scale * CHAQ/HAQ disability index with a minimum value of 0.25 * Elevation of at least one of the muscle enzymes \[which includes creatine kinase (CK), aldolase, lactate dehydrogenase (LDH), alanine aminotransferase (ALT) and aspartate aminotransferase (AST)\] at a minimum level of 1.3 x the upper limit of normal. * Global extra-muscular disease activity score with a minimum value of 2.0 cm on a 10 cm VAS scale Exclusion Criteria: 1. Drug-induced myositis (myositis in patients taking medications known to induce myositis-like syndromes, including, but not limited to, statin agents, fibric acid derivatives, and colchicine). 2. Juvenile polymyositis; inclusion body myositis; cancer-associated myositis, defined as the diagnosis of myositis within 2 years of the diagnosis of cancer except basal or squamous cell skin cancer or carcinoma in situ of the cervix if at least 5 years since excision 3. Myositis in overlap with another connective tissue disease (CTD) that precludes the accurate assessment of a treatment response (for example, difficulty in assessing muscle strength in a scleroderma patient with associated myositis) 4. History of receiving a live vaccine 4 weeks prior to initiation of study treatment 5. Joint disease, severe calcinosis, or other musculoskeletal condition, which precludes the ability to quantitate muscle strength. 6. Wheelchair bound patients. 7. Known hypersensitivity to abatacept or prior receipt of abatacept 8. Concomitant illness that would prevent adequate patient assessment or in the investigators opinion pose an added risk for study participants: 9. Recurrent or chronic infections, including HIV, tuberculosis, hepatitis B and C, or TB infection, including contact with a household contact with active tuberculosis (TB) and who did not receive appropriate and documented prophylaxis for TB. (a documented negative Hepatitis B surface antigen and Hepatitis C antibody completed at the screening visit or within 6 weeks prior to screening visit is required) 10. Have had symptomatic herpes zoster or herpes simplex infection (not including simple oral HSV lesions) within 12 weeks prior to entry or during screening period 11. Have a history of disseminated/complicated herpes zoster (for example, multi-dermatomal involvement, ophthalmic zoster, central nervous system (CNS) involvement, post-herpetic neuralgia) 12. Known liver disease (i.e. cirrhosis or other conditions compromising the synthetic function of the liver) 13. Disorders that would preclude accurate assessment of neuromuscular function 14. Cardiomyopathy or arrhythmias that in the investigators opinion poses an additional risk for study participants 15. New York Heart Association Classification III or IV for congestive heart failure 16. Psychiatric illness that precludes compliance or neuromuscular assessment 17. Serum creatinine \> 2.0mg/dl 18. Pregnant females or nursing mothers 19. Life threatening illness that would interfere with the patient's ability to complete the study. 20. Known or suspected history of drug or alcohol abuse within the past 6 months as determined by the medical record or patient interview 21. Anticipated poor compliance 22. Participation in another clinical experimental therapeutic study within 30 days of screening visit. 23. Any history or evidence of severe illness or any other condition that would make the patient, in the opinion of the investigator unsuitable for the study. 24. Low total WBC \<2.000, platelets \< 100,000/mm3; hemoglobin \<10 gm/dl 25. History of recurrent infection including active skin infections with calcinosis 26. Subjects with a history of cancer in the last 5 years 27. Subjects who have at any time received treatment with any investigational drug within 28 days (or less than 5 terminal half-lives of elimination) of the day 1 dose of study drug, including waiting until CD19 returns to a detectable level and IgG level is within normal limits (normal serum levels of IgG per reference lab: 7-9 years ≥572 mg/dl, 10-11 yrs ≥698 mg/dl, 12-13 yrs ≥759 mg/dl, 14-15 yrs ≥716 mg/dl,16-19 yrs ≥549 mg/dl, \>19 yrs ≥700 mg/dl) for those patients who have received rituximab 28. Concomitant treatment with anti-TNF therapies, rituximab or anakinra or other biologic therapies. 29. Initiation of colchicine and hydroxychloroquine as new drugs during study participation is not allowed. 30. Initiation of statins or fibric acid derivatives during study participation is not allowed. 31. Initiation of an exercise program within 4 weeks of the screening visit. Only a stretching program may be initiated during the study (See section 5.4 Other Restrictions) 32. Prisoners or subjects who are involuntarily incarcerated. 33. Subjects who are compulsorily detained for treatment of either a psychiatric or physical (e.g., infectious disease) illness.