Inclusion Criteria:

Patients will be included in the trial based on the following criteria:

1. Definite or probable PM or DM by the criteria of Bohan and Peter (as modified by IMACS) in adults over the age of 18. We will also allow enrollment of JDM patients (considered to have DM) over the age of 18 who otherwise meet the inclusion criteria stipulated below.
2. Subjects must either have the classic rash(es) of DM (heliotrope, Gottron sign or Gottron papules), possess one of the myositis-associated autoantibodies (e.g. anti-synthetase, anti-SRP, anti-Mi-2, anti-PM-Scl, anti TIF1-γ etc.), or have the diagnosis of PM agreed upon by a 3-member Adjudication Committee consisting of a rheumatologist, neurologist and neuromuscular pathologist.
3. Refractory myositis patients are defined (see Section 3.1.1) as having failed (or considered intolerant to) an adequate course of glucocorticoids or having failed glucocorticoids and at least one other immunosuppressive (IS) or immunomodulatory agent (e.g. methotrexate, azathioprine, cyclosporine, tacrolimus, mycophenolate mofetil, cyclophosphamide, IVIg, anti-TNF agents, and rituximab).
4. Subjects with an MMT-8 score ≤ 66 (see Appendix B) must also have at least 2 other core set measures meeting the criteria listed below.
5. Subjects with an MMT-8 score > 66 must have at least 3 other core set measures meeting the criteria listed below and a global extramuscular VAS score on the MDAAT ≥ 5cm on a 10cm scale.
6. Criteria for core set measures for study entry:
   1. Patient global VAS with a minimum value of 2.0cm on a 10cm scale.
   2. MD global VAS with a minimum value of 2.0cm on a 10cm scale.
   3. HAQ disability index with a minimum value of 0.25
   4. Elevation of at least one of the muscle enzymes (CK, AST, ALT, aldolase, LDH) at a minimum level of 1.3x the ULN.
   5. Global extramuscular disease activity score with a minimum value of 1.0cm on a 10cm VAS scale on the Myositis Disease Activity Assessment Tool (MDAAT).
7. If on prednisone, the dose must be stable for 4 weeks prior to the screening visit. Tapering of the prednisone dose will only be allowed after the subject meets the DOI or if safety/toxicity issues supervene.
   1. Prednisone Tapering: Prednisone should be held constant without tapering or escalation (unless there is a serious adverse event or disease flare) until the subject has achieved the DOI. Then, tapering of prednisone may commence using a schedule approximating a 20-25% taper of the existing dose every 4 weeks based on the clinical judgment of the clinical site investigator. Prednisone tapering using the aforementioned guidelines can be commenced at any time if: (a) the patient achieves the DOI or (b) there are complications or circumstances that, in the clinical site investigator's opinion, necessitate the tapering of corticosteroids.
   2. Prednisone at Entry: It is also recommended that patients be on less than 1mg/kg/day of prednisone at study entry.
   3. Prednisone Dosing During Flare: If in the clinical site investigator's opinion there are complications or worsening of disease that necessitate an increase in the prednisone dose then the smallest reasonable increase should be considered.
8. If an IS agent was discontinued prior to the screening visit there may be a washout as stipulated below or individualized according to the patients treating physician:
   1. 4 week washout for methotrexate
   2. 8 week washout for any other IS agent (e.g. azathioprine, cyclosporine, tacrolimus, mycophenolate mofetil)
   3. 3 month washout for leflunomide, IVIg or cyclophosphamide
   4. 6 month washout for rituximab
   5. 8 week washout for infliximab or adalimumab
   6. 2 week washout for etanercept
   7. 1 week washout for anakinra
9. If an IS agent is continued, the dose must remain stable for 4 weeks prior to enrollment and at least until the DOI is met or if safety/toxicity issues supervene. Concomitant IS medications permitted include methotrexate, azathioprine, cyclosporine, mycophenolate mofetil, and tacrolimus. IVIg will also be allowed as an immunomodulatory agent. Careful patient safety monitoring along with ACR guidelines for monitoring these medications will be employed if those toxicity monitoring laboratory studies are not already being assessed as part of this trial. No concomitant biologic agents are allowed (rituximab, anti-TNFs, abatacept) as well as cyclophosphamide or tofacitinib as concomitant immunosuppressive agents. Investigators will be certain to assess and classify adverse events as being secondary to either study drug as well as any concomitant immunosuppressive agent(s). That is, there should be attribution of the AE to the appropriate agent.
10. Normal organ function, except if abnormal due to the disease under investigation
11. Men and women of reproductive potential must agree to use an acceptable method of birth control during treatment and for twelve months after completion of treatment.
12. Subject has provided written informed consent.

Exclusion Criteria:

A patient will be excluded if any of the following criteria are met:

1. Subjects under the age of 18.
2. Severe muscle damage defined as a global muscle damage score >5 on a 10cm VAS scale on the Muscle Damage Index (MDI).
3. Evidence of active malignant disease, malignancies diagnosed within the previous 5 years (including hematological malignancies and solid tumors, except basal and squamous cell carcinoma of the skin or carcinoma in situ of the cervix uteri that has been excised and cured), or breast cancer diagnosed within the previous 10 years unless related to primary disease under investigation.
4. Known active current or history of recurrent bacterial, viral, fungal, mycobacterial or other infections (including but not limited to tuberculosis and atypical mycobacterial disease, Hepatitis B and C, and herpes zoster, but excluding fungal infections of nail beds).
5. Any major episode of infection requiring hospitalization or treatment with IV antibiotics within 4 weeks of screening or oral antibiotics within 2 weeks prior to screening.
6. Active TB requiring treatment within the previous 3 years. Patients should be screened for latent TB and, if positive, treated following local practice guidelines prior to initiating TCZ. Patients treated for tuberculosis with no recurrence in 3 years are permitted.
7. Primary or secondary immunodeficiency (history of or currently active) unless related to primary disease under investigation.
8. Pregnant women or nursing (breast feeding) mothers.
9. Patients with reproductive potential not willing to use an effective method of contraception.
10. History of alcohol, drug or chemical abuse within 1 year prior to screening or any medical condition or physical or psychological state that the PI feels would not allow the subject to safely complete the study.
11. Initiation of an exercise program for muscle strengthening within 4 weeks of the screening visit or initiation of a muscle strengthening exercise program during the study.
12. Major surgery (including joint surgery) within 8 weeks prior to screening or planned major surgery within 6 months following randomization.
13. Treatment with any investigational agent within 4 weeks (or 5 half-lives of the investigational drug, whichever is longer) of screening.
14. Previous treatment with the following cell-depleting therapies, including investigational agents or approved therapies: CAMPATH, anti-CD4, anti-CD5, and anti¬CD3.
15. Immunization with a live/attenuated vaccine within 4 weeks prior to baseline.
16. Previous treatment with TCZ.
17. History of severe allergic or anaphylactic reactions to human, humanized or murine monoclonal antibodies.
18. Evidence of serious uncontrolled concomitant cardiovascular, nervous system, pulmonary (including obstructive pulmonary disease), renal, hepatic, endocrine (include uncontrolled diabetes mellitus) or gastrointestinal disease (including complicated diverticulitis, ulcerative colitis, or Crohn's disease.)
19. Patients with lack of peripheral venous access.
20. Body weight of > 150 kg.
21. Abnormal laboratory values noted below:
    1. Serum creatinine > 1.6 mg/dL in female patients and > 1.9 mg/dL in male patients. Patients with serum creatinine values exceeding limits may be eligible for the study if their estimated glomerular filtration rates (GFR) are >30.
    2. Platelet count < (100,000/mm3); hemoglobin < 8.5 g/dl and WBC count < 3000/mm3; Absolute Neutrophil Count < 2.0 x 109/L (2000/mm3); Absolute Lymphocyte Count < 0.5 x 109/L (500/mm3)
22. Positive hepatitis BsAg or hepatitis C antibody