Inclusion Criteria:

Subjects must meet all of the following criteria:

• Age 18 years or older at the time of screening.
• Written informed consent and any locally required authorization (eg, HIPAA in the USA, EU Data Privacy Directive in the EU) obtained from the subject/legal representative prior to performing any protocol-related procedures, including screening evaluations.
• Female subjects of childbearing potential who are sexually active must use must use 2 effective methods of avoiding pregnancy from screening, and must agree to continue using such precautions for 12 weeks after the final dose of investigational product.
• Females or female partners not of childbearing potential must have been surgically sterilized (eg. hysterectomy, bilateral tube ligation, or bilateral oophorectomy) or 2 years postmenopausal (defined as at least 2 years since last regular menses).
• Non-sterilized males who are sexually active with a female of child-bearing potential must use 2 effective methods of birth control from screening until 12 weeks after the final dose of investigational product.
• Must have qualified for and received investigational product (sifalimumab or placebo) from one of the following sifalimumab clinical studies: MI-CP151, MI-CP152, or MI‑CP179.
• If female, unless cervix has been surgically removed, have had a Pap smear with no evidence of malignancy within 6 months of baseline (defined as Day 1).
• Ability to complete the study period through the Day 1177.
• Willing to forego other forms of experimental drug treatment during the study.

Exclusion Criteria:

Any of the following would exclude the subject from participation in the study:

• Discontinued investigational product (sifalimumab) for safety reasons from any previous sifalimumab clinical study.
• Employees of the clinical study site or any other individuals involved with the conduct of the study, or immediate family members of such individuals

• For subjects with SLE:

  • Active severe or unstable neuropsychiatric SLE, that in the opinion of the investigator, would make the subject unsuitable for the study or unable to fully understand the informed consent
  • Active severe or unstable renal disease that in the opinion of the investigator would make the subject unsuitable for this study

• For subjects with DM or PM:

  · Inclusion body myositis, cancer-associated myositis, myositis associated with another connective tissue disease, environmentally-associated myositis, or drug-related myopathy, a history of or a family history of non-inflammatory myopathy, scapular winging, atrophy, or hypertrophy of the calf muscles

• Active Hepatitis A, confirmed positive tests for hepatitis B surface antigen (HbsAg) and hepatitis B core antibody (HbcAb) or hepatitis C serology. Isolated HbcAb positivity will be explored with additional reflex testing to determine eligibility.
• Evidence of active tuberculosis (TB), either treated or untreated, or latent TB without completion of an appropriate course of treatment or appropriate ongoing prophylactic treatment. Evaluation will be according to the local standard of care and may consist of history and physical examination and/or chest X-ray, and/or TB skin test (eg, purified protein derivative [PPD]) testing, with the standard of care as determined by local guidelines. If a PPD is required per local standard of care and has not been obtained in past 90 days, it must be repeated.
• History of severe viral infection, such as disseminated herpes, herpes encephalitis, or ophthalmic herpes.
• Herpes zoster infection within 3 months before entry into the study
• Deep space/tissue infection (eg, fasciitis, abscess, osteomyelitis, or infected joint replacements) within 1 year before entry into the study.

• Any of the following within 28 days before entry into the study:

  • Clinically significant active infection, including ongoing, chronic infection (Chronic nail bed fungal infections are not considered an exclusion)
  • Any infection requiring hospitalization or treatment with IV anti-infectives
• Completion of a course of oral anti-infectives within 14 days before entry into the study.
• Receipt of any live or attenuated vaccine within 28 days before entry into the study or anticipated to be given during the study or within 3 months of the last dose of investigational product administration.
• History of cancer, apart from basal cell carcinoma, treated with apparent success with curative therapy ≥ 1 year prior to entry into the study.
• History of primary immunodeficiency or underlying condition that predisposes the subject to infection (eg, splenectomy).
• Lactating or pregnant females or females who intend to become pregnant during the study or within 84 days of last dose of investigational product administration.
• Current evidence for alcohol, drug or chemical abuse, or a recent history of such abuse < 1 year before entry into the study.
• Major surgery within 8 weeks before entry into the study or elective surgery planned during the study period.
• History of allergy to any component of the sifalimumab formulation.
• B cell-depleting therapies within 12 months before screening.
• Any investigational drug therapy or non-B-cell depleting biologic therapy within 28 days of Day 1 (or within 5 half-lives whichever is longer).

• Any of the following medications within 6 months before entry into the study:

  • Leflunomide > 20 mg/day
  • Cyclophosphamide (or any other alkylating agent)

• Any of the following medications within 28 days before entry into the study:

  • Prednisone or equivalent > 30 mg/day or > 0.5 mg/kg, whichever is the lesser amount
  • Cyclosporine at any dose
  • Thalidomide at any dose
  • Interferon alpha 2b
  • Hydroxychloroquine > 600 mg/day
  • Mycophenolate mofetil > 3 g/day
  • Methotrexate > 25 mg/week
  • Azathioprine > 3 mg/kg/day
  • Combination of leflunomide and methotrexate

• Nonstable doses of one or more of the following medications within 28 days before entry into the study:

  • Hydroxychloroquine
  • Mycophenolate mofetil
  • Methotrexate
  • Azathioprine

• At screening blood tests (within 28 days before entry into the study), any of the following:

  • Total bilirubin > upper limit of normal (ULN)
  • Neutrophil count < 1,500/μL (or < 1.5 × 109/L)
  • Platelet count < 60,000/μL (or < 60 × 109/L)
  • Hemoglobin (Hgb) < 7 g/dL (or < 70 g/L)
  • Hemoglobin A1c (HbA1c) > 8% (or > 0.08) at screening (diabetic subjects only)
• Evidence of any disease or history of any disease, any finding upon physical examination or any clinically significant laboratory or radiographic abnormality that, in the opinion of the investigator, designated health care provider, or medical monitor may compromise the safety of the subject in the study.
• Any condition that, in the opinion of the investigator, would interfere with evaluation of the investigational product or interpretation of subject safety or study results.
• Concurrent enrollment in another clinical study.