The objective of this study is to assess the safety and tolerability of sifalimumab in adult participants with active systemic lupus erythematosus (SLE) or active dermatomyositis (DM) or polymyositis (PM) who participated in the following clinical studies: MI-CP151, MI-CP152, or MI-CP179.

The primary objective of this study is to evaluate the long-term safety and tolerability of multiple intravenous (IV) doses of sifalimumab in adult participants with active SLE or DM or PM who were previously treated with investigational product (sifalimumab or placebo) in one of the following sifalimumab clinical studies: MI-CP151, MI-CP152, or MI-CP179.

Inclusion Criteria:

• Age 18 years or older at the time of screening.
• Written informed consent and any locally required authorization [example, Health Insurance Portability and Accountability Act (HIPAA) in the United States of America (USA), European Union (EU) Data Privacy Directive in the EU] obtained from the participant/legal representative prior to performing any protocol-related procedures, including Screening evaluations.
• Female participants 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 26 weeks after the final dose of investigational product.
• Males, unless surgically sterile, must use 2 effective methods of birth control with a female partner and must agree to continue using such contraceptive precautions from Screening until 26 weeks after the final dose of investigational product. 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).
• Must have qualified for and received investigational product (sifalimumab or placebo) and completed the treatment period plus follow-up (through Day 266 for participants from MI-CP151 and MI-CP152 or through Day 168 for participants from MI-CP179) in one of the following sifalimumab clinical studies: MI-CP151, MI-CP152, or MI-CP179, ability to complete the study period through the final visit, willing to forego other forms of experimental drug treatment during the study.

Exclusion Criteria:

• Discontinued investigational product (sifalimumab) for safety reasons from any previous sifalimumab clinical study.
• For participants with systemic lupus erythematosus (SLE): Active severe or unstable neuropsychiatric SLE, that in the opinion of the investigator, would make the participant 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 participant unsuitable for this study
• For participants with dermatomyositis (DM) or polymyositis (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, history of severe viral infection, such as disseminated herpes, herpes encephalitis, or ophthalmic herpes.
• Any of the following medications within 6 months before entry into the study: Leflunomide greater than (>) 20 milligram/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 gram/day, Methotrexate > 25 mg/week, Azathioprine > 3 mg/kilogram (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/microliter (mcl) (or < 1.5 × 109/L), Platelet count < 60,000/microliter (mcl) (or < 60 × 109/L), Hemoglobin (Hgb) < 7 gram per decilitre (g/dL) (or < 70 g/L).

MedImmune has 60 days to review results communications prior to public release and may delete information that compromises ongoing studies or is considered proprietary. This restriction is not intended to compromise the objective scientific integrity of the manuscript, it being understood that results shall be published regardless of outcome. The PIs also agree for data to be presented first as a joint, multi-center publication.