Milatuzumab will be given subcutaneously at different dose levels once (depending on the dose level) for 4 weeks to determine if milatuzumab helps to control lupus (SLE).

Milatuzumab or placebo will be given subcutaneously once weekly for 4 weeks to determine if milatuzumab helps to control lupus (SLE). The treatment portion of the study lasts 4 weeks. Then patients are followed for disease activity for at least 12 weeks. If patients respond to the study drug, they may be eligible for one course of retreatment, again followed by 12 weeks of follow-up. Patients who showed a response will continue to be followed at timepoints up to one year after treatment to assess how long the response lasts.

Inclusion Criteria:

• Male or female ≥ 18 years old
• Signed written informed consent before study entry
• Diagnosis of SLE by American College of Rheumatology revised criteria (meets ≥ 4 criteria)
• Positive ANA (titer ≥ 1:80) at study entry
• At least 1 BILAG A or 2 BILAG B scores in any organ/body system and ≥ 6 SELENA-SLEDAI score
• Receiving at least 5.0 mg/day oral prednisone (or equivalent) at stable doses for at least 4 weeks prior to study entry
• If receiving immunosuppressives or antimalarial agents, at stable doses for at least 4 weeks prior to study entry

Exclusion Criteria:

• Pregnant or lactating women. Women of childbearing potential must have a negative pregnancy test.
• Women of childbearing potential and fertile men not practicing or unwilling to practice birth control during the study
• Rituximab, belimumab, other prior antibody, investigational or experimental therapy within 6 months
• Allergic to murine, chimeric, humanized or human antibodies
• Hematologic abnormalities not attributed to lupus: hemoglobin < 8.0 mg/dL, WBC < 2000/L, ANC < 1500/L, platelets < 50,000/L,
• AST, ALT or alkaline phosphatase > 3 times upper limit of normal and not attributed to lupus
• Serum creatinine > 2.5 mg/dL, proteinuria > 3.5 g/day
• Received live vaccine within 4 weeks
• Thrombosis, spontaneous or induced abortion, stillbirth or live birth within 4 weeks
• Antiphospholipid antibodies AND a history of thromboembolic events
• On oral anticoagulants (not including NSAIDs) within 4 weeks
• Active infection with antibiotics within 7 days
• Infection requiring hospitalization or herpes zoster treatment within 4 weeks
• Long-term infectious diseases (tuberculosis, fungal infections) active within 2 years
• Malignancy (except squamous or basal cell carcinoma, cervical CIS) within 3 years (unless approved by the medical monitor)
• History of recurrent abortions (2 or more)
• Known HIV, hepatitis B or C, other immunosuppressive states
• Other concurrent medical conditions that, in the investigator's opinion, could affect the patient's ability to tolerate or complete the study will not be eligible for the study.