The purpose of this study is to demonstrate the efficacy and safety of belimumab 10mg/kg administered intravenously (IV) every 4 weeks compared to placebo, in patients with SLE when added to standard of care therapy, as measured by the SLE Responder Index (SRI) at 52 weeks, defined by a composite endpoint using SELENA SLEDAI score, Physician's Global Assessment (PGA) and BILAG A and B organ domain scores.