The purpose of this study is to assess whether co-administration of belimumab and a single cycle of rituximab will optimize treatment with belimumab, which will result in improvements of clinical status with a favorable safety profile, by comparing subjects randomized to belimumab plus rituximab versus belimumab plus rituximab-placebo. Approximately 200 subjects will be randomized in a 1:2:1 ratio to 1 of 3 treatment arms; belimumab plus rituximab-placebo (Arm A, control), belimumab plus rituximab (Arm B, combination), or belimumab plus standard therapy (Arm C, reference). Arm A and Arm B will be double-blinded and Arm C will be open-labeled. The open-label arm will provide a qualitative reference to standard therapy. Belimumab will be administered as subcutaneous (SC) and rituximab-placebo or rituximab will be administered by intravenous (IV) infusions. The total duration of the study is for 104 weeks. Double-blinded phase will be for 52-weeks with subjects from Arm A and B who cannot tolerate discontinuation of immunosuppressant or taper of corticosteroids. Subjects in Arm C will continue to receive belimumab SC and their stable immunosuppressant during Weeks 53 through 104.