Belimumab in Remission of VASculitis

Key Inclusion Criteria: * Clinical diagnosis Wegener's granulomatosis or microscopic polyangiitis by Chapel Hill criteria. * Disease flare in the past 26 weeks requiring treatment with high dose corticosteroids and 1 of the following medications: rituximab, oral cyclophosphamide OR IV cyclophosphamide. * Tested positive for anti-proteinase 3 (anti-PR3) or anti-myeloperoxidase (anti-MPO) antibodies at any time prior to enrollment. * Achieve remission no more than 26 weeks after first dose of induction treatment. Remission is defined as a Birmingham Vasculitis Activity (BVAS) score of 0 and receiving less than 10 mg/day of oral prednisone (or equivalent) on 2 consecutive visits 21 to 35 days apart. * Maintenance therapy on this study must start no more than 2 weeks after confirmation of remission. Key Exclusion Criteria: * Pregnant or nursing. * Receipt of a B cell targeted therapy (other than rituximab) at anytime * Receipt of an investigational biological agent within the past 60 days. * Required management of acute or chronic infections within the past 60 days. * Current drug or alcohol abuse or dependence. * Current or past positive test for human immunodeficiency virus (HIV), hepatitis B, or hepatitis C. * History of severe allergic reaction to contrast agents or biological medicines.