A Study of Belimumab in Treating Symptomatic Waldenstroms Macroglobulinaemia

Inclusion Criteria: * At least 18 years of age. * Diagnosis of WM histologically confirmed on bone marrow biopsy. * Detectable IgM paraprotein \>5 g/L * Less than 3 lines of prior therapy for WM * Full blood count within 4 weeks prior to screening shows ANC \>1.0 x109/l AND platelet count \>50 x109/l * Therapy indicated due to development of one or more of the following: 1. symptomatic anaemia 2. hyperviscosity symptoms 3. rapidly rising paraprotein of \>25% or \>5g/l over 3 months 4. splenomegaly 5. bulky lymphadenopathy 6. B symptoms or paraneoplastic phenomena, which, in the opinion of the investigator are the result of progressive WM. * Life expectancy \>12 months * ECOG \< 3 * Able to provide informed consent * Ability to understand the requirements of the study, provide written informed consent, including consent for the use and disclosure of research-related health information, and comply with the protocol procedures, including required study visits. * Subjects of child bearing potential must agree to use effective contraception throughout the study and for 3 months after the last dose of belimumab Exclusion Criteria: * Prior therapy with belimumab. * Pregnant or breast feeding * Chemotherapy, immunotherapy or biological therapy within 4 weeks of enrolment. Therapeutic plasma exchange can continue- see section 3.1.4. * Creatinine clearance (calculated by Cockcroft-Gault) \< 60ml/min * Bilirubin \>2x ULN, ALT \>2x ULN. * History of an allergic or anaphylactic reaction to parenteral administration of contrast agents, human or murine proteins or monoclonal antibodies, a history of severe allergic reaction to drugs, food, or insects requiring medical intervention, or a history of hypersensitive triad (having all 3 features of allergic rhinitis with nasal polyps, asthma, and aspirin sensitivity). * Prior opportunistic infection including tuberculosis or atypical mycobacterial infection, multi-dermatome Herpes Zoster or Pneumocystis pneumonia or invasive fungal infection (not including oral or vaginal candidiasis or superficial dermatophytes) . * Active infection with hepatitis B, hepatitis C or HIV or historically positive test or test positive at screening for HIV antibody, hepatitis B surface antigen, or hepatitis C antibody. * History of organ transplant (eg, heart, lung, kidney, liver) or hematopoietic stem cell/marrow transplant. * Planned surgical procedure during the treatment period of this study or a history of any other medical disease (eg, cardiopulmonary), laboratory abnormality, or condition that, in the opinion of the principal investigator, makes the subject unsuitable for the study. * Hospitalization for treatment of infection within 60 days of Day 1. * Use of parenteral (IV or IM) antibiotics (antibacterials, antivirals, anti-fungals, or anti parasitic agents) within 60 days of Day 1. * Current drug or alcohol abuse or dependence, or a history of drug or alcohol abuse or dependence within 364 days prior to Day 1.