InclusionRelapsed or refractory WM who have received ≥1 prior lines of therapy
Presenceof measurable disease (defined as a serum IgM level of >0.5 g/L)
ECOG performance status 0-2
Adequate renal, liver, and bone marrow function
Exclusion
Refractory to rituximab
Clinically significant cardiac disease within 6months prior to registration
History of significant cerebrovascular disease inlast 6 months
Known central nervous system involvementof WM / Lymphoplasmacytic Lymphoma (LPL)
Prior history of haemolyticanaemia (either warm or cold)
History of colitis or pneumonitis
Received a T cell depletingantibody (e.g. alemtuzumab) within 3 months prior to starting treatment
Prior therapy with an anti-PD-1,anti-PD-L1or CTLA4 monoclonal antibody
Prior allogeneic bone marrowtransplantation
Known orsuspected hypersensitivity to components of pembrolizumab and/or rituximab (orother CD20 monoclonal antibody)