A therapeutically effective amount of an extracellular domain of a transmembrane activator and calcium modulator and cyclophilin ligand-interactor (TACI) polypeptide comprising a secreted form of SEQ ID NO:23 and an anti-CD-20 agent comprising rituximab, for use in reducing B cell numbers in a mammal, wherein said therapeutically effective amount results in a synergistic reduction in the number of memory B cells.