[1][2][3][4] Since approval of a depleting anti-CD20 monoclonal antibody (mAb) (rituximab [Genentech, South San Francisco, CA; Biogen-IDEC, Cambridge, MA; Roche, Basel, Switzerland)] in 1997 for the treatment of non-Hodgkin lymphoma (NHL), almost 1 million patients have been treated with rituximab as a first or second line therapy, either alone or in combination with chemotherapy. In addition, rituximab maintenance therapy significantly prolongs tumor remission and patient survival in patients with indolent B-cell NHL or chronic lymphocytic leukemia (CLL). 5,6 More recently, rituximab has demonstrated clinical benefit in a variety of autoimmune diseases including rheumatoid arthritis, pemphigus vulgaris, immune thrombocytopenia and autoimmune hemolytic anemia. 4,7,8 As a result, understanding the contribution of B lymphocytes to human autoimmune diseases received revived interest, and several mechanisms have been postulated to participate in disease pathogenesis, including autoantibody production, B-cell antigen presentation, cytokine generation, and lymphorganogenesis. 2,3,9,10 Inhibition of different combinations of these mechanisms is probably responsible for clinical benefit.The success of anti-CD20 B-cell immunotherapy spearheaded a large number of preclinical and clinical efforts to understand in vivo mechanisms of drug activity. Direct elimination of malignant B cells through antigen-dependent cell-mediated cytotoxicity (ADCC), complement-mediated cytotoxicity (CDC), and apoptosis have been demonstrated as the main mechanisms of action in a variety of systems including mouse xenotumors and normal mouse and nonhuman primate (NHP) B-cell subsets. 11-15 Two major determinants affecting normal mouse B-cell depletion have been identified. 13,16 First, the kinetics of B-cell recirculation determines the speed and magnitude of anti-CD20 mAb-mediated B-cell depletion. Cells with higher recirculatory kinetics from blood, lymph nodes, and spleen follicular areas are depleted faster and more completely than cells with lower recirculatory kinetics (eg, peritoneal cavity [PEC], marginal zone [MZ], germinal centers [GC]). Second, the local microenvironment influences the extent of B-cell depletion. Marginal zone, Peyer patches (PP), germinal center, memory, and peritoneal cavity B cells exhibit greater resistance to depletion in mice and nonhuman primates. Reduced recruitment of effector mechanisms in the peritoneal cavity as well as intrinsic B1 B cells properties appear to cause the slower kinetics and B-cell reduction after anti-CD20 mAb treatment. 16 Differences across mouse strains and epitopes recognized by anti-CD20 antibodies used for depletion might explain the differential effects seen on mouse splenic marginal zone B cells. 13,16 Small but consistent numbers of residual B cells can be detected in most lymphoid organs in mice and primates treated with anti-CD20 mAbs. One possibility of achieving a more complete B-cell reduction would be to block B-cell survival signals in addition to Submitted April 30, 2007...