Effective therapies for non-Hodgkin's B-cell lymphoma aim to deplete the B-cell population in patients. However, the precise mechanisms by which the humanized immunoglobulin G1 (IgG1) antibody therapy rituximab kills B cells was previously unknown. Jungi Uchida et al. now reveal the mechanism involved.

Rituximab, which targets a B-cell-specific antigen called CD20, could affect many aspects of the immune response, including antibody, effector-cell (macrophage and natural killer cell) and complement-dependent cytoxicity; the disruption of CD20 signalling pathways; and the induction of apoptosis. Previous studies have looked at the mechanisms in vitro or in circulating human B cells only. So, the authors developed a mouse model for anti-CD20 immunotherapy using 12 mouse anti-mouse CD20 monoclonal antibodies (mAbs) to study each of the possible mechanisms. All these antibodies bound to B cells in the CD20 wild-type mice and depleted both the circulating and splenic B-cell compartments. The effectiveness of mAb-induced B-cell depletion correlated closely with mAb isotype — a single injection of an IgG2a mAb (MB20-11) depleted more than 95% of blood B cells and more than 93% of splenic B cells. None of the antibodies had any effect in Cd20−/− mice.

Immune effector cells express three different Fc receptor classes for IgG; FcγRI is the highest-affinity receptor and binding of IgG to it triggers phagocytosis by macrophages and cytotoxicity by natural killer cells. Although treatment of mice deficient in either FcγRI or FcγRIII with MB20-11 did deplete B cells, treatment of mice deficient in both FcγRI and FcγRIII did not deplete B cells. This shows that binding to one of these receptors is important for efficacy of anti-CD20 mAbs. Next, the authors looked at complement-deficient mice to assess the role of complement in B-cell depletion by anti-CD20 mAbs. In vitro, the antibodies caused B-cell lysis and apoptosis only in the presence of complement. However, in vivo, there was no difference in the ability of any mAb to induce B-cell killing in the wild-type or complement-knockout mice.

So, Fc receptors are crucial for the efficiency of anti-CD20 mAbs; but what are the effectors of this response? When mice lacking T cells or natural killer cells were treated with MB20-11, more than 96% of B cells were depleted. However, similar treatment of macrophage-deficient mice did not cause significant depletion of circulating or splenic B cells.

The authors conclude that a likely mechanism of B-cell depletion by anti-CD20 mAbs is FcγR-mediated phagocytosis of mAb-coated B cells by macrophages. This knowledge should help to understand the response and resistance to rituximab therapy and the development of effective methods to enhance the benefits of therapies for non-Hodgkin's lymphoma.