Immunotherapeutic mechanisms of anti-CD20 monoclonal antibodies

Abstract

The anti-CD20, B-cell-specific mAb rituximab (RTX) has been approved for treatment of non-Hodgkin's B cell lymphoma and rheumatoid arthritis. Under conditions of high B cell burden, exhaustion of the body's effector mechanisms, for example, NK-cell-mediated killing, may lead to substantial decreases in the immunotherapeutic efficacy of this mAb. Moreover, RTX treatment of patients with chronic lymphocytic leukemia and high levels of circulating B cells can lead to removal of CD20 from the cells, thus allowing them to persist and resist clearance. RTX therapy for several autoimmune diseases has proven to be effective, but in numerous instances there has been little correlation between reductions in disease activity and changes in titers of pathogenic autoantibodies. This paradox may be explained by a separate mechanism: Binding of RTX to B cells generates immune complexes that act as decoys to attract monoycte/macrophages and thus reduce their inflammatory activity in certain autoantibody-mediated diseases. Several second-generation anti-CD20 mAbs with enhanced cytotoxic action have been developed and are being tested in the clinic for treatment of cancer and autoimmune diseases. The application of these mAbs, potentially in combination with immune effector modifying drugs, may successfully address the shortcomings of current anti-CD20 immunotherapy.

Figure 1

The Immune Complex Decoy Hypothesis [5*]. A. In ITP, autoantibodies bind to platelets, and the IgG-opsonized platelets are removed and destroyed in the spleen (and possibly in the liver) due to interaction with FcγR on fixed tissue macrophages. Infusion of RTX generates IgG-opsonized cells that bind to the macrophages, thus sparing the platelets. Modified from [5*]. B. Alternatively, in RA and in SLE respectively, immune complexes in the joints and concentrated in the synovium, or associated with the kidneys, interact with FcγR on monocyte/macrophages which initiates an inflammatory cascade, leading to the release of numerous inflammatory factors, ultimately leading to local tissue destruction. After RTX infusion, diversion of the monocyte/macrophages to other sites, due to interaction of FcγR with RTX-opsonized B cells, spares the synovium or kidney.