Antigenic modulation and rituximab resistance.

Several types of B-cell lymphoma have been successfully treated with rituximab, and approval by the US Food and Drug Administration for use of rituximab in the treatment of rheumatoid arthritis has increased interest in targeting CD20 on B cells for other indications. Although large amounts of rituximab can be infused into humans with no apparent dose-limiting toxicity, recent evidence suggests that the body's effector mechanisms, including complement-mediated cytotoxicity and natural killer (NK) cell-mediated killing, can be saturated or exhausted at high burdens of rituximab-opsonized B cells. One of the consequences of this saturation phenomenon is that the opsonized B cells are instead processed by a different pathway mediated by FcgammaR on effector cells. In this alternative pathway, both rituximab and CD20 are removed ("shaved") from the B cells and are taken up by monocytes/macrophages. This process, formerly called antigenic modulation, appears to occur in several compartments in the body and may play a key role in the development of resistance to rituximab therapy.