Rituximab therapy and autoimmune disorders: prospects for anti-B cell therapy

In recent years, advances in our understanding of the regulation of the immune system have enabled the identification of cellular and molecular targets that could potentially affect the pathogenesis of many autoimmune diseases. In particular, the demonstration that B lymphocytes could play a central role in pathogenesis suggests that their elimination may be a highly beneficial therapeutic goal in a variety of diseases. Hybridoma antibody technology has been applied as a first step toward developing such specific agents. One of the initial applications of this technology was the characterization of the surface molecules on lymphocytes, to enable the discrimination of each type of lymphocyte. These early studies identified CD20 as a specific marker for B cells (1). CD20 has been found to be highly expressed on the surface of pre–B lymphocytes as well as on both resting and activated mature B lymphocytes, whereas it is not expressed by hematopoietic stem cells, pro–B cells, or other normal tissues (Figure 1). However, despite extensive investigation, the role of CD20 in B cell physiology has remained a mystery; there is no known natural ligand, and CD20-knockout mice are without a discernible immunologic phenotype (2). From the characterization of its encoding gene, CD20 has been predicted to be a 33–37-kd membraneassociated phosphoprotein, with a structure of 4 transmembrane regions, a 44–amino acid extracellular loop, and cytoplasmic Nand C-termini (3). Based on structural homologies, CD20 has been postulated to function as a calcium channel subunit (for review, see ref. 4), which may explain the finding that ligation of CD20 can affect B cell activation, differentiation, and cell cycle progression from the G1 to the S phase (5,6). Important for its use as a therapeutic target, binding of CD20 does not modulate its expression or result in substantial internalization. CD20 is also not shed, and there are no other known membrane or secreted analogs to interfere with its use for B cell targeting (7). In recent reports, the properties of different monoclonal antibodies against human CD20 have been investigated (8). The clinical potential of CD20-targeted therapy derives, in large part, from the unexpected finding that treatment with an antibody to CD20 induces the death of B lymphocytes, even without the need to conjugate the antibody to a toxin. The best-exploredmechanism has been the properties of rituximab (trade name Rituxan), a chimeric monoclonal antibody specific for human CD20, comprising the variable regions of a murine anti-human CD20 B cell hybridoma fused to human IgGand -constant regions (9) (see Figure 1). Rituximab has a binding affinity for human CD20 of 8 nM. The clearance of B cells is, in part, mediated through induction of complement-mediated activities and triggering of antibody-dependent cellular cytotoxicity. This latter activity is dependent on interactions with cellular receptors for the IgG1-constant regions (i.e., Fc receptors), especially Fc receptor type IIIa, which is expressed on a variety of cells including phagocytic cells (10). Rituximab has also been shown to directly trigger intracellular pathways for apoptotic B cell death that involve the activation of phospholipase C , interruption of the signal transducer and activator of transcription 3/interleukin-6 pathway, down-regulation of c-myc, and up-regulation of the proapoptosis molecule Bax, a member of the Bcl-2 family (11,12). This sequence of molecular events results in the activation of Supported by grants AI-40305, AR-47360, and AI-46637 from the NIH, and a grant from the Alliance for Lupus Research. Gregg J. Silverman, MD, Stuart Weisman, MD: University of California, San Diego. Address correspondence and reprint requests to Gregg J. Silverman, MD, Department of Medicine, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0663. E-mail: gsilverman@ucsd.edu. Submitted for publication September 16, 2002; accepted in revised form January 28, 2003.