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Results. Rituximab was well tolerated in this patient population, with most experiencing no significant adverse effects. Only 3 serious adverse events, which were thought to be unrelated to rituximab administration, were noted. A majority of patients (11 of 17) had profound B cell depletion (to <5 CD19؉ B cells/ l). In these patients, the SLAM score was significantly improved at 2 and 3 months compared with baseline (P ‫؍‬ 0.0016 and P ‫؍‬ 0.0022, respectively, by paired t-test). This improvement persisted for 12 months, despite the absence of a significant change in anti-doublestranded DNA antibody and complement levels. Six patients developed human antichimeric antibodies (HACAs) at a level >100 ng/ml. These HACA titers were associated with African American ancestry, higher baseline SLAM scores, reduced B cell depletion, and lower levels of rituximab at 2 months after initial infusion.Conclusion. Rituximab therapy appears to be safe for the treatment of SLE and holds significant therapeutic promise, at least for the majority of patients experiencing profound B cell depletion. Based on these results, controlled trials of rituximab appear to be warranted.

Objective. Despite wide use of the anti-CD20 monoclonal antibody rituximab in the treatment of B cell lymphomas, the mechanism by which it causes B cell depletion remains a subject of controversy. As part of an ongoing phase I/II trial of rituximab in the treatment of systemic lupus erythematosus (SLE), we sought to determine whether the effectiveness of B cell depletion was influenced by polymorphisms of Fc receptors (FcR) on effector cells.Methods. During rituximab treatment of 12 SLE patients, B cell depletion was monitored as a function of the serum rituximab level and Fc␥RIIa and Fc␥RIIIa genotypes at baseline and at 1 month and 2 months after treatment. FcR genotypes were determined by polymerase chain reaction. Serum levels of rituximab were measured by enzyme-linked immunosorbent assay (ELISA). B lymphocyte percentages were assessed by flow cytometry.Results. B cell depletion was highly variable in this patient cohort, with B cell percentages at the 1-2-month posttreatment nadir ranging from undetectable (<0.1 cell/ l) to 16% (ϳ30 cells/ l) of the total peripheral blood lymphocytes. At 2 months posttreatment, B cell percentages were highly correlated with both the serum rituximab level and the Fc␥RIIIa genotype (R 2 ‫؍‬ 0.75, P ‫؍‬ 0.002). The Fc␥RIIIa genotype was a significant independent predictor of the efficacy of B cell depletion (P ‫؍‬ 0.019). Conclusion. These results highlight the potential variability of B cell depletion by rituximab in the treatment of autoimmune disease and indicate that Fc receptors are an important determinant of that variability. The findings further suggest the importance of antibody-dependent cell-mediated cytotoxicity and/or apoptosis induction via Fc␥RIIIa-expressing effector cells in the mechanism of B cell depletion by this widely used monoclonal antibody.Rituximab is a chimeric mouse-human monoclonal antibody against the B cell-specific antigen CD20, a cell surface protein believed to function in B cell cycle initiation and differentiation (1-3). CD20 is first expressed in the early pre-B cell stage, and it remains present until terminal differentiation into plasma cells. The CD20 antigen represents an ideal target for immunotherapy of B cell lymphomas and, more recently, B cell-mediated autoimmune diseases due to its high and relatively sustained expression on malignant and normal B cells (4). Rituximab can effectively deplete B cells for several months and, as such, represents an effective treatment of B cell lymphoma (5).Several potential mechanisms have been proposed as mediators of the effects of rituximab (6). Thus, in vitro studies have demonstrated that rituximab bound to CD20ϩ cells induces complement-dependent cytotoxicity (CDC) and, in the presence of effector cells, antibody-dependent cell-mediated cytotoxicity (ADCC) (5). Additionally, rituximab can induce B cell apoptosis when crosslinked by Fc␥ receptor (Fc␥R)-bearing cells (7). The relative importance of antibody-mediated apoptosis, CDC, and ADCC in the depletion of B cells by rituximab remains to be...

We describe a system to evaluate the function of lymphocyte-specific and generafly expressed genes in the differentiation and/or function of lymphocytes. RAG-2 (recombination-activating gene 2)-deficient mice have no mature B and T lymphocytes due to the inability to initiate VDJ recombination. Blastocysts from RAG-2-deficient mice generate animals with no mature B and T cells foOlowing implantation into foster mothers. However, i 'ection of normal ES

B lymphocyte differentiation is characterized by an ordered series of Ig gene assembly and expression events. In the majority of normal B cells, assembly and expression of Ig heavy (H) chain genes precedes that of light (L) chain genes. To determine the role of the Ig heavy chain protein in B cell development and L chain gene rearrangement, we have generated mice that cannot assemble Ig H chain genes as a result of targeted deletion of the JH gene segments in embryonic stem cells. Mice homozygous for this deletion are devoid of slg+ B cells in the bone marrow and periphery. B cell differentiation in these mice is blocked at the large, CD43+ precursor stage. However, these precursor B cells do assemble kappa L chain genes at a low level in the absence of mu H chain proteins. These data demonstrate that rearrangement and expression of the mu H chain gene is not absolutely required for kappa L chain gene rearrangement in vivo. Expression of mu chains may facilitate either efficient L chain gene rearrangement or the survival of cells that have rearranged light chain genes by promoting the differentiation of large, CD43+ to small, CD43- pre-B cells.

The ability of lymphocyte receptor V, D and J gene segments to rearrange generates much of the receptor diversity that is the hallmark of the immune system. Naturally, the mechanisms of immunoglobulin and T-cell receptor gene recombination are of enormous interest. Here, Fred Alt and colleagues review current understanding of the process and speculate on future findings.

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To study the influence of immunoglobulin heavy-chain (HC) and light-chain (LC) expression in promoting