Antibody specificity controls in vivo effector mechanisms of anti-CD20 reagents

Radioimmunotherapy for B-cell lymphoma: Y90 ibritumomab tiuxetan and I(131) tositumomab

Radioimmunotherapy, targeting the CD20 antigen, in B-cell lymphoma has clearly demonstrated efficacy and tolerability over the preceding 15 years. As a result, two products are available with Food and Drug Administration approval for marketing - Y(90) ibritumomab tiuxetan and I(131) tositumomab, given as the Zevalin and Bexxar therapeutic regimens, respectively. Both demonstrate high-response rates and durability of remission in the relapsed/refractory disease setting. Data are emerging regarding their utility as initial therapy, and furthermore, they are been investigated for use sequentially with chemotherapy, and in the myeloablative setting. As yet however, how to best use these agents in the clinical disease course remains uncertain.

CD20-specific antibody-targeted chemotherapy of non-Hodgkin's B-cell lymphoma using calicheamicin-conjugated rituximab

Tumor-targeted delivery of a potent cytotoxic agent, calicheamicin, using its immunoconjugates is a clinically validated therapeutic strategy. Rituximab is a human CD20-specific chimeric antibody extensively used in B-NHL therapy. We investigated whether conjugation to calicheamicin can improve the anti-tumor activity of rituximab against human B-cell lymphoma (BCL) xenografts in preclinical models. BCL cells were cultured with rituximab or its calicheamicin conjugates and their in vitro growth was monitored. BCL cells were injected s.c. to establish localized xenografts in nude mice or i.v. to establish disseminated BCL in severe combined immunodeficient (scid) mice. I.p. treatment with rituximab or its calicheamicin conjugates was initiated and its effect on s.c. BCL growth or survival of mice with disseminated BCL was monitored. Conjugation of calicheamicin to rituximab vastly enhanced its growth inhibitory activity against BCL in vitro. Conjugation to calicheamicin had no deleterious effect on the effector functional activity of rituximab. Calicheamicin conjugated to rituximab with an acid-labile linker exhibited greater anti-tumor activity against s.c. BCL xenografts and improved survival of mice with disseminated BCL over that of unconjugated rituximab. Anti-tumor activities of rituximab conjugated to calicheamicin via an acid-stable linker were similar to that of unconjugated rituximab. Superior anti-tumor efficacy exhibited by a calicheamicin immunoconjugate of rituximab with an acid-labile linker over that of rituximab demonstrates the therapeutic potential of CD20-specific antibody-targeted chemotherapy strategy in the treatment of B-NHL.

The CCL3 Family of Chemokines and Innate Immunity Cooperate In Vivo in the Eradication of an Established Lymphoma Xenograft by Rituximab

The therapeutic mAb rituximab induced the expression of the CCL3 and CCL4 chemokines in the human lymphoma line BJAB following binding to the CD20 Ag. Induction of CCL3/4 in vitro was specific, was observed in several cell lines and freshly isolated lymphoma samples and also took place at the protein level in vitro and in vivo. To investigate the role of these beta-chemokines in the mechanism of action of rituximab, we synthesized a N-terminally truncated CCL3 molecule CCL3(11-70), which had antagonist activity on chemotaxis mediated by either CCL3 or BJAB supernatant. We also set up an established s.c. BJAB tumor model in athymic mice. Rituximab, given weekly after tumors had reached 250 mm2, led to complete disappearance of the lymphoma within 2-3 wk. Treatment of mice with cobra venom factor showed that complement was required for rituximab therapeutic activity. Treatment of BJAB tumor bearing mice every 2 days with the CCL3(11-70) antagonist, starting 1 wk before rituximab treatment, had no effect on tumor growth by itself, but completely inhibited the therapeutic activity of the Ab. To determine whether CCL3 acts through recruitment/activation of immune cells, we specifically depleted NK cells, polymorphonuclear cells, and macrophages using mAbs, clodronate treatment, or Rag2-/-cgamma-/- mice. The data demonstrated that these different cell populations are involved in BJAB tumor eradication. We propose that rituximab rapidly activates complement and induces beta-chemokines in vivo, which in turn activate the innate immunity network required for efficient eradication of the bulky BJAB tumor.

Enhanced binding affinity for FcgammaRIIIa of fucose-negative antibody is sufficient to induce maximal antibody-dependent cellular cytotoxicity

Antibody-dependent cellular cytotoxicity (ADCC) is considered to be an important therapeutic function for clinical efficacy of monoclonal antibodies. Recent studies have revealed two methods to increase binding affinity for FcgammaRIIIa and enhance ADCC more efficiently for antibodies: (i) fucose removal from antibody N-linked complex oligosaccharides and (ii) amino acid mutations in the antibody Fc region. In this study, we compare the biological activities of the methods of generating high ADCC antibodies. We used a fucose-negative antibody and two antibodies with sets of mutations, demonstrated previously to optimally enhance ADCC using the chimeric anti-CD20 antibody, rituximab, as the model. Both amino acid mutant antibodies showed a significantly higher affinity for recombinant FcgammaRIIIa than fucose-negative antibody when compared using biosensor analysis. The removal of fucose from the antibodies bearing amino acid mutations exhibited a further enhancement of binding to recombinant FcgammaRIIIa and significantly increased binding to natural killer (NK) cells. Despite the differences manifested in binding for the FcgammaR, ADCCs were indistinguishable between methods and even when the methods were combined. These results indicate that the affinity of binding to FcgammaRIIIa does not predict ADCC beyond a certain threshold and that each method alone is sufficient to induce maximal ADCC of the antibody.

Apoptosis assays with lymphoma cell lines: problems and pitfalls

Much attention has been focused on the manner in which tumour cells die after treatment with cytotoxic agents. The basic question is whether cells die via apoptosis or via direct damage from the toxic agent. Various assays have been used to make this distinction. However, we show herein that some of the widely used assays for apoptosis do not in fact distinguish between apoptosis and other forms of cell death. More specifically: (1) A sub-G₁ DNA content, identified by propidium iodide staining, does not distinguish between apoptotic and necrotic cells; (2) loss of mitochondrial membrane potential does not distinguish between apoptotic and necrotic cells, unless combined with an assay for an intact cell membrane; (3) subcellular fragments that arise from dead cells or from apoptotic bodies can interfere with some assays for apoptosis such as annexin V staining, as they may be close to the size of intact cells, making it difficult to decide where to set the size threshold; (4) irradiated cells display a large increase in nonspecific Ab binding. This may be partly due to an increase in cell size, but, regardless of the cause, it can lead to a mistaken conclusion that there is an increase in a particular antigen if appropriate control reagents are not tested; and (5) experiments utilising Ab crosslinking have neglected the role of cell aggregation, which can cause multiple problems including death from mechanical stress when cells are handled. Consideration of these factors will improve our ability to determine the mode of cell death.

In Vitro Antitumoral Activity of Baculovirus-expressed Chimeric Recombinant Anti-CD4 Antibody 13B8.2 on T-cell Lymphomas

A baculovirus-expressed chimeric recombinant IgG1 (rIgG1) antibody, with Cgamma1 and Ckappa human constant domains, was derived from the murine monoclonal antibody 13B8.2, which is specific for the CDR3-like loop of the CD4 molecule. The recombinant IgG1 antibody 13B8.2 was previously shown to inhibit HIV-1 replication and to abrogate the one-way mixed-lymphocyte reaction and block proliferation of CD3-stimulated peripheral blood CD4 lymphocytes from healthy donors. Before testing this recombinant anti-CD4 antibody in in vivo preclinical trials, in vitro mechanisms of action of rIgG1 13B8.2 were assessed using various CD4 T-cell lymphomas. The baculovirus-expressed rIgG1 13B8.2 antibody led to 14% to 40% proliferation inhibition of the lymphoblastic leukaemia-derived SUP-T1, the acute T lymphoma-derived CCRF-CEM and Jurkat, and the cutaneous T-Cell lymphoma-derived HUT-78 cell lines, but it did not affect the cell cycle nor induce cell apoptosis. rIgG1 antibody 13B8.2 bound the C1q fraction, leading to 9% to 17% complement-mediated lysis of the HUT-78, H9, Sup-T1, and the CCRF-CEM cell lines. No correlation was observed between cell sensitivity to rIgG1 13B8.2-triggered complement-dependent lysis and CD35-, CD46-, CD55-, and CD59-surface expression on T lymphoma cells. Using fluorescence-activated cell sorter analysis, the antibody was shown to bind to FcgammaRI/CD64-transfected IIA1.6, FcgammaRII/CD32-transfected CDw32L, and FcgammaRIII/CD16-transfected Jurkat CD16 cell lines. In correlation with these findings, rIgG1 13B8.2 induced 11% to 31% antibody-dependent cell-mediated cytotoxicity of the CCRF-CEM, SUP-T1, A2.01 CD4, and Jurkat cell lines. These convincing results on the activity of the recombinant chimeric anti-CD4 antibody 13B8.2 have led us to perform in vivo preclinical study in a murine xenograft model of CD4 lymphomas.

Follicular NHL: from antibodies and vaccines to graft-versus-lymphoma effects

Monoclonal antibody therapy with rituximab in combination with standard chemotherapy has improved the survival of patients with advanced-stage follicular lymphoma (FL). A series of next-generation anti-CD20 antibodies may be less immunogenic and have even greater antitumor activity through augmented interactions with host effector mechanisms responsible for tumor cell kill. Additional approaches with patient-specific immunoglobulin idiotype vaccines; novel monoclonal antibodies binding to biologically active cell-surface antigen are also demonstrating early clinical activity. Antibodies targeting radioisotopes, toxins or drugs are also slowly entering clinical trials and practice. Last, allogeneic stem cell transplantation following reduced-intensity conditioning provides graft-versus-tumor immune responses that may be able to control FL and allow this risky but potentially curative treatment option to older patents or those with comorbidities.

Fcgamma receptor-dependent effector mechanisms regulate CD19 and CD20 antibody immunotherapies for B lymphocyte malignancies and autoimmunity

Immunotherapy using Rituximab, an unconjugated CD20 monoclonal antibody, has proven effective for treating non-Hodgkin's lymphoma and autoimmune disease. CD19 antibody immunotherapy is also effective in mouse models of lymphoma and autoimmunity. In both cases, mouse models have demonstrated that effector cell networks effectively deplete the vast majority of circulating and tissue B lymphocytes through Fcgamma receptor-dependent pathways. In mice, B cell depletion is predominantly, if not exclusively, mediated by monocytes. CD20 mAbs rapidly deplete circulating and tissue B cells in an antibody isotype-restricted manner with a hierarchy of antibody effectiveness: IgG2a/c > IgG1 > IgG2b >> IgG3. Depending on antibody isotype, mouse B cell depletion is regulated by FcgammaRI-, FcgammaRII-, FcgammaRIII-, and FcgammaRIV-dependent pathways. The potency of IgG2a/c mAbs for B cell depletion in vivo results from FcgammaRIV interactions, with likely contributions from high-affinity FcgammaRI. IgG1 mAbs induce B cell depletion through preferential, if not exclusive, interactions with low-affinity FcgammaRIII, while IgG2b mAbs interact preferentially with intermediate-affinity FcgammaRIV. By contrast, inhibitory FcgammaRIIB-deficiency significantly increases CD20 mAb-induced B cell depletion at low mAb doses by enhancing monocyte function. Thus, isotype-specific mAb interactions with distinct FcgammaRs contribute significantly to the effectiveness of CD20 mAbs in vivo. These results provide a molecular basis for earlier observations that human FcgammaRII and FcgammaRIII polymorphisms correlate with the in vivo effectiveness of CD20 antibody therapy. That the innate monocyte network depletes B cells through FcgammaR-dependent pathways during immunotherapy has important clinical implications for CD19, CD20, and other antibody-based therapies for the treatment of diverse B cell malignancies and autoimmune disease.

CD32B, the human inhibitory Fc-γ receptor IIB, as a target for monoclonal antibody therapy of B-cell lymphoma

Human CD32B (FcγRIIB), the low-affinity inhibitory receptor for IgG, is the predominant Fc receptor (FcR) present on B cells. Immunohistochemical and expression studies have identified CD32B expression in a variety of B-cell malignancies, suggesting that CD32B is a potential immunotherapeutic target for B-cell malignancies. A high-affinity monoclonal antibody (mAb 2B6), from a novel panel of anti–human CD32B–specific mAbs, was chimerized (ch2B6) and humanized (hu2B6-3.5). Both ch2B6 and hu2B6-3.5 were capable of directing cytotoxicity by peripheral blood mononuclear cells and monocyte-derived macrophages against B-lymphoma lines in vitro. In a human B-cell lymphoma mouse xenograft model, administration of ch2B6 or hu2B6-3.5 reduced tumor growth rate and improved tumor-free survival. Both the in vitro and in vivo activities of 2B6 required an intact Fc, suggesting an FcR-mediated mechanism of action. These data support the hypothesis that CD32B is a viable target for mAb treatment of B-cell lymphoproliferative disorders.

The Fc Portion of UV3, an Anti-CD54 Monoclonal Antibody, Is Critical for its Antitumor Activity in SCID Mice With Human Multiple Myeloma or Lymphoma Cell Lines

UV3 is a monoclonal antibody that recognizes human CD54 (intercellular adhesion molecule-1), and it was generated for the therapy of human multiple myeloma. In a severe combined immunodeficient (SCID) xenograft model of human multiple myeloma, UV3 significantly prolonged the survival of mice with either early or advanced stages of disease. However, the mechanism by which UV3 exerted its antitumor effect remained unknown. As reported previously UV3 could mediate antibody-dependent cell-mediated cytotoxicity or complement-dependent cytotoxicity in vitro. F(ab)'2 fragments of UV3 had therapeutic efficacy in vivo, suggesting that effector functions were not critical. The purpose of this study was to further define the importance of the Fc portion of UV3 for its antitumor activity in vivo. To this end, we examined the effect of an "ultrapure" preparation of UV3 F(ab)'2 to treat SCID mice xenografted with either ARH-77 cells, a human multiple myeloma cell line, or Daudi cells, a human Burkitt's lymphoma cell line. In addition, we evaluated different doses of UV3 immunoglobulin G (IgG) in these mice to determine the minimum amount of IgG that would produce a therapeutic effect. Data obtained from this study suggest that (1) the Fc portion of UV3 is critical for its antitumor activity in vivo, (2) low levels of UV3 IgG in a preparation of F(ab)'2 fragments account for all of its in vivo activity in multiple myeloma and most of its activity in lymphoma, and (3) UV3 IgG significantly prolongs the survival of SCID/ARH-77 mice as well as SCID/Daudi mice.

Comparing antibody and small-molecule therapies for cancer

The 'magic bullet' concept of specifically targeting cancer cells at the same time as sparing normal tissues is now proven, as several monoclonal antibodies and targeted small-molecule compounds have been approved for cancer treatment. Both antibodies and small-molecule compounds are therefore promising tools for target-protein-based cancer therapy. We discuss and compare the distinctive properties of these two therapeutic strategies so as to provide a better view for the development of new drugs and the future direction of cancer therapy.

Rituximab but not Other anti-CD20 Antibodies Reverses Multidrug Resistance in 2 B lymphoma Cell Lines, Blocks the Activity of P-glycoprotein (P-gp), and Induces P-gp to Translocate out of Lipid Rafts

The objective of this study was to investigate the ability of the anti-CD20 antibody, Rituximab (RTX), to inhibit the activity of P-glycoprotein (P-gp), and reverse multidrug resistance (MDR) in 2 P-gp/CD20 lymphoma cell lines. We determined whether RTX would chemosensitize the 2 P-gp cell lines in vitro, and inhibit the ability of the cells to efflux Rhodamine 123. One cell line was infected with an MDR1 vector and the other was generated by drug selection. We also determined whether RTX induced P-gp to translocate out of lipid rafts. RTX chemosensitized 2 different MDR cell lines, inhibited the activity of P-gp in both, and induced P-gp to translocate out of lipid rafts in the 1 cell line that was studied in greater detail. In contrast, 3 other anti-CD20 antibodies did not chemosensitize, inhibit the activity of P-gp, or induce it to translocate out of rafts, despite the fact that 1 antibody recognized the same epitope on CD20. Our results suggest that RTX can chemosensitize 2 CD20/P-gp cell lines in vitro by inhibiting the activity of the P-gp pump. The inhibition of P-gp activity correlated with the ability of RTX to induce P-gp to translocate out of lipid rafts. Although the mechanisms by which RTX effects P-gp translocation and activity are not yet known, they are not associated with acid-sphingomyelinase activation in raft microdomains, as described for the antiproliferative activity of RTX.

Anti-CD20 monoclonal antibody with enhanced affinity for CD16 activates NK cells at lower concentrations and more effectively than rituximab

Growing evidence indicates that the affinity of monoclonal antibodies (mAbs) for CD16 (FcgammaRIII) plays a central role in the ability of the mAb to mediate antitumor activity. We evaluated how CD16 polymorphisms, and mAb with modified affinity for target antigen and CD16, affect natural killer (NK) cell phenotype when CD20(+) malignant B cells were also present. The mAb consisted of rituximab (R), anti-CD20 with enhanced affinity for CD20 (AME-B), and anti-CD20 with enhanced affinity for both CD20 and CD16 (AME-D). Higher concentrations of mAb were needed to induce CD16 modulation, CD54 up-regulation, and antibody-dependent cellular cytotoxicity (ADCC) on NK cells from subjects with the lower affinity CD16 polymorphism. The dose of mAb needed to induce NK activation was lower with AME-D irrespective of CD16 polymorphism. At saturating mAb concentrations, peak NK activation was greater for AME-D. Similar results were found with measurement of CD16 modulation, CD54 up-regulation, and ADCC. These data demonstrate that cells coated with mAb with enhanced affinity for CD16 are more effective at activating NK cells at both low and saturating mAb concentrations irrespective of CD16 polymorphism, and they provide further evidence for the clinical development of such mAbs with the goal of improving clinical response to mAb.

The anti-CD20 antibody rituximab augments the immunospecific therapeutic effectiveness of an anti-CD19 immunotoxin directed against human B-cell lymphoma

The chimaeric anti-CD20 antibody rituximab (Rituxan) sensitises lymphoma cells to small molecule cytotoxic drugs and to protein toxins. We have explored the augmentive effect of rituximab on the anti-CD19 immunotoxin BU12-SAPORIN in a model of human lymphoma. Intact rituximab and its F(ab)2 derivative both augmented the immunospecific protein synthesis inhibitory effects of BU12-SAPORIN in a complement-independent manner. A combination of rituximab + BU12-SAPORIN completely abolished the proliferation of Ramos cells in vitro and also induced a significantly greater degree of apoptosis in these cells. Treatment with rituximab, BU12-SAPORIN or a combination of both induced poly(ADPribose) polymerase and caspase 3 cleavage, although this was always consistently greater in combination-treated cells. zVAD almost completely inhibited apoptosis in rituximab- or BU12-SAPORIN-treated cells but only partially in combination-treated cells. In severe combined immunodeficient (SCID)-Ramos mice the combination of rituximab + BU12-SAPORIN was significantly better therapeutically than either single agent. The immunological fidelity of the therapeutic effect because of combination treatment was demonstrated through the failure of rituximab to augment an irrelevant anti-CD7 immunotoxin. The therapeutic efficacy of rituximab and combination treatment was reduced in SCID-Ramos mice depleted of serum complement while natural killer cell depletion failed to show any convincing role for antibody-dependent cellular cytotoxicity. This study shows a clear therapeutic advantage from using rituximab to immunospecifically augment immunotoxin cytotoxicity warranting further investigation.

Antitumor Antibodies in the Treatment of Cancer: Fc Receptors Link Opsonic Antibody with Cellular Immunity

Engineered antibody therapeutics have provided new treatment options in cancer. Genetic evidence in man and in the mouse suggests that Fc receptor (FcR) engagement contributes mechanistically to the therapeutic activity of naked antibodies. Preferential activation of activating FcRs and limited engagement of inhibitory FcRs enhance tumor responses in mouse models. Thus, engineered Fc domains with favorable affinities for specific FcR types may prove to be clinically superior.

Potent antibody therapeutics by design

Antibodies constitute the most rapidly growing class of human therapeutics and the second largest class of drugs after vaccines. The generation of potent antibody therapeutics, which I review here, is an iterative design process that involves the generation and optimization of antibodies to improve their clinical potential.

Complement Activated by Chimeric Anti–Folate Receptor Antibodies Is an Efficient Effector System to Control Ovarian Carcinoma

Two chimeric monoclonal antibodies (mAb), cMOV18 and cMOV19, recognizing distinct epitopes of folate receptor highly expressed on epithelial ovarian cancer (EOC) cells were analyzed for their ability to activate complement (C) as a means to enhance their antitumor activity. The individual cMOVs failed to activate C on six EOC cell lines as documented by the marginal deposition of C components and the negligible C-dependent cytotoxicity (CDC). Conversely, the mixture of cMOVs was more effective, although the percentage of cell killing did not exceed 25%. Fluorescence-activated cell sorting analysis of EOC cells for surface expression of the membrane C regulatory proteins (mCRP) revealed high levels of CD46, variable expression of CD59, and absence of CD55. This finding was confirmed in tumor tissue specimens obtained from advanced-stage EOC patients and analyzed for the expression of mCRPs mRNA using a cDNA microarray and for the presence of proteins by immunohistochemistry. Incubation of EOC cells with neutralizing mAbs to CD46 and CD59 led to a significant increase in the CDC from 10%-20% to 45%-50%. The relative contribution of antibody-dependent cell cytoxicity (ADCC) and C-dependent killing of two EOC cell lines induced by the mixture of cMOV18 and cMOV19 was about 15% and 25%-35%, respectively, bringing the total killing to about 40%-50%. This value increased to 60%-70% after neutralization of CD46 and CD59 without an appreciable change of ADCC. These results suggest that C is the major contributor to the killing of EOC cells induced by the mixture of cMOV18 and cMOV19.

Biological activities on T lymphocytes of a baculovirus-expressed chimeric recombinant IgG1 antibody with specificity for the CDR3-like loop on the D1 domain of the CD4 molecule

A baculovirus-expressed chimeric recombinant IgG1 (rIgG1) antibody, with Cgamma1 and Ckappa human constant domains, was derived from the murine monoclonal antibody (mAb) 13B8.2, which is specific for the CDR3-like loop of the CD4 molecule and which inhibits HIV-1 replication. Chimeric rIgG1 antibody 13B8.2 blocked, in a dose-dependent manner, antigen presentation through inhibition of subsequent IL-2 secretion by stimulated T cells. The one-way mixed lymphocyte reaction was abrogated by previous addition of baculovirus-produced rIgG1 13B8.2 in the T-cell culture. Anti-proliferative activity of rIgG1 was demonstrated on CD3-activated CD4+ T lymphocytes from healthy donors, such effect being associated with reduced IL-2 secretion of activated T cells. On the other hand, no proliferation inhibition was observed on CD4+ T lymphocytes activated with phorbol ester plus ionomycin, suggesting that rIgG1 13B8.2 preferentially acts on a proximal TCR-induced signaling pathway. Treatment of DBA1/J human CD4-transgenic mice with 100 microg of recombinant antibody for three consecutive days led to in vivo recovery of rIgG1 antibody 13B8.2 both coated on murine T lymphocytes and free in mouse serum, without CD4 depletion or down-modulation. These findings predict that the baculovirus-expressed chimeric rIgG1 anti-CD4 antibody 13B8.2 is a promising candidate for immunotherapy.

Caspase-independent killing of Burkitt lymphoma cell lines by rituximab

Caspase-independent cell death may have a critical role to play in the therapeutic destruction of tumours. Recently it has been suggested that one of the mechanisms by which rituximab, a therapeutic anti-CD20 antibody, kills B cells is caspase-independent. In this study we show that rituximab can induce death in a variety of Burkitt lymphoma derived cell lines. Rituximab-treated cells show leakage of adenylate kinase, surface expression of phosphatidylserine, upregulation of the cellular stress protein HSP70, phosphorylation of the survival protein Akt, and depolarisation of the mitochondrial membrane but no loss of cytochrome c or apoptosis inducing factor. Caspase inhibitors do not block these events. In support of these data there is no cleavage of caspases 3, 8 and 9, poly(ADP-ribose) polymerase, BH3 interacting domain death agonist or genomic DNA. Morphologically, cells show nuclear enlargement and cytoplasmic vacuolisation. Triggering of receptor mediated death in CD95 responsive lines results in "classical" apoptosis indicating that the internal machinery necessary for apoptosis is intact in these lines. The results suggest that rituximab can kill human B cells via a caspase-independent form of programmed cell death that shares features of apoptosis and necrosis. This pathway may be relevant to the clinical efficacy of rituximab.

Anti-CD70 antibodies: a potential treatment for EBV+ CD70-expressing lymphomas

A monoclonal antibody (Rituximab) directed against the B-cell surface antigen, CD20, is increasingly used as a therapy for B-cell lymphomas. However, CD20 is expressed on all normal mature B cells and hence is not a specific tumor target. In contrast, CD70 is expressed on highly activated lymphocytes as well as on many B-cell and T-cell lymphomas but is not expressed on the great majority of B cells and T cells. In this report, we have explored the potential utility of anti-CD70 monoclonal antibodies for treatment of CD70+ EBV+ B-cell lymphomas. Using two Burkitt's lymphoma lines (Raji and Jijoye) that express surface CD70 and a CD70- Burkitt's lymphoma line (Akata), we show that two different monoclonal antibodies directed against human CD70 allow rabbit and human complement to kill EBV+ B cells in a CD70-dependent manner in vitro. In the absence of complement, neither anti-CD70 antibody induced in vitro killing of CD70+ cell lines. Importantly, i.p. injection of anti-CD70 antibodies also inhibited the growth of CD70+ Burkitt's lymphoma cells in severe combined immunodeficient mice but did not inhibit the growth of CD70- Burkitt's lymphoma cells. These results suggest that anti-CD70 antibodies may be useful for the treatment of CD70+ B-cell lymphomas.

The Shaving Reaction: Rituximab/CD20 Complexes Are Removed from Mantle Cell Lymphoma and Chronic Lymphocytic Leukemia Cells by THP-1 Monocytes

Clinical investigations have revealed that infusion of immunotherapeutic mAbs directed to normal or tumor cells can lead to loss of targeted epitopes, a phenomenon called antigenic modulation. Recently, we reported that rituximab treatment of chronic lymphocytic leukemia patients induced substantial loss of CD20 on B cells found in the circulation after rituximab infusion, when rituximab plasma concentrations were high. Such antigenic modulation can severely compromise therapeutic efficacy, and we postulated that B cells had been stripped (shaved) of the rituximab/CD20 complex by monocytes or macrophages in a reaction mediated by FcgammaR. We developed an in vitro model to replicate this in vivo shaving process, based on reacting rituximab-opsonized CD20(+) cells with acceptor THP-1 monocytes. After 45 min at 37 degrees C, rituximab and CD20 are removed from opsonized cells, and both are demonstrable on acceptor THP-1 cells. The reaction occurs equally well in the presence and absence of normal human serum, and monocytes isolated from peripheral blood also promote shaving of CD20 from rituximab-opsonized cells. Tests with inhibitors and use of F(ab')(2) of rituximab indicate transfer of rituximab/CD20 complexes to THP-1 cells is mediated by FcgammaR. Antigenic modulation described in previous reports may have been mediated by such shaving, and our findings may have profound implications for the use of mAbs in the immunotherapy of cancer.

Improved antigen binding by a CD20-specific single-chain antibody fragment with a mutation in CDRH1

We have prepared single-chain immunoglobulin Fv fragments from the CD20-specific hybridoma HB13d. One scFv clone demonstrated strong binding to a CD20-derived peptide by ELISA and to CD20-positive cells by flow cytometry, a second had reduced binding, and a third clone did not bind the target antigen. Sequence analysis showed that all three constructs contained shared and unique amino acid changes when compared to the nearest germline match. Molecular modelling of the scFv variants revealed that several of the mutations are located in regions predicted to contact antigen, including a mutation in the heavy chain CDR1 of the strongest binding scFv construct. No similar mutation is present in the highly conserved protein sequences of a number of CD20-specific monoclonal antibodies. BIACORE analysis demonstrated that the mutated scFv had approximately three-fold greater antigen-binding activity than another clone. Competition studies showed that the scFv is able to compete with intact CD20 monoclonal antibody for binding to the target antigen. The improved antigen binding of this scFv will permit the construction of novel CD20-specific reagents for the therapy of lymphomas.

Establishment and characterization of a new erythroblastic leukemia cell line, EEB: phosphatidylglucoside-mediated erythroid differentiation and apoptosis

A new erythroblastic leukemia cell line (EEB) was established from a patient with early erythroblastic leukemia. The cells had features of immature erythroblasts, including an agranular basophilic cytoplasm and CD36, CD71, CD175s (sialyl-Tn) and CD235a (glycophorin A) expression without CD41 expression, myeloperoxidase activity and platelet-peroxidase activity. The cells were confirmed to be of the erythroid lineage based on expression of the gamma-globin message. They were induced to differentiate into benzidine-positive cells by hemin and delta-amino levulinic acid (delta-ALA). An analysis of cell membrane lipids showed that EEB cells contain a type of glycerolipid, phosphatidylglucose (PhGlc), but not unbranched type 2 chains, i antigens. GL-7 which is a recombinant Fab fragment of GL-2 and binds to PhGlc, induced production of hemoglobin F (HbF) associated with accumulation of the gamma-globin (gamma-globin) message in EEB cells. The GL-7-mediated erythroid differentiation was associated with apoptosis. These results suggest that direct signaling to PhGlc mediates erythroid differentiation and apoptosis in EEB cells.

How does B cell depletion therapy work, and how can it be improved?

The past few years have seen a surge of interest in B cell depletion therapy for patients with rheumatoid arthritis. This paper outlines the possible mechanism(s) by which B cell depletion therapy works. It is likely there is more than one mechanism and the relative importance of each mechanism depends on the target cell. These include CD20-induced apoptosis, complement dependent cytotoxicity, antibody dependent cell-mediated cytotoxicity, and selective targeting and depletion of B cell subsets. The implications of these mechanisms in the further improvement of B cell depletion therapy in rheumatoid arthritis and other autoimmune diseases are discussed.

A DeImmunized chimeric anti-C3b/iC3b monoclonal antibody enhances rituximab-mediated killing in NHL and CLL cells via complement activation

Complement-dependent cytotoxicity (CDC) is a key mechanism of Rituximab (RTX) action in killing non-Hodgkin's lymphoma (NHL) cells both in vitro and probably in vivo. A DeImmunized, mouse/human chimeric monoclonal antibody (Mab), H17, specific for cell-associated complement C3 cleavage products, C3b and iC3b, was generated to enhance RTX-mediated killing of target cells by CDC. When NHL cell lines were treated with RTX and H17 in the presence of complement for 1 h, there was 40-70% more cell death than that observed with RTX alone. The enhancing effect of H17 was also seen over longer treatment periods. H17 was tested ex vivo against primary cells from NHL and chronic lymphocytic leukemia (CLL) patients. In RTX-resistant NHL samples, H17 enhanced RTX-mediated killing; in the remaining samples RTX + complement alone promoted more than 80% killing, and no significant enhancement was observed. The H17 antibody also increased RTX-mediated killing in four out of nine CLL samples. H17 may have therapeutic applications in NHL and CLL treatment as an adjunctive therapy to RTX. It might also enhance the activity of other therapeutic antibodies that work through CDC.

Cholesterol depletion inhibits src family kinase-dependent calcium mobilization and apoptosis induced by rituximab crosslinking

The monoclonal antibody (mAb) rituximab produces objective clinical responses in patients with B-cell non-Hodgkin's lymphoma and antibody-based autoimmune diseases. Mechanisms mediating B-cell depletion by rituximab are not completely understood and may include direct effects of signalling via the target antigen CD20. Like most but not all CD20 mAbs, rituximab induces a sharp change in the solubility of the CD20 protein in the non-ionic detergent Triton-X-100, reflecting a dramatic increase in the innate affinity of CD20 for membrane raft signalling domains. Apoptosis induced by rituximab hypercrosslinking has been shown to require src family kinases (SFK), which are enriched in rafts. In this report we provide experimental evidence that SFK-dependent apoptotic signals induced by rituximab are raft dependent. Cholesterol depletion prevented the association of hypercrosslinked CD20 with detergent-insoluble rafts, and attenuated both calcium mobilization and apoptosis induced with rituximab. CD20 cocapped with the raft-associated transmembrane adaptor LAB/NTAL after hypercrosslinking with CD20 mAbs, regardless of their ability to induce a change in the affinity of CD20 for rafts. Taken together, the data demonstrate that CD20 hypercrosslinking via rituximab activates SFKs and downstream signalling events by clustering membrane rafts in which antibody-bound CD20 is localized in a high-affinity configuration.

Rituximab (chimeric anti-CD20) sensitizes B-NHL cell lines to Fas-induced apoptosis

Rituximab (chimeric anti-CD20 monoclonal antibodies) is currently being used in the treatment of B non-Hodgkin's lymphoma (NHL). We have recently reported that rituximab triggers and modifies various intracellular signaling pathways in NHL B-cell lines, resulting in reverting the chemoresistant phenotype to a sensitive phenotype. This study investigated whether rituximab also modifies intracellular signaling pathways resulting in the sensitization of NHL cells to Fas-induced apoptosis. Treatment of the Fas-resistant NHL cell lines (2F7, Ramos and Raji) with rituximab sensitized the cells to CH-11 (FasL agonist mAb)-induced apoptosis and synergy was achieved. Fas expression was upregulated by rituximab as early as 6 h post-treatment as determined by flow cytometry, reverse transcriptase-polymerase chain reaction and Western blot. Rituximab inhibited both the expression and activity of the transcription repressor Yin-Yang 1 (YY1) that negatively regulates Fas transcription. Inhibition of YY1 resulted in the upregulation of Fas expression and sensitization of the tumor cells to CH-11-induced apoptosis. The downregulation of YY1 expression was the result of rituximab-induced inhibition of both the p38 mitogen-activated protein kinase (MAPK) signaling pathway and constitutive nuclear factor kappa of B cells (NF-kappaB) activity. The involvement of NF-kappaB and YY1 in the regulation of Fas expression was corroborated by the use of Ramos cells with a dominant-active inhibitor of NF-kappaB (Ramos IkappaB-estrogen receptor (ER) mutant) and by silencing YY1 with YY1 siRNA, respectively. Further, the role of rituximab-mediated inhibition of the p38 MAPK/NF-kappaB/YY1 pathway in the regulation of Fas and sensitization to CH-11-induced apoptosis was validated by the use of specific chemical inhibitors of this pathway and which mimicked rituximab-mediated effects. These findings provide a novel mechanism of rituximab-mediated activity by sensitizing NHL cells to Fas-induced apoptosis.

In vivo tumouricidal effects of LAD-1 monoclonal antibody on murine RL-male-1 lymphoma mediated by enhanced phagocytosis

We have reported previously that the LAD-4 monoclonal antibody (mAb) directed against a fibronectin receptor (FNR) on RL-male-1 T lymphoma cells in BALB/c mice partially inhibited their migration to the liver. In the present study, we examined the mechanism by which another anti-FNR mAb, LAD-1, exerts its antitumourigenic effects. Administration of LAD-1 significantly prolonged survival of BALB/c mice challenged previously with RL-male-1 cells. LAD-1 enhanced phagocytosis of RL-male-1 cells by hepatic macrophages and clodronate-mediated macrophage depletion abrogated the antitumour activity of LAD-1. In vitro experiments revealed that a pan-caspase inhibitor, zVAD-fmk, did not affect the ability of LAD-1 to inhibit the proliferation of RL-male-1 cells. These data suggest that the antitumour effects of LAD-1 may be dependent on stimulation of tumour cell phagocytosis and are apoptosis-independent. Thus, LAD-1-induced phagocytosis of lymphoma cells by hepatic macrophages in mice may, at least in part, be responsible for the prolonged survival of the mice.

Cell membrane-specific epitopes on CD30: Potentially superior targets for immunotherapy

Because CD30 is highly expressed on Hodgkin's lymphoma and anaplastic large cell lymphoma, it is a promising target for immunotherapy. Soluble CD30, the extracellular domain of CD30 that is shed from the cells, can reduce the effects of CD30-targeting agents by competitive binding. In this study, we identified two epitopes on membrane-associated CD30 that are missing on soluble CD30 probably because of a conformational change upon shedding. These epitopes are potentially superior targets for immunotherapy because targeting them should be free from the competitive effects of soluble CD30. We studied 27 anti-native CD30 mAbs that were assigned to 8 different topographical epitopes. Soluble CD30 was prepared from culture supernatants of L540 cells or Karpas 299 cells. In an ELISA, the mAbs to two epitopes, Ep2 (amino acids 107-153) and Ep7 (amino acids 282-338), showed less than a 2% average cross-reactivity to soluble CD30 compared with a CD30-Fc fusion protein. In addition, these mAbs bound to CD30 on cells in the presence of an excess of soluble CD30. These epitopes (Ep2 and Ep7) are, therefore, more efficiently presented on cell-associated CD30 than on soluble CD30 (membrane-specific epitopes). Also, soluble CD30 in the sera of mice bearing L540 tumors did not form immune complexes with the membrane-specific mAbs analyzed by size-exclusion chromatography. In contrast, mAbs to the other epitopes reacted with both soluble CD30 and membrane CD30. Our results suggest that it may be possible to find membrane-specific epitopes on other immunotherapy target molecules.

Long-term results of the R-CHOP study in the treatment of elderly patients with diffuse large B-cell lymphoma: a study by the Groupe d'Etude des Lymphomes de l'Adulte

PURPOSE

To analyze the long-term outcome of patients included in the Lymphome Non Hodgkinien study 98-5 (LNH98-5) comparing cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP) to rituximab plus CHOP (R-CHOP) in elderly patients with diffuse large B-cell lymphoma.

PATIENTS AND METHODS

LNH98-5 was a randomized study that included 399 previously untreated patients, age 60 to 80 years, with diffuse large B-cell lymphoma. Patients received eight cycles of classical CHOP (cyclophosphamide 750 mg/m(2), doxorubicin 50 mg/m(2), vincristine 1.4 mg/m(2), and prednisone 40 mg/m(2) for 5 days) every 3 weeks. In R-CHOP, rituximab 375 mg/m(2) was administered the same day as CHOP. Survivals were analyzed using the intent-to-treat principle.

RESULTS

Median follow-up is 5 years at present. Event-free survival, progression-free survival, disease-free survival, and overall survival remain statistically significant in favor of the combination of R-CHOP (P = .00002, P < .00001, P < .00031, and P < .0073, respectively, in the log-rank test). Patients with low-risk or high-risk lymphoma according to the age-adjusted International Prognostic Index have longer survivals if treated with the combination. No long-term toxicity appeared to be associated with the R-CHOP combination.

CONCLUSION

Using the combination of R-CHOP leads to significant improvement of the outcome of elderly patients with diffuse large B-cell lymphoma, with significant survival benefit maintained during a 5-year follow-up. This combination should become the standard for treating these patients.

A review of tositumomab and I131tositumomab radioimmunotherapy for the treatment of follicular lymphoma

The CD20 antigen has become a major therapeutic target in the management of follicular and other B cell non-Hodgkin's lymphomas. The murine monoclonal antibody, tositumomab, on binding CD20, is able to induce antibody-dependent cellular cytotoxicity, complement-dependent cytotoxicity and apoptosis. In addition, when radioiodinated, the antibody exploits the tumour's sensitivity to ionising radiation by direct targeting of the malignant cell. Tositumomab and Iodine (I(131)) tositumomab (Bexxar, GlaxoSmithKline, Philadelphia, PA, USA) is administered in two steps. The dosimetric step determines individual patient pharmacokinetics, allowing a patient- specific dose to be calculated. This is followed by the therapeutic step, with administration of the therapeutic dose between 7 and 14 days after the dosimetric dose. Over a decade's worth of experience in clinical trials has determined the efficacy and safety of the regimen in a variety of clinical circumstances; establishment of exactly where the regimen fits amongst the algorithm for the management of follicular lymphoma continues.

Unique toxicities and resistance mechanisms associated with monoclonal antibody therapy

Anti-CD20 therapy has had a truly dramatic impact on treatment and outcome of patients with follicular lymphoma. Unfortunately, the majority of responses to single-agent rituximab are incomplete, and all patients with follicular lymphoma will experience disease progression at some point following rituximab therapy. Rituximab has multiple mechanisms of inducing in vivo cytotoxicity, including antibody-dependent cell-mediated cytotoxicity, complement-dependent cytotoxicity, direct apoptotic signaling, and possible vaccinal effects. The cellular microenvironment within follicular lymphoma has a profound impact on which mechanism is dominant, and confers resistance in many situations. Both tumor-associated and host-associated factors also contribute to rituximab resistance. There are multiple potential approaches to overcoming rituximab resistance, including rational biologic combination immunotherapy, engineered antibodies, and radioimmunoconjugates. Improved ability to overcome resistance will require further elucidation of critical signaling pathways involved in rituximab induced cytotoxicity and a comprehensive understanding of interactions between its multiple mechanisms of action.

Efficacy and safety of tositumomab and iodine-131 tositumomab (Bexxar) in B-cell lymphoma, progressive after rituximab

PURPOSE

To determine overall response (OR) and complete response (CR) rates, response duration, progression-free (PFS) and overall survival and safety with the tositumomab and iodine-131 tositumomab ((131)I tositumomab) therapeutic regimen in patients with indolent, follicular large-cell, or transformed B-cell lymphoma, progressive after rituximab.

PATIENTS AND METHODS

From July 1998 to November 1999, 40 patients (24 rituximab nonresponders: 11 with response < 6 months, and five with response > or = 6 months) received a therapeutic dose (0.65 to 0.75 Gy per platelet count) of (131)I tositumomab based on total-body dosimetry in this prospective phase II study. The median number of prior treatments was four; 59% of patients were chemotherapy-resistant.

RESULTS

Confirmed OR (65%) and CR (38%) rates were not significantly associated with prior rituximab response. With a median follow-up of 3.3 years, the median PFS was 10.4 months, 24.5 months for responders, and not reached for CR patients. Among follicular grade 1 or 2 patients with tumors < or = 7 cm (n = 21), the OR and CR rates were 86% and 57%. Estimated 3-year PFS in this subgroup was 48%, compared with 11% for all others (P = .002). Transient grade 3 to 4 marrow toxicity was seen in 50% of patients. Two patients, one of whom received two subsequent chemotherapy regimens, developed secondary myelodysplasia.

CONCLUSION

(131)I tositumomab is effective in CD20-positive lymphoma progressive after rituximab, with a 65% OR rate and median PFS of 24.5 months for responders. Patients with follicular grade 1 or 2 histology and tumors < or = 7 cm achieved very high OR and CR rates, with 48% PFS at 3 years.

Antibody-targeted therapy: a paradigm of innovative treatment strategies in indolent and aggressive B-cell non-Hodgkin lymphoma

PURPOSE OF REVIEW

This review outlines the principles of treatment of indolent and aggressive B-cell lymphoma based on current knowledge on the classification of hematologic malignancies and the rationale to implement new antibody-targeted immunotherapeutic approaches.

RECENT FINDINGS

An update is provided on the use of antibody-targeted therapies in clinical trials, with emphasis on new, emerging strategies of immunotherapy in B-cell non-Hodgkin lymphoma.

SUMMARY

The success of immune-mediated therapies has encouraged studies on antibody-targeted therapy in B-cell non-Hodgkin lymphoma. Promising new approaches combine classical dose-intense chemotherapy with "tumor-specific" antibody targeting during several phases of the disease. The safety and efficacy of anti-CD20 in the treatment of indolent and aggressive B-cell non-Hodgkin lymphoma at any stage of disease, either as a single agent or as part of multimodality regimes, as an unconjugated antibody or as radioimmunoconjugate have changed dramatically our treatment strategies. Increasing insights into basic molecular biology and immunology of B-cell non-Hodgkin lymphoma may identify subgroups of patients categorized in current classification systems who may benefit from tailored approaches with new modality antibody-targeted therapy in near future.

From the bench to the bedside: ways to improve rituximab efficacy

Rituximab (MabThera, Rituxan) is a chimeric IgG1 monoclonal antibody that specifically targets the CD20 surface antigen expressed on normal and neoplastic B-lymphoid cells. Rituximab is currently used in the treatment of both follicular and aggressive B-cell non-Hodgkin lymphomas. Despite its demonstrated clinical effectiveness, its in vivo mechanisms of action remain unknown and could differ by subtype of lymphoma. Rituximab has been shown to induce apoptosis, complement-mediated lysis, and antibody-dependent cellular cytotoxicity in vitro, and some evidence points toward an involvement of these mechanisms in vivo. Rituximab also has a delayed therapeutic effect as well as a potential “vaccinal” effect. Here, we review the current understanding of the mechanism of action of rituximab and discuss approaches that could increase its clinical activity. A better understanding of how rituximab acts in vivo should make it possible to develop new and more effective therapeutic strategies. (Blood. 2004;104:2635-2642)

Rapid grouping of monoclonal antibodies based on their topographical epitopes by a label-free competitive immunoassay

Topography of epitopes of monoclonal antibodies (MAbs), identified as the mutual competition of the MAbs, can be valuable indicators for the biological functions of MAbs. However, the determination of topographical epitopes is not performed before the functional screening of MAbs, because the requirement for purifying and labeling of MAbs makes the mapping experiment difficult, particularly in the early stage of MAb production. Here we describe a new label-free competitive enzyme-linked immunosorbent assay (LFC-ELISA) for the rapid grouping of MAbs based on the topography of their epitopes. In the LFC-ELISA, the immune complex formed by a competitor, MAb#2, and an antigen is challenged by an indicator, MAb#1 that had been captured on the ELISA plate through a secondary antibody. The MAb#2-antigen immune complex is trapped by MAb#1 only if MAb#1 reacts with an epitope different from that of MAb#2. The immune complex (MAb#2-antigen-MAb#1) is detected with an enzyme-labeled reagent specific to a tag on the antigen. Our experiments using different anti-CD30 MAbs and a CD30-Fc fusion protein as the antigen revealed that the LFC-ELISA performed well with MAbs of different isotypes (IgG1, IgG2a, and IgG2b), and in a practical range of MAb concentrations (0.3-10 microg/ml) and affinities (0.9-13 nM of Kd). We obtained pairwise competition data from all 26 anti-CD30 MAbs. We then utilized a cluster analysis and a bootstrap method to analyze the competition data for grouping of the MAbs. This objective and automated analysis identified eight distinct topographical epitopes on CD30. The reactivity of the anti-CD30 MAbs in immunoblot, and their inhibiting activity on CD30-CD30-ligand binding correlated with the topographical epitopes. The results show that the LFC-ELISA combined with cluster analysis is a useful new method for grouping MAbs based on their topographical epitopes and can be used in the early stage of MAb production. One useful application is to identify MAbs reacting with different epitopes from a large number of MAbs so that the most appropriate MAbs can be selected for therapeutic use.

A new anti-idiotype antibody capable of binding rituximab on the surface of lymphoma cells

The chimeric anti-CD20 monoclonal antibody (mAb), rituximab, is an established part of the management of many non-Hodgkin lymphomas. The in vivo action of rituximab remains elusive, and this partially reflects a lack of highly specific reagents to detect rituximab binding at the cell surface. Here we report a new high-affinity mAb (MB2A4) with fine specificity for the idiotype of rituximab. It is able to detect rituximab in vitro, in the presence of high levels of human immunoglobulin G (IgG), in the serum of patients receiving rituximab therapy, and, surprisingly, when rituximab is bound to CD20 on the cell surface. We propose that the anti–idiotype (Id) binds to rituximab molecules bound univalently at the cell surface, facilitated by the relatively high off-rate of rituximab. This reagent provides new insights into the binding of rituximab at the cell surface and demonstrates a mode of binding that could be exploited for the surface detection of other mAbs with clinical and biologic applications.

Empowering targeted therapy: lessons from rituximab

Rituximab, a monoclonal antibody directed against the B cell-specific protein CD20, has revolutionized lymphoma treatment by providing a highly effective form of therapy with relatively mild toxic side effects. Effective as a single agent against some forms of B cell lymphoma, rituximab also has a chemosensitizing effect, enhancing the efficacy of chemotherapy against other forms of the disease. Although the mechanisms whereby rituximab achieves its effects remain incompletely understood, these seem to involve at least three distinct phenomena: (i) antibody-dependent cell-mediated cytotoxicity, (ii) complement-mediated cell lysis, and (iii) stimulation of apoptosis in target cells. The latter occurs through interaction of complexes of rituximab and CD20 in lipid rafts, with elements of a signaling pathway involving Src kinases. Effector molecules trigger various gene expression events, leading to sensitization of malignant cells to proapoptotic stimuli. Lessons learned from the research on rituximab may be applied to the rational development of antibody-based therapies against other forms of cancer.

The innate mononuclear phagocyte network depletes B lymphocytes through Fc receptor-dependent mechanisms during anti-CD20 antibody immunotherapy

Anti-CD20 antibody immunotherapy effectively treats non-Hodgkin's lymphoma and autoimmune disease. However, the cellular and molecular pathways for B cell depletion remain undefined because human mechanistic studies are limited. Proposed mechanisms include antibody-, effector cell-, and complement-dependent cytotoxicity, the disruption of CD20 signaling pathways, and the induction of apoptosis. To identify the mechanisms for B cell depletion in vivo, a new mouse model for anti-CD20 immunotherapy was developed using a panel of twelve mouse anti-mouse CD20 monoclonal antibodies representing all four immunoglobulin G isotypes. Anti-CD20 antibodies rapidly depleted the vast majority of circulating and tissue B cells in an isotype-restricted manner that was completely dependent on effector cell Fc receptor expression. B cell depletion used both FcgammaRI- and FcgammaRIII-dependent pathways, whereas B cells were not eliminated in FcR common gamma chain-deficient mice. Monocytes were the dominant effector cells for B cell depletion, with no demonstrable role for T or natural killer cells. Although most anti-CD20 antibodies activated complement in vitro, B cell depletion was completely effective in mice with genetic deficiencies in C3, C4, or C1q complement components. That the innate monocyte network depletes B cells through FcgammaR-dependent pathways during anti-CD20 immunotherapy has important clinical implications for anti-CD20 and other antibody-based therapies.