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

Increasing the efficacy of CD20 antibody therapy through the engineering of a new type II anti-CD20 antibody with enhanced direct and immune effector cell-mediated B-cell cytotoxicity

CD20 is an important target for the treatment of B-cell malignancies, including non-Hodgkin lymphoma as well as autoimmune disorders. B-cell depletion therapy using monoclonal antibodies against CD20, such as rituximab, has revolutionized the treatment of these disorders, greatly improving overall survival in patients. Here, we report the development of GA101 as the first Fc-engineered, type II humanized IgG1 antibody against CD20. Relative to rituximab, GA101 has increased direct and immune effector cell-mediated cytotoxicity and exhibits superior activity in cellular assays and whole blood B-cell depletion assays. In human lymphoma xenograft models, GA101 exhibits superior antitumor activity, resulting in the induction of complete tumor remission and increased overall survival. In nonhuman primates, GA101 demonstrates superior B cell-depleting activity in lymphoid tissue, including in lymph nodes and spleen. Taken together, these results provide compelling evidence for the development of GA101 as a promising new therapy for the treatment of B-cell disorders.

CD20-depleting therapy in autoimmune diseases: from basic research to the clinic

The B lymphocyte-associated antigen CD20 is becoming an important immunotherapy target for autoimmune diseases, although its biological function has not been defined. Besides rheumatoid arthritis, growing experience with B cell-depleting therapy indicates that it may be effective in Sjögren's syndrome, dermatomyositis-polymyositis, systemic lupus erythematosus and some types of vasculitides. However, controlled clinical trials are still lacking for some of these indications. Infection has not been seen as a major limitation to this therapy, but reports of progressive multifocal leukoencephalopathy in an extremely small number of patients are of concern. Here, we review the therapeutic actions of anti-CD20 antibodies, and the recent and ongoing clinical trials with CD20-depleting therapy in autoimmune diseases.

The effect of rituximab on humoral and cell mediated immunity and infection in the treatment of autoimmune diseases

Depletion of B lymphocytes using the anti-CD20 monoclonal antibody rituximab has wide-spread use in the treatment of patients with autoimmune disorders. As haematopoietic progenitor cells and only a fraction of differentiated plasma express CD20, the effect of rituximab on immune function appears to be minimal. However, hypogammagobulinaemia can occur with repeated doses and emerging data from large studies suggest a subtle increase in the risk of infection. Reactivation of latent JC virus, resulting in progressive multifocal leucoencephalopathy, and hepatitis B virus, resulting in hepatoxicity, have been documented in patients receiving rituximab; although confounding effects of concomitant immunosuppressive therapies and immune dysregulation due to the underlying disease make causal associations of infections problematic. This review discusses the efficacy of B cell depletion therapy in the treatment of autoimmune diseases, the effect of B cell depletion on infection and immunity including the role of the B cell in autoimmunity, and identifies areas of controversy.

Targeting B cells in immune-mediated inflammatory disease: a comprehensive review of mechanisms of action and identification of biomarkers

B cell-depletion therapy, particularly using anti-CD20 treatment, has provided proof of concept that targeting B cells and the humoral response may result in clinical improvements in immune-mediated inflammatory disease. In this review, the mechanisms of action of B cell-targeting drugs are investigated, and potential biomarkers associated with response to treatment in patients with autoimmune diseases are identified. Most available data relate to B cell depletion using anti-CD20 therapy (rituximab) in patients with rheumatoid arthritis (RA). Treatment leads to significant clinical benefit, but apparently fails to deplete long-lived plasma cells, and discontinuation is associated with relapse. Biomarkers commonly used in studies of B cell-targeted therapies include rheumatoid factor, anti-citrullinated peptide antibodies, and immunoglobulin (Ig) levels. More recently, there has been interest in markers such as B cell phenotype analysis, and B lymphocyte stimulator (BLyS)/a proliferation-inducing ligand (APRIL), the latter particularly in studies of the IgG Fc-transmembrane activator and CAML interactor (TACI) fusion protein (atacicept) and anti-BLyS therapy (belimumab). Data from clinical trials of B cell-depleting agents in RA suggest that specific autoantibodies, BLyS, APRIL, and circulating and synovial B lineage cell levels may have potential as biomarkers predictive of response to treatment. Further trials validating these markers against clinical outcomes in RA are required. In patients with systemic lupus erythematosus, Fc receptors and levels of circulating immune cells (including B cells and natural killer cells) may be relevant markers.

Characterization of a humanized anti-CD20 antibody with potent antitumor activity against B-cell lymphoma

Despite the effectiveness of the anti-CD20 chimeric antibody (mAb), rituximab, in treating B-cell lymphomas, its efficacy remains variable and often modest. In this study, a humanized anti-CD20 antibody, hu8E4, was generated by complementarity-determining region grafting method. Hu8E4 was as effective as rituximab in mediating antibody-dependent cellular cytotoxicity and inducing apoptosis in B-lymphoma cells, but it exhibited much more potent complement-dependent cytotoxicity than rituximab. Immunotherapeutic studies showed that hu8E4 was significantly more effective than rituximab in prolonging the survival of severe combined immunodeficient mice bearing human B-cell lymphomas, suggesting that it might be a promising therapeutic agent for B-cell lymphomas.

Anti-CD20 monoclonal antibodies: historical and future perspectives

Antibodies to CD20 have confirmed the hypothesis that monoclonal reagents can be given in vivo to alleviate human diseases. The targeting of CD20 on normal, malignant and auto-immune B-lymphocytes by rituximab has demonstrated substantial benefits for patients with a variety of B-cell lymphomas, as well as some with autoimmune disorders. There has been a notable increase in the survival rates from B-cell lymphoma in the decade since anti-CD20 therapy was introduced.

Evolution of anti-CD20 monoclonal antibody therapeutics in oncology

Approval of an anti-CD20 chimeric monoclonal antibody, rituximab, has revolutionized cancer treatment and also validated CD20 targeting for providing benefit and improvement of overall response rate in B cell malignancies. Although many patients have benefited from the treatment of rituximab, there are still significant numbers of patients who are refractory or develop resistance to the treatment. Here we discuss pre-clinically well-defined potential mechanisms of action for rituximab and review the ways next generation anti-CD20 monoclonal antibodies can potentially exploit them to further enhance the treatment of B cell malignancies. Although the relative importance of each of these mechanism remains to be established in the clinic, well-designed clinical trials will help to define the efficacy and understanding of which effector activity of modified next generation anti-CD20 mAb will be important in the treatment of B-cell malignancies.

Characterization of a rituximab variant with potent antitumor activity against rituximab-resistant B-cell lymphoma

Despite widespread use of the anti-CD20 monoclonal antibody (mAb), rituximab, in treating B-cell lymphomas, its efficacy remains variable and often modest. A better understanding of rituximab-mediated killing mechanisms is essential to develop more effective therapeutic agents. In this study, we modulated the binding property of rituximab by introducing several point mutations in its complementarity-determining regions. The data showed that changing the binding avidity of rituximab in the range from 10−8 to 10−10 M could regulate its antibody-dependent cellular cytotoxicity but not affect its complement-dependent cytotoxicity and apoptosis-inducing activity in B-lymphoma cells. Contradictory to previous findings, we found that the complement-dependent cytotoxicity potency of CD20 mAb was independent of the off-rate. Despite still being a type I CD20 mAb, a rituximab triple mutant (H57DE/H102YK/L93NR), which had a similar binding avidity to a double mutant (H57DE/H102YK), was unexpectedly found to have extremely potent apoptosis-inducing activity. Moreover, this triple mutant, which was demonstrated to efficiently initiate both caspase-dependent and -independent apoptosis, exhibited potent in vivo therapeutic efficacy, even in the rituximab-resistant lymphoma model, suggesting that it might be a promising therapeutic agent for B-cell lymphomas.

Engineered anti-CD20 antibodies with enhanced complement-activating capacity mediate potent anti-lymphoma activity

One of the major issues in current antibody therapy is insufficient efficacy. Various biological factors relating to the host's immune system or tumor cells have been suggested to reduce the efficacy of anti-CD20 therapy in B-cell malignancies. In this study, we characterized the in vitro anti-lymphoma activity of anti-CD20 antibodies having a novel engineered heavy chain with enhanced complement-dependent cytotoxicity (CDC). Anti-CD20 antibodies having a variant heavy constant region of mixed IgG1/IgG3 isotype, which have previously been found to enhance CDC, were investigated for their in vitro CDC against lymphoma cells and whole blood B-cell depletion activity. Use of the variant constant region greatly increased the CDC of an anti-CD20 antibody having variable regions identical to those of rituximab to the level shown by an IgG1 antibody of ofatumumab. Although the whole blood assay showed different cytotoxicity patterns among individual blood donors, the CDC-enhancing variant of rituximab showed higher activity than the parent IgG1 and consistently showed maximized activity when further combined with antibody-dependent cellular cytotoxicity (ADCC)-enhancing modification by fucose removal from Fc-linked oligosaccharides. In addition, the rituximab variant showed potent CDC against transfectant cells with lower CD20 expression and chronic lymphocytic leukemia-derived cell lines with higher complement regulatory proteins. These findings suggest that CDC enhancement, both alone and in combination with ADCC enhancement, increases the anti-lymphoma activity of anti-CD20 antibodies irrespective of individual differences in effector functions, and renders current anti-CD20 therapy capable of overcoming the potential resistance mechanisms.

Depletion of the C3 component of complement enhances the ability of rituximab-coated target cells to activate human NK cells and improves the efficacy of monoclonal antibody therapy in an in vivo model

Growing evidence indicates antibody-dependent cellular cytotoxicity (ADCC) contributes to the clinical response to monoclonal antibody (mAb) therapy of lymphoma. Recent in vitro analysis suggests C3b can inhibit mAb-induced natural killer (NK)-cell activation and ADCC. Further studies were conducted to assess the effect of C3 depletion on mAb-induced NK activation and therapy of lymphoma. Normal human serum inhibited the ability of rituximab-coated lymphoma cells to activate NK cells as previously reported. Serum did not inhibit NK-cell activation when it was preincubated with cobra venom factor (CVF) to deplete C3. Similar results were found when transudative pleural fluid or nonmalignant ascites was used as surrogates for extravascular fluid, suggesting the inhibitory effect of complement may be present in the extravascular compartment, in which many malignant lymphocytes reside. In vivo, C3 was depleted before mAb treatment in a syngeneic murine model of lymphoma. Survival of lymphoma-bearing mice after treatment with CVF plus mAb and with a human C3 derivative with CVF-like functions (HC3-1496) plus mAb was both superior to that of mAb alone. These studies show that complement depletion enhances NK-cell activation induced by rituximab-coated target cells and improves the efficacy of mAb therapy in a murine lymphoma model.

Molecular mechanisms of resistance to Rituximab and pharmacologic strategies for its circumvention

The introduction of Rituximab has greatly improved therapeutic options for patients with B-cell non-Hodgkin lymphoma (B-NHL). However, a substantial fraction of patients with aggressive B-NHL fails first-line therapy, and most patients with relapsing indolent B-NHL eventually acquire Rituximab resistance. Molecular understanding of the underlying mechanisms facilitates the development of pharmacologic strategies to overcome resistance. Rituximab exerts its activity on CD20-expressing B-cells by indirect and direct effector mechanisms. Indirect mechanisms are complement-dependent cytotoxicity (CDC), and antibody-dependent cell-mediated cytotoxicity (ADCC). Direct activities, such as growth inhibition, induction of apoptosis and chemosensitisation, have been reported, but are less defined. Moreover, the relative contribution of CDC, ADCC and direct mechanisms to the activity of Rituximab in vivo is unclear. Down-regulation of CD20 and expression of complement inhibitors have been described as escape mechanisms in B-NHL. Recent reports suggest that deregulated phosphoinositide-3-kinase (PI3K)/Akt, mitogen-activated kinases (MAPK) and nuclear-factor kappaB (NF-kappaB), as well as up-regulation of anti-apoptotic proteins may determine the efficacy of Rituximab to kill B-NHL cells in vitro and in vivo. The latter signalling pathways are attractive targets for pharmacologic modulation of resistance to Rituximab. With the advent of new inhibitors and antibodies, rationally designed clinical trials addressing Rituximab resistance are feasible.

Induction of apoptosis by cross-linking antibodies bound to human B-lymphoma cells: expression of Annexin V binding sites on the antibody cap

There are many reports that cross-linking antibodies (Abs) bound to the surface of B-lymphoma cells can induce apoptosis and/or cell death, especially with anti-CD20 Abs. This study was intended to extend our understanding of these effects. To determine if CD20 is a unique target in this respect, or whether Abs to other antigens would have similar effects, six Abs were tested, with and without cross-linking with a secondary Ab, on three target cell lines. We utilized assays that distinguish between apoptotic, dead, and viable cells. Two assays were used: Annexin V plus propidium iodide, and JC-1 plus SYTOX green (Molecular Probes, Eugene, OR). Most of the Abs tested induced a low level of apoptosis and cell death in Ramos cells, but not in the other two cell lines (Raji and RL). In general, the level of toxicity was correlated with the level of antigen expression, with Abs to high-density antigens having the strongest effects. However, since the majority of Ramos cells continued to multiply, it is questionable whether toxicity at this level can provide a significant clinical benefit. Unexpectedly, there was also a population of cells that stained weakly with Annexin V. These cells were distinct from classical apoptotic cells, and appeared to belong to the viable cell population. In these cells, Annexin V stained the region of the Ab cap, in contrast to the ringed staining of classical apoptotic cells.

IN CONCLUSION

1) Low-level induction of apoptosis was not unique for anti-CD20 Abs, but occurred similarly with other Abs, and 2) results of Annexin V staining experiments may need to be reevaluated. Further studies are required to explain why Annexin V binding sites are exposed in the region of an Ab cap.

Monoclonal antibodies directed to CD20 and HLA-DR can elicit homotypic adhesion followed by lysosome-mediated cell death in human lymphoma and leukemia cells

mAbs are becoming increasingly utilized in the treatment of lymphoid disorders. Although Fc-FcgammaR interactions are thought to account for much of their therapeutic effect, this does not explain why certain mAb specificities are more potent than others. An additional effector mechanism underlying the action of some mAbs is the direct induction of cell death. Previously, we demonstrated that certain CD20-specific mAbs (which we termed type II mAbs) evoke a nonapoptotic mode of cell death that appears to be linked with the induction of homotypic adhesion. Here, we reveal that peripheral relocalization of actin is critical for the adhesion and cell death induced by both the type II CD20-specific mAb tositumomab and an HLA-DR-specific mAb in both human lymphoma cell lines and primary chronic lymphocytic leukemia cells. The cell death elicited was rapid, nonapoptotic, nonautophagic, and dependent on the integrity of plasma membrane cholesterol and activation of the V-type ATPase. This cytoplasmic cell death involved lysosomes, which swelled and then dispersed their contents, including cathepsin B, into the cytoplasm and surrounding environment. The resulting loss of plasma membrane integrity occurred independently of caspases and was not controlled by Bcl-2. These experiments provide what we believe to be new insights into the mechanisms by which 2 clinically relevant mAbs elicit cell death and show that this homotypic adhesion-related cell death occurs through a lysosome-dependent pathway.

Development of a new fully human anti-CD20 monoclonal antibody for the treatment of B-cell malignancies

Despite the widespread use of rituximab, a chimeric monoclonal antibody with demonstrated efficacy in the treatment of non-Hodgkin's lymphomas, there is a recognized need to develop new agents with improved efficacy. Towards this end, using XenoMouse technology, a fully human IgG1 anti-CD20 monoclonal antibody was generated. This antibody, denoted mAb 1.5.3, evoked enhanced pro-apoptotic activity in vitro, as compared to rituximab, in the Ramos lymphoma cell line. Also, mAb 1.5.3 mediated both complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC) similar to rituximab in human B-lymphoma lines. Interestingly, mAb 1.5.3 demonstrated superior ADCC compared to rituiximab when FcgammaRIIIa F/F allotype donors were profiled and superior cytolytic activity across multiple human B-lymphoma and chronic B-cell leukemia lines in an in vitro whole blood assay. Furthermore, mAb 1.5.3 exhibited enhanced anti-tumor activity in Ramos, Daudi, and Namalwa tumour xenograft models. Lastly, mAb 1.5.3 produced a superior B-cell depletion profile in lymph node organs and bone marrow as compared to rituximab in a primate pharmacodynamic (PD) model. These findings underscore the potential of mAb 1.5.3 to exhibit improved clinical activity in the treatment of B-cell malignancies compared to rituximab.

Engaging the lysosomal compartment to combat B cell malignancies

The combination of rituximab, a type I anti-CD20 mAb, with conventional chemotherapy has significantly improved the outcome of patients with B cell malignancies. Regardless of this success, many patients still relapse with therapy-resistant disease, highlighting the need for the development of mAbs with higher capacity to induce programmed cell death. The so-called type II anti-CD20 mAbs (e.g., tositumomab) that trigger caspase-independent B cell lymphoma cell death in vitro and show superior efficacy as compared with rituximab in eradicating target cells in mouse models are emerging as the next generation of therapeutic anti-CD20 mAbs. In this issue of the JCI, Ivanov and colleagues identify the lysosomal compartment as a target for type II mAbs (see the related article beginning on page 2143). These data encourage the further clinical development of type II mAbs as well as other lysosome-targeting drugs in the treatment of B cell malignancies.

Binding of submaximal C1q promotes complement-dependent cytotoxicity (CDC) of B cells opsonized with anti-CD20 mAbs ofatumumab (OFA) or rituximab (RTX): considerably higher levels of CDC are induced by OFA than by RTX

The CD20 mAb ofatumumab (OFA) is more effective than rituximab (RTX) in promoting complement-dependent cytotoxicity (CDC) of B cells via the classical pathway (CP) of complement. CP activation is initiated by C1q binding to cell-bound IgG. Therefore, we examined the role of C1q in the dynamics of complement activation and CDC of B cell lines and primary cells from patients with chronic lymphocytic leukemia, reacted with OFA or RTX. C1q binding, complement activation, and colocalization of C1q with cell-bound mAbs were determined by flow cytometry and high-resolution digital imaging. C1q binds avidly to OFA-opsonized Raji and Daudi cells (K(D) = 12-16 nM) and colocalizes substantially with cell-bound OFA. Cells opsonized with OFA undergo high levels of complement activation and CDC in C1q-depleted serum supplemented with low concentrations of C1q. Under comparable conditions, RTX-opsonized cells bind less C1q; in addition, even when higher concentrations of C1q are used to achieve comparable C1q binding to RTX-opsonized cells, less complement activation and CDC are observed. Greater CDC induced by OFA may occur because C1q is bound in close proximity and with high avidity to OFA, resulting in effective CP activation. Moreover, OFA binds to the small, extracellular CD20 loop, placing the mAb considerably closer to the cell membrane than does RTX. This may facilitate effective capture and concentration of activated complement components closer to the cell membrane, potentially shielding them from inactivation by fluid phase agents and promoting efficient generation of the membrane attack complex.

Understanding and circumventing resistance to anticancer monoclonal antibodies

With the widespread use of therapeutic monoclonal antibodies in the treatment of patients with cancer, resistance to these agents has become a major issue. Preclinical models of drug action or resistance have contributed to unravel the main mechanisms of resistance, involving both tumor-associated and host related factors. However our understanding of how a monoclonal antibody destroys cancer cells in a patient and why it one day stops being effective are still far from being complete. This review focuses on the available data on mechanisms of action and resistance to rituximab and includes some additional information for other monoclonal antibodies. Innovative approaches designed to overcome resistance, such as combination immunotherapy, costimulation with cytokines or growth factors are presented.

Whole lymphoma B cells allow efficient cross-presentation of antigens by dendritic cells

BACKGROUND

In order to compensate for the paucity of defined tumor antigens (Ag) in non-Hodgkin's lymphomas, a promising approach might be the use of whole tumor cells as a source of tumor Ag to pulse antigen-presenting cells (APC). However, it is not presently known how the tumor cells should be delivered to APC to optimize the cross-presentation of tumor Ag to anti-tumor CD8 T cells. We aimed to compare CD20-opsonized, apoptotic and necrotic human tumor cells for their capacity to induce endocytosis and cross-presentation of tumor-associated Ag by dendritic cells (DC) or macrophages.

METHODS

Endocytosis of human tumor-derived material by macrophages or DC was monitored by flow cytometry. We used a previously described influenza model and studied cross-presentation of viral Ag as cellular surrogate tumor-associated Ag by APC after endocytosis of lymphoma B cells treated by inactivated influenza virus.

RESULTS

Optimal endocytosis was obtained when tumor cells were opsonized by an anti-CD20 antibody and, as expected, macrophages were more phagocytic than DC. However, Ag from opsonized, apoptotic and live cells, but not from necrotic lymphoma cells, were efficiently cross-presented by DC but not by macrophages.

DISCUSSION

We have developed a new model with human primary lymphoma cells to study the cross-presentation of tumor-associated Ag by APC. The results we have obtained support the use of whole lymphoma cells from patients to pulse DC to induce an anti-tumor immune response.

Update on the rational use of tositumomab and iodine-131 tositumomab radioimmunotherapy for the treatment of non-Hodgkin's lymphoma

Targeted radioimmunotherapy in non-Hodgkin’s B-cell lymphoma (NHL) offers an efficacious therapy and minimal toxicity compared to conventional chemotherapy. Iodine 131 tositumomab (¹³¹I-TST) is a murine monoclonal antibody against the CD20 cell surface protein and is directly covalently conjugated to ¹³¹I, a radioactive β and γ emitter. While initially approved for use in relapsed, refractory, or transformed low grade B-cell NHL, investigational uses with promising results include autologous stem cell transplant, intermediate grade NHL, and the frontline management of indolent NHL. This review summarizes the ¹³¹I-TST literature on mechanism of action, treatment indications, treatment delivery, efficacy, investigational uses, and future prospects.

Engineering therapeutic monoclonal antibodies

During last two decades, the chimerization and humanization of monoclonal antibodies (mAbs) have led to the approval of several for the treatment of cancer, autoimmune diseases, and transplant rejection. Additional approaches have been used to further improve their in vivo activity. These include combining them with other modalities such as chemotherapy and redesigning them for improved pharmacokinetics, effector function, and signaling activity. The latter has taken advantage of new insights emerging from an increased understanding of the cellular and molecular mechanisms that are involved in the interaction of immunoglobulin G with Fc receptors and complement as well as the negative signaling resulting from the hypercrosslinking of their target antigens. Hence, mAbs have been redesigned to include mutations in their Fc portions, thereby endowing them with enhanced or decreased effector functions and more desirable pharmacokinetic properties. Their valency has been increased to decrease their dissociation rate from cells and enhance their ability to induce apoptosis and cell cycle arrest. In this review we discuss these redesigned mAbs and current data concerning their evaluation both in vitro and in vivo.

New insights into the mechanisms of action of radioimmunotherapy in lymphoma

The exquisite sensitivity of haematological malignancies to targeted radiation alongside the impressive results achieved by the pioneers in this field suggests that radioimmunotherapy is likely to be a productive area for future clinical research. Recent experimental work has demonstrated that the combination of targeted radiation and antibody effector mechanisms are critical to long-term clearance of tumour. This review provides the background of clinical and biological insights into the mechanisms of action of radioimmunotherapy.

Complete eradication of human B-cell lymphoma xenografts using rituximab in combination with the immunocytokine L19-IL2

The antibody-mediated delivery of therapeutic agents to sites of angiogenesis is an attractive strategy for anticancer therapy, but is largely unexplored in hematologic malignancies. In the present study, we show that the extra domain B (EDB) of fibronectin, a marker of angiogenesis, is expressed in B-cell non-Hodgkin lymphoma (NHL) and that the human monoclonal anti-EDB antibody L19 can selectively localize to the lymphoma-associated subendothelial extracellular matrix. In vivo, the preferential accumulation of the antibody at the tumor site was confirmed by quantitative biodistribution analyses with radioiodinated antibody preparations. The fusion protein L19-IL2, which mediates the delivery of interleukin-2 (IL-2) to the neovasculature, displayed a superior antilymphoma activity compared with unconjugated IL-2 in localized and systemic xenograft models of NHL. When coadministered with rituximab, L19-IL2 induced complete remissions of established localized lymphomas and provided long-lasting protection from disseminated lymphoma. The combined use of rituximab and L19-IL2, which dramatically increases the infiltration of immune effector cells in lymphomas, may deserve clinical investigations, facilitated by the fact that L19-IL2 is currently being studied in phase II clinical trials in patients with solid tumors.

Internalisation of uncross-linked rituximab is not essential for the induction of caspase-independent killing in Burkitt lymphoma cell lines

Characterising the mechanisms underpinning caspase-independent programmed cell death (CI-PCD) induction by uncross-linked rituximab in B-cells may positively impact upon the treatment of disease states in which the classical apoptotic pathway is disabled. The necessity of rituximab internalisation for CI-PCD induction was investigated by flow cytometry and confocal microscopy in human BL cell lines with (e.g. Mutu I) and without (Mutu III) susceptibility to rituximab-induced killing. Flow cytometry demonstrated small, significant and similar amounts of rituximab internalisation by Mutu I cells after 1, 2, 4 and 24 h (p < 0.03, n = 5) and Mutu III cells after 0.5, 2, 4 and 24 h (p < 0.05, n = 4). Confocal microscopy confirmed this. Cytochalasin B and latrunculin A significantly inhibited rituximab-induced CI-PCD (p < or = 0.04, n = 6 and p = 0.01, n = 6, respectively) and internalisation (p = 0.02, n = 5 and p = 0.0002, n = 6, respectively) in Mutu I cells, but confocal microscopy showed no correlation between internalised rituximab and phosphatidylserine exposure. We conclude that rituximab internalisation is not essential for CI-PCD induction in BL cell lines.

Complement and cellular cytotoxicity in antibody therapy of cancer

The effective and practical use of mAbs in cancer therapy became a reality with the development of the chimeric anti-CD20 mAb, rituximab. Several additional mAbs have since been approved for clinical use. Despite these successes, the mechanisms by which mAbs mediate antitumor activity are still unclear. Preclinical studies indicate complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC) both can contribute to mAb-induced tumor cell lysis. However, evidence related to the relative clinical importance of each mechanism, and whether they are synergistic or antagonistic, is conflicting. New ways to enhance both CDC and ADCC are being developed in attempt to develop a more effective anticancer mAb. Continued research on the mechanisms of mAb therapy will be necessary if we are to take optimal advantage of the current mAbs and develop more effective mAbs in the future.

Radiation therapy with tositumomab (B1) anti-CD20 monoclonal antibody initiates extracellular signal-regulated kinase/mitogen-activated protein kinase-dependent cell death that overcomes resistance to apoptosis

PURPOSE

The use of targeted radiation therapy (RT) in conjunction with anti-CD20 monoclonal antibodies (mAb) delivers high clinical response rates in B-cell lymphomas as part of radioimmunotherapy. The mechanisms underlying these impressive responses, particularly in patients whose lymphomas have become refractory to chemotherapy, are poorly understood.

EXPERIMENTAL DESIGN

In this study, we have investigated the signaling pathways and mode of cell death induced in B-cell lymphoma cells after the combination of RT and either type I (rituximab) or type II (tositumomab/B1) anti-CD20 mAb.

RESULTS

Increased tumor cell death was observed when RT was combined with tositumomab, but not rituximab. This additive cell death was found to be mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK)-dependent and could be reversed with mitogen-activated protein/extracellular signal-regulated kinase kinase (MEK) inhibitors, as well as small interfering RNA targeting MEK1/2. Furthermore, we found that this increased death was associated with ERK1/2 nuclear accumulation after tositumomab treatment, which was enhanced in combination with RT. Importantly, although Bcl-2 overexpression resulted in resistance to RT-induced apoptosis, it had no effect on the tumor cell death induced by tositumomab plus RT, indicating a nonapoptotic form of cell death.

CONCLUSIONS

These findings indicate that RT and type II anti-CD20 mAb combine to stimulate a prodeath function of the MEK-ERK1/2 pathway, which is able to overcome apoptotic resistance potentially explaining the efficacy of this modality in treating patients with chemoresistant disease.

Type II (tositumomab) anti-CD20 monoclonal antibody out performs type I (rituximab-like) reagents in B-cell depletion regardless of complement activation

Anti-CD20 monoclonal antibodies (mAbs) are classified into type I (rituximab-like) or type II (tositumomab-like) based on their ability to redistribute CD20 molecules in the plasma membrane and activate various effector functions. To compare type I and II mAbs directly in vivo and maximize Fc effector function, we selected and engineered mAbs with the same mouse IgG(2)a isotype and assessed their B-cell depleting activity in human CD20 transgenic mice. Despite being the same isotype, having similar affinity, opsonizing activity for phagocytosis, and in vivo half-life, the type II mAb tositumomab (B1) provided substantially longer depletion of B cells from the peripheral blood compared with the type I mAb rituximab (Rit m2a), and 1F5. This difference was also evident within the secondary lymphoid organs, in particular, the spleen. Failure to engage complement did not explain the efficacy of the type II reagents because type I mAbs mutated in the Fc domain (K322A) to prevent C1q binding still did not display equivalent efficacy. These results give support for the use of type II CD20 mAbs in human B-cell diseases.

MS4A12 is a colon-selective store-operated calcium channel promoting malignant cell processes

Using a data mining approach for the discovery of new targets for antibody therapy of colon cancer, we identified MS4A12, a sequence homologue of CD20. We show that MS4A12 is a cell surface protein. Expression analysis and immunohistochemistry revealed MS4A12 to be a colonic epithelial cell lineage gene confined to the apical membrane of colonocytes with strict transcriptional repression in all other normal tissue types. Expression is maintained upon malignant transformation in 63% of colon cancers. Ca(2+) flux analyses disclosed that MS4A12 is a novel component of store-operated Ca(2+) entry in intestinal cells. Using RNAi-mediated gene silencing, we show that loss of MS4A12 in LoVo colon cancer cells attenuates epidermal growth factor receptor-mediated effects. In particular, proliferation, cell motility, and chemotactic invasion of cells are significantly impaired. Cancer cells expressing MS4A12, in contrast, are sensitized and respond to lower concentrations of epidermal growth factor. In summary, these findings have implications for both the physiology of colonic epithelium as well as for the biology and treatment of colon cancer.

Lymphoma depletion during CD20 immunotherapy in mice is mediated by macrophage FcgammaRI, FcgammaRIII, and FcgammaRIV

Despite the demonstrated clinical efficacy of CD20 monoclonal antibody (mAb) for lymphoma therapy, the in vivo mechanisms of tumor depletion remain controversial and variable. To identify the molecular mechanisms responsible for lymphoma killing by CD20 mAb in a homologous system amenable to mechanistic studies and genetic manipulation, a mouse lymphoma model was developed using primary tumor cells from a C57BL/6 Emicro-cMyc transgenic mouse and mouse antimouse CD20 mAbs. CD20 mAb treatment of syngeneic mice with adoptively transferred lymphomas prevented tumor development or significantly prolonged mouse survival depending on tumor volume, mAb dose, and treatment timing. Cooperative FcgammaRIV, FcgammaRIII, and FcgammaRI interactions mediated optimal lymphoma depletion by CD20 mAb in vivo, whereas clodronate-mediated depletion of macrophages eliminated the therapeutic benefit of CD20 mAb. Although CD20 mAbs activated complement in vitro and in vivo, normal and malignant B-cell depletion was induced through C1q- and C3-independent mechanisms. Thus, the ability of CD20 mAbs to deplete malignant B cells in vivo required FcgammaR-dependent use of the innate mononuclear cell immune system. These findings allow for mechanism-based predictions of the biologic outcome of CD20 mAb therapy and treatment optimization.

Enhanced anti-tumor immunity generated by Rituximab-coated tumor cell vaccine

Rituximab is a chimeric monoclonal antibody against CD20, and has been used to treat malignant tumors derived from B cell. We designed a tumor cell vaccine modified by Rituximab, and evaluated anti-tumor effect in human CD20 gene transfected mice tumor model in vivo, and in human CTLs induction by mixed lymphocyte tumor cell culture in vitro. The results demonstrated that the Rituximab-coated tumor cell vaccine had a significant therapeutic effect against tumor pulmonary metastasis formation. Antibody depletion experiments showed CD8+ T Cells were essential for the anti-tumor effect but not NK cells. Capture rate of tumor cells by DCs, which were detected by flow cytometry, was increased by adding Rituximab. The tumor specific cytolysis could be induced by Rituximab-coated tumor cell in human in vitro assay. This therapeutic strategy provides a simple way to potentialize CTLs function to combat cancer and may promote more clinical consideration in immunotherapy for tumors. Rituximab-coated tumor cell vaccine also expanded the clinical Rituximab applications.

Generation of tumour-rejecting anti-carbohydrate monoclonal antibodies using melanoma modified with Fas ligand

Carbohydrate antigens such as glycolipids and glycoproteins are over-expressed in a variety of cancers and have therefore been identified as ideal candidates for tumour vaccines. Detection of anti-carbohydrate antibodies is associated with a good prognosis in cancer patients. However, generation of an efficient adaptive immune response has been hampered by the low immunogenicity of carbohydrates due to tolerance. Here, we describe a method by which tumour-rejecting antibodies directed against carbohydrates can be elicited in two different melanoma mouse models. Thus, using the murine melanoma B16F10 over-expressing Fas ligand (FasL), we have generated mAbs against cancer carbohydrate antigens expressed by the melanoma. Importantly, passive transfer of mAbs resulted in rejection of melanoma in vivo. Their protective effect in vivo was dependent on FcR and in vitro antibody-dependent cellular phagocytosis. They were also able to delay tumour growth when injected after the tumour was established. FasL-expressing tumours as an adjuvant are a novel way to generate anti-carbohydrate antibodies able to reject tumours in vivo.

High level expression of a functional human/mouse chimeric anti-CD20 monoclonal antibody in milk of transgenic mice

Rituximab, a chimeric anti-CD20 monoclonal antibody, is one of the most successful biomedicines and has been used to treat at least 370,000 patients with indolent, aggressive non-Hodgkin's lymphoma and other malignant diseases. However, the global demand for rituximab and other therapeutic monoclonal antibodies is exponentially increasing and barely able to be met by current manufacturing capacities of mammalian cell culture. The mammary gland bioreactor has been regarded as an ideal substitute for mammalian cell culture to mass-produce recombinant monoclonal antibodies at the lowest possible cost. Here, we show a feasible model to produce recombinant anti-CD20 antibodies in the mammary glands of transgenic animals. Six lines of transgenic mice were generated by co-microinjection of the two expression cassettes that can specially express the chimeric light and heavy chain of anti-CD20 mAbs in the milk of transgenic animals. The recombinant antibodies were detected in the milk of transgenic mice with the highest expression level up to 17 microg/mul and could specifically bind the CD20 surface antigens on human B-lymphoma cells.

Cooperation of the proapoptotic receptor agonist rhApo2L/TRAIL with the CD20 antibody rituximab against non-Hodgkin lymphoma xenografts

Recombinant human rhApo2L/TRAIL selectively stimulates apoptosis in various cancer cells through its receptors DR4 and DR5, and is currently in clinical trials. Preclinical studies have established antitumor activity of rhApo2L/TRAIL in models of epithelial cancers; however, efficacy in non-Hodgkin lymphoma (NHL) models is not well studied. Of 7 NHL cell lines tested in vitro, rhApo2L/TRAIL stimulated apoptosis in BJAB, Ramos RA1, and DoHH-2 cells. Rituximab, a CD20 antibody used to treat certain types of NHL, augmented rhApo2L/TRAIL-induced caspase activation in Ramos RA1 and DoHH2 but not BJAB or SC-1 cells, through modulation of intrinsic rather than extrinsic apoptosis signaling. In vivo, rhApo2L/TRAIL and rituximab cooperated to attenuate or reverse growth of tumor xenografts of all 4 of these cell lines. Depletion of natural killer (NK) cells or serum complement substantially reduced combined efficacy against Ramos RA1 tumors, suggesting involvement of antibodydependent cell- and complement-mediated cytotoxicity. Both agents exhibited greater activity against disseminated than subcutaneous BJAB xenografts, and worked together to inhibit or abolish disseminated tumors and increase survival. Moreover, rhApo2L/TRAIL helped circumvent acquired rituximab resistance of a Ramos variant. These findings provide a strong rationale for clinical investigation of rhApo2L/TRAIL in combination with rituximab as a novel strategy for NHL therapy.

In vivo targeting of human neutralizing antibodies against CD55 and CD59 to lymphoma cells increases the antitumor activity of rituximab

An in vivo model of human CD20+ B-lymphoma was established in severe combined immunodeficiency mice to test the ability of human neutralizing miniantibodies to CD55 and CD59 (MB55 and MB59) to enhance the therapeutic effect of rituximab. The miniantibodies contained single-chain fragment variables and the hinge-CH2-CH3 domains of human IgG(1). LCL2 cells were selected for the in vivo study among six B-lymphoma cell lines for their high susceptibility to rituximab-dependent complement-mediated killing enhanced by MB55 and MB59. The cells injected i.p. primarily colonized the liver and spleen, leading to the death of the animals within 30 to 40 days. Thirty percent of mice receiving biotin-labeled rituximab (25 microg) i.p. on days 4 and 11 after cell injection survived to 120 days. Administration of biotin-labeled rituximab, followed by avidin (40 microg) and biotin-labeled MB55-MB59 (100 microg) at 4-h intervals after each injection resulted in the survival of 70% of mice. Surprisingly, 40% of mice survived after the sole injection of avidin and biotin-labeled MB55-MB59, an observation consistent with the in vitro data showing that the miniantibodies induced killing of approximately 25% cells through antibody-dependent cell cytotoxicity. In conclusion, MB55 and MB59 targeted to tumor cells represent a valuable tool to enhance the therapeutic effect of rituximab and other complement-fixing antitumor antibodies.

Estimation of dose requirements for sustained in vivo activity of a therapeutic human anti-CD20 antibody

We evaluated the dose requirements for sustained in vivo activity of ofatumumab, a human anti-CD20 antibody under development for the treatment of B cell-mediated diseases. In a mouse xenograft model, a single dose, resulting in an initial plasma antibody concentration of 5 microg/ml, which was expected to result in full target saturation, effectively inhibited human B-cell tumour development. Tumour growth resumed when plasma concentrations dropped below levels that are expected to result in half-maximal saturation. Notably, tumour load significantly impacted antibody pharmacokinetics. In monkeys, initial depletion of circulating and tissue residing B cells required relatively high-dose levels. Re-population of B-cell compartments, however, only became detectable when ofatumumab levels dropped below 10 microg/ml. We conclude that, once saturation of CD20 throughout the body has been reached by high initial dosing, plasma concentrations that maintain target saturation on circulating cells (5-10 microg/ml) are probably sufficient for sustained biological activity. These observations may provide a rationale for establishing dosing schedules for maintenance immunotherapy following initial depletion of CD20 positive (tumour) cells.

NK-cell activation and antibody-dependent cellular cytotoxicity induced by rituximab-coated target cells is inhibited by the C3b component of complement

Antibody-dependent cellular cytotoxicity (ADCC) and complement fixation both appear to play a role in mediating antitumor effects of monoclonal antibodies (mAbs), including rituximab. We evaluated the relationship between rituximab-induced complement fixation, natural killer (NK)-cell activation, and NK cell-mediated ADCC. Down-modulation of NK- cell CD16 and NK-cell activation induced by rituximab-coated target cells was blocked by human serum but not heat-inactivated serum. This inhibition was also observed in the absence of viable target cells. C1q and C3 in the serum were required for these inhibitory effects, while C5 was not. An antibody that stabilizes C3b on the target cell surface enhanced the inhibition of NK-cell activation induced by rituximab-coated target cells. Binding of NK cells to rituximab-coated plates through CD16 was inhibited by the fixation of complement. C5-depleted serum blocked NK cell-mediated ADCC. These data suggest that C3b deposition induced by rituximab-coated target cells inhibits the interaction between the rituximab Fc and NK-cell CD16, thereby limiting the ability of rituximab-coated target cells to induce NK activation and ADCC. Further studies are needed to define in more detail the impact of complement fixation on ADCC, and whether mAbs that fail to fix complement will be more effective at mediating ADCC.

Mechanisms of killing by anti-CD20 monoclonal antibodies

CD20 is a cell-surface marker expressed on mature B cells and most malignant B cells, but not stem or plasma cells. It is an ideal target for monoclonal antibodies (mAb), such as rituximab and ofatumumab, as it is expressed at high levels on most B-cell malignancies, but does not become internalized or shed from the plasma membrane following mAb treatment. This allows mAb to persist on the cell surface for extended periods and deliver sustained immunological attack from complement and FcR-expressing innate effectors, particularly macrophages. CD20 can also generate transmembrane signals when engaged by certain mAb which, although unproven, might provide an important element of the therapeutic success of anti-CD20 mAb. These favourable characteristics have led to anti-CD20 mAb being developed and exploited for use in immunotherapy, where they have proven remarkably efficacious in both the treatment of malignant disease and autoimmune disorders by deleting malignant or normal B cells, respectively. In this review, we discuss how these mAb have driven research in the immunotherapy field over the last decade, detail their likely modes of action and their limitations in terms of effector exhaustion, and explore ways in which they might be enhanced and further exploited in the future.

Treating autoimmune demyelination by augmenting lymphocyte apoptosis in the central nervous system

The elimination of autoreactive T cells from the central nervous system (CNS) by apoptosis plays an important role in switching off autoimmune attack. B-cell apoptosis in the CNS probably also has a key role in downregulating autoimmunity. Augmenting lymphocyte apoptosis in the CNS is a potential strategy for treating autoimmune CNS diseases such as multiple sclerosis. These strategies involve modulation of the physiological pro-apoptotic and anti-apoptotic pathways that control lymphocyte fate in the CNS. In the case of T cells, apoptosis can be augmented by enhancing activation-induced T-cell apoptosis through the CD95 (Fas) pathway and by inhibiting costimulation-induced anti-apoptotic pathways mediated through BCL-2 and BCL-X L.

Rituximab-CD20 Complexes Are Shaved from Z138 Mantle Cell Lymphoma Cells in Intravenous and Subcutaneous SCID Mouse Models

Infusion of standard-dose rituximab (RTX) in chronic lymphocytic leukemia (CLL) patients promotes rapid complement activation and deposition of C3 fragments on CLL B cells. However, immediately after RTX infusions, there is substantial loss (shaving) of CD20 from circulating malignant cells. Because shaving can compromise efficacies of anticancer immunotherapeutic mAbs, we investigated whether shaving occurs in SCID mouse models. Z138 cells, a B cell line derived from human mantle cell lymphoma, were infused i.v. or s.c. The i.v. model recapitulates findings we previously reported for therapeutic RTX in CLL: i.v. infused RTX rapidly binds to Z138 cells in lungs, and binding is accompanied by deposition of C3 fragments. However, within 1 h targeted cells lose bound RTX and CD20, and these shaved cells are still demonstrable 40 h after RTX infusion. Z138 cells grow in tumors at s.c. injection sites, and infusion of large amounts of RTX (0.50 mg on each of 4 days) leads to considerable loss of CD20 from these cells. Human i.v. Ig blocked shaving, suggesting that FcgammaRI on cells of the mononuclear phagocytic system promote shaving. Examination of frozen tumor sections from treated mice by immunofluorescence revealed large areas of B cells devoid of CD20, with CD20 intact in adjacent areas; it is likely that RTX had opsonized Z138 cells closest to capillaries, and these cells were shaved by monocyte/macrophages. The shaving reaction occurs in neoplastic B cells in tissue and in peripheral blood, and strategies to enhance therapeutic targeting and block shaving are under development.

Recombinant anti-CD4 antibody 13B8.2 blocks membrane-proximal events by excluding the Zap70 molecule and downstream targets SLP-76, PLC gamma 1, and Vav-1 from the CD4-segregated Brij 98 detergent-resistant raft domains

The biological effects of rIgG(1) 13B8.2, directed against the CDR3-like loop on the D1 domain of CD4, are partly due to signals that prevent NF-kappaB nuclear translocation, but the precise mechanisms of action, particularly at the level of membrane proximal signaling, remain obscure. We support the hypothesis that rIgG(1) 13B8.2 acts by interfering with the spatiotemporal distribution of signaling or receptor molecules inside membrane rafts. Upon cross-linking of Jurkat T lymphocytes, rIgG(1) 13B8.2 was found to induce an accumulation/retention of the CD4 molecule inside polyoxyethylene-20 ether Brij 98 detergent-resistant membranes at 37 degrees C, together with recruitment of TCR, CD3zeta, p56 Lck, Lyn, and Syk p70 kinases, linker for activation of T cells, and Csk-binding protein/phosphoprotein associated with glycosphingolipid adaptor proteins, and protein kinase Ctheta, but excluded Zap70 and its downstream targets Src homology 2-domain-containing leukocyte protein of 76 kDa, phospholipase Cgamma1, and p95(vav). Analysis of key upstream events such as Zap70 phosphorylation showed that modulation of Tyr(292) and Tyr(319) phosphorylation occurred concomitantly with 13B8.2-induced Zap70 exclusion from the membrane rafts. 13B8.2-induced differential raft partitioning was epitope, cholesterol, and actin dependent but did not require Ab hyper-cross-linking. Fluorescence confocal imaging confirmed the spatiotemporal segregation of the CD4 complex inside rafts and concomitant Zap70 exclusion, which occurred within 10-30 s following rIgG(1) 13B8.2 ligation, reached a plateau at 1 min, and persisted until the end of the 1-h experiment. The differential spatiotemporal partitioning between the CD4 receptor and the Zap70-signaling kinase inside membrane rafts interrupts the proximal signal cross-talk leading to subsequent NF-kappaB nuclear translocation and explains how baculovirus-expressed CD4-CDR3-like-specific rIgG(1) 13B8.2 acts to induce its biological effects.