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First Published Online September 8, 2008
The Oncologist, Vol. 13, No. 9, 954-966, September 2008; doi:10.1634/theoncologist.2008-0089
© 2008 AlphaMed Press

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Lymphoma

The Role of Complement in the Mechanism of Action of Rituximab for B-Cell Lymphoma: Implications for Therapy

Xuhui Zhoua, Weiguo Hub,c, Xuebin Qinb,c

aChangzheng Hospital, Second Military Medical University, Shanghai, China; bDepartment of Medicine, Brigham and Women's Hospital, Boston, Massachusetts, USA; cHarvard Medical School, Laboratory for Translational Research, Cambridge, Massachusetts, USA

Key Words. Rituximab • CD20 • Mechanism • Resistance • Complement • B-cell non-Hodgkin's lymphoma

Correspondence: Xuebin Qin, M.D., Ph.D., Harvard Medical School, One Kendall Square, Building 600, 3rd Floor, Cambridge, Massachusetts 02139, USA. Telephone: 617-621-6102; Fax: 617-621-6148; e-mail: xuebin_qin{at}hms.harvard.edu

Received April 9, 2008; accepted for publication August 5, 2008; first published online in THE ONCOLOGIST Express on September 8, 2008.

Disclosure: The content of this article has been reviewed by independent peer reviewers to ensure that it is balanced, objective, and free from commercial bias. No financial relationships relevant to the content of this article have been disclosed by the authors, planners, independent peer reviewers, or staff managers.


   ABSTRACT
Top
 Abstract
Introduction
 Rituximab Targeting CD20
 CDC
 ADCC
 Apoptosis
 Direct Growth Arrest
 Conclusion
 Author Contributions
 Acknowledgments
 References
 
Rituximab, a genetically engineered chimeric monoclonal antibody specifically binding to CD20, was the first antibody approved by the U.S. Food and Drug Administration for the treatment of cancer. Rituximab significantly improves treatment outcome in relapsed or refractory, low-grade or follicular B-cell non-Hodgkin's lymphoma (NHL). However, there are also some challenges for us to overcome: why ~50% of patients are unresponsive to rituximab in spite of the expression of CD20, and why some responsive patients develop resistance to further treatment. Although the antitumor mechanisms of rituximab are not completely understood, several distinct antitumor activities of rituximab have been suspected, including complement-dependent cytotoxicity (CDC), antibody-dependent cellular cytotoxicity (ADCC), apoptosis, and direct growth arrest. To counteract resistance to rituximab therapy, several strategies have been developed to: (a) augment the CDC effect by increasing CD20 expression, heteroconjugating rituximab to cobra venom factor and C3b, and inhibiting membrane complement regulatory protein, especially CD59, function; (b) enhance the ADCC effect through some immunomodulatory cytokines and CR3-binding β-glucan; and (c) reduce the apoptotic threshold or induce apoptotic signaling on the tumor. Extensive studies indicate that rituximab combined with these approaches is more effective than a single rituximab approach. Herein, the mechanism of action of and resistance to rituximab therapy in B-cell NHL, in particular, the involvement of the complement system, are extensively reviewed.


   INTRODUCTION
Top
 Abstract
 Introduction
Rituximab Targeting CD20
 CDC
 ADCC
 Apoptosis
 Direct Growth Arrest
 Conclusion
 Author Contributions
 Acknowledgments
 References
 
Non-Hodgkin's lymphoma (NHL) is a diverse group of lymphatic malignancies distinct from Hodgkin's lymphoma on the basis of their epidemiology, clinical manifestation, therapy, and prognosis, as well as laboratory tests, including pathological features. The incidence of NHL has been steadily rising, currently ranking fifth in cancer mortality in the U.S. The annual incidence of NHL in 2000–2004 was 19.3 cases per 100,000 persons, and approximately 66,120 new cases of NHL were expected to be diagnosed in 2008, representing 5% and 4% of all cancers in males and females, respectively [1]. NHL is classified as either B-cell or T-cell NHL. B-cell NHL is much more common than T-cell NHL. We focus on reviewing the current treatment of B-cell NHL, but not T-cell NHL.

In the past 10 years or more, significant progress has been made in specifically treating B-cell NHL. While traditional nonspecific cancer therapies such as chemotherapy and radiotherapy mainly focus on killing rapidly dividing cancer cells, they also kill rapidly dividing normal cells, thereby causing obvious multiple side effects. In contrast, specific targeted therapies, including immunotherapy, mainly kill cancer cells, with no or minimal effects on normal cells. Therefore, specific targeted therapies have obvious advantages and are increasingly becoming the preferred tools to conquer cancer. Immunotherapy that targets receptors using monoclonal antibodies (mAbs) has an established role in the treatment of certain leukemias, lymphomas, and breast cancers by specifically recognizing cancer cells [2]. Not only can mAbs conjugate with cytotoxic compounds, radionuclides, and toxins, but unconjugated mAbs can exert antitumor activity alone. To date, five mAbs, including three unconjugated mAbs, have been approved for cancer therapy by the U.S. Food and Drug Administration (FDA). Among the three unconjugated mAbs, the chimeric antibody rituximab, the first mAb approved by the FDA, has been successfully used in the treatment of B-cell NHL via targeting the membrane molecule of CD20 on B-cell lymphoma, with a great improvement in outcome [3], and is also regarded as the driving force in our current focus on immunotherapy of B-cell NHL. Herein, the mechanism of action of and resistance to rituximab therapy in B-cell NHL, in particular, the involvement of the complement system, are described.


   RITUXIMAB TARGETING CD20
Top
 Abstract
 Introduction
 Rituximab Targeting CD20
CDC
 ADCC
 Apoptosis
 Direct Growth Arrest
 Conclusion
 Author Contributions
 Acknowledgments
 References
 
Rituximab (IDEC-C2B8, Rituxan®; IDEC Pharmaceuticals, San Diego, CA, and Genentech, Inc, San Francisco, CA) is a genetically engineered chimeric mAb that specifically binds to CD20, which contains human {gamma}-1 and {kappa} constant regions with murine variable regions [4]. Rituximab therapy alone or in combination with chemotherapy has remarkably improved the treatment outcome of patients with B-cell NHL. Its single-agent efficacy with low toxicity was proven by an overall response rate of 50% with a median time to progression in responders of 13.2 months, but some responsive patients develop resistance to further rituximab treatment [5, 6]. The mechanism of rituximab unresponsiveness remains unclear, and rituximab resistance requires more extensive investigation.

The rituximab-targeted molecule, CD20, a 32-kDa nonglycosylated phosphoprotein, provides a universal target for immunotherapy, which is especially expressed on the surface of normal precursor and mature B cells and, importantly, not on early pre-B cells, stem cells, or antigen-presenting dendritic reticulum cells [7]. More than 90% of B-cell NHLs express this surface protein [8, 9]. The CD20 protein has four transmembrane domains and does not modulate from the cell surface in response to antibody binding, thus providing an excellent target for immunotherapeutic strategies [4]. So far, the natural ligand for CD20 has not been identified, and the biological function of CD20 remains unclear. CD20 knockout mice show normal B-cell development and function [10]. Via phage display libraries, Binder et al. [11] showed that rituximab binds a discontinuous conformational epitope on CD20, comprising (170)ANPS(173) and (182)YCYSI(185), with both strings brought in steric proximity by a disulfide bridge between C(167) and C(183). This structural interaction was further proven by crystal structure analysis of the complex of rituximab Fab fragment and CD20 fragment (aa163-aa187) [12]. Several distinct antitumor activities of rituximab have been suspected in B-cell NHL therapy, including complement-dependent cytotoxicity (CDC), antibody-dependent cellular cytotoxicity (ADCC), apoptosis, and direct growth arrest [13]. In addition, complement-dependent cellular cytotoxicity (CDCC) has often been included in ADCC [14].


   CDC
Top
 Abstract
 Introduction
 Rituximab Targeting CD20
 CDC
ADCC
 Apoptosis
 Direct Growth Arrest
 Conclusion
 Author Contributions
 Acknowledgments
 References
 
The Complement System
The complement system, a central innate system, is the effector of adaptive immunity. It spontaneously identifies any potential pathogens and efficiently protects the host from intruding pathogen attack through a wide range of cellular responses [15–18]. This system is composed of >30 soluble plasma proteins and membrane proteins that can trigger three distinct protease cascades known as the classical, mannose-binding lectin (MBL), and alternative pathways (Fig. 1). The classical pathway is triggered by antigen-bound antibody molecules and is initiated by the binding of a specific part of the antibody (Fc) to C1q [15]. The MBL pathway is initiated when plasma MBL in complex MBL and MBL-associated protease (MASP)-1/2 bind to arrays of carbohydrates (specifically mannose and fucose residues) on the surface of many pathogens such as bacteria, viruses, fungi, yeasts, and parasites; MASP-1/2 then cleaves C4 and triggers the subsequent complement cascade [16]. The alternative pathway is capable of spontaneous autoactivation, termed the "tickover" of C3 at a rate of ~1% of total C3 per hour [19, 20], thereby identifying any potential pathogens. All three pathways converge at the C3 level, which allows C3b to target the pathogens, and at the C5 level, which leads to polymerization of C9 by C5b-8 binding and to assembly of a membrane attack complex (MAC) with a diameter of 5–10 nm (Fig. 1) [15]. If the MAC inserted into the cell membrane remains open, it will directly induce targeted cell lysis through subsequent influx of ions and water that leads to lethal colloid-osmotic swelling, CDC. The CDC effect is characterized by swelling of mitochondria, dilation of the rough endoplasmic reticulum, disruption of the Golgi complex and of the plasma and nuclear membranes, and heterochromatin disappearance at the ultrastructural level involving the generation of reactive oxygen species [21, 22]. For targeted cell lysis, a single MAC is enough for erythrocytes but not for nucleated cells, because nucleated cells can endocytose MAC and repair the damage unless multiple MACs (12- to 16-mers) are assembled [23].


Figure 1
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  		Figure 1. Three complement activation cascades.

Abbreviations: C1 INH, C1 inhibitor; C4BP, C4 binding protein; CR, complement receptor; DAF, decay-accelerating factor; MAC, membrane attack complex; MBL, mannose-binding lectin; MCP, membrane cofactor protein; MASP, MBL-associated proteases.

 
The byproducts produced in complement activation, such as C1q, C3b, iC3b, and C4b, are critical opsonins for host defense against infection and for disposal of immune complexes and dead cell debris by the phagocytosis/lysis effect of the immune effector cells (macrophages, neutrophils, natural killer [NK] cells, etc.) through their surface receptor binding to these byproducts (Fig. 1 and 2). On the other hand, the small fragment byproducts such as C3a, C4a, and C5a (Fig. 1), termed anaphylatoxins, also play an important role in inflammation and especially in host defense against parasites. These anaphylatoxins can cause mast cell and basophil degranulation, with the release of histamine and other substances that increase vascular permeability, stimulate smooth muscle constriction, and induce chemotaxis. The anaphylatoxins can also activate these immune effector cells by binding to cell surface receptors [24, 25]. Among these anaphylatoxins, C5a has the most potent biological activity.


Figure 2
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  		Figure 2. Rituximab-mediated CDC effect and strategies for enhancing the CDC effect. C1q interacts with the rituximab Fc region exposed after binding to CD20 on the B-cell surface, thus activating the classical complement cascade, and a MAC is inserted into the cell membrane, with multiple MACs (12- to 16-mers) leading to cytolysis. Strategies to overcome resistance to the CDC effect include: (a) inhibitors of the mCRPs DAF and especially CD59, which leads to more complement activation and more MAC formation on the cell surface; (b) heteroconjugates of rituximab to CVF or C3b, and other Ag-Ab complexes targeting tumor cells, which enhance complement activation; and (c) other drugs that can upregulate the expression of CD20, which include the histone deacetylase inhibitor trichostatin A and protein kinase C activator bryostatin-1.

Abbreviations: Ag-Ab, antigen-antibody; CDC, complement-dependent cytotoxicity; CVF, cobra venom factor; DAF, decay-accelerating factor; MAC, membrane attack complex; MCP, membrane cofactor protein; mCRP, membrane complement regulatory protein.

 
To prevent the potentially harmful effect of complement activation on autologous cells, >10 plasma- and membrane-bound inhibitory proteins have evolved for restricting complement activation at different stages of activation pathways (Fig. 1 and 2) [26, 27]. The soluble plasma complement regulatory proteins include C1 inhibitor, which regulates C1; factor H and factor I, which regulate the cleavage of C3b and C3/C5 convertases; C4 binding protein, which splits C4 convertase and assists factor I in the cleavage of C4b; and S-protein, clusterin, and serum lipids, which compete with membrane lipids for reacting with nascent C5b67 [15]. Membrane complement regulatory proteins (mCRPs) include: (a) CD46 (membrane cofactor protein), which regulates C3 activation by functioning as a cofactor protein for factor I–mediated cleavage of C3b and C4b [28]; (b) CD55 (decay-accelerating factor [DAF]), which inactivates the convertases of C3 (C4b2a and C3bBb) and C5 (C4b2a3b and C3bBb3b) by preventing the formation of new and accelerating the decay of activated convertase via binding to C3b and C4b [29]; (c) CD59 (membrane inhibitor of reactive lysis), which restricts MAC assembly on the cell membrane through binding to C8{alpha} and C9; (d) CD35 (complement receptor 1), which has both CD46 and CD55 functions in humans [26, 27, 30] and is a major immune adherence receptor that plays a role in immune-complex processing and clearance [31]; and (e) complement-receptor 1-related protein y, present only in rats and mice, which possesses both CD46 and CD55 biological functions [32–35]. CD55 and CD59 attach to the cell surface via a glycosylphosphatidylinositol (GPI) anchor, whereas CD46 and CD35 associate with the plasma membrane via their C-terminal transmembrane domains [36]. Structurally, CD55, CD46, and CD35 belong to the regulators of complement activation (RCA) family and contain a variable number of short consensus repeat domains. CD59 is a much smaller protein with no sequence or structural resemblance to the RCA family of proteins [27]. Several lines of evidence from human and animal studies indicate that CD59 is more relevant than CD55 and CD46 in protecting normal cells from MAC formation and MAC-induced phenomena [37–42].

There is a delicate balance between complement activation and regulation on autologous cells that is subject to perturbation by either increased complement activation or decreased regulation, which may cause a variety of immune diseases. For example, immune-complex activation of the classical complement pathway results in autoimmune diseases such as systemic lupus erythematosus and glomerulonephritis [30, 43]. Conversely, deficiency of GPI-anchored DAF and CD59 in circulating cells—resulting from an acquired somatic PIG-A gene mutation—is responsible for the hemolytic anemia and thrombosis that characterize paroxysmal nocturnal hemoglobinuria [44–46]. However, the high expression level of mCRPs also confers protection on cancer cells, invading microorganisms including HIV for complement attack, and may lead to resistance to antibody therapy.

Rituximab-Mediated CDC
The formation of MACs and subsequent cytolysis are usually considered as the activity of CDC via activation of the classical pathway initiated by C1q binding to antibody Fc fragment in rituximab therapy (Fig. 2). The consequence of the formation of MACs generally depends on the copy numbers assembled on the targeted tumor cells. Only at high copy numbers (12- to 16-mers) per cell (a lytic dose), can MACs induce a loss of membrane integrity and rapid necrotic-type cell death, possibly by a caspase-independent mechanism [21–23, 47]. At low copy numbers of MACs per cell (nonlytic or sublytic dose), MACs exhibit a wide range of effects leading to cellular responses such as secretion, adherence, aggregation, chemotaxis, and even cell proliferation activity through various cell-signaling pathways [48–50].

There is much evidence to highlight the importance of CDC in rituximab therapy of B-cell NHL. Complement depletion by cobra venom factor (CVF) significantly reduced the antitumor activity of rituximab in severe combined immunodeficiency (SCID), athymic nude mice [51, 52]. This activity was not affected by depletion of macrophages, NK cells, and/or neutrophils, which are irrelevant to CDC, in either a disseminated or s.c. tumor mouse model. It was also completely abolished in a C1q-deficient syngeneic mouse model and in a CVF complement-depleted mouse model [53, 54]. Furthermore, use of inhibitors abrogating mCRP (such as CD55 and CD59) functions can facilitate rituximab therapy in B-cell NHL, confirming the importance of CDC. However, many groups have claimed that ADCC plays a pivotal role in the antiproliferative effect of rituximab. B-cell lymphomas were depleted in vivo mainly by innate monocytes/macrophages in an Fc{gamma}R-dependent manner of isotype-specific mAb interactions with distinct Fc{gamma}Rs. This rituximab-mediated ADCC effect on B-cell lymphomas is completely effective in C3-, C4-, or C1q-deficient mice [55–57]. The explanation for the discrepancy between rituximab-mediated CDC and ADCC is complex and far from convincing [3]. A number of factors, including tumor status, mCRP expression, tumor cell type, tumor-inoculating methods, and tumor growth period, may influence the contributions of CDC and ADCC differently. However, it is widely accepted that ADCC and CDC synergistically affect cytotoxicity in cancer cells through the ability of complement to promote inflammation and change the activation status of innate effectors [3]. The byproduct of anaphylatoxin C5a during CDC is pivotal in activating these effector cells and can upregulate the expression of activating Fc{gamma}R and downregulate the expression of inhibitory Fc{gamma}RIIb via interaction with the C5a receptor, which enhances the ADCC effect in tumors [58–61]. Moreover, CDC-resistant cells are sensitive to ADCC and vice versa [62].

The potency of CDC in rituximab therapy is partially correlated with the level of CD20 expression on the B-cell NHL cell membrane [22, 63]. The higher the CD20 level, the more MACs can be assembled. Subsequently, targeted cells are directly lysed. With B-cell NHL cells freshly isolated from patients, a level of CD20 expression of >50 x 103 CD20 molecules per cell is a prerequisite for the lytic response arising from the mechanism of CDC [22]. A sigmoidal or linear correlation between the CD20 expression level and the rituximab-mediated killing effect further demonstrates the important role of CDC in the rituximab-mediated anticancer effect [22, 62, 63]. Other reports indicate that the efficacy of CDC in rituximab therapy may be determined by the expression levels of CD20 and mCRPs (CD55, and especially CD59) [64], but not of individual CD20 or mCRPs [65, 66].

Overcoming Resistance to Rituximab Through Enhancing the CDC Effect

Abrogating mCRP Function
The mCRPs CD46, CD55, and CD59 play a critical role in tumor resistance to rituximab-mediated CDC (Fig. 2). These mCRPs are expressed widely in almost all cancer cells independent of their tissue of origin [13]. The various expression levels of mCRPs may be regulated by the selective stress of complement attack [67], the stage of differentiation [68, 69], and host factors of neighboring tumor or stromal cells. These mCRPs in tumor cells restrict complement activation cascades in distinct stages and reduce MAC formation, thereby profoundly protecting the tumor from rituximab-mediated CDC (Fig. 1 and 2). The rituximab-resistant B-cell NHL cell lines of Raji and Ramos generated under the stress of rituximab and normal human serum (complement resource) exhibit upregulation of CD52 and the complement regulators CD55 and CD59, as well as downregulation of CD20 [67, 70]. The use of neutralizing antibodies abrogating the function of CD46, CD55, and CD59 markedly enhanced the antitumor activity of rituximab in vitro and in vivo [13, 14, 22, 63, 71, 72]. However, these antibodies are not applicable to clinical treatment because they would initiate undesired complement attack by binding to the host cells expressing mCRPs. To overcome this problem, Tedesco's group [73] developed two miniantibodies (MBs), MB-55 (against CD55) and MB-59 (against CD59), containing the human IgG1 hinge-CH2-CH3 domains, two single-chain variable fragments (scFv) without Fc fragment, a region necessary for activating the classical pathway. Furthermore, they generated two heteroconjugates of bio-MB55 and bio-MB59 [74] through a three-step biotin–avidin system, which is currently employed in patients to target radionuclides to cancer cells for enhancing the anticancer effect [75–77]. They documented that these modified antibodies against CD55 and CD59 significantly increased the antitumor activity of rituximab in vitro [73] and in vivo [74]. Alternatively, enhanced susceptibility can be achieved by downregulating the expression of mCRPs. Knockdown of CD55 expression via siRNA transfection attenuated the resistance of B-cell lymphoma or breast cancer cells to complement-mediated cytolysis by treatment with rituximab or trastuzumab [78]. The chemotherapeutic drug fludarabine showed a synergistic effect with rituximab treatment in NHL tumors, likely through the downmodulation of the membrane expression of CD55 [78, 79]. Consistent with the critical role of CD59 for restricting MAC in normal cells, CD59 is also more relevant to protect tumor cells from immunotherapy including rituximab than CD46 and CD55. Therefore, it is imperative to develop effective and practicable inhibitors against mCRPs, especially CD59, for facilitating rituximab and even other antibodies in treating tumors. For this reason, we have searched for an inhibitor for human CD59. Intermedilysin (ILY), a cytolytic pore-forming toxin secreted by Streptococcus intermedius, lyses human cells exclusively because the toxin binds to human CD59 (hCD59) with high specificity [80–82]. The specificity for hCD59 is derived from binding of ILY domain 4 (ILYd4) to amino acids 42–58 in hCD59, which also participate in binding to C8 and C9 [81]. Thus, we hypothesize that truncated ILYd4 would abrogate hCD59 function and thereby increase antibody-mediated CDC in cancer cells. If this is the case, ILYd4 may represent an innovative adjuvant for cancer immunotherapy in general and for antibody-resistant cancers in particular.

Increasing CD20 Expression
It is of note that the level of CD20 expression is related to the number of MACs formed on rituximab-targeted lymphocytes via binding to CD20, which determines whether targeted tumor cells can be lysed by CDC or not. Thus, an increased level of CD20 expression might be a means to overcome resistance to CDC (Fig. 2). However, rituximab–CD20 complex formed by rituximab binding to CD20 on the targeted cell membrane can be shaved by monocytes/macrophages expressing Fc{gamma}RI, a phenomenon called antigen modulation [83–85] (Fig. 2). This shaving reaction starts within 1–40 hours after rituximab infusion, which could substantially compromise the therapeutic efficacy of rituximab [85]. Interestingly, the histone deacetylase inhibitor suberoylanilide hydroxamic acid modulated the expression of apoptosis-related genes [86], and another histone, deacetylase inhibitor trichostatin A, could epigenetically increase CD20 mRNA and protein expression in an established CD20-negative cell line to sensitize rituximab therapy [87]. Bryostatin-1, a protein kinase C activator, can also enhance expression of CD20 at the level of both mRNA and protein in human tumor B cells through extracellular signal–related kinase (ERK)-dependent mechanisms, which in turn increases the susceptibility of the tumor to rituximab [88]. Moreover, synthetic CpG oligodeoxynucleotides are currently being tested in clinical trials as a vaccine adjuvant and for rituximab immunotherapy of B-cell NHL [89]. Synthetic CpG oligodeoxynucleotides resembling sequences found in bacterial DNA specifically increase primary malignant B-cell expression of CD20, thereby resulting in enhanced sensitivity to rituximab treatment [89, 90].

Enhancing Complement Activation
Besides abrogating mCRP function and increasing the CD20 expression level, an alternative strategy that can be employed to overcome resistance to CDC is to directly enhance complement activation on tumor cells. A complement-activating protein such as CVF or C3b can be conjugated to the antitumor antibody for the enhancement of complement activation [91] (Fig. 2). This effect was achieved in vitro by either anti–epithelial cell adhesion molecule–CVF heteroconjugates targeted to colorectal cancer cells [92] or anti–GD2-CVF heteroconjugates targeted to neuroblastoma cells [93, 94]. Additionally, an antibody against iC3b was demonstrated to increase iC3b deposition, which allowed it to interact with effector cells containing both Fc and complement receptors, and therefore significantly enhancing rituximab-mediated killing of Raji and DB cells in a cynomologous monkey model [95] (Fig. 2).


   ADCC
Top
 Abstract
 Introduction
 Rituximab Targeting CD20
 CDC
 ADCC
Apoptosis
 Direct Growth Arrest
 Conclusion
 Author Contributions
 Acknowledgments
 References
 
Effect of ADCC in Rituximab Therapy
Together with CDC, ADCC also plays an important role in rituximab antitumor activity. ADCC triggers tumor cell killing through interaction between the Fc region of CD20 binding rituximab and Fc{gamma}Rs, particularly Fc{gamma}RI and Fc{gamma}RIII, activating receptors expressed on immune effector cells such as monocytes/macrophages, granulocytes/neutrophils, and NK cells (Fig. 3). This interaction, mediated by rituximab on activated effector cells, initiates a series of signaling pathways that lead to the release of inflammatory and/or cytotoxic immune modulators including cytokines, chemokines, proteases, and reactive oxygen species. Eventually, the activated monocytes/macrophages and granulocytes/neutrophils phagocytose the targeted cancer cells, whereas activated NK cells eliminate targeted lymphoma cells using the granzyme-perforin system. Generally, the resting effector cells where the inhibitory Fc{gamma}RIIB is dominantly expressed cannot function unless activated. The inhibitory Fc{gamma}RIIB is a potent regulator of ADCC in vivo, modulating the activity of Fc{gamma}RIII on effector cells [96]. The antitumor activity of rituximab is greatly reduced in Fc{gamma}RI/Fc{gamma}RIII-deficient mice, whereas disruption of the gene that encodes the inhibitory receptor Fc{gamma}RIIB substantially enhanced antitumor activity. In addition, rituximab can induce the β-chemokine CCL3 in vivo, activating innate immune cells such as NK cells, macrophages, and polymorphonuclear cells, which increases the ADCC effect [52]. These results suggest that rituximab-mediated ADCC is important for killing cancer cells [96, 97].


Figure 3
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  		Figure 3. Rituximab-mediated ADCC and strategies to enhance the ADCC effect. CDCC is also classified here, which normally has little efficacy in rituximab therapy. This kind of cellular cytotoxicity can occur after binding of either the rituximab Fc region (in ADCC) or C1q, C3b, C4b, and iC3b (in CDCC) to their respective receptors, resulting in either phagocytosis or cell-mediated lysis of B-lymphocytes, depending on the effector cell type. The Fc region of cell-bound rituximab is recognized principally by either the activating receptors Fc{gamma}RI/Fc{gamma}RIII or the inhibitory receptor Fc{gamma}RIIB, whereas the byproducts of complement activation are recognized by C1qR, CR1, or CR3 on effector cell surfaces. The strategies to overcome resistance to the ADCC/CDCC effect include: (a) anaphylatoxins (C5a, C3a); (b) some cytokines (IL-2, IL-12, M-CSF) that are able to activate effector cells; and (c) CR3-specific polysaccharides such as β-glucan, which primes CR3 and therefore triggers cellular cytotoxicity.

Abbreviations: ADCC, antibody-dependent cellular cytotoxicity; CDCC, complement-dependent cellular cytotoxicity; CR, complement receptor; IL, interleukin.

 
CDCC may also be engaged in destroying invading pathogens. In CDCC, deposition of C3b, iC3b, C1q, and C4b bound on the surface of targeted cells is able to opsonize phagocytosis through binding to their receptors on monocytes/macrophages, NK cells, and polymorphonuclear leukocytes [95, 98, 99] (Fig. 3). This process can be further enhanced by chemotactic and cell-activating anaphylatoxins such as C5a and C3a through their receptors on the above phagocytes [24, 25], which are pivotal mediators of the host defense against infection and of the disposal of immune complexes and products of inflammatory injury [16]. To date, there has been little research on the effect of CDCC on rituximab therapy, perhaps because of its relative ineffectiveness [14].

Enhancing ADCC
Antibody-targeted tumor resistance to innate immune cells such as macrophages, NK cells, and neutrophils in ADCC may arise from the suppression of these immune effectors after long-term expansion of the tumor. Resistance to rituximab in some patients may be linked to a polymorphism in Fc{gamma}RIIIa affecting isotype preference [100]. Genetically modified NK cells carrying a chimeric antigen receptor that consists of a CD20-specific scFv antibody fragment conferred enhanced cytotoxic activity and could overcome NK-cell resistance of lymphoma and leukemia cells [101]. In addition, the effect of some cytokines on ADCC-related resistance was tested. M-CSF in vitro enhanced the cytotoxicity of monocytes on Daudi B-cell lymphomas through upregulation of monocyte Fc{gamma}RI and Fc{gamma}RIII by 1.5-fold and 1.6-fold, respectively, whereas the expression of Fc{gamma}RII remained unchanged [102]. Interleukin (IL)-2 is a pleiotropic cytokine that activates selective immune effector cell responses associated with antitumor activity, including ADCC. Golay et al. [103] demonstrated in vitro that IL-2–activated NK cells strongly enhanced the therapeutic activity of rituximab through ADCC in primary B-cell NHL cells freshly isolated from patients. Furthermore, IL-2 synergistically enhanced rituximab efficiency in killing B-cell NHL cells in a xenograft model, in part, through activation and trafficking of monocytes and NK cells to tumors [104]; a similar result was obtained with an anti-CD20–IL-2 immunocytokine [105]. Additionally, IL-12 synergizes the rituximab ADCC effect through upregulating {gamma}-interferon and interferon-inducible protein 10 expression and increasing NK cell lytic activity in vitro [106] (Fig. 3). The concomitant use of IL-12 and rituximab had only a modest effect in treating patients with B-cell NHL. The response rate in patients treated with IL-12 in combination with rituximab did not seem to be better than that seen with rituximab alone. Also, the sequential administration of IL-12 after rituximab did not result in additional clinical responses [107].

To enhance the CDCC effect in rituximab treatment, β-glucan can be applied to promote CR3-dependent cellular cytotoxicity. Generally, β-glucan is naturally exposed on the cell wall of yeasts and fungi. Moreover this CR3-mediated cytotoxicity that is responsible for host protection against yeast and fungus infection requires the binding of CR3 to both iC3b and β-glucan. Normally, this CDCC effect does not happen in rituximab therapy because tumor cells lack β-glucan [108]. However, together with deposited iC3b generated by rituximab-mediated complement activation, the administration of β-glucan can trigger cytotoxic phagocytosis and degranulation of iC3b-coated tumor cells [109, 110], thus regressing tumor growth [110, 111] (Fig. 3). The application of β-glucan has consistently been documented to markedly increase the therapeutic efficacy of rituximab in B-cell NHL-inoculated SCID mice [112].


   APOPTOSIS
Top
 Abstract
 Introduction
 Rituximab Targeting CD20
 CDC
 ADCC
 Apoptosis
Direct Growth Arrest
 Conclusion
 Author Contributions
 Acknowledgments
 References
 
Effect of Apoptosis in Rituximab Therapy
Deans et al. [113–116] demonstrated that CD20 is associated with Src family tyrosine kinases, Lyn, Fyn, and Lck, which are involved in rituximab-induced apoptosis. CD20-associated PAG (phosphoprotein associated with GEMs [glycosphingolipid-enriched membrane microdomains]), also known as Csk-binding protein, is a ubiquitous, highly tyrosine-phosphorylated adaptor protein localized exclusively in lipid rafts [117, 118]. PAG normally recruits Csk to lipid rafts to maintain resident Src family tyrosine kinases such as Lyn, Fyn, and Lck in an inactive state. After rituximab specifically binds to CD20, it redistributes lipid rafts, subsequently transactivates the above Src family tyrosine kinase, and initiates downstream signaling pathways resulting in apoptosis (Fig. 4).


Figure 4
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  		Figure 4. Rituximab-induced apoptosis in therapy and strategies to enhance apoptosis. PAG normally recruits Csk to maintain the Src-family kinases Lyn, Fyn, and Lck in an inactivated state. CD20–rituximab crosstalking can redistribute lipid rafts and thus transactivate these kinases and initiate downstream signaling pathways resulting in apoptosis. The redistribution of lipid rafts also induces apoptosis by Fas molecule clustering, which leads to the formation of DISC and the subsequent recruitment of FADD and caspase-8 into DISC, followed by the activation of the downstream apoptosis pathway. Meanwhile, the redistribution of lipid rafts can also inhibit the p38 MAPK, ERK-1/2, NF-{kappa}B, and Akt signaling pathways and result in the inhibition of both transcription and expression of many genes, particularly the antiapoptotic genes Bcl-2, Bcl-xL, XIAP, and Mcl-1, thereby making B-lymphocytes susceptible to apoptosis. In addition, inhibition of the NF-{kappa}B pathway downregulates the transcription factor YY1, enhancing the transcription of Fas and DR5 and facilitating FasL- and TRAIL-induced apoptosis. Evidence suggests a caspase-independent apoptosis pathway, whose mechanism needs more investigation. Strategies to enhance apoptosis after rituximab treatment include: (a) scFvRit:sFasL; (b) mapatumumab, a humanized monoclonal antibody targeting TRAIL receptor 1; (c) a recombinant protein, Apo2L/TRAIL, all three drugs target DRs to trigger the apoptosis pathway; and (d) other means such as antisense oligonucleotides of related apoptotic molecules and bortezomib, a proteasome inhibitor that induces apoptosis.

Abbreviations: DISC, death-inducing signaling complex; DR, death receptor; ERK, extracellular signal–related kinase; FADD, Fas-associated death domain protein; MAPK, mitogen-activated protein kinase; Mcl-1, myeloid cell leukemia sequence 1; NF-{kappa}B, nuclear factor {kappa}B; PAG, phosphoprotein associated with GEMs (glycosphingolipid-enriched membrane microdomains); TNF, tumor necrosis factor; TRAIL, tumor necrosis factor–related apoptosis-inducing ligand; XIAP, X-linked inhibitor of apoptosis protein; YY1, Yin Yang 1.

 
On the other hand, this redistribution of lipid rafts can also induce apoptosis by Fas molecule clustering. This clustering further induces the formation of the death-inducing signaling complex (DISC) and downstream activation of the death receptor (DR) pathway in Ramos B-cell NHL cells. Upon stimulation with rituximab and subsequent crosstalking with CD20, Fas-associated death domain protein (FADD) and caspase-8 were recruited into the DISC. This DISC was further confirmed by the identification of Fas, caspase-8, and FADD together with CD20 in sucrose-gradient raft fractions [119]. Therefore, rituximab can sensitize lymphoma B cells to Fas-induced apoptosis in a caspase-8–dependent manner (Fig. 4).

Rituximab preferentially inhibits the expression of the antiapoptotic gene products Bcl-2/Bcl-xL, X-linked inhibitor of apoptosis protein, and myeloid cell leukemia sequence 1 in B-cell NHL cells through downregulating the p38 mitogen-activated protein kinase (MAPK), nuclear factor (NF)-{kappa}B, ERK-1/2, and Akt survival pathways (Fig. 4). The inhibition of these antiapoptosis-related pathways sensitizes B-cell NHL to undergo apoptosis and even to chemotherapy for killing drug-resistant B-cell NHL cell lines [6, 120–122]. Furthermore, the inhibition of the NF-{kappa}B pathway by rituximab treatment can subsequently block the downstream transcription repressor Yin Yang 1, thereby resulting in enhancement of the transcription of Fas-induced and DR5-induced apoptosis [123, 124] (Fig. 4).

Rituximab also enhances apoptosis by an as yet unclear caspase-independent mechanism in B-cell NHL cells [125–128]. A broad-spectrum caspase inhibitor, zVAD-fmk, prevented processing of caspase-9, caspase-3, and poly(ADP-ribose) polymerase (PARP) as well as DNA fragmentation, but did not block apoptosis as measured by annexin V staining, cell size, and membrane integrity in the rituximab response, which was also independent of Fas resistance and Bcl-2 overexpression. Rituximab-treated cells show leakage of adenylate kinase, surface expression of phosphatidylserine, upregulation of the cellular stress protein heat shock protein-70, phosphorylation of the survival protein Akt, and depolarization of the mitochondrial membrane but no loss of cytochrome c or apoptosis-inducing factor, which could not be blocked by caspase inhibitors. Furthermore, in cytoplasts that lack mitochondria and in Bcl-2–transfected cells, phosphatidylserine was still translocated to the cell surface after CD20 stimulation. Consistently, in support of these data, there is no cleavage of caspase-3, caspase-8, caspase-9, PARP, BH3-interacting domain death agonist, or genomic DNA.

Enhancing Apoptosis in Rituximab Therapy
Recently, the synergistic effect of apoptosis-inducing drugs in combination with rituximab in B-cell NHL treatment has received much attention and interest. A genetically engineered fusion protein, scFvRit:sFasL, containing a rituximab-derived antibody fragment and Fas ligand was successfully used in malignant B cells [129] (Fig. 4). Mapatumumab, a humanized mAb that targets/activates tumor necrosis factor–related apoptosis-inducing ligand receptor 1 (TRAIL-R1) was able to augment the antitumor activity of rituximab via significant apoptosis [130] (Fig. 4). Another recombinant human mAb, Apo2 ligand (Apo2L)/TRAIL, which was in clinical trials for selectively stimulating apoptosis in other cancer cells through its proapoptotic receptors DR4 and/or DR5, was strongly demonstrated to augment rituximab antitumor efficacy in mice inoculated with several different B-cell NHL cell lines [131] (Fig. 4). Alternatively, the downregulation of the apoptosis-signaling molecule survivin or Bfl-1 expression by antisense oligonucleotides also sensitized rituximab-mediated B-cell NHL cells to apoptosis [132, 133] (Fig. 4). Research on patients suffering from relapsed or refractory cutaneous B-cell lymphoma disclosed a strong upregulation of the antiapoptotic molecule Bcl-2 compared with pretherapeutic levels, which suggests that upregulation of Bcl-2 plays a major role in therapy resistance [134]. Furthermore, Jazirehi et al. [135] generated rituximab-resistant B-cell NHL cell lines by increasing the concentration of rituximab in the culture medium, and then found lower expression of CD20 in these resistant clones. Interestingly, these clones exhibited hyperactivation of the p38 MAPK, NF-{kappa}B, and ERK-1/2 pathways and overexpression of Bcl-2/Bcl-xL, indicating that the hyperactivation of these signaling pathways may be related to resistance. Therefore, it is reasonable to assume that the efficacy of rituximab will be enhanced in combination with antisense Bcl-2 oligonucleotides in SCID mouse/human lymphoma xenografts [136]. In addition, interference with hyperactivated signal pathways with various pharmacological and proteasome inhibitors reversed resistance (Fig. 4). Bortezomib, a proteasome inhibitor, induced apoptosis earlier and sensitized resistant clones to various cytotoxic drugs and rituximab by preventing the degradation of proapoptotic factors and then activating caspase-8, caspase-9, caspase-3, and PARP [135, 137]. The development/acquisition of rituximab resistance may have arisen from perturbations of tumor cell survival signaling pathways and failure of rituximab to alter these pathways [6].


   DIRECT GROWTH ARREST
Top
 Abstract
 Introduction
 Rituximab Targeting CD20
 CDC
 ADCC
 Apoptosis
 Direct Growth Arrest
Conclusion
 Author Contributions
 Acknowledgments
 References
 
The binding of rituximab to CD20 can also induce direct growth arrest on lymphocytes. Bezombes et al. [138] demonstrated in vitro the direct inhibition of tumor growth by rituximab in Daudi and RL B-lymphoma cells. Rituximab induced moderate accumulation of tumor cells in the G1 phase and significant loss of clonogenic potential without apoptosis. They observed that treatment with rituximab resulted in a rapid and transient increase in acid-sphingomyelinase activity and concomitant cellular ceramide generation in raft microdomains. This result suggests that rituximab-induced growth inhibition may be mediated through a ceramide-triggered signaling pathway, leading to the induction of cell cycle–dependent kinase inhibitors such as p27Kip1 through an MAPK-dependent mechanism. Therefore, rituximab mediates direct growth arrest, which may also contribute to its anticancer activity.


   CONCLUSION
Top
 Abstract
 Introduction
 Rituximab Targeting CD20
 CDC
 ADCC
 Apoptosis
 Direct Growth Arrest
 Conclusion
Author Contributions
 Acknowledgments
 References
 
Although rituximab significantly improves the treatment outcome of B-cell NHL, it still has two challenges for us: unresponsiveness and resistance. Generally, the antitumor activity of rituximab is attributed to CDC, ADCC, apoptosis, and possible direct growth arrest, and CDC and ADCC are considered as the main effects. However, the contributions of CDC and ADCC are very controversial and require further investigation.

On the basis of the mechanisms described above, various strategies may be designed for triumphing over resistance to rituximab therapy. For example, complement activation in CDC can be enhanced by either increasing CD20 expression or heteroconjugating rituximab with CVF or C3b, or inhibiting mCRP (especially CD59) function. In particular, CD59 is a key inhibitor for MAC formation to escape the immunoresponse. Sublytic MAC is not sufficient to induce cytolysis, but may induce cell resistance by potentially increasing survival of cell-signaling pathways. Thus, it is imperative to develop an inhibitor of CD59 function. To enhance the ADCC effect, some immunomodulatory cytokines such as M-CSF, IL-2, and IL-12, as well as CR3-binding β-glucan, may offer effective strategies to increase the antitumor activity of rituximab. In addition, reduction of the apoptotic threshold or the induction of apoptotic signaling may also increase tumor susceptibility to rituximab treatment through Bcl-2 inhibitors and bortezomib as well as molecules targeting the tumor necrosis factor family receptor. In conclusion, although the rational combination of rituximab treatment with the above strategies toward clinical application requires further evaluation, this approach has been extensively shown to be more effective than rituximab treatment alone.


   AUTHOR CONTRIBUTIONS
Top
 Abstract
 Introduction
 Rituximab Targeting CD20
 CDC
 ADCC
 Apoptosis
 Direct Growth Arrest
 Conclusion
 Author Contributions
Acknowledgments
 References
 
Conception/design: Xuhui Zhou, Weiguo Hu, Xuebin Qin

Financial support: Xuebin Qin

Manuscript writing: Xuhui Zhou, Weiguo Hu, Xuebin Qin

Final approval of manuscript: Xuhui Zhou, Weiguo Hu, Xuebin Qin


   ACKNOWLEDGMENTS
Top
 Abstract
 Introduction
 Rituximab Targeting CD20
 CDC
 ADCC
 Apoptosis
 Direct Growth Arrest
 Conclusion
 Author Contributions
 Acknowledgments
References
 
This work was supported by the U.S. National Institutes of Health grant RO1 AI061174, Harvard Technology Development Accelerator Fund, and Scientist Development grant 0435483N from the American Heart Association. The authors take full responsibility for the content of the paper but thank Jaylyn Olivo from the Brigham and Women's Hospital Editorial Service for her assistance in editing the manuscript.

X.Z. and W.H. contributed equally to the paper.


   REFERENCES
Top
 Abstract
 Introduction
 Rituximab Targeting CD20
 CDC
 ADCC
 Apoptosis
 Direct Growth Arrest
 Conclusion
 Author Contributions
 Acknowledgments
 References
 

  1. Jemal A, Siegel R, Ward E et al. Cancer statistics, 2008. CA Cancer J Clin 2008;58:71–96.[Abstract/Free Full Text]
  2. Dillman RO. Monoclonal antibodies in the treatment of malignancy: Basic concepts and recent developments. Cancer Invest 2001;19:833–841.[CrossRef][Medline]
  3. Glennie MJ, French RR, Cragg MS et al. Mechanisms of killing by anti-CD20 monoclonal antibodies. Mol Immunol 2007;44:3823–3837.[CrossRef][Medline]
  4. Maloney DG, Liles TM, Czerwinski DK et al. Phase I clinical trial using escalating single-dose infusion of chimeric anti-CD20 monoclonal antibody (IDEC-C2B8) in patients with recurrent B-cell lymphoma. Blood 1994;84:2457–2466.[Abstract/Free Full Text]
  5. White CA, Larocca A, Grillo-Lopez AJ. Anti-CD20 monoclonal antibodies as novel treatments for non-Hodgkin's lymphoma. Pharm Sci Technolo Today 1999;2:95–101.[CrossRef][Medline]
  6. Bonavida B. Rituximab-induced inhibition of antiapoptotic cell survival pathways: Implications in chemo/immunoresistance, rituximab unresponsiveness, prognostic and novel therapeutic interventions. Oncogene 2007;26:3629–3636.[CrossRef][Medline]
  7. Schriever F, Freedman AS, Freeman G et al. Isolated human follicular dendritic cells display a unique antigenic phenotype. J Exp Med 1989;169:2043–2058.[Abstract/Free Full Text]
  8. Anderson KC, Bates MP, Slaughenhoupt BL et al. Expression of human B cell-associated antigens on leukemias and lymphomas: A model of human B cell differentiation. Blood 1984;63:1424–1433.[Abstract/Free Full Text]
  9. Nadler LM, Ritz J, Hardy R et al. A unique cell surface antigen identifying lymphoid malignancies of B cell origin. J Clin Invest 1981;67:134–140.[Medline]
  10. O'Keefe TL, Williams GT, Davies SL et al. Mice carrying a CD20 gene disruption. Immunogenetics 1998;48:125–132.[CrossRef][Medline]
  11. Binder M, Otto F, Mertelsmann R et al. The epitope recognized by rituximab. Blood 2006;108:1975–1978.[Abstract/Free Full Text]
  12. Du J, Wang H, Zhong C et al. Structural basis for recognition of CD20 by therapeutic antibody rituximab. J Biol Chem 2007;282:15073–15080.[Abstract/Free Full Text]
  13. Fishelson Z, Donin N, Zell S et al. Obstacles to cancer immunotherapy: Expression of membrane complement regulatory proteins (mCRPs) in tumors. Mol Immunol 2003;40:109–123.[CrossRef][Medline]
  14. Harjunpää A, Junnikkala S, Meri S. Rituximab (anti-CD20) therapy of B-cell lymphomas: Direct complement killing is superior to cellular effector mechanisms. Scand J Immunol 2000;51:634–641.[CrossRef][Medline]
  15. Morgan BP. Regulation of the complement membrane attack pathway. Crit Rev Immunol 1999;19:173–198.[Medline]
  16. Walport MJ. Complement. First of two parts. N Engl J Med 2001;344:1058–1066.[Free Full Text]
  17. Qin X, Ferris S, Hu W et al. Analysis of the promoters and 5'-UTR of mouse Cd59 genes, and of their functional activity in erythrocytes. Genes Immun 2006;7:287–297.[CrossRef][Medline]
  18. Qin X, Goldfine A, Krumrei N et al. Glycation inactivation of the complement regulatory protein CD59: A possible role in the pathogenesis of the vascular complications of human diabetes. Diabetes 2004;53:2653–2661.[Abstract/Free Full Text]
  19. Müller-Eberhard HJ. Molecular organization and function of the complement system. Annu Rev Biochem 1988;57:321–347.[CrossRef][Medline]
  20. Thurman JM, Holers VM. The central role of the alternative complement pathway in human disease. J Immunol 2006;176:1305–1310.[Abstract/Free Full Text]
  21. Papadimitriou JC, Drachenberg CB, Shin ML et al. Ultrastructural studies of complement mediated cell death: A biological reaction model to plasma membrane injury. Virchows Arch 1994;424:677–685.[CrossRef][Medline]
  22. Bellosillo B, Villamor N, Lopez-Guillermo A et al. Complement-mediated cell death induced by rituximab in B-cell lymphoproliferative disorders is mediated in vitro by a caspase-independent mechanism involving the generation of reactive oxygen species. Blood 2001;98:2771–2777.[Abstract/Free Full Text]
  23. Ollert MW, Kadlec JV, David K et al. Antibody-mediated complement activation on nucleated cells. A quantitative analysis of the individual reaction steps. J Immunol 1994;153:2213–2221.[Abstract]
  24. Gerard C, Gerard NP. C5A anaphylatoxin and its seven transmembrane-segment receptor. Annu Rev Immunol 1994;12:775–808.[CrossRef][Medline]
  25. Chen NJ, Mirtsos C, Suh D et al. C5L2 is critical for the biological activities of the anaphylatoxins C5a and C3a. Nature 2007;446:203–207.[CrossRef][Medline]
  26. Hourcade D, Holers VM, Atkinson JP. The regulators of complement activation (RCA) gene cluster. Adv Immunol 1989;45:381–416.[Medline]
  27. Miwa T, Song WC. Membrane complement regulatory proteins: Insight from animal studies and relevance to human diseases. Int Immunopharmacol 2001;1:445–459.[CrossRef][Medline]
  28. Liszewski MK, Post TW, Atkinson JP. Membrane cofactor protein (MCP or CD46): Newest member of the regulators of complement activation gene cluster. Annu Rev Immunol 1991;9:431–455.[CrossRef][Medline]
  29. Lublin DM, Atkinson JP. Decay-accelerating factor: Biochemistry, molecular biology, and function. Annu Rev Immunol 1989;7:35–58.[CrossRef][Medline]
  30. Mollnes TE, Song WC, Lambris JD. Complement in inflammatory tissue damage and disease. Trends Immunol 2002;23:61–64.[CrossRef][Medline]
  31. Ahearn JM, Fearon DT. Structure and function of the complement receptors, CR1 (CD35) and CR2 (CD21). Adv Immunol 1989;46:183–219.[Medline]
  32. Holers VM, Kinoshita T, Molina H. The evolution of mouse and human complement C3-binding proteins: Divergence of form but conservation of function. Immunol Today 1992;13:231–236.[CrossRef][Medline]
  33. Li B, Sallee C, Dehoff M et al. Mouse Crry/p65. Characterization of monoclonal antibodies and the tissue distribution of a functional homologue of human MCP and DAF. J Immunol 1993;151:4295–4305.[Abstract]
  34. Kurtz CB, Paul MS, Aegerter M et al. Murine complement receptor gene family. II. Identification and characterization of the murine homolog (Cr2) to human CR2 and its molecular linkage to Crry. J Immunol 1989;143:2058–2067.[Abstract]
  35. Paul MS, Aegerter M, O'Brien SE et al. The murine complement receptor gene family. Analysis of mCRY gene products and their homology to human CR1. J Immunol 1989;142:582–589.[Abstract]
  36. Kim DD, Song WC. Membrane complement regulatory proteins. Clin Immunol 2006;118:127–136.[CrossRef][Medline]
  37. Telen MJ, Green AM. The Inab phenotype: Characterization of the membrane protein and complement regulatory defect. Blood 1989;74:437–441.[Abstract/Free Full Text]
  38. Gossling J. Occurrence and pathogenicity of the Streptococcus milleri group. Rev Infect Dis 1988;10:257–285.[Medline]
  39. Reid ME, Mallinson G, Sim RB et al. Biochemical studies on red blood cells from a patient with the Inab phenotype (decay-accelerating factor deficiency). Blood 1991;78:3291–3297.[Abstract/Free Full Text]
  40. Qian YM, Qin X, Miwa T et al. Identification and functional characterization of a new gene encoding the mouse terminal complement inhibitor CD59. J Immunol 2000;165:2528–2534.[Abstract/Free Full Text]
  41. Qin X, Krumrei N, Grubissich L et al. Deficiency of the mouse complement regulatory protein mCd59b results in spontaneous hemolytic anemia with platelet activation and progressive male infertility. Immunity 2003;18:217–227.[CrossRef][Medline]
  42. Akatsu H, Yamada T, Okada N et al. Unique expression of HRF20 (CD59) in human nervous tissue. Microbiol Immunol 1997;41:321–329.[Medline]
  43. Walport MJ. Complement. Second of two parts. N Engl J Med 2001;344:1140–1144.[Free Full Text]
  44. Takeda J, Miyata T, Kawagoe K et al. Deficiency of the GPI anchor caused by a somatic mutation of the PIG-A gene in paroxysmal nocturnal hemoglobinuria. Cell 1993;73:703–711.[CrossRef][Medline]
  45. Hillmen P, Richards SJ. Implications of recent insights into the pathophysiology of paroxysmal nocturnal haemoglobinuria. Br J Haematol 2000;108:470–479.[CrossRef][Medline]
  46. Hillmen P, Young NS, Schubert J et al. The complement inhibitor eculizumab in paroxysmal nocturnal hemoglobinuria. N Engl J Med 2006;355:1233–1243.[Abstract/Free Full Text]
  47. Koski CL, Ramm LE, Hammer CH et al. Cytolysis of nucleated cells by complement: Cell death displays multi-hit characteristics. Proc Natl Acad Sci U S A 1983;80:3816–3820.[Abstract/Free Full Text]
  48. Bohana-Kashtan O, Ziporen L, Donin N et al. Cell signals transduced by complement. Mol Immunol 2004;41:583–597.[CrossRef][Medline]
  49. Shin ML, Carney DF. Cytotoxic action and other metabolic consequences of terminal complement proteins. Prog Allergy 1988;40:44–81.[Medline]
  50. Morgan BP. Complement membrane attack on nucleated cells: Resistance, recovery and non-lethal effects. Biochem J 1989;264:1–14.[Medline]
  51. Cragg MS, Glennie MJ. Antibody specificity controls in vivo effector mechanisms of anti-CD20 reagents. Blood 2004;103:2738–2743.[Abstract/Free Full Text]
  52. Cittera E, Leidi M, Buracchi C et al. The CCL3 family of chemokines and innate immunity cooperate in vivo in the eradication of an established lymphoma xenograft by rituximab. J Immunol 2007;178:6616–6623.[Abstract/Free Full Text]
  53. Golay J, Cittera E, Di Gaetano N et al. The role of complement in the therapeutic activity of rituximab in a murine B lymphoma model homing in lymph nodes. Haematologica 2006;91:176–183.[Abstract/Free Full Text]
  54. Di Gaetano N, Cittera E, Nota R et al. Complement activation determines the therapeutic activity of rituximab in vivo. J Immunol 2003;171:1581–1587.[Abstract/Free Full Text]
  55. Uchida J, Hamaguchi Y, Oliver JA et al. The innate mononuclear phagocyte network depletes B lymphocytes through Fc receptor-dependent mechanisms during anti-CD20 antibody immunotherapy. J Exp Med 2004;199:1659–1669.[Abstract/Free Full Text]
  56. Hamaguchi Y, Xiu Y, Komura K et al. Antibody isotype-specific engagement of Fcgamma receptors regulates B lymphocyte depletion during CD20 immunotherapy. J Exp Med 2006;203:743–753.[Abstract/Free Full Text]
  57. Hamaguchi Y, Uchida J, Cain DW et al. The peritoneal cavity provides a protective niche for B1 and conventional B lymphocytes during anti-CD20 immunotherapy in mice. J Immunol 2005;174:4389–4399.[Abstract/Free Full Text]
  58. Shushakova N, Skokowa J, Schulman J et al. C5a anaphylatoxin is a major regulator of activating versus inhibitory FcgammaRs in immune complex-induced lung disease. J Clin Invest 2002;110:1823–1830.[CrossRef][Medline]
  59. Schmidt RE, Gessner JE. Fc receptors and their interaction with complement in autoimmunity. Immunol Lett 2005;100:56–67.[CrossRef][Medline]
  60. Huber-Lang M, Sarma JV, Zetoune FS et al. Generation of C5a in the absence of C3: A new complement activation pathway. Nat Med 2006;12:682–687.[CrossRef][Medline]
  61. Kumar V, Ali SR, Konrad S et al. Cell-derived anaphylatoxins as key mediators of antibody-dependent type II autoimmunity in mice. J Clin Invest 2006;116:512–520.[CrossRef][Medline]
  62. van Meerten T, van Rijn RS, Hol S et al. Complement-induced cell death by rituximab depends on CD20 expression level and acts complementary to antibody-dependent cellular cytotoxicity. Clin Cancer Res 2006;12:4027–4035.[Abstract/Free Full Text]
  63. Golay J, Lazzari M, Facchinetti V et al. CD20 levels determine the in vitro susceptibility to rituximab and complement of B-cell chronic lymphocytic leukemia: Further regulation by CD55 and CD59. Blood 2001;98:3383–3389.[Abstract/Free Full Text]
  64. Manches O, Lui G, Chaperot L et al. In vitro mechanisms of action of rituximab on primary non-Hodgkin lymphomas. Blood 2003;101:949–954.[Abstract/Free Full Text]
  65. Perz J, Topaly J, Fruehauf S et al. Level of CD 20-expression and efficacy of rituximab treatment in patients with resistant or relapsing B-cell prolymphocytic leukemia and B-cell chronic lymphocytic leukemia. Leuk Lymphoma 2002;43:149–151.[CrossRef][Medline]
  66. Weng WK, Levy R. Expression of complement inhibitors CD46, CD55, and CD59 on tumor cells does not predict clinical outcome after rituximab treatment in follicular non-Hodgkin lymphoma. Blood 2001;98:1352–1357.[Abstract/Free Full Text]
  67. Takei K, Yamazaki T, Sawada U et al. Analysis of changes in CD20, CD55, and CD59 expression on established rituximab-resistant B-lymphoma cell lines. Leuk Res 2006;30:625–631.[CrossRef][Medline]
  68. Koretz K, Bruderlein S, Henne C et al. Expression of CD59, a complement regulator protein and a second ligand of the CD2 molecule, and CD46 in normal and neoplastic colorectal epithelium. Br J Cancer 1993;68:926–931.[Medline]
  69. Inoue H, Mizuno M, Uesu T et al. Distribution of complement regulatory proteins, decay-accelerating factor, CD59/homologous restriction factor 20 and membrane cofactor protein in human colorectal adenoma and cancer. Acta Med Okayama 1994;48:271–277.[Medline]
  70. Cruz RI, Hernandez-Ilizaliturri FJ, Olejniczak S et al. CD52 over-expression affects rituximab-associated complement-mediated cytotoxicity but not antibody-dependent cellular cytotoxicity: Preclinical evidence that targeting CD52 with alemtuzumab may reverse acquired resistance to rituximab in non-Hodgkin lymphoma. Leuk Lymphoma 2007;48:2424–2436.[CrossRef][Medline]
  71. Treon SP, Mitsiades C, Mitsiades N et al. Tumor cell expression of CD59 is associated with resistance to CD20 serotherapy in patients with B-cell malignancies. J Immunother 2001;24:263–271.[CrossRef][Medline]
  72. Golay J, Zaffaroni L, Vaccari T et al. Biologic response of B lymphoma cells to anti-CD20 monoclonal antibody rituximab in vitro: CD55 and CD59 regulate complement-mediated cell lysis. Blood 2000;95:3900–3908.[Abstract/Free Full Text]
  73. Ziller F, Macor P, Bulla R et al. Controlling complement resistance in cancer by using human monoclonal antibodies that neutralize complement-regulatory proteins CD55 and CD59. Eur J Immunol 2005;35:2175–2183.[CrossRef][Medline]
  74. Macor P, Tripodo C, Zorzet S et al. In vivo targeting of human neutralizing antibodies against CD55 and CD59 to lymphoma cells increases the antitumor activity of rituximab. Cancer Res 2007;67:10556–10563.[Abstract/Free Full Text]
  75. Goldenberg DM, Sharkey RM, Paganelli G et al. Antibody pretargeting advances cancer radioimmunodetection and radioimmunotherapy. J Clin Oncol 2006;24:823–834.[Abstract/Free Full Text]
  76. Paganelli G, Bartolomei M, Grana C et al. Radioimmunotherapy of brain tumor. Neurol Res 2006;28:518–522.[CrossRef][Medline]
  77. Paganelli G, Magnani P, Zito F et al. Three-step monoclonal antibody tumor targeting in carcinoembryonic antigen-positive patients. Cancer Res 1991;51:5960–5966.[Abstract/Free Full Text]
  78. Terui Y, Sakurai T, Mishima Y et al. Blockade of bulky lymphoma-associated CD55 expression by RNA interference overcomes resistance to complement-dependent cytotoxicity with rituximab. Cancer Sci 2006;97:72–79.[CrossRef][Medline]
  79. Di Gaetano N, Xiao Y, Erba E et al. Synergism between fludarabine and rituximab revealed in a follicular lymphoma cell line resistant to the cytotoxic activity of either drug alone. Br J Haematol 2001;114:800–809.[CrossRef][Medline]
  80. Nagamune H, Ohnishi C, Katsuura A et al. Intermedilysin, a novel cytotoxin specific for human cells secreted by Streptococcus intermedius UNS46 isolated from a human liver abscess. Infect Immun 1996;64:3093–3100.[Abstract/Free Full Text]
  81. Giddings KS, Zhao J, Sims PJ et al. Human CD59 is a receptor for the cholesterol-dependent cytolysin intermedilysin. Nat Struct Mol Biol 2004;11:1173–1178.[CrossRef][Medline]
  82. Hu W, Ferris SP, Tweten RK et al. Rapid conditional targeted ablation of cells expressing human CD59 in transgenic mice by intermedilysin. Nat Med 2008;14:98–103.[CrossRef][Medline]
  83. Haidar JH, Shamseddine A, Salem Z et al. Loss of CD20 expression in relapsed lymphomas after rituximab therapy. Eur J Haematol 2003;70:330–332.[CrossRef][Medline]
  84. Beum PV, Kennedy AD, Williams ME et al. The shaving reaction: Rituximab/CD20 complexes are removed from mantle cell lymphoma and chronic lymphocytic leukemia cells by THP-1 monocytes. J Immunol 2006;176:2600–2609.[Abstract/Free Full Text]
  85. Li Y, Williams ME, Cousar JB et al. Rituximab-CD20 complexes are shaved from Z138 mantle cell lymphoma cells in intravenous and subcutaneous SCID mouse models. J Immunol 2007;179:4263–4271.[Abstract/Free Full Text]
  86. Zhao WL, Wang L, Liu YH et al. Combined effects of histone deacetylase inhibitor and rituximab on non-Hodgkin's B-lymphoma cells apoptosis. Exp Hematol 2007;35:1801–1811.[CrossRef][Medline]
  87. Tomita A, Hiraga J, Kiyoi H et al. Epigenetic regulation of CD20 protein expression in a novel B-cell lymphoma cell line, RRBL1, established from a patient treated repeatedly with rituximab-containing chemotherapy. Int J Hematol 2007;86:49–57.[CrossRef][Medline]
  88. Wojciechowski W, Li H, Marshall S et al. Enhanced expression of CD20 in human tumor B cells is controlled through ERK-dependent mechanisms. J Immunol 2005;174:7859–7868.[Abstract/Free Full Text]
  89. Jahrsdorfer B, Muhlenhoff L, Blackwell SE et al. B-cell lymphomas differ in their responsiveness to CpG oligodeoxynucleotides. Clin Cancer Res 2005;11:1490–1499.[Abstract/Free Full Text]
  90. Jahrsdörfer B, Hartmann G, Racila E et al. CpG DNA increases primary malignant B cell expression of costimulatory molecules and target antigens. J Leukoc Biol 2001;69:81–88.[Abstract/Free Full Text]
  91. Reiter Y, Fishelson Z. Targeting of complement to tumor cells by heteroconjugates composed of antibodies and of the complement component C3b. J Immunol 1989;142:2771–2777.[Abstract]
  92. Gelderman KA, Kuppen PJ, Bruin W et al. Enhancement of the complement activating capacity of 17–1A mAb to overcome the effect of membrane-bound complement regulatory proteins on colorectal carcinoma. Eur J Immunol 2002;32:128–135.[CrossRef][Medline]
  93. Juhl H, Petrella EC, Cheung NK et al. Complement killing of human neuroblastoma cells: A cytotoxic monoclonal antibody and its F(ab')2-cobra venom factor conjugate are equally cytotoxic. Mol Immunol 1990;27:957–964.[CrossRef][Medline]
  94. Juhl H, Petrella EC, Cheung NK et al. Additive cytotoxicity of different monoclonal antibody-cobra venom factor conjugates for human neuroblastoma cells. Immunobiology 1997;197:444–459.[Medline]
  95. Kennedy AD, Solga MD, Schuman TA et al. An anti-C3b(i) mAb enhances complement activation, C3b(i) deposition, and killing of CD20+ cells by rituximab. Blood 2003;101:1071–1079.[Abstract/Free Full Text]
  96. Clynes RA, Towers TL, Presta LG et al. Inhibitory Fc receptors modulate in vivo cytoxicity against tumor targets. Nat Med 2000;6:443–446.[CrossRef][Medline]
  97. Hernandez-Ilizaliturri FJ, Jupudy V, Ostberg J et al. Neutrophils contribute to the biological antitumor activity of rituximab in a non-Hodgkin's lymphoma severe combined immunodeficiency mouse model. Clin Cancer Res 2003;9:5866–5873.[Abstract/Free Full Text]
  98. Kennedy AD, Beum PV, Solga MD et al. Rituximab infusion promotes rapid complement depletion and acute CD20 loss in chronic lymphocytic leukemia. J Immunol 2004;172:3280–3288.[Abstract/Free Full Text]
  99. Gorter A, Meri S. Immune evasion of tumor cells using membrane-bound complement regulatory proteins. Immunol Today 1999;20:576–582.[CrossRef][Medline]
  100. Cartron G, Dacheux L, Salles G et al. Therapeutic activity of humanized anti-CD20 monoclonal antibody and polymorphism in IgG Fc receptor FcgammaRIIIa gene. Blood 2002;99:754–758.[Abstract/Free Full Text]
  101. Mller T, Uherek C, Maki G et al. Expression of a CD20-specific chimeric antigen receptor enhances cytotoxic activity of NK cells and overcomes NK-resistance of lymphoma and leukemia cells. Cancer Immunol Immunother 2008;57:411–423.[CrossRef][Medline]
  102. Shimadoi S, Takami A, Kondo Y et al. Macrophage colony-stimulating factor enhances rituximab-dependent cellular cytotoxicity by monocytes. Cancer Sci 2007;98:1368–1372.[CrossRef][Medline]
  103. Golay J, Manganini M, Facchinetti V et al. Rituximab-mediated antibody-dependent cellular cytotoxicity against neoplastic B cells is stimulated strongly by interleukin-2. Haematologica 2003;88:1002–1012.[Abstract/Free Full Text]
  104. Lopes de Menezes DE, Denis-Mize K, Tang Y et al. Recombinant interleukin-2 significantly augments activity of rituximab in human tumor xenograft models of B-cell non-Hodgkin lymphoma. J Immunother 2007;30:64–74.[CrossRef][Medline]
  105. Gillies SD, Lan Y, Williams S et al. An anti-CD20-IL-2 immunocytokine is highly efficacious in a SCID mouse model of established human B lymphoma. Blood 2005;105:3972–3978.[Abstract/Free Full Text]
  106. Ansell SM, Witzig TE, Kurtin PJ et al. Phase 1 study of interleukin-12 in combination with rituximab in patients with B-cell non-Hodgkin lymphoma. Blood 2002;99:67–74.[Abstract/Free Full Text]
  107. Ansell SM, Geyer SM, Maurer MJ et al. Randomized phase II study of interleukin-12 in combination with rituximab in previously treated non-Hodgkin's lymphoma patients. Clin Cancer Res 2006;12:6056–6063.[Abstract/Free Full Text]
  108. Ross GD. Regulation of the adhesion versus cytotoxic functions of the Mac-1/CR3/alphaMbeta2-integrin glycoprotein. Crit Rev Immunol 2000;20:197–222.[Medline]
  109. Vetvicka V, Thornton BP, Ross GD. Soluble beta-glucan polysaccharide binding to the lectin site of neutrophil or natural killer cell complement receptor type 3 (CD11b/CD18) generates a primed state of the receptor capable of mediating cytotoxicity of iC3b-opsonized target cells. J Clin Invest 1996;98:50–61.[Medline]
  110. Xia Y, Vetvicka V, Yan J et al. The beta-glucan-binding lectin site of mouse CR3 (CD11b/CD18) and its function in generating a primed state of the receptor that mediates cytotoxic activation in response to iC3b-opsonized target cells. J Immunol 1999;162:2281–2290.[Abstract/Free Full Text]
  111. Hong F, Hansen RD, Yan J et al. Beta-glucan functions as an adjuvant for monoclonal antibody immunotherapy by recruiting tumoricidal granulocytes as killer cells. Cancer Res 2003;63:9023–9031.[Abstract/Free Full Text]
  112. Modak S, Koehne G, Vickers A et al. Rituximab therapy of lymphoma is enhanced by orally administered (1-->3),(1-->4)-D-beta-glucan. Leuk Res 2005;29:679–683.[CrossRef][Medline]
  113. Deans JP, Schieven GL, Shu GL et al. Association of tyrosine and serine kinases with the B cell surface antigen CD20. Induction via CD20 of tyrosine phosphorylation and activation of phospholipase C-gamma 1 and PLC phospholipase C-gamma 2. J Immunol 1993;151:4494–4504.[Abstract]
  114. Deans JP, Robbins SM, Polyak MJ et al. Rapid redistribution of CD20 to a low density detergent-insoluble membrane compartment. J Biol Chem 1998;273:344–348.[Abstract/Free Full Text]
  115. Deans JP, Li H, Polyak MJ. CD20-mediated apoptosis: Signalling through lipid rafts. Immunology 2002;107:176–182.[CrossRef][Medline]
  116. Deans JP, Kalt L, Ledbetter JA et al. Association of 75/80-kDa phosphoproteins and the tyrosine kinases Lyn, Fyn, and Lck with the B cell molecule CD20. Evidence against involvement of the cytoplasmic regions of CD20. J Biol Chem 1995;270:22632–22638.[Abstract/Free Full Text]
  117. Brdicka T, Pavlistova D, Leo A et al. Phosphoprotein associated with glycosphingolipid-enriched microdomains (PAG), a novel ubiquitously expressed transmembrane adaptor protein, binds the protein tyrosine kinase Csk and is involved in regulation of T cell activation. J Exp Med 2000;191:1591–1604.[Abstract/Free Full Text]
  118. Kawabuchi M, Satomi Y, Takao T et al. Transmembrane phosphoprotein Cbp regulates the activities of Src-family tyrosine kinases. Nature 2000;404:999–1003.[CrossRef][Medline]
  119. Stel AJ, Ten Cate B, Jacobs S et al. Fas receptor clustering and involvement of the death receptor pathway in rituximab-mediated apoptosis with concomitant sensitization of lymphoma B cells to Fas-induced apoptosis. J Immunol 2007;178:2287–2295.[Abstract/Free Full Text]
  120. Byrd JC, Kitada S, Flinn IW et al. The mechanism of tumor cell clearance by rituximab in vivo in patients with B-cell chronic lymphocytic leukemia: Evidence of caspase activation and apoptosis induction. Blood 2002;99:1038–1043.[Abstract/Free Full Text]
  121. Pedersen IM, Buhl AM, Klausen P et al. The chimeric anti-CD20 antibody rituximab induces apoptosis in B-cell chronic lymphocytic leukemia cells through a p38 mitogen activated protein-kinase-dependent mechanism. Blood 2002;99:1314–1319.[Abstract/Free Full Text]
  122. Suzuki E, Umezawa K, Bonavida B. Rituximab inhibits the constitutively activated PI3K-Akt pathway in B-NHL cell lines: Involvement in chemosensitization to drug-induced apoptosis. Oncogene 2007;26:6184–6193.[CrossRef][Medline]
  123. Vega MI, Huerta-Yepaz S, Garban H et al. Rituximab inhibits p38 MAPK activity in 2F7 B NHL and decreases IL-10 transcription: Pivotal role of p38 MAPK in drug resistance. Oncogene 2004;23:3530–3540.[CrossRef][Medline]
  124. Huerta-Yepez S, Vega M, Jazirehi A et al. Nitric oxide sensitizes prostate carcinoma cell lines to TRAIL-mediated apoptosis via inactivation of NF-kappa B and inhibition of Bcl-xl expression. Oncogene 2004;23:4993–5003.[CrossRef][Medline]
  125. van der Kolk LE, Evers LM, Omene C et al. CD20-induced B cell death can bypass mitochondria and caspase activation. Leukemia 2002;16:1735–1744.[CrossRef][Medline]
  126. Chan HT, Hughes D, French RR et al. CD20-induced lymphoma cell death is independent of both caspases and its redistribution into triton X-100 insoluble membrane rafts. Cancer Res 2003;63:5480–5489.[Abstract/Free Full Text]
  127. Skvortsova I, Skvortsov S, Popper BA et al. Rituximab enhances radiation-triggered apoptosis in non-Hodgkin's lymphoma cells via caspase-dependent and -independent mechanisms. J Radiat Res (Tokyo) 2006;47:183–196.[CrossRef][Medline]
  128. Daniels I, Abulayha AM, Thomson BJ et al. Caspase-independent killing of Burkitt lymphoma cell lines by rituximab. Apoptosis 2006;11:1013–1023.[CrossRef][Medline]
  129. Bremer E, ten Cate B, Samplonius DF et al. Superior activity of fusion protein scFvRit:sFasL over cotreatment with rituximab and Fas agonists. Cancer Res 2008;68:597–604.[Abstract/Free Full Text]
  130. Maddipatla S, Hernandez-Ilizaliturri FJ, Knight J et al. Augmented antitumor activity against B-cell lymphoma by a combination of monoclonal antibodies targeting TRAIL-R1 and CD20. Clin Cancer Res 2007;13:4556–4564.[Abstract/Free Full Text]
  131. Daniel D, Yang B, Lawrence DA et al. Cooperation of the proapoptotic receptor agonist rhApo2L/TRAIL with the CD20 antibody rituximab against non-Hodgkin lymphoma xenografts. Blood 2007;110:4037–4046.[Abstract/Free Full Text]
  132. Ansell SM, Arendt BK, Grote DM et al. Inhibition of survivin expression suppresses the growth of aggressive non-Hodgkin's lymphoma. Leukemia 2004;18:616–623.[CrossRef][Medline]
  133. Brien G, Trescol-Biemont MC, Bonnefoy-Bérard N. Downregulation of Bfl-1 protein expression sensitizes malignant B cells to apoptosis. Oncogene 2007;26:5828–5832.[CrossRef][Medline]
  134. Wobser M, Voigt H, Eggert AO et al. Bcl-2 expression in rituximab refractory cutaneous B-cell lymphoma. Br J Cancer 2007;96:1540–1543.[CrossRef][Medline]
  135. Jazirehi AR, Vega MI, Bonavida B. Development of rituximab-resistant lymphoma clones with altered cell signaling and cross-resistance to chemotherapy. Cancer Res 2007;67:1270–1281.[Abstract/Free Full Text]
  136. Smith MR, Jin F, Joshi I. Enhanced efficacy of therapy with antisense BCL-2 oligonucleotides plus anti-CD20 monoclonal antibody in SCID mouse/human lymphoma xenografts. Mol Cancer Ther 2004;3:1693–1699.[Abstract/Free Full Text]
  137. Wang M, Han XH, Zhang L et al. Bortezomib is synergistic with rituximab and cyclophosphamide in inducing apoptosis of mantle cell lymphoma cells in vitro and in vivo. Leukemia 2008;22:179–185.[CrossRef][Medline]
  138. Bezombes C, Grazide S, Garret C et al. Rituximab antiproliferative effect in B-lymphoma cells is associated with acid-sphingomyelinase activation in raft microdomains. Blood 2004;104:1166–1173.[Abstract/Free Full Text]



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