The association between CD20 and Src-family Tyrosine kinases requires an additional factor

Apoptosis-promoting effect of rituximab-conjugated magnetic nanoprobes on malignant lymphoma cells with CD20 overexpression

Background

Cancer targeting nanoprobes with precisely designed physicochemical properties may show enhanced pharmacological targeting and therapeutic efficacy. As a widely used commercialized antibody, rituximab has been in clinical use for three decades and has lengthened or even saved thousands of lives. However, many people cannot benefit from rituximab treatment because of drug resistance or side effects.

Methods

In this study, a 13-nm rituximab-conjugated magnetic nanoparticle was developed as a therapeutic nanoprobe targeting CD20 overexpressing malignant lymphoma cells to enhance the treatment effects of rituximab. The magnetic cores (2,3-dimercaptosuccinicacid modified Fe₃O₄ nanoparticles, Fe₃O₄@DMSA) of the nanoprobes with an average diameter of 6.5 nm were synthesized using a co-precipitation method. Rituximab was then conjugated on the surface of Fe₃O₄@DMSA using a cross-linking agent (carbodiimide/N-hydroxysulfosuccinimide sodium salt). Based on theoretical calculations, approximately one antibody was coupled with one nanoparticle, excluding the multivalent antibody effect.

Results

Cell targeting experiments and magnetic resonance (MR) signal and T2 measurements showed that the Fe₃O₄@DMSA@Ab nanoprobes have specific binding affinity for CD20-positive cells. Compared to rituximab and Fe₃O₄@DMSA, Fe₃O₄@DMSA@Ab nanoprobes significantly reduced cell viability and promoted Raji cell apoptosis. Initiating events of apoptosis, including increased intracellular calcium and reactive oxygen species, were observed in nanoprobe-treated Raji cells. Nanoprobe-treated Raji cells also showed the most drastic decrease in mitochondrial membrane potential and Bcl-2 expression, compared to rituximab and Fe₃O₄@DMSA-treated Raji cells.

Conclusion

These results indicate that Fe₃O₄@DMSA@Ab nanoprobes have the potential to serve as MRI tracers and therapeutic agents for CD20-positive cells.

Rituximab effectively reverses Tyrosine kinase inhibitors (TKIs) resistance through inhibiting the accumulation of rictor on mitochondria-associated ER-membrane (MAM)

Tyrosine kinase inhibitors (TKIs), a novel group of target-specific anti lung cancer drugs, have recently been found to resistant to some NSCLC cells which have the T790M EGFR mutation. However, recent investigations on the therapies of resistance to EGFR-TKIs are very limited. Therefore, it is important to develop more effective therapies to reverse EGFR-TKIs resistance. In our present study, erlotinib was used as the TKIs drug and the effects of the erlotinib on cell growth were evaluated. Cell viability and concentration dependent studies were performed using HCI-H1975 and HCI-H1299 cells alone with erlotinib, respectively. Further combined with rituximab, the results showed that erlotinib and rituximab were significantly inhibited the cell growth. Furthermore, the combination of erlotinib and rituximab greatly decreased the expression of p-mTOR and p-EGFR. Additional results from western blotting and immunofluorescence assays demonstrated that the accumulation of rictor was also decreased on MAM. Thus, all these results suggested that EGFR-TKIs combined with CD20 mono-antibody significantly decrease the cell growth of H1975 cells and H1299, with T790M EGFR mutation, and inhibit the localization of the key mTOR pathway proteins to MAM. So, it may be a promising strategy for overcoming EGFR TKI resistance in NSCLC patients.

A Phase II Trial of Rituximab Combined With Pegfilgrastim in Patients With Indolent B-cell Non-Hodgkin Lymphoma

BACKGROUND

To explore the role of augmenting neutrophil function in B-cell lymphoma, we conducted a phase II study evaluating the safety and clinical efficacy of pegfilgrastim and rituximab in low-grade CD20⁺ B-cell non-Hodgkin lymphoma (B-NHL).

PATIENTS AND METHODS

Twenty patients with indolent B-NHL were treated with rituximab (375 mg/m²) every other week for 4 doses, followed by every 2 months for 4 additional doses. Pegfilgrastim was administered subcutaneously 3 days before each dose of rituximab. Clinical activity and tolerability were assessed using standard criteria. Biologic monitoring included phenotype characteristics of the host neutrophils, changes in oxidative burst, and functional assays.

RESULTS

The patient demographics included median age of 64 years, 70% were male, 70% had follicular lymphoma, and 90% had stage III-IV disease. The median number of previous therapies was 2 (range, 0-5); 90% had received previous anti-CD20 monoclonal antibody therapy. The addition of pegfilgrastim to rituximab did not increase rituximab-related toxicities. The overall response rate was 60% (12 of 20), with a complete response (CR) rate of 35% (7 of 20). The median progression-free survival (PFS) duration was 17.9 months (95% confidence interval, 9.9-27.6 months); the median overall survival was not reached. A shorter time-to-peak oxidative burst after the first dose of pegfilgrastim was associated with greater CR rates (P = .04) and longer PFS (P = .03).

CONCLUSION

The pegfilgrastim-rituximab combination was well tolerated, with favorable outcomes compared with historical controls. A shorter time-to-peak oxidative burst was associated with higher CR rates and longer PFS. Our results support further evaluation of strategies that enhance the innate immune system to improve rituximab activity in B-NHL.

Non-Hodgkin's B-cell lymphoma: advances in molecular strategies targeting drug resistance

Non-Hodgkin's lymphoma (NHL) is a heterogeneous class of cancers displaying a diverse range of biological phenotypes, clinical behaviours and prognoses. Standard treatments for B-cell NHL are anthracycline-based combinatorial chemotherapy regimens composed of cyclophosphamide, doxorubicin, vincristine and prednisolone. Even though complete response rates of 40-50% with chemotherapy can be attained, a substantial proportion of patients relapse, resulting in 3-year overall survival rates of about 30%. Relapsed lymphomas are refractory to subsequent treatments with the initial chemotherapy regimen and can exhibit cross-resistance to a wide variety of anticancer drugs. The emergence of acquired chemoresistance thus poses a challenge in the clinic preventing the successful treatment and cure of disseminated B-cell lymphomas. Gene-expression analyses have increased our understanding of the molecular basis of chemotherapy resistance and identified rational targets for drug interventions to prevent and treat relapsed/refractory diffuse large B-cell lymphoma. Acquisition of drug resistance in lymphoma is in part driven by the inherent genetic heterogeneity and instability of the tumour cells. Due to the genetic heterogeneity of B-cell NHL, many different pathways leading to drug resistance have been identified. Successful treatment of chemoresistant NHL will thus require the rational design of combinatorial drugs targeting multiple pathways specific to different subtypes of B-cell NHL as well as the development of personalized approaches to address patient-to-patient genetic heterogeneity. This review highlights the new insights into the molecular basis of chemorefractory B-cell NHL that are facilitating the rational design of novel strategies to overcome drug resistance.

Rituximab activates Syk and AKT in CD20-positive B cell lymphoma cells dependent on cell membrane cholesterol levels

The introduction of rituximab, an anti-CD20 monoclonal antibody, has dramatically improved the treatment outcomes of patients with B cell lymphoma. Nevertheless, the clinical response to rituximab varies, and a subpopulation of patients does not respond well to this antibody. Although several molecular events have been shown to be involved in the mechanism of action of rituximab, recent studies have demonstrated that intracellular signaling pathways and the direct effects of rituximab on cell membrane components are responsible for the antilymphoma action of this drug. In the present study, we demonstrated that rituximab activated Syk and Akt, molecules with antiapoptotic functions, in several CD20-positive lymphoma cell lines. Notably, rituximab activated Syk and Akt in all the tested primary lymphoma samples from six patients. Our results show that the cholesterol levels in lymphoma cell membranes have a crucial role in the regulation of Syk and Akt. The depletion of cholesterol from the cell membrane completely blocked rituximab-induced Syk and Akt activation. Simvastatin, an inhibitor of cholesterol synthesis, also abrogated rituximab-mediated Syk and Akt activation. Finally, we report that rituximab inhibited the apoptosis induced by chemotherapeutic drugs, which was observed solely in Akt-activated cells. This work demonstrates for the first time that rituximab paradoxically works to suppress apoptosis under certain conditions in a manner that is dependent on the cell membrane cholesterol level. Our observations provide novel insights and suggest that the cell membrane cholesterol level represents a new biomarker for predicting patient response to rituximab. Furthermore, the modulation of lipid rafts could provide a new strategy for enhancing the antilymphoma action of rituximab.

Analysis of innate and acquired resistance to anti-CD20 antibodies in malignant and nonmalignant B cells

The anti-CD20 monoclonal antibody, rituximab, provides a significant therapeutic benefit for patients with B-cell disorders. However, response to therapy varies and relapses are common, so an understanding of both inherited and acquired rituximab resistance is needed. In order to identify mechanisms of inherited resistance, sensitive versus resistant individuals were selected from a survey of 92 immortalized lymphoblastoid B-cell lines from normal individuals. Levels of CD20 protein and surface expression were lower in the resistant group. In contrast, CD20 mRNA levels were not correlated with susceptibility, suggesting regulation at a post-transcriptional level. To examine acquired resistance, resistant sublines were selected from both lymphoblastoid as well as lymphoma cell lines. Confirming previous findings, there was significant down-regulation of CD20 protein expression in all the resistant sublines. CD20 mRNA splice variants are reported to be associated with development of resistance. Three splice variants were observed in our cell lines, each lacking the binding epitope for rituximab, but none were associated with rituximab resistance. The second generation anti-CD20 mAb, ofatumumab, was more active compared with rituximab in vitro in the survey of all B-cell lines, mirroring results that have been reported previously with malignant B-cells. These studies show that normal B-lymphoblastoid cell lines can be used to model both innate and acquired mechanisms of resistance. They validate the important role of CD20 expression and enable future genetic studies to identify additional mediators of anti-CD20 mAb resistance.

Anti-CD20 monoclonal antibodies in chronic lymphocytic leukemia

INTRODUCTION

The last decade has witnesd immense progress in the treatment of chronic lymphocytic leukemia (CLL). Chemoimmunotherapy (CIT) combining rituximab and fludarabine with cyclophosphamide (FCR) in the frontline setting has clearly been shown to improve outcomes in patients with CLL. Building on the success achieved with rituximab, other anti-CD20 monoclonal antibodies (mAbs) are being investigated. Novel bioengineering techniques have helped in the development of anti-CD20 mAbs. One antibody, ofatumumab, was recently approved for the treatment of refractory CLL. A type II anti-CD20 mAb, GA-101 (obinutuzumab), is currently in clinical trials. This short review focuses on ongoing clinical trials of anti-CD20 mAbs in CLL.

AREAS COVERED

Literature search was performed using PubMed ( www.clinicaltrials.gov (till August 2012)), and recent American Society of Clinical Oncology (ASCO), American Society of Hematology (ASH), European Hematology association (EHA), International workshop on CLL (iwCLL) abstracts, using the primary search terms 'anti-CD20 monoclonal antibody' with/without CLL. Articles were chosen on the basis of relevance of anti-CD20 mAbs to CLL therapy.

EXPERT OPINION

Rituximab, the prototype anti-CD20 mAb, forms the core of CIT in CLL. The success of rituximab and ofatumumab has led investigators to evaluate other anti-CD20 mAbs in the treatment of CLL.

Strategies to enhance rituximab anti-tumor activity in the treatment of CD20-positive B-cell neoplasms

Rituximab is a chimeric monoclonal anti-CD20 antibody and was the first monoclonal antibody (mAb) therapy approved by FDA (Food and Drug Administration) for the treatment of B-cell lymphoma. It has revolutionized the treatment of patients with CD20-positive non-Hodgkin's lymphoma and CLL. Rituximab is currently being used in virtually all patients with B-cell lymphomas either alone or in combination with chemotherapy. Despite its excellent safety and efficacy profile, only a small portion of B-cell lymphoma patients treated with rituximab as a single agent have sustained complete remissions. Combining rituximab with standard chemotherapy regimens is associated with higher response rates, and improved survival in a subset of patients. Unfortunately, a significant percentage of patients who initially respond to rituximab eventually relapse, and there are patients that demonstrate intrinsic resistance to initial therapy. In the last decade, ongoing scientific research has led to a better understanding of rituximab-associated cytotoxic mechanisms against lymphoma target cells. Scientific efforts are increasingly being focused in developing new strategies to improve mAb activity. Various strategies include the following: combining rituximab with different biologic agents (e.g. cytokines, immunomodulatory drugs); developing novel antibody constructs (including bi-specific antibodies); and/or inhibiting signaling pathways associated with lymphomagenesis and immuno-chemotherapy resistance. In this review article, we will provide an overview of various rituximab-associated cytotoxic mechanisms and novel strategies to improve mAb activity against B-cell lymphoma.

Clinical consequences of defects in B-cell development

Abnormalities in humoral immunity typically reflect a generalized or selective failure of effective B-cell development. The developmental processes can be followed through analysis of cell-surface markers, such as IgM, IgD, CD10, CD19, CD20, CD21, and CD38. Early phases of B-cell development are devoted to the creation of immunoglobulin and testing of B-cell antigen receptor signaling. Failure leads to the absence of B cells and immunoglobulin in the blood from birth. As the developing B cells begin to express a surface B-cell receptor, they become subject to negative and positive selection pressures and increasingly depend on survival signals. Defective signaling can lead to selective or generalized hypogammaglobulinemia, even in the presence of normal numbers of B cells. In the secondary lymphoid organs some B cells enter the splenic marginal zone, where preactivated cells lie ready to rapidly respond to T-independent antigens, such as the polysaccharides that coat some microorganisms. Other cells enter the follicle and, with the aid of cognate follicular T cells, divide to help form a germinal center (GC) after their interaction with antigen. In the GC B cells can undergo the processes of class switching and somatic hypermutation. Failure to properly receive T-cell signals can lead to hyper-IgM syndrome. B cells that leave the GC can develop into memory B cells, short-lived plasma cells, or long-lived plasma cells. The latter ultimately migrate back to the bone marrow, where they can continue to produce protective antigen-specific antibodies for decades.

Identification of an alternative CD20 transcript variant in B-cell malignancies coding for a novel protein associated to rituximab resistance

Human CD20 is a B-cell lineage–specific marker expressed by normal and leukemic B cells from the pre-B to the plasma-cell stages and is a target for rituximab (RTX) immunotherapy. A CD20 reverse transcriptase–polymerase chain reaction (PCR) on B-cell lines cDNA yielded a short PCR product (ΔCD20) corresponding to a spliced mRNA transcript linking the exon 3 and exon 7 ends. We established here that this novel, alternatively spliced CD20 transcript is expressed and detectable at various levels in leukemic B cells, lymphoma B cells, in vivo tonsil- or in vitro CD40L-activated B cells, and Epstein-Barr virus (EBV)–transformed B cells, but not in resting CD19+- or CD20+-sorted B cells from peripheral blood or bone marrow of healthy donors. The truncated CD20 sequence is within the reading frame, codes a protein of 130 amino acids (∼ 15-17 kDa) lacking large parts of the 4 transmembrane segments, suggesting that ΔCD20 is a nonanchored membrane protein. We demonstrated the translation into a ΔCD20 protein which is associated with the membrane CD20 protein and showed its involvement in RTX resistance. Study of patient samples before and after RTX resistance or escape confirms our in vitro findings.

Rituximab sensitizes a Burkitt lymphoma cell line to cell killing by X-irradiation

Clinical trials with rituximab in combination with chemotherapeutic regimens have shown promising results. Data on the effects of rituximab treatment in combination with irradiation are, however, limited and inconsistent. This study aims to investigate the effects of rituximab (R) on cell death induced by X-irradiation in Raji lymphoma cells and to evaluate its mechanisms. We found the cell growth inhibition by irradiation was enhanced by additional rituximab exposure both in cells precultured with rituximab followed by irradiation (R + irradiation) or in cells treated in the reverse sequence (irradiation + R). R + irradiation combination treatment induced more apoptotic cells than irradiation and irradiation + R treatment as early as 12 h after treatment. At 24 h, both combination treatments, R + irradiation and irradiation + R, showed apoptotic cells, which were significantly different from irradiation alone. G2/M cell cycle arrest was observed after irradiation alone and the combination treatment. The combination treatment revealed an elevation in reactive oxygen species (ROS) generation in a radiation dose-dependent manner. In addition, rituximab enhanced the cell growth inhibition and apoptotic cell death induced by the oxidative agent, H(2)O(2). We propose that rituximab mediates a significant in vitro radiosensitizing effect and induces cell cycle changes and apoptosis in Raji cells. ROS probably play an important role in these events.

Acquirement of rituximab resistance in lymphoma cell lines is associated with both global CD20 gene and protein down-regulation regulated at the pretranscriptional and posttranscriptional levels

Acquirement of resistance to rituximab has been observed in lymphoma patients. To define mechanisms associated with rituximab resistance, we developed various rituximab-resistant cell lines (RRCL) and studied changes in CD20 expression/structure, lipid raft domain (LRD) reorganization, calcium mobilization, antibody-dependent cellular cytotoxicity, and complement-mediated cytotoxicity (CMC) between parental and RRCL. Significant changes in surface CD20 antigen expression were shown in RRCL. Decreased calcium mobilization and redistribution of CD20 into LRD were found in RRCL. Western blotting identified a unique 35 kDa protein band in RRCL, which was not seen in parental cells and was secondary to an increase in surface and cytoplasmic expression of IgM light chains. CD20 gene expression was decreased in RRCL. In vitro exposure to PS341 increased CD20 expression in RRCL and minimally improved the sensitivity to rituximab-associated CMC. Our data strongly suggest that the acquisition of rituximab resistance is associated with global gene and protein down-regulation of the CD20 antigen affecting LRD organization and downstream signaling. CD20 expression seems to be regulated at the pretranscriptional and posttranscriptional levels. Proteasome inhibition partially reversed rituximab resistance, suggesting the existence of additional mediators of rituximab resistance. Future research is geared to identify drugs and/or biological agents that are effective against RRCL.

Immunomodulatory drug CC-5013 or CC-4047 and rituximab enhance antitumor activity in a severe combined immunodeficient mouse lymphoma model

New thalidomide derivatives CC-5013 and CC-4047 (immunomodulatory drugs, IMiD) are up to 10,000 times more potent than Thalidomide. The biological effects of IMiDs are presumed to be mediated by (a) activation of some components of the innate [natural killer (NK) cells] or adoptive immune system (T cells), (b) modification of cytokine microenvironment in the tumor bed, or by (c) inhibition of angiogenesis. In this article, we tested an innovative combination strategy involving rituximab and IMiDs in aggressive lymphoma cell lines and human lymphoma xenografts. Treatment of non-Hodgkin's lymphoma cells with CC-5013 resulted in a 40% to 70% growth inhibition when compared with controls (P < 0.05). Exposure of lymphoma cells to CC-4047 resulted in a lesser degree of growth inhibition. Induction of apoptosis was shown in 10% to 26% of lymphoma cells 24 hours following exposure to either IMiD. In vivo studies in severe combined immunodeficient mice showed synergistic activity between CC-4047 (and to a lesser degree, CC-5013) plus rituximab. Animals treated with the CC-4047/rituximab combination had a median survival of 74 days (P = 0.0012) compared with 58 days (P = 0.167) in CC-5013/rituximab-treated animals compared with 45 days in rituximab monotherapy-treated animals. The synergistic effect between IMiDs and rituximab in our mouse model was attributed to NK cell expansion. The enhancement of rituximab activity by IMiDs was abrogated by in vivo depletion of NK cells. Augmenting NK cell function by CC-4047 or CC-5013 exposure may increase the antitumor effects of rituximab against B-cell lymphomas and warrants further exploration in the context of a clinical trial.

CD20: A target antigen for immunotherapy of autoimmune diseases

This article reviews the role of CD20 antigen in B cell function and the effectiveness and limits of passive immunotherapy with anti-CD20 monoclonal antibody (Rituximab) in the treatment of autoimmune (or immune-mediated) diseases. Active immunotherapy is a more feasible way to control these chronic diseases. A peptide that mimics the CD20 epitope recognized by Rituximab is employed to stimulate the host immune response against CD20.

Circulating CD20 and CD52 in patients with non-Hodgkin's lymphoma or Hodgkin's disease

The cell surface proteins CD20 and CD52 differ significantly in their structures and are expressed on the majority of B cells. Both circulating CD20 (cCD20) and circulating CD52 (cCD52) have been recently documented in patients with chronic lymphocytic leukaemia. A retrospective study to establish whether cCD20 and/or cCD52 were detectable in patients with lymphoma, and the clinical associations of these soluble antigens if detected, was conducted. cCD20 and cCD52 levels were analysed in a cohort of 65 patients with non-Hodgkin's lymphoma (NHL) and 37 with Hodgkin's disease (HD). Patients with NHL had elevated pretherapy levels of cCD20 and cCD52 compared with normal individuals. Patients with HD had significantly lower than normal pretherapy levels of both cCD20 and cCD52. cCD20 levels were marginally elevated post-therapy in NHL patients while in patients with HD, cCD20 levels remained significantly lower than normal after therapy. Serum cCD52 levels became significantly lower than normal post-therapy in NHL patients, and remained significantly lower than normal in HD patients. No predictive effects were found for pretherapy or post-therapy levels of cCD52 on survival for either cohort of patients. Post-therapy cCD20 levels independently highly correlated with survival in patients with NHL. Prospective evaluation will be required to establish if cCD20 and cCD52 may be used as biomarkers in the diagnosis, prognostic categorization, and monitoring of the clinical course in patients with lymphoma. The clinical significance of circulating antigen in patients receiving monoclonal antibody therapy directed against CD20 and/or CD52 warrants study.

Transient down-modulation of CD20 by rituximab in patients with chronic lymphocytic leukemia

Lymphoid cells in most patients with chronic lymphocytic leukemia (CLL), when treated with rituximab, become CD20-. This is thought to be due to masking of CD20 by rituximab. We used specific antimouse immunoglobulin antibodies to detect rituximab on the surface of CLL lymphocytes and we demonstrate that rituximab is rarely detectable after therapy. Only 3 of 65 patients with CLL had rituximab detectable on their lymphocytes after rituximab therapy despite the fact that most had no detectable CD20 expression. In vitro mixing of CLL or Raji cells with rituximab demonstrated that rituximab was detectable on the surface of cells due to its binding to CD20. However, the addition of plasma led to the down-modulation of CD20 expression, and the rituximab became undetectable. This down-modulation of CD20 protein expression was associated with a down-modulation of CD20 mRNA. CLL cells that lost their CD20 expression regained CD20 expression after 24 hours in culture. These data suggest that rituximab therapy leads to a substantial but transient down-modulation of CD20 expression and that negativity for CD20 in cells from patients treated with rituximab is not necessarily due to CD20 masking. The importance of this down-modulation in the efficacy of current therapy with rituximab needs further investigation.

A cholesterol-dependent CD20 epitope detected by the FMC7 antibody

Accurate diagnosis of lymphoid malignancies is essential for appropriate therapeutic intervention. In conjunction with other diagnostic determinants, immunophenotypic analysis of differentially expressed cell surface markers, such as CD5, CD20, CD23 and FMC7, is useful in the subclassification of lymphomas and leukemias arising from the B-cell lineage. Recent evidence suggesting that CD20 predicts FMC7 expression has prompted reappraisal of the utility of monitoring both markers. Here, we report that the FMC7 monoclonal antibody (mAb) specifically and strongly recognized CD20 ectopically expressed in hematopoietic and nonhematopoietic cell lines. The reactivity of FMC7 was abolished by mutations in the extracellular domain of CD20. These data confirm the CD20 specificity of FMC7. Like other CD20 mAbs, FMC7 binding was temperature dependent and induced detergent insolubility of CD20. Of significant interest, the CD20 epitope recognized by FMC7 was unusual in that it was exceptionally sensitive to membrane cholesterol. Cholesterol depletion profoundly reduced expression of the FMC7 epitope, whereas cholesterol enrichment enhanced its expression. FMC7 mAb binding thus appears to be a sensitive indicator of the level of plasma membrane cholesterol and reveals a conformational state of CD20 that is regulated by cholesterol.

Immunotherapy with anti-CD20 compounds

The B-cell surface antigen CD20 is currently the prime target for near-selective treatment of mature B-cell malignancies and a range of reactive B-cell associated disorders (including virus-associated lymphoproliferation or autoimmune conditions). CD20 is strongly and homogeneously expressed on the majority of mature B-cell neoplasms except chronic lymphocytic leukaemia cells, and on all mature reactive B-cells. This review will summarise the modes of action of various reagents targeting CD20. Treatment results following their use in single and combination therapy for B-cell disorders are reviewed.

Circulating CD20 is detectable in the plasma of patients with chronic lymphocytic leukemia and is of prognostic significance

CD20 is a 33- to 36-kDa transmembrane phosphoprotein involved in the activation, proliferation, and differentiation of B lymphocytes. The predicted amino acid sequence of the CD20 suggests 4 transmembrane-spanning regions with both N- and C-termini located in the cytoplasm. We demonstrate herein that significant levels of circulating CD20 (cCD20) can be detected in the plasma of patients with chronic lymphocytic leukemia (CLL) and that cCD20 interferes with the binding of rituximab, a humanized anti-CD20 monoclonal antibody, to CLL cells. An enzyme-linked immunosorbent assay (ELISA) was developed to measure circulating cCD20 levels in the plasma. We measured cCD20 levels in the plasma of 180 patients with CLL and correlated these levels with clinical characteristics and outcome. Circulating CD20 levels correlated positively with beta(2)-microglobulin level (p =.006) and percentage of CD38(+) cells (p =.03) and negatively with platelet count (p =.004) and hemoglobin level (p =.02). Patients with advanced Rai (III/IV) or Binet (C) stage disease had significantly higher levels of cCD20 than did patients with earlier-stage disease (P =.01 and P =.006, respectively). There was no correlation between cCD20 level and age, lymphocyte count, or white blood cell count. Using a recursive classification method, we found that patients with a cCD20 level more than 1875 nM/L had significantly shorter survival than those with cCD20 1875 nM/L or below (P =.01). The prognostic value of cCD20 was independent of Rai staging or hemoglobin level. Prospective evaluation is indicated to establish whether rituximab dosing should be adjusted according to cCD20 levels.

In vitro and in vivo purging of B lymphoma cells from stem-cell products using anti-CD20 Abs

BACKGROUND

Autologous stem-cell transplantation has proved curative therapy for relapsed NHL. However, recurrence of underlying disease remains the major cause of treatment failure in this setting.

METHODS

Development of effective MAb therapy directed against the B cell surface antigen CD20 has added a valuable tool of clearing contaminating lymphoma cells from stem-cell products by either in vitro or in vivo application.

RESULTS

Transplantation of successfully in vitro purged bone marrow using Mabs has been correlated with prolonged survival in large Phase-II study. So far, no randomized trial could demonstrate a therapeutic benefit for in vitro purging. The anti-CD20 Mab rituximab has been used for in vivo purging at the time of stem cell collection or peritransplantation. This method has been shown to be safe and feasible. In the majority of patients the combination of rituximab with anti-lymphoma chemotherapy meant the collected stem cell products were free of molecularly-detectable lymphoma cells.

DISCUSSION

The increasing ability to kill all lymphoma cells in vivo by regimens including myeloablative therapy renders contaminating lymphoma cells of the autologous stem cell product the main source for disease recurrence. Clearing of these cells remains a prerequisite for curative stem-cell transplantation. Establishment of safe and effective therapeutic schedules using Mabs will enhance the chance for collection of lymphoma-free hematopoietic stems cells.

CD20-mediated apoptosis: signalling through lipid rafts

CD20 is an effective target for therapeutic B-cell depletion with monoclonal antibodies. One proposed mechanism of action is direct cytotoxicity mediated via tyrosine kinase-dependent signalling pathways activated upon CD20 cross-linking. The association of CD20 with membrane microdomains known as lipid rafts, enriched in src-family tyrosine kinases and other signalling effectors, suggests an indirect mechanism of anti-CD20-induced apoptosis in which activation of src-family kinases occurs as a consequence of lipid raft clustering.

Anti-CD20-based therapy of B cell lymphoma: state of the art

Over the last 5 years, studies applying the chimeric anti-CD20 MAb have renewed enthusiasm and triggered world-wide application of anti-CD20 MAb-based therapies in B cell non-Hodgkin's lymphoma (NHL). Native chimeric anti-CD20 and isotope-labeled murine anti-CD20 MAbs are currently employed with encouraging results as monotherapy or in combination with conventional chemotherapy and in consolidation of remission after treatments with curative intent (ie after/ in combination with high-dose chemotherapy and hematopoietic stem cell rescue). On the available experience, anti-CD20 MAb-based therapeutic strategies will be increasingly integrated in the treatment of B cell NHL and related malignancies.

Rituximab: mechanism of action and resistance

Rituximab (Rituxan; Genentech, Inc, South San Francisco, CA, and IDEC Pharmaceuticals, San Diego, CA)-mediated killing of CD20-positive tumor cells is likely caused by a combination of immune-mediated effects including complement-mediated lysis and antibody-dependent cell-mediated cytotoxicity and direct effects induced by CD20 ligation. In vivo, the clearance of damaged or preapoptotic cells through specific receptors for phosphatidylserine translocated to the outer cell membrane may also be important. Direct effects, including growth inhibition and apoptosis, have been shown in vitro; however, their contribution to the clinical effect is not known. Currently, most data suggest that the predominant effector mechanism is antibody-dependent cell-mediated cytotoxicity, with a minor role of complement. With treatment, resistance to rituximab-mediated killing may emerge. Little is known regarding the molecular pathogenesis of this resistance. In rare cases, the CD20 antigen may be lost. Complement-resistance proteins may also increase, but it is not clear that this is the reason for loss of sensitivity. A better understanding of these mechanisms should allow combination therapy with agents capable of augmenting antibody-based killing.

Lymphoma immunophenotyping: a new era in paraffin-section immunohistochemistry

Recent advances in immunohistochemistry have made it possible to investigate lymphomas for the expression of a wide range of antigens in fixed tissues. Epitope retrieval, sensitive detection methods, and the availability of new monoclonal antibodies have all contributed to one's ability to perform detailed immunophenotyping that previously could only be done in cryostat sections or by flow cytometry. Current lymphoma classifications make use of characteristic immunophenotypic profiles that aid in the reproducible diagnosis and subcassification of these neoplasms. The following is a review of the current state of immunophenotyping for lymphoid neoplasms in fixed tissues.

Clinical trials of antibody therapy

Much of the 25 years since Kohler and Milstein first described making monoclonal antibodies (mAbs) has been spent trying to develop these reagents to treat human disease. Until recently, progress has been frustratingly slow and by 1994 only one mAb, anti-CD3 (OKT3), had been licensed for clinical use. In the past five years, however, the situation has changed dramatically, with numerous mAbs now showing clinical potential, and a further seven approved for human treatment. Furthermore, all indications are that this upward trend will continue, with a quarter of all new biological products currently undergoing clinical development being antibody based.

Biologic response of B lymphoma cells to anti-CD20 monoclonal antibody rituximab in vitro: CD55 and CD59 regulate complement-mediated cell lysis

The chimeric anti-CD20 MAb rituximab has recently become a treatment of choice for low-grade or follicular non-Hodgkin's lymphomas (FL) with a response rate of about 50%. In this report, we have investigated the mechanism of action of rituximab on 4 FL and 1 Burkitt's lymphoma (BL) cell lines, 3 fresh FL samples and normal B cells in vitro. Rituximab efficiently blocks the proliferation of normal B cells, but not that of the lymphoma lines. We did not detect significant apoptosis of the cell lines in response to rituximab alone. All cell lines were targets of antibody-dependent cellular cytotoxicity (ADCC). On the other hand, human complement-mediated lysis was highly variable between cell lines, ranging from 100% lysis to complete resistance. Investigation of the role of the complement inhibitors CD35, CD46, CD55, and CD59 showed that CD55, and to a lesser extent CD59, are important regulators of complement-mediated cytotoxicity (CDC) in FL cell lines as well as in fresh cases of FL: Blocking CD55 and/or CD59 function with specific antibodies significantly increased CDC in FL cells. We conclude that CDC and ADCC are major mechanisms of action of rituximab on B-cell lymphomas and that a heterogeneous susceptibility of different lymphoma cells to complement may be at least in part responsible for the heterogeneity of the response of different patients to rituximab in vivo. Furthermore, we suggest that the relative levels of CD55 and CD59 may become useful markers to predict the clinical response.

Biologic response of B lymphoma cells to anti-CD20 monoclonal antibody rituximab in vitro: CD55 and CD59 regulate complement-mediated cell lysis

The chimeric anti-CD20 MAb rituximab has recently become a treatment of choice for low-grade or follicular non-Hodgkin's lymphomas (FL) with a response rate of about 50%. In this report, we have investigated the mechanism of action of rituximab on 4 FL and 1 Burkitt's lymphoma (BL) cell lines, 3 fresh FL samples and normal B cells in vitro. Rituximab efficiently blocks the proliferation of normal B cells, but not that of the lymphoma lines. We did not detect significant apoptosis of the cell lines in response to rituximab alone. All cell lines were targets of antibody-dependent cellular cytotoxicity (ADCC). On the other hand, human complement-mediated lysis was highly variable between cell lines, ranging from 100% lysis to complete resistance. Investigation of the role of the complement inhibitors CD35, CD46, CD55, and CD59 showed that CD55, and to a lesser extent CD59, are important regulators of complement-mediated cytotoxicity (CDC) in FL cell lines as well as in fresh cases of FL: Blocking CD55 and/or CD59 function with specific antibodies significantly increased CDC in FL cells. We conclude that CDC and ADCC are major mechanisms of action of rituximab on B-cell lymphomas and that a heterogeneous susceptibility of different lymphoma cells to complement may be at least in part responsible for the heterogeneity of the response of different patients to rituximab in vivo. Furthermore, we suggest that the relative levels of CD55 and CD59 may become useful markers to predict the clinical response.

Clustered CD20 induced apoptosis: src-family kinase, the proximal regulator of tyrosine phosphorylation, calcium influx, and caspase 3-dependent apoptosis

Anti-CD20 antibodies may reduce or eliminate non-Hodgkin's lymphoma B cells in patients, although the mechanism of action is not clear. To explore mechanism(s), we examined the induction of signal transduction events using anti-CD20 monoclonal antibodies (mAb) in the human non-Hodgkin's lymphoma Ramos B cell line. We found that while Rituximab (a human-mouse hybrid mAb) alone induced apoptotic cell death, other murine anti-CD20 mAbs induced apoptosis of Ramos B cells only upon clustering with a secondary antibody. CD20 clustering was accompanied by activation of tyrosine protein kinase activity, PLCgamma2 phosphorylation, influx of Ca(2+), and activation of caspase 3. All signaling events, as well as the subsequent apoptosis, were blocked by PP2, a selective inhibitor of Src-family kinases. Treatment of Ramos with EGTA and BAPTA to block changes in cytoplasmic Ca(2+) likewise prevented CD20-induced apoptosis. Our findings support a model in which CD20 clustering activates members of the Src family of protein tyrosine kinases, leading to phosphorylation of PLCgamma2 and increased cytoplasmic Ca(2+). These early signal transduction events activate caspase 3 to promote apoptotic cell death of NHL B cells.

CD40 associates with the MHC class II molecules on human B cells

The MHC class II and CD40 molecules are two major components of the immune system that are involved in cell-cell interactions and signal transduction. Data obtained in the course of the present investigation show that these two molecules are physically associated on the surface of various human B cell lines and on normal tonsilar B cells. The CD40 / MHC class II complexes were not detected on the germinal center B cell line Ramos. However, stimulation of these cells via CD40 or MHC class II triggered their association, suggesting that the formation of the complex is related to the activation status of the cells. The formation of these complexes did not alter the interaction of MHC class II molecules with one of their natural ligands, the staphylococcal enterotoxin A (SEA), as evidenced by the ability of SEA to bind MHC class II / CD40 complexes. Cross-linking of MHC class II or CD40 molecules leads to the association as well as the co-association of both molecules to the NP-49-insoluble cellular matrix. Such association allowed us to demonstrate that only a fraction of these molecules can be physically associated on the cell surface. Based on previous observations and those presented here, it is highly possible that the CD40 / MHC class II complexes may have an important role in signal(s) induced via both molecules and during T / B cells interactions.

Signaling antibodies in cancer therapy

Over the past 10-15 years, genetic engineering of monoclonal antibodies has greatly improved their utility in humans and in particular their ability to recruit immunological effectors such as natural killer cells and macrophages. Clinical results now confirm that these new reagents, when directed at the appropriate tumor markers (e.g. CD20 or Her-2), can control disease without untoward side effects. However, despite such success it is still unclear exactly how monoclonal antibodies (mAbs) destroy tumors in vivo. The ability of mAbs to crosslink membrane receptors and generate intracellular signals is part of the mechanism by which they control tumor growth. New data show that such 'signaling' mAbs can be used to sensitize tumors to the action of conventional DNA-damaging drugs.