CD20-induced B cell death can bypass mitochondria and caspase activation

CD3xCD19 DART molecule treatment induces non-apoptotic killing and is efficient against high-risk chemotherapy and venetoclax-resistant chronic lymphocytic leukemia cells

BACKGROUND

Bispecific antibodies are promising new therapeutics in B cell malignancies. Whether they lead to potent T cell activation despite described T cell dysfunction in chronic lymphocytic leukemia (CLL), and are able to effectively target high-risk or venetoclax-resistant samples, is currently unknown.

METHODS

CD19⁺ cell lines or primary (high-risk) CLL were cocultured in vitro with healthy donor (HD) or CLL-derived T cells in the presence of a CD3xCD19 dual affinity retargeting molecule (CD3xCD19 DART). Cell cytotoxicity, T cell activation, proliferation and effector molecule production were analyzed using flow cytometry.

RESULTS

Here, we report that a bispecific CD3xCD19 DART mediates efficient killing by HD T cells of CD19⁺ cell-lines and primary CLL cells, regardless of immunoglobulin heavy chain variable region (IGHV) mutational status TP53 status or chemotherapy, ibrutinib or venetoclax sensitivity. Whereas TCR stimulation of CLL-derived T cells resulted in dysfunctional T cell activation and proliferation, treatment with CD3xCD19 DART led to a similar activation profile in CLL-derived and HD-derived T cells. Consistently, co-culture of CLL derived T cells with JeKo-1 or CLL cells in the presence of CD3xCD19 DART resulted in significant cytotoxicity by both CD4⁺ and CD8⁺ T cells. On stimulation of CLL cells with CD40L, CLL cells become resistant to the specific inhibitor of anti-apoptotic Bcl-2 protein venetoclax, due to upregulation of Bcl-2 family members such as Bcl-XL. Nevertheless, CD40L stimulated CLL cells were as efficiently lysed on CD3xCD19 DART treatment as unstimulated CLL cells. Further examination of the mechanism of CD3xCD19 DART mediated killing showed that lysis was dependent on granules, but was independent of BAX/BAK or caspase activity, indicating non-apoptotic cell death.

CONCLUSIONS

These data show that CD3xCD19 DART in CLL leads to robust T cell activation and lysis of high-risk venetoclax resistant CLL cells through a non-apoptotic mechanism.

Mechanisms of Resistance to Monoclonal Antibodies (mAbs) in Lymphoid Malignancies

PURPOSE OF REVIEW

Passive immunotherapy with therapeutic monoclonal antibodies (mAbs) has revolutionized the treatment of cancer, especially hematological malignancies over the last 20 years. While use of mAbs has improved outcomes, development of resistance is inevitable in most cases, hindering the long-term survival of cancer patients. This review focuses on the available data on mechanisms of resistance to rituximab and includes some additional information for other mAbs currently in use in hematological malignancies.

RECENT FINDINGS

Mechanisms of resistance have been identified that target all described mechanisms of mAb activity including altered antigen expression or binding, impaired complement-mediated cytotoxicity (CMC) or antibody-dependent cellular cytotoxicity (ADCC), altered intracellular signaling effects, and inhibition of direct induction of cell death. Numerous approaches to circumvent identified mechanisms of resistance continue to be investigated, but a thorough understanding of which resistance mechanisms are most clinically relevant is still elusive. In recent years, a deeper understanding of the tumor microenvironment and targeting the apoptotic pathway has led to promising breakthroughs. Resistance may be driven by unique patient-, disease-, and antibody-related factors. Understanding the mechanisms of resistance to mAbs will guide the development of strategies to overcome resistance and re-sensitize cancer cells to these biological agents.

Association of rituximab with graphene oxide confers direct cytotoxicity for CD20-positive lymphoma cells

Non-Hodgkin lymphoma (NHL) is one of the most common hematologic malignancies among adults for which the chimeric monoclonal anti-CD20 antibody (Ab) rituximab (RTX) is used as first-line therapy. As RTX itself is not directly cytotoxic but relies on host immune effector mechanisms or chemotherapeutic agents to attack target cells, its therapeutic capacity may become limited when host effector mechanisms are compromised. Currently, refractory disease and relapse with NHL are still common, highlighting the need for novel anti-CD20 antibody strategies with superior therapeutic efficacy over current protocols. We hypothesized that making RTX directly cytotoxic might improve the therapeutic efficacy. Graphene oxide (GO) has recently emerged as a highly attractive nanomaterial for biomedical applications; and several studies have reported cytotoxic effect of GO on benign and malignant cells in vitro. Herein, we report that RTX can be stably associated with GO, and that GO-associated RTX (RTX/GO) demonstrates remarkably high avidity for CD20. Binding of GO-associated RTX to CD20-positive lymphoma cells induces CD20 capping and target cell death through an actin dependent mechanism. In vivo, GO-associated RTX, but not free RTX, quickly eliminates high-grade lymphomas in the absence of host effector mechanisms in a xenograft lymphoma mouse model. Our findings represent the first demonstration of using GO-associated antibody as effective cytotoxic therapy for human B cell malignancies in the absence of chemotherapy, and these findings could have important clinical implications.

Unexpected cross-reactivity of anti-cathepsin B antibodies leads to uncertainties regarding the mechanism of action of anti-CD20 monoclonal antibody GA101

GA101, also known as obinutuzumab or Gazyva (Gazyvaro), is a glycoengineered type II humanized antibody that targets the CD20 antigen expressed at the surface of B-cells. This novel anti-CD20 antibody is currently assessed in clinical trials with promising results as a single agent or as part of therapeutic combinations for the treatment of B-cell malignancies. Detailed understanding of the mechanisms of GA101-induced cell death is needed to get insight into possible resistance mechanisms occurring in patients. Although multiple in vitro and in vivo mechanisms have been suggested to describe the effects of GA101 on B-cells, currently available data are ambiguous. The aim of our study was to clarify the cellular mechanisms involved in GA101-induced cell death in vitro, and more particularly the respective roles played by lysosomal and mitochondrial membrane permeabilization. Our results confirm previous reports suggesting that GA101 triggers homotypic adhesion and caspase-independent cell death, two processes that are dependent on actin remodeling and involve the production of reactive oxygen species. With respect to lysosomal membrane permeabilization (LMP), our data suggest that lack of specificity of available antibodies directed against cathepsin B may have confounded previously published results, possibly challenging current LMP-driven model of GA101 action mode.

Drug-Free Macromolecular Therapeutics--A New Paradigm in Polymeric Nanomedicines

This review highlights a unique research area in polymer-based nanomedicine designs. Drug-free macromolecular therapeutics induce apoptosis of malignant cells by the crosslinking of surface non-internalizing receptors. The receptor crosslinking is mediated by the biorecognition of high-fidelity natural binding motifs (such as antiparallel coiled-coil peptides or complementary oligonucleotides) that are grafted to the side chains of polymers or attached to targeting moieties against cell receptors. This approach features the absence of low-molecular-weight cytotoxic compounds. Here, we summarize the rationales, different designs, and advantages of drug-free macromolecular therapeutics. Recent developments of novel therapeutic systems for B-cell lymphomas are discussed, as well as relevant approaches for other diseases. We conclude by pointing out various potential future directions in this exciting new field.

Photo-extracellular synthesis of gold nanoparticles using Baker's yeast and their anticancer evaluation against Ehrlich ascites carcinoma cells

Novel gold nanoparticles capped by yeast extract, showing highly effective cytotoxicity towards cancer cells under visible light.

Reformatting Rituximab into Human IgG2 and IgG4 Isotypes Dramatically Improves Apoptosis Induction In Vitro

The direct induction of cell death, or apoptosis, in target cells is one of the effector mechanisms for the anti CD20 antibody Rituximab. Here we provide evidence that Rituximab’s apoptotic ability is linked to the antibody IgG isotype. Reformatting Rituximab from the standard human IgG1 heavy chain into IgG2 or IgG4 boosted in vitro apoptosis induction in the Burkitt’s lymphoma B cell line Ramos five and four-fold respectively. The determinants for this behavior are located in the hinge region and CH1 domain of the heavy chain. By transplanting individual IgG2 or IgG4 specific amino acid residues onto otherwise IgG1 like backbones, thereby creating hybrid antibodies, the same enhancement of apoptosis induction could be achieved. The cysteines at position 131 of the CH1 domain and 219 in the hinge region, involved in IgG2 and IgG4 disulfide formation, were found to be of particular structural importance. Our data indicates that the hybrid antibodies possess a different CD20 binding mode than standard Rituximab, which appears to be key in enhancing apoptotic ability. The presented work opens up an interesting engineering route for enhancing the direct cytotoxic ability of therapeutic antibodies.

POLYMERIC BIOMATERIALS AND NANOMEDICINES

This overview intends to demonstrate the close relationship between the design of smart biomaterials and water-soluble polymer-drug conjugates. First, the discovery and systematic studies of hydrogels based on crosslinked poly(meth)acrylic acid esters and substituted amides is described. Then, the lessons learned for the design of water-soluble polymers as drug carriers are highlighted. The current state-of-the-art in water-soluble, mainly poly[N-(2-hydroxypropyl)methacylamide (HPMA), polymer-drug conjugates is shown including the design of backbone degradable HPMA copolymer carriers. In the second part, the modern design of hybrid hydrogels focuses on the self-assembly of hybrid copolymers composed from the synthetic part (backbone) and biorecognizable grafts (coiled-coil forming peptides or morpholino oligonucleotides) is shown. The research of self-assembling hydrogels inspired the invention and design of drug-free macromolecular therapeutics - a new paradigm in drug delivery where crosslinking of non-internalizating CD20 receptors results in apoptosis in vitro and in vivo. The latter is mediated by biorecognition of complementary motifs; no low molecular weight drug is needed.

Rituximab: modes of action, remaining dispute and future perspective

Less than two decades ago, immunotherapy joined chemotherapy and radiotherapy as an effective approach for the treatment of cancer. The anti-CD20 monoclonal antibody, rituximab, is now used to treat almost all types of non-Hodgkin's B-cell lymphomas, and it could be useful in the treatment of other diseases with B-cell involvement. Upon binding, rituximab induces death of the target cells. It seems to act not only by activating immune system defense mechanisms such as complement-dependent and antibody-dependent cellular cytotoxicity, but also by inducing direct cell death. In this paper, we review current knowledge on rituximab mechanisms of action, with particular attention to its direct effects, and also highlight potential future avenues of research.

Action and resistance of monoclonal CD20 antibodies therapy in B-cell Non-Hodgkin Lymphomas

Anti-CD20 monoclonal antibodies (mAbs) have improved patient's survival with Non-Hodgkin Lymphoma, when combined with chemotherapy. Several mechanisms of action have been reported, including antibody-dependent cellular cytotoxicity, complement-dependent cytotoxicity and induction of apoptosis. Despite the large amount of studies and published data, the role each mechanism played in vivo is not fully understood. Furthermore, the reason why a significant percentage of patients are refractory or resistant remains unknown. Several activated intracellular signaling pathways have been implicated in the mechanisms of resistance of rituximab. In the present manuscript, we review those mechanisms and new anti-CD20 mAbs, as well as the efforts being accomplished to overcome it, focusing on new drugs targeting pathways implicated in resistance to rituximab.

Drug-free macromolecular therapeutics induce apoptosis of patient chronic lymphocytic leukemia cells

A new drug-free nanotherapeutic approach for B-cell malignancies was developed. Exposure of B-cells to an anti-CD20 Fab'-morpholino oligonucleotide1 (MORF1) conjugate decorated the cell surface with MORF1; further exposure of the decorated cells to multivalent polymer-oligonucleotide2 conjugates (P-MORF2) resulted in CD20 clustering at the cell surface with induction of apoptosis. We evaluated this concept in chronic lymphocytic leukemia (CLL) cells isolated from 10 patients. Apoptosis and cytotoxicity were observed in eight samples, including 2 samples with the 17p13 deletion, which suggested a p53-independent mechanism of apoptosis induction. When compared to an anti-CD20 monoclonal antibody (mAb), the nanotherapeutic showed significantly more potent apoptosis-inducing activity and cytotoxicity. This was due to the multivalency effect (8 binding sites per polymer chain) of our design in comparison to the divalent mAb. In conclusion, we have developed a novel and potent therapeutic system against CLL and other B-cell malignancies with significant advantages over conventional chemo-immunotherapy.

A Two-Step Pretargeted Nanotherapy for CD20 Crosslinking May Achieve Superior Anti-Lymphoma Efficacy to Rituximab

The use of rituximab, an anti-CD20 mAb, in combination with chemotherapy is the current standard for the treatment of B-cell lymphomas. However, because of a significant number of treatment failures, there is a demand for new, improved therapeutics. Here, we designed a nanomedicine that crosslinks CD20 and directly induces apoptosis of B-cells without the need for toxins or immune effector functions. The therapeutic system comprises a pretargeting component (anti-CD20 Fab' conjugated with an oligonucleotide1) and a crosslinking component (N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer grafted with multiple complementary oligonucleotide2). Consecutive treatment with the two components resulted in CD20 clustering on the cell surface and effectively killed malignant B-cells in vivo. To enhance therapeutic efficacy, a two-step pretargeting approach was employed. We showed that the time lag between the two doses can be optimized based on pharmacokinetics and biodistribution of the Fab'-oligonucleotide1 conjugate. In a mouse model of human non-Hodgkin lymphoma (NHL), increasing the time lag from 1 h to 5 h resulted in dramatically improved tumor growth inhibition and animal survival. When the 5 h interval was used, the nanotherapy was more efficacious than rituximab and led to complete eradication of lymphoma cells with no signs of metastasis or disease recurrence. We further evaluated the nanomedicine using patient mantle cell lymphoma cells; the treatment demonstrated more potent apoptosis-inducing activity than rituximab hyper-crosslinked with secondary antibodies. In summary, our approach may constitute a novel treatment for NHL and other B-cell malignancies with significant advantages over conventional chemo-immunotherapy.

Rituximab increases the cytotoxicities of vincristine and hydroxyurea through caspase-dependent and caspase-independent cell death, respectively

In the treatment of B cell non-Hodgkin's lymphoma, rituximab is used in combination with different chemotherapeutics to improve its efficacy, but the mechanisms involved are not fully understood. The authors examined the mechanism by which rituximab combined with hydroxyurea or vincristine induces cell death in the human Burkitt's lymphoma Ramos cell line. Cell death was analyzed by phosphatidylserine exposure, caspase activation, and mitochondrial membrane changes. Their results indicate that the cell death initiated by the combination of rituximab and hydroxyurea is caspase-independent. In contrast, preincubation of the cells with the same concentrations of caspase inhibitors used with hydroxyurea eliminated the synergistic effect of the rituximab and vincristine combination. This was confirmed by the presence of the active fragment of caspase-3 in vincristine-treated cells. These preliminary results demonstrate that rituximab can activate different downstream signals to induce direct cell effects. Furthermore, the findings support the important role of mitochondria in the regulation of both pathways.

Antitumor activity of an anti-CD98 antibody

CD98 is expressed on several tissue types and specifically upregulated on fast-cycling cells undergoing clonal expansion. Various solid (e.g., nonsmall cell lung carcinoma) as well as hematological malignancies (e.g., acute myeloid leukemia) overexpress CD98. We have identified a CD98-specific mouse monoclonal antibody that exhibits potent preclinical antitumor activity against established lymphoma tumor xenografts. Additionally, the humanized antibody designated IGN523 demonstrated robust tumor growth inhibition in leukemic cell-line derived xenograft models and was as efficacious as standard of care carboplatin in patient-derived nonsmall lung cancer xenografts. In vitro studies revealed that IGN523 elicited strong ADCC activity, induced lysosomal membrane permeabilization and inhibited essential amino acid transport function, ultimately resulting in caspase-3 and -7-mediated apoptosis of tumor cells. IGN523 is currently being evaluated in a Phase I clinical trial for acute myeloid leukemia (NCT02040506). Furthermore, preclinical data support the therapeutic potential of IGN523 in solid tumors.

The status of radioimmunotherapy in CD20+ non-Hodgkin's lymphoma

Rituximab, the CD20-directed antibody, has become a standard component of treatment regimens for patients with B cell non-Hodgkin's lymphoma (NHL). The use of rituximab has resulted in greatly improved response and survival rates with less toxicity relative to standard chemotherapeutic regimes. However, relapse and recurrence is common, particularly in indolent varieties which remain incurable, requiring alternate therapeutic options. The subsequent coupling of β-emitting isotopes such as (131)I and (90)Y to anti-CD20 monoclonal antibodies (mAbs), including rituximab, has been steadily growing over the last decade and demonstrates even greater therapeutic efficacy with more durable responses. (177)Lutetium-labelled rituximab offers a number of convenient advantages over (131)I and (90)Y anti-CD20 mAbs for treatment of NHL, and a number of alpha-emitting isotopes lie at the frontier of consolidation therapy for residual, micrometastatic disease.

The anti-lymphoma activity of APO866, an inhibitor of nicotinamide adenine dinucleotide biosynthesis, is potentialized when used in combination with anti-CD20 antibody

APO866 is an inhibitor of nicotinamide adenine dinucleotide (NAD) biosynthesis that exhibits potent anti-lymphoma activity. Rituximab (RTX), an anti-CD20 antibody, kills lymphoma cells by direct apoptosis and antibody- and complement-dependent cell-mediated cytotoxicities, and has clinical efficacy in non-Hodgkin cell lymphomas. In the present study, we evaluated whether RTX could potentiate APO866-induced human B-lymphoma cell death and shed light on death-mediated mechanisms associated with this drug combination. We found that RTX significantly increases APO866-induced death in lymphoma cells from patients and lines. Mechanisms include enhancement of autophagy-mediated cell death, activation of caspase 3 and exacerbation of mitochondrial depolarization, but not increase of reactive oxygen species (ROS) production, when compared with those induced by each drug alone. In vivo, combined administration of APO866 with RTX in a laboratory model of human aggressive lymphoma significantly decreased tumor burden and prolonged survival over single-agent treatment. Our study demonstrates that the combination of RTX and APO866 optimizes B-cell lymphoma apoptosis and therapeutic efficacy over both compounds administered separately.

Expression and biological characterization of an anti-CD20 biosimilar candidate antibody: a case study

The CD20 molecule is a non-glycosylated protein expressed mainly on the surface of B lymphocytes. In some pathogenic B cells, it shows an increased expression, thus becoming an attractive target for diagnosis and therapy. Rituximab is a chimeric antibody that specifically recognizes the human CD20 molecule. This antibody is indicated for the treatment of non-Hodgkin lymphomas and autoimmune diseases, such as rheumatoid arthritis and systemic lupus erythematosus. In this work, we describe the stable expression and biological evaluation of an anti-CD20 biosimilar antibody. While rituximab is produced in fed-batch culture of recombinant Chinese hamster ovary (CHO) cells, our biosimilar antibody expression process consists of continuous culture of recombinant murine NS0 myeloma cells. The ability of the purified biosimilar antibody to recognize the CD20 molecule on human tumor cell lines, as well as on peripheral blood mononuclear cells from humans and primates, was demonstrated by flow cytometry. The biosimilar antibody induced complement-dependent cytotoxicity, antibody-dependent cell-mediated cytotoxicity and apoptosis on human cell lines with high expression of CD20. In addition, this antibody depleted CD20-positive B lymphocytes from peripheral blood in monkeys. These results indicate that the biological properties of the biosimilar antibody compare favorably with those of the innovator product, and that it should be evaluated in future clinical trials.

CD40 stimulation sensitizes CLL cells to lysosomal cell death induction by type II anti-CD20 mAb GA101

Sensitivity of chronic lymphocytic leukemia (CLL) cells to anti-CD20 mAbs is low and, therefore, the efficacy of monotherapy with current anti-CD20 mAbs is limited. At present, it is not known whether sensitivity of CLL cells to CD20 mAbs is modulated by microenvironmental stimuli. We have shown previously that in vitro CD40 stimulation of peripheral blood-derived CLL cells results in resistance to cytotoxic drugs. In the present study, we show that, in contrast, CD40 stimulation sensitizes CLL cells to the recently described novel type II anti-CD20 mAb GA101. Cell death occurred without cross-linking of GA101 and involved a lysosome-dependent mechanism. Combining GA101 with various cytotoxic drugs resulted in additive cell death, not only in CD40-stimulated CLL cells, but also in p53-dysfunctional CLL cells. Our findings indicate that GA101 has efficacy against chemoresistant CLL, and provide a rationale for combining cytotoxic drugs with anti-CD20 mAbs.

Rituximab resistance

Rituximab has become a ubiquitous component of treatment regimens for follicular non-Hodgkin lymphoma. Despite widespread clinical use, the mechanisms by which tumor cells resist rituximab-mediated destruction remain unclear. Rituximab relies in part on immune effector mechanisms for its antitumor effect, and thus resistance may be mediated not only by intrinsic tumor-cell alterations but also by the host immunological environment. In this article, we explore the mechanisms of action of rituximab, the incidence of rituximab resistance, and potential mechanisms of resistance. Finally, we discuss novel approaches to modulate the antibody, the tumor cell, and the host immunologic environment to overcome rituximab resistance. Further research into the mechanisms of rituximab resistance will be essential to improving the efficacy of anti-CD20 therapy in NHL, and may also pay dividends in the optimization of monoclonal antibody therapy across a wide range of diseases.

Novel type II anti-CD20 monoclonal antibody (GA101) evokes homotypic adhesion and actin-dependent, lysosome-mediated cell death in B-cell malignancies

The anti-CD20 mAb rituximab has substantially improved the clinical outcome of patients with a wide range of B-cell malignancies. However, many patients relapse or fail to respond to rituximab, and thus there is intense investigation into the development of novel anti-CD20 mAbs with improved therapeutic efficacy. Although Fc-FcγR interactions appear to underlie much of the therapeutic success with rituximab, certain type II anti-CD20 mAbs efficiently induce programmed cell death (PCD), whereas rituximab-like type I anti-CD20 mAbs do not. Here, we show that the humanized, glycoengineered anti-CD20 mAb GA101 and derivatives harboring non-glycoengineered Fc regions are type II mAb that trigger nonapoptotic PCD in a range of B-lymphoma cell lines and primary B-cell malignancies. We demonstrate that GA101-induced cell death is dependent on actin reorganization, can be abrogated by inhibitors of actin polymerization, and is independent of BCL-2 overexpression and caspase activation. GA101-induced PCD is executed by lysosomes which disperse their contents into the cytoplasm and surrounding environment. Taken together, these findings reveal that GA101 is able to potently elicit actin-dependent, lysosomal cell death, which may potentially lead to improved clearance of B-cell malignancies in vivo.

Molecular profiling of cytomegalovirus-induced human CD8+ T cell differentiation

CD8+ T cells play a critical role in the immune response to viral pathogens. Persistent human cytomegalovirus (HCMV) infection results in a strong increase in the number of virus-specific, quiescent effector-type CD8+ T cells with constitutive cytolytic activity, but the molecular pathways involved in the induction and maintenance of these cells are unknown. We show here that HCMV infection induced acute and lasting changes in the transcriptomes of virus-reactive T cells collected from HCMV-seropositive patients at distinct stages of infection. Enhanced cell cycle and metabolic activity was restricted to the acute phase of the response, but at all stages, HCMV-specific CD8+ T cells expressed the Th1-associated transcription factors T-bet (TBX21) and eomesodermin (EOMES), in parallel with continuous expression of IFNG mRNA and IFN-γ-regulated genes. The cytolytic proteins granzyme B and perforin as well as the fractalkine-binding chemokine receptor CX3CR1 were found in virus-reactive cells throughout the response. During HCMV latency, virus-specific CD8+ T cells lacked the typical features of exhausted cells found in other chronic infections. Persistent effector cell traits together with the permanent changes in chemokine receptor usage of virus-specific, nonexhausted, long-lived CD8+ T cells may be crucial to maintain lifelong protection from HCMV reactivation.

Endocytosis of PEGylated agents enhances cancer imaging and anticancer efficacy

PEGylated nanoparticles and macromolecules are increasingly used in cancer imaging and anticancer treatment. The role of receptor-mediated endocytosis in the efficacy of these agents, however, has not been clearly defined. Here, we developed a matched pair of endocytic and nonendocytic receptors to directly and unambiguously assess this issue. The ligand-binding domains of the low-density lipoprotein receptor (LDLR) or a truncated LDLR lacking the NPXY endocytosis motif (DeltaLDLR) were replaced with an anti-polyethylene glycol antibody (alphaPEG) to form endocytic alphaPEG-LDLR and nonendocytic alphaPEG-DeltaLDLR receptors. The receptors were stably expressed at similar levels on the surface of HCC36 cells. HCC36/alphaPEG-LDLR cells, but not HCC36/alphaPEG-DeltaLDLR cells, rapidly endocytosed PEG-quantum dots and PEG-liposomal doxorubicin (Lipo-Dox) in vitro and in vivo. Lipo-Dox was significantly more cytotoxic to HCC36/alphaPEG-LDLR cells than to HCC36/alphaPEG-DeltaLDLR cells. HCC36/alphaPEG-LDLR tumors also accumulated significantly more PEGylated near-IR probes (PEG-NIR797) and PEG-liposomal-(111)In than HCC36/alphaPEG-DeltaLDLR tumors in vivo. Furthermore, Lipo-Dox more significantly suppressed the growth of established HCC36/alphaPEG-LDLR tumors as compared with HCC36/alphaPEG-DeltaLDLR tumors. Our data show that endocytosis of PEGylated probes and drugs enhances both cancer imaging and anticancer efficacy, indicating that endocytic receptors are superior targets for the design of cancer imaging probes and immunoliposomal drugs.

The autophilic anti-CD20 antibody DXL625 displays enhanced potency due to lipid raft-dependent induction of apoptosis

Despite widespread use of anti-CD20 antibodies as therapeutic agents for oncologic and autoimmune indications, precise descriptions of killing mechanisms remain incomplete. Complement-dependent cytolysis and antibody-dependent cell-mediated cytotoxicity are indicated as modes of target cell depletion; however, the importance of apoptosis induction is controversial. Studies showing that the therapeutic anti-CD20 antibody rituximab (Rituxan) mediates apoptosis of tumor cell targets in vitro after cross-linking by anti-Fc reagents suggest that enhancement strategies applied to Fc-independent activities for anti-CD20 antibodies could improve therapeutic efficacy. An anti-CD20 antibody designated DXL625, with autophilic properties such as increased binding avidity, is shown here to independently induce caspase-mediated apoptosis of an established B-cell lymphoma line in vitro. Depletion of membrane cholesterol or chelation of extracellular calcium abrogated the pro-apoptotic activity of DXL625, indicating that intact lipid rafts and calcium are required for this activity. The Fc-mediated complement-dependent and antibody-dependent cellular killing mechanisms are maintained by DXL625 despite conjugation of the parental Rituxan antibody to the autophilic DXL peptide sequence. This study shows a strategy for improving anti-CD20 immunotherapy by endowing therapeutic antibodies with self-interacting properties.

Unique epitopes on C epsilon mX in IgE-B cell receptors are potentially applicable for targeting IgE-committed B cells

Membrane-bound IgE (mIgE) is part of the IgE-BCR and is essential for generating isotype-specific IgE responses. On mIgE(+) B cells, the membrane-bound epsilon-chain (mepsilon) exists predominantly in the long isoform, mepsilon(L), containing an extra 52 aa CepsilonmX domain between CH4 and the C-terminal membrane-anchoring segment; the short isoform of mepsilon, mepsilon(S), exists in minor proportions. CepsilonmX thus provides an attractive site for immunologic targeting of mIgE(+) B cells. In this study, we show that nine newly prepared CepsilonmX-specific mAbs, as well as the previously reported a20, bound to mIgE.Fc(L)-expressing CHO cells, while only 4B12 and 26H2 bound to mIgE.Fc(L)-expressing B cell line Ramos cells. The mAb 4B12 bound to the N-terminal part, 26H2 the middle part, and all others the C-terminal part of CepsilonmX. Expression of Igalpha and Igbeta on the mIgE.Fc(L)-CHO cells reduces the binding of a20 to CepsilonmX as compared with that of 4B12 and 26H2. The chimeric mAbs c4B12 and c26H2, when cross-linked by secondary antibodies, lysed mIgE.Fc(L)-Ramos cells by apoptosis through a BCR-dependent caspase pathway. Using PBMCs as the source of effector cells, c4B12 and c26H2 demonstrated Ab-dependent cellular cytotoxicity toward mIgE.Fc(L)-Ramos cells in a dose-dependent fashion. In cultures of PBMCs from atopic dermatitis patients, c4B12 and c26H2 inhibited the synthesis of IgE driven by anti-CD40 and IL-4. These results suggest that 4B12 and 26H2 and an immunogen using the peptide segments recognized by these mAbs are potentially useful for targeting mIgE(+) B cells to control IgE production.

CD20 deficiency in humans results in impaired T cell-independent antibody responses

CD20 was the first B cell differentiation antigen identified, and CD20-specific mAbs are commonly used for the treatment of B cell malignancies and autoantibody-mediated autoimmune diseases. Despite this the role of CD20 in human B cell physiology has remained elusive. We describe here a juvenile patient with CD20 deficiency due to a homozygous mutation in a splice junction of the CD20 gene (also known as MS4A1) that results in "cryptic" splicing and nonfunctional mRNA species. Analysis of this patient has led us to conclude that CD20 has a central role in the generation of T cell-independent (TI) antibody responses. Key evidence to support this conclusion was provided by the observation that although antigen-independent B cells developed normally in the absence of CD20 expression, antibody formation, particularly after vaccination with TI antigens, was strongly impaired in the patient. Consistent with this, TI antipolysaccharide B cell responses were severely impeded in CD20-deficient mice. Our study therefore identifies what we believe to be a novel type of humoral immunodeficiency caused by CD20 deficiency and characterized by normal development of antigen-independent B cells, along with a reduced capacity to mount proper antibody responses.

The involvement of mitochondria and the caspase-9 activation pathway in rituximab-induced apoptosis in FL cells

Despite the wide use of anti-CD20 antibody rituximab in the cancer treatment of B cell malignancies, the signalling pathways of CD20-induced apoptosis are still not understood. By using dominant negative (DN)-caspase-9 overexpressing follicular lymphoma cells we demonstrated that the activation of caspase-9 was essential for rituximab-mediated apoptosis. The death receptor pathway mediated by caspase-8 activation was not involved in rituximab-mediated apoptosis since overexpression of FLIP(short) or FLIP(long) proteins, inhibitors of caspase-8 activation, could not inhibit rituximab-induced apoptosis. However, the death receptor pathway activation by anti-Fas antibodies showed an additive effect on rituximab-induced apoptosis. The stabilisation of the mitochondrial outer membrane by Bcl-x(L) overexpression inhibited cell death, showing the important role of mitochondria in rituximab-induced apoptosis. Interestingly, the rituximab-induced release of cytochrome c and collapse of mitochondrial membrane potential were regulated by caspase-9. We suggest that caspase-9 and downstream caspases may feed back to mitochondria to amplify mitochondrial disruption during intrinsic apoptosis.

Understanding and circumventing resistance to anticancer monoclonal antibodies

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

Novel designs of multivalent anti-CD20 humanized antibodies as improved lymphoma therapeutics

Multivalent antibodies, either monospecific or bispecific, may improve the efficacy of current therapeutic interventions involving a single monoclonal antibody (mAb). We have applied the Dock-and-Lock (DNL) method, a new platform technology for the site-specific and covalent assembly of modular components into stably tethered complexes of defined composition, to prepare a hexavalent, anti-CD20 antibody, designated Hex-hA20, which comprises six Fabs with one Fc. We show that Hex-hA20 retains the binding activity of all six Fabs, associates with CD20 in lipid rafts, affects antibody-dependent cell-mediated cytotoxicity, but not complement-dependent cytotoxicity, and inhibits proliferation of Daudi, Raji, and Ramos cells in vitro at subnanomolar concentrations without the need for a cross-linking antibody. In addition, Hex-hA20 induces strong homotypical adhesion and is inefficient in stimulating calcium mobilization. Thus, Hex-hA20 exhibits biological properties attributable to both type I and type II anti-CD20 mAbs, as exemplified by rituximab and tositumomab, respectively. Although Hex-hA20 has a short serum half-life, it shows antitumor efficacy in tumor-bearing mice comparable with veltuzumab at equivalent doses. The versatile DNL method was also applied to generate two other multivalent anti-CD20 antibodies without the Fc region, Tri-hA20 and Tetra-hA20, comprising three and four Fabs of veltuzumab, respectively. Similar to Hex-hA20, these were purified to near homogeneity and shown to have potent antiproliferative activity in vitro, thus indicating the need for clustering three or more CD20 molecules on the cell surface to induce growth inhibition.

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

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

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

PURPOSE

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

EXPERIMENTAL DESIGN

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

RESULTS

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

CONCLUSIONS

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

A requirement for calcium in the caspase-independent killing of Burkitt lymphoma cell lines by Rituximab

The therapeutic monoclonal antibody rituximab has previously been shown to kill B cells in a caspase-independent manner. The signalling pathways underpinning this novel death pathway are unknown. The present study showed that rituximab treatment of Burkitt lymphoma cell lines induced a slow rise in intracellular calcium ([Ca(2+)](i)). This rise was only witnessed in cell lines that were killed by antibody, suggesting a critical role for Ca(2+) in mediating rituximab-driven caspase-independent cell death. Inhibition of the two main intracellular store-located Ca(2+) channels, i.e. the ryanodine and inositol-1,4,5-triphosphate receptor channels by dantrolene and xestospongen-c respectively did not prevent the rise in Ca(2+) seen with rituximab or protect cells from subsequent death. In sharp contrast, inhibition of Ca(2+) entry via plasma membrane channels with (2-aminoethoxy) diphenylborate or SKF-96365 or complete chelation of extracellular Ca(2+) with ethyleneglycol bis (aminoethylether) tetra-acetate inhibited the rise in [Ca(2+)](i) and increased cell viability. Together, these data suggest that ligation of the CD20 receptor with rituximab allows a slow sustained influx of Ca(2+) from the external environment that under certain conditions can lead to cell death.

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

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

The expanding role of rituximab and radioimmunotherapy in the treatment of B-cell lymphomas

The role of rituximab in the treatment of B-cell lymphomas has rapidly emerged from the relapsed setting to first-line combination regimens across the broad range of histologic subtypes. The role of maintenance rituximab in indolent lymphomas after first-line therapy needs to be defined along with the integration of radioimmunotherapy into the first-line therapeutic regimens. As mechanisms of action/resistance to monoclonal antibody therapy are better understood, approaches to predicting response and optimizing combination therapy to overcoming primary and acquired resistance may be developed.

Mechanisms of killing by anti-CD20 monoclonal antibodies

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

Lym-1-induced apoptosis of non-Hodgkin's lymphomas produces regression of transplanted tumors

Lym-1 was one of the first antibodies to be used successfully for the radioimmunotherapy of the human malignant lymphomas. This antibody, which recognizes the HLA-DR10 antigen preferentially expressed in B-cell lymphomas, was recently shown to induce apoptosis upon binding to lymphoma cells. In this study, Lym-1-induced apoptosis was studied to identify the potential molecular pathways of programmed cell death and to demonstrate the clinical potential of this antibody in the treatment of the human malignant lymphomas. Immunofluorescence microscopy revealed that Lym-1 stained focal areas of the cell surface, consistent with the fact that the HLA-DR10 antigen is associated with lipid rafts, a known prerequisite for apoptosis signaling. Likewise, Annexin V/propidium iodide staining and TUNEL assays demonstrated that both murine Lym-1 and chimeric Lym-1 induced both early and late apoptosis, respectively, unlike anti-CD20 rituximab. Furthermore, Lym-1 was found to produce a rapid loss of mitochondrial membrane potential and mitochondrial release of cytochrome C 14 hours post-Lym-1 treatment. Although it was found to activate caspase-3, inhibitors of caspase pathways showed that the Lym-1-induced apoptosis in lymphoma cell lines is independent of caspase induction. Finally, treatment studies in vivo demonstrated that, compared with murine anti-CD20 (2B8), Lym-1 was more effective in inducing the regression of human lymphoma xenografts. Based upon these results, chimeric Lym-1 should be especially effective in treating lymphoma patients, as, in addition to being able to elicit immune effector functions such as chimeric anti-CD20, it can also induce apoptosis directly upon cell binding.

Recombinant therapeutic monoclonal antibodies: mechanisms of action in relation to structural and functional duality

Naked therapeutic recombinant monoclonal antibodies (mAbs) are bifunctional molecules. On the one hand, they recognize their antigen through the variable regions of the antigen binding portion (Fab). The recombinant mAb binding to a soluble or a membrane antigen may interfere with one or several functions of this antigen, leading to the therapeutic effect. On the other hand, since their crystalisable portion (Fc) is humanized (usually IgG1), they interact efficiently with human Fc-binding molecules, such as C1q and receptors for the Fc portion of IgG (FcgammaR). Thus, they initiate the classical pathway of complement and activate FcgammaR-expressing cells. The recruitment of these patient immune effector functions is essential in the therapeutic effect of several recombinant mAbs used in oncology. The aim of this review is to describe the main mechanisms of action of recombinant mAbs in relation to this structural and functional duality.

Crosstalk among Bcl-2 family members in B-CLL: seliciclib acts via the Mcl-1/Noxa axis and gradual exhaustion of Bcl-2 protection

Seliciclib (R-roscovitine) is a cyclin-dependent kinase inhibitor in clinical development. It triggers apoptosis by inhibiting de novo transcription of the short-lived Mcl-1 protein, but it is unknown how this leads to Bax/Bak activation that is required for most forms of cell death. Here, we studied the effects of seliciclib in B-cell chronic lymphocytic leukemia (B-CLL), a malignancy with aberrant expression of apoptosis regulators. Although seliciclib-induced Mcl-1 degradation within 4 h, Bax/Bak activation occurred between 16 and 20 h. During this period, no transcriptional changes in apoptosis-related genes occurred. In untreated cells, prosurvival Mcl-1 was engaged by the proapoptotic proteins Noxa and Bim. Upon drug treatment, Bim was quickly released. The contribution of Noxa and Bim as a specific mediator of seliciclib-induced apoptosis was demonstrated via RNAi. Significantly, 16 h after seliciclib treatment, there was accumulation of Bcl-2, Bim and Bax in the 'mitochondria-rich' insoluble fraction of the cell. This suggests that after Mcl-1 degradation, the remaining apoptosis neutralizing capacity of Bcl-2 is gradually overwhelmed, until Bax forms large multimeric pores in the mitochondria. These data demonstrate in primary leukemic cells hierarchical binding and crosstalk among Bcl-2 members, and suggest that their functional interdependence can be exploited therapeutically.

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

Ab binding to CD20 has been shown to induce apoptosis in B cells. In this study, we demonstrate that rituximab sensitizes lymphoma B cells to Fas-induced apoptosis in a caspase-8-dependent manner. To elucidate the mechanism by which Rituximab affects Fas-mediated cell death, we investigated rituximab-induced signaling and apoptosis pathways. Rituximab-induced apoptosis involved the death receptor pathway and proceeded in a caspase-8-dependent manner. Ectopic overexpression of FLIP (the physiological inhibitor of the death receptor pathway) or application of zIETD-fmk (specific inhibitor of caspase-8, the initiator-caspase of the death receptor pathway) both specifically reduced rituximab-induced apoptosis in Ramos B cells. Blocking the death receptor ligands Fas ligand or TRAIL, using neutralizing Abs, did not inhibit apoptosis, implying that a direct death receptor/ligand interaction is not involved in CD20-mediated cell death. Instead, we hypothesized that rituximab-induced apoptosis involves membrane clustering of Fas molecules that leads to formation of the death-inducing signaling complex (DISC) and downstream activation of the death receptor pathway. Indeed, Fas coimmune precipitation experiments showed that, upon CD20-cross-linking, Fas-associated death domain protein (FADD) and caspase-8 were recruited into the DISC. Additionally, rituximab induced CD20 and Fas translocation to raft-like domains on the cell surface. Further analysis revealed that, upon stimulation with rituximab, Fas, caspase-8, and FADD were found in sucrose-gradient raft fractions together with CD20. In conclusion, in this study, we present evidence for the involvement of the death receptor pathway in rituximab-induced apoptosis of Ramos B cells with concomitant sensitization of these cells to Fas-mediated apoptosis via Fas multimerization and recruitment of caspase-8 and FADD to the DISC.

Antibody and immunomodulatory agents in treatment of indolent non-Hodgkin's lymphoma

Immunomodulatory agents, including cytokines, monoclonal antibodies, and CpG oligonucleotides, have properties that suggest they have the ability to augment rituximab in the treatment of non-Hodgkin's lymphoma. Although several clinical trials have promising results, no randomized trials of reasonable size have been performed to date, limiting the ability to discern whether combinations of immunomodulatory agents with rituximab impact clinical outcome. Until such trials are mature, we do not recommend using these agents in combination outside of the research setting.

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

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

Rituximab enhances radiation-triggered apoptosis in non-Hodgkin's lymphoma cells via caspase-dependent and - independent mechanisms

Rituximab (RTX), a chimeric human anti-CD20 monoclonal antibody, is currently employed in the treatment of malignant non-Hodgkin's lymphoma (NHL) either alone or in combination with other cytotoxic approaches. The present study examines the effects of ionizing radiation in combination with RTX on proliferation and apoptosis development in B-lymphoma RL and Raji cells. RTX was used at a concentration of 10 microg/mL 24 hours prior to irradiation at a single dose of 9 Gy. CD20 expression, cell viability, apoptosis, mitochondrial membrane potential and apoptosis-related proteins were evaluated in the treated B cells. The constitutive level of CD20 expression in RL and Raji lymphoma cells did not play an essential role in RTX-induced cell growth delay. Both lymphoma cells showed similar inhibition of cell proliferation without apoptosis development in response to RTX treatment. Exposure to ionizing radiation induced cell growth delay and apoptosis in RL cells, whereas Raji cells showed moderate radio-resistance and activation of cell growth at 24 hours after irradiation, which was accompanied by increased radiation-triggered CD20 expression. The simultaneous exposure of lymphoma cells to ionizing radiation and RTX abrogated radioresistance of Raji cells and significantly enhanced cell growth delay and apoptosis in RL cells. X-linked inhibitor of apoptosis protein (XIAP) and the inducible form of heat shock protein 70 (Hsp70) were positively modulated by RTX in combination with ionizing radiation in order to induce apoptosis. Furthermore, it was demonstrated that mitochondrial membrane potential dissipation is not an essential component to induce apoptosis-inducing factor (AIF) maturation and apoptosis. Our results show that RTX-triggered enhancement of radiation-induced apoptosis and cell growth delay is achieved by modulation of proteins involved in programmed cell death.

Caspase-independent killing of Burkitt lymphoma cell lines by rituximab

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

Generation of rituximab polymer may cause hyper-cross-linking-induced apoptosis in non-Hodgkin's lymphomas

PURPOSE

Although Rituximab has produced significant tumor regressions in lymphoma patients, only 50% respond. Clinically, it has been shown that the major mechanism of action of Rituximab is antibody-dependent cytotoxicity requiring presentation by Fc-bearing cells. To improve the clinical efficacy of Rituximab for the treatment of CD20+ lymphomas, we now describe a new formulation of Rituximab, which, on direct binding to target, can induce apoptosis.

METHODS

In this report, enhanced apoptosis was observed by treating CD20+ lymphoma cells with a new polymer formulation of Rituximab. The polymer was produced by formation of a peptide bond using the sugar moiety of dextran (MW 6,000) to generate a clinically relevant reagent for use in vivo.

RESULTS

Comparison of Rituximab with a previously described dimer and the newly generated polymer shows that the polymer induced apoptosis more effectively in CD20+ cells as shown by the terminal deoxyribonucleotidyl transferase-mediated dUTP nick end labeling assay (Rituximab, 3%; dimer, 3%; polymer, 58%). Consistent with these results, the polymer produced marked regression in CD20+ lymphoma xenografts, whereas the dimer and monomer reagents showed little effect. In addition, we were able to show that the level of apoptosis induced in human lymphoma cell lines was in accordance with the extent of both surface CD20 clustering and caspase-3 activation.

CONCLUSIONS

These data suggest that hyper-cross-linking-induced apoptosis can be simulated by the use of a dextran polymer of Rituximab, which, when used in vivo, can directly kill CD20+ lymphoma cells and improve the clinical efficacy of this important therapeutic for human B-cell lymphomas.

Underlying mechanism of hypoxic preconditioning decreasing apoptosis induced by anoxia in cultured hippocampal neurons

It is known that hypoxic preconditioning (HP, a brief period of sublethal hypoxia) provides neuroprotection against subsequent severe anoxia, but the mechanisms of this increased tolerance have not been fully elucidated. A hypoxic preconditioning model was established by exposing a 4-day hippocampal culture to 1% O(2) for 20 min/day for 8 days. The preconditioning significantly decreased the number of apoptotic neurons at reoxygenation 24 h after 4 h of severe anoxia (0% O(2)). Further study demonstrated that the degradation of mitochondrial membrane potential (MMP) was greatly inhibited and the expression of B-cell lymphoma protein-2 (Bcl-2) was increased considerably after severe anoxia in the HP groups. These results indicate that the increased anoxic tolerance, which is induced by HP in cultured hippocampal cells, may be correlated with Bcl-2 overexpression and enhanced stability of MMP, which ultimately reduces apoptosis 24 h after reoxygenation.