Conjugation of Blocked Ricin to an Anti-CD19 Monoclonal Antibody Increases Antibody-Induced Cell Calcium Mobilization and CD19 Internalization

Activating Autophagy Enhanced the Antitumor Effect of Antibody Drug Conjugates Rituximab-Monomethyl Auristatin E

Background

Antibody drug conjugate (ADC) showed potent therapeutic efficacy in several types of cancers. The role of autophagy in antitumor effects of ADC remains unclear.

Methods

In this study, the ADC, Rituximab-monomethyl auristatin E (MMAE) with a Valine–Citrulline cleavable linker, was designed to investigate its therapeutic efficacy against non-Hodgkin lymphoma (NHL) as well as the underlying mechanisms. Methylthiazolyldiphenyl-tetrazolium bromide (MTT) was used to detect growth inhibition in B-cell lymphoma cell lines, Ramos and Daudi cells, which were treated by Rituximab-MMAE alone or combined with autophagy conditioner. Apoptosis was detected by flow cytometry and immunohistochemistry, and apoptosis inhibitor was employed to discover the relationship between autophagy and apoptosis during the Rituximab-MMAE treatment. Autophagy was determined by three standard techniques which included confocal microscope, transmission electron microscope, and western blots. Ramos xenograft tumors in BALB/c nude mice were established to investigate the antitumor effect of combination use of Rituximab-MMAE and autophagy conditioner in B-NHL therapy.

Results

Our results showed that Rituximab-MMAE elicited caspase-3-dependent apoptosis in NHL cells and exhibited potent therapeutic efficacy in vivo. Autophagy, which was characterized by upregulated light chain 3-II expression, and accumulation of autophagosomes, was triggered during the Rituximab-MMAE treatment. Meanwhile, inactivation of Akt/mTOR pathway was shown to be involved in the autophagy triggered by Rituximab-MMAE, indicating a probable mechanism of the ADC-initiated autophagy. Importantly, inhibition of autophagy by chloroquine suppressed the Rituximab-MMAE-induced apoptosis, while activating autophagy by rapamycin significantly enhanced the therapeutic effect of Rituximab-MMAE both in vitro and in vivo.

Conclusion

Our data elucidated the critical relationship between autophagy and apoptosis in Rituximab-MMAE-based therapy and highlighted the potential approach for NHL therapy by combined administration of the ADC and autophagy activator.

Sortase A-Generated Highly Potent Anti-CD20-MMAE Conjugates for Efficient Elimination of B-Lineage Lymphomas

Antibody-drug conjugate (ADC) targeting antigens expressed on the surface of tumor cells are an effective approach for delivering drugs into the cells via antigen-mediated endocytosis. One of the well-known tumor antigens, the CD20 of B-lymphocyte, has long been suggested to be noninternalizing epitope, and is thus not considered a desirable target for ADCs. Here, sortase A (srtA)-mediated transpeptidation is used to specifically conjugate triple glycine-modified monomethyl auristatin E (MMAE), a highly toxic antimitotic agent, to anti-CD20 ofatumumab (OFA) equipped with a short C-terminal LPETG (5 amino acids) tag at heavy chain (HL), which generates ADCs that show extremely strong potency in killing CD20 positive cancer cells. One of the srtA-generated ADCs with a cleavable dipeptide linker (valine-citrulline, vc), OFA-HL-vcMMAE, shows IC50 values ranging from 5 pg mL^-1 to 4.1 ng mL^-1 against CD20+ lymphoma cells. Confocal laser scanning microscopy confirms that OFA-HL-vcMMAE internalization by Ramos cells is significantly improved compared to OFA alone, consistent with the high antitumor activity of the new ADC. OFA-HL-vcMMAE, at 5 mg kg^-1 dose, is able to eliminate tumors with mean volume ≈400 mm³ while no obvious drug-related toxicity is observed. The results show that srtA-generated OFA-MMAE conjugate system provides a viable strategy for targeting CD20+ B lineage lymphomas.

Global Plant Stress Signaling: Reactive Oxygen Species at the Cross-Road

Current technologies have changed biology into a data-intensive field and significantly increased our understanding of signal transduction pathways in plants. However, global defense signaling networks in plants have not been established yet. Considering the apparent intricate nature of signaling mechanisms in plants (due to their sessile nature), studying the points at which different signaling pathways converge, rather than the branches, represents a good start to unravel global plant signaling networks. In this regard, growing evidence shows that the generation of reactive oxygen species (ROS) is one of the most common plant responses to different stresses, representing a point at which various signaling pathways come together. In this review, the complex nature of plant stress signaling networks will be discussed. An emphasis on different signaling players with a specific attention to ROS as the primary source of the signaling battery in plants will be presented. The interactions between ROS and other signaling components, e.g., calcium, redox homeostasis, membranes, G-proteins, MAPKs, plant hormones, and transcription factors will be assessed. A better understanding of the vital roles ROS are playing in plant signaling would help innovate new strategies to improve plant productivity under the circumstances of the increasing severity of environmental conditions and the high demand of food and energy worldwide.

How validated receptor occupancy flow cytometry assays can impact decisions and support drug development

Because of the pressure of significant attrition in drug development, demonstration of target engagement after drug administration enables dose and regimen optimization, patient selection, and stratification from the earliest stages of drug development. The determination of receptor occupancy (RO) can support these efforts. Flow cytometry is one of the preferred technologies to be used based on the important advances in the technology over the last years enabling the simultaneous determination on target cells, of multi intra or surface cell parameters with adequate precision in a regulated environment. Nevertheless, compared to other platforms using the same antigen-antibody binding concept, the flow cytometry approach has faced several challenges, not only due to the technology per se and the diversity of receptor occupancy approaches, but also related to the nature of the matrix where the determination is performed. To illustrate these points, three case studies (antibody-drug conjugate and naked antibody) are provided here to highlight the importance of the choice of the right antibody pair to measure both receptor density (RD) and occupancy by the drug on cancer cells in blood and in bone marrow and the possibility to circumvent the lack of a critical reagent with an innovative approach. In addition, the use of RO data to determine the minimum anticipated biological effect level (MABEL) with translational data from preclinical to human studies, selection of starting dose for the first in man study will be discussed.

Preclinical studies of targeted therapies for CD20-positive B lymphoid malignancies by Ofatumumab conjugated with auristatin

Utilization of antibodies to deliver highly potent cytotoxic agents to corresponding antigen-overexpressed tumor cells is a clinically validated therapeutic strategy. Ofatumumab (OFA, trade name Arzerra) is a fully human CD20-specific antibody that is active against CD20-positive B-cell lymphoma/chronic lymphocytic leukemia cells. In order to further enhance the anticancer effect of OFA, anti-CD20 OFA has been conjugated with highly cytotoxic monomethyl auristatin E (MMAE) through a cathepsin-B-cleavable valine-citrulline (vc) dipeptide linkage to form OFA-vcMMAE and the anti-tumor activity of OFA-vcMMAE against CD20-positive B lymphoma cells are then evaluated in vitro and in vivo. As a result, conjugation of OFA with MMAE has kept the initial effector functional activities of OFA such as binding affinity, complement-dependent cytotoxicity (CDC) as well as antibody-dependent cell-mediated cytotoxicity (ADCC). In addition, the conjugation of MMAE significantly improved the cytotoxic activity of OFA against CD20-positive cells (i.e., Raji, Daudi and WIL2-S cells) but not against CD20-negative K562 cells. On the other hand, OFA-vcMMAE was modulated from the CD20-positive cell surface and then entered the lysosomes by receptor-mediated endocytosis, underwent proteolytic degradation and released active drug MMAE to induce apoptotic cell death through a caspase-3-like protease-dependent pathway. Surprisingly, OFA-vcMMAE completely inhibited the growth of CD20-positive Daudi and Ramos lymphoma xenografts in vivo, and exhibited greater anti-tumor activity than unconjugated OFA, suggesting that the anti-tumor activity of anti-CD20 antibody can be enhanced by conjugation with MMAE. In the near future, this new approach might be used as a clinical treatment of CD20-positive B lymphoid malignancies.

Bioengineering a unique deimmunized bispecific targeted toxin that simultaneously recognizes human CD22 and CD19 receptors in a mouse model of B-cell metastases

A drug of high potency and reduced immunogenicity is needed to develop a targeted biological drug that when injected systemically can penetrate to malignant B cells. Therefore, a novel deimmunized bispecific ligand-directed toxin targeted by dual high-affinity single-chain Fvs (scFv) spliced to PE38 with a KDEL COOH-terminus was genetically engineered. The aims were to reduce toxin immunogenicity using mutagenesis, measure the ability of mutated drug to elicit antitoxin antibody responses, and show that mutated drug was effective against systemic B-cell lymphoma in vivo. Both human anti-CD22 scFv and anti-CD19 scFv were cloned onto the same single-chain molecule with truncated pseudomonas exotoxin (PE38) to create the drug. Site-specific mutagenesis was used to mutate amino acids in seven key epitopic toxin regions that dictate B-cell generation of neutralizing antitoxin antibodies. Bioassays were used to determine whether mutation reduced potency, and ELISAs were done to determine whether antitoxin antibodies were reduced. Finally, a powerful genetically altered luciferase xenograft model was used that could be imaged in real time to determine the effect on systemic malignant human B-cell lymphoma, Raji-luc. Patient B-lineage acute lymphoblastic leukemia, B-cell chronic lymphocytic leukemia, and B lymphoma were high in CD22 and CD19 expression. 2219KDEL7mut was significantly effective against systemic Raji-luc in mice and prevented metastatic spread. Mutagenesis reduced neutralizing antitoxin antibodies by approximately 80% with no apparent loss in in vitro or in vivo activity. Because 2219KDEL7mut immunogenicity was significantly reduced and the drug was highly effective in vivo, we can now give multiple drug treatments with targeted toxins in future clinical trials.

Genetic alteration of a bispecific ligand-directed toxin targeting human CD19 and CD22 receptors resulting in improved efficacy against systemic B cell malignancy

A bispecific ligand-directed toxin (BLT) called DT2219ARL consisting of two scFv ligands recognizing CD19 and CD22 and catalytic DT390 was genetically enhanced for superior in vivo anti-leukemia activity. Genetic alterations included reverse orienting VH-VL domains and adding aggregation reducing/stabilizing linkers. In vivo, these improvements resulted in previously unseen long-term tumor-free survivors measured in a bioluminescent xenograft imaging model in which the progression of human Raji Burkitt's lymphoma could be tracked in real time and in a Daudi model as well. Studies showed DT2219ARL was potent (IC50s 0.06-0.2 nM range) and selectively blockable. Imaging studies indicated the highly invasive nature of this B cell malignancy model and showed it likely induced pre-terminal hind limb paralysis because of metastasis to spinal regions prevented by DT2219ARL. DT2219ARL represents a new class of bispecific biological that can be continually improved by genetic mutation.

Anti-B4 Blocked Ricin Post Chemotherapy in Patients with Chronic Lymphocytic Leukemia--Long-term Follow-up of a Monoclonal Antibody-based Approach to Residual Disease

Anti-B4-blocked ricin is an immunotoxin consisting of anti-B4 murine monoclonal antibody and "blocked ricin" toxin. The anti-B4 monoclonal antibody is directed against the CD19 antigen, which is expressed on B-lymphocytes. A phase II study of anti-B4 blocked ricin toxin in patients with B-cell chronic lymphocytic leukemia (CLL) with residual disease after chemotherapy was conducted. Eleven patients received anti-B4 blocked ricin at 30 microg/kg lean body mass (LBM) daily by continuous infusion for 7 days followed with repeat infusion administered at 14-day intervals. No patient achieved an objective response. The major reasons for failure to respond were the presence of adenopathy and residual marrow disease. Three patients achieved immunophenotypic response in marrow and peripheral blood. Three of 6 patients with rearranged IgH and/or Ig kappa were germline after anti-B4 blocked ricin. The median follow-up of surviving patients is 8.6 years. The median survival is 5.8 years (range, 0.0-8.8). All patients have progressed. The median time to progression was 0.8 years (range, 0.3-3.0). Infusion-related toxicities were all grade 1-2. The most common toxicity was transaminitis. Human antimouse antibody (HAMA) and/or human antiricin antibody (HARA) development was documented in 2 patients. Anti-B4 blocked ricin was well tolerated but had limited activity in patients with residual CLL after chemotherapy.

New therapeutic strategies in acute lymphoblastic leukemia

While cure rates of over 80% are achieved in contemporary pediatric acute lymphoblastic leukemia (ALL) protocols, most adults with ALL succumb to their disease, and little progress has been made in the treatment of refractory and relapsed ALL. Moreover, the burden of therapy is high in a significant number of newly diagnosed patients, and in all those with relapse. Early response to therapy measured by minimal residual disease evaluation has proven the single most important prognostic factor and is increasingly used in risk stratification. However, as the benefit from intensification of frontline therapy becomes limiting, it becomes increasingly challenging to rescue patients who fail on contemporary risk-adapted protocols. New therapeutic strategies are needed, not only in salvage regimens but also in frontline protocols for patients who are at high risk of relapse. Current novel approaches include new formulations of existing chemotherapeutic agents, new antimetabolites and nucleoside analogs, monoclonal antibodies against leukemic-associated antigens, cellular immunotherapy, and molecular therapeutics. Some have already been adopted into standard regimens, while others remain in early stages of development. This review summarizes the current status of these novel therapies as they get integrated into ALL regimens.

High CD21 expression inhibits internalization of anti-CD19 antibodies and cytotoxicity of an anti-CD19-drug conjugate

CD19 and CD21 (CR2) are co-receptors found on B-cells and various B-cell lymphomas, including non-Hodgkin lymphoma. To evaluate their suitability as targets for therapy of such lymphomas using internalization-dependent antibody-drug conjugates [such as antibody-4-(N-maleimidomethyl)cyclohexane-1-carboxylate, (N^2′-deacetyl-N^2′-(3-mercapto-1-oxopropyl)-maytansine) (MCC-DM1) conjugates, which require lysosomal degradation of the antibody moiety for efficacy], we examined uptake of antibodies to CD19 and CD21 in a panel of B-cell lines. Anti-CD21 antibodies were not sufficiently internalized even in the highest CD21-expressing Raji cells, resulting in lack of efficacy with anti-CD21-MCC-DM1 conjugates. Anti-CD19 antibody uptake was variable, and was unexpectedly negatively correlated with CD21 expression. Thus, high CD21-expressing Raji, ARH77 and primary B-cells only very slowly internalized anti-CD19 antibodies, while CD21-negative or low expressing cells, including Ramos and Daudi, rapidly internalized these antibodies in clathrin-coated vesicles followed by lysosomal delivery. Anti-CD19-MCC-DM1 caused greater cytotoxicity in the faster anti-CD19-internalizing cell lines, implying that the rate of lysosomal delivery and subsequent drug release is important. Furthermore, transfection of Ramos cells with CD21 impeded anti-CD19 uptake and decreased anti-CD19-MCC-DM1 efficacy, suggesting that CD21-negative tumours should respond better to such anti-CD19 conjugates. This may have possible clinical implications, as anti-CD21 immunohistochemistry revealed only approximately 30% of 54 diffuse large B-cell lymphoma patients lack CD21 expression.

New therapeutic strategies for the treatment of acute lymphoblastic leukaemia

Although contemporary treatments cure more than 80% of children with acute lymphoblastic leukaemia (ALL), some patients require intensive treatment and many patients still develop serious acute and late complications owing to the side effects of the treatments. Furthermore, the survival rate for adults with ALL remains below 40%. Therefore, new treatment strategies are needed to improve not only the cure rate but also the quality of life of these patients. Here, we discuss emerging new treatments that might improve the clinical outcome of patients with ALL. These include new formulations of existing chemotherapeutic agents, new antimetabolites and nucleoside analogues, monoclonal antibodies against leukaemia-associated antigens, and molecular therapies that target genetic abnormalities of the leukaemic cells and their affected signalling pathways.

Characterization of a humanized IgG4 anti-HLA-DR monoclonal antibody that lacks effector cell functions but retains direct antilymphoma activity and increases the potency of rituximab

HLA-DR is under investigation as a target for monoclonal antibody (mAb) therapy of malignancies. Here we describe a humanized IgG4 form of the anti-HLA-DR mAb L243, hL243gamma4P (IMMU-114), generated to provide an agent with selectivity toward neoplastic cells that can kill without complement-dependent cytotoxicity (CDC) or antibody-dependent cellular-cytotoxicity (ADCC), so as to reduce reliance on intact immunologic systems in the patient and effector mechanism-related toxicity. In vitro studies show that replacing the Fc region of hL243gamma1, a humanized IgG1 anti-HLA-DR mAb, with the IgG4 isotype abrogates the effector cell functions of the antibody (ADCC and CDC) while retaining its antigen-binding properties, antiproliferative capacity (in vitro and in vivo), and the ability to induce apoptosis concurrent with activation of the AKT survival pathway. Growth inhibition was evaluated compared with and in combination with the anti-CD20 mAb rituximab, with the combination being more effective than rituximab alone in inhibiting proliferation. Thus, hL243gamma4P is indistinguishable from hL243gamma1 and the parental murine mAb in assays dependent on antigen recognition. The abrogation of ADCC and CDC, which are believed to play a major role in side effects of mAb therapy, may make this antibody an attractive clinical agent. In addition, combination of hL243gamma4P with rituximab offers the prospect for improved patient outcome.

Lysosomal trafficking and cysteine protease metabolism confer target-specific cytotoxicity by peptide-linked anti-CD30-auristatin conjugates

The chimeric anti-CD30 monoclonal antibody cAC10, linked to the antimitotic agents monomethyl auristatin E (MMAE) or F (MMAF), produces potent and highly CD30-selective anti-tumor activity in vitro and in vivo. These drugs are appended via a valine-citrulline (vc) dipeptide linkage designed for high stability in serum and conditional cleavage and putative release of fully active drugs by lysosomal cathepsins. To characterize the biochemical processes leading to effective drug delivery, we examined the intracellular trafficking, internalization, and metabolism of the parent antibody and two antibody-drug conjugates, cAC10vc-MMAE and cAC10vc-MMAF, following CD30 surface antigen interaction with target cells. Both cAC10 and its conjugates bound to target cells and internalized in a similar manner. Subcellular fractionation and immunofluorescence studies demonstrated that the antibody and antibody-drug conjugates entering target cells migrated to the lysosomes. Trafficking of both species was blocked by inhibitors of clathrin-mediated endocytosis, suggesting that drug conjugation does not alter the fate of antibody-antigen complexes. Incubation of cAC10vc-MMAE or cAC10vc-MMAF with purified cathepsin B or with enriched lysosomal fractions prepared by subcellular fractionation resulted in the release of active, free drug. Cysteine protease inhibitors, but not aspartic or serine protease inhibitors, blocked antibody-drug conjugate metabolism and the ensuing cytotoxicity of target cells and yielded enhanced intracellular levels of the intact conjugates. These findings suggest that in addition to trafficking to the lysosomes, cathepsin B and perhaps other lysosomal cysteine proteases are requisite for drug release and provide a mechanistic basis for developing antibody-drug conjugates cleavable by intracellular proteases for the targeted delivery of anti-cancer therapeutics.

Efficient elimination of B-lineage lymphomas by anti-CD20-auristatin conjugates

The anti-CD20 antibody rituximab is useful in the treatment of certain B-cell malignancies, most notably non-Hodgkin's lymphoma. Its efficacy has been increased when used in combination with chemotherapy, yet anti-CD20 monoclonal antibodies (mAbs) directly conjugated with drugs such as doxorubicin (Dox) have failed to deliver drug or to demonstrate antitumor activity. We have produced anti-CD20 antibody-drug conjugates that possess potent antitumor activity by using the anti-mitotic agent, monomethyl auristatin E (MMAE), linked via the lysosomally cleavable dipeptide, valine-citrulline (vc). Two anti-CD20 conjugates, rituximab-vcMMAE and 1F5-vcMMAE, were selectively cytotoxic against CD20(+) B-lymphoma cell lines, with IC(50) values ranging from 50 ng/mL to 1 microg/mL. Unlike rituximab, which showed diffuse surface localization, rituximab-vcMMAE capped and was internalized within 4 hours after binding to CD20(+) B cells. Internalization of rituximab-vcMMAE was followed by rapid G(2)-M phase arrest and onset of apoptosis. Anti-CD20 antibody-drug conjugates prepared with Dox were internalized and localized as with rituximab-vcMMAE, yet these were not effective for drug delivery (IC(50) > 50 microg/mL). Consistent with in vitro activity, rituximab-vcMMAE showed antitumor efficacy in xenograft models of CD20-positive lymphoma at doses where rituximab or rituximab-Dox conjugates were ineffective. These data indicate that anti-CD20-based antibody-drug conjugates are effective antitumor agents when prepared with a stable, enzyme-cleavable peptide linkage to highly potent cytotoxic agents such as MMAE.

In vitro internalization, intracellular transport, and clearance of an anti-CD11a antibody (Raptiva) by human T-cells

Efalizumab (Raptiva) is a humanized CD11a-specific monoclonal antibody that was recently approved for the treatment of moderate to severe psoriasis. In psoriasis patients, the rate of efalizumab clearance from serum is related to T-cell surface expression of CD11a, suggesting a receptor-mediated clearance model for efalizumab (Bauer et al., 1999). However, limited experimental data are available to explain how the interaction with CD11a results in the systemic clearance of efalizumab. The following studies were designed to test the hypothesis that one mechanism of anti-CD11a antibody clearance is mediated in part by cellular internalization. This was tested in vitro using purified mouse and human T-cells as a model to study the cellular uptake and clearance of anti-CD11a antibodies. Data from these studies suggest that anti-CD11a antibodies are internalized by purified T-cells. Upon internalization, the antibodies appeared to be targeted to lysosomes and were cleared from within the cells in a time-dependent manner. CD11a-mediated internalization and lysosomal targeting of efalizumab may constitute one pathway by which this antibody is cleared in vivo.

Selective targeting of immunoliposomal doxorubicin against human multiple myeloma in vitro and ex vivo

Circulating malignant CD19(+) B cells have been implicated in the pathogenesis and relapse of multiple myeloma (MM). This study investigated the therapeutic applicability of using long-circulating liposome-encapsulated doxorubicin (DXR) targeted against the internalizing CD19 antigens present on human MM cells. In vitro binding studies using the CD19(+) MM cell line ARH77 demonstrated that CD19-directed immunoliposomes (SIL[anti-CD19]) specifically attached to these cells. Formulations of immunoliposomal doxorubicin (DXR-SIL[anti-CD19]) showed a higher association with, and higher cytotoxicity against, ARH77 cells than did non-targeted liposomal doxorubicin (DXR-SL) or isotype-matched controls (DXR-NSIL[IgG2a]). By using the pH-sensitive fluorophore, 1-hydroxypyrene-3,6, 8-trisulfonic acid, binding of SIL[anti-CD19] to CD19 antigens was shown to trigger receptor-mediated internalization of the antibody-antigen complexes into endosomes. Targeting of SIL[anti-CD19] to CD19(+) B cells was also demonstrated in a heterogeneous mixture of peripheral blood mononuclear cells (PBMC) from MM patients. A decrease in cellular DNA (which is an indicator of apoptosis) caused by the cytotoxicity of DXR-SIL[anti-CD19] to myeloma PBMC was determined by using flow cytometry. While PBMC treatment with free DXR resulted in non-specific cytotoxicity to both B and T cells, DXR-SL were only minimally cytotoxic to either. In contrast, DXR-SIL[anti-CD19] were selectively cytotoxic for B cells in PBMC, indicating that this treatment may be effective in eliminating circulating malignant B cells in MM patients.

Processing of preproricin in transgenic tobacco

The plant protein toxin ricin has found widespread application as a potential therapeutic agent for many human diseases and in disease-model systems such as those involving apoptosis. Genetic engineering and expression of the complete two-polypeptide chain toxin have only been possible in plants, specifically in transgenic tobacco carrying the preproricin gene under the control the cauliflower mosaic virus 35S promoter. Production of modified ricin for altered controllable activity and/or fusion therapeutics to target delivery requires knowledge of the heterologous processing that occurs when preproricin is expressed in tobacco. Here, recombinant ricin from transgenic tobacco was purified using lectin affinity chromatography and characterized using various biochemical and biophysical techniques. Coomassie blue staining of an SDS-PAGE gel of lactose-agarose purified material identified predominant proteins of 30 and 35 kDa molecular weight. Western analysis using anti-ricin a- and b-chain antibodies confirmed the expression and purification of recombinant ricin, with identical protein banding profiles to that of authentic castor-bean-derived ricin. High-resolution gel filtration chromatography characterized the lactose binding complex as a 66-kDa native molecular weight protein which could be separated into 30- and 35-kDa proteins upon incubation with the reducing agent dithiothreitol. N-terminal sequencing of the recombinant ricin a-chain revealed that an equimolar ratio of two alternately processed peptides was present, which varied by an additional amino acid derived from the signal peptide. Similar analysis of ricin b-chain again identified two forms of this polypeptide as well; however, full-length ricin b-chain and b-chain missing the first alanine residue were present at 11:1 molar ratios. Transgenic tobacco plants expressing ricin were used to develop a stable cell suspension culture system from callus induced with the growth regulators 2,4-dichlorophenoxyacetic acid and 6-benzylaminopurine. Double sandwich enzyme-linked immunosorbent assay using anti-ricin b-chain antibodies and Western analysis identified soluble ricin in the media of the cultures, indicating that cell cultures provide a safe and simple means to produce properly processed recombinant ricin.