Immunotoxins

Monovalent and polyvalent carbohydrate inhibitors of ricin binding to a model of the cell-surface receptor

A selection of galactose and lactose analogues was evaluated for their potency in inhibiting the binding of ricin to immobilised asialofetuin, which is a model of the cell-surface receptor for ricin. The aim was to identify compounds that could be used as antagonists of ricin toxicity in vivo, and as more selective, and therefore safer, antitoxins. Although one of these analogues had been identified by molecular modelling in a previous study as a potentially potent inhibitor, it and the other carbohydrates studied were less effective than galactose and lactose themselves (I(50) = 1.39 and 0.74 mM, respectively). In an attempt to increase the potency of carbohydrate-based inhibitors, galactose was coupled to the surface of dendrimers. No synergistic interactions were observed from this multivalent approach. Encouraging results, however, were obtained with a self-assembled lyotropic mesophase gel containing novel synthetic galactose-based surfactants, which was able to sequester ricin from aqueous solution in a 2-phase system.

Monoclonal antibody-based enzyme immunoassay for detection of ricin

A sensitive and specific enzyme-linked immunoadsorbent assay (ELISA) was developed to detect ricin in biological fluids. The assay is based on the sandwich format using monoclonal antibodies (MAbs) of two distinct specificities. An affinity-purified anti-ricin B chain MAb (1G7) is utilized to adsorb ricin from solution and the second anti-ricin A chain MAb (5E11) conjugated with peroxidase is then used to form a sandwich, and peroxidase allows color development and measurement of optical density at 450 nm. Standard curves were linear over the range of 2.5-100 ng/mL ricin. The limit of detection was below 5 ng/mL in assay buffer as well as in a 1:10 dilution of urine or 1:50 dilution of human serum spiked with ricin.

Listeriolysin O potentiates immunotoxin and bleomycin cytotoxicity

Antitumor immunotoxins were formed by covalently attaching the ribosome-inactivating protein ricin A chain (RA) to the antitumor antibodies BR96 and L6. In vitro cytotoxicity assays established that BR96-RA was cytotoxic to H2987 human lung adenocarcinoma cells (IC50 = 6 nM), while L6-RA exhibited very low levels of cytotoxic activity (18% cell kill at 67 nM). The virulence factor from the intracellular pathogen Listeria monocytogenes, listeriolysin O (LLO), was able to potentiate the cytotoxicity of BR96-RA and L6-RA by 120- and > 1340-fold, respectively, resulting in IC50 values of approximately 50 pM. LLO also potentiated the cytotoxicity of the peptide anticancer drug bleomycin by a factor of > 2500 but had no effect on the cytotoxic activities of the anticancer drugs cytarabine and etoposide phosphate. In addition, LLO did not potentiate the cytotoxic activity of unconjugated ricin A chain or L6-RA on H2987 cells that were saturated with L6 prior to conjugate treatment. These results are attributed to LLO-induced alteration of the intracellular trafficking of molecules that are incorporated into acidic vesicles.

Construction, expression, and characterization of BD1-G28-5 sFv, a single-chain anti-CD40 immunotoxin containing the ribosome-inactivating protein bryodin 1

The major limitation to the use of immunotoxins in the clinic is the toxicity associated with the toxin moiety. BD1-G28-5 single-chain Fv (sFv) is a single-chain immunotoxin targeted to human CD40 and consists of bryodin 1 (BD1), a plant ribosome-inactivating protein that is 20-30-fold less toxic in animals than commonly used toxins, fused to the sFv region of the anti-CD40 monoclonal antibody G28-5. This immunotoxin was expressed in Escherichia coli and purified from refolded inclusion bodies. BD1-G28-5 sFv retained the full protein synthesis inhibition activity of recombinant BD1 and specifically bound to CD40 with a binding affinity, kd, of 1.5 nM, within 10-fold of the bivalent parental monoclonal antibody. BD1-G28-5 sFv was potently cytotoxic against CD40-expressing B lineage non-Hodgkin's lymphoma and multiple myeloma cell lines, with EC50 values in the ng/ml range, but not against a CD40-negative T cell line. Interestingly, BD1-G28-5 sFv was not cytotoxic against CD40-expressing carcinoma cell lines that were sensitive to a BD1-based immunotoxin conjugate targeted to the Ley carbohydrate antigen. These data represent the first report indicating that BD1 can be used in the construction of potent single-chain immunotoxins. Additionally, although BD1-G28-5 sFv effectively killed CD40-expressing hematologic malignancies, its lack of activity against CD40-expressing carcinomas suggests that CD40-mediated trafficking of BD1 differs in the two cancer types.

Expression and characterization of bryodin 1 and a bryodin 1-based single-chain immunotoxin from tobacco cell culture

Bryodin 1 (BD1) is a potent ribosome-inactivating protein (RIP) isolated from the plant Bryonia dioica. It is relatively nontoxic in rodents (LD50 > 40 mg/kg) and represents a potential improvement over other RIPs and bacterial toxins that have been used in immunotoxins. Recombinant BD1, expressed in Escherichia coli, localizes to insoluble inclusion bodies necessitating denaturation and refolding steps to generate active protein. In this report, BD1 was expressed as a soluble recombinant protein in tobacco cell culture (ntBD1) and purified to near homogeneity with yields of up to 30 mg/(L of culture). The protein synthesis inhibition activity of ntBD1 was identical to that of both native BD1 isolated from the roots of B. dioica and recombinant BD1 expressed in E. coli. Toxicology analysis showed that ntBD1 was well tolerated in rats at doses that cannot be achieved with most other toxin components of immunotoxins. Additionally, a single-chain immunotoxin composed of BD1 fused to the single-chain Fv region of the anti-CD40 antibody G28-5 (ntBD1-G28-5 sFv) was expressed in tobacco tissue culture as a soluble protein and was specifically cytotoxic toward CD40 expressing non-Hodgkin's lymphoma cells in vitro. These data indicate that tobacco tissue culture is a viable system for soluble expression of BD1 and BD1-containing immunotoxins.

Structure of beta 2-bungarotoxin: potassium channel binding by Kunitz modules and targeted phospholipase action

BACKGROUND

beta-bungarotoxin is a heterodimeric neurotoxin consisting of a phospholipase subunit linked by a disulfide bond to a K+ channel binding subunit which is a member of the Kunitz protease inhibitor superfamily. Toxicity, characterized by blockage of neural transmission, is achieved by the lipolytic action of the phospholipase targeted to the presynaptic membrane by the Kunitz module.

RESULTS

The crystal structure at 2.45 A resolution suggests that the ion channel binding region of the Kunitz subunit is at the opposite end of the module from the loop typically involved in protease binding. Analysis of the phospholipase subunit reveals a partially occluded substrate-binding surface and reduced hydrophobicity.

CONCLUSIONS

Molecular recognition by this Kunitz module appears to diverge considerably from more conventional superfamily members. The ion channel binding region identified here may mimic the regulatory interaction of endogenous neuropeptides. Adaptations of the phospholipase subunit make it uniquely suited to targeting and explain the remarkable ability of the toxin to avoid binding to non-target membranes. Insight into the mechanism of beta-bungarotoxin gained here may lead to the development of therapeutic strategies against not only pathological cells, but also enveloped viruses.

Translocation of ricin A-chain into proteoliposomes reconstituted from Golgi and endoplasmic reticulum

Translocation to the cytosol is an essential and rate-limiting step in the cytotoxicity of the potent plant toxin ricin. In an attempt to study the mechanism of ricin A-chain translocation in a cell-free assay, we have partially purified Golgi and endoplasmic reticulum from Jurkat cells by discontinuous sucrose gradient fractionation. The membranes of the organelle fractions were solubilized by the addition of sodium cholate and reconstituted into proteoliposomes by dialyzing out the detergent. The resulting vesicles supported cell-free translocation of RTA (as assessed by an enzyme protection assay) at a rate which was linearly dependent on the concentration of the vesicle preparation. Ricin B-chain (RTB) neither translocated into the vesicles, nor increased the efficiency of RTA translocation. Liposomes prepared from purified phospholipids were not capable of supporting RTA translocation. Furthermore, protease treatment of concanavalin A adsorption of proteins from lysates prior to vesicle reconstitution resulted in abrogation of the translocation process, suggesting that the protein components of organelle membranes are required for RTA translocation. Reconstitution of translocation-competent proteoliposomes from detergent-solubilized membranes of endoplasmic reticulum- and Golgi-enriched fractions provides a convenient cell-free system to study the mechanism of RTA translocation.

Detection of ricin by colorimetric and chemiluminescence ELISA

A highly sensitive and specific ELISA was developed to detect ricin in biological fluids. The assay utilizes an affinity-purified goat polyclonal antibody to adsorb ricin from solution. The same antibody (biotinylated) is then used to form a sandwich, and avidin-linked alkaline phosphatase allows color development and measurement of optical density at 405 nm. Our routine assay uses a standard curve over the range of 0-10 ng/ml ricin, with accurate quantitation below 1 ng/ml (100 pg/well) in assay buffer as well as in a 1:10 dilution of human urine or 1:50 dilution of human serum spiked with ricin. Ricin measured in spiked samples demonstrated accuracy typically within 5% of the expected value in all matrices. The coefficient of variation ranged from 3-10% at 10 ng/ml to 8-25% at 2.5 ng/ml. Two variations on the routine assay were also investigated. First, lengthened incubation times and additional time for color development allowed accurate quantitation in serum dilutions as low as 1:2. Second, increased concentrations of biotinylated antibody and avidin-linked enzyme from 1:250 to 1:70 enhanced the sensitivity of the assay 10-fold, achieving a detection limit of at least 100 pg/ml (10 pg/well). The assay was also configured to a format based upon chemiluminescence, which allowed quantitation in the 0.1-1 ng/ml range, but was subject to slightly greater variability than the colorimetric assay.

Antibody-induced modulation and intracellular transport of CD10 and CD19 antigens in human malignant B cells

Antibody-induced antigenic modulation (AIAM) is a complex biological phenomenon closely resembling other receptor-ligand interactions. Following exposure to specific antibodies, surface antigens are usually rapidly redistributed on the cell surface and internalized. A subsequent intracellular processing results in dissociation of the antigen-antibody complexes, degradation, exocytosis and recycling. AIAM plays an important role in MoAb-targeted therapy of hematopoietic malignancies contributing to escape of tumor cells from immunodestruction. On the other hand, internalization of MoAbs used as carriers of toxins and drugs is a prerequisite of therapeutic efficacy. Even though MoAbs directed against CD10 and CD19 have been used in immunotherapy of B cell malignancies, some aspects regarding AIAM of these Ags are not yet fully understood. Both Ags are modulated by specific MoAbs and internalized through the same pathway, however, the kinetics of AIAM vary from one Ag to another and from one cell type to another. Recent studies with malignant B-cell lines show that, under certain experimental conditions, the extent and rate of surface clearing, uptake and intracellular transport are considerably higher in the case of CD19 than in CD10 and higher in less mature cells compared with more mature cells. These observations may be useful in the selection of MoAbs for immunotherapy, although they need to be confirmed with fresh malignant B cells.

Modulation and intracellular transport of CD20 and CD21 antigens induced by B1 and B2 monoclonal antibodies in RAJI and JOK-1 cells--an immunofluorescence and immunoelectron microscopy study

By fluorescence microscopy (FM), flow cytometry (FCM) and immunoelectron microscopy (IEM) we have shown that B1 and B2 monoclonal antibodies (MoAbs) were able to induce modulation of CD20 and CD21 in RAJI and JOK-1 cell lines. Redistribution and internalization of both antigens (Ags) after binding with MoAbs was readily demonstrated by FM, and by IEM CD20 and CD21 were found to be processed by the pathway of receptor-mediated endocytosis. The rate of intracellular transport varied: CD21 > CD20 and RAJI > JOK-1. Approximately 65 and 55% of CD20 and 60 and 45% of CD21 were cleared from the surface of RAJI and JOK-1 cells, respectively (FCM and IEM). These values, however, clearly exceeded those corresponding to internalization (11, 9, 24 and 16%) indicating shedding of Ag-MoAb complexes. No evidence of recycling was found. The present data support the hypothesis that the kinetics of modulation vary from one Ag to another and probably also reflect the stage of differentiation of the malignant B-cells. The results are discussed in the context of the possible usefulness of B1 and B2 MoAbs in the therapy of B-cell malignancies.

Antibody-targeted drugs for the therapy of cancer

The advent of monoclonal antibodies has revitalised the concept of magic bullets and various agents (eg. drugs, toxins and isotopes) have been conjugated to monoclonal antibodies for selective delivery to tumours. Preclinical studies in mouse tumour models have been impressive and have lead to several clinical trials. These phase I trials have been less impressive. However, keeping in mind the aim of Phase I trials, the safety of using these conjugates in humans have been established. Several, major problems still remain to be overcome before these agents may be useful for the treatment of cancer. These problems stem from the nature of tumour vasculature, cytotoxic activity of the moiety linked to antibody and the targeted tumour antigen expressed on the cell surface. This review will deal with these various aspects described above and possible approaches to overcome these obstacles with a definite bias towards drug-monoclonal antibody conjugates. However, these concepts are equally applicable for improved targeting of other agents.

Endocytosis of ricin by rat liver cells in vivo and in vitro is mainly mediated by mannose receptors on sinusoidal endothelial cells

Upon intravenous injection into rats, the plant toxin ricin was rapidly cleared from the circulation by the liver. Among the different liver cell populations, most of the injected ricin associated with the sinusoidal endothelial cells (EC), whereas the liver parenchymal cells (PC) and Kupffer cells (KC) yielded minor contributions to the total liver uptake in vivo. Co-injection of mannan strongly inhibited ricin uptake by the EC, showing that it was mediated by mannose receptors. On the other hand, co-injection of lactose, which inhibits the galactose-specific association of ricin with cells, enhanced ricin uptake by the EC. The carbohydrate-dependency of the EC contribution to the uptake of ricin in vivo was reflected in the carbohydrate-dependency of the uptake in vivo by whole liver. In vitro, the EC also endocytosed ricin more efficiently than did the PC or KC. Whereas uptake in vitro in the EC was mainly mannose-specific, uptake in the two other cell types was mainly galactose-specific. Western blotting showed that the mannose receptors of liver non-parenchymal cells are identical with the mannose receptor previously isolated from alveolar macrophages. The mannose receptors are expressed at a higher level in EC than in KC. Ligand blotting showed that, in the presence of lactose, the mannose receptor is the only protein in the EC that binds ricin, and the binding is mannose-specific and Ca(2+)-dependent.

Engineering antibodies for therapy

Success in the generation of an antibody-based therapeutic requires careful consideration of the binding site, to achieve specificity and high affinity; of the effector, to produce the desired therapeutic effect; of the means of attachment of the effector to the binding site; production of the end product; and the response made by the patient to the administered compound. Each of these areas is receiving attention by antibody-engineering techniques. The number of potentially useful monoclonal antibodies developed over the last 10 years, and currently in clinical trials or preregistration, is now being increased by these engineered newcomers. It will be interesting to see over the next few years how many of these antibodies, and of which kind, emerge as products.

High-level expression and simplified purification of recombinant ricin A chain

Ricin toxin is a glycoprotein which catalytically inactivates eukaryotic ribosomes by depurination of a single adenosine residue from the 28S ribosomal RNA. The enzymatic activity is present in the A chain of the toxin molecule, whereas the B chain contains two binding sites for galactose. Since it is highly potent in inhibiting protein synthesis, the A chain is used to prepare cytotoxic conjugates effective against tumor cells. Such chimeric proteins are highly selective and have a wide range of clinical applications. Extensive preclinical studies on these conjugates require large amounts of purified A chain. Native ricin A chain is heterogeneous, since plants produce a number of isoforms of ricin toxin. Purified, native preparations often contain two types of ricin A chain which differ in the extent of glycosylation. By cloning and expressing the gene of A chain, one could obtain homogeneous toxin molecules devoid of carbohydrates. In addition, structural changes in the toxin polypeptide could be introduced by in vitro mutagenesis, which can improve the pharmacological properties and antitumor activity. Earlier methods of expression strategies using Escherichia coli have yielded only moderate levels of expression. In the present study, the coding region of ricin A chain was cloned into pET3b, a high-level expression vector under the control of the T7 promoter. Recombinant ricin A chain produced by this construct has an additional 14 amino acid residues at the NH2 terminus. Subsequently, a NdeI site was created at the 5' end of the gene by oligonucleotide-directed mutagenesis. The modified fragment was then introduced into pET3b vector to produce toxin polypeptide identical to the native sequence.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo and in vitro antitumor activity of mitomycin C conjugates at 7-N position through a linker containing thiocarbamate bond with CD10 monoclonal antibody

Through a linker containing thiocarbomate bound to the 7-N position of mitomycin C (MMC), conjugates with a monoclonal antibody to CD10 (NL-1) were prepared, and their antitumor activities were examined. All five conjugates, except one, showed in vitro cytotoxicity to two CD10+ lymphoid cell lines superior to MMC. The conjugate displaying the highest cytotoxicity was selected and further tested against three CD10+ and two CD10- lymphoid cell lines in vitro. The conjugate with NL-1 antibody demonstrated higher cytotoxic activity against CD10+ tumor cells than the control conjugate with normal immunoglobulin, while there was no significant difference, when tested against CD10- tumors. The cytotoxic activity of the NL-1 conjugate to CD10+ tumors was significantly blocked by NL-1 antibody. In vivo antitumor activity of the NL-1 conjugate was then tested against a CD10+ tumor transplanted to nude mice, and side effects were recorded. The NL-1 conjugate (4 mg/kg) showed an in vivo antitumor effect similar to MMC (2 mg/kg), which is at nearly maximal tolerable dose; the latter induced decreases in numbers of leukocytes and platelets, while the former did not, suggesting less side effect by the NL-1 conjugate. Since MMC demonstrates a broad spectrum of antitumor activity, the conjugate, as such, may be applicable for the treatment of cancer patients.

Neural lesioning with ribosome-inactivating proteins: suicide transport and immunolesioning

Toxic lectins, plant proteins that inactivate ribosomes, irreversibly inhibit protein synthesis with high efficiency. After intraneural (subepineurial) microinjection, these agents are taken up by axons and are retrogradely transported to the perikarya, where they result in cell death. These 'suicide transport' toxins can produce pathway-specific lesions that are useful in several types of experiment, including cellular localization of neurotransmitter receptors. The toxins can be coupled to monoclonal antibodies to produce immunotoxins: reagents that can make highly selective lesions of specific types of neurons. Central or peripheral neurons that express the low-affinity NGF receptor are selectively destroyed by the immunotoxin 192 IgG-saporin. Development of other anti-neuronal immunotoxins should provide a variety of powerful selective lesioning tools.