Protein biosensors based on biofunctionalized conical gold nanotubes

There is increasing interest in the concept of using nanopores as the sensing elements in biosensors. The nanopore most often used is the alpha-hemolysin protein channel, and the sensor consists of a single channel embedded within a lipid bilayer membrane. An ionic current is passed through the channel, and analyte species are detected as transient blocks in this current associated with translocation of the analyte through the channel-stochastic sensing. While this is an extremely promising sensing paradigm, it would be advantageous to eliminate the very fragile lipid bilayer membrane and perhaps to replace the biological nanopore with an abiotic equivalent. We describe here a new family of protein biosensors that are based on conically shaped gold nanotubes embedded within a mechanical and chemically robust polymeric membrane. While these sensors also function by passing an ion current through the nanotube, the sensing paradigm is different from the previous devices in that a transient change in the current is not observed. Instead, the protein analyte binds to a biochemical molecular-recognition agent at the mouth of the conical nanotube, resulting in complete blockage of the ion current. Three different molecular-recognition agents, and correspondingly three different protein analytes, were investigated: (i) biotin/streptavidin, (ii) protein-G/immunoglobulin, and (iii) an antibody to the protein ricin with ricin as the analyte.

Introduction of macromolecules into cultured mammalian cells by osmotic lysis of pinocytic vesicles

We have developed a new procedure for introducing macromolecules into cultured mammalian cells based on osmotic lysis of pinocytic vesicles. Cells are first incubated in culture medium containing 0.5 M sucrose, 10% polyethylene glycol 1000 and the macromolecule to be transferred. Cells are then placed in medium diluted with 0.66 parts water. Most pinocytic vesicles formed in the presence of sucrose burst in hypotonic medium, thereby releasing the enclosed macromolecule. L929 cells remain fully viable after a single hypertonic sucrose treatment, and a majority survives four successive rounds of osmotic lysis. This procedure, termed osmotic lysis of pinosomes, has been used to transfer substantial amounts of horseradish peroxidase, antiricin antibodies and dextran 70,000 into the cytosol of L929 cells. Direct comparison of the degree of ricin resistance conferred by transfer of antiricin antibodies revealed pinosome lysis to be equal, if not superior, to injection mediated by red blood cells.

Colloidal gold-based immunochromatographic assay for detection of ricin

A rapid immunochromatographic assay was developed to detect ricin. The assay was based on the sandwich format using monoclonal antibodies (Mabs) of two distinct specificities. One anti-ricin B chain Mab (1G7) was immobilized to a defined detection zone on a porous nitrocellulose membrane, while the other anti-ricin A chain Mab (5E11) was conjugated to colloidal gold particles which served as a detection reagent. The ricin-containing sample was added to the membrane and allowed to react with Mab (5E11)-coated particles. The mixture was then passed along the porous membrane by capillary action past the Mab (1G7) in the detection zone, which will bind the particles that had ricin bound to their surface, giving a red color within this detection zone with an intensity proportional to ricin concentration. In the absence of ricin, no immunogold was bound to the solid-phase antibody. With this method, 50 ng/ml of ricin was detected in less than 10 min. The assay sensitivity can be increased by silver enhancement to 100 pg/ml.

Facile generation of heat-stable antiviral and antitoxin single domain antibodies from a semisynthetic llama library

Llamas possess a class of unconventional immunoglobulins that have only heavy chains; unpaired heavy variable domains are responsible for antigen binding. These domains have previously been cloned and expressed as single domain antibodies (sdAbs); they comprise the smallest known antigen binding fragments. SdAbs have been shown to bind antigens at >90 degrees C and to refold after being denatured. To take advantage of the remarkable properties of sdAbs, we constructed a large, semisynthetic llama sdAb library. This library facilitated the rapid selection of binders to an array of biothreat targets. We selected sdAb specific for live vaccinia virus (a smallpox virus surrogate), hen egg lysozyme, cholera toxin, ricin, and staphylococcal enterotoxin B. The selected sdAb possessed high specificity as well as enhanced thermal stability in comparison to conventional IgG and scFv antibodies. We also determined equilibrium dissociation constants as well as demonstrated the use of several antitoxin sdAbs as effective capture and reporter molecules in sandwich assays on the Luminex instrument. The ability to rapidly select such rugged antibodies will enhance the reliability of immunoassays by extending shelf life and the capacity to function in hostile environments.

Ex-vivo treatment of donor bone marrow with anti-T-cell immunotoxins for prevention of graft-versus-host disease

Two patients undergoing marrow transplantation for acute lymphocytic leukaemia in third remission received from histocompatible siblings marrow pretreated with a mixture of three anti-T-cell immunotoxins, consisting of murine monoclonal antibodies covalently linked to ricin. This marrow processing effectively eliminated functional T-cell responses while preserving the marrow precursors necessary for sustained haematoimmunopoietic engraftment. No post-transplant immunoprophylaxis was administered. Both patients showed prompt peripheral engraftment and were discharged from hospital within a month of transplantation. No toxic effects or graft-versus-host disease were apparent after the administration of immunotoxin-treated marrow. Ex-vivo immunotoxin pretreatment appears a safe and simple procedure which deserves further clinical testing as a sole method of graft-versus-host disease prophylaxis.

Anti-B4-blocked ricin: a phase I trial of 7-day continuous infusion in patients with B-cell neoplasms

PURPOSE

This phase I trial was undertaken to determine the maximum-tolerated dose (MTD) and dose-limiting toxicities (DLTs) of the B-cell-restricted immunotoxin anti-B4-blocked ricin (anti-B4-bR) when it is administered by 7-day continuous infusion.

PATIENTS AND METHODS

Thirty-four patients with relapsed and refractory B-cell neoplasms (26 non-Hodgkin's lymphoma [NHL], four chronic lymphocytic leukemia [CLL], four acute lymphoblastic leukemia [ALL]) received 7-day continuous infusion anti-B4-bR. Successive cohorts of at least three patients were treated at doses of 10 to 70 micrograms/kg/d for 7 days with the dose increased by 10 micrograms/kg/d for each cohort. The initial three cohorts of patients (10, 20, and 30 micrograms/kg/d x 7 days) also received a bolus infusion of 20 micrograms/kg before beginning the continuous infusion.

RESULTS

The MTD was reached at 50 micrograms/kg/d x 7 days. The DLTs were National Cancer Institute Common Toxicity Criteria (NCI CTC) grade IV reversible increases in AST and ALT, and grade IV decreases in platelet counts. Adverse reactions included fevers, nausea, headaches, myalgias, hypoalbuminemia, dyspnea, edema, and capillary leak syndrome. Potentially therapeutic serum levels of anti-B4-bR could be sustained for 4 days in patients treated at the MTD. Two complete responses (CRs), three partial responses (PRs), and 11 transient responses (TRs) were observed.

CONCLUSION

Anti-B4-bR can be administered safely by 7-day continuous infusion with tolerable, reversible toxicities to patients with relapsed B-cell neoplasms. Although occasional responses were seen, future trials will use anti-B4-bR in patients with lower tumor burdens to circumvent the obstacle of immunotoxin delivery to bulk disease.

Anti-T-cell reagents for human bone marrow transplantation: ricin linked to three monoclonal antibodies

Three new reagents that react against human T cells were synthesized by covalently linking the toxin ricin to monoclonal antibodies recognizing differentiation antigens on the surface of T lymphocytes. Each of these immunotoxins selectively inhibited T-cell proliferation when the cells were incubated in the presence of lactose. Multipotent human stem cells were inhibited only at much higher concentrations. Mixtures of all three immunotoxins were more effective than any one alone. These reagents have the potential for preventing graft-versus-host disease in man.

Immunological characteristics associated with the protective efficacy of antibodies to ricin

A/B toxins, produced by bacteria and plants, are among the deadliest molecules known. The B chain binds the cell, whereas the A chain exerts the toxic effect. Both anti-A chain and anti-B chain Abs can neutralize toxins in vivo and in vitro. B chain Abs block binding of the toxin to the cell. It is not known how anti-A chain Abs function. Working with ricin toxin, we demonstrate that immunization with A chain induces greater protection than immunization with B chain. A panel of mAbs, binding to A chain, B chain, or both chains, has been produced and characterized. Immunologic characteristics evaluated include isotype, relative avidity, and epitope specificity. The ability to inhibit ricin enzymatic or cell binding activity was studied, as was the ability to block ricin-mediated cellular cytotoxicity on human and murine cell lines. Finally, the in vivo protective efficacy of the Abs in mice was studied. The Ab providing the greatest in vivo protective efficacy was directed against the A chain. It had the greatest relative avidity and the greatest ability to block enzymatic function and neutralize cytotoxicity. Interestingly, we also obtained an anti-A chain Ab that bound with high avidity, blocked enzymatic activity, did not neutralize cytotoxicity, and actually enhanced the in vivo toxicity of ricin. Anti-A chain Abs with moderate avidity had no in vivo effect, nor did any anti-B chain Abs.

Ricin: Mechanism of Toxicity, Clinical Manifestations, and Vaccine Development. A Review

Ricin is one of the most potent plant toxins known, and the castor plant from which it is derived, Ricinus communis, is ubiquitous. The harvesting of castor beans exceeds one million tons annually, and ricin is easier to produce than either anthrax or botulinum. As a result, ricin is a convenient, potent, and available toxin for terrorist acts. This paper will review the mechanism of toxicity, major clinical manifestations, treatment, current methods of detection, and vaccine development.

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.

Blockade of the galactose-binding sites of ricin by its linkage to antibody. Specific cytotoxic effects of the conjugates

A method is described for preparing specific cytotoxic agents by linking intact ricin to antibodies in a manner that produces obstruction of the galactose-binding sites on the B chain of the toxin and so diminishes the capacity of the conjugate to bind non-specifically to cells. The conjugates were synthesised by reacting iodoacetylated ricin with thiolated immunoglobulin and the components of conjugate with reduced galactose-binding capacity were separated by affinity chromatography on Sepharose (a beta-galactosyl matrix) and asialofetuin-Sepharose. Fluorescence-activated cell sorter (FACS) analyses revealed that the fraction of a monoclonal anti-Thy1.1-ricin conjugate that passed through a Sepharose column had markedly diminished capacity to bind non-specifically to Thy1.2-expressing CBA thymocytes and EL4 lymphoma cells. The fraction of conjugate that passed through an asialofetuin-Sepharose column displayed no detectable non-specific binding. Both fractions of conjugate were potent cytotoxic agents for Thy1.1-expressing AKR-A lymphoma cells in tissue culture. They reduced the [3H]leucine incorporation of the cells by 50% at a concentration of 2-5 pM. Comparable inhibition of EL4 cells was only achieved with 3000-7500-fold greater concentrations of conjugate. By contrast, the fraction of anti-Thy1.1-ricin that retained Sepharose-binding capacity showed marked non-specific binding and toxicity to EL4 cells. A conjugate with diminished galactose-binding capacity was also prepared from the W3/25 monoclonal antibody which recognises an antigen upon helper T-lymphocytes in the rat. It elicited powerful and specific toxic effects upon W3/25 antigen-expressing rat T-leukaemia cells. This finding is of particular importance because isolated ricin A-chain disulphide-linked to W3/25 antibody is not cytotoxic. The property of the B-chain in intact ricin conjugates that facilitates delivery of the A-chain to the cytosol thus appears to be independent of galactose recognition. It is concluded that the 'blocked' ricin conjugates combine the advantages of high potency, which is often lacking in antibody-A-chain conjugates, with high specificity, which previously was lacking in intact ricin conjugates.

RiVax, a recombinant ricin subunit vaccine, protects mice against ricin delivered by gavage or aerosol

Ricin is a plant toxin that is a CDC level B biothreat. Our recombinant ricin A chain vaccine (RiVax), which contains mutations in both known toxic sites, has no residual toxicity at doses at least 800 times the immunogenic dose. RiVax without adjuvant given intramuscularly (i.m.) protected mice against intraperitoneally administered ricin. Furthermore the vaccine without alum was safe and immunogenic in human volunteers. Here we describe the development of gavage and aerosol ricin challenge models in mice and demonstrate that i.m. vaccination protects mice against ricin delivered by either route. Also RiVax protects against aerosol-induced lung damage as determined by histology and lung function tests.

A novel recombinant vaccine which protects mice against ricin intoxication

Ricin toxin (RT) is a plant-derived toxin of extraordinary toxicity; a single molecule successfully internalized into the cytoplasm of a cell is lethal for that cell. An estimated dose of 1-10 microg/kg is lethal to humans, making aerosolized ricin a potential agent for bioterrorism. Vaccination against ricin using either denatured toxin or its modified A chain subunit (RTA) has been successful in experimental animals but both vaccines have potential toxicities. Recombinant (r) RTA has not been evaluated as a vaccine. However, the advantage of such a vaccine is that these potential toxicities can be deleted by appropriate mutations. In this study we have generated three mutants and shown that two lack toxicity as compared to the wild type rRTA. These mutants induce protective humoral immune responses in mice. One or both should be considered for use in humans.

Detection of Functional Ricin by Immunoaffinity and Liquid Chromatography−Tandem Mass Spectrometry

The toxin ricin is a biological weapon that may be used for bioterrorist purposes. As a member of the group of ribosome-inactivating proteins (RIPs), ricin has an A-chain possessing N-glycosidase activity which irreversibly inhibits protein synthesis. In this paper, we demonstrate that provided appropriate sample preparation is used, this enzymatic activity can be exploited for functional ricin detection with sensitivity similar to the best ELISA and specificity allowing application to environmental samples. Ricin is first captured by a monoclonal antibody directed against the B chain and immobilized on magnetic beads. Detection is then realized by determination of the adenine released by the A chain from an RNA template using liquid chromatography coupled to tandem mass spectrometry. The immunoaffinity step combined with the enzymatic activity detection leads to a specific assay for the entire functional ricin with a lower limit of detection of 0.1 ng/mL (1.56 pM) after concentration of the toxin from a 500 microL sample size. The variability of the assay was 10%. Finally, the method was applied successfully to milk and tap or bottled water samples.

A pilot clinical trial of a recombinant ricin vaccine in normal humans

Ricin, a highly potent toxin produced by castor beans, is classified by the Centers for Disease Control and Prevention as a level B biothreat because it is easily produced, readily available, and highly stable. There have been >750 cases of documented ricin intoxication in humans. There is no approved vaccine for ricin. Ricin contains a lectin-binding B chain and a ribotoxic A chain (RTA). In addition to its ribotoxic site, we have identified a separate site on RTA that is responsible for inducing vascular leak syndrome (VLS) in humans. We have generated a recombinant RTA with two amino acid substitutions that disrupt its ribotoxic site (Y80A) and its VLS-inducing site (V76M). This mutant recombinant RTA (named RiVax) was expressed and produced in Escherichia coli and purified. When RiVax was injected i.m. into mice it protected them against a ricin challenge of 10 LD50s. Preclinical studies in both mice and rabbits demonstrated that RiVax was safe. Based on these results, we have now conducted a pilot clinical trial in humans under an investigational new drug application submitted to the Food and Drug Administration. In this study, three groups of five normal volunteers were injected three times at monthly intervals with 10, 33, or 100 mug of RiVax. The vaccine was safe and elicited ricin-neutralizing Abs in one of five individuals in the low-dose group, four of five in the intermediate-dose group, and five of five in the high-dose group. These results justify further development of the vaccine.

Identification and characterization of a monoclonal antibody that neutralizes ricin toxicity in vitro and in vivo

We wanted to identify and characterize MAbs with specificity for the toxic lectin ricin, which could serve as detection reagents in elucidating mechanisms and tissue distribution. Neutralizing MAbs could be developed into immunotherapeutics to reverse clinical intoxications from immunotoxin or to counteract the use of ricin as a terrorist or biological warfare weapon. Two hybridomas, UNIVAX 70 and 138, producing MAbs against ricin were identified by Western blot strip analysis. The antibodies were IgG1 and were specific for the ricin A chain with no ricin B chain cross-reactivity. The MAbs neutralized ricin in vitro in an EL-4 mouse leukemia cell assay and in an in vivo mouse model. The two antibodies recognized the same epitope or overlapping epitopes, based on a competition with one another. All further characterization proceeded on the assumption that they were the same. The MAb UNIVAX 70/138 was characterized in vivo by titrating it against an 18 micrograms/kg (> six LD50) i.v. challenge and by titrating the i.v. toxin challenge against a constant dose of 100 micrograms of passive antibody per mouse. A 4:1 molar ratio of MAb to ricin led to neutralization of > or = 90% of the toxin in vitro. The MAb recognized ricin toxoid prepared by formaldehyde treatment and after conjugation of low molecular weight haptens (based on ELISA) equally as well as it recognized ricin and ricin A chain. The affinity and specificity of UNIVAX 70/138 give it excellent reagent potential, and the toxin-neutralizing capacity makes it at least a log and a half better than the next best candidate immunotherapeutic.

Potent and specific killing of human malignant brain tumor cells by an anti-transferrin receptor antibody-ricin immunotoxin

Immunotoxins are hybrid molecules which combine the exquisite selectivity of monoclonal antibodies with the potent toxicity of protein toxins. An immunotoxin was constructed by linking a murine monoclonal antibody against the human transferrin receptor (TR) to the plant toxin, ricin. The cytotoxic activity of the anti-TR-ricin immunotoxin was tested in vitro and demonstrated highly potent and cell type-specific killing of cells derived from human glioblastoma, medulloblastoma, and leukemia. The anti-TR-ricin immunotoxin killed more than 50% of "target" cells at a concentration of 5.6 X 10(-13) M after an 18-hour incubation with the ionophore, monensin. This potency exceeds that of any other anti-TR immunotoxin reported in the literature. When the activity of the anti-TR-ricin immunotoxin against "target" tumor-derived cells was compared with the immunotoxin's activity against "non-target" cells, it could be predicted that a selective toxicity of anti-TR-ricin immunotoxin between tumor cells and normal brain was more than 150- to 1380-fold. Solid-phase indirect radioimmunoassay techniques were used to demonstrate significantly higher levels of TR in the glioblastoma- and medulloblastoma-derived cell lines, as well as in surgical tissue samples of medulloblastoma and glioblastoma, as compared to normal brain. Immunotoxins targeted to the TR may possess sufficient specificity to be of therapeutic importance, particularly to treat neoplastic disease of the central nervous system involving compartments (such as intrathecal, intraventricular, or cystic) where delivery of immunotoxins to tumor would not require transvascular transport.

Characterization of immunotoxins active against ovarian cancer cell lines

The purpose of the present study was to develop immunotoxins directed against human ovarian carcinoma cells. Four monoclonal antibodies (260F9, 454C11, 280D11, and 245E7) were chosen because they were found to bind to various ovarian carcinoma cell lines. These antibodies were covalently linked to either Pseudomonas exotoxin (PE) or ricin A chain (RTA), and the conjugates were tested against five ovarian cancer cell lines (OVCAR-2, -3, -4, -5; A1847). The ability of the immunotoxins to inhibit both protein synthesis and colony formation was evaluated. Qualitatively similar results were obtained for both types of assays. Usually, PE conjugates were more toxic than their corresponding RTA conjugates. 454C11-PE was very toxic for all ovarian carcinoma lines, whereas 454C11-RTA had low activity. Both 260F9-PE and 260F9-RTA were active in all OVCAR cell lines but not in A1847 cells. 280D11-PE was toxic for OVCAR-4; otherwise, 280D11-PE and RTA conjugates of both 280D11 and 245E7 had little activity. Specificity of immunotoxin action was shown by competition by excess antibody, nontoxicity in nontarget cells, and inactivity of an irrelevant immunotoxin. To investigate the basis of antibody-dependent differences in activity of the various immunotoxins, antibody uptake was studied in OVCAR-2 cells, and the results indicate that antibody internalization is one important factor in the activity of immunotoxins.

Vaccines against the category B toxins: Staphylococcal enterotoxin B, epsilon toxin and ricin

The threat of bioterrorism worldwide has accelerated the demand for the development of therapies and vaccines against the Category B toxins: staphylococcal enterotoxin B (SEB), epsilon toxin (ETX) produced by Clostridium perfringens types B and D, and ricin, a natural product of the castor bean. The diverse and unique nature of these toxins poses a challenge to vaccinologists. While formalin-inactivated toxins can successfully induce antibody-mediated protection in animals, their usefulness in humans is limited because of potential safety concerns. For this reason, research is now aimed at developing recombinant, attenuated vaccines based on a detailed understanding of the molecular mechanisms by which these toxins function. Vaccine development is further complicated by the fact that as bioterrorism agents, SEB, ETX and ricin would most likely be disseminated as aerosols or in food/water supplies. Our understanding of the mechanisms by which these toxins cross mucosal surfaces, and importance of mucosal immunity in preventing toxin uptake is only rudimentary.