The use of anti-ricin antibodies to protect mice intoxicated with ricin

Ricin and Ricinus communis in pharmacology and toxicology-from ancient use and "Papyrus Ebers" to modern perspectives and "poisonous plant of the year 2018"

While probably originating from Africa, the plant Ricinus communis is found nowadays around the world, grown for industrial use as a source of castor oil production, wildly sprouting in many regions, or used as ornamental plant. As regards its pharmacological utility, a variety of medical purposes of selected parts of the plant, e.g., as a laxative, an anti-infective, or an anti-inflammatory drug, have been described already in the sixteenth century BC in the famous Papyrus Ebers (treasured in the Library of the University of Leipzig). Quite in contrast, on the toxicological side, the native plant has become the "poisonous plant 2018" in Germany. As of today, a number of isolated components of the plant/seeds have been characterized, including, e.g., castor oil, ricin, Ricinus communis agglutinin, ricinin, nudiflorin, and several allergenic compounds. This review mainly focuses on the most toxic protein, ricin D, classified as a type 2 ribosome-inactivating protein (RIP2). Ricin is one of the most potent and lethal substances known. It has been considered as an important bioweapon (categorized as a Category B agent (second-highest priority)) and an attractive agent for bioterroristic activities. On the other hand, ricin presents great potential, e.g., as an anti-cancer agent or in cell-based research, and is even explored in the context of nanoparticle formulations in tumor therapy. This review provides a comprehensive overview of the pharmacology and toxicology-related body of knowledge on ricin. Toxicokinetic/toxicodynamic aspects of ricin poisoning and possibilities for analytical detection and therapeutic use are summarized as well.

Generation of Highly Efficient Equine-Derived Antibodies for Post-Exposure Treatment of Ricin Intoxications by Vaccination with Monomerized Ricin

Ricin, a highly lethal toxin derived from the seeds of Ricinus communis (castor beans) is considered a potential biological threat agent due to its high availability, ease of production, and to the lack of any approved medical countermeasure against ricin exposures. To date, the use of neutralizing antibodies is the most promising post-exposure treatment for ricin intoxication. The aim of this work was to generate anti-ricin antitoxin that confers high level post-exposure protection against ricin challenge. Due to safety issues regarding the usage of ricin holotoxin as an antigen, we generated an inactivated toxin that would reduce health risks for both the immunizer and the immunized animal. To this end, a monomerized ricin antigen was constructed by reducing highly purified ricin to its monomeric constituents. Preliminary immunizing experiments in rabbits indicated that this monomerized antigen is as effective as the native toxin in terms of neutralizing antibody elicitation and protection of mice against lethal ricin challenges. Characterization of the monomerized antigen demonstrated that the irreversibly detached A and B subunits retain catalytic and lectin activity, respectively, implying that the monomerization process did not significantly affect their overall structure. Toxicity studies revealed that the monomerized ricin displayed a 250-fold decreased activity in a cell culture-based functionality test, while clinical signs were undetectable in mice injected with this antigen. Immunization of a horse with the monomerized toxin was highly effective in elicitation of high titers of neutralizing antibodies. Due to the increased potential of IgG-derived adverse events, anti-ricin F(ab')₂ antitoxin was produced. The F(ab')₂-based antitoxin conferred high protection to intranasally ricin-intoxicated mice; ~60% and ~34% survival, when administered 24 and 48 h post exposure to a lethal dose, respectively. In line with the enhanced protection, anti-inflammatory and anti-edematous effects were measured in the antitoxin treated mice, in comparison to mice that were intoxicated but not treated. Accordingly, this anti-ricin preparation is an excellent candidate for post exposure treatment of ricin intoxications.

Protein Structure Facilitates High-Resolution Immunological Mapping

Select agents (SA) pose unique challenges for licensing vaccines and therapies. In the case of toxin-mediated diseases, HHS assigns guidelines for SA use, oversees vaccine and therapy development, and approves animal models and approaches to identify mechanisms for toxin neutralization. In this commentary, we discuss next-generation vaccines and therapies against ricin toxin and botulinum toxin, which are regulated SA toxins that utilize structure-based approaches for countermeasures to guide rapid response to future biothreats.

Recent advances in the development of vaccines against ricin

Several promising subunit vaccines against ricin toxin (RT) have been developed during the last decade and are now being tested for safety and immunogenicity in humans and for efficacy in nonhuman primates. The incentive to develop a preventive vaccine as a countermeasure against RT use as a bioweapon is based on the high toxicity of RT after aerosol exposure, its environmental stability, abundance, and ease of purification. RT is the second most lethal biological toxin and is considered a "universal toxin" because it can kill all eukaryotic cells through binding to ubiquitous cell surface galactosyl residues. RT has two subunits conjoined by a single disulfide linkage: RTB, which binds galactosyl residues and RTA which enzymatically inactivates ribosomes intracellularly by cleavage ribosomal RNA. Attenuation of toxicity by elimination of the active site or introduction of other structural mutations of RTA has generated two similar clinical subunit vaccine candidates which induce antibodies in both humans and nonhuman primates. In rhesus macaques, inhaled RT causes rapid lung necrosis and fibrosis followed by death. After parenteral vaccination with RTA vaccine, macaques can be protected against aerosol RT exposure, suggesting that circulating antibodies can protect lung mucosa. Vaccination induces RT-neutralizing antibodies, the most likely correlate of protection. Macaques responded to conformational determinants in an RTA vaccine formulation, indicating preservation of RTA structure during initial manufacture. Comparative mapping studies have also demonstrated that macaques and humans recognize the same epitopes, significant in the study of macaques as a model during development of vaccines which cannot be tested for efficacy in humans.

Monoclonal antibody, mAb 4C13, an effective detoxicant antibody against ricin poisoning

Ricin is a glycoprotein produced in castor seeds and consists of two polypeptide chains named Ricin Toxin A Chain (RTA) and Ricin Toxin B Chain (RTB), linked via a disulfide bridge. Due to its high toxicity, ricin is regarded as a high terrorist risk for the public. However, antibodies can play a pivotal role in neutralizing the toxin. In this research, the anti-toxicant effect of mAb 4C13, a monoclonal antibody (mAb) established using detoxicated ricin as the immunized antigen, was evaluated. Compared with mAb 4F2 and mAb 5G6, the effective mechanism of mAb 4C13 was analyzed by experiments relating to its cytotoxicity, epitope on ricin, binding kinetics with the toxin, its blockage on the protein synthesis inhibition induced by ricin and the intracelluar tracing of its complex with ricin. Our result indicated that mAb 4C13 could recognize and bind to RTA, RTB and exert its high affinity to the holotoxin. Both cytotoxicity and animal toxicity of ricin were well blocked by pre-incubating the toxin with mAb 4C13. By intravenous injection, mAb 4C13 could rescue the mouse intraperitoneally (ip) injected with a lethal dose of ricin (20μg/kg) even at 6h after the intoxication and its efficacy was dependent on its dosage. This research indicated that mAb 4C13 could be an excellent candidate for therapeutic antibodies. Its potent antitoxic efficiency was related to its recognition on the specific epitope with very high affinity and its blockage of protein synthesis inhibition in cytoplasm followed by cellular internalization with ricin.

Differential neutralizing activities of a single domain camelid antibody (VHH) specific for ricin toxin's binding subunit (RTB)

Ricin, a member of the A-B family of ribosome-inactivating proteins, is classified as a Select Toxin by the Centers for Disease Control and Prevention because of its potential use as a biothreat agent. In an effort to engineer therapeutics for ricin, we recently produced a collection of alpaca-derived, heavy-chain only antibody V_H domains (V_HH or “nanobody”) specific for ricin’s enzymatic (RTA) and binding (RTB) subunits. We reported that one particular RTB-specific V_HH, RTB-B7, when covalently linked via a peptide spacer to different RTA-specific V_HHs, resulted in heterodimers like V_HH D10/B7 that were capable of passively protecting mice against a lethal dose challenge with ricin. However, RTB-B7 itself, when mixed with ricin at a 1∶10 toxin:antibody ratio did not afford any protection in vivo, even though it had demonstrable toxin-neutralizing activity in vitro. To better define the specific attributes of antibodies associated with ricin neutralization in vitro and in vivo, we undertook a more thorough characterization of RTB-B7. We report that RTB-B7, even at 100-fold molar excess (toxin:antibody) was unable to alter the toxicity of ricin in a mouse model. On the other hand, in two well-established cytotoxicity assays, RTB-B7 neutralized ricin with a 50% inhibitory concentration (IC₅₀) that was equivalent to that of 24B11, a well-characterized and potent RTB-specific murine monoclonal antibody. In fact, RTB-B7 and 24B11 were virtually identical when compared across a series of in vitro assays, including adherence to and neutralization of ricin after the toxin was pre-bound to cell surface receptors. RTB-B7 differed from both 24B11 and V_HH D10/B7 in that it was relatively less effective at blocking ricin attachment to receptors on host cells and was not able to form high molecular weight toxin:antibody complexes in solution. Whether either of these activities is important in ricin toxin neutralizing activity in vivo remains to be determined.

Mechanistic insights into the neutralization of cytotoxic abrin by the monoclonal antibody D6F10

Abrin, an A/B toxin obtained from the Abrus precatorius plant is extremely toxic and a potential bio-warfare agent. Till date there is no antidote or vaccine available against this toxin. The only known neutralizing monoclonal antibody against abrin, namely D6F10, has been shown to rescue the toxicity of abrin in cells as well as in mice. The present study focuses on mapping the epitopic region to understand the mechanism of neutralization of abrin by the antibody D6F10. Truncation and mutational analysis of abrin A chain revealed that the amino acids 74-123 of abrin A chain contain the core epitope and the residues Thr112, Gly114 and Arg118 are crucial for binding of the antibody. In silico analysis of the position of the mapped epitope indicated that it is present close to the active site cleft of abrin A chain. Thus, binding of the antibody near the active site blocks the enzymatic activity of abrin A chain, thereby rescuing inhibition of protein synthesis by the toxin in vitro. At 1∶10 molar concentration of abrin:antibody, the antibody D6F10 rescued cells from abrin-mediated inhibition of protein synthesis but did not prevent cell attachment of abrin. Further, internalization of the antibody bound to abrin was observed in cells by confocal microscopy. This is a novel finding which suggests that the antibody might function intracellularly and possibly explains the rescue of abrin's toxicity by the antibody in whole cells and animals. To our knowledge, this study is the first report on a neutralizing epitope for abrin and provides mechanistic insights into the poorly understood mode of action of anti-A chain antibodies against several toxins including ricin.

Antibody to ricin a chain hinders intracellular routing of toxin and protects cells even after toxin has been internalized

Background

Mechanisms of antibody-mediated neutralization are of much interest. For plant and bacterial A-B toxins, A chain mediates toxicity and B chain binds target cells. It is generally accepted and taught that antibody (Ab) neutralizes by preventing toxin binding to cells. Yet for some toxins, ricin included, anti-A chain Abs afford greater protection than anti-B. The mechanism(s) whereby Abs to the A chain neutralize toxins are not understood.

Methodology/Principal Findings

We use quantitative confocal imaging, neutralization assays, and other techniques to study how anti-A chain Abs function to protect cells. Without Ab, ricin enters cells and penetrates to the endoplasmic reticulum within 15 min. Within 45–60 min, ricin entering and being expelled from cells reaches equilibrium. These results are consistent with previous observations, and support the validity of our novel methodology. The addition of neutralizing Ab causes ricin accumulation at the cell surface, delays internalization, and postpones retrograde transport of ricin. Ab binds ricin for >6hr as they traffic together through the cell. Ab protects cells even when administered hours after exposure.

Conclusions/Key Findings

We demonstrate the dynamic nature of the interaction between the host cell and toxin, and how Ab can alter the balance in favor of the cell. Ab blocks ricin’s entry into cells, hinders its intracellular routing, and can protect even after ricin is present in the target organelle, providing evidence that the major site of neutralization is intracellular. These data add toxins to the list of pathogenic agents that can be neutralized intracellularly and explain the in vivo efficacy of delayed administration of anti-toxin Abs. The results encourage the use of post-exposure passive Ab therapy, and show the importance of the A chain as a target of Abs.

Immunity to ricin: fundamental insights into toxin-antibody interactions

Ricin toxin is an extraordinarily potent inducer of cell death and inflammation. Ricin is also a potent provocateur of the humoral immune system, eliciting a mixture of neutralizing, non-neutralizing and even toxin-enhancing antibodies. The characterization of dozens of monoclonal antibodies (mAbs) against the toxin's enzymatic (RTA) and binding (RTB) subunits has begun to reveal fundamental insights into the underlying mechanisms by which antibodies neutralize (or fail to neutralize) ricin in systemic and mucosal compartments. This information has had immediate applications in the design, development and evaluation of ricin subunit vaccines and immunotherapeutics.

Ricinus communis intoxications in human and veterinary medicine-a summary of real cases

Accidental and intended Ricinus communis intoxications in humans and animals have been known for centuries but the causative agent remained elusive until 1888 when Stillmark attributed the toxicity to the lectin ricin. Ricinus communis is grown worldwide on an industrial scale for the production of castor oil. As by-product in castor oil production ricin is mass produced above 1 million tons per year. On the basis of its availability, toxicity, ease of preparation and the current lack of medical countermeasures, ricin has gained attention as potential biological warfare agent. The seeds also contain the less toxic, but highly homologous Ricinus communis agglutinin and the alkaloid ricinine, and especially the latter can be used to track intoxications. After oil extraction and detoxification, the defatted press cake is used as organic fertilizer and as low-value feed. In this context there have been sporadic reports from different countries describing animal intoxications after uptake of obviously insufficiently detoxified fertilizer. Observations in Germany over several years, however, have led us to speculate that the detoxification process is not always performed thoroughly and controlled, calling for international regulations which clearly state a ricin threshold in fertilizer. In this review we summarize knowledge on intended and unintended poisoning with ricin or castor seeds both in humans and animals, with a particular emphasis on intoxications due to improperly detoxified castor bean meal and forensic analysis.

Protective immunity to ricin toxin conferred by antibodies against the toxin's binding subunit (RTB)

The B subunit (RTB) of ricin toxin is a galactose-/N-acetyl galactosamine-specific lectin that promotes attachment and entry of ricin into host cells. RTB is also the archetype of the so-called R-type lectin family, whose members include haemagglutinins of botulinum neurotoxin (BoNT) progenitor toxins, as well as the binding subunits of cytolethal distending toxins. Although RTB is an appealing subunit vaccine candidate, as well as a potential target for immunotherapeutics, the degree to which RTB immunization elicits protective antibodies against ricin toxin remains unresolved. To address this issue, groups of mice were immunized with RTB and then challenged with 5×LD(50)s of ricin administered intraperitoneally. Despite high RTB-specific serum antibody titers, groups of RTB immunized mice were only partially immune to ricin challenge. Analysis of a collection of RTB-specific B cell hybridomas suggested that only a small fraction of antibodies against RTB have demonstrable neutralizing activity. Two RTB-specific neutralizing monoclonal IgG(1) antibodies, 24B11 and SylH3, when passively administered to mice, were sufficient to protect the animals against a 5×LD(50) dose of ricin. Both 24B11 and SylH3 blocked ricin attachment to terminal galactose residues and prevented toxin binding to the surfaces of bone marrow-derived macrophages (BMM), suggesting that they function by steric hindrance and recognize epitopes located on RTB's carbohydrate recognition sub-domains (1α or 2γ). These data raise the possibility of using specific RTB sub-domains, rather than RTB itself, as antigens to more efficiently elicit neutralizing antibodies and protective immunity against ricin.

AB toxins: a paradigm switch from deadly to desirable

To ensure their survival, a number of bacterial and plant species have evolved a common strategy to capture energy from other biological systems. Being imperfect pathogens, organisms synthesizing multi-subunit AB toxins are responsible for the mortality of millions of people and animals annually. Vaccination against these organisms and their toxins has proved rather ineffective in providing long-term protection from disease. In response to the debilitating effects of AB toxins on epithelial cells of the digestive mucosa, mechanisms underlying toxin immunomodulation of immune responses have become the focus of increasing experimentation. The results of these studies reveal that AB toxins may have a beneficial application as adjuvants for the enhancement of immune protection against infection and autoimmunity. Here, we examine similarities and differences in the structure and function of bacterial and plant AB toxins that underlie their toxicity and their exceptional properties as immunomodulators for stimulating immune responses against infectious disease and for immune suppression of organ-specific autoimmunity.

Intradermal administration of RiVax protects mice from mucosal and systemic ricin intoxication

Ricin toxin is a CDC level B biothreat. We have developed a ricin vaccine, RiVax, which is a recombinant mutant of ricin A chain. RiVax is safe, immunogenic and protective in mice when administered intramuscularly (IM). We have now attempted to increase the utility and immunogenicity of RiVax by administering it intradermally (ID) with or without alum. Without alum, Rivax administered by the ID and IM routes was equally immunogenic and protective. With alum, ID vaccinations were more immunogenic and protective against both systemic and mucosal challenge with ricin and superior in protecting animals from ricin-induced lung damage.

A lyophilized formulation of RiVax, a recombinant ricin subunit vaccine, retains immunogenicity

Ricin is a CDC level B biothreat. Our recombinant ricin A chain vaccine (RiVax) contains two mutations, rendering it non-toxic at high doses. Frozen or alum formulations of RiVax protected mice against ricin administered by injection, gavage or aerosol. Without alum, RiVax was safe and immunogenic in rabbits and human volunteers. For military use, the predominant target group, it would be optimal not to require a cold chain for transport and storage. We have now developed a lyophilized formulation and demonstrated stability and efficacy for at least 1 year stored refrigerated or at room temperature administered with or without alum.

[Recombinant antibodies against bioweapons]

The threat posed by bioweapons (BW) could lead to the re-emergence of such deadly diseases as plague or smallpox, now eradicated from industrialized countries. The development of recombinant antibodies allows tackling this risk because these recombinant molecules are generally well tolerated in human medicine, may be utilized for prophylaxis and treatment, and because antibodies neutralize many BW. Recombinant antibodies neutralizing the lethal toxin of anthrax, botulinum toxins and the smallpox virus have in particular been isolated recently, with different technologies. Our approach, which uses phage-displayed immune libraries built from non-human primates (M. fascicularis) to obtain recombinant antibodies, which may later be super-humanized (germlinized), has allowed us to obtain such BWs-neutralizing antibodies.

Broad recognition of ricin toxins prepared from a range of Ricinus cultivars using immunochromatographic tests

INTRODUCTION

It is extremely important that diagnosis of persons exposed to ricin be achieved in a timely fashion for triage and appropriate treatment. A sensitive and specific immunochromatographic test (ICT) for ricin has been developed for this purpose and reported recently. This ICT detected ricin of a single cultivar, by naked eye, at concentrations as low as 1 ng/mL. However, there are many cultivars of the Ricinus communis plant, each producing ricin. Further, significant differences in ricin toxicity between two cultivars have been formerly demonstrated. Correspondingly, ricins extracted from different cultivars might exhibit variations in primary sequence or glycosylation, leading to differences in the limits of detectability. This was the purpose of this study.

METHODS

The ICT was used against solutions of ricin samples extracted from a wide range of cultivars to determine whether it could detect all the samples and to determine whether there were differences between the limits of visible detection using the ICT and also in the detection limits using a densitometer.

RESULTS

The ICT successfully identified ricins prepared from 19 Ricinus cultivars, all with a limit of visible detection between 1 and 2.5 ng/mL in buffer. This depended to some extent on whether the operator was experienced or not in reading the test.

DISCUSSION

This rapid and sensitive test provides a platform for the rapid diagnosis of ricin poisoning using relevant biological samples and will now be evaluated in animal models of inhalational exposure to ricin.

CONCLUSION

This sensitive and rapid test may provide an early diagnosis to inform the administration of a ricin antitoxin currently in development at Defence Science and Technology Laboratory in the United Kingdom.

Isolation of a human-like antibody fragment (scFv) that neutralizes ricin biological activity

Background

Ricin is a lethal toxin that inhibits protein synthesis. It is easily extracted from a ubiquitously grown plant, Ricinus communis, and thus readily available for use as a bioweapon (BW). Anti-ricin antibodies provide the only known therapeutic against ricin intoxication.

Results

In this study, after immunizing a non-human primate (Macaca fascicularis) with the ricin chain A (RTA), a phage-displayed immune library was built (2 × 10⁸ clones), that included the λ light chain fragment. The library was screened against ricin, and specific binders were sequenced and further analyzed. The best clone, 43RCA, was isolated using a new, stringent neutralization test. 43RCA had a high, picomolar affinity (41 pM) and neutralized ricin efficiently (IC₅₀ = 23 ± 3 ng/ml, corresponding to a [scFv]/[ricin] molar ratio of 4). The neutralization capacity of 43RCA compared favourably with that of polyclonal anti-deglycosylated A chain (anti-dgRCA) IgGs, obtained from hyperimmune mouse serum, which were more efficient than any monoclonal at our disposal. The 43RCA sequence is very similar to that for human IgG germline genes, with 162 of 180 identical amino acids for the VH and VL (90% sequence identity).

Conclusion

Results of the characterization studies, and the high degree of identity with human germline genes, altogether make this anti-ricin scFv, or an IgG derived from it, a likely candidate for use in humans to minimize effects caused by ricin intoxication.

A mouse model for ricin poisoning and for evaluating protective effects of antiricin antibodies

BACKGROUND. Ricin is a potential bioterrorism agent and no specific antidote or treatment exists for ricin poisoning. For this reason, we developed ricin-specific antibodies that were tested in a murine model of ricin poisoning for use as antidotes against symptoms of ricin poisoning.

METHODS

Mice were poisoned with a lethal dose of ricin (5 microg) and their temperature and general condition were monitored for determination of a surrogate and humane end point. Mice were then treated with injections of ricin and different combinations of polyclonal anti-ricin antibodies. Antibody effect was evaluated for various doses and using various time points from ricin to antibody injection. Also, the effect of adjuvant symptomatic treatment was examined. Brain, heart, intestines, kidney, liver, lung, pancreas, spleen, and stomach tissues were sampled for histopathological analysis.

RESULTS

The mouse model was reproducible and easy to use. A clear protective effect of both anti-ricin A-chain and anti-ricin B-chain antibodies-but not of irrelevant antibodies-was demonstrated with no added effect of symptomatic treatment.

CONCLUSIONS

These data suggest that specific polyclonal antibodies against ricin A- and B-chain may reproducibly protect mice against ricin poisoning, even when the antibodies are administered up to 1.5 h after poisoning.

Post-exposure targeting of specific epitopes on ricin toxin abrogates toxin-induced hypoglycemia, hepatic injury, and lethality in a mouse model

Effects in the liver of fatal intoxication with the binary toxin ricin are unclear. We report a robust neutrophil influx into the liver of C57BL/6 mice after lethal parenteral ricin challenge, occurring in peri-portal and centro-lobular hepatic areas within 2 h, followed by the abrupt disappearance of hepatic macrophages/Kupffer cells. Chemokine profiles determined by microarray, ribonuclease protection assays, northern blotting, and enzyme-linked immunosorbent assays showed rapid (2 h) upregulation and persistence of those for neutrophils (CXCL1/KC, CXCL2/MIP-2) and monocytes (CCL2/MCP-1). Red blood cell pooling (8-12 h), loss of hepatocyte glycogen (8-48 h) associated with progressive hypoglycemia, fibrin deposition (24-48 h), and death (72-96 h) followed. Monoclonal antibody to ricin A chain, administered intravenously, blunted hypoglycemia, and abrogated death. This outcome was observed when anti-ricin antibody was given before toxin exposure as well as when administered approximately 10 h after toxin exposure. Targeting antibody to specific amino-acid sequences on the ricin A chain (HAEL and QXXWXXA) was critical to the therapeutic effect. Re-emergence of liver macrophages/Kupffer cells and replenishment of glycogen in previously depleted hepatocytes preceded full recovery of the host. These data identify critical events for liver injury and healing in ricin intoxication, as well as a new means and specific targets for post-exposure therapeutic intervention.

Inhalation Toxicology of Ricin Preparations: Animal Models, Prophylactic and Therapeutic Approaches to Protection

Ricin is a toxin and seed protein produced by the castor oil plant, Ricinus communis. The toxin is a dimeric protein consisting of an enzymic A chain and a B chain with lectin properties aiding the uptake of the whole molecule into cells. Ricin has been considered a possible military threat for several decades and is now also of some concern as a terrorist agent. The inhalation route is of primary concern in these situations, although previous attacks with ricin have used other approaches. Medical countermeasures against ricin are urgently required and the strategy adopted has been first to understand the nature of the problem, in this case the inhalation toxicology of ricin, followed by the preparation of vaccine antigens. Toxoided ricin and modified recombinant A chain components have been examined in terms of efficacy as potential vaccine candidates in protection of animal models against inhaled ricin, primarily in laboratories both in the United Kingdom and in the United States. One recombinant A chain vaccine has been taken through to clinical trials in the United States and should become commercially available in the next few years. Toxoided ricin has also been used as an antigen to prepare antitoxin antibodies for therapeutic treatment following poisoning. In this review, a synopsis of the inhalation toxicology of ricin and approaches to medical prophylaxis and therapy of poisoning is given, based on work conducted at our laboratory and at other research institutes.

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 Neutralizing Monoclonal Antibody Against Both Ricin Toxin A and Ricin Toxin B, and Application of a Rapid Sandwich Enzyme-Linked Immunosorbent Assay

Two strains of neutralizing monoclonal antibodies (MAbs) anti-ricin, both B chain and A chain, named 3D74 and 4C13, were generated efficiently. The two antibodies recognized different epitopes located in a separated toxin structure domain characterized by enzyme-linked immunosorbent assay (ELISA) and Western blotting. 3D74 recognized space conformation epitope, whereas 4C13 recognized linearity epitope of ricin. 4C13 possessed a novel neutralizing ability; the safe period for intraperitoneal injection of 100 microg of antibody was 30 min after intraperitoneal injection of 2 microg of ricin (10 times LD50). Using 3D74 and 4C13 as pair-matching antibodies, we established sandwich ELISA and rapid ELISA for detection of ricin. The detection sensitivities were 31.3 and 156 pg/ml, respectively. Rapid sandwich ELISA was sufficiently sensitive and was performed easily for ricin detection without any special instruments.

Ricin: current understanding and prospects for an antiricin vaccine

Ricin is a potent cytotoxin that can be rapidly internalized into mammalian cells leading to cell death. The ease in obtaining the toxin and its deadly nature combine to implicate ricin as a convenient agent for bioterrorism. Research into the mechanism of toxicity, as well as strategies for treatment and protection from the toxin has been widely undertaken for a number of years. This article reviews the current understanding of the mechanism of action of the toxin, the clinical effects of ricin intoxication and how these relate to current and continuing prospects for vaccine development.

A novel neutralizing monoclonal antibody against cell-binding polypeptide of ricin

One strain of neutralizing monoclonal antibody (MAb) against cell-binding polypeptide of ricin, named 3E1, was generated efficiently. The antibody recognized the linearity epitope of RTB located in a toxin structure domain characterized by Western blotting. The safe period of mice for intraperitoneal injection of 100 microg of antibody was 20 min after intraperitoneal injection of 2 microg of Ricin (10 times LD50). The neutralizing MAb we obtained could be developed into an immunotherapeutic agent to counteract the use of ricin as a terrorist or biological warfare weapon. It might be useful, as well, for antibody-based prophylaxis.

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.

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.

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.

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.

Prediction of a conserved, neutralizing epitope in ribosome-inactivating proteins

The secondary structures, side-chain solvent accessibilities, and superpositioned crystal structures of the A-chain of ricin and four other plant rRNA N-glycosidases (ribosome-inactivating proteins, RIPs) were examined. Previously, a 26-residue fragment from the A-chain of ricin was determined to bind to a neutralizing monoclonal antibody. The region in the native ricin A-chain, to which this peptide corresponds, is solvent-exposed and contains a negatively charged residue that has been hypothesized to participate in the toxin's function, namely, rRNA binding and/or enzymatic activity. This region appears to be conserved in all of the structurally defined plant RIPs examined. Moreover, other plant RIPs, whose tertiary structures are, as yet, unknown, were predicted to have an analogous, solvent-exposed region containing a conserved, negatively charged residue. By analogy, these conserved structural and functional features lead to the suggestion that this exposed region represents a logical starting point for experiments designed to locate neutralizing epitopes in these RIPs. In contrast, the tertiary structure of the analogous region in a bacteria-derived RIP (Shiga toxin) is a less solvent-exposed, truncated loop and is a structure that is not as likely to be a neutralizing epitope. Because most of the amino acid residues are not conserved within this exposed region, these RIPs are predicted to be antigenically distinct.

Ricin toxin contains three lectin sites which contribute to its in vivo toxicity

Ricin intoxication of mammalian cells is initiated by B chain (RTB) binding to cell surface galactosides. Recombinant insect-derived RTB mutants with modifications in lectin-site subdomains 2 gamma, 1 alpha/2 gamma, and 1 alpha/1 beta/2 gamma were reassociated with plant RTA and tested for lethality in C57B1/6 6-8 weeks old mice. The LD50 of intraperitoneally injected castor bean ricin was 75 ng per 18 g mouse. The LD50 of single-site 2 gamma mutant heterodimer was 100 ng: the LD50 of the double-site 1 alpha/2 gamma mutant heterodimer was 500 ng, and the LD50 of the triple-site 1 alpha/1 beta,2 gamma mutant heterodimer was > 10 micrograms. Plant RTA alone had an LD50 of 300 micrograms. Animals died between 1 and 10 days post-injection. Histopathological examination of morbid animals receiving an LD50 dose of each toxin revealed only apoptosis in the thymus and spleen. The present data provide clear evidence for participation of three lectin sites in ricin in vivo toxicity. These results suggest an origin for some of the normal tissue toxicities observed with clinical trials of doubly blocked ricin conjugates and suggest modification of at least three RTB subdomains will be necessary in genetically engineered ricin fusion proteins.

Aerosolized specific antibody protects mice from lung injury associated with aerosolized ricin exposure

Parenteral vaccination with ricin toxoid, although protective against death after a lethal aerosol ricin challenge, only partially protects against lung lesions. Therefore, we tested whether passive protection with aerosolized specific anti-ricin IgG (goat polyclonal, affinity-purified) could protect against both lethality and lung lesions in unvaccinated mice. Healthy CD-l mice were administered antibody (Ab) by small particle aerosol. Group 1 received non-specific control Ab (2160 mg/min/m3), and groups 2 and 3 received anti-ricin IgG (960 and 3280 mg/min/m3, respectively). Each group was challenged with a lethal dose of aerosolized ricin 1 hr after Ab exposure. All group 1 (control Ab) mice developed diffuse airway epithelial necrosis, with severe interstitial edema and inflammation involving all lung lobes, and died 48-96 hr post-challenge (PC). In contrast, in groups 2 and 3 at 24 hr PC, lung lesions were absent to very mild although there was rare epithelial necrosis in the upper airways in both groups. By 48 hr PC, necrosis of the tracheal epithelium and peritracheal inflammation were noted in some group 3 mice only. By 4 days PC, lungs and airways did not differ from cage controls in most group 2 and 3 mice. Weight gain in group 2 and 3 mice paralleled that of control mice. At 14 days PC, lungs were no different in controls than in group 3 mice. However, two non-survivors in group 3 had obstructions due to proximal airway epithelial damage. All group 2 mice survived, although a mild lymphoplasmacytic perivasculitis was present at 14 days PC which was not noted in the group 3 mice.

Protection of mice from inhaled ricin by vaccination with ricin or by passive treatment with heterologous antibody

Mice were vaccinated subcutaneously with 25 micrograms kg-1 of ricin in the presence of Freund's complete adjuvant or Ribi adjuvant, followed by a boost 14 days later with 50 micrograms kg-1 ricin in Freund's incomplete adjuvant or Ribi adjuvant, respectively. Three subsequent boosts at 28-day intervals with 25 micrograms kg-1 ricin yielded high anti-ricin antibody titres as determined by ELISA. Vaccinated animals were exposed to an aerosolized LD99 dose of ricin. With the exception of one death not attributable to ricin intoxication, all vaccinated mice survived the lethal aerosol exposure. In addition, a passive protection regimen was evaluated in mice pretreated with 100 micrograms purified goat anti-ricin IgG administered intravenously, and then challenged with ricin intravenously. All were resistant to 125 micrograms kg-1 of ricin, a dose greater than 25 times the intravenous lethal dose. Mice injected intravenously with 5 mg of the same IgG were protected from a lethal aerosol challenge. These results indicated that it is possible to protect animals from inhaled ricin by vaccination or passive administration of specific antibodies.

Anti-ricin antibody protects against systemic toxicity without affecting suicide transport

Ricin is an effective suicide transport agent which reliably destroys sensory and motor neurons in anatomically selective fashion after peripheral nerve application. In the present study, we gave subcutaneous injections of commercially available antiricin antibody (ARA) at the time of ricin application to peripheral nerves. The ARA protected rats against systemic ricin poisoning without altering the suicide transport activity of ricin. This approach removes a significant barrier to the routine use of ricin and should prove useful when ricin is employed to ablate large peripheral nerves. However, antibody protection permits use of such large ricin doses that the toxin may diffuse within the CNS destroying neurons adjacent to those that project through the injected nerve. Consequently, antibody and ricin doses must be optimized for any particular experimental situation.

The preparation of deglycosylated ricin by recombination of glycosidase-treated A- and B-chains: effects of deglycosylation on toxicity and in vivo distribution

Deglycosylation of ricin may be necessary to prevent the entrapment of antibody-ricin conjugates in vivo by cells of the reticuloendothelial system which have receptors that recognise the oligosaccharide side chains on the A- and B-chains of the toxin. Carbohydrate-deficient ricin was therefore prepared by recombining the A-chain, which had been treated with alpha-mannosidase, with the B-chain, which had been treated with endoglycosidase H or alpha-mannosidase or both. By recombining treated and untreated chains, a series of ricin preparations was made having different carbohydrate moieties. The removal of carbohydrate from the B-chain did not affect the ability of the toxin to agglutinate erythrocytes, and alpha-mannosidase treatment of the A-chain did not affect its ability to inactivate ribosomes. The toxicity of ricin to cells in culture was only reduced in those preparations containing B-chain that had been treated with alpha-mannosidase, when a 75% decrease in toxicity was observed. The toxicity of the combined ricin preparation to mice varied from double to half that of native ricin, depending on the chain(s) treated and the enzymes used. Removal of carbohydrate greatly reduced the hepatic clearance of the toxin and the levels of toxin in the blood were correspondingly higher. These results suggest that antibody-ricin conjugates prepared from deglycosylated ricin would be cleared more slowly by the liver, inflict less liver damage, and have greater opportunity to reach their target.

The removal of carbohydrates from ricin with endoglycosidases H, F and D and alpha-mannosidase

Recently, several investigators have explored the possibility of targeting ricin to designated cell types in animals by its linkage to specific antibodies. There is evidence, however, that the mannose-containing oligosaccharide chains on ricin are recognised by reticuloendothelial cells in the liver and spleen and so cause the immunotoxins to be removed rapidly from the blood stream. In the present study we analysed the carbohydrate composition of ricin and examined enzymic methods for removing the carbohydrate. The carbohydrate analysis ricin A-chain revealed the presence of one residue of xylose and one of fucose in addition to mannose and N-acetylglucosamine which had been detected previously. The B-chain contained only mannose and N-acetylglycosamine. Ricin A-chain is heterogeneous containing two components of molecular weight 30 000 and 32 000. Strong evidence was found that the heavier form of the A-chain contains an extra carbohydrate unit which is heterogeneous with respect to concanavalin A binding and sensitivity to endoglycosidase H. The lower molecular weight form of A-chain did not bind concanavalin A and was insusceptible to endoglycosidases. Only one of the two high mannose oligosaccharide units on the isolated B-chain could be removed by endoglycosidases H or F, whereas both were removable after denaturation of the polypeptide by SDS. Both the isolated A- and B-chains were sensitive to alpha-mannosidase. Intact ricin was resistant to endoglycosidase treatment and was only slightly sensitive to alpha-mannosidase. The addition of SDS allowed endoglycosidase H to remove both of the B-chain oligosaccharides from intact ricin and increased the toxin's sensitivity to alpha-mannosidase. In conclusion, extensive enzymic deglycosylation of ricin may only be possible if the A- and B-chains are first separated, treated with enzymes and then recombined to form the toxin.