Protection against inhalation toxicity of ricin and abrin by immunisation

Gaps in forensic toxicological analysis: The veiled abrin

Abrus precatorius is an herbaceous, flowering plant that is widely distributed in tropical and subtropical regions. Its toxic component, known as abrin, is classified as one of the potentially significant biological warfare agents and bioterrorism tools due to its high toxicity. Abrin poisoning can be utilized to cause accidents, suicides, and homicides, which necessitates attention from clinicians and forensic scientists. Although a few studies have recently identified the toxicological and pharmacological mechanisms of abrin, the exact mechanism remains unclear. Furthermore, the clinical symptoms and pathological changes induced by abrin poisoning have not been fully characterized, and there is a lack of standardized methods for identifying biological samples of the toxin. Therefore, there is an urgent need for further toxicopathologic studies and the development of detection methods for abrin in the field of forensic medicine. This review provides an overview of the clinical symptoms, pathological changes, metabolic changes, toxicologic mechanisms, and detection methods of abrin poisoning from the perspective of forensic toxicology. Additionally, the evidence on abrin in the field of forensic toxicology and forensic pathology is discussed. Overall, this review serves as a reference for understanding the toxicological mechanism of abrin, highlighting the clinical applications of the toxin, and aiding in the diagnosis and forensic identification of toxin poisoning.

Characterization of Lung Injury following Abrin Pulmonary Intoxication in Mice: Comparison to Ricin Poisoning

Abrin is a highly toxic protein obtained from the seeds of the rosary pea plant Abrus precatorius, and it is closely related to ricin in terms of its structure and chemical properties. Both toxins inhibit ribosomal function, halt protein synthesis and lead to cellular death. The major clinical manifestations following pulmonary exposure to these toxins consist of severe lung inflammation and consequent respiratory insufficiency. Despite the high similarity between abrin and ricin in terms of disease progression, the ability to protect mice against these toxins by postexposure antibody-mediated treatment differs significantly, with a markedly higher level of protection achieved against abrin intoxication. In this study, we conducted an in-depth comparison between the kinetics of in vivo abrin and ricin intoxication in a murine model. The data demonstrated differential binding of abrin and ricin to the parenchymal cells of the lungs. Accordingly, toxin-mediated injury to the nonhematopoietic compartment was shown to be markedly lower in the case of abrin intoxication. Thus, profiling of alveolar epithelial cells demonstrated that although toxin-induced damage was restricted to alveolar epithelial type II cells following abrin intoxication, as previously reported for ricin, it was less pronounced. Furthermore, unlike following ricin intoxication, no direct damage was detected in the lung endothelial cell population following abrin exposure. Reduced impairment of intercellular junction molecules following abrin intoxication was detected as well. In contrast, similar damage to the endothelial surface glycocalyx layer was observed for the two toxins. We assume that the reduced damage to the lung stroma, which maintains a higher level of tissue integrity following pulmonary exposure to abrin compared to ricin, contributes to the high efficiency of the anti-abrin antibody treatment at late time points after exposure.

A Novel Humanized Anti-Abrin A Chain Antibody Inhibits Abrin Toxicity In Vitro and In Vivo

Abrin, a type-II ribosome inactivating protein from the seed of Abrus precatorius, is classified as a Category B bioterrorism warfare agent. Due to its high toxicity, ingestion by animals or humans will lead to death from multiple organ failure. Currently, no effective agents have been reported to treat abrin poisoning. In this study, a novel anti-abrin neutralizing antibody (S008) was humanized using computer-aided design, which possessed lower immunogenicity. Similar to the parent antibody, a mouse anti-abrin monoclonal antibody, S008 possessed high affinity and showed a protective effect against abrin both in vitro and in vivo, and protected mice that S008 was administered 6 hours after abrin. S008 was found that it did not inhibit entry of abrin into cells, suggesting an intracellular blockade capacity against the toxin. In conclusion, this work demonstrates that S008 is a high affinity anti-abrin antibody with both a neutralizing and protective effect and may be an excellent candidate for clinical treatment of abrin poisoning.

Induction of Acute Lung Injury after Intranasal Administration of Toxin Botulinum A Complex

The inhalation of aerozolized botulinum toxin may represent a potential significant hazard to both military and civilian personnel. Since the lung is the primary target organ for inhaled toxin, the investigation reported herein was conducted to examine lung function in mice exposed to botulinum toxin A complex by intranasal route. Data includes lethality, symptomatology, measurement of respiratory function (minute ventilation, respiratory frequency, and tidal volume), and histopathology of the lungs. The clinical signs of intoxication are similar to those observed in foodborne botulism. Plethysmography revealed severe impairment of all respiratory parameters tested from 7 hours postexposure. Severe lung lesions, possibly secondary to the intoxication, were observed in mice who survived 14 days after the toxin challenge. These included intra-alveolar hemorrhage and interstitial edema. Mice immunized by the pentavalent (ABCDE) toxoid were protected against the neurotoxin (4 LD50) as revealed by the decrease of lethality and severity of nervous signs of intoxication, but not against histopathological changes in the lungs. These effects are nonspecific and require further experiments in order to specify the relationships between the pathology and the inflammatory process in the lung due to mediators such as cytokines, and possibly permanent physiological sequelae.

A novel recombinant vaccine protecting mice against abrin intoxication

Abrin toxin (AT) consisting of an A chain and a B chain is a potential agent for bioterrorism and an effective vaccine against AT poisoning is urgently required. In this study, AT B chain (ATB) was successfully expressed in the Escherichia coli (E. coli) and assessed the protection capacity against AT intoxication. The recombinant ATB (rATB) subunit induces a good immune response after 4 immunizations. All BALB/c mice immunized intraperitoneally (i.p.) with the purified rATB protein survived after challenged with 5 × LD₅₀ of AT. Transfusion of sera from immunized mice provided passive protection in naive mice. Furthermore, histological findings showed that immunization with rATB decreased the severity of toxin-related tissue damage. This work indicates that the rATB protein may be a promising vaccine candidate against human exposure to AT.

Truncated abrin A chain expressed in Escherichia coli: a promising vaccine candidate

Abrin toxin (AT) is a highly potent toxin, and is classified as one of the most important biological warfare and bioterrorism agents. There is currently no approved vaccine for AT. Therefore, the development of an effective vaccine is important in the prevention of intoxication by abrin. In this study, five vectors containing different gene of truncated abrin toxin A chain (tATA) fragments were constructed, and two of them (tATA1(1-126), tATA4(1-188)) were successfully expressed as a soluble form in E.coli strain. Both of the two tATA retained most of their immunogenicity with either low or no toxic effects as determined by both in vitro and in vivo assays. They were used to immunize BALB/c mice three times at an interval of three weeks apart. As a result, the tATA1 can elicite 80% protective efficacy against i.p. challenge of 5×LD50 of abrin, and the tATA4 provides a better protection, which can elicite 100% protective efficacy against intraperitoneal challenge of 40×LD50 of abrin. The superior fragment (tATA4(1-188)) should be considered as a promising vaccine candidate for further investigations.

A recombinant chimeric protein containing B chains of ricin and abrin is an effective vaccine candidate

Both ricin toxin (RT) and abrin toxin (AT) are 2 important toxin agents as potantial bioweapons. A dual subunit vaccine against RT and AT exposure is a promising option for developing prophylactic vaccination. In this study, we constructed a dual vaccine with RT B chain and AT B chain named RTB-ATB. The RTB-ATB chimeric protein was expressed in Escherichia coli (E. coli), and the purified protein was used to evaluate the immune response by a 2 × 2 × 2 × 2 factorial design. The main effects included dose of RTB-ATB, route of immunization injection, immunization time interval, and dose of native toxins challenge. For 2 × LD(50) challenge of RT or AT, 100% of the RTB-ATB immunized mice survived and regained or exceeded their initial weights within 10 days. For 4 × LD(50) challenge, different routes of immunization injection caused significant difference (P < 0.05), intraperitoneal (i.p.) administration of immunogen protected mice better than the subcutaneous (s.c.) administration. In conclusion, when administered i.p. to mice with 25 μg per mouse and immunization time interval Π in the absence of adjuvant, the chimeric protein elicited a stronger immune response and protected the animals from a dose of native toxins which was 4 times higher than their LD(50) in unvaccinated mice. Besides, the RTB-ATB chimeric protein could induce specific neutralizing antibodies against these 2 toxins. We anticipate that this study will open new possibilities in the preparation of RTB-ATB dual subunit vaccine against the exposure to deadly RT and AT.

Structural insights into the neutralization mechanism of monoclonal antibody 6C2 against ricin

Ricin belongs to the type II ribosome-inactivating proteins that depurinate the universally conserved α-sarcin loop of rRNA. The RNA N-glycosidase activity of ricin also largely depends on the ribosomal proteins that play an important role during the process of rRNA depurination. Therefore, the study of the interaction between ricin and the ribosomal elements will be better to understand the catalysis mechanism of ricin. The antibody 6C2 is a mouse monoclonal antibody exhibiting unusually potent neutralizing ability against ricin, but the neutralization mechanism remains unknown. Here, we report the 2.8 Å crystal structure of 6C2 Fab in complex with the A-chain of ricin (RTA), which reveals an extensive antigen-antibody interface that contains both hydrogen bonds and van der Waals contacts. The complementarity-determining region loops H1, H2, H3, and L3 form a pocket to accommodate the epitope on the RTA (residues Asp(96)-Thr(116)). ELISA results show that Gln(98), Glu(99), Glu(102), and Thr(105) (RTA) are the key residues that play an important role in recognizing 6C2. With the perturbation of the 6C2 Fab-RTA interface, 6C2 loses its neutralization ability, measured based on the inhibition of protein synthesis in a cell-free system. Finally, we propose that the neutralization mechanism of 6C2 against ricin is that the binding of 6C2 hinders the interaction between RTA and the ribosome and the surface plasmon resonance and pulldown results confirm our hypothesis. In short, our data explain the neutralization mechanism of mAb 6C2 against ricin and provide a structural basis for the development of improved antibody drugs with better specificity and higher affinity.

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.

Passive and active vaccination strategies to prevent ricin poisoning

Ricin toxin (RT) is derived from castor beans, produced by the plant Ricinus communis. RT and its toxic A chain (RTA) have been used therapeutically to arm ligands that target disease-causing cells. In most cases these ligands are cell-binding monoclonal antibodies (MAbs). These ligand-toxin conjugates or immunotoxins (ITs) have shown success in clinical trials [1]. Ricin is also of concern in biodefense and has been classified by the CDC as a Class B biothreat. Virtually all reports of RT poisoning have been due to ingestion of castor beans, since they grow abundantly throughout the world and are readily available. RT is easily purified and stable, and is not difficult to weaponize. RT must be considered during any "white powder" incident and there have been documented cases of its use in espionage [2,3]. The clinical syndrome resulting from ricin intoxication is dependent upon the route of exposure. Countermeasures to prevent ricin poisoning are being developed and their use will depend upon whether military or civilian populations are at risk of exposure. In this review we will discuss ricin toxin, its cellular mode of action, the clinical syndromes that occur following exposure and the development of pre- and post-exposure approaches to prevent of intoxication.

Mucosal vaccines for biodefense

Bioterrorism is the deliberate release of biological toxins, pathogenic viruses, bacteria, parasites, or other infectious agents into the public sphere with the objective of causing panic, illness, and/or death on a local, regional, or possibly national scale. The list of potential biological agents compiled by the Centers for Disease Control and Prevention is long and diverse. However, a trait common to virtually all the potential bioterrorism agents is the fact that they are likely to be disseminated by either aerosol or in food/water supplies with the intention of targeting the mucosal surfaces of the respiratory or gastrointestinal tracts, respectively. In some instances, inhalation or ingestion would mimic the natural route by which humans are exposed to these agents. In other instances, (e.g., the inhalation of a toxin is normally associated with food borne illness), it would represent an unnatural route of exposure. For most potential bioterrorism agents, the respiratory or gastrointestinal mucosa may simply serve as a route of entry by which they gain access to the systemic compartment where intoxication/replication occurs. For others, however, the respiratory or gastrointestinal mucosa is the primary tissue associated with pathogenesis, and therefore, the tissue for which countermeasures must be developed.

Ricin vaccine development

In this chapter we discuss vaccines to protect against the highly toxic plant-derived toxin, ricin. Due to its prevalence, ease of use, and stability it has been used in sporadic incidents of espionage. There is also concern that it will be used as an agent of bioterrorism. As a result there has been a great deal of interest in developing a safe vaccine or antidote to protect humans, and in particular soldiers and first responders. Although multiple types of vaccines have been tested, at this time two recombinant vaccines are the leading candidates for the national vaccine stockpile. In terms of passive post-exposure protection, monoclonal neutralizing antibodies that passively protect animals are also under development. These vaccines and antibodies are discussed in the context of the toxicity and structure of ricin.

Purification, characterization and toxicity profile of ricin isoforms from castor beans

The castor seed contains the toxin ricin, one of the most poisonous naturally occurring toxins. The whole of the plant is poisonous, however the seeds are considered the major source of ricin. Ricin exists in different forms in beans of different origin. We investigated the presence of ricin in different isoforms and elucidate some of their structural and biological features isolated from the castor seeds. The isoforms were sub fractionated into ricin I, II and III by chromatography. Their molecular weights lie between 60-65 kDa with difference in their relative electrophoretic mobility. An acidic native PAGE of ricin isoforms at pH 2.9 was performed. Ricin I, II and III are highly cytotoxic against Vero cell line with IC(50) values of 60, 30 and 8 ng/ml respectively. Difference in cytotoxicity of isoforms was confirmed through hemagglutination assay, ricin III caused high degree of hemolysis. The preliminary in vivo toxicity studies showed that ricin III is highly toxic. Immunological studies revealed that anti-ricin I and II antibodies are cross reactive with all the ricin variants, whereas the anti-ricin III antibody is highly specific. The present study shows that anti-ricin I and II antibodies can be used for detection of entire ricin isoforms.

Novel therapeutic strategies for acute lung injury induced by lung damaging agents: the potential role of growth factors as treatment options

The increasing threat from terrorism has brought attention to the possible use of toxic industrial compounds (TICs) and other lung-damaging agents as weapons against civilian populations. The way in which these agents could be used favours the development of generic countermeasures. Improved medical countermeasures would increase survivability and improve the quality of recovery of lung damaged casualties. It is evident that there is a dearth of therapeutic regimes available to treat those forms of lung damage that currently require intensive care management. It is quite possible that mass casualties from a terrorist incident or major industrial accident involving the release of large quantities of inhaled TICs would place a severe burden on already scarce intensive care facilities. The development of effective pharmacological approaches to assist the recovery of casualties suffering from acute lung injury (ALI)/acute respiratory distress syndrome (ARDS) may improve the prognosis of such patients (which is currently poor) and would ideally be used as a means of preventing subjects from developing the pulmonary oedema characteristic of ALI/ARDS. Many promising candidate pharmacological treatments have been evaluated for the treatment of ALI/ARDS, but their clinical value is often debatable. Thus, despite improvements in ventilation strategies, pharmacological intervention for ALI/ARDS remains problematical. A new approach is clearly required for the treatment of patients with severely compromised lungs. Whilst the pathology of ALI/ARDS associated with exposure to a variety of agents is complex, numerous experimental studies suggest that generic therapeutic intervention directed at approaches that aim to upregulate repair of the damaged alveolar blood/air barrier of the lung may be of value, particularly with respect to chemical-induced injury. To this end, keratinocyte growth factor (KGF), epithelial growth factor (EGF) and basic fibroblast growth factor (bFGF) are emerging as the most important candidates. Hepatocyte growth factor (HGF) does not have epithelial specificity for lung tissue. However, the enhanced effects of combinations of growth factors, such as the synergistic effect of HGF upon vascular endothelial growth factor (VEGF)-mediated endothelial cell activity, and the combined effect of HGF and KGF in tissue repair should be investigated, particularly as the latter pair of growth factors are frequently implicated in processes associated with the repair of lung damage. Synergistic interactions also occur between trefoil factor family (TFF) peptides and growth factors such as EGF. TFF peptides are most likely to be of value as a short term therapeutic intervention strategy in stimulating epithelial spreading activities which allow damaged mucosal surfaces to be rapidly covered by epithelial cells.

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.

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.

Detection of abrin in food using enzyme-linked immunosorbent assay and electrochemiluminescence technologies

Abrin is a toxic ribosome-inactivating protein present in beans of Abrus precatorius, also known as rosary peas. The possibility that abrin could be used to adulterate food has made the development of assays for the detection of abrin a priority. Rabbit-derived polyclonal antibodies and mouse monoclonal antibodies were prepared against a mixture of abrin isozymes. The specificity and cross-reactivity of the antibodies were evaluated against a challenge library of 40 grains, nuts, legumes, and foods. An enzyme-linked immunosorbent assay (ELISA) and an electrochemiluminescence (ECL)-based assay were assembled and optimized. Polyclonal (capture) and polyclonal (detection) ELISAs, polyclonal and monoclonal ELISAs, and polyclonal and monoclonal ECL assays had limits of detection (LODs) of 0.1 to 0.5 ng/ml for abrin in buffer. The LOD for abrin dissolved into juices, dairy products, soda, chocolate drink, and condiments and analyzed with the ECL assay ranged from 0.1 to 0.5 ng/ml in the analytical sample. In contrast, the LODs for the ELISAs ranged from 0.5 to 10 ng/ml in the analytical sample.

Oropharyngeal aspiration of ricin as a lung challenge model for evaluation of the therapeutic index of antibodies against ricin A-chain for post-exposure treatment

To investigate the effectiveness of passive antibody treatment as post-exposure therapy for ricin, we had developed an oropharyngeal aspiration model for ricin lethal challenge and antibody administration. When polyclonal anti-deglycosylated ricin A-chain antibody (dgA Ab) was administered between 1-18 hr after ricin challenge, all animals survived while delayed treatment to 24 hr resulted in 30% survival. The protective effects of dgA Ab correlated with inhibition of apoptosis in the lungs in vivo and in RAW264.7 macrophage and Jurkat T cells in vitro. In addition, ricin-induced cell cytotoxicity was inhibited by both dgA Ab and RAC18 monoclonal antibody against ricin A-chain. Administration of RAC18 monoclonal antibody at 4, 18, and 24 hr after ricin exposure resulted in 100%, 60% and 50% protection, respectively, suggesting that the therapeutic window for passive vaccination extended to at least 24 hr post-ricin lung challenge.

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.

Intrapulmonary delivery of ricin at high dosage triggers a systemic inflammatory response and glomerular damage

In view of the possibility that ricin may be used as a bioweapon against human populations, we examined the pathological consequences that occur in mice after introduction of ricin into the pulmonary system. Intratracheal instillation of a lethal dose of ricin (20 microg/100 g body weight) resulted in a hemorrhagic inflammatory response in multiple organs, accompanied by activation of mitogen-activated protein kinases, increased synthesis of proinflammatory RNA transcripts, and increased levels of circulating cytokines and chemokines. A sublethal dose of instilled ricin (2 microg/100 g body weight) induced a similar response in lungs but did not cause detectable damage in other organs. Lungs of mice that recovered from a sublethal dose of ricin displayed evidence of fibrosis and residual damage. A lethal dose of ricin caused accumulation of proinflammatory RNA transcripts and substantial damage to 28S rRNA of multiple organs, including lung, kidney, spleen, liver, and blood, demonstrating that instilled ricin gained access to the circulation. The kidneys of mice instilled with a lethal dose of ricin showed accumulation of fibrin/fibrinogen in glomerular capillaries, increased numbers of glomerular leukocytes, and impairment of kidney function. A sublethal dose of ricin failed to induce damage to 28S rRNA in kidney or other extrapulmonary organs.

Ribosome depurination is not sufficient for ricin-mediated cell death in Saccharomyces cerevisiae

The plant toxin ricin is one of the most potent and lethal substances known. Ricin inhibits protein synthesis by removing a specific adenine from the highly conserved alpha-sarcin/ricin loop in the large rRNA. Very little is known about how ricin interacts with ribosomes and the molecular mechanism by which it kills cells. To gain insight to the mechanism of ricin-induced cell death, we set up yeast (Saccharomyces cerevisiae) as a simple and genetically tractable system to isolate mutants defective in cytotoxicity. Ribosomes were depurinated in yeast cells expressing the precursor form of the A chain of ricin (pre-RTA), and these cells displayed apoptotic markers such as nuclear fragmentation, chromatin condensation, and accumulation of reactive oxygen species. We conducted a large-scale mutagenesis of pre-RTA and isolated a panel of nontoxic RTA mutants based on their inability to kill yeast cells. Several nontoxic RTA mutants depurinated ribosomes and inhibited translation to the same extent as wild-type RTA in vivo. The mutant proteins isolated from yeast depurinated ribosomes in vitro, indicating that they were catalytically active. However, cells expressing these mutants did not display hallmarks of apoptosis. These results provide the first evidence that the ability to depurinate ribosomes and inhibit translation does not always correlate with ricin-mediated cell death, indicating that ribosome depurination and translation inhibition do not account entirely for the cytotoxicity of ricin.

Down-regulation of gene transcripts associated with ricin tolerance in human RPMI 2650 cells

The present study sought to determine if novel therapeutic approaches against ricin intoxication could be identified from human respiratory tract cells selected for increased resistance to this toxin. Initial studies indicated that the RPMI 2650 line was an appropriate model, owing to its sensitivity to ricin. Tolerant cultures were developed by exposing cells to a graded series of ricin concentrations from 6 to 192 pM. This resulted in the generation of cultures whose LC(50) values were increased by up to 4-fold following exposure to up to 96 pM ricin and by up to 6-fold following exposure to up to 192 pM ricin, compared to control cultures. DNA microarrays were employed to determine the gene transcript expression profile of cultures with increased resistance to ricin to investigate which gene products mediate ricin resistance. Transcripts (10) were identified that were greater than 2-fold down-regulated in the cells tolerant to 96 pM ricin, whereas 48 transcripts were seen to be down-regulated in cultures tolerant to 192 pM ricin. Gene transcripts (5) were up-regulated 2-fold or more in the 192 pM tolerant cultures in comparison to unexposed cells. The results indicate that ricin tolerance is the product of complex changes in gene expression profiles, most of which were found to involve down-regulation of transcript expression. It may be possible to modulate the gene expression profiles associated with ricin tolerance for potential therapeutic purposes using drugs and antisense technologies.

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.

Monoclonal antibodies to ricin: in vitro inhibition of toxicity and utility as diagnostic reagents

Monoclonal antibodies (MAbs) against ricin toxin (RT) and its subunits were produced in mice. The MAbs were initially selected based upon the ability to either bind ricin or the individual subunits in a solid-phase enzyme-linked immunosorbent assay (ELISA). Several candidates were selected for further evaluation, including their ability to inhibit ricin intoxication in vitro and their utility as immunodiagnostic reagents. Although their ability to capture antigen when bound to the solid phase was poor, some MAbs demonstrated potential utility as detection reagents in solid-phase immunoassays. Several MAbs were also able to inhibit ricin-mediated eukaryotic cell cytotoxicity in vitro. These MAbs may prove useful for preventing and/or treating ricin intoxication.

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.

Production of vaccines against leading biowarfare toxins can utilize DNA scientific technology

There are a significant number of different natural toxins that are potential biological warfare agents against which a vaccine is needed. DNA science has been a key to the development of potential vaccines against the top threat toxin and should contribute such effects for other toxin's vaccines. Several different DNA technological scientific techniques have been used to accomplish the general goals of (1) cloning of the toxin or large toxin fragments, (2) altering the specific gene sequence to obtain high level expression of vaccine candidate production in alternate species (3) placement of the vaccine gene in very different presentation types of species.

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.

Mechanism of ricin-induced apoptosis in human cervical cancer cells

The mechanism of ricin-induced apoptosis in human cervical cancer cell line HeLa was studied. The present study demonstrated that ricin induces apoptosis of human cervical cancer cells (HeLa) in a time dependent manner with an IC(50) for cell viability of 1 microg/ml. Ricin treatment resulted in a time dependent increase in LDH leakage, DNA fragmentation, percent apoptotic cells, generation of reactive oxygen species and depletion of intracellular glutathione levels. DNA agarose gel electrophoresis showed typical oligonucleosomal length DNA fragmentation. Additionally, DNA diffusion assay was performed to confirm DNA damage and apoptosis. Ricin activated caspase-3 as evidenced by both proteolytic cleavage of procaspase-3 into 20 and 18 kDa subunits, and increased protease activity. Caspase activity was maximum at 4h and led to the cleavage of 116 kDa poly(ADP-ribose) polymerase (PARP), resulting in the 85 kDa cleavage product. Ricin-induced caspase-3 activation also resulted in cleavage of DNA fragmentation factor-45 (DFF45/ICAD) and DFF40 or caspase-activated DNase in HeLa cells. Activation of caspase-3, cleavage of PARP and DNA fragmentation was blocked by pre-treatment with caspase-3 specific inhibitor Ac-DEVD-CHO (100 microM) and broad-spectrum caspase inhibitor Z-VAD-FMK (40 microM). Ricin-induced DNA fragmentation was inhibited by pre-treatment with PARP inhibitors 3-aminobenzamide (100 microM) and DPQ (10 microM). Our results indicate that ricin-induced cell death was mediated by generation of reactive oxygen species and subsequent activation of caspase-3 cascade followed by down stream events leading to apoptotic mode of cell death.

Ribosome-inactivating proteins

The main results of the research performed in the last 30 years on ribosome-inactivating proteins (RIPs) are reviewed, with emphasis on the new, controversial and uncertain aspects. The nature, distribution, mechanism of action and properties of these proteins are briefly reported, together with their possible applications. A pattern appears of a still largely unexplored subject, whose role in nature is probably important, and not limited to the biology of plants, since RIPs have been found also in other organisms.

Insertional mutagenesis of ricin A chain: a novel route to an anti-ricin vaccine

The insertion of a specific 25-residue internal peptide into ricin toxin A chain (RTA) reduced the catalytic activity of this protein approximately 300-fold. Directed proteolytic cleavage of the peptide insert essentially restored catalytic activity of the resulting two peptide A chain to normal levels. Ricin holotoxin containing unprocessed mutant A chain was not toxic to cultured mammalian cells, due to enhanced proteasomal degradation, nor was it toxic when injected into rats at a concentration that is lethal in the case of native ricin. Rats treated in this way were completely resistant to native ricin when subsequently challenged with a potentially lethal dose of the toxin. These ricin-resistant animals had a significant anti-ricin antibody titer, indicating that this approach has potential for developing an effective vaccine against this toxin.

Ricin Poisoning

Ricin is a naturally occurring toxin derived from the beans of the castor oil plant Ricinus communis. It is considered a potential chemical weapon. Ricin binds to cell surface carbohydrates, is internalised then causes cell death by inhibiting protein synthesis. Oral absorption is poor and absorption through intact skin most unlikely; the most hazardous routes of exposure being inhalation and injection. Features of toxicity mainly reflect damage to cells of the reticuloendothelial system, with fluid and protein loss, bleeding, oedema and impaired cellular defence against endogenous toxins. It has been estimated that in man, the lethal dose by inhalation (breathing in solid or liquid particles) and injection (into muscle or vein) is approximately 5-10 micrograms/kg, that is 350-700 micrograms for a 70 kg adult. Death has ensued within hours of deliberate subcutaneous injection. Management is supportive. Prophylactic immunisation against ricin toxicity is a developing research initiative, although presently not a realistic option in a civilian context.

Abrin Poisoning

Abrin is a toxic protein obtained from the seeds of Abrus precatorius (jequirity bean), which is similar in structure and properties to ricin. Abrin is highly toxic, with an estimated human fatal dose of 0.1-1 microgram/kg, and has caused death after accidental and intentional poisoning. Abrin can be extracted from jequirity beans using a relatively simple and cheap procedure. This satisfies one criterion of a potential chemical warfare agent, although the lack of large scale production of jequirity seeds means that quantity is unavailable for ready mass production of abrin for weapons. This contrasts with the huge cultivation of Ricinus seeds for castor oil production. At the cellular level, abrin inhibits protein synthesis, thereby causing cell death. Many of the features observed in abrin poisoning can be explained by abrin-induced endothelial cell damage, which causes an increase in capillary permeability with consequent fluid and protein leakage and tissue oedema (the so-called vascular leak syndrome). Most reported cases of human poisoning involve the ingestion of jequirity beans, which predominantly cause gastrointestinal toxicity. Management is symptomatic and supportive. Experimental studies have shown that vaccination with abrin toxoid may offer some protection against a subsequent abrin challenge, although such an approach is unlikely to be of benefit in a civilian population that in all probability would be unprotected.

Biological consequences of multiple vaccine and pyridostigmine pretreatment in the guinea pig

An investigation of the possible interactions between combinations of vaccines and pyridostigmine bromide (PB) has been undertaken in the guinea pig. This study is part of a research programme funded by the UK Government to determine any effects of the pretreatment regimes given to UK Forces during the Persian Gulf conflict of 1990-1991. The study was designed to simulate PB administration and to model multiple vaccination protocols that were experienced by UK Forces, modelling a "worst case" situation in which all ten vaccines and PB were administered within a short period of time. Seven of the vaccines were health and hygiene (H+H) vaccines given to protect against endemic diseases and two vaccines to protect against the biological warfare agents anthrax and plague. In addition, pertussis vaccine was administered as an adjuvant to reduce the time to achieve immunity against anthrax. Four groups of eight animals were treated with 1/20th, 1/10th or 1/5th human doses of vaccines or vehicles, respectively. The PB or saline was delivered by implanted 28 day mini-osmotic pumps to achieve a mean red blood cell acetylcholinesterase (AChE) inhibition of around 30%. Body weight, temperature, immunological response, biochemical indices and spontaneous activity were monitored for 72 days. Although immunological responses to bacterial vaccines were observed, there were no remarkable findings in the parameters measured other than minor changes in body weight (4.9% decrease at the 1/5th human dose of vaccines) and temperature increases in response to vaccination. Animals in all groups remained generally healthy and active without visible adverse signs throughout the study. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.

Comparison of the quality of protection elicited by toxoid and peptide liposomal vaccine formulations against ricin as assessed by markers of inflammation

Ricin is a very toxic substance which inhibits protein synthesis and produces severe tissue damage and inflammation. It is very potent when inhaled as an aerosol and protection has been examined in a series of studies using vaccine candidates including a formaldehyde inactivated ricin toxoid and the A chain of ricin, a polypeptide equivalent to half of the toxin molecule. Initially, subcutaneous injections of both compounds were found to protect against inhaled ricin but not without some subsequent adverse signs. Intra-pulmonary vaccination using liposomal formulations of these compounds was investigated with a view to improving lung condition following challenge. Using the humoral and local pulmonary immune responses as indices of vaccine performance, no significant difference between toxoid or peptide vaccines was found. In the third and current study, the quality of lung protection by vaccines was assessed using markers of inflammation. Thus, the profiles of inflammatory cell and protein influx into the lung were determined following intratracheal (i.t.) challenge with ricin of rats treated with liposomal vaccine formulations. Results showed that liposomal ricin toxoid offered a better quality of protection with a significantly lower influx of polymorphonuclear leucocytes (neutrophils) and little pulmonary oedema compared with the A chain/liposome formulation. Further, there was no significant difference between the quality of protection offered by the A chain when administered subcutaneously or locally into the lung by i.t. instillation. Liposomal ricin toxoid is a good candidate vaccine and optimised pulmonary delivery by inhalation should be further examined.

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.