Antibody/doxycycline combined therapy for pulmonary ricinosis: Attenuation of inflammation improves survival of ricin-intoxicated mice

Short- and long-term outcomes of pulmonary exposure to a sublethal dose of ricin in mice

Ricin, an extremely potent toxin produced from the seeds of castor plant, Ricinus communis, is ribosome-inactivating protein that blocks cell-protein synthesis. It is considered a biological threat due to worldwide availability of castor beans, massive quantities as a by-product of castor oil production, high stability and ease of production. The consequence of exposure to lethal dose of ricin was extensively described in various animal models. However, it is assumed that in case of aerosolized ricin bioterror attack, the majority of individuals would be exposed to sublethal doses rather than to lethal ones. Therefore, the purpose of current study was to assess short- and long-term effects on physiological parameters and function following sublethal pulmonary exposure. We show that in the short-term, sublethal exposure of mice to ricin resulted in acute lung injury, including interstitial pneumonia, cytokine storm, neutrophil influx, edema and cellular death. This damage was manifested in reduced lung performance and physiological function. Interestingly, although in the long-term, mice recovered from acute lung damage and restored pulmonary and physiological functionality, the reparative process was associated with lasting fibrotic lesions. Therefore, restriction of short-term acute phase of the disease and management of long-term pulmonary fibrosis by medical countermeasures is expected to facilitate the quality of life of exposed survivors.

Caspase-3/Gasdermin E-mediated pyroptosis contributes to Ricin toxin-induced inflammation

Ricin toxin (RT) is highly cytotoxic and can release a considerable amount of pro-inflammatory factors due to depurination, causing excessive inflammation that may aggravate the harm to the body. Pyroptosis, a type of gasdermin-mediated cell death, is a contributor to the exacerbation of inflammation. Accumulating evidence indicate that pyroptosis plays a significant role in the pathogen infection and tissue injury, suggesting a potential correlation between pyroptosis and RT-induced inflammation. Here, we aim to demonstrate this correlation and explore its molecular mechanisms. Results showed that RT triggers mouse alveolar macrophage MH-S cells pyroptosis by activating caspase-3 and cleaving Gasgermin E (GSDME). In contrast, inhibition of caspase-3 with Z-DEVD-FMK (inhibitor of caspase-3) or knockdown of GSDME attenuates this process, suggesting the essential role of caspase-3/GSDME-mediated pyroptosis in contributing to RT-induced inflammation. Collectively, our study enhances our understanding of a novel mechanism of ricin cytotoxicity, which may emerge as a potential target in immunotherapy to control the RT-induced inflammation.

Long-Term Pulmonary Damage in Surviving Antitoxin-Treated Mice following a Lethal Ricin Intoxication

Ricin, a highly potent plant-derived toxin, is considered a potential bioterrorism weapon due to its pronounced toxicity, high availability, and ease of preparation. Acute damage following pulmonary ricinosis is characterized by local cytokine storm, massive neutrophil infiltration, and edema formation, resulting in respiratory insufficiency and death. A designated equine polyclonal antibody-based (antitoxin) treatment was developed in our laboratory and proved efficacious in alleviating lung injury and increasing survival rates. Although short-term pathogenesis was thoroughly characterized in antitoxin-treated mice, the long-term damage in surviving mice was never determined. In this study, long-term consequences of ricin intoxication were evaluated 30 days post-exposure in mice that survived antitoxin treatment. Significant pulmonary sequelae were demonstrated in surviving antitoxin-treated mice, as reflected by prominent histopathological changes, moderate fibrosis, increased lung hyperpermeability, and decreased lung compliance. The presented data highlight, for the first time to our knowledge, the possibility of long-term damage development in mice that survived lethal-dose pulmonary exposure to ricin due to antitoxin treatment.

Clinically Evaluated COVID-19 Drugs with Therapeutic Potential for Biological Warfare Agents

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) outbreak resulted in hundreds of millions of coronavirus cases, as well as millions of deaths worldwide. Coronavirus Disease 2019 (COVID-19), the disease resulting from exposure to this pathogen, is characterized, among other features, by a pulmonary pathology, which can progress to "cytokine storm", acute respiratory distress syndrome (ARDS), respiratory failure and death. Vaccines are the unsurpassed strategy for prevention and protection against the SARS-CoV-2 infection. However, there is still an extremely high number of severely ill people from at-risk populations. This may be attributed to waning immune response, variant-induced breakthrough infections, unvaccinated population, etc. It is therefore of high importance to utilize pharmacological-based treatments, despite the progression of the global vaccination campaign. Until the approval of Paxlovid, an efficient and highly selective anti-SARS-CoV-2 drug, and the broad-spectrum antiviral agent Lagevrio, many pharmacological-based countermeasures were, and still are, being evaluated in clinical trials. Some of these are host-directed therapies (HDTs), which modulate the endogenic response against the virus, and therefore may confer efficient protection against a wide array of pathogens. These could potentially include Biological Warfare Agents (BWAs), exposure to which may lead to mass casualties due to disease severity and a possible lack of efficient treatment. In this review, we assessed the recent literature on drugs under advanced clinical evaluation for COVID-19 with broad spectrum activity, including antiviral agents and HDTs, which may be relevant for future coping with BWAs, as well as with other agents, in particular respiratory infections.

Autophagic Degradation Is Involved in Cell Protection against Ricin Toxin

Autophagy is a complex and highly regulated degradative process, which acts as a survival pathway in response to cellular stress, starvation and pathogen infection. Ricin toxin is a plant toxin produced by the castor bean and classified as a category B biothreat agent. Ricin toxin inhibits cellular protein synthesis by catalytically inactivating ribosomes, leading to cell death. Currently, there is no licensed treatment for patients exposed to ricin. Ricin-induced apoptosis has been extensively studied; however, whether its intoxication via protein synthesis inhibition affects autophagy is not yet resolved. In this work, we demonstrated that ricin intoxication is accompanied by its own autophagic degradation in mammalian cells. Autophagy deficiency, by knocking down ATG5, attenuates ricin degradation, thus aggravating ricin-induced cytotoxicity. Additionally, the autophagy inducer SMER28 (Small Molecule Enhancer 28) partially protects cells against ricin cytotoxicity, an effect not observed in autophagy-deficient cells. These results demonstrate that autophagic degradation acts as a survival response of cells against ricin intoxication. This suggests that stimulation of autophagic degradation may be a strategy to counteract ricin intoxication.

Csf2ra deletion attenuates acute lung injuries induced by intratracheal inoculation of aerosolized ricin in mice

Specific therapeutics are not available for acute lung injury (ALI) induced by ricin toxin (RT). Inhibiting the host immune response in the course of pulmonary ricinosis is hypothesized to be of benefit and can be achieved by impairing granulocyte-macrophage colony-stimulating factor (GM-CSF) signaling, thereby reducing the pro-inflammatory response to exogenous foreign body invasion. However, it is unknown whether mice with impaired GM-CSF signaling can survive after RT inhalation. To test this, colony stimulating factor 2 receptor alpha (Csf2ra) knockout (KO) mice that lack GM-CSF signaling and wild-type (WT) mice models of intratracheal exposure to a lethal dose (2× LD50) of RT were established. Survival was greater in Csf2ra KO mice 21 days after RT inhalation compared with WT mice. Highly co-expressed genes that probably attenuated the pro-inflammatory response in the lung of Csf2ra KO mice were identified. Bioinformatics analysis revealed that transcriptome changes involved mostly inflammation-related genes after RT exposure in both Csf2ra KO mice and WT mice. However, the activity levels of pro-inflammatory pathways, such as the TNF signaling pathway and NF-κB signaling pathway, in Csf2ra KO mice were significantly decreased and the degree of neutrophil chemotaxis and recruitment inhibited after RT-exposure relative to WT mice. RT-qPCR and flow cytometry validated results of RNA-Seq analysis. This work provides potential avenues for host-directed therapeutic applications that can mitigate the severity of ALI-induced by RT.

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.

Pathophysiological profile of awake and anesthetized pigs following systemic exposure to the highly lethal ricin toxin

Ricin, a plant-derived toxin originating from the seeds of Ricinus communis (castor bean plant), is one of the most lethal toxins known. To date, no in-depth study of systemic exposure to ricin in a standardized large animal model has been reported. This study details for the first time the pathophysiological hemodynamic profile following systemic/intramuscular exposure to the ricin toxin in a porcine model by comprehensive cardiorespiratory monitoring of awake and anesthetized pigs. Unlike respiratory exposure to ricin, which is characterized by the development of acute respiratory distress syndrome, following intramuscular exposure to ricin respiratory parameters were grossly unaffected, however the hemodynamics of both awake and anesthetize pigs were unsustainably compromised. We show that in the early phase until approximately 24 h post-exposure, cardiac output is not impaired although one of its components, stroke volume, is relatively low. This is due to compensatory increase in heart rate, which eventually becomes insufficient. Later, distributive shock develops, characterized by severe vasodilatation (decreased systemic vascular resistance), low central venous oxygen saturation and elevation of venous-to-arterial carbon dioxide difference indicating increase in tissue oxygen demand not met by cardiac supply. These findings serve as a basis for further studies to evaluate the ability of supportive treatments such as vasoactive and inotropic drugs, to postpone the hemodynamic deterioration and thus expand the therapeutic window for the anti-ricin treatment. Such studies are of crucial importance for judicious treatment of victims of acts of bioterrorism or of intentional self-poisoning.

Mice with induced pulmonary morbidities display severe lung inflammation and mortality following exposure to SARS-CoV-2

Mice are normally unaffected by SARS coronavirus 2 (SARS-CoV-2) infection since the virus does not bind effectively to the murine version of the angiotensin-converting enzyme 2 (ACE2) receptor molecule. Here, we report that induced mild pulmonary morbidities rendered SARS-CoV-2–refractive CD-1 mice susceptible to this virus. Specifically, SARS-CoV-2 infection after application of low doses of the acute lung injury stimulants bleomycin or ricin caused severe disease in CD-1 mice, manifested by sustained body weight loss and mortality rates greater than 50%. Further studies revealed markedly higher levels of viral RNA in the lungs, heart, and serum of low-dose ricin–pretreated mice compared with non-pretreated mice. Furthermore, lung extracts prepared 2–3 days after viral infection contained subgenomic mRNA and virus particles capable of replication only when derived from the pretreated mice. The deleterious effects of SARS-CoV-2 infection were effectively alleviated by passive transfer of polyclonal or monoclonal antibodies generated against the SARS-CoV-2 receptor binding domain (RBD). Thus, viral cell entry in the sensitized mice seems to depend on viral RBD binding, albeit by a mechanism other than the canonical ACE2-mediated uptake route. This unique mode of viral entry, observed over a mildly injured tissue background, may contribute to the exacerbation of coronavirus disease 2019 (COVID-19) pathologies in patients with preexisting morbidities.

Ricin Antibodies' Neutralizing Capacity against Different Ricin Isoforms and Cultivars

Ricin, a highly toxic protein from Ricinus communis, is considered a potential biowarfare agent. Despite the many data available, no specific treatment has yet been approved. Due to their ability to provide immediate protection, antibodies (Abs) are an approach of choice. However, their high specificity might compromise their capacity to protect against the different ricin isoforms (D and E) found in the different cultivars. In previous work, we have shown the neutralizing potential of different Abs (43RCA-G1 (anti ricin A-chain) and RB34 and RB37 (anti ricin B-chain)) against ricin D. In this study, we evaluated their protective capacity against both ricin isoforms. We show that: (i) RB34 and RB37 recognize exclusively ricin D, whereas 43RCA-G1 recognizes both isoforms, (ii) their neutralizing capacity in vitro varies depending on the cultivar, and (iii) there is a synergistic effect when combining RB34 and 43RCA-G1. This effect is also demonstrated in vivo in a mouse model of intranasal intoxication with ricin D/E (1:1), where approximately 60% and 40% of mice treated 0 and 6 h after intoxication, respectively, are protected. Our results highlight the importance of evaluating the effectiveness of the Abs against different ricin isoforms to identify the treatment with the broadest spectrum neutralizing effect.

Disease-modifying treatment of chemical threat agent-induced acute lung injury

Acute respiratory distress syndrome (ARDS) is a highly morbid lung pathology induced by exposure to chemical warfare agents, including vesicants, phosgene, chlorine, and ricin. In this review, we describe the pathology associated with the development of ARDS in humans and experimental models of acute lung injury following animal exposure to these high-priority threat agents. Potential future approaches to disease-modifying treatment used in preclinical animal studies, including antioxidants, anti-inflammatories, biologics, and mesenchymal stem cells, are also described. As respiratory pathologies, including ARDS, are the major cause of morbidity and mortality following exposure to chemical threat agents, understanding mechanisms of disease pathogenesis is key to the development of efficacious therapeutics beyond the primary intervention principle, which remains mechanical ventilation.

Post-Exposure Anti-Ricin Treatment Protects Swine Against Lethal Systemic and Pulmonary Exposures

Ricin, a plant-derived toxin originating from the seeds of Ricinus communis (castor bean plant), is one of the most lethal toxins known. To date, there is no approved post-exposure therapy for ricin exposures. This work demonstrates for the first time the therapeutic efficacy of equine-derived anti-ricin F(ab’)₂ antibodies against lethal pulmonary and systemic ricin exposures in swine. While administration of the antitoxin at 18 h post-exposure protected more than 80% of both intratracheally and intramuscularly ricin-intoxicated swine, treatment at 24 h post-exposure protected 58% of the intramuscular-exposed swine, as opposed to 26% of the intratracheally exposed animals. Quantitation of the anti-ricin neutralizing units in the anti-toxin preparations confirmed that the disparate protection conferred to swine subjected to the two routes of exposure stems from variance between the two models. Furthermore, dose response experiments showed that approximately 3 times lesser amounts of antibody are needed for high-level protection of the intramuscularly compared to the intratracheally intoxicated swine. This study, which demonstrates the high-level post-exposure efficacy of anti-ricin antitoxin at clinically relevant time-points in a large animal model, can serve as the basis for the formulation of post-exposure countermeasures against ricin poisoning in humans.

Equal Neutralization Potency of Antibodies Raised against Abrin Subunits

Abrin and ricin are potent AB toxins, which are considered biological threats. To date, there are no approved treatments against abrin or ricin intoxications. Previously, we showed that the administration of polyclonal anti-abrin antibodies to mice that were intranasally exposed to abrin, even very late post-exposure, conferred an exceedingly high-level of protection, while following ricin intoxication, similar treatment with anti-ricin antibodies resulted in negligible survival rates. To probe this unexpected difference in protection ability, we first examined whether the efficient anti-abrin-induced protection was due to neutralization of the A-subunit responsible for the catalytic effect, or of the B-subunit, which enables binding/internalization, by evaluating the protection conferred by antibodies directed against one of the two subunits. To this end, we generated and immunized rabbits with chimeric toxins containing a single abrin subunit, A_abrinB_ricin in which abrin A-subunit was linked to ricin B-subunit, and A_ricinB_abrin in which ricin A-subunit is linked to abrin B-subunit. Here, we show that antibodies raised against either A_abrinB_ricin or A_ricinB_abrin conferred exceptionally high protection levels to mice following intranasal exposure to a a lethal dose of abrin, suggesting that the high level of protection conferred by anti-abrin antibodies is not related to the neutralization of a particular subunit.

Intramuscular Ricin Poisoning of Mice Leads to Widespread Damage in the Heart, Spleen, and Bone Marrow

Ricin, a lethal toxin derived from castor oil beans, is a potential bio-threat due to its high availability and simplicity of preparation. Ricin is prepared according to simple recipes available on the internet, and was recently considered in terrorist, suicide, or homicide attempts involving the parenteral route of exposure. In-depth study of the morbidity developing from parenteral ricin poisoning is mandatory for tailoring appropriate therapeutic measures to mitigate ricin toxicity in such instances. The present study applies various biochemical, hematological, histopathological, molecular, and functional approaches to broadly investigate the systemic effects of parenteral intoxication by a lethal dose of ricin in a murine model. Along with prompt coagulopathy, multi-organ hemorrhages, and thrombocytopenia, ricin induced profound morpho-pathological and functional damage in the spleen, bone marrow, and cardiovascular system. In the heart, diffuse hemorrhages, myocyte necrosis, collagen deposition, and induction in fibrinogen were observed. Severe functional impairment was manifested by marked thickening of the left ventricular wall, decreased ventricular volume, and a significant reduction in stroke volume and cardiac output. Unexpectedly, the differential severity of the ricin-induced damage did not correlate with the respective ricin-dependent catalytic activity measured in the various organs. These findings emphasize the complexity of ricin toxicity and stress the importance of developing novel therapeutic strategies that will combine not only anti-ricin specific therapy, but also will target ricin-induced indirect disturbances.

Neutrophil mediated inflammatory lung damage following single Sub lethal inhalation exposure to plant protein toxin abrin in mice

Abrin, a highly toxic plant protein found in the seeds of Abrus precatorius plant. To date, there is no antidote against abrin intoxication. Abrin is toxic by all routes of exposure, but inhalation exposure is the most toxic of all routes. Present study was conducted to evaluate the acute inhalation toxicity of aerosolized abrin in BALB/c mice. Animals were exposed to 0.2 and 0.8LC50 doses of aerosolized abrin and evaluated at 1 and 3 day post toxin exposure. Bronchoalveolar fluid from lungs was used for evaluation of markers for lung injury. Abrin inhalation exposure caused rise in LDH activity, protein content, increase in β-glucuronidase and myeloperoxidase activity. Increase in CRP activity, MMP-9 expression and recruitment of CD11b + inflammatory cells in lungs was also observed which was associated with severe inflammation and lung damage. Histopathological findings support the lung damage after abrin exposure. Our results indicate lung injury after single aerosol inhalation exposure, associated with excessive inflammation, oxidative stress, pulmonary edema followed by lung damage. These results could supplement treatment strategies and planning for therapeutic approaches against aerosolized abrin inhalation exposure.

Ricin: An Ancient Story for a Timeless Plant Toxin

The castor plant (Ricinus communis L.) has been known since time immemorial in traditional medicine in the pharmacopeia of Mediterranean and eastern ancient cultures. Moreover, it is still used in folk medicine worldwide. Castor bean has been mainly recommended as anti-inflammatory, anthelmintic, anti-bacterial, laxative, abortifacient, for wounds, ulcers, and many other indications. Many cases of human intoxication occurred accidentally or voluntarily with the ingestion of castor seeds or derivatives. Ricinus toxicity depends on several molecules, among them the most important is ricin, a protein belonging to the family of ribosome-inactivating proteins. Ricin is the most studied of this category of proteins and it is also known to the general public, having been used for several biocrimes. This manuscript intends to give the reader an overview of ricin, focusing on the historical path to the current knowledge on this protein. The main steps of ricin research are here reported, with particular regard to its enzymatic activity, structure, and cytotoxicity. Moreover, we discuss ricin toxicity for animals and humans, as well as the relation between bioterrorism and ricin and its impact on environmental toxicity. Ricin has also been used to develop immunotoxins for the elimination of unwanted cells, mainly cancer cells; some of these immunoconjugates gave promising results in clinical trials but also showed critical limitation.

Characterization of MicroRNA and Gene Expression Profiles Following Ricin Intoxication

Ricin, derived from the castor bean plant, is a highly potent toxin, classified as a potential bioterror agent. Current methods for early detection of ricin poisoning are limited in selectivity. MicroRNAs (miRNAs), which are naturally occurring, negative gene expression regulators, are known for their tissue specific pattern of expression and their stability in tissues and blood. While various approaches for ricin detection have been investigated, miRNAs remain underexplored. We evaluated the effect of pulmonary exposure to ricin on miRNA expression profiles in mouse lungs and peripheral blood mononuclear cells (PBMCs). Significant changes in lung tissue miRNA expression levels were detected following ricin intoxication, specifically regarding miRNAs known to be involved in innate immunity pathways. Transcriptome analysis of the same lung tissues revealed activation of several immune regulation pathways and immune cell recruitment. Our work contributes to the understanding of the role of miRNAs and gene expression in ricin intoxication.

Whole-Cell Multiparameter Assay for Ricin and Abrin Activity-Based Digital Holographic Microscopy

Ricin and abrin are ribosome-inactivating proteins leading to inhibition of protein synthesis and cell death. These toxins are considered some of the most potent and lethal toxins against which there is no available antidote. Digital holographic microscopy (DHM) is a time-lapse, label-free, and noninvasive imaging technique that can provide phase information on morphological features of cells. In this study, we employed DHM to evaluate the morphological changes of cell lines during ricin and abrin intoxication. We showed that the effect of these toxins is characterized by a decrease in cell confluence and changes in morphological parameters such as cell area, perimeter, irregularity, and roughness. In addition, changes in optical parameters such as phase-shift, optical thickness, and effective-calculated volume were observed. These effects were completely inhibited by specific neutralizing antibodies. An enhanced intoxication effect was observed for preadherent compared to adherent cells, as was detected in early morphology changes and confirmed by annexin V/propidium iodide (PI) apoptosis assay. Detection of the dynamic changes in cell morphology at initial stages of cell intoxication by DHM emphasizes the highly sensitive and rapid nature of this method, allowing the early detection of active toxins.

Early disruption of the alveolar-capillary barrier in a ricin-induced ARDS mouse model: neutrophil-dependent and -independent impairment of junction proteins

Irrespective of its diverse etiologies, acute lung injury (ALI)/acute respiratory distress syndrome (ARDS) leads to increased permeability of the alveolar-capillary barrier, which in turn promotes edema formation and respiratory failure. We investigated the mechanism of ALI/ARDS lung hyperpermeability triggered by pulmonary exposure of mice to the highly toxic plant-derived toxin ricin. One prominent hallmark of ricin-mediated pulmonary intoxication is the rapid and massive influx of neutrophils to the lungs, where they contribute to the developing inflammation yet may also cause tissue damage, thereby promoting ricin-mediated morbidity. Here we show that pulmonary exposure of mice to ricin results in the rapid diminution of the junction proteins VE-cadherin, claudin 5, and connexin 43, belonging, respectively, to the adherens, tight, and gap junction protein families. Depletion of neutrophils in ricin-intoxicated mice attenuated the damage caused to these junction proteins, alleviated pulmonary edema, and significantly postponed the time to death of the intoxicated mice. Inhibition of matrix metalloproteinase (MMP) activity recapitulated the response to neutrophil depletion observed in ricin-intoxicated mice and was associated with decreased insult to the junction proteins and alveolar-capillary barrier. However, neutrophil-mediated MMP activity was not the sole mechanism responsible for pulmonary hyperpermeability, as exemplified by the ricin-mediated disruption of claudin 18, via a neutrophil-independent mechanism involving tyrosine phosphorylation. This in-depth study of the early stage mechanisms governing pulmonary tissue integrity during ALI/ARDS is expected to facilitate the tailoring of novel therapeutic approaches for the treatment of these diseases.

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.

TRAIL (CD253) Sensitizes Human Airway Epithelial Cells to Toxin-Induced Cell Death

Ricin toxin is a biothreat agent that is particularly damaging to lung tissue following inhalation. A hallmark of ricin exposure is widespread inflammation and concomitant destruction of the airway epithelium. In this study, we investigated the possible interaction between ricin and known proinflammatory cytokines associated with lung tissue. Using an established human airway epithelial cell line, we demonstrate that epithelial cell killing by ricin is significantly enhanced in the presence of the proinflammatory cytokine known as TRAIL (CD253). Moreover, epithelial cells that are simultaneously exposed to ricin and TRAIL produced large amounts of secondary proinflammatory signals, including IL-6, which in the context of the lung would be expected to exacerbate toxin-induced tissue damage. Our results suggest that therapies designed to neutralize proinflammatory cytokines such as TRAIL and IL-6 may limit the bystander damage associated with ricin exposure.

Inhalation of ricin toxin is associated with the onset of acute respiratory distress syndrome (ARDS), characterized by hemorrhage, inflammatory exudates, and tissue edema, as well as the nearly complete destruction of the lung epithelium. Here we report that the Calu-3 human airway epithelial cell line is relatively impervious to the effects of ricin, with little evidence of cell death even upon exposure to microgram amounts of toxin. However, the addition of exogenous soluble tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL; CD253) dramatically sensitized Calu-3 cells to ricin-induced apoptosis. Calu-3 cell killing in response to ricin and TRAIL exposure was partially inhibited by caspase-8 and caspase-3/7 inhibitors, consistent with involvement of extrinsic apoptotic pathways in cell death. We employed nCounter Technology to define the transcriptional response of Calu-3 cells to ricin, TRAIL, and the combination of ricin plus TRAIL. An array of genes associated with inflammation and cell death were significantly upregulated upon treatment with ricin toxin and were further amplified upon addition of TRAIL. Of particular note was interleukin-6 (IL-6), whose expression in Calu-3 cells increased 300-fold upon ricin treatment and more than 750-fold upon ricin and TRAIL treatment. IL-6 secretion by Calu-3 cells was confirmed by cytometric bead array analysis. On the basis of these finding, we speculate that the severe airway epithelial cell damage observed in animal models following ricin exposure is a result of a positive-feedback loop driven by proinflammatory cytokines such as TRAIL and IL-6.

IMPORTANCE Ricin toxin is a biothreat agent that is particularly damaging to lung tissue following inhalation. A hallmark of ricin exposure is widespread inflammation and concomitant destruction of the airway epithelium. In this study, we investigated the possible interaction between ricin and known proinflammatory cytokines associated with lung tissue. Using an established human airway epithelial cell line, we demonstrate that epithelial cell killing by ricin is significantly enhanced in the presence of the proinflammatory cytokine known as TRAIL (CD253). Moreover, epithelial cells that are simultaneously exposed to ricin and TRAIL produced large amounts of secondary proinflammatory signals, including IL-6, which in the context of the lung would be expected to exacerbate toxin-induced tissue damage. Our results suggest that therapies designed to neutralize proinflammatory cytokines such as TRAIL and IL-6 may limit the bystander damage associated with ricin exposure.

Fine-Specificity Epitope Analysis Identifies Contact Points on Ricin Toxin Recognized by Protective Monoclonal Antibodies

Ricin is a fast-acting protein toxin classified by the Centers for Disease Control and Prevention as a biothreat agent. In this report, we describe five new mouse mAbs directed against an immunodominant region, so-called epitope cluster II, on the surface of ricin's ribosome-inactivating enzymatic subunit A (RTA). The five mAbs were tested alongside four previously described cluster II-specific mAbs for their capacity to passively protect mice against 10× LD₅₀ ricin challenge by injection. Only three of the mAbs (LE4, PH12, and TB12) afforded protection over the 7-d study period. Neither binding affinity nor in vitro toxin-neutralizing activity could fully account for LE4, PH12, and TB12's potent in vivo activity relative to the other six mAbs. However, epitope mapping studies by hydrogen exchange-mass spectrometry revealed that LE4, PH12, and TB12 shared common contact points on RTA corresponding to RTA α-helices D and E and β-strands d and e located on the back side of RTA relative to the active site. The other six mAbs recognized overlapping epitopes on RTA, but none shared the same hydrogen exchange-mass spectrometry profile as LE4, PH12, and TB12. A high-density competition ELISA with a panel of ricin-specific, single-domain camelid Abs indicated that even though LE4, PH12, and TB12 make contact with similar secondary motifs, they likely approach RTA from different angles. These results underscore how subtle differences in epitope specificity can significantly impact Ab functionality in vivo. ImmunoHorizons, 2018, 2: 262-273.

Improved antibody-based ricin neutralization by affinity maturation is correlated with slower off-rate values

While potent monoclonal antibodies against ricin were introduced over the years, the question whether increasing antibody affinity enables better toxin neutralization was not fully addressed yet. The aim of this study was to characterize the contribution of antibody affinity to the ricin neutralization potential of the antibody. cHD23 monoclonal antibody that targets the toxin B-subunit and interferes with its binding to membranal receptors, was isolated. In order to create antibody clones with improved affinity toward ricin, a scFv-phage display library containing mutated versions of the variable regions of cHD23 was constructed and clones with improved binding of ricin were isolated. Structural modeling of these mutants suggests that the inserted mutations may increase the antibody conformational flexibility thus improving its ability to bind ricin. While it was found that the selected clones exhibited improved neutralization of ricin, the correlation between the KD values and potency was only minor (r = 0.55). However, a positive correlation (r = 0.84) exist between the off-rate values (koff) of the affinity matured clones and their ability to neutralize ricin. As cell membranes display inordinately large amounts of potential surface binding sites for ricin, it is suggested that antibodies with improved off-rate values block the ability of the toxin to bind to target receptors, in a highly efficient manner. Currently, antibody-based therapy is the most effective treatment for ricin intoxication and it is anticipated that the findings of this study will provide useful information and a possible strategy to design an improved antibody-based therapy for the toxin.

Production, Characterisation and Testing of an Ovine Antitoxin against Ricin; Efficacy, Potency and Mechanisms of Action

Ricin is a type II ribosome-inactivating toxin that catalytically inactivates ribosomes ultimately leading to cell death. The toxicity of ricin along with the prevalence of castor beans (its natural source) has led to its increased notoriety and incidences of nefarious use. Despite these concerns, there are no licensed therapies available for treating ricin intoxication. Here, we describe the development of a F(ab')₂ polyclonal ovine antitoxin against ricin and demonstrate the efficacy of a single, post-exposure, administration in an in vivo murine model of intoxication against aerosolised ricin. We found that a single dose of antitoxin afforded a wide window of opportunity for effective treatment with 100% protection observed in mice challenged with aerosolised ricin when given 24 h after exposure to the toxin and 75% protection when given at 30 h. Treated mice had reduced weight loss and clinical signs of intoxication compared to the untreated control group. Finally, using imaging flow cytometry, it was found that both cellular uptake and intracellular trafficking of ricin toxin to the Golgi apparatus was reduced in the presence of the antitoxin suggesting both actions can contribute to the therapeutic mechanism of a polyclonal antitoxin. Collectively, the research highlights the significant potential of the ovine F(ab')₂ antitoxin as a treatment for ricin intoxication.

Treatments for Pulmonary Ricin Intoxication: Current Aspects and Future Prospects

Ricin, a plant-derived toxin originating from the seeds of Ricinus communis (castor beans), is one of the most lethal toxins known, particularly if inhaled. Ricin is considered a potential biological threat agent due to its high availability and ease of production. The clinical manifestation of pulmonary ricin intoxication in animal models is closely related to acute respiratory distress syndrome (ARDS), which involves pulmonary proinflammatory cytokine upregulation, massive neutrophil infiltration and severe edema. Currently, the only post-exposure measure that is effective against pulmonary ricinosis at clinically relevant time-points following intoxication in pre-clinical studies is passive immunization with anti-ricin neutralizing antibodies. The efficacy of this antitoxin treatment depends on antibody affinity and the time of treatment initiation within a limited therapeutic time window. Small-molecule compounds that interfere directly with the toxin or inhibit its intracellular trafficking may also be beneficial against ricinosis. Another approach relies on the co-administration of antitoxin antibodies with immunomodulatory drugs, thereby neutralizing the toxin while attenuating lung injury. Immunomodulators and other pharmacological-based treatment options should be tailored according to the particular pathogenesis pathways of pulmonary ricinosis. This review focuses on the current treatment options for pulmonary ricin intoxication using anti-ricin antibodies, disease-modifying countermeasures, anti-ricin small molecules and their various combinations.

Total Body Irradiation Mitigates Inflammation and Extends the Therapeutic Time Window for Anti-Ricin Antibody Treatment against Pulmonary Ricinosis in Mice

Ricin, a highly toxic plant-derived toxin, is considered a potential weapon in biowarfare and bioterrorism due to its pronounced toxicity, high availability, and ease of preparation. Pulmonary exposure to ricin results in the generation of an acute edematous inflammation followed by respiratory insufficiency and death. Massive neutrophil recruitment to the lungs may contribute significantly to ricin-mediated morbidity. In this study, total body irradiation (TBI) served as a non-pharmacological tool to decrease the potential neutrophil-induced lung injury. TBI significantly postponed the time to death of intranasally ricin-intoxicated mice, given that leukopenia remained stable following intoxication. This increase in time to death coincided with a significant reduction in pro-inflammatory marker levels, and led to marked extension of the therapeutic time window for anti-ricin antibody treatment.

Extended therapeutic window for post-exposure treatment of ricin intoxication conferred by the use of high-affinity antibodies

The plant toxin ricin is considered a potential bioterror agent against which there is no available antidote. To date, neutralizing antibodies are the most promising post-exposure treatment for ricin intoxication, yet so far they were shown to be effective only when given within several hours post exposure. As part of an ongoing effort to develop efficient ricin-countermeasures, we tested whether high-affinity antibodies that were previously isolated from immunized non-human primates, may confer effective post-exposure therapy for ricin-intoxicated mice treated at late time-points after exposure. While each antibody is capable of providing high protection rate by itself, a formulation consisting of three neutralizing antibodies that target different epitopes was tested to provide therapeutic coverage against different variants of the malicious pathogen. Indeed, the tri-antibody based cocktail was highly effective, its administration resulting in very high survival rates (>70%) when animals were treated as late as 48 h post exposure and significant protection (>30%) even at 72 h. This study establishes for the first time that anti-ricin antibodies can serve as a highly effective antidote at such late time-points after exposure. From the clinical point of view, the extended therapeutic window documented here is of high importance allowing adequate time to accurately identify the causative agent and may permit initiation of life-saving treatment with these antibodies even after the onset of clinical signs.

A novel swine model of ricin-induced acute respiratory distress syndrome

Pulmonary exposure to the plant toxin ricin leads to respiratory insufficiency and death. To date, in-depth study of acute respiratory distress syndrome (ARDS) following pulmonary exposure to toxins is hampered by the lack of an appropriate animal model. To this end, we established the pig as a large animal model for the comprehensive study of the multifarious clinical manifestations of pulmonary ricinosis. Here, we report for the first time, the monitoring of barometric whole body plethysmography for pulmonary function tests in non-anesthetized ricin-treated pigs. Up to 30 h post-exposure, as a result of progressing hypoxemia and to prevent carbon dioxide retention, animals exhibited a compensatory response of elevation in minute volume, attributed mainly to a large elevation in respiratory rate with minimal response in tidal volume. This response was followed by decompensation, manifested by a decrease in minute volume and severe hypoxemia, refractory to oxygen treatment. Radiological evaluation revealed evidence of early diffuse bilateral pulmonary infiltrates while hemodynamic parameters remained unchanged, excluding cardiac failure as an explanation for respiratory insufficiency. Ricin-intoxicated pigs suffered from increased lung permeability accompanied by cytokine storming. Histological studies revealed lung tissue insults that accumulated over time and led to diffuse alveolar damage. Charting the decline in PaO2/FiO2 ratio in a mechanically ventilated pig confirmed that ricin-induced respiratory damage complies with the accepted diagnostic criteria for ARDS. The establishment of this animal model of pulmonary ricinosis should help in the pursuit of efficient medical countermeasures specifically tailored to deal with the respiratory deficiencies stemming from ricin-induced ARDS.

Potent Antiedematous and Protective Effects of Ciprofloxacin in Pulmonary Ricinosis

The plant toxin ricin is considered a biological threat agent of concern and is most toxic when inhaled. Pulmonary exposure to a lethal dose of ricin can be redressed by treatment with antiricin antibodies; however, late antitoxin intervention is of limited efficacy. This limitation is associated with overt lung damage, clinically manifested as severe pulmonary inflammation, which develops over time. Increased evidence indicates that ciprofloxacin, a broad-spectrum antimicrobial agent, possesses immunomodulatory properties. Here we demonstrate that while antiricin antibody administration at late hours after intranasal exposure to ricin confers limited protection to mice, highly efficient protection can be achieved by adding ciprofloxacin to the antibody treatment. We further demonstrate that parameters associated with lung injury, in particular, pulmonary proinflammatory cytokine production, neutrophil migration, and edema, are sharply reduced in ricin-intoxicated mice that were treated with ciprofloxacin. The presented data highlight the potential clinical application of ciprofloxacin as a beneficial immunomodulatory agent in the course of ricin intoxication.

Quantitative profiling of the in vivo enzymatic activity of ricin reveals disparate depurination of different pulmonary cell types

The plant-derived toxins ricin and abrin, operate by site-specific depurination of ribosomes, which in turn leads to protein synthesis arrest. The clinical manifestation following pulmonary exposure to these toxins is that of a severe lung inflammation and respiratory insufficiency. Deciphering the pathways mediating between the catalytic activity and the developing lung inflammation, requires a quantitative appreciation of the catalytic activity of the toxins, in-vivo. In the present study, we monitored truncated cDNA molecules which are formed by reverse transcription when a depurinated 28S rRNA serves as template. We found that maximal depurination after intranasal exposure of mice to 2LD50 ricin was reached 48h, where nearly 40% of the ribosomes have been depurinated and that depurination can be halted by post-exposure administration of anti-ricin antibodies. We next demonstrated that the effect of ricin intoxication on different cell types populating the lungs differs greatly, and that outstandingly high levels of damage (80% depurination), were observed in particular for pulmonary epithelial cells. Finally, we found that the magnitude of depurination induced by the related plant-derived toxin abrin, was significantly lower in comparison to ricin, and can be attributed mostly to reduced depurination of pulmonary epithelial cells by abrin. This study provides for the first time vital information regarding the scope and timing of the catalytic performance of ricin and abrin in the lungs of intact animals.

Highly Stable Lyophilized Homogeneous Bead-Based Immunoassays for On-Site Detection of Bio Warfare Agents from Complex Matrices

This study shows the development of dry, highly stable immunoassays for the detection of bio warfare agents in complex matrices. Thermal stability was achieved by the lyophilization of the complete, homogeneous, bead-based immunoassay in a special stabilizing buffer, resulting in a ready-to-use, simple assay, which exhibited long shelf and high-temperature endurance (up to 1 week at 100 °C). The developed methodology was successfully implemented for the preservation of time-resolved fluorescence, Alexa-fluorophores, and horse radish peroxidase-based bead assays, enabling multiplexed detection. The multiplexed assay was successfully implemented for the detection of Bacillus anthracis, botulinum B, and tularemia in complex matrices.

A biolayer interferometry-based assay for rapid and highly sensitive detection of biowarfare agents

Biolayer interferometry (BLI) is an optical technique that uses fiber-optic biosensors for label-free real-time monitoring of protein-protein interactions. In this study, we coupled the advantages of the Octet Red BLI system (automation, fluidics-free, and on-line monitoring) with a signal enhancement step and developed a rapid and sensitive immunological-based method for detection of biowarfare agents. As a proof of concept, we chose to demonstrate the efficacy of this novel assay for the detection of agents representing two classes of biothreats, proteinaceous toxins, and bacterial pathogens: ricin, a lethal plant toxin, and the gram-negative bacterium Francisella tularensis, the causative agent of tularemia. The assay setup consisted of biotinylated antibodies immobilized to the biosensor coupled with alkaline phosphatase-labeled antibodies as the detection moiety to create nonsoluble substrate crystals that precipitate on the sensor surface, thereby inducing a significant wavelength interference. It was found that this BLI-based assay enables sensitive detection of these pathogens (detection limits of 10 pg/ml and 1 × 10(4) pfu/ml ricin and F. tularensis, respectively) within a very short time frame (17 min). Owing to its simplicity, this assay can be easily adapted to detect other analytes in general, and biowarfare agents in particular, in a rapid and sensitive manner.

Isolation of Anti-Ricin Protective Antibodies Exhibiting High Affinity from Immunized Non-Human Primates

Ricin, derived from the castor bean plant Ricinus communis, is one of the most potent and lethal toxins known, against which there is no available antidote. To date, the use of neutralizing antibodies is the most promising post-exposure treatment for ricin intoxication. The aim of this study was to isolate high affinity anti-ricin antibodies that possess potent toxin-neutralization capabilities. Two non-human primates were immunized with either a ricin-holotoxin- or subunit-based vaccine, to ensure the elicitation of diverse high affinity antibodies. By using a comprehensive set of primers, immune scFv phage-displayed libraries were constructed and panned. A panel of 10 antibodies (five directed against the A subunit of ricin and five against the B subunit) was isolated and reformatted into a full-length chimeric IgG. All of these antibodies were found to neutralize ricin in vitro, and several conferred full protection to ricin-intoxicated mice when given six hours after exposure. Six antibodies were found to possess exceptionally high affinity toward the toxin, with K_D values below pM (k_off < 1 × 10⁻⁷ s⁻¹) that were well correlated with their ability to neutralize ricin. These antibodies, alone or in combination, could be used for the development of a highly-effective therapeutic preparation for post-exposure treatment of ricin intoxication.

Ricin-Holotoxin-Based Vaccines: Induction of Potent Ricin-Neutralizing Antibodies

Ricin is one of the most potent and lethal toxins known to which there is no available antidote. Currently, the most promising therapy is based on neutralizing antibodies elicited by active vaccination or given passively. Here, detailed protocols are provided for the production of two ricin holotoxin-based vaccines: monomerized subunit-based vaccine, and a formaldehyde-based ricin toxoid vaccine. Both vaccines were found to be stable with no toxic activity reversion even after long-term storage while eliciting high anti-ricin antibody titers possessing a potent neutralizing activity. The use of these vaccines is highly suitable for both the production of sera that can be used in passive protection experiments and immunization aimed to isolate potent anti-ricin monoclonal antibodies.

Diverse profiles of ricin-cell interactions in the lung following intranasal exposure to ricin

Ricin, a plant-derived exotoxin, inhibits protein synthesis by ribosomal inactivation. Due to its wide availability and ease of preparation, ricin is considered a biothreat, foremost by respiratory exposure. We examined the in vivo interactions between ricin and cells of the lungs in mice intranasally exposed to the toxin and revealed multi-phasic cell-type-dependent binding profiles. While macrophages (MΦs) and dendritic cells (DCs) displayed biphasic binding to ricin, monophasic binding patterns were observed for other cell types; epithelial cells displayed early binding, while B cells and endothelial cells bound toxin late after intoxication. Neutrophils, which were massively recruited to the intoxicated lung, were refractive to toxin binding. Although epithelial cells bound ricin as early as MΦs and DCs, their rates of elimination differed considerably; a reduction in epithelial cell counts occurred late after intoxication and was restricted to alveolar type II cells only. The differential binding and cell-elimination patterns observed may stem from dissimilar accessibility of the toxin to different cells in the lung and may also reflect unequal interactions of the toxin with different cell-surface receptors. The multifaceted interactions observed in this study between ricin and the various cells of the target organ should be considered in the future development of efficient post-exposure countermeasures against ricin intoxication.

Antibody treatment against pulmonary exposure to abrin confers significantly higher levels of protection than treatment against ricin intoxication

Abrin, a potent plant-derived toxin bearing strong resemblance to ricin, irreversibly inactivates ribosomes by site-specific depurination, thereby precipitating cessation of protein synthesis in cells. Due to its high availability and ease of preparation, abrin is considered a biological threat, especially in context of bioterror warfare. To date, there is no established therapeutic countermeasure against abrin intoxication. In the present study, we examined the progress of pulmonary abrin intoxication in mice, evaluated the protective effect of antibody-based post-exposure therapy, and compared these findings to those observed for ricin intoxication and therapy. Salient features of abrin intoxication were found to be similar to those of ricin and include massive recruitment of neutrophils to the lungs, high levels of pro-inflammatory markers in the bronchoalveolar lavage fluid and damage of the alveolar-capillary barrier. In contrast, the protective effect of anti-abrin antibody treatment was found to differ significantly from that of anti-ricin treatment. While anti-ricin treatment efficiency was quite limited even at 24h post-exposure (34% protection), administration of polyclonal anti-abrin antibodies even as late as 72h post-exposure, conferred exceedingly high-level protection (>70%). While both anti-toxin antibody treatments caused neutrophil and macrophage levels in the lungs to revert to normal, only anti-abrin treatment brought about a significant decline in the pulmonary levels of the pro-inflammatory cytokine IL-6. The differential ability of the anti-toxin treatments to dampen inflammation caused by the two similar toxins, abrin and ricin, could explain the radically different levels of protection achieved following antibody treatment.

Rapid assessment of antibody-induced ricin neutralization by employing a novel functional cell-based assay

Ricin is one of the most potent and lethal toxins known against which there is no available antidote. Currently, the most promising countermeasures against the toxin are based on neutralizing antibodies elicited by active vaccination or administered passively. A cell-based assay is widely applied for the primary screening and evaluation of anti-ricin antibodies, yet such assays are usually time-consuming (18-72 h). Here, we report of a novel assay to monitor ricin activity, based on HeLa cells that stably express the rapidly-degraded ubiquitin-luciferase (Ub-FL, half-life of 2 min). Ricin-induced arrest of protein synthesis could be quantified within 3 to 6h post intoxication (IC90 of 300 and 100 ng/ml, respectively). Furthermore, by stabilizing the intracellular levels of Ub-FL in the last hour of the assay, a 3-fold increase in the assay sensitivity was attained. We applied this assay to monitor the efficacy of a ricin holotoxin-based vaccine by measuring the formation of neutralizing antibodies throughout the immunization course. The potency of anti-ricin monoclonal antibodies (directed to either subunit of the toxin) could also be easily and accurately measured in this assay format. Owing to its simplicity, this assay may be implemented for high-throughput screening of ricin-neutralizing antibodies and for identification of small-molecule inhibitors of the toxin, as well as other ribosome-inactivating toxins.

Characterization and epitope mapping of the polyclonal antibody repertoire elicited by ricin holotoxin-based vaccination

Ricin, one of the most potent and lethal toxins known, is classified by the Centers for Disease Control and Prevention (CDC) as a select agent. Currently, there is no available antidote against ricin exposure, and the most promising therapy is based on neutralizing antibodies elicited by active vaccination or that are given passively. The aim of this study was to characterize the repertoire of anti-ricin antibodies generated in rabbits immunized with ricin toxoid. These anti-ricin antibodies exhibit an exceptionally high avidity (thiocyanate-based avidity index, 9 M) toward ricin and an apparent affinity of 1 nM. Utilizing a novel tissue culture-based assay that enables the determination of ricin activity within a short time period, we found that the anti-ricin antibodies also possess a very high neutralizing titer. In line with these findings, these antibodies conferred mice with full protection against pulmonary ricinosis when administered as a passive vaccination. Epitope mapping analysis using phage display random peptide libraries revealed that the polyclonal serum contains four immunodominant epitopes, three of which are located on the A subunit and one on the B subunit of ricin. Only two of the four epitopes were found to have a significant role in ricin neutralization. To the best of our knowledge, this is the first work that characterizes these immunological aspects of the polyclonal response to ricin holotoxin-based vaccination. These findings provide useful information and a possible strategy for the development and design of an improved ricin holotoxin-based vaccine.