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

Potency determination of ricin toxin using a monoclonal antibody-based competition assay

Mouse challenge studies with death as an endpoint remain the gold standard in assessing the potency of ricin toxin, a Category B biothreat agent derived from the castor bean (Ricinus communis). However, animal studies are expensive, time consuming and ethically concerning. In an effort to reduce reliance on animals in vaccine development, we developed a monoclonal antibody (MAb)-based ricin competition ELISA (RiCoE) that indicates conformation integrity of ricin toxin. In forced degradation (heat-denaturation) experiments with native ricin holotoxin, we demonstrate a correlation between the decline in MAb reactivity in RiCoE and a corresponding loss of toxin potency in Vero cells (IC₅₀) and mice (LD₅₀). The RiCoE assay was applied to differentially sourced commercial lots of ricin toxin derived from R. communis blends and compared to toxin potency in mice. There was near perfect congruence between RiCoE values with two different MAbs (PB10, SyH7) and ricin potency in the mouse model using morbidity as an endpoint. In conclusion, we propose that RiCoE can serve as a rapid and sensitive substitute to mouse lethal dose challenge studies as a means to determine ricin toxin potency and will be valuable at various stages of vaccine development.

Endpoint and epitope-specific antibody responses as correlates of vaccine-mediated protection of mice against ricin toxin

The successful licensure of vaccines for biodefense is contingent upon the availability of well-established correlates of protection (CoP) in at least two animal species that can be applied to humans, without the need to assess efficacy in the clinic. In this report we describe a multivariate model that combines pre-challenge serum antibody endpoint titers (EPT) and values derived from an epitope profiling immune-competition capture (EPICC) assay as a predictor in mice of vaccine-mediated immunity against ricin toxin (RT), a Category B biothreat. EPICC is a modified competition ELISA in which serum samples from vaccinated mice were assessed for their ability to inhibit the capture of soluble, biotinylated (b)-RT by a panel of immobilized monoclonal antibodies (mAbs) directed against four immunodominant toxin-neutralizing regions on the enzymatic A chain (RTA) of RT. In a test cohort of mice (n = 40) vaccinated with suboptimal doses of the RTA subunit vaccine, RiVax®, we identified two mAbs, PB10 and SyH7, which had EPICC inhibition values in pre-challenge serum samples that correlated with survival following a challenge with 5 × LD₅₀ of RT administered by intraperitoneal (IP) injection. Analysis of a larger cohort of mice (n = 645) revealed that a multivariate model combining endpoint titers and EPICC values for PB10 and SyH7 as predictive variables had significantly higher statistical power than any one of the independent variables alone. Establishing the correlates of vaccine-mediated protection in mice represents an important steppingstone in the development of RiVax® as a medical countermeasure under the United States Food and Drug Administration's "Animal Rule."

A New Method for Extraction and Analysis of Ricin Samples through MALDI-TOF-MS/MS

We report for the first time the efficient use of accelerated solvent extraction (ASE) for extraction of ricin to analytical purposes, followed by the combined use of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), and MALDI-TOF MS/MS method. That has provided a fast and unambiguous method of ricin identification for in real cases of forensic investigation of suspected samples. Additionally, MALDI-TOF MS was applied to characterize the presence and the toxic activity of ricin in irradiated samples. Samples containing ricin were subjected to ASE, irradiated with different dosages of gamma radiation, and analyzed by MALDI-TOF MS/MS for verification of the intact protein signal. For identification purposes, samples were previously subjected to SDS-PAGE, for purification and separation of the chains, followed by digestion with trypsin, and analysis by MALDI-TOF MS/MS. The results were confirmed by verification of the amino acid sequences of some selected peptides by MALDI-TOF MS/MS. The samples residual toxic activity was evaluated through incubation with a DNA substrate, to simulate the attack by ricin, followed by MALDI-TOF MS/MS analyses.

Plant Ribosome-Inactivating Proteins: Progesses, Challenges and Biotechnological Applications (and a Few Digressions)

Plant ribosome-inactivating protein (RIP) toxins are EC3.2.2.22 N-glycosidases, found among most plant species encoded as small gene families, distributed in several tissues being endowed with defensive functions against fungal or viral infections. The two main plant RIP classes include type I (monomeric) and type II (dimeric) as the prototype ricin holotoxin from Ricinus communis that is composed of a catalytic active A chain linked via a disulphide bridge to a B-lectin domain that mediates efficient endocytosis in eukaryotic cells. Plant RIPs can recognize a universally conserved stem-loop, known as the α-sarcin/ ricin loop or SRL structure in 23S/25S/28S rRNA. By depurinating a single adenine (A4324 in 28S rat rRNA), they can irreversibly arrest protein translation and trigger cell death in the intoxicated mammalian cell. Besides their useful application as potential weapons against infected/tumor cells, ricin was also used in bio-terroristic attacks and, as such, constitutes a major concern. In this review, we aim to summarize past studies and more recent progresses made studying plant RIPs and discuss successful approaches that might help overcoming some of the bottlenecks encountered during the development of their biomedical applications.

Progress and challenges associated with the development of ricin toxin subunit vaccines

The past several years have seen major advances in the development of a safe and efficacious ricin toxin vaccine, including the completion of two Phase I clinical trials with two different recombinant A subunit (RTA)-based vaccines: RiVax™ and RVEc™ adsorbed to aluminum salt adjuvant, as well as a non-human primate study demonstrating that parenteral immunization with RiVax elicits a serum antibody response that was sufficient to protect against a lethal dose aerosolized ricin exposure. One of the major obstacles moving forward is assessing vaccine efficacy in humans, when neither ricin-specific serum IgG endpoint titers nor toxin-neutralizing antibody levels are accepted as definitive predictors of protective immunity. In this review we summarize ongoing efforts to leverage recent advances in our understanding of RTA-antibody interactions at the structural level to develop novel assays to predict vaccine efficacy in humans.

Development of a drug delivery system for efficient alveolar delivery of a neutralizing monoclonal antibody to treat pulmonary intoxication to ricin

The high toxicity of ricin and its ease of production have made it a major bioterrorism threat worldwide. There is however no efficient and approved treatment for poisoning by ricin inhalation, although there have been major improvements in diagnosis and therapeutic strategies. We describe the development of an anti-ricin neutralizing monoclonal antibody (IgG 43RCA-G1) and a device for its rapid and effective delivery into the lungs for an application in humans. The antibody is a full-length IgG and binds to the ricin A-chain subunit with a high affinity (KD=53pM). Local administration of the antibody into the respiratory tract of mice 6h after pulmonary ricin intoxication allowed the rescue of 100% of intoxicated animals. Specific operational constraints and aerosolization stresses, resulting in protein aggregation and loss of activity, were overcome by formulating the drug as a dry-powder that is solubilized extemporaneously in a stabilizing solution to be nebulized. Inhalation studies in mice showed that this formulation of IgG 43RCA-G1 did not induce pulmonary inflammation. A mesh nebulizer was customized to improve IgG 43RCA-G1 deposition into the alveolar region of human lungs, where ricin aerosol particles mostly accumulate. The drug delivery system also comprises a semi-automatic reconstitution system to facilitate its use and a specific holding chamber to maximize aerosol delivery deep into the lung. In vivo studies in monkeys showed that drug delivery with the device resulted in a high concentration of IgG 43RCA-G1 in the airways for at least 6h after local deposition, which is consistent with the therapeutic window and limited passage into the bloodstream.

Novel Ricin Subunit Antigens With Enhanced Capacity to Elicit Toxin-Neutralizing Antibody Responses in Mice

RiVax is a candidate ricin toxin subunit vaccine antigen that has proven to be safe in human phase I clinical trials. In this study, we introduced double and triple cavity-filling point mutations into the RiVax antigen with the expectation that stability-enhancing modifications would have a beneficial effect on overall immunogenicity of the recombinant proteins. We demonstrate that 2 RiVax triple mutant derivatives, RB (V81L/C171L/V204I) and RC (V81I/C171L/V204I), when adsorbed to aluminum salts adjuvant and tested in a mouse prime-boost-boost regimen were 5- to 10-fold more effective than RiVax at eliciting toxin-neutralizing serum IgG antibody titers. Increased toxin neutralizing antibody values and seroconversion rates were evident at different antigen dosages and within 7 days after the first booster. Quantitative stability/flexibility relationships analysis revealed that the RB and RC mutations affect rigidification of regions spanning residues 98-103, which constitutes a known immunodominant neutralizing B-cell epitope. A more detailed understanding of the immunogenic nature of RB and RC may provide insight into the fundamental relationship between local protein stability and antibody reactivity.

Strong protection against ricin challenge induced by a novel modified ricin A-chain protein in mouse model

Ricin toxin (RT) is an extremely potent toxin derived from the castor bean plant. As a possible bioterrorist weapon, it was categorized as a level B agent in international society. With the growing awareness and concerns of the “white powder incident” in recent years, it is indispensable to develop an effective countermeasure against RT intoxication. In this study we used site-directed mutagenesis and polymerase chain reaction (PCR) techniques to modify the gene of ricin A-chain (RTA). As a result, we have generated a mutated and truncated ricin A-chain (mtRTA) vaccine antigen by E.coli strain. The cytotoxicity assay was used to evaluate the safety of the as-prepared mtRTA antigen, and the results showed that there was no residual toxicity observed when compared to the recombinant RTA (rRTA) or native RT. Furthermore, BALB/c mice were subcutaneously (s.c.) vaccinated with mtRTA 3 times at an interval of 2 weeks, and then the survivals were evaluated after intraperitoneal (i.p.) or intratracheal challenge of RT. The vaccinated mice developed a strong protective immune response that was wholly protective against 40 × LD₅₀ of RT i.p. injection or 20 × LD₅₀ of RT intratracheal spraying. The mtRTA antigen has great potential to be a vaccine candidate for future application in humans.

Enhancement of humoral immunity by the type II heat-labile enterotoxin LT-IIb is dependent upon IL-6 and neutrophils

LT-IIb, a type II heat-labile enterotoxin produced by Escherichia coli, is a potent intradermal adjuvant that enhances immune responses to coadministered antigens. Although the immune mechanisms that promote this augmented immune response have not been well defined, prior intradermal immunization experiments suggested that early cellular and immunomodulatory events at the site of immunization modulated the augmentation of antigen-specific immune responses by LT-IIb. To investigate that hypothesis, mice were intradermally immunized with a recombinant ricin vaccine, a prospective toxin subunit antigen, in the presence and absence of LT-IIb. Analysis of tissue-fluid collection, coupled with histologic sections from the site of intradermal immunization, revealed that a single dose of LT-IIb induced local production of interleukin 6 and promoted a regional infiltration of neutrophils. The adjuvant effects of LT-IIb were abrogated in interleukin 6-deficient mice and when mice were depleted of neutrophils by pretreatment with anti-Ly6G. Overall, these data firmly demonstrated that LT-IIb, when used as an intradermal adjuvant, recruits neutrophils and is a potent rapid inducer of interleukin 6.

Recent advances in the development of vaccines against ricin

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

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

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

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

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

Near-infrared laser adjuvant for influenza vaccine

Safe and effective immunologic adjuvants are often essential for vaccines. However, the choice of adjuvant for licensed vaccines is limited, especially for those that are administered intradermally. We show that non-tissue damaging, near-infrared (NIR) laser light given in short exposures to small areas of skin, without the use of additional chemical or biological agents, significantly increases immune responses to intradermal influenza vaccination without augmenting IgE. The NIR laser-adjuvanted vaccine confers increased protection in a murine influenza lethal challenge model as compared to unadjuvanted vaccine. We show that NIR laser treatment induces the expression of specific chemokines in the skin resulting in recruitment and activation of dendritic cells and is safe to use in both mice and humans. The NIR laser adjuvant technology provides a novel, safe, low-cost, simple-to-use, potentially broadly applicable and clinically feasible approach to enhancing vaccine efficacy as an alternative to chemical and biological adjuvants.

LT-IIb(T13I), a non-toxic type II heat-labile enterotoxin, augments the capacity of a ricin toxin subunit vaccine to evoke neutralizing antibodies and protective immunity

Currently, there is a shortage of adjuvants that can be employed with protein subunit vaccines to enhance protection against biological threats. LT-IIb(T13I) is an engineered nontoxic derivative of LT-IIb, a member of the type II subfamily of heat labile enterotoxins expressed by Escherichia coli, that possesses potent mucosal adjuvant properties. In this study we evaluated the capacity of LT-IIb(T13I) to augment the potency of RiVax, a recombinant ricin toxin A subunit vaccine, when co-administered to mice via the intradermal (i.d.) and intranasal (i.n.) routes. We report that co-administration of RiVax with LT-IIb(T13I) by the i.d. route enhanced the levels of RiVax-specific serum IgG antibodies (Ab) and elevated the ratio of ricin-neutralizing to non-neutralizing Ab, as compared to RiVax alone. Protection against a lethal ricin challenge was also augmented by LT-IIb(T13I). While local inflammatory responses elicited by LT-IIb(T13I) were comparable to those elicited by aluminum salts (Imject®), LT-IIb(T13I) was more effective than aluminum salts at augmenting production of RiVax-specific serum IgG. Finally, i.n. administration of RiVax with LT-IIb(T13I) also increased levels of RiVax-specific serum and mucosal Ab and enhanced protection against ricin challenge. Collectively, these data highlight the potential of LT-IIb(T13I) as an effective next-generation i.d., or possibly i.n. adjuvant for enhancing the immunogenicity of subunit vaccines for biodefense.

Comparative efficacy of two leading candidate ricin toxin a subunit vaccines in mice

The two leading ricin toxin vaccine candidates, RVEc and RiVax, are recombinant derivatives of the toxin's 267-amino-acid enzymatic A chain (RTA). RVEc is truncated at the C terminus (residues 199 to 267) to improve protein thermostability, while RiVax has two point mutations (V76M and Y80A) that eliminate the RNA N-glycosidase activity of RTA, as well as its ability to induce vascular leak syndrome. The two vaccines have never been directly compared in terms of their ability to stimulate RTA-specific antibodies (Abs), toxin-neutralizing activity (TNA), or protective immunity. To address this issue, groups of female BALB/c mice were immunized two or three times with Alhydrogel-adsorbed RiVax or RVEc at a range of doses (0.3 to 20 μg) and then challenged with 10 50% lethal doses (LD(50)s) of ricin. We found that the vaccines were equally effective at eliciting protective immunity at the doses tested. There were, however, quantitative differences in the antibody responses. RVEc tended to elicit higher levels of ricin-specific RTA IgG and TNA than did RiVax. Pepscan analysis revealed that serum Abs elicited by RVEc were skewed toward a solvent-exposed immunodominant α-helix known to be the target of potent toxin-neutralizing Abs. Finally, immunodepletion experiments suggest that the majority of toxin-neutralizing Abs elicited by RiVax were confined to residues 1 to 198, possibly explaining the equal effectiveness of RVEc as a vaccine.

Pilot phase IB clinical trial of an alhydrogel-adsorbed recombinant ricin vaccine

There is no FDA-approved vaccine for the potent plant toxin ricin. We have developed a recombinant ricin vaccine, RiVax. Without adjuvant it is safe and immunogenic in mice, rabbits, and humans. Based on our studies in mice, we now report the results of a small clinical trial with Alhydrogel-adsorbed RiVax.

The need for continued development of ricin countermeasures

Ricin toxin, an extremely potent and heat-stable toxin produced from the bean of the ubiquitous Ricinus communis (castor bean plant), has been categorized by the US Centers for Disease Control and Prevention (CDC) as a category B biothreat agent that is moderately easy to disseminate. Ricin has the potential to be used as an agent of biological warfare and bioterrorism. Therefore, there is a critical need for continued development of ricin countermeasures. A safe and effective prophylactic vaccine against ricin that was FDA approved for “at risk” individuals would be an important first step in assuring the availability of medical countermeasures against ricin.

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.

Structure of RiVax: a recombinant ricin vaccine

RiVax is a recombinant protein that is currently under clinical development as part of a human vaccine to protect against ricin poisoning. RiVax includes ricin A-chain (RTA) residues 1-267 with two intentional amino-acid substitutions, V76M and Y80A, aimed at reducing toxicity. Here, the crystal structure of RiVax was solved to 2.1 Å resolution and it was shown that it is superposable with that of the ricin toxin A-chain from Ricinus communis with a root-mean-square deviation of 0.6 Å over 258 C(α) atoms. The RiVax structure is also compared with the recently determined structure of another potential ricin-vaccine immunogen, RTA 1-33/44-198 R48C/T77C. Finally, the locations and solvent-exposure of two toxin-neutralizing B-cell epitopes were examined and it was found that these epitopes are within or near regions predicted to be involved in catalysis. The results demonstrate the composition of the RiVax clinical material and will guide ongoing protein-engineering strategies to develop improved immunogens.

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.

Vaccine-induced intestinal immunity to ricin toxin in the absence of secretory IgA

The RNA N-glycosidase ribosome inactivating proteins (RIPs) constitute a ubiquitous family of plant- and bacterium-derived toxins that includes the category B select agents ricin, abrin and shiga toxin. While these toxins are potent inducers of intestinal epithelial cell death and inflammation, very little is known about the mechanisms underlying mucosal immunity to these toxins. In the present study, we report that secretory IgA (SIgA) antibodies are not required for intestinal immunity to ricin, as evidenced by the fact that mice devoid of SIgA, due to a mutation in the polymeric immunoglobulin receptor, were impervious to the effects of intragastric toxin challenge following ricin toxoid immunization. Furthermore, parenteral administration of ricin-specific monoclonal IgGs, directed against either ricin's enzymatic subunit (RTA) or ricin's binding subunit (RTB), to wild type mice was as effective as monoclonal IgAs with comparable specificities in imparting intestinal immunity to ricin. These data are consistent with reports from others demonstrating that immunization of mice by routes known not to induce mucosal antibody responses (e.g., intramuscular and intradermal) is sufficient to elicit protection against both systemic and mucosal ricin challenges.

Ribosome-inactivating proteins: from plant defense to tumor attack

Ribosome-inactivating proteins (RIPs) are EC3.2.32.22 N-glycosidases that recognize a universally conserved stem-loop structure in 23S/25S/28S rRNA, depurinating a single adenine (A4324 in rat) and irreversibly blocking protein translation, leading finally to cell death of intoxicated mammalian cells. Ricin, the plant RIP prototype that comprises a catalytic A subunit linked to a galactose-binding lectin B subunit to allow cell surface binding and toxin entry in most mammalian cells, shows a potency in the picomolar range. The most promising way to exploit plant RIPs as weapons against cancer cells is either by designing molecules in which the toxic domains are linked to selective tumor targeting domains or directly delivered as suicide genes for cancer gene therapy. Here, we will provide a comprehensive picture of plant RIPs and discuss successful designs and features of chimeric molecules having therapeutic potential.