Oral immunization of mice with ricin toxoid vaccine encapsulated in polymeric microspheres against aerosol challenge

A chimeric protein of abrin and Abrus precatorius agglutinin that neutralizes abrin mediated lethality in mice

Abrin, a type II ribosome inactivating protein from the Abrus precatorius plant, is extremely toxic. It has been shown to be 75 times more potent than its infamous sister toxin, ricin and their potential use in bio-warfare is a cause of major concern. Although several vaccine candidates are under clinical trials for ricin, none are available against abrin. The present study proposes a chimeric protein, comprising of 1-123 amino acids taken from the A chain of abrin and 124-175 amino acids from Abrus precatorius agglutinin A chain, as a vaccine candidate against abrin intoxication. The design was based on the inclusion of the immunogenic region of the full length protein and the minimal essential folding domains required for inducing neutralizing antibody response. The chimera also contains the epitope for the only two neutralizing antibodies; D6F10 and A7C4, reported against abrin till now. Active immunization with the chimera protected all the mice challenged with 45 X LD₅₀ of abrin. Also, passive transfer of antibodies raised against the chimera rescued all mice challenged with 50 X LD₅₀ of toxin. Hence the chimeric protein appears to be a promising vaccine candidate against abrin induced lethality.

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.

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.

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.

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.

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.

Protective effect of two recombinant ricin subunit vaccines in the New Zealand white rabbit subjected to a lethal aerosolized ricin challenge: survival, immunological response, and histopathological findings

Ricin, isolated from the castor bean plant Ricinus communis, is included on the Centers for Disease Control and Prevention Category B list of bioterrorism agents, indicating that the toxin is moderately easy to disseminate and could result in moderate morbidity rates. This study evaluated two promising recombinant ricin subunit vaccines, one made using an Escherichia coli codon-optimized gene and the other using a yeast codon-optimized gene in E. coli-based fermentations. Rabbits were vaccinated four times over a period of 6 months and challenged with ∼10 to 30 times the median lethal dose of aerosolized ricin. All unvaccinated control rabbits were either found dead or humanely euthanized within 30 h postchallenge, while the rabbits vaccinated with either vaccine survived the exposure without adverse clinical signs. When the protective antibody responses were analyzed, no significant difference was seen between the two vaccines. However, there was a significant difference in the immune response over time for both vaccines tested. Although clinical pathology was unremarkable, significant histological lesions in the control animals included fibrinonecrotic pneumonia, acute necrotizing lesions in the upper respiratory tract, and necrotizing lymphadenitis in the lymph nodes draining the upper and lower respiratory tract. Vaccine-treated rabbits exhibited resolving lesions associated with ricin exposure, namely chronic inflammation in the upper respiratory tract and lungs, fibrosis, type II pneumocyte hyperplasia, and bronchiolitis obliterans. This study confirmed the safety and efficacy of two recombinant ricin subunit vaccines in rabbits, offering potential protection to warfighters and select populations.

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.

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.

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.

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

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

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

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

Evidence for widespread epithelial damage and coincident production of monocyte chemotactic protein 1 in a murine model of intestinal ricin intoxication

The development of small-animal models is necessary to understand host responses and immunity to emerging infectious diseases and potential bioterrorism agents. In this report we have characterized a murine model of intestinal ricin intoxication. Ricin administered intragastrically (i.g.) to BALB/c mice at doses ranging from 1 to 10 mg/kg of body weight induced dose-dependent morphological changes in the proximal small intestine (i.e., duodenum), including widespread villus atrophy and epithelial damage. Coincident with epithelial damage was a localized increase in monocyte chemotactic protein 1, a chemokine known to be associated with inflammation of the intestinal mucosa. Immunity to intestinal ricin intoxication was achieved by immunizing mice i.g. with ricin toxoid and correlated with elevated levels of antitoxin mucosal immunoglobulin A (IgA) and serum IgG antibodies. We expect that this model will serve as a valuable tool in identifying the inflammatory pathways and protective immune responses that are elicited in the intestinal mucosa following ricin exposure and will prove useful in the evaluation of antitoxin vaccines and therapeutics.

Microparticle-based technologies for vaccines

Microparticles have been effectively used for many years as delivery systems for drugs and therapeutic proteins. Their application to the delivery of vaccines is not as extensive, but is growing. Utility has been demonstrated for the delivery of various types of vaccines (e.g., recombinant proteins, plasmid DNA, and peptides) and other vaccine components (e.g., immune potentiators). With respect to delivery of immune potentiators, synergistic effects are often observed whereby much more potent immune responses are induced with a combination than with either component alone. Hence, the prospects for broad application of microparticle-based delivery systems for vaccines are excellent.

Immunoglobulin A antibodies against ricin A and B subunits protect epithelial cells from ricin intoxication

Epithelial cells of the respiratory and gastrointestinal tracts are extremely vulnerable to the cytotoxic effects of ricin, a Shiga-like toxin with ribosome-inactivating properties. While mucosal immunity to ricin correlates with secretory immunoglobulin A (IgA) antibody levels in vivo, the potential of IgA to protect epithelial cells from ricin in vitro has not been examined due to the unavailability of well-defined antitoxin IgA antibodies. Here we report the characterization of four monoclonal IgA antibodies (IgA MAbs) produced from the Peyer's patches and mesenteric lymph nodes of BALB/c mice immunized intragastrically with ricin toxoid. Two IgA MAbs (33G2 and 35H6) were active against ricin's lectin subunit (RTB), and two (23D7 and 25A4) reacted with the toxin's enzymatic subunit (RTA). All four IgA MAbs neutralized ricin in a Vero cell cytotoxicity assay, blocked toxin-induced interleukin-8 release by the human monocyte/macrophage cell line 28SC, and protected polarized epithelial cell monolayers from ricin-mediated protein synthesis inhibition. 33G2 and 35H6 reduced ricin binding to the luminal surfaces of human intestinal epithelial cells to undetectable levels in tissue section overlay assays, whereas 23D7 had no effect on toxin attachment. 23D7 and 25A4 did, however, reduce ricin transcytosis across MDCK II cell monolayers, possibly by interfering with intracellular toxin transport. We conclude that IgA antibodies against RTA and RTB can protect mucosal epithelial cells from ricin intoxication.

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.

Validation of ELISA for the determination of anti-ricin immunoglobulin G concentration in mouse sera

An enzyme-linked immunosorbent assay (ELISA) for the determination of anti-ricin immunoglobulin G (IgG) concentration in mouse sera was systematically validated. The results obtained throughout the validation process strongly demonstrated that the ELISA was reliable, reproducible, and suitable for its intended use. The assay had a high level of precision within and between runs, was specific for the anti-ricin IgG, and showed no interference with a number of different serum matrices. The assay exhibited excellent accuracy, linearity, and stability. The mean recovery of four test samples with different known concentrations was 100.9+/-11.3%, 102.7+/-10.8%, 99.0+/-7.2%, and 95.9+/-11.3%, respectively (n=10). The mean recovery of the observed anti-ricin IgG concentration of three quality control samples run on 73 plates to their nominal concentrations was 100.1+/-7.3%, 100.2+/-5.8%, and 103.7+/-8.1%; and the coefficient of variation (CV) was 7.3%, 5.8%, and 7.8%, respectively. The back-calculated anti-ricin IgG concentration, %CV, and relative error of seven standards from the calibration curves run in the entire validation study were analyzed (n=7 x 73). The results indicated that the four-parameter logistic (4PL) equation, y=(a-d)/(1+(x/c)b)+d, provided an accurate representation of a sigmoidal relationship between the measured response and the logarithm of observed concentration of anti-ricin IgG in mouse sera for this ELISA. The lower limit of quantification and upper limit of quantification of the calibration curve were 3.3 ng/ml and 82.8 ng/ml, respectively. The measurable range of the assay would cover all possible anti-ricin IgG concentrations in mouse sera stimulated with a ricin vaccine candidate, when the test sera are measured at a 1:800 starting dilution followed by four additional fourfold serial dilutions.

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.

Interaction of various pectin formulations with porcine colonic tissues

Pectins of low and high degrees of esterification, as well as pectin derivatives carrying primary amines, were investigate for gel forming ability with mucosal tissues. The combination of scanning electronic microscopy and small deformation dynamic mechanical studies revealed that pectins with higher net electrical charges are more bioadhesive than the less charged ones. Both the negatively charged pectin formulation, P-25, and the positively charged formulation, P-N, were able to synergize with the mucus to produce rheologically strengthened gels. The highly esterified pectin, P-94, also synergized with the mucosal glycoproteins to form a gel structure via coil entanglements. The ex vivo studies further confirmed the microstructures of mucus gel networks with adsorbed pectins. When incubated with porcine intestinal mucus membrane, P-94 gels were found generally bound to the lumen area, P-25 gels were able to penetrate deeply near the wall area, P-N gels interacted with mucins via electrostatic bonding and dispersed into the whole area from the lumen to the wall. Hence, both P-N and P-94, by enhancing the protective barrier properties of mucus systems, may be useful alternatives for the treatment of mucus related irritation and infection. In drug-delivery systems, P-N and P-25 would deliver incorporated drugs mainly by pectin dissolution, while a diffusion mechanism would release drugs from P-94 gels.

Microparticles for oral delivery of vaccines

Nonreplicating antigens are poorly immunogenic when given orally, either due to their degradation in the gastrointestinal tract or because they are not efficiently taken up in the gut. Studies in laboratory animals have clearly demonstrated that microparticles can significantly improve the immunogenicity of orally administered antigens. However, the oral delivery of vaccines using microparticles has not been explored extensively in humans and large animals. In this article the progress in oral microparticle antigen delivery will be reviewed and, where possible, studies in humans and large animals will be highlighted. In addition, possible approaches that have the potential to significantly improve microparticle delivery of oral vaccines will be suggested.

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.

Microencapsulated subunit vaccine approach to enterotoxigenic Escherichia coli and other mucosal pathogens

Infections of the intestinal, urogenital, and respiratory tracts are serious health problems worldwide from both a morbidity and mortality perspective. Mucosal pathogens attach to surfaces of mucosa as a prerequisite for colonization and subsequent pathogenesis. By expressing various surface adhesins (colonization factors, CF) they are able to bind to specific mucosal receptors. Enterotoxigenic Escherichia coli (ETEC) can express numerous CF that allow them to attach to a variety of hosts. Mucosal immunity directed against pathogenic microorganisms is critical in host protection with secretory IgA being particularly important in preventing microoganisms from colonizing host cells. M cells likewise have an important immunological function in the small intestines by binding and transporting antigens to lymphocytes and macrophages thus enhancing the immune response. The use of subunit vaccines, such as antigen encapsulated microspheres, can act to effectively deliver specific antigens so as to optimize their immunological response. With the threat of bioterrorism becoming a reality in recent years, the miroencapsulation of antigens from potential bioterrorist agents may be an effective method of delivery so as to induce a level of protection in at risk individuals. The encapsulation of ETEC colonization factors in microspheres and their subsequent administration in small animals and humans has been conducted for many years. Evidence suggests that this type of delivery system for ETEC antigens may enhance their immunogenicity and provide protection against this microorganism.

Particulate delivery systems for biodefense subunit vaccines

Expanding identification of potentially protective subunit antigens and correlates of protection has provided a basis for the introduction of safer vaccines. Despite encouraging results in animal models, the significant potential of particulate delivery systems in vaccine design has not yet translated into effective vaccines available for use in humans. This review article will focus on the current status of the development of particulate vaccines, mainly liposomes and bio-degradable polymers, against potential agents for biowarfare: plague, anthrax, botulinum, and smallpox; and filoviruses: Marburg and Ebola.

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.

Immune response after oral administration of the encapsulated malaria synthetic peptide SPf66

The synthetic peptide SPf66 adsorbed on alum is one of the few Plasmodium falciparum vaccines which have been tested in field trials. We previously reported that subcutaneous administration of SPf66 loaded PLGA microparticles (MP) enhances the antibody response to this antigen compared to the conventional alum formulation. We now evaluate the suitability of polymeric formulations to obtain systemic immune responses by gastric intubation of Balb/c mice. Formulations composed of 1:1 mixtures of PLGA 50:50 and 75:25 (lactic:glycolic) microparticles were administered by the oral route, and when animals were boosted 3 weeks later significant systemic IgG antibody responses were elicited, comparable to alum triple shot and superior to the aqueous vaccine given by the oral route. The finding of IgG2a isotype for PLGA-vaccinated mice compared to the absent levels of this isotype for the alum-vaccinated group could be interpreted as a sign of Th1-like immune response and cellular immune response activation. Our results confirm that using the appropriate schedule the oral administration of PLGA particles is suitable to obtain systemic immune responses to the carried antigen.

Microparticles as vaccine adjuvants and delivery systems

Adjuvants can be broadly divided into two groups, based on their principal mechanisms of action: vaccine delivery systems and immunostimulatory adjuvants. Vaccine delivery systems are generally particulate (e.g., emulsions, microparticles, immunostimulatory complexes and liposomes) and function mainly to target associated antigens into antigen-presenting cells. However, increasingly, more complex formulations are being developed in which delivery systems are exploited both for the delivery of antigens and also for the delivery of coadministered immunostimulatory adjuvants. The rationale for this approach is to ensure that both antigen and adjuvant are delivered into the same population of antigen-presenting cells. In addition, delivery systems can focus the effect of the adjuvants onto the key cells of the immune system and limit the systemic distribution of the adjuvant, to minimize its potential to induce adverse effects. The formulation and delivery of potent adjuvants in microparticles may allow the development of prophylactic and therapeutic vaccines against cancers and chronic infectious diseases, which are currently poorly controlled. In addition, microparticle formulations may also allow vaccines to be delivered mucosally.

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.