RT is a disulfide-linked heterodimeric glycoprotein consisting of the toxic RTA and the cell-binding B chain (RTB). RTA is an N-glycosidase that specifically cleaves the 28S ribosomal RNA at adenine 4324. RTB is a galactose-specific lectin, that binds to cell-surface glycolipids and glycoproteins found on all vertebrate cells. The toxin is extracted from castor beans, where it consists of 3–5% of the bean’s dry weight [2]. Ricinus communis grows worldwide in warm temperate and tropical climates. It is cultivated as an ornamental plant and used commercially for its oil; it also grows as a weed. Castor oil is extracted for its utility as a high temperature lubricant. Following the extraction of the oil, the toxin can be purified from the remaining mash by sodium hydroxide precipitation (crude preparation) or by chromatography. However, even crude RT is highly toxic; pulverized beans are a potential bioweapon. The purified toxin is a white powder, and is quite stable at temperatures below 60°. However, even when boiled, high does of RT can be lethal (Vitetta et al., unpublished). The ubiquity of castor beans, ease of extraction, and chemical stability, make ricin an attractive and inexpensive agent for scientifically unsophisticated individuals or nations to produce in large quantities. The LD₅₀ of ricin toxin varies according to the route of exposure, 5–15 µg/kg by aerosol or parenteral administration, 25–100 mg/kg orally [2,4,5] or 5–15 µg/kg by gastric gavage following a period of fasting [6]. While RT is 100-fold less toxic than botulinum toxin, even smaller scale events in civilian populations could lead to panic and economic disruption, which are the basic objectives of terrorism [7]. Given the history of RT-related events [2,3,7], there is no question that it will at some point be used in an act of terrorism. Indeed caches of RT have been found throughout the world [7] and in August, 2011 it was reported by the New York Times that Al-Qaeda was experimenting with ricin bombs.

RT is a biochemically simple molecule, which must be routed to specific sites in the cell to exert its toxicity [8]. As a result, the processing of RT has been well studied and informative regarding unique trafficking pathways in the cell, notably “reverse transport”. There are several excellent review articles that describe the intracellular processing of RT [2,9,10,11,12,13]. This review will focus on those aspects of RT that can be targeted by antibodies or other inhibitors and the reader is referred to the cited reviews for biochemical details.

RTB binds to cells via its lectin receptors [9]. Because many cell surface glycoproteins and glycolipids display terminal galactosyl residues, RT binds promiscuously to virtually all cell types. Because each RT monomer can bind to two galactose-containing residues, RT can also cross-link some cell surface molecules. The binding of the toxin to cells is a target for intervention, by both antibodies and competitive ligands [14]. We routinely use 0.1 M lactose or galactose solutions to block the cellular cytotoxicity of RT in vitro. Milk contains a similar concentration of lactose, and thus we use the milk-based Blocker ^TM BLOTTO to block non-specific binding of glycoproteins to ricin in immunoassays. One wonders whether the antidote to ricin ingestion might not be a simple glass of milk, assuming it is given within a short period of time. To our knowledge this has not been investigated. It is also possible that microbial polysaccharides bind to RT at mucosal sites, and thus hinder its binding to human cells. This could account for the low toxicity of orally administered RT.

RT is internalized from the cell surface via a variety of mechanisms, both clathrin dependent and independent, and dynamin dependent and independent. By cross-linking cell surface molecules, and signaling through cell surface kinases, it is possible that RT can upregulate its own uptake into cells [9]. Because RT can bind to a number of cell surface glycan-containing structures, as well as glycoproteins in the serum and tissues, it is likely to be internalized by the full gamut of uptake mechanisms, including phagocytosis and micropinocytosis. Since the mechanisms of intake of RT differ in different cells as well as within the same cell, it can be deposited in different intracellular sites.

Much of the initial intracellular routing of RT involves shuttling among endosomal compartments. Ricin may follow three paths from the endosomes. The first involves exocytosis and expulsion from the cell, likely involving blebbing. Figure 1 shows micrographs of this process. The blebbing might indicate an attempt by the cell to expel the toxin. It might also represent a potential way for active ricin to be transferred to other cells in a tissue. The second pathway leads to lysosomal degradation. It is only by the third, retrograde, route that RT reaches its intracellular site of action. RT traverses the same set of organelles involved in the secretion of proteins, but does so in the reverse direction used by secreted proteins, moving from the endosomes, through the Golgi, and into the ER. Initially it was believed that this retrograde path was unique to toxins, but it has been found to be a regular feature of intracellular trafficking [9,10,13]. Drugs that specifically inhibit this pathway have been developed and have been show to be effective anti-RT agents, both in cells and in animals, albeit at very high concentrations [8].

Once RT reaches the endoplasmic reticulum, it must be translocated across the ER membrane into the cytosol, where it exerts its toxicity. Within the ER, the holotoxin is reduced, and RTA is released. The RTA then unfolds, crosses the ER membrane and translocates into the cytosol [15,16]. To successfully accomplish this, the unfolded A chain must avoid ubiquitination and the ER-associated degradation pathway, while still utilizing protein conducting translocons that are part of the degradation pathways. This is accomplished through the use of chaperones including Hsc70 and Hsp90 [16], as well as by a temperature-dependent structural alteration in RTA, involving the loss of alpha-helical structures and insertion of the C-terminus into the membrane lipid bilayer of the ER [15].

Once at its site of action, RTA acts as an N-glycosidase, depurinating ribosomal RNA by removing adenine 4324 in the RTA/sarcin loop of the 28S rRNA. The enzyme inactivates 1500 ribosomes per minute. The high catalytic rate has been found to be due, in part, to movement of the RNA structure itself [17]. It has been proposed that a helical domain of RTA, (amino acids 99–106) also plays a key role in depurination, and that the function of this domain might be hindered by an antibody (Ab)-mediated attack [18]. Chemical analogues that mimic RNA’s transition states during catalytic cleavage have been studied as potential inhibitors of RT [19,20,21,22].

It has been estimated that for each functional RT molecule that reaches its site of action, 10,000 other internalized molecules have either been degraded, eliminated by exocytosis, or sequestered in functionally irrelevant compartments in the cell [23]. Cell death results from inhibition of protein synthesis and is primarily by apoptosis.

The authors of this review first studied RT because of its therapeutic potential, primarily as the toxic moiety of MAb-based immunotoxins (ITs). ITs are part of a larger class of agents, termed immunoconjugates (ICs), that consist of a targeting moiety linked to a cytotoxic moiety. If the toxic moiety is a protein toxin or its active subunit, these ICs are called ITs. Projects under active investigation in our laboratories include ITs to treat lymphomas and leukemias [24,25,26], ITs as immunomodulatory agents [27], and ITs to eliminate the latent reservoir of HIV that remains following antiretroviral treatment [28,29]. The clinical efficacy of ITs and immunoconjugates has clearly been demonstrated in human clinical trials [30]. While there was initial concern regarding the widespread inherent toxicity of ITs, this was not the case; the major-dose limiting toxicity in early trials was vascular leak syndrome (VLS).

When compared to cytotoxic agents used to treat cancer, RT is far more potent. Figure 2 shows agents that target CD4+ lymphoma cells. RT is 4 logs more potent (on a molar basis) than the most active cytotoxic drugs. Although RT has a relatively high molecular weight, MAbs armed with 1–2 molecules of RT retain antigen binding. For the smaller cytotoxic agents, that number is approximately 10 molecules bound to each MAb. Thus even by conjugating more cytotoxic drug per molecule of MAb, it is unlikely that a cytotoxic drug-conjugate will achieve the toxicity of the most potent RT/RTA-based ITs. We have coupled the same anti-HIV MAb to RTA and to doxyrubicin, and found excellent in vitro and in vivo killing with the RTA IT, but only non-specific killing with the doxyrubicin conjugate (C. Coyne and SHP, unpublished).

Given the high degree of toxicity, as well as its general reputation as an agent of bioterrorism, it is no surprise that there have been concerns about the therapeutic use of RT. Considerable effort has gone into reducing the non-specific toxic side-effects of RTA-based ITs. VLS, hepatotoxicy, and nephrotoxicty were the major dose-limiting effects of very early ITs. It was subsequently found that mannose residues on the plant-derived RTA led to binding of RTA to mannose receptors on liver cells [31,32,33,34] and thus chemically or enzymatically deglycosylated RTA (dgRTA) was subsequently used. The molecular basis of VLS has been studied [35,36], and it may be reduced with anti-inflammatory drugs [37] or by altering the three amino acids in RTA that bind to endothelial cells and cause VLS. [36].

RTA is a proinflammatory and immunogenic molecule. RT and RTA can in rare cases be allergens. The presence of antibodies against RTA leads to rapid clearance of RTA-ITs from the blood, such that its efficacy is reduced. Hence, instead of a half-life of several days, it can be cleared in hours. However, if one waits 2–8 weeks, titers of antibody drop to baseline and further doses can be given. Several approaches have been taken to specifically suppress the development of anti-IT immune responses, including the concomitant administration of immunosuppressive agents such as cyclosporine, deoxyspergualin, anti-CD4 antibodies, or CTLA-4-IgG [38,39,40,41,42,43,44], or the development of tolerance with agents such as polyethylene glycol [45,46,47].

RT is of concern in the area of biodefense because it is readily available to individuals or groups with little technical expertise or funding. Its source is a ubiquitous weed and a cultivated crop in many countries. Crude RT made from pulverized castor beans is toxic and a 2-step extraction with readily available chemicals yields >95% pure toxin. The toxin is chemically stable and can be stored unrefrigerated for long periods of time with little loss of activity. Over the past several decades, there have been well-documented instances of RT prepared for nefarious uses, and at least one well-documented assassination with RT [2,3,7]. Although some have underplayed the risk from RT, primarily because it is unlikely to cause mass casualties [7], we should not underestimate the panic and disruption that can result from a biological attack initiated by a lone lunatic, even if it only involves a small number of individuals. Public assurances that an antidote is at hand will greatly allay anxiety and act as a deterrent.

Measures designed to protect people from the lethal effects of RT will be different for civilian and military populations. For civilians, any one individual is at very low risk of exposure, but there is a high likelihood that there will eventually be an attack against the public that will likely involve a limited number of individuals. In this case, post-exposure treatment is most appropriate. In addition to developing appropriate therapies, effective use of post-exposure strategies will require recognition of the event by alert first responders, rapid confirmation of an attack via specific assays, obtaining the appropriate therapeutic agents, and administering them, all within a limited window of time (<24 h). Since the symptoms of RT poisoning do not appear for hours, and they are difficult to distinguish from many other common infections, this will be challenging. A factor that strongly mitigates the risk of exposure by a large population is the difficulty in delivering RT in uniformly lethal quantities. As discussed below, aerosol exposure is most likely to produce serious symptoms. Delivery of either dissolved toxin or milled powder requires particle size less than 3 microns [48], larger particles rapidly settle and cannot penetrate the pulmonary system as deeply. Particles < 1 micron may remain suspended as aerosols indefinitely. Hence some technical expertise would be needed to produce optimally weaponized aerosols, although this could be done by trained individuals working for an enemy group. As noted earlier, however, larger quantities of crude ricin could be effectively used as well.

For the military, entire personnel units are likely to be exposed as a group. However, in this setting, RT has limited utility. The amount required for aerosol toxicity is large (10 µg/kg), compared to botulinum toxin (100 ng/kg), for example. Dispersion over a battlefield or military encampment is highly dependent upon weather conditions and technical expertise. Nevertheless, if the military were facing an enemy thought to have ricin in its arsenal, then pre-exposure preventative measures, which might include physical barriers, chemical detoxification agents, and immunization [6,49,50,51,52], should be planned.

The route by which one is exposed to RT is a key determinant of the clinical symptoms observed. Military groups are most likely to be exposed via the aerosol route, whereas attacks against civilians could be by aerosol, ingestion, or possibly even injection. RT could be used in solution or in its powdered form. Even if aerosolized particles are not small enough to be fully inhaled into pulmonary spaces, damage to mucosal tissues is still possible. There are a number of unresolved issues regarding the clinical syndromes associated with RT exposure, among them: Does the oral route pose a significant risk to humans? Clearly the ingestion of castor beans can be fatal, but this might be due to the protective effect of the seed coat in the upper GI. It is not clear what the effect of pure toxin either in suspension or as a powder will be in humans. Several excellent reviews of the clinical effects of ricin have been written [2,3,4,5,7]. However, it should be emphasized that there are almost no human data regarding exposure to the pure toxin, so that the signs and symptoms attributed to RT exposure syndrome have, for the most part, been extrapolated from animal data.

Different populations at risk require distinct approaches to protection. For military units facing an enemy that is believed to have RT in its arsenal, immunization is the most logical approach. For civilians, where the likelihood of any one individual being exposed to the toxin is low, the emphasis needs to be placed on post-exposure treatment. Civilians will not accept the use of a vaccine unless there are continuing attacks. Unfortunately, post-treatment therapy requires early recognition of intoxication, rapid confirmation of the exposure, and easy access to specific treatment. This is difficult since the early symptoms can mimic those of many diseases including influenza.

In this section we will discuss approaches to the prevention and treatment of RT toxicosis. Both specific and non-specific measures will be discussed. The authors of this article believe that antibodies currently offer the greatest potential for the development of specific RT inhibitors. Perhaps this is no surprise, since we are immunologists. This section will close with a discourse on passive and active antibody therapies. Active immunization will be of interest to the military and perhaps civilian first responders, whereas passive therapies are optimal for post-exposure treatment in any population.

Since several passive and active vaccines appear promising, it is important that the best of these reach the national stockpile for future use. Because RT is considered by many not to be a high priority biothreat the funding needed to accomplish this is difficult to obtain. Considering the efforts and resources already spent developing RT vaccines and antibodies, and the relative certainty that an incident will occur in the future, we view this as short-sighted and hope that government officials will rethink this policy.