Identification and quantification of ricin toxin in animal tissues using ELISA

A Proof-of-Concept, Two-Tiered Approach for Ricin Detection Using Ambient Mass Spectrometry

Ricin is a naturally occurring, highly potent toxin native to castor bean plants that has recently been used as a biological weapon in cases of bioterrorism and suicide attempts. Difficulties with direct detection arise from large heterogeneities in ricin glycosylation, which leads to markedly different bioactivity, and the fact that carefully developed and laborious sample preparation steps are required to maintain the activity of the protein during analysis. Herein, we present an alternative, two-tiered approach to identify the presence of ricin by detecting ricinoleic acid and ricinine, which are co-extracted with the protein. This direct mass spectrometric-based technique takes as little as 2 minutes, and we determined its sensitivity to be in the parts-per-trillion range. Our method is applicable to paper substrates from suspected contaminated envelopes and biofluids from at-risk patients. The fact that prior sample preparations are not needed in this procedure means that analysis can be performed in the field for emergency cases.

Ricin is a naturally occurring, highly potent toxin native to castor bean plants that has recently been used as a biological weapon in cases of bioterrorism and suicide attempts.

Improved Sensitivity for the Qualitative and Quantitative Analysis of Active Ricin by MALDI-TOF Mass Spectrometry

Ricin is a highly toxic protein which causes cell death by blocking protein synthesis and is considered a potential bioterrorism agent. Rapid and sensitive detection of ricin toxin in various types of sample matrices is needed as an emergency requirement for public health and antibioterrorism response. An in vitro MALDI TOF MS-based activity assay that detects ricin mediated depurination of synthetic substrates was improved through optimization of the substrate, reaction conditions, and sample preparation. In this method, the ricin is captured by a specific polycolonal antibody followed by hydrolysis reaction. The ricin activity is determined by detecting the unique cleavage product of synthetic oligomer substrates. The detection of a depurinated substrate was enhanced by using a more efficient RNA substrate and optimizing buffer components, pH, and reaction temperature. In addition, the factors involved in mass spectrometry analysis, such as MALDI matrix, plate, and sample preparation, were also investigated to improve the ionization of the depurinated product and assay reproducibility. With optimized parameters, the limit of detection of 0.2 ng/mL of ricin spiked in buffer and milk was accomplished, representing more than 2 orders of magnitude enhancement in assay sensitivity. Improving assay's ruggeddness or reproducibility also made it possible to quantitatively detect active ricin with 3 orders of magnitude dynamic range.

An International Proficiency Test to Detect, Identify and Quantify Ricin in Complex Matrices

While natural intoxications with seeds of Ricinus communis (R. communis) have long been known, the toxic protein ricin contained in the seeds is of major concern since it attracts attention of those intending criminal, terroristic and military misuse. In order to harmonize detection capabilities in expert laboratories, an international proficiency test was organized that aimed at identifying good analytical practices (qualitative measurements) and determining a consensus concentration on a highly pure ricin reference material (quantitative measurements). Sample materials included highly pure ricin as well as the related R. communis agglutinin (RCA120) spiked into buffer, milk and meat extract; additionally, an organic fertilizer naturally contaminated with R. communis shred was investigated in the proficiency test. The qualitative results showed that either a suitable combination of immunological, mass spectrometry (MS)-based and functional approaches or sophisticated MS-based approaches alone successfully allowed the detection and identification of ricin in all samples. In terms of quantification, it was possible to determine a consensus concentration of the highly pure ricin reference material. The results provide a basis for further steps in quality assurance and improve biopreparedness in expert laboratories worldwide.

Recommended Immunological Assays to Screen for Ricin-Containing Samples

Ricin, a toxin from the plant Ricinus communis, is one of the most toxic biological agents known. Due to its availability, toxicity, ease of production and absence of curative treatments, ricin has been classified by the Centers for Disease Control and Prevention (CDC) as category B biological weapon and it is scheduled as a List 1 compound in the Chemical Weapons Convention. An international proficiency test (PT) was conducted to evaluate detection and quantification capabilities of 17 expert laboratories. In this exercise one goal was to analyse the laboratories’ capacity to detect and differentiate ricin and the less toxic, but highly homologuous protein R. communis agglutinin (RCA120). Six analytical strategies are presented in this paper based on immunological assays (four immunoenzymatic assays and two immunochromatographic tests). Using these immunological methods “dangerous” samples containing ricin and/or RCA120 were successfully identified. Based on different antibodies used the detection and quantification of ricin and RCA120 was successful. The ricin PT highlighted the performance of different immunological approaches that are exemplarily recommended for highly sensitive and precise quantification of ricin.

Characterization of Ricin and R. communis Agglutinin Reference Materials

Ricinus communis intoxications have been known for centuries and were attributed to the toxic protein ricin. Due to its toxicity, availability, ease of preparation, and the lack of medical countermeasures, ricin attracted interest as a potential biological warfare agent. While different technologies for ricin analysis have been established, hardly any universally agreed-upon "gold standards" are available. Expert laboratories currently use differently purified in-house materials, making any comparison of accuracy and sensitivity of different methods nearly impossible. Technically challenging is the discrimination of ricin from R. communis agglutinin (RCA120), a less toxic but highly homologous protein also contained in R. communis. Here, we established both highly pure ricin and RCA120 reference materials which were extensively characterized by gel electrophoresis, liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI MS/MS), and matrix-assisted laser desorption ionization-time of flight approaches as well as immunological and functional techniques. Purity reached >97% for ricin and >99% for RCA120. Different isoforms of ricin and RCA120 were identified unambiguously and distinguished by LC-ESI MS/MS. In terms of function, a real-time cytotoxicity assay showed that ricin is approximately 300-fold more toxic than RCA120. The highly pure ricin and RCA120 reference materials were used to conduct an international proficiency test.

Evaluation of ricinine, a ricin biomarker, from a non-lethal castor bean ingestion

A case is presented of the attempted suicide of a 58-year-old man using castor beans. The patient came to the emergency room complaining of nausea, vomiting and diarrhea for nine hours following the ingestion of six castor beans. Urine samples were taken throughout the hospital stay and submitted to the Centers for Disease Control and Prevention for analysis of ricinine, a castor bean component. The samples were found to be positive for ricinine, with a maximum concentration of 674 µg/g-creatinine excreted approximately 23 h post-exposure. Subsequent samples demonstrated lower ricinine concentrations, with the final sample taken at 62 h post-exposure at a concentration of 135 µg/g-creatinine of ricinine. The estimated urinary excretion half-life was approximately 15 h and the recovery of ricinine in the urine over the three days was estimated to be less than 10%. The patient fully recovered with supportive care and was discharged from the hospital six days after admission.

Simultaneous detection of ricin and abrin DNA by real-time PCR (qPCR)

Ricin and abrin are two of the most potent plant toxins known and may be easily obtained in high yield from the seeds using rather simple technology. As a result, both toxins are potent and available toxins for criminal or terrorist acts. However, as the production of highly purified ricin or abrin requires sophisticated equipment and knowledge, it may be more likely that crude extracts would be used by non-governmental perpetrators. Remaining plant-specific nucleic acids in these extracts allow the application of a real-time PCR (qPCR) assay for the detection and identification of abrin or ricin genomic material. Therefore, we have developed a duplex real-time PCR assays for simultaneous detection of ricin and abrin DNA based on the OmniMix HS bead PCR reagent mixture. Novel primers and hybridization probes were designed for detection on a SmartCycler instrument by using 5'-nuclease technology. The assay was thoroughly optimized and validated in terms of analytical sensitivity. Evaluation of the assay sensitivity by probit analysis demonstrated a 95% probability of detection at 3 genomes per reaction for ricin DNA and 1.2 genomes per reaction for abrin DNA. The suitability of the assays was exemplified by detection of ricin and abrin contaminations in a food matrix.

A novel xyloglucan film-based biosensor for toxicity assessment of ricin in castor seed meal

Oil from the seed of the castor plant (Ricinus communis L.) is an important commodity for a number of industries, ranging from pharmaceuticals to renewable energy resources. However, the seed and subsequent seed meal contain ricin (RCA60), a potent cytotoxin, making it an unusable product for animal feed. In order to investigate the efficiency of reducing the toxicity of the seed meal, a biosensor is proposed by exploring the lectin-carbohydrate binding. A gold electrode was assembled with a film of Xyloglucan (XG) extracted from Hymenaea courbaril L. The analytical response to RCA60 was obtained using a polyclonal antibody against RCA60 conjugated to peroxidase. The current responses were generated by reaction with H2O2 and amplified with hydroquinone as chemical mediator. Voltammetric studies showed that the XG film was tightly bound to the gold electrode. This biosensor allows discriminate lectins in native and denatured forms. The limit of detection of native RCA60 was 2.1 μg mL(-1). This proposed biosensor showed to be a potential and accurate method for toxicity assessment of the ricin in castor seed meal by simple polysaccharide film-electrode strategy.

Understanding ricin from a defensive viewpoint

The toxin ricin has long been understood to have potential for criminal activity and there has been concern that it might be used as a mass-scale weapon on a military basis for at least two decades. Currently, the focus has extended to encompass terrorist activities using ricin to disrupt every day activities on a smaller scale. Whichever scenario is considered, there are features in common which need to be understood; these include the knowledge of the toxicity from ricin poisoning by the likely routes, methods for the detection of ricin in relevant materials and approaches to making an early diagnosis of ricin poisoning, in order to take therapeutic steps to mitigate the toxicity. This article will review the current situation regarding each of these stages in our collective understanding of ricin and how to defend against its use by an aggressor.

Ricinus communis intoxications in human and veterinary medicine-a summary of real cases

Accidental and intended Ricinus communis intoxications in humans and animals have been known for centuries but the causative agent remained elusive until 1888 when Stillmark attributed the toxicity to the lectin ricin. Ricinus communis is grown worldwide on an industrial scale for the production of castor oil. As by-product in castor oil production ricin is mass produced above 1 million tons per year. On the basis of its availability, toxicity, ease of preparation and the current lack of medical countermeasures, ricin has gained attention as potential biological warfare agent. The seeds also contain the less toxic, but highly homologous Ricinus communis agglutinin and the alkaloid ricinine, and especially the latter can be used to track intoxications. After oil extraction and detoxification, the defatted press cake is used as organic fertilizer and as low-value feed. In this context there have been sporadic reports from different countries describing animal intoxications after uptake of obviously insufficiently detoxified fertilizer. Observations in Germany over several years, however, have led us to speculate that the detoxification process is not always performed thoroughly and controlled, calling for international regulations which clearly state a ricin threshold in fertilizer. In this review we summarize knowledge on intended and unintended poisoning with ricin or castor seeds both in humans and animals, with a particular emphasis on intoxications due to improperly detoxified castor bean meal and forensic analysis.

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

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

Denatured ricin can be detected as native ricin by immunological methods, but nontoxic in vivo

Ricin is a glycoprotein from Ricinus communis seeds. It is known to have diverse toxic effects on cells of different visceral organs. In the present study, we purified and denatured ricin in a boiling water bath for different time intervals. We further made an attempt to identify native and denatured ricin by immunobased detection systems. All the antigen/antibody-based assays identified native and denatured ricin. On SDS-PAGE, only native ricin was observed. In western blotting, ricin boiled for 3.75 min gave a strong band on X-ray film. On native polyacryl amide gel electrophoresis, native and denatured ricin gave ricin band in 60-kDa region. The denatured ricin did not [corrected] cause mortality up to 25 mg/kg, while 5 and 10 microg/kg of native ricin caused 50% and 100% mortality, respectively. Detection of native and denatured ricin is very difficult, and the investigating agencies, forensic scientists, and analysts should be very careful while interpreting the results.

Comparison of an electrochemiluminescence assay in plate format over a colorimetric ELISA, for the detection of ricin B chain (RCA-B)

An electrochemiluminescence (ECL) assay for the detection of the B chain of ricin (RCA-B) in a 96-well plate format was developed in parallel with a colorimetric ELISA utilizing the same pair of antibodies. Sensitivity results were interpreted with the ANOVA and Tukey statistical tests, allowing a direct comparison between the two technologies, that can probably be extended to other protein antigens such as toxins. Reproducibility, repeatability and rapidity of the two techniques were also compared. The ELISA assay utilized an alkaline phosphatase conjugate for signal generation. After optimization, its limit of detection was 400 pg of RCA-B per ml buffer, with an intra-day standard deviation (SD) of 2.2% of the mean and an inter-day SD of 5.1%. The ECL assay utilized ruthenylated antibodies for detection. The ECL measurement was carried out using a Sector PR 400 plate reader. After optimization, its limit of detection was 50 pg of RCA-B per ml buffer, with an intra-day SD of 4.1% of the mean and an inter-day SD of 4.3%. Starting from a pre-coated plate, the ELISA assay was completed in 7 h and the ECL assay took 2.5 h. While reproducibility and repeatability of the two assays were equivalent, this ECL assay in plate format had an 8-fold better sensitivity for RCA-B detection than the colorimetric ELISA in buffer and in various matrices. The ECL assay was also three times faster, and retained the robustness and convenience of the 96-well plate format.

Aptamer-based detection and quantitative analysis of ricin using affinity probe capillary electrophoresis

The ability to detect sub-nanomolar concentrations of ricin using fluorescently tagged RNA aptamers is demonstrated. Aptamers rival the specificity of antibodies and have the power to simplify immunoassays using capillary electrophoresis. Under nonequilibrium conditions, a dissociation constant, Kd, of 134 nM has been monitored between the RNA aptamer and ricin A-chain. With use of this free-solution assay, the detection of 500 pM (approximately 14 ng/mL) or 7.1 amol of ricin is demonstrated. The presence of interfering proteins such as bovine serum albumin and casein do not inhibit this interaction at sub-nanomolar concentrations. When spiked with RNAse A, ricin can still be detected down to 1 nM concentrations despite severe aptamer degradation. This approach offers a promising method for the rapid, selective, and sensitive detection of biowarfare agents.

Ricin detection by biological signal amplification in a well-in-a-well device

This paper presents a ricin detection method based on ricin's inhibitory effects on protein synthesis. Biological synthesis (expression) of a protein includes the steps of gene transcription (DNA --> RNA) and protein translation (RNA --> proteins); these reactions can be coupled into a one-step operation and carried out in a cell-free medium. Ricin is known to inhibit protein synthesis by interacting with 28S ribosome RNA; the inhibitory effect is exploited as the sensing mechanism in this work. For each copy of DNA, thousands of copies of proteins can be produced. As a result, the inhibitory effects of ricin are amplified, leading to a significantly enhanced detection signal (the difference between the positive control and samples). An array of protein expression units is developed to accommodate positive/negative controls and multiple samples. The array device contains a solution without any reagent captured on a solid surface, offering flexibility without comprising the activities of biomolecules. The miniaturized well-in-a-well design possesses a mechanism to supply nutrients continuously and remove byproducts, leading to higher protein expression yields and thus larger detection signals (lower detection limit) when ricin is present. We demonstrate the production of green fluorescent protein and luciferase in the device. A calibration curve has been obtained between the luciferase expression yield and the ricin concentration, showing a detection limit of 0.01 nM (0.3 ng/mL) ricin. The nested-well device is also used for measuring the toxicity level of ricin after physical or chemical treatment.

Detection of Functional Ricin by Immunoaffinity and Liquid Chromatography−Tandem Mass Spectrometry

The toxin ricin is a biological weapon that may be used for bioterrorist purposes. As a member of the group of ribosome-inactivating proteins (RIPs), ricin has an A-chain possessing N-glycosidase activity which irreversibly inhibits protein synthesis. In this paper, we demonstrate that provided appropriate sample preparation is used, this enzymatic activity can be exploited for functional ricin detection with sensitivity similar to the best ELISA and specificity allowing application to environmental samples. Ricin is first captured by a monoclonal antibody directed against the B chain and immobilized on magnetic beads. Detection is then realized by determination of the adenine released by the A chain from an RNA template using liquid chromatography coupled to tandem mass spectrometry. The immunoaffinity step combined with the enzymatic activity detection leads to a specific assay for the entire functional ricin with a lower limit of detection of 0.1 ng/mL (1.56 pM) after concentration of the toxin from a 500 microL sample size. The variability of the assay was 10%. Finally, the method was applied successfully to milk and tap or bottled water samples.

Detection of ricin and other ribosome-inactivating proteins by an immuno-polymerase chain reaction assay

Ribosome-inactivating proteins (RIPs) are plant proteins with enzymatic activity, classified as type 1 (single chain) or type 2 (two chains). They are identified as rRNA N-glycosidases (EC 3.2.2.22) and cause an irreversible inhibition of protein synthesis. Among type 2 RIPs, there are potent toxins (ricin is the best known) that are considered as potential biological weapons. The development of a fast and sensitive method for the detection of biological agents is an important tool to prevent or deal with the consequences of intoxication. In this article, we describe a very sensitive immuno-polymerase chain reaction (IPCR) assay for the detection of RIPs-a type 1 RIP (dianthin) and a type 2 RIP (ricin)-that combines the specificity of immunological analysis with the exponential amplification of PCR. The limit of detection (LOD) of the technique was compared with the LODs of the conventional immunological methods enzyme-linked immunosorbent assay (ELISA) and fluorescent immunosorbent assay (FIA). The LOD of IPCR was more than 1 million times lower than that of ELISA, allowing the detection of 10 fg/ml of dianthin and ricin. The possibility to detect ricin in human serum was also investigated, and a similar sensitivity was observed (10 fg/ml). IPCR appears to be the most sensitive method for the detection of ricin and other RIPs.

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.

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.

Colloidal gold-based immunochromatographic assay for detection of ricin

A rapid immunochromatographic assay was developed to detect ricin. The assay was based on the sandwich format using monoclonal antibodies (Mabs) of two distinct specificities. One anti-ricin B chain Mab (1G7) was immobilized to a defined detection zone on a porous nitrocellulose membrane, while the other anti-ricin A chain Mab (5E11) was conjugated to colloidal gold particles which served as a detection reagent. The ricin-containing sample was added to the membrane and allowed to react with Mab (5E11)-coated particles. The mixture was then passed along the porous membrane by capillary action past the Mab (1G7) in the detection zone, which will bind the particles that had ricin bound to their surface, giving a red color within this detection zone with an intensity proportional to ricin concentration. In the absence of ricin, no immunogold was bound to the solid-phase antibody. With this method, 50 ng/ml of ricin was detected in less than 10 min. The assay sensitivity can be increased by silver enhancement to 100 pg/ml.

Monoclonal antibody-based enzyme immunoassay for detection of ricin

A sensitive and specific enzyme-linked immunoadsorbent assay (ELISA) was developed to detect ricin in biological fluids. The assay is based on the sandwich format using monoclonal antibodies (MAbs) of two distinct specificities. An affinity-purified anti-ricin B chain MAb (1G7) is utilized to adsorb ricin from solution and the second anti-ricin A chain MAb (5E11) conjugated with peroxidase is then used to form a sandwich, and peroxidase allows color development and measurement of optical density at 450 nm. Standard curves were linear over the range of 2.5-100 ng/mL ricin. The limit of detection was below 5 ng/mL in assay buffer as well as in a 1:10 dilution of urine or 1:50 dilution of human serum spiked with ricin.

Fiber optic-based biosensor for ricin

We report on an evanescent wave fiber-optic biosensor for detecting a potently toxic protein, ricin, in the picograms per milliliter range. A sandwich immunoassay scheme was used to detect ricin. First, an anti-ricin IgG was immobilized onto the surface of an optical fiber in two different ways. In the first method, the antibody was directly coated to the silanized fiber using a crosslinker. Second, avidin-coated fibers were incubated with biotinylated anti-ricin IgG to immobilize the antibody using an avidin-biotin bridge. The assay using the avidin-biotin linked antibody demonstrated higher sensitivity and wider linear dynamic range than the assay using antibody directly conjugated to the surface. The linear dynamic range of detection for ricin in buffer using the avidin-biotin chemistry is 100 pg/ml-250 ng/ml. The limits of detection for ricin in buffer solution and river water are 100 pg/ml and 1 ng/ml, respectively. At higher concentrations of ricin (> 50 ng/ml), we observe a strong interaction of ricin with the avidin coated on the surface of the fibers. We have demonstrated that this interaction is primarily due to the lectin activity of ricin and is significantly reduced using fibers coated with neutravidin or by adding galactose to the ricin samples.

Detection of ricin by colorimetric and chemiluminescence ELISA

A highly sensitive and specific ELISA was developed to detect ricin in biological fluids. The assay utilizes an affinity-purified goat polyclonal antibody to adsorb ricin from solution. The same antibody (biotinylated) is then used to form a sandwich, and avidin-linked alkaline phosphatase allows color development and measurement of optical density at 405 nm. Our routine assay uses a standard curve over the range of 0-10 ng/ml ricin, with accurate quantitation below 1 ng/ml (100 pg/well) in assay buffer as well as in a 1:10 dilution of human urine or 1:50 dilution of human serum spiked with ricin. Ricin measured in spiked samples demonstrated accuracy typically within 5% of the expected value in all matrices. The coefficient of variation ranged from 3-10% at 10 ng/ml to 8-25% at 2.5 ng/ml. Two variations on the routine assay were also investigated. First, lengthened incubation times and additional time for color development allowed accurate quantitation in serum dilutions as low as 1:2. Second, increased concentrations of biotinylated antibody and avidin-linked enzyme from 1:250 to 1:70 enhanced the sensitivity of the assay 10-fold, achieving a detection limit of at least 100 pg/ml (10 pg/well). The assay was also configured to a format based upon chemiluminescence, which allowed quantitation in the 0.1-1 ng/ml range, but was subject to slightly greater variability than the colorimetric assay.

Demonstration of ricin within the mammalian para-aortic lymph node. I. Comparison of the localization, after intramuscular injection, with three immunocytochemical methods

Following a supralethal injection of ricin into thigh muscle of the adult rat, the toxin was demonstrated post-mortem in the para-aortic lymph node, ipsilateral to the side of injection. The relative merits of two immunoenzyme methods, peroxidase anti-peroxidase (PAP) and avidin-biotin-peroxidase complex (ABC) and a silver-enhanced immunogold method (IGSS) were assessed in the detection of ricin in the lymph node tissue. The toxin was clearly seen to be located in association with histiocytes found both within and lining the sinuses of the nodes and also, in some cases, in the subcapsular sinus of the node; the toxin was not demonstrable within lymphoid follicles by light microscopy. However, using electron microscopy and the IGSS technique, cells carrying discrete particles of gold could be visualized within follicular areas. The IGSS and ABC-peroxidase methods were both found to give excellent results without background staining at the light microscopy level. However, when these techniques were used prior to embedding and viewing by electron microscopy, the IGSS technique proved to be far superior.

Distribution of ricin within the mammalian para-aortic lymph node. II. Comparison of the localization, after intramuscular dosage of colloidal gold-labelled ricin in vivo, with in vitro binding characteristics of the native toxin

Previous work has shown that, following an intramuscular injection of ricin, the toxin becomes localized within histiocytes in the sinuses of lymph nodes draining the 'wound' site. When ricin labelled with colloidal gold was similarly injected, it was found within the same lymphoid cells as seen with native ricin. Biologically inert Indian ink apparently follows a similar fate, as demonstrated by the appearance of carbon particles within sinus histiocytes, as soon as 1 h after intramuscular injection. When the binding in vitro of Indian ink or ricin toxin to sections of lymph node was examined, ricin was seen to bind to the surfaces of the same sinusoidal cells and also, with a much lower frequency, to follicular lymphocytes, whereas Indian ink failed to bind. This indicated an interaction between ricin and cell membrane components. Moreover, this binding was inhibited markedly by the galactose-containing disaccharide, lactose, a target sugar specified by the lectin binding site of ricin and to a much lesser extent by the monosaccharide mannose.