The popliteal lymph node assay: results of a preliminary interlaboratory validation study

Iosimenol, a new non-ionic dimeric contrast medium, does not induce immunoreactivity in the popliteal lymph node assay

Animal studies in mice were conducted to determine the potential immunoreactivity of the new non-ionic dimeric contrast medium (CM) iosimenol using the PLNA and flow cytometric analyses. Comparative studies were performed with iodixanol. The known immune-reactive substance strepozotocin (STZ) and vehicle injections served as positive and negative controls, respectively. Our experiments did not show any immunological effect of iosimenol, concluding that the new CM iosimenol may be beneficial for use in high-risk patients.

Assessment of sensitization potential of monoterpenes using the rat popliteal lymph node assay

The popliteal lymph node assay (PLNA) has been proposed as a screening test for detecting chemicals with potential of inducing allergic and auto-immune-like reactions in humans. In the present study, we used the rat PLNA to evaluate the immuno-sensitizing potential of 10 monoterpenes found in the essential oils of a variety of aromatic, edible and medicinal plants. The primary or direct PLNA was performed with the monoterpenes, and chlorpromazine (CPZ) and barbital were used as positive and negative controls, respectively. Female, 7-8 week-old Wistar rats were injected subcutaneously (50 microL) with the test substance (0.5, 2.5 or 5mg) into the right hind footpad while the contralateral footpad was injected with the vehicle (DMSO) alone. Weight (WI) and cellularity (CI) indices for draining PLNs were determined 7 days after treatment. PLNA was positive (WI >or= 2 and CI >or= 5) for CPZ, citral, alpha-terpinene, beta-myrcene and (-)-alpha-pinene, and negative for barbital, DMSO, (-)-menthol, 1,8-cineole, (+/-) citronellal, (+)-limonene, (+/-) camphor and terpineol. A secondary PLNA, a T-cell priming test, was carried out with the four substances that had been positive in the primary assay. Six weeks after being locally primed with 5 mg/paw, rats were sc injected into the same footpad with a dose (0.5 mg/paw) of the substance that had been previously found to be insufficient to cause a positive response. WI and CI were then calculated 4 and 7 days after the second injection. CPZ was also positive in the secondary assay thereby confirming that it is a sensitizing agent. Citral, alpha-terpinene, beta-myrcene and (-)-alpha-pinene, however, were negative in the secondary assay. In summary, citral, alpha-terpinene, beta-myrcene and (-)-alpha-pinene induced a clear immuno-stimulatory response due to their irritant properties but no monoterpene proved to be a sensitizing agent in the PLNA.

Methods of evaluating immunotoxicity

Immunotoxicology is an important aspect of the safety evaluation of drugs and chemicals. Immunosuppression, (unspecific) immunostimulation, hypersensitivity and autoimmunity are the four types of immune-mediated adverse effects. However, the nonclinical assessment of immunotoxicity is at present often restricted to animal models and assays to predict unexpected immunosuppression. There is, however, no general consensus that a variety of assays can be considered depending on the compound to be tested. A major issue is whether histological examination of the thymus, spleen, lymphoid organs and Peyer's patches is a reliable predictor of immunosuppression or whether immune function should also be assessed. A T-dependent antibody response assay, either the plaque-forming cell assay or anti-keyhole limpet haemocyanin enzyme-linked immunosorbant assay, is recommended as a first-line assay. A variety of assays, including lymphocyte subset analysis, natural killer-cell activity, lymphocyte proliferation, delayed-type hypersensitivity, cytotoxic T-lymphocyte activity and macrophage/neutrophil function assays, can also be used. In certain circumstances, host resistance assays can be considered. With the exception of contact sensitisation, very few animal models and assays can reliably predict the potential for (unspecific) immunostimulation, hypersensitivity or autoimmunity. A major limitation of immunotoxicity risk assessment is the lack of human data. Immunological end points and clinical criteria to be included in clinical trials and epidemiological studies have to be carefully standardised and validated.

Popliteal lymph node (PLN) assay to study adjuvant effects on respiratory allergy

Different variants of the popliteal lymph node (PLN) assay have been published. Here we describe the adjuvant popliteal lymph node assay, an immune response assay to study the adjuvant activity of soluble substances as well as particulate matter. The substance to be studied for adjuvant activity is injected into the hind footpad of mice or rats together with an antigen. Adjuvant activity is determined as the increase in PLN weight and cell numbers in animals receiving antigen together with the substance under study, compared with PLN weight and cell numbers in animals given the antigen without the substance in question, and animals given the putative adjuvant alone. Because lymph node weight and cell numbers are immunologically non-specific parameters, specific immune response assays like serum antibody responses or antibody-forming cell numbers should additionally be performed. Different antigens and immune response assays may be used, depending on the research question asked. In relation to respiratory (or food) allergy, the assays should as a minimum include determination of specific IgE in serum, and preferably also IgG1 (mouse). Serum specific IgG2a antibody determination may be added to get an indication of the Th1-Th2-balance of the response. The adjuvant PLN assay, with cellular response assays performed in the draining popliteal lymph node and antibody determinations in serum, requires small amounts of test material. The assay offers a practical, sensitive and reproducible method to determine the adjuvant activity of soluble substances as well as particulate material, with the possibility to also perform mechanistic studies.

Popliteal lymph node assay: facts and perspectives

The popliteal lymph node assay (PLNA) derives from the hypothesis that some supposedly immune-mediated adverse effects induced by certain pharmaceuticals involve a mechanism resembling a graft-versus-host reaction. The injection of many but not all of these compounds into the footpad of mice or rats produces an increase in the weight and/or cellularity of the popliteal lymph node in the treated limb (direct PLNA). Some of the compounds known to cause these adverse effects in humans, however, failed to induce a positive PLNA response, leading to refinements of the technique to include pretreatment with enzyme inducers, depletion of CD4(+) T cells or additional endpoints such as histological examination, lymphocyte subset analysis and cytokine fingerprinting. Alternative approaches have been used to improve further the predictability of the assay. In the secondary PLNA, the test compound is injected twice in order to illicit a greater secondary response, thus suggesting a memory-specific T cell response. In the adoptive PLNA, popliteal lymph node cells from treated mice are injected into the footpad of naive mice; a marked response to a subsequent footpad challenge demonstrates the involvement of T cells. Finally, the reporter antigens TNP-Ficoll and TNP-ovalbumin are used to differentiate compounds that induce responses involving neo-antigen help or co-stimulatory signals (modified PLNA). The PLNA is increasingly considered as a tool for detection of the potential to induce both sensitization and autoimmune reactions. A major current limitation is validation. A small inter-laboratory validation study of the direct PLNA found consistent results. No such study has been performed using an alternative protocol. Other issues include selection of the optimal protocol for an improved prediction of sensitization vs autoimmunity, and the elimination of false-positive responses due to primary irritation. Finally, a better understanding of underlying mechanisms is essential to determine the most relevant endpoints. The confusion resulting from use of the PLNA to predict autoimmune-like reactions as well as sensitization should be clarified. Interestingly, most drugs that were positive in the direct PLNA are also known to cause drug hypersensitivity syndrome in treated patients. This observation is expected to open new avenues of research.

Iosimenol, a low-viscosity nonionic dimer: preclinical physicochemistry, pharmacology, and pharmacokinetics

RATIONALE AND OBJECTIVES

Newer radiologic techniques require fast bolus injections and thus low-viscosity, high-concentration, well-tolerated contrast media (CM), especially in vulnerable patients. To this end, we designed and developed iosimenol, a novel isotonic nonionic dimer, and have conducted tests to enable its clinical evaluation.

METHODS

Standard physicochemical methods were used. Effects on erythrocyte morphology and coagulation were investigated in human and rat blood. Neural tolerance was assessed by behavioral tests in rats after intracisternal injection. Immunosensitizing potential was evaluated by the skin sensitization test in guinea pigs and by the popliteal lymph node assay in rats. Pharmacokinetics and biotransformation were investigated in rats and dogs.

RESULTS

Iosimenol is extremely hydrophilic, it is less viscous than any other isotonic CM, has little effect on erythrocytes and blood coagulation, and has good neural tolerance. No immunosensitizing effect was found in validated animal models. Pharmacokinetics are identical with other angio- and urographic CM.

CONCLUSIONS

Iosimenol is the only CM which, although isotonic, affords, unlike current nonionic dimers, at the same iodine concentration the low viscosity of monomeric, nonionic agents, which are all hypertonic. Iosimenol's pharmacologic characteristics closely resemble those of iotrolan and iodixanol.

Cytokine release does not improve the sensitivity and specificity of the direct popliteal lymph node assay

The popliteal lymph node assay (PLNA) is being considered as a tool to predict the potential of drugs for inducing systemic autoimmune and hypersensitivity reactions. Despite the use of different technical approaches and the evaluation of over 130 compounds, the sensitivity and specificity of the PLNA are still debatable due to many false positive and negative responses. In this study, cytokine production was assessed as a possible endpoint to improve the direct (primary) PLNA. Diclofenac, imipramine, hydralazine, glafenin and minocycline were tested using the classical procedure. TH1 cytokines (IL-2 and IFN-gamma), TH2 cytokines (IL-4 and IL-5) and pro-inflammatory cytokines (IL-6, TNF-alpha, monocyte chemoattractant protein-1 (MCP-1), IL-12p70 and IL-10) were measured in the serum and in suspensions of popliteal lymph node cells of female Balb/c mice by flow cytometry 7 days after drug administration. Only diclofenac and imipramine induced a cellularity index above 5 (considered as a positive response). Of the five tested drugs, only diclofenac induced a slight increase in TH1 cytokines, but there were no effects on TH2 cytokine production whatever the drug tested. Diclofenac increased the production of pro-inflammatory cytokines, whereas the production of MCP-1 was increased by minocycline and decreased by imipramine. No changes in serum cytokine levels were evident. These results suggest that measuring cytokine release is unlikely to improve the sensitivity and specificity of the direct PLNA.

Popliteal lymph node responses to acetone and ethanol differ from those induced by streptozotocin

The popliteal lymph node (PLN) assay was proposed to detect the potential of immunotoxicants for inducing systemic autoimmune-like reactions, but also xenobiotics that are sensitizing or exert immunostimulatory properties. Results on over 100 chemicals, mostly pharmaceuticals, are available with the PLN assay and show many correlations between rodent data and the clinical experience. A major issue is that the mechanisms involved have not been fully elucidated. In order to provide mechanistic clues to improve the predictability of the PLN assay, the effects of streptozotocin (STZ) were compared to those of ethanol and acetone in normal C57Bl/6 mice as well as mice depleted in CD4+ or CD8+ T-cells by treatment with specific monoclonal antibodies. STZ, ethanol and acetone gave similar positive responses in normal mice. Neither CD4+ nor CD8+ T-cell depletion influenced the PLN responses to ethanol or acetone, whereas CD8+ in contrast to CD4+ T-cell depletion abolished the response to STZ. There was an increase in the production of IL-6 and IFN-gamma mRNAs measured by RT-PCR in STZ-, but not in ethanol- or acetone-treated normal mice. The production of TNFalpha, IL-1alpha, IL-1beta, IL-2R and IL-12 mRNAs was increased whatever the treatment, but increases were 2- to 3-fold greater after STZ than ethanol or acetone. These results suggest that PLN responses to primary irritants such as ethanol and acetone essentially reflect non-specific inflammation, whereas PLN responses to an autoimmunogenic compound such as STZ involve CD8+ T lymphocytes and the production of IFN-gamma and IL-6. These findings may prove useful to improve the predictability of the PLN assay.

Tritiated thymidine incorporation does not enhance sensitivity of the popliteal lymph node assay

The popliteal lymph node (PLN) assay has been proposed as a tool to predict drugs and chemicals with the potential to induce systemic autoimmune reactions in man. In this assay, weight and cellularity indices typically are the measured endpoints. The present study was conducted to test whether incorporation of tritiated thymidine could improve sensitivity of the PLN assay. Male and female Balb/c mice were injected with 20 microCi of [3H]-methyl-thymidine intravenously 7 days after receiving 0.5, 1 or 2 mg of diphenylhydantoin, streptozotocin, sulfamethoxazole, ofloxacin, phenobarbital, or metformin intradermally. Results obtained with incorporation of tritiated thymidine were compared to weight indices. No consistent or marked differences in these endpoints were noted whatever the compound used. This study shows that incorporation of tritiated thymidine does not improve sensitivity of the PLN assay.

Nonclinical evaluation of therapeutic cytokines: immunotoxicologic issues

Cytokines are a diverse family of immunoregulatory proteins whose function has been the focus of intensive research for the past 25 years. Modern molecular biology techniques have resulted in production of these proteins in quantities sufficient to consider them as candidates for drug development. Due to the often pleiotropic nature of their action, in order to design meaningful nonclinical safety assessment studies, the practicing toxicologist must be able to differentiate between an exaggerated, but expected pharmacologic response and toxicity associated with their administration. Species specificity, nontraditional dose-response relationships, antigenicity, and unanticipated immunotoxicity are a few issues that must be considered when designing study protocols and interpreting data. In addition, despite validated methodology to examine the immune system as a potential target organ for toxicity, limitations exist in the ability to predict the autoimmune and allergenic potential of proteins such as these. Despite these challenges and limitations, over the past decade, many of these molecules have found clinical utility to treat a wide variety of diseases.

Investigating the TNP-OVA and direct popliteal lymph node assays for the detection of immunostimulation by drugs associated with anaphylaxis in humans

Using current animal models, it is not possible to identify low-molecular-weight compounds (LMWCs) that are likely to be associated with anaphylaxis. It is generally accepted that the ultimate effector mechanism involves drug-induced IgE antibody. The objective of the present study was to determine if diclofenac, zomepirac and glafenine, which are associated with anaphylaxis in humans, have immunostimulating potential in the murine TNP-OVA (trinitrophenyl-ovalbumin) popliteal lymph node assay (PLNA), and more specifically to determine if the immunostimulation caused by these LMWCs results in IgE antibody production. These LMWCs were chosen because both zomepirac and glafenine were removed from the market due to high association with anaphylaxis, and diclofenac, which remains on the market, is frequently associated with anaphylaxis. In addition to conducting a TNP-OVA PLNA, the immunostimulating potential of these compounds was examined in the direct PLNA. When co-administered with TNP-OVA, all three LMWCs caused dose-dependent (0.25, 0.50, 1.00 and 1.25 mg) increases in popliteal lymph node (PLN) weight and cellularity that were observed beginning with the 0.25-mg dose. In addition, beginning with the 0.25-mg dose, all three compounds caused dose-dependent increases in TNP-OVA specific IgM and IgG(1) antibody-forming cells (AFCs). Diclofenac induced an isotype switch and caused a dose-dependent increase in the number of IgE AFCs with no detectable IgG(2a) AFCs and minimal high-dose-only IgG(2b) AFCs. Zomepirac induced IgE, IgG(2a) and IgG(2b) AFCs following the injection of 0.50 mg only, and glafenine induced IgE, IgG(2a) and IgG(2b) AFCs following the injection of 0.50-1.00 mg. In the direct PLNA, diclofenac caused dose-dependent increases in PLN weight and cellularity that were observed beginning with dose of 0.50 mg, whereas zomepirac failed to increase any PLN parameter and glafenine only increased the PLN weight. These results suggest that diclofenac, zomepirac and glafenine are immunostimulating LMWCs in the TNP-OVA PLNA with the potential to induce IgE antibody against a co-administered hapten-conjugate. Furthermore, these results suggest that the TNP-OVA PLNA offered significant advantages over the direct PLNA. Although it is not realistic to suggest that a single assay, based on a low number of test compounds, can identify all LMWCs with the potential to cause anaphylaxis in humans, these observations do demonstrate the potential utility of the PLNAs in examining LMWC-induced immunomodulation and support further development and investigation of the assays.

Application of the popliteal lymph node assay in immunotoxicity testing: complementation of the direct popliteal lymph node assay with flow cytometric analyses

The popliteal lymph node assay (PLNA) has been proposed to measure the immunosensitizing potential of chemicals. The direct PLNA detects an immunomodulating effect but does not give insight into the mode of action of the chemical under test. Modifications of this test have been proposed, but they are difficult to perform in routine toxicity testing and require many animals. In the present investigation the direct PLNA was extended with the flow cytometric determinations of: (a) lymphoblasts; (b) the phenotyping of lymphoid subpopulations; (c) the determination of expression of proliferation/activation markers CD25, CD69 and CD62L/CD44 and (d) the analysis of intracellular cytokines interferon gamma, interleukin 2 and interleukin 4. Streptozotocin, hydrazine, HgCl2 and trinitrobenzene sulfonic acid were used as model chemicals. The different mode of action of these substances was well documented by the techniques applied. As the proposed flow cytometric methods can easily be performed and do not require additional test animals this complementation of the direct PLNA seems a promising approach in immunotoxicity testing.

Positive responses to imipramine in the popliteal lymph node assay are due to primary irritation

The popliteal lymph node (PLN) assay has long been proposed as a tool to detect immunotoxicants with the potential to induce systemic autoimmunity. A major problem hampering the further validation of this assay is the need to rule out irritants that cause false-positive PLN responses. The anti-depressant, imipramine, has not been reported to induce systemic autoimmune reactions in treated patients, but has been repeatedly found positive in the PLN assay, suggesting that this is a false-positive response. To test this hypothesis, the effects of imipramine were compared to those of 50% ethanol in C57B1/6 mice. Footpad edema was evidenced in the few days after injection of both ethanol and imipramine. T-cell depletion using monoclonal antibodies against either CD4+ or CD8+ T-lymphocytes prior to the PLN assay did not influence the responses to either ethanol or imipramine. Cytokine (TNFalpha, IL-1alpha, IL-1beta, IL-2R, IL-6, IL-12 and IFN-gamma) fingerprinting of the PLNs after injection of ethanol and imipramine evidenced the same pattern of responses. These results indicate a closely similar pattern of responses following the footpad injection of either imipramine or ethanol. The conclusion can be drawn that imipramine induces positive responses in the PLN assay via primary (nonspecific) irritation.

The local lymph node assay and potential application to the identification of drug allergens

The local lymph node assay (LLNA) is a method for the identification of skin sensitization hazard. The method is based upon measurement of proliferative responses induced in draining lymph nodes following topical exposure of mice to the test chemical. More recently the LLNA has also been used for the evaluation of relative skin sensitization potency in the context of risk assessment. Idiosyncratic drug reactions resulting from the stimulation of allergic or autoimmunogenic responses are poorly understood but represent an important clinical problem. In this article, the potential utility of the LLNA, either in a conventional modified configuration, to provide information of value in assessment the potential for systemic allergenicity is considered.

Pre-clinical methods for detecting the hypersensitivity potential of pharmaceuticals: regulatory considerations

Immune-based systemic hypersensitivities account for a significant number of adverse drug reactions. There appear to be no adequate nonclinical models to predict systemic hypersensitivity to small molecular weight drugs. Although there are very good methods for detecting drugs that can induce contact sensitization, these have not been successfully adapted for prediction of systemic hypersensitivity. Several factors have made the development of adequate models difficult. The term systemic hypersensitivity encompases many discrete immunopathologies. Each type of immunopathology presumably is the result of a specific cluster of immunologic and biochemical phenomena. Certainly other factors, such as genetic predisposition, metabolic idiosyncrasies, and concomitant diseases, further complicate the problem. Therefore, it may be difficult to find common mechanisms upon which to construct adequate models to predict specific types of systemic hypersensitivity reactions. There is some reason to hope, however, that adequate methods could be developed for at least identifying drugs that have the potential to produce signs indicative of a general hazard for immune-based reactions.

Increased production of interferon-gamma, but not IL-4 mRNA, by streptozotocin in the popliteal lymph node assay

The popliteal lymph node (PLN) assay has been proposed as a tool to predict systemic autoimmune reactions induced by medicinal products and chemicals, the mechanisms of which are poorly understood. To determine whether PLN responses involved Th1 or Th2 cell control, or both, the effects of streptozotocin (STZ), a prototypic immunotoxic compound, were analysed on the production of interferon-gamma (IFN-gamma) and interleukin-4 (IL-4) mRNA by lymph node cells after injection into the hind footpad of C57 BL/6 mice. Streptozotocin induced a dramatic increase in IFN-gamma mRNA production, which correlated with PLN responses as evidenced by augmented weight and cellularity indices. No effect on IL-4 mRNA synthesis was noted. These results suggest that a Th1 response is involved in the PLN response to STZ.

The popliteal lymph node response to streptozotocin is under type 1, MHC class-I restricted, CD8(+) T-cell control

The popliteal lymph node (PLN) assay has been proposed to predict the 'autoimmunogenic' potential of xenobiotics. A better understanding of the processes involved in PLN responses is needed to establish the value of this assay for preclinical safety evaluation. In order to determine whether PLN responses involve CD4(+) or CD8(+) T-cells, the effects of streptozotocin (STZ), a prototypic immunotoxic compound, were analyzed after injection into the hind footpad of C57 BL/6 mice and major histocompatibility complex (MHC) class I or II deficient mice. The involvement of type 1 or type 2 cell control on the production of cytokine mRNAs was analyzed in lymph node cells by quantitative RT-PCR, together with the analysis of a wide range of cytokine mRNAs after STZ injection (IL-1alpha, IL-1beta, TNF-alpha, IFN-gamma, IL-2, IL-2 receptor, IL-4, IL-5, IL-6, IL-10 and IL-12). We have found that mice depleted in CD8(+) T-cells did not respond to STZ, whereas mice depleted in CD4(+) T-cells exhibited the expected positive PLN responses, with increased weight and cellularity indices. STZ induced a low production of interleukin (IL)-2 mRNAs, a mild increase in IL-1alpha and IL-6 mRNAs production, and a dramatic increase in IFN-gamma, IL-1beta, TNF-alpha, IL-12 and IL-2 receptor mRNAs, which correlated with positive PLN responses. No effects on IL-4, IL-5 and IL-10 mRNAs synthesis were noted. In CD8(+) T-cell deficient mice, there was no production of IFN-gamma or IL-6 mRNAs. These results suggest that PLN responses to STZ are under the control of type 1, MHC class-I-restricted, CD8(+) T-cells. This is in accordance to the known physiopathology of STZ-induced diabetes. Additional studies are necessary to establish the mechanism of CD8+ T-cell activation.

Autoimmunity and toxicity testing

Major difficulties when addressing autoimmunity today within the context of regulatory toxicology are the following: (i) the incidence of auto-immunity related to chemical exposure is not known; (ii) the mechanisms involved are not understood in most instances; and (iii) no fully validated models are available. Although no relationship has been firmly established between the serum levels of autoantibodies and the development and/or the severity of autoimmune diseases, these are considered the hallmarks of autoimmunity. It has proved impossible so far to detect reproducibly autoantibodies indicative of organ-specific autoimmune reactions induced by chemicals in conventional toxicity testing. The detection of autoantibodies suggestive of a systemic autoimmune reaction has been successful with very few compounds only, and most often using non-conventional strains of animals. Genetically-deficient animal strains were sometimes helpful, but these models should be standardised and validated. This also applies to experimental autoimmune disease models. The popliteal lymph node (PLN) assay is potentially helpful, but more research and validation efforts are warranted. From a regulatory toxicology perspective, the search for serum autoantibodies, as it is performed today, does not seem to be a reliable tool, and progress is more likely to be expected from the design, standardisation and validation of dedicated models.