Local lymph node assay: use in hazard and risk assessment

Chemical respiratory sensitization-Current status of mechanistic understanding, knowledge gaps and possible identification methods of sensitizers

Respiratory sensitization is a complex immunological process eventually leading to hypersensitivity following re-exposure to the chemical. A frequent consequence is occupational asthma, which may occur after long latency periods. Although chemical-induced respiratory hypersensitivity has been known for decades, there are currently no comprehensive and validated approaches available for the prospective identification of chemicals that induce respiratory sensitization, while the expectations of new approach methodologies (NAMs) are high. A great hope is that due to a better understanding of the molecular key events, new methods can be developed now. However, this is a big challenge due to the different chemical classes to which respiratory sensitizers belong, as well as because of the complexity of the response and the late manifestation of symptoms. In this review article, the current information on respiratory sensitization related processes is summarized by introducing it in the available adverse outcome pathway (AOP) concept. Potentially useful models for prediction are discussed. Knowledge gaps and gaps of regulatory concern are identified.

Peptide reactivity assays for skin sensitisation - scope and limitations

The direct peptide reactivity assay (DPRA) is an OECD test guideline method that aims to determine if a chemical is reactive enough to be a skin sensitiser. It involves incubation of the test chemical at 5 mMolar concentration for 24 h with a cysteine-based peptide at 0.5 mMolar concentration and measurement of the percentage depletion (DP) of the peptide. The kinetic direct peptide reactivity assay (kDPRA) is derived from the DPRA and involves incubating the peptide with the test chemical at a range of concentrations and incubation times to produce a data matrix of DP values, which is analysed to give a reactivity parameter logk_max that assigns chemicals to the 1A potency class (high potency) if logk_max reaches the threshold value of -2. Here the DPRA, with a threshold of 47% DP, is compared against the kDPRA for their abilities to distinguish between the 1A and non-1A potency classes. It is found that they perform very similarly against a dataset of 157 chemicals with known potency, with only marginal differences in predictive performance. The thresholds of -2.0 (kDPRA) and 47% DP (DPRA) to distinguish 1A sensitisers are not scientific absolutes but the best compromises for a heterogenous set of data containing classes of chemicals for which different thresholds would be applicable. It is concluded that although the kDPRA represents a major advance towards predicting skin sensitisation potency on a continuous basis without animal testing, it offers no significant advantage over the DPRA for the purpose of 1A classification.

Pre-validation of SENS-IS assay for in vitro skin sensitization of medical devices

According to ISO 10993-1:2018, the skin sensitization potential of all medical devices must be evaluated, and for this endpoint ISO 10993-10:2010 recommends the use of in vivo assays. The goal of the present study was to determine if the in vitro SENS-IS assay could be a suitable alternative to the current in vivo assays. The SENS-IS assay uses the Episkin Large and SkinEthic RHE reconstructed human epidermis models to evaluate marker genes. In our study, the SENS-IS assay correctly identified 13 sensitizers spiked in a non-polar solvent. In a subsequent analysis six medical device silicone samples previously impregnated with sensitizers were extracted with polar and non-polar solvents. The SENS-IS assay correctly identified five of these extracts, while a sixth extract, which contained the weak sensitizer phenyl benzoate, was classified as negative. However, when this extract was concentrated, or a longer exposure time was used, the assay was able to detect phenyl benzoate. The SENS-IS assay was transferred to a naïve laboratory which correctly identified sensitizers in six blinded silicone samples, including the one containing phenyl benzoate. In light of these results, we conclude that the SENS-IS assay is able to correctly identify the presence of sensitizers in medical devices extracts.

Evaluation of the skin-sensitizing potential of gold nanoparticles and the impact of established dermal sensitivity on the pulmonary immune response to various forms of gold

Gold nanoparticles (AuNP) are largely biocompatible; however, many studies have demonstrated their potential to modulate various immune cell functions. The potential allergenicity of AuNP remains unclear despite the recognition of gold as a common contact allergen. In these studies, AuNP (29 nm) dermal sensitization potential was assessed via Local Lymph Node Assay (LLNA). Soluble gold (III) chloride (AuCl₃) caused lymph node (LN) expansion (SI 10.9), whereas bulk particles (Au, 942 nm) and AuNP did not. Next, the pulmonary immune effects of AuNP (10, 30, 90 µg) were assessed 1, 4, and 8 days post-aspiration. All markers of lung injury and inflammation remained unaltered, but a dose-responsive increase in LN size was observed. Finally, mice were dermally-sensitized to AuCl₃ then aspirated once, twice, or three times with Au or AuNP in doses normalized for mass or surface area (SA) to assess the impact of existing contact sensitivity to gold on lung immune responses. Sensitized animals exhibited enhanced responsivity to the metal, wherein subsequent immune alterations were largely conserved with respect to dose SA. The greatest increase in bronchoalveolar lavage (BAL) lymphocyte number was observed in the high dose group - simultaneous to preferential expansion of BAL/LN CD8+ T-cells. Comparatively, the lower SA-based doses of Au/AuNP caused more modest elevations in BAL lymphocyte influx (predominantly CD4+ phenotype), exposure-dependent increases in serum IgE, and selective expansion/activation of LN CD4+ T-cells and B-cells. Overall, these findings suggest that AuNP are unlikely to cause sensitization; however, established contact sensitivity to gold may increase immune responsivity following pulmonary AuNP exposure.

Evaluation of 4-methylcyclohexanemethanol (MCHM) in a combined irritancy and Local Lymph Node Assay (LLNA) in mice

4-Methylcyclohexanemethanol (MCHM) is a flotation reagent used in fine coal beneficiation. On January 9, 2014, crude MCHM, a mixture containing predominantly MCHM, was inadvertently released into the Elk River, a municipal water source that serves about 300,000 people in the Charleston, WV area, resulting in temporary contamination of 15 percent of the state's tap water and causing significant dermal exposure. The current studies were undertaken to determine whether crude MCHM or MCHM has the potential to produce dermal irritancy and/or sensitization. BALB/c female mice were treated daily for 3 consecutive days by direct epicutaneous application of 25 μL of various concentrations of crude MCHM or MCHM to the dorsum of each ear. A mouse ear-swelling test was used to determine irritancy potential and was undertaken in combination with the standardized Local Lymph Node Assay (LLNA) to determine skin sensitizing potential. MCHM was found to produce skin irritation at concentrations above 20% and did not produce sensitization. Crude MCHM also produced irritation, although weaker, and in addition was found to be a weak to moderate skin sensitizer. The results are discussed in terms of potential human health hazard.

The use of peptide reactivity assays for skin sensitisation hazard identification and risk assessment

Over the past 20 years or more, investigators have been developing non-animal test methods for use in assessing the skin sensitisation potential of chemicals. In parallel with this effort, the key biological events of skin sensitisation have been well-characterised in an Adverse Outcome Pathway (AOP) proposed by the Organisation for Economic Co-operation and Development (OECD). The key molecular initiating event of this AOP is haptenation or covalent modification of epidermal proteins. In this review, the strengths and limitations of the Direct Peptide Reactivity Assay (DPRA) are described, and the more recently developed Peroxidase Peptide Reactivity Assay (PPRA). The DPRA has been formally validated and incorporated into an OECD Test Guideline (TG442C). The DPRA shows promise for assisting in hazard identification as well as for assessing skin sensitisation potency when used in an integrated testing strategy.

Ethanol and Diethyl Phthalate: Vehicle Effects in the Local Lymph Node Assay

The vehicle in which an allergen is presented to the skin has been recognized to have an effect on the skin-sensitizing potency of the allergen. Typical vehicles used to evaluate the skin sensitization potential of fragrance materials include ethanol, diethyl phthalate, or a combination of the two. The authors conducted a series of studies to evaluate each of these vehicles for their utility in the murine local lymph node assay and to investigate the potential differences in skin sensitization resulting from their use. Four fragrance materials were tested in four different vehicles. The test materials were p-t-butyl- α-methylhydrocinnamic aldehyde, geraniol, eugenol, and hydroxycitronellal. The vehicles were diethyl phthalate, 1:3 ethanol:diethyl phthalate, 3:1 ethanol:diethyl phthalate, and ethanol. Each of the fragrance materials was tested at five dose levels ranging from 0.3% to 50% w/v. In all four vehicles, each material tested elicited positive responses, exhibiting weak to moderate skin sensitization potential. Overall, p-t-butyl- α-methylhydrocinnamic aldehyde exhibited the most potency, followed by eugenol, geraniol, and hydroxycitronellal. The sensitization potential of both p-t-butyl- α-methylhydrocinnamic aldehyde and geraniol was greatest when the vehicle was ethanol. The sensitization potential of eugenol was greatest in 3:1 ethanol:diethyl phthalate, but the sensitization potential for hydroxycitronellal was greatest in 1:3 ethanol:diethyl phthalate. The strength of the sensitization response was observed to vary with the vehicle; however, the results did not show any clear pattern of one vehicle over another regarding skin sensitization.

Skin Sensitization: Modeling Based on Skin Metabolism Simulation and Formation of Protein Conjugates

A quantitative structure-activity relationship (QSAR) system for estimating skin sensitization potency has been developed that incorporates skin metabolism and considers the potential of parent chemicals and/or their activated metabolites to react with skin proteins. A training set of diverse chemicals was compiled and their skin sensitization potency assigned to one of three classes. These three classes were, significant, weak, or nonsensitizing. Because skin sensitization potential depends upon the ability of chemicals to react with skin proteins either directly or after appropriate metabolism, a metabolic simulator was constructed to mimic the enzyme activation of chemicals in the skin. This simulator contains 203 hierarchically ordered spontaneous and enzyme controlled reactions. Phase I and phase II metabolism were simulated by using 102 and 9 principal transformations, respectively. The covalent interactions of chemicals and their metabolites with skin proteins were described by 83 reactions that fall within 39 alerting groups. The SAR/QSAR system developed was able to correctly classify about 80% of the chemicals with significant sensitizing effect and 72% of nonsensitizing chemicals. For some alerting groups, three-dimensional (3D)-QSARs were developed to describe the multiplicity of physicochemical, steric, and electronic parameters. These 3D-QSARs, so-called pattern recognition-type models, were applied each time a latent alerting group was identified in a parent chemical or its generated metabolite(s). The concept of the mutual influence amongst atoms in a molecule was used to define the structural domain of the skin sensitization model. The utility of the structural model domain and the predictability of the model were evaluated using sensitization potency data for 96 chemicals not used in the model building. The TIssue MEtabolism Simulator (TIMES) software was used to integrate a skin metabolism simulator and 3D-QSARs to evaluate the reactivity of chemicals thus predicting their likely skin sensitization potency.

Contact dermatitis: in pursuit of sensitizer's molecular targets through proteomics

Protein haptenation, i.e., the modification of proteins by small reactive chemicals, is the key step in the sensitization phase of allergic contact dermatitis (ACD). Despite the research effort in past decades, the identification of immunogenic hapten-protein complexes that trigger a relevant pathogenic immune response in ACD, as well as the haptenation reaction molecular site, and the elements of a potentially conditioning environment during each of these stages, remain poorly understood. These questions led us to employ a proteomics-based approach to identify modified proteins in the dendritic-like cell line THP-1 sensitized with fluorescein isothiocyanate (FITC), through a combination of 2D-gel electrophoresis, nano-LC and mass spectrometry. A specific set of 39 targeted proteins was identified and comprised proteins from various cellular locations and biological functions. One of FITC targets was identified as MLK, a member of the mixed-lineage kinase family known to act as a mitogen-activated protein kinase kinase kinase and to control the activity of specific mitogen-activated protein kinase pathways, namely p38 and JNK pathways. Haptenated in the vicinity of its active site, our results point to MLK being a relevant target due to a consistent non-activation at early time points of these pathways upon FITC sensitization in THP-1 cells. Moreover, FITC pre-treatment significantly decrease phospho-p38 and phospho-JNK levels induced upon exposure to a classical activator such as lipopolysaccharide or to the sensitizer 2,4-dinitrofluorobenzene. Overall, our data point to specific amino acid residues haptenation within critical proteins as the key step in the subsequent signaling pathways modulation responsible for DC activation and maturation events.

Mechanism-Based QSAR Modeling of Skin Sensitization

Many chemicals can induce skin sensitization, and there is a pressing need for non-animal methods to give a quantitative indication of potency. Using two large published data sets of skin sensitizers, we have allocated each sensitizing chemical to one of 10 mechanistic categories and then developed good QSAR models for the seven categories that have a sufficient number of chemicals to allow modeling. Both internal and external validation checks showed that each model had good predictivity.

Effect of mouse strain in a model of chemical-induced respiratory allergy

The inhalation of many types of chemicals is a leading cause of allergic respiratory diseases, and effective protocols are needed for the detection of environmental chemical–related respiratory allergies. In our previous studies, we developed a method for detecting environmental chemical–related respiratory allergens by using a long-term sensitization–challenge protocol involving BALB/c mice. In the current study, we sought to improve our model by characterizing strain-associated differences in respiratory allergic reactions to the well-known chemical respiratory allergen glutaraldehyde (GA). According to our protocol, BALB/c, NC/Nga, C3H/HeN, C57BL/6N, and CBA/J mice were sensitized dermally with GA for 3 weeks and then challenged with intratracheal or inhaled GA at 2 weeks after the last sensitization. The day after the final challenge, all mice were euthanized, and total serum IgE levels were assayed. In addition, immunocyte counts, cytokine production, and chemokine levels in the hilar lymph nodes (LNs) and bronchoalveolar lavage fluids (BALF) were also assessed. In conclusion, BALB/c and NC/Nga mice demonstrated markedly increased IgE reactions. Inflammatory cell counts in BALF were increased in the treated groups of all strains, especially BALB/c, NC/Nga, and CBA/J strains. Cytokine levels in LNs were increased in all treated groups except for C3H/HeN and were particularly high in BALB/c and NC/Nga mice. According to our results, we suggest that BALB/c and NC/Nga are highly susceptible to respiratory allergic responses and therefore are good candidates for use in our model for detecting environmental chemical respiratory allergens.

Contact sensitizing potential of pyrogallol and 5-amino-o-cresol in female BALB/c mice

Hair dye components such as pyrogallol and cresol have been shown previously to promote allergic reactions such as rashes, dermal inflammation, irritation and dermatitis. The objective of this study was to determine the contact sensitization potential of pyrogallol (PYR) and 5-amino-o-cresol (AOC) when applied dermally to female BALB/c mice. Measurement of the contact hypersensitivity response was initially accomplished using the local lymph node assay. For PYR, significant increases in the proliferation of lymph node cells were observed at concentrations of 0.5% (w/v) and higher. For AOC, borderline increases, albeit significant, in auricular lymph node cell proliferation were observed at 5% and 10%. Results from the irritancy assay suggested that PYR, but not AOC, was an irritant. To further delineate whether PYR was primarily an irritant or a contact sensitizer, the mouse ear swelling test (MEST) was conducted. A significant increase in mouse ear thickness was observed at 72h following challenge with 0.5% PYR in mice that had been sensitized with 5% PYR. In contrast, no effects were observed in the MEST in mice sensitized and challenged with the highest achievable concentration of AOC (10%). Additional studies examining lymph node subpopulations and CD86 (B7.2) expression by B cells further support the indication that PYR was a sensitizer in BALB/c mice. The results demonstrate that PYR is both a sensitizer and an irritant in female BALB/c mice. However, the contact sensitization potential of AOC is minimal in this strain of mouse.

Allergy-related disorders in the construction industry

Working conditions in the construction industry have improved in many industrialized countries, but heavy physical work with recurrent exposure to chemical agents, dust, and climatic influences still represents considerable risk for construction workers and may affect their health. The aim of this review is to analyze available data of the literature on allergy-related respiratory and skin disorders with emphasis on a preventive appraisal in order to produce statements and recommendations based on research evidence. The most common agents involved in the construction industry as a cause of occupational asthma (OA) in industrialized countries are isocyanates, wood dust, resins, glues, cobalt, and chromium. Allergic contact dermatitis (ACD) is an immunologic cell-mediated response to a sensitizing agent and the most common sensitizing agents associated with construction workers are epoxy resins, thiurams and thiazoles, and chromates. Medical surveillance must consider individual risk factors such as differences in individual susceptibility and sensitization to agents at workplace. Once work-related disorder is confirmed, adequate fitness for work should be assessed for the worker impaired by health condition. A reliable diagnosis of an index case is a sentinel event that may reveal risks for workers with similar exposure, leading to a revised risk assessment at the workplace that should reduce the risk and prevent further cases.

Current topics in occupational asthma

The study of occupational asthma (OA) provides insights into asthma in general, as the cause is known. The relationships between the cause and response can be measured and modifying factors can be identified and their influence quantified. Developing OA has much more serious consequences for the patient than new onset asthma unrelated to work exposures, as the patient's livelihood is nearly always affected. Many healthcare professionals are more ready to accept and act on asthmatic symptoms when they are unrelated to work than when work may be the cause; antagonism can also occur in the workplace. This article reviews some of the areas where development and controversy enrich the study of OA. It makes no attempt to be comprehensive.

Identification of phenolic dermal sensitizers in a wound closure tape

A latex-allergic patient presented with a severe local reaction to a non-latex wound closure bandage following surgery. Extracts of the bandage were analyzed by gas chromatograph-electron impact-mass spectrometry (GC EI-MS) in the total ion monitoring mode. Components were identified by their ion mass fingerprint and elution time as a corresponding standard from the GC column. The chemicals identified were 4,4'-thiobis-(6-tert-butyl-m-cresol) (TBBC), 6-tert-Butyl-m-cresol (BC), 2,4-di-tert-butylphenol (BP) and erucamide (EA). Sensitization potential of these chemicals was evaluated using two quantitative structure-activity relationship (QSAR) programs. The phenol 2,6-di-tert-butyl-4-(hydroxymethyl)phenol (BHP) was also included in the test series. It was initially thought to be present in the bandage but detectable levels could not be confirmed. The potential for TBBC to induce a sensitization response was predicted by both Derek for Windows and TOPKAT 6.2. The potential for BC and BP to induce a sensitization response was predicted by Derek for Windows, but not TOPKAT. BHP and EA were not predicted to be sensitizers by either QSAR program. Local lymph node assay (LLNA) analysis of the chemicals identified TBBC, BP, and BC as potential sensitizers with EC3 values between 0.2 and 4.5%. None of the animals exhibited body weight loss or skin irritation at the concentrations tested. In agreement with the toxicological modeling, BHP did not induce a sensitization response in the LLNA. Following a positive LLNA response, TBBC, BP, and BC were further characterized by phenotypic analysis of the draining lymph nodes. A positive LLNA result coupled with a lack of increase in B220(+)IgE(+) cell and serum IgE characterize these chemicals as Type IV sensitizers. These studies used a multidisciplinary approach combining clinical observation, GC-EI-MS for chemical identification, QSAR modeling of chemicals prior to animal testing, and the LLNA for determination of the sensitization potential of chemicals in a manufactured product.

Biodegradable Polymers Induce CD54 on THP-1 Cells in Skin Sensitization Test

Currently, nonanimal methods of skin sensitization testing for various chemicals, biodegradable polymers, and biomaterials are being developed in the hope of eliminating the use of animals. The human cell line activation test (h-CLAT) is a skin sensitization assessment that mimics the functions of dendritic cells (DCs). DCs are specialized antigen-presenting cells, and they interact with T cells and B cells to initiate immune responses. Phenotypic changes in DCs, such as the production of CD86 and CD54 and internalization of MHC class II molecules, have become focal points of the skin sensitization test. In this study, we used h-CLAT to assess the effects of biodegradable polymers. The results showed that several biodegradable polymers increased the expression of CD54, and the relative skin sensitizing abilities of biodegradable polymers were PLLG (75 : 25) < PLLC (40 : 60) < PLGA (50 : 50) < PCG (50 : 50). These results may contribute to the creation of new guidelines for the use of biodegradable polymers in scaffolds or allergenic hazards.

The LLNA: A Brief Review of Recent Advances and Limitations

Allergic contact dermatitis is the second most commonly reported occupational illness, accounting for 10% to 15% of all occupational diseases. This highlights the importance of developing rapid and sensitive methods for hazard identification of chemical sensitizers. The murine local lymph node assay (LLNA) was developed and validated for the identification of low molecular weight sensitizing chemicals. It provides several benefits over other tests for sensitization because it provides a quantitative endpoint, dose-responsive data, and allows for prediction of potency. However, there are also several concerns with this assay including: levels of false positive responses, variability due to vehicle, and predictivity. This report serves as a concise review which briefly summarizes the progress, advances and limitations of the assay over the last decade.

Chemical allergy: translating biology into hazard characterization

The induction by chemicals of allergic sensitization and allergic disease is an important and challenging branch of toxicology. Skin sensitization resulting in allergic contact dermatitis represents the most common manifestation of immunotoxicity in humans, and many hundreds of chemicals have been implicated as skin sensitizers. There are far fewer chemicals that have been shown to cause sensitization of the respiratory tract and asthma, but the issue is no less important because hazard identification remains a significant challenge, and occupational asthma can be fatal. In all areas of chemical allergy, there have been, and remain still, intriguing challenges where progress has required a close and productive alignment between immunology, toxicology, and clinical medicine. What the authors have sought to do here is to exemplify, within the framework of chemical allergy, how an investment in fundamental research and an improved understanding of relevant biological and biochemical mechanisms can pay important dividends in driving new innovations in hazard identification, hazard characterization, and risk assessment. Here we will consider in turn three specific areas of research in chemical allergy: (1) the role of epidermal Langerhans cells in the development of skin sensitization, (2) T lymphocytes and skin sensitization, and (3) sensitization of the respiratory tract. In each area, the aim is to identify what has been achieved and how that progress has impacted on the development of new approaches to toxicological evaluation. Success has been patchy, and there is still much to be achieved, but the journey has been fascinating and there have been some very important developments. The conclusion drawn is that continued investment in research, if coupled with an appetite for translating the fruits of that research into imaginative new tools for toxicology, should continue to better equip us for tackling the important challenges that remain to be addressed.

Nickel sensitisation in mice: a critical appraisal

BACKGROUND

The market release of new domestic and industrial chemical and metal products requires certain safety certification, including testing for skin sensitisation. Although various official guidelines have described how such testing is to be done, the validity of the available test models are in part dubious, for which reason regulatory agencies and research aim to further improve and generalise the models for testing of skin sensitisation.

OBJECTIVE

We applied a recently published murine model of nickel allergy as to test its applicability in a regulatory setting and to study and better understand the events leading to type-IV hypersensitivity. Nickel was chosen as model hapten since it induces allergic contact dermatitis with high incidence in the general population.

METHOD

Typically, C57BL/6 mice were sensitised and challenged by intradermal applications of nickel, and cutaneous inflammation was analysed by the mouse ear-swelling test, by histology, and by lymphocyte reactivity in vitro.

RESULT

Surprisingly, the study suggested that the skin reactions observed were results of irritant reactions rather than of adaptive immune responses. Non-sensitised mice responded with cutaneous inflammation and in vitro lymphocyte reactivity which were comparable with nickel-sensitised mice. Furthermore, histological examinations as well as experiments in T-cell deficient mice demonstrated that lymphocytes were not involved and that nickel caused an irritant contact dermatitis rather a true allergic type-IV contact dermatitis.

CONCLUSION

The authors question the validity of the described murine model of nickel allergy.

The local lymph node assay and skin sensitization testing

The mouse local lymph node assay (LLNA) is a method for the identification and characterization of skin sensitization hazards. In this context the method can be used both to identify contact allergens, and also determine the relative skin sensitizing potency as a basis for derivation of effective risk assessments.The assay is based on measurement of proliferative responses by draining lymph node cells induced following topical exposure of mice to test chemicals. Such responses are known to be causally and quantitatively associated with the acquisition of skin sensitization and therefore provide a relevant marker for characterization of contact allergic potential.The LLNA has been the subject of exhaustive evaluation and validation exercises and has been assigned Organization for Economic Cooperation and Development (OECD) test guideline 429. Herein we describe the conduct and interpretation of the LLNA.

Development of topical microbicides to prevent the sexual transmission of HIV

Women comprise almost 50% of the population of people living with HIV and the majority of these women contracted the virus through sexual transmission in monogamous relationships in the developing world. In these environments, where women are not empowered to protect themselves through the negotiation of condom use, effective means of preventing HIV transmission are urgently needed. In the absence of an approved and effective vaccine, microbicides have become the strategy of choice to provide women with the ability to prevent HIV transmission from their infected partners. Topical microbicides are agents specifically developed and formulated for use in either the vaginal or rectal environment that prevent infection by sexually transmitted infectious organisms, including pathogenic viruses, bacteria and fungi. Although a microbicidal product will have many of the same properties as other anti-infective agents and would be similarly developed through human clinical trials, microbicide development bears its own challenges related to formulation and delivery and the unique environment in which the product must act, as well as the requirement to develop a product that is acceptable to the user. Herein, perspectives based on preclinical and clinical microbicide development experience, which have led to an evolving microbicide development algorithm, will be discussed. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of anti-retroviral drug discovery and development, Vol 85, issue 1, 2010.

Effect of Prolonged Repeated Exposure to Formaldehyde Donors with Doses Below the EC3 Value on Draining Lymph Node Responses

In the Local Lymph Node Assay (LLNA), a stimulation index of 3 (SI = 3) is established as a threshold value for hazard identification of sensitization. The corresponding EC3 value, the effective concentration inducing a threefold increase compared to controls, can possibly predict threshold levels for sensitization in humans. Exposure to a dose below the threshold dose would not result in an induction of an immune response. Each repeated contact would be considered and viewed as a new contact and as long as the dose is below the threshold there will be no response, even after repeated exposures. However, repeated exposure may result in local accumulation eventually resulting in a dose that induces a response above the threshold for immunization. We investigated lymph node responses after short and prolonged exposure to formaldehyde donors, chemicals that are highly reactive with proteins and may thus persist in the skin. The studies were performed with formaldehyde and formaldehyde releasers (formaldehyde, paraformaldehyde, Quaternium-15, 2-Chloro-N-(hydroxymethyl)acetamide, and hexamethylenetetramine), at concentrations that induce a SI = 2, i.e., below the threshold for hazard identification. For all test chemicals investigated enhanced lymph node responses were obtained when comparing long-term prolonged exposure to short-term exposure, while three of five chemicals induced responses above SI = 3. Our results show that repeated and prolonged exposure to doses below the EC3 value can induce reactions above the SI = 3, the hazard identification threshold for sensitization in mice. So, when discussing the possible use of the EC3 as benchmark for risk assessment, one should consider duration of exposure and the possibility of local accumulation of the chemical under investigation.

Engineering tissue alternatives to animals: applying tissue engineering to basic research and safety testing

The focus for the rapid progress in the field of tissue engineering has been the clinical potential of the technology to repair, replace, maintain or enhance the function of a particular tissue or organ. However, tissue engineering has much wider applicability in basic research and safety testing, which is often not recognized owing to the clinical focus of tissue engineers. Using examples from a recent National Centre for the Replacement, Refinement and Reduction of Animals in Research/Biotechnology and Biological Sciences Research Council symposium, which brought together tissue engineers and scientists from other research communities, this review highlights the potential of tissue engineering to provide scientifically robust alternatives to animals to address basic research questions and improve drug and chemical development in the pharmaceutical and chemical industries.

Potency values from the local lymph node assay: application to classification, labelling and risk assessment

Hundreds of chemicals are contact allergens but there remains a need to identify and characterise accurately skin sensitising hazards. The purpose of this review was fourfold. First, when using the local lymph node assay (LLNA), consider whether an exposure concentration (EC3 value) lower than 100% can be defined and used as a threshold criterion for classification and labelling. Second, is there any reason to revise the recommendation of a previous ECETOC Task Force regarding specific EC3 values used for sub-categorisation of substances based upon potency? Third, what recommendations can be made regarding classification and labelling of preparations under GHS? Finally, consider how to integrate LLNA data into risk assessment and provide a rationale for using concentration responses and corresponding no-effect concentrations. Although skin sensitising chemicals having high EC3 values may represent only relatively low risks to humans, it is not possible currently to define an EC3 value below 100% that would serve as an appropriate threshold for classification and labelling. The conclusion drawn from reviewing the use of distinct categories for characterising contact allergens was that the most appropriate, science-based classification of contact allergens according to potency is one in which four sub-categories are identified: 'extreme', 'strong', 'moderate' and 'weak'. Since draining lymph node cell proliferation is related causally and quantitatively to potency, LLNA EC3 values are recommended for determination of a no expected sensitisation induction level that represents the first step in quantitative risk assessment.

Chemical allergens--what are the issues?

Chemical allergy describes the adverse health effects that may result when exposure to a chemical elicits an immune response. Allergy develops in two phases. In the first phase, exposure of an inherently susceptible subject results in stimulation of an immune response or immunological priming. If the then sensitised subject is exposed on a subsequent occasion to the same chemical then an accelerated and more aggressive secondary immune response will be provoked resulting in inflammation and the signs and symptoms of a clinically discernible allergic reaction. The two forms of chemical allergy of greatest relevance for occupational toxicology are skin sensitisation resulting in allergic contact dermatitis, and sensitisation of the respiratory tract associated with occupational rhinitis and asthma. In this brief survey we identify what we believe currently represent the key issues and key challenges in these areas.

Allergic reaction induced by dermal and/or respiratory exposure to low-dose phenoxyacetic acid, organophosphorus, and carbamate pesticides

Several types of pesticides, such as organophosphates, phenoxyacetic acid, and carbamate have a high risk of affecting human health, causing allergic rhinitis and bronchial asthma-like diseases. We used our long-term sensitization method and a local lymph node assay to examine the allergic reactions caused by several types of pesticides. BALB/c mice were topically sensitized (9 times in 3 weeks), then challenged dermally or intratracheally with 2,4-D, BRP, or furathiocarb. One day post-challenge, the mice were processed to obtain biologic materials for use in assays of total IgE levels in serum and bronchoalveolar lavage fluid (BALF); differential cell counts and chemokine levels in BALF; lymphocyte counts and surface antigen expression on B-cells within regional lymph nodes (LNs); and, ex situ cytokine production by cells from these LNs. 2,4-D-induced immune responses characteristic of immediate-type respiratory reactions, as evidenced by increased total IgE levels in both serum and BALF; an influx of eosinophils, neutrophils, and chemokines (MCP-1, eotaxin, and MIP-1beta) in BALF; increased surface antigen expression on B-cells IgE and MHC class II production) in both auricular and the lung-associated LNs; and increased Th2 cytokine production (IL-4, IL-5, IL-10, and IL-13) in both auricular and the lung-associated LN cells. In contrast, BRP and furathiocarb treatment yielded, at most, non-significant increases in all respiratory allergic parameters. BRP and furathiocarb induced marked proliferation of MHC Class II-positive B-cells and Th1 cytokines (IL-2, TNF-alpha, and IFN-gamma) in only auricular LN cells. These results suggest that 2,4-D is a respiratory allergen and BRP and furathiocarb are contact allergens. As our protocol detected classified allergic responses to low-molecular-weight chemicals, it thus may be useful for detecting environmental chemical-related allergy.

Detection of low-level environmental chemical allergy by a long-term sensitization method

Multiple chemical sensitivity (MCS) is characterized by various signs, including neurological disorders and allergy. Exposure may occur through a major event, such as a chemical spill, or from long-term contact with chemicals at low levels. We are interested in the allergenicity of MCS and the detection of low-level chemical-related hypersensitivity. We used long-term sensitization followed by low-dose challenge to evaluate sensitization by well-known Th2 type sensitizers (trimellitic anhydride (TMA) and toluene diisocyanate (TDI)) and a Th1 type sensitizer (2,4-dinitrochlorobenzene (DNCB)). After topically sensitizing BALB/c mice (9 times in 3 weeks) and challenging them with TMA, TDI or DNCB, we assayed their auricular lymph nodes (LNs) for number of lymphocytes, surface antigen expression of B cells, and local cytokine production, and measured antigen-specific serum IgE levels. TMA and TDI induced marked increases in levels of antigen-specific serum IgE and of Th2 cytokines (IL-4, IL-5, IL-10, and IL-13) produced by ex vivo restimulated lymph node cells. DNCB induced a marked increase in Th1 cytokine (IL-2, IFN-gamma, and TNF-alpha) levels, but antigen-specific serum IgE levels were not elevated. All chemicals induced significant increases in number of lymphocytes and surface antigen expression of B cells. Our mouse model enabled the identification and characterization of chemical-related allergic reactions at low levels. This long-term sensitization method would be useful for detecting environmental chemical-related hypersensitivity.

Local lymph node assay (LLNA) for detection of sensitization capacity of chemicals

The local lymph node assay (LLNA) is a murine model developed to evaluate the skin sensitization potential of chemicals. The LLNA is an alternative approach to traditional guinea pig methods and in comparison provides important animal welfare benefits. The assay relies on measurement of events induced during the induction phase of skin sensitization, specifically lymphocyte proliferation in the draining lymph nodes which is a hallmark of a skin sensitization response. Since its introduction the LLNA has been the subject of extensive evaluation on a national and international scale, and has been successfully validated and incorporated worldwide into regulatory guidelines. Experience gained in recent years has demonstrated that adherence to published procedures and guidelines for the LLNA (e.g., with respect to dose and vehicle selection) is critical for the successful conduct and eventual interpretation of the data. In addition to providing a robust method for skin sensitization hazard identification, the LLNA has proven very useful in assessing the skin sensitizing potency of test chemicals, and this has provided invaluable information to risk assessors. The primary method to make comparisons of the relative potency of chemical sensitizers is to use linear interpolation to estimate the concentration of chemical required to induce a stimulation index of three relative to concurrent vehicle-treated controls (EC3). In certain situations where there are available less than optimal dose response data a log-linear extrapolation method can be used to estimate an EC3 value which can reduce significantly the need for repeat testing of chemicals. The LLNA, when conducted according to published guidelines, provides a robust method for skin sensitization testing that not only provides reliable hazard identification information but also data necessary for effective risk assessment and risk management.

Assessment of the U937 cell line for the detection of contact allergens

The human myeloid cell line U937 was evaluated as an in vitro test system to identify contact sensitizers in order to develop alternatives to animal tests for the cosmetic industry. Specific culture conditions (i.e., presence of interleukin-4, IL-4) were applied to obtain a dendritic cell-like phenotype. In the described test protocol, these cells were exposed to test chemicals and then analyzed by flow cytometry for CD86 expression and by quantitative real-time reverse transcriptase-polymerase chain reaction for IL-1beta and IL-8 gene expressions. Eight sensitizers, three non-sensitizers and five oxidative hair dye precursors were examined after 24-, 48- and 72-h exposure times. Test item-specific modulations of the chosen activation markers (CD86, IL-1beta and IL-8) suggest that this U937 activation test could discriminate test items classified as contact sensitizers or non-sensitizers in the local lymph node assay in mice (LLNA). More specifically, a test item can be considered as a potential sensitizer when it significantly induced the upregulation of the expression of at least two markers. Using this approach, we could correctly evaluate the dendritic cell (DC) activation potential for 15 out of 16 tested chemicals. We conclude that the U937 activation test may represent an useful tool in a future in vitro test battery for predicting sensitizing properties of chemicals.

Evaluation of anex vivo murine local lymph node assay: multiple endpoint comparison

The local lymph node assay (LLNA) is used to assess the skin sensitization potential of chemicals. In the standard assay, mice are treated topically on the dorsum of both ears with test substance for 3 days. Following 2 days of rest, the initiation of the hypersensitivity response is evaluated by injecting (3)H-thymidine into a tail vein, and then measuring the levels of radioisotope incorporated into the DNA of lymph node cells draining the ears. In the current study, BALB/c mice were treated with the contact sensitizers hexylcinnamic aldehyde (HCA) and oxazolone, and the nonsensitizer methyl salicylate. The proliferative response of lymph node cells was evaluated in an ex vivo assay, in which isolated cells were cultured in vitro with (3)H-thymidine. Treatment of mice with HCA at 5-50% resulted in concentration-related increases in (3)H-thymidine incorporation, with stimulation indices ranging from 3 to 14. Low animal-to-animal variability was seen in three replicate assays testing HCA at 25%. As anticipated, the proliferative response induced by the potent sensitizer oxazolone at 0.25% was greater than HCA at all concentrations tested. Stimulation indices of 1.5 and 3 were seen in two independent experiments with methyl salicylate. These equivocal findings were likely due to the irritancy properties of the compound. Importantly, measuring ex vivo (3)H-thymidine incorporation was more sensitive than evaluating lymph node weight and cellularity, and in vitro bromodeoxyuridine incorporation. Furthermore, the results of the ex vivo LLNA were comparable to the standard assay. This study provided evidence that supports the use of an ex vivo LLNA for hazard assessment of contact hypersensitivity.

Evaluation of lymphocyte subpopulations in draining lymph node cells following allergen and irritant

The murine local lymph node assay (LLNA) has been developed as an alternative to guinea pig models for the assessment of the contact sensitization potential. However, there is a need to develop a non-radioisotopic endpoint for the LLNA, because of the radioisotopic method's requiring the use of special facilities. In this study, we investigated to evaluate the lymphocyte subpopulations in the lymph node cells following allergen and irritant treatment. Female Balb/c mice were treated by the topical application on the dorsum of both ears with sensitizers, 2,4-dinitrochlorobenzene (DNCB), toluene diisocyanate (TDI), and α-hexylcinnamaldehyde (HCA), and an irritant, sodium lauryl sulfate (SLS), once daily for three consecutive days. The lymph node (LN) cells were harvested 72h after the final treatment. Phenotypic analysis of lymphocytes subsets was performed with a flow cytometry. The allergens DNCB, TDI, and HCA and an irritant, SLS increased cell number compared to the vehicle. Mice were treated with DNCB, HCA, and TDI showed a preferential increase in the percentage of B220+CD40+ cells compared with vehicle and irritant-treated mice. There was an increase in B220+CD86+ cells of mice treated with DNCB, TDI, and HCA, but no significant increases were observed in mice treated with SLS. Mice were treated with DNCB and TDI showed an increase in the percentage of B220+CD23+ cells compared with vehicle and irritant-treated mice. These results suggest that analysis of B cell activation marker, CD40 on B cells may be useful in differentiating allergen and irritant responses in the draining lymph nodes of chemically treated mice.

Dendritic cells and skin sensitisation hazard assessment

Allergic contact dermatitis is an important occupational and environmental health disease. There is a need, therefore, to identify skin sensitisation hazard, and to assess accurately likely risks to human health. During the past 15 years very significant advances have been made in our understanding of the cellular and molecular mechanisms that serve to initiate and regulate cutaneous immune responses, including the acquisition of skin sensitisation. This has facilitated parallel advances in the identification and characterisation of skin sensitising chemicals and the development of more robust approaches to risk assessment. It is relevant to consider whether advances in immunobiology provide opportunities also for the design of alternative approaches to the toxicological evaluation of skin sensitisation, including the development of in vitro methods. Here we review the potential use of strategies based on analysis of responses induced in Langerhans cells and dendritic cells; professional antigen processing and presenting cells that are known to play pivotal roles during the induction phase of adaptive immune responses.

Classification of contact allergens according to potency: proposals

It is clear that contact allergens vary substantially with regard to the relative potency with which they are able to induce skin sensitisation. Considerations of potency will in the future become a significant factor in the classification of skin sensitising chemicals. It is therefore appropriate to establish what is known of potency and thresholds in the induction of skin sensitisation and the elicitation of allergic contact dermatitis, and to identify approaches that might be available for assessment of relative potency for the purposes of categorising chemical allergens. This paper was prepared by an ECETOC (European Centre for Ecotoxicology and Toxicology) Task Force that had the objective of recommending approaches for the measurement of potency and definition of thresholds for both the induction and elicitation of contact sensitisation. The deliberations recorded here build upon recommendations made previously by an ECETOC Task Force that considered the conduct of standard skin sensitisation test methods for the purposes of hazard identification and risk assessment (ECETOC, Monograph No. 29, Brussels, 2000). The emphasis in this present paper is also on standard and accepted methods for the assessment of skin sensitisation, and for which OECD guidelines are available: the local lymph node assay (LLNA), the guinea pig maximisation test and the occluded patch test of Buehler. For various reasons, discussed in detail herein, attention focused primarily upon consideration of categorisation of chemical allergens and the identification of thresholds with respect to the induction of skin sensitisation, rather than the elicitation of allergic contact dermatitis. It is concluded that although the LLNA is the method of choice for the determination of skin sensitisation potency for the purposes of categorisation, if data are already available from appropriate guinea pig tests then their judicious interpretation may provide information of value in determinations of potency and categorisation. Included here are detailed and specific recommendations for how best the results of the three test methods considered can be used for the categorisation of chemical allergens as a function of skin sensitisation potency.