Human T cell priming assay (hTCPA) for the identification of contact allergens based on naive T cells and DC – IFN-γ and TNF-α readout

Chemical-Specific T Cell Tests Aim to Bridge a Gap in Skin Sensitization Evaluation

T cell activation is the final key event (KE4) in the adverse outcome pathway (AOP) of skin sensitization. However, validated new approach methodologies (NAMs) for evaluating this step are missing. Accordingly, chemicals that activate an unusually high frequency of T cells, as does the most prevalent metal allergen nickel, are not yet identified in a regulatory context. T cell reactivity to chemical sensitizers might be especially relevant in real-life scenarios, where skin injury, co-exposure to irritants in chemical mixtures, or infections may trigger the heterologous innate immune stimulation necessary to induce adaptive T cell responses. Additionally, cross-reactivity, which underlies cross-allergies, can only be assessed by T cell tests. To date, several experimental T cell tests are available that use primary naïve and memory CD4+ and CD8+ T cells from human blood. These include priming and lymphocyte proliferation tests and, most recently, activation-induced marker (AIM) assays. All approaches are challenged by chemical-mediated toxicity, inefficient or unknown generation of T cell epitopes, and a low throughput. Here, we summarize solutions and strategies to confirm in vitro T cell signals. Broader application and standardization are necessary to possibly define chemical applicability domains and to strengthen the role of T cell tests in regulatory 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.

Drug hypersensitivity reactions: review of the state of the science for prediction and diagnosis

Drug hypersensitivity reactions (DHRs) are a type of adverse drug reaction that can occur with different classes of drugs and affect multiple organ systems and patient populations. DHRs can be classified as allergic or non-allergic based on the cellular mechanisms involved. Whereas nonallergic reactions rely mainly on the innate immune system, allergic reactions involve the generation of an adaptive immune response. Consequently, drug allergies are DHRs for which an immunological mechanism, with antibody and/or T cell, is demonstrated. Despite decades of research, methods to predict the potential for a new chemical entity to cause DHRs or to correctly attribute DHRs to a specific mechanism and a specific molecule are not well-established. This review will focus on allergic reactions induced by systemically administered low-molecular weight drugs with an emphasis on drug- and patient-specific factors that could influence the development of DHRs. Strategies for predicting and diagnosing DHRs, including potential tools based on the current state of the science, will also be discussed.

A blinded in vitro analysis of the intrinsic immunogenicity of hepatotoxic drugs: implications for preclinical risk assessment

In vitro preclinical drug-induced liver injury (DILI) risk assessment relies largely on the use of hepatocytes to measure drug-specific changes in cell function or viability. Unfortunately, this does not provide indications toward the immunogenicity of drugs and/or the likelihood of idiosyncratic reactions in the clinic. This is because the molecular initiating event in immune DILI is an interaction of the drug-derived antigen with MHC proteins and the T-cell receptor. This study utilized immune cells from drug-naïve donors, recently established immune cell coculture systems and blinded compounds with and without DILI liabilities to determine whether these new methods offer an improvement over established assessment methods for the prediction of immune-mediated DILI. Ten blinded test compounds (6 with known DILI liabilities; 4 with lower DILI liabilities) and 5 training compounds, with known T-cell-mediated immune reactions in patients, were investigated. Naïve T-cells were activated with 4/5 of the training compounds (nitroso sulfamethoxazole, vancomycin, Bandrowski's base, and carbamazepine) and clones derived from the priming assays were activated with drug in a dose-dependent manner. The test compounds with DILI liabilities did not stimulate T-cell proliferative responses during dendritic cell-T-cell coculture; however, CD4+ clones displaying reactivity were detected toward 2 compounds (ciprofloxacin and erythromycin) with known liabilities. Drug-responsive T-cells were not detected with the compounds with lower DILI liabilities. This study provides compelling evidence that assessment of intrinsic drug immunogenicity, although complex, can provide valuable information regarding immune liabilities of some compounds prior to clinical studies or when immune reactions are observed in patients.

Evaluation of Delayed-Type Hypersensitivity to Antineoplastic Drugs-An Overview

Nowadays, clinical practice encounters the problem of delayed-type hypersensitivity (DTH) induced by several drugs. Antineoplastic treatments are among the drugs which show an elevated proportion of DHT reactions, leading to the worsening of patients' quality of life. The range of symptoms in DHT reactions can vary from mild, such as self-limiting maculopapular eruptions, to severe, such as Stevens-Johnson Syndrome. The development of these reactions supposes a negative impact, not only by limiting patients' quality of life, but also leading to economic loss due to market withdrawal of the affected drugs and high hospitalization costs. However, despite this problem, there are no available standard in vitro or in vivo methods that allow for the evaluation of the sensitizing potential of drugs in the preclinical phase. Therefore, the aim of this review is to summarize the skin reactions caused by the different antineoplastic families, followed by a comprehensive evaluation of the in vitro and in vivo methods used to detect DTHs and that could be suitable to test antineoplastic hypersensitivity reactions.

New Approach Methodologies in Immunotoxicology: Challenges and Opportunities

To maintain the integrity of an organism, a well-functioning immune system is essential. Immunity is dynamic, with constant surveillance needed to determine whether to initiate an immune response or to not respond. Both inappropriate immunostimulation and decreased immune response can be harmful to the host. A reduced immune response can lead to high susceptibility to cancer or infections, whereas an increased immune response can be related to autoimmunity or hypersensitivity reactions. Animal testing has been the gold standard for hazard assessment in immunotoxicity but a lot of efforts are ongoing to develop non-animal-based test systems, and important successes have been achieved. The term "new approach methodologies" (NAMs) refer to the approaches which are not based on animal models. They are applied in hazard and risk assessment of chemicals and include approaches such as defined approaches for data interpretation and integrated approaches to testing and assessment. This review aims to summarize the available NAMs for immunotoxicity assessment, taking into consideration both inappropriate immunostimulation and immunosuppression, including implication for cancer development.

Immunotoxicity In Vitro Assays for Environmental Pollutants under Paradigm Shift in Toxicity Tests

With the outbreak of COVID-19, increasingly more attention has been paid to the effects of environmental factors on the immune system of organisms, because environmental pollutants may act in synergy with viruses by affecting the immunity of organisms. The immune system is a developing defense system formed by all metazoans in the course of struggling with various internal and external factors, whose damage may lead to increased susceptibility to pathogens and diseases. Due to a greater vulnerability of the immune system, immunotoxicity has the potential to be the early event of other toxic effects, and should be incorporated into environmental risk assessment. However, compared with other toxicity endpoints, e.g., genotoxicity, endocrine toxicity, or developmental toxicity, there are many challenges for the immunotoxicity test of environmental pollutants; this is due to the lack of detailed mechanisms of action and reliable assay methods. In addition, with the strong appeal for animal-free experiments, there has been a significant shift in the toxicity test paradigm, from traditional animal experiments to high-throughput in vitro assays that rely on cell lines. Therefore, there is an urgent need to build high-though put immunotoxicity test methods to screen massive environmental pollutants. This paper reviews the common methods of immunotoxicity assays, including assays for direct immunotoxicity and skin sensitization. Direct immunotoxicity mainly refers to immunosuppression, for which the assays mostly use mixed immune cells or isolated single cells from animals with obvious problems, such as high cost, complex experimental operation, strong variability and so on. Meanwhile, there have been no stable and standard cell lines targeting immune functions developed for high-throughput tests. Compared with direct immunotoxicity, skin sensitizer screening has developed relatively mature in vitro assay methods based on an adverse outcome pathway (AOP), which points out the way forward for the paradigm shift in toxicity tests. According to the experience of skin sensitizer screening, this paper proposes that we also should seek appropriate nodes and establish more complete AOPs for immunosuppression and other immune-mediated diseases. Then, effective in vitro immunotoxicity assay methods can be developed targeting key events, simultaneously coordinating the studies of the chemical immunotoxicity mechanism, and further promoting the paradigm shift in the immunotoxicity test.

Alternative Methods for Skin-Sensitization Assessment

Skin sensitization is a term used to refer to the regulatory hazard known as allergic contact dermatitis (ACD) in humans or contact hypersensitivity in rodents, an important health endpoint considered in chemical hazard and risk assessments. Information on skin sensitization potential is required in various regulatory frameworks, such as the Directive of the European Parliament and the Council on Registration, Evaluation and Authorization of Chemicals (REACH). The identification of skin-sensitizing chemicals previously required the use of animal testing, which is now being replaced by alternative methods. Alternative methods in the field of skin sensitization are based on the measurement or prediction of key events (KE), i.e., (i) the molecular triggering event, i.e., the covalent binding of electrophilic substances to nucleophilic centers in skin proteins; (ii) the activation of keratinocytes; (iii) the activation of dendritic cells; (iv) the proliferation of T cells. This review article focuses on the current state of knowledge regarding the methods corresponding to each of the key events in skin sensitization and considers the latest trends in the development and modification of these methods.

Fish allergens of turbot (Scophthalmus maximus) parvalbumin triggers food allergy via inducing maturation of bone marrow derived dendritic cells and driving Th2 immune response

Aquatic food allergy has become a key food safety problem and therefore it is urgent to study the mechanism of aquatic food allergy. Turbot parvalbumin (PV) is a major marine food allergen that could cause allergic reactions but the cellular and molecular mechanisms remain to be defined. In this study, we used flow cytometry and ELISA, a coupled co-culture system of dendritic cells and T cells, and revealed that PV could promote the maturation of dendritic cells, mainly by inducing bone marrow-derived dendritic cells (BMDCs) to express MHC II and CD86, and promote the cytokines/chemokines IL-6, IFN-γ, IL-23, and IL-12p70, whereas inhibiting TNF-α expression. Our results suggested that murine BMDCs play a crucial role in the effect of PV on the induction of Th2 responses.

Frequencies and TCR Repertoires of Human 2,4,6-Trinitrobenzenesulfonic Acid-specific T Cells

Allergic contact dermatitis is a widespread T cell-mediated inflammatory skin disease, but in vitro monitoring of chemical-specific T cells remains challenging. We here introduce short-term CD154/CD137 upregulation to monitor human T cell responses to the experimental sensitizer 2,4,6-trinitrobenzenesulfonic acid (TNBS). Peripheral blood mononuclear cells (PBMC) from healthy donor buffy coats were TNBS-modified and incubated with unmodified PBMC. After 5 and 16 h, we detected TNBS-specific activated CD154+CD4+ and CD137+CD8+ T cells by multi-parameter flow cytometry, respectively. Activated cells were sorted for restimulation and bulk T cell receptor (TCR) high-throughput sequencing (HTS). Stimulation with TNBS-modified cells (3 mM) induced CD154 expression on 0.04% of CD4+ and CD137 expression on 0.60% of CD8+ memory T cells, respectively (means, n = 11-17 donors). CD69 co-expression argued for TCR-mediated activation, which was further supported by TNBS-specific restimulation of 10/13 CD154+CD4+ and 11/15 CD137+CD8+ T cell clones and lines. Major histocompatibility complex (MHC) blocking antibodies prevented activation, illustrating MHC restriction. The high frequencies of TNBS-specific T cells were associated with distinct common changes in the TCR β-chain repertoire. We observed an overrepresentation of tryptophan and lysine in the complementarity determining regions 3 (CDR3) (n = 3-5 donors), indicating a preferential interaction of these amino acids with the TNBS-induced epitopes. In summary, the detection of TNBS-specific T cells by CD154/CD137 upregulation is a fast, comprehensive and quantitative method. Combined with TCR HTS, the mechanisms of chemical allergen recognition that underlie unusually frequent T cell activation can be assessed. In the future, this approach may be adapted to detect T cells activated by additional chemical sensitizers.

In Vitro Monitoring of Human T Cell Responses to Skin Sensitizing Chemicals-A Systematic Review

Background: Chemical allergies are T cell-mediated diseases that often manifest in the skin as allergic contact dermatitis (ACD). To prevent ACD on a public health scale and avoid elicitation reactions at the individual patient level, predictive and diagnostic tests, respectively, are indispensable. Currently, there is no validated in vitro T cell assay available. The main bottlenecks concern the inefficient generation of T cell epitopes and the detection of rare antigen-specific T cells. Methods: Here, we systematically review original experimental research papers describing T cell activation to chemical skin sensitizers. We focus our search on studies published in the PubMed and Scopus databases on non-metallic allergens in the last 20 years. Results: We identified 37 papers, among them 32 (86%) describing antigen-specific human T cell activation to 31 different chemical allergens. The remaining studies measured the general effects of chemical allergens on T cell function (five studies, 14%). Most antigen-specific studies used peripheral blood mononuclear cells (PBMC) as antigen-presenting cells (APC, 75%) and interrogated the blood T cell pool (91%). Depending on the individual chemical properties, T cell epitopes were generated either by direct administration into the culture medium (72%), separate modification of autologous APC (29%) or by use of hapten-modified model proteins (13%). Read-outs were mainly based on proliferation (91%), often combined with cytokine secretion (53%). The analysis of T cell clones offers additional opportunities to elucidate the mechanisms of epitope formation and cross-reactivity (13%). The best researched allergen was p-phenylenediamine (PPD, 12 studies, 38%). For this and some other allergens, stronger immune responses were observed in some allergic patients (15/31 chemicals, 48%), illustrating the in vivo relevance of the identified T cells while detection limits remain challenging in many cases. Interpretation: Our results illustrate current hardships and possible solutions to monitoring T cell responses to individual chemical skin sensitizers. The provided data can guide the further development of T cell assays to unfold their full predictive and diagnostic potential, including cross-reactivity assessments.

Drug-induced skin toxicity: gaps in preclinical testing cascade as opportunities for complex in vitro models and assays

Skin is the largest organ of the body and serves as the principle barrier to the environment. Composed of multiple cell types arranged in stratified layers with highly specialized appendages, it serves sensory and immune surveillance roles in addition to its primary mechanical function. Several complex in vitro models of skin (i.e. microphysiological systems (MPS) including but not limited to 3D tissues, organ-on-a-chip, organoids), have been developed and assays validated for regulatory purposes. As such, skin is arguably the most advanced organ with respect to model development and adoption across industries including chemical, cosmetic, and to a somewhat lesser extent, pharmaceutical. Early adoption of complex skin models and associated assays for assessment of irritation and corrosion spurred research into other areas such as sensitization, absorption, phototoxicity, and genotoxicity. Despite such considerable advancements, opportunities remain for immune capabilities, inclusion of appendages such as hair follicles, fluidics, and innervation, among others. Herein, we provide an overview of current complex skin model capabilities and limitations within the drug development scheme, and recommendations for future model development and assay qualification and/or validation with the intent to facilitate wider adoption of use within the pharmaceutical industry.

The Overt and Hidden Use of Animal-Derived Products in Alternative Methods for Skin Sensitisation: A Systematic Review

In vitro methods that can replace animal testing in the identification of skin sensitisers are now a reality. However, as cell culture and related techniques usually rely on animal-derived products, these methods may be failing to address the complete replacement of animals in safety assessment. The objective of this study was to identify the animal-derived products that are used as part of in vitro methods for skin sensitisation testing. Thus, a systematic review of 156 articles featuring 83 different in vitro methods was carried out and, from this review, the use of several animal-derived products from different species was identified, with the use of fetal bovine serum being cited in most of the methods (78%). The use of sera from other animals, monoclonal antibodies and animal proteins were also variously mentioned. While non-animal alternatives are available and methods free of animal-derived products are emerging, most of the current methods reported used at least one animal-derived product, which raises ethical and technical concerns. Therefore, to deliver technically and ethically better in vitro methods for the safety assessment of chemicals, more effort should be made to replace products of animal origin in existing methods and to avoid their use in the development of new method protocols.

The 21st Century movement within the area of skin sensitization assessment: From the animal context towards current human-relevant in vitro solutions

In a regulatory context, skin sensitization hazard and risk evaluations of manufactured products and their ingredients (e.g. cosmetics) are mandatory in several regions. Great efforts have been made within the field of 21st Century Toxicology to provide non-animal testing approaches to assess the skin allergy potential of materials (e.g. chemicals, mixtures, nanomaterials, particles). Mechanistic understanding of skin sensitization process through the adverse outcome pathway (AOP) has promoted the development of in vitro methods, demonstrating accuracies superior to the traditional animal testing. These in vitro testing approaches are based on one of the four AOP key events (KE) of skin sensitization: formation of immunogenic hapten-protein complexes (KE-1 or the molecular initiating event, MIE), inflammatory keratinocyte responses (KE-2), dendritic cell activation (KE-3), and T-lymphocyte activation and proliferation (KE-4). This update provides an overview of the historically used in vivo methods as well as the current in chemico and in cell methods with and without OECD guideline designations to analyze the progress towards human-relevant in vitro test methods for safety assessment of the skin allergenicity potential of materials. Here our focus is to review 96 in vitro testing approaches directed to the KEs of the skin sensitization AOP.

Identification and Characterization of Circulating Naïve CD4+ and CD8+ T Cells Recognizing Nickel

Allergic contact dermatitis caused by contact sensitizers is a T-cell-mediated inflammatory skin disease. The most prevalent contact allergens is nickel. Whereas, memory T cells from nickel-allergic patients are well-characterized, little is known concerning nickel-specific naïve T-cell repertoire. The purpose of this study was to identify and quantify naïve CD4+ and CD8+ T cells recognizing nickel in the general population. Using a T-cell priming in vitro assay based on autologous co-cultures between naïve T cells and dendritic cells loaded with nickel, we were able to detect a naïve CD4+ and CD8+ T-cell repertoire for nickel in 10/11 and 7/8 of the tested donors. We calculated a mean frequency of 0.49 nickel-specific naïve CD4+ T cells and 0.37 nickel-specific naïve CD8+ T cells per million of circulating naïve T cells. The activation of these specific T cells requires MHC molecules and alongside IFN-γ production, some nickel-specific T-cells were able to produce granzyme-B. Interestingly, nickel-specific naïve CD4+ and CD8+ T cells showed a low rate of cross-reactivity with cobalt, another metallic hapten, frequently mixed with nickel in many alloys. Moreover, naïve CD4+ T cells showed a polyclonal TCRβ composition and the presence of highly expanded clones with an enrichment and/or preferentially expansion of some TRBV genes that was donor and T-cell specific. Our results contribute to a better understanding of the mechanism of immunization to nickel and propose the T-cell priming assay as a useful tool to identify antigen-specific naïve T cells.

Application of proteomics in the elucidation of chemical-mediated allergic contact dermatitis

Allergic contact dermatitis (ACD) is a widespread hypersensitivity reaction of the skin. The cellular mechanisms underlying its development are complex and involve close interaction of different cell types of the immune system. It is this very complexity which has long prevented straightforward replacement of the corresponding regulatory in vivo tests. Recent efforts have already resulted in the development of several in vitro testing alternatives that address key steps of ACD. Yet identification of suitable biomarkers is still a subject of intense research. Search strategies for the latter encompass transcriptomics, proteomics as well as metabolomics approaches. The scope of this review shall be the application and use of proteomics in the context of ACD. This includes highlighting relevant aspects of the molecular and cellular mechanisms underlying ACD, the exploitation of these mechanisms for testing and biomarkers (e.g., in the context of the OECD's adverse outcome pathway initiative) as well as an outlook on emerging proteome targets, for example during the allergen-induced activation of dendritic cells (DCs).

State of the art in non-animal approaches for skin sensitization testing: from individual test methods towards testing strategies

The hazard assessment of skin sensitizers relies mainly on animal testing, but much progress is made in the development, validation and regulatory acceptance and implementation of non-animal predictive approaches. In this review, we provide an update on the available computational tools and animal-free test methods for the prediction of skin sensitization hazard. These individual test methods address mostly one mechanistic step of the process of skin sensitization induction. The adverse outcome pathway (AOP) for skin sensitization describes the key events (KEs) that lead to skin sensitization. In our review, we have clustered the available test methods according to the KE they inform: the molecular initiating event (MIE/KE1)-protein binding, KE2-keratinocyte activation, KE3-dendritic cell activation and KE4-T cell activation and proliferation. In recent years, most progress has been made in the development and validation of in vitro assays that address KE2 and KE3. No standardized in vitro assays for T cell activation are available; thus, KE4 cannot be measured in vitro. Three non-animal test methods, addressing either the MIE, KE2 or KE3, are accepted as OECD test guidelines, and this has accelerated the development of integrated or defined approaches for testing and assessment (e.g. testing strategies). The majority of these approaches are mechanism-based, since they combine results from multiple test methods and/or computational tools that address different KEs of the AOP to estimate skin sensitization potential and sometimes potency. Other approaches are based on statistical tools. Until now, eleven different testing strategies have been published, the majority using the same individual information sources. Our review shows that some of the defined approaches to testing and assessment are able to accurately predict skin sensitization hazard, sometimes even more accurate than the currently used animal test. A few defined approaches are developed to provide an estimate of the potency sub-category of a skin sensitizer as well, but these approaches need further independent evaluation with a new dataset of chemicals. To conclude, this update shows that the field of non-animal approaches for skin sensitization has evolved greatly in recent years and that it is possible to predict skin sensitization hazard without animal testing.

In vitro Models to Evaluate Drug-Induced Hypersensitivity: Potential Test Based on Activation of Dendritic Cells

Hypersensitivity drug reactions (HDRs) are the adverse effect of pharmaceuticals that clinically resemble allergy. HDRs account for approximately 1/6 of drug-induced adverse effects, and include immune-mediated ("allergic") and non-immune-mediated ("pseudo allergic") reactions. In recent years, the severe and unpredicted drug adverse events clearly indicate that the immune system can be a critical target of drugs. Enhanced prediction in preclinical safety evaluation is, therefore, crucial. Nowadays, there are no validated in vitro or in vivo methods to screen the sensitizing potential of drugs in the pre-clinical phase. The problem of non-predictability of immunologically-based hypersensitivity reactions is related to the lack of appropriate experimental models rather than to the lack of -understanding of the adverse phenomenon. We recently established experimental conditions and markers to correctly identify drug associated with in vivo hypersensitivity reactions using THP-1 cells and IL-8 production, CD86 and CD54 expression. The proposed in vitro method benefits from a rationalistic approach with the idea that allergenic drugs share with chemical allergens common mechanisms of cell activation. This assay can be easily incorporated into drug development for hazard identification of drugs, which may have the potential to cause in vivo hypersensitivity reactions. The purpose of this review is to assess the state of the art of in vitro models to assess the allergenic potential of drugs based on the activation of dendritic cells.

Biology-inspired microphysiological system approaches to solve the prediction dilemma of substance testing

The recent advent of microphysiological systems - microfluidic biomimetic devices that aspire to emulate the biology of human tissues, organs and circulation in vitro - is envisaged to enable a global paradigm shift in drug development. An extraordinary US governmental initiative and various dedicated research programs in Europe and Asia have led recently to the first cutting-edge achievements of human single-organ and multi-organ engineering based on microphysiological systems. The expectation is that test systems established on this basis would model various disease stages, and predict toxicity, immunogenicity, ADME profiles and treatment efficacy prior to clinical testing. Consequently, this technology could significantly affect the way drug substances are developed in the future. Furthermore, microphysiological system-based assays may revolutionize our current global programs of prioritization of hazard characterization for any new substances to be used, for example, in agriculture, food, ecosystems or cosmetics, thus, replacing laboratory animal models used currently. Thirty-six experts from academia, industry and regulatory bodies present here the results of an intensive workshop (held in June 2015, Berlin, Germany). They review the status quo of microphysiological systems available today against industry needs, and assess the broad variety of approaches with fit-for-purpose potential in the drug development cycle. Feasible technical solutions to reach the next levels of human biology in vitro are proposed. Furthermore, key organ-on-a-chip case studies, as well as various national and international programs are highlighted. Finally, a roadmap into the future is outlined, to allow for more predictive and regulatory-accepted substance testing on a global scale.

In vitro methods for hazard assessment of industrial chemicals - opportunities and challenges

Allergic contact dermatitis (ACD) is a delayed-type hypersensitivity immune reaction mediated by T-lymphocytes as a result of repeated exposure of an allergen primarily on skin. ACD accounts for up to 95% of occupational skin diseases, with epoxy resins implicated as one of the most common causes of ACD. Efficient high-throughput in vitro screening for accurate identification of compounds and materials that may pose hazardous risks in the workplace is crucial. At present, the murine local lymph node assay is the 'method of choice' for predicting the sensitizing potency of contact allergens. As the 3Rs principles of reduction, refinement, and replacement in animal testing has gained political and economic momentum, several in vitro screening methods have been developed for identifying potential contact allergens. To date, these latter methods have been utilized primarily to assess the skin sensitizing potential of the chemical components of cosmetic products with scant research attention as to the applicability of these methods to industrial chemicals, particularly epoxy resins. Herein we review the currently utilized in vitro methods and identify the knowledge gaps with regard to assessing the generalizability of in vitro screening methods for assessing the skin sensitizing potential of industrial chemicals.

In vitro diagnosis of delayed-type drug hypersensitivity: mechanistic aspects and unmet needs

Several laboratories use the lymphocyte transformation test for the diagnosis of delayed-type drug hypersensitivity reactions. Recently, the availability of multiple readouts has improved our ability to diagnose reactions. It is important to note that most published studies characterizing the usefulness of diagnostic tests utilize blood samples from well-defined test and control patient groups. The purpose of this article is to briefly summarize the cellular and chemical basis of delayed-type drug hypersensitivity reactions and to review in vitro assays that are available for drug hypersensitivity diagnosis.

Evaluation of combinations of in vitro sensitization test descriptors for the artificial neural network-based risk assessment model of skin sensitization

The skin sensitization potential of chemicals has been determined with the use of the murine local lymph node assay (LLNA). However, in recent years public concern about animal welfare has led to a requirement for non-animal risk assessment systems for the prediction of skin sensitization potential, to replace LLNA. Selection of an appropriate in vitro test or in silico model descriptors is critical to obtain good predictive performance. Here, we investigated the utility of artificial neural network (ANN) prediction models using various combinations of descriptors from several in vitro sensitization tests. The dataset, collected from published data and from experiments carried out in collaboration with the Japan Cosmetic Industry Association (JCIA), consisted of values from the human cell line activation test (h-CLAT), direct peptide reactivity assay (DPRA), SH test and antioxidant response element (ARE) assay for chemicals whose LLNA thresholds have been reported. After confirming the relationship between individual in vitro test descriptors and the LLNA threshold (e.g. EC3 value), we used the subsets of chemicals for which the requisite test values were available to evaluate the predictive performance of ANN models using combinations of h-CLAT/DPRA (N = 139 chemicals), the DPRA/ARE assay (N = 69), the SH test/ARE assay (N = 73), the h-CLAT/DPRA/ARE assay (N = 69) and the h-CLAT/SH test/ARE assay (N = 73). The h-CLAT/DPRA, h-CLAT/DPRA/ARE assay and h-CLAT/SH test/ARE assay combinations showed a better predictive performance than the DPRA/ARE assay and the SH test/ARE assay. Our data indicates that the descriptors evaluated in this study were all useful for predicting human skin sensitization potential, although combinations containing h-CLAT (reflecting dendritic cell-activating ability) were most effective for ANN-based prediction.

Systemic drugs inducing non-immediate cutaneous adverse reactions and contact sensitizers evoke similar responses in THP-1 cells

Contact sensitizers induce phenotypic and functional changes in dendritic cells (DC) that enhance their antigen-presenting capacity and, ultimately, modulate the T cell response. To evaluate if there is a similar effect of drugs causing T-cell-mediated cutaneous adverse drug reactions (CADR), we studied the in vitro effect of drugs on THP-1 cells, a cell line widely used to evaluate the early molecular and cellular events triggered by contact sensitizers. The effect of allopurinol, oxypurinol, ampicillin, amoxicillin, carbamazepine and sodium valproate, at EC30 concentrations, was evaluated on p38 MAPK activation, by Western Blot, and on the expression of genes coding for DC maturation markers, pro-inflammatory cytokine/chemokines and hemeoxygenase 1 (HMOX1), by real-time RT-PCR. Results were compared with lipopolysaccharide (LPS), a DC maturation stimulus, and the strong contact sensitizer, 1-fluoro-2,4-dinitrobenzene (DNFB). All drugs studied significantly upregulated HMOX1 gene transcription and all, except the anticonvulsants, also upregulated IL8. Allopurinol and oxypurinol showed the most intense effect, in a magnitude similar to DNFB and superior to betalactams. Transcription of CD40, IL12B and CXCL10 genes by drugs was more irregular. Moreover, like DNFB, all drugs activated p38 MAPK, although significantly only for oxypurinol. Like contact sensitizers, drugs that cause non-immediate CADR activate THP-1 cells in vitro, using different signalling pathways and affecting gene transcription with an intensity that may reflect the frequency and severity of the CADR they cause. Direct activation of antigen-presenting DC by systemic drugs may be an important early step in the pathophysiology of non-immediate CADR.

Assessment of the skin sensitising potency of the lower alkyl methacrylate esters

There is continued interest in, and imperatives for, the classification of contact allergens according to their relative skin sensitising potency. However, achieving that end can prove problematic, not least when there is an apparent lack of concordance between experimental assessments of potency and the prevalence allergic contact dermatitis as judged by clinical experience. For the purpose of exploring this issue, and illustrating the important considerations that are required to reach sound judgements about potency categorisation, the lower alkyl methacrylate esters (LAM) have been employed here as a case study. Although the sensitising potential of methyl methacrylate (MMA) has been reviewed previously, there is available new information that is relevant for assessment of skin sensitising potency. Moreover, for the purposes of this article, analyses have been extended to include also other LAM for which relevant data are available: ethyl methacrylate (EMA), n-butyl methacrylate (nBMA), isobutyl methacrylate (iBMA), and 2-ethylhexyl methacrylate (EHMA). In addressing the skin sensitising activity of these chemicals and in drawing conclusions regarding relative potency, a number of sources of information has been considered, including estimates of potency derived from local lymph node assay (LLNA) data, the results of guinea pig assays, and data derived from in silico methods and from recently developed in vitro approaches. Moreover, clinical experience of skin sensitisation of humans by LAM has also been evaluated. The conclusion drawn is that MMA and other LAM are contact allergens, but that none of these chemicals has any more than weak skin sensitising potency. We have also explored here the possible bases for this modest sensitising activity. Finally, the nature of exposure to LAM has been reviewed briefly and on the basis of that information, together with an understanding of skin sensitising potency, a risk assessment has been prepared.

Correlation of contact sensitizer potency with T cell frequency and TCR repertoire diversity

Allergic contact dermatitis is a T cell-mediated skin disease. Many hundreds of organic chemicals and some metal ions are contact sensitizers. They induce an innate inflammatory immune response in the skin that results in the priming of contact sensitizer-specific T cells by dendritic cells in the draining lymph nodes. The factors that determine the strength of this T cell response and thereby define the potency of a contact sensitizer are largely unknown. This chapter highlights different variables such as precursor frequency of antigen-specific T cells, possible bystander activation, and T cell receptor diversity or avidity of the TCR/peptide-MHC interactions, which might impact the quality and strength of T cell responses to contact sensitizers. In addition, different methods available to determine both the frequency of antigen-specific T cells and T cell receptor repertoires are discussed. Identification of the factors determining potency may allow for the development of suitable in vitro assays for potency assessment of contact sensitizers.

Tools and methods for identification and analysis of rare antigen-specific T lymphocytes

T lymphocytes are essential as effector and memory cells for immune defense against infections and as regulatory T cells in the establishment and maintenance of immune tolerance. However, they are also involved in immune pathology being effectors in autoimmune and allergic diseases or suppressors of immunity in cancer, and they often cause problems in transplantation. Therefore, strategies are being developed that allow the in vivo amplification or isolation, in vitro expansion and genetic manipulation of beneficial T cells for adoptive cell therapies or for the tolerization, or elimination of pathogenic T cells. The major goal is to make use of the exquisite antigen specificity of T cells to develop targeted strategies and to develop techniques that allow for the identification and depletion or enrichment of very often rare antigen-specific naïve as well as effector and memory T cells. Such techniques are very useful for immune monitoring of T cell responses in diagnostics and vaccination and for the development of T cell-based assays for the replacement of animal testing in immunotoxicology to identify contact allergens and drugs that cause adverse reactions.

Human T cell priming assay: depletion of peripheral blood lymphocytes in CD25(+) cells improves the in vitro detection of weak allergen-specific T cells

To develop an in vitro assay that recapitulates the key event of allergic contact dermatitis (ACD), that is the priming of effector T cells by hapten-presenting dendritic cells, and then allows for the sensitive detection of chemical allergens represents a major challenge. Classical human T cell priming assays (hTCPA) that have been developed in the past, using hapten-loaded monocyte-derived dendritic cells (MDDCs) as antigen-presenting cells and peripheral blood lymphocytes (PBLs) as responding cells, were not efficient to prime T cells to common allergens with moderate/weak sensitizing properties. Recent progress in the understanding of the effector and regulatory mechanisms of ACD have shown that T cell priming requires efficient uptake of allergens by immunogenic DCs and that it is controlled by several subsets of regulatory cells including CD25(+) Tregs. We therefore analyzed various parameters involved in allergen-specific T cell activation in vitro and showed that priming of allergen-specific T cells is hampered by several subsets of immune cells comprising CD1a(neg) DCs, CD25(+) T cells, and CD56(+) regulatory cells.CD4(+)CD25(+)FoxP3(+) Tregs prevented the in vitro T cell priming to moderate/weak allergens, and depletion of human PBLs in CD25(+) cells significantly increased specific T cell proliferation and IFN-γ secretion. CD56(+) cells exerted an additional control of T cell priming since co-depletion of both CD56(+) and CD25(+) cells improved the magnitude of chemical-specific T cell activation. Finally, CD1a(low) MDDCs were able to inhibit T cell activation obtained by allergen-pulsed CD1a(high) MDDC. Moreover, we showed that uptake by DC of allergen-encapsulated nanoparticles significantly increased their activation status and their ability to prompt specific T cell activation. Hence, by combining the different strategies, i.e., depletion of CD25(+) and CD56(+) cells, use of CD1a(high) MDDC, and nanoparticle encapsulation of allergens, it was possible to induce T cell priming to most of the moderate/weak allergens, including lipophilic molecules highly insoluble in culture media. Therefore, the present optimized in vitro human T cell priming assay is a valuable method to detect the sensitizing properties of chemical allergens.

Allergic contact dermatitis: xenoinflammation of the skin

Many xenobiotic chemicals cause sterile inflammation. This xenoinflammation is often induced by protein reactive contact allergens resulting in allergic contact dermatitis (ACD). Recent findings reveal that these chemicals mimick infection by triggering innate immune responses via pattern recognition receptors (PRRs) and endogenous danger signals. The emerging cellular responses in ACD are mediated by various innate effector cells. Here, an important role for mast cells has now been recognized. Eventually, chemical specific T cells such as CD8+ and CD4+ Tc1/Th1 as well as Tc17/Th17 cells are activated. Langerhans cells may serve a tolerogenic function. The mechanisms of tolerance induction by ultraviolet irradiation or by very low doses of contact allergen are now understood in much greater detail.