Skin and respiratory chemical allergy: confluence and divergence in a hybrid adverse outcome pathway

Identification of toxicity-induced biomarkers in human non-immune airway cells exposed to respiratory sensitizers: A mechanistic approach

Occupational asthma covers a group of work-related diseases whose clinical manifestations include airway hyperresponsiveness and airflow limitation. Although the chemical respiratory allergy (CRA) induced by Low Molecular Weight (LMW) sensitizers is a major concern, especially in terms of the regulatory framework, to date there are no methods available for preclinically addressing this toxicological outcome, as its mechanistic background is not fully understood at molecular or cellular levels. This paper proposes a mechanistic study applying New Approach Methodologies (NAM) of the pro-inflammatory and functional effects triggered by LMW respiratory allergens in different respiratory tract cell lines, including bronchial epithelial (BEAS-2B), lung fibroblast (MRC-5), and endothelial cells (EA.hy926), and an analysis of the capacity of such chemicals to interact with the mucin protein, to address certain toxicodynamic aspects of such compounds. The results showed that some of the sensitizers evaluated interact with mucin, the main protein mucus component, but the toxicant-mucin complex formation does not seem to be a common feature of different chemical classes of allergens. At a cellular level, sensitizers promoted an increase in IL-8, IL-6, and IL-1β production in the evaluated cell types. It also impaired the MUC1 expression by bronchial cells and activated endothelial cells, thereby increasing the ICAM-I surface expression. Taken together, our results showed that these aforementioned cell types participate in the CRA Adverse Outcome Pathway and must be considered when developing preclinical testing strategies, particularly investigating danger signal production after exposure to LMW sensitizers in different tissue compartments.

Exposure limits for indoor volatile substances concerning the general population: The role of population-based differences in sensory irritation of the eyes and airways for assessment factors

Assessment factors (AFs) are essential in the derivation of occupational exposure limits (OELs) and indoor air quality guidelines. The factors shall accommodate differences in sensitivity between subgroups, i.e., workers, healthy and sick people, and occupational exposure versus life-long exposure for the general population. Derivation of AFs itself is based on empirical knowledge from human and animal exposure studies with immanent uncertainty in the empirical evidence due to knowledge gaps and experimental reliability. Sensory irritation in the eyes and airways constitute about 30-40% of OELs and is an abundant symptom in non-industrial buildings characterizing the indoor air quality and general health. Intraspecies differences between subgroups of the general population should be quantified for the proposal of more 'empirical' based AFs. In this review, we focus on sensitivity differences in sensory irritation about gender, age, health status, and vulnerability in people, based solely on human exposure studies. Females are more sensitive to sensory irritation than males for few volatile substances. Older people appear less sensitive than younger ones. However, impaired defense mechanisms may increase vulnerability in the long term. Empirical evidence of sensory irritation in children is rare and limited to children down to the age of six years. Studies of the nervous system in children compared to adults suggest a higher sensitivity in children; however, some defense mechanisms are more efficient in children than in adults. Usually, exposure studies are performed with healthy subjects. Exposure studies with sick people are not representative due to the deselection of subjects with moderate or severe eye or airway diseases, which likely underestimates the sensitivity of the group of people with diseases. Psychological characterization like personality factors shows that concentrations of volatile substances far below their sensory irritation thresholds may influence the sensitivity, in part biased by odor perception. Thus, the protection of people with extreme personality traits is not feasible by an AF and other mitigation strategies are required. The available empirical evidence comprising age, lifestyle, and health supports an AF of not greater than up to 2 for sensory irritation. Further, general AFs are discouraged for derivation, rather substance-specific derivation of AFs is recommended based on the risk assessment of empirical data, deposition in the airways depending on the substance's water solubility and compensating for knowledge and experimental gaps. Modeling of sensory irritation would be a better 'empirical' starting point for derivation of AFs for children, older, and sick people, as human exposure studies are not possible (due to ethical reasons) or not generalizable (due to self-selection). Dedicated AFs may be derived for environments where dry air, high room temperature, and visually demanding tasks aggravate the eyes or airways than for places in which the workload is balanced, while indoor playgrounds might need other AFs due to physical workload and affected groups of the general population.

Characterization and applicability of a novel physiologically relevant 3D-tetraculture bronchial model for in vitro assessment of respiratory sensitization

Chemical Respiratory Allergy (CRA) is triggered after exposure to Low Molecular Weight (LMW) sensitizers and manifests clinically as asthma and rhinitis. From a risk/toxicity assessment point of view, there are few methods, none of them validated, for evaluating the respiratory sensitization potential of chemicals once the in vivo-based models usually employed for inhalation toxicity addressment do not comprise allergenicity endpoints specifically. Based on that, we developed, characterized, and evaluated the applicability of a 3D-tetraculture airway model reconstructed with bronchial epithelial, fibroblasts, endothelial and monocytic cell lines. Moreover, we exposed the tissue to maleic anhydride (MA) aerosols to challenge the model and subsequently assessed inflammatory and functional aspects of the tissue. The reconstructed tissue presented phenotypic biomarkers compatible with human bronchial epithelium, and MA aerosol exposure triggered an increased IL-8 and IL-6 production, reactive oxygen species (ROS) formation, and apoptosis of epithelial cells. Besides, augmented IL-8 production by monocytic cells was also found, correlating with dendritic cell activation within the co-culture model after MA exposure. Our results demonstrated that the 3D-tetraculture bronchial model presents hallmarks related to human airways' structure and function. Additionally, exposure to a respiratory sensitizer induced inflammatory and functional alterations in the reconstructed tissue, rendering it a valuable tool for exploring the mechanistic framework of chemically induced respiratory sensitization.

Identification of true chemical respiratory allergens: Current status, limitations and recommendations

Asthma in the workplace is an important occupational health issue. It comprises various subtypes: occupational asthma (OA; both allergic asthma and irritant-induced asthma) and work-exacerbated asthma (WEA). Current regulatory paradigms for the management of OA are not fit for purpose. There is therefore an important unmet need, for the purposes of both effective human health protection and appropriate and proportionate regulation, that sub-types of work-related asthma can be accurately identified and classified, and that chemical respiratory allergens that drive allergic asthma can be differentiated according to potency. In this article presently available strategies for the diagnosis and characterisation of asthma in the workplace are described and critically evaluated. These include human health studies, clinical investigations and experimental approaches (structure-activity relationships, assessments of chemical reactivity, experimental animal studies and in vitro methods). Each of these approaches has limitations with respect to providing a clear discrimination between OA and WEA, and between allergen-induced and irritant-induced asthma. Against this background the needs for improved characterisation of work-related asthma, in the context of more appropriate regulation is discussed.

Development of an In Vitro Sensitisation Test Using a Coculture System of Human Bronchial Epithelium and Immune Cells

Chemical respiratory sensitisation is a serious health problem. However, to date, there are no validated test methods available for identifying respiratory sensitisers. The aim of this study was to develop an in vitro sensitisation test by modifying the human cell line activation test (h-CLAT) to detect respiratory sensitisers and distinguish them from skin sensitisers. THP-1 cells were exposed to the test chemicals (two skin sensitisers and six respiratory sensitisers), either as monocultures or as cocultures with air-liquid interface-cultured reconstructed human bronchial epithelium. The responses were analysed by measuring the expression levels of surface markers on THP-1 cells (CD86, CD54 and OX40L) and the concentrations of cytokines in the culture media (interleukin (IL)-8, IL-33 and thymic stromal lymphopoietin (TSLP)). The cocultures exhibited increased CD54 expression on THP-1 cells; moreover, in the cocultures but not in the monocultures, exposure to two uronium salts (i.e. respiratory sensitisers) increased CD54 expression on THP-1 cells to levels above the criteria for a positive h-CLAT result. Additionally, exposure to the respiratory sensitiser abietic acid, significantly increased IL-8 concentration in the culture medium, but only in the cocultures. Although further optimisation of the method is needed to distinguish respiratory from skin sensitisers by using these potential markers (OX40L, IL-33 and TSLP), the coculture of THP-1 cells with bronchial epithelial cells offers a potentially useful approach for the detection of respiratory sensitisers.

Biological effects triggered by chemical respiratory sensitizers on THP-1 monocytic cells

Respiratory sensitization encompasses a group of diseases that manifest through airway hyperresponsiveness and airflow limitation. Although the concerns regarding human health, to date there are still no validated methods for preclinical assessment of this class of toxicants once the chemical respiratory allergy mechanistic framework is not fully understood. As Dendritic Cells (DCs) are the bridging elements between innate and adaptative immune responses, we preliminarily investigated the biological alterations triggered by seven different LMW respiratory allergens in the DC model THP-1. The results have shown that exposure to respiratory allergens promoted alterations in DCs maturation/activation status and triggered pro-inflammatory changes in these cells through increased expression for the CD86/HLA-DR/CD11c surface biomarkers and enhancement in IL-8 and IL-6 production by exposed THP-1 cells. Therefore, evidence was found to support the startpoint for chemical respiratory allergy pathogenesis elucidation, subsidizing the contribution of dendritic cells in such pathomechanisms.

Challenges in the classification of chemical respiratory allergens based on human data: Case studies of 2-hydroxyethylmethacrylate (HEMA) and 2-hydroxypropylmethacrylate (HPMA)

Occupational asthma resulting from workplace exposure to chemical respiratory allergens is an important disease. No widely accepted or formally validated tests for the identification of chemical respiratory sensitizers. Consequently, there is a heavy reliance on human data from clinical examinations. Unfortunately, however, although such investigations are critical for the diagnosis of occupational asthma, and in guiding remedial actions, they do not reliably identify specific chemicals within the workplace that are the causative agents. There are several reasons for this, including the fact that specific inhalation tests conducted as part of clinical investigations are frequently performed with complex mixtures rather than single substances, that sometimes inhalation challenges are conducted at concentrations above the OEL and STEL, where effects may be confounded by irritation, and that involvement of immune mechanisms cannot be assumed from the observation of late asthmatic reactions. Further, caution should be taken when implicating substances on lists of "recognised" asthmagens unless they have undergone a formal weight of evidence assessment. Here the limitations of clinical investigations as currently performed for the purposes of regulatory classification and decision making are explored by reference to previously published case studies that implicate 2-hydroxyethylmethacrylate (HEMA) and/or 2-hydroxypropylmethacrylate (HPMA) as respiratory allergens.

A Hydrophilic Sulfated Resveratrol Derivative for Topical Application: Sensitization and Anti-Allergic Potential

Resveratrol (RSV), a naturally occurring metabolite, is widely used in skincare products, but its hydrophobicity impairs its own incorporation into cosmetic formulations. RSV-GS is a synthetic hydrophilic sulfated glycosylated derivative inspired by marine natural products that present a lower cytotoxicity than RSV while exhibiting similar levels of bioactivity. Herein, we predict the skin sensitization potential of this new compound using an in vitro approach based on the OECD 442E guideline. Furthermore, the anti-allergic potential of RSV-GS was also disclosed. The monocyte THP-1 cell line was stimulated with RSV and RSV-GS in the presence or absence of the extreme skin allergen 1-fluoro-2,4-dinitrobenzene (DNFB). The results demonstrated that RSV-GS alone (500 µM) evoked a relative fluorescence index (RFI) lower than the thresholds established by the OECD guideline for CD54 (200%) and CD86 (150%), indicating the absence of a skin sensitization potential. Interestingly, in the presence of the skin allergen DNFB, RSV-GS exhibited the ability to rescue the DNFB-induced maturation of THP-1 cells, with RFI values lower than those for RSV, suggesting the potential of RSV-GS to mitigate skin sensitization evoked by allergens and, consequently, allergic contact dermatitis. These results open new avenues for the use of RSV-GS as a safe and anti-allergic active cosmetic ingredient.

Propylene glycol, skin sensitisation and allergic contact dermatitis: A scientific and regulatory conundrum

Propylene glycol (PG) has widespread use in pharmaceuticals, cosmetics, fragrances and personal care products. PG is not classified as hazardous under the Globally Harmonised System of Classification and Labelling of Chemicals (GHS) but poses an intriguing scientific and regulatory conundrum with respect to allergic contact dermatitis (ACD), the uncertainty being whether and to what extent PG has the potential to induce skin sensitisation. In this article we review the results of predictive tests for skin sensitisation with PG, and clinical evidence for ACD. Patch testing in humans points to PG having the potential to be a weak allergen under certain conditions, and an uncommon cause of ACD in subjects without underlying/pre-disposing skin conditions. In clear contrast PG is negative in predictive toxicology tests for skin sensitisation, including guinea pig and mouse models (e.g. local lymph node assay), validated in vitro test methods that measure various key events in the pathway leading to skin sensitisation, and predictive methods in humans (Human Repeat Insult Patch and Human Maximisation Tests). We here explore the possible scientific basis for this intriguing inconsistency, recognising there are arguably no known contact allergens that are universally negative in, in vitro, animal and human predictive tests methods.

Chemical- and Drug-Induced Allergic, Inflammatory, and Autoimmune Diseases Via Haptenation

Haptens are small molecules that only elicit an immune response when bound to proteins. Haptens initially bind to self-proteins and activate innate immune responses by complex mechanisms via inflammatory cytokines and damage-associated molecular patterns and the subsequent upregulation of costimulatory signals such as cluster of differentiation 86 (CD86) on dendritic cells. Subsequent interactions between CD86 and CD28 on T cells are critically important for properly activating naive T cells and inducing interleukin 2 production, leading to the establishment of adaptive immunity via effector and memory T cells. Accumulating evidence revealed the involvement of haptens in the development of various autoimmune-like diseases such as allergic, inflammatory, and autoimmune diseases including allergic contact dermatitis, atopy, asthma, food allergy, inflammatory bowel diseases, hemolytic anemia, liver injury, leukoderma, and even antitumor immunity. Therefore, the development of in vitro testing alternatives to evaluate in advance whether a substance might lead to the development of these diseases is highly desirable. This review summarizes and discusses recent advances in chemical- and drug-induced allergic, inflammatory, and autoimmune diseases via haptenation and the possible molecular underlying mechanisms, as well as in vitro testing alternatives to evaluate in advance whether a substance might cause the development of these diseases.

Computer-Aided Discovery and Redesign for Respiratory Sensitization: A Tiered Mechanistic Model to Deliver Robust Performance Across a Diverse Chemical Space

Asthma is among the most common occupational diseases with considerable public health and economic costs. Chemicals that induce hypersensitivity in the airways can cause respiratory distress and comorbidities with respiratory infections such as COVID. Robust predictive models for this end point are still elusive due to the lack of an experimental benchmark and the over-reliance of existing in silico tools on structural alerts and structural (vs chemical) similarities. The Computer-Aided Discovery and REdesign (CADRE) platform is a proven strategy for providing robust computational predictions for hazard end points using a tiered hybrid system of expert rules, molecular simulations, and quantum mechanics calculations. The recently developed CADRE model for respiratory sensitization is based on a highly curated data set of structurally diverse chemicals with high-fidelity biological data. The model evaluates absorption kinetics in lung mucosa using Monte Carlo simulations, assigns reactive centers in a molecule and possible biotransformations via expert rules, and determines subsequent reactivity with cell proteins via quantum-mechanics calculations using a multi-tiered regression. The model affords an accuracy above 0.90, with a series of external validations based on literature data in the range of 0.88-0.95. The model is applicable to all low-molecular-weight organics and can inform not only chemical substitution but also chemical redesign to advance development of safer alternatives.

New approach methods for assessing indoor air toxicity

Indoor air is typically a mixture of many chemicals at low concentrations without any adverse health effects alone, but in mixtures they may cause toxicity and risks to human health. The aim of this study was by using new approach methods to assess the potential toxicity of indoor air condensates. In specific, different in vitro test methods including cyto-and immunotoxicity, skin sensitization and endocrine disruption were applied. In addition to biological effects, the indoor air samples were subjected to targeted analysis of 25 volatile organic compounds (VOCs) and Genapol X-80 (a nonionic emulsifier) suspected to be present in the samples, and to a non-targeted "total chemical scan" to find out whether the chemical composition of the samples is associated with the biological effects. The results confirm that assessing health risks of indoor air by analysing individual chemicals is not an adequate approach: We were not able to detect the VOCs and Genapol X-80 in the indoor air samples, yet, several types of toxicity, namely, cytotoxicity, immunotoxicity, skin sensitization and endocrine disruption were detected. In the non-targeted total chemical scan of the indoor air samples, a larger number of compounds were found in the cytotoxic samples than in the non-cytotoxic samples supporting the biological findings. If only one biological method would be selected for the screening of indoor air quality, THP-1 macrophage/WST-1 assay would best fit for the purpose as it is sensitive and serves as a good representative for different sub-toxic end points, including immunotoxicity, (skin) sensitization and endocrine disruption.

Consensus on the Key Characteristics of Immunotoxic Agents as a Basis for Hazard Identification

BACKGROUND

Key characteristics (KCs), properties of agents or exposures that confer potential hazard, have been developed for carcinogens and other toxicant classes. KCs have been used in the systematic assessment of hazards and to identify assay and data gaps that limit screening and risk assessment. Many of the mechanisms through which pharmaceuticals and occupational or environmental agents modulate immune function are well recognized. Thus KCs could be identified for immunoactive substances and applied to improve hazard assessment of immunodulatory agents.

OBJECTIVES

The goal was to generate a consensus-based synthesis of scientific evidence describing the KCs of agents known to cause immunotoxicity and potential applications, such as assays to measure the KCs.

METHODS

A committee of 18 experts with diverse specialties identified 10 KCs of immunotoxic agents, namely, 1) covalently binds to proteins to form novel antigens, 2) affects antigen processing and presentation, 3) alters immune cell signaling, 4) alters immune cell proliferation, 5) modifies cellular differentiation, 6) alters immune cell-cell communication, 7) alters effector function of specific cell types, 8) alters immune cell trafficking, 9) alters cell death processes, and 10) breaks down immune tolerance. The group considered how these KCs could influence immune processes and contribute to hypersensitivity, inappropriate enhancement, immunosuppression, or autoimmunity.

DISCUSSION

KCs can be used to improve efforts to identify agents that cause immunotoxicity via one or more mechanisms, to develop better testing and biomarker approaches to evaluate immunotoxicity, and to enable a more comprehensive and mechanistic understanding of adverse effects of exposures on the immune system. https://doi.org/10.1289/EHP10800.

Framework for sensitization assessment of extractables and leachables in pharmaceuticals

During the toxicological assessment of extractables and leachables in drug products, localized hazards such as irritation or sensitization may be identified. Typically, because of the low concentration at which leachables occur in pharmaceuticals, irritation is of minimal concern; therefore, this manuscript focuses on sensitization potential. The primary objective of performing a leachable sensitization assessment is protection against Type IV induction of sensitization, rather than prevention of an elicitation response, as it is not possible to account for the immunological state of every individual. Sensitizers have a wide range of potencies and those which induce sensitization upon exposure at a low concentration (i.e. strong, or extreme sensitizers) pose the highest risk to patients and should be the focus of the risk assessment. The Extractables and Leachables Safety Information Exchange (ELSIE) consortium has reviewed the status of dermal, respiratory, and systemic risk assessment in cosmetic and pharmaceutical industries, and proposes a framework to evaluate the safety of known or potential dermal sensitizers in pharmaceuticals. Due to the lack of specific regulatory guidance on this topic, the science-driven risk-based approach proposed by ELSIE encourages consistency in the toxicological assessment of extractables and leachables to maintain high product quality and ensure patient safety.

Exploring structure/property relationships to health and environmental hazards of polymeric polyisocyanate prepolymer substances-2. Dermal sensitization potential in the mouse local lymph node assay

The sensitization potencies of twenty custom-designed monomer-depleted polymeric polyisocyanate prepolymer substances and their associated toluene diisocyanate (TDI), methylene diphenyl diisocyanate (MDI), hexamethylene diisocyanate (HDI), and isophorone diisocyanate (IPDI) monomer precursors were investigated by means of the mouse Local Lymph Node Assay (LLNA). These polymeric prepolymers were designed to represent the structural features and physical-chemical properties exhibited by a broad range of commercial polymeric polyisocyanate prepolymers that are produced from the reaction of aromatic and aliphatic diisocyanate monomers with aliphatic polyether and polyester polyols. The normalization of LLNA responses to the applied (15-45-135 mM) concentrations showed that the skin sensitization potency of polymeric polyisocyanate prepolymers is at least 300 times less than that of the diisocyanate monomers from which they are derived. The sensitization potency of the prepolymers was shown to be mainly governed by their hydrophobicity (as expressed by the calculated octanol-water partition coefficient, log K_ow) and surfactant properties. Neither hydrophilic (log K_ow <0) nor very hydrophobic (log K_ow >25) prepolymers stimulated lymphocyte proliferation beyond that of the dosing vehicle control. The findings of this investigation challenge the generally held assumption that all isocyanate (-N=C=O) bearing substances are potential skin (and respiratory) sensitizers. Further, these findings can guide the future development of isocyanate chemistries and associated polyurethane applications toward reduced exposure and health hazard potentials.

Methyl methacrylate and respiratory sensitisation: a comprehensive review

Methyl methacrylate (MMA) is classified under GHS as a weak skin sensitiser and a skin and respiratory irritant. It has recently been proposed that MMA be classified as a respiratory sensitiser (a designation that in a regulatory context embraces both true respiratory allergens, as well as chemicals that cause asthma through non-immunological mechanisms). This proposal was based primarily upon the interpretation of human data. This review, and a detailed weight of evidence analysis, has led to another interpretation of these data. The conclusion drawn is that persuasive evidence consistent with the designation of MMA as a respiratory sensitiser is lacking. It is suggested that one reason for different interpretations of these data is that occupational asthma poses several challenges with respect to establishing causation. Among these is that it is difficult to distinguish between allergic asthma, non-allergic asthma, and work-related exacerbation of pre-existing asthma. Moreover, there is a lack of methods for the identification of true chemical respiratory allergens. The characterisation and causation of occupational asthma is consequently largely dependent upon interpretation of human data of various types. Recommendations are made that are designed to improve the utility and interpretation of human data for establishing causation in occupational asthma.

A brief overview of properties and reactions of diisocyanates

By way of introduction to the special issue on d iisocyanates and their corresponding diamines, this brief overview presents, for the most commonly used diisocyanate monomers, a selection of physical-chemical properties that are relevant to exposure in the workplace and in the general environment, as well as a concise overview of diisocyanate reactions and some of their toxicological implications.

Replacement of animal testing by integrated approaches to testing and assessment (IATA): a call for in vivitrosi

Alternative methods to animal use in toxicology are evolving with new advanced tools and multilevel approaches, to answer from one side to 3Rs requirements, and on the other side offering relevant and valid tests for drugs and chemicals, considering also their combination in test strategies, for a proper risk assessment.

While stand-alone methods, have demonstrated to be applicable for some specific toxicological predictions with some limitations, the new strategy for the application of New Approach Methods (NAM), to solve complex toxicological endpoints is addressed by Integrated Approaches for Testing and Assessment (IATA), aka Integrated Testing Strategies (ITS) or Defined Approaches for Testing and Assessment (DA). The central challenge of evidence integration is shared with the needs of risk assessment and systematic reviews of an evidence-based Toxicology. Increasingly, machine learning (aka Artificial Intelligence, AI) lends itself to integrate diverse evidence streams.

In this article, we give an overview of the state of the art of alternative methods and IATA in toxicology for regulatory use for various hazards, outlining future orientation and perspectives. We call on leveraging the synergies of integrated approaches and evidence integration from in vivo, in vitro and in silico as true in vivitrosi.

Absorption, distribution, metabolism, and excretion of methylene diphenyl diisocyanate and toluene diisocyanate: Many similarities and few differences

Methylene diphenyl diisocyanate (MDI) and toluene diisocyanate (TDI) are high production volume chemicals used for the manufacture of polyurethanes. For both substances, the most relevant adverse health effects after overexposure in the workplace are isocyanate-induced asthma, lung function decrement and, to a much lesser extent, skin effects. Over the last two decades many articles have addressed the reactivity of MDI and TDI in biological media and the associated biochemistry, which increased the understanding of their biochemical and physiological behavior. In this review, these new insights with respect to similarities and differences concerning the adsorption, distribution, metabolism, and excretion (ADME) of these two diisocyanates and the implications on their toxicities are summarized. Both TDI and MDI show very similar behavior in reactivity to biological macromolecules, distribution, metabolism, and excretion. Evidence suggests that the isocyanate (NCO) group is scavenged at the portal-of-entry and is not systemically available in unbound reactive form. This explains the lack of other than portal-of-entry toxicity observed in repeated-dose inhalation tests.

Evaluation of animal toxicity studies with diisocyanates regarding presence of thresholds for induction and elicitation of respiratory allergy by quantitative weight of evidence

Animal toxicity studies on diisocyanates were evaluated using quantitative weight of evidence (QWoE) to test the hypothesis that the dose-response curve shows a threshold for the induction and/or elicitation of respiratory sensitization. A literature search identified 59 references that included at least two concentration groups of the diisocyanate and a vehicle-exposed concurrent control in the study design. These studies were subjected to a QWoE-assessment applying scoring criteria for quality and relevance/strength of effects relevant to the selected endpoint of respiratory sensitization. Overall, the studies assessing dose/concentration-response for diisocyanates with the endpoint, respiratory sensitization, were heterogenous regarding study design, animal models used, endpoints assessed, and quality. Only a limited number of the studies subjected to the QWoE-assessment allowed drawing conclusions about possible thresholds for respiratory sensitization. Highest quality and relevance/strength of effects scores were obtained by a series of studies specifically designed to investigate a potential threshold for elicitation of respiratory sensitization in the Brown Norway (BN) rat. These studies applied an elaborate study design to optimize induction of respiratory sensitization and reduce interference by respiratory tract irritation. In summary, the available studies provided moderate to good support for the existence of a threshold for elicitation and limited to moderate support for a threshold regarding induction of respiratory allergy by diisocyanates in experimental animals. However, a quantitative extrapolation of threshold values established in rodents to humans remains complex.

Identifying a reference list of respiratory sensitizers for the evaluation of novel approaches to study respiratory sensitization

The induction of immunological responses that trigger bio-physiological symptoms in the respiratory tract following repeated exposure to a substance, is known as respiratory sensitization. The inducing compound is known as a respiratory sensitizer. While respiratory sensitization by high molecular weight (HMW) materials is recognized and extensively studied, much less information is available regarding low molecular weight (LMW) materials as respiratory sensitizers. Variability of symptoms presented in humans from such exposures, limited availability of (and access to) documented reports, and the absence of standardized and validated test models, hinders the identification of true respiratory sensitizers. This review aims to sort suspected LMW respiratory sensitizers based on available compelling, reasonable, inadequate, or questionable evidence in humans from occupational exposures and use this information to compose a reference list of reported chemical respiratory sensitizers for scientific research purposes. A list of 97 reported respiratory sensitizers was generated from six sources, and 52 LMW organic chemicals were identified, reviewed, and assigned to the four evidence categories. Less than 10 chemicals were confirmed with compelling evidence for induction of respiratory sensitization in humans from occupational exposures. Here, we propose the reference list for developing novel research on respiratory sensitization.

Enzymes and sensitization via skin exposure: A critical analysis

Some proteins, including enzymes, can induce allergic sensitization of various types, including allergic sensitization of the respiratory tract. There is now an increased understanding of the role that the skin plays in the development of IgE-mediated allergy and this prompts the question whether topical exposure to enzymes used widely in consumer cleaning products could result in allergic sensitization. Here, the evidence that proteins can interact with the skin immune system and the way they do so is reviewed, together with a consideration of the experience gained over decades of the use of enzymes in laundry and cleaning products. The conclusion drawn is that although transcutaneous sensitization to proteins can occur (typically through compromised skin) resulting in IgE antibody-mediated allergy, in practice such skin contact with enzymes used in laundry and cleaning products does not appear to pose a significant risk of allergic disease. Further, the evidence summarized in this publication support the view that proteins do not pose a risk of allergic contact dermatitis.

A new cytometry-based method reveals an accumulation of Nrf2 in dendritic cells exposed to two respiratory sensitizers

The mechanisms underlying chemical respiratory sensitization are incompletely understood. One of the major cell types involved in this pathology are dendritic cells. In this study, the mechanisms of the NRF2-Keap1 pathway were studied using a bone marrow-derived dendritic cell model exposed to two respiratory sensitizers: ammonium hexachloroplatinate (HCP) and ammonium tetrachloroplatinate (ATCP). Expression levels for two Nrf2-regulated genes, hmox1 and srxn1, were analyzed by real time-quantitative polymerase chain reaction. A flow cytometry-based method was also developed to measure intracellular Nrf2 accumulation in dendritic cells following exposure. Exposure to HCP and ATCP increased both hmox1 and srxn1 gene expression, and was associated with accumulation of Nrf2 protein in cells. Overall, these results show that the respiratory sensitizers, in addition to skin sensitizers, can also induced markers associated with NRF2-Keap1 pathway activation in dendritic cells. This study contributes to a better understanding of the adverse outcome of respiratory sensitization.

Critical Evaluation of Low-Molecular Weight Respiratory Sensitizers and Their Protein Reactivity Potential Toward Lysine Residues

Interest in the development of methods to evaluate the respiratory sensitization potential of low-molecular weight chemicals continues, but no method has yet been generally accepted or validated. A lack of chemical reference standards, together with uncertainty regarding relevant immunological mechanisms, has hampered method development. The first key event in the development of either skin or respiratory sensitization is the formation of stable adducts of the chemical with host proteins. This event is measured in the Direct Peptide Reactivity Assay using cysteine- and lysine-containing model peptides. It is hypothesized that protein reactivity and subsequent adduct formation may represent the earliest point of divergence in the pathways leading to either skin or respiratory sensitization. Direct Peptide Reactivity Assay data for 200 chemicals were compiled and grouped into respiratory, skin and nonsensitizers. Chemicals grouping was based on extensive literature research and expert judgment. To evaluate if chemical groups represent different peptide reactivity profiles, peptide reactivity data were clustered and compared with information on protein binding mechanisms and chemical categories available via the Organization for Economic Co-operation and Development. Toolbox. Respiratory sensitizers (n = 15) showed a significant (3-fold) higher lysine reactivity than skin sensitizers (n = 129). However, this difference was driven largely by the high representation of acid anhydrides among the respiratory sensitizers that showed clear lysine selectivity. Collectively, these data suggest that preferential reactivity for either cysteine or lysine is associated primarily with chemical structure, and that lysine preference is not a unifying characteristic of chemical respiratory allergens.

Chemical Modifications Induced by Phthalic Anhydride, a Respiratory Sensitizer, in Reconstructed Human Epidermis: A Combined HRMAS NMR and LC-MS/MS Proteomic Approach

Chemical skin and respiratory allergies are becoming a major health problem. To date our knowledge on the process of protein haptenation is still limited and mainly derived from studies performed in solution using model nucleophiles. In order to better understand chemical interactions between chemical allergens and the skin, we have investigated the reactivity of phthalic anhydride 1 (PA), a chemical respiratory sensitizer, toward reconstructed human epidermis (RHE). This study was performed using a new approach combining HRMAS NMR to investigate the in situ chemical reactivity and LC-MS/MS to identify modified epidermal proteins. In RHE, the reaction of PA appeared to be quite fast and the major product formed was phthalic acid. Two amide type adducts on lysine residues were observed and after 8h of incubation, we also observed the formation of an imide type cyclized adducts with lysine. In parallel, RHE samples topically exposed to phthalic anhydride (¹³C)-1 were analyzed using the shotgun proteomics method. Thus, 948 different proteins were extracted and identified, 135 of which being modified by PA, i.e., 14.2% of the extracted proteome. A total of 211 amino acids were modified by PA and validated by fragmentation spectra. We thus identified 154 modified lysines, 22 modified histidines, 30 modified tyrosines, and 5 modified arginines. The rate of modified residues, as a proportion of the total number of modifiable nucleophilic residues in RHE, was rather low (1%). At the protein level, modified proteins were mainly type I and type II keratins and other proteins which are abundant in the epidermis such as protein S100A, Caspase 14, annexin A2, serpin B3, fatty-acid binding protein 5, histone H2, H3, H4, etc. However, the most modified protein, mainly on histidine residues, was filaggrin, a protein that is of low abundance (0.0266 mol %) and rich in histidine.

Beyond dermal exposure: The respiratory tract as a target organ in hazard assessments of cosmetic ingredients

Dermal contact is the main route of exposure for most cosmetics; however, inhalation exposure could be significant for some formulations (e.g., aerosols, powders). Current cosmetic regulations do not require specific tests addressing respiratory irritation and sensitisation, and despite the prohibition of animal testing for cosmetics, no alternative methods have been validated to assess these endpoints to date. Inhalation hazard is mainly determined based on existing human and animal evidence, read-across, and extrapolation of data from different target organs or tissues, such as the skin. However, because of mechanistic differences, effects on the skin cannot predict effects on the respiratory tract, which indicates a substantial need for the development of new approach methodologies addressing respiratory endpoints for inhalable chemicals in general. Cosmetics might present a particularly significant need for risk assessments of inhalation exposure to provide a more accurate toxicological evaluation and ensure consumer safety. This review describes the differences in the mechanisms of irritation and sensitisation between the skin and the respiratory tract, the progress that has already been made, and what still needs to be done to fill the gap in the inhalation risk assessment of cosmetic ingredients.

Classification of chemicals as respiratory allergens based on human data: Requirements and practical considerations

Occupational asthma is an important health problem that can include exacerbation of existing asthma, or induce new asthma either through allergic sensitisation, or non-immunological mechanisms. While allergic sensitisation of the respiratory tract can be acquired to proteins, or to low molecular weight chemicals (chemical respiratory allergens) this article is on the latter exclusively. Chemical respiratory allergy resulting in occupational asthma is associated with high levels of morbidity and there is a need, therefore, that chemicals which can cause sensitisation of the respiratory tract are identified accurately. However, there are available no validated, or even widely accepted, predictive test methods (in vivo, in vitro or in silico) that have achieved regulatory acceptance for identifying respiratory sensitising hazards. For this reason there is an important reliance on human data for the identification of chemical respiratory allergens, and for distinguishing these from chemicals that cause occupational asthma through non-immunological mechanisms. In this article the reasons why it is important that care is taken in designating chemicals as respiratory allergens are reviewed. The value and limitations of human data that can aid the accurate identification of chemical respiratory allergens are explored, including exposure conditions, response characteristics in specific inhalation challenge tests, and immunological investigations.

Mapping Chemical Respiratory Sensitization: How Useful Are Our Current Computational Tools?

Chemical respiratory sensitization is an immunological process that manifests clinically mostly as occupational asthma and is responsible for 1 in 6 cases of adult asthma, although this may be an underestimate of the prevalence, as it is under-diagnosed. Occupational asthma results in unemployment for roughly one-third of those affected due to severe health issues. Despite its high prevalence, chemical respiratory sensitization is difficult to predict, as there are currently no validated models and the mechanisms are not entirely understood, creating a significant challenge for regulatory bodies and industry alike. The Adverse Outcome Pathway (AOP) for respiratory sensitization is currently incomplete. However, some key events have been identified, and there is overlap with the comparatively well-characterized AOP for dermal sensitization. Because of this, and the fact that dermal sensitization is often assessed by in vivo, in chemico, or in silico methods, regulatory bodies are defaulting to the dermal sensitization status of chemicals as a proxy for respiratory sensitization status when evaluating chemical safety. We identified a data set of known human respiratory sensitizers, which we used to investigate the accuracy of a structural alert model, Toxtree, designed for skin sensitization and the Centre for Occupational and Environmental Health (COEH)'s model, a model developed specifically for occupational asthma. While both models had a reasonable level of accuracy, the COEH model achieved the highest balanced accuracy at 76%; when the models agreed, the overall accuracy was 87%. There were important differences between the models: Toxtree had superior performance for some structural alerts and some categories of well-characterized skin sensitizers, while the COEH model had high accuracy in identifying sensitizers that lacked identified skin sensitization reactivity domains. Overall, both models achieved respectable accuracy. However, neither model addresses potency, which, along with data quality, remains a hurdle, and the field must prioritize these issues to move forward.

The activity of methacrylate esters in skin sensitisation test methods II. A review of complementary and additional analyses

Allergic contact dermatitis is an important occupational health issue, and there is a need to identify accurately those chemicals that have the potential to induce skin sensitisation. Hazard identification was performed initially using animal (guinea pig and mouse) models. More recently, as a result of the drive towards non-animal methods, alternative in vitro and in silico approaches have been developed. Some of these new in vitro methods have been formally validated and have been assigned OECD Test Guideline status. The performance of some of these recently developed in vitro methods, and of 2 quantitative structure-activity relationships (QSAR) approaches, with a series of methacrylate esters has been reviewed and reported previously. In this article that first review has been extended further with additional data and complementary analyses. Results obtained using in vitro methods (Direct Peptide Reactivity Assay, DPRA; ARE-Nrf2 luciferase test methods, KeratinoSens and LuSens; Epidermal Sensitisation Assay, EpiSensA; human Cell Line Activation Test, h-CLAT, and the myeloid U937 Skin Sensitisation test, U-SENS), and 2 QSAR approaches (DEREK™-nexus and TIMES-SS), with 11 methacrylate esters and methacrylic acid are reported here, and compared with existing data from the guinea pig maximisation test and the local lymph node assay. With this series of chemicals it was found that some in vitro tests (DPRA and ARE-Nrf2 luciferase) performed well in comparison with animal test results and available human skin sensitisation data. Other in vitro tests (EpiSensA and h-CLAT) proved rather more problematic. Results with DEREK™-nexus and TIMES-SS failed to reflect accurately the skin sensitisation potential of the methacrylate esters. The implications for assessment of skin sensitising activity are discussed.

Allergic contact dermatitis: From pathophysiology to development of new preventive strategies

As the body's first line of defense, the skin is the organ most frequently exposed to chemicals present in personal hygiene products, household products, or materials used in the work environment. In this context, skin disorders account for more than 40 % of all occupational and work-related diseases, constituting a significant public health burden. Among skin disorders, allergic contact dermatitis (ACD) is the most prevalent occupational disease and the most common form of immunotoxicity in humans. ACD is a T-cell-mediated skin inflammation resulting from the priming and expansion of allergen-specific CD4⁺ and CD8⁺ T cells. The clinical condition is characterized by local skin rash, itchiness, redness, swelling, and lesions, being mainly diagnosed by the patch test. Upon ACD diagnosis, avoiding the exposure to the triggering allergen is the mainstay of treatment to prevent future flares. In cases where avoidance is not possible, the use of a standard of care interim treatments such as steroid creams or ointments, barrier creams, and moisturizers are strongly recommended to alleviate symptoms. In this review, we sought to provide the reader with an overview of the pathophysiology of ACD as well as the currently available pharmacological treatment options. Furthermore, a comprehensive outline of several preventive strategies is also provided.

Cutaneous lewisite exposure causes acute lung injury

Lewisite is a strong vesicating and chemical warfare agent. Because of the rapid transdermal absorption, cutaneous exposure to lewisite can also elicit severe systemic injury. Lewisite (2.5, 5.0, and 7.5 mg/kg) was applied to the skin of Ptch1^+/- /SKH-1 mice and acute lung injury (ALI) was assessed after 24 hours. Arterial blood gas measurements showed hypercapnia and hypoxemia in the lewisite-exposed group. Histological evaluation of lung tissue revealed increased levels of proinflammatory neutrophils and a dose-dependent increase in structural changes indicative of injury. Increased inflammation was also confirmed by altered expression of cytokines, including increased IL-33, and a dose-dependent elevation of CXCL1, CXCL5, and GCSF was observed in the lung tissue. In the bronchoalveolar lavage fluid of lewisite-exposed animals, there was a significant increase in HMGB1, a damage-associated molecular pattern molecule, as well as elevated CXCL1 and CXCL5, which coincided with an influx of neutrophils to the lungs. Complete blood cell analysis revealed eosinophilia and altered neutrophil-lymphocyte ratios as a consequence of lewisite exposure. Mean platelet volume and RBC distribution width, which are predictors of lung injury, were also increased in the lewisite group. These data demonstrate that cutaneous lewisite exposure causes ALI and may contribute to mortality in exposed populations.

Adverse outcome pathways for chemical toxicity and their applications to workers' health: a literature review

OBJECTIVE AND METHODS

Various papers related to the application of adverse outcome pathways (AOPs) for the prevention of occupational disease were reviewed. The Internet was used as the primary tool to search for the necessary research data and information, using such online resources as Google Scholar, ScienceDirect, Scopus, NDSL, and PubMed. The key search terms were "adverse outcome pathway," "toxicology," "risk assessment," "human," "worker," "occupational safety and health," and so on.

RESULTS AND CONCLUSION

The aim of this paper is to explain the use of AOP for the understanding of chemical toxicity as a conceptual means and to predict the toxic mechanism. The tools of AOP have emerged as a forward-looking alternative to the existing chemical risk assessment paradigm. AOP is being applied to the assessment of acute toxicity and to chronic toxic chemicals in the workplace. Not only can it lead to breakthroughs in occupational and environmental cancer prevention, it is also widely used in chemical risk assessment and has led to breakthroughs in the prevention of occupational disease in the workplace.

How to assess respiratory sensitization of low molecular weight chemicals?

There are no validated and regulatory accepted (animal) models to test for respiratory sensitization of low molecular weight (LMW) chemicals. Since several decades such chemicals are classified as respiratory sensitizers almost exclusively based on observations in workers. However, both respiratory allergens (in which process the immune system is involved) as well as asthmagens (no involvement of the immune system) may induce the same type of respiratory symptoms. Correct classification is very important from a health's perspective point of view. On the other hand, over-classification is not preferable in view of high costs to overdue workplace engineering controls or the chemical ultimately being banned due to Authorities' decisions. It would therefore be very beneficial if respiratory sensitizers can be correctly identified and distinguished from skin sensitizers and non-sensitizers/respiratory irritants. The purpose of this paper is to consider whether LMW chemicals can be correctly identified based on the currently available screening methods in workers, and/or via in silico, in vitro and/or in vivo testing. Collectively, based on the available information further effort is still needed to be able to correctly identify respiratory sensitizers and to distinguish these from skin sensitizers and irritants, not at least because of the far-reaching consequences once a chemical is classified as a respiratory sensitizer.

Application of the direct peptide reactivity assay (DPRA) to inorganic compounds: a case study of platinum species

The in chemico Direct Peptide Reactivity Assay (DPRA) was developed as a non-animal, relatively high throughput, screening tool for skin sensitization potential. Although the Adverse Outcome Pathway (AOP) for respiratory sensitization remains to be fully elucidated, it is recognized that the molecular initiation event for both skin and respiratory sensitization to low molecular weight chemicals involves haptenation with proteins. The DPRA examines the reactivity of a test compound to two model peptides (containing either cysteine or lysine) and consequently is able to screen for both skin and respiratory sensitization potential. The DPRA was primarily developed for and validated with organic compounds and assessment of the applicability of the assay to metal compounds has received only limited attention. This paper reports the successful application of the DPRA to a series of platinum compounds, including hexachloroplatinate and tetrachloroplatinate salts, which are some of the most potent chemical respiratory sensitizers known. Eleven platinum compounds were evaluated using the DPRA protocol as detailed by Lalko et al., with only minor modification. Two palladium compounds with structures similar to that of the platinum species studied and cobalt chloride were additionally tested for comparison. The hexachloroplatinate and tetrachloroplatinate salts showed exceptionally high reactivity with the cysteine peptide (EC₁₅ values of 1.4 and 14 μM, respectively). However, for platinum compounds (e.g. hydrogen hexahydroxyplatinate and tetraammineplatinum) where clinical and epidemiological evidence indicates limited sensitization potential, the cysteine DPRA showed only minor or no reactivity (EC₁₅ values of 24 600 and >30 000 μM, respectively). The outcomes of the lysine peptide assays were less robust and where EC₁₅ was measurable, values were substantially higher than the corresponding results from the cysteine assay. This work supports the value of in chemico peptide reactivity as a metric for assessment of platinum sensitization potential and therefore in screening of new platinum compounds for low or absent sensitization potential. Additional studies are required to determine whether the DPRA may be successfully applied to other metals. We provide details on method modifications and precautions important to the success of the DPRA in the assessment of metal reactivity.

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.

Fragrance inhalation and adverse health effects: The question of causation

The toxicology of fragrance materials is largely well understood. Although most are benign, a minority have the potential to cause adverse health effects, notably allergic contact dermatitis resulting from skin sensitization. As a consequence, industry guidelines have banned certain materials and strictly limited the use of others. Recently, data have been published that have been interpreted to suggest that inhalation of fragrances is associated with the occurrence of a variety of health effects, ranging from headaches to asthma attacks. In this review, the evidence basis for these assertions is examined critically and the biological basis and mechanistic plausibility for causation by fragranced products of these health effects is explored. This review concludes that respiratory effects, including irritation and allergy appear highly unlikely to occur by this route. While some sensory/psychosomatic effects are possible, this does not explain the very high rates of adverse effects reported in the recently published questionnaire studies, which this review concludes are more likely to be attributed to methodological weaknesses. Ultimately, it is concluded that adverse health effects arising from fragrance inhalation are uncommon and remain to be identified and confirmed by methodologically rigorous epidemiological investigations supported by a convincing biological and mechanistic basis.