Current knowledge on biomarkers for contact sensitization and allergic contact dermatitis

Subclinical immune responses to nickel in sensitized individuals-a dose-response study

BACKGROUND

Nickel is the leading cause of contact allergy in Europe, with 14.5% of the adult population being sensitized. Despite regulations limiting nickel release from consumer items, the incidence and prevalence of nickel allergy remain high.

OBJECTIVE

To investigate the clinical and subclinical immune response to low-dose nickel exposure on nickel pre-exposed skin to assess the adequacy of current regulatory limits.

METHOD

Nickel-allergic and healthy controls were patch tested with nickel twice with a 3-4 weeks interval. The first exposure used the diagnostic concentration of 2000 μg/cm2 nickel sulphate, and the same skin areas were then re-exposed to 0.2, 0.5, 12.8 and 370 μg/cm2 nickel sulphate. After 48 h, the patch reactions were examined for clinical signs of eczema, and skin biopsies were collected. The transcriptomic immune profile was analysed with Nanostring nCounter and quantitative polymerase chain reaction.

RESULTS

Two nickel-allergic participants (15%) had clinical reactions to the regulatory limiting doses for nickel (0.2/0.5 μg/cm2) following re-exposure. There was immune activation in all skin areas following re-exposure to nickel, predominantly mediated by up-regulation of cytokines and chemokines. In all nickel re-exposed skin areas, 81 genes were up-regulated independent from the clinical response. In skin areas exposed to 0.2 μg/cm2, 101 immune-related genes were differentially expressed, even when no clinical response was observed. Healthy controls showed up-regulation of three genes in response to nickel re-exposures without any clinical reactions.

CONCLUSION

Immune activation can be induced in skin with local memory to nickel upon challenge with nickel doses within the regulatory limits. Our findings suggest that the regulatory limits in the European nickel regulation may not provide sufficient protection for consumers against low-dose exposures.

CD1a and skin T cells: a pathway for therapeutic intervention

The CD1 and MR1 protein families present lipid antigens and small molecules to T cells, complementing well-studied major histocompatibility complex-peptide mechanisms. The CD1a subtype is highly and continuously expressed within the skin, most notably on Langerhans cells, and has been demonstrated to present self and foreign lipids to T cells, highlighting its cutaneous sentinel role. Alteration of CD1a-dependent T-cell responses has recently been discovered to contribute to the pathogenesis of several inflammatory skin diseases. In this review, we overview the structure and role of CD1a and outline the current evidence implicating CD1a in the development of psoriasis, atopic dermatitis and allergic contact dermatitis.

BIOSKIN: A Protocol for the Copenhagen Translational Skin Immunology Biobank and Research Programme

INTRODUCTION

Psoriasis, atopic dermatitis and contact dermatitis are common chronic inflammatory skin diseases that have a significant impact on individuals and society.

METHODS AND ANALYSIS

The Copenhagen Translational Skin Immunology Biobank and Research Programme (BIOSKIN) is a translational biobank and research study that aims to prospectively collect high-quality biological samples and clinical data from 3000 patients with psoriasis, atopic dermatitis and contact dermatitis over a minimum period of 5 years. The longitudinal open design allows participants to enter and leave the study at different time points depending on their disease and treatment course. At every visit, the investigator collects biological samples, conducts interviews and assembles self-reported questionnaires on disease-specific and general health-related information. Clinical examination and biological sampling will be conducted at enrolment, during and after disease flare, before and after initiation of new treatment and at least once per year. The clinical examination includes dermatological verification of diagnosis, evaluation of disease severity and detailed information on phenotype. The biological samples include blood and when accessible and relevant, skin biopsies, tape strips and skin swabs. The data collected will undergo rigorous statistical analysis using appropriate analytical methods. As of December 2023, 825 patients have been enrolled in the study.

ETHICS AND DISSEMINATION

The study is approved by the Scientific Ethical Committee of the Capital Region (H-21032986) and the Danish Data Protection Agency. Results will be published in peer-reviewed scientific journals and presented at national and international conferences.

New Perspectives in the Management of Chronic Hand Eczema: Lessons from Pathogenesis

Chronic hand eczema (CHE) is a common inflammatory skin condition that significantly impacts the quality of life. From work-related disabilities to social embarrassment, pain, and financial costs, the burden on society is substantial. Managing this condition presents challenges such as long-term treatment, poor patient compliance, therapy side effects, and economic feasibility. As a result, significant efforts have been made in this field in recent years. Specifically, the broader understanding of CHE pathogenesis has led to the development of new drugs, both topical and systemic. The aim of this narrative review is to summarize the current available data on hand eczema pathophysiology and explore the resulting developments in drugs for its treatment. A comprehensive search on PubMed and the other main scientific databases was conducted using keywords related to CHE and its pathogenesis. The most relevant pathways targeted by therapies include the JAK-STAT cascade, IL-4, and IL-13 axis, phosphodiesterase 4 enzyme, and chemo-attractant cytokines. In the near future, physicians will have a plethora of therapeutic alternatives. Consequently, they should be well-trained not only in how to use these alternatives but also how to combine these treatments to address the ongoing challenges related to efficacy, tolerability, and safety.

How Do Classical Subtypes Correspond to Endotypes in Atopic Dermatitis?

Since atopic dermatitis (AD) is a heterogeneous condition, the subtyping of AD is a crucial issue. The classical subtypes of AD are represented by extrinsic and intrinsic subtypes, European-American and Asian subtypes, and adult and pediatric subtypes. While the subtyping of AD was historically conducted based on the phenotype, recent findings on the mechanisms of AD have revealed the importance of the endotype, which can characterize individual patients more accurately. Considering the current development of AD therapies, AD endotyping is a prerequisite for a personalized therapeutic choice. Endotypes of AD can be stratified from different viewpoints, including cytokine expression patterns, allergen properties, epidermal barrier conditions, ceramide variation, the involvement of innate immunity, and serum biomarkers. Among them, the cytokine-based endotype seems to be the most useful one and is categorized into type 2 cytokine (IL-4, IL-13 and IL-31)-high, type 1 cytokine (IFN-γ)-high, and/or type 3 cytokine (IL-22 and IL-17)-high, or mixed subtypes. Recently proposed biomarker endotyping aims at individualized treatment options, although the daily clinical use of endotypes is a future issue. To better understand the endotypes for clinicians, attempts to adjust each of the classical subtypes to endotypes are required. This review will discuss the correspondence of the classical subtypes to the various endotypes that have recently been proposed.

A comparative proteomics analysis of four contact allergens in THP-1 cells shows distinct alterations in key metabolic pathways

Allergic contact dermatitis (ACD) is the predominant form of immunotoxicity in humans. The sensitizing potential of chemicals can be assessed in vitro. However, a better mechanistic understanding could improve the current OECD-validated test battery. The aim of this study was to get insights into toxicity mechanisms of four contact allergens, p-benzoquinone (BQ), 2,4-dinitrochlorobenzene (DNCB), p-nitrobenzyl bromide (NBB) and NiSO4, by analyzing differential proteome alterations in THP-1 cells using two common proteomics workflows, stable isotope labeling by amino acids in cell culture (SILAC) and label-free quantification (LFQ). Here, SILAC was found to deliver more robust results. Overall, the four allergens induced similar responses in THP-1 cells, which underwent profound metabolic reprogramming, including a striking upregulation of the TCA cycle accompanied by pronounced induction of the Nrf2 oxidative stress response pathway. The magnitude of induction varied between the allergens with DNCB and NBB being most potent. A considerable overlap between transcriptome-based signatures of the GARD assay and the proteins identified in our study was found. When comparing the results of this study to a previous proteomics study in human primary monocyte-derived dendritic cells, we found a rather low share in regulated proteins. However, on pathway level, the overlap was high, indicating that affected pathways rather than single proteins are more eligible to investigate proteomic changes induced by contact allergens. Overall, this study confirms the potential of proteomics to obtain a profound mechanistic understanding, which may help improving existing in vitro assays for skin sensitization.

Circulating biomarkers are associated with disease severity of chronic hand eczema and atopic dermatitis

BACKGROUND

Although chronic hand eczema (CHE) is a highly prevalent and disabling skin disease, it is currently unknown if CHE is associated with systemic inflammation.

OBJECTIVES

To characterize the plasma inflammatory signature of CHE.

METHODS

Using Proximity Extension Assay technology, we assessed 266 inflammatory and cardiovascular disease risk proteins in the plasma of 40 healthy controls, 57 patients with atopic dermatitis (AD) with active lesions, 11 with CHE and a history of AD (CHEPREVIOUS_AD), and 40 with CHE and no history of AD (CHENO_AD). Filaggrin gene mutation status was also assessed. Protein expression was compared between groups and according to disease severity. Correlation analyses for biomarkers, and clinical- and self-reported variables, were performed.

RESULTS

Very severe CHENO_AD was associated with systemic inflammation when compared with controls. Levels of T helper (Th)2- and Th1-, general inflammation and eosinophil activation markers increased with severity of CHENO_AD, primarily being significantly increased in very severe disease. Significant, positive correlations were found between markers from these pathways and severity of CHENO_AD. Moderate-to-severe but not mild AD displayed systemic inflammation. The Th2 markers C-C motif chemokine (CCL)17 and CCL13 (also known as monocyte chemotactic protein 4) were the top differentially expressed proteins in both very severe CHENO_AD and moderate-to-severe AD, showing a higher fold change and significance in AD. CCL17 and CCL13 levels further correlated positively with disease severity in both CHENO_AD and AD.

CONCLUSIONS

Systemic Th2-driven inflammation is shared between very severe CHE with no history of AD, and moderate-to-severe AD, suggesting that Th2 cell targeting could be effective in several CHE subtypes.

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.

Autophagy involvement in T lymphocyte signalling induced by nickel with quantitative phosphoproteomic analysis

Nickel-induced allergic contact dermatitis (ACD) is a common skin disease. The mechanism by which nickel causes ACD is not clear. There is no treatment for it, only symptomatic therapy. However, due to the lifetime sensitization characteristics, the recurrence rate in patients is high. T lymphocytes play a key role in nickel-induced ACD. Elucidating the potential mechanism underlying nickel-induced T lymphocyte signalling might make it possible to achieve targeted treatment of nickel-induced ACD. In our study, a phosphoproteomic approach based on tandem mass tag (TMT) labelling and LCMS/MS analyses was employed. An animal model of nickel allergy was established. Splenic T lymphocytes were purified for quantitative phosphoproteomic analysis. The numbers of phosphoproteins, phosphopeptides and phosphosites identified in this study were 3072, 7977 and 10,200, respectively. Comprehensive gene ontology (GO) analysis combined with Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis revealed that nickel can significantly affect the phosphorylation of the mTOR signalling pathway in T lymphocytes. Western blotting analysis was used to detect changes in the expression of autophagy-related proteins (Beclin 1, LC3II, and p62). Nickel allergy changed autophagy-related protein expression (p < 0.05). It has been demonstrated that nickel causes autophagy of T lymphocytes in the spleen. Using autophagy inhibitors to intervene, it was found that Th1 differentiation was inhibited, and the expression of Th1-related inflammatory factors was downregulated. Overall, the identification of relevant signalling pathways yielded new insights into the molecular mechanisms underlying nickel allergy and might help in the discovery and development of mechanism-based drugs.

Claudin targeting as an effective tool for directed barrier modulation of the viable epidermis

Tight junction (TJ) formation is vital for epidermal barrier function. We aimed to specifically manipulate TJ barriers in the reconstructed human epidermis (RHE) by claudin-1 and -4 knockdown (KD) and by claudin-binding fusion proteins of glutathione S-transferase and modified C-terminal fragments of Clostridium perfringens enterotoxin (GST-cCPE). Impedance spectroscopy and tracer permeability imaging were employed for functional barrier assessment and investigation of claudin contribution. KD of claudin-1, but not claudin-4, impaired the paracellular barrier in vitro. Similarly, claudin-binding GST-cCPE variants weakened the paracellular but not the stratum corneum barrier. Combining both TJ targeting methods, we found that claudin-1 targeting by GST-cCPE after claudin-4 KD led to a marked decrease in paracellular barrier properties. Conversely, after claudin-1 KD, GST-cCPE did not further impair the barrier. Comparison of GST-cCPE variants with different claudin-1/claudin-4 affinities, NHS-fluorescein tracer detection, and immunostaining of RHE paraffin sections showed that GST-cCPE variants bind to extrajunctional claudin-1 and -4, which are differentially distributed along the stratum basale-stratum granulosum axis. GST-cCPE binding blocks these claudins, thereby specifically opening the paracellular barrier of RHE. The data indicate a critical role for claudin-1 in regulating paracellular permeability for ions and small molecules in the viable epidermis. Claudin targeting is presented as a proof-of-concept for precise barrier modulation.

Allergies caused by textiles: control, research and future perspective in the medical field

Textile production forms one of the most polluting industries worldwide. However, other than damaging environmental effects, chemical waste products, such as formaldehyde or thiazolinone, are problematic for human health, as allergic potential is present in these compounds. Mostly, contact dermatitis occurs when human skin is exposed to textiles. Moreover, non-eczemous variants are mainly associated to textiles. In order to diagnose the possible allergy of the patient towards these compounds, in vivo and in vitro methods ca be performed, such as patch testing or cytokine detection assays, respectively. Newest research focuses on medical textiles such as garments or sutures to help in diagnosis, therapy and recovery of the patients. Sutures and dressings with antimicrobial properties, with the release of oxygen and growth factors offer greater properties. In this review, state of the art in the field as well as future perspectives will be discussed, which are based on the smart textiles that are going to become more important and probably widespread after the current limits exceeded.

Efficacy and Safety of an Antioxidant-Enriched Medical Device for Topical Use in Adults with Eczematous Dermatitis

INTRODUCTION

Emollients reduce the severity of dermatitis-associated symptoms. Antioxidant supplementation may be helpful to control inflammatory processes and consequential skin damage. The clinical performance and safety of an emollient medical device for topical treatment enriched with antioxidant ingredients in adults with mild-to-moderate dermatitis is presented in this manuscript.

METHODS

We performed a monocenter, open-label, uncontrolled clinical trial. Participants applied the product twice a day for 28 days. No other medication or moisturizer was allowed. Changes in dermatitis severity were assessed at days 14 and 28 by study investigators. Subjects self-assessed pruritus, Dermatology Life Quality Index, and product satisfaction. At the end of the study, a global evaluation of the product was done both by patient-reported outcomes and investigators' evaluations.

RESULTS

Forty subjects were enrolled in the study (mean age 35 years). Treatment success was achieved in 87.5% of subjects (p < 0.0001) after 28 days. Mean Investigator's Global Assessment (IGA) and Eczema Area and Severity Index (EASI) scores decreased at days 14 and 28 (p < 0.0001). Subjects reported a reduction in pruritus severity and improvement of quality of life (p < 0.0001), along with satisfaction with the product. At the end of the study, skin condition improved in more than 90% of subjects. No safety issues were identified.

CONCLUSION

The medical device studied for topical use in this clinical trial is considered safe and reduces pruritus in adults with atopic and contact dermatitis.

Haptenation of Macrophage Migration Inhibitory Factor: A Potential Biomarker for Contact Hypersensitivity

The immunological response in contact hypersensitivity is incited by small electrophilic compounds, known as haptens, that react with endogenous proteins after skin absorption. However, the identity of hapten-modified proteins seen as immunogenic remains as yet largely unknown. In a recent study, we have for the first time identified a hapten-modified protein in the local lymph nodes of mice treated topically with the model hapten tetramethylrhodamine isothiocyanate (TRITC). The TRITC modification was located on the N-terminal proline of the protein macrophage migration inhibitory factor (MIF). The focus of the current study was to investigate the presence of the same hapten-protein conjugate in blood samples from mice treated topically with TRITC. Furthermore, TRITC modifications of the two major blood proteins, namely hemoglobin (Hb) and albumin (Alb), as well as TRITC modifications of MIF other than the N-terminal proline, were examined. Following incubation with different molar ratios of TRITC, a proteomic approach was applied to characterize conjugate formation of the three aforementioned proteins, using high resolution mass spectrometry (HRMS). The targeted screening of the TRITC-treated mice blood and lymph node samples for these sites led to the identification of only the same TRITC-MIF conjugate previously detected in the lymph nodes. No Hb and Alb conjugates were detected. Quantification of both the TRITC-modified and unmodified N-terminal peptide of MIF in blood and lymph node samples gave interesting insights of MIF’s role in murine contact hypersensitivity. Incubation of MIF with four different haptens encompassing different reactivity mechanisms and potencies, showed adduct formation at different amino acid residues, suggesting that MIF can be the preferred target for a wide variety of haptens. The present study provides essential progress toward understanding of hapten-protein conjugate formation in contact hypersensitivity and identifies hapten-modified MIF as a potential biomarker for this condition. Further investigation of MIF as a target protein can be a next step to determine if MIF is a biomarker that can be used to develop better diagnostic tools and targeted therapeutics for individuals with allergic contact dermatitis.

Immunological Mechanisms in Allergic Contact Dermatitis

Purpose of the review

The understanding of the cellular and molecular pathogenesis of allergic contact dermatitis (ACD) has increased dramatically.

Recent findings

Besides CD4+ and CD8+ T cells, other cell types such as innate lymphoid cells, natural killer T cells (NKT), natural killer cells, and T regulatory cells have emerged as crucial key players. New immunological insights have unravelled that the predominant effector cell type determines the clinical pictures. Hence, a better understanding of the involvement of distinct effector cells has shed light on the diversity of ACD reactions and subsequent clinical pictures. Another new perspective has arisen in the elicitation phase. Here, Langerhans cells can play a role in the development of immune tolerance and not, as previously thought, exclusively in the allergen-driven hypersensitivity reaction. B cells also appear to play an important role in triggering ACD by secreting IgM antibodies in response to interleukin (IL)-4 produced by NKT cells, leading to complement activation and chemotaxis of immune cells.

Summary

Allergic contact dermatitis is a delayed-type hypersensitivity reaction triggered by skin contact with the chemical of interest in individuals previously sensitised to the same or a chemically related substance. The understanding of the cellular and molecular pathogenesis of allergic contact dermatitis has improved considerably. In addition to CD4+ and CD8+ T cells, other cell types such as natural killer T cells (NKT) and regulatory T cells have emerged as important participants. The binding of haptens is the first step in the development of allergic contact dermatitis. Haptens are low molecular weight (mostly <500 Dalton) chemicals that are able to penetrate the stratum corneum of the skin or can enter the body upon systemic administration. Haptens are not immunogenic per se but can be effectively recognised by the immune system after binding to a protein carrier. In the clinically inapparent sensitisation phase, Langerhans cells and dendritic cells initiate an adaptive immune response by capturing and processing antigens and presenting them to naïve T cells in the paracortical regions of the lymph nodes. In the elicitation phase, the clinical manifestations of allergic contact dermatitis are the result of a T cell-mediated inflammatory response that occurs in the skin upon re-exposure to the bite and is mediated by the activation of bite-specific T cells in the skin or other organs.

Novel insights into contact dermatitis

Contact dermatitis is a common disease, caused by repeated skin contact to contact allergens or irritants, resulting in allergic contact dermatitis (ACD) and/or irritant contact dermatitis. Attempts have been made to identify biomarkers to distinguish irritant and allergic patch test reactions, which could aid diagnosis. Some promising candidates have recently been identified, but verification and validation in clinical cases still need to be done. New causes of ACD are constantly recognized. In this review, 10 new contact allergens from recent years, were identified, several relating to anti-age products. Frequent allergens causing considerable morbidity in the population such as the preservative methylisothiazolinone (MI) have been regulated in EU. A significant drop in cases has been seen, while high rates are still occurring in other areas such as North America. Other frequent causes are fragrance allergens especially widely used terpenes and acrylates used in medical devises for control of diabetes. These represent unsolved problems. Recent advances in immunology have opened for a better understanding of the complexity of contact dermatitis, especially ACD. The disease may be more heterogenous that previous understood with several subtypes. With the rapidly evolving molecular understanding of the disease, the potential for development of new drugs for personalized treatment of contact dermatitis is considerable.

Exposure to the immunomodulatory chemical triclosan differentially impacts immune cell populations in the skin of haired (BALB/c) and hairless (SKH1) mice

Workers across every occupational sector have the potential to be exposed to a wide variety of chemicals, and the skin is a primary route of exposure. Furthermore, exposure to certain chemicals has been linked to inflammatory and allergic diseases. Thus, understanding the immune responses to chemical exposures on the skin and the potential for inflammation and sensitization is needed to improve worker safety and health. Responses in the skin microenvironment impact the potential for sensitization; these responses may include proinflammatory cytokines, inflammasome activation, barrier integrity, skin microbiota, and the presence of immune cells. Selection of specific mouse strains to evaluate skin effects, such as haired (BALB/c) or hairless (SKH1) mice, varies dependent on experimental design and needs of a study. However, dermal chemical exposure may impact reactions in the skin differently depending on the strain of mouse. Additionally, there is a need for established methods to evaluate immune responses in the skin. In this study, exposure to the immunomodulatory chemical triclosan was evaluated in two mouse models using immunophenotyping by flow cytometry and gene expression analysis. BALB/c mice exposed to triclosan (2%) had a higher number and frequency of neutrophils and lower number and frequency of dendritic cells in the skin compared to controls. Although these changes were not observed in SKH1 mice, SKH1 mice exposed to triclosan had a higher number and frequency of type 2 innate lymphoid cells in the skin. Taken together, these results demonstrate that exposure to an immunomodulatory chemical, triclosan, differentially impacts immune cell populations in the skin of haired and hairless mice. Additionally, the flow cytometry panel reported in this manuscript, in combination with gene expression analysis, may be useful in future studies to better evaluate the effect of chemical exposures on the skin immune response. These findings may be important to consider during strain selection, experimental design, and result interpretation of chemical exposures on the skin.

Validation of Quantitative Structure-Activity Relationship (QSAR) and Quantitative Structure-Property Relationship (QSPR) approaches as alternatives to skin sensitization risk assessment

The aim of this study was conducted to validate the physicochemical properties of a total of 362 chemicals [305 skin sensitizers (212 in the previous study + 93 additional new chemicals), 57 non-skin sensitizers (38 in the previous study + 19 additional new chemicals)] for skin sensitization risk assessment using quantitative structure-activity relationship (QSAR)/quantitative structure-property relationship (QSPR) approaches. The average melting point (MP), surface tension (ST), and density (DS) of the 305 skin sensitizers and 57 non-sensitizers were used to determine the cutoff values distinguishing positive and negative sensitization, and correlation coefficients were employed to derive effective 3-fold concentration (EC3 (%)) values. QSAR models were also utilized to assess skin sensitization. The sensitivity, specificity, and accuracy were 80, 15, and 70%, respectively, for the Toxtree QSAR model; 88, 46, and 81%, respectively, for Vega; and 56, 61, and 56%, respectively, for Danish EPA QSAR. Surprisingly, the sensitivity, specificity, and accuracy were 60, 80, and 64%, respectively, when MP, ST, and DS (MP+ST+DS) were used in this study. Further, MP+ST+DS exhibited a sensitivity of 77%, specificity 57%, and accuracy 73% when the derived EC3 values were classified into local lymph node assay (LLNA) skin sensitizer and non-sensitizer categories. Thus, MP, ST, and DS may prove useful in predicting EC3 values as not only an alternative approach to animal testing but also for skin sensitization risk assessment.

Direct assessment of individual skin barrier components by electrical impedance spectroscopy

BACKGROUND

Expression of the tight junction proteins Cldn1 and 4 is altered in skin diseases such as atopic dermatitis, and Cldn1 deficiency affects skin barrier formation. Impedance spectroscopy (IS) has been proven to allow detection of alterations in the skin barrier but is currently unable to separate effects on viable epidermis (VE) and stratum corneum (SC).

METHODS

Effects of siRNA-mediated Cldn1 and 4 knockdown in reconstructed human epidermis (RHE) on VE and SC barrier function were investigated with Ussing chamber-based IS. Barrier components were sequentially altered, employing iron oxide nanoparticles and EGTA, to identify their contribution to the impedance spectrum. Resistance changes due to apically applied hyperosmolar electrolyte were used to identify barrier defects non-invasively.

RESULTS

IS of RHE yielded two relaxation frequencies, representing the barrier properties of the SC (~1000 Hz) and VE (~100 Hz). As proof of concept, it was shown that the Cldn1 knockdown-induced resistance drop arises from the impairment of both SC and VE, indicated by a shift of both relaxation frequencies. Hyperosmolar electrolyte penetration allowed non-invasive detection of Cldn1 knockdown via time-dependent frequency shifts. The absence of Cldn4 knockdown-induced changes revealed the weaknesses of transepithelial electrical resistance analysis.

CONCLUSION

In conclusion, the present technique allows to separately measure the barrier properties of SC and VE and further evaluate the Cldn1 and 4 knockdown impact on the skin barrier. As the measurement with agarose-embedded electrolyte allowed non-invasive identification of the Cldn1 knockdown, this opens the way to detailed in vivo skin barrier assessment.

Molecular perspective of efficiency and safety problems of chemical enhancers: bottlenecks and recent advances

Chemical penetration enhancer (CPE) is a preferred approach to improve drug permeability through the skin, due to its unique advantages of simple use and high compatibility. However, CPEs efficiency and safety problems frequently arise, which greatly restrains the further application in transdermal drug delivery systems (TDDS). To get access to the root of problems, the efficiency and safety of CPEs are reviewed especially from molecular perspectives, which include (1) the possible factors of CPEs low efficiency; (2) the possible contribution of CPEs in the evolution of safety problems such as skin irritation and allergic reaction; (3) the interactive relationship between CPEs efficiency and safety, as well as the bottlenecks of achieving their balance. More importantly, based on these, recent advances are summarized in improving efficiency or safety of CPEs, which offers a guidance of rationally selecting CPEs in future research.

Characterization of the Class I MHC Peptidome Resulting From DNCB Exposure of HaCaT Cells

Skin sensitization following the covalent modification of proteins by low molecular weight chemicals (haptenation) is mediated by cytotoxic T lymphocyte (CTL) recognition of human leukocyte antigen (HLA) molecules presented on the surface of almost all nucleated cells. There exist 3 nonmutually exclusive hypotheses for how haptens mediate CTL recognition: direct stimulation by haptenated peptides, hapten modification of HLA leading to an altered HLA-peptide repertoire, or a hapten altered proteome leading to an altered HLA-peptide repertoire. To shed light on the mechanism underpinning skin sensitization, we set out to utilize proteomic analysis of keratinocyte presented antigens following exposure to 2,4-dinitrochlorobenzene (DNCB). We show that the following DNCB exposure, cultured keratinocytes present cysteine haptenated (dinitrophenylated) peptides in multiple HLA molecules. In addition, we find that one of the DNCB modified peptides derives from the active site of cytosolic glutathione-S transferase-ω. These results support the current view that a key mechanism of skin sensitization is stimulation of CTLs by haptenated peptides. Data are available via ProteomeXchange with identifier PXD021373.

Contact dermatitis

Contact dermatitis (CD) is among the most common inflammatory dermatological conditions and includes allergic CD, photoallergic CD, irritant CD, photoirritant CD (also called phototoxic CD) and protein CD. Occupational CD can be of any type and is the most prevalent occupational skin disease. Each CD type is characterized by different immunological mechanisms and/or requisite exposures. Clinical manifestations of CD vary widely and multiple subtypes may occur simultaneously. The diagnosis relies on clinical presentation, thorough exposure assessment and evaluation with techniques such as patch testing and skin-prick testing. Management is based on patient education, avoidance strategies of specific substances, and topical treatments; in severe or recalcitrant cases, which can negatively affect the quality of life of patients, systemic medications may be needed.

Impact of mono-culture vs. Co-culture of keratinocytes and monocytes on cytokine responses induced by important skin sensitizers

Sensitization to a contact allergen brings with it a lifelong risk to develop allergic contact dermatitis. Inflammation is an important part of the skin sensitizing mechanism, and understanding how different haptens stimulate the immune system, as well as the role played by different cell types present in skin, may be helpful for developing optimized in vitro models for risk assessment of new chemicals or mixtures. The aim of this study was to compare the cytokine profile following exposure of cells representing keratinocytes (HaCaT), monocytes (THP-1) and a co-culture of these cells to three clinically important skin sensitizers: cobalt (II) chloride (CoCl₂), methylisothiazolinone (MI) and p-phenylenediamine (PPD). Secretion of ten pro-inflammatory cytokines was measured using multiplexing. The results showed that the cytokine response differed substantially between the three cell assays. CoCl₂ caused an increase of IL-8 in HaCaT cells, while the induction of also IL-13 and IL-1β was observed in THP-1 cells and co-cultures. MI induced six cytokines in HaCaT cells but only IL-1β in the THP-1 cells and four cytokines in the co-culture. Interestingly, the IL-1β response was massive in the co-culture. PPD caused release of IL-1β in all three models as well as IL-8 in the co-culture. Control experiments with two non-sensitizers and irritants (lactic acid and sodium dodecyl sulfate) showed no effect on IL-8 or IL-1β in the co-culture. Taken together, results from this exploratory analysis show unique cytokine profiles dependent on the type of hapten and cell model. Importantly, all three haptens triggered secretion of IL-1β and IL-8 in a co-culture of HaCaT cells and THP-1 cells, representing the most robust test system.

Machine-learning-driven biomarker discovery for the discrimination between allergic and irritant contact dermatitis

Contact dermatitis tremendously impacts the quality of life of suffering patients. Currently, diagnostic regimes rely on allergy testing, exposure specification, and follow-up visits; however, distinguishing the clinical phenotype of irritant and allergic contact dermatitis remains challenging. Employing integrative transcriptomic analysis and machine-learning approaches, we aimed to decipher disease-related signature genes to find suitable sets of biomarkers. A total of 89 positive patch-test reaction biopsies against four contact allergens and two irritants were analyzed via microarray. Coexpression network analysis and Random Forest classification were used to discover potential biomarkers and selected biomarker models were validated in an independent patient group. Differential gene-expression analysis identified major gene-expression changes depending on the stimulus. Random Forest classification identified CD47, BATF, FASLG, RGS16, SYNPO, SELE, PTPN7, WARS, PRC1, EXO1, RRM2, PBK, RAD54L, KIFC1, SPC25, PKMYT, HISTH1A, TPX2, DLGAP5, TPX2, CH25H, and IL37 as potential biomarkers to distinguish allergic and irritant contact dermatitis in human skin. Validation experiments and prediction performances on external testing datasets demonstrated potential applicability of the identified biomarker models in the clinic. Capitalizing on this knowledge, novel diagnostic tools can be developed to guide clinical diagnosis of contact allergies.

Stratum corneum levels of inflammatory mediators and natural moisturizing factor in patch test reactions to thiurams and fragrances and their possible role in discrimination between irritant and allergic reactions to hapten mixtures

BACKGROUND

Patch test (PT) reactions to thiuram mix (TM) and fragrance mix (FM) I or II without concomitant reactions to their single constituents are potentially caused by the irritant properties of the mixes.

OBJECTIVE

Comparing inflammatory profiles of PT reactions to TM, FM I, FM II, and their constituents and assessing their potential in discrimination of irritant and allergic reactions.

PATIENTS AND METHODS

Levels of 14 cytokines and natural moisturizing factor (NMF) were determined in stratum corneum samples collected from PT reactions to TM, FM I or II, their constituents, and petrolatum (pet.) control sites in 36 individuals.

RESULTS

Levels of interleukin (IL)-16, chemokine (CXC motif) ligand (CXCL) 8, CXCL10, chemokine (CC motif) ligand (CCL) 17, and CCL22 were significantly increased in reactions (+, ++) to thiurams and fragrances compared to their petrolatum. controls, except for PT reactions to FM I/II with negative breakdown testing in which, however, decreased levels of NMF were observed. In doubtful reactions to FM I/II with negative breakdown testing, NMF was significantly lower than in petrolatum controls.

CONCLUSIONS

PT reactions to thiurams and fragrances indicate a Th2-skewed inflammation. The inflammatory profiles suggest that weak or doubtful FM I/II reactions without accompanying reaction to a constituent were irritant. IL-16 might be suitable to distinguish irritant from allergic reaction.

Update of the risk assessment of nickel in food and drinking water

The European Commission asked EFSA to update its previous Opinion on nickel in food and drinking water, taking into account new occurrence data, the updated benchmark dose (BMD) Guidance and newly available scientific information. More than 47,000 analytical results on the occurrence of nickel were used for calculating chronic and acute dietary exposure. An increased incidence of post-implantation loss in rats was identified as the critical effect for the risk characterisation of chronic oral exposure and a BMDL 10 of 1.3 mg Ni/kg body weight (bw) per day was selected as the reference point for the establishment of a tolerable daily intake (TDI) of 13 μg/kg bw. Eczematous flare-up reactions in the skin elicited in nickel-sensitised humans, a condition known as systemic contact dermatitis, was identified as the critical effect for the risk characterisation of acute oral exposure. A BMDL could not be derived, and therefore, the lowest-observed-adverse-effect-level of 4.3 μg Ni/kg bw was selected as the reference point. The margin of exposure (MOE) approach was applied and an MOE of 30 or higher was considered as being indicative of a low health concern. The mean lower bound (LB)/upper bound (UB) chronic dietary exposure was below or at the level of the TDI. The 95th percentile LB/UB chronic dietary exposure was below the TDI in adolescents and in all adult age groups, but generally exceeded the TDI in toddlers and in other children, as well as in infants in some surveys. This may raise a health concern in these young age groups. The MOE values for the mean UB acute dietary exposure and for the 95th percentile UB raises a health concern for nickel-sensitised individuals. The MOE values for an acute scenario regarding consumption of a glass of water on an empty stomach do not raise a health concern.

Characterizing Adduct Formation of Electrophilic Skin Allergens with Human Serum Albumin and Hemoglobin

Skin (contact) allergy, the most predominant form of immunotoxicity in humans, is caused by small electrophilic compounds (haptens) that modify endogenous proteins. Approximately 20% of the general population in the Western world is affected by contact allergy. Although the importance of the hapten–protein conjugates is well established in the initiation of the immunological reaction, not much progress has been made regarding identification of these conjugates in vivo or exploration of their potential as diagnostic tools. In this study, the human serum albumin (HSA) and human hemoglobin (Hb) adductome for three representative contact allergens with different chemical properties, 1-chloro-2,4-dinitrobenzene (DNCB), 1,2-epoxy-3-phenoxypropane (PGE), and 2-bromo-2-(bromomethyl)glutaronitrile (MDBGN), were studied. Plasma and red blood cell lysate were used as a source for HSA and Hb, respectively. The Direct Peptide Reactivity Assay was used to investigate adduct formation of MDBGN with nucleophilic moieties and revealed that MDGBN is converted to 2-methylenepentanedinitrile in the presence of sulfhydryl groups prior to adduct formation. Following incubation of HSA and Hb with haptens, an Orbitrap Q Exactive high-resolution mass spectrometer was used to perform an initial untargeted analysis to screen for adduct formation, followed by confirmation by targeted Parallel Reaction Monitoring analysis. Although a subset of adducted sites was confirmed by targeted analysis, only some of the adducted peptides showed an increase in the relative amount of the adducted peptide with an increased concentration of hapten. In total, seven adduct sites for HSA and eight for Hb were confirmed for DNCB and PGE. These sites are believed to be the most reactive. Further, three of the HSA sites (Cys₃₄, Cys₆₂, and Lys₁₉₀) and six of the Hb sites (subunit α: Val₁, His₄₅, His₇₂; subunit β: Cys₉₃, His₉₇, and Cys₁₁₂) were haptenated already at the lowest level of hapten to protein molar ratio (0.1:1), indicating that these sites are the most likely to be modified in vivo. To the best of our knowledge, this is the first time that the adductome of Hb has been studied in the context of contact allergens. Identification of the most reactive sites of abundant proteins, such as HSA and Hb, is the first step toward identification of contact allergy biomarkers that can be used for biomonitoring and to develop better diagnostic tools based on a blood sample.

CD44 expression in human skin: High expression in irritant and allergic contact dermatitis and moderate expression in psoriasis lesions in comparison with healthy controls

BACKGROUND

Previous research using animal models demonstrated that CD44 expression may contribute to directing inflammatory cells into skin lesions during inflammation development in allergic contact dermatitis (ACD).

OBJECTIVES

To examine CD44 expression in patients with ACD and irritant contact dermatitis (ICD), and to compare it to patients with psoriatic lesions and healthy controls' (HCs) skin.

METHODS

This study included 200 patients comprising four groups of 50 each: ACD, ICD, psoriasis vulgaris, and HCs. CD44 expression was determined by immunohistochemical analysis using an optical microscope, and the results were visualized semiquantitatively by determining the percentage of immunoreactive cells in the epidermis, dermis, and on lymphocytes.

RESULTS

The highest CD44 expression was found in ICD, followed by ACD, psoriasis vulgaris, and lastly, the HCs (P < .001). Epidermal CD44 expression was significantly higher in contact dermatoses (especially in ICD) compared with psoriasis and healthy skin (P < .001). Similarly, CD44 expression in the dermis and on lymphocytes was strongest in ICD, although less pronounced than in the epidermis.

CONCLUSIONS

Because significantly elevated CD44 expression in ICD might be related to its function in maintaining and preserving the skin barrier in affected patients, further research on disease pathogenesis and new treatment options is needed.

Metal nanomaterials: Immune effects and implications of physicochemical properties on sensitization, elicitation, and exacerbation of allergic disease

The recent surge in incorporation of metallic and metal oxide nanomaterials into consumer products and their corresponding use in occupational settings have raised concerns over the potential for metals to induce size-specific adverse toxicological effects. Although nano-metals have been shown to induce greater lung injury and inflammation than their larger metal counterparts, their size-related effects on the immune system and allergic disease remain largely unknown. This knowledge gap is particularly concerning since metals are historically recognized as common inducers of allergic contact dermatitis, occupational asthma, and allergic adjuvancy. The investigation into the potential for adverse immune effects following exposure to metal nanomaterials is becoming an area of scientific interest since these characteristically lightweight materials are easily aerosolized and inhaled, and their small size may allow for penetration of the skin, which may promote unique size-specific immune effects with implications for allergic disease. Additionally, alterations in physicochemical properties of metals in the nano-scale greatly influence their interactions with components of biological systems, potentially leading to implications for inducing or exacerbating allergic disease. Although some research has been directed toward addressing these concerns, many aspects of metal nanomaterial-induced immune effects remain unclear. Overall, more scientific knowledge exists in regards to the potential for metal nanomaterials to exacerbate allergic disease than to their potential to induce allergic disease. Furthermore, effects of metal nanomaterial exposure on respiratory allergy have been more thoroughly-characterized than their potential influence on dermal allergy. Current knowledge regarding metal nanomaterials and their potential to induce/exacerbate dermal and respiratory allergy are summarized in this review. In addition, an examination of several remaining knowledge gaps and considerations for future studies is provided.

[Pathogenesis of hand eczema]

BACKGROUND

The pathogenesis of hand eczema is multifactorial. Exogenous factors such as skin irritation and contact sensitization contribute to this as well as an endogenous atopic predisposition.

OBJECTIVE

This article provides a review of the pathogenetic factors relevant to the development of hand eczema.

MATERIAL AND METHODS

An evaluation of the current literature on the pathogenesis of hand eczema was carried out.

RESULTS

Impairment of the epidermal barrier function plays an essential role for the development of hand eczema. The molecular biological basis and the possible significance of a genetic predisposition beyond atopy are not yet fully understood. Immunological processes involved in the pathogenesis of allergic contact dermatitis and atopic eczema are likely to play a role in the development of certain subtypes of hand eczema. This might contribute to an expansion of the therapeutic armamentarium for hand eczema in the future.

CONCLUSION

The exact understanding of the individual pathogenesis in single hand eczema patients is essential in order to provide specific advice on allergen avoidance, skin protection and basic treatment and to initiate appropriate therapeutic measures.

Factors related to skin health in hairdressing apprentices from two Croatian regions

BACKGROUND

Hairdressing apprentices are at high risk for developing occupational contact dermatitis.

OBJECTIVES

The objective of this study was to analyse whether there are differences in skin health, skin protection and health-related quality of life between apprentices from two Croatian regions (Zagreb and Dalmatia).

MATERIALS AND METHODS

Final-year hairdressing apprentices from Zagreb (n = 101) and Dalmatia (n = 77) were enrolled in the study. Clinical examination of hands was performed by use of the Osnabrück Hand Eczema Severity Index. Transepidermal water loss (TEWL) was measured on forearm and hand. A history of self-reported skin symptoms, work-related exposure to skin hazards and health-related quality of life were assessed with validated questionnaires.

RESULTS

Apprentices from Zagreb reported having been engaged in practical work more days per month than Dalmatian apprentices (median 12 days vs 8 days, respectively, P < 0.001), and washing their hands >20 times per day more frequently (28% vs 10%, respectively, P = 0.004), and consequently more frequently having dry hands (37% vs 16%, respectively, P = 0.002), and significantly higher TEWL values on the hand and forearm, which was confirmed by the results obtained with multiple linear regression models.

CONCLUSIONS

A higher extent of exposure to skin hazards was related to poorer skin barrier function, which confirms the need for improvement of appropriate safety practices in hairdressing schools.

IL-24 contributes to skin inflammation in Para-Phenylenediamine-induced contact hypersensitivity

Para-Phenylenediamine (PPD) is an aromatic amine used in hair dyes and in temporary black henna tattoos, which is a frequent cause of allergic contact dermatitis (ACD). ACD is a skin inflammatory reaction characterized by modifications such as spongiosis, exocytosis and acanthosis. The aim of this study is to characterize the expression and the role of IL-20-related cytokines, including IL-19, IL-20, IL-22 and IL-24, in ACD. The expression of IL19, IL20, IL22 and IL24 is increased in affected skin from PPD allergic patients compared with uninvolved skin. In addition, the expression of these cytokines positively correlates with clinical symptoms. To assess their role in ACD, we set up a mouse model of PPD-induced allergic contact dermatitis and we showed that, in contrast to Il22-deficient mice, Il22ra1-, Il20rb- and Il24-deficient mice are partially protected against development of PPD-induced contact hypersensitivity. These mice have decreased ear thickening and less acanthosis compared with WT mice after PPD treatment. In addition, the absence of IL-22R, IL-20R2 or IL-24 affects the recruitment of neutrophils into the skin but not the total IgE production. Taken together, these results demonstrate the implication of IL-24 via the IL-20R type II receptor in the inflammatory process of ACD.

Contact Allergy to Fragrances: In Vitro Opportunities for Safety Assessment

The majority of cosmetic products contain fragrances to make products more pleasant to the consumer, as we all like goods that smell nice. Unfortunately, contact allergy to fragrance compounds is among the most frequent findings in patients with suspected allergic contact dermatitis. In order to revert this and to reduce contact allergy to cosmetics, it is imperative to improve safety assessment of cosmetic products for skin sensitization. In the era of animal ban for cosmetic ingredients, this represents a challenge. Luckily, in the last decades, substantial progress has been made in the understanding of the mechanism of chemical-induced contact allergy and several in vitro methods are available for hazard identification. The purpose of this manuscript is to explore the possibility of non-animal testing for quantitative risk assessment of fragrance-induced contact allergy, essential for cosmetic products, which cannot be tested on animals.

Sample Treatment for Tissue Proteomics in Cancer, Toxicology, and Forensics

Since the birth of proteomics science in the 1990, the number of applications and of sample preparation methods has grown exponentially, making a huge contribution to the knowledge in life science disciplines. Continuous improvements in the sample treatment strategies unlock and reveal the fine details of disease mechanisms, drug potency, and toxicity as well as enable new disciplines to be investigated such as forensic science.This chapter will cover the most recent developments in sample preparation strategies for tissue proteomics in three areas, namely, cancer, toxicology, and forensics, thus also demonstrating breath of application within the domain of health and well-being, pharmaceuticals, and secure societies.In particular, in the area of cancer (human tumor biomarkers), the most efficient and multi-informative proteomic strategies will be covered in relation to the subsequent application of matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) and liquid extraction surface analysis (LESA), due to their ability to provide molecular localization of tumor biomarkers albeit with different spatial resolution.With respect to toxicology, methodologies applied in toxicoproteomics will be illustrated with examples from its use in two important areas: the study of drug-induced liver injury (DILI) and studies of effects of chemical and environmental insults on skin, i.e., the effects of irritants, sensitizers, and ionizing radiation. Within this chapter, mainly tissue proteomics sample preparation methods for LC-MS/MS analysis will be discussed as (i) the use of LC-MS/MS is majorly represented in the research efforts of the bioanalytical community in this area and (ii) LC-MS/MS still is the gold standard for quantification studies.Finally, the use of proteomics will also be discussed in forensic science with respect to the information that can be recovered from blood and fingerprint evidence which are commonly encountered at the scene of the crime. The application of proteomic strategies for the analysis of blood and fingerprints is novel and proteomic preparation methods will be reported in relation to the subsequent use of mass spectrometry without any hyphenation. While generally yielding more information, hyphenated methods are often more laborious and time-consuming; since forensic investigations need quick turnaround, without compromising validity of the information, the prospect to develop methods for the application of quick forensic mass spectrometry techniques such as MALDI-MS (in imaging or profiling mode) is of great interest.

IRRITANT AND ALLERGIC CONTACT DERMATITIS - SKIN LESION CHARACTERISTICS

SUMMARY – Contact skin lesions may be the consequences of contact with various irritants or allergens, or due to other factors (e.g., UV radiation, microbials), intrinsic factors (e.g., in autoimmune responses), or even their combination. There are many substances related to irritant contact dermatitis (CD), causing irritant or toxic effects, e.g., chemical and physical agents, plants, phototoxic agents, airborne irritants, etc. Impaired barrier function (e.g., aberrancies in epidermal pH buffering capabilities) also participates by promoting bacterial biofilms and creating an environment favoring sensitization. Development of allergic CD skin lesions includes complex immune pathways and inflammatory mediators, influenced by both genetic (predominantly filaggrin mutations) and environmental triggers. In the pathogenesis of allergic CD, antimicrobial peptides play a prominent role; they are produced by various skin cells (e.g., keratinocytes, sebocytes) and move to inflamed lesions during an inflammation process. Also, in allergic CD skin lesions, the skin shows different types of immune responses to individual allergens, although clinical manifestations do not depend on the causative allergen type, e.g., nickel stimulates immune activation primarily of the Th1/Th17 and Th22 components. Also important are alarmins, proteases, immunoproteomes, lipids, natural moisturizing factors, tight junctions, smoking, etc. We expect that future perspectives may reveal new pathogenetic factors and scientific data important for the workup and treatment of patients with CD.

Application of red light phototherapy in the treatment of radioactive dermatitis in patients with head and neck cancer

BACKGROUND

To observe the effect of red light phototherapy (RLPT) on radioactive dermatitis (RD) caused by radiotherapy in patients with head and neck cancer (HNC).

METHODS

Sixty patients with HNC admitted to our hospital were randomly divided into experimental group and control group, 30 patients in each group. The control group received routine daily care during radiotherapy treatment. In the experimental group, in addition to routine daily care during radiotherapy treatment, photon therapy apparatus RLPT was added, 10 min/time, 2 times/day, and lasted until the end of radiotherapy. The pain and conditions of the patients' skin were assessed daily, and the skin pain and dermatitis grades of the two groups were compared.

RESULTS

In terms of the reaction degree of RD, experimental group was mainly grade 0-2, and control group was mainly grade 2-3, with a significant difference (P < 0.05). In terms of skin pain, according to the pain records at week 2, 3, and 4, the pain degree increased with time. However, the score of wound pain in experimental group was significantly lower than that in control group, and there was a significant difference between the two groups (P < 0.05).

CONCLUSIONS

The application of RLPT in the treatment of RD can help accelerate wound healing and significantly shorten healing time. It can not only reduce wounds pain of patients, promote inflammation and ulcer healing, but also ensure the smooth progress of patients' radiotherapy and improve their quality of lives, which is worth popularization and application in the clinical practice.

Chemical-induced contact allergy: from mechanistic understanding to risk prevention

Chemical allergens are small molecules able to form a sensitizing complex once they bound to proteins. One of the most frequent manifestations of chemical allergy is contact hypersensitivity, which can have serious impact on quality of life. Allergic contact dermatitis is a predominantly CD8 + T cell-mediated immune disease, resulting in erythema and eczema. Chemical allergy is of considerable importance to the toxicologist, who has the responsibility of identifying and characterizing the allergenic potential of chemicals, and estimating the risk they pose to human health. This review aimed at exploring the phenomena of chemical-induced contact allergy starting from a mechanistic understanding, immunoregulatory mechanisms, passing through the potency of contract allergen until the hazard identification, pointing out the in vitro models for assessing contact allergen-induced cell activation and the risk prevention.