Reporter cell lines for skin sensitization testing

Novel three-dimensional live skin-like in vitro composite for bioluminescence reporter gene assay

We genetically manipulated HaCaT cells, a spontaneously immortalised normal keratinocyte cell line, to stably express two different coloured luciferase reporter genes, driven by interleukin 8 (IL-8) and ubiquitin-C (UBC) promoters, respectively. Subsequently, we generated a three-dimensional (3D) skin-like in vitro composite (SLIC) utilising these cells, with the objective of monitoring bioluminescence emitted from the SLIC. This SLIC was generated on non-woven silica fibre membranes in differentiation medium. Immunohistochemical analyses of skin differentiation markers in the SLIC revealed the expression of keratins 2 and 10, filaggrin, and involucrin, indicating mature skin characteristics. This engineered SLIC was employed for real-time bioluminescence monitoring, allowing the assessment of time- and dose-dependent responses to UV stress, as well as to hydrophilic and hydrophobic chemical loads. Notably, evaluation of responses to hydrophobic substances has been challenging with conventional 2D cell culture methods, suggesting the need for a new approach, which this technology could address. Our observations suggest that engineered SLIC with constitutively expressing reporters driven by selected promoters which are tailored to specific objectives, significantly facilitates assays exploring the physiological functions of skin cells based on genetic response mechanisms. It also highlights new avenues for evaluating the physiological impacts of various compounds designed for topical application to human skin.

Pluripotent stem cells for target organ developmental toxicity testing

Prenatal developmental toxicity research focuses on understanding the potential adverse effects of environmental agents, drugs, and chemicals on the development of embryos and fetuses. Traditional methods involve animal testing, but ethical concerns and the need for human-relevant models have prompted the exploration of alternatives. Pluripotent stem cells (PSCs) are versatile cells with the unique ability to differentiate into any cell type, serving as a foundational tool for studying human development. Two-dimensional (2D) PSC models are often chosen for their ease of use and reproducibility for high-throughput screening. However, they lack the complexity of an in vivo environment. Alternatively, three-dimensional (3D) PSC models, such as organoids, offer tissue architecture and intercellular communication more reminiscent of in vivo conditions. However, they are complicated to produce and analyze, usually requiring advanced and expensive techniques. This review discusses recent advances in the use of human PSCs differentiated into brain and heart lineages and emerging tools and methods that can be combined with PSCs to help address important scientific questions in the area of developmental toxicology. These advancements and new approach methods align with the push for more relevant and predictive developmental toxicity assessment, combining innovative techniques with organoid models to advance regulatory decision-making.

Assessment of antibacterial properties and skin irritation potential of anodized aluminum impregnated with various quaternary ammonium

The importance of the inert environment in the transmission of pathogens has been reassessed in recent years. To reduce cross-contamination, new biocidal materials used in high touch surfaces (e.g., stair railings, door handles) have been developed. However, their impact on skin remains poorly described. The present study aimed to evaluate the antibacterial properties and the risk of skin irritation of two materials based on hard-anodized aluminum (AA) impregnated with quaternary ammonium compound solutions (QAC#1 or QAC#2). The QAC#1 or QAC#2 solutions vary in composition, QAC#2 being free of dioctyl dimethyl ammonium chloride (Dio-DAC) and octyl decyl dimethyl ammonium chloride (ODDAC). Unlike AA used as a control, both AA-QAC#1 and AA-QAC#2 had excellent and rapid antibacterial efficacy, killing 99.9 % of Staphylococcus aureus and Escherichia coli bacteria, in 15 s and 1 min, respectively. The impregnation solutions (QAC#1 and QAC#2) did not show any skin sensitizing effect on transformed human keratinocytes. Nevertheless, these solutions as well as the materials (AA-QAC#1, AA-QAC#2), and the liquid extracts derived from them, induced a very rapid cytotoxicity on L929 murine fibroblasts (>70 % after 1 h of contact) as shown by LDH, MTS and neutral red assays. This cytotoxicity can be explained by the fast QACs release occurring when AA-QAC#1 and AA-QAC#2 were immersed in aqueous medium. To overcome the limitation of assays based on liquid condition, an in vitro skin irritation assay on reconstructed human epidermis (RHE) was developed. The effect of the materials upon their direct contact with the epidermis grown at the liquid-air interface was determined by evaluating tissue viability and quantifying interleukin-1 alpha (IL-1α) which is released in skin during injury or infection. AA-QAC#1 induced a significant decrease in RHE viability, close to OECD and ISO 10993-10 acceptability thresholds and enhanced the pro-inflammatory IL-1α secretion compared with AA-QAC#2. Finally, these results were corroborated by in vivo assays on mice using erythema and edema visual scores, histological observations, and epidermal thickness measurement. AA had no effect on the skin, while a stronger irritation was induced by AA-QAC#1 compared with AA-QAC#2. Hence, these materials were classified as moderate and slight irritants, respectively. In summary, this study revealed that AA-QAC#2 without Dio-DAC and ODDAC could be a great candidate for high touch surface applications, showing an extremely effective and rapid bactericidal activity, without inducing adverse effects for skin tissue.

Application of ARE-reporter systems in drug discovery and safety assessment

The human body is continuously exposed to xenobiotics and internal or external oxidants. The health risk assessment of exogenous chemicals remains a complex and challenging issue. Alternative toxicological test methods have become an essential strategy for health risk assessment. As a core regulator of constitutive and inducible expression of antioxidant response element (ARE)-dependent genes, nuclear factor erythroid 2-related factor 2 (Nrf2) plays a critical role in maintaining cellular redox homeostasis. Consistent with the properties of Nrf2-mediated antioxidant response, Nrf2-ARE activity is a direct indicator of oxidative stress and thus has been used to identify and characterize oxidative stressors and redox modulators. To screen and distinguish chemicals or environmental insults that affect the cellular antioxidant activity and/or induce oxidative stress, various in vitro cell models expressing distinct ARE reporters with high-throughput and high-content properties have been developed. These ARE-reporter systems are currently widely applied in drug discovery and safety assessment. In the present review, we provide an overview of the basic structures and applications of various ARE-reporter systems employed for discovering Nrf2-ARE modulators and characterizing oxidative stressors.

Development of a 96-Well Electrophilic Allergen Screening Assay for Skin Sensitization Using a Measurement Science Approach

The Electrophilic Allergen Screening Assay (EASA) has emerged as a promising in chemico method to detect the first key event in the adverse outcome pathway (AOP) for skin sensitization. This assay functions by assessing the depletion of one of two probe molecules (4-nitrobenzenethiol (NBT) and pyridoxylamine (PDA)) in the presence of a test compound (TC). The initial development of EASA utilized a cuvette format resulting in multiple measurement challenges such as low throughput and the inability to include adequate control measurements. In this study, we describe the redesign of EASA into a 96-well plate format that incorporates in-process control measurements to quantify key sources of variability each time the assay is run. The data from the analysis of 67 TCs using the 96-well format had 77% concordance with animal data from the local lymph node assay (LLNA), a result consistent with that for the direct peptide reactivity assay (DPRA), an OECD test guideline (442C) protein binding assay. Overall, the measurement science approach described here provides steps during assay development that can be taken to increase confidence of in chemico assays by attempting to fully characterize the sources of variability and potential biases and incorporate in-process control measurements into the assay.

Genetic variants affecting chemical mediated skin immunotoxicity

The skin is an immune-competent organ and this function may be impaired by exposure to chemicals, which may ultimately result in immune-mediated dermal disorders. Interindividual variability to chemical-induced skin immune reactions is associated with intrinsic individual characteristics and their genomes. In the last 30-40 years, several genes influencing susceptibility to skin immune reactions were identified. The aim of this review is to provide information regarding common genetic variations affecting skin immunotoxicity. The polymorphisms selected for this review are related to xenobiotic-metabolizing enzymes (CYPA1 and CYPB1 genes), antioxidant defense (GSTM1, GSTT1, and GSTP1 genes), aryl hydrocarbon receptor signaling pathway (AHR and ARNT genes), skin barrier function transepidermal water loss (FLG, CASP14, and SPINK5 genes), inflammation (TNF, IL10, IL6, IL18, IL31, and TSLP genes), major histocompatibility complex (MHC) and neuroendocrine system peptides (CALCA, TRPV1, ACE genes). These genes present variants associated with skin immune responses and diseases, as well as variants associated with protecting skin immune homeostasis following chemical exposure. The molecular and association studies focusing on these genetic variants may elucidate their functional consequences and contribution in the susceptibility to skin immunotoxicity. Providing information on how genetic variations affect the skin immune system may reduce uncertainties in estimating chemical hazards/risks for human health in the future.

Extraction With Sweat-Sebum Emulsion as a New Test Method for Leachables in Patch-Based Medical Devices, Illustrated by Assessment of Isobornylacrylate (IBOA) in Diabetes Products

BACKGROUND

The increasing offering of patch-based medical devices is accompanied by growing numbers of reported adverse skin reactions. Procedures for testing leachables according to ISO 10993 may not be optimal for lipophilic substances that can be mobilized on skin by sweat and sebum. We propose an improved extraction method for targeted analysis of leachables using low volumes of a sweat-sebum emulsion. The approach is illustrated by the analysis of isobornylacrylate (IBOA), a compound found in some devices and suspected for allergenic potential.

METHOD

Three patch-based products were tested: an implantable device for continuous glucose monitoring (CGM), an intermittently scanned CGM (isCGM) device, and a micro-insulin pump. Quantification of IBOA was performed by gas chromatography and allergenic potential of IBOA levels was assessed by the KeratinoSens cell assay. Different combinations were used for extraction solvent (isopropanol, 5% ethanol-water solution, and sweat-sebum emulsion), extraction volumes (complete immersion vs partial immersion in 2 mm of solvent), and extraction time (3, 5, and 14 days).

RESULTS

Isobornylacrylate was only found in the isCGM device. About 20 mg/L IBOA were eluted after 3 days in isopropanol but only about 1 mg/L in ethanol-water. Sweat-sebum emulsion dissolves IBOA better and gives a more stable solution than ethanol-water. Decomposition of IBOA solutions requires adjusted extraction timing or correction of results. In the sweat-sebum extract, IBOA levels were about 20 mg/L after 3 days and about 30 mg/L after 5 days, clearly above the threshold found in the KerationSens assay for keratinocyte activation (10 mg/L).

CONCLUSION

Extraction by low volumes of sweat-sebum emulsion can be a superior alternative for the targeted simulating-use assessment of leachables in patch-based medical devices.

The IL-1 promoter-driven luciferase reporter cell line THP-G1b can efficiently predict skin-sensitising chemicals

IL-1 functions as an essential pro-inflammatory mediator for the sensitisation of allergic contact dermatitis (ACD). However, studies conducted to date have typically used a limited number of haptens and examined their effects only on murine ACD or murine dendritic cells (DCs). It therefore remains unclear whether IL-1α and/or IL-1β is produced in ACD induced by haptens other than those commonly used in mouse ACD models, and whether they are essential for sensitisation leading to ACD in humans. In addition, it is unclear whether human DCs also produce IL-1α or IL-1β after stimulation by haptens in general. Here, we first demonstrated that 10 haptens (3 extreme, 1 strong, 3 moderate and 3 weak) increased both IL-1α mRNA and IL-1β mRNA expression by the human monocyte cell line THP-1, a commonly used surrogate of DCs in in vitro skin sensitisation tests. Next, we constructed an in vitro skin sensitisation test using a stable IL-1β reporter cell line, THP-G1b, and evaluated whether 88 haptens and 34 non-haptens increase IL-1β reporter activity. We found that 94% of 77 haptens evaluated after considering their applicability domain and solubility in the chosen media stimulated reporter activity. These studies demonstrated that most haptens, irrespective of their potency, increased IL-1β mRNA expression by THP-1 cells, confirming that human DCs also produce IL-1β after stimulation by most haptens. The luciferase assay using THP-G1b cells is thus another skin sensitisation test based on the adverse outcome pathway with reasonable performance.

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.

Continuous long-term cytotoxicity monitoring in 3D spheroids of beetle luciferase-expressing hepatocytes by nondestructive bioluminescence measurement

Background

Three-dimensional (3D) spheroids are frequently used in toxicological study because their morphology and function closely resemble those of tissue. As these properties are maintained over a long term, repeated treatment of the spheroids with a test object is possible. Generally, in the repeated treatment test to assess cytotoxicity in the spheroids, ATP assay, colorimetric measurement using pigments or high-content imaging analysis is performed. However, continuous assessment of cytotoxicity in the same spheroids using the above assays or analysis is impossible because the spheroids must be disrupted or killed. To overcome this technical limitation, we constructed a simple monitoring system in which cytotoxicity in the spheroids can be continuously monitored by nondestructive bioluminescence measurement.

Results

Mouse primary hepatocytes were isolated from transchromosomic (Tc) mice harboring a mouse artificial chromosome (MAC) vector expressing beetle luciferase Emerald Luc (ELuc) under the control of cytomegalovirus immediate early enhancer/chicken β-actin promoter/rabbit β-globin intron II (CAG) promoter, and used in 3D cultures. We confirmed that both luminescence and albumin secretion from the spheroids seeded in the 96-well format Cell-able^TM were maintained for approximately 1 month. Finally, we repetitively treated the luminescent 3D spheroids with representative hepatotoxicants for approximately 1 month, and continuously and nondestructively measured bioluminescence every day. We successfully obtained daily changes of the dose-response bioluminescence curves for the respective toxicants.

Conclusions

In this study, we constructed a monitoring system in which cytotoxicity in the same 3D spheroids was continuously and sensitively monitored over a long term. Because this system can be easily applied to other cells, such as human primary cells or stem cells, it is expected to serve as the preferred platform for simple and cost-effective long-term monitoring of cellular events, including cytotoxicity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-017-0374-1) contains supplementary material, which is available to authorized users.

Systems Toxicology: Real World Applications and Opportunities

Systems Toxicology aims to change the basis of how adverse biological effects of xenobiotics are characterized from empirical end points to describing modes of action as adverse outcome pathways and perturbed networks. Toward this aim, Systems Toxicology entails the integration of in vitro and in vivo toxicity data with computational modeling. This evolving approach depends critically on data reliability and relevance, which in turn depends on the quality of experimental models and bioanalysis techniques used to generate toxicological data. Systems Toxicology involves the use of large-scale data streams ("big data"), such as those derived from omics measurements that require computational means for obtaining informative results. Thus, integrative analysis of multiple molecular measurements, particularly acquired by omics strategies, is a key approach in Systems Toxicology. In recent years, there have been significant advances centered on in vitro test systems and bioanalytical strategies, yet a frontier challenge concerns linking observed network perturbations to phenotypes, which will require understanding pathways and networks that give rise to adverse responses. This summary perspective from a 2016 Systems Toxicology meeting, an international conference held in the Alps of Switzerland, describes the limitations and opportunities of selected emerging applications in this rapidly advancing field. Systems Toxicology aims to change the basis of how adverse biological effects of xenobiotics are characterized, from empirical end points to pathways of toxicity. This requires the integration of in vitro and in vivo data with computational modeling. Test systems and bioanalytical technologies have made significant advances, but ensuring data reliability and relevance is an ongoing concern. The major challenge facing the new pathway approach is determining how to link observed network perturbations to phenotypic toxicity.

Testing Human Skin and Respiratory Sensitizers-What Is Good Enough?

Alternative methods for accurate in vitro assessment of skin and respiratory sensitizers are urgently needed. Sensitization is a complex biological process that cannot be evaluated accurately using single events or biomarkers, since the information content is too restricted in these measurements. On the contrary, if the tremendous information content harbored in DNA/mRNA could be mined, most complex biological processes could be elucidated. Genomic technologies available today, including transcriptional profiling and next generation sequencing, have the power to decipher sensitization, when used in the right context. Thus, a genomic test platform has been developed, denoted the Genomic Allergen Rapid Detection (GARD) assay. Due to the high informational content of the GARD test, accurate predictions of both the skin and respiratory sensitizing capacity of chemicals, have been demonstrated. Based on a matured dendritic cell line, acting as a human-like reporter system, information about potency has also been acquired. Consequently, multiparametric diagnostic technologies are disruptive test principles that can change the way in which the next generation of alternative methods are designed.

Nrf2 Activation as a Key Event Triggered by Skin Sensitisers: The Development of the Stable KeratinoSens Reporter Gene Assay

The 21st century paradigm for toxicology and the adverse outcome pathway concept envisage a future toxicology largely based on mechanistic in vitro assays and relying mainly on cellular models. In the skin sensitisation field, this concept was not intuitive at the beginning. Given the high structural diversity of skin sensitising molecules, classical receptor binding as the molecular initiating event in a cell-based assay could be excluded from the start, leaving the question of how cells could sense potential skin sensitising chemicals and be able to differentiate them from non-sensitisers. When we entered this field in 2006, we realised that, in another emerging field of toxicology, detailed work on the antioxidant/electrophile sensing pathway Keap1/Nrf2/ARE was being performed. We postulated that, based on their intrinsic electrophilicity, a large structural variety of skin sensitisers would activate this pathway. This was demonstrated in a preliminary pilot study with an existing, breast cancer-derived reporter cell line. Broader confirmation of this initial hypothesis then came from a multitude of genome-wide studies, in which sensitiser-induced changes to the transcriptome were investigated. The results showed that this regulatory pathway is indeed the most common regulatory pathway activated by sensitisers at the gene expression level, and the underlying event in keratinocytes has become formalised as a Key Event in the Organisation for Economic Co-operation and Development (OECD) Adverse Outcome Pathway for sensitisation. These studies led to the development of the KeratinoSens® assay, which became the first cell-based in vitro test for skin sensitisation to be endorsed by a European Union Reference Laboratory for Alternatives to Animal Testing (EURL ECVAM) statement and an OECD Test Guideline. More recently, a number of studies have further developed this approach into 3-D skin models. Here, we review the underlying mechanism and the development of the KeratinoSens assay. We also present data on the stability of the assay over time, which is a key requirement for a cell-based biological assay to be endorsed in a regulatory context.

Immune-competent human skin disease models

All skin diseases have an underlying immune component. Owing to differences in animal and human immunology, the majority of drugs fail in the preclinical or clinical testing phases. Therefore animal alternative methods that incorporate human immunology into in vitro skin disease models are required to move the field forward. This review summarizes the progress, using examples from fibrosis, autoimmune diseases, psoriasis, cancer and contact allergy. The emphasis is on co-cultures and 3D organotypic models. Our conclusion is that current models are inadequate and future developments with immune-competent skin-on-chip models based on induced pluripotent stem cells could provide a next generation of skin models for drug discovery and testing.

Intra- and inter-laboratory reproducibility and accuracy of the LuSens assay: A reporter gene-cell line to detect keratinocyte activation by skin sensitizers

Several non-animal methods are now available to address the key events leading to skin sensitization as defined by the adverse outcome pathway. The KeratinoSens assay addresses the cellular event of keratinocyte activation and is a method accepted under OECD TG 442D. In this study, the results of an inter-laboratory evaluation of the "me-too" LuSens assay, a bioassay that uses a human keratinocyte cell line harboring a reporter gene construct composed of the rat antioxidant response element (ARE) of the NADPH:quinone oxidoreductase 1 gene and the luciferase gene, are described. Earlier in-house validation with 74 substances showed an accuracy of 82% in comparison to human data. When used in a battery of non-animal methods, even higher predictivity is achieved. To meet European validation criteria, a multicenter study was conducted in 5 laboratories. The study was divided into two phases, to assess 1) transferability of the method, and 2) reproducibility and accuracy. Phase I was performed by testing 8 non-coded test substances; the results showed a good transferability to naïve laboratories even without on-site training. Phase II was performed with 20 coded test substances (performance standards recommended by OECD, 2015). In this phase, the intra- and inter-laboratory reproducibility as well as accuracy of the method was evaluated. The data demonstrate a remarkable reproducibility of 100% and an accuracy of over 80% in identifying skin sensitizers, indicating a good concordance with in vivo data. These results demonstrate good transferability, reliability and accuracy of the method thereby achieving the standards necessary for use in a regulatory setting to detect skin sensitizers.