Potential method to determine irritant potency in vitro – Comparison of two reconstructed epidermal culture models with different barrier competency

PhotoSENSIL-18 assay development: Enhancing the safety testing of cosmetic raw materials and finished products to support the in vitro photosensitization assessment?

Although photosensitization remains a major toxicological endpoint for the safety assessment of cosmetic products and their raw materials, there is no validated in vitro method available for the evaluation of this adverse effect so far. Given that previous studies have proposed that the Interleukine-18 (IL-18) plays a key role in keratinocyte-driven pro-inflammatory responses specific of the skin sensitization process, we hypothesize that IL-18 might be used as a specific biomarker for in vitro photosensitization assessment. The aim of the present study was the set-up of a new in vitro assay using IL-18 as a biomarker for the identification of photosensitizers in a reconstructed human epidermis (RHE) model. EpiCS™ RHE were incubated with a set of 16 known sensitising / phototoxic / photosensitizing substances and exposed to ultra-violet (UV) irradiation. Then, the cell viability was analysed by MTT assay, while the IL-18 secretion was quantified by ELISA. Preliminary assays have shown that 1 h of incubation followed by a recovery period of 23 h induced the highest IL-18 production in response to UV exposure. This protocol was used to test 16 substances and a ratio of IL-18 production (UV+/UV- ratio) was then generated. Our data shows that the cut-off of 1.5 (UV+/UV- ratio) is the most predictive model among the tested conditions, being capable of identifying true positive photosensitizers (8 of 9) with a good prediction in comparison with in vivo data. In a nutshell, our data suggests that the PhotoSENSIL-18 is a promising in vitro method for identification of photosensitizing substances. Although further studies are necessary to optimize the model, we foresee that the PhotoSENSIL-18 assay can be used in the context of an Integrative Approach to Testing and Assessment (IATA) of chemicals.

Assessment of cytotoxicity and sensitization potential of intradermally injected tattoo inks in reconstructed human skin

Background

The number of people within the European population having at least one tattoo has increased notably, and with it the number of tattoo‐associated clinical complications. Despite this, safety information and testing regarding tattoo inks remain limited.

Objective

To assess cytotoxicity and sensitization potential of 16 tattoo inks after intradermal injection into reconstructed human skin (RHS).

Methods

Commercially available tattoo inks were injected intradermally into RHS (reconstructed epidermis on a fibroblast‐populated collagen hydrogel) using a permanent makeup device. RHS biopsies, tissue sections, and culture medium were assessed for cytotoxicity (thiazolyl blue tetrazolium bromide assay [MTT assay]), detrimental histological changes (haematoxylin and eosin staining), and the presence of inflammatory and sensitization cytokines (interleukin [IL]‐1α, IL‐8, IL‐18; enzyme‐linked immunosorbent assay).

Results

Varying degrees of reduced metabolic activity and histopathological cytotoxic effects were observed in RHS after ink injection. Five inks showed significantly reduced metabolic activity and enhanced sensitization potential compared with negative controls.

Discussion

Using the RHS model system, four tattoo inks were identified as highly cytotoxic and classified as potential sensitizers, suggesting that allergic contact dermatitis could emerge in individuals carrying these inks. These results indicate that an RHS‐based assessment of cytotoxicity and sensitization potential by intradermal tattoo ink injection is a useful analytical tool to determine ink‐induced deleterious effects.

Progress on Reconstructed Human Skin Models for Allergy Research and Identifying Contact Sensitizers

Contact with the skin is inevitable or desirable for daily life products such as cosmetics, hair dyes, perfumes, drugs, household products, and industrial and agricultural products. Whereas the majority of these products are harmless, a number can become metabolized and/or activate the immunological defense via innate and adaptive mechanisms resulting in sensitization and allergic contact dermatitis upon following exposures to the same substance. Therefore, strict safety (hazard) assessment of actives and ingredients in products and drugs applied to the skin is essential to determine I) whether the chemical is a potential sensitizer and if so II) what is the safe concentration for human exposure to prevent sensitization from occurring. Ex vivo skin is a valuable model for skin penetration studies but due to logistical and viability limitations the development of in vitro alternatives is required. The aim of this review is to give a clear overview of the organotypic in vitro skin models (reconstructed human epidermis, reconstructed human skin, immune competent skin models incorporating Langerhans Cells and T-cells, skin-on-chip) that are currently commercially available or which are being used in a laboratory research setting for hazard assessment of potential sensitizers and for investigating the mechanisms (sensitization key events 1-4) related to allergic contact dermatitis. The limitations of the models, their current applications, and their future potential in replacing animals in allergy-related science are discussed.

Skin Sensitization Tests: The LLNA and the RhE IL-18 Potency Assay

Contact allergy is of considerable importance to the toxicologist, and regulatory authorities worldwide require testing for skin sensitization potential and appropriate hazard labeling to enable management of the risk to human health. Although traditionally the identification of skin-sensitizing chemicals has been carried out using animal models, in Europe legislative changes have promoted, and now require, the use of non-animal methods (i.e., Cosmetic Directive, REACH). Several in vitro alternatives for hazard identification have now been validated, but do not provide information on the potency of a skin sensitizer. Here, we describe an animal model, the local lymph node assay (LLNA), and an in vitro model, the RhE IL-18 potency assay, in the context of the identification and potency classification of skin sensitizers. These two assays have been chosen among the different available tests as representative of an alternative in vivo model (the LLNA) and a promising in vitro method with the potential of both hazard identification and potency classification.

Contribution of Palmitic Acid to Epidermal Morphogenesis and Lipid Barrier Formation in Human Skin Equivalents

The outermost barrier layer of the skin is the stratum corneum (SC), which consists of corneocytes embedded in a lipid matrix. Biosynthesis of barrier lipids occurs de novo in the epidermis or is performed with externally derived lipids. Hence, in vitro developed human skin equivalents (HSEs) are developed with culture medium that is supplemented with free fatty acids (FFAs). Nevertheless, the lipid barrier formation in HSEs remains altered compared to native human skin (NHS). The aim of this study is to decipher the role of medium supplemented saturated FFA palmitic acid (PA) on morphogenesis and lipid barrier formation in HSEs. Therefore, HSEs were developed with 100% (25 μM), 10%, or 1% PA. In HSEs supplemented with reduced PA level, the early differentiation was delayed and epidermal activation was increased. Nevertheless, a similar SC lipid composition in all HSEs was detected. Additionally, the lipid organization was comparable for lamellar and lateral organization, irrespective of PA concentration. As compared to NHS, the level of monounsaturated lipids was increased and the FFA to ceramide ratio was drastically reduced in HSEs. This study describes the crucial role of PA in epidermal morphogenesis and elucidates the role of PA in lipid barrier formation of HSEs.

Depilatory chemical thioglycolate affects hair cuticle and cortex, degrades epidermal cornified envelopes and induces proliferation and differentiation responses in keratinocytes

Thioglycolate is a potent depilatory agent. In addition, it has been proposed to be useful as a penetration enhancer for transepidermal drug delivery. However, the effects on hair structure and stress responses it elicits in epidermal keratinocytes have not been fully characterised. We have used label-free confocal and fluorescence lifetime imaging supported by electron microscopy to demonstrate how thioglycolate damages hair cuticle cells by generating breakages along the endocuticle and leading to swelling of cortex cells. Maleimide staining of free SH-groups and a decrease in the average fluorescence lifetime of endogenous fluorophores demonstrate a specific change in protein structure in both hair cuticle and cortex. We found that the thioglycolate damages cornified envelopes isolated from the stratum corneum of the epidermis. However, thioglycolate-treated epidermal equivalent cultures recover within 48 hours, which highlights the reversibility of the damage. HaCaT keratinocytes respond to thioglycolate by increased proliferation, onset of differentiation and expression of the chaperone protein Hsp 70, but not Hsp 27. Up-regulation of involucrin can be blocked by an application of c-Jun N-terminal kinase (JNK) inhibitor, but the up-regulation of Hsp 70 takes place regardless of the presence of the JNK inhibitor.

Allergens of permanent hair dyes induces epidermal damage, skin barrier loss and IL-1 α increase in epidermal in vitro model

Allergic and irritant skin reactions caused by topical exposure to permanent hair dyes are a common problem. For regulatory and ethnical purposes, it is required to perform chemical safety assessment following the replacement, reduction, and refinement of animal testing (3Rs). Permanent hair dyes are formed by a mixture of ingredients that vary from low to extreme skin sensitizing potency and that inter-react to form unknown by-products. Because of the complex reaction, this cytotoxic mechanism has not yet been elucidated and is the subject of this study. Here, we topically exposed p-phenylenediamine (PPD), Resorcinol (RES), Hydrogen Peroxide (H₂O₂) alone or as a mixture to RhE and evaluated parameters related to skin irritation such as epidermal viability, keratinocytes damage, barrier loss and IL-1 α. Our data indicates that ingredients tested alone did not lead to an increase of cytotoxic parameters related to skin irritation. However, when the mixture of PPD/H₂O₂/RES and PPD/H₂O₂ was applied to the RhE, some of the parameters such as morphological changes including the presence of apoptotic cells, barrier loss and increased IL- 1 α release were observed. The results indicate that the mixture of ingredients used in permanent hair dyes have an irritant effect in RhE while the ingredients alone not.

Characterization of reconstructed human skin containing Langerhans cells to monitor molecular events in skin sensitization

Human cell-based approaches to assess defined key events in allergic contact dermatitis (ACD) are well-established, but lack cutaneous penetration and biotransformation as well as cellular cross-talk. Herein, we integrated in vitro-generated immature MUTZ-3-derived Langerhans-like cells (MUTZ-LCs) or monocyte-derived LC-like cells (MoLCs) into reconstructed human skin (RHS), consistent of a stratified epidermis formed by primary keratinocytes on a dermal compartment with collagen-embedded primary fibroblasts. LC-like cells were mainly localized in the epidermal compartment and distributed homogenously in accordance with native human skin. Topical application of the strong contact sensitizer 2,4-dinitrochlorobenzene (DNCB) induced IL-6 and IL-8 secretion in RHS with LC-like cells, whereas no change was observed in reference models. Increased gene expression of CD83, PD-L1, and CXCR4 in the dermal compartment indicated LC maturation. Importantly, exposure to DNCB enhanced mobility of the LC-like cells from epidermal to dermal compartments. In response to the moderate sensitizer isoeugenol and irritant sodium dodecyl sulphate, the obtained response was less pronounced. In summary, we integrated immature and functional MUTZ-LCs and MoLCs into RHS and provide a unique comparative experimental setting to monitor early events during skin sensitization.

Preliminary performance data of the RHE/IL-18 assay performed on SkinEthic™ RHE for the identification of contact sensitizers

OBJECTIVE

The purpose of this study was to evaluate the performances of the RHE/IL-18 assay using the SkinEthic^™ RHE model for the identification of contact sensitizers.

METHODS

A set of 18 substances and mixtures was tested on this epidermal model, following the RHE/IL-18 protocol. The final results of the assay were obtained following 5 interpretation schemes, to determine the optimal prediction model for this assay with this specific test system. The data were analysed with a special focus on the basal level of IL-18 release and on the performance obtained with respect to three different gold standards: LLNA, HRIPT and an integrated reference, constructed from all available results.

RESULTS

No important differences were found in the performance levels depending on the three gold standards. The performances obtained with the SkinEthic^™ RHE model support that this model may be considered as an alternative to different reconstructed epidermis models (EpiDERM^™ , EpiCS^™ and VUMC-EE) for the performance of RHE/IL-18 assays.

CONCLUSION

The prediction model to be used was refined, and more substances have to be tested in order to gather enough data for this evaluation and to determine the right criteria applicable for this assay using the SkinEthic^™ RHE test system.

Role of Ceramide from Glycosphingolipids and Its Metabolites in Immunological and Inflammatory Responses in Humans

Glycosphingolipids (GSLs) are composed of hydrophobic ceramide and hydrophilic sugar chains. GSLs cluster to form membrane microdomains (lipid rafts) on plasma membranes, along with several kinds of transducer molecules, including Src family kinases and small G proteins. However, GSL-mediated biological functions remain unclear. Lactosylceramide (LacCer, CDw17) is highly expressed on the plasma membranes of human phagocytes and mediates several immunological and inflammatory reactions, including phagocytosis, chemotaxis, and superoxide generation. LacCer forms membrane microdomains with the Src family tyrosine kinase Lyn and the Gαi subunit of heterotrimeric G proteins. The very long fatty acids C24:0 and C24:1 are the main ceramide components of LacCer in neutrophil plasma membranes and are directly connected with the fatty acids of Lyn and Gαi. These observations suggest that the very long fatty acid chains of ceramide are critical for GSL-mediated outside-in signaling. Sphingosine is another component of ceramide, with the hydrolysis of ceramide by ceramidase producing sphingosine and fatty acids. Sphingosine is phosphorylated by sphingosine kinase to sphingosine-1-phosphate, which is involved in a wide range of cellular functions, including growth, differentiation, survival, chemotaxis, angiogenesis, and embryogenesis, in various types of cells. This review describes the role of ceramide moiety of GSLs and its metabolites in immunological and inflammatory reactions in human.

Pseudomonas-derived ceramidase induces production of inflammatory mediators from human keratinocytes via sphingosine-1-phosphate

Ceramide is important for water retention and permeability barrier functions in the stratum corneum, and plays a key role in the pathogenesis of atopic dermatitis (AD). A Pseudomonas aeruginosa-derived neutral ceramidase (PaCDase) isolated from a patient with AD was shown to effectively degrade ceramide in the presence of Staphylococcus aureus-derived lipids or neutral detergents. However, the effect of ceramide metabolites on the functions of differentiating keratinocytes is poorly understood. We found that the ceramide metabolite sphingosine-1-phosphate (S1P) stimulated the production of inflammatory mediators such as TNF-α and IL-8 from three-dimensionally cultured human primary keratinocytes (termed "3D keratinocytes"), which form a stratum corneum. PaCDase alone did not affect TNF-α gene expression in 3D keratinocytes. In the presence of the detergent Triton X-100, which damages stratum corneum structure, PaCDase, but not heat-inactivated PaCDase or PaCDase-inactive mutant, induced the production of TNF-α, endothelin-1, and IL-8, indicating that this production was dependent on ceramidase activity. Among various ceramide metabolites, sphingosine and S1P enhanced the gene expression of TNF-α, endothelin-1, and IL-8. The PaCDase-enhanced expression of these genes was inhibited by a sphingosine kinase inhibitor and by an S1P receptor antagonist VPC 23019. The TNF-α-binding antibody infliximab suppressed the PaCDase-induced upregulation of IL-8, but not TNF-α, mRNA. PaCDase induced NF-κB p65 phosphorylation. The NF-κB inhibitor curcumin significantly inhibited PaCDase-induced expression of IL-8 and endothelin-1. VPC 23019 and infliximab inhibited PaCDase-induced NF-κB p65 phosphorylation and reduction in the protein level of the NF-κB inhibitor IκBα. Collectively, these findings suggest that (i) 3D keratinocytes produce S1P from sphingosine, which is produced through the hydrolysis of ceramide by PaCDase, (ii) S1P induces the production of TNF-α via S1P receptors, and (iii) released TNF-α stimulates the production of inflammatory mediators such as IL-8.

In vitro biocompatibility of solid lipid nanoparticles

This study was undertaken to address the current deficient knowledge of cellular response to solid lipid nanoparticles (SLNs) exposure. We investigated the cytotoxicity of several SLNs formulations in two fibroblast cell lineages, Vero and MDCK. Several methods were used to explore the mechanisms involved in this cytotoxic process, including cell viability assays, flow cytometry and ROS generation assessment. Among nanoparticles tested, two of them (F4 and F5) demonstrated more cytotoxic effects in both cell lineages. The cell viability assays suggested that F4 and F5 interfere in cell mitochondrial metabolism and in lysosomal activity. In addition, F5 decreased the percentage of MDCK cells in G0/G1 and G2/M phases, with a marked increase in the Sub/G1 population, suggesting DNA fragmentation. Regarding F4, although IC(50) was higher (~700 μg/mL), this formulation affected mitochondrial membrane potential for Vero cells. However, the IC(50) of F5 was around 250 μg/mL, suggesting the effect of SDS (sodium dodecyl sulfate) present in the formulation. In summary, the nanoparticles tested here appears to be biocompatible, with the exception of F5. Further studies are required to elucidate the in vivo effects of these nanoscale structures, in order to evaluate or predict the connotation of their increased and widespread use.

Transfer of a two-tiered keratinocyte assay: IL-18 production by NCTC2544 to determine the skin sensitizing capacity and epidermal equivalent assay to determine sensitizer potency

At present, the identification of potentially sensitizing chemicals is carried out using animal models. However, it is very important from ethical, safety and economic point of view to have biological markers to discriminate allergy and irritation events, and to be able to classify sensitizers according to their potency, without the use of animals. Within the Sens-it-iv EU Frame Programme 6 funded Integrated Project (LSHB-CT-2005-018681), a number of in vitro, human cell based assays were developed which, when optimized and used in an integrated testing strategy, may be able to distinguish sensitizers from non-sensitizers. This study describes two of these assays, which when used in a tiered strategy, may be able to identify contact sensitizers and also to quantify sensitizer potency. Tier 1 is the human keratinocyte NCTC2544 IL-18 assay and tier 2 is the Epidermal Equivalent potency assay. The aim of this study is to show the transferability of the two-tiered approach with training chemicals: 3 sensitizers (DNCB, resorcinol, pPD) and 1 non sensitizer (lactic acid) in tier 1 and 2 sensitizers with different potency in tier 2 (DNCB; extreme and resorcinol; moderate). The chemicals were tested in a non-coded fashion. Here we describe the transferability to naïve laboratories, the establishment of the standard operating procedure, critical points, acceptance criteria and project management. Both assays were successfully transferred to laboratories that had not performed the assays previously. The two tiered approach may offer an unique opportunity to provide an alternative method to the Local Lymph Node Assay (LLNA). These assays are both based on the use of human keratinocytes, which have been shown over the last two decades, to play a key role in all phases of skin sensitization.

Decylglucoside-based microemulsions for cutaneous localization of lycopene and ascorbic acid

Cutaneous delivery of combinations of antioxidants offers the possibility of enhanced protection against UV-radiation. In this study, we investigated the potential of sugar-based microemulsions containing monoglycerides to promote simultaneous cutaneous delivery of lycopene and ascorbic acid, and increase tissue antioxidant activity. Lycopene and ascorbic acid were incorporated (0.04% and 0.2% (w/w), respectively) in decylglucoside-based microemulsions containing isopropyl myristate mixed with monocaprylin (ME-MC), monolaurin (ME-ML) or monoolein (ME-MO) as oil phase. The microemulsions increased lycopene delivery into porcine ear skin by 3.3- to 8-fold compared to a drug solution. The effect of microemulsions on ascorbic acid cutaneous delivery was more modest (1.5-3-fold), and associated with an approximately 2-fold increase in transdermal delivery. According to their penetration-enhancing ability, the microemulsions were ranked ME-MC>ME-MO>ME-ML. This superiority of ME-MC coincided with a stronger effect in decreasing skin electrical resistance. After 18 h of treatment, the viability of bioengineered skin treated with ME-MC was 2.2-times higher compared to Triton-X100 (moderate irritant), demonstrating that ME-MC is less cytotoxic. Skin treatment with ME-MC containing both antioxidants increased the tissue antioxidant activity by 10.2-fold, but no synergism between the antioxidants was observed.

Guiding principles for the implementation of non-animal safety assessment approaches for cosmetics: skin sensitisation

Characterisation of skin sensitisation potential is a key endpoint for the safety assessment of cosmetic ingredients especially when significant dermal exposure to an ingredient is expected. At present the mouse local lymph node assay (LLNA) remains the 'gold standard' test method for this purpose however non-animal test methods are under development that aim to replace the need for new animal test data. COLIPA (the European Cosmetics Association) funds an extensive programme of skin sensitisation research, method development and method evaluation and helped coordinate the early evaluation of the three test methods currently undergoing pre-validation. In May 2010, a COLIPA scientific meeting was held to analyse to what extent skin sensitisation safety assessments for cosmetic ingredients can be made in the absence of animal data. In order to propose guiding principles for the application and further development of non-animal safety assessment strategies it was evaluated how and when non-animal test methods, predictions based on physico-chemical properties (including in silico tools), threshold concepts and weight-of-evidence based hazard characterisation could be used to enable safety decisions. Generation and assessment of potency information from alternative tools which at present is predominantly derived from the LLNA is considered the future key research area.

Alternative (non-animal) methods for cosmetics testing: current status and future prospects-2010

The 7th amendment to the EU Cosmetics Directive prohibits to put animal-tested cosmetics on the market in Europe after 2013. In that context, the European Commission invited stakeholder bodies (industry, non-governmental organisations, EU Member States, and the Commission's Scientific Committee on Consumer Safety) to identify scientific experts in five toxicological areas, i.e. toxicokinetics, repeated dose toxicity, carcinogenicity, skin sensitisation, and reproductive toxicity for which the Directive foresees that the 2013 deadline could be further extended in case alternative and validated methods would not be available in time. The selected experts were asked to analyse the status and prospects of alternative methods and to provide a scientifically sound estimate of the time necessary to achieve full replacement of animal testing. In summary, the experts confirmed that it will take at least another 7-9 years for the replacement of the current in vivo animal tests used for the safety assessment of cosmetic ingredients for skin sensitisation. However, the experts were also of the opinion that alternative methods may be able to give hazard information, i.e. to differentiate between sensitisers and non-sensitisers, ahead of 2017. This would, however, not provide the complete picture of what is a safe exposure because the relative potency of a sensitiser would not be known. For toxicokinetics, the timeframe was 5-7 years to develop the models still lacking to predict lung absorption and renal/biliary excretion, and even longer to integrate the methods to fully replace the animal toxicokinetic models. For the systemic toxicological endpoints of repeated dose toxicity, carcinogenicity and reproductive toxicity, the time horizon for full replacement could not be estimated.

A potential in vitro epidermal equivalent assay to determine sensitizer potency

Most in vitro assays aim to distinguish sensitizers from non-sensitizers. Few aim to classify sensitizers according to potency. Here, we describe a potential method for classifying sensitizers according to their irritant potency with the aid of in house epidermal equivalents (EE). Sixteen sensitizers were applied topically in a dose response to EE for 24h. The EE-EC(50) value (effective chemical concentration required to reduce cell viability by 50%) and the EE-IL-1α(10)(×) value (chemical concentration which increases IL-1α secretion by 10-fold) were calculated. From 16 sensitizers, EE-EC(50) and/or EE-IL-1α(10×) values were obtained from 12 skin sensitizers. EE-EC(50) and IL-1α(10×) values decreased in proportion to increasing sensitizer potency. The in vitro assay correlated with existing in vivo mouse and human sensitization data (LLNA, HRIPT), and showed low intra- and inter-experimental variability. Additionally DNCB and resorcinol were correctly assessed as extreme and moderate sensitizers using commercial EE (EST1000™ and RHE™). In conclusion, our data supports the view that irritancy may in part be a factor determining sensitizer potency. Since this assay does not distinguish sensitizers from non-sensitizers, its potential application is in a tiered strategy, where Tier 1 identifies sensitizers which may then tested in Tier 2, this assay, which determines sensitizer potency.