Gene expression changes in peripheral blood-derived dendritic cells following exposure to a contact allergen

Preliminary discovery of novel markers for human cell line activation test (h-CLAT)

The human cell line activation test (h-CLAT) is an OECD approved (Test No. 442E) assay to identify novel skin sensitizers. h-CLAT simulates dendritic cell activation in the skin sensitization pathway and is based on the measurement of CD54 and CD86 overexpression on monocytic, leukemic THP-1 cells. However, the current h-CLAT markers show inconsistent results with moderate and weak sensitizers. Moreover, these markers have accessory roles in cell adhesion and signaling rather than a direct role in cellular inflammation. Therefore, we have explored other inflammation-related markers in this study. PBMCs comprises a mixture of cells that resemble the complex immunological milieu in adults and were primarily used to identify markers. PBMCs (n = 10) and THP-1 cells were treated with 1-chloro-2,4-dinitrobenzene (DNCB, strong) and NiCl₂ (Ni, moderate) sensitizers or DMSO (control) and incubated for 24 h. The samples were subjected to RNA sequencing to obtain log₂fold change in gene expression. DNCB and NiCl₂ significantly upregulated 80 genes in both cell types. Of these, CD109, CD181, CD183, CLEC5A, CLEC8A & CD354 were experimentally validated. DNCB and Ni but not isopropyl alcohol (non-sensitizer) significantly induced the expression of all novel markers except CLEC8A. Moreover, the percentage induction of all novel markers except CLEC8A satisfied the OECD acceptance criteria. In summary, we identified five novel markers that may supplement the current repertoire of h-CLAT markers.

Proteomics analysis of dendritic cell activation by contact allergens reveals possible biomarkers regulated by Nrf2

Allergic contact dermatitis is a widespread disease with high clinical relevance affecting approximately 20% of the general population. Typically, contact allergens are low molecular weight electrophilic compounds which can activate the Keap1/Nrf2 pathway. We performed a proteomics study to reveal possible biomarkers for dendritic cell (DC) activation by contact allergens and to further elucidate the role of Keap1/Nrf2 signaling in this process. We used bone marrow derived dendritic cells (BMDCs) of wild-type (nrf2^+/+) and Nrf2 knockout (nrf2^-/-) mice and studied their response against the model contact sensitizers 2,4-dinitrochlorobenzene (DNCB), cinnamaldehyde (CA) and nickel(II) sulfate by 2-dimensional polyacrylamide gel electrophoresis (2D-PAGE) in combination with electrospray ionization tandem mass spectrometry (ESI-MS/MS). Sodium dodecyl sulfate (SDS, 100μM) served as irritant control. While treatment with nickel(II) sulfate and SDS had only little effects, CA and DNCB led to significant changes in protein expression. We found 18 and 30 protein spots up-regulated in wild-type cells treated with 50 and 100μM CA, respectively. For 5 and 10μM DNCB, 32 and 37 spots were up-regulated, respectively. Almost all of these proteins were not differentially expressed in nrf2^-/- BMDCs, indicating an Nrf2-dependent regulation. Among them proteins were detected which are involved in oxidative stress and heat shock responses, as well as in signal transduction or basic cellular pathways. The applied approach allowed us to differentiate between Nrf2-dependent and Nrf2-independent cellular biomarkers differentially regulated upon allergen-induced DC activation. The data presented might contribute to the further development of suitable in vitro testing methods for chemical-mediated sensitization.

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.

Towards AOP application--implementation of an integrated approach to testing and assessment (IATA) into a pipeline tool for skin sensitization

Since the OECD published the Adverse Outcome Pathway (AOP) for skin sensitization, many efforts have focused on how to integrate and interpret nonstandard information generated for key events in a manner that can be practically useful for decision making. These types of frameworks are known as Integrated Approaches to Testing and Assessment (IATA). Here we have outlined an IATA for skin sensitization which focuses on existing information including non testing approaches such as QSAR and read-across. The IATA was implemented into a pipeline tool using OASIS technology to provide a means of systematically collating and compiling relevant information which could be used in an assessment of skin sensitization potential. A test set of 100 substances with available skin sensitization information was profiled using the pipeline IATA. In silico and in chemico profiling information alone was able to correctly predict skin sensitization potential, with a preliminary accuracy of 73.85%. Information from other relevant endpoints (e.g., Ames mutagenicity) was found to improve the accuracy (to 87.6%) when coupled with a reaction chemistry mechanistic understanding. This pipeline platform could be useful in the assessment of skin sensitization potential and marks a step change in how non testing approaches can be practically applied.

Gene expression changes induced by skin sensitizers in the KeratinoSens™ cell line: Discriminating Nrf2-dependent and Nrf2-independent events

The KeratinoSens™ assay is an in vitro screen for the skin sensitization potential of chemicals. It is based on a luciferase reporter gene under the control of the antioxidant response element of the aldoketoreductase gene AKR1C2. The transferability, reproducibility, and predictivity of the KeratinoSens™ assay have been investigated in detail and it is currently under assessment at the European Center for Validation of Alternatives to animal testing (ECVAM). Here we investigate the sensitizer-induced gene expression in the KeratinoSens™ cell line at the mRNA level and discriminate Nrf2-dependent and Nrf2-independent events by using siRNA to better characterize this test system at the molecular level. The results show that (i) the sensitizer-induced luciferase signal in KeratinoSens™ cells is completely dependent on Nrf2. The same holds true for the luciferase induction observed for the false positive chemical Tween80, indicating that the false positive result is not due to recruitment of an alternative transcription factor. (ii) Luciferase induction parallels the induction of endogenous Nrf2-dependent genes, indicating that the luciferase signal is representative for the sensitizer-induced Nrf2-response. (iii) The induction by sensitizers of additional genetic markers related to heat shock proteins and cellular stress could be reproduced in the KeratinoSens™ cell line and they were shown to be Nrf2-independent. These results confirm that the KeratinoSens™ cell line is a rapid and adequate screening tool to assess the sensitizer-induced Nrf2-response in keratinocytes.

Development of a new in vitro skin sensitization assay (Epidermal Sensitization Assay; EpiSensA) using reconstructed human epidermis

Recent changes in regulatory requirements and social views on animal testing have accelerated the development of reliable alternative tests for predicting skin sensitizing potential of chemicals. In this study, we aimed to develop a new in vitro skin sensitization assay using reconstructed human epidermis, RhE model, which is expected to have broader applicability domain rather than existing in vitro assays. Microarray analysis revealed that the expression of five genes (ATF3, DNAJB4, GCLM, HSPA6 and HSPH1) related to cellular stress response were significantly up-regulated in RhE model after 6h treatment with representative skin sensitizers, 1-fluoro-2,4-dinitrobenzene and oxazolone, but not a non-sensitizer, benzalkonium chloride. The predictive performance of five genes was examined with eight skin sensitizers (e.g., cinnamic aldehyde), four non-sensitizers (e.g., sodium lauryl sulfate) and four pre-/pro-haptens (e.g., p-phenylenediamine, isoeugenol). When the positive criteria were set to obtain the highest accuracy with the animal testing (LLNA), ATF3, DNAJB4 and GCLM exhibited a high predictive accuracy (100%, 93.8% and 87.5%, respectively). All tested pre-/pro-haptens were correctly predicted by both ATF3 and DNAJB4. These results suggested that the RhE-based assay, termed epidermal sensitization assay (EpiSensA), could be an useful skin sensitization assay with a broad applicability domain including pre-/pro-haptens.

Allergic skin inflammation induced by chemical sensitizers is controlled by the transcription factor Nrf2

Allergic contact dermatitis (ACD) is induced by low-molecular weight electrophilic chemicals and metal ions. Chemical contact sensitizers trigger reactive oxygen species production and provoke electrophilic stress, leading to the accumulation of the transcription factor nuclear-related factor 2 (Nrf2) in innate immune cell types. The objective of this work was to identify the role of Nrf2 in the regulation of ACD. We used the local lymph node assay (LLNA) and the mouse ear swelling test (MEST) to study the role of Nrf2 in both the sensitization and elicitation phase in nrf2 knockout (nrf2(-/-)) and wild-type (nrf2(+/+)) mice. Five chemicals were used: two compounds known to react with cysteine residues, 2,4-dinitrochlorobenzene (DNCB) and cinnamaldehyde (CinA); one sensitizer known to exhibit mixed reactivity to cysteine and lysine residues, isophorone diisocyanate; and one reacting specifically with lysine residues, trimellitic anhydride and croton oil, a well-known irritant. In the MEST assay, DNCB (1 and 2%) induced a significant increase in ear thickness in nrf2(-/-) compared with nrf2(+/+) mice, suggesting a role for Nrf2 in the control of the inflammatory process. When DNCB was used at 0.25 and 0.5% or when mice were treated with CinA, inflammation was found only in nrf2(-/-) mice. In the LLNA, all chemical sensitizers induced an increase of lymphocyte proliferation in nrf2(-/-) compared with nrf2(+/+) mice for the same chemical concentration. These results reveal an important role for Nrf2 in controlling ACD and lymphocyte proliferation in response to sensitizers.

Role of oxidative stress in chemical allergens induced skin cells activation

Allergic contact dermatitis (ACD) is an important occupational and environmental disease caused by topical exposure to chemical allergens. It describes the adverse effects that may results when exposure to a chemical elicits a T cell-mediated inflammatory skin disease. The ability of contact sensitizers to induce the oxidative stress pathway in keratinocytes and dendritic cells has been confirmed by several authors. Reactive oxygen species (ROS) can serve as essential second messengers mediating cellular responses resulting in immune cells activation. Oxidative stress may be the starter point, as it leads to the activation of transcription factors and signaling pathways, including NF-kB and p38 MAPK, which leads to the release of cytokines and chemokines. ROS are also involved in the activation of the NLRP3/NALP3 inflammasome, which is required to direct the proteolytic maturation of inflammatory cytokines such as IL-1β and IL-18, which are all integral to the process of dendritic cells mobilization, migration and functional maturation. Moreover, emerging evidence correlates ROS to changes in the constitution of the extracellular microenvironment found to facilitate ACD. The purpose of this review is to provide both conceptual and technical frameworks on the role of oxidative stress in chemical allergy.

Development and validation of a new in vitro assay designed to measure contact allergen-triggered oxidative stress in dendritic cells

BACKGROUND

Selected contact allergens are known to induce phenotypic and functional maturation of dendritic cells (DCs). Such changes occurring in DCs have been employed as assay readouts to predict skin-sensitizing potentials of small chemicals.

OBJECTIVE

To respond to the urgent needs for reliable in vitro tests to identify contact allergens, we sought to develop a DC-based assay designed to detect early change(s) induced by sensitizers.

METHODS

Signature gene expression profiles of skin sensitization were determined by GeneChip and quantitative RT-PCR analyses of RNA samples harvested from mouse skin and XS106 DC line after exposure to dinitrofluorobenzene (DNFB). Production of reactive oxygen species (ROS) was examined indirectly by measuring the level of oxidative stress-XS106 DCs were labeled with a fluorescent dye, CM-H(2)DCFDA, exposed to test chemicals, and then examined for fluorescence signals by flow cytometer.

RESULTS

DNFB induced abundant mRNA expression of several redox regulatory genes in both mouse skin and XS106DCs. Expression of these genes was inducible by hydrogen peroxide and blocked by a ROS inhibitor, diphenyleneiodonium. Rapid and significant ROS production was induced by 25 of the 28 tested skin sensitizers, but only by 3 of the 21 tested skin irritants.

CONCLUSIONS

Our small-scale validation study demonstrates the practical utility of our DC-based ROS production assay to detect structurally diverse contact allergens with varying sensitizing potencies. It is tempting to speculate that ROS production in DCs may represent an early event during the sensitization phase.

Non-animal test methods for predicting skin sensitization potentials

Contact allergies are complex diseases, and it is estimated that 15-20 % of the general population suffers from contact allergy, with increasing prevalence. Evaluation of the sensitization potential of a substance is usually carried out in animal models. Nowadays, there is much interest in reducing and ultimately replacing current animal tests. Furthermore, as of 2013, the EU has posed a ban on animal testing of cosmetic ingredients that includes skin sensitization. Therefore, predictive and robust in vitro tests are urgently needed. In order to establish alternatives to animal testing, the in vitro tests must mimic the very complex interactions between the sensitizing chemical and the different parts of the immune system. This review article summarizes recent efforts to develop in vitro tests for predicting skin sensitizers. Cell-based assays, in chemico methods and, to a lesser extent, in silico methods are presented together with a discussion of their current status. With considerable progress having been achieved during the last years, the rationale today is that data from different non-animal test methods will have to be combined in order to obtain reliable hazard and potency information on potential skin sensitizers.

Chemical sensitization and allergotoxicology

Chemical sensitization remains an important environmental and occupational health issue. A wide range of substances have been shown to possess the ability to induce skin sensitization or respiratory sensitization. As a consequence, there is a need to have appropriate methods to identify sensitizing agents. Although a considerable investment has been made in exploring opportunities to develop methods for hazard identification and characterization, there are, as yet, no validated nonanimal methods available. A state of the art of the different in vitro approaches to identify contact and respiratory capacity of chemicals is covered in this chapter.

Allergic asthma: influence of genetic and environmental factors

Allergic asthma is a chronic airway inflammatory disease in which exposure to allergens causes intermittent attacks of breathlessness, airway hyper-reactivity, wheezing, and coughing. Allergic asthma has been called a "syndrome" resulting from a complex interplay between genetic and environmental factors. Worldwide, >300 million individuals are affected by this disease, and in the United States alone, it is estimated that >35 million people, mostly children, suffer from asthma. Although animal models, linkage analyses, and genome-wide association studies have identified numerous candidate genes, a solid definition of allergic asthma has not yet emerged; however, such studies have contributed to our understanding of the multiple pathways to this syndrome. In contrast with animal models, in which T-helper 2 (T(H)2) cell response is the dominant feature, in human asthma, an initial exposure to allergen results in T(H)2 cell-dependent stimulation of the immune response that mediates the production of IgE and cytokines. Re-exposure to allergen then activates mast cells, which release mediators such as histamines and leukotrienes that recruit other cells, including T(H)2 cells, which mediate the inflammatory response in the lungs. In this minireview, we discuss the current understanding of how associated genetic and environmental factors increase the complexity of allergic asthma and the challenges allergic asthma poses for the development of novel approaches to effective treatment and prevention.

Genetic factors in contact allergy--review and future goals

The genetics of contact allergy are still only partly understood, despite decades of research; this might be a consequence of inadequately defined phenotypes used in the past. A recommendation is to study an extreme phenotype, namely, polysensitization (sensitization to three or more unrelated allergens). Another approach to unravel the genetics of contact allergy is the study of candidate genes. In this review, we summarize studies on the associations between genetic variation (e.g. single-nucleotide polymorphisms) in certain candidate genes and contact allergy. Polymorphisms and mutations affecting the following proteins were studied: (i) filaggrin; (ii) N-acetyltransferase (NAT) 1 and 2; (iii) glutathione-S-transferase (GST) M and T; (iv) manganese superoxide dismutase; (v) angiotensin-converting enzyme (ACE); (vi) tumour necrosis factor (TNF); and (vii) interleukin-16 (IL-16). The polymorphisms of NAT1, NAT2, GSTM, GSTT, ACE, TNF and IL-16 were shown to be associated with an increased risk of contact allergy. In one of our studies, the increased risk conferred by the TNF and IL-16 polymorphisms was confined to polysensitized individuals. Other relevant candidate genes may be identified by studying diseases related to contact allergy in terms of clinical symptoms, a more general pathology (inflammation), and possibly an overlapping genetic background, such as irritant contact dermatitis.

The intra- and inter-laboratory reproducibility and predictivity of the KeratinoSens assay to predict skin sensitizers in vitro: results of a ring-study in five laboratories

Due to regulatory constraints and ethical considerations, research on alternatives to animal testing to predict the skin sensitization potential of novel chemicals has gained a high priority. Accordingly, different in vitro, in silico and in chemico approaches have been described in the scientific literature to achieve this goal. To replace regulatory approved animal tests, these alternatives need to be transferable to other labs, their within and between laboratory reproducibility must be assured, and their predictivity should be high. The KeratinoSens assay is a cell-based reporter gene assay to screen substances with a full dose-response assessment. It is based on a stable transgenic keratinocyte cell line. The induction of a luciferase gene under the control of the antioxidant response element (ARE) derived from the human AKR1C2 gene is determined. Here we report on the results of a ring-study with five laboratories performing the KeratinoSens assay on a set of 28 test substances. The assay was found to be easily transferable to all laboratories. Overall both the qualitative (sensitizer/non-sensitizer categorization) and the quantitative (concentration for significant gene induction) results were reproducible between laboratories. A detailed analysis of the transferability, the within- and between laboratory reproducibility and the predictivity is presented.

T-cell recognition of chemicals, protein allergens and drugs: towards the development of in vitro assays

Chemicals can elicit T-cell-mediated diseases such as allergic contact dermatitis and adverse drug reactions. Therefore, testing of chemicals, drugs and protein allergens for hazard identification and risk assessment is essential in regulatory toxicology. The seventh amendment of the EU Cosmetics Directive now prohibits the testing of cosmetic ingredients in mice, guinea pigs and other animal species to assess their sensitizing potential. In addition, the EU Chemicals Directive REACh requires the retesting of more than 30,000 chemicals for different toxicological endpoints, including sensitization, requiring vast numbers of animals. Therefore, alternative methods are urgently needed to eventually replace animal testing. Here, we summarize the outcome of an expert meeting in Rome on 7 November 2009 on the development of T-cell-based in vitro assays as tools in immunotoxicology to identify hazardous chemicals and drugs. In addition, we provide an overview of the development of the field over the last two decades.

Chemical allergy: translating biology into hazard characterization

The induction by chemicals of allergic sensitization and allergic disease is an important and challenging branch of toxicology. Skin sensitization resulting in allergic contact dermatitis represents the most common manifestation of immunotoxicity in humans, and many hundreds of chemicals have been implicated as skin sensitizers. There are far fewer chemicals that have been shown to cause sensitization of the respiratory tract and asthma, but the issue is no less important because hazard identification remains a significant challenge, and occupational asthma can be fatal. In all areas of chemical allergy, there have been, and remain still, intriguing challenges where progress has required a close and productive alignment between immunology, toxicology, and clinical medicine. What the authors have sought to do here is to exemplify, within the framework of chemical allergy, how an investment in fundamental research and an improved understanding of relevant biological and biochemical mechanisms can pay important dividends in driving new innovations in hazard identification, hazard characterization, and risk assessment. Here we will consider in turn three specific areas of research in chemical allergy: (1) the role of epidermal Langerhans cells in the development of skin sensitization, (2) T lymphocytes and skin sensitization, and (3) sensitization of the respiratory tract. In each area, the aim is to identify what has been achieved and how that progress has impacted on the development of new approaches to toxicological evaluation. Success has been patchy, and there is still much to be achieved, but the journey has been fascinating and there have been some very important developments. The conclusion drawn is that continued investment in research, if coupled with an appetite for translating the fruits of that research into imaginative new tools for toxicology, should continue to better equip us for tackling the important challenges that remain to be addressed.

Assessment of chemical skin-sensitizing potency by an in vitro assay based on human dendritic cells

The skin-sensitizing potential of chemicals is an important concern for public health and thus a significant end point in the hazard identification process. To determine skin-sensitizing capacity, large research efforts focus on the development of assays, which do not require animals. As such, an in vitro test has previously been developed based on the differential expression of CREM and CCR2 transcripts in CD34(+) progenitor-derived dendritic cells (CD34-DC), which allows to classify chemicals as skin (non-)sensitizing. However, skin sensitization is not an all-or-none phenomenon, and up to now, the assessment of relative potency can only be derived using the in vivo local lymph node assay (LLNA). In our study, we analyzed the feasibility to predict the sensitizing potency, i.e., the LLNA EC3 values, of 15 skin sensitizers using in vitro data from the CD34-DC-based assay. Hereto, we extended the in vitro-generated gene expression data set by an additional source of information, the concentration of the compound that causes 20% cell damage (IC20) in CD34-DC. We statistically confirmed that this IC20 is linearly independent from the gene expression changes but that it does correlate with LLNA EC3 values. In a further analysis, we applied a robust linear regression with both IC20 and expression changes of CREM and CCR2 as explanatory variables. For 13 out of 15 compounds, a high linear correlation was established between the in vitro model and the LLNA EC3 values over a range of four orders of magnitude, i.e., from weak to extreme sensitizers.

DNA microarrays provide new options for allergen testing

Microarray studies are increasingly used for toxicological research and even for the development of new toxicological test methods. Since gene-expression changes in cultured cells can be conveniently measured with microarrays, this method might be of use for in vitro toxicity testing, for example, in the field of contact sensitization. Allergic contact dermatitis, the clinical manifestation of contact sensitization, may occur when sensitizing chemicals enter the skin and get in contact with epidermal and dermal antigen-presenting cells. The resulting maturation process in these cells can be measured by employing gene-expression analysis. Biomarkers currently known seem to be insufficient to identify all kinds of contact sensitizers, which may partly activate different signaling pathways (e.g., metal or organic sensitizers). Therefore, genome-wide screenings using whole-genome DNA microarrays and extensive data analysis can be performed in order to identify additional genes. Ultimately, marker genes detected in whole-genome experiments can be included in small-scale-targeted microarrays in order to establish the final test method.

Detection call algorithms for high-throughput gene expression microarray data

Extensive methodological research has been conducted to improve gene expression summary methods. However, in addition to quantitative gene expression summaries, most platforms, including all those examined in the MicroArray Quality Control project, provide a qualitative detection call result for each gene on the platform. These detection call algorithms are intended to render an assessment of whether or not each transcript is reliably measured. In this paper, we review uses of these qualitative detection call results in the analysis of microarray data. We also review the detection call algorithms for two widely used gene expression microarray platforms, Affymetrix GeneChips and Illumina BeadArrays, and more clearly formalize the mathematical notation for the Illumina BeadArray detection call algorithm. Both algorithms result in a P-value which is then used for determining the qualitative detection calls. We examined the performance of these detection call algorithms and default parameters by applying the methods to two spike-in datasets. We show that the default parameters for qualitative detection calls yield few absent calls for high spike-in concentrations. When genes of interest are expected to be present at very low concentrations, spike-in datasets can be useful for appropriately adjusting the tuning parameters for qualitative detection calls.

The Nrf2-Keap1-ARE toxicity pathway as a cellular sensor for skin sensitizers--functional relevance and a hypothesis on innate reactions to skin sensitizers

With the tight deadlines set both by the public and by the regulatory authorities to replace animal tests for toxicological endpoints relevant to the development of cosmetic products, a large number of research projects have recently focused on cellular endpoints affected by skin sensitizing compounds. The general aim stated in these projects was to find "markers" for skin sensitizers, be it at the level of the transcriptome or at the protein level. Rather than talking of "cellular markers," the new paradigm "Toxicity testing in the 21st century" formulated by the National Academy of Sciences in the United States focuses on "Toxicity pathways." A specific marker for any given toxicological endpoint can only exist, if specific toxicity pathways, comprising specific sensors, are linked to this endpoint. In the context of skin sensitization, one has to ask whether there is an innate cellular signaling pathway activated by skin sensitizers. Here a significant body of evidence, mainly accumulated in the last 20 months, is reviewed, indicating that indeed the Nrf2-Keap1-ARE regulatory pathway is such a toxicity pathway activated by cysteine-reactive skin sensitizers. Whereas first indications on the in vivo relevance are available, key questions remain open and can now specifically be addressed. A minority of sensitizers, more specifically reacting with lysine residues, appears not to activate the Nrf2-Keap1-ARE pathway and might trigger yet another unknown toxicity pathway.

Toxicogenomics: transcription profiling for toxicology assessment

Toxicogenomics, the application of transcription profiling to toxicology, has been widely used for elucidating the molecular and cellular actions of chemicals and other environmental stressors on biological systems, predicting toxicity before any functional damages, and classification of known or new toxicants based on signatures of gene expression. The success of a toxicogenomics study depends upon close collaboration among experts in different fields, including a toxicologist or biologist, a bioinformatician, statistician, physician and, sometimes, mathematician. This review is focused on toxicogenomics studies, including transcription profiling technology, experimental design, significant gene extraction, toxicological results interpretation, potential pathway identification, database input and the applications of toxicogenomics in various fields of toxicological study.

HMOX1 and NQO1 genes are upregulated in response to contact sensitizers in dendritic cells and THP-1 cell line: role of the Keap1/Nrf2 pathway

Electrophilicity is one of the most common features of skin contact sensitizers and is necessary for protein haptenation. The Keap1 (Kelch-like ECH-associated protein 1)/Nrf2 -signaling pathway is dedicated to the detection of electrophilic stress in cells leading to the upregulation of genes involved in protection or neutralization of chemical reactive species. Signals provided by chemical stress could play an important role in dendritic cell activation and the aim of this work was to test whether contact sensitizers were specific activators of the Keap1/Nrf2 pathway. CD34-derived dendritic cells (CD34-DC) and the THP-1 myeloid cell line were treated by a panel of sensitizers (Ni, 1-chloro 2,4-dinitrobenzene, cinnamaldehyde, 7-hydroxycitronellal, 1,4-dihydroquinone, alpha-methyl-trans-cinnamaldehyde, 2-4-tert-(butylbenzyl)propionaldehyde or Lilial, and 1,4-phenylenediamine), irritants (sodium dodecyl sulfate, benzalkonium chloride), and a nonsensitizer molecule (chlorobenzene). Three well-known Nrf2 activators (tert-butylhydroquinone, lipoic acid, sulforaphane) were also tested. Expression of hmox1 and nqo1 was measured using real-time PCR and cellular accumulation of Nrf2 was assessed by Western blot. Our results showed an increased expression at early time points of hmox1 and nqo1 mRNAs in response to sensitizers but not to irritants. Accumulation of the Nrf2 protein was also observed only with chemical sensitizers. A significant inhibition of the expression of hmox1 and nqo1 mRNAs and CD86 expression was found in 1-chloro 2,4-dinitrobenzene-treated THP-1 cells preincubated with N-acetyl cysteine, a glutathione precursor. Altogether, these data suggested that the Keap1/Nrf2-signaling pathway was activated by electrophilic molecules including sensitizers in dendritic cells and in the THP-1 cell line. Monitoring of this pathway may provide new biomarkers (e.g., Nrf2, hmox1) for the detection of the sensitization potential of chemicals.

Cell types involved in allergic asthma and their use in in vitro models to assess respiratory sensitization

This review first describes the mechanism and cell types involved in allergic asthma, which is a complex clinical disease characterized by airway obstruction, airway inflammation and airway hyperresponsiveness to a variety of stimuli. The development of allergic asthma exists of three phases, namely the induction phase, the early-phase asthmatic reaction (EAR) and the late-phase asthmatic reaction (LAR). In the induction phase, antigen-presenting cells play a major role. Most important cells in the EAR are mast cells, and during the LAR, various cell types, such as eosinophils, neutrophils, T cells, macrophages, dendritic cells (DCs), and cells that endow structure are involved. In occupational asthma, this immunological mechanism is involved in 90% of the cases. The second part of this review gives an overview of in vitro models to assess the hazardous potential of high- and low-molecular weight chemicals on the respiratory system. In order to develop a good in vitro model for respiratory allergy, the choice of appropriate cell types is important. Epithelial cells, macrophages and DCs are currently the most used models in this field of research.

Microarray-Based In Vitro Test System for the Discrimination of Contact Allergens and Irritants: Identification of Potential Marker Genes

Background: Animal tests have been used to characterize the potential of chemicals to produce allergic contact dermatitis, but this approach is increasingly a matter of public and political concern. Our aim was to develop and validate an alternative in vitro test that can identify contact allergens.

Methods: We developed a targeted microarray containing oligonucleotide probes for 66 immune-relevant genes and analyzed gene expression in monocyte-derived dendritic cells (Mo-DCs) treated with 1 irritant (SDS) and 2 prominent contact allergens, nickel and Bandrowski’s base (BB), which is the oxidation product of the most important hair dye allergen, p-phenylenediamine.

Results: Comparing RNA amounts in chemical-treated and solvent-treated cells, we identified significant changes in the expression of 21 genes and 10 genes after exposure of immature DCs (iDCs) to nickel and BB, respectively, but not after exposure to SDS. Eight genes were differentially expressed after application of both nickel and BB. Real-time PCR was used to confirm the results for selected genes.

Conclusion: We propose a microarray-based in vitro test that might allow the identification of contact allergens. Independently from donor variability, several immune-relevant genes were up- or downregulated after the application of the tested sensitizers to iDCs, therefore presenting potential marker genes. While reducing the number of laboratory animals used, this test would also enable reliable analysis of chemicals using a human system.

The contact allergen dinitrochlorobenzene (DNCB) and respiratory allergy in the Th2-prone Brown Norway rat

All LMW respiratory allergens known to date can also induce skin allergy in test animals. The question here was if in turn skin allergens can induce allergy in the respiratory tract. Respiratory allergy was tested in Th2-prone Brown Norway (BN) rats by dermal sensitization with the contact allergen dinitrochlorobenzene (DNCB; 1%, day 0; 0.5%, day 7) and a head/nose-only inhalation challenge of 27mg/m3 of DNCB (15 min, day 21), using a protocol that successfully identified chemical respiratory allergens. Skin allergy to DNCB was examined in BN rats and Th1-prone Wistar rats in a local lymph node assay followed by a topical patch challenge of 0.1% DNCB. Sensitization of BN rats via the skin induced DNCB-specific IgG in serum, but not in all animals, and an increased number of CD4+ cells in the lung parenchyma. Subsequent inhalation challenge with DNCB did not provoke apneas or allergic inflammation (signs of respiratory allergy) in the BN rats. However, microarray analysis of mRNA isolated from the lung revealed upregulation of the genes for Ccl2 (MCP-1), Ccl4 (MIP-1beta), Ccl7 and Ccl17. Skin challenge induced considerably less skin irritation and allergic dermatitis in the BN rat than in the Wistar rat. In conclusion, the Th2-prone BN rat appeared less sensitive to DNCB than the Wistar rat; nevertheless, DNCB induced allergic inflammation in the skin of BN rats but even a relatively high challenge concentration did not induce allergy in the respiratory tract, although genes associated with allergy were upregulated in lung tissue.

Pre-filtering improves reliability of Affymetrix GeneChips results when used to analyze gene expression in complex tissues

Affymetrix GeneChip represents a very reliable and standardized technology for genome-wide gene expression screening. However, in experiments carried out on complex biological samples (e.g. brain tissues composed of several diverse cell types), significant noise can arise due to important transcripts being expressed in a relatively small number of cells. This noise results in many observations coming from unreliable hybridization reactions. Here we propose a method for pre-filtering Affymetrix data according to measures of hybridization reliability. We used our pre-filtering method on a microarray dataset obtained from the brains of rats chronically treated with a psychostimulant drug. Our pre-filter protocol facilitates selection of biologically relevant candidate genes, which could be validated by real-time PCR with a rate of 98%.

Gene Expression Time Course in the Human Skin during Elicitation of Allergic Contact Dermatitis

Genes involved in the inflammatory response resulting in allergic contact dermatitis (ACD) are only partly known. In this study, we introduce the use of high-density oligonucleotide arrays for gene expression profiling in human skin during the elicitation of ACD. Skin biopsies from normal and nickel-exposed skin were obtained from seven nickel-allergic patients and five nonallergic controls at four different time points during elicitation of eczema. Each gene expression profile was analyzed by hybridization to high-density oligonucleotide arrays. Cluster analysis of 74 genes found to be differentially expressed in the patients over time revealed that the patient samples may be categorized into two groups: an early time-point group (no clinical reaction) and a late time-point group (clinical reaction). Bioinformatics analyses unraveled the potential involvement of signal transducers and activator of transcription and small/mothers against decepentaplegic (SMAD) transcription factors in the late time-point gene expression patterns. Concerning specific genes, the homeostatic chemokine CCL19 was unexpectedly found to be highly expressed in cells scattered in the deep dermis of the late time-point samples. Taken together, these findings suggest hitherto unknown roles of SMAD transcription factors and of CCL19 in the elicitation phase of ACD.