Signal transduction profile of chemical sensitisers in dendritic cells: an endpoint to be included in a cell-based in vitro alternative approach to hazard identification?

Nrf2 regulates the activation of THP-1 cells induced by chloral hydrate

Trichloroethylene (TCE) triggers a severe hypersensitivity syndrome in the occupational population dependent on dendritic cells (DCs). Chloral hydrate (CH), the major oxidative metabolite of TCE, has been proved to be the culprit causative substance of TCE-induced hypersensitivity by human patch tests. Because redox imbalance is essential for chemical sensitizers-induced maturation of DCs, we predicted that CH would activate DCs by the nuclear factor E2-related factor 2 (Nrf2)-mediated antioxidant response. This study selected THP-1 cells as the in vitro DC model, and we evaluated the cell activation markers, intracellular oxidative stress, and Nrf2 pathway related genes expression in response to CH in THP-1 cells. CH displayed significant stimulation of THP-1 cells activation, including CD54 and CD86 expression, IL-8 release, and cell migration, and damaged the redox balance by triggering ROS generation, GSH consumption, and antioxidase activities modulation. The levels of Nrf2 and its downstream genes (HO-1 and NQO1) in mRNA and protein expressions were upregulated by CH, and CH also promoted the nuclear translocation of Nrf2. Subsequently, we investigated the effects of antioxidant on Nrf2-mediated cell defense in CH treated cells. Pretreatment with curcumin dramatically reduced cell activation and oxidative stress triggered by CH in THP-1 cells. We also confirmed the specific role of Nrf2 in CH-induced cell activation using NRF2-knockout cells. Deficiency of Nrf2 inhibited cell activation and downregulated HO-1 and NQO1 expression in CH-challenged cells. These findings suggest that Nrf2-dependent redox homeostasis plays a pivotal role in CH-induced activation of THP-1 cells, thereby providing new knowledge of the allergen as well as the molecular mechanism involving in TCE-induce hypersensitivity syndrome.

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.

Differential signaling patterns of stimulated bone marrow-derived dendritic cells under α1-antitrypsin-enriched conditions

Dendritic cells (DCs) mature upon an inflammatory trigger. However, an inflammatory trigger can lead to a semi-mature phenotype, allowing DCs to evoke tolerance and expedite the resolution of inflammation. This duality likely involves context-dependent modulation of inflammatory signaling. Human α1-antitrypsin (hAAT) promotes semimature DCs. We examined changes in a wide spectrum of signaling cascades in stimulated murine bone marrow-derived cells with hAAT. Upon stimulation by IL-1β+IFNγ, hAAT-treated cells depicted an attenuated calcium flux. Disrupting PKA or NF-κB pathways revoked only some hAAT-mediated outcomes. hAAT-treated cells exhibited a distict pattern of kinase phosphorylation. hAAT-mediated increase in Treg cells in-vitro required intact inflammatory signaling pathways. Taken together, hAAT appears to require a stimulated microenvironment to promote inflammatory resolution, setting it aside from classical anti-inflammatory agents. Further studies are required to identify the specific molecules targeted by hAAT that mediate these and other outcomes.

Adenosine diphosphate involvement in THP-1 maturation triggered by the contact allergen 1-fluoro-2,4-dinitrobenzene

Dendritic cells' (DC) activation is considered a key event in the adverse outcome pathway for skin sensitization elicited by covalent binding of chemicals to proteins. The mechanisms underlying DC activation by contact sensitizers are not completely understood. However, several "danger signals" are pointed as relevant effectors. Among these extra-cellular early danger signals, purines may be crucial for the development of xenoinflammation and several reports indicate their involvement in contact allergic reactions. In the present work we used the DC-surrogate monocytic cell line THP-1, cultured alone or co-cultured with the human keratinocyte cell line HaCaT, to explore the contribution of extracellular adenine nucleotides to THP-1 maturation triggered by the extreme contact sensitizer, 1-fluoro-2,4-dinitrobenzene (DNFB). We found that THP-1 maturation induced by DNFB is impaired after purinergic signaling inhibition, and that the transcription of the purinergic metabotropic receptors P2Y2 and P2Y11 is modulated by the sensitizer. We also detected that THP-1 cells only partially hydrolyse extracellular adenosine triphosphate, leading to accumulation of the mono-phosphate derivative, AMP. We detected different and non-overlapping activation patterns of mitogen activated protein kinases by DNFB and extracellular nucleotides. Overall, our results indicate that THP-1 maturation induced by DNFB is strongly modulated by extracellular adenine nucleotides through metabotropic purinergic receptors. This knowledge unveils a molecular toxicity pathway evoked by sensitizers and involved in THP-1 maturation, a DC-surrogate cell line thoroughly used in in vitro tests for the identification of skin allergens.

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.

Oxidative stress-dependent activation of the eIF2α–ATF4 unfolded protein response branch by skin sensitizer 1-fluoro-2,4-dinitrobenzene modulates dendritic-like cell maturation and inflammatory status in a biphasic manner [corrected]

The pathogenesis of allergic contact dermatitis, the most common manifestation of immunotoxicity in humans, is intimately connected to hapten-induced maturation of dendritic cells (DC). The molecular mechanisms driving this maturational program are not completely known; however, initial danger signals such as the generation of reactive oxygen species (ROS) were shown to play a critical role. Recent evidence linking ROS production, endoplasmic reticulum (ER) stress, and the pathogenesis of several inflammatory diseases led us to analyze, in the present work, the ability of the skin sensitizer 1-fluoro-2,4-dinitrobenzene (DNFB) to evoke ER stress in DC-like THP-1 cells and the concomitant consequences to their immunobiology. We found that DNFB triggers a ROS-dependent activation of the PERK-eIFα-ATF4 unfolded protein response (UPR) branch conferring cytoprotection and modulating the maturation/proinflammatory cell status in a biphasic manner. Early DNFB induction of ATF4 positively modulates autophagy-related genes MAP1LC3B and ATG3 and stabilizes the transcription factor Nrf2, causing a strong induction of the HMOX1-detoxifying gene. Moreover, we observed that in a first phase, DNFB-induced ATF4 upregulates IL8 mRNA levels while blocking CD86, IL1B, IL12B, and CXL10 transcription. Later, following ATF4 decay, HMOX1 and IL8 transcription drastically decrease and CD86, IL1B, and Il12B are upregulated. Overall, our results evidence a connection between sensitizer-induced redox imbalance and the establishment of ER stress in DC-like cells and provide new insights into the role of UPR effectors such as ATF4 to the complex DC maturational program.

Systemic drugs inducing non-immediate cutaneous adverse reactions and contact sensitizers evoke similar responses in THP-1 cells

Contact sensitizers induce phenotypic and functional changes in dendritic cells (DC) that enhance their antigen-presenting capacity and, ultimately, modulate the T cell response. To evaluate if there is a similar effect of drugs causing T-cell-mediated cutaneous adverse drug reactions (CADR), we studied the in vitro effect of drugs on THP-1 cells, a cell line widely used to evaluate the early molecular and cellular events triggered by contact sensitizers. The effect of allopurinol, oxypurinol, ampicillin, amoxicillin, carbamazepine and sodium valproate, at EC30 concentrations, was evaluated on p38 MAPK activation, by Western Blot, and on the expression of genes coding for DC maturation markers, pro-inflammatory cytokine/chemokines and hemeoxygenase 1 (HMOX1), by real-time RT-PCR. Results were compared with lipopolysaccharide (LPS), a DC maturation stimulus, and the strong contact sensitizer, 1-fluoro-2,4-dinitrobenzene (DNFB). All drugs studied significantly upregulated HMOX1 gene transcription and all, except the anticonvulsants, also upregulated IL8. Allopurinol and oxypurinol showed the most intense effect, in a magnitude similar to DNFB and superior to betalactams. Transcription of CD40, IL12B and CXCL10 genes by drugs was more irregular. Moreover, like DNFB, all drugs activated p38 MAPK, although significantly only for oxypurinol. Like contact sensitizers, drugs that cause non-immediate CADR activate THP-1 cells in vitro, using different signalling pathways and affecting gene transcription with an intensity that may reflect the frequency and severity of the CADR they cause. Direct activation of antigen-presenting DC by systemic drugs may be an important early step in the pathophysiology of non-immediate CADR.

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.

Reactivity of chemical sensitizers toward amino acids in cellulo plays a role in the activation of the Nrf2-ARE pathway in human monocyte dendritic cells and the THP-1 cell line

Allergic contact dermatitis resulting from skin sensitization is an inflammatory skin disease linked to the use of chemicals termed haptens. Chemical reactivity is necessary for a chemical to be a sensitizer, allowing both covalent binding to proteins and maturation of dendritic cells (DCs) by mimicking "danger signals." The aim of this study was to evaluate how the reactivity of chemical sensitizers toward amino acids translates into a biological response using the activation of the nuclear factor-erythroid 2-related factor 2 (Nrf2) pathway, which was assessed by the induction of three Nrf2 target genes (ho-1, nqo1, and il-8) and Nrf2 protein accumulation. Nrf2 activation is known to play a role in numerous detoxification mechanisms that could regulate danger signal outcomes in myeloid cells. Monocyte-derived DCs and THP-1 cells were exposed to (a) haptens with cysteine, lysine, or cysteine/lysine reactivity, (b) pro-/prehaptens, and (c) nonsensitizing molecules with reducing or oxidative properties (17 molecules in total). Chemicals were classified as "Nrf2 pathway activators" when at least two Nrf2 target genes associated with Nrf2 protein expression were induced. Results showed that most chemical sensitizers having cysteine and cysteine/lysine affinities were inducers of the Nrf2 pathway in both cell models, whereas lysine-reactive chemicals were less efficient. In THP-1 cells, the Nrf2 pathway was also activated by pro-/prehaptens. Regression analysis revealed that ho-1 and nqo1 expressions were found to be associated with chemical sensitizer reactivity to cysteine, providing evidence of the importance of chemical reactivity, as a part of danger signals, in DC biology.

Dendritic cells and the assessment in vitro of skin sensitizing potential

It is now well established that dendritic cells (DC) play pivotal roles in the initiation and orchestration of adaptive immune responses, including cutaneous immune responses to chemical allergens that drive the acquisition of skin sensitization. It is not unexpected, therefore, that a large number, and wide variety, of proposed approaches for the identification of skin sensitizing chemicals in vitro are based upon the use of cultured DC or DC-like cells. The use of DC in this context is legitimate. However, with our rapidly increasing understanding of the diversity of cutaneous DC with respect to both phenotype and function, it is timely now to review briefly the potential limitations and interpretive difficulties that are associated with the use of DC-based assays. Among the important considerations are the fact that chemical-induced changes in the characteristics and function of cultured DC will not necessarily reflect accurately the events that that support the development of skin sensitization in vivo. In addition, most DC-based assays are predicated on a view that cutaneous DC have as their primary function the initiation of adaptive immune responses. However, it is now appreciated that cutaneous DC, and in particular epidermal Langerhans cells (LC), may also play important immunoregulatory roles that serve to limit and contain skin immune responses. Notwithstanding these considerations there is reason to believe that at least some in vitro DC-based assays are of value, and indeed some are currently the subject of a formal validation process. However, it is appropriate that such assays are configured and interpreted carefully, and with an appreciation of the complexity of DC biology.

Isoeugenol destabilizes IL-8 mRNA expression in THP-1 cells through induction of the negative regulator of mRNA stability tristetraprolin

We previously demonstrated in the human promyelocytic cell line THP-1 that all allergens tested, with the exception of the prohapten isoeugenol, induced a dose-related release of interleukin-8 (IL-8). In the present study, we investigated whether this abnormal behavior was regulated by the AU-rich element-binding proteins HuR and tristetraprolin (TTP) or by the downstream molecule suppressor of cytokine signaling (SOCS)-3. The contact allergens isoeugenol, diethylmaleate (DEM), and 2,4-dinitrochlorobenzene (DNCB), and the irritant salicylic acid were used as reference compounds. Chemicals were used at concentrations that induced a 20% decrease in cell viability as assessed by propidium iodide staining, namely 100 μg/ml (0.61 mM) for isoeugenol, 100 μg/ml (0.58 mM) for DEM, 3 μg/ml (14.8 μM) for DNCB, and 250 μg/ml (1.81 mM) for salicylic acid. Time course experiments of IL-8 mRNA expression and assessment of IL-8 mRNA half-life, indicated a decreased IL-8 mRNA stability in isoeugenol-treated cells. We could demonstrate that a combination and regulation of HuR and TTP following exposure to contact allergens resulted in a different modulation of IL-8 mRNA half-life and release. The increased expression of TTP in THP-1 cells treated with isoeugenol results in destabilization of the IL-8 mRNA, which can account for the lack of IL-8 release. In contrast, the strong allergen DNCB failing to up-regulate TTP, while inducing HuR, resulted in longer IL-8 mRNA half-life and protein release. SOCS-3 was induced only in isoeugenol-treated cells; however, its modulation did not rescue the lack of IL-8 release, indicating that it is unlikely to be involved in the lack of IL-8 production. Finally, the destabilization effect of isoeugenol on IL-8 mRNA expression together with SOCS-3 expression resulted in an anti-inflammatory effect, as demonstrated by the ability of isoeugenol to modulate LPS or ionomycin-induced cytokine release.