The skin sensitization adverse outcome pathway: exploring the role of mechanistic understanding for higher tier risk assessment

For over a decade, the skin sensitization Adverse Outcome Pathway (AOP) has served as a useful framework for development of novel in chemico and in vitro assays for use in skin sensitization hazard and risk assessment. Since its establishment, the AOP framework further fueled the existing efforts in new assay development and stimulated a plethora of activities with particular focus on validation, reproducibility and interpretation of individual assays and combination of assay outputs for use in hazard/risk assessment. In parallel, research efforts have also accelerated in pace, providing new molecular and dynamic insight into key events leading to sensitization. In light of novel hypotheses emerging from over a decade of focused research effort, mechanistic evidence relating to the key events in the skin sensitization AOP may complement the tools currently used in risk assessment. We reviewed the recent advances unraveling the complexity of molecular events in sensitization and signpost the most promising avenues for further exploration and development of useful assays.

Alternative Methods for Skin-Sensitization Assessment

Skin sensitization is a term used to refer to the regulatory hazard known as allergic contact dermatitis (ACD) in humans or contact hypersensitivity in rodents, an important health endpoint considered in chemical hazard and risk assessments. Information on skin sensitization potential is required in various regulatory frameworks, such as the Directive of the European Parliament and the Council on Registration, Evaluation and Authorization of Chemicals (REACH). The identification of skin-sensitizing chemicals previously required the use of animal testing, which is now being replaced by alternative methods. Alternative methods in the field of skin sensitization are based on the measurement or prediction of key events (KE), i.e., (i) the molecular triggering event, i.e., the covalent binding of electrophilic substances to nucleophilic centers in skin proteins; (ii) the activation of keratinocytes; (iii) the activation of dendritic cells; (iv) the proliferation of T cells. This review article focuses on the current state of knowledge regarding the methods corresponding to each of the key events in skin sensitization and considers the latest trends in the development and modification of these methods.

Heat Shock Protein 90's Mechanistic Role in Contact Hypersensitivity

Despite the known dangers of contact allergens and their long-lasting use as models in immunology, their molecular mode of action largely remains unknown. In this study, we report that a contact allergen, 1-chloro-2,4-dinitrobenzene (DNCB), elicits contact hypersensitivity through binding the protein we identify. Starting from an unbiased sampling of proteomics, we found nine candidate proteins with unique DNCB-modified peptide fragments. More than half of these fragments belonged to heat shock protein 90 (HSP90), a common stress-response protein and a damage-associated molecular pattern, and showed the highest probability of incidence. Inhibition and short hairpin RNA knockdown of HSP90 in human monocyte cell line THP-1 suppressed the potency of DNCB by >80%. Next, we successfully reduced DNCB-induced contact hypersensitivity in HSP90-knockout mice, which confirmed our findings. Finally, we hypothesized that DNCB-modified HSP90 activates the immune cells through HSP90's receptor, CD91. Pretreatment of CD91 in THP-1 cell lines and BALB/c mice attenuated the potency of DNCB, consistent with the result of HSP90-knockout mice. Altogether, our data show that DNCB-HSP90 binding plays a role in mediating DNCB-induced contact hypersensitivity, and the activation of CD91 by DNCB-modified HSP90 proteins could mediate this process.

An isoflavone extract from soybean cake suppresses 2,4-dinitrochlorobenzene-induced contact dermatitis

ETHNOPHARMACOLOGICAL RELEVANCE

Numerous epidemiological and clinical studies have demonstrated the protective role of dietary isoflavones against development of several chronic diseases. ISO-1, one fraction of isoflavone powders derived from soybean cake, is reported to attenuate inflammation and photodamage.

AIM OF THE STUDY

Contact dermatitis is a common inflammatory skin disease, which accounts for most occupational skin disorders. Instead of oral administration, we aimed to explore the effects of topical ISO-1 application on contact dermatitis by using 2,4-dinitrochlorobenzene (DNCB)-stimulated HaCaT keratinocytes and DNCB-induced mouse dermatitis as models.

MATERIALS AND METHODS

In the in vitro study, we first evaluated the biologic effects of DNCB on HaCaT keratinocytes. HaCaT keratinocytes were treated with 2,4-dinitrochlorobenzene (DNCB), and cell viability was measured by MTT assay. Then, we detect the prominent induction of IL-8 mRNA expression after DNCB and ISO-1 treatment by reverse transcription polymerase chain reaction (RT-PCR), and release of IL-8 from HaCaT keratinocytes was measured by ELISA assay. HaCaT keratinocytes were pretreated with ISO-1 and then treated with DNCB, phosphorylation of JNK, p38, ERK and IκBα was analyzed by western blot. In the in vivo study, the hairless mice were used for an induced contact dermatitis model. The surface changes in the dorsal skin after DNCB and ISO-1 treatment were recorded using photography, and TEWL, erythema were measured using an MPA-580 cutometer. Blood was also collected from mice for measurement of white blood cell counts.

RESULTS

Results showed ISO-1 inhibited DNCB-induced IL-8 production and also suppressed DNCB-induced phosphorylation of JNK and p38, and IκBα in HaCaT. In the animal model of DNCB-induced contact dermatitis, topical ISO-1 treatment significantly decreased DNCB-induced erythema and transepidermal water loss (TEWL) in mouse skin. ISO-1 also reduced DNCB-induced skin thickening and increase of white blood cell count.

CONCLUSIONS

ISO-1 is promising for improvement of DNCB-induced inflammation and skin barrier impairment, suggesting the potential application of topical ISO-1 for inflammatory dermatoses.

In vitro identification of drugs inducing systemic hypersensitivity reactions known in vivo to be associated with specific HLA genotypes

Hypersensitivity drug reactions (HDRs) are common among drugs, despite this, there are no validated in vitro or in vivo methods for screening the sensitizing potential of drugs in the preclinical phase. We previously developed the THP-1 activation assay, based on CD86 upregulation and IL-8 production, for the in vitro identification of drugs able to induce selective dendritic cell activation. In this paper, we investigated the predictive capacity of the method toward drugs associated with HDRs for which a correlation with specific human leukocyte antigens (HLA) have been demonstrated. For that purpose, abacavir, carbamazepine and clozapine were used. Metformin was used as negative control. Dose- and time-course experiments were conducted. The surface markers CD86, CD54 and HLA-DR were evaluated by flow cytometry analysis, whereas IL-8 release by ELISA. Abacavir, carbamazepine and clozapine gave positive results with CD86 upregulation and/or IL-8 release, with abacavir also inducing HLA-DR. The test reveals the ability of drugs to induce dendritic cell activation (signals 1/2), that preceded the adaptive immune response, which will be manifested only in a minority of patients carrying the specific HLA genotypes. The idea is to integrate this simple method during drug development to identify the potential of drugs to induce hypersensitivity reactions in the pre-clinical phase.

The Overt and Hidden Use of Animal-Derived Products in Alternative Methods for Skin Sensitisation: A Systematic Review

In vitro methods that can replace animal testing in the identification of skin sensitisers are now a reality. However, as cell culture and related techniques usually rely on animal-derived products, these methods may be failing to address the complete replacement of animals in safety assessment. The objective of this study was to identify the animal-derived products that are used as part of in vitro methods for skin sensitisation testing. Thus, a systematic review of 156 articles featuring 83 different in vitro methods was carried out and, from this review, the use of several animal-derived products from different species was identified, with the use of fetal bovine serum being cited in most of the methods (78%). The use of sera from other animals, monoclonal antibodies and animal proteins were also variously mentioned. While non-animal alternatives are available and methods free of animal-derived products are emerging, most of the current methods reported used at least one animal-derived product, which raises ethical and technical concerns. Therefore, to deliver technically and ethically better in vitro methods for the safety assessment of chemicals, more effort should be made to replace products of animal origin in existing methods and to avoid their use in the development of new method protocols.

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.

Zinc ions have a potential to attenuate both Ni ion uptake and Ni ion-induced inflammation

Nickel ions (Ni²⁺) are eluted from various metallic materials, such as medical devices implanted in human tissues. Previous studies have shown that Ni²⁺ enters inflammatory cells inducing inflammation. However, the regulation of Ni²⁺ uptake in cells has not yet been reported in detail. In the present study, we investigated the effects of various divalent cations on Ni²⁺ uptake and Ni²⁺-induced interleukin (IL)-8 production in the human monocytic cell line, THP-1. We demonstrated that ZnCl_2, MnCl₂, and CoCl₂ inhibited the Ni²⁺ uptake, while CuCl₂, FeCl₂, MgCl₂, and divalent metal transporter (DMT)-1 inhibitor, Chlorazol Black, did not. Furthermore, ZnCl₂ inhibited Ni²⁺-induced IL-8 production, correlating with the inhibition of Ni²⁺ uptake. These results suggested that Ni²⁺ uptake occurred through Zn²⁺, Mn²⁺, and Co²⁺-sensitive transporters and that the inhibition of Ni²⁺ uptake resulted in the inhibition of IL-8 production. Furthermore, using an Ni wire-implanted mouse model, we found that Ni wire-induced expression of mouse macrophage inflammatory protein-2 (MIP-2) and cyclooxygenase-2 (COX-2) mRNA in the skin tissue surrounding the wire were enhanced by low Zn conditions. These results suggested that the physiological concentration of Zn²⁺ modulates Ni²⁺ uptake by inflammatory cells, and a Zn deficient state might increase sensitivity to Ni.

Nature and kinetics of redox imbalance triggered by respiratory and skin chemical sensitizers on the human monocytic cell line THP-1

Low molecular weight reactive chemicals causing skin and respiratory allergies are known to activate dendritic cells (DC), an event considered to be a key step in both pathologies. Although generation of reactive oxygen species (ROS) is considered a major danger signal responsible for DC maturation, the mechanisms leading to cellular redox imbalance remain poorly understood. Therefore, the aim of this study was to unveil the origin and kinetics of redox imbalance elicited by 1-fluoro-2,4-dinitrobenzene (DNFB) and trimellitic anhydride chloride (TMAC), two golden standards of skin and chemical respiratory allergy, respectively. To track this goal, we addressed the time course modifications of ROS production and cellular antioxidant defenses as well as the modulation of MAPKs signaling pathways and transcription of pathophysiological relevant genes in THP-1 cells. Our data shows that the thiol-reactive sensitizer DNFB directly reacts with cytoplasmic glutathione (GSH) causing its rapid and marked depletion which results in a general increase in ROS accumulation. In turn, TMAC, which preferentially reacts with amine groups, induces a delayed GSH depletion as a consequence of increased mitochondrial ROS production. These divergences in ROS production seem to be correlated with the different extension of intracellular signaling pathways activation and, by consequence, with distinct transcription kinetics of genes such as HMOX1, IL8, IL1B and CD86. Ultimately, our observations may help explain the distinct DC phenotype and T-cell polarizing profile triggered by skin and respiratory sensitizers.

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Distinctive ROS origin and kinetics elicited by skin and respiratory sensitizers.

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ROS production elicited by DNFB results primarily from direct GSH haptenation.

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Distinct expression of genes involved in DC maturation and T-cell polarizing capacity.

Application of proteomics in the elucidation of chemical-mediated allergic contact dermatitis

Allergic contact dermatitis (ACD) is a widespread hypersensitivity reaction of the skin. The cellular mechanisms underlying its development are complex and involve close interaction of different cell types of the immune system. It is this very complexity which has long prevented straightforward replacement of the corresponding regulatory in vivo tests. Recent efforts have already resulted in the development of several in vitro testing alternatives that address key steps of ACD. Yet identification of suitable biomarkers is still a subject of intense research. Search strategies for the latter encompass transcriptomics, proteomics as well as metabolomics approaches. The scope of this review shall be the application and use of proteomics in the context of ACD. This includes highlighting relevant aspects of the molecular and cellular mechanisms underlying ACD, the exploitation of these mechanisms for testing and biomarkers (e.g., in the context of the OECD's adverse outcome pathway initiative) as well as an outlook on emerging proteome targets, for example during the allergen-induced activation of dendritic cells (DCs).

Optimization of the IL-8 Luc assay as an in vitro test for skin sensitization

We previously reported a dataset of the IL-8 Luc assay covering reference chemicals published by ECVAM, in which the effects of chemicals on IL-8 promoter activity were evaluated by an IL-8 reporter cell line, THP-G8 cells. To clarify its performance, we created another dataset of 88 sensitizers and 34 non-sensitizers. Simultaneously, to improve its performance, we changed the incubation time from 5 h to 16 h, deleted the criterion regarding the effects of N-acetylcysteine, and set an exclusion criterion for detergents. These modifications significantly improved its performance. In addition, we examined the following three criteria to judge chemicals as sensitizers: Criterion 1: Fold induction of SLO luciferase activity (FlnSLO-LA)⩾1.4, Criterion 2: the lower limit of the 95% confidence interval of FInSLO-LA⩾1.0, Criterion 3: the intersection of criteria 1 and 2. Among them, Criterion 1 produced the best performance, demonstrating that the accuracy, sensitivity and specificity were 81%, 79%, and 90%, respectively. In addition, we found that the IL-8 Luc assay solubilizing chemicals with X-VIVO substantially improved its performance. Finally, the IL-8 Luc assay combined with DPRA and DEREK could improve substantially its performance. These data suggest that the IL-8 Luc assay is a promising test method to screen skin sensitizers.

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.

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.

An in vitro test to screen skin sensitizers using a stable THP-1-derived IL-8 reporter cell line, THP-G8

Several studies have suggested that interleukin (IL)-8 can serve as a biomarker for discrimination of skin sensitizers from nonsensitizers. We established a stable THP-1-derived IL-8 reporter cell line, THP-G8, which harbors SLO and SLR luciferase genes under the control of IL-8 and glyceraldehyde 3-phosphate dehydrogenase promoters, respectively. After 6 h treatment with chemicals, normalized SLO luciferase activity (nSLO-LA) was calculated by dividing SLO-LA by SLR-LA, and the fold induction of nSLO-LA (FInSLO-LA) was calculated by dividing nSLO-LA of chemically treated cells by that of nontreated cells. The nSLO-LA of THP-G8 cells increased in response to lipopolysaccharide (LPS) and several sensitizers. The FInSLO-LA in THP-G8 cells induced by LPS or sensitizers positively correlated with their induction of IL-8 messenger RNA in THP-1 cells. The nSLO-LA value of THP-G8 cells was significantly increased (FInSLO-LA ≥ 1.4) by 13 of the 15 sensitizers as well as by 5 of the 7 nonsensitizers. Interestingly, pretreatment with N-acetylcysteine suppressed the increase in FInSLO-LA induced by all sensitizers (inhibition index (II) ≤ 0.8) but did not suppress that induced by most of the nonsensitizers. We then evaluated the performance of this assay using values of FInSLO-LA ≥ 1.4 and II ≤ 0.8 in at least two of three independent experiments as the criteria of a sensitizer, which resulted in test accuracies of 82% for the 22 chemicals used and of 88% for the chemicals proposed by European Center for the Validation of Alternative Methods. This newly developed assay is a candidate replacement for animal tests of skin sensitization because of its accuracy, convenience, and high throughput performance.

Perspectives on Non-Animal Alternatives for Assessing Sensitization Potential in Allergic Contact Dermatitis

Skin sensitization remains a major environmental and occupational health hazard. Animal models have been used as the gold standard method of choice for estimating chemical sensitization potential. However, a growing international drive and consensus for minimizing animal usage have prompted the development of in vitro methods to assess chemical sensitivity. In this paper, we examine existing approaches including in silico models, cell and tissue based assays for distinguishing between sensitizers and irritants. The in silico approaches that have been discussed include Quantitative Structure Activity Relationships (QSAR) and QSAR based expert models that correlate chemical molecular structure with biological activity and mechanism based read-across models that incorporate compound electrophilicity. The cell and tissue based assays rely on an assortment of mono and co-culture cell systems in conjunction with 3D skin models. Given the complexity of allergen induced immune responses, and the limited ability of existing systems to capture the entire gamut of cellular and molecular events associated with these responses, we also introduce a microfabricated platform that can capture all the key steps involved in allergic contact sensitivity. Finally, we describe the development of an integrated testing strategy comprised of two or three tier systems for evaluating sensitization potential of chemicals.

Hybrid shell engineering of animal cells for immune protections and regulation of drug delivery: towards the design of "artificial organs"

Background

With the progress in medicine, the average human life expectancy is continuously increasing. At the same time, the number of patients who require full organ transplantations is augmenting. Consequently, new strategies for cell transplantation are the subject of great interest.

Methodology/Principal Findings

This work reports the design, the synthesis and the characterisation of robust and biocompatible mineralised beads composed of two layers: an alginate-silica composite core and a Ca-alginate layer. The adequate choice of materials was achieved through cytotoxicity LDH release measurement and in vitro inflammatory assay (IL-8) to meet the biocompatibility requirements for medical purpose. The results obtained following this strategy provide a direct proof of the total innocuity of silica and alginate networks for human cells as underscored by the non-activation of immune defenders (THP-1 monocytes). The accessible pore size diameter of the mineralised beads synthesized was estimated between 22 and 30 nm, as required for efficient immuno-isolation without preventing the diffusion of nutrients and metabolites. The model human cells, HepG2, entrapped within these hybrid beads display a high survival rate over more than six weeks according to the measurements of intracellular enzymatic activity, respiration rate, as well as the “de novo” biosynthesis and secretion of albumin out of the beads.

Conclusions/Significance

The current study shows that active mammalian cells can be protected by a silica-alginate hybrid shell-like system. The functionality of the cell strain can be maintained. Consequently, cells coated with an artificial and a biocompatible mineral shell could respond physiologically within the human body in order to deliver therapeutic agents in a controlled fashion (i.e. insulin), substituting the declining organ functions of the patient.

Active transport of contact allergens in human monocyte-derived dendritic cells is mediated by multidrug resistance related proteins

The multidrug resistance related proteins (MRPs) function as efflux transporters of a variety of large organic anions or their conjugates. In recent studies we demonstrated that antigen-presenting cells express a specific pattern of MRPs. MRP-mediated efflux activity of human monocyte-derived dendritic cells (moDCs) was analyzed using an in vitro transport assay. The efflux transport of radiolabeled contact allergens was inhibited using the specific MRP inhibitor indomethacin. Treatment with indomethacin increased intracellular concentration of [³H] eugenol and [³H] isoeugenol in moDCs. In addition by using MRP1 expressing inside-out membrane vesicles we revealed that the transport of eugenol is mediated by MRP1. Human DCs were employed to assess the sensitizing potential of contact allergens and alters their cytokine gene expression profile. Hence, to survey the functionality of indomethacin after stimulation with contact allergens IL-8 and TRIM16 regulation was measured by a DC-based in vitro assay. Incubation with isoeugenol after pre-treatment with indomethacin leads to increased IL-8 and TRIM16 gene expression. These results strongly support the functional role of MRPs in the active efflux of contact allergens also in antigen-presenting cells like moDCs, a novel mechanism which could possibly play a role in the pathogenesis of contact allergy.

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?

The development of non-animal testing methods for the assessment of skin sensitisation potential is an urgent challenge within the framework of existing and forthcoming legislation. Efforts have been made to replace current animal tests, but so far no alternative methods have been developed. It is widely recognised that alternatives to animal testing cannot be accomplished with a single approach, but rather will require the integration of results obtained from different in vitro and in silico assays. The argument subjacent to the development of in vitro dendritic cell (DC)-based assays is that sensitiser-induced changes in the DC phenotype can be differentiated from those induced by irritants. This assumption is derived from the unique capacity of DC to convert environmental signals encountered at the skin into a receptor expression pattern (MHC class II molecules, co-stimulatory molecules, chemokine receptors) and a soluble mediator release profile that will stimulate T lymphocytes. Since signal transduction cascades precede changes in surface marker expression and cytokine/chemokine secretion, these phenotypic modifications are a consequence of a signal transduction profile that is specifically triggered by sensitisers and not by irritants. A limited number of studies have addressed this subject and the present review attempts to summarise and highlight all of the signalling pathways modulated by skin sensitisers and irritants. Furthermore, we conclude this review by focusing on the most promising strategies suitable for inclusion into a cell-based in vitro alternative approach to hazard identification.

Allergen particle binding by human primary bronchial epithelial cells is modulated by surfactant protein D

Background

Allergen-containing subpollen particles (SPP) are released from whole plant pollen upon contact with water or even high humidity. Because of their size SPP can preferentially reach the lower airways where they come into contact with surfactant protein (SP)-D. Our previous work demonstrated that SP-D increases the uptake of SPP by alveolar macrophages. In the present study, we investigated the uptake of SPP in human primary epithelial cells and the potential modulation by SP-D. The patho-physiological consequence was evaluated by measurement of pro-inflammatory mediators.

Methods

SPP were isolated from timothy grass and subsequently fluorescently labelled. Human primary bronchial epithelial cells were incubated with SPP or polystyrene particles (PP) in the presence and absence of surfactant protein D. In addition, different sizes and surface charges of the PP were studied. Particle uptake was evaluated by flow cytometry and confocal microscopy. Soluble mediators were measured by enzyme linked immunosorbent assay or bead array.

Results

SPP were taken up by primary epithelial cells in a dose dependent manner. This uptake was coincided with secretion of Interleukin (IL)-8. SP-D increased the fraction of bronchial epithelial cells that bound SPP but not the fraction of cells that internalized SPP. SPP-induced secretion of IL-8 was further increased by SP-D. PP were bound and internalized by epithelial cells but this was not modulated by SP-D.

Conclusions

Epithelial cells bind and internalize SPP and PP which leads to increased IL-8 secretion. SP-D promotes attachment of SPP to epithelial cells and may thus be involved in the inflammatory response to inhaled allergen.

Oxidative stress and inflammation response after nanoparticle exposure: differences between human lung cell monocultures and an advanced three-dimensional model of the human epithelial airways

Combustion-derived and manufactured nanoparticles (NPs) are known to provoke oxidative stress and inflammatory responses in human lung cells; therefore, they play an important role during the development of adverse health effects. As the lungs are composed of more than 40 different cell types, it is of particular interest to perform toxicological studies with co-cultures systems, rather than with monocultures of only one cell type, to gain a better understanding of complex cellular reactions upon exposure to toxic substances. Monocultures of A549 human epithelial lung cells, human monocyte-derived macrophages and monocyte-derived dendritic cells (MDDCs) as well as triple cell co-cultures consisting of all three cell types were exposed to combustion-derived NPs (diesel exhaust particles) and to manufactured NPs (titanium dioxide and single-walled carbon nanotubes). The penetration of particles into cells was analysed by transmission electron microscopy. The amount of intracellular reactive oxygen species (ROS), the total antioxidant capacity (TAC) and the production of tumour necrosis factor (TNF)-alpha and interleukin (IL)-8 were quantified. The results of the monocultures were summed with an adjustment for the number of each single cell type in the triple cell co-culture. All three particle types were found in all cell and culture types. The production of ROS was induced by all particle types in all cell cultures except in monocultures of MDDCs. The TAC and the (pro-)inflammatory reactions were not statistically significantly increased by particle exposure in any of the cell cultures. Interestingly, in the triple cell co-cultures, the TAC and IL-8 concentrations were lower and the TNF-alpha concentrations were higher than the expected values calculated from the monocultures. The interplay of different lung cell types seems to substantially modulate the oxidative stress and the inflammatory responses after NP exposure.