Aerosol generation and characterization of multi-walled carbon nanotubes exposed to cells cultured at the air-liquid interface

Modular air-liquid interface aerosol exposure system (MALIES) to study toxicity of nanoparticle aerosols in 3D-cultured A549 cells in vitro

We present a novel lung aerosol exposure system named MALIES (modular air-liquid interface exposure system), which allows three-dimensional cultivation of lung epithelial cells in alveolar-like scaffolds (MatriGrids®) and exposure to nanoparticle aerosols. MALIES consists of multiple modular units for aerosol generation, and can be rapidly assembled and commissioned. The MALIES system was proven for its ability to reliably produce a dose-dependent toxicity in A549 cells using CuSO4 aerosol. Cytotoxic effects of BaSO4- and TiO2-nanoparticles were investigated using MALIES with the human lung tumor cell line A549 cultured at the air-liquid interface. Experiments with concentrations of up to 5.93 × 105 (BaSO4) and 1.49 × 106 (TiO2) particles/cm3, resulting in deposited masses of up to 26.6 and 74.0 µg/cm2 were performed using two identical aerosol exposure systems in two different laboratories. LDH, resazurin reduction and total glutathione were measured. A549 cells grown on MatriGrids® form a ZO-1- and E-Cadherin-positive epithelial barrier and produce mucin and surfactant protein. BaSO4-NP in a deposited mass of up to 26.6 µg/cm2 resulted in mild, reversible damage (~ 10% decrease in viability) to lung epithelium 24 h after exposure. TiO2-NP in a deposited mass of up to 74.0 µg/cm2 did not induce any cytotoxicity in A549 cells 24 h and 72 h after exposure, with the exception of a 1.7 fold increase in the low exposure group in laboratory 1. These results are consistent with previous studies showing no significant damage to lung epithelium by short-term treatment with low concentrations of nanoscale BaSO4 and TiO2 in in vitro experiments.

Assessing the in vitro toxicity of airborne (nano)particles to the human respiratory system: from basic to advanced models

Several studies have been conducted to address the potential adverse health risks attributed to exposure to nanoscale materials. While in vivo studies are fundamental for identifying the relationship between dose and occurrence of adverse effects, in vitro model systems provide important information regarding the mechanism(s) of action at the molecular level. With a special focus on exposure to inhaled (nano)particulate material toxicity assessment, this review provides an overview of the available human respiratory models and exposure systems for in vitro testing, advantages, limitations, and existing investigations using models of different complexity. A brief overview of the human respiratory system, pathway and fate of inhaled (nano)particles is also presented.

The State of the Art and Challenges of In Vitro Methods for Human Hazard Assessment of Nanomaterials in the Context of Safe-by-Design

The Safe-by-Design (SbD) concept aims to facilitate the development of safer materials/products, safer production, and safer use and end-of-life by performing timely SbD interventions to reduce hazard, exposure, or both. Early hazard screening is a crucial first step in this process. In this review, for the first time, commonly used in vitro assays are evaluated for their suitability for SbD hazard testing of nanomaterials (NMs). The goal of SbD hazard testing is identifying hazard warnings in the early stages of innovation. For this purpose, assays should be simple, cost-effective, predictive, robust, and compatible. For several toxicological endpoints, there are indications that commonly used in vitro assays are able to predict hazard warnings. In addition to the evaluation of assays, this review provides insights into the effects of the choice of cell type, exposure and dispersion protocol, and the (in)accurate determination of dose delivered to cells on predictivity. Furthermore, compatibility of assays with challenging advanced materials and NMs released from nano-enabled products (NEPs) during the lifecycle is assessed, as these aspects are crucial for SbD hazard testing. To conclude, hazard screening of NMs is complex and joint efforts between innovators, scientists, and regulators are needed to further improve SbD hazard testing.

Human lung cell models to study aerosol delivery - considerations for model design and development

Human lung tissue models range from simple monolayer cultures to more advanced three-dimensional co-cultures. Each model system can address the interactions of different types of aerosols and the choice of the model and the mode of aerosol exposure depends on the relevant scenario, such as adverse outcomes and endpoints of interest. This review focuses on the functional, as well as structural, aspects of lung tissue from the upper airway to the distal alveolar compartments as this information is relevant for the design of a model as well as how the aerosol properties determine the interfacial properties with the respiratory wall. The most important aspects on how to design lung models are summarized with a focus on (i) choice of appropriate scaffold, (ii) selection of cell types for healthy and diseased lung models, (iii) use of culture condition and assembly, (iv) aerosol exposure methods, and (v) endpoints and verification process. Finally, remaining challenges and future directions in this field are discussed.

U.S. Federal Agency interests and key considerations for new approach methodologies for nanomaterials

Engineered nanomaterials (ENMs) come in a wide array of shapes, sizes, surface coatings, and compositions, and often possess novel or enhanced properties compared to larger sized particles of the same elemental composition. To ensure the safe commercialization of products containing ENMs, it is important to thoroughly understand their potential risks. Given that ENMs can be created in an almost infinite number of variations, it is not feasible to conduct in vivo testing on each type of ENM. Instead, new approach methodologies (NAMs) such as in vitro or in chemico test methods may be needed, given their capacity for higher throughput testing, lower cost, and ability to provide information on toxicological mechanisms. However, the different behaviors of ENMs compared to dissolved chemicals may challenge safety testing of ENMs using NAMs. In this study, member agencies within the Interagency Coordinating Committee on the Validation of Alternative Methods were queried about what types of ENMs are of agency interest and whether there is agency-specific guidance for ENM toxicity testing. To support the ability of NAMs to provide robust results in ENM testing, two key issues in the usage of NAMs, namely dosimetry and interference/bias controls, are thoroughly discussed.

Impact of Air Pollution in Airway Diseases: Role of the Epithelial Cells (Cell Models and Biomarkers)

Biomedical research is multidisciplinary and often uses integrated approaches performing different experimental models with complementary functions. This approach is important to understand the pathogenetic mechanisms concerning the effects of environmental pollution on human health. The biological activity of the substances is investigated at least to three levels using molecular, cellular, and human tissue models. Each of these is able to give specific answers to experimental problems. A scientific approach, using biological methods (wet lab), cell cultures (cell lines or primary), isolated organs (three-dimensional cell cultures of primary epithelial cells), and animal organisms, including the human body, aimed to understand the effects of air pollution on the onset of diseases of the respiratory system. Biological methods are divided into three complementary models: in vitro, ex vivo, and in vivo. In vitro experiments do not require the use of whole organisms (in vivo study), while ex vivo experiments use isolated organs or parts of organs. The concept of complementarity and the informatic support are useful tools to organize, analyze, and interpret experimental data, with the aim of discussing scientific notions with objectivity and rationality in biology and medicine. In this scenario, the integrated and complementary use of different experimental models is important to obtain useful and global information that allows us to identify the effect of inhaled pollutants on the incidence of respiratory diseases in the exposed population. In this review, we focused our attention on the impact of air pollution in airway diseases with a rapid and descriptive analysis on the role of epithelium and on the experimental cell models useful to study the effect of toxicants on epithelial cells.

In Vitro Toxicity of Industrially Relevant Engineered Nanoparticles in Human Alveolar Epithelial Cells: Air-Liquid Interface versus Submerged Cultures

Diverse industries have already incorporated within their production processes engineered nanoparticles (ENP), increasing the potential risk of worker inhalation exposure. In vitro models have been widely used to investigate ENP toxicity. Air-liquid interface (ALI) cell cultures have been emerging as a valuable alternative to submerged cultures as they are more representative of the inhalation exposure to airborne nano-sized particles. We compared the in vitro toxicity of four ENP used as raw materials in the advanced ceramics sector in human alveolar epithelial-like cells cultured under submerged or ALI conditions. Submerged cultures were exposed to ENP liquid suspensions or to aerosolised ENP at ALI. Toxicity was assessed by determining LDH release, WST-1 metabolisation and DNA damage. Overall, cells were more sensitive to ENP cytotoxic effects when cultured and exposed under ALI. No significant cytotoxicity was observed after 24 h exposure to ENP liquid suspensions, although aerosolised ENP clearly affected cell viability and LDH release. In general, all ENP increased primary DNA damage regardless of the exposure mode, where an increase in DNA strand-breaks was only detected under submerged conditions. Our data show that at relevant occupational concentrations, the selected ENP exert mild toxicity to alveolar epithelial cells and exposure at ALI might be the most suitable choice when assessing ENP toxicity in respiratory models under realistic exposure conditions.

An Integrated Approach to Testing and Assessment to Support Grouping and Read-Across of Nanomaterials After Inhalation Exposure

Introduction: Here, we describe the generation of hypotheses for grouping nanoforms (NFs) after inhalation exposure and the tailored Integrated Approaches to Testing and Assessment (IATA) with which each specific hypothesis can be tested. This is part of a state-of-the-art framework to support the hypothesis-driven grouping and read-across of NFs, as developed by the EU-funded Horizon 2020 project GRACIOUS.

Development of Grouping Hypotheses and IATA: Respirable NFs, depending on their physicochemical properties, may dissolve either in lung lining fluid or in acidic lysosomal fluid after uptake by cells. Alternatively, NFs may also persist in particulate form. Dissolution in the lung is, therefore, a decisive factor for the toxicokinetics of NFs. This has led to the development of four hypotheses, broadly grouping NFs as instantaneous, quickly, gradually, and very slowly dissolving NFs. For instantaneously dissolving NFs, hazard information can be derived by read-across from the ions. For quickly dissolving particles, as accumulation of particles is not expected, ion toxicity will drive the toxic profile. However, the particle aspect influences the location of the ion release. For gradually dissolving and very slowly dissolving NFs, particle-driven toxicity is of concern. These NFs may be grouped by their reactivity and inflammation potency. The hypotheses are substantiated by a tailored IATA, which describes the minimum information and laboratory assessments of NFs under investigation required to justify grouping.

Conclusion: The GRACIOUS hypotheses and tailored IATA for respiratory toxicity of inhaled NFs can be used to support decision making regarding Safe(r)-by-Design product development or adoption of precautionary measures to mitigate potential risks. It can also be used to support read-across of adverse effects such as pulmonary inflammation and subsequent downstream effects such as lung fibrosis and lung tumor formation after long-term exposure.

Cytotoxicity analysis of biomass combustion particles in human pulmonary alveolar epithelial cells on an air-liquid interface/dynamic culture platform

BACKGROUND

Exposure to indoor air pollution from solid fuel combustion is associated with lung diseases and cancer. This study investigated the cytotoxicity and molecular mechanisms of biomass combustion-derived particles in human pulmonary alveolar epithelial cells (HPAEpiC) using a platform that combines air-liquid interface (ALI) and dynamic culture (DC) systems.

METHODS

HPAEpiC were cultured on the surface of polycarbonate (PC) membranes on the ALI-DC platform. The cells were sprayed with an aerosolized solution of biomass combustion soluble constituents (BCSCs) and simultaneously nourished with culture medium flowing beneath the permeable PC membranes. The ALI-DC method was compared with the traditional submerged culture approach. BCSC particle morphology and dosages deposited on the chip were determined for particle characterization. Flow cytometry, scanning electron microscopy, and transmission electron microscopy were used to investigate the apoptosis rate of HPAEpiC and changes in the cell ultrastructure induced by BCSCs. Additionally, the underlying apoptotic pathway was examined by determining the protein expression levels by western blotting.

RESULTS

Scanning electron microscope images demonstrated that the sample processing and delivering approach of the ALI-DC platform were suitable for pollutant exposure. Compared with the submerged culture method, a significant decline in cell viability and increase in apoptosis rate was observed after BCSC exposure on the ALI-DC platform, indicating that the ALI-DC platform is a more sensitive system for investigating cytotoxicity of indoor air pollutants in lung cells. The morphology and ultrastructure of the cells were damaged after exposure to BCSCs, and the p53 pathway was activated. The Bcl-2/Bax ratio was reduced, upregulating caspase-9 and caspase-3 expression and subsequently inducing apoptosis of HPAEpiC. The addition of N-acetyl cysteine antioxidant significantly alleviated the cytotoxicity induced by BCSCs.

CONCLUSION

A novel ALI-DC platform was developed to study the cytotoxicity of air pollutants on lung cells. Using the platform, we demonstrated that BCSCs could damage the mitochondria, produce reactive oxygen species, and activate p53 in HPAEpiC, ultimately inducing apoptosis.

Use of Cause-and-Effect Analysis to Optimize the Reliability of In Vitro Inhalation Toxicity Measurements Using an Air-Liquid Interface

In vitro inhalation toxicology methods are increasingly being used for research and regulatory purposes. Although the opportunity for increased human relevance of in vitro inhalation methods compared to in vivo tests has been established and discussed, how to systematically account for variability and maximize the reliability of these in vitro methods, especially for assays that use cells cultured at an air-liquid interface (ALI), has received less attention. One tool that has been used to evaluate the robustness of in vitro test methods is cause-and-effect (C&E) analysis, a conceptual approach to analyze key sources of potential variability in a test method. These sources of variability can then be evaluated using robustness testing and potentially incorporated into in-process control measurements in the assay protocol. There are many differences among in vitro inhalation test methods including the use of different types of biological test systems, exposure platforms/conditions, substances tested, and end points, which represent a major challenge for use in regulatory testing. In this manuscript, we describe how C&E analysis can be applied using a modular approach based on the idea that shared components of different test methods (e.g., the same exposure system is used) have similar sources of variability even though other components may differ. C&E analyses of different in vitro inhalation methods revealed a common set of recommended exposure systems and biological in-process control measurements. The approach described here, when applied in conjunction with Good Laboratory Practices (GLP) criteria, should help improve the inter- and intralaboratory agreement of in vitro inhalation test results, leading to increased confidence in these methods for regulatory and research purposes.

Air-liquid interface cultures of the healthy and diseased human respiratory tract: promises, challenges and future directions

Air-liquid interface (ALI) culture models currently represent a valid instrument to recreate the typical aspects of the respiratory tract in vitro in both healthy and diseased state. They can help reducing the number of animal experiments, therefore, supporting the 3R principle. This review discusses ALI cultures and co-cultures derived from immortalized as well as primary cells, which are used to study the most common disorders of the respiratory tract, in terms of both pathophysiology and drug screening. The article displays ALI models used to simulate inflammatory lung diseases such as chronic obstructive pulmonary disease (COPD), asthma, cystic fibrosis, lung cancer, and viral infections. It also includes a focus on ALI cultures described in literature studying respiratory viruses such as SARS-CoV-2 causing the global Covid-19 pandemic at the time of writing this review. Additionally, commercially available models of ALI cultures are presented. Ultimately, the aim of this review is to provide a detailed overview of ALI models currently available and to critically discuss them in the context of the most prevalent diseases of the respiratory tract.

Non-Animal Strategies for Toxicity Assessment of Nanoscale Materials: Role of Adverse Outcome Pathways in the Selection of Endpoints

Faster, cheaper, sensitive, and mechanisms-based animal alternatives are needed to address the safety assessment needs of the growing number of nanomaterials (NM) and their sophisticated property variants. Specifically, strategies that help identify and prioritize alternative schemes involving individual test models, toxicity endpoints, and assays for the assessment of adverse outcomes, as well as strategies that enable validation and refinement of these schemes for the regulatory acceptance are needed. In this review, two strategies 1) the current nanotoxicology literature review and 2) the adverse outcome pathways (AOPs) framework, a systematic process that allows the assembly of available mechanistic information concerning a toxicological response in a simple modular format, are presented. The review highlights 1) the most frequently assessed and reported ad hoc in vivo and in vitro toxicity measurements in the literature, 2) various AOPs of relevance to inhalation toxicity of NM that are presently under development, and 3) their applicability in identifying key events of toxicity for targeted in vitro assay development. Finally, using an existing AOP for lung fibrosis, the specific combinations of cell types, exposure and test systems, and assays that are experimentally supported and thus, can be used for assessing NM-induced lung fibrosis, are proposed.

Invited review: human air-liquid-interface organotypic airway tissue models derived from primary tracheobronchial epithelial cells-overview and perspectives

The lung is an organ that is directly exposed to the external environment. Given the large surface area and extensive ventilation of the lung, it is prone to exposure to airborne substances, such as pathogens, allergens, chemicals, and particulate matter. Highly elaborate and effective mechanisms have evolved to protect and maintain homeostasis in the lung. Despite these sophisticated defense mechanisms, the respiratory system remains highly susceptible to environmental challenges. Because of the impact of respiratory exposure on human health and disease, there has been considerable interest in developing reliable and predictive in vitro model systems for respiratory toxicology and basic research. Human air-liquid-interface (ALI) organotypic airway tissue models derived from primary tracheobronchial epithelial cells have in vivo–like structure and functions when they are fully differentiated. The presence of the air-facing surface allows conducting in vitro exposures that mimic human respiratory exposures. Exposures can be conducted using particulates, aerosols, gases, vapors generated from volatile and semi-volatile substances, and respiratory pathogens. Toxicity data have been generated using nanomaterials, cigarette smoke, e-cigarette vapors, environmental airborne chemicals, drugs given by inhalation, and respiratory viruses and bacteria. Although toxicity evaluations using human airway ALI models require further standardization and validation, this approach shows promise in supplementing or replacing in vivo animal models for conducting research on respiratory toxicants and pathogens.

An In Vitro Lung System to Assess the Proinflammatory Hazard of Carbon Nanotube Aerosols

In vitro three-dimensional (3D) lung cell models have been thoroughly investigated in recent years and provide a reliable tool to assess the hazard associated with nanomaterials (NMs) released into the air. In this study, a 3D lung co-culture model was optimized to assess the hazard potential of multiwalled carbon nanotubes (MWCNTs), which is known to provoke inflammation and fibrosis, critical adverse outcomes linked to acute and prolonged NM exposure. The lung co-cultures were exposed to MWCNTs at the air-liquid interface (ALI) using the VITROCELL^® Cloud system while considering realistic occupational exposure doses. The co-culture model was composed of three human cell lines: alveolar epithelial cells (A549), fibroblasts (MRC-5), and macrophages (differentiated THP-1). The model was exposed to two types of MWCNTs (Mitsui-7 and Nanocyl) at different concentrations (2–10 μg/cm²) to assess the proinflammatory as well as the profibrotic responses after acute (24 h, one exposure) and prolonged (96 h, repeated exposures) exposure cycles. The results showed that acute or prolonged exposure to different concentrations of the tested MWCNTs did not induce cytotoxicity or apparent profibrotic response; however, suggested the onset of proinflammatory response.

Use of EpiAlveolar Lung Model to Predict Fibrotic Potential of Multiwalled Carbon Nanotubes

Expansion in production and commercial use of nanomaterials increases the potential human exposure during the lifecycle of these materials (production, use, and disposal). Inhalation is a primary route of exposure to nanomaterials; therefore it is critical to assess their potential respiratory hazard. Herein, we developed a three-dimensional alveolar model (EpiAlveolar) consisting of human primary alveolar epithelial cells, fibroblasts, and endothelial cells, with or without macrophages for predicting long-term responses to aerosols. Following thorough characterization of the model, proinflammatory and profibrotic responses based on the adverse outcome pathway concept for lung fibrosis were assessed upon repeated subchronic exposures (up to 21 days) to two types of multiwalled carbon nanotubes (MWCNTs) and silica quartz particles. We simulate occupational exposure doses for the MWCNTs (1-30 μg/cm²) using an air-liquid interface exposure device (VITROCELL Cloud) with repeated exposures over 3 weeks. Specific key events leading to lung fibrosis, such as barrier integrity and release of proinflammatory and profibrotic markers, show the responsiveness of the model. Nanocyl induced, in general, a less pronounced reaction than Mitsui-7, and the cultures with human monocyte-derived macrophages (MDMs) showed the proinflammatory response at later time points than those without MDMs. In conclusion, we present a robust alveolar model to predict inflammatory and fibrotic responses upon exposure to MWCNTs.

Environmental and toxicological assessment of nanodiamond-like materials derived from carbonaceous aerosols

Carbonaceous aerosols (CAs) are ubiquitous and among the most significant environmental materials found in ambient air, mainly derived from anthropogenic sources (biomass burning, industrial activity, vehicle emissions, etc.). Elemental carbon (black carbon) and organic carbons are the major constituents of CAs. Due to their toxic effects, they are considered as high-risk compounds for human health. The key objective of the present work is to conduct a feasibility study for the conversion of CAs (TSP and PM₁₀) into a value-added carbon nanostructured product by using a chemical method. High resolution-transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), Raman spectroscopy, Fourier transforms infrared spectroscopy (FT-IR), X-ray photoelectron spectrometer (XPS), ultraviolet-visible spectroscopy (UV-visible), fluorescence spectroscopy (FL), and Zeta potential analyses indicated the formation of carbon nanomaterials with crystalline phases, which exhibit the characteristics of nanodiamonds (NDs). The HR-TEM image analysis showed that the nominal size of the CAs-derived NDs ranged from 4 to 17 nm composed of mainly carbon and oxygen. The FT-IR and XPS analysis indicated that the NDs are highly functionalized with an oxygen-containing functional group. The CAs-derived NDs showed the property of blue-fluorescence with excitation dependent. In the cytotoxicity and genotoxicity study, the NDs obtained was observed to be biocompatible and suitable for bioimaging applications. This result provides a new avenue for the conversion of CAs to high-value products leading to the mitigation of atmospheric pollution.

Development of a novel aerosol generation system for conducting inhalation exposures to ambient particulate matter (PM)

In this study, we developed a novel method for generating aerosols that are representative of real-world ambient particulate matter (PM) in terms of both physical and chemical characteristics, with the ultimate objective of using them for inhalation exposure studies. The protocol included collection of ambient PM on filters using a high-volume sampler, which were then extracted with ultrapure Milli-Q water using vortexing and sonication. As an alternative approach for collection, ambient particles were directly captured into aqueous slurry samples using the versatile aerosol concentration enrichment system (VACES)/aerosol-into-liquid collector tandem technology. The aqueous samples from both collection protocols were then re-aerosolized using commercially available nebulizers. The physical characteristics (i.e., particle size distribution) of the generated aerosols were examined by the means of a scanning mobility particle sizer (SMPS) connected to a condensation particle counter (CPC) at different compressed air pressures of the nebulizer, and dilution air flow rates. In addition, the collected PM samples (both ambient and re-aerosolized) were chemically analyzed for water-soluble organic carbon (WSOC), elemental and organic carbon (EC/OC), inorganic ions, polycyclic aromatic hydrocarbons (PAHs), and metals and trace elements. Using the aqueous filter extracts, we were able to effectively recover the water-soluble components of ambient PM (e.g., water-soluble organic matter, and water-soluble inorganic ions); however, this method was deficient in recovering some of the important insoluble components such as EC, PAHs, and many of the redox-active trace elements and metals. In contrast, using the VACES/aerosol-into-liquid collector tandem technology for collecting ambient PM directly into water slurry, we were able to preserve the water-soluble and water-insoluble components very effectively. These results illustrate the superiority of the VACES/aerosol-into liquid collector tandem technology to be used in conjunction with the re-aerosolization setup to create aerosols that fully represent ambient PM, making it an attractive choice for application in inhalation exposure studies.

An Air-Liquid Interface Organ-Level Lung Microfluidics Platform for Analysis on Molecular Mechanisms of Cytotoxicity Induced by Cancer-Causing Fine Particles

Fine particulate matter less than 2.5 μm in diameter (PM_2.5) is regarded as a carcinogenic factor, but the mechanism has been left unexplored. Our goal was to reveal the carcinogenic mechanism at the gene and protein level under the inhalational air-liquid interface (ALI) condition. Herein, we developed an ALI organ-level lung microfluidic platform (ALI-OLMP) carrying lung epithelial cell line BEAS-2B and human pulmonary microvascular endothelial cells (HPMEC); the cell viability was above 98% within 14 days on this system, which was used to mimic the practical alveolar microenvironment for the multiomics analysis, to identify the global gene and protein expression after exposure to PM_2.5 in Shanghai, China from 2014 to 2015. The combined RNA-Seq and iTRAQ analysis indicated that the unique set was 2532 genes at 10 μg/cm² of PM_2.5, and there were also at least 25 identical activated signal transduction cascades including bladder cancer, transcriptional dysregulation in cancer, the TP53 (p53) signaling pathway, Jak-STAT signaling pathway, and PI3K-Akt signaling pathway, which could lead to blocking of differentiation, cell proliferation and survival, and sustained angiogenesis. The images obtained by the transmission electron microscopy (TEM) showed that the particles could enter the mitochondria, and even get into the nucleus. The Pearson's correlation coefficient test elucidated that inorganics (EC), organics (OC, PAHs, and alkane), and metals (Cr, Mn, and Sb) were significantly correlated to the dysregulated oncoproteins (VEGF, IL6, MDM2, AKT1, STAT, and P53). The findings may to some extent explain the molecular mechanism of carcinogenicity caused by fine-particle exposure.

Pathway-based predictive approaches for non-animal assessment of acute inhalation toxicity

New approaches are needed to assess the effects of inhaled substances on human health. These approaches will be based on mechanisms of toxicity, an understanding of dosimetry, and the use of in silico modeling and in vitro test methods. In order to accelerate wider implementation of such approaches, development of adverse outcome pathways (AOPs) can help identify and address gaps in our understanding of relevant parameters for model input and mechanisms, and optimize non-animal approaches that can be used to investigate key events of toxicity. This paper describes the AOPs and the toolbox of in vitro and in silico models that can be used to assess the key events leading to toxicity following inhalation exposure. Because the optimal testing strategy will vary depending on the substance of interest, here we present a decision tree approach to identify an appropriate non-animal integrated testing strategy that incorporates consideration of a substance's physicochemical properties, relevant mechanisms of toxicity, and available in silico models and in vitro test methods. This decision tree can facilitate standardization of the testing approaches. Case study examples are presented to provide a basis for proof-of-concept testing to illustrate the utility of non-animal approaches to inform hazard identification and risk assessment of humans exposed to inhaled substances.

Through the Looking Glass: In Vitro Models for Inhalation Toxicology and Interindividual Variability in the Airway

With 7 million deaths reported annually from air pollution alone, it is evident that adverse effects of inhaled toxicant exposures remain a major public health concern in the 21st century. Assessment and characterization of the impacts of air pollutants on human health stems from epidemiological and clinical studies, which have linked both outdoor and indoor air contaminant exposure to adverse pulmonary and cardiovascular health outcomes. Studies in animal models support epidemiological findings and have been critical in identifying systemic effects of environmental chemicals on cognitive abilities, liver disease, and metabolic dysfunction following inhalation exposure. Likewise, traditional monoculture systems have aided in identifying biomarkers of susceptibility to inhaled toxicants and served as a screening platform for safety assessment of pulmonary toxicants. Despite their contributions, in vivo and classic in vitro models have not been able to accurately represent the heterogeneity of the human population and account for interindividual variability in response to inhaled toxicants and susceptibility to the adverse health effects. Development of new technologies that can investigate genetic predisposition, are cost and time efficient, and are ethically sound, will enhance elucidation of mechanisms of inhalation toxicity, and aid in the development of novel pharmaceuticals and/or safety evaluation. This review will describe the classic and novel cell-based inhalation toxicity models and how these emerging technologies can be incorporated into regulatory or nonregulatory testing to address interindividual variability and improve overall human health.

The impact of nanomaterial characteristics on inhalation toxicity

During the last few decades, nanotechnology has evolved into a success story, apparent from a steadily increasing number of scientific publications as well as a large number of applications based on engineered nanomaterials (ENMs). Its widespread uses suggest a high relevance for consumers, workers and the environment, hence justifying intensive investigations into ENM-related adverse effects as a prerequisite for nano-specific regulations. In particular, the inhalation of airborne ENMs, being assumed to represent the most hazardous type of human exposure to these kinds of particles, needs to be scrutinized. Due to an increased awareness of possible health effects, which have already been seen in the case of ultrafine particles (UFPs), research and regulatory measures have set in to identify and address toxic implications following their almost ubiquitous occurrence. Although ENM properties differ from those of the respective bulk materials, the available assessment protocols are often designed for the latter. Despite the large benefit ensuing from the application of nanotechnology, many issues related to ENM behavior and adverse effects are not fully understood or should be examined anew. The traditional hypothesis that ENMs exhibit different or additional hazards due to their "nano" size has been challenged in recent years and ENM categorization according to their properties and toxicity mechanisms has been proposed instead. This review summarizes the toxicological effects of inhaled ENMs identified to date, elucidating the modes of action which provoke different mechanisms in the respiratory tract and their resulting effects. By linking particular mechanisms and adverse effects to ENM properties, grouping of ENMs based on toxicity-related properties is supposed to facilitate toxicological risk assessment. As intensive studies are still required to identify these "ENM classes", the need for alternatives to animal studies is evident and advances in cell-based test systems for pulmonary research are presented here. We hope to encourage the ongoing discussion about ENM risks and to advocate the further development and practice of suitable testing and grouping methods.

Application of a direct aerosol exposure system for the assessment of biological effects of cigarette smoke and novel tobacco product vapor on human bronchial epithelial cultures

Recent advancements in in vitro exposure systems and cell culture technology enable direct exposure to cigarette smoke (CS) of human organotypic bronchial epithelial cultures. MucilAir organotypic bronchial epithelial cultures were exposed, using a Vitrocell exposure system, to mainstream aerosols from the 3R4F cigarette or from a recently developed novel tobacco vapor product (NTV). The exposure aerosol dose was controlled by dilution flow and the number of products smoked; there were five exposure conditions for 3R4F smoke and three for NTV vapor. The amount of nicotine delivered to the tissues under each condition was analyzed and that of the total particulate matter (TPM) was estimated using nicotine data. The nicotine dose was similar for the two products at the highest dose, but the estimated TPM levels from the NTV were 3.7 times the levels from the 3R4F. Following 3R4F smoke exposure, a dose dependent increase was observed in cytotoxicity, cytokine secretion, and differential gene expression. However, no changes were detected in these endpoints following NTV vapor exposure, suggesting the biological effects of NTV vapor are lower than those of conventional combustible CS. Our study design, which includes collection of biological data and dosimetry data, is applicable to assessing novel tobacco products.

Carbon Nanotubes and Other Engineered Nanoparticles Induced Pathophysiology on Mesothelial Cells and Mesothelial Membranes

Nanoparticles have great potential for numerous applications due to their unique physicochemical properties. However, concerns have been raised that they may induce deleterious effects on biological systems. There is accumulating evidence that, like asbestos, inhaled nanomaterials of >5 μm and high aspect ratio (3:1), particularly rod-like carbon nanotubes, may inflict pleural disease including mesothelioma. Additionally, a recent set of case reports suggests that inhalation of polyacrylate/nanosilica could in part be associated with inflammation and fibrosis of the pleura of factory workers. However, the adverse outcomes of nanoparticle exposure to mesothelial tissues are still largely unexplored. In that context, the present review aims to provide an overview of the relevant pathophysiological implications involving toxicological studies describing effects of engineered nanoparticles on mesothelial cells and membranes. In vitro studies primarily emphasize on simulating cellular uptake and toxicity of nanotubes on benign or malignant cell lines. On the other hand, in vivo studies focus on illustrating endpoints of serosal pathology in rodent animal models. From a molecular aspect, some nanoparticle categories are shown to be cytotoxic and genotoxic after acute treatment, whereas chronic incubation may lead to malignant-like transformation. At an organism level, a number of fibrous shaped nanotubes are related with features of chronic inflammation and MWCNT-7 is the only type to consistently inflict mesothelioma.

Alternative approaches for acute inhalation toxicity testing to address global regulatory and non-regulatory data requirements: An international workshop report

Inhalation toxicity testing, which provides the basis for hazard labeling and risk management of chemicals with potential exposure to the respiratory tract, has traditionally been conducted using animals. Significant research efforts have been directed at the development of mechanistically based, non-animal testing approaches that hold promise to provide human-relevant data and an enhanced understanding of toxicity mechanisms. A September 2016 workshop, "Alternative Approaches for Acute Inhalation Toxicity Testing to Address Global Regulatory and Non-Regulatory Data Requirements", explored current testing requirements and ongoing efforts to achieve global regulatory acceptance for non-animal testing approaches. The importance of using integrated approaches that combine existing data with in vitro and/or computational approaches to generate new data was discussed. Approaches were also proposed to develop a strategy for identifying and overcoming obstacles to replacing animal tests. Attendees noted the importance of dosimetry considerations and of understanding mechanisms of acute toxicity, which could be facilitated by the development of adverse outcome pathways. Recommendations were made to (1) develop a database of existing acute inhalation toxicity data; (2) prepare a state-of-the-science review of dosimetry determinants, mechanisms of toxicity, and existing approaches to assess acute inhalation toxicity; (3) identify and optimize in silico models; and (4) develop a decision tree/testing strategy, considering physicochemical properties and dosimetry, and conduct proof-of-concept testing. Working groups have been established to implement these recommendations.

Direct fluorescent labeling for efficient biological assessment of inhalable particles

Labeling of aerosol particles with a radioactive, magnetic, or optical tracer has been employed to confirm particle localization in cell compartments, which has provided useful evidence for correlating toxic effects of inhaled particles. However, labeling requires several physicochemical steps to identify functionalities of the inner or outer surfaces of particles, and moreover, these steps can cause changes in size, surface charge, and bioactivity of the particles, resulting in misinterpretations regarding their toxic effects. This study addresses this challenging issue with a goal of introducing an efficient strategy for constantly supplying labeled aerosol particles in a single-pass configuration without any pre- or post-physicochemical batch treatments of aerosol particles. Carbon black (CB, simulating combustion-generated soot) or calcium carbonate (CC, simulating brake-wear fragments) particles were constantly produced via spark ablation or bubble bursting, respectively. These minute particles were incorporated with fluorescein isothiocyanate-poly(ethylene glycol) 2-aminoethyl ether acetic acid solution at the orifice of a collison atomizer to fabricate hybrid droplets. The droplets successively entered a diffusion dryer containing 254-nm UV irradiation; therefore, the droplets were dynamically stiffened by UV to form fluorescent probes on particles during solvent extraction in the dryer. Particle size distributions, morphologies, and surface charges before and after labeling were measured to confirm fluorescence labeling without significant changes in the properties. In vitro assays, including confocal imaging, were conducted to confirm the feasibility of the labeling approach without inducing significant differences in bioactivity compared with untreated CB or CC particles.

A multi-faceted approach to achieving the global acceptance of animal-free research methods

In 2015, the PETA International Science Consortium Ltd. was awarded the Lush Training Prize for its broad approach to education and training on the effective use of human-relevant, non-animal research techniques. The prize was awarded for work that included hosting workshops and webinars, initiating in-person training sessions and developing educational resources. The Consortium works closely with industry and regulatory agencies to identify and overcome barriers to the validation and use of alternatives to animal testing, by using an approach that identifies, promotes and verifies the implementation of these methods. The Consortium's recent activities toward replacing tests on animals for nanomaterials, pesticides and medical devices, are described, as examples of projects with broad applicability aimed at large-scale regulatory change.