Quartz crystal microbalances (QCM) are suitable for real-time dosimetry in nanotoxicological studies using VITROCELL®Cloud cell exposure systems

Pro-inflammatory and genotoxic responses by metal oxide nanomaterials in alveolar epithelial cells and macrophages in submerged condition and air-liquid interface: An in vitro-in vivo correlation study

Studies on in vitro-in vivo correlations of inflammatory and genotoxic responses are needed to advance new approach methodologies. Here, we assessed pro-inflammatory and genotoxic responses by 13 nanosized metal oxides (nMeOx) and quartz (DQ12) in alveolar epithelial cells (A549) and macrophages (THP-1a) exposed in submerged conditions, and in A549:THP-1a co-cultures in air-liquid interface (ALI) system. Soluble nMeOx produced the highest IL-8 expression in A549 and THP-1a cells in submerged conditions (≥2-fold, p < 0.05), whereas only CuO caused a strong response in co-cultures exposed in the ALI system (13-fold, p < 0.05). IL-8 expression in A549 cells with concentrations as nMeOx specific surface area (SSA) correlated with neutrophil influx in mice (r = 0.89-0.98, p < 0.05). Similarly, IL-8 expression in THP-1a cell with concentrations as mass and SSA (when excluding soluble nMeOx) correlated with neutrophil influx in mice (r = 0.81-0.84, p < 0.05). DNA strand breaks (SB) was measured by the comet assay. We used a scoring system that categorizes effects in standard deviation units for comparison of genotoxicity in different models. Concordant genotoxicity was observed between SB levels in vitro (A549 and co-culture) and in vivo (broncho-alveolar lavage fluid cells and lung tissue). In conclusion, this study shows in vitro-in vivo correlations of nMeOx-induced inflammatory and genotoxic responses.

Determining the toxicological effects of indoor air pollution on both a healthy and an inflammatory-comprised model of the alveolar epithelial barrier in vitro

Exposure to indoor air pollutants (IAP) has increased recently, with people spending more time indoors (i.e. homes, offices, schools and transportation). Increased exposures of IAP on a healthy population are poorly understood, and those with allergic respiratory conditions even less so. The objective of this study, therefore, was to implement a well-characterised in vitro model of the human alveolar epithelial barrier (A549 + PMA differentiated THP-1 incubated with and without IL-13, IL-5 and IL-4) to determine the effects of a standardised indoor particulate (NIST 2583) on both a healthy lung model and one modelling a type-II (stimulated with IL-13, IL-5 and IL-4) inflammatory response (such as asthma).Using concentrations from the literature, and an environmentally appropriate exposure we investigated 232, 464 and 608ng/cm2 of NIST 2583 respectively. Membrane integrity (blue dextran), viability (trypan blue), genotoxicity (micronucleus (Mn) assay) and (pro-)/(anti-)inflammatory effects (IL-6, IL-8, IL-33, IL-10) were then assessed 24 h post exposure to both models. Models were exposed using a physiologically relevant aerosolisation method (VitroCell Cloud 12 exposure system).No changes in Mn frequency or membrane integrity in either model were noted when exposed to any of the tested concentrations of NIST 2583. A significant decrease (p < 0.05) in cell viability at the highest concentration was observed in the healthy model. Whilst cell viability in the "inflamed" model was decreased at the lower concentrations (significantly (p < 0.05) after 464ng/cm2). A significant reduction (p < 0.05) in IL-10 and a significant increase in IL-33 was seen after 24 h exposure to NIST 2583 (464, 608ng/cm2) in the "inflamed" model.Collectively, the results indicate the potential for IAP to cause the onset of a type II response as well as exacerbating pre-existing allergic conditions. Furthermore, the data imposes the importance of considering unhealthy individuals when investigating the potential health effects of IAP. It also highlights that even in a healthy population these particles have the potential to induce this type II response and initiate an immune response following exposure to IAP.

Analysis of the response to cigarette smoke exposure in cell coculture and monoculture based on bionic-lung microfluidic chips

BACKGROUND

In vitro toxicity assessment studies with various experimental models and exposure modalities frequently generate diverse outcomes. In the prevalent experimental, aerosol pollutants are dissolved in culture medium through capture for exposure to two-dimensional planar cellular models in multiwell plates via immersion. However, this approach can generate restricted and inconclusive experimental data, significantly constraining the applicability of risk assessment outcomes. Herein, the in vitro cocultivation of lung epithelial and/or vascular endothelial cells was performed using self-designed bionic-lung microfluidic chip housing a gas-concentration gradient generator (GCGG) unit. Exposure experiments involving a concentration gradient of cigarette smoke (CS) aerosol were then conducted through an original assembled real-time aerosol exposure system.

RESULTS

Transcriptomic analysis revealed a potential involvement of the cGMP-signaling pathway following online CS aerosol exposure on different cell culture models. Furthermore, distinct responses to different concentrations of CS aerosol exposure on different culture models were highlighted by detecting inflammation- and oxidative stress-related biomarkers (i.e., cell viability, reactive oxygen species, nitric oxide, IL-6, IL-8, TNF-α, GM-CSF, malondialdehyde, and superoxide dismutase).

SIGNIFICANT

The results underscore the importance of improving chip biomimicry while addressing multi-throughput demands, given the substantial influence of the coculture model on cellular responses triggered by CS. Furthermore, the coculture model exhibited a mutually beneficial protective effect on cells at low CS concentrations within the GCGG unit, yet revealed a mutually amplified damaging effect at higher CS concentrations in contrast to the monoculture model.

The use of a complex tetra-culture alveolar model to study the biological effects induced by gold nanoparticles with different physicochemical properties

A substantial increase in engineered nanoparticles in consumer products has been observed, heightening human and environmental exposure. Inhalation represents the primary route of human exposure, necessitating a focus on lung toxicity studies. However, to avoid ethical concerns the use of in vitro models is an efficient alternative to in vivo models. This study utilized an in vitro human alveolar barrier model at air-liquid-interface with four cell lines, for evaluating the biological effects of different gold nanoparticles. Exposure to PEGylated gold nanospheres, nanorods, and nanostars did not significantly impact viability after 24 h, yet all AuNPs induced cytotoxicity in the form of membrane integrity impairment. Gold quantification revealed cellular uptake and transport. Transcriptomic analysis identified gene expression changes, particularly related to the enhancement of immune cells. Despite limited impact, distinct effects were observed, emphasizing the influence of nanoparticles physicochemical parameters while demonstrating the model's efficacy in investigating particle biological effects.

How to use an in vitro approach to characterize the toxicity of airborne compounds

As part of the development of new approach methodologies (NAMs), numerous in vitro methods are being developed to characterize the potential toxicity of inhalable xenobiotics (gases, volatile organic compounds, polycyclic aromatic hydrocarbons, particulate matter, nanoparticles). However, the materials and methods employed are extremely diverse, and no single method is currently in use. Method standardization and validation would raise trust in the results and enable them to be compared. This four-part review lists and compares biological models and exposure methodologies before describing measurable biomarkers of exposure or effect. The first section emphasizes the importance of developing alternative methods to reduce, if not replace, animal testing (3R principle). The biological models presented are mostly to cultures of epithelial cells from the respiratory system, as the lungs are the first organ to come into contact with air pollutants. Monocultures or cocultures of primary cells or cell lines, as well as 3D organotypic cultures such as organoids, spheroids and reconstituted tissues, but also the organ(s) model on a chip are examples. The exposure methods for these biological models applicable to airborne compounds are submerged, intermittent, continuous either static or dynamic. Finally, within the restrictions of these models (i.e. relative tiny quantities, adhering cells), the mechanisms of toxicity and the phenotypic markers most commonly examined in models exposed at the air-liquid interface (ALI) are outlined.

The BioCascade-VIVAS system for collection and delivery of virus-laden size-fractionated airborne particles

The size of virus-laden particles determines whether aerosol or droplet transmission is dominant in the airborne transmission of pathogens. Determining dominant transmission pathways is critical to implementing effective exposure risk mitigation strategies. The aerobiology discipline greatly needs an air sampling system that can collect virus-laden airborne particles, separate them by particle diameter, and deliver them directly onto host cells without inactivating virus or killing cells. We report the use of a testing system that combines a BioAerosol Nebulizing Generator (BANG) to aerosolize Human coronavirus (HCoV)-OC43 (OC43) and an integrated air sampling system comprised of a BioCascade impactor (BC) and Viable Virus Aerosol Sampler (VIVAS), together referred to as BC-VIVAS, to deliver the aerosolized virus directly onto Vero E6 cells. Particles were collected into four stages according to their aerodynamic diameter (Stage 1: >9.43 μm, Stage 2: 3.81-9.43 μm, Stage 3: 1.41-3.81 μm and Stage 4: <1.41 μm). OC43 was detected by reverse-transcription quantitative polymerase chain reaction (RT-qPCR) analyses of samples from all BC-VIVAS stages. The calculated OC43 genome equivalent counts per cm3 of air ranged from 0.34±0.09 to 70.28±12.56, with the highest concentrations in stage 3 (1.41-3.81 μm) and stage 4 (<1.41 μm). Virus-induced cytopathic effects appeared only in cells exposed to particles collected in stages 3 and 4, demonstrating the presence of viable OC43 in particles <3.81 μm. This study demonstrates the dual utility of the BC-VIVAS as particle size-fractionating air sampler and a direct exposure system for aerosolized viruses. Such utility may help minimize conventional post-collection sample processing time required to assess the viability of airborne viruses and increase the understanding about transmission pathways for airborne pathogens.

Impact of mucus modulation by N-acetylcysteine on nanoparticle toxicity

Human respiratory mucus is a biological hydrogel that forms a protective barrier for the underlying epithelium. Modulation of the mucus layer has been employed as a strategy to enhance transmucosal drug carrier transport. However, a drawback of this strategy is a potential reduction of the mucus barrier properties, in particular in situations with an increased exposure to particles. In this study, we investigated the impact of mucus modulation on its protective role. In vitro mucus was produced by Calu-3 cells, cultivated at the air-liquid interface for 21 days and used for further testing as formed on top of the cells. Analysis of confocal 3D imaging data revealed that after 21 days Calu-3 cells secrete a mucus layer with a thickness of 24 ± 6 μm. Mucus appeared to restrict penetration of 500 nm carboxyl-modified polystyrene particles to the upper 5-10 μm of the layer. Furthermore, a mucus modulation protocol using aerosolized N-acetylcysteine (NAC) was developed. This treatment enhanced the penetration of particles through the mucus down to deeper layers by means of the mucolytic action of NAC. These findings were supported by cytotoxicity data, indicating that intact mucus protects the underlying epithelium from particle-induced effects on membrane integrity. The impact of NAC treatment on the protective properties of mucus was probed by using 50 and 100 nm amine-modified and 50 nm carboxyl-modified polystyrene nanoparticles, respectively. Cytotoxicity was only induced by the amine-modified particles in combination with NAC treatment, implying a reduced protective function of modulated mucus. Overall, our data emphasize the importance of integrating an assessment of the protective function of mucus into the development of therapy approaches involving mucus modulation.

Vaping product exposure system (VaPES): a novel in vitro aerosol deposition system

OBJECTIVE

Due to recent increases in the use of vaping devices, there is a high demand for research addressing the respiratory health effects of vaping products. Given the constantly changing nature of the vaping market with new devices, flavors, metals, and other chemicals rapidly emerging, there is a need for inexpensive and highly adaptable vaping device exposure systems. Here, we describe the design and validation of a novel in vitro aerosol exposure system for toxicity testing of vaping devices.

MATERIALS AND METHODS

We developed an inexpensive, open-source in vitro vaping device exposure system that produces even deposition, can be adapted for different vaping devices, and allows for experiments to be performed under physiological conditions. The system was then validated with deposition testing and a representative exposure with human bronchial epithelial cells (hBECs).

RESULTS

The Vaping Product Exposure System (VaPES) produced sufficient and uniform deposition for dose-response studies and was precise enough to observe biological responses to vaping exposures. VaPES was adapted to work with both pod and cartridge-based vaping devices.

CONCLUSION

We have designed and validated a novel vaping device exposure system that will eliminate the need to use high-cost commercial exposure systems, lowering the barrier to entry of physiologically relevant vaping studies.

Aptamer-Protein Interactions: From Regulation to Biomolecular Detection

Serving as the basis of cell life, interactions between nucleic acids and proteins play essential roles in fundamental cellular processes. Aptamers are unique single-stranded oligonucleotides generated by in vitro evolution methods, possessing the ability to interact with proteins specifically. Altering the structure of aptamers will largely modulate their interactions with proteins and further affect related cellular behaviors. Recently, with the in-depth research of aptamer-protein interactions, the analytical assays based on their interactions have been widely developed and become a powerful tool for biomolecular detection. There are some insightful reviews on aptamers applied in protein detection, while few systematic discussions are from the perspective of regulating aptamer-protein interactions. Herein, we comprehensively introduce the methods for regulating aptamer-protein interactions and elaborate on the detection techniques for analyzing aptamer-protein interactions. Additionally, this review provides a broad summary of analytical assays based on the regulation of aptamer-protein interactions for detecting biomolecules. Finally, we present our perspectives regarding the opportunities and challenges of analytical assays for biological analysis, aiming to provide guidance for disease mechanism research and drug discovery.

Assessment of Scanning Mobility Particle Sizer (SMPS) for online monitoring of delivered dose in an in vitro aerosol exposure system

Real-time monitoring of dosimetry is critical to mitigating the constraints of offline measurements. To address this need, the use of the Scanning Mobility Particle Sizer (SMPS) to estimate the dose delivered through the Dosimetric Aerosol in Vitro Inhalation Device (DAVID) was assessed. CuO nanoparticles suspended in ethanol at different concentrations (0.01-10 mg/mL) were aerosolized using a Collison nebulizer and diluted with air at a ratio of either 1:3 (setup 1) or 1:18 (setup 2). From the aerosol volume concentrations measured by the SMPS, density of CuO (6.4 g/cm3), collection time (5-30 min), flow rate (0.5 LPM) and deposition area (0.28 cm2), the mass doses (DoseSMPS) were observed to increase exponentially over time and ranged from 0.02 ± 0.001 to 84.75 ± 3.49 μg/cm2. The doses calculated from the Cu concentrations determined by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) (DoseICP) also increased exponentially over time (0.01 ± 0.01-97.25 ± 1.30 μg/cm2). Regression analysis between DoseICP and DoseSMPS showed R2 ≥ 0.90 for 0.1-10 mg/mL. As demonstrated, the SMPS can be used to monitor the delivered dose in real-time, and controlled delivery of mass doses with a 226-fold range can be attained in ≤30 min in DAVID by adjusting the nebulizer concentration, dilution air and time.

Transferability and reproducibility of exposed air-liquid interface co-culture lung models

BACKGROUND

The establishment of reliable and robust in vitro models for hazard assessment, a prerequisite for moving away from animal testing, requires the evaluation of model transferability and reproducibility. Lung models that can be exposed via the air, by means of an air-liquid interface (ALI) are promising in vitro models for evaluating the safety of nanomaterials (NMs) after inhalation exposure. We performed an inter-laboratory comparison study to evaluate the transferability and reproducibility of a lung model consisting of the human bronchial cell line Calu-3 as a monoculture and, to increase the physiologic relevance of the model, also as a co-culture with macrophages (either derived from the THP-1 monocyte cell line or from human blood monocytes). The lung model was exposed to NMs using the VITROCELL® Cloud12 system at physiologically relevant dose levels.

RESULTS

Overall, the results of the 7 participating laboratories are quite similar. After exposing Calu-3 alone and Calu-3 co-cultures with macrophages, no effects of lipopolysaccharide (LPS), quartz (DQ12) or titanium dioxide (TiO2) NM-105 particles on the cell viability and barrier integrity were detected. LPS exposure induced moderate cytokine release in the Calu-3 monoculture, albeit not statistically significant in most labs. In the co-culture models, most laboratories showed that LPS can significantly induce cytokine release (IL-6, IL-8 and TNF-α). The exposure to quartz and TiO2 particles did not induce a statistically significant increase in cytokine release in both cell models probably due to our relatively low deposited doses, which were inspired by in vivo dose levels. The intra- and inter-laboratory comparison study indicated acceptable interlaboratory variation for cell viability/toxicity (WST-1, LDH) and transepithelial electrical resistance, and relatively high inter-laboratory variation for cytokine production.

CONCLUSION

The transferability and reproducibility of a lung co-culture model and its exposure to aerosolized particles at the ALI were evaluated and recommendations were provided for performing inter-laboratory comparison studies. Although the results are promising, optimizations of the lung model (including more sensitive read-outs) and/or selection of higher deposited doses are needed to enhance its predictive value before it may be taken further towards a possible OECD guideline.

Development and Characterization of a 96-Well Exposure System for Safety Assessment of Nanomaterials

In this study, a 96-well exposure system for safety assessment of nanomaterials is developed and characterized using an air-liquid interface lung epithelial model. This system is designed for sequential nebulization. Distribution studies verify the reproducible distribution over all 96 wells, with lower insert-to-insert variability compared to non-sequential application. With a first set of chemicals (TritonX), drugs (Bortezomib), and nanomaterials (silver nanoparticles and (non-)fluorescent crystalline nanocellulose), sequential exposure studies are performed with human lung epithelial cells followed by quantification of the deposited mass and of cell viability. The developed exposure system offers for the first time the possibility of exposing an air-liquid interface model in a 96-well format, resulting in high-throughput rates, combined with the feature for sequential dosing. This exposure system allows the possibility of creating dose-response curves resulting in the generation of more reliable cell-based assay data for many types of applications, such as safety analysis. In addition to chemicals and drugs, nanomaterials with spherical shapes, but also morphologically more complex nanostructures can be exposed sequentially with high efficiency. This allows new perspectives on in vivo-like and animal-free approaches for chemical and pharmaceutical safety assessment, in line with the 3R principle of replacing and reducing animal 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.

Investigating nanoplastics toxicity using advanced stem cell-based intestinal and lung in vitro models

Plastic particles in the nanometer range-called nanoplastics-are environmental contaminants with growing public health concern. As plastic particles are present in water, soil, air and food, human exposure via intestine and lung is unavoidable, but possible health effects are still to be elucidated. To better understand the Mode of Action of plastic particles, it is key to use experimental models that best reflect human physiology. Novel assessment methods like advanced cell models and several alternative approaches are currently used and developed in the scientific community. So far, the use of cancer cell line-based models is the standard approach regarding in vitro nanotoxicology. However, among the many advantages of the use of cancer cell lines, there are also disadvantages that might favor other approaches. In this review, we compare cell line-based models with stem cell-based in vitro models of the human intestine and lung. In the context of nanoplastics research, we highlight the advantages that come with the use of stem cells. Further, the specific challenges of testing nanoplastics in vitro are discussed. Although the use of stem cell-based models can be demanding, we conclude that, depending on the research question, stem cells in combination with advanced exposure strategies might be a more suitable approach than cancer cell lines when it comes to toxicological investigation of nanoplastics.

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.

Emerging trends in the methodology of environmental toxicology: 3D cell culture and its applications

Human diseases and health concerns caused by environmental pollutants are globally emerging. Therefore, rapid and efficient evaluation of the effects of environmental pollutants on human health is essential. Due to the significant differences between humans and animals and the lack of physiologically related environments, animal models and two-dimensional (2D) culture cannot accurately describe toxicological effects and predict actual in vivo responses. To make up for the limitations of traditional environmental toxicology screening, three-dimensional (3D) culture has been developed. The 3D culture could provide a good organizational structure comparable to the complex internal environment of humans and produce a more realistic response to environmental pollutants, which has been used in drug development, toxicity evaluation, personalized therapy and biological mechanism research. The goal of environmental toxicology is to provide clues and support for the risk assessment and management of environmental pollutants. With the development of 3D culture that can reproduce specific physiological aspects loaded with specific cells that reflect human biology, interactions between pollutants and target tissues and organs can be explored to assess the acute and chronic adverse health effects of exposure to various environmental toxins. The 3D culture with great potential shows broad prospects in toxicology research and is expected to bridge the gap between 2D culture and animal models eventually. In this sense, we strongly recommend that 3D culture be used to identify and understand environmental toxins, which will greatly facilitate the public's comprehensive understanding of environmental toxins.

Different Sensitivity of Advanced Bronchial and Alveolar Mono- and Coculture Models for Hazard Assessment of Nanomaterials

For the next-generation risk assessment (NGRA) of chemicals and nanomaterials, new approach methodologies (NAMs) are needed for hazard assessment in compliance with the 3R's to reduce, replace and refine animal experiments. This study aimed to establish and characterize an advanced respiratory model consisting of human epithelial bronchial BEAS-2B cells cultivated at the air-liquid interface (ALI), both as monocultures and in cocultures with human endothelial EA.hy926 cells. The performance of the bronchial models was compared to a commonly used alveolar model consisting of A549 in monoculture and in coculture with EA.hy926 cells. The cells were exposed at the ALI to nanosilver (NM-300K) in the VITROCELL^® Cloud. After 24 h, cellular viability (alamarBlue assay), inflammatory response (enzyme-linked immunosorbent assay), DNA damage (enzyme-modified comet assay), and chromosomal damage (cytokinesis-block micronucleus assay) were measured. Cytotoxicity and genotoxicity induced by NM-300K were dependent on both the cell types and model, where BEAS-2B in monocultures had the highest sensitivity in terms of cell viability and DNA strand breaks. This study indicates that the four ALI lung models have different sensitivities to NM-300K exposure and brings important knowledge for the further development of advanced 3D respiratory in vitro models for the most reliable human hazard assessment based on NAMs.

Liposomes or Extracellular Vesicles: A Comprehensive Comparison of Both Lipid Bilayer Vesicles for Pulmonary Drug Delivery

The rapid and non-invasive pulmonary drug delivery (PDD) has attracted great attention compared to the other routes. However, nanoparticle platforms, like liposomes (LPs) and extracellular vesicles (EVs), require extensive reformulation to suit the requirements of PDD. LPs are artificial vesicles composed of lipid bilayers capable of encapsulating hydrophilic and hydrophobic substances, whereas EVs are natural vesicles secreted by cells. Additionally, novel LPs-EVs hybrid vesicles may confer the best of both. The preparation methods of EVs are distinguished from LPs since they rely mainly on extraction and purification, whereas the LPs are synthesized from their basic ingredients. Similarly, drug loading methods into/onto EVs are distinguished whereby they are cell- or non-cell-based, whereas LPs are loaded via passive or active approaches. This review discusses the progress in LPs and EVs as well as hybrid vesicles with a special focus on PDD. It also provides a perspective comparison between LPs and EVs from various aspects (composition, preparation/extraction, drug loading, and large-scale manufacturing) as well as the future prospects for inhaled therapeutics. In addition, it discusses the challenges that may be encountered in scaling up the production and presents our view regarding the clinical translation of the laboratory findings into commercial products.

A multiplex inhalation platform to model in situ like aerosol delivery in a breathing lung-on-chip

Prolonged exposure to environmental respirable toxicants can lead to the development and worsening of severe respiratory diseases such as asthma, chronic obstructive pulmonary disease (COPD) and fibrosis. The limited number of FDA-approved inhaled drugs for these serious lung conditions has led to a shift from in vivo towards the use of alternative in vitro human-relevant models to better predict the toxicity of inhaled particles in preclinical research. While there are several inhalation exposure models for the upper airways, the fragile and dynamic nature of the alveolar microenvironment has limited the development of reproducible exposure models for the distal lung. Here, we present a mechanistic approach using a new generation of exposure systems, the Cloud α AX12. This novel in vitro inhalation tool consists of a cloud-based exposure chamber (VITROCELL) that integrates the breathing ^AXLung-on-chip system (AlveoliX). The ultrathin and porous membrane of the AX12 plate was used to create a complex multicellular model that enables key physiological culture conditions: the air-liquid interface (ALI) and the three-dimensional cyclic stretch (CS). Human-relevant cellular models were established for a) the distal alveolar-capillary interface using primary cell-derived immortalized alveolar epithelial cells (^AXiAECs), macrophages (THP-1) and endothelial (HLMVEC) cells, and b) the upper-airways using Calu3 cells. Primary human alveolar epithelial cells (^AXhAEpCs) were used to validate the toxicity results obtained from the immortalized cell lines. To mimic in vivo relevant aerosol exposures with the Cloud α AX12, three different models were established using: a) titanium dioxide (TiO2) and zinc oxide nanoparticles b) polyhexamethylene guanidine a toxic chemical and c) an anti-inflammatory inhaled corticosteroid, fluticasone propionate (FL). Our results suggest an important synergistic effect on the air-blood barrier sensitivity, cytotoxicity and inflammation, when air-liquid interface and cyclic stretch culture conditions are combined. To the best of our knowledge, this is the first time that an in vitro inhalation exposure system for the distal lung has been described with a breathing lung-on-chip technology. The Cloud α AX12 model thus represents a state-of-the-art pre-clinical tool to study inhalation toxicity risks, drug safety and efficacy.

Predicting nanomaterials pulmonary toxicity in animals by cell culture models: Achievements and perspectives

Animal experiments are highly relevant models for the assessment of toxicological effects of engineered nanomaterials (ENMs), due to lack of biomonitoring and epidemiological studies. However, the expanding number of ENMs with different physico-chemical properties strains this approach, as there are ethical concerns and economical challenges with the use of animals in toxicology. There is an urgent need for cell culture models that predict the level of toxicological responses in vivo, consequently reducing or replacing the use of animals in nanotoxicology. However, there is still a limited number of studies on in vitro-in vivo correlation of toxicological responses following ENMs exposure. In this review, we collected studies that have compared in vitro and in vivo toxic effects caused by ENMs. We discuss the influence of cell culture models and exposure systems on the predictability of in vitro models to equivalent toxic effects in animal lungs after pulmonary exposure to ENMs. In addition, we discuss approaches to qualitatively or quantitatively compare the effects in vitro and in vivo. The magnitude of toxicological responses in cells that are exposed in submerged condition is not systematically different from the response in cells exposed in air-liquid interface systems, and there appears to be similar ENMs hazard ranking between the two exposure systems. Overall, we show that simple in vitro models with cells exposed to ENMs in submerged condition can be used to predict toxic effects in vivo, and identify future strategies to improve the associations between in vitro and in vivo ENMs-induced pulmonary toxicity. This article is categorized under: Toxicology and Regulatory Issues in Nanomedicine > Toxicology of Nanomaterials.

An inter-laboratory effort to harmonize the cell-delivered in vitro dose of aerosolized materials

Air-liquid interface (ALI) lung cell models cultured on permeable transwell inserts are increasingly used for respiratory hazard assessment requiring controlled aerosolization and deposition of any material on ALI cells. The approach presented herein aimed to assess the transwell insert-delivered dose of aerosolized materials using the VITROCELL® Cloud12 system, a commercially available aerosol-cell exposure system. An inter-laboratory comparison study was conducted with seven European partners having different levels of experience with the VITROCELL® Cloud12. A standard operating procedure (SOP) was developed and applied by all partners for aerosolized delivery of materials, i.e., a water-soluble molecular substance (fluorescence-spiked salt) and two poorly soluble particles, crystalline silica quartz (DQ₁₂) and titanium dioxide nanoparticles (TiO₂ NM-105). The material dose delivered to transwell inserts was quantified with spectrofluorometry (fluorescein) and with the quartz crystal microbalance (QCM) integrated in the VITROCELL® Cloud12 system. The shape and agglomeration state of the deposited particles were confirmed with transmission electron microscopy (TEM). Inter-laboratory comparison of the device-specific performance was conducted in two steps, first for molecular substances (fluorescein-spiked salt), and then for particles. Device- and/or handling-specific differences in aerosol deposition of VITROCELL® Cloud12 systems were characterized in terms of the so-called deposition factor (DF), which allows for prediction of the transwell insert-deposited particle dose from the particle concentration in the aerosolized suspension. Albeit DF varied between the different labs from 0.39 to 0.87 (mean (coefficient of variation (CV)): 0.64 (28%)), the QCM of each VITROCELL® Cloud 12 system accurately measured the respective transwell insert-deposited dose. Aerosolized delivery of DQ₁₂ and TiO₂ NM-105 particles showed good linearity (R² > 0.95) between particle concentration of the aerosolized suspension and QCM-determined insert-delivered particle dose. The VITROCELL® Cloud 12 performance for DQ₁₂ particles was identical to that for fluorescein-spiked salt, i.e., the ratio of measured and salt-predicted dose was 1.0 (29%). On the other hand, a ca. 2-fold reduced dose was observed for TiO₂ NM-105 (0.54 (41%)), which was likely due to partial retention of TiO₂ NM-105 agglomerates in the vibrating mesh nebulizer of the VITROCELL® Cloud12. This inter-laboratory comparison demonstrates that the QCM integrated in the VITROCELL® Cloud 12 is a reliable tool for dosimetry, which accounts for potential variations of the transwell insert-delivered dose due to device-, handling- and/or material-specific effects. With the detailed protocol presented herein, all seven partner laboratories were able to demonstrate dose-controlled aerosolization of material suspensions using the VITROCELL® Cloud12 exposure system at dose levels relevant for observing in vitro hazard responses. This is an important step towards regulatory approved implementation of ALI lung cell cultures for in vitro hazard assessment of aerosolized materials.

Dynamic Fluid Flow Exacerbates the (Pro-)Inflammatory Effects of Aerosolised Engineered Nanomaterials In Vitro

The majority of in vitro studies focusing upon particle-lung cell interactions use static models at an air-liquid interface (ALI). Advancing the physiological characteristics of such systems allows for closer resemblance of the human lung, in turn promoting 3R strategies. PATROLS (EU Horizon 2020 No. 760813) aimed to use a well-characterised in vitro model of the human alveolar epithelial barrier to determine how fluid-flow dynamics would impact the outputs of the model following particle exposure. Using the QuasiVivo^TM (Kirkstall Ltd., York, UK) system, fluid-flow conditions were applied to an A549 + dTHP-1 cell co-culture model cultured at the ALI. DQ₁₂ and TiO₂ (JRCNM01005a) were used as model particles to assess the in vitro systems' sensitivity. Using a quasi- and aerosol (VitroCell Cloud12, VitroCell Systems, Waldkirch, Germany) exposure approach, cell cultures were exposed over 24 h at IVIVE concentrations of 1 and 10 (DQ₁₂) and 1.4 and 10.4 (TiO₂) µg/cm², respectively. We compared static and fluid flow conditions after both these exposure methods. The co-culture was subsequently assessed for its viability, membrane integrity and (pro-)inflammatory response (IL-8 and IL-6 production). The results suggested that the addition of fluid flow to this alveolar co-culture model can influence the viability, membrane integrity and inflammatory responses dependent on the particle type and exposure.

Measuring TiO2N and AgHEC Airborne Particle Density during a Spray Coating Process

Effective particle density is a key parameter for assessing inhalation exposure of engineered NPs in occupational environments. In this paper, particle density measurements were carried out using two different techniques: one based on the ratio between mass and volumetric particle concentrations; the other one based on the ratio between aerodynamic and geometric particle diameter. These different approaches were applied to both field- and laboratory-scale atomization processes where the two target NPs (N-doped TiO₂, TiO₂N and AgNPs capped with a quaternized hydroxyethylcellulose, AgHEC) were generated. Spray tests using TiO₂N were observed to release more and bigger particles than tests with AgHEC, as indicated by the measured particle mass concentrations and volumes. Our findings give an effective density of TiO₂N particle to be in a similar range between field and laboratory measurements (1.8 ± 0.5 g/cm³); while AgHEC particle density showed wide variations (3.0 ± 0.5 g/cm³ and 1.2 + 0.1 g/cm³ for field and laboratory campaigns, respectively). This finding leads to speculation regarding the composition of particles emitted because atomized particle fragments may contain different Ag-to-HEC ratios, leading to different density values. A further uncertainty factor is probably related to low process emissions, making the subtraction of background concentrations from AgHEC process emissions unreliable.

Quartz crystal microbalance and atomic force microscopy to characterize mimetic systems based on supported lipids bilayer

Quartz Crystal Microbalance (QCM) with dissipation and Atomic Force Microscopy (AFM) are two characterization techniques that allow describing processes taking place at solid-liquid interfaces. Both are label-free and, when used in combination, provide kinetic, thermodynamic and structural information at the nanometer scale of events taking place at surfaces. Here we describe the basic operation principles of both techniques, addressing a non-specialized audience, and provide some examples of their use for describing biological events taking place at supported lipid bilayers (SLBs). The aim is to illustrate current strengths and limitations of the techniques and to show their potential as biophysical characterization techniques.

Advanced Respiratory Models for Hazard Assessment of Nanomaterials—Performance of Mono-, Co- and Tricultures

Advanced in vitro models are needed to support next-generation risk assessment (NGRA), moving from hazard assessment based mainly on animal studies to the application of new alternative methods (NAMs). Advanced models must be tested for hazard assessment of nanomaterials (NMs). The aim of this study was to perform an interlaboratory trial across two laboratories to test the robustness of and optimize a 3D lung model of human epithelial A549 cells cultivated at the air–liquid interface (ALI). Potential change in sensitivity in hazard identification when adding complexity, going from monocultures to co- and tricultures, was tested by including human endothelial cells EA.hy926 and differentiated monocytes dTHP-1. All models were exposed to NM-300K in an aerosol exposure system (VITROCELL^® cloud-chamber). Cyto- and genotoxicity were measured by AlamarBlue and comet assay. Cellular uptake was investigated with transmission electron microscopy. The models were characterized by confocal microscopy and barrier function tested. We demonstrated that this advanced lung model is applicable for hazard assessment of NMs. The results point to a change in sensitivity of the model by adding complexity and to the importance of detailed protocols for robustness and reproducibility of advanced in vitro models.

A New Immortalized Human Alveolar Epithelial Cell Model to Study Lung Injury and Toxicity on a Breathing Lung-On-Chip System

The evaluation of inhalation toxicity, drug safety and efficacy assessment, as well as the investigation of complex disease pathomechanisms, are increasingly relying on in vitro lung models. This is due to the progressive shift towards human-based systems for more predictive and translational research. While several cellular models are currently available for the upper airways, modelling the distal alveolar region poses several constraints that make the standardization of reliable alveolar in vitro models relatively difficult. In this work, we present a new and reproducible alveolar in vitro model, that combines a human derived immortalized alveolar epithelial cell line (^AXiAEC) and organ-on-chip technology mimicking the lung alveolar biophysical environment (^AXlung-on-chip). The latter mimics key features of the in vivo alveolar milieu: breathing-like 3D cyclic stretch (10% linear strain, 0.2 Hz frequency) and an ultrathin, porous and elastic membrane. ^AXiAECs cultured on-chip were characterized for their alveolar epithelial cell markers by gene and protein expression. Cell barrier properties were examined by TER (Transbarrier Electrical Resistance) measurement and tight junction formation. To establish a physiological model for the distal lung, ^AXiAECs were cultured for long-term at air-liquid interface (ALI) on-chip. To this end, different stages of alveolar damage including inflammation (via exposure to bacterial lipopolysaccharide) and the response to a profibrotic mediator (via exposure to Transforming growth factor β1) were analyzed. In addition, the expression of relevant host cell factors involved in SARS-CoV-2 infection was investigated to evaluate its potential application for COVID-19 studies. This study shows that ^AXiAECs cultured on the ^AXlung-on-chip exhibit an enhanced in vivo-like alveolar character which is reflected into: 1) Alveolar type 1 (AT1) and 2 (AT2) cell specific phenotypes, 2) tight barrier formation (with TER above 1,000 Ω cm²) and 3) reproducible long-term preservation of alveolar characteristics in nearly physiological conditions (co-culture, breathing, ALI). To the best of our knowledge, this is the first time that a primary derived alveolar epithelial cell line on-chip representing both AT1 and AT2 characteristics is reported. This distal lung model thereby represents a valuable in vitro tool to study inhalation toxicity, test safety and efficacy of drug compounds and characterization of xenobiotics.

Aerosol-Cell Exposure System Applied to Semi-Adherent Cells for Aerosolization of Lung Surfactant and Nanoparticles Followed by High Quality RNA Extraction

Nanoparticle toxicity assessments have moved closer to physiological conditions while trying to avoid the use of animal models. An example of new in vitro exposure techniques developed is the exposure of cultured cells at the air-liquid interface (ALI), particularly in the case of respiratory airways. While the commercially available VITROCELL^® Cloud System has been applied for the delivery of aerosolized substances to adherent cells under ALI conditions, it has not yet been tested on lung surfactant and semi-adherent cells such as alveolar macrophages, which are playing a pivotal role in the nanoparticle-induced immune response.

OBJECTIVES

In this work, we developed a comprehensive methodology for coating semi-adherent lung cells cultured at the ALI with aerosolized surfactant and subsequent dose-controlled exposure to nanoparticles (NPs). This protocol is optimized for subsequent transcriptomic studies.

METHODS

Semi-adherent rat alveolar macrophages NR8383 were grown at the ALI and coated with lung surfactant through nebulization using the VITROCELL^® Cloud 6 System before being exposed to TiO₂ NM105 NPs. After NP exposures, RNA was extracted and its quantity and quality were measured.

RESULTS

The VITROCELL^® Cloud system allowed for uniform and ultrathin coating of cells with aerosolized surfactant mimicking physiological conditions in the lung. While nebulization of 57 μL of 30 mg/mL TiO₂ and 114 μL of 15 mg/mL TiO₂ nanoparticles yielded identical cell delivered dose, the reproducibility of dose as well as the quality of RNA extracted were better for 114 μL.

How Structured Metadata Acquisition Contributes to the Reproducibility of Nanosafety Studies: Evaluation by a Round-Robin Test

It has been widely recognized that nanosafety studies are limited in reproducibility, caused by missing or inadequate information and data gaps. Reliable and comprehensive studies should be performed supported by standards or guidelines, which need to be harmonized and usable for the multidisciplinary field of nanosafety research. The previously described minimal information table (MIT), based on existing standards or guidelines, represents one approach towards harmonization. Here, we demonstrate the applicability and advantages of the MIT by a round-robin test. Its modular structure enables describing individual studies comprehensively by a combination of various relevant aspects. Three laboratories conducted a WST-1 cell viability assay using A549 cells to analyze the effects of the reference nanomaterials NM101 and NM110 according to predefined (S)OPs. The MIT contains relevant and defined descriptive information and quality criteria and thus supported the implementation of the round-robin test from planning, investigation to analysis and data interpretation. As a result, we could identify sources of variability and justify deviating results attributed to differences in specific procedures. Consequently, the use of the MIT contributes to the acquisition of reliable and comprehensive datasets and therefore improves the significance and reusability of nanosafety studies.

In vitro genotoxicity of dibutyl phthalate on A549 lung cells at air-liquid interface in exposure concentrations relevant at workplaces

The ubiquitous use of phthalates in various materials and the knowledge about their potential adverse effects is of great concern for human health. Several studies have uncovered their role in carcinogenic events and suggest various phthalate-associated adverse health effects that include pulmonary diseases. However, only limited information on pulmonary toxicity is available considering inhalation of phthalates as the route of exposure. While in vitro studies are often based on submerged exposures, this study aimed to expose A549 alveolar epithelial cells at the air-liquid interface (ALI) to unravel the genotoxic and oxidative stress-inducing potential of dibutyl phthalate (DBP) with concentrations relevant at occupational settings. Within this scope, a computer modeling approach calculating alveolar deposition of DBP particles in the human lung was used to define in vitro ALI exposure conditions comparable to potential occupational DBP exposures. The deposited mass of DBP ranged from 0.03 to 20 ng/cm² , which was comparable to results of a human lung particle deposition model using an 8 h workplace threshold limit value of 580 μg/m³ proposed by the Scientific Committee on Occupational Exposure Limits for the European Union. Comet and Micronucleus assay revealed that DBP induced genotoxicity at DNA and chromosome level in sub-cytotoxic conditions. Since genomic instability was accompanied by increased generation of the lipid peroxidation marker malondialdehyde, oxidative stress might play an important role in phthalate-induced genotoxicity. The results highlight the importance of adapting in vitro studies to exposure scenarios relevant at occupational settings and reconsidering occupational exposure limits for DBP.

Emerging Technologies for In Vitro Inhalation Toxicology

Respiratory toxicology remains a major research area in the 21st century since current scenario of airborne viral infection transmission and pollutant inhalation is expected to raise the annual morbidity beyond 2 million. Clinical and epidemiological research connecting human exposure to air contaminants to understand adverse pulmonary health outcomes is, therefore, an immediate subject of human health assessment. Important observations in defining systemic effects of environmental contaminants on inhalation metabolic dysfunction, liver health, and gastrointestinal tract have been well explored with in vivo models. In this review, a framework is provided, a paradigm is established about inhalation toxicity testing in vitro, and a brief overview of breathing Lungs-on-Chip (LoC) as design concepts is given. The optimized bioengineering approaches and microfluidics with their fundamental pros, and cons are presented. There are different strategies that researchers apply to inhalation toxicity studies to assess a variety of inhalable substances and relevant LoC approaches. A case study from published literature and frame arguments about reproducibility as well as in vitro/in vivo correlations are discussed. Finally, the opportunities and challenges in soft robotics, systems inhalation toxicology approach integrating bioengineering, machine learning, and artificial intelligence to address a multitude model for future toxicology are discussed.

Advances in Smoking Related In Vitro Inhalation Toxicology: A Perspective Case of Challenges and Opportunities from Progresses in Lung-on-Chip Technologies

The inhalation toxicology of multifaceted particulate matter from the environment, cigarette smoke, and e-cigarette liquid vapes is a major research topic concerning the adverse effect of these items on lung tissue. In vitro air-liquid interface (ALI) culture models hold more potential in an inhalation toxicity assessment. Apropos to e-cigarette toxicity, the multiflavor components of the vapes pose a complex experimental bottleneck. While an appropriate ALI setup has been one part of the focus to overcome this, parallel attention towards the development of an ideal exposure system has pushed the field forward. With the advent of microfluidic devices, lung-on-chip (LOC) technologies show enormous opportunities in in vitro smoke-related inhalation toxicity. In this review, we provide a framework, establish a paradigm about smoke-related inhalation toxicity testing in vitro, and give a brief overview of breathing LOC experimental design concepts. The capabilities with optimized bioengineering approaches and microfluidics and their fundamental pros and cons are presented with specific case studies. The LOC model can imitate the structural, functional, and mechanical properties of human alveolar-capillary interface and are more reliable than conventional in vitro models. Finally, we outline current perspective challenges as well as opportunities of future development to smoking lungs-on-chip technologies based on advances in soft robotics, machine learning, and bioengineering.

A Complete In Vitro Toxicological Assessment of the Biological Effects of Cerium Oxide Nanoparticles: From Acute Toxicity to Multi-Dose Subchronic Cytotoxicity Study

Engineered nanomaterials (ENMs) are of significant relevance due to their unique properties, which have been exploited for widespread applications. Cerium oxide nanoparticles (CeO₂-NPs) are one of most exploited ENM in the industry due to their excellent catalytic and multi-enzyme mimetic properties. Thus, the toxicological effects of these ENMs should be further studied. In this study, the acute and subchronic toxicity of CeO₂-NPs were assessed. First, an in vitro multi-dose short-term (24 h) toxicological assessment was performed in three different cell lines: A549 and Calu3 were used to represented lung tissue and 3T3 was used as an interstitial tissue model. After that, a sub-chronic toxicity assessment (90 days) of these NPs was carried out on a realistic and well-established reconstituted primary human airway epithelial model (MucilAir™), cultured at the Air–Liquid Interface (ALI), to study the long-term effects of these particles. Results showed minor toxicity of CeO₂-NPs in acute exposures. However, in subchronic exposures, cytotoxic and inflammatory responses were observed in the human airway epithelial model after 60 days of exposure to CeO₂-NPs. These results suggest that acute toxicity approaches may underestimate the toxicological effect of some ENMs, highlighting the need for subchronic toxicological studies in order to accurately assess the toxicity of ENM and their cumulative effects in organisms.

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.

A Custom-Made Device for Reproducibly Depositing Pre-metered Doses of Nebulized Drugs on Pulmonary Cells in vitro

The deposition of pre-metered doses (i.e., defined before and not after exposition) at the air-liquid interface of viable pulmonary epithelial cells remains an important but challenging task for developing aerosol medicines. While some devices allow quantification of the deposited dose after or during the experiment, e.g., gravimetrically, there is still no generally accepted way to deposit small pre-metered doses of aerosolized drugs or pharmaceutical formulations, e.g., nanomedicines. Here, we describe a straightforward custom-made device, allowing connection to commercially available nebulizers with standard cell culture plates. Designed to tightly fit into the approximately 12-mm opening of either a 12-well Transwell^® insert or a single 24-well plate, a defined dose of an aerosolized liquid can be directly deposited precisely and reproducibly (4.8% deviation) at the air-liquid interface (ALI) of pulmonary cell cultures. The deposited dose can be controlled by the volume of the nebulized solution, which may vary in a range from 20 to 200 μl. The entire nebulization-deposition maneuver is completed after 30 s and is spatially homogenous. After phosphate-buffered saline (PBS) deposition, the viability and barrier properties transepithelial electrical resistance (TEER) of human bronchial epithelial Calu-3 cells were not negatively affected. Straightforward in manufacture and use, the device enables reproducible deposition of metered doses of aerosolized drugs to study the interactions with pulmonary cell cultures grown at ALI conditions.

Stem Cells for Next Level Toxicity Testing in the 21st Century

The call for a paradigm change in toxicology from the United States National Research Council in 2007 initiates awareness for the invention and use of human-relevant alternative methods for toxicological hazard assessment. Simple 2D in vitro systems may serve as first screening tools, however, recent developments infer the need for more complex, multicellular organotypic models, which are superior in mimicking the complexity of human organs. In this review article most critical organs for toxicity assessment, i.e., skin, brain, thyroid system, lung, heart, liver, kidney, and intestine are discussed with regards to their functions in health and disease. Embracing the manifold modes-of-action how xenobiotic compounds can interfere with physiological organ functions and cause toxicity, the need for translation of such multifaceted organ features into the dish seems obvious. Currently used in vitro methods for toxicological applications and ongoing developments not yet arrived in toxicity testing are discussed, especially highlighting the potential of models based on embryonic stem cells and induced pluripotent stem cells of human origin. Finally, the application of innovative technologies like organs-on-a-chip and genome editing point toward a toxicological paradigm change moves into action.

A Bioinspired in vitro Lung Model to Study Particokinetics of Nano-/Microparticles Under Cyclic Stretch and Air-Liquid Interface Conditions

Evolution has endowed the lung with exceptional design providing a large surface area for gas exchange area (ca. 100 m²) in a relatively small tissue volume (ca. 6 L). This is possible due to a complex tissue architecture that has resulted in one of the most challenging organs to be recreated in the lab. The need for realistic and robust in vitro lung models becomes even more evident as causal therapies, especially for chronic respiratory diseases, are lacking. Here, we describe the Cyclic InVItroCell-stretch (CIVIC) “breathing” lung bioreactor for pulmonary epithelial cells at the air-liquid interface (ALI) experiencing cyclic stretch while monitoring stretch-related parameters (amplitude, frequency, and membrane elastic modulus) under real-time conditions. The previously described biomimetic copolymeric BETA membrane (5 μm thick, bioactive, porous, and elastic) was attempted to be improved for even more biomimetic permeability, elasticity (elastic modulus and stretchability), and bioactivity by changing its chemical composition. This biphasic membrane supports both the initial formation of a tight monolayer of pulmonary epithelial cells (A549 and 16HBE14o⁻) under submerged conditions and the subsequent cell-stretch experiments at the ALI without preconditioning of the membrane. The newly manufactured versions of the BETA membrane did not improve the characteristics of the previously determined optimum BETA membrane (9.35% PCL and 6.34% gelatin [w/v solvent]). Hence, the optimum BETA membrane was used to investigate quantitatively the role of physiologic cyclic mechanical stretch (10% linear stretch; 0.33 Hz: light exercise conditions) on size-dependent cellular uptake and transepithelial transport of nanoparticles (100 nm) and microparticles (1,000 nm) for alveolar epithelial cells (A549) under ALI conditions. Our results show that physiologic stretch enhances cellular uptake of 100 nm nanoparticles across the epithelial cell barrier, but the barrier becomes permeable for both nano- and micron-sized particles (100 and 1,000 nm). This suggests that currently used static in vitro assays may underestimate cellular uptake and transbarrier transport of nanoparticles in the lung.