Name: assessment of the acute inhalation toxicity of airborne particles by exposing cultivated human lung cells at the air-liquid interface
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This work was supported by the German Federal Ministry of Education and Research (Bundesministerium f&#252;r Bildung und Forschung, BMBF, Germany (Grant 031A581, sub-project A-D)) and by the German Research Foundation (Deutsche Forschungsgesellschaft, DFG, Research Training Group GRK 2338).

NOTE: The protocol of one exposure experiment covers a period of three days.
Day 1
1. General preparations and cultivation of cells
NOTE: The human lung adenocarcinoma epithelial cell line A549 was used for exposure experiments. Cells must be handled under sterile conditions. Other cell lines that are suitable for cultivation at the ALI can be used.
<ol>
	<li>Prepare the growth medium (Dulbecco&#39;s Minimum Essential Medium (DMEM), sup...

<ol>
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D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+useful+in+vitro+model+for+transport+studies+of+alveolar+epithelial+barrier.">A useful in vitro model for transport studies of alveolar epithelial barrier.</a> Pharmaceutical Research. 18 (3), 253-255 (2001).</li><li>OECD. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Test+No.+403:+Acute+Inhalation+Toxicity.">Test No. 403: Acute Inhalation Toxicity.</a> OECD Guidelines for the Testing of Chemicals, Section 4. , (2009).</li><li>OECD. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Test+No.+436:+Acute+Inhalation+Toxicity+–+Acute+Toxic+Class+Method.">Test No. 436: Acute Inhalation Toxicity – Acute Toxic Class Method.</a> OECD Guidelines for the Testing of Chemicals, Section 4. , (2009).</li><li>Tsoutsoulopoulos, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Validation+of+the+CULTEX+Radial+Flow+System+for+the+assessment+of+the+acute+inhalation+toxicity+of+airborne+particles.">Validation of the CULTEX Radial Flow System for the assessment of the acute inhalation toxicity of airborne particles.</a> Toxicology In vitro. 58, 245-255 (2019).</li><li>Tsoutsoulopoulos, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+novel+exposure+system+generating+nebulized+aerosol+of+sulfur+mustard+in+comparison+to+the+standard+submerse+exposure.">A novel exposure system generating nebulized aerosol of sulfur mustard in comparison to the standard submerse exposure.</a> Chemico-Biological Interactions. 298, 121-128 (2019).</li><li>Tsoutsoulopoulos, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optimization+of+the+CULTEX+radial+flow+system+for+in+vitro+investigation+of+lung+damaging+agents.">Optimization of the CULTEX radial flow system for in vitro investigation of lung damaging agents.</a> Toxicology Letters. 244, 28-34 (2016).</li><li>Osman, J. 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Many non-animal inhalation toxicity testing models have been developed in recent years in order to gain information about the acute inhalation hazard of inhalable particles and to reduce and replace animal experiments according to the 3R principle25.
In terms of cell culture models, exposure of cells can be done under submerged conditions or at the ALI. Exposing cells under submerged conditions may affect the physico-chemical properties and thus, the toxic properties of...

Inhalation of toxic airborne particles is a public health concern, leading to a multitude of health risks worldwide and many millions of deaths annually1,2. Climate change, the ongoing industrial development and the rising demand for energy, agricultural and consumer products have contributed to the increase of pulmonary diseases over the last years3,4,5,6. Knowledge and evaluation of inhalable substances regarding their acute inhalation toxicity provide the basis for hazard assessment and risk management, but this information is still lacking for a wide range of these substances7,8. Since 2006, the EU chemical legislation REACH (Registration, Evaluation, Authorization and Restriction of Chemicals) requires that already existing and newly introduced products undergo a toxicological characterization including the inhalation route before being placed on the market. Therefore, REACH focuses on alternative and animal-free methods, the implementation of the &#34;3R&#34; principle (Replacement, Refinement, and Reduction of animal experiments) and the use of appropriate in vitro models9. In recent years, many different and adequate non-animal inhalation toxicity testing models (e.g., in vitro cell cultures, lung-on-a-chip models, precision cut lung slices (PCLS)) have been developed in order to assess the acute inhalation toxicity of airborne particles5,7,10,11. In terms of in vitro cell culture models, cultivated cells can be exposed under submerged conditions or at the ALI (Figure 1). However, the validity of submerged exposure studies is limited with regard to the evaluation of the toxicity of airborne compounds especially particles. Submerged exposure techniques do not correspond to the human in vivo situation; the cell culture medium covering the cells may affect the physico-chemical properties and thus, the toxic properties of a test substance12,13. ALI in vitro inhalation models allow the direct exposure of cells to the test substances without interference of the cell culture medium with test particles, thus, mimicking human exposure with higher physiological and biological similarity than submerged exposures12,14.
For regulatory processes such as REACH, however, only animal models are available in the field of acute inhalation toxicology, as no alternative in vitro methods have been sufficiently validated and officially accepted so far14. For this purpose, test models have to be validated according to the requirements of the European Union Reference Laboratory for Alternatives to Animal Testing (EURL-ECVAM) principles on test validity15.
A former pre-validation study and a recently completed validation study successfully demonstrated the application area of the CULTEX RFS exposure system and its transferability, stability, and reproducibility13. This exposure system is an in vitro cell-based exposure system that enables homogenous exposure of cells to gases, particles or complex atmospheres (e.g., cigarette smoke) at the ALI due to its radial aerosol distribution concept and the conduction of the test aerosol in a continuous flow over the cells16. The basic module of this radial flow system consists of the inlet adapter, the aerosol guiding module with a radial aerosol distribution, the sampling and socket module, and a locking module with a hand wheel (Figure 2). The generated particles reach the cells via the inlet adapter and the aerosol guiding module and are deposited on the cell culture inserts, which are located in the three radially arranged exposure chambers of the sampling module. The aerosol guiding module as well as the sampling module can be heated by connecting to an external water bath17.
Within the framework of both studies, A549 cells were used for all exposure experiments. The cell line A549 is a human immortalized epithelial cell line that is very well-characterized and has been used as an in vitro model for type II alveolar epithelial cells in numerous toxicological studies. The cells are characterized by lamellar bodies, the production of surfactant and a number of inflammation-relevant factors18. They also show properties of bronchial epithelial cells due to their mucus production19. Moreover, they can be cultured at the ALI. Although this cell line is deficient in building cell-cell contacts, the cultivation of these cells is much more convenient, less cost expensive and results derived thereof are donor-independent compared to primary cells20.
A549 cells were seeded in 6-well cell culture inserts (PET membrane, 4.67 cm2, pore size 0.4 mm) with a density of 3.0 x 105 cells per insert and cultivated for 24 h under submerged conditions. Cells were then exposed in three independent laboratories to clean air and three different exposure doses (25, 50, and 100 &#181;g/cm2) of 20 test substances at the ALI. The exposure dose is correlated to the deposition time resulting in a constant particle rate of 25 &#181;g/cm2, 50 &#181;g/cm2 and 100 &#181;g/cm2 onto the cells after 15, 30 or 60 min, respectively. The deposited particles, however, were not washed off after deposition, but remained on the cells for 24 h. The deposition times of the particles were therefore 15, 30 and 60 min, but the exposure of the cells lasted for 24 h in total. The deposition rate of the test substances was determined in preliminary experiments according to previous methods17.
Cell viability as an indicator of toxicity was assessed 24 h after particle deposition using a cell viability assay. Special focus was set on the quality of clean air controls, the optimization and refinement of the exposure protocol, the intra- and inter-laboratory reproducibility and the establishment of a prediction model (PM). Substances that led to a decrease of cell viability below 50% (PM 50%) or 75% (PM 75%) at any of the three exposure doses were considered to exert an acute inhalation hazard. Results were then compared to existing in vivo data (based on at least one reliable study according to OECD test guideline (TG) 403 or TG 43621,22), leading to an overall concordance of 85%, with a specificity of 83% and a sensitivity of 88%23.
Besides the measurement of cell viability, other endpoints such as cytokine release, examination of the cell lysate or membrane integrity via LDH assay can be assessed but were not required for the validation study. Thus, the exposure system (e.g., CULTEX RFS) was proven as a predictive screening system for the qualitative assessment of the acute inhalation toxicity of the tested airborne particles, representing a promising alternative method to animal testing. The following protocol is recommended for exposure experiments to airborne particles using this exposure system.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Cells</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>A549</td>
			<td>ATCC</td>
			<td>CCL-185</td>
			<td></td>
		</tr>
		<tr>
			<td>Cell culture medium and supplies</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>DMEM</td>
			<td>Biochrom, Berlin, Germany</td>
			<td>FG 0415</td>
			<td>used as growth medium</td>
		</tr>
		<tr>
			<td>DMEM</td>
			<td>Gibco-Invitrogen, Darmstadt, Germany</td>
			<td>22320</td>
			<td>used as exposure medium</td>
		</tr>
		<tr>
			<td>FBS superior</td>
			<td>Biochrom, Berlin, Germany</td>
			<td>S 0615</td>
			<td></td>
		</tr>
		<tr>
			<td>Gentamycin (10mg/mL)</td>
			<td>Biochrom, Berlin, Germany</td>
			<td>A 2710</td>
			<td></td>
		</tr>
		<tr>
			<td>HEPES 1M</td>
			<td>Th. Geyer, Renningen, Germany</td>
			<td>L 0180</td>
			<td></td>
		</tr>
		<tr>
			<td>PBS</td>
			<td>Biochrom, Berlin, Germany</td>
			<td>L 1825</td>
			<td></td>
		</tr>
		<tr>
			<td>Trypsin/EDTA (0.05%/0.02%)</td>
			<td>Biochrom, Berlin, Germany</td>
			<td>L 2143</td>
			<td></td>
		</tr>
		<tr>
			<td>Cell culture material</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>CASY Cups</td>
			<td>Roche Diagnostic GmbH, Mannheim, Germany</td>
			<td>REF 05651794</td>
			<td></td>
		</tr>
		<tr>
			<td>Cell culture plates</td>
			<td>Corning, Wiesbaden, Germany</td>
			<td>3516</td>
			<td>6&#173;-well plates</td>
		</tr>
		<tr>
			<td>Corning Transwell cell culture inserts</td>
			<td>Corning, Wiesbaden, Germany</td>
			<td>3450</td>
			<td>24mm inserts; 6-&#173;well plates; 0.4 &#181;m</td>
		</tr>
		<tr>
			<td>Chemicals</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>CASYton</td>
			<td>Roche Diagnostic GmbH, Mannheim, Germany</td>
			<td>REF 05651808001</td>
			<td></td>
		</tr>
		<tr>
			<td>Compressed Air (DIN EN 12021)</td>
			<td>Linde Gas Therapeutics GmbH, Oberschlei&#223;heim, Germany</td>
			<td>2290152</td>
			<td></td>
		</tr>
		<tr>
			<td>WST-1</td>
			<td>Abcam, Cambridge, United Kingdom</td>
			<td>ab155902</td>
			<td></td>
		</tr>
		<tr>
			<td>Instruments + equipment</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>CASY Cell Counter</td>
			<td>Sch&#228;rfe System GmbH, Reutlingen, Germany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Circulation thermostat</td>
			<td>LAUDA, Lauda-K&#246;nigshofen, Germany</td>
			<td>Ecoline RE 100</td>
			<td></td>
		</tr>
		<tr>
			<td>CULTEX HyP - Hydraulic Press</td>
			<td>Cultex&#174; Technology GmbH, Hannover, Gemany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>CULTEX insert sleeve</td>
			<td>Cultex&#174; Technology GmbH, Hannover, Gemany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>CULTEX RFS - Radial Flow System Type 2 (module for particle exposure)</td>
			<td>Cultex&#174; Technology GmbH, Hannover, Gemany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>CULTEX RFS - Radial Flow System Type 2 (module for clean air exposure)</td>
			<td>Cultex&#174; Technology GmbH, Hannover, Gemany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>CULTEX supply</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Flow controller 0-30 ml/min (IQ-Flow)</td>
			<td>Bronkhorst Deutschland Nord GmbH</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Flow controller 0-1,5 l/min (EL-Flow)</td>
			<td>Bronkhorst Deutschland Nord GmbH</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Filters (large)</td>
			<td>Munktell &#38; Filtrak GmbH, Sachsen, Germany</td>
			<td>LP-050</td>
			<td>Munktell Sterile Filter; Particle retention efficiency &#62; 99,999%</td>
		</tr>
		<tr>
			<td>Filters (small)</td>
			<td>Parker Hannifin Corporation, Mainz, Germany</td>
			<td>9933-05-DQ</td>
			<td>Balston disposable filter</td>
		</tr>
		<tr>
			<td>Medium pump</td>
			<td>Cole-Parmer GmbH, Wertheim, Germany</td>
			<td>Ismatec IPC High Precision Multichannel Dispenser</td>
			<td>digital peristaltic pump</td>
		</tr>
		<tr>
			<td>Microplate Reader Infinite M200 Pro</td>
			<td>Tecan Deutschland GmbH, Crailsheim, Germany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Vakuum pump</td>
			<td>KNF, Freiburg, Germany</td>
			<td>N86 KT.18</td>
			<td></td>
		</tr>
		<tr>
			<td>V&#246;gtlin mass flow controller 0,2-10 l/min</td>
			<td>TrigasFI GmbH</td>
			<td>V&#246;gtlin red-y compact regulator, Typ-Nr.: GCR-C3SA-BA20</td>
			<td></td>
		</tr>
		<tr>
			<td>Water Bath</td>
			<td>LAUDA, Lauda-K&#246;nigshofen, Germany</td>
			<td>Ecoline Staredition RE 104</td>
			<td></td>
		</tr>
	</tbody>
</table>

assessment of the acute inhalation toxicity of airborne particles by exposing cultivated human lung cells at the air-liquid interface

Here, we present a specially designed modular in vitro exposure system that enables the homogenous exposure of cultivated human lung cells at the ALI to gases, particles or complex atmospheres (e.g., cigarette smoke), thus providing realistic physiological exposure of the apical surface of the human alveolar region to air. In contrast to sequential exposure models with linear aerosol guidance, the modular design of the radial flow system meets all requirements for the continuous generation and transport of the test atmosphere to the cells, a homogenous distribution and deposition of the particles and the continuous removal of the atmosphere. This exposure method is primarily designed for the exposure of cells to airborne particles, but can be adapted to the exposure of liquid aerosols and highly toxic and aggressive gases depending on the aerosol generation method and the material of the exposure modules.
Within the framework of a recently completed validation study, this exposure system was proven as a transferable, reproducible and predictive screening method for the qualitative assessment of the acute pulmonary cytotoxicity of airborne particles, thereby potentially reducing or replacing animal experiments that would normally provide this toxicological assessment.

The CULTEX RFS is a specially designed modular in vitro exposure system that enables the direct and homogenous exposure of cells at the ALI. Within a former pre-validation study, the general applicability of this exposure system and its transferability, stability and reproducibility were successfully demonstrated. In a recent research project funded by the German Federal Ministry of Education and Research, the exposure system was successfully validated and established as a prediction mode...

Watch this Scientific Journal Video about Assessment of the Acute Inhalation Toxicity of Airborne Particles by Exposing Cultivated Human Lung Cells at the Air-Liquid Interface at JoVE.com

Assessment of the Acute Inhalation Toxicity of Airborne Particles by Exposing Cultivated Human Lung Cells at the Air-Liquid Interface

We present a robust, transferable and predictive in vitro exposure system for the screening and monitoring of airborne particles concerning their acute pulmonary cytotoxicity by exposing cultivated human lung cells at the air-liquid interface (ALI).

Here, we present a specially designed modular in vitro exposure system that enables the homogenous exposure of cultivated human lung cells at the ALI to ...

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Medicine

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