Application of a Quartz Crystal Microbalance to Measure the Mass Concentration of Combustion Particle Suspensions

Ozone and particulate matter co-exposure at the air-liquid interface: Establishing an approach to assess pollutant interactions in vitro

BACKGROUND

Air pollution is a complex mixture of gases and particulates that varies spatially and temporally, making attribution of adverse effects to specific individual air pollutants a challenge. To disentangle effects of mixtures in a controlled setting, reproducible and realistic co-exposures of human-relevant models to gaseous and particulate pollutants are needed. Although air-liquid interface (ALI) exposures offer considerable promise as non-animal models for inhalation toxicity testing, a lack of studies comparing individual and co-exposures to gaseous and particulate pollutants has thus far prevented assessment of their strengths and limitations for disentangling effects of pollutant mixtures.

METHODS

Using an integrated ALI exposure system, we characterized the interaction between ozone and particles (25 nm fluorescent polystyrene beads) to assess effects on and reproducibility of critical physical endpoints including temperature, humidity, ozone concentration and particle deposition. Particle deposition and concentration were assessed via three independent methods: fluorescence, quartz crystal microbalance (QCM), and airborne particle count. To evaluate the acute biological effects of an air pollutant mixture in vitro, human lung type 2 epithelial-like cells (A549) were exposed at the ALI to air, ozone (O3), particles, and O3 + particles (co-exposure) for 1 h (n = 4 independent repeats/exposure type). Cell injury and inflammation were quantified by extracellular lactate dehydrogenase (LDH) activity and release of proinflammatory cytokines (interleukin (IL)-8 and IL-6) respectively 0 and 24 h post-exposure.

RESULTS

Exposures were effective at delivering targeted O3 exposures under controlled temperature and relative humidity. In-well particle deposition and airborne concentration exiting the exposure system, quantified through parallel methods, were consistent, and increased in relation to aerosolized particle concentration. Levels of each pollutant were effectively maintained in the presence of the other. O3 alone, and co-exposure to O3 and particles, increased LDH release from A549 cells, indicating pollutant-specific cytotoxicity. In contrast, IL-8 and IL-6 release (24 h > 0 h) were not changed by exposure to the individual pollutants, but tended to increase following co-exposure.

CONCLUSION

The present work establishes the utility of ALI exposure systems to disentangle individual effects of pollutants from a mixture, and highlights the importance of direct experimental characterization of dosimetry and exposure conditions.

Key challenges for in vitro testing of tobacco products for regulatory applications: Recommendations for dosimetry

The Institute for In Vitro Sciences (IIVS) is sponsoring a series of workshops to develop recommendations for optimal scientific and technical approaches for conducting in vitro assays to assess potential toxicity within and across tobacco and various next-generation products (NGPs) including heated tobacco products (HTPs) and electronic nicotine delivery systems (ENDSs). This publication was developed by a working group of the workshop members in conjunction with the sixth workshop in that series entitled "Dosimetry for conducting in vitro evaluations" and focuses on aerosol dosimetry for aerosol exposure to combustible cigarettes, HTP, and ENDS aerosolized tobacco products and summarizes the key challenges as well as documenting areas for future research.

Comparison of biological responses between submerged, pseudo-air-liquid interface, and air-liquid interface exposure of A549 and differentiated THP-1 co-cultures to combustion-derived particles

Air liquid interface (ALI) exposure systems are gaining interest, and studies suggest enhanced response of lung cells exposed to particles at ALI as compared to submerged exposure, although the results have been somewhat inconsistent. Previous studies have used monocultures and measured particle deposition using assumptions including consistent particle deposition, particle density, and shape. This study exposed co-cultures of A549 and differentiated THP-1 cells to flame-generated particles using three exposure methods: ALI, pseudo-ALI, and submerged. The dose at ALI was measured directly, reducing the need for assumptions about particle properties and deposition. For all exposure methods an enhanced pro-inflammatory response (TNFα) and Cytochrome P450 (CYP1A1) gene expression, compared to their corresponding negative controls, was observed. ALI exposure induced a significantly greater TNFα response compared to submerged exposure. The submerged exposures exhibited greater induction of CYP1A1 than other exposure methods, although not statistically significant. Some of the factors behind the observed difference in responses for the three exposure methods include differences in physicochemical properties of particles in suspending media, delivered dose, and potential contribution of gas-phase species to cellular response in ALI exposure. However, given the difficulty and expense of ALI exposures, submerged exposure may still provide relevant information for particulate exposures.

Determining real-time mass deposition with a quartz crystal microbalance in an electrostatic, parallel-flow, air-liquid interface exposure system

In vitro studies are the first step toward understanding the biological effects of particulate matter. As a more realistic exposure strategy than submerged culture approaches, air-liquid interface (ALI) in vitro exposure systems are gaining interest. One challenge with ALI systems is determining accurate particle mass deposition. Although a few commercially available ALI systems are equipped with online mass deposition monitoring, most studies use indirect methods to estimate mass doses. These different indirect methods may contribute to inconsistencies in the results from in vitro studies of aerosolized nanoparticles. This study explored the effectiveness of using a commercially available Quartz Crystal Microbalance (QCM) to estimate the real-time, particle-mass deposition inside an electrostatic, parallel-flow, ALI system. The QCM system required minor modifications, including custom-designed and fabricated headers. Three QCM systems were simultaneously placed in three of the six wells in the ALI exposure chamber to evaluate the uniformity of particle deposition. The measurements from fluorescein dosimetry and QCM revealed an uneven deposition between these six wells. The performance of the QCM system was also evaluated using two different methods. First, using fluorescein deposition in one well, depositions in three other wells were estimated, which was then compared to the actual QCM readings. Second, using the QCM measured deposition in one well, the deposition in three other wells was estimated and compared to deposition measured by fluorescein dosimetry. For both methods, the expected and actual deposition yields a linear fit with the slope ~1. This good fit suggests that QCM systems can be used to measure real-time mass deposition in an electrostatic ALI system.

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

Background

Accurate knowledge of cell−/tissue-delivered dose plays a pivotal role in inhalation toxicology studies, since it is the key parameter for hazard assessment and translation of in vitro to in vivo dose-response. Traditionally, (nano-)particle toxicological studies with in vivo and in vitro models of the lung rely on in silio computational or off-line analytical methods for dosimetry. In contrast to traditional in vitro testing under submerged cell culture conditions, the more physiologic air-liquid interface (ALI) conditions offer the possibility for real-time dosimetry using quartz crystal microbalances (QCMs). However, it is unclear, if QCMs are sensitive enough for nanotoxicological studies. We investigated this issue for two commercially available VITROCELL®Cloud ALI exposure systems.

Results

Quantitative fluorescence spectroscopy of fluorescein-spiked saline aerosol was used to determine detection limit, precision and accuracy of the QCMs implemented in a VITROCELL®Cloud 6 and Cloud 12 system for dose-controlled ALI aerosol-cell exposure experiments. Both QCMs performed linearly over the entire investigated dose range (200 to 12,000 ng/cm²) with an accuracy of 3.4% (Cloud 6) and 3.8% (Cloud 12). Their precision (repeatability) decreased from 2.5% for large doses (> 9500 ng/cm²) to values of 10% and even 25% for doses of 1000 ng/cm² and 200 ng/cm², respectively. Their lower detection limit was 170 ng/cm² and 169 ng/cm² for the Cloud 6 and Cloud 12, respectively. Dose-response measurements with (NM110) ZnO nanoparticles revealed an onset dose of 3.3 μg/cm² (or 0.39 cm²/cm²) for both cell viability (WST-1) and cytotoxicity (LDH) of A549 lung epithelial cells.

Conclusions

The QCMs of the Cloud 6 and Cloud 12 systems show similar performance and are highly sensitive, accurate devices for (quasi-) real-time dosimetry of the cell-delivered particle dose in ALI cell exposure experiments, if operated according to manufacturer specifications. Comparison with in vitro onset doses from this and previously published ALI studies revealed that the detection limit of 170 ng/cm² is sufficient for determination of toxicological onset doses for all particle types with low (e.g. polystyrene) or high mass-specific toxicity (e.g. ZnO and Ag) investigated here. Hence, in principle QCMs are suitable for in vitro nanotoxciological studies, but this should be investigated for each QCM and ALI exposure system under the specific exposure conditions as described in the present study.