Research focus: Using QCM-D in particulate toxicity assessments

Despite the reduction in harmful gas emissions due to automobile catalytic converters, the concentration of airborne particles from catalytic metals like Palladium is approaching unsafe levels. This highlights the need to evaluate nanotoxicology and improve our understanding of the mechanisms behind the harmful effects of such airborne particles, which is crucial for better risk assessments. Curious to learn more about this area and how QSense QCM-D is used for such nanotoxicology assessments, I reached out to Karin Fromell, Associate Professor in experimental clinical immunology at the Department of Immunology, Genetics and Pathology at Uppsala University. Assoc. Prof. Fromell’s area of expertise is the mechanisms behind activation of innate immune system, i.e., the complement and coagulation/contact systems and their cross talk.

Recently, Assoc. Prof. Fromell and her team conducted a study¹ to investigate the effect of airborne Palladium nanoparticles on human lung cells, endothelium and blood using three in vitro models. In this post, I share what I learned from the interview with Assoc. Prof. Fromell.

Air pollution may cause harmful inflammatory processes

There is a growing concern about how particulate air pollution affects our health, Assoc. Prof. Fromell says. The activation of the complement and coagulation systems may lead to a serious condition called thromboinflammation. There are many possible reasons why this type of activation occurs, ranging from various diseases and pathogens to foreign material exposures. One such example is particulate air pollution, which we actually are exposed to every day, Assoc. Prof. Fromell continues. These small particles and nanoparticles may initiate harmful inflammatory processes resulting in chronic inflammation, cardiovascular diseases, cancer and lung dysfunction. The most important preventive measure is to reduce the harmful exposures, but to do that we must first identify the most toxic particles and understand how and why they lead to serious health effects. Therefore, an important corner stone in our research is to study how different surface properties affect important key proteins in the innate immune system in order to identify which chemical and biochemical mechanisms are responsible for the observed activation.

Developing a new model system for particulate toxicity assessments

Although animal models can be used to assess particulate toxicity, there are limitations with these in vivo studies such as ethical aspects and species dependent results distortion, Assoc. Prof Fromell says. Therefore, there is a great need to develop new methods for toxicity assessment. In this study,¹ we combined three in vitro exposure systems to model the human lung and the surrounding blood and blood vessels for evaluation of pathological events triggered by airborne palladium (Pd) nanoparticles (NP:s).

The three model systems were:

1. lung system with human lung cells directly exposed to airborne NP:s (ALI)
2. human vascular endothelial cell system, and
3. human whole blood loop system

In contrast to many other in vitro - systems, where only a few purified components are included, all components ranging from the lung cell to the blood and blood vessel are present in our model, Assoc. Prof. Fromell says. Exactly the same Pd NP:s were generated, characterized and exposed for all three parts of the model.

Analysis indicates that Pd NP:s are less aggressive than TiO₂

In the ALI lung cell exposure system, we found that the Pd NP:s induced clear reduction in Human bronchial epithelial (BEAS-2B) cell growth, an effect that also remained over longer term and in the endothelial cell model. We also found that Pd NP:s induced apoptosis, but not as potent as the more aggressive TiO₂. In the same way, Pd NP:s triggered coagulation and contact system activation in the blood, but at a significantly lower level compared to the very thrombogenic TiO₂ NP:s.

Using QSense QCM-D to analyze surface triggered coagulation activation

When coagulation activation is triggered by a foreign surface, it is usually the intrinsic pathway that is involved, Assoc. Prof. Fromell explains. Activation is then initiated by coagulation factor XII, also referred to as ‘FXII’, which by binding to the surface changes conformation leading to autoactivation.

The surface activated FXII can trigger two different activation pathways:

1) cleavage of coagulation factor XI (FXI), through a series of events which finally leads to thrombus formation, and
2) activation of prekallikrein, which will generate bradykinin, a proinflammatory signal peptide, inducing inflammation.

To find out if the Pd and TiO₂ surfaces activate FXII directly upon contact, adsorption of FXII to both the Pd and TiO₂ materials was studied with QCM-D. In brief, the experiments were executed as follows:

• FXII was first allowed to bind to both Pd and TiO₂ coated QCM-D sensors to evaluate its ability to bind to the two materials
• Next, the serine protease inhibitor C1-inhibitor was added to the FXII bound to the respective surfaces

Since only the activated form of FXII can form a complex with the C1-inhibitor, we were thus able to find out whether FXII had been autoactivated upon contact with the respective surface. It turned out that FXII was bound to both Pd and TiO₂, although slightly more to TiO₂. However, only FXII on the TiO₂ surface was activated whereas FXII on the Pd surface more or less retained its inactive form.

In this way, we were able to confirm with QCM-D the results from the whole blood model, which showed that Pd NP:s are less aggressive compared to TiO₂ and that this already occurs in contact with FXII.

Concluding remarks

Although automobile catalytic converters have effectively reduced harmful gas emissions, the concentration of airborne particles from catalytic metals such as Pd is reaching unsafe levels and the impact of particulate air pollution on our health is becoming a growing concern, Assoc. Prof. Fromell says. To overcome ethical aspects and limitations associated with animal models traditionally used for particulate toxicity testing, we have developed a new model which could be used to study health issues caused by tiny airborne particles; a 3-step in vitro model that mimics the human lung and surrounding blood vessels. In this study,¹ we particularly wanted to use the model system to assess the toxicity of Pd NP:s derived from automobile catalytic converters, Assoc. Prof. Fromell says. QCM-D analysis was used to study the activation of the initiator molecule FXII on Pd- and TiO₂ surfaces, and the results indicated a clear difference in contact activation between the two materials. In summary, the study suggests that Pd NPs cause a response, but that they appear to be less aggressive than TiO₂ NP:s, Assoc. Prof. Fromell concludes.