Carbon nanotubes (CNTs) have attracted interest in many industries because of their novel and beneficial properties. Future exposure to public by inhalation is expected with the dramatic increase in manufacture and use of CNTs. Concurrently urban air pollution, including the contribution of diesel exhaust particles (DEPs), is already a heavy burden on public health on a global scale.
The overarching hypothesis of the study was that the physicochemical properties of carbon-based nanoparticles would determine their bioreactivity and toxicity at the alveolar unit of the lung. As I was studying both CNTs and DEPs, I had two more specific hypotheses:
1) The cellular responses would be dependent on the source of exhaust emissions which determines the size and composition of the resultant DEPs.
2) The surface charge of multi-walled carbon nanotubes (MWCNTs) would determine the magnitude of the bioreactive responses in alveolar cells.
Simple and complex in vitro models utilising the human alveolar type 1 cell line (TT1) and human primary cells: alveolar epithelial type 2 (AT2), alveolar macrophage (AM) and microvascular endothelial cells (HPMVEC) were used to test the hypotheses.
The development and use of a complex in vitro cell model was a vital aspect of this study as it was imperative to understand the interaction not only between nanoparticles (NPs) and cells but also to gain an insight into the interaction between cells. Performing studies in simple and complex models comparatively allowed for greater awareness of the cell-cell interactions which inform toxicological outcomes following particle uptake including reactive oxygen species (ROS) formation and inflammation.
Diesel particulate matter (PM) and carbon black induced moderate endothelial cell activation characterised by IL-6 release and a pro-thrombotic response. Recently generated automobile diesel PM and biodiesel PM were notably less reactive at the epithelium than forklift diesel PM and carbon black; however as all of the NPs were internalised by the epithelial type 1 cells and macrophages, the potential chronic and systemic effects of all of the particles should be considered. The conspicuous cytotoxicity of carbon black was likely a combination of its smaller size and surface activity.

The MWCNTs were acutely reactive at the alveolar epithelium, inducing cytotoxicity, inflammation (including anti-protease production at even the lowest concentrations, 0.1 µg/ml) and ROS generation. All of the MWCNTs were internalised by the alveolar cells, by active and passive uptake mechanisms, however the positively charged CNT had distinctly more interaction with the alveolar cells and thus greater uptake, and even translocation in the epithelial type 1 cells. Of the MWCNTs, MAA and APTAC induced the smallest and greatest bioreactive responses respectively, in the cellular (simple and complex) models utilised, confirming the hypothesis.
The association between nanoparticle exposure in humans to a number of adverse respiratory outcomes is well-established however the mechanisms by which these objects influence human health are still only partially understood. My study demonstrated that nanoparticle toxicity is dependent on their physicochemical properties, specifically charge in the case of MWCNTs, and generally size, as well as the cell models used. Understanding the toxicity of carbon nanomaterials, both anthropogenic and engineered, which are major contaminants of our environment and determinants of cardiopulmonary toxicity, is essential to generating safer nanomaterials and limiting airborne exposure to protect human health.