The role of carbon nanotube structure in their retention and pathogenicity in the pleural cavity

• Main Author: Murphy, Fiona A
• Format: Dissertation
• Language: English
• Published: University of Edinburgh 2012

Carbon nanotubes (CNT) are hexagonal arrangements of carbon atoms built up to form fibres with diameters in the nanometre range but lengths which may extend up to hundreds of microns. The physiochemical properties of CNT are advantageous for a variety of industrial applications leading to CNT becoming one of the major products in the burgeoning field of nanotcchnology. However their structural similarity to asbestos has raised concerns that they may also pose an occupational inhalation hazard and cause diseases of the lung or pleura. Several decades of fibre toxicology have lead to the development of a robust structure/activity model, the fibre pathogenicity paradigm (FPP), which identifies length, thinness and biopersistence as the critical properties a fibrous particle must possess if it is to be pathogenic. The purpose of this study was to examine the pathogenicity of CNT in relation to the FPP by examining the effect of CNT in the pleural space, a target tissue for asbestosrelated disease. In order to address this aim a method of injection directly into the pleura cavity of mice was employed. Direct instillation of long and short CNT into the pleural cavity produced length-dependent responses characterized by acute inflammation leading to progressive fibrosis on the parietal pleura which mirrored the pleura response to asbestos. Furthermore examination of the size-restricted clearance mechanisms from the pleural cavity confirmed the hypothesis that the pathogenicity of long CNT and other fibres, arises as a result of length-dependent retention at the stomata on the parietal pleura. The cellular interactions leading to an inflammatory response in the pleural cavity were also examined in an in vitro study which tested the CNT for their ability to stimulate the release of the acute phase cytokines from both mesothelial cells and macrophages. Direct exposure to CNT resulted in significant cytokine release from the macrophages but not mesothelial cells. This pro-inflammatory response was length dependent but modest and was shown to be a function of frustrated phagocytosis. Furthermore the indirect actions of the CNT were examined by treating the mesothelial cells with conditioned media from CNT-treated macrophages. This resulted in dramatic amplification of cytokine release from the mesothelial cells. We therefore hypothesise that long fibres elicit an inflammatory response in the pleural cavity via frustrated phagocytosis in pleural macrophages. The activated macrophages then stimulate an amplified pro-inflammatory cytokine response from the adjacent pleural mesothelial cells. A further aim was to investigate the relationship between the length-dependent pathogenicity of a fibre sample and the surface of the fibre. By using different forms of functional groups attached to the surface of a pathogenic CNT we tested if the level of inflammation and fibrosis triggered in vivo can be altered by simple alteration of the surface. Our results showed that, although the surface modification of CNT did not alter the acute inflammogenicity of the CNT, the chronic fibrotic response was significantly attenuated. The specific role surface chemistry played in the modification of the CNT pathogenicity however was obfuscated by the apparent lack of biopersistcncc of the functionalised CNT compared with the pristine sample. Although direct injection into the pleural space is a convenient model to assess the hazard of fibres to the mesothelium it is not a physiologically relevant route by which workers may be exposed to CNT. Therefore we examined the inflammatory potential of CNT on the lungs and pleural cavity following pharyngeal aspiration into the airspaces. A length-dependent inflammatory response in the lungs was observed where only the long CNT sample caused acute neutrophilic inflammation at one week and progressive interstitial thickening of the alveolar septa by six weeks post exposure. Furthermore we report the induction of a length-dependent inflammatory response in the pleural cavity after exposure to CNT via the lung airspaces with concomitant evidence for the translocation of CNT from the lung into in the pleural cavity and subsequent retention along the parietal pleura. In summary the results presented here demonstrate the length-dependent pathogenicity of CNT in the pleural cavity and highlights the necessity for risk assessment for people likely to be exposed in the workplace. We also explored mechanistic aspects of the inflammatory response to long CNT which has implications for the general understanding of fibre-related pleural disease and may prove useful for the design of safe nanofibres.