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Exposure to iron-laden particulate matter impacts on respiratory health

Williams, LJ ORCID: 0000-0002-4808-9580 2019 , 'Exposure to iron-laden particulate matter impacts on respiratory health', PhD thesis, University of Tasmania.

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Abstract

Introduction
Ambient air pollution causes more than 4 million premature deaths globally every year. This excess mortality is primarily driven by inhalation of particulate matter (PM). The physicochemical properties of PM are important in determining the respiratory health outcomes but are typically not considered when setting air quality guidelines which are primarily based on PM size. The physicochemical properties of PM vary depending on the source. For example, urban PM is dominated by combustion-derived PM; particularly diesel exhaust particles. PM from these sources has been extensively studied in terms of its negative impact(s) on health. In contrast, geogenic (earth-derived) particles often affect populations in rural and arid areas, and our understanding of the respiratory health impacts of PM from these sources is far more limited. Exposure to geogenic PM is correlated with increased mortality and hospital admissions. Geogenic PM may be particularly relevant to the health of disadvantaged populations such as Australian Aboriginal children living in remote communities where geogenic PM levels are high and there is a disproportionate prevalence of chronic respiratory disease. Several studies suggest that inhalation of geogenic PM10 has the capacity to cause inflammation and exacerbate existing lung disease. It is also clear that the physicochemical characteristics of the particles have a significant impact on the response. However, research on the respiratory impacts of Fe, an important constituent of geogenic PM, is lacking.
Bronchiectasis, which is prevalent in some communities exposed to high levels of geogenic PM, is predominantly driven by respiratory infection. Bronchiectasis requires an impaired host response and/or increased pathogenicity of the bacteria. Geogenic PM has been shown to increase the severity of viral infection in vivo and NTHi adhesion and invasion in vitro. This suggests that inorganic PM can reduce the host’s ability to defend against infection. However, there are no data on the effect of these particles on bacterial growth or any comparative analysis of the effect of NTHi strain on respiratory cell invasion and how this is modified by silica and iron oxide. Thus, there is a need for further studies on the environmental contributions to susceptibility to bacterial infection as a potential explanation for the increased prevalence of bronchiectasis in Aboriginal Australian communities.
Methodology
To assess the impact of silica and iron oxide on inflammation and cytotoxicity in lung epithelial cells (Chapter 2), bronchial epithelial (BEAS-2B) and alveolar epithelial (A549) cell lines were exposed to increasing concentrations of silica (quartz; SiO2) and/or iron oxide (haematite; Fe2O3 or magnetite; Fe3O4). Cytotoxicity was assessed by lactate dehydrogenase (LDH) assay and the production of the inflammatory cytokines interleukin (IL)-1β, IL-6, IL-8 and tumour necrosis factor-α (TNF-α) were measured by enzyme-linked immunosorbent assay (ELISA). To assess the effects of particles on inflammation and the response to a bacterial stimulus in macrophages (Chapter 3), THP-1 monocyte-derived macrophages were exposed to increasing concentrations of silica and/or iron oxide with or without lipopolysaccharide (LPS). The response was assessed by LDH assay and ELISA. To evaluate whether the PM could directly impact bacterial growth (Chapter 4), 9 clinical isolates of the iron-dependent bacteria Non-typeable Haemophilus influenzae (NTHi) were exposed to silica or iron oxide under a range of free iron growth conditions. Growth cycles of each isolate were evaluated for growth rate and maximal growth using spectral absorbance and agar plate counts. Finally, to evaluate the impacts of these particles on the invasion of NTHi in lung cells and on phagocytosis by macrophages (Chapter 5), epithelial and macrophage cell lines and primary human peripheral blood mononuclear cell (PBMC)-derived macrophages were exposed to silica and/or iron oxide. Invasion of cells by NTHi was quantified by flow cytometry. Cellular and bacterial responses associated with bacterial invasion were assessed by flow cytometry, qPCR and whole genome sequencing.
Results
Silica caused mild cytotoxicity and a robust pro-inflammatory response in epithelial cells. Comparatively, iron oxide was not cytotoxic and had no effect on cytokine production in BEAS-2B cells but did increase IL-8 production in A549 cells (Chapter 2). In contrast, both silica and magnetite caused cytotoxicity in THP-1 cells. All particles caused an inflammatory cytokine response; although the silica response was more potent. Interestingly, haematite and magnetite augmented the IL-1β, IL-6 and IL-8 response to LPS to a similar extent as silica (Chapter 3). In the absence of cells, exposure to magnetite increased maximal growth in several isolates of NTHi. Comparatively, the response to silica and haematite was variable with some isolates exhibiting enhanced growth and others reduced growth (Chapter 4). The effect of particles on bacterial invasion (Chapter 5) varied depending on the PM source, cell type and NTHi isolate. Surprisingly, silica either had no effect on, or reduced, the invasiveness of NTHi in epithelial cells. In contrast, haematite significantly increased the invasion of most NTHi isolates in BEAS-2B cells. Conversely, both haematite and magnetite, but not silica, significantly reduced the phagocytotic capacity of macrophages in response to most NTHi isolates.
Discussion
Previous studies have shown a link between exposure to iron-laden geogenic PM and prevalence of bronchiectasis. In this Thesis, it has been shown that inflammatory response of epithelial cells and macrophages to iron oxide alone is unlikely to be making a major contributor to this association. However, iron oxide PM clearly enhanced the macrophage response to bacterial stimulation (LPS) and modified the growth of NTHi in a chemical (magnetite vs haematite) and isolate dependent manner. What was most striking, was the capacity of iron oxides to promote NTHi invasion in epithelial cells and impair macrophage phagocytosis of NTHi. The presented data provides insight into one of the mechanisms that may be contributing to excessively high burdens of chronic respiratory disease in regional communities, particularly bronchiectasis in Australian Aboriginals, and those in arid environments exposed to high levels of geogenic PM. In particular, this Thesis highlights that iron oxide PM may have profound impacts on the respiratory susceptibility to the severe bacterial infections.

Item Type: Thesis - PhD
Authors/Creators:Williams, LJ
Keywords: Respiratory health, Inflammation, Non-typeable Haemophilus influenzae, Iron oxide, Bronchiectasis.
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