Pollutants and immune regulation in the human airway: Modulation of dendritic cell function by environmental particulate matter
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Ambient air pollution, including airborne particulate matter (PM) derived from combustion of fossil fuels (FF) or biomass (BM), has detrimental inflammatory effects on human health. Myeloid antigen presenting cells, including dendritic cells (DCs) regulate immune responses in the airway and sample inhaled PM. This study tests the hypothesis that PM interacts with multiple environmental sensing pathways in DCs with outcomes that depend on particle size and composition as determined by combustion source. The effects of different sized PM (<10μm, PM10; <2.5μm, PM2.5), derived from the combustion of FF or BM, on human monocyte-derived or ex vivo sputum DCs, were examined. DC activation status, cytokine production and aryl hydrocarbon receptor (AhR) signalling were assessed by flow-cytometry, multiplex ELISA and qRT-PCR, following exposure to PM. Pathway-specific antagonists were used to explore underlying mechanisms. Particle size and combustion source influenced the effects of PM on DCs. Irrespective of combustion source, PM10 but not PM2.5, induced MoDC maturation and stimulated production of inflammatory cytokines, including IL-1β and IL-18, indicative of inflammasome activation. These responses were dependent, at least in part, on TLR4 as was the induction of IDO by PM10. AhR signalling was induced by PM in both MoDC and ex vivo sputum DC. It was stimulated by both PM10 and PM2.5 and was induced more strongly by BM-derived PM. AhR activation was independent of DC maturation and TLR4 signalling. Additionally, BM- but not FF-derived PM increased NADH levels in DC suggestive of altered metabolism. Thus, PM induces a complex programme of DC activation, influenced by size and combustion source, which includes classical maturation, inflammasome dependent cytokine release and AhR signalling as well as potential metabolic changes. In the airway, exposure to different PM and the changes in DCs induced by them may lead to altered responses to inhaled antigen.
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