Radical containing combustion derived particulate matter enhance pulmonary Th17 inflammation via the aryl hydrocarbon receptor.

Background: Pollutant particles containing environmentally persistent free radicals (EPFRs) are formed during many combustion processes (e.g. thermal remediation of hazardous wastes, diesel/gasoline combustion, wood smoke, cigarette smoke, etc.). Our previous studies demonstrated that acute exposure to EPFRs results in dendritic cell maturation and Th17-biased pulmonary immune responses. Further, in a mouse model of asthma, these responses were enhanced suggesting exposure to EPFRs as a risk factor for the development and/or exacerbation of asthma. The aryl hydrocarbon receptor (AHR) has been shown to play a role in the differentiation of Th17 cells. In the current study, we determined whether exposure to EPFRs results in Th17 polarization in an AHR dependent manner.

Results: Exposure to EPFRs resulted in Th17 and IL17A dependent pulmonary immune responses including airway neutrophilia. EPFR exposure caused a significant increase in pulmonary Th17 cytokines such as IL6, IL17A, IL22, IL1β, KC, MCP-1, IL31 and IL33. To understand the role of AHR activation in EPFR-induced Th17 inflammation, A549 epithelial cells and mouse bone marrow-derived dendritic cells (BMDCs) were exposed to EPFRs and expression of Cyp1a1 and Cyp1b1, markers for AHR activation, was measured. A significant increase in Cyp1a1 and Cyp1b1 gene expression was observed in pulmonary epithelial cells and BMDCs in an oxidative stress and AHR dependent manner. Further, in vivo exposure of mice to EPFRs resulted in oxidative stress and increased Cyp1a1 and Cyp1b1 pulmonary gene expression. To further confirm the role of AHR activation in pulmonary Th17 immune responses, mice were exposed to EPFRs in the presence or absence of AHR antagonist. EPFR exposure resulted in a significant increase in pulmonary Th17 cells and neutrophilic inflammation, whereas a significant decrease in the percentage of Th17 cells and neutrophilic inflammation was observed in mice treated with AHR antagonist.

Conclusion: Exposure to EPFRs results in AHR activation and induction of Cyp1a1 and in vitro this is dependent on oxidative stress. Further, our in vivo studies demonstrated a role for AHR in EPFR-induced pulmonary Th17 responses including neutrophilic inflammation.