Rationale Chronic obstructive pulmonary disease (COPD) is a leading global cause of morbidity and mortality. Cigarette smoking (CS) is the main risk factor for the development of COPD and most COPD exacerbations are caused by respiratory infections including influenza. Influenza A virus (IAV) infections are more severe in smokers with increased morbidity and mortality. The mechanism of increased susceptibility to infections in smokers is likely due to alteration of immunologic host defenses. Our group has shown that Retinoic acid-inducible protein I (RIG-I) induction in the lung is inhibited by CS in our human lung organ culture model and two animal models.
Methods We investigated RIG-I, TLR3 and interferon (IFN) induction by IAV in human bronchial epithelial cells (HBEC) isolated from smokers or non-smokers. Subcultured HBECs were infected with A/Puerto Rico/8/1934 (PR8) influenza virus at an MOI of 1. Virus-free diluents (mock) were used as negative controls. After 24 h infection, cells and supernatants were collected for qRT-PCR, immunoblot or ELISA to determine RIG-I, TLR3 and IFN expression levels.
Results As expected, we found IAV PR8 exposure induced a vigorous IFN-β, IFN-λ 1 and IFN-λ 2/3 antiviral response in HBECs from nonsmokers as well as high RIG-I and TLR3 induction by virus. In cells from smokers, viral RIG-I and TLR3 mRNA induction was reduced 87% and 79% compared to the response from nonsmokers. CS inhibited viral induction of the IFN-β, IFN-λ1 and IFN-λ 2/3 mRNA response 85%, 96% and 95%, respectively, from that seen in HBEC from nonsmokers. However, preincubation with the demethylating agent 5-Aza-2-deoxycytidine reversed the immunosuppressive effects of CS exposure in HBEC since normal induction of all three IFNs was restored. We also demonstrated, even without virus infection, that IFN-β-mediated induction of RIG-I and TLR3 was also suppressed in cells from smokers.
Conclusions Our results suggest that active smoking reduces expression of antiviral cytokines in primary HBECs. This effect likely occurs via downregulation of RIG-I and TLR3 through epigenetic modifications. Reduction in lung epithelial cell RIG-I and TLR3 responses may be one major mechanism contributing to the increased incidence of viral respiratory infections in smokers and to viral induction of acute exacerbations of COPD.
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