Cigarette smoke exposure modulates the inflammatory responses during pandemic H1N1 and H9N2 infection

Abstract

Cigarette smoke displays both immunoactivating and immunosuppressive activities. Although the health risks of cigarette smoke are widely reported, the impact and mechanism of cigarette smoke exposure on inflammatory diseases are still unclear. Innate immune responses could be activated shortly after influenza A virus infection and then trigger inflammation in the lung. Until now, the effect of prior cigarette smoke on the inflammation caused by the following influenza A virus infection has not been fully understood and there is no study about the role of cigarette smoke in 2009 pandemic H1N1 (pdmH1N1) and avian H9N2 (H9N2/G1) infection.

In this study, C57/B6N mice were whole body exposed to 4% cigarette smoke 4 hours per day for 21 days and then infected with pdmH1N1 or H9N2 virus. Some mice were exposed to room air in parallel as the control. Nicotine was also used to mimic the effect of the cigarette smoke. Human lung adenocarcinoma epithelial cell line A549 or human monocyte-derived macrophages were pre-treated with 10μM nicotine, and then infected with pdmH1N1 or H9N2 virus. The expression of cytokines and chemokines was examined in the supernatant. C57/B6N mice were treated with 24mg/kg/day nicotine or sterile water subcutaneously for 21 days and then infected with pdmH1N1 or H9N2 virus. Survival, body weight changes, lung viral loads, inflammatory responses and immune cells that infiltrated into the lung were analyzed.

Cigarette smoke exposure alone significantly up-regulated the lung inflammation, confirmed by the dramatically decreased body weight gain and increased inflammatory response compared with the control mice. Such prior cigarette smoke exposure significantly reduced the disease severity induced by subsequent pdmH1N1 or H9N2 virus infection. For pdmH1N1 infection, cigarette smoke exposure group had a significantly lower mortality than the control group, which might be due to the lower inflammatory response at day 5 post virus infection. Similarly, after H9N2 virus infection, the mice in cigarette smoke exposure group displayed a significantly milder disease, as evidenced by less weight loss, weaker inflammatory response as well as lower number of immune cells infiltrating into the lung compared to the control group. There was no difference for the lung viral loads between the two groups upon both pdmH1N1 and H9N2 virus infection.

Besides, pre-treating the A549 and primary human macrophages with nicotine decreased the expression of some cytokines and chemokines after pdmH1N1 or H9N2 virus infection. The mice treated with nicotine displayed significantly less weight loss and lower inflammatory response than control mice treated with sterile water upon pdmH1N1 or H9N2 virus infection.

Collectively, our study demonstrated that the immunosuppressive effect of cigarette smoke was responsible for amelioration of pdmH1N1 and H9N2 pathogenicity by dampening the hyper-reaction of inflammatory response without any direct effect on viral replication. Nicotine, an anti-inflammatory factor, was the key component in cigarette smoke that was responsible for this immunosuppressive effect. Our study provided the first in vivo evidence that cigarette smoke, mostly due to nicotine, could protect against the pathogenicity of not only pdmH1N1, but also H9N2 virus.