Nitrogen Dioxide Exposure Attenuates Cigarette Smoke-Induced Cytokine Production in Mice

Abstract

Cigarette smoke is the most important cause for the development of chronic obstructive pulmonary disease (COPD). Since only a minority of smokers and some nonsmokers develop COPD, other factors must be involved as well. NO₂ is an important air pollutant associated with respiratory symptoms in humans and emphysema development in animal models. We hypothesized that combined exposure to NO₂ and cigarette smoke will enhance pulmonary inflammation and emphysema development. Mice were exposed to 20 ppm NO₂ for 17 h/day, to 24 puffs of cigarette smoke 2 times per day, to their combination, or to control air for 5 days/wk during 4 wk. Following the last NO₂ exposure and within 24 h after the last smoke exposure the mice were sacrificed. Lungs were removed and analyzed for several inflammatory parameters and emphysema. Cigarette smoke exposure increased eosinophil numbers and levels of tumor necrosis factor (TNF)-α, KC, monocyte chemoattractant protein (MCP)-1, and interleukin (IL)-6. NO₂ exposure increased goblet cells, eosinophils, and the levels of IL-6, while it decreased the levels of IL-10. Four weeks of NO₂, cigarette smoke, or their combination was not sufficient to induce significant emphysema, nor did it lead to increased numbers of lymphocytes, neutrophils, or macrophages in lung tissue. Instead, NO₂ exposure attenuated the smoke-induced increases in levels of TNF-α, KC, and MCP-1. These dampening effects of NO₂ may be due to modulating effects of NO₂ on cytokine production by macrophages and epithelial cells, which have been reported earlier. The next step is to translate these findings of combined, controlled exposure in animals to the human situation.