Prior acute ozone injury modulates inflammatory responses to subsequent repetitive ozone exposures in mice

Ground-level ozone (O₃), a criteria air pollutant, can cause significant adverse effects on lung health, including airway inflammation, compromised lung function, and increased susceptibility to lung infections. This study was designed to determine whether a history of acute lung injury from high-concentration O₃ modulates respiratory tract inflammatory responses to repetitive low-concentration O₃ exposures. Accordingly, we hypothesized that mice acutely exposed to a high concentration of O₃ would exhibit a modulated type-2 (Th2)-like inflammatory response to subsequent, repetitive O₃ exposures, even after full recovery from non-Th2 acute inflammation caused by the initial exposure. Ten-week-old C57BL6/J mice were exposed to either filtered air (FA) or 2 ppm O₃ for 3 h. After a three-week recovery period, the mice received daily exposures to FA or O₃ for 4 h per day over nine days. We evaluated immune cells and inflammatory mediators in cell-free bronchoalveolar lavage fluid (BALF), and examined mucous cell metaplasia (MCM) and epithelial cell injury in lung tissue sections. As expected, mice in the FA-FA group, which were not exposed to O₃, showed no overt signs of injury or inflammation. Mice in the O₃-FA group, which received acute exposure to 2 ppm O₃ and allowed to recover for 3 weeks, exhibited homeostatic immune cell composition within lung airspaces and no evidence of MCM, suggesting almost complete recovery from non-Th2 acute inflammation caused by the initial exposure. In contrast, mice in the FA-O₃ group, which received only repetitive exposures to O₃, exhibited elevated eosinophil counts and prominent MCM, suggesting Th2 inflammatory responses. These mice also displayed increased numbers of FIZZ1⁺ epithelial cells in the lungs. Compared with the FA-O₃ group, mice in the O₃-O₃ group, which received acute exposure to 2 ppm O₃, were allowed to recover for 3 weeks, and then repetitively exposed to 1 ppm O₃, exhibited attenuated Th2 inflammatory responses, as evidenced by diminished eosinophil and lymphocyte counts and attenuated MCM. Compared with the FA-O₃ group, bronchoalveolar lavage fluid (BALF) from the O₃-O₃ group showed lower levels of eotaxin, IL-1α, IL-1β, and IL-4, but higher levels of G-CSF, KC, IL-6, and IL-12, with a trend toward higher IL-10. Furthermore, lung sections in the O₃-O₃ group also displayed reduced numbers of mast cells and a trend toward fewer FIZZ1⁺ epithelial cells. These findings suggest that prior exposure to acute, high-concentration O₃ modulates inflammatory and remodeling responses induced by subsequent repeated exposures to low-concentration O₃. This study highlights the health impacts of pollution, particularly for populations experiencing intermittent high-level exposures to O₃.