Prior acute ozone injury dampens Th2 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 conceived to determine whether a history of high-concentration O₃-induced acute lung injury responsiveness of respiratory tract to low-concentration repetitive O₃ exposures. Accordingly, we hypothesized that prior acute O₃ exposure would modulate the lung's Th2 responses to subsequent repetitive O₃ exposures. We exposed 8-10-week-old C57BL6/J mice to either filtered air (FA) or 2 ppm O₃ for three hours, followed by a three-week recovery period, after which the mice received daily exposures to FA or 1 ppm O₃ for four hours over 9 days. We evaluated immune cell recruitment, inflammatory mediators in cell-free bronchoalveolar lavage fluid (BALF) and examined mucous cell metaplasia and epithelial cell injury in lung tissue sections. As expected, FA-FA (no O₃ exposure) mice did not exhibit any signs of injury or inflammation. The O₃-FA (acute O₃ exposure only) groups exhibited baseline immune cell populations and no evidence of MCM suggesting almost complete recovery from acute lung injury. In contrast, FA-O₃ group (repetitive O₃ exposure only) demonstrated loss of body weight, marked immune cell recruitment, and prominent MCM. These mice also displayed elevated BALF levels of eotaxin, IL-1α, IL-1β, and IL-4, along with increased number of mast cells and FIZZ1⁺ epithelial cells in the lungs. Compared with FA-O₃ group, the O₃-O₃ group (both acute and repetitive O₃) exhibited attenuated responses, as evidenced by diminished eosinophil and lymphocyte counts, and attenuated MCM. BALF analysis revealed lower levels of eotaxin, IL-1α, IL-1β, and IL-4, but elevated levels of G-CSF, KC, IL-6, IL-10, and IL-12 in the O₃-O₃ group. Furthermore, these mice displayed reduced numbers of mast cells and FIZZ1⁺ epithelial cells. These findings suggest that prior exposure to acute high-concentration O₃ modulates inflammatory and remodeling responses induced by subsequent repeated low-concentration O₃ exposures. The findings from this study highlight the health impacts of O₃ pollution, particularly in populations experiencing intermittent high-level exposures.