Prior acute ozone injury dampens Th2 responses to subsequent repetitive ozone exposures in mice

Ground-level ozone (O ₃ ), 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 O ₃ -induced acute lung injury responsiveness of respiratory tract to low-concentration repetitive O ₃ exposures. Accordingly, we hypothesized that prior acute O ₃ exposure would modulate the lung’s Th2 responses to subsequent repetitive O ₃ exposures. We exposed 8-10-week-old C57BL6/J mice to either filtered air (FA) or 2 ppm O ₃ for three hours, followed by a three-week recovery period, after which the mice received daily exposures to FA or 1 ppm O ₃ 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 O ₃ exposure) mice did not exhibit any signs of injury or inflammation. The O ₃ -FA (acute O ₃ exposure only) groups exhibited baseline immune cell populations and no evidence of MCM suggesting almost complete recovery from acute lung injury. In contrast, FA-O ₃ group (repetitive O ₃ 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-O ₃ group, the O ₃ -O ₃ group (both acute and repetitive O ₃ ) 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 O ₃ -O ₃ group. Furthermore, these mice displayed reduced numbers of mast cells and FIZZ1 ⁺ epithelial cells. These findings suggest that prior exposure to acute high-concentration O ₃ modulates inflammatory and remodeling responses induced by subsequent repeated low-concentration O ₃ exposures. The findings from this study highlight the health impacts of O ₃ pollution, particularly in populations experiencing intermittent high-level exposures.