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

## TL;DR

Prior high ozone exposure reduces lung inflammation from later low ozone exposures in mice.

## Contribution

The study reveals that prior acute ozone exposure modulates subsequent Th2 immune responses to repeated ozone exposure.

## Key findings

- Mice with prior acute ozone exposure showed reduced immune cell recruitment and inflammation after repeated low-dose ozone.
- BALF analysis showed lower levels of Th2-related cytokines like eotaxin and IL-4 in the O3-O3 group.
- O3-O3 mice had elevated levels of anti-inflammatory cytokines like IL-10 and IL-12.

## Abstract

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.