Cigarette smoke exposure
The animals in different groups were caged separately in a series of transparent chambers (each sized 28 × 21 × 17 cm3) that were arranged in parallel and connected to a central CS supply, where smoke from the ignited cigarettes (Hongjinglong™, Wuhan, China) was automatically pumped evenly into the chambers. Each cigarette contained 10 mg of tar, 0.8 mg of nicotine, and 12 mg of carbon monoxide. During each 60-minutes episode of smoke exposure, eight cigarettes without filters were combusted in turn, generating an air mixture containing 8% CS (the PM 2.5 particles in the air mixture was 45 ± 8.37 mg/m3). The mice allocated to the sham CS exposure were caged in the same environment but exposed only to fresh air for 60 minutes. These 60 minutes CS exposure or sham -CS exposure episodes were conducted twice a day, 5 days a week, for 30 weeks.