Study On The Mechanism Of Air Fine Particles PM_2.5-induced Alveolar Epithelial Injury And Repair

PM2.5,as a fine particle with a diameter of less than 2.5 microns,is a great harm to the human body,especially the lung.Most studies based on cell lines suggest that PM2.5 inhalation causes lung redox reactions to cause tissue damage,but lung epithelial mucosa has an ability to repair itself after injury.How this regenerative mechanism responds to PM2.5-induced lung injury has not been reported.Type Ⅱ alveolar epithelial stem cells,also called as Alveolar type 2 cells(AT2 cells),are able to self-renew,proliferate,and differentiate into type I alveolar epithelial(AT1)cells,thereby maintaining the homeostasis of alveolar epithelia.After alveolar epithelial injury,the repair program also plays a vital role.Therefore,this study focuses on the important environmental issues of PM2.5-induced acute lung injury,and aims to explore the role of AT2 cells in it,and adopts a microfluidic platform to reveal the mechanism of PM2.5induced lung injury.In this research,experiments were carried out at the in vivo and in vitro levels.The wholebody exposure animal model was used to mimic the PM2.5 particle exposure conditions in people’s daily life.The composition of the inhaled PM2.5 components and particle size were analyzed.The amount of PM2.5 inhaled in the mice was estimated.H&E staining and immunofluorescence staining were used to evaluate the lung damage and AT2 cell proliferation after PM2.5 inhalation.The results showed that the composition of PM2.5 includes both watersoluble cations,water-insoluble carbon substances and heavy metals.The size of collected PM2.5 particles was around 1 micrometer.After one month of continuous exposure to PM2.5,the total cumulative exposure dose of each mouse inhaled was 33.6 mg/(m3h),which caused an oxidative stress response in the lung.There was a significant increase in the abundance of macrophages in the lungs of mice.AT2 proliferation was accelerated,suggesting that continuous PM2.5 exposure can easily cause inflammation in the lungs and activate AT2 cells to initiate repair.After PM2.5 continuous exposure for one month,mice were then switched to intermittent exposure for additional three months.The abundance of macrophages in the lungs of mice and the proliferation ability of AT2 cells returned to normal levels,suggesting that continuous PM2.5 exposure-induced lung injury was reversed during the following intermittent exposure.After continuous and intermittent exposure to PM2.5,the mice were kept for additional six months,and the abundances of lung macrophages and AT2 cells were similar to those of the control group.The above studies suggest that continuous exposure to PM2.5 can cause lung damage,but the damage was gradually diminished without long-term sequelae when exposure frequency was decreased,which is related to the initiation and repair of the alveolar regeneration mechanism.To be estimated,one-month in vivo PM2.5 exposure dose is relevant to 0.01 mg/mL in the in vitro experiment.In vitro 3D organoid culture suggested that PM2.5 at 0.01mg/mL stimulated the proliferation of mouse AT2 cells,which was consistent with in vivo experiments.Increasing the concentration of PM2.5 was found to impair the proliferation of AT2 cells.In order to further explore the possible factors of PM2.5-induced changes in AT2 cell proliferation,transmission electron microscopy and microfluidic chip were used.The results showed that the stiffness of AT2 cells decreased slightly under PM2.5 stimulation,but the cell electrical impedance reduced significantly with the increase of PM2.5 concentration,suggesting that PM2.5 induced AT2 cell senescence.Finally,the effect of PM2.5 on the MLg fibroblast cells,supporting niche for AT2 cells,was studied,and the results showed that PM2.5 did not promote nitric oxide(NO)production in fibroblasts.This research combines the in vivo and in vitro studies,and initially explores the direct and indirect effects of PM2.5 particles on alveolar stem cells,demonstrated the involvement of lung repairing program after PM2.5-induced lung injury,broadens the research in this field,and provides cross-platform tools to understand PM2.5-induced injury to human body.