Immune Regulation And Antioxidant Defense Responses Induced By The Exposure Of Urban Atmospheric Particulate Matter In Mice

With the increasingly serious problem of atmospheric particulate pollution,the health hazard caused by particulate matter?PM?has attracted more and more attention from the government,the public and scholars.Numerous studies have shown that most of the adverse health effects induced by PM exposure are closely associated with the maladjustment of immune defense system mediated by inflammatory response and oxidative stress.In recent years,China has achieved remarkable results in controlling air pollution,and the PM concentration in ambient air has significantly decreased.However,there is still a large gap between the concentration of atmospheric particulate matter and the WHO guidelines?WHO Air Auality Guidelines of 2005?,and the frequent occurrence of haze in autumn and winter has not been fundamentally solved.In-depth study on the interaction mechanism between PM exposure and immune defense system will help to assess the harm of current atmospheric particulate pollution objectively.This study can also provide reference for the prevention and control of air pollution as well as the making of relevant policies.In this paper,male C57BL/6 mice were selected as experimental animals,and different exposure models were constructed to investigate the response of immune defense system to actual PM exposure.Firstly,a 12-week continuous exposure experiment was conducted to investigate the periodic response of the immune defense system of mice exposured to PM and identify the sensitive target organs.The mice were randomly divided into two groups and exposed to ambient air or filtered air.After 4,8,and 12 weeks of exposure time,biological samples of lung,heart,liver,and hippocampus were collected,and inflammatory cytokines?including pro-inflammatory factors such as TNF?,IL-1?,and IL-6;anti-inflammatory factors such as IL-10 and IL-1RA?,redox biomarkers?including oxidation markers of ROS and NO;antioxidant enzymes of Sod,Cat and GPx?and histomorphology?hematoxylin-eosin staining and electron microscopy?were analyzed.Then,in order to further reveal the driving factors for different modes of immune defense response and thier related mechanism,a combined exposure model of atmospheric particles was constructed.In the pre-exposure stage of the combined exposure model,the mice were randomly assigned to three groups and exposed for 38 days to whole ambient air,ambient air containing a low PM concentration(PM_2.5?75?g/m³)and filtered air,respectively.Thereafter,all mice underwent a 3-day haze exposure followed by a 7-day exposure to filtered air.At the end of the pre-exposure,all mice experienced three days of exposure to smog and seven days of exposure to filtered air.Lung tissues of mice were collected after pre-exposure and haze-exposure as well as 1 day,3 days and 7 days after haze exposure.The dynamic responses of immune regulation and antioxidant defense ability were evaluated according to the measurements of gene expression,protein content,signal pathway and tissue morphology.The main results of this paper are as follows:?1?Continuous PM exposure disturbedthe homeostasis of the pro-and anti-inflammatory cytokines in multiple mouse organs,and the impacts varied with exposure time and target organs.Compared with the clean air control,the expression levels of pro-inflammatory factors?TNF?,IL-1?and IL-6?in the lungs,heart,liver and hippocampus of the mice gradually increased with the prolonged PM exposure.The expression of IL-10 was up-regulated at 8 weeks in multiple organs of the ambient air group,which may help to inhibit the excessive release of pro-inflammatory cytokines and restore immune system balance.However,the pronounced down-regulation of IL-10,and therefore a high pro-/anti-inflammatory cytokine ratio,in lung and hippocampus at 12 weeks implied a disrupted modulation of cytokine balance.?2?Continuous PM exposure resulted in the disturbances of antioxidant defense homeostasis in multiple mouse organs.Compared with the clean air control,the levels of oxidative products?ROS and NO?and the activities of antioxidant enzymes?Sod,Cat and GPx?in the lungs,heart and liver of the mice were increased synchronously at the initial stage of exposure,indicating that the body initiated the antioxidant defense mechanism.After 12 weeks of exposure,Sod and GPx activities in lung were significantly inhibited,but Sod activity in heart and liver maintained at a high level.?3?Extraneousparticles were expected in the lung and hippocampus of mice exposed to ambient air,including single or aggregated particulate carbon,polygonal silicoaluminate minerals,and metal microbeads suspected to be generated by high temperature combustion.No similar particles were found in the heart and liver.Therefore,the imbalance of immune defense system and irreversible tissue damage in the lung and hippocampus may be related to the fact that atmospheric particles can reach these two organs through various channels.?4?In the combined PM exposure model,the ability of immune regulation to haze exposure was weakest in mice in the whole-air group.Specifically,the number of inflammatory cytokines involved in the response were few and the their up-regulations were lower compared with those in the other two groups.After leaving from haze exposure,the up-regulated inflammatory cytokines faded slowly,and the expression levels of the genes involved in the regression or termination of inflammation were not significantly increased.?5?In the combined PM exposure model,the antioxidant defense response capacity to haze exposure in mice in the whole-air group was impaired.In the primary antioxidant system,only a slight increase of Sod2 was observed after haze exposure.In the secondary antioxidant defense system,most of the genes related to the scavenging of damaged proteins didn't response to haze exposure and the proteasome activity was significantly inhibited.The signaling pathways related to antioxidant defense in ambient air were not significantly activated.?6?When the mice were exposed to a low concentration of particulate matter(PM_2.5?75?g/m³),the responses of immune regulation and antioxidant defense were effectively activated,and most of these biomarkers quickly subsided after leaving from haze exposure.In contrast,the immune defense system of the lung in the filtered-air group responded strongly to the exposure of haze,but the regulation of pro-and anti-inflammatory,oxidation and anti-oxidation was disordered,presenting a"panic"immune defense response mode.According to the above results,the immune regulation and antioxidant defense system can initiate compensatory protection mechanism during the PM exposure process.However,this positive regulation can be gradually weakened with the extension of exposure time,and finally resulting in the imbalanced homeostasis of immune defense system and tissue damage.Repeated exposure to high concentrations of atmospheric particulates may be a key environmental factor for the PM-induced toxicity on immune defense system.Repeated exposure to a high PM concentration compromised the immune defense response to haze exposure,which temporarily protected the tissues from excessive inflammation,also leading to the incomplete removal of extraneous PM and accumulation of oxidative damage.Therefore,the impairment of immune defense response capacity caused by repeated exposure by high level of PM may be an important mechanism driving the transformation of immune defense system from compensatory response to imbalanced homeostasis.