1. Effects of PM on the Epithelial Barrier and 2 Graduate School of Global Environmental Studies, Kyoto University 1 Department of Environmental Engineering, Graduate School of Engineering, Kyoto University 3 Inflammation and Immunology, Graduate School of Medical Science, Kyoto Prefectural University of Medicine Innate and Acquired Immune Systems Global Environmental Research 27/2023 21-26 printed in Japan C Cluster, Kyoto-Daigaku-Katsura, Nishikyo-ku, Kyoto, 615-8540, Japan *E-mail: Key words: acquired immune system, allergy, bronchitis, COVID-19, epithelial barrier, innate immune system Particulate matter (PM) is an aggregate of particles and chemical/biological components. Some of these components include harmful substances, and fine PM easily reaches the bronchioli and alveoli when inhaled, causing health challenges in the respiratory and immune systems. Exacerbation of respiratory diseases, such as bronchitis and bronchial asthma, is one of the representative cases. There is also concern that PM exacerbates coronavirus disease 2019 (COVID-19). Here, we introduce the health effects of PM and its components and the underlying mechanisms, focusing on the respiratory and immune systems from the perspective of an experimental approach. Air pollutants in the atmosphere surrounding us affect our respiratory and immune systems adversely. Particulate matter (PM) in the atmosphere, especially PM2.5 (particulate matter with aerodynamic diameters ≤2.5 µm), reaches deep into the lungs (Heyder et al., 1986). PM2.5 is generated by human activities (combustion, industrial production, etc.) and natural activities (sand dust, volcanic activity, etc.) and contains hydrocarbons, metals, ions, endotoxins and β-glucan (Chowdhury et al., 2018). Compared to large particles, PM2.5 easily reaches the bronchioli and alveoli when inhaled, and some of these various components contain harmful substances; therefore, PM2.5 can contribute to respiratory and immune diseases such as bronchitis and bronchial asthma (Ko and Kyung, 2022). Airway epithelial cells are constantly exposed to environmental first physicochemical contact point between xenobiotics and the respiratory system. These cells physically remove xenobiotics by mucociliary transport (Fig. 1). The respiratory mucosa also contains antimicrobial peptides and proteins, contributing to bacterial eradication contaminants and are the Akiko HONDA1* and Tomoya SAGAWA2, 3 Abstract (Beentjes et al., 2022). When PM and its components induce damage and inflammation in airway epithelial cells, xenobiotics can easily enter the airway. Phagocytes, including neutrophils and macrophages, and group 2 innate lymphoid cells (ILC2) are activated in innate immunity, while dendritic cells/macrophages that have antigen-presenting abilities and phagocytic functions, lymphocytes including T cells and B cells, along with eosinophils and mast cells are involved in acquired immunity. Diesel exhaust particles (DEP), representative of PM2.5, and H2O2 produced from the photolysis of ozone or ambient PM2.5, have been reported to reduce airway mucociliary motility (Bayram et al., 1998; Honda et al., 2014) and weaken epithelial tight junctions (Smyth et al., 2020; He et al., 2021). These are associated with dysfunction of the epithelial barrier. PM2.5 collected from Asian countries, such as Japan, Bangkok, Taiwan and Singapore, the release of pro-inflammatory molecules such as interleukin (IL)-6 or IL-8 from airway epithelial cells (Chowdhury et al., 2019, Honda et al., 2022). IL-6 and IL-8 play important roles in inflammation of inducing neutrophil recruitment, up-regulating mucin secretion and stimulating lymphocytes (Bautista et al., 2009; Chen et al., 2003; Levine et al., 1993; Thacker, 2006). PM2.5 also the induces respiratory system by ©2023 AIRIES 21 Biological Mechanisms of Respiratory Effects of Particulate Matter and its Interaction with COVID-19

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