The Impact of PM2.5 on the Host Defense of Respiratory System

1Exposure to air pollution, including gaseous pollution and particulate matter (PM) pollution, is a leading contributor to the Global Burden of Disease. In recent years, more and more attention has been paid to the impact of PM pollution on public health. PM is a complex mixture of solids and liquids suspended in the air, which can be classified by its aerodynamic diameter as PM10 (<10 μm, inhalable particulate matter), PM2.5 (<2.5 μm, fine particulate matter) and PM0.1 (<0.1 μm, ultrafine particulate matter). PM originates from natural sources (such as dust, sea salt, and wildfires) and anthropogenic emissions (such as vehicles, household wood and coal burning as well as power plants and industry), and the latter accounts for most of the PM pollution. The components of PM are extremely complex, including inorganic components (such as heavy and transition metals, elemental carbon, and sulfuric/nitric/ammonia salts), organic components (such as polycyclic aromatic hydrocarbons) and biological components (such as fungi, spores, and viruses). There are certain differences in the source and composition of different types of particulate matter, and the harm to public health varies as well (Figure 1). Among them, PM2.5 was considered to be the most harmful one. PM2.5 has a large surface area and can adsorb a variety of toxic and harmful substances. Because of its small particle size, it can penetrate deep into the lungs and deposit in the terminal bronchioles and alveoli with breath, and even enter the circulatory system through the gas-blood barrier. Exposure to PM2.5 can endanger multiple organs in the body, and even lead to systemic adverse effects. Among them, the most common are the respiratory and cardiovascular systems. This review focuses on the respiratory system, the primary target organ for PM2.5 exposure.

2PM2.5 may be the environmental risk factor that poses the greatest public health hazard. The GBD 2015 ranks PM2.5 as the fifth leading risk factor for death, with exposure to PM2.5 causing 4.2 million deaths (7.6% of global deaths) and loss of 10.31 million disability-adjusted life-years (DALYs) (4.2% of global DALYs). As claimed by the World Health Organization (WHO), 9 out of every 10 people in urban areas are exposed to high levels of PM2.5 (annual average concentration >10 μg/m3) from outdoor air pollution, and about 3 billion people using non-renewable fuels are exposed to serious indoor air pollution. According to the Air Quality Life Index (AQLI), sustained exposure to an additional 10 μg/m3 of PM2.5 reduces life expectancy by 0.98 years1. In addition, an increase of 10 μg/m3 per day of PM2.5 concentration increases 0.29% of overall non-accidental mortality and 0.22% of respiratory disease mortalities.

3A large number of epidemiological studies have shown that PM2.5 exposure is closely related to a variety of respiratory diseases. It is noteworthy that exposure to PM2.5 increases the susceptibility to respiratory infections. For instance, numerous clinical studies have found that PM2.5 exposure is positively correlated with the number of outpatient visits, emergency visits, and hospitalizations for acute upper or lower respiratory infections. Related animal models also support the notion that PM2.5 exposure increases susceptibility to the lung infection. However, the underlying mechanisms remain elusive. We speculate that PM2.5 exposure may impair the host defense of the respiratory system, making the body more susceptible to infection. The purpose of this article is to review the existing epidemiological and experimental evidence to support the above hypothesis and to summarize the possible underlying mechanisms.

4The respiratory system is the primary route for inhaled PM2.5. Exposure to PM2.5 can cause the development and progression of acute and chronic lung diseases, such as tracheal and pulmonary inflammation, asthma and its acute exacerbations, chronic obstructive pulmonary disease (COPD) and its acute exacerbations. We are concerned with the fact that PM2.5 exposure increases the susceptibility to respiratory infections.

5Outpatient, emergency, and hospitalization-related data on respiratory infections confirmed that PM2.5 exposure was positively associated with the increased respiratory infections. studied the effects of air pollution on outpatients’ acute respiratory outcomes and their study indicated that PM2.5 exposure was positively correlated with outpatient visits for upper respiratory tract infection (URTI). A study in the U.S. state of Georgia also showed that pediatric emergency visits for URTI were associated with PM2.5 concentrations. Another study reported that PM2.5 was significantly associated with emergency room visits for respiratory diseases, particularly for URTI and lower respiratory tract infection (LRTI). Recently, confirmed that PM2.5 exposure was positively correlated with emergency visits for multiple respiratory diseases, including respiratory infections. In addition, studied the association between different types of air pollution and respiratory hospitalization and found that short-term exposure of PM2.5 was positively correlated with the number of hospitalizations for acute respiratory infections. Similarly, another four studies also supported a significant positive correlation between PM2.5 and the number of hospitalizations for URTI and LRTI.

6The effect of PM2.5 exposure on respiratory infections is not immediate, but there is a certain lag effect. For example, PM2.5 exposure was positively associated with an increase in hospitalization for acute respiratory infections, but this correlation was delayed by 7–13 days. analyzed the time curve of PM2.5 concentration and human influenza in Beijing urban area from 2008 to 2011, and proved that there was a significant correlation between PM2.5 exposure and influenza, but this correlation showed a 1–2 months delay. In a study of 150,000 cases, investigated the relationship between PM2.5 and the health status of patients with acute LRTI and discovered that the association of PM2.5 exposure with respiratory syncytial virus (RSV) infection occurred in 2–4 weeks. The hysteresis effects varied in different studies. Therefore, when investigating the association between PM2.5 exposure and respiratory infections, researchers may need to choose the appropriate follow-up time.

7Exposure to PM2.5 increases the susceptibility to respiratory infections, especially in children and the elderly, as well as the vulnerable groups with hereditary or underlying diseases. A study on the relationship between PM2.5 and the health status of patients with acute LRTI revealed that about 77% of the subjects were infants aged 0–2 years. RSV and influenza virus were the main pathogens. Another study on risks of respiratory infections associated with air pollution in China showed that children under the age of 14 were the predominantly susceptible population of acute respiratory infections caused by air pollution, accounting for 80% of hospitalized cases of respiratory system infections in the study. Similarly, found that students aged 6–17 years were more vulnerable to PM2.5 exposure. The maternal exposure to air pollution before birth may result in an impaired lung development and increase the risk of respiratory system infections. In addition, by analyzing the association between PM2.5 pollution and hospital emergency room visits for total and cause-specific respiratory diseases in urban areas in Beijing, found that people over 60 years of age demonstrated a higher risk of respiratory disease (including URTI and LRTI) after PM2.5 exposure. Exposure to PM2.5 is more likely to cause respiratory infections in people with congenital immune deficiencies due to hereditary diseases. For instance, exposure to PM2.5 in patients with cystic fibrosis has been reported to be associated with the acquisition of Pseudomonas aeruginosa (P. aeruginosa). During the follow-up period, each additional PM2.5 exposure of 10 μg/m3 increased the risk of 24% P. aeruginosa acquisition. Another study has found that each additional PM2.5 exposure of 10 μg/m3 increased the risk of methicillin-resistant Staphylococcus aureus (MRSA) by 68%.

8In vivo studies have shown that as a risk factor for respiratory infection, PM2.5 exposure, can prime the lung for greater susceptibility to pathogens by impairing the respiratory host defense. found that PM exposure suppressed macrophage function and slowed the pulmonary clearance of Listeria monocytogenes (L. monocytogenes) in rats. Another research discovered that the colony-forming units (CFUs) of P. aeruginosa detected in the lung were significantly greater in the PM-exposed mice compared to the control mice. found that prior PM2.5 exposure markedly increased the susceptibility of rats to subsequent Staphylococcus aureus (S. aureus) infection. Similarly, found that PM2.5 exposure increased the susceptibility of rats to Klebsiella pneumoniae (K. pneumoniae) infection. and established an exposure model in mice and subsequently infected with Streptococcus pneumoniae (S. pneumoniae). They found that PM exposure reduced bacterial clearance in the lungs of mice. In addition, discovered that exposure to PM2.5 lowers influenza virus resistance. We summarized the in vivo experimental studies of PM2.5 on respiratory host defense (Table 1).

9In vitro experiments have also confirmed that PM2.5 exposure increased the susceptibility of respiratory infection. For example, PM2.5-pretreated A549 cells have a significantly increased risk of infection with Mycobacterium tuberculosis (M. tuberculosis), and PM can disrupt the airway epithelium through oxidative burst to promote P. aeruginosa infection. Similarly, found that PM suppressed airway antibacterial defense, causing an increased susceptibility to P. aeruginosa. In addition, adhesion is the key to microbial invasion of the respiratory tract. PM increased the binding of S. pneumoniae to both primary alveolar macrophages (AMs) and the murine macrophage cell line J774 A.1 but decreased internalization of bacteria. have discovered that urban PM increased the adhesion of S. pneumoniae to human tracheal epithelial cells. We also summarized the in vitro experimental studies of PM2.5 on respiratory host defense (Table 2).

10PM2.5 exposure impairs the host defense of respiratory system causing the body more susceptible to infection. We dissect the underlying mechanisms from the following three aspects: defective airway epithelial host defense functions, alterations in respiratory microecology, insufficiency and dysfunction of immune cells (Figure 2).

11At the interface between the external environment and the host, the airway epithelium serves as the first line of host defense against pathogens. The airway epithelial host defense functions mainly include mucociliary clearance, the barrier functions of the epithelium, and the secretion of a number of proteins and peptides with antimicrobial activities (Figure 3A). Mucociliary clearance in the airway epithelium is a critical protective function and is essential for the clearance of respiratory pathogens. In general, most foreign bodies inhaled into the lungs can be removed in time by the mucociliary clearance system. However, it has been reported that PM2.5 exposure decreased bacterial clearance by impairing the bronchial mucociliary system. Mucin hyperproduction or hypersecretion is a common reason for decreased mucociliary clearance. found that the expression of MUC5AC, one of the predominant mucins produced by the airway epithelium, was upregulated via the epidermal growth factor receptor (EGFR) pathway after PM2.5 exposure in mice (Figure 3B).

12The adhesion of pathogens to host cells is a prerequisite for infection. A study has reported that exposure to urban PM increased the adhesion of S. pneumoniae to human airway epithelial cells, and the addition of N-acetylcysteine (NAC, an antioxidant) reversed this process, possibly be related to reactive oxygen species (ROS) produced by oxidative stress. In addition, reported that ROS induced by PM2.5 activated the AKT/STAT3/NF-κB pathway through IL-6 paracrine signaling, which then upregulated the expression of intercellular adhesion molecule-1 (ICAM-1, an important glycoprotein on the cell surface) in the lung to increase the adhesion of pathogens to the airway epithelium (Figure 3B). also found that PM2.5 could enhance the adhesion of P. aeruginosa to epithelial cells, the mechanism of which depended on the increased bacterial surface hydrophobicity and damaged human cell plasma membrane by PM2.5.

13Tight junctions (TJs) are the significant protein complexes at cell-cell interfaces that connect adjacent cells with each other to form lung epithelial barrier against pathogens. Lack of an intact TJs structure, the airway epithelial barrier cannot keep tight. It will allow pathogens to translocate across the barrier, making the lungs more susceptible to infection. A recent study reported that PM impaired TJs of airway epithelial barrier via oxidative stress to promote P. aeruginosa infection. Claudin-1 is a major structural protein of TJs. Similarly, another study also discovered that exposure to PM downregulated claudin-1 expression in human airway cells via the ERK1/2 signaling pathway (Figure 3B).

14The airway epithelial cells are covered with a very thin fluid layer (airway surface liquid, ASL), which is an important component of the respiratory innate immunity. Antimicrobial peptides (AMPs) content is a significant and indispensable factor affecting the antibacterial effect of ASL. AMPs include salivary agglutinin (SAG), beta-defensins, lactoferrin, secretory IgA, and surfactant protein D (SPD). found that PM2.5 exposure attenuated the antibacterial activity of airways by down-regulating the expression of SAG. In addition, several studies indicated that PM2.5 exposure down-regulated airway β-defensin expression levels through oxidative stress. Collectively, these studies suggested that PM2.5 could compromise the host defense function of airway epithelial cells by downregulating the expression of AMPs (Figure 3B).

15In healthy humans, the low respiratory tract is usually sterile, and without permanent bacterial colonization, while the upper respiratory tract (especially oropharynx) has a normal bacterial flora, which is also an important component of respiratory tract’s natural immune defense, providing a biological barrier against foreign matter or pathogenic microorganisms througha space-occupying effect, nutritional competition, and secretion of bacteriostatic or bactericidal substances. The oropharyngeal microecosystems of rats changed following PM exposure, including a decline of indigenous microflora and an increase of the content of conditional pathogenic microorganisms. By analyzing 16S rRNA sequencing of respiratory tract lavage fluid, a recent study confirmed that there is a link between PM2.5 exposure and alterations of the respiratory tract microecology.

16A variety of immune cells are resident in the respiratory tract, including AMs, polymorphonuclear granulocytes (PMNs), lymphocytes, etc. Their numbers and functions are essential to protect against pathogen invasion. found that the reduction of phagocytic phagocytosis caused by PM2.5 exposure was related to a decrease of NKs in a PM2.5 exposure rat model and subsequently infected with S. aureus. Another research found that PM2.5 exposure can trigger a Th2-type immune response and reduce the phagocytic capacity of AMs, which may be related to Toll-like receptor 2 (TLR2) and Toll-like receptor 4 (TLR4). In addition, found that long-term exposure to PM2.5 lowered influenza virus resistance via down-regulating pulmonary macrophage Kdm6a and mediated histones modification in IL-6 and IFN-β promoter regions. Another two studies indicated that PM2.5 can be used as an immunity inhibitor to reduce the phagocytic capacity of macrophages, thereby increasing the susceptibility to S. pneumoniae infection. PM exposure has also been indicated to decrease the ability of macrophages to produce antimicrobial oxidants in response to L. monocytogenes, which may play a role in the increased susceptibility of rats to respiratory infection.

17In summary, there are sufficient epidemiological evidences from outpatient, emergency and hospitalization-related data that exposure to PM2.5 increases susceptibility to respiratory infections. However, studies on the association between PM2.5 exposure and age, gender, and specific pathogens remain controversial. In the future, a meta-analysis of existing research can be attempted to further confirm the susceptible population of PM2.5 exposure to respiratory infections. We notice that there is a lag effect in the association between PM2.5 exposure and respiratory infections. This may be due to the variations of the average incubation period of different pathogens. We also note that only a few epidemiological studies have reported the relationship between PM2.5 exposure and the infection rates of specific pathogens associated with respiratory infections. Is there a specific pathogen preference for respiratory infections caused by PM2.5 exposure? This still requires the unremitting efforts of the researchers.

18In vivo and in vitro studies have shown that PM2.5 exposure is beneficial to the adhesion, colonization and growth of microorganisms, but it is not conducive to the removal from the body. However, most of the current in vivo experiments establish a PM2.5 exposure animal model by intratracheal instillation. This model usually affects the lower respiratory tract and is prone to uneven distribution in the lung lobes. Concentration and enrichment of PM2.5 aerosol combined with the oral-inhalation or whole-body exposure system may be the best model for simulating human exposure. In recent years, an emerging technology has appeared, combing versatile aerosol concentration enrichment system (VACES) with oral-inhalation or whole-body exposure system. The advantages and disadvantages of these two models are shown in Table 3. Furthermore, current in vitro experiments are limited to epithelial cells- and alveolar macrophage-mediated innate immune responses, lacking attention to adaptive immune responses. Either in vivo or in vitro, the exposure doses currently used in the study are usually based on the corresponding toxicity experiments. How can such exposure levels represent real-world or clinical scenarios?

19PM2.5 exposure may impair the host defense system of respiratory system, making the organism susceptible to infection. In brief, the possible mechanisms include defective airway epithelial host defense functions, alterations of the respiratory tract microecology, and insufficient number or dysfunction of immune cells (Figure 2). However, there is still a lack of robust research on molecular mechanisms. Further efforts are desperately needed to elucidate the underlying mechanisms at the molecular level.

20XT conceived and designed the manuscript. LY, CL, and XT wrote the manuscript and critically revised it. CL and XT generated the figures. XT provided guidance and edited the manuscript.

21The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.