Immunopathological features of air pollution and its impact on inflammatory airway diseases (IAD)

The presence in the air of one or more natural or anthropogenic substances at a concentration, or location, for a duration, above their natural levels with the potential to cause an adverse health effect defines air pollution. Indoor air pollution refers to chemical, biological, and physical exposure of air pollutants in homes, schools, and workplaces. Similar to indoor air pollution, ambient (outdoor) air pollution can result from chemical substances or biologically derived contaminants modified by climate change or human activity such as bioaerosols and aeroallergens. Air quality guidelines endorsed by the World Health Organization (WHO) aim to provide clean air in and around the home. Air pollution reduced life expectancy in 2017 by 1 year and 8 months on average worldwide. WHO has linked 4.3 million deaths globally in 2012 to household cooking using coal, wood and biomass stoves. Outdoor air pollution in the same year caused an estimated 3.7 million deaths. In inflammatory airway disease (IAD) patients an estimated 7–11% increased risk in asthma-related mortality was commensurate with a rise in ambient pollutant concentrations such as NO2, PM2.5, or ozone when computed few days prior to asthma death. Similar but smaller increments in chronic obstructive pulmonary disease (COPD)-related mortality were attributed to pollution and ranged from 0.78% to 1.78%.

In the respiratory tract, air pollution can impact wellness in healthy people and patients with IAD, irrespective of their atopic status. Hence, the airway mucosal barrier may be disrupted by immunopathological mechanisms resulting from effects of pollution and IAD. The co-occurrence of IAD phenotypes (allergic rhinitis, and chronic rhinosinusitis, COPD and asthma) within an individual increases the likelihood of pollutant induced exacerbation of disease or infection.

We reviewed the following IADs in relation to air pollution. Allergic rhinitis (AR), is an IgE mediated inflammatory disease generated by a spectrum of outdoor aeroallergens like pollens or indoor aeroallergens such as dust mites, cockroaches, cat allergens, or molds. CRS represents multiple overlapping rhinosinusitis phenotypes with different endotypes. Asthma is characterized by chronic atopic or non-atopic inflammation of the airway with superimposed episodes of acute exacerbations. The majority of exacerbations are triggered by respiratory viral infections, most commonly human rhinovirus.^, Other triggers include allergens and atmospheric pollutants.^, COPD, another chronic inflammatory airway disease, is characterized by airflow limitation and cough. Acute exacerbation of COPD, like in the upper airway, can be triggered by infection and inhalation of irritants.^,

Chemical pollutants are health-damaging atmospheric aerosol and non-aerosol particles originating from a variety of natural (eg, volcanic eruptions) or anthropogenic sources (eg, biomass burning, fossil fuel combustion, or traffic related particles). Primary pollutants such as particulate matter (PM) and volatile organic compounds are aerosol particles directly emitted as solid or liquid droplets in the air. In the atmosphere, natural gas-to-particle conversion can culminate in secondary chemical pollutant particles like are ozone and PM.

Allergens can pollute indoor and outdoor air and exacerbate AR and asthma. Indoor allergenic pollutants can be derived from skin scales of pets (eg, cats, dogs), urine of rodents (eg, mice), molds, or from fecal material of arthropods such as house dust mites and cockroaches. Outdoor allergens are aeroallergens originating from grasses, trees, weeds, or molds. Outdoor pollen also modulates indoor aeroallergen concentration. The concentration of aeroallergens in the indoor environment is governed by complex bioaerosol dynamics. For example, airborne cat allergen (Fel d1) is mostly associated with large particles (>9 μm), but around ¼ of Fel d1 are carried on particles less than 4 micra in diameter. Thus Fel d1 can be deposited in the alveoli but most importantly suspended for several days in the air favoring distribution of the allergen in the environment.^{,  ,}  Also, the 33 groups of mite allergens listed in the WHO nomenclature of allergens are composed of particles ranging in diameter from 10 to 40 μm. Hence, they can become airborne upon disturbance and can be carried on house dust that becomes a vector for exposure. How dust mite allergen particles can induce and trigger asthma in lower airways remains to be determined.

The diversity and functioning of the normal microbiome are crucial for maintaining the health of the host. While the effects of PM on human health are well established, the impact of infectious particles on bacterial ecosystems has been overlooked.

In vitro studies suggest black carbon, a major component of PM, is strongly implicated in predisposition to respiratory infectious diseases,^, and induces structural and functional changes in the biofilms of both Streptococcus pneumonia and Staphylococcus aureus. This is manifested by increase in biofilm thickness and tolerance to degradation by proteolytic enzymes, thereby promoting colonization of the respiratory tract. Similarly, evidence suggests indoor and outdoor dust modifies microbial growth, virulence, and biofilm formation of opportunistic pathogens. By exposing 3 opportunistic bacteria (Pseudomonas aeruginosa, Escherichia coli, and Enterococcus faecalis) to progressively increasing concentrations of indoor and outdoor dust, a differential growth pattern of pathogens was noted. This was commensurate with increased biofilm formation and sensitivity to oxidative stress following hydrogen peroxide challenge. Consequently, the detrimental impact of particulate pollutants on human health is not only due to direct effects on the host but also may involve the effect on bacterial behavior in the host.

Oxidative stress is a disproportionate generation of free radicals beyond the body antioxidant capacity. It translates into a non-IgE mediated Th2 airway inflammation following exposure to a pollutant. In brief, reactive oxygen species (ROS), generated naturally as by-product of cell growth and metabolism, can be produced following pollutant exposure.^, ROS include oxygen radicals (eg, superoxide, hydroxyl, hydroperoxyl) and certain non-radicals (eg, H₂O₂, ozone, singlet oxygen) that are easily converted into radicals. ROS have a pivotal role in cell signaling in the oxidation/reduction cascades following exposure and ultimately generation of anti-oxidant mechanisms thru nrf-2, activator protein 1, and nuclear factor-kappa B.^{,  ,  ,}  Antioxidants are scavengers of ROS and can be enzymatic or non-enzymatic systems, constitutive or de novo synthesized by activated gene expression, according to ROS load. The inflammatory phase of oxidative stress involves cytokines- and chemokines-mediated activation and recruitment of inflammatory cells secondary to direct effect of pollutants on airway epithelial cells. This can propagate oxidative stress further and augment the inflammatory response and tissue damage. Alternatively, ROS can contribute directly to cell injury and apoptosis by disrupting cellular and nuclear membranes in the epithelial barrier wall and altering the function of cellular enzymes.^, A different mechanism by which environmental pollution can trigger disease in the nose is via a neurogenic mechanism. Another component of oxidative pathway is the exposure-driven adjuvant effect on atopy where environmental pollution acts as an exacerbating factor for allergic airway disease by enhancement of allergic airway hypersensitivity in atopic individuals. The evidence emerges from experimental protocols involving inhalation of pollutants and allergen challenge which show pollutants can act synergistically to heighten the allergic response with increased expression of Th2 inflammatory biomarkers.^, This is in contrast to healthy individuals which express either Th1 or a mixed Th1/Th2 profile in controlled exposure studies.

Epidemiological studies suggest pollution modulates AR,^{,  ,  ,  ,  ,  ,}  rhinosinusitis, and asthma.^, Other studies suggest a positive association between exposure and prevalence of AR and asthma^{,,  ,  ,}  in children and adults predominately in reports on short-term exposure and residential proximity studies to sources of traffic pollution.^, However, other long-term exposure studies provided evidence to the contrary.^{,  ,}  This could be due to differences in study design, methods of exposure assessment, and complex nature of studied pollutants.^,,,

InIn- vivovivo studies in both human and animal models suggest pollutant exposure induces inflammatory changes in normal, chronically diseased and allergic nasal and sinonasal tissues (Table 1). The cytokine profile of affected tissues suggests activation of the oxidative inflammatory pathways.^{,  ,}  Moreover, there is compelling evidence for involvement of oxidative stress inflammatory pathways following pollutant exposure in the pathogenesis of rhinitis, CRS, and asthma irrespective of atopic status. This stems from an abundance of literature on oxidative stress biomarkers studied under natural or experimental allergen exposure both in seasonal and perennial AR described in Table 1. In fact, dust mite or ragweed allergic patients exposed to diesel exhaust particlesDEPs in climate chamber expressed higher nasal symptom scores following dust mite or ragweed challenge, respectively, when compared to non-exposed but allergen-challenged patients.^, Also in the lower airways, short-term natural increase in ambient air ozone was associated with deteriorating lungh function tests in atopic asthmatics despite use of proper asthma controller therapy. Similarly, an ozone exposure protocol revealed atopic asthmatics expressed depressed spirometry testing results compared to healthy volunteers. Along with this, climate chamber studies revealed (ozone) exposure of healthy or allergic asthmatics induces a neutrophilic or a mixed neutrophilic and eosinophilic inflammatory profile in the lower airways, respectively. Furthermore, gene expression profiles of sputum cells recovered from healthy volunteers and allergic asthmatic patients also confirmed significant difference in inflammatory response to ozone exposure.

Analysis of biomarkers activity greatly improved our understanding of cascade and signal pathways involved in atopic and non-atopic phenotypes of airway disease following exposure. Although most oxidative stress biomarkers require tissue specimen collection, some studies suggest an analysis of biomarkers can be determined non-invasively in exhaled breath condensates or blood.^{,  ,  ,  ,  ,}  Natural allergen exposure reverses oxidative and antioxidative status compared to asymptomatic period, with a persistent oxidative state outside pollination season in allergic patients when compared to healthy controls. AR and asthma comorbidity in children does not seem to augment oxidative stress markers compared to AR alone, although adult patients with seasonal AR and asthma manifest an exaggerated stress response during natural allergen exposure compared to AR alone. Clinically, oxidative stress correlates with nasal symptom scores in children with perennial AR and can predict AR severity independent of total IgE. Additionally, ROS status does not correlate with atopic skin sensitization in children with perennial AR. Furthermore, dust mite challenge in asthmatics or sensitized mice resulted in oxidative damage to nucleic acids as well as lipids and proteins and subsequently triggered DNA repair pathways. Further blockage of DNA repair proteins resulted in increased production of DNA double-strand breaks and cell apoptotic enzymes suggesting importance of DNA repair in suppressing airway inflammation.

Endogenous antioxidant response in atopic respiratory diseases is complex and oxidative stress response to anti-inflammatory drugs isare poorly understood. Antioxidant enzymes mostly studied in atopic respiratory diseases include heme oxygenase 1 and 2,, NADPH oxidases, catalase,^, superoxide dismutase,^,, dual oxidases 1 and 2 (in CRS patients), paraoxonase, and glutathione peroxidase.^, Antioxidant activity can also be measured by serum thiol-SH and total antioxidant status (Table 1). In this respect, evidence suggests oxidative stress decreases antioxidant enzyme activity or total antioxidant status in atopic children^,, or in human in vitro controlled exposure studies, whereas other studies present evidence to the contrary. For example, heme oxygenase antioxidant (iso)enzyme-1 activity was preferentially increased in a human in vitro model of perennial AR, and upregulated in a human exposure model of COPD aggravated by infection;; also dual oxidase antioxidant (iso)enzymes showed preferential upregulation in different phenotypes and endotypes of CRS.^, Contrary to this, antioxidant enzymes can be downregulated in asthma and rhinitis irrespective of atopic status, and in vitro animal exposure models challenged by an infectious insult. Importantly, genetic polymorphism in antioxidant/detoxifying genes like GSTM1 and GSTP1 can alter oxidative stress response in patients with COPD and those with AR following exposure.^,

Exogenous (dietary) antioxidants are scavengers of oxygen free radicals and can act on different levels of defensive antioxidation pathways.^, Epidemiologic, in vivo^, and in vitro studies suggest a beneficial role of exogenous antioxidants in patients with IAD or in controlled exposure studies of healthy sinonasal epithelial cells. However, lack of clinical trials data clearly supporting their efficacy, in addition to their potential role in skewing Th1/Th2 balance towards a Th2-type immunity as suggested in vitro, renders their indication restricted to special situations such as over exposure to environmental pollutants, among others. N-acetylcysteine maintains a potent antioxidant effect in in vitro studies or in ovalbumin-sensitized rats by downregulating tumor necrosis factor-alpha in recruited inflammatory cells. Along these lines, intranasal steroids can exhibit an exogenous antioxidant regulatory role in seasonal AR by decreasing exhaled breath condensates of leukotriene B4 and 8-Isoprostane, although no effect was seen on exhaled carbon monoxide and nitrogen oxide. In another study involving children with AR and asthma, no effect of topical nasal steroid therapy was noted on measured lipid peroxidation oxidative stress biomarkers and antioxidant enzymes. Data on potential antioxidant effect of inhaled steroids in adult asthmatics is scarce. Epidemiological studies suggested prior intake of oral or inhaled steroids in adult asthmatic patients had no effect on asthma control, as measured by clinical symptoms and FEV1 testing, with PM and ozone exposure. Other similar studies noted increased consumption of asthma controller therapy (bronchodilators, inhaled corticosteroids, or both) with PM10 or NO₂ exposure in adults. Moreover, in children inhaled steroid therapy downregulated induced expression of heme oxygenase-1 in non-smoking patients with bronchiectasis but had no effect on exhaled carbon monoxide. Furthermore, desloratadine can exert an antioxidant effect in children with perennial AR by increasing antioxidant enzyme activities (catalase and superoxide dismutase) and decreasing lipid peroxidation marker (malonaldehyde) although no effect was seen on total antioxidant status. When compared to placebo, fexofenadine improved nasal symptom scores in ragweed AR patients following ragweed challenge and DEP controlled exposure.

The majority of controlled human exposure studies to ambient pollutants have been conducted in climate chambers on healthy individuals.^{,  ,  ,}  For example, relative to clean air, mixtures of VOCs increased ratings of nasal irritation, odor intensity and cognitive symptoms (memory loss, dizziness), and a two-fold increase in polymorphonuclear cells in nasal lavage immediately following exposure. Similar studies using different pollutants showed no detectable effects on nasal symptom scores or markers of nasal inflammation.^, Additionally, healthy subjects exposed to room air, nanoparticles, or O₃/terpene showed no significant changes in inflammatory biomarkers in blood, sputum or nasal secretions and pulmonary function tests. However, only nanoparticles exposure increased significantly high frequency variability in heart rate, thereby indicating a shift in autonomic balance to a more parasympathetic tone. Low level ozone exposure in healthy subjects resulted in increased sputum production of airway inflammatory cells such as neutrophils, monocytes, and dendritic cells, and modification of cell surface phenotypes of antigen presenting cells. Using a similar protocol the reported decrement in lung spirometry testing (FEV1) of healthy subjects was associated with increased neutrophilic airway inflammation following exposure; the latter likely being more pronounced in healthy individuals with GSTM1 null genotype. More importantly, comparing healthy controls to atopic asthmatics, exposure to high levels of ultrafine particles in a climate chamber was associated with a small but significant fall in arterial oxygen saturation, a fall in forced expired volume over 1 s (FEV1) the morning after exposure, and a transient slight decrease in low frequency (sympathetic) power during quiet rest. These controversial results can be related partly to the nature and concentration of the investigated pollutant or its experimental duration of exposure keeping in mind brief exposure to a single pollutant in a climate chamber does not reflect chronic exposure to multiple pollutants in real life. Controlled exposure studies in atopic patients involving allergen challenge revealed more consistent results. For example, dust mite allergic patients reported worsening nasal symptom scores following intranasal dust mite challenge and DEP exposure commensurate with increased histamine levels in nasal washes, all suggestive of induced mast-cell degranulation. Similarly, controlled exposure studies in ragweed allergic patients challenged with DEP and ragweed outside their pollen season reported higher total nasal symptoms scores or increased levels of specific IgE and expression of Th2 inflammatory cytokines, when compared to ragweed challenged alone.

Taken together, controlled airway exposure studies to ambient pollutants in healthy individuals show small but significant negative health effects whereas exposure studies in allergic patients support the role of pollutants in increasing atopic airway hypersensitivity. Large scale translational studies are needed to correlate the bio-cellular toxic effects of pollution with epidemiological studies.

Signal and cascade pathways triggered across the airway mucosal barrier at first encounter of pollutants are complex (see Fig. 1). Airway mucosal cells can recognize pollutants through an epithelial toll-like receptors (TLR)-mediated mechanism either directly or indirectly by the intermediary of pattern recognition receptors (see below). More precisely, pollutants such as PM, cigarette smoke, and ozone can present themselves directly to subclasses of surface TLRs, namely TLR2 and TLR4, which can serve as ligands for these pollutants. Alternatively, pollutants can be bound to pattern recognition receptors, a collective conglomerate of receptors which encompasses TLRs and normally can recognize conserved molecular structures derived from microbial agents or released by damaged non-microbial cells. Once triggered, pattern recognition receptors and TLRs attract antigen presenting cells and leukocytes to the site of inflammation resulting in priming of the airway to subsequent mucosal infectious insults. Afterwards, when eventuated by an infectious challenge, alveolar macrophages mount a heightened inflammatory response aimed at containing and clearing bacteria while producing minimal collateral tissue damage.^, The immunological “storm” resulting from co-exposure and infection is studied in different clinical models of respiratory cells and also in patients with IAD such as COPD (Table 2). Another signal pathway is mediated by submucosal innate lymphoid cells (ILCs) which can differentiate into adaptive subsets. ILC1s relates to immune reactions in CRS without nasal polyps, COPD, and some viral and bacterial infections; whereas ILC2s becomes important in regulating type 2 immunity and some helminthic and viral infections.^, Other immunologic and antimicrobial responses to pollutant exposure modulate expression of host defense peptides and antiviral mechanisms, impair mucus production crucial for capturing pollutants or weaken tight junctions essential for the epithelial airway defense barrier.^,