Air Pollution and Climate Change Effects on Allergies in the Anthropocene: Abundance, Interaction, and Modification of Allergens and Adjuvants

Air pollution and climate change are potential drivers for the increasing burden of allergic diseases. The molecular mechanisms by which air pollutants and climate parameters may influence allergic diseases, however, are complex and elusive. This article provides an overview of physical, chemical and biological interactions between air pollution, climate change, allergens, adjuvants and the immune system, addressing how these interactions may promote the development of allergies. We reviewed and synthesized key findings from atmospheric, climate, and biomedical research. The current state of knowledge, open questions, and future research perspectives are outlined and discussed. The Anthropocene, as the present era of globally pervasive anthropogenic influence on planet Earth and, thus, on the human environment, is characterized by a strong increase of carbon dioxide, ozone, nitrogen oxides, and combustion- or traffic-related particulate matter in the atmosphere. These environmental factors can enhance the abundance and induce chemical modifications of allergens, increase oxidative stress in the human body, and skew the immune system toward allergic reactions. In particular, air pollutants can act as adjuvants and alter the immunogenicity of allergenic proteins, while climate change affects the atmospheric abundance and human exposure to bioaerosols and aeroallergens. To fully understand and effectively mitigate the adverse effects of air pollution and climate change on allergic diseases, several challenges remain to be resolved. Among these are the identification and quantification of immunochemical reaction pathways involving allergens and adjuvants under relevant environmental and physiological conditions.


INTRODUCTION AND MOTIVATION
Allergies are hypersensitivities initiated by specific immunologic mechanisms (abnormal adaptive immune responses). 1−3 They constitute a major health issue in most modern societies, and related diseases, such as allergic rhinitis, atopic asthma, eczema (atopic dermatitis), and food allergies, have strongly increased during the past decades. 4−12 While some of the perceived rise in allergies may be due to improved diagnosis, the prevalence of allergic diseases has genuinely increased with industrialization preindustrial times, especially in densely populated and industrialized areas but also in agricultural environments and around the globe. 38,47,65−69 For example, the average mixing ratios of ozone in continental background air have increased by factors of 2−4 from around 10−20 ppb from the beginning of the 19th century to 30−40 ppb in the 21st century, and the number and mass concentrations of aerosol particles in polluted urban air are typically by 1−2 orders of magnitude higher than in pristine air of remote continental regions (∼10 2 −10 3 cm −3 and ∼1−10 μg m −3 vs ∼10 3 −10 5 cm −3 and ∼10−100 μg m −3 ). 38,70 Numerous studies indicate that ozone and air particulate matter have strong effects on human health and mortality as well as on agricultural crop yields. 71−80 In view of these findings, it appears unlikely that the strong environmental changes of the Anthropocene would have no effect on the interaction of the human immune system with environmental stimuli, including allergens and adjuvants. Indeed, it seems necessary to address the question whether human activities are creating a hazardous atmosphere that may severely affect public health. 35,37,38,81,82 Figure 2 illustrates how climate parameters and air pollutants can exert proinflammatory and immunomodulatory effects. 8 As detailed in the following sections, both air pollutants and climate parameters can influence the environmental abundance of allergenic bioparticles and the release of allergenic proteins and biogenic adjuvants. Moreover, air pollutants can chemically modify and agglomerate allergenic proteins, and they can act as adjuvants inducing epithelial damage and inflammation.
Several reviews have addressed the general determinants of allergenicity 3−8,83−85 and various environmental risk factors for allergic diseases. 4,9,12,34,36,86−101 In this Critical Review, we attempt to summarize, update, and synthesize the different perspectives and most relevant findings reported in earlier reviews and recent research articles addressing the effects of air pollutants and climate parameters on allergies. A central aim of this article is to review and outline both proven and potential effects of the globally pervasive environmental changes that are characteristic for the Anthropocene; a holistic view of environmentally caused changes in the abundance, interaction, and modification of allergens and related substances is provided. Our target audience comprises physical, chemical, and biomedical scientists interested in environmental effects on public health. Sections 2−4 deal with specific environmental processes and air pollutants that are likely to affect the development of allergies in the Anthropocene, that is, in an environment strongly influenced by human activity. Section 5 provides an outlook identifying key questions and promising directions of future research. For orientation of readers not familiar with the basics of allergic sensitization and response, section S1 outlines key features of the immunochemical interactions involved in IgE-mediated allergies (type I hypersensitivities) 3−5,14−16,84,102−136 on which this article is mainly focused and which usually involve Th2 cell-mediated inflammation 137,138 ( Figure S2).

ABUNDANCE AND RELEASE OF ALLERGENS AND
ADJUVANTS Environmental allergens are mostly proteins derived from plants, animals, and fungi that can trigger chemical and biological reaction cascades in the immune system leading to allergic sensitization and formation of IgE antibodies (section S1). 8  birch pollen (Bet v 1), timothy grass pollen (Phl p 1), ragweed (Ambrosia, Amb a 1), molds (Alternaria alternata, Alt a 1, Cladosporium herbarum, Cla h 1, Aspergillus f umigatus, Asp f 1), and dust mites (Der p 1). 4,139,140 Besides allergens, also adjuvants and their interaction with the immune system play a critical role in the development of allergies. Here, we use the term adjuvant generically for substances that are promoting pro-allergic immune responses. Adjuvants can trigger the immune system by inducing tissue damage and subsequent enhanced uptake of allergens, by inducing oxidative stress and activation of immune cells, by coexposure with the allergen favoring Th2 responses, or by modification of allergens enhancing their allergic potential. An overview of biogenic and anthropogenic adjuvants, including particulate matter as well as trace gases, and their effects on the immune system is given in Table 1.
Climate change is influencing vegetation patterns and plant physiology through spatial and temporal changes in temperature and humidity (Figure 1), 141−143 and increasing atmospheric carbon dioxide (CO 2 ) affects plant biology by supplying more carbon for photosynthesis, biomass production, and growth (CO 2 fertilization). 144,145 These factors can influence the spread of invasive plants, the beginning, duration, and intensity of pollination, the fruiting patterns and sporulation of fungi, as well as the allergen content and allergenicity of pollen grains, fungal spores, and other biological aerosol particles (Figure 2). 12,90,93,96−98,145−162 Specific examples of climate change effects on allergenic plants and fungi are outlined in Table 2. Climate and land use change are also expected to influence the composition and spread of microbial surface communities (cryptogamic covers), from which allergenic cyanobacteria and other microbial allergens or adjuvants can be emitted to the atmosphere. 163−174 Moreover, the frequency and intensity of dust storms are expected to increase, 141,175−179 and dust particles are known to carry biological and organic components with pathogenic, allergenic, and adjuvant activity. 152,154,180−187 Dust storms have been shown to cause and aggravate respiratory disorders including atopic asthma and allergic rhinitis. 181,188−191 So-called "thunderstorm asthma" is characterized by acute asthma exacerbations possibly caused by the dispersion of inhalable allergenic particles derived from plant pollen and fungal spores by osmotic rupture. 145,192 On the other hand, climate changerelated regional enhancements of outdoor humidity and indoor home dampness may also lead to an increase of respiratory symptoms and atopic asthma induced by allergenic and adjuvant substances from fungi, other microbes, and mite. 12,193−196 Pollen grains generally belong to the coarse fraction of air particulate matter (particle diameters >10 μm), but fungal spores and pollen fragments are also found in fine particulate matter (<2.5 μm; PM2.5), which can penetrate deep into the human respiratory tract and alveolar regions of the lung. 152,153,197−203 Allergenic proteins can be released from pollen and spores after cell damage or under humid conditions. 204 In particular, pollen rupture due to an osmotic shock during rain can lead to outbreaks of thunderstorm asthma. 145,192,205,206 Furthermore, peaks of high concentrations of pollen, fungal spores, and other primary biological aerosol (PBA) particles have been observed at the onset of heavy rain Figure 2. Pathways through which climate parameters and air pollutants can influence the release, potency, and effects of allergens and adjuvants: temperature (T), relative humidity (RH), ultraviolet (UV) radiation, particulate matter (PM), ozone and nitrogen oxides (O 3 , NO x ), reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, pollen-associated lipid mediators (PALMs), damage-associated molecular patterns (DAMPs), pattern recognition receptors (PRR), type 2 T helper (Th2) cells, immunoglobulin E (IgE), allergenic proteins (green dots), and chemical modifications (red dots). and moist weather conditions; 200,207,208 and increased concentrations of free allergen molecules in fine air particulate matter have been observed after rainfall. 209 Prominent airborne fungi, such as Cladosporium herbarum, Alternaria alternata, and Aspergillus f umigatus, have been found to release higher amounts of allergens after germination under humid conditions, 210 and certain allergens are expressed only following germination. 210,211 Air pollutants, such as ozone, nitrogen oxides, and acids, can also interact with PBA particles, damage their envelope, and facilitate the release of allergenic substances, such as cytoplasmic granules from pollen ( Figure S3). 205,212,213 Besides allergenic proteins, pollen and fungal spores also release other compounds that can act as adjuvants (Table 1). In particular, the release of nonallergenic, bioactive, pollenassociated lipid mediators (PALMs) with pro-inflammatory and immunomodulatory effects can trigger and enhance allergies ( Figure 2). 8,109,214−217 For example, skin prick tests of pollen allergens elicited larger wheals when tested together with low molecular weight compounds extracted from pollen. 218 The release of these substances can be influenced by climatic conditions and air pollution, and significantly higher levels were found for pollen collected near roads with heavy traffic. 205 Leukotriene-like PALMs (oxylipins) have the potential to attract and activate innate immune cells like neutrophils and eosinophils. 214,217 Other PALMs such as phytoprostanes (lipophilic counterparts of prostaglandins) are water-soluble and can inhibit the production of interleukin 12 (IL-12) by dendritic cells in the lower respiratory tract, thus favoring an allergenic Th2 T cell response. 8, 215 A recent study showed that the low-molecular-weight fraction of phytopros-  466,467 DEP and cigartette smoke can increase thymic stromal lymphopoietin (TSLP) expression in epithelial cells 468,469 DEP induce permeability of epithelial cells; disrupt tight junctions by a ROS-mediated pathway 470,471 PM increase the expression of costimulatory molecules on DCs (MHC class II, CD40, CD80, CD86) 86,469 ultrafine particles (UFP < 100 nm) and DEP alter soluble protein levels (e.g., surfactant protein D, complement protein C3), increase levels of e.g., glycerin-aldehyde-3-phosphate-dehydrogenase (GADPH), manganese superoxide dismutase (MnSOD), or mitochondrial heat shock protein (Hsp 90) 472,473 PM2.5 and DEP activate complement proteins (C3) 474,475 black carbon (BC) and DEP induce epigenetic effects: DNA methylation in genes associated with Th2 polarization 476−478 DEP and cigarette smoke induce epithelial damage, oxidatitive stress, and inflammation 460 prenatal and postnatal exposure to environmental tobacco smoke (EST) is associated with asthma and wheezing 34,479,480 transition metals and other redox-active compounds (organic peroxides, quinones) induce ROS production and inflammation via Fenton-like reactions 38 tane E1 (PPE1) in ragweed pollen extract specifically enhanced IgE production in Th2 primed B cells. It was suggested that pollen-derived nonallergenic substances might be responsible for aggravation of IgE-mediated allergies. 219 Fine aerosol particles and a wide range of inorganic, organic and biological substances from both natural and anthropogenic sources (e.g., secondary organic material, sulfuric and nitric acid, microbial compounds) can agglomerate and accumulate on the surface of pollen, fungal spores, and other PBA particles as illustrated in Figure S3. 152,205,220−223 An overview of reported air pollutant effects on the allergenic potential of plant pollen and fungal spores is given in Table  S1. 38,205,221,224−240 Moreover, free allergens and adjuvants can bind to particulate pollutants, such as dust, soot, black/ elemental carbon (BC/EC), and diesel exhaust particles (DEP) carrying the allergens and adjuvants into peripheral and deep airways. 241−243 The colocalization of allergens and adjuvants on particle surfaces (sorption layers, protein coronas) might also promote allergic sensitization and response by providing multiple/multivalent epitopes that facilitate receptor crosslinking (similar to parasitic organisms, against which IgE is naturally deployed). 244,245 During recent years, great progress has been made in the development and application of efficient sampling and measurement methods for bioaerosol particles and components, including microscopic, spectroscopic, mass spectrometric, genomic, and proteomic analyses. 152,246−253 These and related advances in measurement and modeling techniques for health and climate relevant air contaminants (aerosols and trace gases) are expected to enable comprehensive characterization and forecasting of allergenic and adjuvant substances, as well as their mixing state in outdoor and indoor air. 38,70,254−268 Note that indoor air quality is usually influenced by both outdoor air pollutants (O 3 , NO x , PM2.5, etc.) and additional pollutants from indoor sources (e.g., formaldehyde and other organic compounds). 35,37,265,269−274 The data from ambient and individual monitoring and modeling of aeroallergen and adjuvant exposure can then be applied in epidemiological studies to better understand the risk factors of allergic sensitization and disease. 74−76, 275−280 Several epidemiological studies and meta-analyses reported that respiratory allergies and atopic dermatitis are associated with exposure to traffic-related air pollution (TRAP), but different results were obtained for different diseases and locations/studies. 281−293 TRAP is a complex mixture comprising variable proportions of particulate matter and gaseous pollutants from traffic-related primary emissions, as well as secondary pollutants formed by chemical reactions in the atmosphere. 283 Among the pollutants from primary emissions (combustion and noncombustion sources) are road dust, tire and break wear, soot/DEP, BC/EC, metals, polycyclic aromatic

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Critical Review hydrocarbons (PAH), and nitrogen oxides (NO x ); among the secondary pollutants are ozone, nitrates, and secondary organic aerosols (SOA). 38,70,273,283 A recent review concluded that epidemiological studies were restricted by imprecise methods of assessing both TRAP exposure and related health effects. 283 Accordingly, several studies called for more comprehensive investigations of TRAP markers, personal exposure, and lifetime outcomes. 281,294,295 The application of improved measurement and modeling techniques as outlined above should enable refined epidemiological studies and more targeted testing of hypotheses by resolving different types of TRAP (e.g., freshly emitted DEP vs resuspended road dust; soot and polycyclic hydrocarbons vs trace metals; ozone vs nitrogen oxides; etc.).

CHEMICAL MODIFICATION OF PROTEINS AND AMINO ACIDS
Chemical modification by air pollutants can lead to changes in the structure of protein macromolecules (amino acid oxidation, peptide backbone cleavage, conformational changes, crosslinking, and oligomerization), and affect protein stability and other properties, such as hydrophobicity and acidity of binding sites. 296−303 These and other posttranslational protein modifications may induce multiple effects in the molecular interaction of allergens with the immune system: (1) stability effects influencing the accumulation and degradation of allergenic proteins, the duration of exposure times to cellular receptors, and the process of antigen presentation via major histocompatibility complex (MHC) class II; 304,305 (2) epitope effects, that is, generation of new epitopes or modification of existing epitopes, changing the binding properties of antibodies and receptors, by direct chemical modification or as a result of conformational changes; (3) adjuvant effects, that is, generation of new adjuvant functions or modification of existing adjuvant functions such as lipid-binding capacities due to modified ligand binding sites; and (4) agglomeration effects, that is, multiplication or shielding of epitopes or adjuvant functions by cross-linking (oligomerization) of allergenic proteins, which may enhance the cross-linking of effector cell receptors (FcεRI) or sterically hinder molecular and cellular interactions. 307,308306229 In the atmosphere, reactive oxygen and nitrogen species (ROS/RNS) are generated via photochemistry and gas-phase, heterogeneous, and multiphase reactions involving atmospheric oxidants and aerosol particles. In the human body, ROS/RNS can be formed upon exposure to air pollutants 38,309−312 or radiation (UV, X-rays, γ-rays), 313 and by regular physiological reactions. 314 For example, ROS/RNS are generated during oxidative metabolism as well as in cellular responses to foreign or danger signals (cytokines, xenobiotics, bacterial invasion). 315 Low amounts of ROS/RNS are involved in intra-and intercellular redox signaling processes, for example, oxidizing low molecular mass thiols and protein thiols ( Figure 3). 316,317 An imbalance between oxidants and antioxidants in favor of oxidants (e.g., induced by air pollutants) can lead to irreversible damage of cellular lipids, proteins, nucleic acids, and carbohydrates, eventually resulting in cell death. 38,317,318 Rising levels of atmospheric oxidants and air particulate matter may lead to protein modifications in the atmosphere, as well as in the human body because of elevated oxidative stress levels, especially in the epithelial lining fluid (section 4). 38 Moreover, air pollutants and climatic stress factors, such as UV radiation, drought, salinity, and temperature extremes, can also induce higher ROS/RNS levels inside plants, which may lead to chemical modification of plant proteins, including allergens. 38,142,143 In the course of the Anthropocene, the ambient concentrations of many ROS/RNS have strongly increased because of emissions from traffic and combustion sources, as well as other industrial and agricultural activities like nitrogen fertilization of soils. 37,38,82,319,320 In the following, we focus on irreversible modifications of allergenic proteins, such as oxidation, nitration, and crosslinking The reaction of proteins with nitrating agents leads mainly to the nitration of the aromatic amino acid tyrosine forming 3nitrotyrosine (NTyr). 333 The addition of the rather bulky NO 2 group at the ortho position of the aromatic ring induces a significant shift in the pK a value of the tyrosine residue (Tyr) from ∼10 to ∼7, thus increasing the acidity of the hydroxyl group. These structural and chemical changes of the amino acid can affect the conformation and function of proteins. 334,335 For example, the modification of tyrosine residues can influence cell signaling through the important role of receptor tyrosine kinases, which are key regulators of cellular processes. 336 Moreover, nitrotyrosine has been reported as a biomarker for oxidative stress, inflammation, and a wide range of diseases. 296,301,337,338 Early immunological studies already suggested that dinitrophenyl derivatives of proteins and peptides evade immune tolerance and boost immune responses. 339,340 As early as 1934, the allergic reaction to dinitrophenol was described, 341 and dinitrophenyl haptens became very popular reagents for the experimental induction of allergies. 342−344 Thus, nitrated aromatics and especially nitrophenols can be considered corner stones in the field of allergy research, suggesting that protein nitration by air pollutants might play a role in the development of allergies. 330 Indeed, several studies showed enhanced allergenic potentials for nitrated pollen allergens, 229,305,306 nitrated fungal allergens, 237 and nitrated food allergens. 304,345 For example, the most efficiently nitrated tyrosine residue in the food allergen ovalbumin (OVA) is part of human and murine IgE epitopes and also belongs to a human T cell epitope. 304 Recent studies suggest that nitration may also affect the allergenic potential and adjuvant activity of α-amylase/trypsin inhibitors (ATIs) from wheat and other gluten-containing grains, which act as aeroallergens in baker's asthma and are involved in hyper-

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Critical Review sensitivities and chronic inflammation of the gastrointestinal tract. 346−351 Nitrated variants of the major birch pollen allergen Bet v 1 induced enhanced levels of specific IgE in murine models, possibly because of the formation of neo-epitopes. 229 Nitration of Bet v 1 also increased the presentation of allergenderived peptides by antigen presenting cells (APC). 305 Moreover, increased proteolytic stability, up-regulation of CCL17 (Th2-associated chemokine secreted by dendritic cells, DC), and alterations of T cell proliferation and stimulatory capacities have been observed for nitrated Bet v 1. 306 Nitrated proteins also have been observed to modulate the antioxidant levels in murine pneumocytes. 352 In a recent study, in vivo fumigation of ragweed pollen with NO 2 resulted in an altered proteomic pattern including nitrosylation products and the treated pollen showed higher IgE recognition in immunoblots. 239 Enhanced allergenic potential was also observed for Betula pendula, Ostrya carpinifolia, and Carpinus betulus pollen after NO 2 exposure (Table S1). 236 Reaction product studies and kinetic experiments have shown that environmentally relevant O 3 and NO 2 concentrations can induce protein nitration on tyrosine residues. 237,328−330,333,353−355 This is in line with earlier observations that atmospheric oxidation and nitration processes leads to the formation of nitrophenols and dinitrophenols, 356 and that nitration is an important reaction pathway particularly in the atmospheric aqueous phase. 357,358 Especially, aromatic amino acids like tyrosine and tryptophan can react with atmospheric nitrating agents, such as ozone/NO 2 mixtures or peroxyacetylnitrate (PAN). 330,359 Under photochemical smog conditions in polluted urban environments (high O 3 and NO 2 concentrations), proteins on the surface of aerosol particles can be efficiently nitrated within minutes to hours. 328,330 The reaction kinetics also depends strongly on ambient relative humidity: At high relative humidity and especially during aqueous phase processing (when aerosol particles are activated as cloud or fog droplets), nitration may proceed efficiently also within the particle bulk. 328,360,361 Mechanistically, the reaction between O 3 /NO 2 and tyrosine involves the formation of long-lived reactive oxygen intermediates (ROI), likely via hydrogen abstraction from the phenolic OH group, yielding tyrosyl radicals (phenoxy radical derivatives of tyrosine) that can further react with NO 2 to form nitrotyrosine residues as shown in Figure 4. 329,362,363 The twostep protein nitration by air pollutants is similar to the endogenous nitration of proteins by peroxynitrite (ONOO − ) 298,328,364 formed from nitrous oxide (NO) and superoxide anions (O 2 − ). 301,365,366 For endogeneous protein nitration by ONOO − , both radical and electron transfer reaction pathways have been proposed. 367 Besides nitration, tyrosyl radicals can also undergo hydroxylation or self-reaction (cross-linking) to form dityrosine derivatives (Figure 4). 368 The site selectivity of protein nitration is influenced by the molecular structure of the protein, the nitrating agent, and the reaction conditions. For example, different preferred reaction sites were observed for the birch pollen allergen Bet v 1, the egg allergen ovalbumin, and bovine serum albumin. 304,328,333,354 Upon exposure of Bet v 1 to atmospherically relevant concentrations of O 3 /NO 2 and physiologically relevant concentrations of ONOO − , the preferred sites of nitration were tyrosine residues with high solvent accessibility and within a hydrophobic environment. Accordingly, nitrated tyrosine residues occurred mainly in the C-terminal helix and in the hydrophobic cavity ( Figure S4). 328 Both are key positions for the binding of specific IgE, 369 as well as ligands like fatty acids, cytokines, and flavonoids. 370−372 The binding of such ligands may be involved in allergic and inflammatory immune responses by stabilizing Bet v 1 against endo/lysosomal degradation. 373 Moreover, nitration-related changes in ligandbinding capacity might influence the interaction of allergenic proteins like Bet v 1 with adjuvant substances like lipopolysaccharide (LPS) and induce a shift from Th1 to Th2 responses, thus resulting in increased allergenicity. 306 Dimerization and oligomerization are supposed to have a strong influence on the immunogenicity of allergenic proteins and are common features of major allergens like Bet v 1. 307,308 The cross-linking of IgE receptors (FcεRI) on effector cells is a key element of allergic reactions and requires IgE antibody clustering on the cell surface, 374,375 which may be facilitated by multivalent allergens, such as oligomers of allergenic proteins providing multiple epitopes of the same kind. 122,376 Moreover, cross-linking can make proteins less susceptible to enzymatic proteolysis and influence immune responses. 313,373,377 Indeed, immune responses to oligomers and aggregates of certain allergenic proteins were found to be enhanced compared to the monomeric form of the allergenic protein. 307,308,378−380 The clustering of allergenic proteins on nanoparticle surfaces (protein coronas) can also modulate allergic respones depending on protein and particle properties. 244 Accordingly, the investigation and effects of allergen colocalization on the surface of inhalable ambient particles, such as pollen fragments or soot (DEP), are potentially important research perspectives.
Oxidative protein cross-linking can occur upon (a) tyrosyl radical coupling through dityrosine cross-links, (b) Schiff-base coupling of oxidation-derived protein carbonyl groups with the ε-amino groups of lysine residues, and (c) intermolecular . Posttranslational modification of proteins exposed to ozone (O 3 ) and nitrogen dioxide (NO 2 ). The initial reaction with O 3 leads to the formation of reactive oxygen intermediates (ROI, tyrosyl radicals), which can further react with each other to form cross-linked proteins (dityrosine) or with NO 2 to form nitrated proteins (nitrotyrosine). The shown protein is Bet v 1.0101 (PDB accession code 4A88, 370 created with the PDB protein workshop 3.9 498 ), for which nitration and cross-linking were found to influence the immunogenicity and allergenic potential. 229,305,306,328 Red dot indicates a tyrosyl radical; red bar indicates dityrosine cross-link.

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Critical Review disulfide coupling. 381 Recently, protein cross-linking and oligomerization upon exposure to atmospherically relevant concentrations of O 3 have been shown to proceed via the formation dityrosine cross-links as outlined in Figure 4, yielding up to ∼10% of dimers, trimers, and higher oligomers of a model protein within minutes to hours of exposure under summer smog conditions. 368 Similar reaction mechanisms involving reactive oxygen intermediates may also be responsible for the protein cross-linking observed upon reaction with physiological and synthetic nitrating agents like ONOO − and tetranitromethane, respectively. 306,313,382,383 Cross-linking upon reaction with tetranitromethane was suggested to alter the immunogenicity and enhance the allergenicity of Bet v 1 through decreased endolysosomal degradation leading to extended MHC class II antigen presentation. 306 On the other hand, oligomerization of allergens induced by modification with glutaraldehyde, that is, formation of glutaraldehyde bridges between nucleophilic amino acid residues (in particular lysine), was suggested to reduce immunogenicity and allergenicity due to delayed allergen uptake and presentation by dendritic cells. 384,385 As illustrated in Figure S2, the processes of allergic sensitization and response involve a wide range of interactions between protein molecules dissolved in liquids (blood, lymph, etc.) and embedded in semisolid structures (membranes, cells, tissues), which can be regarded as protein multiphase chemistry. 38 Protein reactions with ROS/RNS are generally pH-dependent and yield a mixture of hydroxylated, nitrated, cross-linked, aggregated or degraded products. 386−391 To assess immune responses to specific posttranslational modifications of proteins, it is necessary to carefully characterize the investigated samples and avoid artifacts or misinterpretations that might arise from interferences between different reaction products and pathways, for example, nitration vs dimerization or oligomerization of proteins exposed to oxidizing and nitrating agents (Figure 4).

EPITHELIAL SURFACE INTERACTIONS
The deposition of particles in the respiratory tract is sizedependent and deposited particles are removed by a number of physical, chemical, and biological clearance processes, including

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Critical Review mucociliary movement, endo-and phagocytosis, dissolution, leaching, and protein binding. 201 Thus, the first step of an inhaled allergen-carrying particle is evading the mechanical defenses of the respiratory tract and passing, for example, alveolar macrophages, which prevent inappropriate immune activation by removing inhaled allergens via phagocytosis. 392−394 The epithelial surface is a protective barrier, which protects the underlying tissue from many inhaled substances. The epithelial cells are covered by a viscous mucosal lining rich in immune cells and soluble components, such as antioxidants, complement proteins, and surfactant proteins. 201, 395,396 As the epithelium is more than a passive protective barrier, it recruits and activates more specialized immune cells and promotes inflammatory responses, 397 allergy is also discussed to be an epithelial barrier disease. 15,131,398−400 For example, nasal epithelium is clearly different between healthy and allergic subjects and only in allergic subjects the transport of Bet v 1 is caveolar-mediated. 401 Air pollutants interacting with epithelial surfaces can act as adjuvants promoting pro-allergic innate and adaptive immune reactions as outlined in Table 2 and section S1. For example, they can induce inflammation and disrupt epithelial barriers, facilitating the access of allergens to immunogenic effector cells. 8,86 In particular, air particulate matter can trigger ROS production through Fenton-like reactions and the activation of macrophages, mitochondria and enzymes related to the oxidant/antioxidant balance (e.g., NADPH oxidase, glutathione peroxidase). 309,310,402−405 Additionally, pollution-derived ROS can induce proinflammatory responses by the production of damage associated molecular patterns (DAMPs oxidized phospholipids, hyaluronic acid, etc.) and trigger immune reactions leading to acute or chronic inflammation, 29,406 for example, through feedback cycles involving Toll-like receptors (TLR) and other pattern recognition receptors (PRR) ( Figure  S5). 407 Ozone and particulate matter can prime the airways for pro-allergic responses, and TLR signaling plays an important role in pollutant-induced inflammation. 408,409 During inflammation, inducible nitric oxide synthase (iNOS) that is mainly expressed in innate immune cells (monocytes, macrophages, dendritic cells) provides high amounts of nitrogen oxide (NO), which can react with superoxide radicals to form peroxynitrite (ONOO − ), a central endogenous nitrating agent for proteins. 301 In addition, particulate and gaseous pollutants may also drive pro-allergic inflammation through the generation of oxidative stress involving elevated levels of ONOO − . 410 As illustrated in Figure 5A, epithelial surfaces are interfaces coupling the atmospheric and the physiological production, cycling, and effects of ROS/RNS. 38 Specific interactions of atmospheric ROS/RNS with antioxidants in the epithelial lining fluid are shown in Figure 5B. An increase of ozone from typical background concentration levels (∼30 ppb) to summer smog conditions (>100 ppb) reduces the chemical half-life of antioxidants from days to hours, 309 which may be comparable or shorter than the physiological replenishment rates. 411 Furthermore, the adjuvant effect of ambient ultrafine particles was correlated with their oxidant potential. 412 Major contributors to the redox properties of ambient particles are transition metals, polycyclic aromatic hydrocarbons, and derivatives (PAH, nitro/oxy-PAH), and semiquinones. 38,312,412−415 In addition, the deposition of acidic particles may reduce the pH of the epithelial lining fluid (ELF). For healthy people the mean pH is ∼7.4, while in people with diseases (e.g., asthma, acid reflux) it can be as low as ∼4. 416,417 Oxidant exposure and changes of pH can alter reaction pathways of antioxidants 418 and also decrease the activities of antioxidant-related enzymes in the ELF, which are also reduced in smokers and people suffering from lung diseases. 419−421 Recent studies yielded chemical exposure-response relations between ambient concentrations of air pollutants and the production rates and concentrations of ROS in the ELF of the human respiratory tract. 309 As illustrated in Figure 6, the total concentration of ROS generated by redox-active substances contained in fine particulate matter (PM2.5) deposited in the ELF ranges from ∼10 nmol L −1 under clean conditions up to almost ∼250 nmol L −1 under highly polluted conditions. Thus, the inhalation of PM2.5 can increase ROS concentrations in the ELF to levels that exceed physiological background levels (50− 200 nmol L −1 ) and are characteristic for respiratory diseases. 309,422 In addition to the effects of PM2.5, ambient ozone readily saturates the ELF and can enhance oxidative stress by depleting antioxidants and surfactants. 309 Ozone also reacts with skin lipids (e.g., squalene) and generates organic compounds (e.g., mono-and dicarbonyls) that can act as irritants. 269 These and related organic compounds were found to act as adjuvants in the development of respiratory allergies as well as atopic dermatitis. 270,271,423,424 Some air pollutants and

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Critical Review chemical reaction products formed at epithelial interfaces are sufficiently long-lived and mobile to diffuse through membranes and interact with the neural, cardiovascular, and immune system networks of the human body. 314,425−429 Through these and related physiological interactions involving DAMPs, inflammatory mediators, cytokines, leukocytes etc., oxidative stress and inflammation caused by air pollutants may propagate from the respiratory tract and skin to other parts of the human organism and exert systemic influence on the development of allergies, reaching also the gastrointestinal tract. 38,429 A wide variety of commensal, symbiotic, and pathogenic microorganisms are found on the epithelial surfaces of the human body, such as the skin, lungs, and the gastrointestinal tract. Recent research suggests that the human microbiome is important to maintain physiological functions and to induce immune regulation by balancing the activities of Th1 and Th2 cells. 430−433 Normal microbial colonization in early life can promote tolerance to aeroallergens via induced regulatory T cells. 434 The development and composition of the human microbiome are influenced by many factors such as diet, infections, medical treatment, and also environmental factors. 435 For example, air pollutants and climatic stress factors may disturb microbial communities through oxidative stress, inflammation, and changes in environmental biodiversity. 4, 36 Modifications in the composition of the gastrointestinal and lung microbiome can in turn affect the development of allergies in accordance with the "hygiene hypothesis" 36,436−440 and may also promote pathogenic species that can contribute to these diseases. 4,441−443 Recent studies revealed differences in the structure and composition of microbiota in the lower airways of healthy and asthmatic people: Bacteroidetes, Firmicutes, and Proteobacteria are the most common phyla found in airways of healthy subjects, whereas increased concentrations of pathogenic Proteobacteria, such as Haemophilus, Moraxella, and Neisseria spp., were found in asthma patients. 442,443 Moreover, viral infections can exacerbate allergies. 31 It is still unclear, however, if these changes are a cause or a consequence of the disease. Moreover, it has been suggested that air pollutants, especially air particulate matter, ingested together with food can trigger and accelerate the development of gastrointestinal inflammatory diseases by altering the gastrointestinal microbiome and immune functions. 444 Besides the human microbiome, also microbes associated with allergenic pollen (pollen microbiome) and other aeroallergens may act as adjuvants when deposited on epithelial surfaces. 235,445

CONCLUSIONS AND OUTLOOK
As the globally pervasive anthropogenic influence continues to shape planet Earth and the human environment in the Anthropocene, it becomes increasingly important to understand and assess the potential effects of environmental change on human health. The widespread increase of allergies and their complex dependence on multiple influencing factors, including environmental pollution, indicate that allergic diseases are a major challenge with regard to maintaining and improving public health.
Anthropogenic emissions of atmospheric trace substances are affecting air quality and climate on local, regional, and global scales. Changes in atmospheric aerosol composition, oxidant concentrations, and climate parameters can induce chemical modifications of allergens, increase oxidative stress in the human body, and skew the immune system toward allergic reactions. In particular, air pollutants can act as adjuvants and alter the immunogenicity of allergenic proteins, while climate change affects the abundance and properties of bioaerosols as carriers of aeroallergens. The production, release and properties of allergens and adjuvants are subject to various human interferences with the biosphere and climate system, including air pollutant interactions with natural and agricultural vegetation, fertilization and land-use change, as well as plant breeding and genetic engineering.
The following key questions remain to be resolved to understand and mitigate potential effects of air pollution and climate change on the observed increase and future develop-  Table S2, building on and extending suggestions given in related review and perspective articles (e.g., refs 8, 12, 93, and 280). Beyond addressing the above questions, it appears worthwhile to explore which components of the immune system could be modulated to prevent adverse effects of air pollution, for example, whether therapeutic monoclonal antibodies against relevant cytokines (e.g., IL-4, IL-5, IL-13) or IgE antibodies could make a difference. Further information about ongoing efforts and future perspectives of mitigating the health effects of climate change and air pollution is available from various national and international government agencies, medical institutions and related organizations (e.g., refs 4, 37, and 446). For efficient scientific progress, it will be important to combine and optimize state-of-the-art methods and results of environmental, immunological and epidemiological studies, tightly coupling physical, chemical, biological, and medical techniques and knowledge. One of the challenges consists in identifying and quantifying the mechanisms and feedback loops of immunochemical reactions in response to environmental influencing factors, including chemical modifications and interactions of allergens and adjuvants under realistic environmental and physiological conditions. For this purpose, the results of laboratory experiments and monitoring networks with improved detection methods for allergens, adjuvants and reactive intermediates should be used to design and inform epidemiological studies targeting the effects of different types and combinations of air pollutants and climate parameters.

Environmental Science & Technology
Critical Review