Inflammatory mechanisms underlying potentiation of effects of inhaled aeroallergens in response to nitrogen dioxide in allergic airways disease

Among the oxides of nitrogen, nitrogen dioxide (NO2) is considered to be the most important pollutant and has been linked to the morbidity and mortality of asthma and susceptibility to childhood respiratory tract infections [ 1 Advisory Group on Medical Aspects of Air Pollution Episodes. ‘Third Report: Oxides of Nitrogen’. London: HMSO 234]. It is produced as a result of a direct reaction between nitrogen and oxygen during high temperature combustion and consequently is present both in the indoor as well as in the outdoor atmosphere from sources such as gas cookers and road traffic emissions, respectively [ 1 Advisory Group on Medical Aspects of Air Pollution Episodes. ‘Third Report: Oxides of Nitrogen’. London: HMSO ]. Outdoor levels may peak up to 0.4 p.p.m. at kerb sites in the urban areas during ‘rush hours’ but the peak indoor levels are higher and may even reach up to 4 p.p.m. in skating ice rinks, silage plants, fertilizer and explosive factories [ 1 Advisory Group on Medical Aspects of Air Pollution Episodes. ‘Third Report: Oxides of Nitrogen’. London: HMSO ]. Several studies have shown that short-term controlled exposure to NO2 can induce a small but significant increase in bronchial reactivity to nonspecific agents such as cold air, histamine and carbachol in healthy and asthmatic airways, but does not have any major effects on lung function parameters [ 1 Advisory Group on Medical Aspects of Air Pollution Episodes. ‘Third Report: Oxides of Nitrogen’. London: HMSO , 5678]. However, prior exposure to nitrogen dioxide enhances both early and late responses to inhaled allergen in sensitive asthmatics [ 9]. Tunnicliffe, Burge and Ayres have shown that when mild Dermatophagoides pteronnysinus (house dust mite)-sensitive asthmatics were exposed to 0.4 p.p.m. NO2 and filtered for 1 h on separate occasions and subsequently challenged with D. Pteronnysinus, exposure to NO2 significantly enhanced both the early and late responses to D. Pteronnysinus [ 9]. Devalia et al. exposed 10 atopic mild asthmatics sensitive to D. Pteronnysinus to 0.4 p.p.m. NO2, 0.2 p.p.m. SO2, O.4 p.p.m. NO2 plus 0.2 p.p.m. SO2 and filtered air for 6 h followed by bronchial challenge with D. Pteronnysinus on four separate occasions [ 10]. They found enhanced responses to inhaled allergen only following a combined exposure to the two gases but not after exposure to these gases singly. In a further study Rusznak, Devalia and Davies demonstrated that the potentiation of the response to inhaled allergen following a combined exposure to NO2 and SO2 can last up to 48 h and is maximal at 24 h following exposures [ 11]. Although the pattern of exposures occurring in the natural environment is much more complex, the concentrations employed in these studies are relevant to those occurring in the ambient atmosphere. Similar studies have been carried out in patients with allergic rhinitis. Wang et al. studied the effects of short-term exposure to 0.4 p.p.m. NO2 and filtered air for 6 h on the upper airways of two randomly allocated groups of patients with seasonal allergic rhinitis (SAR) [ 12]. Whilst the first control group was challenged with NO2 and filtered air alone on separate occasions, the second index group underwent local nasal allergen challenge with mixed grass pollen allergen extract immediately after exposure to NO2 and filtered air. In the control group nasal airway lavage (NAL) was performed immediately following exposure while the index group NAL was performed 0.5 h following allergen challenge. The results demonstrated that while exposure to NO2 alone had no effect on any of the biomarkers measured in the NAL fluid (including IL-8, myeloperoxidase, tryptase and eosinophil cationic protein [ECP]) and nasal airway resistance, allergen challenge after exposure to both air and NO2 significantly increased mast cell tryptase levels. Furthermore, a significant increase in ECP was observed only after exposure to NO2 and allergen suggesting that NO2 exposure may ‘prime’ the eosinophils already present in the nasal mucosa for subsequent activation by allergen. No data on cellular influx was presented in this study. The observations have clear implication for the induction of symptoms in susceptible atopic asthmatics exposed to outdoor allergens followed by relevant indoor and outdoor exposures to NO2. Until now, there have been no studies addressing whether conventional treatment with inhaled corticosteroids attenuate this response. The same group of workers have published a report in this issue of Clinical and Experimental Allergy on the inflammatory mechanisms underlying potentiation of inhaled responses to aeroallergen in the upper airways of patients with SAR and effects of regular treatment with fluticasone propionate nasal spray on these responses [ 13]. They have confirmed their earlier observations that prior exposure to NO2 potentiates the responses of nasal airways to local allergen challenge with mixed grass pollen allergen and that this response is mediated by influx and activation of eosinophils. In addition, this response is significantly attenuated by regular treatment with fluticasone nasal spray highlighting the potential role of inhaled corticosteroids in protecting the airways from the adverse effects of NO2 and other oxidant pollutants though further studies are required to confirm these observations. Currently, there is no data available on the effects of regular corticosteroid therapy on the lower airways of patients with asthma and chronic obstructive pulmonary disease following exposure to oxidant pollutants such as ozone, NO2 and diluted diesel exhaust. Such data will be of utmost importance in devising suitable strategies for treating patients with asthma during air pollution episodes and could be an important step forward for reducing asthma morbidity and mortality. The two studies by Wang and coworkers have shown that NO2 has an effect on eosinophils but does not induce an acute inflammatory response (polymorphonuclear neutrophils [PMN] influx and activation) in the nasal airways of patients with allergic airways disease. This is in contrast with a similar study in perennially allergic asthmatics carried out by Peden, Setzer Jr and Devlin [ 14]. They reported that, in addition to priming of eosinophils as observed following exposure to NO2, ozone also induces an acute inflammatory response seen as an influx of PMNs in NAL fluid. Vagaggini et al. have studied the effects of acute exposure to NO2 at peak outdoor concentrations (0.3 p.p.m. NO2/filtered air for 1 h) on the upper and lower airways of mild atopic asthmatics [ 15]. They performed NAL and sputum induction 2 h following exposures. No significant alterations were seen in lung function parameters, NAL and sputum differential cell counts in that study. Similary Jörres and coworkers did not detect PMN influx in the bronchoalveolar fluid of mild atopic asthmatics immediately following a 2-h exposure to 1 p.p.m. NO2 [ 16]. These observations are in contrast with those seen in response to ozone in the lower airways of asthmatics where such an exposure induces bronchoconstriction, influx and activation of PMNs and eosinophils [ 17]. The absence of PMN influx in these studies could be explained at least in part due to early sampling time, but the more likely explanation is the lesser potency of NO2 as compared with ozone as an oxidant pollutant. Several mechanisms can be proposed to explain the potentiation of responses to inhaled aeroallergen in individuals with allergic airways disease exposed to NO2. Bronchial epithelium is an important source of several cytokines and mediators including IL-8, Gro-α, TNF-α, GM-CSF, RANTES, sICAM and products of the arachidonate pathway [ 18]. Devalia and coworkers have shown that exposure of cultured human bronchial epithelial cells in vitro to 0.4–0.8 p.p.m. NO2 results in the release of TNF-α, IL-8 and GM-CSF which could play an important role in the recruitment and activation of eosinophils as described above in the context of exposure to a combination of NO2 and allergen in allergic airways disease. In addition, short-term exposure to NO2 has been shown to induce ciliary dyskinesia, epithelial damage and increase epithelial permeability, thereby contributing to increased penetration of allergen into submucosal sites [ 19]. Bronchial epithelial damage is a characteristic feature of asthma and recent studies have clearly shown that experimental rhinovirus infections can cause further damage [ 20, 21]. Thus, it follows that a combination of viruses and NO2 could prove extremely detrimental to the airways in allergic disease and could provide a biologically plausible explanation linking NO2 and asthma morbidity during pollution episodes. Figure 1 illustrates some of the plausible mechanisms contributing to the response to a combination of NO2 and allergen in allergic airways disease. . IIlustrating the plausible inflammatory mechanisms contributing to the responses to the combination of NO2 and aeroallergen in allergic airways disease. Exposure to ozone and NO2 is known to induce damage to cell membranes by an oxidative reaction and generate reactive oxygen intermediates (ROIs), cytokines and preformed cell mediators from inflammatory cells in the airways [ 1 Advisory Group on Medical Aspects of Air Pollution Episodes. ‘Third Report: Oxides of Nitrogen’. London: HMSO , 22, 23]. Activation of cytoplasmic NF-κB can be brought about by a variety of stimuli including reactive oxygen intermediates (ROIs), cytokines, mitogens, bacteria and their products [ 24, 25]. These lead to phosphorylation of bound inhibitory IκB protein and this reaction is catalysed by kinases. Loss of inhibitory IκB protein bound to Rel protein complex p50-p65 is followed by rapid translocation of p50-p65 heterodimer to the nucleus where it binds to the cis-acting κB sites in the enhancer and promoter regions of inflammatory genes such as those encoding for ICAM-1, E-selectin, IL-8 and iNOS leading to gene transcription [ 24, 25]. Although recent epidemiological studies do not support the view that air pollution per se contributes to the rising asthma prevalence, recent studies have provided some biologically plausible explanations linking exposure to diesel exhaust particles (DEPs) and development of allergic disease [ 26, 27]. Diaz Sanchez et al. have shown that in a group of ragweed-sensitive subjects, challenge with DEP and ragweed induced significantly higher amounts of ragweed specific IgE as opposed to challenge with ragweed alone [ 28]. In addition, in a further study they have shown that challenge with DEP plus ragweed suppressed mRNA expression of TH1 type cytokines (IFN-γ, IL-2) and stimulated mRNA expression of TH2-type cytokines (IL-4, -5, -6, -10, -13), a pattern characteristic of asthma and allergic disease [ 29]. Although speculative, similar mechanisms could also operate following exposure to NO2, but these remain to be confirmed. Our current understanding of the inflammatory mechanisms contributing to potentiation of responses to inhaled aeroallergen in the lower airways have until recently been hindered by a lack of noninvasive methods for sampling the lower airways. Sputum induction has proven to be an inexpensive, safe and valuable tool to investigate the inflammatory responses in the lower airways of patients with moderate to severe asthma, and future studies should focus on the inflammatory responses to a combination of air pollutants and aeroallergen using this method. There is now mounting evidence for the protective role of antioxidants in the epithelial lining fluid against oxidant pollutant-induced airways injury [ 30, 31]. Future studies must investigate this in allergic airways disease.