Background
Asthma is characterized by airflow obstruction primarily due to bronchospasms, chronic inflammation and remodeling of the airway wall. Glucocorticosteroids (GC) are the most effective drugs used to control asthma symptoms and exacerbation. However, neutrophils the predominant cells within the airways of a subset of asthmatic patients, are recognized as poor responders to GC treatment even with high doses of inhaled glucocorticosteroids (ICS). Indeed, GC significantly decrease the recruitment and activation of several inflammatory cells (mast cells, eosinophils, macrophages and T lymphocytes) in asthmatic airways, yet pulmonary neutrophilia remains unchanged [
1‐
3]. However, blood neutrophils are sensitive to the effects of glucocorticoids in vitro, suggesting that the activation stimuli occurring within in airways are responsible for the lack of inhibition of airway neutrophilia observed in asthma under GC treatment [
4].
Recently, NETs have emerged as a new microbial and cytotoxic mechanism of the immune response to infections and injuries [
5,
6]. NETs are formed by decondensed chromatin and bactericidal proteins released by granules of neutrophils, through a process called NETosis [
7]. This release is induced by several agents (reactive oxygen species, bacteria, fungi, viruses, antigen–antibody complexes, microbial components and lipopolysaccharide) [
8‐
13] and leads to tissue injury [
5,
6]. Several other factors, such as
N-formyl-methionyl-leucyl-phenylalanine (fMLP) can induce neutrophil activation without leading to NETs formation [
14]. Indeed, unlike previous beliefs, neutrophils have been shown to modulate their responses according to the mediators present in their environment [
15], and therefore the contribution of disease processes is complex. NETs are associated with inflammation and disease severity in chronic airway diseases such as asthma [
16], and cause airway obstruction [
17]. In sputum of patients with chronic obstructive pulmonary disease
(COPD), however, NETs formation is present whether the patients are in exacerbation or not [
18]. While GC (dexamethasone) did not prevent NETs formation induced by stimulating peripheral blood neutrophils with phorbol myristate acetate (PMA) in vitro [
19], no study, to the best of our knowledge, has investigated NETs formation in vivo during treatment using these drugs.
Despite a recent interest in the role of NETs in disease pathogenesis, the molecular mechanisms resulting in NETs formation and their regulation remain unknown [
20]. It was recently shown in an immune-mediated disorder, that neutrophils release IL-17 through NETs formation [
21]. IL-17 is considered to play a central role in severe asthma and to contribute to neutrophilic inflammation and steroid insensitivity [
22], particularly in asthmatics with frequent exacerbations [
23]. We previously found that this cytokine induces the activation of equine neutrophils and importantly, that it is not inhibited by GC [
24]. However, IL-17’s contribution to NETs formation and its insensitivity to GC have not been studied in asthma. We therefore hypothesized that NETs formation in asthmatic airways would be resistant to GC through an IL-17 mediated pathway. We also postulated that the IL-17 rich lung microenvironment in asthma would contribute to this insensitivity. We investigated NETs in both airway and blood neutrophils of asthmatic horses before and during GC administration. We studied horses with severe neutrophilic asthma (severe equine asthma, also known as heaves), a condition that shares marked similarities with human neutrophilic asthma [
25], including the remodeling of the ASM layer [
26] and extracellular matrix [
27]. Furthermore, human and equine neutrophils have similar biology [
4,
24,
28].
Discussion
NETs participate in antimicrobial defense; however, their persistence in tissues can result in host damage. NETosis has been shown to contribute to lung pathology by promoting airway obstruction, alveolar capillary damage and disruption of host proteins and cellular matrix [
31,
32]. The recently recognized neutrophilic asthma phenotype is considered a major health concern as affecting >50% of severe human asthmatics [
33,
34]. These patients are often poorly responsive to therapy, have fixed airflow obstruction and an accelerated decline in lung function. Severe equine asthma shares numerous similarities with human neutrophilic asthma [
35], and was studied here to perform experiments not possible in humans because of ethical considerations. Interestingly, NETs are found to be sensitive to non-steroidal anti-inflammatory drugs in vitro but were reported to be unaffected by GC [
19]. As in vitro findings do not always reflect in vivo effects, we evaluated whether NETs formation is insensitive to GC through an IL-17 pathway in an equine model of asthma. However, to the contrary, we observed that GC decreases PMA-induced NETs formation in vitro and also in vivo in the lungs of severe asthmatic horses. This effect was observed despite the persistence of neutrophils within the airways and was independent of the activation state of neutrophils and the persistence of IL-17 in asthmatic airways. Taken together, these results suggest that NETs formation is sensitive to GC and independently regulated by neutrophil recruitment within the airways and their activation.
We first confirmed the presence of NETs in the airways of asthmatic horses [
36], as observed in human asthmatic patients [
37]. Extracellular DNA quantification was difficult to determine in BALF using usual fluorometric methods or agarose gel electrophoresis since DNA binds to tenacious mucus that is difficult to dissolve without damaging the NETs (data not shown). Considering this, we developed and adapted a score to quantify NETs in airway secretions, estimating the surface of the microscopic field occupied by NETs, with unbiased point counting, as this parameter seems to be an indicator of inflammation [
38]. In BAL cytospin, NETS were identified only in the peripheral areas of the sildes. We suggest that neutrophils undergo morphological changes with NETs secretion, that lead to their accumulation in the peripheral areas on the slides with the centrifugal force. As the NETs score was zero for the central areas, we only reported NETs scores for peripheral ones.
Importantly, we observed that dexamethasone inhibits NETs formation in both asthmatic and control horses. To the best of our knowledge, this is the first study investigating the role of GC in NETosis in vivo. In agreement with this finding, and contrary to a previous report [
19], dexamethasone inhibited in vitro the PMA-induced NETs formation in the present study. While species differences may have contributed to these contrasting results, our experimentation allowed us to detect only NETs neoformation following a shorter PMA simulation period. After one hour, we observed, chronologically, adherence and flattening, vacuolization, and intracellular chromatin decondensation. Nevertheless, as NETs formation was not attenuated after one week of treatment, this delayed response, combined with the previous in vitro findings [
19], suggests an indirect effect of GC on the lung and that the inflammatory microenvironment in the airways may be contributing to the delayed effect of GC.
The migration of neutrophils from the blood to the inflammation site involves a complex regulation of surface adhesion proteins and expressions of activation markers. The tight adhesion and transendothelial migration of granulocytes mediated by β2 integrins, including CD11b (CR3, or Mac1) and aminopeptidase CD13 [
39], are required for blood neutrophils to reach the airway lumen. We, therefore, investigated whether NETs inhibition was specific or resulting from an overall downregulation of neutrophil activation by GC. We found that dexamethasone reduced the expression of CD11b in blood neutrophils but not in pulmonary neutrophils of asthmatic horses, and that a similar trend occurred for CD13. As CD11b [
40] and CD13 [
41] are cell surface markers typically expressed upon neutrophil activation, these results suggest a distinct pro-inflammatory milieu for neutrophils in airways and blood, regardless of the disease status. These findings also support the previous report [
42] stating that following the bronchial instillation of endotoxins, dexamethasone has a more potent anti-inflammatory effect in the blood than in the lungs, in healthy human volunteers.
IL-17 mRNA was increased in BAL cells of asthmatic horses and was unaffected by GC. In human studies, GC had conflicting effects on IL-17 production (6, 32) and, therefore, our IL-17 mRNA results within the lung during a GC therapy need to be investigated at the protein level. Furthermore, to our knowledge, while IL-17 has been implicated in steroid-resistant neutrophilic asthma [
43‐
45], its possible contribution to NETs formation in asthma has not been investigated. In this study, we observed that IL-17 decreased NETs formation. These results were unexpected, as IL-17 was shown to be associated with NETs formation in psoriasis lesions [
46]. These findings point to a link between IL-17, NETosis and GC insensitivity in severe neutrophilic asthma [
46], which is intriguing since we reported that IL-17 activates and increases the survival of neutrophils in vitro, which are unresponsive to GC [
24]. Thus, considering that IL-17 decreases NETs formation and that neutrophils also die by NETosis
, it remains possible that the IL-17 pathway resistant under GC contributes to the persistence of neutrophils in the asthmatic airways. Also, although IL-17 decreases NETs formation in vitro, it also promotes the production of IL-8 within the lung tissue [
24]. As IL-8 induces NETs formation [
47], the balance of IL-8 inducing NETs release may be opposed by the IL-17 inhibitory response. However, the effects IL-17 have on NETs formation in vivo likely depend on the stimuli applied and therefore, the response within the lung microenvironment may differ from the inhibition we observed in vitro.
Our results also demonstrate that GC decrease the apoptosis of airway neutrophils. Studies have reported that while in vitro
, neutrophil apoptosis is delayed by GC [
3,
48,
49], no studies have compared the level of apoptosis in vivo before and following GC treatment in asthmatic and in healthy subjects. As dexamethasone did not affect the activation markers of pulmonary neutrophils, but decreased neutrophil apoptosis in both healthy and asthmatic animals, neutrophils’ response to GC appear to be specific and not influenced by asthmatic inflammation. However, apoptosis levels are relatively low (around 5%) and probably contribute minimally to the airway neutrophilia persistence in asthmatic animals. Further investigations using larger sample sizes need to be performed to confirm these results.