Background
Smoking is the most prominent risk factor for the development of inflammatory lung diseases such as chronic obstructive pulmonary disease (COPD), Pulmonary Langerhans Cell Histiocytosis (PLCH) or Respiratory Bronchiolitis Interstitial Lung Disease (RB-ILD) [
1]. In patients with asthma, smoking reduces asthma control and the therapeutic response to inhaled contricosteroids (ICS) [
2]. There is growing evidence that a pathologic accumulation and activation of Dendritic cells (DCs) within the lung plays a central pathogenetic role in these diseases [
3‐
5]. Airway DCs are professional antigen presenting cells which control pulmonary immune responses by regulating the expansion of specific pro-inflammatory or anti-inflammatory T-cell subsets [
3]. Under physiological conditions, DCs migrate to the draining lymph nodes in order to present antigenic information to lymphocytes. However, mainly under pathological conditions, DCs can also present antigens locally to lymphocytes within the lung parenchyma [
6]. DCs are subdivided into myeloid DCs (mDCs) and plasmacytoid DCs (pDCs). The mDCs initiate and promote inflammation in various diseases of the airways, including asthma and respiratory infections [
7,
8]. The functional role of pDCs in humans is less clear. Current concepts postulate that pDCs might have anti-inflammatory properties and that pDCs play a crucial role in the defense against infections [
7,
8].
In the airways of smokers, a characteristic expansion of Langerhans cells (a subset of mDCs) can be observed [
9]. In addition, airway mDCs of smokers are characterised by changes in function-associated surface molecules, such as an increased expression of the co-stimulatory molecules CD80 and CD86, and an increased expression of antigen recognition receptors such as BDCA-1 (CD1c) and MMR (Macrophage Mannose Receptor) [
9]. Smoke-related lung diseases such as COPD and PLCH are accompanied by a local accumulation of DCs in the lungs which exceeds DC numbers in smoking controls [
1,
10‐
12]. In addition, phenotypic characteristics of DCs appear to differ between “healthy” smokers and patients with smoke-related lung diseases [
13]. Although there is a substantial body of evidence that corticosteroids influence airway DCs in patients with allergic asthma and allergic rhinitis [
14‐
16], there is currently no information on the effect of ICS on airway DCs in cigarette smokers with or without smoke-related lung diseases. Therefore, it was the aim of this trial to investigate the influence of ICS inhalation on airway DCs in smokers for the first time. Participant selection was limited to smokers with normal spirometry, because the local ethics committee of Rostock (Germany) stated that it is ethically inappropriate to treat patients with COPD (of any severity) with placebo only. Using an established flow cytometric method to analyze DCs in human bronchoalveolar lavage (BAL) fluid [
17‐
19], we measured airway DCs in smokers prior to and after 4 weeks of inhalation of fluticasone, and compared the findings with the effects of placebo treatment and of combination therapy (fluticasone plus salmeterol), in a randomized, double-blind, placebo-controlled clinical trial.
Discussion
DCs are crucial players in pulmonary diseases caused by tobacco smoke, but knowledge on the pharmacologic modulation of airway DCs in smokers is sparse. This study demonstrates for the first time that inhalation of the ICS fluticasone reduces pDC numbers in the airways of cigarette smokers. In addition, it demonstrates that only a combination therapy of fluticasone with the long-acting beta-agonist (LABA) salmeterol reduces airway mDCs in smokers. Notably, neither inhalation of fluticasone alone nor a combination therapy modulated the phenotype of airway DCs. Thus, our study adds important new evidence to the ongoing discussion on the role of ICS in the treatment of smokers.
Corticosteroids can influence the number, phenotype and function of human mDCs [
22,
23]. Several studies have demonstrated that ICS reduce mDCs in the airways of healthy subjects and of patients with allergic asthma [
14,
15,
24]. Our study is the first to investigate ICS effects on airway DCs in smokers. In contrast to the findings in healthy subjects and patients with asthma [
14,
15,
24], there was no reduction in endobronchial mDCs after fluticasone monotherapy. In addition, there were no changes in the expression of function-associated mDC surface molecules. There are two hypotheses to explain these findings. On the one hand, airway mDCs of smokers might be ICS resistant, according to the concept that smokers exhibit a relative corticosteroid resistance compared to non-smokers [
25]. On the other hand, the 4-week inhalation period might be too short or the ICS dose chosen too low to cause effects on endobronchial mDCs in smokers. However, a previous report using the same fluticasone dose as in our study demonstrated a reduction in airway mDCs in patients with asthma and in healthy controls in a comparable time period [
14]. Therefore, the lacking effects of fluticasone monotherapy on airway mDCs in smokers might be rather due to a resistance to the drug than due to the dose of fluticasone or the duration of the treatment.
An abnormal response of the innate and adaptive immune system to noxious particles and gases is the key pathogenetic feature of chronic obstructive pulmonary disease (COPD) [
3]. Only combined therapy of ICS with LABA, but not ICS monotherapy, is an effective treatment option for COPD. This clinical experience is supported by molecular evidence suggesting that the crosstalk between ICS and LABA potentiates the anti-inflammatory effect of ICS [
26]. Accordingly, ICS resistance of patients with COPD can be reversed by inhalation of a LABA [
27‐
29]. Our study confirms these data by showing that only smokers treated with fluticasone plus salmeterol, but not smokers treated with fluticasone alone, displayed a decrease in endobronchial macrophages and neutrophils. In addition, we demonstrate for the first time that only inhalation of fluticasone plus salmeterol reduces endobronchial mDCs in smokers. Myeloid DCs have been postulated to be key drivers of inflammation in smoke-related lung diseases [
3,
30]. Thus, we hypothesize that the mDC reduction induced by the inhalation of fluticasone plus salmeterol might lead to a sustainable reduction of inflammation in the airways of cigarette smokers. It has been postulated that specific mDC subsets might play a major role in the pathogenesis of smoke-related lung diseases [
31]. However, we did not observe a preferential decrease in one specific subset (such as CD1a+ mDCs or BDCA-1+ mDCs) following combination therapy. Thus, combination therapy might not only reduce those mDC subsets which induce the pathology of smoke-related lung diseases, but also other mDC subsets which might be beneficial. In addition, it has to be noted that mDCs also play a role in the protection against infections [
8]. A reduction of mDCs in the airways may, therefore, predispose for infections. Thus, further studies are needed to clarify the precise role of mDC subsets in smoke-related lung diseases.
In additional DC-T-cell co-culture experiments using DCs from peripheral blood of active cigarette smokers we observed that the supression of DC-induced T-cell proliferation by fluticasone is not dependent on the presence of salmeterol. Thus, it appears that some aspects of DC pathophysiology can be influenced by ICS alone (such as the induction of T-cell proliferation), while others can only be influenced by a combination of ICS plus LABA (such as the number of mDCs in the airways).
Another effect of fluticasone independent of salmeterol co-medication was the reduction of endobronchial pDCs after 4 weeks of treatment. Plasmacytoid DCs are thought to have beneficial effects in chronic inflammatory airway diseases due to anti-inflammatory properties and due to their crucial role in the defense against infections [
7,
8]. However, pDCs are very sensitive to corticosteroids. Corticosteroids such as prednisolone or dexamethasone inhibit the differentiation and induce apoptosis of human pDCs [
32,
33]. It has been suggested that a corticosteroid-induced reduction of endobronchial pDCs might be related to the increased risk of recurrent pneumonias [
18]. An increased risk of pneumonia has also been identified as a side effect of ICS therapy in patients with COPD [
34], although the underlying mechanisms for this clinical observation are still unknown. The reduction of airway pDCs by fluticasone observed in our study might be one contributing mechanism. However, studies in patients with COPD are needed to further clarify this issue.
A limitation of the study is the fact that only smokers with normal spirometry were included in the trial. Therefore, the relevance of the findings of this study for patients with COPD or other smoking-related lung diseases remains open. It was necessary to include a placebo group in this trial in order to exclude artifacts due to the endoscopic intervention and to exclude a possible physiologic variation of DC concentrations in the airways of smokers over the chosen time period. The scientific decision to include a placebo group led to the statement of the ethics committee that smokers with COPD (of any severity) must be excluded from the study because it was felt that treatment of patients with COPD with placebo only was ethically inappropriate. There is an ongoing debate about the design of clinical trials in the era of new COPD treatment options. The availability of potent drugs for this devastating disease makes it ethically impossible to perform controlled trials including a group of patients treated with placebo only. This results in the difficulty to distinguish between direct effects of the studied compound on the disease and complex drug-drug interactions with baseline therapy.
Competing interests
UK, LT, PS AB, SK, KG, MK, KB have no conflicts of interest. ML and JCV served in advisory boards and/or received lecture fees from the following companies: Astra Zeneca, Boehringer Ingelheim, Chiesi, GSK, Janssen, MSD, Novartis.
Authors’ contributions
ML and JCV designed and supervised the study, wrote the proposals for the authorities, analyzed the data and wrote the manuscript draft; UK, LT and PS recruited and characterized the participants, and collected the clinical data; AB and ML performed the bronchoscopies; SK designed the study and wrote the proposals for the authorities; KG, MK and KB performed flow cytometric analyses, cell purifications and cell cultures. All authors read and approved the final manuscript.