We report here that IL-17A expression in the bronchial submucosa was increased in mild to moderate asthma, and IL-17F was increased in mild to moderate and severe asthma. In contrast, in ex-smoking subjects with mild to moderate COPD the number of IL-17A+ cells in the bronchial submucosa, but not IL-17F, was increased compared with nonsmoking control subjects. In asthma there was a weak relationship between the number of IL-17A+ cells and the FEV1% predicted, and the sputum neutrophil count. There was no association between neutrophilic inflammation and IL-17A or F expression in the bronchial submucosa. However, there was a good correlation between the number of IL-17F+ cells and eosinophils in the bronchial submucosa. In COPD there was a highly significant, albeit small, increase in sputum IL-17 compared with asthma, and this was correlated with airflow obstruction and lung function impairment, but not airway inflammation. Our study therefore supports our hypothesis that IL-17 may play a role in asthma and COPD.
There is an increasing body of evidence supporting a role for Th17 cells in asthma.13,14
Animal models of asthma suggest that Th17 cytokines promote neutrophilic inflammation,15,16
which in concert with Th2 cells is important in the development of airway hyperresponsiveness.17
Previous reports have demonstrated increased IL-17A and IL-17F bronchial submucosa expression in moderate to severe asthma,17,18
which were attenuated by systemic corticosteroids.17
The data for sputum IL-17 is conflicting with one study demonstrating no differences in the sputum IL-17 concentration between asthma and healthy volunteers,29
whereas in another study IL-17 was increased in moderate to severe asthma.30
In the latter study sputum IL-17 was measured before and after inhaled corticosteroids. Importantly, consistent with our findings previous studies have found that the sputum IL-17 concentration in sputum is low and in some studies samples have needed to be concentrated to detect IL-17. We found that IL-17A and IL-F were increased in mild to moderate disease, whereas IL-17F but not IL-17A was increased in severe asthma. Our subjects with severe asthma were treated with high-dose inhaled and/or oral corticosteroids, and therefore this may have attenuated the IL-17A expression in this group. IL-17A and IL-17F expression was not associated with smoking status but importantly this study was not powered to fully explore the effects of smoking in asthma. We report for the first time, to our knowledge, the relationship between IL-17A and F expression and neutrophilic inflammation in tissue and sputum. We found that there was no association between IL-17A and IL-17F expression neutrophilic inflammation in tissue. Intriguingly, there was a weak inverse correlation between submucosal IL-17A expression and the sputum neutrophil count. Corticosteroids attenuate IL-17 expression in tissue17
but do not modulate sputum neutrophil counts.31,32
Sputum IL-17 was also markedly attenuated by inhaled corticosteroids, but the sputum neutrophil count was unchanged, questioning the role of IL-17 in persistent neutrophilic airway inflammation. However, others have demonstrated an association between IL-17 and IL-8 mRNA in sputum cells and the number of sputum neutrophils,33
suggesting that the site of IL-17 expression may be important. Interestingly, we also found a weak relationship between FEV1
% predicted and IL-17A expression. Consistent with our finding in a recent comparison between subjects with severe asthma with and without persistent airflow obstruction, IL-17 was among the dominant mediators associated with the absence of persistent airflow obstruction.34
Together these findings do not support a role for IL-17 in the development of fixed airflow obstruction in asthma.
To date there are few data examining the expression of IL-17A or IL-17F in COPD. Neutrophilic inflammation is a common feature of COPD.7,11
Hence, it is predictable that IL-17 may play an important role in this disease. There is a single report of IL-17A and F expression in bronchoscopic biopsies obtained across the spectrum of COPD disease severity compared with smoking and nonsmoking control subjects.15
IL-17A was increased equally across the severity of COPD compared with the nonsmoking control subjects, but was not different from the smoking control subjects. IL-17F was not significantly different across all the disease and control groups. In this study the number of neutrophils in tissue was increased in the COPD groups with a marked increase in severe COPD. There was no relationship between IL-17A and IL-F expression and neutrophilic inflammation. Our findings of increased IL-17A expression in mild to moderate COPD compared with nonsmoking control subjects, but not smoking control subjects, is therefore entirely consistent with these earlier reports. We found that the sputum IL-17 concentration was increased in COPD compared with asthma. However, we were unable to detect IL-17 by ELISA, and with a more sensitive mesoscale device platform a substantial proportion still remained below the limit of detection. The difference between COPD and asthma was highly significant, but small, questioning whether this difference is biologically important. Interestingly, the sputum IL-17 concentration in COPD was related to the degree of airflow limitation and obstruction. Therefore, the potential role of IL-17, particularly in severe disease, and its synergy with the development of airway inflammation in response to cigarette smoke exposure or alternatively through interactions with other cytokines, such as tumor necrosis factor35
needs to be further explored.
Neutrophils were the predominant source of IL-17 as evidenced by colocalization using immunohistochemistry. However, there was no correlation between the number of IL-17+
cells and neutrophils in the bronchial submucosa. In contrast to the established role of IL-17A and IL-17F in the initiation of neutrophilic inflammation, neither has been implicated in the development of eosinophilic inflammation. Indeed, IL-17F transgenic mice develop an airway neutrophilia after allergen challenge, whereas knockout mice have an impaired neutrophilic response and enhanced Th2 cytokine production.16
Therefore, we were surprised by the relationship between the number of IL-17F+
cells and eosinophils in the bronchial submucosal. However, a similar relationship has been described between sputum IL-17 mRNA and IL-5 mRNA,33
possibly suggesting a hitherto undescribed association between eosinophilic airway inflammation and the Th17 axis. Eosinophils express the IL-17 receptors and have enhanced cytokine release after IL-17F activation.37
Thus, the potential interactions between eosinophilic inflammation and up-regulation of the Th17 axis require further study.
Our study has a number of possible criticisms. This is a cross-sectional observational study. Whether IL-17A and IL-17F expression is related to longitudinal clinical outcomes, such as disease progression, lung function decline, and exacerbations, requires further examination. Similarly, we are unable to determine whether differences observed between mild and severe asthma reflect disease severity or are a consequence of differences in treatment. Current literature suggests that IL-17A is sensitive to corticosteroid therapy,17,30
in contrast to neutrophilic inflammation, which is corticosteroid resistant.31,32
Importantly, the number of IL-17A or IL-17F cells was expressed as cells/mm2
of submucosa rather than as a proportion of the total number of cells in the submucosa. Therefore, whether the changes in cell number between groups is a reflection of increased expression or an increase in the total number of cells needs to be examined in future studies. We are confident that our assays to assess IL-17 were robust as recovery was unaffected by the mucolytic DTT and recovery of exogenous spikes of IL-17 to sputum samples was good. However, a number of samples were below the limit of detection; therefore, the sputum data do need to be interpreted cautiously. Furthermore, no corrections for multiple comparisons were made. Therefore, findings with a marginal level of significance also need to be interpreted with caution.
In conclusion, we found that IL-17A expression in bronchial submucosa was increased in mild to moderate asthma and in COPD, although this was not independent of smoking. IL-17F expression was increased in mild to moderate and severe asthma, but not in COPD. The increased IL-17A and IL-17F expression was not associated with increased neutrophilic inflammation. Sputum IL-17 was increased in COPD and was related to airflow obstruction. Our findings therefore do support a potential role for IL-17A and IL-17F in asthma and possibly COPD. Efficacy studies of therapeutic strategies targeted at the IL-17 axis are eagerly awaited that will further define the functional importance of IL-17A and IL-17F in airways disease.