Chronic obstructive pulmonary disease (COPD) is an inflammatory airway disease characterized by non-reversible airflow limitation[¹]. Airflow limitation is usually progressive and associated with an abnormal inflammatory response of the lungs to noxious particles or gasses. The treatment options in COPD are still limited and current efforts focus on therapy targeted to particular phenotypes of the disease[¹]. A non-invasive, standardised way to measure and monitor airway inflammation in COPD is hypertonic saline-induced sputum[²]. Analysis of induced sputum provides information about cell counts (eosinophils, neutrophils, lymphocytes, macrophages) and cell activity by mediator concentrations (e.g. ECP, MPO and IL-8).

In COPD patients the identification of sputum eosinophilia has shown to be of clinical value as it predicts a response to corticosteroids[^3-5]. Furthermore, guiding inhaled steroid therapy by sputum eosinophil counts leads to a reduction in exacerbations in COPD, without an increase in steroid dose[⁶]. These observations demonstrate the value of identifying inflammatory subphenotypes in the treatment of COPD. However, the application of sputum analysis is somewhat limited by the requirement of lab facilities and the not-directly available results. Therefore, there is a need for adequate surrogate markers of airway inflammation in COPD.

Airway hyperresponsiveness (AHR) may serve as a surrogate measure of airway inflammation, since it is associated with the presence of inflammatory cells and release of mediators in the airways[⁷]. In particular, this holds for indirect challenges, amongst which dry powder mannitol challenge is relatively easy to apply[⁸,⁹]. Local mannitol deposition results in an osmotic change, likely to induce the release of mediators from inflammatory cells in the airways[¹⁰]. Studies in asthma showed that AHR to mannitol is indeed related to the degree of eosinophilic airway inflammation and is sensitive to treatment with inhaled corticosteroids[^11-13]. Interestingly, a proof of concept study demonstrated that mannitol challenge might also be useful in identifying COPD patients who will most likely benefit from inhaled corticosteroids[¹⁴]. This may suggest that AHR to mannitol identifies the degree of eosinophilic inflammation in COPD.

We postulated that AHR to mannitol captures eosinophilic airway inflammation in adults with mild to moderate COPD. Our aim was to test this hypothesis by examining the relationship between AHR to mannitol and markers of inflammation in hypertonic saline-induced sputum, blood and exhaled air. As secondary aim, we investigated whether similar relations can be observed when using spontaneously produced sputum during or directly after the mannitol challenge itself. Finally, we constructed receiver operating characteristic (ROC) curves using AHR against sputum eosinophilia in COPD.

Twenty-eight of the 32 patients completed the study (table 1). Four patients dropped out for reasons of: non-compliance with medication restrictions (n = 1), lost to follow up (n = 1), FEV₁ <1.2 litre prior to challenge (n = 1) and inability to perform all techniques necessary to measure lung function (n = 1). Two out of 28 mannitol challenges were not completed for reasons of coughing (n = 1) and tiredness (n = 1), but these patients were included since this was not an exclusion criterion. Hypertonic saline-induced sputum was collected in 28 patients and mannitol-induced sputum in 21 patients.

In this group of mild to moderate COPD patients, AHR to inhaled mannitol was consistently associated with eosinophil counts in hypertonic saline- as well as mannitol-induced sputum. In addition, we observed associations between AHR to mannitol and soluble markers of inflammation in sputum. Our results suggest that mannitol challenge identifies inflammatory subphenotypes in COPD, in particular those patients without eosinophilic inflammation due to the high specificity of the test. This may facilitate individualized treatment in COPD.

To our knowledge, this is the first study assessing the relationship between airway hyperresponsiveness to inhaled mannitol and markers of airway inflammation in sputum and exhaled air in patients with COPD. These observations extend previous findings in COPD using adenosine 5'-monophosphate (AMP), in which a correlation between AHR to AMP and eosinophils in sputum was observed[²⁴]. Interestingly, a similar correlation between RDR to mannitol and sputum eosinophils was recently reported in patients with asthma, also showing absence of eosinophilia in patients without mannitol hyperresponsiveness[¹³]. Hence, mannitol challenge appears to provide valuable information on the inflammatory profile in both patients with COPD and asthma.

In our study, particular attention was paid to methodological aspects such as selection of COPD patients, design and methods. The patients were derived from a clinical population rather than an epidemiological one, in order to strengthen the applicability of our findings. All patients were well characterised by using subjective and objective criteria. This included the presence of symptoms, fixed airway obstruction and smoking history. The full range in sputum eosinophils counts was 0.1 to 7.4%, which is similar to previous studies in COPD[^3-5]. To exclude any confounding effects of inhaled corticosteroids on mannitol challenge, the patients who used inhaled corticosteroids stopped this medication for 4 weeks[⁹,¹¹]. In order to answer the research question accurately, we performed mannitol challenge and sputum induction on separate days. In addition, we examined sputum expectorated after the mannitol itself, which confirmed our results. Furthermore, as inflammatory markers we used both, the presence of inflammatory cells and markers of cell activation. This provided consistent associations.

Nevertheless, our study has limitations. First, we could not obtain adequate sputum samples in all patients at all time points. Even though the power of the study was adequate to address the primary objectives, it may not have been adequate to examine our secondary objective. Second, we can not exclude that our COPD group included patients who also had asthma. We excluded those with a previous history of asthma, but this may not have sufficed. However, all patients had a smoking history, fixed airflow limitation, met the COPD GOLD criteria, and were diagnosed and treated as COPD patients. Third, the patients needed to stop the inhaled corticosteroids in order to examine unbiased disease markers. Therefore, the test performance cannot be generalized to COPD patients on inhaled steroids. This will require a separate study. Finally, we did not include a second mannitol challenge for examining reproducibility of our results, which is a limitation of our design.

How can we interpret these results? Mannitol is an osmotic stimulus that causes airway narrowing by release of bronchoconstrictor mediators such as leukotrienes, prostaglandins and histamine[²⁵,²⁶]. The source of these mediators is likely to be mast cells and eosinophils in the airways as both these cell types release mediators in vitro in response to mannitol[¹⁰,¹¹,²⁷]. Mast cells and eosinophils are not unimportant in COPD and may contribute to the fluctuations of airways obstruction as observed e.g. during exacerbations[^28-31]. We did not observe associations of mannitol responsiveness with neutrophil counts in sputum or blood, but did found significant correlations with sputum IL-8 and MPO. This may suggest that epithelial cell and neutrophil activity are also involved in determining the airway narrowing to inhaled mannitol in COPD. Interestingly, mannitol responsiveness was more strongly associated with FeNO than with sputum eosinophils. However, we did not find a significant association between the latter two parameters. This is in keeping with the data by Siva et al. [⁶]. Our results suggest that mannitol responsiveness is a better marker of eosinophilic inflammation than FeNO in COPD.

Notably, we observed that most COPD patients produced adequate sputum samples during the mannitol challenge. This occurred even in absence of encouraging the patients to expectorate. Therefore, the success rate of obtaining mannitol-induced sputum may well be improved by adjusting the standard operating procedure of the test. Our findings extend a recent study in asthma, showing adequate sputum samples after mannitol challenge[³²]. Inhaled mannitol changes osmolarity and reduces viscoelasticity, surface tension, contact angle and the solids content of sputum[³³]. This may explain why 75% of the patients gave up sputum during mannitol challenge. Our results suggest that mannitol activated eosinophils, neutrophils and epithelial cells. Hence, even though AHR to mannitol was associated with eosinophilic airway inflammation, it is likely to be a more pleiotropic stimulus within the airways.

What are the clinical implications of our study? Eosinophilic airway inflammation predicts the response of COPD patients to systemic and inhaled corticosteroids[⁴,⁵]. In addition, inhaled steroid therapy guided by sputum eosinophils reduces exacerbation rate in patients with COPD[⁶]. Our results suggest that mannitol challenge can identify COPD patients without eosinophilic airway inflammation, who not likely to benefit from inhaled steroid therapy[⁶]. This subphenotype of patients cannot be distinguished from other patients with COPD on clinical grounds or lung function criteria. Therefore, mannitol challenge may qualify as a feasible alternative in the monitoring of anti-inflammatory therapy in COPD. The high specificity (100%) in combination with limited sensitivity indicates that mannitol responsiveness is particularly suitable to exclude sputum eosinophilia in COPD. Indeed, inhaled steroids appear to be ineffective in COPD patients with the lowest responsiveness to mannitol[¹⁴]. Therefore, mannitol responsiveness may support decisions to refrain from inhaled steroid treatment, thereby potentially preventing overtreatment of COPD. This requires a randomized controlled study in COPD comparing a treatment strategy based on AHR to mannitol with the currently recommended treatment strategy based on clinical markers only. It remains to be established whether mannitol challenge can also be an outcome measure of the efficacy of steroids in COPD, as has been shown in asthma[¹²]. Finally, our data suggest that the assessment of AHR and airway inflammation in COPD can be combined in a single test. This would have large practical advantages, not only in clinical research, but also regarding the guidance and monitoring of anti-inflammatory therapy in clinical practice.

We conclude that airway responsiveness to mannitol can be used to rule out eosinophilic airway inflammation in patients with mild to moderate COPD who are not treated with inhaled corticosteroids. These finding suggests that mannitol challenge is a candidate for the guidance and monitoring of individualized, anti-inflammatory therapy in COPD, as an alternative to sputum eosinophils.

AHR: airway hyperresponsiveness; ECP: eosinophil cationic protein; FeNO: fraction exhaled nitric oxide; FEV₁: forced expiratory volume in one second; FEV₁/FVC: forced vital capacity divided by the forced expiratory volume in one second; FVC: forced vital capacity; ICS: inhaled corticosteroids; IL-8: interleukin-8; MPO: myeloperoxidase; PD₁₅: provocation dose to cause a fall in FEV₁ >15%; RDR: response dose ratio; ROC: receiver operating characteristic.

The authors declare that they have no competing interests.

SdN was the main author of the paper and developed the study design and subject recruitment, collected study data and performed statistical analysis. All other authors contributed significantly to the design of the study, the collection and assessment of clinical data and development of this paper. All authors contributed significantly to the development of the manuscript and all have seen and approved the final version and take responsibility for the content.

We would like to thank the participants in the study and Tamara Dekker for determining the inflammatory parameters. This study was financially supported by the Academic Medical Centre, Amsterdam, without external sponsor.