Exhaled biomarkers in adults with non-productive cough

In this first explorative study on exhaled biomarkers from peripheral airways of individuals with a non-productive cough, we found through proteomics analysis that 75 proteins were significantly altered, compared to individuals without cough. The magnitude of associations, and the common pathways involved for the identified proteins, suggest that these findings represent true associations between altered biochemical processes in the distal airways and non-productive cough.

Various immune system pathways have been suggested to be involved in the pathogenesis of cough, both eosinophilic (Th2 inflammation), neutrophilic, and lymphocytic [12], suggesting that there are different pathological mechanisms involved and that chronic cough is a heterogenous disease. Currently, it is unclear which of these pathways are most important, or if they mostly associate with certain subgroups of patients with chronic cough. Certain biomarkers may be related to the phenotype of cough, when identifiable. Indeed, a previous study found substance P to be specifically elevated in exhaled breath condensate of patients with gastroesophageal reflux and cough [11], and patients with chronic cough and elevated fractional exhaled nitric oxide (FeNO, reflecting eosinophilic inflammation) respond better to inhaled corticosteroids than those with low FeNO [15].

We did not identify any study on airway proteomics in chronic cough. In our data, we found Tenascin-C, a biomarker associated with chronic inflammation [26], to be more commonly identified in PEx among participants with non-productive cough, and Contactin-1 to be elevated in PEx. Contactin-1 is implicated in asthma pathogenesis, as a signal molecule in airway epithelium-derived exosomes, inducing a Th2 inflammation [27]. Thus, we found some support for a role of Th2 inflammation in chronic cough.

We also found NOTCH3 to be more often identified above the limit of detection in PEx samples among participants with cough compared to those without (50% vs 22%, respectively). NOTCH3 is centrally involved in epithelial homeostasis and regeneration [28], but its role in chronic cough has not been studied.

The complement system, a part of the innate immune system, and the coagulation system are known to crosstalk [29]. Both the coagulation- and complement systems are also known to be affected in some airway diseases, such as asthma [30]. In our study, we found numerous proteins involved in both the complement and coagulation systems to be affected, suggesting a role for the innate immune system in non-productive cough. Indeed, as cough is essentially a first-line mechanism to clear debris from the airways, it is perhaps not surprising that the innate immune system—a first-line immune response system—may affect the cough reflex.

The complement system has not been extensively studied in chronic cough. In one study on COPD patients, the complement factors C3 and C4 were found to be lower in serum among those with cough and expectoration [31]. We did not identify other studies on chronic cough and complement activation. Further studies are needed to further explore the role of the complement and coagulation systems in cough.

Fibrinogen was found to be increased, a protein which is both implicated in formation of extracellular matrix, and involved in inflammation and coagulation in the blood [32, 33]. Fibrinogen has previously been described to be increased in blood samples among patients with asthma and cough, compared to patients with asthma but without cough [32]. Also, the extracellular matrix protein fibronectin was increased among those with non-productive cough, a protein that also has been implicated in the pathogenesis of lung fibrosis [34, 35].

Some proteins identified deserve specific attention with regard to the cough hypersensitivity theory. For example, plasma kallikrein, which we found to be elevated, is a protein that cleaves kininogen to form bradykinin. Bradykinin is well known to be implicated in chronic cough [36]. Other studies have also indicated this possible association, where plasma kallikrein may be implicated in inducing cough through activating bradykinins [37, 38]. Furthermore, the kallikrein-kinin system (including bradykinin) has been implicated to play a role in neuroinflammation, both centrally as in Alzheimer's disease, as well as affect the peripheral nervous system. Our data therefore support the cough hypersensitivity theory as a plausible mechanism in chronic cough.

Another interesting aspect is the potential impairment of tight junctions, which may lead to increased permeability and susceptibility of airway nerve receptors to external stimuli [8, 9]. In our data, the protein ESAM, which contributes to the integrity of tight junctions [39], was significantly lowered in PEx samples of participants with non-productive cough. Unfortunately, many of the tight junction proteins analysed in the SOMAscan platform were below the detection limit in the present study. We did not identify other studies directly addressing epithelial permeability in chronic cough, but the issue has been discussed in asthma, where increased epithelial permeability is suggested to be a part of the pathogenesis [8].

Strengths and weaknesses

Some of the novelties and main strengths of this study are the combination of a non-invasive collection of non-diluted biosamples from the small airways, and the unbiased proteomic approach. However, some methodological issues need to be discussed. First of all, the sample size is small and an independent validation cohort is needed to confirm our findings. We also observed variability in the protein-profile depending on the investigator performing the PEx sampling, possibly explained by differences in how the breathing maneuver was performed, and as a result the statistical analysis had to be adjusted accordingly. Choosing to include proteins in the analysis with RFU values > LOD in more than 50% of samples could be considered another possible limitation. However, due to the exploratory nature of the study, we chose to select 50% as the limit instead of other commonly used percentages such as 70–80%, to open the analytical window and explore proteins that could potentially be missed otherwise. For that same reason and also due to the small sample size, a more inclusive approach was chosen and differences in protein abundance were considered significant at p-value < 0.05.

Some potentially interesting biomarkers, such as the neuroinflammatory marker Substance P which is implicated in cough [12], were not among the proteins analysed by the SOMAscan platform. Also, in part because of the relatively low sample volume, some proteins were not easily identified, such as Cadherin-1 which was only detected above LOD in 6% of all samples.

Because of the exploratory nature of this study, strong conclusions cannot be made, even though the results show interesting findings. The clear differences in abundance of many proteins between participants with non-productive cough and those without cough, merit larger studies to validate these findings, preferably including clinical patients with chronic cough.

留言 (0)

沒有登入
gif