There are several potential reasons explaining why inhaled HMW-HA does not lead to increased inflammation in this population. First, even though there was apparent degradation of inhaled HMW-HA, there were still appreciable amounts of HMW-HA in induced sputum, suggesting that the degradation does not affect the entire inhaled dose. Thus, there may still be HMW-HA available to antagonize sHA. Second, sHA generated from the degradation of HMW-HA in the airway compartment may not reach interstitial immune cells in high enough concentrations to cause their activation. Lastly, sHA may not contribute significantly to inflammation in the context of pre-existing severe airway inflammation, although it is a proinflammatory agonist in the naïve lung. Interestingly, 100–150 kDa HA fragments (i.e. sHA) appear to be beneficial in the treatment of tobacco-induced airway disease, [
6,
7] whereas they induce airway inflammation when given to naïve mice [
8]. Thus, the theoretical concern about sHA aggravating airway disease may not be relevant in the real-world setting of pre-existing airway inflammation such as in cystic fibrosis or other chronic inflammatory diseases.
Although there was no apparent harm from inhaled HMW-HA in this population, there was also no apparent benefit in lung function or inflammatory cytokine expression. This particular formulation of inhaled HMW-HA is used to improve tolerability of HTS in CF [
4,
9], not as an anti-inflammatory agent; nevertheless, HMW-HA has been previously shown to improve lung function in asthma and COPD [
10‐
12] and inflammation in an animal model of CF [
3]. Inhaled glycosaminoglycan formulations may have a role in airway disease. Recently, an exciting study reported significant improvements from inhaled heparin in patients with severe COPD. Inhaled HA has also been used successfully in COPD [
6,
7,
13‐
15], and two clinical trials (NCT00993707 and NCT02674880) are currently underway evaluating efficacy of inhaled HA in COPD. However, HA effects in asthma and COPD may not parallel CF, where inflammation is more pronounced, and may overwhelm HMW-HA effects; furthermore, there are higher concentrations of sHA in CF [
16] compared to asthma [
17] which may antagonize HMW-HA; the thickness and tenacity of CF mucus may prevent HA from penetrating sufficiently into the airway cells layer; finally, lung function changes and inflammatory changes in CF have different kinetics than the dynamic airway obstruction in asthma, and thus the observation period in this study may not have been sufficiently long to detect changes. It is worth noting, that heparin demonstrated in vitro mucolytic effects in CF sputum [
18], but inhaled heparin did not show significant improvement in lung function in a pilot study of CF patients [
19]. Furthermore, some mechanisms of action of HA, such as inhibition of elastases and matrix breakdown [
13,
14,
20] are relevant in COPD but not in CF. In aggregate, we believe that glycosaminoglycans like heparin and HA are likely to be effective in COPD and asthma, but that prolonged dosing, or different formulations may be needed for CF patients.
An interesting finding of our study involves the dichotomous effect of inhaled HMW-HA on HA induced sputum levels depending on initial concentration. In patients with low pre-treatment HA concentrations (defined as concentration < 100 ng/ml), initiation of treatment with HMW-HA led to an increase in measured levels. This may be reflecting the addition of inhaled HMW-HA to the airway fluid, and would suggest that inhaled HMW-HA does contribute to the HA pool in the airways. Given that HA is extremely hydrophilic, such increase may help hydrate the airways and contribute to bacterial clearance and improved ciliary function. More intriguing is the effect of inhaled HMW-HA when patients had initially high endogenous HA levels (>100 ng/ml): in this case there is an apparent decrease in sputum HA, which is evident both via ELISA and gel electrophoresis assays. It is possible that exogenous HMW-HA exerts a negative feedback effect in sHA generation, thus indirectly improving HA homeostasis towards less sHA and more HMW-HA.