Cystic fibrosis (CF) is the most common life-limiting hereditary condition of Caucasian populations and is characterised by chronic airways inflammation driving progressive structural lung damage. Despite tremendous advances in the treatment of CF and concomitant increased life expectancy for patients, chronic lung disease remains the main cause of morbidity and mortality among CF patients. While universal restoration of cystic fibrosis transmembrane conductance regulator activity remains a future hope, novel therapies aimed at reducing or preventing chronic airways inflammation and progressive structural lung damage are required. It is well-established that proteolytic enzymes are important in the CF lung beyond the basic turnover of proteins and intracellular degradation of pathogens. When secreted, these enzymes play key roles in extracellular substrate modification implicated in important biological processes such as matrix and airway remodelling, goblet cell metaplasia and mucus hypersecretion, immune cell recruitment and dysregulation of epithelial ion channels. Importantly, the burden of proteases in the CF lung is significantly elevated, overwhelming the endogenous antiprotease shield. Indeed, free protease activity has emerged as a major risk factor of the onset and progression of bronchiectasis and lung function decline in patients with CF. Recent research has highlighted the importance of new players such as cathepsin S and matrix metalloprotease-12, as well as the membrane-associated activity of key proteases such as neutrophil elastase on the surface of neutrophils. Here, we review the current knowledge and emerging concepts of the role of host proteases in the pathogenesis of CF lung disease and their potential as therapeutic targets.

Dysregulated expression and activity of cathepsin S (CTSS), a lysosomal protease and a member of the cysteine cathepsin protease family, is linked to the pathogenesis of multiple diseases, including a number of conditions affecting the lungs. Extracellular CTSS has potent elastase activity and by processing cytokines and host defense proteins, it also plays a role in the regulation of inflammation. CTSS has also been linked to G-coupled protein receptor activation and possesses an important intracellular role in major histocompatibility complex class II antigen presentation. Modulated CTSS activity is also associated with pulmonary disease comorbidities, such as cancer, cardiovascular disease, and diabetes. CTSS is expressed in a wide variety of immune cells and is biologically active at neutral pH. Herein, we review the significance of CTSS signaling in pulmonary diseases and associated comorbidities. We also discuss CTSS as a plausible therapeutic target and describe recent and current clinical trials examining CTSS inhibition as a means for treatment. Proteases in pulmonary diseases Research in the last 60 years has demonstrated that proteases are critical contributors to pulmonary disease pathophysiology. Initially known as protein-degrading enzymes with a restricted spectrum of substrates, recent studies have revealed that the diversity of protease substrates and biological effects triggered by their processing is vast [1, 2]. Proteases are primarily known for their matrix degradation capabilities, but also play significant roles in other biological mechanisms such as angiogenesis, growth factor bioavailability, cytokine processing, receptor shedding and activation, as well as cellular processes including migration, proliferation, invasion, and survival [3]. Importantly, protease activity requires tight regulation, and disruption of the close interplay between proteases, substrates and inhibitors may contribute to the pathogenesis and progression of a variety of pulmonary diseases, including muco-inflammatory diseases such as cystic fibrosis (CF) and chronic obstructive pulmonary disease (COPD), idiopathic pulmonary fibrosis (IPF), as well as infection [4]. In pulmonary diseases with a high neutrophil burden such as CF, a protease:antiprotease imbalance is frequently observed. The activity of proteases such as neutrophil elastase (NE) in the respiratory tract is regulated by antiproteases, such as α1-antitrypsin (A1AT) [5], secretory leukoprotease inhibitor (SLPI) [6] and elafin [7]. However, in diseases like CF, the antiproteases are overburdened by their cognate proteases and this imbalance can result in chronic airway inflammation, decreased mucociliary clearance, mucus obstruction, extracellular matrix (ECM) remodeling, increased susceptibility to infection and impaired immune responses [8].

Chronic obstructive pulmonary disease (COPD) is the integrated form of chronic obstructive bronchitis and pulmonary emphysema, characterized by persistent small airway inflammation and progressive irreversible airflow limitation. COPD is characterized by acute pulmonary exacerbations and associated accelerated lung function decline, hospitalization, readmission and an increased risk of mortality, leading to huge social-economic burdens. Recent evidence suggests ∼50% of COPD acute exacerbations are connected with a range of respiratory viral infections. Nevertheless, respiratory viral infections have been linked to the severity and frequency of exacerbations and virus-induced secondary bacterial infections often result in a synergistic decline of lung function and longer hospitalization. Here, we review current advances in understanding the cellular and molecular mechanisms underlying the pathogenesis of COPD and the increased susceptibility to virus-induced exacerbations and associated immune dysfunction in patients with COPD. The multiple immune regulators and inflammatory signaling pathways known to be involved in host-virus responses are discussed. As respiratory viruses primarily target airway epithelial cells, virus-induced inflammatory responses in airway epithelium are of particular focus. Targeting virus-induced inflammatory pathways in airway epithelial cells such as Toll like receptors (TLRs), interferons, inflammasomes, or direct blockade of virus entry and replication may represent attractive future therapeutic targets with improved efficacy. Elucidation of the cellular and molecular mechanisms of virus infections in COPD pathogenesis will undoubtedly facilitate the development of these potential novel therapies that may attenuate the relentless progression of this heterogeneous and complex disease and reduce morbidity and mortality.

The arrival of cystic fibrosis transmembrane conductance regulator (CFTR) modulators as a new class of treatment for cystic fibrosis (CF) in 2012 represented a pivotal advance in disease management, as these small molecules directly target the upstream underlying protein defect. Further advancements in the development and scope of these genotype-specific therapies have been transformative for an increasing number of people with CF (PWCF). Despite clear improvements in CFTR function and clinical endpoints such as lung function, body mass index (BMI), and frequency of pulmonary exacerbations, current evidence suggests that CFTR modulators do not prevent continued decline in lung function, halt disease progression, or ameliorate pathogenic organisms in those with established lung disease. Furthermore, it remains unknown whether their restorative effects extend to dysfunctional CFTR expressed in phagocytes and other immune cells, which could modulate airway inflammation. In this review, we explore the effects of CFTR modulators on airway inflammation, infection, and their influence on the impaired pulmonary host defences associated with CF lung disease. We also consider the role of inflammation-directed therapies in light of the widespread clinical use of CFTR modulators and identify key areas for future research.

Helminths (worms) are one of the most successful organisms in nature given their ability to infect millions of humans and animals worldwide. Their success can be attributed to their ability to modulate the host immune response for their own benefit by releasing excretory-secretory (ES) products. Accordingly, ES products have been lauded as a potential source of immunomodulators/biotherapeutics for an array of inflammatory diseases. However, there is a significant lack of knowledge regarding the specific interactions between these products and cells of the immune response. Many different compounds have been identified within the helminth “secretome,” including antioxidants, proteases, mucin-like peptides, as well as helminth defense molecules (HDMs), each with unique influences on the host inflammatory response. HDMs are a conserved group of proteins initially discovered in the secretome of the liver fluke, Fasciola hepatica . HDMs interact with cell membranes without cytotoxic effects and do not exert antimicrobial activity, suggesting that these peptides evolved specifically for immunomodulatory purposes. A peptide generated from the HDM sequence, termed FhHDM-1, has shown extensive anti-inflammatory abilities in clinically relevant models of diseases such as diabetes, multiple sclerosis, asthma, and acute lung injury, offering hope for the development of a new class of therapeutics. In this review, the current knowledge of host immunomodulation by a range of F. hepatica ES products, particularly FhHDM-1, will be discussed. Immune regulators, including HDMs, have been identified from other helminths and will also be outlined to broaden our understanding of the variety of effects these potent molecules exert on immune cells.