Another good year for biomarkers

Identification of a systemic molecular signature facilitating early diagnosis and enabling targeted therapy is the Holy Grail for the investigators of many pulmonary diseases, particularly those with heterogeneous presentation such as chronic obstruction pulmonary disease (COPD) or idiopathic pulmonary fibrosis (IPF). Genome-wide gene expression profiling of large groups of patients has revealed a clear gene signature for elastogenesis during COPD and identified fibulin-5 as a potential molecular target.1 Genetic regulation of gene expression patterns was investigated by integrative genomics which identified cystatin C (CST3) and CD22 as causal genes for airflow obstruction.2 The fact that these gene expression patterns in COPD are dynamic during treatment, reflecting disease severity, highlights the potential of this approach for the development of molecular phenotype-driven therapy for COPD.3 Similarly, a coordinated approach was used to assess the relationship between gene expression patterns, pathological features and systemic biomarkers to identify indicators of disease severity in patients with IPF. MMP3 and CXCL13 emerged as systemic biomarkers that reflect brochiolisation and lymphoid aggregates and are predicted to be of clinical value as prognostic or surrogate markers of disease severity in these patients.4 An alternative method of predicting cancer risk in patients presenting with endobronchial squamous metaplasia is the identification of DNA copy number aberration. This technique has proved a highly accurate biomarker for assessment of disease progression and could assist in deciding which patients with endobronchial lesions may benefit from more aggressive treatment or closer surveillance.5

Smoking and steroids

Further evidence that smoking is bad for you—and your immune system—stems from a series of papers examining the effect of cigarette smoke on innate immune function. Tobacco smoke blunted secretion of key innate cytokines such as interferon γ and tumour necrosis factor α and impaired the ability of macrophages derived from peripheral blood or the airways to contain mycobacteria within their cell membranes.6 This subversion of key innate immune pathways has the potential to increase risk for tuberculosis (TB), particularly in those with underlying pulmonary conditions. In fact, innate immune pathways have been shown to correlate with the severity of stable COPD.7 Exposure to cigarette smoke also reduces the effectiveness of steroids, commonly prescribed medications in COPD and asthma.8 Although budesonide protects against oxidative stress-induced epithelial dysfunction, cigarette smoke reduces the responsiveness of pulmonary epithelial cells to budesonide, perhaps explaining why steroids do not work in many patients. An elegant series of experiments involving human cells and mice exposed to cigarette smoke revealed a novel mechanism whereby transient receptor potential (TRP) and pannexin channels contribute to smoke-triggered ATP release.9 These TRP channels have also been shown to be important in mediating virus-induced cough, thus increasing their attractiveness as potential novel therapeutic targets in COPD and other respiratory diseases.10

Innate immune mechanisms

Innate immunity is vital for host defence, and dysfunction in components of this immune network can lead to enhanced pathogen growth and increased inflammation. IL-27 is critical for the control of sepsis-induced impairment of lung antibacterial host defence. Mice deficient in IL-27 were more resistant to secondary bacterial pneumonia and showed significantly lower bacterial burdens and improved survival, leading to the suggestion that manipulation of the IL-27 pathway has therapeutic potential.11 Neutrophils provide host immunity to Streptococcus pneumonia via a number of pathways. However, a series of studies in patients with community-acquired pneumonia and mechanistic studies in a mouse model have revealed that one such pathway has been subverted by the bacteria to facilitate bacterial growth rather than clearance.12 In contrast to previous reports which outline a host protective role for the neutrophil-derived myeloid related protein (MRP) 8/14, Achouiti and colleagues determined that patients with community-acquired pneumonia (CAP) showed high levels of MRP 8/14 and that mice deficient in MRP14 exhibited reduced bacterial growth and improved survival—a prime example of how a bacterium can manipulate the immune system for its own benefit.

Resident lung cells such as epithelial cells and alveolar macrophages provide vital defence against pathogens in the airways and the consequence of impaired function was demonstrated in exacerbation-prone COPD patients. Macrophage responses from these patients were suboptimal in terms of cytokine production and TLR signalling on exposure to respiratory pathogens when compared to macrophages donated from non-exacerbation-prone COPD patients.13 Although previous studies have implied that rhinovirus-induced interferon production from epithelial cells is suboptimal in patients with asthma, a new study has shown that defective production of type I and III interferons during rhinovirus infection is not universal but rather may be a facet of asthma control.14

Manipulation of the immune system

A sizeable proportion of the space in the Basic Science section of this journal is taken up by studies which use in vivo and in vitro systems to assess the efficacy of potential therapies. Manipulation of immune pathways to improve diseases is the goal for many researchers, particularly those who seek to promote tolerance to common aeroallergens such as pollen. Investigation into how the immune system reacts to Timothy grass identified an immunodominant epitope that promoted reduced T cell responses when delivered as a peptide in vivo in a mouse model.15 Importantly, the epitope was conserved across a range of grasses. Sustained suppression of ragweed-induced allergic inflammation was also achieved using a limited regimen of CpG-containing oligodeoxynucleotide—known to be potent inhibitors of Th2-mediated allergic responses.16 However, controlling the immune system may not be as simple as shifting from a Th2 response. Exposure to farm environments has been shown to protect against a Th2 response and the development of allergy, but Robbe and colleagues determined in a mouse model and in agricultural workers that chronic farm exposure shifted T cell response away from a Th2 towards a Th17 phenotype, perhaps increasing the risk for other chronic respiratory conditions, which wins the Thorax Bronze Medal.17

There is increasing recognition of the role that proteases play in regulating immune responses. Imbalance in protease networks has been associated with a range of different pulmonary conditions, particularly those where tissue remodelling is a component of the pathology. Increased alveolar apoptosis has been linked to proteolysis via protease-mediated ectodomain shedding of lung epithelial cell adhesion molecule 1 (CADM1).18 Targeting particular protease pathways is a potential avenue for therapy. Protease activated receptor (PAR)-1 is a G-protein coupled receptor and interacts with several protease systems involved in tissue remodelling, such as thrombin and members of the matrix metalloproteinase family. A specific PAR-1 antagonist was found to ameliorate the features of bleomycin-induced fibrosis, even when delivered in a therapeutic regimen after fibrosis was established.19

Genes and stem cells

Innovative strategies and systems to manipulate disease progression are vital to determine the in vivo function of particular molecules and gauge the effect on physiological function and disease outcome. Gene manipulation in vivo can be achieved using delivery via viral vectors—an approach which has led to successful reduction of MCP-1 and structural remodelling in a chronic heart failure model.20 Similarly, human mesenchymal stem cells (MSCs) engineered to express tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) successfully incorporate into malignant pleural mesothelioma and induce cancer cell death.21 MSCs were tracked in vivo by bioluminescent and fluorescent imaging to delineate the location of the cells delivered to the lungs, highlighting the need for systemic rather than topical application of cells.

Mice, horses and men

Mouse models are arguably invaluable for the development and testing of hypotheses and potential therapeutic pathways. However, they incur the inevitable criticism of how they relate to human disease. Investigators have tried to mitigate these criticisms by developing models of disease in larger animals: in sheep, whose lungs are more similar to those of humans, and in horses which naturally develop a form of asthma.22 However, particular emphasis should be given to innovative ways of investigating disease mechanisms in humans. A study employed autofluorescent bronchoscopy to track the migration of cancerous cells and, together with molecular profiling of biopsies, monitored lesions longitudinally, documenting histological and molecular relationships and winning the Thorax Silver Medal.23 Using this combined approach, the authors determined that cancerous cells were able to migrate across histologically normal epithelium to establish new clonal lesions. Summers and colleagues have used gamma scintigraphy to track the migration of ¹¹¹indium-labelled neutrophils through the lungs of healthy volunteers and are our Thorax Gold Medal winners.24 They determined that the healthy human lung is able to retain primed neutrophils, and facilitate their depriming and release into the general circulation. This mechanism appeared to be defective in patients with acute respiratory distress syndrome which results in remote organs being exposed to primed neutrophils, contributing to organ damage.