An investigation into the mechanisms driving spontaneous cough in a preclinical model of idiopathic pulmonary fibrosis

Abstract

Idiopathic Pulmonary Fibrosis (IPF) is a terminal lung disease characterised by a progressive deposition of scar tissue within the lung interstitium. The disease is made more troubling for patients by being frequently accompanied by a chronic, sputumless cough, which has a severe impact on their quality of life. The pathophysiology behind cough in IPF remains poorly understood, and as such, it has proven refractory to treatment; the aim of this thesis was to elucidate the mechanisms behind this distressing symptom. We sought to model IPF-associated chronic cough using a bleomycin-driven model of fibrosis in guinea pigs. After having characterised the inflammatory and fibrotic features of this model, we discovered, in a preclinical first, that bleomycin-treated guinea pigs cough spontaneously, much like IPF patients. We believe that this model more closely recapitulates the cough seen in patients than other provoked cough challenge models. In search for the mediators that were driving cough within this preclinical model, we quantified the levels of different biomarkers in bronchoalveolar lavage fluid (BALF) that are reportedly elevated in the IPF lung: extracellular ATP, mast cell tryptase and 8-isoprostane (a biomarker for oxidative stress). We found evidence that all three of these were more highly present in the lungs of bleomycin-treated animals than the vehicle control group. Further to this, we measured these biomarkers in BALF donated by IPF patients at Royal Brompton Hospital and found that 8- isoprostane was significantly more concentrated in IPF BALF than in healthy volunteer BALF. We then utilised a variety of in vitro and in vivo techniques to further investigate how these lung milieus might elicit cough. After showing that H2O2, a reactive oxygen species, was capable of generating oxidative stress in guinea pig vagal ganglia, we further determined that it caused vagus nerve depolarisation through the ion channel TRPA1. We also discovered that activating the PAR2 receptor, a target of mast cell tryptase, also caused airway sensory nerve activation through the ion channel TRPV4; TRPV4 activation has previously been shown to cause cough via the extracellular release of ATP. Finally, we report on the efficacy of GSK2798745, a selective TRPV4 channel antagonist, that is capable of abrogating TRPV4-induced cough. In the near future, we intend to use a variety of pharmacological tools, including this one, in our novel in vivo model of fibrosis-induced spontaneous cough, with the end goal of finding an effective antitussive therapy for IPF patients.Open Acces