Date of Award
Doctor of Philosophy (PhD)
Dr. Jack Gauldie
Idiopathic pulmonary fibrosis is a chronic inflammatory disease of the lung characterized by pathologic alterations with fibroblast proliferation and disordered deposition of extracellular matrix resulting in impairment of gas exchange often leading to respiratory failure. In the pulmonary interstitium, the fibroblast and the extracellular matrix products they produce, play a pivotal role in maintaining the structural and functional integrity of the lung.
In the study of lung fibrosis and other fibrosing diseases much attention has been directed at events which alter fibroblast activities. In this construct however, the lung fibroblast is viewed merely as a homogeneous target cell altering its behaviour as a result of immune and inflammatory events. Importantly, it has become apparent over the years that (1) fibroblasts are themselves effector cells capable of releasing a variety of growth factors and mediators and (2) that fibroblasts comprise a heterogeneous population, both within and between tissues. Therefore, an alternative, but not a mutually exclusive hypothesis considers the heterogeneous nature of the fibroblast population and the potential contribution of various subpopulations to disease expression.
Our first report that fibroblasts derived from chronically activated and inflamed human lung tissue behaved differently than normal fibroblasts suggested that fibroblast populations present in fibrotic lung tissue exhibited accelerated growth rates. In order to further examine the growth characteristics of fibrotic lung fibroblasts, a soft agarose culture system was established in which we examined the ability of lung fibroblast primary lines to form colonies under anchorage-independent growth conditions, possibly equating to aggressive or "transformed" behaviour. Fibroblasts derived from the Iungs of patients with idiopathic pulmonary fibrosis exhibited anchorage-independent growth in soft agarose culture whereas fibroblast lines derived from normal adult tissue did not. These colonies were fibroblast-like according to morphology and immunohistochemical stain characteristics, and the ability to grow under semi-solid growth conditions was maintained by the fibrotic derived cells even after selection and expansion of single colonies. Interestingly, fibroblast cell lines derived from neonatal lung tissue also exhibited the ability to grow as colonies under soft agarose growth conditions suggesting newly differentiated fibroblast populations may be prevalent in fibrotic lung tissue.
The effect of various growth and differentiating factors on the modulation of the anchorage-independent colony formation phenotype was examined in vitro. Treatment with various growth factors, including PDGF, FGF, EGF, TGFβ and corticosteroid were able to modify the colony forming abilities of fibrotic and neonatal fibroblast lines. Importantly, none of the above treatments was able to induce fibroblasts derived from normal adult lung tissue to form colonies. The ability of IPF fibroblast lines and neonatal lung fibroblast lines to form colonies under soft agarose growth conditions was inhibited by treatment with retinoids, known differentiating agents, implying the modulation/differentiation of a particular fibroblast phenotype toward a more mature phenotype; one incapable of anchorage-independent growth.
Since the ability to form colonies under soft agarose growth conditions by fibrotic fibroblasts appeared to be a stable and disease specific phenotype and since in vitro we were able to successfully modulate this behaviour with retinoids, we examined the effect of orally administered retinoic acid on the modulation of the fibrotic lung response in a rat model of bleomycin induced pulmonary fibrosis. In this study, we report that the in vivo administration of all-trans retinoic acid significantly decreased the number of fibrotic lesions and dramatically altered the pattern of collagen deposition in the bleomycin treated lung.
These studies have confirmed and extended our earlier hypothesis concerning the contribution of fibroblast heterogeneity to the pathogenesis of pulmonary fibrosis and suggests avenues of intervention that may lead to alteration of the fibrotic tissue and return to normal lung function.
Torry, Diane J., "Fibroblast Heterogeneity and Pulmonary Fibrosis" (1995). Open Access Dissertations and Theses. Paper 2382.