Rationale: The clinical management of idiopathic pulmonary fibrosis (IPF) remains a major challenge due to lack of effective drug therapy or accurate indicators for disease progression. Fibrocytes are circulating mesenchymal cell progenitors that are involved in tissue repair and fibrosis. Objectives: To test the hypothesis that assay of these cells may provide a biomarker for activity and progression of IPF. Methods: Fibrocytes were defined as cells positive for CD45 and collagen-1 by flow cytometry and quantified in patients with stable IPF and during acute exacerbation of the disease. We investigated the clinical and prognostic value of fibrocyte counts by comparison with standard clinical parameters and survival. We used healthy age-matched volunteers and patients with acute respiratory distress syndrome as control subjects. Measurements and Main Results: Fibrocytes were significantly elevated in patients with stable IPF (n = 51), with a further increase during acute disease exacerbation (n = 7; P < 0.001 vs. control subjects). Patients with acute respiratory distress syndrome (n = 10) were not different from healthy control subjects or stable patients with IPF. Fibrocyte numbers were not correlated with lung function or radiologic severity scores, but they were an independent predictor of early mortality. The mean survival of patients with fibrocytes higher than 5% of total blood leukocytes was 7.5 months compared with 27 months for patients with less than 5% (P < 0.0001). Conclusions: Fibrocytes are an indicator for disease activity of IPF and might be useful as a clinical marker for disease progression. This study suggests that quantification of circulating fibrocytes may allow prediction of early mortality in patients with IPF.

Idiopathic pulmonary fibrosis (IPF) can lead to the development of secondary pulmonary hypertension (PH) and ultimately death. Despite this known association, the precise mechanism of disease remains unknown. Using a rat model of IPF, we explored the role of the proangiogenic and antiapoptotic growth factor VEGF in the vascular remodeling that underlies PH. In this model, adenoviral delivery of active TGF-β1 induces pulmonary arterial remodeling, loss of the microvasculature in fibrotic areas, and increased pulmonary arterial pressure (PAP). Immunohistochemistry and mRNA analysis revealed decreased levels of VEGF and its receptor, which were inversely correlated with PAP and endothelial cell apoptosis in both the micro- and macrovasculature. Treatment of IPF rats with adenoviral delivery of VEGF resulted in reduced endothelial apoptosis, increased vascularization, and improved PAP due to reduced remodeling but worsened PF. These data show that experimental pulmonary fibrosis (PF) leads to loss of the microvasculature through increased apoptosis and to remodeling of the pulmonary arteries, with both processes resulting in PH. As administration of VEGF ameliorated the PH in this model but concomitantly aggravated the fibrogenic process, VEGF-based therapies should be used with caution.

Rationale: Recent findings suggesting transforming growth factor (TGF)-β₁ activation by mechanical stimuli in vitro raised the question of whether this phenomenon was relevant in vivo in the context of pulmonary fibrosis. Objectives: To explore the effect of mechanical stress on TGF-β₁ activation and its signaling pathway in rat and human fibrotic lung tissue using a novel ex vivo model. Methods: Rat lung fibrosis was induced using transient gene expression of active TGF-β₁. Lungs were harvested at Day 14 or 21 and submitted to various stimuli in a tissue bath equipped with a force transducer and servo-controlled arm. Measurements and Main Results: Fibrotic lung strips responded to tensile force by releasing active TGF-β₁ from latent stores with subsequent increase in tissue phospho-Smad2/3. In contrast, measurable active TGF-β₁ and phospho-Smad2/3 were not induced by mechanical stress in nonfibrotic lungs. Protease inhibition did not affect the release of active TGF-β₁. A TGF-β₁ receptor inhibitor, Rho-associated protein kinase inhibitor, and αv integrin inhibitor all attenuated mechanical stretch–induced phospho-Smad2/3 in fibrotic lung strips. Furthermore, the induction of phospho-Smad2/3 was enhanced in whole fibrotic rat lungs undergoing ventilation pressure challenge compared with control lungs. Last, tissue slices from human lung with usual interstitial pneumonia submitted to mechanical force showed an increase in TGF-β₁ activation and induction of phospho-Smad2/3 in contrast with human nonfibrotic lungs. Conclusions: Mechanical tissue stretch contributes to the development of pulmonary fibrosis via mechanotransduced activation of TGF-β₁ in rodent and human pulmonary fibrosis.

Adhesion with macrophages promotes the activation of fibroblasts into myofibroblasts and creates a self-sustaining profibrotic niche.

Transient adenovirus-mediated gene transfer of active TGF-β1 (transforming growth factor-β1) induces severe and progressive fibrosis in rodent lung without apparent inflammation. Alternatively, transfer of IL-1β (interleukin 1β) induces marked tissue injury and inflammation, which develops into progressive fibrosis, associated with an increase in TGF-β1 concentrations in lung fluid and tissue. Both vector treatments induce a fibrotic response involving myofibroblasts and progressive matrix deposition starting at the peri-bronchial site of expression and extending over days to involve the entire lung and pleural surface. Administration of the TGF-β1 vector to the pleural space induces progressive pleural fibrosis, which minimally extends into the lung parenchyma. The mechanisms involved in progressive fibrosis need to account for the limitation of fibrosis to specific organs (lung fibrosis and not liver fibrosis or vice versa) and the lack of effect of anti-inflammatory treatments in regulating progressive fibrosis. TGF-β1 is a key cytokine in the process of fibrogenesis, using intracellular signalling pathways involving the ALK5 receptor and signalling molecules Smad2 and Smad3. Transient gene transfer of either TGF-β1 or IL-1β to Smad3-null mouse lung provides little evidence of progressive fibrosis and no fibrogenesis-associated genes are induced. These results suggest that mechanisms of progressive fibrosis involve factors presented within the context of the matrix that define the microenvironment for progressive matrix deposition.