When things go wrong: Exploring possible mechanisms driving the progressive fibrosis phenotype in interstitial lung diseases.

Interstitial lung diseases (ILD) comprise a large and heterogeneous group of disorders of known and unknown etiology characterised by diffuse damage of the lung parenchyma. In the past years, it has become evident that patients with different types of ILD are at risk of developing progressive pulmonary fibrosis known as pulmonary fibrosing ILD (PF-ILD). This is a phenotype behaving similar to idiopathic pulmonary fibrosis, the archetypical example of progressive fibrosis. PF-ILD is not a distinct clinical entity but describes a group of ILD with a similar clinical behavior. This phenotype may occur in diseases displaying distinct etiologies and different biopathology during their initiation and development. Importantly, these entities may have the potential for improvement or stabilisation prior to entering in the progressive fibrosing phase. The crucial questions are (1) why a subset of patients develops a progressive and irreversible fibrotic phenotype even with appropriate treatment, and (2) what the pathogenic mechanisms driving progression possibly are. We here provide a framework highlighting putative mechanisms underlying progression, including genetic susceptibility, aging, epigenetics, the structural fibrotic distortion, the aberrant composition and stiffness of the extracellular matrix, and the emergence of distinct profibrotic cell subsets. Understanding the cellular and molecular mechanisms behind PF-ILD will provide the basis for identifying risk factors and appropriate therapeutical strategies.