Sarcoidosis and interstitial pulmonary fibrosis; two distinct disorders or two ends of the same spectrum

Pulmonary fibrosis is a reparative response characterized by accumulation of extracellular matrix in the lung parenchyma that may be observed in end-stage sarcoidosis. This article will discuss the recent advancements in the understanding of the pathogenesis of pulmonary fibrosis in sarcoidosis in comparison with idiopathic pulmonary fibrosis (IPF)/usual interstitial pneumonia (UIP).

A recent study examined clinical, radiographic, and histopathologic findings of end-stage sarcoidosis patients with lung fibrosis who underwent lung transplantation. The authors found many of the patients to have moderate-to-severe interstitial pneumonitis in some cases with UIP considered to be atypical of end-stage sarcoidosis. Furthermore, these patients had diagnosis of sarcoidosis for a shorter time prior to transplant compared with individuals without interstitial pneumonitis (mean 4.8 years vs. 23.3 years). Another study found a promoter polymorphism in prostaglandin-endoperoxide synthase 2 (PTGS2), -765G>C, to be associated with susceptibility and increased risk for pulmonary fibrosis in sarcoidosis in the white population compared with healthy controls. An altered Sp1/Sp3 binding to the -765 region has been proposed as a possible mechanism for reduced PTGS2 expression.

A subset of patients with sarcoidosis that progresses to pulmonary fibrosis may share some similar mechanistic and morphologic aberrations with IPF/UIP. Future studies are needed to examine the significance of chronic interstitial pneumonitits and UIP pattern in fibrotic sarcoidosis as a potential marker for progressive disease, and the roles of PTGS2 polymorphism in various ethnic groups and Sp1/Sp3 binding in other fibrotic lung diseases.