Macrophages in lung fibrosis.

Pulmonary fibrosis is a disease in which excessive extracellular matrix (ECM) accumulation occurs in the lungs, which induces thickening of the alveolar walls, ultimately leading to the destruction of alveolar structures and respiratory failure. Idiopathic pulmonary fibrosis, the cause of which is unknown, has a poor prognosis with a median survival of 2-4 years after diagnosis. There is currently no known curative treatment. The mechanism underlying pulmonary fibrosis is thought to be initiated by the dysfunction of type II alveolar epithelial cells, which leads to ECM overproduction through the activation of fibroblasts. In addition, it has been suggested that a variety of cells contribute to fibrotic processes. In particular, clinical and basic research findings examining the roles of macrophages suggest that they may be pivotal regulators of pulmonary fibrosis. In this review, we discuss the characteristics, functions and origins of subsets of macrophages involved in pulmonary fibrosis, including resident alveolar, interstitial and monocyte-derived macrophages.