The role of circulating mesenchymal progenitor cells (fibrocytes) in the pathogenesis of pulmonary fibrosis

Pulmonary fibrosis is associated with a number of disorders that affect the lung. Although there are several cellular types that are involved in the pathogenesis pulmonary fibrosis, the resident lung fibroblast has been viewed traditionally as the primary cell involved in promoting the deposition of ECM that culminates in pulmonary fibrosis. However, recent findings demonstrate that a circulating cell (i.e., the fibrocyte) can contribute to the evolution of pulmonary fibrosis. Fibrocytes are bone marrow-derived mesenchymal progenitor cells that express a variety of cell-surface markers related to leukocytes, hematopoietic progenitor cells, and fibroblasts. Fibrocytes are unique in that they are capable of differentiating into fibroblasts and myofibroblasts, as well as adipocytes. In this review, we present data supporting the critical role these cells play in the pathogenesis of pulmonary fibrosis. J. Leukoc. Biol. 86: 1111-1118; 2009.