Fibrocytes in the Pathogenesis of Chronic Fibrotic Lung Disease

Fibrocytes were initially described in 1999 and since that time there has been a growing body of literature to suggest their importance in a number of chronic lung diseases. It is now well established that fibrocytes derive from the bone marrow and circulate within the peripheral blood. However, when injury occurs, fibrocytes can travel to the site of damage via chemokine-mediated recruitment. Recent studies suggest that fibrocyte numbers increase within the lung or circulation during numerous disease processes. Although fibrocytes readily differentiate into fibroblasts in vitro, whether they do so readily in vivo is still unclear. Additionally, while human studies often show evidence of α-smooth muscle actin (SMA) expression in fibrocytes, this is less common in murine studies. A variety of pro-fibrotic mediators that are secreted by fibrocytes make it likely that they can act via paracrine functions to influence the behavior of resident lung cells. This review summarizes recent insights regarding fibrocytes in asthma, scleroderma and IPF.