The fibrocyte in pulmonary hypertension: we seek him here, we seek him there.

T he fibrocyte is one of many populations of bone marrowderived cells that can be detected in the peripheral blood and is considered to represent a mesenchymal progenitor cell [1]. The fibrocyte, as we currently understand this term, was originally described by BUCALA et al. [2] in 1994 and is defined by the expression of a variety of cell surface antigens, including the stem cell marker CD34, the pan-leukocyte marker CD45, and monocyte markers CD14 and CD11. The fibroblast-like nature of these cells is evidenced by their expression of collagen I, collagen II and vimentin. In normal individuals, these cells comprise ,0.5% of circulating leukocytes [3]. In animal models employing surgically implanted wound chambers, it can be shown that ,16% of cells that accumulate over a number of days are putative blood-derived fibrocytes [4]. Similarly, CD34+ cells can be identified in human scar tissue, suggesting that these cells may contribute to tissue remodelling and repair [5]. Importantly, fibrocytes can be isolated and grown in culture [2]. Since these early observations, fibrocytes have been implicated in the tissue remodelling that occurs in a variety of human diseases, including idiopathic pulmonary fibrosis, asthma, pulmonary hypertension, atherosclerosis and renal fibrosis [6]. Their potential role in the pathobiology of these diseases extends beyond the accumulation of fibroblasts in scar tissue and is now thought to include regulation of fibrogenesis by the secretion of cytokines and growth factors, the production of extracellular matrix and secretion of matrix metalloproteinases [7]. In addition, they act as antigen presenting cells and promote angiogenesis [8].