Fibrocytes (Reactive or Reparative)

Human fibrocytes exhibit mixed phenotypic characteristics of haematopoietic stem cells, monocytes and fibroblasts, and originate from a precursor of the monocyte lineage. They constitutively produce chemokines and growth factors that are known to modulate inflammatory reactions or promote angiogenesis and the deposition of extracellular matrix molecules. Upon exposure to transforming growth factor-β₁ and endothelin-1, fibrocytes produce large quantities of extracellular matrix components and acquire a contractile phenotype. Such differentiation of fibrocytes into myofibroblast-like cells occurs at the tissue sites during repair processes and has been found to contribute to wound healing in vivo. Fibrocytes and fibrocyte-derived myofibroblasts are also involved in the pathogenesis of lung disorders characterised by chronic inflammation and extensive remodelling of the bronchial wall, like asthma, or progressive fibrosis with destruction of the pulmonary architecture, like idiopathic pulmonary fibrosis. They participate in tumour-induced stromal reactions and may either promote or inhibit the metastatic progression of cancers. Prevention of excessive extracellular matrix deposition and detrimental tissue remodelling in pulmonary diseases may be achieved by inhibiting the accumulation of fibrocytes in the lungs. Moreover, in vitro expanded fibrocytes may serve as vehicles for the delivery of gene constructs to improve ineffective lung repair or be used in anti-cancer cell therapy.