Peripheral blood fibrocytes: mesenchymal precursor cells and the pathogenesis of fibrosis.

Peripheral blood fibrocytes are a novel population of cells that rapidly enter sites of tissue injury and contribute to connective scar formation. Fibrocytes display a distinct cell surface phenotype (CD34+/CD45+/collagen I+), and are an abundant source of cytokines and growth factors that function to attract and activate inflammatory and connective tissue cells. Fibrocytes also are specialized to activate naïve T cells against foreign antigen, and may play a critical role in the initiation of cognate immunity during the earliest phases of tissue injury. Recently, immunohistochemical studies of fibrotic tissues have shown that fibrocytes co-localize to areas of connective tissue matrix deposition. Peripheral blood fibrocytes thus may participate in the generation of excessive fibroses associated with various autoimmune disorders involving a persistent T-cell activation, as occurs in scleroderma.