CD34+ Fibrocytes: Morphology, Histogenesis and Function

The connective tissue of virtually all human organs harbors huge amounts of resident CD34+ fibrocytes. Recent studies have shown that CD34+ fibrocytes derive from circulating CD14+ monocytes. CD34+ fibrocytes are involved in wound healing, act as antigen presenting cells and secrete a multitude of cytokines. Due to their diverse functions CD34+ fibrocytes play a role in connective tissue diseases, pulmonary fibrosis and tumor associated stromal remodeling. Stromal remodeling precipitated by invasive carcinomas is characterized by a loss of CD34+ expression paralleled by a gain of α-SMA expression in stromal cells resulting in a phenotype change from CD34+ fibrocytes towards α-SMA positive myofibroblasts. This process is very stereotypic and may play an essential role in local tumor invasion and systemic dissemination, since a reduction of antigen presenting CD34+ fibrocytes might constitute a step in escaping the hosts' immune control directed against invasive carcinoma cells.