The origin, morphology, and function of epithelioid cells.

Epithelioid cells are cells of the mononuclear phagocyte system found in certain granulomas mainly associated with intense immunological activity. These cells show little phagocytic activity. In certain experimental granulomas such as those produced in guinea pigs sensitive to zirconium, and at sites of intense inflammatory reaction in man, they may contain varying amounts of rough endoplasmic reticulum ("secretory" epithelioid cells). In other situations such as tuberculoid leprosy and in some cases of sarcoidosis they may have the appearance of activated macrophages or take on a multivesicular appearance ("vesicular" epithelioid cells). It is suggested that "vesicular epithelioid cells could develop from "secretory" epithelioid cells by a process of degeneration. In studies comparing granulomas induced in lymph nodes draining the site of intradermal injection of mycobacteria, epithelioid cell granulomas were produced with BCG vaccine, whereas, the granulomas induced by Mycobacterium leprae contained undifferentiated macrophages that contained phagocytosed organisms. The BCG granulomas were in addition characterised by fibroblast infiltration, the presence of collagen and resolution by fibrosis. M. leprae granulomas showed little evidence of fibroblastic activity. Biochemical studies confirmed that BCG granulomas formed new collagen in vitro, whereas this did not take place with M. leprae granulomas. It is suggested that epithelioid cells could play an important role in fibrosis possibly by the secretion of a fibroblast activating factor.