Regulation of complement protein biosynthesis in mononuclear phagocytes.

Proteins of the complement system (with the exception of the terminal components C6-9) are synthesized in mononuclear phagocytes. The extrahepatic macrophage is therefore an important local source of the complement proteins which may serve as a first-line host defence mechanism. Net synthesis and secretion of complement by these cells is a function of maturation of the mononuclear phagocytic series, the tissue from which the cells are isolated, and the state of macrophage activation. To define some of the mechanisms for regulation of complement gene expression in mononuclear phagocytes, the major histocompatibility complex class III genes and C3 have been investigated. These genes are expressed constitutively in hepatocytes and monocytes/macrophages. In the mononuclear phagocyte, interferon-gamma, at physiological concentrations, effects a dose- and time-dependent increase in factor B and C2 mRNA and a corresponding increase in factor B and C2 biosynthesis. This effect is specific inasmuch as the expression of other genes (e.g. C3) is decreased by interferon-gamma, and interferon-alpha and beta at concentrations one to two logs greater have only a minimal effect on C2 and factor B gene expression. Endotoxin acting directly on monocytes has qualitatively different effects on expression of the complement genes. These complex regulatory mechanisms are being investigated with the use of murine fibroblasts transfected with human DNA bearing the relevent complement genes.