Determination of the inducers of Fc (FcRI) and C3 (C3RI) receptors on myeloid cells in several media from human and mouse origin, and the identification of the macrophage as the cell that produces these factors.

The presence of factors responsible for the induction of Fc receptors (FcRI) and C3 receptors (C3RI) on mouse myeloid cells was determined in postendotoxin serum and in media conditioned by postendotoxin lungs, peritoneal macrophages, peritoneal granulocytes, and lymphocytes from either the lymph nodes, the thymus, or the spleen, FcRI and C3RI were found only in postendotoxin serum and in media conditioned by either postendotoxin lungs or peritoneal macrophages. The apparent molecular weights obtained by molecular sieving chromatography in Sephadex G-100 were 10,500 daltons for FcRI and 35,000 daltons for C3RI. These results suggest the existence of several molecules responsible for induction of Fc and C3 receptors. Under our experimental conditions, the macrophage was the only blood cell capable of FcRI and C3RI production, thus hinting at a possible feedback regulatory mechanism. In order to determine whether both inducing factors are also present in normal individuals, human serum and urine were used. FcRI and C3RI were both found in human serum, while only the FcRI factor was detected in urine. These results indicate that the normal human being produces FcRI and C3RI, and that probably the C3RI does not pass through the kidney barrier. The mechanism of action of these factors can be studied by using mouse cells.