Synthesis and Regulation of Complement Components by Human Monocytes/Macrophages and by Acute Monocytic Leukemia

Abstract

Proteins of the complement system (C2, C3) are synthesized by human monocytes and macrophages, thus providing an important local source of these proteins in vivo which serve as a first-line host defense mechanism. In this study, we investigated the production of complement components C2, C4, and C9 by human monocytes/macrophages and by the pathologic cells of acute monocytic leukemia which represent a source of immature monocytic precursors. Human blood monocytes were collected and purified by cytapheresis and elutriation and leukemic cells by Ficoll gradient. Secretion of complement components was measured by a hemolytic assay. The evaluation of the mRNAs of the various complement components in the cells was performed by polymerase chain reaction (PCR) by adding ³²P labeled deoxycytidinetriphosphate (dCTP) to the amplification step. Functional C2 was found to increase during in vitro maturation of macrophages up to the fourth week of culture. C2 mRNA was detected after amplification and increased during the maturation. Interferon-γ (IFN-γ) mediated a marked increase of the C2 mRNA. We found a decrease in synthesis of C4 mRNA during in vitro differentiation of human monocytes. The effect of IFN-γ resulted in an increase in C4 mRNA. C9 mRNA was not detected although it was detected in the HepG2 hepatoma-derived cell line. Functional C2 was not detected by leukemic cells after 24 h of culture but little functional C4 was present in the cell supernatants. As they were by human monocytes and macrophages, C2 and C4 mRNAs were detected after amplification but C9 mRNAs were not detected. In this study we showed that human monocytes and macrophages synthesize the functional C2 and C4 complement components that are regulated by IFN-γ. On the other hand, they do not express C9 mRNA, indicating that the membrane attack complex of complement cannot be assembled by these cells. According to these results, C2 and C4 gene expression occurs in the pathologic cells of acute monocytic leukemia; however, these cells are unable to assemble the membrane attack complex.