Biosynthesis of complement by human monocytes

Synthesis of complement proteins by mononuclear phagocytes in vitro was examined in long-term cultures. Normal monocytes synthesized functionally active C2 and factors B and D for up to 6 weeks in culture. The kinetics of C2 and factor B production were similar in that a lag period of 3-6 days was noted before C2 or B were secreted. Factor D was secreted within 24 hr of incubation in vitro. There was no evidence for synthesis of active C5, C7, or C9 by monocytes but C5 protein production was readily detectable. Low levels of C6 activity were found in culture media but definitive evidence for synthesis of C6 was not obtained. In culture media, preformed C8 was rapidly catabolized therefore synthesis of C8 could not be studied. These data suggest that monocytes/macrophages may play a role in host defenses by providing complement proteins at tissue sites of injury or infection.