Release of human monocytes protein C and the cofactor protein S in vitro.

Monocytes may contribute to a coagulation process by expression of the tissue factor and by synthesizing factors V, VII, X and XIII. Cultured blood monocytes also express both the tissue plasminogen activator and the plasminogen activator inhibitor I. The present study assesses the ability of human monocytes to secrete protein C and its cofactor protein S, which are potent inhibitors of the clotting cascade. Monocytes derived from the blood of healthy volunteers and prepared according to Boyum were cultured for up to 36 hours with or without lipopolisaccharide from Escherichia coli. After different times of incubation, the concentrations of proteins C and S in the supernatants were measured in order to determine synthesis of proteins, monocytes were cultured in the presence or absence of cycloheximide, the protein synthesis inhibitor. The concentration of protein C was estimated by means of the ELISA Protein C test (Boehringer Mannheim). Protein S concentrations were measured by rocket immunoelectrophoresis according to Laurell, using monospecific antisera (American Diagnostica Inc., N.Y.). The study showed that human monocytes, when stimulated by lipopolisaccharide, release proteins C and S in vitro. The concentration of these proteins in the culture supernatants markedly increased with time during the 36-hour observation. The supernatants obtained from the culture of unstimulated monocytes did not contain detectable quantities of the investigated proteins. The exposure of the cells to cycloheximide did not suppress the release of proteins C and S. In conclusion, our results suggest that monocytes are not able to synthesize proteins C and S. They can only release these factors. Furthermore, monocytes may be responsible for coagulation due to the inactivation of factors Va and VIIIa by protein C.