The monocytic U937 cell line, though immature, has many properties in common with mature monocytes. We have studied the antigenic expression and activity of tissue factor (TF) in the cytosol and on the surface of U937 cells after exposure to GM-CSF and endotoxin (LPS). Following exposure to LPS, both TF phenotype and procoagulant activity (PCA) increased linearly, with peak expression and activity after 18 hours of stimulation. Total PCA (tPCA) increased as early as 6 hours, unlike surface PCA (sPCA) which peaked at 18 hours. A linear correlation was observed between surface TF and both sPCA and tPCA. Incubation of cells with rHuGM-CSF did not effect phenotypic markers of monocyte maturation and had no significant effect on TF expression or PCA. However, cells activated with LPS after rHuGM-CSF priming, demonstrated accelerated expression of TF and PCA, with TF expression peaking at 6 hours and PCA at 2 hours. No increase in the absolute levels of TF were seen after priming with GM-CSF. We conclude that GM-CSF accelerates, but does not increase the magnitude of, the procoagulant response of monocytic cells to endotoxin. We propose that the initial accelerated PCA induction by LPS after rHu-GM-CSF priming, was due to conformational changes in TF and was not due to de novo synthesis of TF protein.
Keywords: LPS-Lipopolysaccharide; PCA-Procoagulant Activity; TF-Tissue Factor; rHuGM-CSF-Recombinant Human Granulocyte-Macrophage Colony-Stimulating Factor; sPCA-Surface PCA; tPCA-Total PCA.