Monocytes, but not endothelial cells, downregulate the anticoagulant activity of activated protein C

Activated protein C (APC) is a natural anticoagulant and inhibits thrombin generation by degrading factors Va and VIIIa. We evaluated the ability of A PC to inhibit blood coagulation triggered by lipopolysaccharide (LPS)-stimu lated [tissue factor (TF)-expressing] human mononuclear cells (MNCs) or umb ilical vein endothelial cells (HUVECs). Using a plasma recalcification assa y, we found that APC (up to 53.3 nmol/l final concentration) had a poor ant icoagulant effect in the presence of LPS-stimulated MNCs, whereas it caused a marked prolongation of clotting time in the presence of LPS-stimulated H UVECs. A poor response to APC was also observed when platelet-free MNCs, mo nocyte-enriched preparations or the monocytoid cell line U937 were tested. Using a TF-independent (FXa-induced) thrombin generation assay, we demonstr ated that both LPS-stimulated and unstimulated MNCs negated the inhibitory activity of APC. Direct determination of FVa activity indicated that MNCs w ere less efficient than HUVECs in promoting FVa inactivation by APC. Togeth er, our results suggest that MNCs, at variance with HUVECs, protect factor Va from inactivation by APC, probably through the expression of a membrane component not present on endothelial cells. These strengthen the importance of monocytes in fibrin deposition associated with pathological conditions characterized by monocyte recruitment and activation.