Platelet activating factor enhancement of lipopolysaccharide-induced tissue factor activity in monocytes: requirement of platelets and granulocytes.

Incubation of platelet activating factor (PAF) with heparinized blood caused no induction of tissue factor activity in monocytes. However, when PAF was added in addition to weak lipopolysaccharide (LPS), it amplified the LPS effect by 80%. By using separated fresh cell populations resuspended in plasma, PAF was shown to have no enhancing effect when added to mononuclear cells incubated with platelet-rich plasma in the presence of LPS. In contrast, when granulocytes also were included, PAF caused an almost 3-fold increase in LPS-induced tissue factor activity. A PAF antagonist blocked this effect and also reduced LPS-induced tissue factor activity of monocytes in whole blood in a dose-dependent manner. In the recombined cell incubation system, the maximal inhibition by the antagonist was observed in the presence of granulocytes. These data provide evidence for an effect of PAF on granulocytes that probably generates a product that, together with platelets, enhances LPS-induced tissue factor activity in monocytes.