Procoagulant activity of human mononuclear leukocytes: dissociation of the effect of mitogens on procoagulant activity and mitogen-stimulated lymphocyte proliferation.

The role of mononuclear cells in generating procoagulant activity was examined by incubating Ficoll-Hypaque-separated mononuclear leukocytes with or without mitogens (phytohemagglutinin, pokeweed mitogen, and concanavalin A). The procoagulant activity was assayed by a modification of a one-stage plasma recalcification time. Significant procoagulant activity developed after 24 hr incubation and was dose dependent; mitogens alone had no effect on the clotting time. The increase in activity was paralleled by the increase in tritiated thymidine incorporation into replication DNA. However, mitomycin C had little inhibitory effect on the development of procoagulant activity, whereas thymidine incorporation was inhibited. The major procoagulant activity was associated with intact cells and not the conditioned supernatant. The removal of adherent mononuclear cells (mostly monocytes) by polystyrene bead columns abolished the procoagulant activity, whereas purification of mononuclear leukocyte populations for monocytes markedly increased the activity as compared to purified lymphocytes. The procoagulant activity was shown to act by the extrinsic limb of the coagulation sequence because of substitution factor VII-deficient plasma for normal plasma resulted in marked depression of procoagulant activity, whereas factor VIII-deficient plasma resulted in a clotting time only minimally longer then normal plasma. Thus, although procoagulant activity in cultures of mononuclear cells is stimulated by the mitogen reagent, these studies suggest that the activity may not be the result of the mitogenic effect on lymphocytes per se. Whether it is a direct effect of the mitogen on the adherent cell or is an effect of a contaminant of the mitogen reagent, such as endotoxin, remains to be determined.