Murine lymphoid procoagulant activity induced by bacterial lipopolysaccharide and immune complexes is a monocyte prothrombinase

Abstract

Murine lymphoid cells respond rapidly to bacterial lipopolysaccharide or antigen-antibody complexes to initiate or accelerate the blood coagulation pathways. The monocyte or macrophage was identified as the cellular source, although lymphocyte collaboration is required for the rapid induction of the procoagulant response. This procoagulant activity is identified in this study as a direct prothrombin activator, i.e., a prothrombinase. Studies with plasmas deficient in single coagulation factors demonstrate that the induced murine procoagulant activity effector molecule does not require factors XII, VIII, VII, X or V, but does require prothrombin to transform fibrinogen to fibrin. This enzyme(s) produces limited proteolysis of prothrombin to yield thrombin or thrombin-like products that are functionally capable of converting fibrinogen to fibrin. The prothrombinase is undetectable in freshly isolated murine peripheral blood mononuclear cells, but increases to maximal levels within 6 h after cells are exposed to bacterial lipopolysaccharide or soluble antigen/antibody complexes. Induced monocytes express the prothrombinase on their surface; the activity is increased 5-fold by cellular disruption, indicating a predominantly intracellular pool of enzymes. The prothrombinase is not secreted into the medium in an active form but is recovered with the cell membranes and small organelles. Cleavage of prothrombin by the monocyte prothrombinase requires the presence of calcium and is inhibited by 2.5 mM DFP, suggesting that it may be a serine protease. Enzymatic activity is proportional to the concentration of monocytes, and the pH and thermal optima are in the physiologic range. This neutral protease exhibits 1st-order kinetics and the cleavage of prothrombin increases linearly with time. Induction of activity appears due to newly initiated synthesis of the enzyme per se rather than neutralization of an inhibitor or activation of a zymogen. Monocytes induced to express prothrombinase by this collaborative pathway are not induced to express plasminogen activator. This procoagulant effector molecule or monokine may be responsible for local fibrin deposition observed at sites of lipopolysaccharide- and antigen/antibody complex-induced inflammation and may contribute to the histogenesis and pathogenesis of these and certain other immunologically mediated tissue lesions.