Monocyte Tissue Factor Expression: Lipopolysaccharide Induction and Roles in Pathological Activation of Coagulation

The coagulation system is a part of the mammalian host defense system. Pathogens and pathogen components, such as bacterial lipopolysaccharide (LPS), induce tissue factor (TF) expression in circulating monocytes that then activates the coagulation protease cascade. Formation of a clot limits dissemination of pathogens, enhances the recruitment of immune cells, and facilitates killing of pathogens. However, excessive activation of coagulation can lead to thrombosis. Here, we review studies on the mechanism of LPS induction of TF expression in monocytes and its contribution to thrombosis and disseminated intravascular coagulation. Binding of LPS to Toll-like receptor 4 on monocytes induces a transient expression of TF that involves activation of intracellular signaling pathways and binding of various transcription factors, such as c-rel/p65 and c-Fos/c-Jun, to the TF promoter. Inhibition of TF in endotoxemia and sepsis models reduces activation of coagulation and improves survival. Studies with endotoxemic mice showed that hematopoietic cells and myeloid cells play major roles in the activation of coagulation. Monocyte TF expression is also increased after surgery. Activated monocytes release TF-positive extracellular vesicles (EVs) and levels of circulating TF-positive EVs are increased in endotoxemic mice and in patients with sepsis. More recently, it was shown that inflammasomes contribute to the induction of TF expression and activation of coagulation in endotoxemic mice. Taken together, these studies indicate that monocyte TF plays a major role in activation of coagulation. Selective inhibition of monocyte TF expression may reduce pathologic activation of coagulation in sepsis and other diseases without affecting hemostasis.

CD11/CD18 and CD14 Share a Common Lipid A Signaling Pathway

The activation of phagocytes by the lipid A moiety of LPS has been implicated in the pathogenesis of Gram-negative sepsis. While two LPS receptors, CD14 and CD11/CD18, have been associated with cell signaling, details of the LPS signal transduction cascade remain obscure. CD14, which exists as a GPI-anchored and a soluble protein, lacks cytoplasmic-signaling domains, suggesting that an ancillary molecule is required to activate cells. The CD11/CD18 integrins are transmembrane proteins. Like CD14, they are capable of mediating LPS-induced cellular activation when expressed on the surface of hamster fibroblasts Chinese hamster ovary (CHO)-K1. The observation that a cytoplasmic deletion mutant is still capable of activating transfected CHO-K1 argues that CD11/CD18 also utilizes an associated signal transducer. We sought to identify further similarities between the signaling systems utilized by CD14 and CD11/CD18. LPS-binding protein, which transfers LPS to CD14, enhanced both LPS-induced cellular activation and binding of Gram-negative bacteria in CD11/CD18-transfected CHO-K1, thus implying that LPS-binding protein can also transfer LPS to CD11/CD18. When synthetic lipid A analogues were analyzed for their ability to function as LPS agonists, or antagonists, in the CHO transfectants, we found the effects were identical regardless of which LPS receptor was expressed. This supports the hypothesis that a receptor distinct from CD14 and CD11/CD18 is responsible for discriminating between the lipid A of LPS and the LPS antagonists. We propose that this receptor, which is the target of the LPS antagonists, functions as the true signal transducer in LPS-induced cellular activation for both CD14 and CD11/CD18.