Tissue-factor-bearing microvesicles arise from lipid rafts and fuse with activated platelets to initiate coagulation

Blood. 2005 Sep 1;106(5):1604-11. doi: 10.1182/blood-2004-03-1095. Epub 2005 Mar 1.


Tissue factor (TF) circulates in plasma, largely on monocyte/macrophage-derived microvesicles that can bind activated platelets through a mechanism involving P-selectin glycoprotein ligand-1 (PSGL-1) on the microvesicles and P-selectin on the platelets. We found these microvesicles to be selectively enriched in both TF and PSGL-1, and deficient in CD45, suggesting that they arise from distinct membrane microdomains. We investigated the possibility that microvesicles arise from cholesterol-rich lipid rafts and found that both TF and PSGL-1, but not CD45, localize to lipid rafts in blood monocytes and in the monocytic cell line THP-1. Consistent with a raft origin of TF-bearing microvesicles, their shedding was significantly reduced with depletion of membrane cholesterol. We also evaluated the interaction between TF-bearing microvesicles and platelets. Microvesicles bound only activated platelets, and required PSGL-1 to do so. The microvesicles not only bound the activated platelets, they fused with them, transferring both proteins and lipid to the platelet membrane. Fusion was blocked by either annexin V or an antibody to PSGL-1 and had an important functional consequence: increasing the proteolytic activity of the TF-VIIa complex. These findings suggest a mechanism by which all of the membrane-bound reactions of the coagulation system can be localized to the surface of activated platelets.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Blood Coagulation / physiology*
  • Blood Platelets / metabolism*
  • Cholesterol / blood
  • Cholesterol / pharmacology
  • Cytoplasmic Vesicles / drug effects
  • Cytoplasmic Vesicles / metabolism*
  • Factor VIIa / metabolism
  • Humans
  • Leukocyte Common Antigens / metabolism
  • Membrane Fusion / physiology*
  • Membrane Glycoproteins / metabolism
  • Membrane Microdomains / metabolism*
  • Platelet Activation / physiology
  • Thromboplastin / metabolism
  • Thromboplastin / physiology*
  • Time Factors


  • Membrane Glycoproteins
  • P-selectin ligand protein
  • Thromboplastin
  • Cholesterol
  • Leukocyte Common Antigens
  • Factor VIIa