Summary
Monocytes/macrophages are important in disease states such as gram-negative sepsis
and coronary artery disease. Following exposure to lipopolysaccharide (LPS), monocytes
express tissue factor (TF), the main initiator of blood coagulation. We previously
demonstrated that human monocytes treated with high concentrations of LPS, or with
LPS and calcium ionophore, displayed higher TF activity than monocytes treated with
only low concentrations of LPS, even though the monocytes under all conditions expressed
similar amounts of cell surface TF antigen. Such restrained TF activity is often referred
to as encryption and its release as de-encryption. We also observed that the increase
in TF activity, de-encryption, coincided with an increase in cell surface phosphatidylserine
(PS) representing apoptosis and necrosis. In the present work, we separated LPS and
LPS and calcium ionophore-treated human monocytes into two populations, one of mainly
viable, PS negative cells, and one of mainly non-viable, PS positive cells, by sorting
flow-cytometry. We observed that non-viable cells expressed considerably less TF antigen
than viable cells. Despite this, non-viable cells were clearly more procoagulant than
viable cells in two different coagulation assays. Procoagulant activity was dependent
on both TF and PS. We consider the higher content of externalized PS in non-viable
monocytes as the major reason for the stronger procoagulant activity of these cells.
Thus, TF de-encryption appears largely to occur on PS positive, non-viable cells under
these conditions. This supports the important role of PS in coagulation, and it suggests
that PS expression signifying cell death, may be clinically relevant.
Keywords
Tissue factor - coagulation - phosphatidylserine - monocyte - sorting flow cytometry