Summary
Tissue factor (TF), the main initiator of blood coagulation, contributes to the manifestation
of disseminated intravascular coagulation following septic shock in meningococcal
infection. Since a direct relationship between disease severity and lipopolysaccharide
(LPS) concentration in the circulation has been shown, we hypothesized that the procoagulant
and cytotoxic effects of endotoxin also in vitro were related to its concentration. In vitro studies, however, have frequently used much higher LPS concentrations than those
observed in clinical samples. Using elutriation-purified human monocytes, we observed
that LPS up to 1000 ng/ml exerted a concentration-dependent increase in TF activity
(tenase activity, fibrin formation in plasma). Although there was a dose-dependent
increase in TF activity, there was not a concomitant increase in TF expression at
LPS concentrations above 1 ng/ml (flow cytometry, Western blotting, TF mRNA). Flow
cytometry revealed that this discrepancy between TF activity and TF expression at
endotoxin concentrations above 1 ng/ml, coincided with an LPS dose-dependent increase
in cell surface phosphatidylserine (PS), considered to promote coagulation. The increased
PS expression was associated with an increased number of 7-AAD-positive cells indicating
cell death. We conclude that enhancement of monocyte procoagulant activity in vitro by high concentrations of LPS may result from increased PS exposure due to apoptosis
and necrosis. Therefore, the LPS concentrations used to examine monocyte procoagulant
activity in vitro, should be carefully chosen.
Keywords
Tissue factor - coagulation - lipopolysaccharide - phosphatidylserine - monocyte