Fibrin down-regulates LPS- and PMA-induced tissue factor expression by blood mononuclear cells

Several studies indicate that fibrin may play a functional role in inflamma tion by modulating a variety of cellular functions. We investigated the eff ect of fibrin on tissue factor (TF) production by blood mononuclear cells ( MNC). Citrated human blood was recalcified and incubated at 37 degrees C fo r 1-4 h. The resulting clot was lysed by the addition of tissue plasminogen activator (t-PA) and MNC were isolated by density gradient centrifugation. A control blood sample was processed in the same way but omitting calcium addition and clot formation. Clot- and blood-derived MNC did not express de tectable TF activity and antigen whatever the incubation time. Clot-derived MNC, however, generated on average 5 fold less TF (activity and antigen) t han control cells, when stimulated with lipopolysaccharide (LPS, 1 mu g/ml) for 3 h at 37 degrees C. A reduced TF response of clot-derived cells was a lso observed at mRNA level as indicated by RT-PCR and in situ hybridization . The effect was dependent on the incubation time within the clot, could no t be reversed by enhancing LPS concentration or by adding serum, and was ma intained if LPS was replaced by the tumor promoter PMA. A reduced TF respon se was also found when washed MNC were incorporated for 1 h at 37 degrees C within purified fibrin bur not when the cells were incubated with fibrinog en, thrombin or fibrin split products alone, indicating that contact with f ibrin was responsible for the inhibition of TF production. Fibrin-induced d own-regulation of TF response to LPS and PMA by MNC may represent a negativ e feed-back aimed at limiting excessive blood clotting activation in immune -inflammatory diseases.