GRANULOCYTES ENHANCE LPS-INDUCED TISSUE FACTOR ACTIVITY IN MONOCYTES VIA AN INTERACTION WITH PLATELETS

In the present study we have investigated the effect of platelets and granulocytes on bacterial lipopolysaccharide (LPS)-induced tissue fact or (TF) activity in monocytes. Experiments were performed on freshly i solated cells resuspended in heparinized plasma and recombined with pl atelet-poor or platelet-rich plasma. In a platelet-dependent reaction the granulocytes enhanced LPS-induced TF activity by an average of 100 %. The effect was dose dependent with regard to the number of both gra nulocytes and platelets, respectively. Granulocytes and/or platelets d id not affect LPS-induced tumor necrosis factor (TNF) secretion from m onocytes. Phorbol myristate acetate (PMA) per se was not able to induc e TF activity in our system. In contrast, the agonist caused a substan tial increase in TF activity induced by LPS. The effect was totally de pendent on the presence of platelets and was shown to be due to stimul ation of both granulocytes and monocytes (the activity rose from 30 +/ - 7 to 83 +/- 12 mU/10(6) cells in the presence of platelets and from 69 +/- 8 to 143 +/- 22 mU/10(6) cells in the presence of platelets and granulocytes). Effects similar to those observed with PMA were obtain ed with physiological concentrations (10 ng/ml) of TNF. A combination of these two agonists gave no further amplification of LPS-induced TF activity compared with the effect of the agonists separately. Low conc entrations of a monoclonal anti-CD15 antibody abolished the stimulator y effects of platelets and granulocytes. Furthermore, the anti-CD15 an tibody neutralized the effect of TNF, whereas the PMA effect was reduc ed by almost 75%. These results were confirmed in a whole-blood system . The inhibitory effect of the antibody may be associated with CD15's role as a complementary ligand for PADGEM. Our study indicates that a close interaction between granulocytes, platelets, and monocytes is es sential for optimal TF activity induced by LPS. It is hypothesized tha t the effect of granulocytes is related to their ability to activate p latelets. We propose that upon activation granulocytes secrete a produ ct that enhances the capacity of platelets to stimulate TF activity in monocytes.