A novel biological effect of platelet factor 4 (PF4): enhancement of LPS-induced tissue factor activity in monocytes.

In a previous study we have shown that granulocytes enhance lipopolysaccharide (LPS)-induced tissue factor (TF) activity in monocytes in a platelet-dependent reaction. The present investigation was undertaken to examine the role of a platelet activation product, platelet factor 4 (PF4), in LPS-induced TF activity in monocytes. Platelet lysate supernatant, purified PF4, and the COOH-terminal tridecapeptide of PF4, termed PF4(58-70), enhanced LPS-induced TF activity in monocytes of whole blood dose dependently. A monoclonal antibody against P-selectin eliminated the enhancing effect of PF4(58-70) on LPS-induced TF activity in monocytes, and PF4(58-70) was shown to act synergistically with tumor necrosis factor alpha (TNF-alpha). However, PF4(58-70) did not enhance TNF-alpha secretion in LPS-stimulated whole blood. The major effect of PF4(58-70) was granulocyte dependent. Our results suggest that PF4 might play an important role in LPS-stimulated monocyte TF activity of whole blood.