Role of platelet P-selectin and CD40 ligand in the induction of monocytic tissue factor expression

Activated platelets can express CD40 Ligand (CD40L) and trigger inflammator y response and tissue factor (TF) expression in endothelial cells through i nteraction with CD40. This pathway is also important for T cell-induced mon ocyte and endothelial cell procoagulant activity. We have studied the poten tial role of the CD40-CD40L pathway in platelet-induced TF expression in a monocytic cell line and in whole-blood monocytes. In vitamin D-3-differenti ated U-937 cells, thrombin-stimulated platelets increased TF expression as measured by mRNA quantification, flow cytometry, and procoagulant activity. Maximum antigen expression occurred after 2 hours. Neutralizing anti-P-sel ectin antibody yielded a 50% suppression of procoagulant activity, whereas antibody to CD40L had no effect. In thrombin receptor activator-stimulated citrated blood, monocytes were up to 77% TF-positive, with peak expression after only 15 minutes. However, no TF mRNA was detectable at that time. Ant i-P-selectin antibody reduced TF by 50%, whereas antibody to CD40L gave a 1 7% reduction. Thus. we conclude that P-selectin exposed on activated platel ets induces the expression of TF in both U-937 cells and whole-blood monocy tes bur by different mechanisms. Platelet CMOL does not display any signifi cant effect on U-937 cells but may be of some importance on whole-blood mon ocytes. This suggests a possible functional difference between U-937 and mo nocyte CD40. Another important finding in this study is the rapid appearanc e of surface TF on monocytes without detectable mRNA formation. This indica tes that TF may be stored intracellularly in these cells and can be exposed on the surface independent of de novo protein synthesis.