S1P enhances thrombin-induced EC surface TF antigen expression. Confluent monolayers of ECs were treated with 1 nM thrombin (dotted line), 4 μM S1P (solid line), or 1 nM thrombin plus 4 μM S1P (bold solid line) for 5 hours. The cells were then harvested and incubated with murine monoclonal anti-TF IgG. FITC-labeled goat antimouse IgG antibody was used as the second antibody. Fluorescence intensity was determined by FACScan as described in “Materials and methods.” Gray histograms depict untreated cells. Similar results were observed in 2 independent experiments.