A B S T R A C T Tissue factor occurs in a dormant state on the surface of cultured normal human fibroblasts and WISH 1 amnion cells. The activity of undisturbed monolayers or cells lifted with brief trypsin treatment (0.125% trypsin for 1 min) increases up to 60-fold upon prolonged digestion with dilute trypsin (0.0025% trypsin for 30 min); activity appears subsequent to cell detachment. Up to 70% of the total cellular tissue factor becomes active under these conditions and is released from the cells. The ruthenium red staining coat of the cells is lost during detachment, but cell viability (more than 90% exclude trypan blue) and cell morphology do not change during the subsequent development of tissue factor activity. Furthermore, less than 10% of four intracellular enzymes and less than 20% of two plasma membrane enzymes are released during this period of time. We therefore conclude that cells in culture do have tissue factor activity, that it exists in a latent form, and that total cell disruption is not necessary for this activity to initiate blood coagulation.

The tissue-factor (thromboplastic) activity of cultured human endothelial cells and fibroblasts is low at time of transfer into fresh medium but increases 3–10 fold. Endothelial cells reach peak activity (400 U/10(5) cells) 5–8 hr after subculture. Activity in fibroblast cultures peaks (3000–12,000 U/10(5) cells) 7–12 hr after subculture. After attaining maximum activity, endothelial and fibroblast tissue- factor content decreases in a time course similar to other cells studied in this laboratory, approaching basal levels by 24–50 hr after subculture. If medium over fibroblasts is changed every 12 hr, activity can be sustained at the peak level for an additional day but cannot be maintained at a high level indefinitely. The kinetics of expression of smooth muscle cell tissue factor are markedly different from other cell types. There is always a pronounced lag (30 hr or more) before the activity increases, and then, in most cases, there is no subsequent decline in activity even though the cells are not refed or restimulated. The activity of each of these cell types is cryptic but becomes available after freeze-thaw disruption of cells.

The tissue-factor (thromboplastic) activity of cultured human endothelial cells and fibroblasts is low at time of transfer into fresh medium but increases 3–10 fold. Endothelial cells reach peak activity (400 U/10(5) cells) 5–8 hr after subculture. Activity in fibroblast cultures peaks (3000–12,000 U/10(5) cells) 7–12 hr after subculture. After attaining maximum activity, endothelial and fibroblast tissue- factor content decreases in a time course similar to other cells studied in this laboratory, approaching basal levels by 24–50 hr after subculture. If medium over fibroblasts is changed every 12 hr, activity can be sustained at the peak level for an additional day but cannot be maintained at a high level indefinitely. The kinetics of expression of smooth muscle cell tissue factor are markedly different from other cell types. There is always a pronounced lag (30 hr or more) before the activity increases, and then, in most cases, there is no subsequent decline in activity even though the cells are not refed or restimulated. The activity of each of these cell types is cryptic but becomes available after freeze-thaw disruption of cells.

Platelets stimulate tissue factor, the initiator of the extrinsic coagulation pathway, and increase fibrinolytic inhibition in fibroblasts grown in vitro. Cellular tissue factor increases an average of 2.8-fold over the control levels after a 6-hr incubation with platelets, and no activity is present in the media. Fibrinolytic inhibition is stimulated in both the fibroblasts and their media in the presence of platelets and accumulates throughout a 24-hr incubation. Neither leukocytes nor erythrocytes stimulate these changes. Both tissue factor and fibrinolytic inhibition increases are dependent on platelet concentration and are blocked by inhibitors of RNA or protein synthesis. Control smooth muscle cells have higher tissue factor and fibrinolytic inhibition than fibroblasts, but their response to the presence of platelets is similar. Confluent monolayers of endothelial cells have very low levels of tissue factor that are not altered by the presence of platelets. However, the ability of endothelial cells to inhibit fibrinolysis is enhanced by the presence of platelets. The fraction that stimulates tissue factor and fibrinolytic inhibition is distinct from platelet-derived growth factor and from the fraction that enhances leukocyte tissue factor. It is associated with an insoluble, nonmitogenic fraction that is not inactivated by phospholipase C, or diisopropylfluorophosphate, nor is it chloroform:methanol extractable. Platelets are a physiologic modulator for both cellular tissue factor and the fibrinolytic system in vitro.