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.

SUMMARY Antibody to tissue factor apoprotein was adsorbed against 7-globulin and coupled to horseradish peroxidase; this complex was applied to various rabbit tissues. The distribution of the peroxidase marker then was observed by electron microscopy. We examined fixed or frozen sections, as well as breis of aorta, vena cava, brain, heart, lung, liver, spleen, kidney, bone marrow, mesothelial gut lining, erythrocytes, leukocytes, and platelets. All endothelial cells that had been exposed to the antibody complex were positive and in all cases only the endothelial cells showed localization of the electron-dense reaction product. Tissues that had been incubated with complexes prepared with 7-globulin from animals not immunized with tissue factor apoprotein showed no staining.Prior treatment of the tissue with uncoupled anti-tissue factor 7-globulin blocked binding by the coupled antibody. In blood vessel preparations that had been specifically designed to expose the media to the anti-tissue factor complex, medial smooth muscle cells and connective tissue showed no reaction product. Parenchyma! cells of the other organs mentioned likewise were devoid of reaction product. Similarly, the leukocytes and platelets occasionally observed in vessel lumens showed no evidence of binding. Platelets adhering to arterial subendothelial structure after injury also were unreactive. These findings suggest that in normal rabbits anti-tissue factorhorseradish peroxidase complex combines selectively with endothelial cells.