AbstractWe investigated the induction of tissue factor by lymphokines in human monoblastic leukemia cell lines (U937) and leukemic cells from AML (acute myelogenous leukemia) patients. After incubation for 24 h, IL-2 enhanced the intracellular tissue factor 15-fold with U937 cells, and GM-CSF enhanced it 6-fold. In contrast, other lymphokines, such as IL-1-α, IL-1-β, IL-3, IL-4 and G-CSF, did not affect the activity of tissue factor.The leukemic blasts, depleted of T-lymphocytes, taken from five out of 16 AML patients showed a 2.5-14-fold increase in the activity of tissue factor per cell following incubation with 200 u/ml of IL-2 for 72 h.The IL-2 induced tissue factor activity more markedly than GM-CSF. Tissue factor stimulation by IL-2 did not correlate with the expression of the IL-2 receptor,Tac, but correlated well with FAB classification of AML cells. IL-2 responders were found in M4 and M5 subtypes only, but not all M4/M5 leukemias responded to IL-2. These findings indicate that IL-2 can mediate the tissue factor induction in the monocytic type of AML and the effect is not mediated byTacreceptors. This may shed a new light on our understanding of hypercoagulability in acute monoblastic leukemia.