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Interleukin-2 activates lymphocytes to become highly cytotoxic for a wide range of tumour cell types in vitro (Iymphokine activated killer or LAK cells), and in animal models. However, only limited therapeutic benefit was observed in clinical trials of LAK cell therapy. This project aimed to investigate the molecular and cellular interactions involved in the production and effector functions of LAK cells, to identify factor(s) which might be responsible for the poor clinical responses observed in LAK cell therapy. Tumour cell lines were heterogeneous in their response to killing by cytokines (TNFα, LT, IFNγ and IL-1β), and purified monocytes or lymphocytes, but were consistently highly sensitive to LAK cell cytotoxicity. Autologous monocytes and lymphocytes were not killed by LAK cells, in contrast to human umbilical vein endothelial cells and fibroblasts. Supernatants from LAK cells were considerably less cytotoxic than the effector cells, and physical separation of effector and target cells resulted in inhibition of killing. Lymphocyte and LAK cell cytotoxicity was associated predominantly with the CD8+ (cytotoxic T-cell) lymphocyte sub-population, and was significantly inhibited by anti-TNFα and anti-LT, demonstrating that these cytokines were the primary effector molecules in this system. LAK cells and A375 melanoma cells showed procoagulant activity, predominantly via the tissue factor pathway, and LAK cells also possessed surface factor V. In addition, A375 cells were highly fibrinolytic. Tumour cell killing by LAK cells was inhibited by plasma, and further experiments determined that polymerised fibrin, but not fibrin monomer was responsible. From these results it was suggested that culture of small numbers of cells from tumour biopsies, and the determination of their sensitivity to cytotoxic drugs, cytokines and effector cells may lead to more effective treatment protocols for immunotherapy of individual tumours. In order to enhance the efficacy of immunotherapy, further in vivo research is required to elucidate the interactions between immune effector cells and the coagulation/fibrinolytic systems.