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The endogenous viral superantigen 7 in DBA/2 mice serves as a target antigen on syngeneic ESb-MP lymphoma cells for allogeneic graft-vs-leukaemia reactive cells. Allogeneic viral superantigen 7 reactive Vβ6[sup +] T cells are able to transfer graft-vsleukaemia reactivity and to kill specifically viral superantigen 7[sup +] ESb-MP tumour cells in vitro. Here we elucidate the mechanism of this superantigen specific cell Iysis. Already 10 min after co-incubation with in vitro stimulated Vβ6[sup +] T cells, viral superantigen 7[sup +] ESb-MP tumour cells show an apoptotic phenotype (Annexin V-positivity, DNA-fragmentation), This extremely rapid type of cel death is not mediated by the death inducing ligands CD95L, TRAIL and TNF but by perforin and granzyme B. Surprisingly, neither mitochondria nor any of the known caspases appear to be involved in this type of tumour cell killing. In contrast, nitric oxide, released by activated macrophages and endothelial cells, induces in the same tumour cells another type of apoptosis which is much slower and involves mitochondria and caspase activation A synergistic effect between the two different effector mechanisms of superantigen reactive donor cytotoxic T lymphocytes and nitric oxide releasing host macrophages ant endothelial cells might explain the effective immune rejection of even advanced metastasised cancer in this graft-vs-leukaemia animal model. [ABSTRACT FROM AUTHOR]