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Hyperthermia in the therapeutic (greater than or equal to 42-43 degrees C) and febrile (less than or equal to 39-40.5 degrees C) ranges modulated the cytotoxic activities of macrophages cocultured with tumour cells and of the monokine tumour necrosis factor (TNF) against tumour cells. These modulatory interactions had clear treatment-sequence dependencies, and some sequences markedly augmented cytotoxic activities. Heated murine bacillus Calmette-Guerin-activated macrophages retained their cytotoxic activities better in coculture with unheated tumour cells if triggering with the endotoxin lipopolysaccharide preceded 1 h of heating at 42 or 43 degrees C than they did if heating to the same extent was concomitant with, or preceded, triggering. Retention of cytotoxicity in coculture during 24 h of 39 or 40.5 degrees C heating was less dependent on pre-heating triggering. The triggering/heating sequence also had modulatory effects on the secretion by heated macrophages of TNF which is involved in cytotoxic manifestations in coculture. Production of TNF by macrophages heated for 1 h at 40.5-43 degrees C or 24 h at 39 or 40.5 degrees C was augmented 1.5- to 6-fold (depending on the heat dose) when triggering preceded heating, whereas sequences in which heating was concomitant with triggering or preceded triggering were detrimental to TNF secretion. Profound treatment-sequence dependencies were also seen when timing of the addition of recombinant human TNF was varied in relation to the heat treatment of tumour cells. Sensitization of TNF-responsive L-929 and TNF-resistant EMT-6 tumour cells occurred if monokine addition preceded heating, whereas the reverse treatment-sequence reduced or eliminated sensitization. Both tumour cell types were also sensitized to TNF if monokine treatment preceded 24 h heating at 40.5 degrees C. These results support the hypothesis that appropriately constructed sequences for either macrophage priming/triggering or monokine treatment of tumour cells, combined with hyperthermia, could augment the cytotoxic actions of macrophages and the cytotoxicity of endogenously added monokines.