SUMMARY: The contribution of activated macrophages to protection against was studied in mice. Mice treated intraperitoneally with killed organisms 1 d prior to challenge showed an increased resistance to intraperitoneal infection with ; the predominant leucocytes in the peritoneal cavity of these animals were polymorphonuclear cells. However, treatment with 4 d prior to challenge induced mainly activated macrophages in the peritoneal cavity and host resistance to the infection was not increased. Activated macrophages from such mice showed both enhanced phagocytic activity and a high degree of intracellular killing of . At the same time these cells became more susceptible to the cytotoxic effect of endotoxin. After challenge with there was a marked decrease in the number of peritoneal macrophages in mice that were treated with 4 d prior to challenge. Increased susceptibility of activated macrophages to the cytotoxic effect of endotoxin could explain the absence of enhanced resistance to infection in such animals.


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