The contribution of activated macrophages to protection against E. coli was studied in mice. Mice treated intraperitoneally with killed C. parvum organisms 1 d prior to challenge showed an increased resistance to intraperitoneal infection with E. coli; the predominant leucocytes in the peritoneal cavity of these animals were polymorphonuclear cells. However, treatment with C. parvum 4d 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 in vivo and a high degree of intracellular killing of E. coli in vitro. At the same time these cells became more susceptible to the cytotoxic effect of endotoxin. After challenge with E. coli there was a marked decrease in the number of peritoneal macrophages in mice that were treated with C. parvum 4d prior to challenge. Increased susceptibility of activated macrophages to the cytotoxic effect of endotoxin could explain the absence of enhanced resistance to E. coli infection in such animals.
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