In previous studies, neutrophil-ingesting macrophages were clearly and easily observed in the peritoneal cavity of guinea pigs after intraperitoneal injection of thioglycolate medium, and phagocytosis of neutrophils by macrophages could be detected in in vitro cultures of peritoneal exudate cells. Using an in vitro system, we examined the effect of bacterial lipopolysaccharide and recombinant human granulocyte colony-stimulating factor on the apoptosis (programmed cell death) of neutrophils and their subsequent ingestion by macrophages. Lipopolysaccharide delayed karyopyknosis and apoptosis of neutrophils, as shown by endogenous endonuclease activity and a high proportion of trypan blue-excluding cells, and subsequent ingestion by autologous macrophages. Granulocyte colony-stimulating factor also delayed neutrophil karyopyknosis and ingestion by macrophages. When a thioglycolate medium was coinjected intraperitoneally with lipopolysaccharide into guinea pigs in the in vivo system, delays in neutrophil disappearance and ingestion by macrophages in the peritoneal cavity were also observed. We suggest that bacterial products and cytokines regulate neutrophil apoptosis and subsequent ingestion by macrophages at inflamed sites.
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