Evaluation of intracellular killing of bacteria by enriched populations of mouse peritoneal exudate neutrophils

Elicited, mouse peritoneal exudate cells were fractionated by centrifugation on discontinuous Percoll density gradients. Two subpopulations of neutrophils, each of greater than 90% purity, were isolated at discontinuous density gradient interfaces different from the region of mononuclear cell enrichment (i.e. 1.0694-1.0871 and 1.0872-1.1002 g/ml for neutrophils and less than 1.0694 g/ml for mononuclear cells). Peritoneal exudate cells were mixed with Proteus mirabilis in the presence of 1% normal mouse serum for 30 min. The mixtures were fractionated on gradients of Percoll diluted with a calcium-free medium. Populations of cells banding at densities greater than 1.0693 g/ml were washed free of gradient material, and neutrophil suspensions containing intracellular bacteria and which were relatively free of extracellular bacteria were isolated. Less than 7% of the total bacteria present was extracellular. The continuing extracellular presence of a heat-labile component of normal mouse serum was essential for maximal intracellular kill of P. mirabilis by mouse peritoneal neutrophils.