Ratio of bacteria to polymorphonuclear neutrophils (PMNs) determines PMN fate

This study was designed to investigate how live Escherichia coil influence the fate of polymorphonuclear neutrophils (PMNs) in vitro. PMNs from 10 hea lthy volunteers were cocultured with or without live E. coli at different r atios. Heat-killed E. coli(Hk) were also added to PMNs at a ratio of 1:10, The PMNs were then analyzed by flow cytometry for cell death, reactive oxyg en intermediates (ROI) production, and CD16 expression. Morphologic feature s were also assessed, PMN apoptosis was confirmed by DNA gel electrophoresi s. Low doses of E. coli (PMN:E. coli ratios of 1:0.01 and 1:0.1) inhibited PMN apoptosis. In contrast, a high dose of E. coli (PMN:E. coli ratio of I: 10) increased PMN necrosis, ROI production was significantly greater at PMN :E. coli ratios of 1:10 and 1:10 (Hk) than at ratios of 1:0.01 and 1:0.1, o r in PMNs cultured alone after a 15 or 30 minute coculture, CD16 expression s were significantly lower in PMNs cocultured with E. coli than in those cu ltured alone after a 4 or 12-h coculture, Tumor necrosis factor-cu, interle ukin (IL)-1 beta and IL-6 levels in cell-free supernatants were also measur ed, The mean percentages of apoptosis at PMN:E. coli ratios of 1:0.01 and I :10 (Hk), and in PMNs cultured alone after a 12-h coculture showed signific ant inverse correlations with these cytokine levels in cell-free supernatan ts at 12 h, Our results demonstrate that low doses of live E, coli inhibits predominantly PMN apoptosis, whereas a high dose of E, coli increases necr osis. Augmented PMN bactericidal function, via inhibition of PMN cell death , may be beneficial for host defense against bacterial infection and/or sep sis.