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In the present study, the ability of Shigella flexneri to activate polymorphonuclear neutrophils (PMN) was examined. The invasive serotype 5 strain M90T induced strong PMN adherence, which was dependent on both the multiplicity of infection and the duration of incubation. When tested under the same experimental conditions, the noninvasive strain BS176 (cured of the 220-kb virulence plasmid) was less efficient. Indeed, incubation of PMN for 2 h with either M90T or BS176 (multiplicity of infection, 100) induced 51.8% +/- 10.5% and 15.2% +/- 4.2% adherence, respectively (n = 3; P < 0.05). Stronger PMN activation by M90T was confirmed by evaluating PMN degranulation induced by the two strains. Whereas M90T triggered significant PMN secretion, BS176 did not. M90T strains with mutations in ipa genes were then analyzed. When PMN were incubated with these mutants, their activation was of the same intensity as that obtained with BS176. These data provide the first evidence for PMN activation induced by S. flexneri, a process which appears to be mediated by Ipa invasins.