Induction of necrosis in human neutrophils by Shigella flexneri requires type III secretion, IpaB and IpaC invasins, and actin polymerization

Infection by Shigella flexneri is characterized by infiltration of neutroph ils in the intestinal mucosa and by a strong inflammatory reaction. Althoug h neutrophils are constitutively programmed to die by apoptosis, we show: t hat isolated human neutrophils undergo necrosis 2 h after infection with vi rulent S. flexneri strain M90T but not with the virulence plasmid-cured str ain BS176. This was demonstrated by the release of azurophil granule protei ns concomitant with the release of lactate dehydrogenase (LDH), disruption of the plasma membrane, and absence of DNA fragmentation. Mutants with the mxiD1 gene, coding for an essential component of the secretion type III mac hinery, or the genes coding for IpaB or IpaC invasins deleted were not cyto toxic. Neutrophil necrosis occurred independently of the bacterial ability to leave phagosomes, and it involved actin polymerization, as the addition of cytochalasin D after phagocytosis of Shigella inhibited the release of L DH. In conclusion, Shigella kills neutrophils by necrosis, a process charac terized by the release of tissue-injurious granular proteins. This probably contributes to disruption of the epithelial barrier, leading to the dysent ery observed in shigellosis and allowing Shigella to enter its host cells.