Shigella flexneri YSH6000 induces two types of cell death, apoptosis and oncosis, in the differentiated human monoblastic cell line U937

Shigella flexneri, but not a non-invasive mutant derivative, rapidly induced cell death in human monoblastic U937 cells as well as in differentiated cells pretreated with interferon-γ (IFNγ) or retinoic acid (RA). We investigated the morphological and biochemical characteristics of bacterial invasion-induced cell death in these differentiated U937 cells. IFNγ-differentiated cells showed morphological changes typical of apoptosis and their DNA was cleaved giving a ladder-like electrophoretic pattern after infection by Shigellae. In contrast, swelling of the cytoplasm and blebbing of the plasma membrane were observed in RA-differentiated and undifferentiated cells invaded by the bacteria. No condensation of nuclei was observed in these cells by light microscopy, and no internucleosomal fragmentation of DNA was detected on agarose gels, which resembled the features of oncosis. Furthermore, cleavage of poly(ADP-ribose) polymerase, a substrate for apoptotic caspases, was seen only in IFNγ-pretreated cells but not in RA-pretreated or undifferentiated cells. These findings suggested that virulent Shigella flexneri induces distinct types of cell death in U937 cells depending on their differentiation state.