LEGIONELLA-PNEUMOPHILA KILLS HUMAN PHAGOCYTES BUT NOT PROTOZOAN HOST-CELLS BY INDUCING APOPTOTIC CELL-DEATH

Legionella pneumophila is a facultative intracellular parasite able to replicate within and to kill a variety of eukaryotic cells. One possi ble killing mechanism is the induction of programmed cell death. Based on electron microscopy and flow cytometry studies using the phosphati dylserine binding protein annexin V, we could demonstrate that L. pneu mophila is able to induce apoptosis in human monocytes which was clear ly dependent on the multiplicity of infection, the time postinfection and the intracellular location of the bacteria. Furthermore, it became evident that Legionella-induced apoptosis does not require the TNF-al pha mediated signal-transduction pathway. By studying infection in Aca nthamoeba castellanii, we found that L. pneumophila is not able to ind uce programmed cell death in their natural host cells indicating that different mechanisms are responsible for host cell killing in protozoa n and human cells. (C) 1998 Federation of European Microbiological Soc ieties. Published by Elsevier Science B.V. All rights reserved.