APOPTOSIS OF EPITHELIAL-CELLS AND MACROPHAGES DUE TO INFECTION WITH THE OBLIGATE INTRACELLULAR PATHOGEN CHLAMYDIA-PSITTACI

We have characterized the cytotoxic activity of the obligate intracell ular bacterium Chlamydia psittaci, which resides within a membrane-bou nd vacuole during the 2-day infection cycle. We have established that infected epithelial cells and macrophages die through apoptosis, which is measurable within 1 day of infection and requires productive infec tion by the bacteria. Inhibition of host cell protein synthesis has no effect on cell death, but blocking bacterial entry or bacterial prote in synthesis prevents apoptosis, implying that bacterial growth is req uired for death of the host cell. Apoptosis was confirmed through the use of electron microscopy, terminal deoxynucleotidyl transferase-medi ated dUTP nick end labeling, gel agarose electrophoresis of fragmented DNA, and propidium-iodide labeling of host cell nuclei. Although infe cted cells died preferentially, both infected and uninfected cells bec ame apoptotic, suggesting that the infected cells may secrete proapopt otic factors, Inhibition of either of two proapoptotic enzymes, caspas e-1 or caspase-3, did not significantly affect Chlamydia-induced apopt osis, These results suggest that, as in the case of apoptosis due to B ax expression or oncogene dysregulation, which initiate the apoptotic program within the cell interior, the Chlamydia infection may trigger an apoptotic pathway that is independent of known caspases, As apoptot ic cells secrete proinflammatory cytokines, Chlamydia-induced apoptosi s may contribute to the inflammatory response of the host.