Apoptosis of Murine Macrophage-like Cells Infected with Orientia tsutsugamushi

ABSTRACT

A large number of bacterial pathogens have been identified as mediators of apoptosis in vitro and the induction of apoptosis might be an important step in the pathogenesis of these bacteria. In this study, we analyzed the interactions of Orientia tsutsugamuchi with J774 murine macrophage-like cells. The J774 cells were infected with Boryong strain of O. tsutsugamushi and the DNA was analyzed with agarose gel electrophoresis. We observed the typical laddering pattern of DNA fragmentation indicative of apoptosis in infected cells but not cells infected with heat-killed O. tsutsugamushi. We performed terminal deoxynucleotidyl transferase (TdT) assay to label the 3'-hydroxy ends of DNA breaks and observed intense brown staining of nuclei of infected macrophages. With Hoechst 33258 for staining nucleus, strong chromatin condensation was observed only in infected J774 cells. We also examined the cytokine secretion pattern of J774 cells during the rickettsial infection. The large amount of TNF-alpha and IL-10 were secreted after 24 hrs of infection, but the secretion of IL-1beta was increased in small amount. These results showed that O. tsutsugamushi induce apoptosis in murine macrophage-like cells by different mechanism from that of shigella which cause secretion of large amount of IL-1beta.