Mitogen-activated protein kinases and tumor necrosis factor alpha responses of macrophages infected with Orientia tsutsugamushi

Abstract

Orientia tsutsugamushi, an obligatory intracellular bacterium, is the causative agent of scrub typhus. The disease is histopathologically characterized by inflammatory manifestations, indicating that orientiae induce mechanisms that amplify the inflammatory response. Here we investigated the transcriptional response of tumor necrosis factor alpha (TNF-α) gene and the host cell responses of nuclear factor-kappa B (NF-κB) and mitogen-activated protein kinases (MAPKs) to O. tsutsugamushi. Induction of the TNF-α gene was not blocked by a eukaryotic protein synthesis inhibitor cycloheximide, suggesting that de novo synthesis of host cell protein is not required for this transcriptional response. The induction of TNF-α mRNA by O. tsutsugamushi was not blocked by the inhibitors of NF-κB activation. Inactivation of Orientia by heat abolished both MAPK activation and TNF-α production, whereas treatment of host cells with cytochalasin D impaired neither NF-κB and MAPK activation nor TNF-α production. In conclusion, our data suggest that O. tsutsugamushi-stimulated TNF-α production and MAPK activation requires cellular attachment by viable orientiae but not bacterial internalization into the host cells.