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Infection and Immunity, July 2006, p. 4246-4253, Vol. 74, No. 7
0019-9567/06/$08.00+0     doi:10.1128/IAI.01620-05
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Exploitation of the Endocytic Pathway by Orientia tsutsugamushi in Nonprofessional Phagocytes

Department of Microbiology and Immunology, Seoul National University College of Medicine, 28 Yongon-Dong, Chongno-Gu, Seoul 110-799, Republic of Korea

Received 4 October 2005/ Returned for modification 21 November 2005/ Accepted 11 April 2006

Orientia tsutsugamushi, a causative agent of scrub typhus, is an obligate intracellular bacterium that requires the exploitation of the endocytic pathway in the host cell. We observed the localization of O. tsutsugamushi with clathrin or adaptor protein 2 within 30 min after the infection of nonprofessional phagocytes. We have further confirmed that the infectivity of O. tsutsugamushi is significantly reduced by drugs that block clathrin-mediated endocytosis but not by filipin III, an inhibitor that blocks caveola-mediated endocytosis. In the present study, with a confocal microscope, O. tsutsugamushi was sequentially colocalized with the early and late endosomal markers EEA1 and LAMP2, respectively, within 1 h after infection. The colocalization of O. tsutsugamushi organisms with EEA1 and LAMP2 gradually disappeared until 2 h postinfection, and then free O. tsutsugamushi organisms were found in the cytoplasm. When the acidification of endocytic vesicles was blocked by treating the cells with NH₄Cl or bafilomycin A, the escape of O. tsutsugamushi organisms from the endocytic pathway was severely impaired, and the infectivity of O. tsutsugamushi was drastically reduced. To our knowledge, this is the first report that the invasion of O. tsutsugamushi is dependent on the clathrin-dependent endocytic pathway and the acidification process of the endocytic vesicles in nonprofessional phagocytes.

Editor: F. C. Fang