Exploitation of the Endocytic Pathway by Orientia tsutsugamushi in Nonprofessional Phagocytes

Abstract

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 NH 4 Cl 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.

Peptidoglycan in obligate intracellular bacteria

“…Rickettsiaceae use a zipper‐like mechanism for uptake into the target cell (Ihn et al ., ; Lee et al ., ; Cho et al ., ). Once inside the cell, they escape from membrane‐enclosed vacuoles in the endolysosomal pathway and undergo growth and replication directly in the host cytosol (Chu et al ., ). The bacteria are therefore directly exposed to autophagy machinery (Choi et al ., ; Ko et al ., ) and cytosolic immune receptors such as Nod1 and Nod2 (Cho et al ., ), and this has likely put selective pressure on these organisms to minimise receptor activation and downstream effectors.…”

Polarized Lung Inflammation and Tie2/Angiopoietin-Mediated Endothelial Dysfunction during SevereOrientia tsutsugamushiInfection

“…Being an obligate intracellular bacterium, O. tsutsugamushi can infect a variety of host cells but primarily replicate in macrophages (MΦs)[8], dendritic cells [9], and endothelial cells (ECs) [6, 8]. The bacteria enter host cells via the phagosome [10] or endosome [11], which they subsequently escape to begin replication in the cytoplasm. Infection-triggered cellular responses, including the activation of activator protein-1 (AP-1) and NF-κB pathways, the production of proinflammatory cytokines (IL-1β, TNF-α, IL-8/CXCL8), and the expression of distinct gene profiles, have been examined in vitro by using primary human umbilical vein endothelial cells (HUVEC) [12], human epithelial/EC-like ECV304 cell line [13], human monocytes or MΦs [14].…”

Evidence for a peptidoglycan‐like structure in Orientia tsutsugamushi

“…Orientia is able to infect a range of cell types, including endothelial, fibroblast, monocyte/macrophage and dendritic cells (Moron et al ., ; Paris et al ., ; Keller et al ., ). Similar to the other Rickettsiaceae , but in contrast to the Chlamydia and Anaplasmataceae that live within remodelled vacuoles (Bastidas et al ., ; Moumène and Meyer, ), Orientia escapes from the endo‐lysosomal pathway shortly after infection and replicates freely in the host cell cytoplasm (Chu et al ., ).…”