Dissemination of Orientia tsutsugamushi and Inflammatory Responses in a Murine Model of Scrub Typhus

Abstract

Central aspects in the pathogenesis of scrub typhus, an infection caused by Orientia (O.) tsutsugamushi, have remained obscure. Its organ and cellular tropism are poorly understood.The purpose of this study was to analyze the kinetics of bacterial dissemination and associated inflammatory responses in infected tissues in an experimental scrub typhus mouse model, following infection with the human pathogenic strain Karp. We provide a thorough analysis of O. tsutsugamushi infection in inbred Balb/c mice using footpad inoculation, which is close to the natural way of infection. By a novel, highly sensitive qPCR targeting the multi copy traD genes, we quantitatively monitored the spread of O. tsutsugamushi Karp from the skin inoculation site via the regional lymph node to the internal target organs. The highest bacterial loads were measured in the lung. Using confocal imaging, we also detected O. tsutsugamushi at the single cell level in the lung and found a predominant macrophage rather than endothelial localization. Immunohistochemical analysis of infiltrates in lung and brain revealed differently composed lesions with specific localizations: iNOS-expressing macrophages were frequent in infiltrative parenchymal noduli, but uncommon in perivascular lesions within these organs. Quantitative analysis of the macrophage response by immunohistochemistry in liver, heart, lung and brain demonstrated an early onset of macrophage activation in the liver. Serum levels of interferon (IFN)-γ were increased during the acute infection, and we showed that IFN-γ contributed to iNOS-dependent bacterial growth control.Our data show that upon inoculation to the skin, O. tsutsugamushi spreads systemically to a large number of organs and gives rise to organ-specific inflammation patterns. The findings suggest an essential role for the lung in the pathogenesis of scrub typhus. The model will allow detailed studies on host-pathogen interaction and provide further insight into the pathogenesis of O. tsutsugamushi infection.

Pulmonary Manifestations of Scrub Typhus: Wisdom May Prevail Obstacles

“…Thus, they concluded that BALT and pleuritic infiltrates could contribute to early bacterial degradation, while solitary infected cells in the parenchyma may have escaped immunosurveillance during the initial phase of infection. Importantly, no bacteria were found in CD31-positive endothelial cells despite a close spatial relationship in this study [25].…”

Toll-Like Receptor 2 Recognizes Orientia tsutsugamushi and Increases Susceptibility to Murine Experimental Scrub Typhus

“…TLRs are crucial mediators of resistance to many infections and mediate resistance by limiting pathogen growth. We thus investigated whether TLR2 is required for survival and the control of O. tsutsugamushi Karp growth during the acute infection phase, using our recently developed mouse footpad infection model, which closely approximates natural inoculation via the dermis (16). All wild-type and tlr2 Ϫ/Ϫ mice survived the acute infection phase until day 21 p.i.…”

“…In order to study the influence of TLR2 on host resistance to O. tsutsugamushi, we first used an experimental mouse model based on the dermal inoculation route via the hind footpad (16). In this model, all tlr2 Ϫ/Ϫ mice survived and were able to control bacterial growth as well as wild-type mice did, as shown by the kinetics of the bacterial loads from draining lymph nodes, spleen, and lung.…”

“…Cryostocks of O. tsutsugamushi Karp inocula were generated as previously described (16). Infections were initiated by inoculation of 5 ϫ 10 3 spot-forming units (SFU) in phosphate-buffered saline (PBS) in a volume of 50 l for dermal inoculation in the right hind footpad or in 200 l as an intraperitoneal (i.p.)…”

“…Our study provides evidence for the requirement of TLR2 in the induction of early inflammation. We also studied the in vivo consequences of TLR2 deficiency in experimental mouse infections (16) and show that TLR2 is largely dispensable for protection. Instead, we demonstrate that TLR2-competent animals were more susceptible to O. tsutsugamushi infections.…”