Lack of the IFN-γ signal leads to lethal Orientia tsutsugamushi infection in mice with skin eschar lesions

Abstract

Scrub typhus is an acute febrile disease due to Orientia tsutsugamushi (Ot) infection and can be life-threatening with organ failure, hemorrhage, and fatality. Yet, little is known as to how the host reacts to Ot bacteria at early stages of infection; no reports have addressed the functional roles of type I versus type II interferon (IFN) responses in scrub typhus. In this study, we used comprehensive intradermal (i.d.) inoculation models and two clinically predominant Ot strains (Karp and Gilliam) to uncover early immune events. Karp infection induced sequential expression of Ifnb and Ifng in inflamed skin and draining lymph nodes at days 1 and 3 post-infection. Using double Ifnar1-/-Ifngr1-/- and Stat1-/- mice, we found that deficiency in IFN/STAT1 signaling resulted in lethal infection with profound pathology and skin eschar lesions, that resembled to human scrub typhus. Further analyses demonstrated that deficiency in IFN-{gamma}, but not IFN-I, resulted in impaired NK cell and macrophage activation and uncontrolled bacterial growth and dissemination, leading to metabolic dysregulation, excessive inflammatory cell infiltration, and exacerbated tissue damage. NK cells were found to be the major cellular source of early IFN-{gamma}, contributing to the initial Ot control. In vitro studies with dendritic cell cultures revealed a superior antibacterial effect offered by IFN-{gamma} than IFN-{beta}. Comparative in vivo studies with Karp- and Gilliam-infection revealed a crucial role of IFN-{gamma} signaling in protection against progression of eschar lesions and Ot infection lethality. Additionally, our i.d. mouse models of lethal infection with eschar lesions are promising tools for immunological study and vaccine development for scrub typhus. SummaryScrub typhus can lead to severe complications and even fatality if not treated properly; however, the early host immune responses to Ot bacterium infection remain unclear. This study focused on the functional roles of IFNs in i.d. inoculation mouse models of scrub typhus. We found that mice lacking IFN receptors were highly susceptible to Ot infection, which resulted in severe pathology and skin eschar lesions that resembled to human scrub typhus. Further investigation revealed that the lack of IFN-{gamma}, but not IFN-I, resulted in dysregulated innate immune responses, leading to uncontrolled bacterial burdens and tissue damage. Using IFN-{gamma} reporter mice and neutralizing antibody treatment, we confirmed that NK cells were the major source of early IFN-{gamma}, and thus played a key role in controlling Ot dissemination. Moreover, our comparative studies with two Ot strains revealed bacterium strain- and dose-dependent eschar formation and disease severity. In conclusion, our study highlights the crucial role of IFN-{gamma} signaling in ensuring host protection against Ot infection. Our mouse models resemble skin eschar lesions and lethal infections observed in human disease, offering potential for future immunological studies on scrub typhus.