Establishment and Improvement of Preclinical Murine Models of Severe Scrub Typhus to Define Immune Signatures and Pathogenic Mechanisms

Scrub typhus is a leading source of febrile illness in endemic countries due to infection with Orientia tsutsugamushi (Ot), a seriously understudied intracellular bacterium. Scrub typhus causes 150,000 deaths per year, with an estimated 1 million cases, and a further 1 billion people at risk. No vaccine is available for this reemerging and severely neglected infection. Frustratingly, scrub typhus can result from infection due to a variety of Ot strains with limited cross protection. However, little is understood about strain-specific virulence factors or host immune response. While there are many established murine models of scrub typhus, differences in route of infection, host resistance, and clinically unrelated pathologic outcomes diminish the usefulness of model studies. Therefore, there is a great need for the development of small animal models that reproducibly mimic human diseases for immunologic investigation and future vaccine studies. Our recent studies have established a new inbred murine model of hematogenous spread. To expand these models, we have established an outbred murine model of hematogenous transmission that mimics tissue tropism and pathologic lesions of scrub typhus patients. Interestingly, we find this outbred model exhibits the same pro-inflammatory/Th1 skewed immune response as seen in our previously established inbred model. We further developed our outbred and inbred models by using two clinically prevalent O. tsutsugamushi strains, Karp and Gilliam, and reveal cellular immune responses in inflamed lungs and potential biomarkers of disease severity. We find that outbred CD-1 mice are highly susceptible to both Karp and Gilliam strains; however, C57BL/6 (B6) mice are susceptible to Karp, but resistant to Gilliam (with self-limiting infection). Our outbred preclinical model is particularly useful for future translational and vaccine studies for severe scrub typhus. This body of work provides the first evidence of differential tissue cellular responses in an inbred model against Karp vs. Gilliam infection, thus offering a framework for future investigation of Ot strain-related mechanisms of disease pathogenesis.