The basis of strain-specific differences in virulence in the human pathogen Orientia tsutsugamushi - PROJECT SUMMARY
Vector-borne obligate intracellular bacteria are an important cause of morbidity and mortality worldwide. One
such pathogen, Orientia tsutsugamushi, causes the neglected tropical disease scrub typhus, which is
estimated to cause infection in at least 1 million people each year, and is associated with high mortality if not
treated promptly and with the appropriate antibiotics. The species O. tsutsugamushi is characterised by high
strain diversity, where different strains exhibit different levels of virulence in patients and animal models. A
major factor in the pathogenicity induced by O. tsutsugamushi is an over-activation of the inflammatory
immune response. However, the detailed mechanisms describing how O. tsutsugamushi induces this response
remain unknown. O. tsutsugamushi is genetically intractable and therefore functional genomics based
approaches to identify virulence factors are not possible. Here we propose to take advantage of strain-specific
natural variation to uncover key pathways important for virulence. In our preliminary work we have quantified
the virulence of two clinical isolate strains of O. tsutsugamushi, Karp and UT176, and shown that they have
significantly different outcomes in a mouse model. We have also used dual RNAseq to identify both bacterial
and host genes that are differentially expressed between these two strains when grown in human cultured cells
(HUVEC), and identified bacterial (surface proteins TSA22, TSA47, TSA56 and Ankyrins Ank6, Ank16, Ank17,
Ank20) and host (SOCS2, IL33, other chemokines and cytokines) factors that strongly correlate with virulence.
We have also recently generated complete genome sequences for 6 strains of O. tsutsugamushi, bringing the
total number of completely sequenced strains up to 8. These strains will all be used in this study. Our
preliminary work and previous reports led us to the hypothesis that strain specific differences in the
virulence of O. tsutsugamushi are driven by differences in expression of bacterial virulence-related
genes, resulting in either pathogen clearance or inflammatory induced pathogenesis. This hypothesis
will be addressed in the following aims. In Aim 1, we will increase the power of our preliminary correlative
analysis by quantifying the virulence and gene expression profiles of a total of 8 strains of O. tsutsugamushi,
and will use this information to identify bacterial and host factors that robustly associate with virulence. In Aim
2, we will use an in vitro immune reporter assay to probe the role of specific bacterial genes in virulence,
focusing on the factors identified in our preliminary studies and in Aim 1. Results of this study will guide future
work on developing novel diagnostic tools and therapies against this pathogen.
