Sensing microbial life or death: Endosomal RNA receptor employment by Orientia tsutsugamushi in the orchestration of immunity and inflammation

• Applicant: Privatdozent Dr. Christian Keller
• Subject Area: Immunology ; Clinical Infectiology and Tropical Medicine ; Medical Microbiology and Mycology, Hygiene, Molecular Infection Biology
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 528601109

Scrub typhus is a neglected tropical disease endemic mainly in Asia. The WHO estimates approximately 1 million acute cases annually, with 1 billion people living in areas at risk. Transmitted by mites, this potentially fatal rickettsiosis is caused by the obligate intracellular bacterium Orientia tsutsugamushi. No vaccination is available. The severity of the disease correlates with excessive production of cytokines, especially TNF-α. However, it is incompletely understood which microbial ligands and host receptors control inflammation and immunity. We have previously shown that Toll-like receptor (TLR) 2 and CCR2-dependent inflammatory monocytes are among the host factors that influence the course of infection with Orientia. Our preliminary data now show that in primary murine dendritic cells and macrophages, endosomal RNA receptors play an important role in innate recognition of Orientia. Interestingly, live and dead Orientia addressed these receptors differently: live Orientia induced cytokines (e.g., TNF-α) via TLR7, a receptor for single-stranded RNA. Heat-inactivated bacteria, on the other hand, triggered cytokine responses via TLR13, a receptor for ribosomal RNA (rRNA). The goal of the proposed project is to decipher the mechanisms by which endosomal RNA receptors drive immunity and inflammation against Orientia. It is hypothesized that TLR7, as a sensor for RNA from live bacteria, may play a role in the activation of the immune response, whereas TLR13, as a sensor for released rRNA after bacteriolysis, may play a role in its termination or in the development of immunopathology. To answer these questions, we will combine cell biological and in vivo approaches. Specifically, we pursue the following goals: In work package 1, we will define the cell types in which recognition of Orientia via TLR7/TLR13 occurs. In addition, we will investigate the dependence of TLR7-/TLR13-dependent recognition on bacterial RNA transcription. Similarly, we address the question of whether recognition of the two recently identified developmental stages of Orientia occurs via different RNA receptors. In work package 2, we will perform Orientia infections in TLR7- and TLR13-deficient mice to elucidate the role of these receptors in immune activation and protection, termination of the immune response, and induction of immunopathology. In work package 3, we will use dual RNAseq to analyze the TLR7-/TLR13-dependent transcriptomes of host and Orientia, and reconstruct regulatory networks. In work package 4, we will investigate the role of human TLR7 and TLR8 in Orientia RNA recognition. The study will improve the understanding of the role of RNA receptors in the pathogenesis of Tsutsugamushi fever, paving the way for new therapeutic and preventive strategies.

• DFG Programme: Research Grants