Tick-borne rickettsial diseases continue to cause severe illness and death in otherwise healthy adults and children worldwide. The incidence rate of rickettsial diseases significantly increased in the past six years with case fatality rate as high as 10%. Patients with rickettsial diseases respond well to doxycycline if the antibiotics are administered early. However, doxycycline is not appropriate for patients in pregnancy or under age of 5 years old. Therefore, host-mechanisms based therapeutic interventions are urgently needed. Our long term goal is to study the key host molecules by which obligately intracellular bacteria, rickettsiae, evolve to establish their infection niche in mammalian host cells. These studies will provide insights into novel immune therapeutic interventions. The objectives of this proposal are to reveal the mechanisms by which R. australis subvert autophagy to facilitate infection in macrophages. The hypothesis to be tested in this proposal is that R. australis modulates and exploits Atg5-dependent autophagy to facilitate infection via inhibiting the maturation of autophagosomes and counteracting host immune controls in macrophages. We will evaluate the potential of autophagy inhibition as a therapeutic strategy against life-threatening rickettsial infection by interrupting the pathways involved in R. australis-induced Atg5-dependent autophagosomes. This project will study this hypothesis utilizing two linked specific aims. Aim 1 will determine the mechanisms by which R. australis induces and subverts Atg5-dependent autophagosomes for the benefit of their infection in macrophages. The role of ULK1 in regulating rickettsiae-induced autophagosomes will be investigated. We will study the involvement of MAVS/IRF7/ISG54 and GTPase Rab 25 in counteracting the antibacterial activity and supporting infection in macrophages. Aim 2 will evaluate the potential of autophagy inhibition as a therapeutic strategy against life-threatening rickettsial infection. We will employ the in vivo mouse model of rickettsial infection to investigate the potential of targeting Atg5 (+) autophagosomes as immune-therapeutic interventions by using specific inhibitors or stimulators of key molecules regulating autophagosomes. It is believed that completion of the specific aims of this proposal may provide novel insights into immuno- therapeutic strategies for treating fatal rickettsioses.