Manipulation of intracellular microbial sensors by the stealth pathogen Coxiella burnetii – ST-Health

Along evolution, intracellular bacterial pathogens have developed elaborated means to counter the immune response of their host in order to establish optimal replicative niches in which they replicate in high numbers. This is achieved by the secretion of bacterial effector proteins that divert the function of host cell proteins to create a pathogen-specific compartment and to dampen the cell response resulting from their invasion. Understanding the host/pathogen interactions that prevent detection and clearance of bacterial pathogens is thus of prime importance to counter infections. Coxiella burnetii is a highly infectious class 3 pathogen causing the zoonosis Q fever. The bacterium develops in the placental cells of contaminated wild animals or cattle without causing any symptoms of disease. Following delivery or stillbirths, Coxiella is shed in the environment and humans usually are infected by inhalation of Coxiella-contaminated particles. Acute Q Fever first develops as a flu-like disease but can evolve to a chronic form that can be lethal, as the bacterium is capable of colonising aortic and endocardial tissues. In humans, Coxiella can invade and develop in alveolar macrophages, hepatocytes and trophoblasts. Key to Coxiella infections is the re-routing of multiple trafficking pathways of the infected cell for the formation of an intracellular replicative niche named Coxiella-Containing Vacuole (CCV) as well as the inhibition of apoptosis. These processes are driven by effector proteins secreted by the bacterium into the host cell cytoplasm via a Dot/Icm Type 4b Secretion System (T4SS). Our laboratory and others have shown that Coxiella can dampen the inflammatory response of infected cells allowing its persistence in the host, as a stealth pathogen. Indeed, Coxiella modulates innate immune signalling in a T4SS-dependent manner, with effectors NopA and IcaA playing a role in the inhibition of nuclear transport of NF-kB and non-canonical inflammasome activity, respectively. Additionally, analysis of our Coxiella mutant library led to the identification of bacterial genes required for cytoprotection of the infected cells.
Among the identified mutants, I focused on Coxiella icaB::Tn mutant and discovered that IcaB is the first Coxiella effector protein capable of interacting with a set of cytosolic sensors of the NOD-like receptors family (NLRs). These NLR proteins play a crucial role in the recognition of intracellular pathogens and the inflammatory response of immune and placental cells. It is thus of prime importance to study the interactions of Coxiella with host NLRs and how this microbe manipulates the inflammasome pathway to evade cytosolic microbial sensors.

With the present project, I aim at: 1) characterising the NLR-interacting effector protein IcaB, 2) determining which NLRs are manipulated by Coxiella during infection, 3) identifying additional bacterial effectors targeting NLRs. This project will help to further understand how innate immune sensing is manipulated during Coxiella infections and unveil the mechanisms that render Coxiella stealth in vivo. It will open the path for the development of antimicrobial molecules to treat Coxiella infections and for the repurposing of bacterial products as new anti-inflammatory molecules.