Interactions between inflammasomes and the stealth pathogen, Coxiella burnetii
Delaney, Martha Ann
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Coxiella burnetii, a zoonotic pathogen and agent of Q fever, is an obligate intracellular bacterium that hijacks host cell machinery to form a large, acidified replicative vacuole from which it injects proteins into the cytosol through a Type IV secretion system. Type IV secretion is required for C. burnetii intracellular growth and modulation of host cell responses, including apoptosis, a non-inflammatory, caspase-mediated programmed cell death, to establish a replicative niche and persistent infection. In contrast to apoptosis, pyroptosis is a pro-inflammatory, caspase-mediated cell death, and plays a key role in the innate immune response to microbial infections. Pyroptosis is regulated by inflammasomes – macromolecular complexes that contain cytosolic sensors, for example NLRP3 and NLRC4, and the effector cysteine protease, caspase-1. Highly adapted pathogenic bacteria have evolved mechanisms to subvert inflammasomes by direct inhibition or avoiding detection. Hosts have counter-adapted strategies as inflammasomes are dispensable to immune responses in some in vivo models. We found that C. burnetii primes but avoids cytosolic detection by NLRP3 inflammasomes and these interactions are independent of Type IV secretion. We established an in vivo mouse model to determine the biological relevance of inflammasomes during immune response to C. burnetii. Following pulmonary infection, mice deficient in caspase-1 or NLRP3 had significantly higher bacterial burdens in the lung than wild-type mice. However, by the end of the 21-day infection, pulmonary bacterial burdens were similarly decreased amongst wild-type and knock-out mice. Furthermore, Nlrp3-/- and Casp1/11-/- mice had similar development and resolution of gross and histologic lesions and macrophage kinetics as for wild-type mice. Together, these data indicate that like other effective pathogens, C. burnetii has developed mechanisms to subvert host inflammasomes. Likewise, hosts utilize other pathways to combat infection further supporting the theory that inflammasomes are not required but are important for the enhancement of adaptive immune responses to pathogens. By identifying mechanisms of inflammasome evasion by C. burnetii, Q fever pathogenesis and pathophysiology is better understood, advancing diagnostic, prophylactic, and treatment strategies.
- Pathology