Inhalation of aerosolized droplets containing Yersinia pestis causes primary pneumonic plague, the most lethal manifestation of plague. The mortality rate of pneumonic plague is nearly 100% within 4 to 7 days after infection without timely antibiotic treatment. Unfortunately, early symptoms are nondescript and thus pneumonic plague is rarely diagnosed in time for effective treatment. Due to high infectivity and mortality rates, the CDC classifies Y. pestis as a Tier 1 Select Agent with potential as a bioterrorism weapon. Pneumonic plague disease progression is biphasic, with an initial asymptomatic pre-inflammatory phase followed by a lethal pro-inflammatory phase. The pro-inflammatory phase is characterized by the rapid expansion of neutrophil-rich lung lesions. The influx of neutrophils to the site infection is a normal defense response, but in the case of pneumonic plague, neutrophils fail to control bacterial growth and are the major cause of pulmonary inflammation. Understanding neutrophil dysfunction during pneumonic plague may suggest host- oriented therapeutic targets for tempering inflammation during severe pneumonia and improving antibiotic efficacy. Upon encountering extracellular bacteria, neutrophils secrete highly pro-inflammatory and anti-microbial granule proteins in a process called degranulation. Because neutrophils cannot control Y. pestis growth in the lungs, degranulation may be inhibited during pneumonic plague.
In Aim 1, I propose to characterize the effect of Y. pestis infection on neutrophil degranulation. My preliminary data reveals that Y. pestis alters neutrophil degranulation by a type III secretion system dependent mechanism. Using T3SS effector deletion Y. pestis strains I have created, I will measure intracellular and extracellular degranulation using flow cytometry and western blotting for granule markers and secreted granule proteins in isolated human neutrophils and during in vivo infection of our mouse model. Our group recently observed a decrease in apoptotic gene expression in inflammatory lung lesions and in neutrophils in vivo. A failure of neutrophils to undergo apoptosis and clear from the lungs contributes to severity of inflammation.
In Aim 2, we propose to uncover the mechanisms by which neutrophil survival is prolonged during Y. pestis infection using our in vivo and in vitro infection models. We will determine how T3SS effectors alter neutrophil cell death during infection and test the therapeutic effect of a pro-apoptotic agent on disease outcome for pneumonic, bubonic, and septicemic plague.
Inhalation of the bacterium Yersinia pestis causes primary pneumonic plague, a highly lethal and rapidly progressing infection that can be fatal within a week of exposure. The severity of pneumonic plague results from a hyper-inflammatory immune response that damages the lung but fails to control bacterial growth. In this proposal, we seek to understand the mechanisms for severe lung inflammation that occurs following Y. pestis inhalation.
