Project Summary/Abstract Pneumonic plague is the deadliest form of disease caused by Yersinia pestis and the basis for its classification as a Tier 1 Select Agent. In the absence of antibiotics mortality rates approach 100% within a week of pulmonary exposure to Y. pestis. The progression of disease begins with an initial “pre-inflammatory” phase in the lung highlighted by a lack of symptoms or detectable host responses. After 36-48 hours there is an abrupt switch into a “pro-inflammatory” phase of disease characterized by the rapid onset of symptoms and the dramatic accumulation of immune infiltrate in the airways. The objective of the work proposed here is to characterize host factors that contribute to the biphasic progression of pneumonic plague by: 1. Identifying mediators of early host/pathogen interactions; 2. Characterizing the role and mechanism of inflammatory damage in the lung during infection; and 3. Targeting lethal host responses to enhance treatment of late-stage disease. Each of these goals will be approached as follows: Identify host mediators of Y. pestis type III secretion (Aim 1): Use a human siRNA library to identify genes that facilitate injection by the type III secretion system. Evaluate host inflammatory damage during the pro-inflammatory disease phase (Aim 2): Determine the role and mechanism of inflammatory damage in the lung during pneumonic plague using inhibitors and mouse lines deficient in key neutrophil-related functions. Optimize treatment for late-stage pneumonic plague (Aim 3): Combine inhibitors of inflammation with antibiotics to optimize treatment during late-stage, lethal pneumonic plague. Characterizing these key steps during disease is essential to fully understanding the pathogenesis of pneumonic plague. This work will yield crucial insight into how dysregulated host inflammatory responses can manifest as severe necrotizing pneumonia.